Microstructure And Properties Of Ti6Al4V Alloy Prepared By Selective Laser Melting

Ti6Al4V alloy with its excellent mechanical properties,good corrosion resistance,high specific strength,low density and other characteristics,and widely used in aerospace,biomedical and automotive manufacturing fields.However,its development is limited by the complex process,low forming accuracy,high manufacturing cost and low material utilization rate.Selective Laser Melting（SLM）technology uses high energy laser beam as heat source and powder as raw material to form components with arbitrary structure and high powder utilization rate,which can directly reduce the cost and make Ti6Al4V alloy more widely used.However,during the preparation of Ti6Al4V alloy by SLM,due to its complex thermal cycle,extremely fast cooling rate and large thermal gradient,the microstructure will appear nonuniform,and the formation of fine acicularα/α’martensite and coarse columnarβphases,respectively.Therefore,on the basis of SLM Ti6Al4V alloy,the anisotropy behavior of microstructures and mechanical properties in the deposited parts,the effect of Cu addition on microstructures and mechanical properties,and the evolution of microstructures and mechanical properties in the alloys under different heat treatment regimes were studied in this paper.The main conclusions are as follows:（1）The effects of laser power and scanning speed on the surface roughness,microstructure,porosity and microhardness of the deposited parts were studied,and the optimal proportions of SLM Ti6Al4V forming process parameters were obtained.On this basis,the anisotropy properties and the internal defects,microstructure and surface hardness of the micro-sized tensile specimens were studied.The experimental results show that the optimum process parameters for preparing Ti6Al4V alloy by SLM are laser power 220W,scanning speed 200mm/s,scanning spacing 70μm and powder layer thickness 30μm.The microstructure and mechanical properties in each micro-zone show obvious anisotropy.The microstructure in the horizontal plane is irregular polygon,while in the vertical plane it is coarse columnar.The elongation of transverse tensile specimens after fracture is as high as14.6%,while that of longitudinal tensile specimens after fracture is only 4.25%,but the tensile strength is similar.（2）In order to reduce the anisotropy of microstructure and mechanical properties of SLM Ti6Al4V deposition in micro-interval,the effect of Cu content on phase composition,microstructure,mechanical properties and wear resistance of SLM Ti6Al4V deposition was studied by Cu doping method.The results show that the XRD pattern of Ti6Al4V+Cu deposition shows the existence of Ti₂Cu phase diffraction peaks,and the intensity of diffraction peaks increases with the increase of Cu content.Theβgrain size of Ti6Al4V+Cu alloy is smaller than that of Ti6Al4V alloy,and the grain size decreases with the increase of Cu content.At the same time,the tensile strength and wear resistance gradually increased with the increase of Cu content,and the tensile plasticity showed the opposite law.The microstructure heterogeneity and mechanical anisotropy of SLM Ti6Al4V deposition are obviously improved when Cu content is 0.5 wt.%.Compared with SLM Ti6Al4V deposition,the tensile strength of Ti6Al4V-0.5Cu deposition is enhanced in different deposition directions,and the elongation after fracture is more than 11%.（3）The internal relationship of"heat treatment system-microstructure-comprehensive properties"in SLM Ti6Al4V/Ti6Al4V-0.5Cu deposition was established,and a better heat treatment regime suitable for the preparation of Ti6Al4V and Ti6Al4V-0.5Cu alloys by SLM was obtained.The experimental results show that when the heat treatment temperature is lower thanβphase transition point temperature,the microstructure coarsing phenomenon occurs in the water-cooled or furnace cooled SLM Ti6Al4V/Ti6Al4V-0.5Cu alloy,and the deposited samples have good mechanical properties,only the tensile strength decreases.When the heat treatment temperature is higher than the temperature ofβphase transformation point,the coarse columnarβgrains change into equiaxed ones,and the tensile strength and elongation of the samples are reduced.However,compared with the deposition,the corrosion resistance of SLM Ti6Al4V/Ti6Al4V-0.5Cu has been improved significantly after heat treatment.The optimal heat treatment system for Ti6Al4V alloys prepared by SLM is 980℃/water cooling,and 980℃/furnace cooling for Ti6Al4V-0.5Cu alloys.