| Ti-6A1-4V(wt.%)alloy is widely used in biomedical and aerospace industries due to its low density,high specific strength,corrosion resistance,high fracture toughness and excellent biocompatibility.However,due to their high melting point and high deformation resistance of titanium alloys are difficult to process,which leads to the high cost of titanium alloy parts and hinders their wide applications.Selective laser melting forming technology(SLM),as a rapid manufacturing process,can directly form complex structural parts by additive method,and it has been paid more and more attention from titanium alloy researchers at home and abroad.Therefore,it is very important to systematically and accurately characterize the relationship between microstructures and mechanical properties of Ti-6Al-4V titanium alloy fabricated by SLM.In this study,Ti-6Al-4V alloy samples with dog-bone and box shapes were fabricated by SLM and using spherical Ti-6Al-4V alloy powders prepared by gas atomization.The phase composition and microstructures of the samples were characterized by X-ray diffractometer,optical microscopy and scanning electron microscopy.The mechanical properties of the samples were measured by a Vickers hardness tester and a universal material testing machine.The effect of heat treatment and sample orientation on microstructure and mechanical properties were investigated.The results show that:The Ti-6Al-4V alloy samples fabricated by SLM have a high relative density of up to 99%.The microstructure is an acicular ?' martensite structure,and the longitudinal section exhibits a distinct columnar crystal structure.This is because there is a large temperature gradient during SLM,and the grains grow preferentially to form columnar crystals.After annealing,?' martensite decomposes and the microstructure is lamellar structure of ?+? phase.After solid solution treatment and aging,the microstructure is a?-transformed matrix with interlaced woven basket-like a structure.The Ti-6Al-4V alloy samples fabricated by SLM have high tensile strength of up to 1060 MPa and an elongation to fracture of up to 10.9%,reaching the mechanical property level of Ti-6Al-4V alloy forgings.Annealing treatment is beneficial to increase the elongation of the samples,but the strength is slightly reduced,while the solution treatment and aging can increase the strength of the samples,but the elongation decreases.It is observed that there are some defects such as pores,lack of fusion and other defects in the samples.These defects can cause stress concentration under tensile stress,producing microcracks and affecting the plasticity of the material.The Ti-6Al-4V alloy samples fabricated by SLM have significant anisotropy of mechanical properties,which is manifested by in the lower post-break elongation perpendicular to the laser scanning direction(longitudinal direction).After annealed,the elongation of the longitudinal samples is very low,only 1.6%,the tensile strength is 1020 MPa,and the fracture exhibits a mixed fracture mainly composed of cleavage fracture.The elongation of the transverse samples is 10.1%,and the tensile strength slightly reduced to 994 MPa,the fracture morphology is ductile fracture.Under the condition of solution and aging,the elongation of the transverse specimen is 9.7%,the tensile strength is 1192 MPa,and the fracture exhibits ductile fracture characteristics.However,the longitudinal samples only undergo a small deformation,and the fracture occurs during the elastic stage,basically without plastic deformation.The fracture morphology can be seen to crack along the a lamella,sometimes passing through the a lamella to form a transgranular fracture.The anisotropy of the mechanical properties of TC4 samples fabricated by SLM can be attributed to the interaction of microstructures and internal defects,but the influence of internal defects is significantly greater than the influence of columnar crystal orientation on the mechanical properties of the materials,thus resulting in the anisotropy in this study. |
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