Microstructure And Properties Of Ti₂AlNb Alloy By Point-mode Forging And Laser Forming And Numerical Simulation Of Stress Field

Ti₂AlNb alloy is a new material with O phase as the main phase.It has good performance in strength at room and high temperature,plasticity,fracture toughness,density and flame retardant properties.Although the as-deposited Ti-22Al-25Nb alloy has ultra-high strength,its low plasticity limits its engineering application in the field of laser additive manufacturing.In this paper,ANSYS finite element software is used to simulate the point forging process,and the influence of forging times and overlap rate on metal deformation and stress field distribution during forging process is explored.The results show that stress concentration occurs around the indenter during single point forging,and the plastic deformation is greater.Excessive forging times may lead to defects inside the metal during forging.With the increase of forging times,the distribution of residual stress and plastic strain along the depth direction increases first and then decreases.During the continuous forging process,the stress concentration phenomenon will also occur at the overlapping position of two adjacent forging points.The stress value decreases with the increase of the overlapping rate,and the plastic strain increases first and then decreases.The processing parameters were determined by referring to the numerical simulation results.Ti-22Al-25Nb alloy samples with different forging times and different scanning rates were formed by point forging laser forming technology.The effects of two process parameters on the original microstructure and properties of the samples were analyzed,and the effects of different solution temperatures and different aging temperatures on the microstructure and properties of the samples after heat treatment were analyzed.The results show that the laser deposited Ti-22Al-25Nb alloy is a coarse columnar crystal.After adding the point forging process,the grain transforms to equiaxed crystal.With the increase of forging times,the grain size in the structure decreases,the microhardness of the material increases,the deeper the dimples observed in the tensile fracture,and the strength of the metal increases and the plasticity decreases.Due to the excessive residual stress of the three-time forging sample,small cracks are generated in the metal,resulting in brittle fracture of the tensile sample.The faster the scanning rate,the smaller the grain size of the sample.The faster the scanning rate,the higher the strength of the material and the lower the plasticity.The original alloy is mainly composed of B2 phase,and the solution treatment will precipitate a large number of lath O phase,α₂ phase and needle O phase in the B2 phase.With the increase of solution temperature,the content of secondary precipitated O phase in the crystal increases,and the strength of the material increases and the plasticity decreases.With the increase of aging temperature,the grain size becomes larger,the volume of precipitated phase becomes larger,the content ofα₂ phase increases,the content of O phase decreases,the strength of the alloy decreases and the plasticity increases.Intergranular fracture characteristics were found in the fracture of the sample with low solution temperature,which was intergranular-cleavage mixed fracture,and the other samples were cleavage fracture.