The Microstructures And Mechanical Properties At Different Forming Positions Of A Box-shaped TC4 Alloy Part Fabricated By Selective Laser Melting

A box-shaped TC4(Ti-6wt%Al-4wt%V)titanium alloy part was fabricated by selective laser melting(SLM)with gas atomized powder,in order to study the variation of microstructures and mechanical properties with the vertical distance from the substrate and the horizontal distance from the centerline.The microstructure of the TC4 titanium alloy fabricated by SLM on the transverse cross-section is composed of a lamellae.On the longitudinal cross-section,the microstructure is composed of α lamellae formed from the original β columnar grains.The length of the a lamellae is in the range of 50～100 μm and the width is in the range of 0.2～2μm.The distribution of Ti,Al and V elements is uniform,and the oxygen content in different areas is 0.12%.The number of pores in the part is in the range of 2～12 μm.Through the XRD and EBSD analysis,preferential orientations of the(102)and(110)planes of the a phase were observed on the cross section perpendicular to the growth direction.The preferred growth of the crystal plane was much more obvious in the top area far from the substrate and the corner area far from the centerline.The microstructures and mechanical properties at different heights from the substrate are different.Because of the higher cooling rate in the top area,the thickness of the a phase lamellae is relatively smaller.Due to the enrichment of heat,the width of the a phase lamellae and the sizes of the α lammela colonies are larger.The top area that far from the substrate exhibited the optimum mechanical properties:the yield strength is 965 MPa,the ultimate tensile strength(UTS)is 1043 MPa and the elongation to fracture is 11.1%.The yield strength and UTS of the bottom area were lower respectively,and the elongation to fracture is clearly lower,being 8.1%.The microstructures and mechanical properties at different horizontal distances from the centerline are not significantly different,and the thickness of the a lamellae in the central,edge and corner areas is basically the same.In the corner area that far from the centerline,the strength is slightly higher,with the UTS approaching 1054 MPa.In the central and edge areas near the centerline,the ductility is slightly higher,with the elongation to fracture being close to 10%.The anisotropy of the mechanical property is affected by the original β-columnar structure generated during the SLM process.The movement of dislocations and the coordinated deformation of α lamellae are hindered by columnar grain boundaries,which make the sample exhibit different mechanical properties in different tensile directions.When being stretched along the direction perpendicular to the crystal growth direction,the strength is higher but the ductility is lower.When being stretched along the crystal growth direction,the strength is lower,but the ductility is fairly good.