Fundamental Research On Laser Additive Manufactured Ti-22Al-25Nb Alloy

The Ti2AlNb-based alloy,which is a new type of high temperature structural material,has excellent comprehensive performances such as high specific strength,high fracture toughness and high temperature creep resistance.It can achieve large weight loss under the premise of ensuring performance,and shows a very broad application prospect in the field of high temperature structural parts of aerospace engines.However,the preparation of this material parts by the traditional technology faces a series of problems such as multi-process,high cost and long processing cycle,restricting the development of the material.This study proposes the direct forming of Ti2AlNb-based alloy parts by laser additive manufacturing technology in less process and shorter processing cycle,which making full use of the advantages of laser additive manufacturing process.But in the present work,the laser additive manufactured Ti2AlNb-based alloy still faces some problems that needs to be solved,like coarse grain size,complex phase transformation,poor performance of the deposited part and high stress during forming process.In this paper,some fundamental research are systematically carried for laser additive manufactured Ti-22Al-25Nb alloy,especially the Ti-22Al-25Nb thin-wall sample.The main research content include the evolution of grain morphology,microstructure,temperature field and stress field during forming process and the subsequent heat treatment.The work completed and mian results are as follows:The laser additive manufactured Ti-22Al-25Nb alloy thin-wall sample exhibits the equaxied grain morphology,and the evolution mechanism are investigated through forming experiment of N layers(N=1,2,3,4)combined with solidification theory.Further research shows that the phase composition of thin-walled samples along the deposition direction changes from three-phase structure to single-phase structure:B2+O+?2?B2.The upper part of the thin wall is the metastable B2 phase retained under the condition of rapid cooling,and the bottom three-phase structure is the result of self-aging during the forming process.In the three-phase structure,the continuous ?2 phase distribute at the grain boundary,and the mixed structure of lath ?2 phase and O phase distribute in the grain.The as-deposited material shows poor plastic owing to the coarse grain,and the bottom of deposited sample exhibits high strength due to the precipitation of a large number of fine ?2/O phases.The finite element method is used to simulate the temperature field during the multi-layer thin-wall forming process.The deposition materials at different position have experienced continuous thermal cycling in different temperature ranges that is the reason why the solid phase transformation happened.The temperature field features along deposition direction,thin-wall thickness direction and horzition direction are analyzed respectively.Also,the microstructure change of the three directions are studied correspondingly.The results shows that the microstructure difference only appears in the deposition direction,and there are no obvious microstructure change in other directions.The phase transformation law for laser additive manufactured Ti-22Al-25Nb alloy in different phase regions are studied in detail.When the three-phase structure is heat-treated in the ?2+B2 phase region,the O phase is completely dissolved,and the content of the ?2 phase gradually increases with the decrease of the heat treatment temperature.Also there is no precipitated phase region of about 3?m wide on both sides of the grain boundary.As the heat treatment temperature is lowered to the a2+B2+0 phase region,the 0 phase on both sides of the grain boundary remains,and the a2GB is slightly coarser than initial structure.After the heat treatment in B2+0 phase region,the grain boundary and the intragranular ?2 phase almost retain the deposited morphology,and the intragranular O phase grows up.As for the single-phase structure,only the ?2 precipitates in a2+B2 phase region and no precipitated phase region still exists.When the single-phase structure is heat-treated in the?2+B2+O phase region,the a2 phase and O are precipitated simultaneously,the ?2GB phase is continuously distributed,and the O phase precipitates on both sides of the grain boundary.The lath O phase newly formed in B2+0 phase region have a certain orientation relationship with the B2 phase.According to the phase transformation law,two heat treatment schemes(pretreatment+solution+aging,solution+aging)are designed.The effects of heat treatment on microstructure and properties are analyzed,and 920?/2 h/AC + 800?/24 h/AC is determined as the best heat treatment scheme.After heat treatment,the tensile strength of the bottom and top of the thin-wall specimens reach 999 MPa and 953 MPa,respectively,and the elongation after fracture is 6.0%and 5.3%,respectively.Compared with the as-deposited sample,the plasticity is greatly improved while ensuring sufficient strength.There is no significant difference in the mechanical properties of thin-walled specimens in different directions.It is shown that the continuous ?2 phase at grain boundary in the as-deposited structure is not the key factor affecting the plasticity for the laser additive manufactured Ti-22Al-25Nb alloy with coarse grain.The plasticity could be adjusted through the coarsening of intragranular O phase.Retaining ?2 phase is good for the improvement of strength.When the size of the precipitated phase in the grain is relatively uniform,the comprehensive mechanical properties are the best.The indirect coupling method is used to simulate the stress field during the forming and cooling process.It is found that the maximum value of the equivalent stress has a good correspondence with the maximum value of the temperature gradient,and the region with a large equivalent stress always locates near the molten pool.Two kinds of stress reduction solutions are proposed in this paper,namely,substrate preheating and slowing down heat dissipation.The simulation results show that the substrate preheating only has obvious stress release effect during the initial deposition process,while slowing down heat dissipation works during the whole forming and cooling process.In addition,based on the research of thin-wall samples,the microstructure and properties of bulk laser additive manufactured Ti-22Al-25Nb alloy sample are analyzed.The grain size of the block sample is coarser,showing a nearly equiaxed crystal structure with the long axis along the deposition direction.The performance after heat treatment shows obvious anisotropy.The tensile strength along the deposition height is low and the plasticity is extremely high.Along the horizontal direction of the scanning,the tensile strength is higher and the plasticity is slightly lower.