Research On The Mechanism Of Electric Pulse Treatment On The Microstructure And Properties Of TiC/Ti6Al4V Formed By Selective Laser Melting

Ti6Al4V is the most widely used Titanium alloy because of its high specific strength and excellent corrosion resistance.However,its wear resistance is poor and its hardness is low,so TiC ceramic particles are often used as reinforcement to improve its strength,wear resistance.Selective Laser Melting(SLM)can achieve net-shape forming of complex parts,which is beneficial to the preparation of Titanium matrix composites.However,the Titanium matrix composites prepared by SLM have the problem of poor plasticity due to defects,brittle second phase and martensite which is produced after quickly cooling.Electric pulse treatment is an efficient post-treatment method for metal materials,which has a certain effect on healing defects and reducing martensite.Therefore,in this paper,TiC/Ti6Al4V composite was prepared by selective laser melting,and the influence of parameters on the microstructure and properties of Ti6Al4V alloy and TiC/Ti6Al4V composite was studied,and the TiC/Ti6Al4V composite was processed by electric pulse treatment.The effect of electric pulse treatment on the microstructure and properties of composites and its mechanism were studied.The results show that:The laser energy density has a great influence on the microstructure and properties of i6Al4V alloy.Increasing the laser power or decreasing the scanning speed can improve the energy density during the forming process,which makes the powder more easily melted and reduces the lack-of-fusion defects.At the same time,the size of the columnar crystal in the Titanium alloy increases.The hardness increases with the increase of energy density due to the decrease of lack-of-fusion defect.The tensile strength of Ti6Al4V alloy increases first and then decreases with the increase of energy density.When the energy density is low,the strength of Titanium alloy is low due to the lack-of-fusion defect,when the energy density is high,the strength begins to decrease due to the increase of the size of the columnar crystal.The Ti6Al4V alloy prepared with laser power of 200 W and scanning speed of 0.8 m/s has an elongation of6.0%and a tensile strength of 1286.8 MPa,which has excellent comprehensive properties.Using the same process parameters to prepare TiC/Ti6Al4V composite,the energy density is not enough to melt the mixed powder well.Therefore,as the TiC content increases,the number of lack-of-fusion increases.As TiC provides nucleation particle,the size ofβTi columnar crystals andαTi inside decreases with increasing TiC content,and a large number of equiaxedβTi grains are formed.In addition to grain refinement strengthening,TiC also improves the hardness and strength of the composites by solution strengthening and second phase strengthening.The tensile strength of TiC/Ti6Al4V composite with a volume fraction of 0.5%TiC prepared with laser power of 200 W and scanning speed of 0.8 m/s reached 1383.5 MPa,and the elongation was only 1.1%.Due to the increase of defects,the plasticity of the composite significantly decreases after the addition of TiC.The TiC/Ti6Al4V composite with a TiC volume fraction of 1.0%also crack without yielding,and its tensile strength is only 1233.9 MPa,even smaller than that of Titanium alloy.As the energy density increases,the defects in the composite gradually decrease,the size ofβTi gradually increases.The hardness of the material decreases as the grain size increases.As the energy density increases from 50 J/mm~3 to 111.11J/mm~3,the defects decrease,and the strength of TiC/Ti6Al4V composite with a TiC volume fraction of 1.0%improved,with a tensile strength exceeding 1450 MPa,significantly higher than that of 0.5%TiC/Ti6Al4V composite.In order to solve the problem of defects in TiC/Ti6Al4V composites prepared by selective laser melting,numerical simulation was used to simulate the temperature and stress of composite samples with defects during electric pulse treatment.The simulation results indicate that due to the current bypassing the defect,a current density difference is generated near the defect,resulting in uneven distribution of heat at the defect,resulting in thermal stress,and plastic deformation near the defect,gradually healing the defect.After increasing the voltage,the temperature and stress increase further,and the degree of defect healing is higher.Firstly,electric pulse treatment experiments were conducted on 1.0%TiC/Ti6Al4V composite.After 90 V electric pulse treatment,defects were reduced and the porosity decreased from 2.23%to 1.41%.Due to the longer processing time,the influence of thermal effects improved,the size of the primaryβTi and internalαTi has increased.The microhardness and elongation of composite increase due to the reduction of lack-of-fusion,while the strength decreases due to the increase in grain size.Using shorter processing time and lower voltage,electric pulse treatment experiments were conducted on the SLM 0.5%TiC/Ti6Al4V composite.It was found that the size of lack-of-fusion defects in the composite after 60 V electric pulse treatment was significantly reduced,and the number and size of pores slightly decreased.After 90 V treatment,the lack-of-fusion defects basically disappeared,and the number and size of pores continued to decrease.In addition,after electric pulse treatment,fine equiaxed structures appeared in the composite,which is due to recrystallization.As the pulse voltage increases,higher temperatures and stresses promote the recrystallization process,resulting in an increase in the number of equiaxed grains.Under the combined effect of defect reduction and grain refinement,the microhardness and elongation of TiC/Ti6Al4V composite continuously increase with the increase of pulse voltage.However,due to the decrease in dislocation density,the final strength of the composite slightly decreased.After 90 V electric pulse treatment,the elongation of 1.0%TiC/Ti6Al4V composite increased from 0%to 3.7%,and the tensile strength decreased from 1233.9 MPa to 1167.9 MPa.The 0.5%TiC/Ti6Al4V composite treated with 90 V electric pulse treatment has excellent comprehensive properties,with a yield strength of 1330.3 MPa,a tensile strength of 1344.5 MPa,and an elongation increase from 1.2%in the deposited state to 4.5%.