Effect Of Heat Treatment On Microstructure And Residual Stress Of 3D Printed TA15 Titanium Alloy

Titanium and titanium alloys have a series of excellent properties,such as high specific strength,great corrosion resistance,stable high temperature mechanical properties,etc.,and are regarded as the most promising new structural materials in the21 st century.However,the mechanical properties of titanium alloy produced by traditional casting process are poor,and the subsequent cutting processing is difficult,which makes the processing cost of complex and precise parts high.Additive manufacturing technology is a "bottom-up" manufacturing method through the accumulation of materials,which makes it possible to direct manufacture complex structural parts.In this paper,TA15 titanium alloy samples were printed by selective laser melting.The effects of printing parameters on the microstructure and mechanical properties of the samples were studied.Then,selecting a group of samples with better comprehensive properties for heat treatment to further optimize their microstructure and properties.The residual stress of the samples formed in the deposition state and after annealing with different process parameters was detected by the keyhole method,and the evolution of residual stress during 3D printing and heat treatment of TA15 titanium alloy was investigated.The main research contents and results are as follows:(1)The effect of laser scanning speed on the microstructure and properties of TA15 titanium alloy was studied.With the increase of scanning speed,unfused holes with different diameters appear in the forming parts,and both strength and plasticity decrease.When the laser power is 400 W,the scanning distance is 0.12 mm,and the spot diameter is 200 ?m,the density of TA15 titanium alloy formed at the scanning speed of 1.2 m/s is close to 100%,and the comprehensive mechanical properties are also good.(2)The effect of annealing treatment on the microstructure of forming parts was studied.After annealing at 950 ?,the microstructure of the sample changes little and the primary ? phase grows up.After annealing at 1000 ? and 1050 ?,the primary ?phase disappeared and the columnar ? grains changed into equiaxed ? grains.During the cooling process,a large number of fine and dispersed acicular martensite ? phases with specific orientation were precipitated from the ? phase.(3)The effect of annealing treatment on the mechanical properties of the formed parts was studied.After annealing at 950 ?,the deposited sample has the best strength and plastic matching,the strength is 1113 MPa and the elongation is 24.9%,which is similar to that of forged TA15 titanium alloy.However,when annealed at 1000 ? and1050 ?,the strength of samples increases slightly but the plasticity decreases rapidly.The annealing treatment has little effect on the grain orientation of the formed parts.The annealing treatment at 950 ? can disperse the texture strength slightly,but with the increase of the annealing temperature,the texture strength in the columnar region will rise to a certain extent.(4)The variation of the residual stress of the forming parts during 3D printing and subsequent heat treatment was studied.The residual stress at the bottom of TA15 titanium alloy formed by selective laser melting is maximum,which is compressive stress,while the residual stress at the middle and top of TA15 titanium alloy is relatively small,which is tensile stress.When the laser scanning speed gradually increases from1.2 m/s,the residual stress at the bottom of the sample increases,but the change is not obvious in the middle and the top of the sample.After the stress relief annealing at600 ?,most of the residual stress of the deposited sample is eliminated,and the decrease is as high as 80%.