Microstructure And Mechanical Properties Of Selective Laser Melted TA15 Titanium Alloy

As the aerospace technology updates quickly,titanium structural components with complex-shape are developing towards high performance and integration.Conventional manufacturing such as casting and forging has limitation somewhat,however the revolutionary invention of additive manufacturing provides a new solution.Selective laser melting(SLM)is one of the commonest additive manufacturing technologies for titanium alloy.It can fabricate complex components with high precision and performance directly by melting powder in the specific area layer by layer.TA15 titanium alloy(nominal composition is Ti-6.5Al-2Zr-1Mo-1V)is a typical near-α titanium alloy.It is widely used as structural material of the aircrafts and engines with the ability of working at 500℃ for a long time.The microstructure and mechanical properties of TA15 titanium alloy fabricated by Selective laser melting before and after post heat treatment together with the effect of TiB in-situ reinforcement are systematically investigated,in order to provide theoretical and experimental support for SLM fabricated TA15 titanium alloy.The main research conclusion are as follows:The microstructure of SLM fabricated TA15 titanium alloy are near equiaxed and columnar prior-β grains perpendicular and parallel to the deposition direction respectively.And there are acicular α’ martensite with high aspect ratio and different orientations staggered arranging interior the prior-β grains.The size of acicular α’ martensite has hierarchical phenomenon obviously,which is considered as a consequence of complex thermal cycles during SLM process.The results of room-temperature and 500℃ tensile tests show extremely high strength but poor plasticity.Single heat treatment,triple heat treatment and hot isostatic pressing(HIP)were conducted to the SLM fabricated samples.Post heat treatment has an apparent effect on the microstructure and mechanical properties of the alloy.After 850～950℃(below theβ-transus temperature)single heat treatment,α’ martensite decomposed and transformed into lamellar α+β.After 1000～1050℃ single heat treatment,the microstructure exhibits typical Widmannstatten structure.After triple heat treatment,near "tri-modal"microstructure composed of equiaxed α phase and lamellar α+β was acquired.And after HIP treatment,it also transformed into lamellar α+β.Overall,after different post heat treatment the tensile strength and yield strength decreased slightly,while the plasticity is obviously improved.The processing parameter,microstructure and mechanical properties of SLM fabricated TA15+0.5 wt.%TiB2 samples were studied tentatively.The laser power has a significant effect on defects rather than the microstructure of samples fabricated by SLM when the scanning speed,hatch spacing and layer thickness are kept constant.The microstructure of TA15+0.5 wt.%TiB2 samples is composed of prior-β grains,acicular α’martensite together with granular and reticular TiB whiskers.Compared with TA15,the grain size of prior-β grains reduced but the average microhardness increased.Due to the addition of B,high residual stress was generated in the material during the SLM process,resulting in a large cracking tendency.