Study On Microstructure And Properties Of SiCw Reinforced Ti6Al4V Matrix Composites Fabricated By Selective Laser Melting

Ti6Al4V alloy is widely used in weapon equipment,aerospace,energy chemical industry,biomedical and other fields because of its advantages of high strength,low density,corrosion resistance and excellent biocompatibility.However,with the development of the aerospace and defense industry and other fields,Ti6Al4V alloy faces a more complex working environment.In order to reduce the limitations of its application,the improvement of demand forces the development of titanium alloys towards better comprehensive properties.Titanium matrix composites（TMC）show higher potential and become a new trend in the development of titanium alloys.In this paper,Ti6Al4V alloy and Ti6Al4V-SiCw composites were formed by selective laser melting（SLM）.The effects of different SiCw content and different heat treatment regimes on microstructure,mechanical properties and corrosion properties of the composites were systematically studied.The strengthening mechanism and corrosion mechanism of Ti6Al4V-SiCw composites were revealed.The main research contents and conclusions are as follows:（1）The effects of pellet ratio,milling time and milling speed on dispersion and sphericity of SiCw in composite powder were studied.The results showed that the dispersion of SiCw was improved with the increase of the energy,but the shape variable of powder was intensified.The optimal parameters of the milling process were obtained by the experimental optimization,the milling time was 4 h,the pellet material ratio was 6:1,and the grinding speed was 210 r/min.（2）The influence of process parameters on the microstructure and defects of Ti6Al4V-SiCw composites formed by SLM was studied.It was found that too small laser energy density will lead to relatively weak temperature gradient,the phase was not uniform distribution,there were a lot of unmelted particles and holes on the surface,and the surface roughness was poor.When the laser energy density was too high,the elements with low melting point tend to evaporate and the large amount of heat released by the chemical reaction in the forming process will cause gas inclusion,and the flow of melt was not enough to fill the residual void,resulting in the generation of holes.When E is 55.56 J/mm³,the produced sample exhibited a more uniform and fine microstructure.（3）The effects of different content of SiCw on microstructure and mechanical properties of composites were studied.The transformation of cross-sectional plate-likeαgrains and longitudinal-sectionalβcolumnar grains to equiaxed grains could be achieved by adding a trace amount of SiC_w,and the weak texture strength microstructure with fine equiaxed grains was obtained.The in situ formed nanosized TiC particles promoted grain refinement through three mechanisms:（i）The diffusion distribution of nanosized TiC particles provided a favorable nucleation position for theαphase,it also limited the growth of theαphase.（ii）Nanosized TiC particles were dispersed to fix dislocations and grain boundaries,inhibit the grain growth.（iii）Nanosized TiC was embedded in the matrix and bound well to the matrix.The Ti matrix was easy to nucleate on some specific crystal planes of primary TiC,which increased the nucleation rate.In addition,the addition of SiC_wsignificantly improved the mechanical properties of Ti6Al4V alloy,compared with that of the Ti6Al4V alloy（360.2 HV）,the microhardness of the sample with 1.0-wt%SiC_w（522.7HV）increased by 45.1%.The addition of 0.5-wt%SiC_w（1445.34 MPa）increased the ultimate tensile strength by 19.4%compared with that of the Ti6Al4V alloy（1210.13 MPa）,and good ductility was maintained.The results show that the microstructure of Ti6Al4V alloy can be significantly improved by adding a trace amount of SiC_w,which provided an effective way for the grain refinement of titanium matrix composites manufactured by SLM.（4）The effect of solution aging heat treatment on microstructure evolution of Ti6Al4V alloy and Ti6Al4V-SiCw composites was studied.With the increase of solution temperature,the metastableα’martensite of SLM formed Ti6Al4V alloy gradually decomposed.Afterβphase（α’→β）precipitation andα’martensite conversion toαphase（α’→α）,α+βbiphase microstructure was finally formed.However,the high temperature results in coarsening and uneven microstructure ofαphase.TiC particles generated in situ in Ti6Al4V-1SiCw composites transformα’martensite into granularαphase,thus forming a random distribution of TiC particles and a more uniformα+βbiphase microstructure.Moreover,the distribution of nano Ti₅Si₃particles at theα/βinterface limited the growth ofαphase,thus forming a finer microstructure.After heat treatment,Ti6Al4V alloy had many coarse columnar crystals,forming a strong fiber texture.The microstructure of the composite is uniform distribution of equiaxed grains,resulting in a scattered texture distribution,which greatly reduces the anisotropy of the material properties.This is because a large number of subgrain boundaries are formed in the composites after heat treatment,thus separating the grains to avoid the formation of coarse columnar grains.（5）The effects of SiCw content and solution aging heat treatment on the corrosion properties of Ti6Al4V alloy and Ti6Al4V-Sic W composites were studied.The results show that Ti6Al4V-1SiCw composite had better corrosion resistance than Ti6Al4V alloy.After solution aging heat treatment at 1050℃,Ti6Al4V-1SiCw composite had better corrosion performance.The main reason for the improvement of corrosion performance of composite materials is that（i）the high-density grain boundaries formed by the grain refinement of composite materials have higher energy,which was easier to react and form stable passivation film.（ii）The structure of Ti6Al4V-SiCw composite was equiaxedαphase,which is easier to form stable passivated film than metastableα’-Ti phase.（iii）The existence of ceramic particles leads to the formation of more microcells in the titanium matrix,which promoted the formation of soluble[TiCl₆]^2-complex.The formation of passivation film on the surface of Ti6Al4V-SiCw composite was accelerated.