Effects of different treatment processes on microstructure and tensile fracture behaviour of selected laser melting TC17 titanium alloy

It is essential to study the tensile fracture mechanism of additively manufactured materials and develop the effective process treatment techniques to improve their fracture resistance. In this paper, the effects of different treatment processes on the microstructure and tensile fracture properties of TC17 titanium alloy melted by laser selective zone melting were investigated. The static tensile fracture morphological characteristics were observed by combining SEM and EDS. The metallographic microstructure after chemical corrosion was observed optically. And the tensile fracture morphology of the three states of TC17 titanium alloy samples at room temperature conditions was investigated. The results show that the metallographic matrix microstructure of TC17 titanium alloy after HT, HIP and HIP-HT treatments was a bimodal structure with α+β phases, i.e., β phase was in the form of a net basket, and α phase was in the form of a coarse bar. The grain sizes of the samples treated by different processes were different, but the difference in grain size of the HT-treated tissues was small, and the difference in grain size of the HIP-HT-treated tissues was large. And the coarse α-phase segregation could be seen at the edge of the samples. The 3D-printed materials had complex changes in anisotropic properties affected by the printing structure and tissue. The printed tissues were brittle and had high internal stresses. These problems were partly improved at high temperatures, but they still existed. The HIP-HT-treated materials had a large α+β-phase bimodal structure. The HT-treated material had coarse grains and precipitation phases, poor room temperature plasticity, and improved high temperature plasticity. After HT treatment, the original printing microstructure changed, the strength and plasticity were significantly improved, but the macroscopic printing structure still had a slight influence on the fracture morphology. When HIP treatment temperature was higher, the influence of macroscopic printing structure basically disappeared, but the grain and microstructure grew up, and the strength and plasticity were slightly lower than that of the HT treatment. HIP process basically eliminated the unfused defects of the three-dimensional morphology, and the local weak bonding zone formed.


Introduction
Titanium alloys play a pivotal role in aerospace applications owing to their exceptional properties such as high specific strength, corrosion resistance, and ability to withstand high temperatures [1][2][3].TC17, a (α+β)-type two-phase titanium alloy with a nominal composition of Ti-5Al-2Sn-2Zr-4Mo-4Cr, stands out as a 'triple-high titanium alloy' due to its exceptional qualities-high strength, toughness, and hardenability.This unique combination aligns seamlessly with design requirements for damage tolerance, structural efficiency, reliability, and cost-effectiveness [4,5].TC17 is primarily available in bar and forging forms, offering versatility in adjusting its mechanical properties through HT treatment.Notably, employing the β forging process yields a refined microstructure, significantly enhancing fracture toughness and fatigue resistance.Its applications span critical components like engine fans, compressor discs, and large cross-section forgings, solidifying its indispensable role in aerospace engineering [6,7].Selective laser melting (SLM) is a significant technology utilizing high-energy laser beams to melt and solidify metal powder, producing parts with high accuracy and superior mechanical properties, making them highly sought after in aerospace applications.Post-treatment plays a crucial role in enhancing the microstructure and properties of printed specimens.Wang Hao et al. [8] examined the impact of heat treatment on the microstructure of TC17 titanium alloy deposited via directed energy.Their investigation revealed that annealing followed by aging treatments triggered the precipitation of secondary needle-like α-phase.As the aging temperature increased, both primary and secondary α-phases grew and coarsened, leading to the disappearance and reappearance of the phase-free precipitation zone.Notably, the diffusion strengthening effect of the α-phase intensified after aging at 580 ℃.Shen Shuxin et al. [9] explored the relationship between microstructure and tensile behavior in dual-phase titanium alloys (TC4, TC11, TC17) fabricated through laser additive manufacturing.Their tests under various temperatures uncovered distinct fracture characteristics: TC4 exhibited ductile fracture at room temperature with minimal α-phase deformation and small tough nest sizes.Conversely, TC11, diffusely strengthened by the secondary α-phase through annealing, displayed high resistance to deformation at room temperature but experienced grain boundary cracking at intermediate temperatures due to the weakening effect on the phase interface.Liu Bingsen et al. [10] extensively studied how interlayer cooling time affects the microstructure and mechanical properties of laser-manufactured TC17 titanium alloy.Longer cooling times prompted a shift from a bimodal to an ultrafine α lamellae net basket microstructure, enhancing strength while reducing plasticity.However, none of the subsequent heat treatments achieved the ideal strengthplasticity balance required for TC17 forging standards (Q/S10-0535-2003).In a related investigation, Chen Bo et al. [11] explored the effects of annealing and solid solution aging on the microstructure and mechanical properties of laser-fused TC17 alloy.Annealing transformed the microstructure into a coarse mesh basket form, improving plasticity but compromising strength.Only a specific solid solution aging treatment achieved the optimal strength-plasticity match, meeting the stringent technical standards for disc tensile testing.This paper examines the impact of various treatments-solid solution-ageing heat treatment, hot isostatic pressing treatment, and hot isostatic pressing-heat treatment (solid solution-ageing)-on the microstructure and tensile fracture properties of TC17 titanium alloy produced through selective laser melting.Additionally, the study delves into the fracture behavior mechanism of TC17 under these diverse treatments.Exploring the impact of different treatments on TC17 titanium alloy offers vital insights for enhancing its properties, crucial for its application in aerospace and related industries.

Test specimens
TC17 titanium alloy powder was prepared using the Argon atomisation (AA method), as depicted in Figure 1, with a nominal composition outlined in Table 1.Utilizing selective laser melting technology (SLM), TC17 titanium alloy samples were fabricated, as illustrated in Figure 2. The entire process was conducted within an inert gas-protected processing room to prevent metal reactions with other gases at elevated temperatures.The EPM 250 machine was employed for this project.The TC17 titanium alloys, Initially laser-printed in the selected area, underwent distinct process treatments: 1) untreated (3D); 2) solid solution-ageing heat treatment (HT) involving solid solution at 800°C, followed by water cooling for 4 hours, and ageing at 630°C with air cooling for 8 hours; 3) hot isostatic pressure treatment (HIP) at 930°C, 140 MPa, held for 4 hours, and then rapidly cooled with argon gas to room temperature; and 4) hot isostatic pressure-heat treatment (HIP-HT), which involved hot isostatic pressure followed by a solid solution-aging process.Upon treatment completion, microstructure analysis of the specimens was conducted, both in the XY and Z directions.Samples were then extracted and processed into plate-type tensile specimens sized as shown in Figure 3. Subsequent tensile tests were performed using the electronic universal materials testing machine (INSTRON 5982).

Experimental device and procedures
Metallographic samples were obtained using the Accutom-5 precision cutting machine and further prepared into specimens by the CcitoPress-20 dual-mode thermal inlay machine.These specimens underwent meticulous sandpaper grinding for surface refinement followed by rough and fine polishing using a Struers polishing machine.Subsequent treatment involved rinsing, drying with water and alcohol, and etching with a corrosive solution comprising 9 ml of HNO3, 5 ml of H2SO4, and 86 ml of HCl.The etched specimens were observed for tissue examination using an Olympus GX51 optical microscope.Post-tensile testing, the fractured specimens underwent ultrasonic cleaning with ethanol solvent for 5-10 minutes, and their fracture morphology was analysed using a field emission scanning electron microscope (Nova Nano 450).

Result and discussion
3.1 Effect of different process treatments on the microstructure of TC17 titanium alloy Figure 4 presents the metallographic microstructure of the TC17 alloy following varied process treatments.In Figure 4(a), at low magnification, the microstructure of the printed state revealed distinct traces of macro-structural features, notably in the Z-direction, indicating irregular grain growth resembling fish scales (as shown in Figure a3 and a4).In the XY-direction, printing streaks intersected, resulting in a uniformly distributed tetragonal shape.The microstructure of the HT state, as depicted in Figure 4(b), showcased coarse, striped β-phase forming a net-basket structure, retaining remnants of the printed macrostructure.Additionally, an α-phase with a ring-like morphology was evident.The Zdirection microstructure exhibited β-phase distribution in needle and bar forms, alongside α-phase presenting as a ring of fish scales.
Moving to Figure 4(c), specimens subjected to constituent zone printing-thermal isostatic pressing displayed similar microstructures in both XY and Z directions: coarse grains with noticeable size disparities.The β-phase structured into a striped mesh basket, while the α-phase exhibited a slightly elongated characteristic, with the printed structure no longer discernible.Finally, Figure 4(d) indicates that the HIP-HT-treated specimens in both XY and Z directions possessed comparable β-phase structures, showcasing coarser grains and microstructure.Figure 5 displays the microstructure variations in the TC17 alloy following distinct process treatments.The initial printed material exhibited an α' microstructure.Post-HT, HIP, and HIP-HT treatments transformed the metallographic microstructure into an (α+β)-phase mesh basket structure, with the αphase mainly distributed in stripe formations.Specifically, the HT-treated microstructure revealed a mesh basket form with evident grain boundary α-phase.Grain distribution in the HT-treated state appeared relatively uniform, with noticeably smaller grain size differences compared to HIP; grain size ranged approximately between 100-130 μm.HIP-treated grains appeared coarser, maintaining a consistent microstructure in both XY and Z directions, presenting an α-phase in a net-basket structure, with grain sizes spanning 100 μm to 1000 μm.Contrastingly, after HIP-HT treatment, deviations from the HIP microstructure were evident, showcasing a mesh basket microstructure with coarser and pronounced α-phase within the matrix.The β-phase distributed in coarsened strips, occasionally exhibiting Weiss microstructure morphology in localized regions.

Effect of different process treatments on the tensile properties of TC17 titanium alloy
Captured in Figure 6 are the macroscopic views of specimens post-tensile testing.Analysis revealed notable instability in the properties of the 3D printed material, exhibiting unpredictable fracture locations, including instances of fracture at the clamping end.Within this state, three data points were invalidated due to their notably low strength.Contrastingly, specimens in the HIP-HT-treated state exhibited the poorest plasticity, evidenced by fractures occurring at the non-working section.This distinction hints at a potential relationship between the material's microstructural properties and its divergent behaviors in these distinct states.Figure 7 presents histograms displaying the tensile results of specimens treated using various processes.Comparatively, the room temperature tensile strength of treated specimens demonstrated a notable increase from the printed state.Specifically, the room temperature tensile strength of HT-treated specimens in the XY-and Z-directions escalated by 25.46% and 38.72%, respectively.Similarly, specimens treated with HIP-HT exhibited an increase of 27.91% and 42.60% in the XY and Z directions, respectively.Despite this enhancement, the 3D printed materials failed to surpass 1000 in tensile strength, with a maximum elongation at break reaching 17.4% in the XY direction.Moreover, materials treated with HT, HIP, and HIP-HT showed substantial improvements in both tensile strength and plasticity compared to their printed counterparts, resulting in a more stable fracture process.Interestingly, the room temperature tensile properties of printed materials varied significantly based on printing direction, with Z-direction tensile strength notably lower than that in the XY-direction.In contrast, after heat treatment, materials in both directions exhibited closely aligned properties, likely attributed to reduced microstructure variability in printed layers and more random crack initiation locations posttreatment.Notably, in certain state specimens, plasticity data exhibited sharp fluctuations despite minor differences in tensile strength.This inconsistency might relate to the presence of printing defects or coarser microstructure in some states.Figure 8 presents the room temperature tensile curves of materials treated through various processes, highlighting substantial differences in the tensile properties between the two printing directions.Along the XY direction, the tensile curve exhibited characteristics typical of ductile materials, with pronounced elongation and clear yield deformation stages.Conversely, the Z-direction curve showed brittle material behavior, lacking evident plastic deformation stages.Materials treated with HIP-HT displayed curves akin to the Z-direction of 3D-printed materials, albeit with observable yield deformation zones.Despite this, the HIP-HT-treated material showed minimal elongation and morphology similar to the Z-direction curve of 3D printed materials.Overall, room temperature tensile properties in printed materials demonstrated significant anisotropy, with brittle behaviour observed in the Z direction despite excellent plasticity in the XY direction, albeit at a lower strength level.HT and HIP-treated materials notably enhanced material strength, plasticity, and isotropy-a desirable performance trait.However, HIP-HTtreated material exhibited a slight strength increase but a drastic reduction in plasticity, resulting in overall poor performance.Consequently, these four states of 3D printed TC17 titanium alloy couldn't address the lack of obvious deformation strengthening and necking phenomenon in the room temperature tensile damage process.Further work is necessary to account for these aspects in safety design considerations.Figure 9 displays the macroscopic image of the specimen following the 400°C high-temperature tensile test.Note that data from specimens fractured beyond the marking distance are considered invalid and are provided for reference purposes only.Results from the 400°C tensile test are outlined in Table 3.Interestingly, the tensile strength of 3D printed materials at 400°C exhibited an abnormal rising trend compared to room temperature, significantly surpassing other states of the alloy.This unusual change might stem from abnormal fracture positions influencing data singularities or internal defects and microstructural alterations within the material.At high temperatures, the HT-treated material displayed notably lower tensile strength than at room temperature, yet it remained relatively high.Additionally, the elongation at break slightly increased, reaching the highest level recorded.Similarly, the HIP-treated material showcased a consistent pattern of decreased strength and slightly elevated plasticity compared to the heat treatment state, albeit with slightly lower strength and plasticity than the HT-treated material, remaining at a high level.Following HIP-HT treatment, the high-temperature strength paralleled that of the HT-treated material, with a significant improvement in elongation at break compared to room temperature, albeit remaining relatively low.Notably, while the Z-direction tensile strength of printed material lagged significantly behind that of the XY-direction, treatment by different processes effectively aligned the material's properties in both directions.Figure 10 illustrates the 400 ℃ tensile curve of the material after undergoing different processes.
Observing Figure 11, the tensile properties of the material in both printing directions during 3D printing tended to exhibit consistency, displaying characteristic features of brittle material with minimal plastic deformation zones.However, for the HIP-HT material, the tensile curve trend contrasted with the printing state, transitioning from near-brittle characteristics to the typical toughness of the material in both printing directions.In contrast, the room temperature tensile curves of other state materials displayed characteristics typical of ductile materials, showcasing deformation strengthening and necking, notably pronounced in HT-treated and HIP-treated materials.Notably, at 400 ℃, all state materials exhibited isotropic properties, except for the 3D printed materials (provided for reference only).These materials demonstrated decreased strength compared to room temperature, but showed increased plasticity.
At elevated temperatures, the 3D-printed materials exhibited fracture behaviors resembling brittle materials.However, the HT-treated materials exhibited the most desirable overall performance in terms of strength and plasticity, followed by HIP-treated materials.HIP-HT-treated materials notably increased in plasticity, approaching the levels of ductile materials, but their overall performance remained comparatively inferior.Furthermore, the HT and HIP-treated 3D printed TC17 titanium alloy at 400 ℃ showcased the onset of deformation strengthening in the tensile damage process, with some instances of necking deformation before fracture.

Effect of different process treatments on the fracture behaviour of TC17 titanium alloy
Figure 12 showcases SEM images depicting room temperature tensile fractures across different specimen groups.In the XY plane of the 3D printed material, a consistently narrow shear lip was observed, accompanied by an unfused print defect at the fracture's lower left corner.The fracture presented a distinct fibrous central area, split between a flat and rough section.Under microscopic scrutiny, the flat region displayed a willow-like tough fossa morphology, while the rough area exhibited irregular granularity and secondary cracks.Conversely, in the Z direction, the fracture occurred on both sides of the through-hole's clamping end without a prominent shear lip.This section displayed a delicate structure with elongated ligamentous fossa along the lamellar microstructure of the fracture.Certain areas featured prismatic lines resembling an irregular porcelain-like cross-section, showing finer ligamentous fossa surfaces and heightened brittleness compared to the XY plane.The elevated plasticity observed in the XY plane of the printed specimen might be attributed to the presence of secondary cracks at the columnar crystal interface.This indicates that the fracture elongation of individual columnar crystals and their interrelation with Z cracks through layer bands and other printed structures significantly influenced the plasticity data, primarily due to the supercooling microstructure's impact.Around the XY tensile fracture of the HT-treated specimens, a distinct shear lip was evident, along with localized visible unfused print defects.Conversely, the Z direction tensile fracture appeared overall flat with microscopic undulations and grid-like patterns, resembling crystal interfaces where secondary cracks were noticeable.The HT treatment gradually homogenized the macro-structure of the printed material's interface organization, significantly enhancing material plasticity.However, the influence of the printed structure on tensile fracture performance persisted, suggesting the significance of the scale effect of the printed structure that warrants attention.The HIP-treated specimens exhibited significant shear lips around both the XY-and Z-direction tensile fractures, displaying coarse tough fossae microscopically and local cracking along lamellar tissue, resembling a deconstructive morphology.However, in the XY direction, the shear lip width lacked uniformity, showing larger undulations in the central section with more apparent overall plastic deformation and consistent fracture characteristics.Post-HIP treatment, the printed macrostructure was replaced by a coarse grain organization, significantly influencing fracture characteristics.Consequently, the tensile fracture properties primarily relied on the final organizational morphology, specifically the scale and morphology of α and β phases.
The HIP-HT-treated specimens in the XY direction displayed a tensile fracture without noticeable necking, featuring low-fold undulations with a coarse crystal fracture morphology.The surface exhibited small tough nests amidst the grain, with weak bonding areas along the lamellar interface resulting in cross-sectional cracking.In the Z direction, the tensile fracture showcased an overall coarse grain appearance with shallow tough nests dominating the surface, revealing localized analogous disintegration along lamellar lines through the crystalline morphology.Post HIP-HT treatment, the abnormal enlargement of grains occurred, accompanied by the appearance of the Weiss tissue pattern, causing a significant reduction in material plasticity.The SEM images in Figure 13 depict the 400°C tensile fractures of each specimen group.In the XY direction of the printed state, the tensile fracture lacked a distinct shear lip, showcasing a rough and irregularly granular cross-section.Various-sized ligamentous fossae morphologies and secondary cracks were visible, with shallow and small surface particle fossae.Conversely, the Z-direction tensile fracture appeared macroscopically flat with uniform undulations and localized edge deformations, revealing irregular faceted morphology and small surface ligament fossae.At 400°C, plastic deformation and fracture characteristics became more pronounced in the printed state, gradually resembling the printed macrostructural features observed at room temperature in heat-treated states.This suggests a transformation of sharply-cooled organization occurring at elevated temperatures.In the HT-treated material, the XY tensile fracture displayed both necking and a shear lip, exhibiting diverse-sized tough nests across the section.Similarly, the Z-direction fracture shared these characteristics, showcasing occasional unfused print defects.At 400°C, the influence of the heat-treated structure on fracture properties diminished, resulting in microplastic fracture morphology similar to room temperature conditions, maintaining comparable plasticity.Conversely, the HIP-treated material displayed yield necking in XY fracture sections, dominated by extensive shear features.Both XY and Z-direction fractures shared microscopic tough nests of various sizes, lacking deconstructive morphology along the lamellar tissue.At 400 °C, the HIP-treated material exhibited similar microplastic fracture morphology to room temperature, with reduced granular features, signifying weakened influences of grain boundaries and coarse phase boundaries at elevated temperatures.In the case of the HIP-HT-treated material, XY tensile fractures exhibited yield deformation primarily through crystal shearing, displaying characteristics of tough fossae and notable secondary cracks.Zdirection fractures resembled XY-directions, occasionally exhibiting coarse grains.At 400°C, the HIP-HT-treated material showed relatively unchanged organizational morphology, yet with weakened influences of grain boundaries and coarse Weinsteinite organization at higher temperatures, thereby restoring some plasticity.The 3D-printed TC17 alloy exhibited complex changes in isotropic properties due to acute cold organization and printing structures, leading to brittleness, internal stresses, and inconsistent tensile fracture behavior.While high temperatures partially improved these issues, plasticity remained poor.HT treatment modified the original print organization, hindering dislocations through diffuse α-phase precipitation [12], releasing internal stresses, and notably enhancing both strength and plasticity.However, the macroscopic print structure still had some impact on fracture morphology, particularly at room temperature.When the HIP treatment conducted at higher temperatures, the printing macrostructures are nearly eliminated.However, due to grain and tissue growth, material strength and plasticity slightly decreased compared to the heat-treated state.HIP-HT treatment resulted in coarse grain (Weiss tissue) and poor room temperature plasticity.At high temperatures, the influence of grain boundaries and coarse organization weakened, partially restoring material plasticity but with overall subpar performance.Hot isostatic pressing effectively eliminated unfused defects in the threedimensional form, forming weak bonding surfaces locally.At 400°C, the fracture behavior drastically impacted both 3D printed and HIP-HT treated materials, fundamentally changing fracture morphology and curve shapes.For HT and HIP treated materials, high-temperature fracture behavior mirrored room temperature trends, aligning with common metallic material behavior by showing decreased strength and a slight increase in plasticity.

Conclusions
Through a thorough analysis of TC17 titanium alloy prepared via laser selective zone melting technology, this study investigated the alterations in metallographic organization following diverse process treatments.It also explored the changing rules of tensile properties and fracture behaviors through both room temperature and high temperature tensile tests.The influence of these treatments on the tensile fracture behavior of laser selective zone melting TC17 titanium alloy was extensively discussed.
(1) The initial printing state primarily exhibited β grains and α' martensite.However, after different treatments, the metallographic organization shifted to an α+β phase net basket structure, with the α phase distributed mainly in stripe-like formations.HIP treatment led to coarsening of the α phase, while HIP-HT treatment resulted in the occurrence of Weiss organizational morphology.
(2) At room temperature, the tensile properties of the printed materials displayed notable anisotropy, marked by high brittleness and low strength.Conversely, HT and HIP-treated materials exhibited isotropic behavior with significantly improved strength and plasticity.However, for HIP-HT materials, despite a slight strength increase, plasticity drastically decreased, resulting in an overall poor performance.Notably, the four states of 3D-printed TC17 alloy didn't exhibit evident deformation strengthening or necking during tensile damage.
(3) At 400 °C, each material state's tensile properties demonstrated isotropic characteristics, except for the 3D printed material, which continued to exhibit brittle fracture.The HT-treated material maintained the most desirable overall performance in terms of strength and plasticity.HIP-HT-treated material showcased a remarkable increase in plasticity, resembling ductile material, but its overall performance remained relatively poor.Additionally, HT and HIP-treated materials began to display deformation strengthening and necking phenomena during the tensile damage process.
(4) Influenced by the original macroscopic printed structure, microstructure, and internal stress, the printed state material showed room temperature anisotropy.However, the sharply cooled organization at 400 °C partially recovered, leading to a reduction in anisotropy.HT and HIP treatments significantly weakened or eliminated this effect, resulting in excellent strength and plasticity at both room and high temperatures.Conversely, HIP-HT-treated materials exhibited evident coarsening, leading to deconvoluted fracture behavior along with brittleness at room temperature.While the plasticity improved at 400 °C, the overall performance remained relatively poor.

Figure 2 .
Figure 2. Schematic diagram of the laser melting process and scanning path in the selected area

Figure 6 .
Figure 6.Macroscopic view of two room temperature tensile specimens.

Figure 7 .
Figure 7. XY-and Z-direction tensile strength of TC17 alloy after different process treatments.

Figure 8 .
Figure 8. Two room temperature tensile curves of TC17 alloy after different process treatments; (a) first stretching in XY direction; (b) first stretching in Z direction; (c) second stretching in XY direction; (d) second stretching in Z direction.

Table 2 .
Room temperature tensile test results.

Table 3 .
High temperature (400°C) tensile test results of TC17 titanium alloy laser-selective printing material.