Microstructure And Properties Of Ti6Al4V Alloy And Its Composites Formed By Selective Laser Melting

Selective Laser Melting(SLM)technology is a process to produce three-dimensional entity by using high-energy laser beam as energy source and metal powder as raw material,building layered part model by software and converting it into digital information,melting and laying metal powder layer by layer according to the program.Ti6Al4V titanium alloy formed by selective laser melting technology has better mechanical properties than casting,and has excellent corrosion resistance.However,its wear resistance and strength are poor,so TiC ceramic particles and other reinforcing phases are often used to improve its strength and wear resistance.In addition,TiC/Ti6Al4V titanium matrix composites formed by selective laser melting technology are widely used in aerospace,ship,automobile,medical materials and other fields because of its advantages of simple forming process,unlimited shape and size of parts,and the ability of regulating the microstructure and composition of composites.However,due to the characteristics of rapid heating and cooling under the action of laser heat source,there are many holes in Ti6Al4V and TiC/Ti6Al4V materials formed by selective laser melting,which reduce their tensile properties.Therefore,high energy pulse current is used to treat Ti6Al4V and TiC/Ti6Al4V formed by selective laser melting technology,and the hole defects in the materials can be reduced and the plasticity of materials can be improved through the thermal effect and electric plasticity of high pulse current.In this thesis,Ti6Al4V material and TiC/Ti6Al4V titanium matrix composites were formed by SLM technology using Ti6Al4V titanium alloy powder and TiC ceramic particles as raw materials and Ti6Al4V board material as the substrate,the formability was measured by porosity,and the influence of different technological parameters on the forming was explored and analyzed;the microstructure observation and performance measurement of the materials under different process parameters were carried out to further explore the influence of forming process parameters on the microstructure and properties of selected laser-melted Ti6Al4V titanium alloy and TiC/Ti6Al4V titanium matrix composites;the microstructure and properties of materials were analyzed after the process of different high-energy electrical pulses with better process parameters of forming.The results show that the porosity of Ti6Al4V alloy is related to laser energy density.When the laser energy density was low,the main defects in the selective laser melting forming Ti6Al4V sample were the lack of fusion defects.With the laser energy density increasing,the powder was further melted,and the lack of fusion defects decreased.However,when the laser energy density was too high,the porosity of the sample caused by keyhole effect also increased.There were coarse columnar grains in the direction perpendicular to the substrate in selective laser melting Ti6Al4V,and the microstructure was mainly composed of??and?phases produced by rapid cooling.Different combinations of laser power and scanning speed were changed,and when the laser power was 200W and the scanning speed was 1m/s,the selective laser melting Ti6Al4V had good mechanical properties.At this time,the tensile strength of the material reached 1283.6MPa,the strain rate was 5.1%,and the hardness was 377.4 HV_1.0.The high-energy pulsed current process was used to treat the Ti6Al4V titanium alloy formed by selective laser melting.Compared with the non-pulse current treatment,the internal pores of the Ti6Al4V sample were reduced,the columnar crystal size was reduced,the material strength and hardness were reduced but the strain was significantly increased.And the effect was more obvious with the increasing of pulse voltage.When the voltage was 90V and the frequency was350Hz,the high energy pulse current treatment for selective laser melting Ti6Al4V had a good effect,which can improve the plasticity of selective laser melting Ti6Al4V.At this time,the tensile strength was 1117.8MPa,the elongation is 13.3%,and the hardness is HV_1.0342.1.TiC/Ti6Al4V titanium matrix composites with 0.5vol.%,2vol.%,5vol.%TiC volume fraction and different particle sizes were formed by selective laser melting technology.When the TiC size was 45?75?m and the volume fraction was 5%,the composite cracked.When the size of TiC less than 10?m and the volume fraction was 2%,the composite still cracked.When the thickness of powder layer was increased from 30?m to 50?m,there was less cracking phenomenon of 2ct%TiC/Ti6Al4V composite.The volume fraction were continued reduced to0.5vol.%,and the TiC/Ti6Al4V composite was formed well without cracks and other defects.The structure of TiC/Ti6Al4V composites was a basket-like??phase and a?phase.A small amount of ceramic TiC dissolved under the action of laser,and according to the different volume fraction of TiC,the microstructure evolution of TiC/Ti6Al4V composites was different:dendritic eutectic TiC and a small amount of granular primary TiC were formed in-situ in TiC/Ti6Al4V composites with 5vol.%while in-situ eutectic TiC structure was formed in TiC/Ti6Al4V composites with volume fraction of 2vol.%and 0.5vol.%.The composites with5%and 2%TiC had brittle fracture during tensile.The tensile strength,elongation and hardness of the composites with 0.5%volume fraction can reach 1401.4 MPa,3.8%and HV_1.0407.6respectively.When 0.5vol.%TiC/Ti6Al4V titanium matrix composites were treated by high energy electric pulse,the pores were reduced and the plasticity of the composites was greatly improved.