Effect Of Cryogenic Treatment On Microstructure And Mechanical Properties Of Ti6Al4V Alloy Fabricated By Additive Manufacturing

Both Electron Beam Selective Melting and Selective Laser Melting are both metal additive manufacturing technologies based on the principle of discrete-stacking and driven by 3D data of parts to selectively scan metal powders by high-energy beams.Thus,the powder melting forming effectively solves the problems of long production cycle and high cost of Ti6Al4V,and has been widely used in aerospace,biomedical,automotive and other fields.However,Ti6Al4V has poor mechanical properties due to its element burning,defects and uneven microstructure during the Electron Beam Selective Melting and Selective Laser Melting process.Cryogenic treatment is a post-treatment process which improves the mechanical properties of materials by adjusting the microstructure of materials under low temperature environment.Therefore,cryogenic treatment can decompose the metastable phase of microstructure in Ti6Al4V fabricated by additive manufacturing,so as to achieve the purpose of regulating the mechanical properties.However,there are few studies on the effects of cryogenic treatment on microstructure and mechanical properties of Ti6Al4V fabricated by additive manufacturing.The effects of cryogenic treatment with different soaking time on microstructure and mechanical properties of Ti6Al4V fabricated by additive manufacturing were studied.SEM,EDS and XRD were used to analyze the evolution of microstructure of Ti6Al4V before and after cryogenic treatment.The mechanical properties of Ti6Al4V before and after cryogenic treatment were evaluated by tensile tests,high cycle fatigue tests,Vickers hardness tests and wear tests.The fracture morphology and wear morphology were observed by SEM,EDS and scanning white-light interferometry profilometer to reveal the mechanism of mechanical properties change of Ti6Al4V before and after cryogenic treatment.The main research contents and results of this paper are as follows:(1)The microstructure of Ti6Al4V prepared by Electron Beam Selective Melting showed typical characteristics of Widmanstatten structure.After cryogenic treatment,the thickness of lamellar?phase decreased gradually with the extension of cryogenic treatment soaking time,and fine granular?phase precipitated.(2)There are obvious dimples in the middle of the tensile fracture of Ti6Al4V prepared by Electron Beam Selective Melting.The tensile fracture mechanism is ductile fracture.The tensile strength,yield strength and elongation of cryogenic treatment for 48 h are increased by 18.64%,18.65%and 91.67%,respectively.After cryogenic treatment.The dimples in the middle of tensile fracture are deepened and the diameter is increased.After cryogenic treatment for 12 h,the average fatigue cycles of Ti6Al4V prepared by Electron Beam Selective Melting increased gradually,and the average fatigue cycles of soaking for 96 h was the highest,which increased by130.20%.(3)The microstructure of Ti6Al4V prepared by Selective Laser Melting is mainly?'phase.After 72 h cryogenic treatment,?'phase was decomposed into lamellar?phase and?phase distributed on the boundary of?phase.(4)Before and after cryogenic treatment,the Vickers hardness of Ti6Al4V prepared by Selective Laser Melting is 390.79 HV_0.2 and 371.34 HV_0.2,respectively.The average coefficient of friction is 0.5497 and 0.4729,and the wear rate is 1.07×10^-12 mm³/Nm and 0.82×10^-12mm³/Nm,respectively.The wear mechanism without cryogenic treatment was abrasive wear,adhesive wear and fatigue wear,while the wear mechanism after cryogenic treatment for 72h was abrasive wear and slight adhesive wear.