Research On TIG Arc Additive Manufacturing Process And Strengthening Mechanism Of TiB₂ Particle Reinforced 7075 Aluminum Matrix Composites

The 7xxx series aluminum alloy processed by additive manufacturing technology（AMT）is widely used in industrial fields such as aerospace,shipbuilding,high-speed rail as well as automotive.However,the 7xxx series high-strength aluminum alloy represented by 7075aluminum alloy fabricated by AMT is difficult in controlling the grain size of the microstructure,which inevitably leads to the problem of low strength with respect to the additive alloys due to coarse grains.It seriously restricts the further application of AMT in the fabrication of 7xxx series high-strength aluminum alloys.Based on the reasons stated above,The article focused on the relevant additive processes and microstructure and performance control mechanisms involved in the TIG arc additive manufacturing of 7075 aluminum alloy.Researches on the TIG arc fusion additive manufacturing process of TiB₂ particle reinforced7075 aluminum matrix composites were conducted,including the thermal process and solidification process of TIG additive manufacturing,the evolution mechanism of as-deposited microstructure,and the heat treatment process of the as-deposited TiB₂/7075aluminum matrix composites.The physical and metallurgical mechanisms related to the microstructure control in arc additive manufacturing of 7075 high-strength aluminum alloy were further revealed.The investigation results obtained in this thesis were expected to provide experimental and theoretical references for the application of AMT on the fabrication of the other 7xxx series high-strength aluminum alloy.（1）Numerical simulations were conducted on the temperature and stress fields during the TIG arc additive process of TiB₂/7075 aluminum matrix composites.The results indicated that the distribution of equivalent stress between different additive layers was extremely uneven,and the maximum equivalent stress occured when the arc heat source was moved to the midpoint of the additive layer.The internal stress was mainly concentrated in the sample cladding layer between connection with the substrate.The heat accumulation could be reduced by controlling the interlayer temperature and appropriately extending the waiting time.thereby improving the residual stress of the sample and the quality of additive forming.By using the proposed process parameters for numerical simulation,lower equivalent stress peaks were obtained,providing a basis for the development of TIG arc additive technology for TiB₂/7075 aluminum matrix composites.（2）The research on the TIG arc additive forming process of TiB₂/7075 aluminum matrix composites were conducted.Through experiments on single pass single-layer and single pass multi-layer additive forming at different parameters,it was found that among the three factors of additive current,wire feeding speed and travelling speed,the wire feeding speed had the greatest impact on the height of the cladding layer,while the traveling speed had the smallest impact.The traveling speed had the greatest impact on the width of the cladding layer,while the wire feeding speed had the least impact.（3）The optimal additive process was determined by studying the microstructure and properties of TiB₂/7075 aluminum matrix composites specimens processed by TIG AMT with different additive parameters.The test results of the optimal additive specimen showed that the microstructure varied in different areas of the additive layer.The top grain size was the smallest,and the microstructure was a smaller equiaxed crystal.The middle grain was a coarser columnar crystal compared to the top grain,and the bottom grain size was the coarsest,which was a coarser columnar crystal.In terms of mechanical properties,the top position of the additive specimen had the best performance by comparing with the middle and bottom positions,with the average value of tensile strength,elongation rate and microhardness of（361.8±4.8）MPa,（8.3±0.9）%,and（145±5）HV.The tensile strength with（279.4±5.3）MPa was increased about 29.5%by comparing with the conventional 7075aluminum alloy arc additive specimen.（4）The influence of TiB₂ particles on the microstructure evolution during the solidification process of TiB₂/7075 aluminum matrix composites processed by TIG AMT was explored,and the strengthening mechanism was analyzed.The results indicated that TiB₂ particles were in a thermally stable state,and were mostly distributed at grain boundaries and a small amount distributed inside the grains during the additive process.The addition of TiB₂ particles changed the evolution process of the additive structure,and provided a nucleation surface for theα-Al matrix as heterogeneous nucleation points.The particles at grain boundaries could inhibit grain growth by hindering the movement of grain boundaries,and refined grain size could be achieved,which contributed to strengthening.（5）The influence of different heat treatment processes on the microstructure and properties of the as-deposited TiB₂/7075 aluminum matrix composites was studied.Research found that T6 heat treatment（solid solution treatment at 470℃with 2 h+aging treatment at120℃with 24 h）could effectively improve the microstructure and mechanical properties of the as-deposited TiB₂/7075 aluminum matrix composites.The average value of tensile strength,microhardness,and elongation rate of T6 state additive specimens reached to（510.2±5.4）MPa,（190±4）HV,and（10.2±0.8）%,respectively.They were increased by 41%,31%,and 22.9%by comparing with the specimen before heat treatment,respectively.Fracture analysis showed that there were many ductile dimples distributed on the fracture surface,and the fracture mode changed from brittle fracture before heat treatment to ductile fracture after heat treatment.（6）The effect of TiB₂ particles on the precipitation behavior of the as-deposited aluminum matrix composites after heat treatment were analyzed.The segregation of elements at grain boundaries during non-equilibrium solidification was improved after heat treatment,and the distribution of elements within and at grain boundaries became more uniform.During the solid solution treatment process,the concentration of vacancies were increased due to the grain boundary area was increased,which made it easier for Zn,Mg,and Cu solute elements to diffuse and transport.At the same time,it increased the number of nucleation of precipitated phases during the aging treatment process.After aging treatment,the TiB₂ particles and MgZn₂second phase should be uniformly dispersed in the matrix,and a large amount of MgZn₂second phase should be formed in the structure,which contributed to precipitation strengthening.