Controlling Mechanism Of Microstructure And Property Of Additively Manufactured Ti6Al4V Alloy And Its Composites

Metal laser additive manufacturing(AM)is one of the most important technical approaches of the current national intelligent manufacturing development strategy and brings a new opportunity,especially for the development of new titanium materials and the corresponding components.However,the comprehensive mechanical properties of titanium alloys and their composites by AM are not ideal and are not enough to meet the needs of strategic equipment in aerospace and national defense fields.For instance,the common issues such as property anisotropy and low ductility are noteworthy.The reason is that the special manufacturing process of rapid cooling and direct deposition leads to anisotropies of microstructure and property,and the research on microstructure regulation,post-processing,and performance optimization is not thorough.Hence this study focused on titanium alloys and their composites and mainly included the innovation of heat treatment schedules and controlling the solidification microstructure of titanium matrix composites by boron addition.With the help of research methods including in-situ tensile investigation,calculation of Schmid Factors et al,the optimization of mechanical property and the controlling mechanism of anisotropy were investigated,which provided theoretical support for solving the common problems in the current additive manufacturing of titanium alloys.The main conclusions are as follows:(1)A new solution and high-temperature aging treatment was put forward which could optimize the mechanical property of the laser powder bed fusion Ti6Al4 V.The as-built L-PBF Ti6Al4 V had a strength of 1185 MPa and an elongation of ～3%.The microstructure fully consisted of ultrafine acicular α′ martensite and was rich in dislocation tanglings and {10(?)1}<10(?)(?)> compressive twins,which accounted for the high strength and poor elongation.This study developed a new solution and high-temperature aging treatment(920℃/1h/water quenching,800℃/2h/furnace cooling)which could decompose the martensite into dual-phaseα+β lamellae and promote the growth of nearly equiaxed grain boundary α with a limited grain coarsening effect resulting in the best comprehensive property.The elongation was dramatically improved to 17.2% and the strength was 1025 MPa.(2)The mechanism of anisotropy of AM Ti6Al4 V and the effect of heat treatment were illustrated.The tensile strength and elongation of the as-built Ti6Al4 V both increased when the tensile direction changed from 90°(vertical to the substrate)to 45° and then 0°,presenting a divergence of 120 MPa.After the heat treatment,the strength anisotropy was remarkably weakened(divergence < 40MPa)while the ductility anisotropy was retained.The weakening of strength anisotropy was due to the removal of the residual stress and the {100}β texture.By the calculation of the most likely active slip system and corresponding the Schmid Factors(SFs),the SFs were found to vary with the loading direction explaining the strength anisotropy.Further,the calculation indicated a distinct difference in overall SFs between the columnar prior-β grains suggesting a variety of strength and deformation capacity.When the tensile direction was 0°,there were most prior-βgrains in the gauge section and the deformation compatibility was best leading to the highest elongation(up to 20%～).(3)The columnar to equiaxed transition(CET)was achieved by the boron addition,which eliminated the anisotropy and improved the mechanical performance,the mechanism of microstructure evolution and the formation of network distributed Ti B was further clarified.This study successfully developed a titanium matrix composite powder embedded with network distributed Ti B and carried out in-situ fabrication of titanium matrix composites(TMCs)by direct laser melting.In the TMCs,the Ti B were distributed in an equiaxed network or columnar dendritic network,located in the middle and bottom of the molten pool,respectively.The transition from columnar prior-β grains to equiaxed prior-β grains was accomplished in the solidification process.The mechanism of CET and network distributed Ti B was further explained and mainly due to the boron-induced constitutional supercooling and nucleation of β grains.In the early stage of solidification,the temperature gradient was steep,and the prior-β grain grew into a columnar morphology and then columnar dendritic grains.Then,the temperature gradient became relatively gentle,and the enrichment of boron led to constitutional undercooling which drove the nucleation of equiaxed prior-β grains.The subsequently formed Ti B whiskers by eutectic reaction were distributed in columnar dendritic or equiaxed network morphologies.The Ti B network could inhibit localized strain and slip,blunt and deflect the cracks.The comprehensive property was better than the conventional TMCs and presented isotropic performance.(4)The mechanism of isotropic strength and the strengthening mechanism by equiaxed transition and network distributed Ti B were clarified.With the increased content of Ti B(1.25/2.5/5vol%Ti B),the network structure showed discontinuous,quasi-continuous,continuous and coarsening morphology,respectively,and the prior-β grains were significantly transformed into equiaxed morphology.The 2.5vol%Ti B/Ti6Al4 V composite had the best comprehensive property: the room-temperature strength was more than 1160 MPa which was greatly improved by 146～197MPa compared to the matrix alloy while the elongation was only reduced by 0.7～1.7%.The divergence of yield strength anisotropy was only 0.37%,and the divergence was0.87% for the ultimate tensile strength anisotropy,which presented strength isotropy.The high-temperature performance had an advantage over the conventional TMCs.The strength isotropy was due to the similar SFs of equiaxed microstructure in different loading directions as well as the load-bearing effect of Ti B in weakening the strength anisotropy induced by <100>β//BD(build direction)texture.The strengthening mechanism was mainly due to the increment of local misorientation induced by the network structure,which made the dislocation strengthening play a leading role.Besides,the network structure could hinder the movement of dislocations.The grain refinement and load-bearing effect of Ti B also contributed to the strengthening.The primary β grains in the low-content reinforcement TMCs(1.25vol%Ti B)were equiaxed and significantly refined,the matrix remained connected,and hence the strength and elongation were both improved.With the increased content of the Ti B(2.5vol%Ti B),the network structure was quasicontinuous,the dislocation strengthening effect was maximum,and the strengthening was the greatest.