Study On The Preparation Of Ti-6Al-4V Powders By EIGA Method And The Microstructure And Properties Of Selective Laser Melted Parts

Titanium alloys are widely used for their excellent specific strength,corrosion resistance and biocompatibility.And the development of selective laser melting?SLM?forming technology has been accelerating its application in biomedical field.The feedstock material of SLM requires high quality,and is often dependent on imports.The high cost restricts the popularity and application of SLM technology.Although Ti-6Al-4V alloy is the most mature titanium alloy,the elastic modulus is much higher than that of human bones,which limit its application as an implant.Ti-6Al-4V alloy powders were prepared by a self-developed electrode induction melting gas atomization?EIGA?equipment.Quantitative characterization of powders was carried out and the SLM formability of it was researched.At the same time,in order to obtain Ti-6Al-4V alloy with a low elastic modulus that fabricated by selective laser melting,a sandwich model of gradient structure is designed in this dissertation.Two types of gas atomizing nozzles,annular-hole and annular-slit,were used in the experiment.Furthermore,the atomization pressure and melting power input were adjusted to design four sets of process parameters and their effect on the properties of powder were studied.The powders were sieved by 270 standard mesh?powder particle size?53?m?,then the size distribution and particle shape were quantitatively characterized by using MASTERSIZE 2000Laser Particle Size Analyzer and Occhio 500NanoXY+HR Particle Size and Shape Analyzer.Results show that optimal results are achieved by the combination of a ring-slit nozzle,atomization pressure of 5 MPa and melting power of 25 kW.A normal distribution of powder size is attained and the D₅₀ is 31.9?m.The mean Circularity and Occhio bluntness are 90.6%and 92.7%,respectively.It is found that more than 80%of powders have Occhio outgrow of 0.Further study shows that the microstructure of the powder is acicular??martensite with random orientations.The amount of hollow powder is extremely small,and the powder composition is practically the same as the ingot with a low oxygen content,which satisfies the requirements of the SLM process.Dumbbell breakup and shear breakup occur in the second stage of the atomization process.In order to verify the forming quality of Ti-6Al-4V alloy powder prepared by EIGA method,the effects of laser power,scanning velocity and scanning strategy on the surface quality,relative density,mechanical properties and microstructure of the as-SLM samples were studied by single factor variable method.Results indicate that the optimum parameters window with the laser power of 275³00W,scanning velocity of 900¹100 mm/s and rotate 90°of adjacent layer in the stripe pattern.The layer thickness and scanning space are fixed at 30?m and 100?m,respectively.The relative density of components fabricated by optimal parameters is 99.4%,and the top surface roughness is 5.32?m.Samples ruptured at a tensile strength of1184.8 MPa and elongation of 5.82%.The samples were destroyed in the mode of mixed toughness and brittleness fracture.The microstructure of samples is prior columnar?grains along the forming direction,the internal of it is basket??martensite.As-SLM Ti-6Al-4V alloy samples fabricated with parameters of higher laser energy density in the optimum parameters window obtained a better corrosion resistance.While all the as-SLM samples possess a worse corrosion resistance than cast sample.As for the sandwich model of gradient structure,three kinds of structure A,B,C with different porosity in middle layer were produced by the laser energy density of 19.6 J/mm³,23.0 J/mm³,and 333.3 J/mm³,respectively.And their microstructure and mechanical properties were investigated.Results show that the primary?columnar grains of the lower dense layer in structure A and B are coarser than that of the upper dense layer.The hardness of lower layer is slightly higher than upper layer as for it contains more brittle phase??.The pores in structure A are connected without obvious orientation.Structure B consists of a large number of isolated holes and a small number of connected hole perpendicular to the forming direction.All pores in structure C are isolated holes with nearly circular sections.Meanwhile,structure A and B contain a few discontinuous prior?columnar grains.While structure C exists coarsened equiaxed grains.Specimens with structure A are wrapped under thermal stress generated in forming process.The tensile strength of samples with structure B can reach 979 MPa combines with elongation of 4.54%.The fracture of dense layer is a mixture of ductile fracture and brittle fracture.The tensile strength of samples with structure C is 1121 MPa,and the elongation is as high as 7.05%.The modulus of A,B and C are 19.7 GPa,31.5 GPa,and 43.7 GPa,respectively.