Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing

FINAL MASTER THESIS Master in Interdisciplinary and Innovative Engineering TEXTURE OPTIMIZATION OF STAINLESS STEEL ALLOYS SURFACES THROUGH VIBRATION-ASSISTED BALL BURNISHING Report and Annex Author: Murat Sarp KOÇAK Supervisor: Ramón JEREZ MESA Department Department of Mechanical Engineering Call: 2024, May Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Abstract This thesis investigates the impact of ultrasonic vibration-assisted ball burnishing (VABB) on the surface properties of VH13IM® (1.2344), a hot work tool steel, compared to traditional ball burnishing methods. The primary objective was to enhance surface quality and determine the most effective combinations of burnishing strategies, lateral offsets, and preload values. This research was driven by the hypothesis that ultrasonic vibrations significantly modify the material's plastic behavior under mechanical stress, facilitating more effective surface deformation and property enhancements. The study utilized a full factorial design to assess various surface roughness parameters under different burnishing strategies and offsets. The findings indicate that VABB, particularly with a 0.05 mm lateral offset and lower preload values, consistently outperforms non-vibrated ball burnishing (NVABB) across all measured parameters. The 0.05 mm offset was found to be most effective in enhancing surface texture, especially in parallel and diagonal strategies, providing superior control over surface roughness and contributing to increased durability and functionality of the components. The conclusions drawn from the analysis recommend the parallel strategy with a 0.05 mm offset as optimal for achieving excellent surface characteristics, essential for applications requiring high precision and minimal deviation. These insights offer a strategic roadmap for optimizing ball burnishing processes in industrial settings, aligning methodological choices with specific performance requirements and industry standards to significantly improve component quality and reliability. This thesis contributes valuable insights into the mechanics of acoustoplasticity and its practical applications in materials engineering, presenting a compelling case for the broader adoption of VABB technologies in manufacturing. 1 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Resum Aquesta tesi investiga l'impacte del brunyit de boles assistit per vibracions ultrasòniques (VABB) en les propietats superficials de l'acer per a eines de treball en calent VH13IM® (1.2344), en comparació amb els mètodes tradicionals de brunyit amb bola. L'objectiu principal és millorar la qualitat superficial i determinar les combinacions més efectives d'estratègies de brunyit, passos laterals i valors de precàrrega. Aquesta recerca va estar impulsada per la hipòtesi que les vibracions ultrasòniques modifiquen significativament el comportament plàstic del material sota estrès mecànic, facilitant una deformació superficial més efectiva i millores en les propietats. L'estudi va utilitzar un disseny factorial complet per avaluar diversos paràmetres de rugositat superficial sota diferents estratègies de brunyit i desplaçaments. Els resultats indiquen que VABB, especialment amb un desplaçament lateral de 0,05 mm i valors de precàrrega més baixos, supera consistentment el brunyit de boles sense vibració (NVABB) en tots els paràmetres mesurats. Es va trobar que el desplaçament de 0,05 mm és el més efectiu per millorar la textura superficial, especialment en estratègies paral·leles i diagonals, proporcionant un control superior sobre la rugositat superficial i contribuint a una major durabilitat i funcionalitat dels components. Les conclusions extretes de l'anàlisi recomanen l'estratègia paral·lela amb un desplaçament de 0,05 mm com a òptima per assolir característiques superficials excel·lents, essencials per a aplicacions que requereixen alta precisió i mínima desviació. Aquestes percepcions ofereixen un mapa estratègic per optimitzar els processos de brunyit de boles en entorns industrials, alineant les opcions metodològiques amb requisits de rendiment específics i normes industrials per millorar significativament la qualitat i la fiabilitat dels components. Aquesta tesi aporta coneixements valuosos sobre la mecànica de l'acoustoplasticitat i les seves aplicacions pràctiques en l'enginyeria de materials, presentant un cas convincent per a l'adopció més àmplia de les tecnologies VABB en la fabricació. 2 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Resumen Esta tesis investiga el impacto del bruñido de bolas asistido por vibraciones ultrasónicas (VABB) en las propiedades superficiales del acero de herramientas para trabajo en caliente VH13IM® (1.2344), en comparación con los métodos tradicionales de bruñido con bola. El objetivo principal esmejorar la calidad superficial y determinar las combinaciones más efectivas de estrategias de bruñido, desplazamientos laterales y valores de precarga. Esta investigación estuvo impulsada por la hipótesis de que las vibraciones ultrasónicas modifican significativamente el comportamiento plástico del material bajo estrés mecánico, facilitando una deformación superficial más efectiva y mejoras en las propiedades. El estudio utilizó un diseño factorial completo para evaluar varios parámetros de rugosidad superficial bajo diferentes estrategias de bruñido y desplazamientos. Los hallazgos indican que VABB, especialmente con un desplazamiento lateral de 0,05 mm y valores de precarga más bajos, supera consistentemente al bruñido de bolas sin vibración (NVABB) en todos los parámetros medidos. Se encontró que el desplazamiento de 0,05 mm es el más efectivo para mejorar la textura superficial, especialmente en estrategias paralelas y diagonales, proporcionando un control superior sobre la rugosidad superficial y contribuyendo a la mayor durabilidad y funcionalidad de los componentes. Las conclusiones extraídas del análisis recomiendan la estrategia paralela con un desplazamiento de 0,05 mm como óptima para lograr características superficiales excelentes, esenciales para aplicaciones que requieren alta precisión y mínima desviación. Estos conocimientos ofrecen un mapa estratégico para optimizar los procesos de bruñido de bolas en entornos industriales, alineando las elecciones metodológicas con requisitos de rendimiento específicos y normas industriales para mejorar significativamente la calidad y fiabilidad de los componentes. Esta tesis aporta valiosos conocimientos sobre la mecánica de la acoustoplasticidad y sus aplicaciones prácticas en la ingeniería de materiales, presentando un caso convincente para la adopción más amplia de las tecnologías VABB en la fabricación. 3 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Appreciations I am deeply grateful for the support and encouragement I have received throughout the course of this thesis, which has been instrumental in the completion of my research. It is a pleasure to extend my heartfelt thanks to all those who made this journey a rewarding and educational experience. First and foremost, I wish to express my profound appreciation to my advisor, Ramón Jerez Mesa, whose expert guidance and insightful critiques have been invaluable. His patience and academic rigor have significantly shaped this work, and his encouragement was crucial in surmounting the challenges I faced. I would also like to acknowledge the hardworking staff and technicians at Universitat Politècnica de Catalunya whose expertise and assistance were essential in the experimental part of my research. Their readiness to help and ability to solve complex problems have been greatly appreciated and have not gone unnoticed. To my friends and classmates, thank you for the camaraderie and peer support throughout our studies together. The discussions we shared and the mutual encouragement we offered each other provided a stimulating academic environment and made my experience at the university all the more enjoyable. Most importantly, I owe my deepest gratitude to my family. Their unwavering support and unconditional love provided the foundation upon which I built my aspirations. Thank you for believing in me, encouraging me to pursue my goals, and providing the support I needed away from home. This thesis would not have been possible without the collective support and encouragement of each individual mentioned and many others who, although not named, have contributed to my academic journey. Thank you all for your part in my path. 4 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Glossary ACRONYMS VABB - Vibration Assisted Ball Burnishing NVABB - Non-Vibration Assisted Ball Burnishing DR - Deep Rolling LPB - Low Plasticity Burnishing NC - Numerical Control CNC - Computer Numerical Control RMS - Root Mean Square AACFA - Areal Autocorrelation Function SYMBOLS mm - Millimeter μm - Micrometer - Depth of penetration ℎ𝑝 - Static Preload 𝐹𝑝 𝐻𝑉 - Vickers hardness of the material 𝑅Radius of the indenter - Spring's elastic constant 𝑘 - Compressed length of the spring ∆𝑧 - Stress concentration factor 𝐾𝑡 5 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing - Maximum stress σ𝑛𝑜𝑚 - Geometrical factor𝑌 - Cracks length𝑎 - Oscillating burnishing force 𝐹𝑏 - Dynamic oscillatory force 𝐹𝑣 - Force experiences variations η - Change of thickness in polarized piezoelectric material ∆𝑡 - the number of piezoelectric disk layers in the stack 𝑛 - Piezoelectric charge coefficient 𝑑33 - Applied voltage across the piezoelectric material 𝑉 np - Number of passes Vf - Feed - Root Mean Square Height 𝑆𝑞 - Skewness 𝑆𝑠𝑘 - Kurtosis 𝑆𝑘𝑢 - Ten-Point Height 𝑆10𝑧 - Arithmetic Mean Height 𝑆𝑎 - Maximum Peak Height 𝑆𝑝 - Maximum Valley Height 𝑆𝑣 - Fastest Decay Autocorrelation Length 𝑆𝑎𝑙 6 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing - Texture Aspect Ratio 𝑆𝑡𝑟 - Developed Interfacial Area Ratio𝑆𝑑𝑟 - Root Mean Square Gradient𝑆𝑑𝑞 - Texture Direction Angle 𝑆𝑡𝑑 - Core Distance 𝑆𝑘 - Reduced Peak Height 𝑆𝑝𝑘 - Reduced Valley Height 𝑆𝑣𝑘 - Peak Material Ratio 𝑀𝑟1 - Valley Material Ratio 𝑀𝑟2 7 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing systematically explore three key variables: burnishing strategy, lateral offset, and preload values. Each of these variables was chosen based on their potential impact on the surface properties of the alloy. Specifically, the scope of this research includes: ●Experimental Design: Conducting experiments exclusively on VH13IM to ensure that the findings are directly applicable to this alloy. The experiments utilize standardized sample dimensions and heat treatment protocols to maintain consistency across all test scenarios. ●Technological Limitations: The use of ultrasonic vibration technology specifically configured for ball burnishing processes. The research is limited to the capabilities and settings of the available ultrasonic equipment, which may affect the generalizability of the results to other forms of ultrasonic or mechanical surface enhancement technologies. ●Data Analysis: Utilization of specific statistical tools and software for data analysis to determine the effects of the experimental variables. The analysis is confined to the data collected from the described experiments. ●Future Research: Recognizing the limitations of this study, suggestions for future research might include expanding the range of materials tested, exploring additional surface properties, or employing different parameters and equipment. 5 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing 3.State of the Art This chapter unfolds the findings of a comprehensive literature review. Initially, the chapter offers a descriptive overview of the burnishing processes and different burnishing techniques, studies of burnishing tools, and tool geometries, an explanation of burnishing parameters, and integration of vibration assistance into the ball burnishing process. Later, more studies on surface integrity parameters will be explained. Finally, the mechanical construction of the burnishing process and experimental setup will be discussed. 3.1.General Aspects of Ball Burnishing In the burnishing process, the material conforming the target surface undergoes plastic deformation, leading to a highly polished surface without removing any material, distinguishing it as a chip-less technique. This aspect offers several benefits over alternative finishing methods such as honing, lapping, and grinding. The process involves cold working the material under a force slightly above its yield strength, causing plastic deformation to a different extent depending on the stress caused by that external force. As a result, not only is the surface finish enhanced, but there are also improvements in wear resistance, fatigue strength, resistance to foreign objects, and the microhardness of the component's surface [2]. In the ball burnishing process, a ball that freely rotates is applied against a workpiece's surface under a normal force, creating compressive stress in the top layer of the material. This action precisely alters the surface's texture by smoothing out its irregularities. Due to the substantial burnishing force, there is a plastic flattening of the raised areas into the recessed ones on the surface. The tools utilized for burnishing can either glide across or roll over the surface to effect this transformation [22]. Figure 3.1: General overview of a vibration-assisted ball burnishing process [8]. 6 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing The ball burnishing process illustrated in Figure 3.1 is done along the workpiece with consecutive passes closely positioned next to each other to treat the designated surface uniformly. The material at the surface's peaks is compressed almost directly downward, and if the applied force is high enough, even the valleys could be altered, reconstructing the overall surface [21]. Since the alterations in topology are very limited and the overall geometry is kept unchanged, the process is used for finishing purposes. This method is generally used for high-precision purposes such as mold production, fileted bearing components, and curved surfaces [11]. There are different ways to classify different styles of burnishing techniques. The intended outcome of the process is one of them. Deep Rolling (DR) and Low Plasticity Burnishing (LPB) are two burnishing methods. Whereas DR aims to induce a high compressive residual stress and enhance the cold work on the surface, LPB aims to reduce the surface roughness and is not oriented to increasing the residual stress [11]. This differentiation between DR and LPB underscores the versatility of burnishing techniques in meeting different manufacturing objectives, from stress improvement to surface smoothing. These different classifications of burnishing systems are categorized by changing the components and factors of the burnishing. Some of these factors include the tool end's shape, the tool's material composition, the force application technique, and the machine used. Choosing the correct components is key to achieving the desired surface finishes. 3.1.1. Burnishing Tool The selection and design of a burnishing tool are crucial when planning to execute a successful burnishing operation. The tools play a pivotal role in shaping the burnishing system and provide control over the key parameters of the burnishing process and ultimately influence the outcome. Understanding different burnishing tool configurations as shown in Figure 3.2 is essential in determining the basic design features for a Vibration Assisted Ball Burnishing (VABB) tool. This involves considering factors such as tool material, geometry, compatibility with machinery, and adjustability of parameters like force, speed, and feed rate, which are instrumental in achieving the desired surface finish and material properties. 7 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Figure 3.2:Burnishing tool configurations on lathe and milling machines [11]. Initially, the first applications of the burnishing process were on very simple geometries such as bores, shafts, filets, etc [20]. The widespread adoption of Numerical Control (NC) machines in manufacturing allowed the finishing of more intricate parts. With the help of automation and robotics, the surface finish improved drastically and significantly broadened the scope of utility and applications. This evolution reflects a crucial shift in manufacturing practices, enabling higher precision and versatility in surface finishing techniques. Another key element that characterizes a burnishing tool is the design of the indenter, the tooltip that compresses the surface of the workpiece. The two main indenters are roller-shaped tools and ball-shaped tools. The roller tools create a fine finish by rolling cylindrical shaped rollers roll over a surface. The ball-shaped tools have a spherical ball on the tip instead of a cylindrical shape. Due to this difference in shape, the roller burnishing tool has higher feed rates and takes less time to finish. Despite this advantage, rollers do not match the adaptability provided by ball indenters, especially when it comes to finishing complex surfaces that are increasingly common in advanced manufacturing sectors [11]. 8 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing A thorough examination of the relevant literature suggests that most studies on ball burnishing have concentrated on applications involving lathes. While there are some similarities between lathe and milling burnishing—such as the comparison of lateral offset in milling to feed in lathe burnishing—the two processes exhibit distinct kinematic behaviors, warranting their consideration as separate entities. For the purpose of this thesis, which is focused on a tool for use in a milling machine, attention will primarily be given to milling setups. This selective approach is also motivated by the inherent limitations of lathe burnishing in addressing the surface of the workpiece that will be used in this dissertation. This distinction underscores the necessity to tailor research and tool development efforts to the specificities of milling burnishing, given its relevance and potential to enhance surface integrity in complex geometrical applications. 3.1.2. Burnishing Parameters In the field of manufacturing engineering, to meet design requirements and get consistent results, it is necessary to meticulously control the process parameters with such precision. This focus on parameter adjustment highlights the critical nature of understanding and controlling the factors that influence the burnishing process, including but not limited to the force applied, the speed of the tool, the feed rate, the hardness of the indenter, and the specific characteristics of the workpiece material. Each of these parameters can significantly affect the quality of the finished product, impacting surface roughness, hardness, and other key attributes. As such, a comprehensive understanding of how to manipulate these variables within optimal ranges is crucial for achieving desired outcomes in burnishing operations. 3.1.2.1. Burnishing Force The burnishing force is the normal force, as shown in Figure 3.3. This normal force is actually the preload that is applied by the burnishing tool onto the surface of the workpiece. Since this preload is the main reason for the plastic deformation which effectively alters the surface characteristic and structural integrity, it stands as the most critical parameter influencing the outcomes of the ball burnishing process. Also, this preload has a vibratory component when the vibration assistance is used during the burnishing process. While the normal force applied in the burnishing process can be controlled, its value tends to fluctuate throughout the operation. This variability primarily stems from the friction encountered as the tool moves across the workpiece surface. 9 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Figure 3.3:Burnishing process [16]. 3.1.2.1.1. Depth of Penetration The depth of penetration method is recognized as one of the first techniques for regulating burnishing force. In this approach, the tool applies force to the workpiece surface by pressing the tooltip against it. The force is adjusted by tightening a screw along the supporting rod where the burnishing ball is located. The distance the screw is tightened is referred to as the depth of penetration ( ), and the ℎ𝑝 resulting preload ( ) is calculated based on the interaction of a rigid sphere sliding over a plastic 𝐹𝑝 semi-plane, involving parameters such as the Vickers hardness of the material (𝐻𝑉) and the radius of the indenter (𝑅) [11]. The resulting equation is given below in Eq 3.1. (3.1) 𝐹𝑝= 𝑅 π 4ℎ𝑝𝐻𝑉 3.1.2.1.2. Calibrated Spring Inside the ball burnishing tools, a spring is positioned that manages the normal force applied during the burnishing process. The working mechanism uses the simple equation of Hooke’s Law, Eq. 3.2. The force that is exerted on the workpiece increases proportionally with the compression of the spring as the ball at the tip of the tool engages with the surface. (3.2) 𝐹𝑝= 𝑘 ∆𝑧 10 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing The equation governing this relationship, Eq 3.2, takes into account the spring's elastic constant (𝑘 ), the axial preload ( ), and the compressed length of the spring ( ), which reflects the𝐹𝑝∆𝑧 intentional compression of the tool [11]. 3.1.2.2. Number of Passes The number of passes in a burnishing process refers to how many times the tool is applied over the same area of the target material. This parameter, in conjunction with the applied preload, significantly influences the extent of plastic deformation achieved on the surface by the end of the operation [8]. Essentially, adjusting the number of passes allows for control over the level of surface smoothing and hardening by increasing the work done on the material. The number of passes is not only relevant for the process's efficiency. Processing time is also taken into account since there is a direct correlation between the number of passes and the required processing time. The more passes are done on the surface, the more time will be needed to finish the process. 3.1.2.3. Ball Diameter Both the diameter and the material of the burnishing ball are critical factors that significantly influence the effectiveness and outcomes of the ball burnishing process. These parameters determine how the force is distributed over the workpiece surface and affect the tool's wear resistance and durability. A larger ball diameter will cover more surface area with each pass, potentially reducing the number of passes required to treat the entire surface. This can lead to a smoother finish as larger balls tend to bridge the valleys on the surface, effectively smoothing out the peaks. They also distribute the applied force over a broader area, resulting in less penetration depth per unit of force, which can be gentler on the surface and reduce the risk of deforming delicate parts. Smaller balls provide more localized pressure and can access tighter spaces and intricate features on the workpiece by concentrating the force onto a smaller area. This can be beneficial for achieving detailed surface finishes or working with complex geometries [6]. 3.1.2.4. Feed The feed parameter, often referred to as feed rate in the context of machining and ball burnishing, denotes the linear speed at which the tool moves across the workpiece. It plays a crucial role in determining the effectiveness and efficiency of the ball burnishing process. This value is usually set within the Numerical Control program of the machine tool that executes the process. 11 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Increasing the feed rate can reduce the contact time between the burnishing ball and the burnishing surface, potentially leading to a less smooth finish. At high speeds, the tool may not adequately deform all surface irregularities, resulting in a finish that might not meet certain smoothness criteria. Decreasing the feed rate allows for more prolonged contact between the tool and the surface, promoting better surface deformation and achieving a smoother finish. The increased interaction time helps in effectively flattening high spots and filling in low spots on the material surface. However, lower feed rates also slow down the process, potentially impacting overall productivity. Higher feed rates accelerate the operation but may compromise the finish quality or even the tool's effectiveness if the speed exceeds an optimal threshold. 3.1.2.5. Lateral Offset The lateral offset in the ball burnishing process is another critical parameter that significantly influences the quality and effectiveness of the surface finish. The lateral offset defines the spacing between consecutive burnishing paths. This parameter determines how thoroughly the surface is covered and how overlaps between passes affect the surface integrity. Setting the lateral offset too narrowly necessitates a greater number of passes to cover the entire surface area which ensures that the passes overlap more. This overlap is crucial for achieving a uniform surface finish, as it helps in consistently smoothing the surface by ensuring that no area is left unprocessed. A larger offset may lead to gaps between successive passes, potentially leaving some areas less processed than others. On the other hand, lateral offset impacts the efficiency of the process. Narrower offsets, while beneficial for achieving finer surfaces, increase the number of passes needed to cover the entire surface, thereby reducing process throughput. Conversely, wider offsets can increase efficiency by reducing the number of passes required, but this may compromise the finish quality. In conclusion, the lateral offset must be carefully optimized based on the specific requirements of the surface finish, the material being processed, and the overall efficiency desired. Striking the right balance is key to maximizing the benefits of the ball-burnishing process. 3.2.Surface Integrity After Ball Burnishing After a ball burnishing process, the bulk material stays unchanged and only the layers that are close to the surface are impacted. As a result, to evaluate the outcome of the ball burnishing application, it is important to make a detailed assessment of the modifications on the surface of the workpiece. Surface integrity is the key concept to studying surface modifications. By analyzing surface integrity, which includes aspects such as surface roughness, microhardness, residual stress distribution, and microstructural changes, one can gain a full understanding of how VABB 12 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing enhances material properties at the surface level, providing valuable insights into its effectiveness as a finishing process. It used to be believed that the properties of the bulk material were the main reason for the failure of engineering components. Nowadays it is known for a fact that surface integrity has a huge impact on failure too. The fatigue performance of a workpiece is significantly influenced by several factors including surface topography, residual stress, work hardening, and changes in metallurgical structure. These elements interact in complex ways that can either enhance or impair the material's ability to withstand cyclic loads without failing [16]. (3.3) 𝐾𝑡= σ𝑛𝑜𝑚 𝑌 π𝑎 As given above in Eq 3.3, the stress concentration factor ( ), which linked the maximum stress ( 𝐾𝑡 ), a geometrical factor (𝑌), and the crack's length (𝑎), serves as an important metric to assess σ𝑛𝑜𝑚 the influence of surface topography on the fatigue performance of materials, offering insights into how geometric discontinuities affect the stress distribution and, consequently, the material's fatigue life. In this equation, the geometrical factor 𝑌incorporates as a variable influenced by surface roughness establishing a critical connection between surface integrity and fatigue failure [11]. Ball burnishing, recognized for its capacity to enhance surface integrity, has been a subject of scholarly interest, particularly in its application to metals such as steel, aluminum, and copper alloys. This focus aligns with the historical use of burnishing in finishing processes for components like shafts, molds, dies, and tubes—applications that demand improved surface characteristics without altering the part's dimensional accuracy. The process's adaptability to various metals and its effectiveness in enhancing surface properties while maintaining the component's geometry make it a valuable technique in manufacturing practices aimed at extending the lifespan and performance of mechanical parts [11]. 3.3.Vibration Assisted Ball Burnishing Engineering is not only creating new devices or procedures. Sometimes the most effective innovations are enhancing pre-existing processes or devices instead of producing a new thing. A prominent strategy for this kind of innovation is known as process assistance. This approach involves augmenting a traditional manufacturing operation which introduces additional effects into the system. These effects can make cutting or deforming the target material more efficient or achieve qualities not possible with the conventional process alone. By leveraging these additional 13 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing energy sources, manufacturers can improve process efficiency, achieve finer control over material properties, and expand the capabilities of existing manufacturing techniques. This thesis will emphasize on the ball burnishing process that is enhanced by the application of ultrasonic vibrations. The force will be applied to the target surface through spring compression. This innovative approach is based on the principle of acoustoplasticity, which suggests that the application of ultrasonic energy can alter the plastic behavior of materials, facilitating deformation and potentially improving surface finish and material properties. The hypothesis posits that ultrasonic vibrations, when superimposed on the mechanical force exerted during ball burnishing, can lead to significant improvements in the process by making the material more receptive to deformation. This could result in enhanced surface integrity, including reduced surface roughness and improved hardness, without compromising the efficiency of the process [11]. 3.3.1. Acoustoplastic Effect The idea that ultrasonic vibrations can enhance the effectiveness of the burnishing process draws from the phenomenon of acoustoplasticity, also known as the Blaha effect. This effect describes the reduction in the quasi-static stress needed to deform a material when vibrations are applied in conjunction with the deforming force [11]. This means that the addition of vibrations may potentially increase the effect of plastic deformation. Since the ball burnishing process relies on plastic deformation, it can be used to improve surface finish and mechanical properties after ball burnishing. It can actually lead to a variety of benefits, including reduced force requirements, enhanced surface characteristics, and potentially even improved material properties due to the altered microstructural effects of the combined mechanical and vibrational energy inputs. By applying this principle, the ball burnishing process could achieve more significant improvements in surface integrity with potentially less effort and energy consumption compared to traditional methods. This innovative approach to manufacturing reflects a broader interest in leveraging physical phenomena to optimize industrial processes, offering new pathways to efficiency and effectiveness in materials engineering. 3.4. Design of the VABB Tool In this section, the design of the VABB tool will be explained in detail. The development of a new tool design is a very crucial part of any manufacturing process. A process is as good as the tool that has been chosen or created for that specific process. The prototype to be chosen is pivotal for the impact of VABB on improving surface integrity. The tool that will be used in this thesis is 14 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing 4.Methodology This chapter outlines the comprehensive approach taken to characterize the Vibratory Assisted Ball Burnishing process, detailing the methodology from preparation to execution. First of all, preparatory tasks for the experimental plan have been done. In this section, initial steps taken to define the experimental plan for VABB are given. It involves selecting the materials and determining the variables to be tested. This foundational stage is crucial for ensuring that the experiments conducted are both systematic and relevant to the research. After, assessment techniques for VABB results on surface integrity will be provided. Here, the various techniques and methods used to evaluate the effects of VABB on surface integrity are explained. These assessments provide insights into how VABB influences the material properties and surface characteristics of the workpiece. The next chapter focuses on characterizing the material selected for the ball burnishing tests. Understanding the material properties is essential for interpreting the results of VABB, as different materials may respond differently to the process. In the final chapter of this section, the experimental execution of tests will be provided. This final section presents the details of how the VABB tests were carried out, including any non-vibratory assisted ball burnishing (NVABB) tests conducted for comparison. It covers the setup of the experiment, the specific conditions under which each test was performed, and any challenges encountered during the process. This documentation is vital for replicability and for understanding the context in which the results were obtained. Overall, this chapter provides a detailed overview of the methodology used to investigate the VABB process, from planning through to execution, ensuring a thorough understanding of its impact on surface integrity. The deliberate decision to separate methodological details from the presentation of experimental results aims to ensure clarity and maintain the reader's focus. By structuring the document to first lay out the experimental design and techniques before delving into the results and their analysis, you help prevent potential confusion or distraction. Such organization enhances the narrative flow and makes it easier to follow the progression from methodology to results, facilitating a deeper engagement with the conclusions drawn from the data. 4.1. Experimental Design 4.1.1. Full Factorial Experimental Design Design of Experiments (DOE) is a systematic method used extensively in engineering and scientific research to investigate the effects of multiple factors on a specific outcome. The process of conducting a DOE typically involves some crucial steps. First, it is essential to identify the factors that will be manipulated or varied during the experiment, as well as the response variables that 21 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing will be measured and evaluated. This step sets the scope of the study by specifying what aspects are under investigation and what outcomes will be analyzed for changes. Once the factors and response variables are defined, the next step is to design the experimental plan. This plan outlines the order and combination in which the factors will be tested. After conducting the experiments according to the plan, the data collected on the response variables are analyzed using statistical methods. The aim here is to quantify the effects of the factors on the responses and to determine which factors are most influential. By following these steps, DOE enables researchers and engineers to gain insights into the complex relationships between multiple variables in a controlled setting. This methodology not only helps in optimizing processes and products by pinpointing critical factors but also contributes to the development of new knowledge and understanding in various fields of study. A full factorial design of experiments will be used for this research. It is a comprehensive method used in research and industry to investigate the interactions and effects of multiple factors across all possible combinations. This approach allows for a detailed analysis of how independent variables influence dependent variables, providing insights into the behavior and characteristics of a system under various conditions. In a full factorial design, every possible combination of factor levels are tested. 4.1.2. Factors Included in the Study To craft the experimental design and ensure the experiments comprehensively cover the necessary degrees of freedom, it's crucial to precisely define the actions of interest and their study levels. Based on a thorough analysis of previous ball burnishing results and a review of relevant literature, three key factors have been identified for investigation. These factors are selected for their potential impact on the outcomes of the ball burnishing process and their relevance to the objectives of this research. The detailed examination of these factors will involve discussing their roles, potential effects on the process, and how they might interact with one another to influence the final results. This approach ensures a focused yet comprehensive exploration of the variables that are most significant to enhancing the understanding and optimization of the ball burnishing process. 4.1.2.1. Preload The preload force ( ) in the ball burnishing process plays a critical role in defining the interaction 𝐹𝑝 between the burnishing ball and the target surface. This force, when combined with the oscillating force ( ) generated by piezoelectric vibrations during ultrasonic burnishing, constitutes the total 𝐹𝑣 burnishing force ( ). By carefully managing this parameter, engineers can optimize the ball 𝐹𝑏 22 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing burnishing process to achieve desired surface characteristics, highlighting the importance of precision in advanced manufacturing settings. 4.1.2.2. Strategy The “Strategy” parameter is critical in determining how the burnishing ball traverses the target surface to achieve the desired number of passes (np) which in the context of this dissertation is equal to two. This involves selecting a trajectory that optimally covers the surface area while considering the initial milling direction. Two passes are done in all of the burnishing tests. The first passes are all in the same direction which is perpendicular to the initial milling trajectory. The second passes are done differently in each strategy. These strategies devised incorporate various directions and protocols to ensure comprehensive coverage and are categorized as follows: “PAR” Strategy: This strategy is the “Parallel” strategy. This approach involves no angular change in trajectory between the two passes, with the tool moving in a direction perpendicular to the initial milling direction, which is considered the main burnishing direction. The second pass offsets a little bit and follows the same trajectory. “PERP” Strategy: This strategy is the “Perpendicular” strategy. In this approach the burnishing direction alternates by ±90° for the second pass. This variation introduces a crosshatch pattern, potentially enhancing surface finish and uniformity. “DIA” Strategies: This strategy is the “Diagonal” strategy. In this approach, the first pass is the same as others perpendicular to the milling direction. The second pass is done diagonally over the first pass. These strategies aim to investigate the effects of directional changes and repeated passes over the same path on the burnishing outcomes. By experimenting with these varied approaches, the study seeks to identify the influence of the burnishing trajectory on surface integrity, offering insights into optimizing the ball burnishing process for improved results. The execution of various strategies in the ball burnishing process, as outlined, involves the precise configuration of NC codes, designed to direct the tool's movement across the surface according to predetermined patterns. These strategies delineated through a combination of linear interpolations, are essential for implementing the diverse LP-St (Lateral Pass-Strategy) pairs that constitute the experimental array. For instance, a specific pair such as “0.2-PAR” might be designed to cover the target surface with Parallel strategy with a 0.2 Lateral offset. Each of these tests follows a unique pattern to assess the impact of the chosen strategy on surface integrity. 23 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Parametrization of the NC codes for all three strategies allows for straightforward adjustments of strategies and preload force, tailoring each test to investigate specific aspects of the burnishing process. This methodical approach enables a comprehensive exploration of how different burnishing strategies affect the finished surface, contributing valuable insights into optimizing the process for enhanced material properties and surface quality. 4.1.2.3. Lateral Offset In the ball burnishing process, a critical factor influencing the outcome is the lateral offset. This parameter is pivotal in ensuring comprehensive surface deformation and achieving a uniform finish. By carefully managing the lateral offset, manufacturers can achieve significant improvements in surface properties In the scope of this dissertation, three different values are chosen and used. These values are 0.2 mm, 0.1 mm, and 0.05 mm. To conclude, there are three different strategies and three different lateral pass values which adds up to nine different tests. Additionally, for all of these different tests, there will be a spectrum of different preload values. 4.1.3. Unvaried factors In the experimental design, while the main factors of the study were varied across different tests to investigate their impact on the outcomes, certain parameters were deliberately kept constant throughout all tests. This decision was made to ensure consistency and to isolate the effects of the variables being studied. Among these constants were the burnishing ball material and diameter, chosen to maintain continuity with the methodologies established in prior research conducted by the group. Keeping these factors unchanged allows for a direct comparison of results and ensures that any observed differences in the outcomes can be attributed to the varied factors rather than changes in the tooling used. This approach is common in experimental research, where controlling variables is crucial to the integrity and reliability of the findings. 4.1.3.1. Number of Passes The "Number of passes" (np) in the context of the ball burnishing process is a critical parameter. By selecting a fixed number of passes for the burnishing process, was aimed to isolate the effects of the parameters being studied. In the end, the chosen value for the number of passes is two. Different strategies can be implemented in the tests and the efficiency of the process is high. 24 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing 4.1.3.2. Feed Maintaining a constant feed rate ensures that all tests are performed under similar conditions, enhancing the consistency and reliability of the results. The objective of this dissertation is to explore the effects of lateral pass, strategy, and preload. Controlling the feed rate allows the study to focus more directly on these areas without the additional layer of complexity. In the experiments, the constant feed rate of 600 mm/min is chosen. 4.1.3.3. Burnishing Ball Using the same type and size of the burnishing ball across all tests ensures consistency in the experimental conditions. This uniformity is crucial for the reproducibility of the results. A ball, made of chrome steel alloy 100Cr6 or F131 with high hardness and resistance to wear, of 10 mm is used on the end of the tool for all of the burnishing operations. 4.2. Measurement of the Surface Integrity Parameters Measuring surface integrity parameters in ball burnishing is crucial to ensure that the process achieves the desired surface characteristics and mechanical properties. Surface integrity encompasses various aspects of the finished surface including its surface topography and surface roughness. 4.2.1. Acquisition of Topological Data The transition from evaluating surface texture through two-dimensional parameters to adopting three-dimensional descriptors has marked a significant advancement in the field of surface metrology, especially in the context of assessing the results of the Vibratory Assisted Ball Burnishing process. This evolution acknowledges that two-dimensional parameters often offer a limited and potentially biased view of surface characteristics, failing to capture the full complexity of engineering surfaces. The equipments given below are used for the setup that is provided in Figure 4.1 to acquire the topological data of the burnished surface area: ●STIL Micromeasure 2 3D Non-Contact Surface Measurement System. ●CCS Prima Confocal Sensor. ●Point Sensor Map Software. 25 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Figure 4.1: Acquisition Setup for topological data. A) STIL Micromeasure 2 3D Non-Contact Surface Measurement System. B) Confocal Sensor and the Workpiece. C) Controller of Micromeasure 2 and Control Box of the Sensor. The CCS Prima Confocal Sensors operate on a sophisticated optical principle using a white light source, which enables precise distance measurements to various surfaces. The sensor uses a broad-spectrum white light, which contains all the colors or wavelengths visible to the human eye. This light source is crucial because each color has a different wavelength, which is central to measuring distances accurately. The white light is directed toward the target surface through an optical system consisting of multiple lenses. These lenses disperse the focused white light into its constituent colors along the measurement axis. This dispersion is key to determining the distance based on color wavelengths. In the calibration of the sensor, each color wavelength is assigned a specific distance. This calibration means that at a particular distance from the sensor, only one wavelength (or color) will be perfectly focused on the surface. The light that reflects back from the target surface passes through a confocal aperture, a narrow opening that allows only the light from the focal point to pass through. This step ensures that the sensor captures only the most relevant spectral information, enhancing measurement accuracy. The light that passes through the confocal aperture reaches a spectrometer. This device identifies which color wavelength is perfectly focused and reflected from the target surface. By identifying this color, the spectrometer can determine the distance to the target based on the preset calibration of wavelengths to distances [3]. 26 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing After the mapping of the burnished surface by the confocal sensor and Point Sensor Map software, the MountainsMap software. MountainsMap software is a specialized application widely used in surface metrology to analyze and visualize surface texture and topography data. It provides advanced tools to process, analyze, and report on measurement data from various surface measuring instruments like profilometers, scanning probe microscopes, and optical microscopes. MountainsMap offers robust visualization tools that help users see and interact with 3D models of surface data. It performs quantitative analysis of surface texture and topography. This includes calculating standard roughness parameters, analyzing functional characteristics, and statistical processing of surface features. The software allows users to create detailed reports with customizable layouts, integrating graphics, analysis results, and annotations to effectively communicate findings [4]. The data that is received from the confocal sensor is raw data. There are specific operators in MountainsMap software to convert this raw data into useful data. For the context of this dissertation, the same operators are used for more reliable analyses for every burnishing surface. The first operator that is crucial is the “Level” operator which is used to correct the tilt of a surface measurement. When measuring surfaces, any slight incline of the sample or measurement device can introduce a slope across the surface data. The Level operator removes this artifact by subtracting the best-fit plane from the surface, ensuring that the analysis is performed on an accurate horizontal base. It's typically used as a preprocessing step before conducting further detailed surface analysis. After leveling the surface, the “Remove Outliers” operator is used. This operator is crucial for cleaning the surface data by identifying and removing anomalous data points that deviate significantly from surrounding data. Outliers can occur due to measurement errors, surface contamination, or anomalies in the material itself. By removing outliers, the data becomes more representative of the actual surface characteristics, leading to more reliable analysis results. Sometimes, “Retouch” and “Correct Lines” operators are used for the same purposes as well. Later, the surface is filtered by the “S Filter” operator. The S Filter is used to separate the roughness and waviness components of the surface texture. The filter operates by applying a cutoff wavelength that discriminates between the shorter wavelength roughness features and the longer wavelength waviness features. This is essential for applications requiring precise control over surface texture properties. The “Metrological Filter” operator is used after the S Filter. Metrological filters are used to ensure that the data analysis conforms to specific metrological standards. These filters correct systematic errors, enhance the reliability of measurements, and ensure compliance with testing protocols. They are applied to raw data to refine the measurement results, ensuring that the analysis output is both accurate and compliant with the required metrological practices [3]. 27 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing The advent of optical systems and their capability to digitize the surface into a mathematical function—where each point on the surface is assigned a unique height value—has paved the way for a more comprehensive analysis. This methodological leap is encapsulated in the ISO 25178 standard, which introduces an extensive suite of three-dimensional parameters [11]. These parameters are organized into two primary groups: the 12 S parameters, which focus on scale-limited surface texture, and the 13 V parameters, dedicated to volumetric features of the surface texture. This classification enables a more nuanced and detailed representation of the surface, allowing for a precise mathematical description of features that influence the functional performance of engineering components. By utilizing these three-dimensional parameters, researchers and engineers can achieve a deeper understanding of how processes like VABB affect material surfaces, facilitating the optimization of manufacturing techniques for improved product quality and functionality. 4.2.1.1. S Parameters The 12 S parameters are a set of three-dimensional descriptors that offer a comprehensive analysis of surface texture by considering the spatial arrangement and heights of surface features. These parameters are calculated from the surface's deviation from a reference plane, providing insights into its roughness, waviness, and form. They include measures of height (such as the average roughness depth), spatial (like texture direction), and hybrid parameters that combine different aspects of the surface's topography. The application of these parameters allows for a detailed characterization of engineering surfaces, supporting the optimization of manufacturing processes, the improvement of product performance, and ensuring compliance with design specifications. Each S parameter has a specific interpretation related to the surface's physical properties, such as its ability to retain lubricant, contact with other surfaces, wear resistance, and aesthetic qualities [8]. For in-depth explanations and the mathematical expressions of each of these 12 S parameters, the ISO 25178 standard itself is the best reference. 4.2.1.1.1. S Amplitude Parameters The 12 S parameters shown in Table 4.1 in surface metrology, including and , serve as 𝑆𝑎𝑆𝑞 statistical descriptors for understanding the complexity of surface textures in three dimensions. These parameters extend the analysis beyond simple two-dimensional measurements, allowing for a more comprehensive assessment of surface characteristics. , the average surface texture, 𝑆𝑎 and , the root mean square (RMS) height, both quantify the general scale of surface 𝑆𝑞 28 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing irregularities, offering insights into the overall roughness and texture uniformity of a surface. , 𝑆𝑞 often referred to in the context of surface texture analysis, corresponds to the second central moment of the surface height distribution. parameter, known for its statistical significance, 𝑆𝑞 offers a robust measure of surface roughness by considering the square root of the average of the squared deviations from the mean plane. The emphasis on RMS over average surface texture in certain analyses stems from its sensitivity to extreme values (peaks and valleys), making it a more comprehensive descriptor for evaluating surface conditions [8]. In the evaluation of surface finishing quality, the root mean square surface texture parameter is 𝑆𝑞 often preferred over the average surface texture as the principal descriptor of amplitude. This 𝑆𝑎 preference is rooted in the statistical robustness of , which takes into account the square of 𝑆𝑞 deviations from the mean height, providing a more sensitive measure of surface variations and roughness. In this research will be considered over . 𝑆𝑞𝑆𝑎 Furthermore, the parameter , which represents the sum of the five largest peak height values 𝑆10𝑧 and the five largest pit depth values within the defined area, provides critical information on the extreme points of the surface texture. This parameter is particularly useful for identifying and assessing the highest peak and deepest valley across a surface, which can be crucial for applications where contact mechanics and wear resistance are of concern. For the assessment of extreme surface features, will be preferred over the peak and valley parameters and . 𝑆10𝑧 𝑆𝑝𝑆𝑣 measures the amplitude of maximum deviations, providing a clear indication of the highest 𝑆10𝑧 peaks and deepest valleys over the sampled area. This makes particularly useful for 𝑆10𝑧 applications where these extreme values play a critical role in the surface's functional performance. 29 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Table 4.1: Names and Equations of the S Parameters. Skewness, , and Kurtosis, , given in Table 4.2 are statistical parameters used in surface 𝑆𝑠𝑘 𝑆𝑘𝑢 metrology to describe the shape and distribution of surface heights as shown in Figure 4.2. Skewness, (the 3rd-order central moment), provides insight into the symmetry of the surface profile around its mean line. A skewness value of 0 indicates a symmetric, balanced profile akin to a Gaussian distribution. Negative skewness implies a surface with more prominent peaks than valleys, often resulting from finishing processes like lapping or plateau honing that aim to create 30 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing ●Reduced Valley Height - 𝑆𝑣𝑘 ●Peak Material Ratio - 𝑆𝑡𝑟 ●Valley Material Ratio - 𝑆𝑡𝑟 4.3. Material Characterization The material that will be used for this research is VH13IM alloy which is equivalent to AISI H13. It is identified as hot work tool steel, known for its good combination of toughness and resistance to high temperatures. This alloy is part of the broader category of tool steels that are crucial in manufacturing applications where resistance to thermal fatigue, shock, and abrasion is essential. The weight composition of the alloy is provided in Table 4.4 below and its properties are provided in Table 4.5 Components Fe Cr Mo Si V C Weight (%) >=90.9 5.13 – 5.25 1.33 – 1.4 1.0 1.0 0.32 – 0.40 Table 4.4: Weight composition of VH13IM. Key Properties: ●Toughness and Hot Resistance: VH13IM alloy is engineered to withstand the demanding conditions of hot work applications where the steel must maintain high strength and toughness at elevated temperatures. ●Versatility: Its alloy composition makes VH13IM a versatile choice for various manufacturing tools, including those used in plastic molding and die-casting of light metals like aluminum. ●Manufacturing Technology: This steel is typically produced in an ingot form, which is a traditional method for making bulk steel intended for further processing. Applications: ●Die Casting: VH13IM is commonly used in dies for casting light metals, particularly aluminum and magnesium, due to its ability to resist thermal fatigue and maintain high surface integrity under the thermal stresses of casting. ●Plastic Molding: Due to its high toughness and ability to withstand wear, VH13IM is also suitable for molds used in high-pressure injection molding processes. 37 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Property Value Density 7.80 g/cc Ultimate Tensile Strength 1990 MPa Yield Tensile Strength 1650 MPa Modulus of Elasticity 210 GPa Bulk Modulus 160 GPa Poisson's Ratio 0.30 Shear Modulus 81 GPa Table 4.5: Properties of VH13IM [1]. VH13IM can be processed using typical methods applicable to hot work tool steels, including machining, milling, and grinding, with its properties optimized through heat treatment processes like hardening and tempering. This combination of properties and the ability to tailor the material through various heat treatments make VH13IM a durable and reliable choice for critical components in manufacturing settings where operational conditions are harsh and the demands on material performance are high. Figure 4.4: Workpiece before burnishing operations. 38 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing 4.4. Experimental Execution All these workpieces are rectangular prisms shown in Figure 4.4 with the dimensions of 32x42 mm. The workpieces require a milling operation before a burnishing operation can be done on them. The workpieces were machined by the provider before. The experimental setup for the burnishing tests was conducted on a milling machine with a comprehensive system for precise measurement and control of the forces exerted during the process. A CNC code was created to perform burnishing passes along the surface. To calculate the burnishing forces exerted on the workpiece, a dynamometer is used. To fix the workpieces for burnishing, a metal piece is drawn and created. These steps will be explained in the next subsections in more detail. 4.4.1. Milling Machine & Burnishing Device The LAGUN 600 milling machine, known for its robust construction and precision in handling complex milling tasks is used for burnishing. The machining table offers more than enough space for the application of this research. The machine provides considerable movement in the X, Y, and Z axes, allowing for precise and extensive machining operations, and is designed for durability and stability, ensuring precision operations with consistent results. Figure 4.5: Burnishing Device, Dynamometer, Workpiece, and the Deck. 39 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing The experimental setup provided in Figure 4.5 also included an external power circuit responsible for supplying and controlling the power to the Vibration Assistance tool. This circuit plays a crucial role in ensuring that the tool is correctly energized and functioning at the optimal parameters for the test. The precise excitation of the tool is crucial for creating the desired vibration force to apply to the workpiece by the burnishing tool. 4.4.2. Burnishing Force Measurement The experimental approach to applying dynamometric measurements involved a comprehensive testing strategy. This testing strategy was designed to capture a broad spectrum of data, enabling a nuanced analysis of how varying the preload affects the effectiveness and efficiency of the VABB process. Such a structured approach offers insights into optimizing the ball burnishing process for improved surface integrity and material properties. The experimental setup shown in Figure 4.6, is positioned adjacent to a milling machine which was equipped with a Kistler 9129AA Multicomponent Dynamometer. The dynamometer is placed under the workpiece to accurately measure the forces applied during the burnishing tests. This configuration allowed for the direct application of compressive forces by the tool onto the sensor. This high-precision instrument is essential for monitoring the mechanical stresses imparted on the workpiece, ensuring that the data collected is both accurate and reliable. The forces measured by the dynamometer were amplified and processed using a Kistler MultiChannel Charge Amplifier Type 5070, which ensures that the signals are accurately captured and enhanced for clearer analysis. The amplified signals were then registered and analyzed using DynoWare software, which is specifically designed for capturing and processing data from dynamometric tests. This software allows for detailed examination and recording of the force dynamics during each test, offering insights into the mechanical interactions between the tool and the workpiece. The equipments given below are used to measure the preload applied by the tool to the workpiece: ●Kistler 9129AA Multicomponent Dynamometer. ●Kistler Multichannel Charge Amplifier Type 5070. ●Connection Cable. ●Dynoware Software. 40 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Figure 4.6: Experimental setup for the dynamometric measurements. The dynamometer records forces according to a Cartesian reference system. As seen in Figure 4.7 represents the burnishing force, which is the primary force exerted directly onto the surface 𝐹𝑧 being burnished. This is the vertical force necessary to deform the surface plastically and achieve the desired finish. and denotes the tangential forces, which occur during the burnishing 𝐹𝑥𝐹𝑦 process. These forces act tangentially to the movement of the burnishing ball and can be significant in understanding the behavior of the tool under operational conditions. and can 𝐹𝑥𝐹𝑦 be described as pseudo-frictional forces because, while it is not a classical friction force resulting from direct surface-to-surface contact, it behaves opposite to the movement of the burnishing ball. This force component is important for analyzing the dynamics of the burnishing process, including how different tool paths, speeds, and exerted forces affect the overall force. 41 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Figure 4.7: An example of dynamometric measurement on Dynoware software showing , , 𝐹𝑥𝐹𝑦 and . 𝐹𝑧 4.4.3. Workpiece Fixation It is critically important to fix the workpieces on the dynamometer to obtain consistent results. To secure the workpieces firmly to the dynamometer and prevent any movement during the burnishing process, a metal deck shown in Figure 4.8 is created. This deck is screwed on top of the dynamometer. This method of attachment provides a stable base for burnishing operations, minimizing external variables that could affect the data quality or the integrity of the test results. Figure 4.8: A) Technical Drawing of the Deck. B) The Deck. 42 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Overall, this experimental setup encapsulates a well-integrated system designed to provide a thorough understanding of the forces involved in the burnishing process and their impact on the workpiece. Such a setup not only enhances the reliability of the experimental results but also contributes significantly to the development and optimization of advanced manufacturing techniques like VABB. 43 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing 5.Result Discussion In this chapter, the results that are obtained from the full factorial experiments on the VH13IM material are presented and discussed. The organization and discussion of the findings are meticulously structured to ensure clarity and a logical flow of information. The results will be investigated under surface integrity parameters. As discussed in section 4.1, the design of the experiments consists of three strategies and three lateral pass values. For every strategy, lateral pass couple, there will be a spectrum of preload values. Also, another study where the VABB and NVABB techniques will be compared and presented. In the analysis of surface integrity, distinguishing between qualitative and quantitative aspects are crucial for setting appropriate optimization targets and defining what constitutes the "best" result. The process of optimizing surface integrity involves a meticulous evaluation of various parameter sets derived from the design of experiments to ascertain which combinations most effectively enhance surface characteristics. As discussed in Section 4.2, not all of the parameters will be used in this section. The parameters that will be used for the analysis are given below in Table 5.1. S Parameters Amplitude Root Mean Square Height - 𝑆𝑞 Skewness - 𝑆𝑠𝑘 Kurtosis - 𝑆𝑘𝑢 Ten-Point Height - 𝑆10𝑧 Spatial Fastest Decay Autocorrelation Length - 𝑆𝑎𝑙 Hybrid Developed Interfacial Area Ratio - 𝑆𝑑𝑟 Miscellaneous Texture Direction Angle - 𝑆𝑡𝑑 V Parameters Areal Parameters Core Distance - 𝑆𝑘 44 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Reduced Peak Height - 𝑆𝑝𝑘 Reduced Valley Height - 𝑆𝑣𝑘 Peak Material Ratio - 𝑀𝑟1 Valley Material Ratio - 𝑀𝑟2 Table 5.1: The Topological Parameters that will be used. 5.1. Parallel Strategy In the exploration of the results of the parallel strategy of vibration-assisted ball burnishing, 23 experiments were meticulously designed and executed. This approach involves the tool moving parallel to a fixed reference line on the surface of the metal alloy, aimed at evaluating how this orientation impacts the surface roughness under varied conditions. The experimental results are presented in Figure 5.1, which employs a color-coded system to enhance the clarity and readability of the data: blue for a lateral offset of 0.05 mm, red for 0.1 mm, and yellow for 0.2 mm. Each test in Figure 5.1 also correlates with a specific spectrum of force values applied during the burnishing process. Figure 5.1: Tests with vibration assistance and parallel strategy. This segment of the study leverages the parallel strategy to potentially uncover optimizations in tool path and force application that could lead to superior surface finishes, reduced 45 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing microstructural damage, or enhanced material properties. By systematically varying and analyzing these parameters, the research aims to develop a deeper understanding of the process dynamics and establish guidelines for achieving optimal results in industrial applications. In the following subsections, the effects of changes in strategies, lateral pass, and forces on the surface roughness in terms of different surface integrity parameters will be presented. 5.1.1. and Amplitude Parameters 𝑆𝑞𝑆10𝑧 In analyzing the effects of lateral offset and preload values on surface roughness in terms of 𝑆𝑞 and , it is found that these two parameters behave similarly. Figure 5.2 and Figure 5.3 below 𝑆10𝑧 show the values for these two parameters. Figure 5.2: values for the tests with vibration assistance and parallel strategy. 𝑆𝑞 The results indicate a pronounced dependency of surface finish quality on the lateral offset. Notably, a lateral offset of 0.05 mm yielded the best surface roughness values. This can be attributed to the reduced spacing between the passes, which likely facilitates a more uniform and consistent smoothing of the surface by minimizing untreated areas and overlap errors. Conversely, when the lateral offset was increased to 0.2 mm, the surface roughness significantly deteriorated. The larger offset may have resulted in insufficient coverage and higher discrepancies in the micro-topography of the treated surface. 46 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Figure 5.10: values for the tests with vibration assistance and parallel strategy. 𝑆𝑡𝑑 These findings collectively emphasize the nuanced impacts of both lateral offset and preload on surface roughness parameters. While and are predominantly influenced by the offset, 's 𝑆𝑎𝑙 𝑆𝑑𝑟 𝑆𝑡𝑑 sensitivity to preload highlights the importance of considering force adjustments to control directional surface properties effectively. 5.1.4. Area Parameters In the analysis of the parallel strategy for ball burnishing, a detailed examination was conducted on the areal surface roughness parameters, providing significant insights into the surface integrity of the workpieces. The results for across all offsets predominantly fell within the 0.4 μm range as seen in Figure 𝑆𝑘 5.11, indicating a generally moderate core roughness depth. Notably, only five tests, consisting lateral offsets of 0.1 mm and 0.05 mm, achieved exceptionally low values around 0.1 μm, 𝑆𝑘 denoting a very fine surface finish. These standout results underscore the potential for achieving superior surface qualities with narrower offsets, though no specific effect of preload was identified as influential in this outcome. 53 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Figure 5.11: values for the tests with vibration assistance and parallel strategy. 𝑆𝑘 For the parameter, while all lateral offset groups provided in Figure 5.12 included tests that 𝑆𝑝𝑘 achieved good results under 0.1 μm, over half of the results for an offset of 0.2 mm exhibited 𝑆𝑝𝑘 values exceeding 0.5 μm, particularly in tests with preloads higher than 270N. This indicates that wider offsets, especially under higher preloads, tend to leave excessive peak heights, potentially compromising surface functionality by increasing wear and reducing fatigue life. Figure 5.12: values for the tests with vibration assistance and parallel strategy. 𝑆𝑝𝑘 54 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing The results given in Figure 5.13 for 0.1 mm and 0.05 mm lateral offsets were predominantly 𝑆𝑣𝑘 below 0.2 μm, with many even lower than 0.1 μm, reflecting very smooth valley surfaces which are ideal for retaining lubricants and enhancing mechanical contact. In contrast, nearly all tests with a 0.2 mm lateral offset reported significantly higher values, suggesting poorer surface 𝑆𝑣𝑘 conditions with deeper valleys that may adversely affect mechanical and tribological performance. Figure 5.13: values for the tests with vibration assistance and parallel strategy. 𝑆𝑣𝑘 Regarding material ratios provided in Figure 5.15 and Figure 5.16, approximately half of the 0.1 mm tests yielded values over 20%, which, while acceptable, is not optimal. Most tests with 𝑀𝑟1 0.2 mm and all the tests with 0.05 mm widths produced better results, staying below this level. For , all results were comfortably above 80%, indicating a high percentage of material 𝑀𝑟2 remaining above the valley depths across all tests, which is beneficial for maintaining surface integrity and operational durability. 55 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Figure 5.14: Surface textures and Abbott-Firestone curves of C12 and B1. Figure 5.14, presents a comparative analysis of surface texture properties for two different tests, B1 and C12, utilizing the Abbott-Firestone curve along with direct texture images. The high in 𝑆𝑣𝑘 test B1 suggests deep valleys within the surface texture, which is indicated by a significant area under the lower part of the Abbott-Firestone curve. Similarly, a high implies a significant core 𝑆𝑘 roughness depth. The texture image for B1 shows a pronounced ruggedness with notable deep valleys. This complex topography is crucial for applications requiring strong mechanical keying or lubricant retention but might be prone to material trapping and increased wear under certain conditions. The low in C12 indicates shallower valleys, shown by less area under the lower part 𝑆𝑝𝑘 of the Abbott-Firestone curve. High and suggest that while the core roughness depth is 𝑆𝑘𝑆𝑝𝑘 significant, the peaks are particularly prominent. The texture image for C12 would generally appear less deep but with sharp, high peaks. This configuration offers a smoother operational interface while maintaining a high level of roughness at the peak regions. 56 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Figure 5.15: values for the tests with vibration assistance and parallel strategy. 𝑀𝑟1 Figure 5.16: values for the tests with vibration assistance and parallel strategy. 𝑀𝑟2 5.1.5. Optimal Parameters for Parallel Strategy The experimental results of the parallel strategy have highlighted several critical insights that can guide future applications and optimizations in industrial processes. 57 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing The parallel strategy in vibration-assisted ball burnishing exhibits a complex interplay between lateral offsets, preload values, and various surface roughness parameters. The results across parameters such as , , , , and have shown that the strategy is robust, delivering 𝑆𝑎𝑙 𝑆𝑑𝑟 𝑆𝑝𝑘 𝑆𝑣𝑘 𝑀𝑟2 consistently good outcomes across all tested lateral offsets. However, a deeper analysis into the amplitude S parameters, including and , reveals significant variations that are critical to 𝑆𝑞𝑆10𝑧 understanding the efficacy of different settings. The 0.05 mm lateral offset consistently outperformed the 0.1 mm and 0.2 mm settings, offering the best overall surface roughness results. This superior performance is especially notable in metrics such as vs , where the proximity to ideal values ( close to 0 and close to 3) 𝑆𝑘𝑢 𝑆𝑠𝑘 𝑆𝑠𝑘 𝑆𝑘𝑢 indicates a highly symmetrical and normally distributed surface profile. In contrast, the 0.2 mm offset displayed the least favorable outcomes, particularly when combined with preload values exceeding 250N. This threshold appears to be a critical point where surface roughness begins to deteriorate, suggesting that higher forces exacerbate the negative impacts of wider lateral passes. Figure 5.17: Texture comparison between good (H2) and bad (C11) results. The impact of lateral offsets and preload values on surface finish is vividly demonstrated in the comparison of two distinct textures provided in Figure 5.17. The left texture, achieved with a 0.05 mm lateral offset and a preload of 203.2N, showcases a superior surface finish characterized by a high degree of uniformity and smoothness. This finer offset allows for more consistent interaction between the burnishing tool and the material surface, effectively smoothing out imperfections and enhancing the microstructural integrity of the surface. Conversely, the right texture, produced with a 0.2 mm lateral offset and a higher preload of 276.2N, exhibits a significantly inferior finish. As it can be seen there are two big hill-like shapes on the surface. This coarser offset, combined with the increased force, likely leads to a more aggressive and less controlled interaction between the tool and the material. The excessive preload can exacerbate this effect, leading to greater 58 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing surface disruption and irregularity. This is evident in the increased roughness and the apparent non-uniformity of the texture. The variability observed with the 0.1 mm offset indicates that while this setting can sometimes produce results comparable to the finer 0.05 mm width, it is also susceptible to producing poorer finishes akin to the 0.2 mm width, depending on specific conditions such as material properties and exact force applied. The observed shift in the parameter after a preload of 267N—from a 𝑆𝑡𝑑 90-degree orientation to a 180-degree orientation—further underscores the sensitivity of surface texturing to preload values. This dramatic change can be attributed to the mechanics of the burnishing process where higher forces may alter the interaction between the tool and the material surface, potentially affecting the directional consistency of the induced texture. These findings highlight the necessity for precise control over both lateral offsets and preload values in optimizing surface finishes through the parallel strategy. The insights suggest that while narrower passes are generally preferable, attention must also be paid to maintaining preload values below a certain threshold to avoid adverse effects on surface quality. Furthermore, the sensitivity of the parameter to preload changes points to the potential for preload adjustments 𝑆𝑡𝑑 to strategically influence texture orientation, which could be leveraged to enhance specific surface properties tailored to different application needs. These findings suggest that precise control over lateral offsets and preload values is essential for optimizing surface roughness characteristics in vibration-assisted ball burnishing. The recommended settings—particularly the narrower offsets and lower preloads—should serve as guidelines for achieving optimal surface finishes, thereby enhancing the performance and lifespan of metal components in various engineering applications. 5.2. Perpendicular Strategy This segment of the study investigates the perpendicular strategy, where the tool movement is oriented perpendicular to a fixed reference line across the surface. A total of 21 tests were conducted using this perpendicular approach, with a focus on examining the effects of three different lateral offsets—0.05 mm, 0.1 mm, and 0.2 mm—each represented by distinct colors in the graphical analysis provided in Figure 5.18 for clarity and ease of interpretation: blue for 0.05 mm, red for 0.1 mm, and yellow for 0.2 mm. 59 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Figure 5.18: Tests with vibration assistance and perpendicular strategy. By varying both the lateral offsets and the preload values, the study seeks to identify how these factors interact under a perpendicular strategy to influence the alloy's surface properties. 5.2.1. and Amplitude Parameters 𝑆𝑞𝑆10𝑧 It was expected to have similar results from the impact of lateral offsets and preload values on surface roughness within the perpendicular strategy on the and parameters. 𝑆𝑞𝑆10𝑧 Figure 5.19: values for the tests with vibration assistance and perpendicular strategy. 𝑆𝑞 60 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing The results indicate a strong influence of lateral offset on values given in Figure 5.19. A width 𝑆𝑞 of 0.05 mm consistently produced the best results, showcasing the effectiveness of finer passes in achieving lower roughness, which is desirable for many precision applications. The 0.1 mm width also yielded good and consistent results, particularly up to a preload of 200N, beyond which the roughness values plateaued and subsequently began to decline slightly. This behavior suggests an optimal range of force application, within which the tool effectively smooths the surface without over-straining the material. In stark contrast, the 0.2 mm width was the least effective, resulting in the highest and most inconsistent values. This suggests that wider offsets in the perpendicular 𝑆𝑞 approach may lead to uneven surface treatment, possibly due to inadequate overlap or excessive spacing between passes. For the parameter, the tests given in Figure 5.21 with a lateral offset of 0.1 mm exhibited a 𝑆10𝑧 similar pattern to , with a plateau around the 200N preload level, followed by a decrease in 𝑆𝑞 values at lower and higher preloads. The results from the 0.05 mm offset showed some favorable outcomes, though without consistency, suggesting that while finer passes can achieve excellent results, they may require more precise control over preload to maintain uniformity across different tests. Figure 5.20: E9) Low and value results. I9) High and value results. 𝑆𝑞𝑆10𝑧 𝑆𝑞𝑆10𝑧 Figure 5.20 showcases two distinct textures for tests E9 and I9, visually representing the impact of and on the surface quality. In test E9, a low value indicates a smoother surface with 𝑆𝑞𝑆10𝑧 𝑆𝑞 minimal deviations from the mean plane. This indicates a consistent and even surface finish with few high peaks or deep valleys, as reinforced by a low value. Its texture shows a relatively flat 𝑆10𝑧 and uniform surface. It appears clean, which is crucial for surfaces that require minimal friction and wear, such as sealing faces or bearing surfaces. A high value in test I9 suggests a rougher 𝑆𝑞 61 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing surface with significant deviations from the mean plane, characterized by the presence of higher peaks and deeper valleys, as evidenced by a high value. Its texture appears more rugged and 𝑆10𝑧 uneven with distinct peaks and valleys. Figure 5.21: values for the tests with vibration assistance and perpendicular strategy. 𝑆10𝑧 The variability and trends observed across different lateral offsets and preload values in the perpendicular strategy highlight the complex interplay between these parameters in influencing surface roughness. These findings underscore the need for careful optimization of both offset and preload to achieve desired surface qualities, particularly in applications where precision and consistency are critical. 5.2.2. and Amplitude Parameters 𝑆𝑠𝑘 𝑆𝑘𝑢 The detailed examination of the roughness parameters and reveals a significant disparity in 𝑆𝑠𝑘 𝑆𝑘𝑢 the results based on lateral offsets, highlighting the critical influence of this variable on achieving optimal surface textures. The outcomes for lateral offsets of 0.2 mm and 0.1 mm were found to be largely unsatisfactory with respect to achieving the desired central area of near 3 and around 0 in the vs 𝑆𝑘𝑢 𝑆𝑠𝑘 𝑆𝑠𝑘 𝑆𝑘𝑢 graph as given in the yellow circle in Figure 5.22. These results suggest that wider passes in the perpendicular orientation may not facilitate the adequate adjustment of surface textures required for a balanced and symmetrical profile. The results consistently fell far from the optimal zone, with 62 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing The values shown in Figure 5.30 were largely below 0.1 μm across all offsets, signifying 𝑆𝑣𝑘 excellent valley smoothness conducive to reducing material trapping and enhancing surface contact properties. Each lateral pass group had one exception where the exceeded 0.2 μm, 𝑆𝑣𝑘 highlighting occasional lapses in achieving optimal valley depths. For 0.05 mm, the results improved steadily until reaching a preload of around 220N, after which the quality slightly started to deteriorate, outlining an optimal operational window for this setting. Figure 5.30: values for the tests with vibration assistance and perpendicular strategy. 𝑆𝑣𝑘 In terms of , the 0.1 mm results given in Figure 5.31 were notably consistent and all fell below 𝑀𝑟1 20%, indicating a favorable material distribution within the surface's core roughness depth. The 0.05 mm tests, while generally presenting good results, showed some inconsistency, and approximately half of the 0.2 mm tests scored above 20%, reflecting poorer and uneven material compression. For Mr2, all results provided in Figure 5.32 maintained levels above 80%, confirming a high retention of material above the valleys, which is beneficial for maintaining structural integrity under operational stresses. 69 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Figure 5.31: values for the tests with vibration assistance and perpendicular strategy. 𝑀𝑟1 Figure 5.32: values for the tests with vibration assistance and perpendicular strategy. 𝑀𝑟2 5.2.5. Optimal Parameters for Perpendicular Strategy The analysis of the perpendicular strategy in vibration-assisted ball burnishing has revealed significant variations in performance across different lateral offsets and preload values. Notably, 70 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing the 0.05 mm lateral offset consistently demonstrates superior performance across a range of parameters, including and , where it significantly outperforms the 0.2 mm setting. This 𝑆𝑞𝑆10𝑧 suggests that finer passes allow for more precise control over the tool’s interaction with the material surface, leading to a smoother finish and more uniform microstructural adjustments. Conversely, the 0.2 mm offset is identified as the least effective, likely due to its broader interaction footprint, which can cause uneven surface engagement and less controlled deformation, leading to increased roughness and variability in surface features. The 0.1 mm offset shows consistent results but not as favorable as the 0.05 mm setting, with a notable plateau at a 200N preload where performance dips significantly for and . This specific reaction could be 𝑆𝑞𝑆10𝑧 due to a threshold in force application where the benefits of the applied preload are overshadowed by the negative impacts such as excessive material displacement or stress concentrations, particularly at this intermediate offset. Interestingly, while and usually 𝑆𝑞𝑆10𝑧 correlated closely, the discrepancy observed between these parameters in the 190N to 270N preload range for the 0.1 and 0.05 mm settings could be attributed to the different aspects of roughness they measure, indicating that certain subtleties in surface peak and valley formations respond differently to the applied conditions. For and , the poor performance for both 0.1 mm and 0.2 mm settings, with visible 𝑆𝑘𝑢 𝑆𝑠𝑘 skewness to the right on the vs graph provided in Figure 5.22, indicates an undesirable 𝑆𝑘𝑢 𝑆𝑠𝑘 deviation from the optimal symmetrical and normally distributed surface profile, which could compromise functional properties of the surface like wear resistance and lubrication retention. The values being consistently below 0.1 mm is favorable, showing fine surface texturing across 𝑆𝑎𝑙 all settings, though again, the 0.2 mm setting is least effective. The 0.05 mm setting exhibits some inconsistency, possibly due to variations in how finely the tool engages with the surface at this smaller width. The parameter showing good results post-205N across all offsets suggests that 𝑆𝑑𝑟 higher preloads may assist in enhancing the effective area of the surface, improving properties such as oil retention and contact mechanics. The absence of correlation in and the notable variability in orientations across tests highlight a 𝑆𝑡𝑑 potential challenge with the perpendicular strategy, where the orientation of the burnishing impacts could be influenced by factors not directly controlled by preload or offset, such as tool wear or machine vibrations. This variability could lead to inconsistent performance in applications requiring directional precision. 71 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing parameter achieving values below 0.1 μm in some tests with a 0.05 mm lateral offset indicates 𝑆𝑘 exceptional control over core surface roughness, offering a smoother finish crucial for high-precision applications. This performance level is not matched in the 0.1 mm and 0.2 mm settings, where results fail to reach this benchmark, likely due to less effective smoothing of core surface irregularities at these wider passes. The poor performance of the 0.2 mm offset in and parameters suggest inadequate peak 𝑆𝑝𝑘 𝑀𝑟1 reduction and suboptimal material distribution at the surface's core, which could negatively impact wear resistance and mechanical load-bearing capacity. Conversely, both 0.05 mm and 0.1 mm offsets show good results, indicating that finer passes more effectively minimize peak heights and optimize the material ratio at critical surface depths, enhancing functional surface properties. and parameters generally show good to excellent results across all offsets, which suggests 𝑆𝑣𝑘 𝑀𝑟2 that the perpendicular strategy, regardless of offset, effectively manages valley depths and maintains a high material ratio above these valleys. Such outcomes are beneficial for ensuring adequate lubricant retention and reducing stress concentrations, crucial for maintaining the durability and operational integrity of the component. Overall, the findings underscore the need for careful optimization of offsets and preload values in the perpendicular strategy to maximize surface finish quality. The superior results with the 0.05 mm offset suggest that finer passes, when appropriately controlled, can significantly enhance the surface characteristics, making this strategy particularly suitable for applications demanding high precision and consistency. 5.3. Diagonal Strategy In this segment, attention is shifted to the results of the diagonal strategy of vibration-assisted ball burnishing, where the tool moves across the surface of the workpiece in a diagonal pattern relative to a fixed reference line. A total of 24 tests were conducted using this strategy, systematically varying the lateral offsets and preload values to gauge their individual and combined effects on the surface roughness of the alloy. The lateral offsets tested were 0.05 mm, 0.1 mm, and 0.2 mm, each represented in the graphical analysis in Figure 5.33 by distinct colors—blue, red, and yellow, respectively. This color coding facilitates a clear visual distinction among the results corresponding to each offset, aiding in the rapid assessment and comparison of their impact. 72 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Figure 5.33: Tests with vibration assistance and diagonal strategy. The preload values, plotted on the y-axis of the column graph in Figure 5.33, span a comprehensive range to understand how varying the force applied by the burnishing tool affects the alloy's surface under diagonal tool movement. The x-axis of the graph enumerates the individual tests, each labeled according to the specific combination of parameters used. This graphical representation not only serves as a straightforward method to display the quantitative data collected but also highlights the distribution and trends of preload effects across different lateral offsets. 5.3.1. and Amplitude Parameters 𝑆𝑞𝑆10𝑧 In the series of experiments employing the diagonal passes strategy, a detailed evaluation of surface roughness parameters of and as provided in Figure 5.34 and Figure 5.36 below 𝑆𝑞𝑆10𝑧 revealed some insights. It was observed that the lateral offset has a significant impact on the outcome of these roughness parameters. Remarkably, a lateral offset of 0.05 mm consistently produced superior results, achieving the lowest values in both and . This suggests that a 𝑆𝑞𝑆10𝑧 narrower offset in the diagonal configuration might facilitate a more effective smoothing action, likely due to the increased contact area and altered dynamics of the tool's interaction with the surface irregularities. Conversely, a lateral offset of 0.2 mm resulted in significantly poorer outcomes, indicating that this particular setting might not provide sufficient overlap or effective distribution of the burnishing force, potentially leading to uneven surface treatment. 73 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Figure 5.34: values for the tests with vibration assistance and diagonal strategy. 𝑆𝑞 Further analysis, excluding the outlying results at 0.2 mm, shows that for lateral offsets of 0.05 mm and 0.1 mm, there exists an optimal range of preload values between 170N and 215N that further enhances the surface quality. Within this range, the preload is likely sufficient to exert the necessary force for effective plastic deformation of the surface asperities, without causing excessive penetration or damage that could lead to increased roughness. Figure 5.35: G5) Low and value results. I8) High and value results. 𝑆𝑞𝑆10𝑧 𝑆𝑞𝑆10𝑧 Figure 5.35 presents a compelling comparison of two textures obtained using the diagonal strategy, showcasing the distinct differences in surface roughness parameters and . A low 𝑆𝑞𝑆10𝑧 𝑆𝑞 74 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing value in test G5 indicates a smoother surface with fewer and less severe peaks and valleys. Accompanying the low , a low value means there is a smaller difference between the 𝑆𝑞𝑆10𝑧 heights of the highest peaks and the lowest valleys. This uniformity results in a more consistent and predictable surface, which is advantageous for many applications requiring tight dimensional tolerances and smooth operational interfaces. Visually, the surface in test G5 appears flatter and even without significant rough or sharp features. This texture is ideal for applications where surface smoothness is critical, such as in sealing faces, bearing surfaces, or aesthetic components where surface finish is crucial. In contrast, a high value for test I8 suggests significant surface 𝑆𝑞 irregularities, with many peaks and deep valleys. A high value indicates a highly textured and 𝑆10𝑧 dynamic surface profile. The surface for test I8 would appear markedly rugged and sharp, with visible peaks and valleys that create a dynamic topographical landscape. Figure 5.36: values for the tests with vibration assistance and diagonal strategy. 𝑆10𝑧 These findings underscore the critical role of both lateral offset and preload in achieving the desired surface finish when employing the diagonal strategy in vibration-assisted ball burnishing. The identified optimal conditions, particularly the combination of a 0.05 mm offset and preloads within 170N to 215N, highlight the importance of tailoring these parameters to harness the full potential of the diagonal tool path. 75 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing 5.3.2. and Amplitude Parameters𝑆𝑠𝑘 𝑆𝑘𝑢 In the examination of surface roughness through the parameters of and , the diagonal 𝑆𝑠𝑘 𝑆𝑘𝑢 strategy reveals intriguing trends and variances influenced by lateral offsets. The graphical representation, depicted in the vs graph given in Figure 5.37, provides a visual assessment 𝑆𝑠𝑘 𝑆𝑘𝑢 of how closely the experimental outcomes align with the optimal surface roughness condition, where is near 0 and approximates 3. 𝑆𝑠𝑘 𝑆𝑘𝑢 Across the spectrum of tests, it is evident that the lateral offset of 0.2 mm does not favor these desired roughness characteristics. All tests at this width resulted in outcomes significantly deviating from the optimal area in the vs graph. This suggests that a wider lateral pass in a 𝑆𝑠𝑘 𝑆𝑘𝑢 diagonal strategy may interfere with achieving a balanced surface profile, potentially due to less effective distribution of the mechanical stress across the surface. Figure 5.37: vs graph for the tests with vibration assistance and diagonal strategy. 𝑆𝑠𝑘 𝑆𝑘𝑢 On the other hand, the tests with lateral offsets of 0.05 mm and 0.1 mm demonstrated better alignment with the desired roughness values as shown in the yellow circled area in Figure 5.37, with 0.05 mm generally yielding the most favorable results. Interestingly, while the results for 0.05 mm varied widely on the graph, indicating a diverse range of surface textures from the same offset, those for 0.1 mm and particularly 0.2 mm showed a consistent grouping. This consistent grouping away from the optimal center for the wider passes suggests a systemic influence of 76 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing offset on the skewness and kurtosis, limiting the ability to achieve the most balanced and normal surface texture distribution. Figure 5.38: Texture examples of 2 tests with optimal vs (E2) and negatively skewed vs 𝑆𝑠𝑘 𝑆𝑘𝑢 𝑆𝑠𝑘 (C5). 𝑆𝑘𝑢 Figure 5.38 showcases contrasting textures between two tests within the diagonal strategy, highlighting how different skewness and kurtosis levels manifest in physical surface characteristics. Test E2, depicted on the left, presents the most optimal results on the vs graph, indicating 𝑆𝑠𝑘 𝑆𝑘𝑢 a balanced and ideally distributed surface texture. Conversely, test C5, shown on the right, displays a negatively skewed surface with a higher , signifying a deviation from the ideal texture 𝑆𝑘𝑢 distribution. In test E2, values near zero signify a symmetric surface profile, with an equal 𝑆𝑠𝑘 distribution of peaks and valleys around the mean line. A value close to 3 indicates that the 𝑆𝑘𝑢 surface texture adheres to a normal distribution, with most surface points lying close to the mean height, reducing extreme deviations. The texture of E2 appears uniformly refined, without excessive peaks or deep valleys. A negative skewness in test C5 indicates an excess of valleys relative to peaks. This asymmetry suggests that the surface has more frequent and potentially deeper indentations than elevations. A higher points to a sharper, more peaked distribution of 𝑆𝑘𝑢 surface features, with more data clustering around the mean but with fatter tails. This can indicate isolated extreme peaks or deep pits, which are not typical for the average surface. The texture in C5 displays a more aggressive and uneven pattern with notable sharp features that disrupt the overall smoothness of the surface. It is also notable that there was no discernible effect of preload on the and values across all 𝑆𝑠𝑘 𝑆𝑘𝑢 tested parameters. This lack of correlation might indicate that within the tested range, preload does not significantly impact the skewness and kurtosis of the surface texture when using the 77 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing diagonal strategy. Instead, the lateral offset appears to be the dominant factor influencing these parameters. 5.3.3. Spatial, and Hybrid and Miscellaneous Parameters 𝑆𝑎𝑙 𝑆𝑑𝑟 𝑆𝑡𝑑 The findings of the tests done with vibration assistance and diagonal strategy in terms of the parameters , , and are not as intriguing as amplitude parameters, but they still provide 𝑆𝑎𝑙 𝑆𝑑𝑟 𝑆𝑡𝑑 some insights. The investigation of the parameter given in Figure 5.39 under the diagonal strategy reveals that 𝑆𝑎𝑙 all tests maintained values below the 0.1 mm threshold, indicating effective surface treatment 𝑆𝑎𝑙 across all configurations. However, the lateral offset of 0.2 mm consistently produced slightly higher values compared to the 0.05 mm and 0.1 mm widths. This suggests that while still 𝑆𝑎𝑙 within acceptable limits, an offset of 0.2 mm is not as effective in reducing the autocorrelation length as the other two, with 0.05 mm yielding the best results. For 0.1 mm offsets, it is noteworthy that the values improved significantly within the preload range of 170N to 270N, 𝑆𝑎𝑙 emphasizing the influence of preload within this offset on achieving finer surface texturing. Figure 5.39: values for the tests with vibration assistance and diagonal strategy. 𝑆𝑎𝑙 For the parameter, which quantifies the relative increase in surface area due to textural 𝑆𝑑𝑟 features, all recorded values were exceptionally low as given in Figure 5.40, under 0.015%, 78 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing orientation impact on the surface, irrespective of the lateral offset or preload values. This stability in orientation is crucial for applications requiring uniform directional properties across the surface, enhancing both aesthetic and functional aspects of the finished product. For parameter the 0.05 mm offset excels particularly below the 200N preload mark, 𝑆𝑘 consistently achieving values around or below 0.05 μm. This indicates a highly effective 𝑆𝑘 smoothing of the core surface, which is critical for reducing wear and enhancing contact performance. While the 0.2 mm offset shows poor performance for parameter, both the 0.1 𝑆𝑝𝑘 mm and 0.05 mm offsets perform exceptionally well, especially 0.05 mm offset below 215N and 0.1 mm offset in the range of 170N to 245N. This excellent performance in reducing peak heights minimizes potential sites for stress concentration and premature wear, which is crucial for extending the lifespan of the component. Similar to , the 0.1 mm and 0.05 mm offset settings 𝑆𝑝𝑘 for exhibit very good responses, ensuring that valley depths are controlled effectively to 𝑆𝑣𝑘 optimize lubricant retention and minimize particulate entrapment. For , the 0.05 mm offset provides the best outcomes, achieving optimal material distribution 𝑀𝑟1 at the surface's core. In contrast, the 0.2 mm setting struggles significantly, potentially compromising the surface's ability to withstand mechanical stresses. Ensuring a high material ratio at the core roughness depth is essential for maintaining structural integrity under operational loads. In case of , all tests across the offsets maintain values above 80%, indicating a consistent 𝑀𝑟2 and robust retention of material above the valley depths, which is advantageous for mechanical durability and resistance to surface fatigue. These findings underscore the importance of precise parameter selection in the diagonal strategy to maximize surface quality. The superior performance of the 0.05 mm setting across most parameters suggests that this offset, when combined with carefully controlled preload, can significantly enhance the microstructural and functional properties of the treated surfaces. 5.4. NVABB vs VABB This section presents the empirical findings from the vibration-assisted and non-vibration-assisted ball burnishing experiments. The experiments were meticulously designed to assess the impact of vibration assistance on the surface finish of the workpiece, utilizing a range of preload values as shown in Figure 5.47. Data collected from the tests focus on multiple surface roughness parameters, Each parameter offers unique insights into the textural characteristics and quality of 85 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing the finish provided by the ball burnishing process. All tests were done using the parallel strategy and 0.2 lateral offset. Figure 5.47: Tests with and without vibration assistance. The following subsections detail the measured values, providing a quantitative evaluation of the effects of vibration assistance compared to the conventional method, across varying operational preloads. Through structured comparison and analysis, these results aim to delineate the enhancements in surface integrity facilitated by the introduction of vibration into the ball burnishing process. 5.4.1. and Amplitude Parameters 𝑆𝑞𝑆10𝑧 The results provided in Figures 5.48 and 5.49 regarding the and parameters respectively, 𝑆𝑞𝑆10𝑧 provide important insights into the effects of vibration assistance in ball burnishing. 86 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Figure 5.48: values for the tests with and without vibration assistance. 𝑆𝑞 In the VABB tests, the application of vibrational energy likely enhances the redistribution and plastic deformation of surface material. This additional energy facilitates a more consistent and controlled interaction between the burnishing tool and the metal surface. As a result, it leads to a reduction in surface irregularities, contributing to lower values of and . 𝑆𝑞𝑆10𝑧 Figure 5.49: values for the tests with and without vibrationtion assistance. 𝑆10𝑧 87 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Without vibration, the application of a preload exceeding 280N in NVABB could lead to excessive mechanical stress on the surface. This stress might exceed the material’s capacity to deform plastically, which results in higher and values. 𝑆𝑞𝑆10𝑧 Figure 5.50: E4) Low and value results. A2) High and value results. 𝑆𝑞𝑆10𝑧 𝑆𝑞𝑆10𝑧 Analyzing the differences in surface texture quality between two different test results, E4 and A2, provides an opportunity to delve into the factors contributing to effective and ineffective ball burnishing outcomes. The presence of vibration in the VABB process aids in the more effective redistribution of material on the surface. The vibrations facilitate a smoother flow of the surface material, leading to decreased roughness parameters and . Vibration can enhance the 𝑆𝑞𝑆10𝑧 plastic deformation capabilities of the tool and the surface interaction. This leads to a more refined texturing of the surface, smoothing out peaks and filling valleys, which is reflected in better roughness values. In the absence of vibrational assistance, the material flow on the surface is less dynamic. This can result in the inability to effectively flatten high peaks or fill in low valleys, thus resulting in higher and values. 𝑆𝑞𝑆10𝑧 5.4.2. and Amplitude Parameters 𝑆𝑠𝑘 𝑆𝑘𝑢 For an in-depth analysis of results concerning the and parameters as displayed in Figure 𝑆𝑠𝑘 𝑆𝑘𝑢 5.51. Skewness quantifies the asymmetry of the surface profile distribution around its mean. A 𝑆𝑠𝑘 value near zero is optimal because it indicates a symmetric distribution of peaks and valleys. Kurtosis assesses the 'peakedness' of the surface profile distribution. A value of approximately 𝑆𝑘𝑢 3, indicative of a normal distribution, is considered ideal. The NVABB tests are primarily clustered just outside of the ideal region and are mostly positively skewed, suggesting that the surface 88 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing nearly achieves a balanced surface profile and are generally characterized by more pronounced peaks than valleys. Figure 5.51: vs graph for the tests with and without vibrationtion assistance. 𝑆𝑠𝑘 𝑆𝑘𝑢 The more scattered distribution of VABB tests around the graph indicates a wider variation in surface texture outcomes. The addition of vibration in VABB could be introducing dynamic forces that alternately compress and relax the surface material, affecting the uniformity of peak and valley formation. Figure 5.52: Textures of 2 VABB tests with optimal vs (A8) and positively skewed vs 𝑆𝑠𝑘 𝑆𝑘𝑢 𝑆𝑠𝑘 𝑆𝑘𝑢 (A12). 89 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Analyzing the differences between two VABB tests, as depicted in Figure 5.52, where texture A8 falls within the optimal vs region and texture A12 shows a positively skewed vs , 𝑆𝑠𝑘 𝑆𝑘𝑢 𝑆𝑠𝑘 𝑆𝑘𝑢 provides insights into the variability and effectiveness of the VABB process under different conditions. For A8 to fall within the optimal region. This optimal setting allows for a balanced distribution of material deformation, leading to a symmetric surface profile with moderate peakiness. The effective use of vibrational energy in A8 might have enhanced the plastic deformation across the surface uniformly, smoothing peaks and filling valleys efficiently. The positive skewness in A12 suggests that the vibrational energy might not have been as effective in uniformly redistributing the surface material. This could result from suboptimal vibration settings that fail to adequately address the natural tendencies of the material to form peaks. A positively skewed indicates a distribution with more pronounced peaks than valleys. 𝑆𝑠𝑘 5.4.3. Spatial, and Hybrid and Miscellaneous Parameters 𝑆𝑎𝑙 𝑆𝑑𝑟 𝑆𝑡𝑑 Both NVABB and VABB of processes, with and without vibration assistance, demonstrate similar auto-correlation lengths with values all below 0.1 mm. This indicates a uniformity in the spacing or periodicity of surface features, suggesting that both processes effectively maintain a consistent surface pattern. The similar performance of NVABB and VABB in terms of values could imply 𝑆𝑎𝑙 that for this specific parameter, the addition of vibration does not significantly alter the fundamental spacing of the surface texture features. Figure 5.53: values for the tests with and without vibrationtion assistance. 𝑆𝑎𝑙 90 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Both NVABB and VABB tests show values below 0.025%, which is considered a good threshold 𝑆𝑑𝑟 for maintaining a low developed surface area relative to the projected area. This indicates that neither process significantly increases the surface roughness or complexity, which is beneficial for applications where minimal texture depth is desired. The low values in both VABB and NVABB 𝑆𝑑𝑟 suggest neither method excessively alters the micro-topography in terms of adding unwanted texture or roughness. Figure 5.54: values for the tests with and without vibrationtion assistance. 𝑆𝑑𝑟 Up to a preload of 265N, both NVABB and VABB show no significant differences in the texture direction. However, beyond this preload level, differences in orientation start to manifest, although there is no consistent trend favoring either VABB or NVABB. The absence of a distinct correlation between the type of burnishing and the changes in texture orientation post-265N suggests that factors other than just vibration may influence . This might include variations in 𝑆𝑡𝑑 material response under higher mechanical loads, tool wear, or even slight misalignments in the tool setup. Similar observations were made in previous studies regarding the impact of lateral offset further supports the notion that variations in could be more strongly influenced by geometric or 𝑆𝑡𝑑 mechanical factors rather than the presence or absence of vibration. 91 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Figure 5.55: values for the tests with and without vibrationtion assistance. 𝑆𝑡𝑑 The results across these three figures suggest that while vibration assistance can significantly impact certain surface texture parameters such as skewness and kurtosis as discussed earlier, its effect on , , and might be more nuanced or dependent on specific conditions. 𝑆𝑎𝑙 𝑆𝑑𝑟 𝑆𝑡𝑑 5.4.4. Area Parameters The observations under different preload conditions with and without vibration assistance provide valuable insights into the effectiveness and specific impacts of these techniques on the surface texture parameters. The VABB tests consistently show lower values compared to NVABB. This suggests that 𝑆𝑘 vibration assistance effectively reduces the core roughness depth, potentially through more uniform material displacement and smoother finishing. In NVABB tests, values significantly 𝑆𝑘 increase once the preload exceeds 280N. This might indicate that beyond this threshold, the mechanical stress imposed by the burnishing tool without the moderating effect of vibration leads to deeper indentations and more pronounced roughness within the core layer of the material. 92 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Figure 5.56: values for the tests with and without vibrationtion assistance. 𝑆𝑘 Vibration assistance slightly improves values, reducing peak heights more effectively than 𝑆𝑝𝑘 NVABB. This can be attributed to the vibrational energy aiding in the more effective flattening of surface peaks. Both VABB and NVABB show a transition in values at a preload of 275N, where 𝑆𝑝𝑘 values increase significantly above this preload. This suggests that both processes can maintain lower peak heights up to a certain mechanical load, beyond which the pressure may cause the tool to generate higher peaks, regardless of vibration assistance. 93 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing Figure 5.57: values for the tests with and without vibrationtion assistance. 𝑆𝑝𝑘 The textures and Abbott-Firestone graphs presented in Figure 5.58 below, which are for tests A11 that is conducted by NVABB, and B1 conducted by VABB, provide a clear contrast in the surface roughness parameters , , and . 𝑆𝑘𝑆𝑝𝑘 𝑆𝑣𝑘 The depth in A11 indicates that the surface has significant deviations from the mean line deep 𝑆𝑘 into the surface layer. This can be attributed to inadequate smoothing of the metal's microstructure, possibly due to the absence of vibrational energy which could have helped redistribute and flatten the material more effectively. suggests that the peaks on the surface 𝑆𝑝𝑘 are pronounced and not adequately flattened. In NVABB, the static pressure applied by the burnishing tool may compact the material peaks without sufficient energy to level them. The low suggests that the valleys are not as deep, which could be because the material displaced by 𝑆𝑣𝑘 the tool is primarily pushed upwards to form peaks rather than spreading out to fill the valleys. The higher in B1 suggests better filling of valleys, likely due to the dynamic action of the 𝑆𝑣𝑘 vibration. The vibrational energy helps in distributing the material more uniformly during the burnishing process. The lower values of and indicate a smoother surface profile with less 𝑆𝑘𝑆𝑝𝑘 pronounced peaks. This can be explained by the vibrational assistance enhancing plastic deformation across the surface, allowing for better leveling of peaks and a more uniform surface finish. 94 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing ACTIVITY START TIME SEP 23 OCT 23 NOV 23 DEC 23 JAN 24 FEB 24 MA R 24 APR 24 PREPARATION SEP 23 2 Research proposal SEP 23 1 Initial literature search and review SEP 23 2 Identification of research gaps SEP 23 2 Selection of materials and methods SEP 23 1 Setting up experimental design SEP 23 2 Preliminary tests OCT 23 1 EXECUTION OF EXPERIMENTS NOV 23 4 Conducting main experiments NOV 23 4 Data gathering from experiments NOV 23 4 Analyzing collected data FEB 23 2 Statistical analysis MAR 23 1 RESULTS MAR 23 2 Drafting methodology section MAR 23 1 Writing results sections APR 23 1 Revising introduction APR 24 1 Analysis of environmental impacts APR 25 1 Economic analysis APR 26 1 Table 5.2: Gantt Chart. 101 Texture optimization of stainless steel alloys surfaces through vibration-assisted ball burnishing References [1] “AISI Type H13 Hot Work Tool Steel.” n.d. 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