Effect Of Laser Selective Melting Process And Subsequent Heat Treatment On Microstructure And Properties Of Ti-6Al-4V

Traditional processing methods of Ti-6A1-4V titanium alloy have disadvantages such as low material utilization,long manufacturing cycle,large energy consumption,and manufacturing difficulties of complex parts,which restrict the application and development of titanium alloys.In this paper,Ti-6A1-4V titanium alloy samples were prepared by selective laser melting technology,and the effects of different laser processes such as laser power,scanning speed,scanning distance,and subsequent heat treatment processes such as stress relief annealing,high temperature annealing,and solution aging on the samples were explored.Due to the heat transfer of the substrate,the structure of the samples is mainly composed of needle-shaped α’phase and a small amount of β phase between α’phases.When the power,scanning speed,and scanning interval are changed,There is the granular near nanometer β phase in some samples.When the laser power is 108W,the scanning speed is 620mm/min,and the scanning interval is 80 μm,the content of near nano β phase is the most.When the laser power is large,the scanning speed is slow or the scanning interval is short,the energy density of the laser will be too large.The time will be too long in the β single-phase area,and the β phase crystal grains will continue to grow.Coarse β phase grains will produce coarse-structured α’ phase during subsequent coolingWhen the laser power is smaller,the scanning speed is faster or the scanning interval is large,and the elements in the samples will not diffuse sufficiently.The uniformity and corrosion resistance of the samples will deteriorate.The insufficient energy density will produce more defects such as unfused defects and unmelted powder between the melt channels,which will affect the density of the samples.Excessive energy density will cause the structure of samples to coarsen,and the hardness,strength,and plasticity will decrease.At the same time,it will cause the molten pool to be unstable,and it will also produce holes,which will affect the density.After stress relief annealing treatment at 600℃,part of the needle-like α’ phase in the samples will be transformed into a and β phases which are more stable.The near-nano β phase disappears basically,but the β phase content rise.After annealing,the residual stress and lattice distortion of the samples are partially eliminated,and the number of granular near-nano β phases that can hinder the movement of dislocations is reduced.The tensile plasticity of the samples is increased,but the strength and hardness are reduced.After high temperature annealing,the needle-like α’ phase transforms into lamellar a and β phases which are more stable,and the crystal grains are coarsened to different degrees.The samples structure annealed at 800℃ is slightly coarsened,while the structure of samples annealed at 900℃ is coarser,and the strength,hardness and plasticity decrease greatly.However,high temperature annealing makes the alloying elements in the samples diffuse sufficiently,the degree of homogenization is improved,and the defects are reduced,so its density and corrosion resistance are increased.Samples after high temperature anneal are suitable for working environments that don’t require very high strength,but require high corrosion resistance.In the 900℃ holding stage of solution treatment,most of α’ phase in the samples will be transformed into β phase.Due to the faster cooling rate of the water quenching subsequently,the β phase undergoes shear-type phase transition to generate metastable martensite,After aging at 500℃ for 6 hours,the metastable martensite will decompose to form more stable lamellar a and β phases.After aging treatment,the residual stress and lattice distortion in the samples are basically eliminated.The tensile strength decreased by 22.5%,the elongation increased by 55.7%.The structure is more uniform,and the corrosion resistance is further improved.