Study On The Microstructure And Properties Of Direct Laser Deposited Ti60 Alloy

Ti60 alloy is a kind of high temperature titanium alloy, designed by our country,which works at 600 ℃ with excellent comprehensive properties. However, the traditional reductive manufacturing or forced forming process of Ti60 alloy is not only complex, but also has many shortcomings, such as low material utilization ratio, long production cycle, high costs, and particularly low machining efficiency. Metal parts additively manufactured by the direct laser deposition(DLD) technology, usually have fine microstructure, uniform composition, excellent mechanical properties, and near-net shaping, which can overcome the shortcomings of the traditional forming methods.In this paper, Ti60 alloys were fabricated on the surface of the forgings by using the DLD technology. The forming characteristics, defects formation mechanism and control measures of DLD Ti60 alloys were investigated, and Ti60 alloys without defects were made on the basis of process optimization. The microstructure and the tensile properties of DLD Ti60 alloys at the room temperature and 600℃ were studied, meanwhile the effects of heat treatment process on them were also investigated in detail, which the heat treatments are respectively solution and aging, double annealing, and hot isostatic pressing(HIP)+ double annealing. The main conclusions and results are as follows.There are two typical kinds of defects in the DLD Ti60 alloys, which are the pores and the lack of fusion. The solidification speed of laser molten pool is so fast that the gases will stay inside the molten pool, and eventually become holes. The lack of fusion defect is mainly influenced by the process parameters. When the lap rate is too large, it is more likely to generate the lack of fusion defect between two adjacent channels. By contrast, when the single increment is too large, the lack of fusion defect emerges between two adjacent layers. The pores and the lack of fusion with small sizes can be eliminated through the HIP treatment.DLD Ti60 alloys have a typical layer structure. There are columnar crystals at the bottom and the middle of DLD Ti60 alloys, while the columnar crystals transform to equiaxed grains on the top. The microstructure of DLD Ti60 alloys is Widmanstatten structure, which is composed of lath-shaped alpha phases and beta phases between the laths. The room temperature and 600℃ high temperature tensile properties of DLD Ti60 alloys show some degree of anisotropic. The transverse strength at the room temperature exceeds the level of the forgings, while the plasticity is lower than the forgings. The longitudinal strength and plasticity are lower than the forgings. Compared with the tensile properties of the forgings at 600℃, the plasticity and the transverse strength are higher, while the longitudinal strength is lower.After solution and aging treatment, the layer structure almost disappears completely, and parts of the original beta grain boundaries degenerate, but the coarsening and discontinuous boundary alpha phases still exist. The microstructure of DLD Ti60 alloys becomes approximate bimodal structure, which is composed of lath-shaped primary alpha phases and beta transformation microstructure. The width of primary alpha phases increases, while the length-width ratio and the volume fraction of the primary alpha phases decrease. Compared with the as-deposited Ti60 alloys, the strength and transverse plasticity decrease, while the longitudinal plasticity increases.Through double annealing treatment, most of the layer structure vanishes, and most of the original beta grain boundaries degenerate. It’s hard to see a continuous grain boundary, while there are still coarsening and discontinuous boundary alpha phases. The lath-shaped alpha phases coarsen apparently, so the length-width ratio decreases. The transverse strength and plasticity of DLD Ti60 alloys after double annealing treatment decrease, and the longitudinal plasticity increases, while the strength decreases dramatically.After HIP + double annealing treatment, the layer structure disappears completely, and most of the original beta grain boundaries degenerate. There are only a small amount of grain boundary alpha phases and alpha clusters, which grow into original beta grains. The lath-shaped alpha phases grow up, and coarsen apparently, so the length-width ratio decreases. The plasticity of DLD Ti60 alloys after HIP + double annealing treatment increases dramatically, while the strength without a significant reduction. DLD Ti60 alloys after HIP + double annealing treatment have such perfect match of strength and plasticity that they show excellent mechanical properties. Therefore the HIP + double annealing treatment is the best heat treatment for DLD Ti60 alloys.