Research On Microstructure Control Of Titanium Alloy And Its Matrix Composites Forming Process By Selective Laser Melting

Selective Laser Melting(SLM)is one of the most promising metal additive manufacturing technologies in laser rapid prototyping technology.It can melt the metal powder to realize rapid and near-net forming of high-performance complex metal components by using high-energy laser beams.The forming component can be used after simple sandblasting treatment.In this paper,TA2,TC4,B₄C/Ti,LaB₆/Ti powder systems are used as the materials,and TA2,TC4,B₄C/Ti,LaB₆/Ti formed samples are prepared by SLM process,and the process parameter optimization and composition control are systematically investigated.With regard to the influence of the surface morphology,relative density,microstructure and macro-mechanical properties of titanium,titanium alloys and titanium-based composites formed by SLM,the changes in the microhardness and tensile strength of the formed samples under different processes and compositions were analyzed,and the strengthening mechanism of the reinforced phase-reinforced titanium matrix composites was discussed.The following conclusions can be obtained:The SLM process successfully prepared the TA2 pure titanium samples under the same laser volume energy density.There is no significant influence on the microhardness,but has a greater impact on the forming quality under the laser process control.With the increase of scanning speed and the decrease of scanning distance,the pores of the longitudinal section surface of the sample show a tendency of obvious first decreasing and then increasing,namely the forming quality increases at first and then decreases;when there is a relatively higher laser power,a faster scanning speed and a smaller scanning distance,the gas in the molten pool won't be able to release and defects such as pores are formed.The SLM process successfully prepared the TC4 titanium alloy formed samples based on the laser multiple scanning strategy.The scanning strategy of multiple laser scanning has influence on the forming quality.A better scanning strategy can significantly improve defects such as forming holes.P₁=240 W,P₂=120 W,v=1200 mm/s,h=0.14 mm and under the process parameters of P₁=320 W,P₂=160 W,v=1200 mm/s,h=0.14 mm,almost dense samples can be formed,and there are basically no pores in the longitudinal section.Since the effect of multiple scans on the grain refinement has not been observed,the increase in microhardness is not significant enough.However,the conclusion that,the microhardness of the titanium alloy increases at first and then decreases under the power within a certain power range during a single scan,is still valid in multiple scan tests.B₄C ceramic particle-reinforced titanium-based composite material powder consumables requires larger laser volume energy density during the forming process,but too higher laser volume energy density will bring out large energy fluctuations,resulting in large fluctuations in the structure of the formed sample and affecting the forming quality.When the laser volume energy density is in the range of 175?233 J/mm³,the forming quality is relatively good.As the laser's volumetric energy density increases from 175 J/mm³to 233 J/mm³and then to 292 J/mm³,the microhardness of the B₄C-reinforced titanium-based composite shows a tendency of the decreasing at first and then increasing.Composition control has a much greater impact on performance than process parameter control.The ceramic particle-reinforced titanium-based composite material's microhardness can reach 513 HV0.5,which is nearly 131.4%higher than the 213.9 HV0.5 of the TA2 matrix,but at the same time,the forming quality deteriorates,resulting in a significant decreasing in plasticity.The in-situ whisker-like Ti B,granular Ti C particles and the refinement of TA2 matrix grains are the main reasons for the enhancement of microhardness.Compared with B₄C ceramic particle reinforced titanium matrix composite powder consumables,LaB₆reinforced titanium matrix composite powder consumables require less laser volume energy density during the forming process.When the laser volume energy density is 107 J/mm³,the sample formed.It has the highest relative density,and the process parameters are P=240 W,v=800 mm/s,and h=0.14 mm.As the added mass of LaB₆increases from 1%to 4%,the relative density decreases to varying degrees,especially under the process parameters of P=280 W,v=1000 mm/s,and h=0.05mm.With the increasing of laser power and scanning speed under a certain range,the relative density of LaB₆reinforced titanium-based composite samples with different compositions shows a trend of increasing at first and then decreasing.When 2%LaB₆was added,the enhancement effect of the reinforcing phase was more significant.Compared with 1%LaB₆,the microhardness increased from 228.2 HV0.5 to 297 HV0.5,an increase of nearly 30.1%.The tensile strength of LaB₆reinforced composite material can reach 420 Mpa at a high temperature of 400?,which is up to 30.4%higher than the 322 Mpa of the TA2 matrix.As the test temperature rises,the performance improvement is more significant than that of the TA2 matrix.So does the service performance.The loading of Ti B whiskers and La₂O₃particles and the refinement of TA2 matrix grains and the strengthening of dislocations are the main reasons for the enhancement of mechanical properties.This paper mainly conducts related researches on SLM forming titanium alloys and titanium-based composite materials,and studies the effects of process parameters and scanning strategies on the surface morphology and forming quality of titanium and titanium alloys.The enhancement phase composition and addition quality have an effect on the in-situ titanium matrix composites.The influence of the surface morphology,microstructure and macro-mechanical properties of composite materials,and through the analysis of microstructure evolution and fracture behavior,reveals the strengthening mechanism of in-situ titanium-based composites,which is for the preparation of high-performance,high-quality titanium-based composites.The materials provide the basis.