Study On High Temperature Deformation Behavior And Microstructure And Properties Of TiC/Ti6Al4V Composites Formed By Selective Laser Melting

Ti6Al4V alloy is a kind ofα+βtype 2-phase titanium alloy has been widely used in aerospace and aerospace fields,such as fans,compressor disc blades and load-bearing components,due to its high strength,high specific strength and good high temperature creep resistance.However,with the development of science and technology,the high temperature performance of Ti6Al4V alloy can not meet the requirements of aerospace field,and it is difficult to produce complex structure parts of Ti6Al4V alloy by traditional forming process.Selective laser melting（SLM）has the advantages of short production cycle,low production cost,high flexibility,high integration,and high material utilization,and has broad application prospects in the aerospace field.In this paper,TiC/Ti6Al4V Ti-based nanocomposites were prepared by in-situ formation of nano-sized TiC particles as reinforcement phase using SLM technology based on Ti6Al4V alloy.The effect of in-situ TiC particles on the microstructures and high-temperature mechanical properties of the composite was systematically studied.The microstructural evolution of Ti6Al4V alloy and TiC/Ti6Al4V composite formed by SLM at high temperature and the effect and strengthening mechanism of TiC particles on the microstructural evolution of composite materials are also discussed.The main research contents and conclusions are as follows:（1）The effects of the microstructures of the SLM in situ synthesized TiC/Ti6Al4V composite and Ti6Al4V alloy on the tensile properties,hardness and friction and wear of the two materials at 400～600℃were studied.The results show that the ultimate tensile strength of the TiC/Ti6Al4V composite at 400,500 and 600℃increases by 13.7%,20.1%,16.5%and17.8%,24.6%,29.9%,respectively.The friction coefficient of the two alloys decreases with the increase of temperature.The wear amount of the specimens increases after reaching a certain temperature.The wear mechanism of both materials at high temperature is oxidation wear,adhesion wear,delamination wear and fatigue wear.Fatigue wear is the main wear mechanism.The fine needle martensite in TiC/Ti6Al4V composites is refined and the TiC particles play the role of diffusion strengthening.The mechanical properties of TiC/Ti6Al4V composites are improved by the combined action of fine grain strengthening mechanism and diffusion strengthening mechanism.（2）The microstructure evolution and strengthening mechanism of Ti6Al4V alloy formed by SLM during isothermal compression deformation at 900℃and 1s^-1 were studied.The results show that the softening process of Ti6Al4V alloy is a DRX process,and the microstructure of Ti6Al4V alloy changes from the severe deformation zone of the sample with a compression amount of 0～60%to the initial acicular martensite to the sphericalαstructure with a compression amount of 15%,and then to the lathαstructure with a compression amount of 30%and 45%to the last 60%recrystallized grain.The change of geometric dislocation density of Ti6Al4V alloy in the process of compression is from 0%to 60%,and the dislocation density increases first and then decreases.Refine the lath formed during compression through DRX mechanismαorganization.In the compression process of Ti6Al4V alloy,there are two kinds of dislocationsαThe internal slip occurs dislocation delivery pinning dislocation slip,so the dislocation increment mechanism during compression is mainly the double cross slip multiplication mechanism during slip and the Frank source dislocation multiplication mechanism caused by dislocation pinning.（3）The microstructure evolution and strengthening mechanism of TiC/Ti6Al4V composites formed by SLM under isothermal compression deformation at 900℃and 1s^-1 were studied.The results show that the compressive strength of TiC/Ti6Al4V composite is 78 MPa higher than that of Ti6Al4V alloy,and the softening process of TiC/Ti6Al4V composite is also a dynamic recrystallization process.The microstructure of Ti6Al4V alloy changes from the initial acicular martensite to the coarsened acicular martensite with the compression amount of15%to the lath structure with the compression amount of 30%αTissue and a small amount of globularαStructure,recrystallized grain and lath structure when the compression amount reaches 45%,lath structure when the compression amount reaches 60%αThe microstructure is refined to form recrystallized grains.The geometric dislocation density of TTiC/Ti6Al4V composite changes in the process of compression of 0～60%,which increases first and then decreases.Because of the grain refinement,the number of grain boundaries increases and the presence of TiC particles,the aggregate dislocation density of the composite under the same conditions is greater than that of Ti6Al4V alloy.The dislocation of TiC/Ti6Al4V composite in the lath shape during compressionαInternal slip,TiC particles pin dislocations,dislocations bypass TiC particles to form dislocation ring and form dislocation accumulation,and dislocations form dislocation accumulation around TiC particles to form dislocation cell and subgrain boundary to accelerate the recrystallization process.It is calculated that the strengthening effect caused by grain refinement is the main one,and the Orovan strengthening formed by TiC particles is the secondary one.