Analysis of Flangeability by Single-Stage SPIF and Press-Working in AA7075-O Sheet

Analysis of Flangeability by Single-Stage SPIF and Press-Working in AA7075-O Sheet Marcos Borrego Assistant Professor Dpt. of Mechanical Engineering and Manufacturing University of Seville Seville, Spain 41092 Email: [email protected] Domingo Morales-Palma∗ Associate Professor Dpt. of Mechanical Engineering and Manufacturing University of Seville Seville, Spain 41092 Email: [email protected] Carp´ oforo Vallellano Full Professor Dpt. of Mechanical Engineering and Manufacturing University of Seville Seville, Spain 41092 Email: carpof[email protected] ABSTRACT Recently, the research interest of hole-flanging has turned from conventional press-working to SPIF as a viable process for small- and medium-sized batches. Both technologies have been studied separately using different approaches and, therefore, most studies cannot be easily compared. Besides, some studies that measured the formability in SPIF using the classical Limiting Forming Ratio (LFR) showed conflicting results that still need to be clarified. Under these circumstances, the aim of this work is to provide a better understanding of the deformation process and the material formability in hole-flanging by critically comparing both forming processes. To this end, a series of experimental tests on AA7075-O sheet of 1.6-mm thickness by press-working and single-stage SPIF, using forming tools with different profile radii, are analysed. The material formability and flange geometry are compared and discussed in detail. The process limits are analysed by using both the Forming Limit Diagram (FLD) and the LFR. The failure modes by necking and fracture are clearly identified and assessed on both processes along with the influence of the bending induced by the tools during the flange forming. Results conclude that the LFR is not an adequate parameter to compare formability between processes other than press-working and, ∗Corresponding author. 1 ASME © 2020. En idUS Licencia Creative Commons CC-BY Accepted version article: Borrego, M., Morales-Palma, D., and Vallellano, C. (October 1, 2020). "Analysis of Flangeability by Single-Stage SPIF and Press-Working in AA7075-O Sheet." ASME. J. Manuf. Sci. Eng. January 2021; 143(1): 011005. https://doi.org/10.1115/1.4047997. accordingly, two additional variables based on either the flange height or the average thickness reduction are proposed to successfully analyse flangeability. Keywords: Hole-flanging, Single-point incremental forming, Formability, Flangeability, Limiting forming ratio Nomenclature PW Press-working. SPIF Single-point incremental forming. SPIF1 SPIF in a single stage. LFR Limiting forming ratio. FLD Forming limit diagram. FFL Fracture forming limit. FLC Forming limit curve (for necking). 1 Introduction Hole-flanging is a very common forming operation in industry, especially in automotive and aeronautics. It aims to strength the hole edge, improve its appearance or provide additional support when joining sheet parts. Traditionally, this operation is performed in a forming press by clamping the sheet blank with a pre-cut hole and deforming the material around the hole in a single punch stroke. This conventional press-working (PW) method needs of large batches to counterbalance the cost of tooling and equipment [1]. In recent years, new forming methods are developed giving answer to an increasing demand of flexible and cost effective tooling compared to conventional methods. In this sense, single-point incremental forming (SPIF) is an acceptable alternative where minimum tooling and common CNC machines are employed. There is an extensive amount of studies in scientific literature addressing the feasibility of hole-flanging by PW due to its long history of use. It is worth to highlight the works dedicated to study the formability of circular hole-flanging using the concept of LFR (limiting forming ratio) [2]. This parameter defines the maximum HER (hole expansion ratio) attainable by the material before the onset of necking as HER =df d0 ,LFR ≡HERmax =df d0max (1) where d0is the pre-cut hole diameter and dfis the inner diameter of the produced hole-flanged part. The recent 2 paper by Paul [3] presents a review of these parameters focused on the effect of punch geometry, the fundamentals of deformation and damage, the impact of edge preparation method, the influence of various uniaxial tensile properties and the effect of microstructure. In particular, the works by Huang and Chien [2, 4] pointed out that failure at the edge of flanged hole is controlled by tensile stress in circumferential direction and the LFR is independent of the punch radius. These conclusions have also been observed in a recent experimental work by Borrego et al. [5], who elucidated the actual influence of the bending effect induced by the punch edge radius as well as the apparent independence of the punch profile on the flange formability. On the other hand, Levy and Tyne [6] and Pathak et al. [7] found a slightly increase of the calculated maximum strain at the hole edge using a conical punch in hole expansion tests. However, these differences in terms of formability are due to the differences in the strain states accomplished in each test type [7]. Although there is a variety of parameters affecting the LFR, such as the mechanical properties of the metal sheet, the punch geometry, the punch-die clearance and others regarding the quality of the pre-cut hole edges, this parameter has proven to be a valid measure of flangeability in PW. On the other hand, SPIF since its patent in 1967, either by Berghahn [US 3316745] [8] or by Leszak [US 3342051] [9], has been the subject of numerous studies due to its flexibility and the apparent enhancement of the sheet formability, among other beneficial features compared with conventional sheet metal forming processes. Thus, there are relevant scientific works addressing the influence of material properties and thickness, tool size, contact area, lubrication, tool rotation and feed rate, step down of the tool path, etc. The paper by Behera et al. [10] describes exhaustively the state of the art of this technology up to 2015, highlighting other main contributions in academia and industry. A newer review by Duflou et al. [11] gives a current overview of the development of the SPIF process and its applications. Due to the particular geometry of the holes, and probably inherited from the PW process, most studies on holeflanging by SPIF have analysed the process formability via the LFR. The pioneering research belongs to Cui and Gao [12] who firstly analysed the formability in terms of the LFR and thickness distribution using three different multi-stage strategies. They found significant differences in the LFR between SPIF strategies for AA1060 sheet. Later, Borrego et al. [13] showed that the LFR also depends on the tool tip size in SPIF, contrary to what happens in PW. Results pointed out that the LFR value increases as the punch radius increases, which leads to an apparent conflict with accepted trends in SPIF (see e.g. the work of Bhattacharya et al. [14]). Besides of the LFR, Borrego et al. [13] also concluded that the failure mechanism in SPIF, either by necking or fracture, is highly influenced by the strain/stress gradient through the sheet thickness, which is typically characterized by the sheet thickness to tool radius ratio. In general, a small tool tip should improve the sheet formability by delaying or suppressing the onset of necking, as reported by Morales-Palma et al. [15] in stretch-bending and 3 Silva et al. [16] and Centeno et al. [17] in SPIF. However, an excessively small tool tip reduces the expected enhancement of formability by blocking the material flow around the forming tool. In this sense, Al-Ghamdi and Hussain [18] pointed out the existence of a threshold tool radius that maximizes the formability in SPIF. Other researches have found that the improvement of formability in SPIF using smaller tools and compared to conventional processes also occurs in terms of fracture limit strains. For instance, Malhotra et al. [19] found for AL5052 sheet a higher fracture strain in SPIF (1.83) that in deep-drawing (1.43). Hirt et al. [1] performed a numerical study of SPIF using A199.5 aluminium sheet and qualitatively found that smaller tool tips produce higher fracture limits. This apparent enhancement of formability was attributed by Martinez-Donaire et al. [20] to the minor levels of stress triaxiality on average in SPIF compared to conventional forming processes. Other discrepancies in the use of the LFR as a flangeability parameter have been found when comparing holeflanging processes by PW and SPIF. Silva et al. [21] and Montanari et al. [22] reported a higher LFR value in PW than in SPIF for Titanium (grade 2) and stainless steel AISI304L sheets, respectively. They concluded that SPIF did not produced the expected enhancement in formability respect to the conventional process. In both studies a small tool tip (4-mm radius) was used. On the other hand, Borrego et al. [13] found similar discrepant results, but only when using small tool tips (6 and 8-mm radius). Conversely, the LFR with a larger tool (10-mm radius) was as expected, that is, greater in SPIF than in PW. In addition to the LFR, just a few authors have applied the Forming Limit Diagram (FLD) as an analysis tool on hole-flanging by SPIF. Among these authors, Centeno et al. [23] analysed the strain distribution along the flange in the FLD for AA1050-H111 sheet. They concluded that a different mode of deformation is triggered in hole flanged parts, as opposed to in parts without holes, involving a combination of in-plane stretching on the flange and local bending induced by the forming tool. This occurrence explains the enhancement of formability in incremental sheet forming compared to conventional processes. Similar analysis in the FLD were carried out by Silva et al. [21] and Montanari et al. [22]. They used CGA (Circle Grid Analysis) to evaluate the flange deformation, however this technique does not allow to accurately measure the deformation near the hole edge which is closely related to the LFR. Theses authors attempted to justify the aforementioned discrepancies in LFR when comparing PW and SPIF by analysing the strain paths in the FLD and the distance from the FLC (Forming Limit Curve for necking in PW) to the FFL (Fracture Forming Limit for fracture in SPIF) curves. However, an analysis based on the LFR might be confusing since this parameter does not provide enough information about material deformation, thickness distribution or the expected onset and type of failure, as pointed out by Borrego et al. [5]. Regarding the SPIF strategy for hole-flanging, most research work have focused on multi-stage operations [12, 21, 22]. However, Borrego et al. [13] reported that a single-stage SPIF (SPIF1) strategy, which reduces consid- 4 erably the production time, is a feasible process to obtain functional flanges. From this point of view, it seems reasonable to evaluate the feasibility of SPIF1, rather than another SPIF variants, as an alternative process to holeflanging by PW. Beyond the modification of tool trajectories, other alternatives have been proposed to speed up the entire process, for example, improving the fixing system, e.g. using electromagnetic devices [24], or reducing computational analysis, e.g. developing simple mechanics-based methodologies to accurately predict the final part geometry [25]. In view of the above, this works aims to perform a critical comparative analysis of the mechanic of deformation in both incremental (SPIF1) and conventional (press-working) hole-flanging processes, contributing to a better comprehension of the factors that control the sheet formability in hole-flanging and its appropriate measure of the flangeabilty regardless of the forming process itself. The main contributions are: (1) to perform a critical comparative analysis of the strain distribution along the flange using DIC (Digital Image Correlation) techniques; (2) to highlight some limitations when using the LFR to quantify the flangeability on incremental processes such as SPIF; (3) to evaluate two new additional geometric parameters to the LFR that are able to quantify the flangeability regardless of the forming process: non-dimensional flange height and theoretical thickness ratio; and (4) to analyse the differences in flange thickness in PW and SPIF. To this end, experimental results of hole-flanging tests by PW and SPIF1 using different forming tools are analysed and compared for AA7075-O sheet. 2 Experimental procedure To perform the comparison between the two manufacturing technologies of hole-flanging, the experimental results carried out by Borrego et al. [5, 13] are analysed. The sheet material was aluminium alloy 7075-O and the manufactured parts by PW and SPIF1 shared the same geometrical characteristics such as the initial and final hole diameters. To induce similar bending conditions in the flange during its deformation, a set of forming tools was designed with the same radii R= 6, 8 and 10 mm, where Ris the edge radius of the punch in the PW process and the tool radius in SPIF. This section presents the material characterization of the metal sheet and summarizes the main results of the experimental hole-flanging tests. 2.1 Material characterization The material used for the experimental work was aluminium alloy 7075-O sheet of 1.6-mm thickness. The mechanical characterization was carried out by means of tensile tests at room temperature, according to standards ASTM E8 [26] and ASTM [27]. Specimens were cut at 0◦, 45◦and 90◦with respect to the sheet rolling direction to evaluate the influence of material anisotropy. Resulting stress–strain curves are represented in Figure 1. The Young’s modulus (E), yield strength (YS), ultimate tensile strength (UTS), total elongation (ef) and Lank- 5 Fig. 1. Mechanical characterization of AA7075-O sheets of thickness 1.6 mm. Table 1. Mechanical properties of AA7075-O sheets. Sheet direction E(GPa) YS (MPa) UTS (MPa) ef(%) r Rolling (0◦) 69.7 110 214 20.6 0.65 Diagonal (45◦) 66.7 108 212 19.6 0.97 Transverse (90◦) 74.1 112 218 17.7 0.81 Average x0+2x45+x90 469.5 109 214 19.4 0.85 ford’s anisotropy coefficients (r) are given in Table 1. According to Figure 1 and Table 1, the normal anisotropic behaviour seems to be not very strong. 2.2 Hole-flanging processes An intensive experimental campaign was planned to evaluate the hole-flanging formability limits by PW and SPIF1. Both hole-flanging processes are schematically represented in Figures 2 and 3. Figure 2 illustrates the conventional forming process by press-working, where the blank is rigidly supported between the blank holder and the backing plate. The tests were carried out using an Erichsen 142–20 machine (drawing force 200 kN, clamping force 120 kN) with a blank holder and a 100-mm diameter die. A series of punches with an equal circumferential diameter df=95.8 mm and different edge radii (R) was used to analyse the effect of the punch profile on the formability. Four edge radii of 6, 8, 10 and 47.9 mm (hemispherical punch) were selected. Figure 3 presents the schema of hole-flanging by SPIF1 as well as the experimental setup and the set of forming tools. The tests were carried out on a EMCO VMC-200 3-axis milling CNC machine (axial force 6 kN, lateral force 4 kN). The SPIF setup comprised a blank holder and a backing plate with a 100-mm diameter 6 Fig. 2. Hole-flanging by PW: (a) schema of the process, and (b) set of punches with different edge radii R. Fig. 3. Hole-flanging by SPIF1: (a) schema of the process, (b) experimental setup, and (c) set of hemispherical tools with different radii R. hole. The sheet blanks were fixed by the holder over the backing plate and were incrementally deformed by a hemispherical tool following a helical trajectory, as illustrated in the schema. Three different tool radii were used (R= 6, 8 and 10 mm) to analyse the effect of the sheet thickness to tool radius ratio (t0/R) on formability. The tool trajectory was modelled and simulated in CATIA® using the machining workbenches. The tool movement was set to 0.2 mm/turn of step down and 1000 mm/min of feed rate. Two tool rotation conditions were tested, 0 rpm (locked tool) and 1000 rpm clockwise. The principal strains at the outer sheet surface were measured using the optical 3D strain analysis system 7 Table 2. Compilation of experimental hole-flanging tests by PW and SPIF1 at 0 and 1000 rpm of spindle speed that resulted in failed (X) and successful (O) flanges. RHF Initial pre-cut hole diameter, d0(mm) (mm) 35 41 55 56 57 57.5 58 59 60 61 62.5 63.5 64.5 65 70 72 82 83 PW X X X O O O O 6 SPIF10X X X X O O O O SPIF11000 XOO PW X X X O O O O 8 SPIF10X X O O O SPIF11000 XOO PW X X O O O O 10 SPIF10X X X O O O O O SPIF11000 X X O O 47.9 PW X X O O O O ARGUS (GOM mbH). Measurements were carried out at the end of the test or after the specimens reached failure. The strains at the hole edge, where ARGUS is not able to provide information [13], were calculated from direct measurement of the final and initial perimeter of the hole and the final and initial sheet thickness, and assuming material volume constancy. Although the material incompressibility is a good assumption for plasticity, it should be noted that it does not apply to ductile fracture since it can be preceded by damage induced by the growth and coalescence of voids [28]. Nevertheless, material volume constancy can be assumed at the hole edge, where no fracture occurs. To analyse the geometric capability of the process, finished flanges were cut to measure the final height and the sheet thickness distribution along the flange. The material anisotropy had a weak influence on the overall geometry of the final flange and could not clearly be assessed. This is in agreement with the normal anisotropic behaviour of the material (see Figure 1 and Table 1). Table 2 presents the results of all experimental tests. The successful and failed flanges (by necking or ductile fracture) are distinguished with Oand X, respectively. Rrepresents either the punch edge radius in PW or the tool radius in SPIF. The sheet blanks were cut out from the supplied sheets with circular holes of different diameters (d0, see Table 2). The central holes were milled and subsequently ground with fine grit sandpaper to eliminate any burrs. For each type of experimental test using a given tool, the hole diameter d0was adjusted in successive tests 8 to identify the LFR value according to Eqn. (1). A representative set of experimental tests was selected in this work to compare the results of both manufacturing technologies PW and SPIF1. There are a priori different selection criteria to carry out the comparison. However, in order to compare geometric features, the analysis of tests using blanks with same geometry seems to be an appropriate criterion. In this regard, specimens with d0=61 and 65 mm are good candidates due to they were successfully tested in a variety of hole-flanging operations, as can be seen in Table 2. Furthermore, the set of tests to be compared was limited to those that used forming tools with the same radius Rto evaluate the bending influence during the flange formation. Notice in Table 2 that successful tests using d0=61 mm and R=8 mm were close to failure in all PW and SPIF1 processes. Indeed, specimens with holes 1 or 2 mm in diameter smaller led to material failure by necking or by ductile fracture. As pointed out by Borrego et al. [13], the use of spindle speed in SPIF provides a better surface quality, a more regular thickness profile along the flange, and it reduces the forming force. However, it does not significantly increase the formability in terms of the minimum pre-cut hole. Therefore, the best candidates of hole-flanging tests to be compared are those formed by PW and SPIF1 at 0 rpm using tools with R= 8 mm, yielding successful and failed parts for d0= 61 and 60 mm, respectively. 3 Results 3.1 Strain analysis The analysis of strain distribution allows to understand the geometrical differences of flanges manufactured by PW and SPIF1. Figure 4 presents the principal major (ε1) and minor (ε2) strain distribution at the outer sheet surface in flanged parts by PW and SPIF1 with d0=61 mm and R=8 mm. The measurements were performed along the flange following a path in the rolling direction, as illustrated schematically in the two specimens. The solid lines represent strains measured with ARGUS. Points represented with a star at the end of the dotted lines were obtained from direct measurements of thickness (to evaluate ε3) and circumferential perimeter in the hole edge (to evaluate ε1, and estimating ε2via volume constancy). As can be seen in Figure 4, the representative loop that describes the strain distribution of specimen tested by SPIF1 contrast with the simpler shape obtained by PW. Actually, the loop shape is a consequence of the inversion of the major and minor strain that occurs along the flange. Indeed, it was observed in all specimens that the major and minor in-plane strains were aligned with the in-plane principal directions of the sheet (i.e. the circumferential and meridional directions), which is due to the axial symmetry of the hole-flanged parts. Thus, the loop shape can be better understood by plotting the strains of the specimen in circumferential (εc) and meridional (εm) directions (see Figure 5), instead of the conventional major and minor strains. As can be seen in Figures 4 and 5, the strain 9 Fig. 11. Thickness distribution along the wall of hole-flanged parts: specimens tested by PW and SPIF1. the middle of the flange. Instead, the flange by PW has a more homogeneous and less thinned thickness as well as a lower height. The local and incremental deformation induced in the material by SPIF1 intensively stretches the material in the meridional direction and, therefore, produce higher and thinner flanges. Conversely, the conventional hole-flanging process provides shorter but thicker and more robust flanges. The practical application and service requirements of the flange would determine the more suitable process. The excessive thinning of the flange by SPIF1 may be improved using a multi-stage SPIF strategy. However, multi-stage strategies have the disadvantage of increasing the production time as well as the process complexity. In this sense, the minimization of the number of stages and the optimisation of the intermediate shape between passes are mandatory to make the SPIF process competitive in practice [29]. Other proposed alternatives to homogenize the thickness of the sheet in SPIFed components are, among others, the use of customized forming tools, especially for hole-flanged parts [30–32]; the application of complex tool paths [33, 34]; or the use of a counter-tool, that is, the variant named Double Sided Incremental Forming (DSIF) [33,35]. 4 Conclusions Hole-flanging processes by PW and SPIF1 have been compared in terms of flangeability and flange geometry. The main conclusions can be summarised as follows: 1. Regarding the strain distribution along the flange, the main difference between both processes is derived from the evolution of the meridional strain. In SPIF1, this strain displays a high positive value around the middle of the flange, which is responsible for a longer and thinner flange than the one in the conventional process. 2. The analysis based on the FLD satisfactorily reflects the failure mechanisms that control the flangeability in 16 both processes. In PW, the failure is triggered by necking at the flange edge. In SPIF1, the failure initiates by ductile fracture in the middle of the flange wall. Both types of failure have been properly described by the FLC and FFL curves. 3. The traditional LFR is not an appropriate parameter for measuring the formability in hole-flanging by SPIF as it does not capture the physics of failure for this process. For this reason, it should be used carefully when comparing flangeability using different SPIF strategies or tool radii. An approximate value of LFR ≈1.6 is obtained for both PW and SPIF1 processes. 4. The non-dimensional flange height (h/df) and thickness ratio (t/t0) are suitable parameters for analysing the flangeability in both PW and SPIF1 processes. In terms of these parameters, SPIF1 exhibits a significant enhancement in flangeability regarding the PW process: up to 47% of flange height and 113% of average thickness reduction. 5. Regarding the flange geometry, SPIF1 produces longer and thinner flanges than PW for a given HER value. However, the lack of homogeneity in thickness by SPIF1 must be solved by using a multi-stage SPIF strategy. Without considering the restrictions induced by the production volume and the thickness distribution, the above results conclude that SPIF would be a suitable process to perform hole flanges with high flexibility in shape. In this case, multi-stage SPIF strategies will allow the flange to be further stretched and thinned in its meridional direction avoiding the appearance of necking and improving flangeability even in terms of LFR. 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