Effect of hot isostatic pressing on the microstructure and mechanical properties of porous Ti-6Al-4V alloy manufactured by laser powder bed fusion

Volume 5 Issue 1 (2026) 1 doi: 10.36922/MSAM025260055 ORIGINAL RESEARCH ARTICLE Effect of hot isostatic pressing on the microstructure and mechanical properties of porous Ti-6Al-4V alloy manufactured by laser powder bed fusion Marketa Strakova1* , Jiri Kubasek1, Jonas Divin1, Jan Pinc2, and Dalibor Vojtech1 1Department of Metals and Corrosion Engineering, University of Chemistry and Technology, Prague, Czech Republic 2FZU-Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic Materials Science in Additive Manufacturing Abstract Laser powder bed fusion (LPBF) enables the production of Ti-6Al-4V alloys with tailored porous structures, which are beneficial for biomedical applications due to their reduced elastic modulus and enhanced bone integration potential. This study examines the effect of hot isostatic pressing (HIP) on the microstructure and mechanical properties of diamond and gyroid porous structures fabricated by LPBF. Solid tensile specimens served as reference materials. HIP significantly reduced porosity, decreased ultimate tensile strength and hardness, but markedly increased ductility (from 6% to 17%). Compressive strengths reached approximately 100 MPa (diamond) and 240 MPa (gyroid), with HIP causing only a slight increase in strain. However, HIP notably improved bending performance, raising the flexural strength of gyroid structures from 280 MPa (as-printed) to 340 MPa (post-HIP). The strength of LPBF-fabricated Ti-6Al-4V porous structures is reduced by HIP, but their ductility and bending performance are enhanced, making them more suitable for biomedical applications. Keywords: Ti-6Al-4V; Laser powder bed fusion; Hot isostatic pressing; Porous material; Mechanical properties; Lattice structures 1. Introduction The aerospace, sporting goods, and petrochemical industries widely use titanium and its alloys, especially Ti-6Al-4V, due to their relatively low density, high strength, and good corrosion resistance.1-4 Furthermore, this alloy is of particular importance in biomedical engineering because of its great mechanical performance. It also has a high strength-to-weight ratio, low modulus, high fatigue strength, and biocompatibility.5 Ti-6Al-4V is particularly useful in the manufacture of artificial joints, bone implants, prosthetics, and surgical instruments, offering a good balance of strength, flexibility, and toughness. Aluminum makes the alloy stronger and lighter, while vanadium adds flexibility and stability. The applicability of Ti-6Al-4V alloy in biomedicine, as well as its potential modification to improve surface properties, has been a major topic in scientific research.1,2,6 *Corresponding author: Marketa Strakova ([email protected]) Citation: Strakova M, Kubasek J, Divin J, Pinc J, Vojtech D. Effect of hot isostatic pressing on microstructure and mechanical properties of porous Ti-6Al-4V alloy manufactured by laser powder bed fusion. Mater Sci Add Manuf. 2026;5(1):025260055. doi: 10.36922/MSAM025260055 Received: June 27, 2025 Revised: July 25, 2025 Accepted: August 11, 2025 Published online: October 13, 2025 Copyright: © 2025 Author(s). This is an Open-Access article distributed under the terms of the Creative Commons Attribution License, permitting distribution, and reproduction in any medium, provided the original work is properly cited. Publisher’s Note: AccScience Publishing remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Materials Science in Additive Manufacturing Effect of HIP on LPBF porous Ti-6Al-4V alloy Volume 5 Issue 1 (2026) 2 doi: 10.36922/MSAM025260055 The production process of titanium is so challenging that it makes it more valuable than steel or aluminum. Given the high material costs, it is advantageous to make parts that need very little post-processing.3 Ti-6Al-4V is also an ideal candidate for innovative production methods, such as powder-based additive manufacturing (AM). In AM, components are fabricated in a net-shape process by fusing successive layers of metallic raw material to a base material using a pre-programmed scanning pattern of a focused laser or electron beam.5,7,8 The powder can be applied either through powder bed fusion (PBF), which involves spreading and selectively melting individual layers of powder, or through direct energy deposition, which delivers powder continuously to the melt pool through coaxial jets.9,10 In theory, AM can produce completely dense, three-dimensional components with highly intricate geometries.11,12 However, AM-fabricated alloy often exhibits relatively high porosity.12 The rapid melting and solidification inherent to AM vary with part geometry and path planning, and the layered nature of the process introduces non-fusion defects at layer and laser pass interfaces. Consequently, it becomes challenging to manufacture fully dense, defect-free components with consistent mechanical properties across multiple batches. Anumber of researchers have used hot isostatic pressing (HIP) to reduce pore size and homogenize microstructural properties through post-processing heat treatment.12,13 Vilaro et al.14 reported that the mechanical behavior of AM Ti-6Al-4V is strongly influenced by its high porosity, with macroscopic ductility strongly affected by pore orientation and shape. Advances in AM technologies have been rapid, with laser PBF (LPBF) emerging as one of the key PBF techniques for producing the demanding α + β Ti-6Al-4V alloy.15,16 Recent studies highlight LPBF’s potential for fabricating multi-material parts with graded layers, significantly affecting interfacial bonding and mechanical integrity.17 LPBF builds three-dimensional metal parts by melting metallic powders layer by layer with a laser, and the high cooling rate promotes rapid solidification and phase transformations. This method produces parts in the desired shape and size directly, minimizing the need for additional machining.5,18,19 Its capability to produce complex 3D medical parts with high precision has driven demand for LPBF in biomedical applications.6 LPBF can also create porous implant models, which can reduce the Young’s modulus, mitigate stress shielding, and improve osseointegration.20 The mechanical response of LPBFfabricated porous structures can be tuned by varying unit cell geometry and orientation, significantly influencing impact resistance and energy absorption.21 The porous part helps to avoid stress shielding and provides biological fixation by allowing tissue ingrowth. In addition, porous structures can act as efficient drug delivery reservoirs to suppress post-surgical inflammatory reactions. Thus, incorporating porous parts into implants can help mimic the behavior of human bone.22 Significant research efforts have been devoted to optimizing both the manufacturing process and surface modification of porous titanium alloys to enhance their performance in medical applications.23 In a study by Gao et al.,24 porous titanium implants with a gradient modulus were found to increase femoral surface deformation by 17.1%, reducing the stress shield effect, bone loss, and aseptic loosening. Verma et al.25 confirmed that the porous architecture reduced the effective modulus of elasticity by more than 95%, thereby reducing the stressshielding effect. Monotonic tensile properties superior to those of conventionally fabricated alloys have been reported for Ti-6Al-4V manufactured by LPBF. However, its fatigue resistance decreases during cyclic loading due to internal porosity.15,16 Even with optimized parameters, it is not possible to achieve fully dense parts.6,26 Recent studies have shown that this issue can be addressed by optimizing the microstructure through precise thermal annealing, which can be especially effective in balancing strength and ductility when performed near the β-transus temperature, typically between 700°C (stress-relieved state) and 1080°C (solution-treated state).14,27-29 In stress-relieved samples, only minimal changes in grain size are observed; however, complete dissolution of martensitic phases occurs during solution treatment, leading to the formation of an equilibrium (α + β) microstructure.19 A comprehensive review of additively manufactured titanium alloys confirms that their microstructure, defect distribution, and mechanical properties are highly dependent on the AM technique employed and the subsequent postprocessing conditions.30 Thermal annealing also produces the expected grain refinement.19 With increasing annealing temperature, tensile strength initially increases and then decreases, with a similar trend observed in elongation data.16 Thermomechanical HIP has also been demonstrated to be effective in minimizing defects associated with porosity in LPBF-printed components.31 HIP is a process that applies high pressure and temperature evenly to materials in a high-pressure container, which helps to remove internal porosity, improve material density, and enhance mechanical properties.32-34 Increasing HIP pressure promotes the formation of the β phase, while higher HIP temperatures lead to a reduction in the amount of α and β phases. Simultaneously, the tensile strength and yield strength of LPBF-produced Ti-6Al-4V Materials Science in Additive Manufacturing Effect of HIP on LPBF porous Ti-6Al-4V alloy Volume 5 Issue 1 (2026) 3 doi: 10.36922/MSAM025260055 decrease as the HIP temperature rises.35 HIP treatment is also known to eliminate microstructural differences across layers in Ti-6Al-4V, resulting in α + β layered structures that enhance energy uptake through plasticity induced by phase transformation.36 Following HIP treatment, the fatigue strength at 8 × 106cycles has been reported to attain 355 MPa, which is on par with that of conventional heat-treated materials.29 Therefore, this study used post-processing involving both heat treatment and HIP to examine the differences in structure and material properties of the Ti-6Al-4V alloy manufactured by LPBF in three configurations: As-built compact structure, diamond structure, and gyroid porous structure. While most reported studies deal with HIP processing of dense AM Ti-6Al-4V alloys, the present study focused on testing HIPed porous structures, which hold promise for use in medical implants. To the best of our knowledge, there is limited information on the HIP processing of porous AM-processed Ti-Al-V alloy. Specifically, diamond and gyroid structures were selected for investigation because they are widely utilized in the medical field. These two types of porous structures differ primarily in the size, shape, and arrangement of struts and in the geometry of interconnected pores between them. Mechanical characterization performed in this study included tensile, compressive, flexural, and hardness testing, accompanied by a detailed structural analysis. 2. Materials and methods 2.1. Materials and processing For this work, Ti-6Al-4V flat bulk tensile samples (70×5×3mm; Figure1) prepared by the LPBF method were used as reference materials (Table1). The samples were printed vertically in the LPBF chamber and examined in both the as-printed and HIPed states. The conditions for HIP process (950°C, 2h, 150 MPa, Argon) were selected according to our preliminary experiments and literature,37 with the temperature chosen to promote transformation of the martensitic phases to the equilibrium (α + β) structure while limiting grain coarsening. The focus of our research was on the porous structure fabricated by LPBF in two types of structural lattices—diamond and gyroid—which belong to the class of triple periodic minimal surfaces (TPMS). All samples were built on a thin, non-porous substrate. Models of these structures, including elementary cells and larger structures, were generated using the MSLattice program38 (Figure2). The computer-aided design (CAD)-generated porosity was set to 70% for both diamond and gyroid types. The experimental macro-scale porosity was evaluated using the gravimetric method to determine the percentage of free space in the porous structures. The base was removed before weighing, and the sample volume was calculated from measured dimensions. Porosity was determined according to Equation (I): ρ ρ = −1 real theoretical Porosity (I) Where the theoretical density corresponds to the intrinsic material density of Ti-6Al-4V, and the actual density is calculated from the sample’s measured mass and geometrical volume. Using this method, the macroporosity values of 69% (diamond) and 67% (gyroid) were obtained, both of which are in good agreement with the designed value of 70%. Any minor deviations are attributed to imperfections in Figure1. Drawing of printed samples for tensile tests (dimensions given in mm). Reprinted from Strakosova etal.18 Table1. Supplementary data on the mechanical properties of as‑printed and HIPed bulk samples, and porous Ti‑6Al‑4V samples Sample YS (MPa) UTS (MPa) E (GPa) ε (%) HV1 Micro‑porosity (%) P_AP 1190±8 1283±19 108±7 3±1 353±11 3.62 P_HIP 930±4 1024±3 110±6 17±1 323±7 0.02 Sample CYS (MPa) UCS (MPa) CD max (%) UBS (MPa) HV0.5 Macro‑porosity (%) D_AP 81±6 99±1 13±1 164±9 395±18 69 D_HIP 68±2 102±2 16±2 214±35 362±7 66 G_AP 162±5 249±3 23±1 261±10 415±14 67 G_HIP 128±13 222±5 34±1 315±15 316±6 63 Abbreviations: AP: As-printed; CYS: Compressive yield strength; CD max: Maximum compressive deformation; D: Diamond structure; E: Young’s modulus; G: Gyroid structure; HIP: Hot isostatic pressing; HV0.5: Vickers hardness (500 g load); HV1: Vickers hardness (1 kg load); UCS: Ultimate compressive strength; UBS: Ultimate bending strength; UTS: Ultimate tensile strength; YS: Yield strength; ε: Elongation. Materials Science in Additive Manufacturing Effect of HIP on LPBF porous Ti-6Al-4V alloy Volume 5 Issue 1 (2026) 4 doi: 10.36922/MSAM025260055 the LPBF manufacturing process, such as partial melting or powder adhesion. Both compact and porous samples were printed using a ConceptLaser M2 Cusing printer (ProSpon, Czech Republic) equipped with a 200 W Yb: YAG fiber laser. The printer has a working area of 250 × 250 × 280 mm3 and operates under a protective argon atmosphere with an oxygen content of up to 0.5% by volume. Continuous mode was used for printing. The scanning speed was 1250mm/s, the layer thickness was 30µm, and the hatch spacing was 80µm. 2.2. Microstructure and porosity Phase composition of the given samples was determined by X-ray diffraction (XRD) using PANalytical X’Pert PRO system (PANanalytical, Holland) equipped with a copper tube (Kα radiation, λ = 0.15406nm), Scans were performed over a 2θ range of 5–89° with a step size of 0.039° and a generator setting of 30mA and 40kV. To examine the microstructure, we used a light optical microscope (Nicon, Czech Republic) and scanning electron microscopes (SEM; Tescan Mira, Czech Republic) equipped with energy dispersive spectroscopy (EDS). Metallographic preparation of samples (Figure 3A) included: (i) Sectioning with a cut-off machine (Q-ATM, Germany); (ii) grinding with SiC abrasive sandpapers (P400–P2500, Q-ATM, Germany); (iii) final polishing with a colloidal silica suspension (Eposil F, 0.1 μm; Q-ATM, Germany); LabTest 5.250SP1-VM universal testing machine (LABORTECH s.r.o., Czech Republic) mixed with 4 parts hydrogen peroxide and 1 part water; and (iv) chemical etching with a Croll’s solution (2mL HNO₃ + 98mL H₂O; Penta, Jersey). Samples for the measurement of volume porosity were cut from the center of the printed discs, with dimensions of 2.5 × 3 × 15mm. These dimensions were selected for the resolution and the ability to detect small pores within the matrix. The volume porosity of the samples was determined using micro-computed tomography (µCT; Zeiss Xradia 610 Versa, Carl Zeiss Microscopy, Germany) at a resolution of 4.5µm/pixel, and the results were analyzed using Dragonfly software (version2022.1.0.1249).36 Matrix material, pores within the matrix (matrix pores), and pores intentionally created by TPMS geometry (structural pores) were identified using histogram segmentation (based on pixel intensity) and classified according to pore size. The volume fractions (vol %) of each phase were determined, and mean Feret diameters were calculated for matrix pores. 2.3. Mechanical properties Mechanical characterization included Vickers hardness (HV1, HV0.5), tensile, compression, and bending tests. Figure2. Porous gyroid and diamond structures. (A) Elementary of the cell gyroid structure. (B) Elementary of the diamond cell structure. (C) Gyroid model with multiple elementary cells. (D) Diamond model with multiple elementary cells. (E) Scanning electron microscopy images of diamond and gyroid structures in their as-printed condition. Reprinted from Školáková et al.39 B C D E A Materials Science in Additive Manufacturing Effect of HIP on LPBF porous Ti-6Al-4V alloy Volume 5 Issue 1 (2026) 5 doi: 10.36922/MSAM025260055 These tests were used to evaluate the influence of processing conditions and resulting structure on mechanical performance. For hardness measurements, a minimum of twenty indentations were made on the cross-sections of bulk samples (aligned with the build direction) at various points on a polished surface (P2500 finish). Measurements were taken both on the surface and in cross-section. An Instron 5882 universal testing machine (Instron Corporation, Germany) with a ±100 kN load cell was used to perform three tensile tests at room temperature for each bulk material. Deformation testing was performed using an MFL 800B extensometer (MF Mess-und Feinwerktechnik GmbH, Velbert, Germany). The specimens were loaded incrementally at a rate of 5 mm/min until failure, after which fracture surfaces were examined using SEM. Compression and bending tests were carried out on porous diamond and gyroid structures using a LabTest 5.250SP1-VM universal testing machine (Eposil F, 0.1 μm; Q-ATM, Germany); LabTest 5.250SP1-VM universal testing machine (LABORTECH s.r.o., Czech Republic) at room temperature. For each geometry, three compression tests (Figure3B) and three bending tests (Figure3C) were performed. Compression properties were evaluated at a constant loading rate of 5 mm/min. Bending tests were conducted using a three-point setup, where two support pins were inserted through the porous section and a third loading pin applied force vertically through the base of the sample (Figure3C). The bending test specimens were cut from printed discs into beams with approximate dimensions of 11–15mm (length) × 3 mm (width) × 3 mm (height). Due to the limited number and small size of samples, a specific ASTM C1161 standard could not be followed for flexural testing. The dense base remained attached and was placed on the compression side (top) during testing. In all cases, the fracture occurred in the porous structure rather than at the base, indicating that the porous region governed the mechanical behavior. The presence of the base may have influenced the stress distribution, which is acknowledged as a limitation of the present test setup. 3. Results 3.1. Microstructure XRD analysis was performed to analyze the phase composition of the samples (Figure 4). In the as-built condition, all samples exhibited the same phase composition, consisting exclusively of the α´-Ti phase—hexagonal martensite formed during rapid cooling from β-Ti. Following HIP, the α´ phase decomposed into α-Ti and β-Ti phases in both bulk samples and the diamond and gyroid structures. The α-Ti phase corresponds to the hcp allotropic modification of Ti and is difficult to distinguish from α´-Ti due to the near coincidence of their diffraction peaks. The alloy’s initial martensitic microstructure after LPBF is shown in Figure5A. The fine martensite is characterized by a typical needle-like structure and is accompanied by a high porosity. For the bulk reference material in the as-printed state, the porosity, determined by the gravimetric method, was 3.62 vol % (Table1), which is Figure3. Schematic representations of the test setups. (A) Cutting a sample to create a metallographic cross-section. (B) Uniaxial compression test setup. (C) Specimen preparation for three-point bending tests B C A Materials Science in Additive Manufacturing Effect of HIP on LPBF porous Ti-6Al-4V alloy Volume 5 Issue 1 (2026) 6 doi: 10.36922/MSAM025260055 in agreement with literature-reported values for standard LPBF parameters. This porosity likely contributed to the reduced ductility observed in mechanical testing. After the application of the HIP process, the microstructure underwent significant changes. The martensite phase transformed into a dual-phase α + β titanium structure (Figure5B). Figure6 shows microstructures of the porous diamond and gyroid structures in the as-printed and HIPed states. In both cross-sections, numerous small internal matrix pores (visible as dark spots) are present within individual struts. HIP transformed the martensite phase into α + β titanium structure and significantly reduced internal porosity to <0.01 vol %. Figure 7 summarizes the quantitative µCT analysis of pores within the metallic matrix. The analysis focused exclusively on pores fully enclosed by the metallic matrix, excluding the designed macroporosity of the TPMS structures. As shown in Figure 7A, the median Feret diameter of matrix pores decreased after HIP by 38% (from 30.7µm to 19.0µm) in the gyroid structure and by 46% (from 30.6µm to 16.3µm) in the diamond structure. The total pore count was also markedly reduced—from ~4,400 to ~400. These conclusions are consistent with the volume ratios of pores shown in Figure7B, illustrating the quantitative description of pores and their mutual relations with the matrix. This comparison is especially important due to potential errors caused by cropping slightly different areas during analysis. For the TPMS structures, the designed macroporosity constituted the largest proportion of sample volume, with minimal impact of the HIP process on this value. In contrast, the matrix pores identified by µCT accounted for up to 1 vol % of the sample volume. These unintended pores were significantly reduced after Figure4. X-ray diffraction (XRD) analysis of the porous samples shows their phase composition Abbreviations: D_AP: Diamond as-printed sample; D_HIP: Diamond hot isostatically pressed sample; G_AP: Gyroid as-printed sample; G_HIP: Gyroid hot isostatically pressed sample; P_AP: Porous as-printed sample; P_HIP: Porous hot isostatically pressed Figure5. Ti-6Al-4V LPBF microstructure (SEM backscatter). (A) LPBF samples with a needle-like martensitic structure in their as-printed condition. (B)LPBF samples after the HIP process at 950°C and 200 MPa Abbreviations: α-Ti: α phase titanium; β: β phase titanium; HIP: Hot isostatic pressing; LPBF: laser powder bed fusion; SEM: Scanning electron microscopy B A Materials Science in Additive Manufacturing Effect of HIP on LPBF porous Ti-6Al-4V alloy Volume 5 Issue 1 (2026) 7 doi: 10.36922/MSAM025260055 HIP, indicating a strong link to improved mechanical properties. However, µCT was not used to evaluate overall macroporosity in bulk samples. 3.2. Mechanical properties It is well established that HIP significantly influences the microstructural evolution of Ti-6Al-4V components, particularly by eliminating internal porosity and transforming the martensitic phase into an α + β equilibrium structure. Since the same HIP process was applied to the additively manufactured porous gyroid and diamond scaffolds, it is methodologically sound to first evaluate its effect on the bulk material. Although tensile testing cannot be directly applied to porous samples due Figure 6. Ti-6Al-4V LPBF porous diamond and gyroid microstructure (SEM back-scattered). Needle-like martensitic structures in the as-printed condition and the corresponding microstructure after HIP at 950°C and 200 MPa Abbreviations: HIP: Hot isostatic pressing; LPBF: Laser powder bed fusion; SEM: Scanning electron microscopy; α-Ti: α phase titanium; β: β phase titanium Materials Science in Additive Manufacturing Effect of HIP on LPBF porous Ti-6Al-4V alloy Volume 5 Issue 1 (2026) 8 doi: 10.36922/MSAM025260055 to their complex geometry and limitations in gripping the sample without causing structural damage, testing the bulk material provides an essential reference. The underlying microstructural mechanisms—porosity closure and phase transformation—are shared between the bulk material and the struts of the porous structures. Therefore, testing the mechanical properties of the bulk material is a relevant indicator of the changes in the mechanical response of the scaffold struts post-HIP. A range of tests was carried out on reference bulk specimens at room temperature to characterize these changes, with the results expressed in terms of hardness measurements (HV1) and uniaxial tensile properties (Figure8A and Table1). The HIP-treated samples exhibited a reduction in hardness from 353 HV1 to 323 HV1, reflecting a phase transformation from a fine martensitic microstructure to a more stable α + β configuration. This transformation was also evident in the tensile properties: The as-built condition exhibited high strength (Ultimate tensile strength [UTS] ≈ 1300 MPa) but limited ductility (elongation ≈ 3%), whereas the HIPed samples displayed reduced strength (UTS ≈ 1024 MPa) and substantially improved ductility (elongation ≈ 17%). The results obtained are in line with the microstructural observations and the hardness trend. Fractographic analysis of the tensile specimens corroborates this interpretation. The fracture surface of the as-built sample exhibited ductile dimples alongside visible, process-induced porosity. In contrast, the HIPed specimen showed uniform ductile fracture morphology with no observable porosity, indicating successful densification of the material (Figure9). The mechanical performance of porous gyroid and diamond structures under compressive and flexural loading is summarized in Figure8B and C, Table1. The gyroid structure demonstrated superior mechanical behavior compared to the diamond structure, due to its continuous curvature and more uniform stress distribution. HIP treatment enhanced both the compressive strength Figure7. Effect of hot isostatic pressing (HIP) on pore size distribution and phase composition in 3D-printed TPMS structures, highlighting changes in Feret diameter and relative fractions of matrix and pores. (A) Box-whisker plot showing the mean Feret diameter of as-printed and HIPed samples. (B) Relative proportions of the matrix, matrix pores, and the intentionally created pores within the printed TPMS structures Abbreviations: HIP: Hot isostatic pressing; TPMS: Triply periodic minimal surface B A Figure8. Mechanical properties of the Ti-6Al-4V alloy. (A) Typical stress-strain curves of bulk LPBF samples. (B) Compressive stress-strain curves of the porous samples. (C) Bending stress-strain curves of the porous samples Abbreviations: D_AP: Diamond as-printed sample; D_HIP: Diamond hot isostatically pressed sample; G_AP: Gyroid as-printed sample; G_HIP: Gyroid hot isostatically pressed sample; LPBF: Laser powder bed fusion; P_AP: Porous as-printed sample; P_HIP: Porous hot isostatically pressed B C A Materials Science in Additive Manufacturing Effect of HIP on LPBF porous Ti-6Al-4V alloy Volume 5 Issue 1 (2026) 9 doi: 10.36922/MSAM025260055 and the plastic deformation capacity of both topologies. For example, the compressive strain of failure increased from 23% to 34% in the gyroid structure. Although there was a slight reduction in ultimate compressive strength (from249 MPa to 222 MPa), which is likely due to reduced dislocation density and stress relaxation, the increase in ductility is important for applications requiring mechanical reliability under cyclic or impact loading. This is particularly important for biomedical implants, where structural integrity and damage tolerance are essential. Differences in elastic modulus were also observed between the two types of scaffolds. The gyroid structure exhibited an elastic modulus approximately 1.6 times higher than the diamond structure in the as-built condition and 1.4times higher after HIP treatment. This reflects the influence of geometric design on stiffness and mechanical anisotropy. Three-point bending tests (Figure8C) confirmed the superior flexural performance of gyroid architecture. HIP processing resulted in a ~30% increase in maximum bending stress for both geometries. However, due to progressive micro-fracturing during loading, the stressstrain curves did not display a distinct linear region, which makes precise modulus determination impractical. SEM of fractured porous samples (Figure10) revealed residual porosity and unmelted particles in the LPBF condition, primarily in the cores of the struts. After HIP, these defects were eliminated, and the fracture surfaces exhibited fully ductile morphologies dominated by equiaxed dimples. Although macroscopic plastic deformation was not visible during the bending test, the higher fracture strain observed in the HIPed samples compared to the as-built ones is consistent with the tensile test data. 4. Discussion 4.1. Microstructure The thermal history of Ti-6Al-4V has a significant impact on its microstructure, which in turn is influenced by the applied temperature and cooling rates.4,40 The LPBF process involves subjecting powdered material to extreme cooling rates (103–106 K/s). This is achieved by rapidly melting the material with a highly energetic laser beam. This induces a fine acicular martensitic α′ microstructure (Figures 5 and 6) and results in the material having a relatively high residual stress. Directional solidification and repeated thermal cycling during layer formation also lead to pronounced anisotropy.37-39 To address these limitations, post-processing methods, such as HIP are commonly employed. HIP simultaneously applies high pressure (>100 MPa) and an elevated temperature (≈920°C for Ti-6Al-4V), which promotes phase transformation (α’ → α + β), heals internal defects, and homogenizes the microstructure.28-30 As demonstrated in previous studies, this process improves ductility and fatigue resistance while reducing anisotropy.31,41 Although grain coarsening during HIP can cause a slight decrease in tensile and yield strength, the process generally produces a more balanced set of mechanical properties compared to as-built LPBF parts.42 However, in the context of porous structures, HIP introduces an additional dimension of complexity: Dimensional stability and structural integrity. Under multiaxial compressive stress and at an elevated temperature, internal pores collapse and surface-connected defects gradually close, leading to a redistribution of material volume. This localized densification can induce slight but significant dimensional changes, particularly in complex porous architectures, such as lattice structures. These changes can affect the geometric fidelity and mechanical response of the final part. Since pore volume and connectivity vary according to unit cell topology (e.g., gyroid vs. diamond), the magnitude of structural shrinkage may also differ. For Figure9. Fracture surfaces of LPBF-produced porous samples before and after HIP treatment. Corresponding high-magnification details of the fracture surfaces are shown below each main image, demonstrating the typical differences in fracture morphology, pore distribution, and surface roughness caused by the HIP Abbreviations: HIP: Hot isostatically pressed; LPBF: Laser powder bed fusion; P_AP: Porous as-printed sample; P_HIP: Porous hot isostatically pressed