Study Of Modification Treatment On The Refinement Mechanism Of Pure Titanium By Laser Additive Manufacturing

Titanium alloys are an important engineering material,and its microstructure is formed in the process of liquid-solid and solid-solid phase transformation.These phase transformations govern the microstructural evolution and exhibit complexity.Traditional manufacturing techniques show lower material utilization and higher manufacturing costs when producing complex structures.In contrast,as a type of metal additive manufacturing(AM)technologies,selective laser melting(SLM)is highly customizable through layer-wise melting,which can fabricate near net-shaped components with complex geometries.However,the inherent high temperature gradient and rapid cooling rate of the SLM process prevent the nucleation before the solidification front and lead to the formation of epitaxial columnar grains with preferred directions along the building direction(BD),resulting in anisotropy of texture and mechanical properties Formation.In this paper,the influence of solute copper(Cu)and lanthanum oxide(La₂O₃)particles on the grain refinement of SLM fabricated commercially pure titanium(CP-Ti),and the relationship between microstructure and mechanical properties is analyzed.In order to maintain the sphericity of the powder and the uniform dispersion of Cu/La₂O₃ powder in CP-Ti,a process based on magnetic stirring was used to prepare the mixed powder of Ti-Cu and CP-Ti+La₂O₃.Based on the optimization of SLM process parameters,a 70×12×2 mm³(Y×Z×X)rectangular sample was formed,and the microstructure characteristics and mechanical properties of the SLM sample were comprehensively studied.Experiments on the addition of 1.25,2.5 and 5 wt.%Cu solutes,the results showed that because the Cu solute have a high constitutional supercooling capacity at the front of the solid-liquid interface during the solidification process,the Ti-5Cu sample manufactured by SLM have obtained completely equiaxed prior-?grains.With increasing Cu content,the morphology of martensite exhibited a transition from parallel-sided plates or laths to acicular plates structure.Due to the accommodation effect of the martensitic transformation strain in the SLM-fabricated Ti-Cu samples,the14?20%of the plate martensite partitioning interfaces was constituted of{10 1}twin boundaries.The microhardness,yield strength and ultimate tensile strength were enhanced synergistically with increasing Cu content,which can be attributed to the combined effects of grain-boundary strengthening,solid-solution strengthening,dislocation strengthening and precipitation strengthening.Experiments on adding 1wt.%of La₂O₃ particles,the results showed that a uniform acicular??martensite microstructure is obtained in the SLM-fabricated CP-Ti+La₂O₃,which contains high-density dislocations and{10 1}twins,and have a weaker crystal texture.Nano-La₂O₃ particles were uniformly distributed along the grain boundaries and inside the grain,exhibiting a highly coherent interface with the CP-Ti matrix.After adding La₂O₃,the equivalent diameter round grain size of CP-Ti is uniformly refined from 2.21?m to 1.47?m.The grain refinement of CP-Ti through La₂O₃ addition can be attributed to a combination of rapid solidification rate during SLM,limitations on grain growth and increased nucleation sites.After grain refinement,the microhardness,yield strength and ultimate tensile strength were enhanced up to 276 HV_0.3,858 MPa and 940MPa,respectively,with the elongation of 8.6%.The enhancement of mechanical properties was mainly attributed to the combined effects of grain refinement strengthening,substructure strengthening and La₂O₃ dispersion strengthening.