Investigation On The Microstructures And Mechanical Properties Of Ti-6Al-4V Parts Fabricated By Electron Beam Melting And Selective Laser Melting

Electron Beam Melting(EBM)and Selective Laser Melting(SLM)are two typical layer-by-layer additive manufacturing technique that shows great promise for fabrication of medical devices and aerospace components.A comprehensive understanding of microstructure and mechanical properties is necessary before its potential can be fully realized.Ti-6Al-4V cylinders with different orientations and part sizes were fabricated by electron beam melting and selective laser melting using Ti-6Al-4V powder.Effects of orientations,part size and hot isostatic pressing(HIP)treatment on microstructure and mechanical properties were investigated in this work.And the microstructure,tensile properties,and fatigue properties of SLM and EBM samples were compared in this study.The effect of part size,orientation on the porosity,microstructure and their contribution to the tensile and fatigue properties were elucidated.The results indicate that the as-fabricated SLM and EBM samples mainly consisted of α’ and α + β phase,respectively.The pores in EBM samples are almost spherical and most of them concentrated in 1×10-4～-1×10-3 mm3;For SLM samples,the pores contained are flat or irregular shape,and most of them concentrated in 1×10-5～1×10-4 mm3 and much more aligned compared to those in EBM samples.EBM samples comprise smaller and less amount of pores compared to SLM samples.The thickness of both SLM and EBM samples decrease with part size whereas the effect in EBM samples are more obvious than that in SLM samples.The a lamellar in EBM samples(-1.6 μm)are much coarser than those of a’ martensite in SLM samples(-0.6 μm).The part size has obvious influence on the microstructure of EBM samples whereas the SLM samples are not so sensitive to the part size.Both SLM and EBM samples along vertical orientation possess higher strength and better ductility than those along horizontal orientation.This is mainly due to that the microstructure of samples have a relationship respected to prior columnar β gain during the rapid solidification and cooling.The growth direction of prior columnar β gain in vertical and horizontal orientation fabricated by both EBM and SLM is parallel to the tensile stress direction of vertical samples and which is vertical to the tensile stress direction of horizontal samples.The tensile strength of SLM samples is significantly greater than that of EBM samples,and tensile properties of EBM samples is comparable to wrought samples,whereas SLM samples are exhibited excellent tensile properties and low plasticity characteristics.And more compact EBM as built samples possess slightly higher fatigue limits compared to SLM as built samples,which can be attributed to the difference in microstructure and internal pores of EBM and SLM samples.By HIP treatment,the fatigue limits of SLM and EBM samples are increased higher than 550 MPa,which is comparable to wrought samples.But after hot isostatic pressing(HIP),the fatigue limit strength of SLM is still lower than that of EBM sample,which is mainly due to that a lamallars in HIPed SLM is still long and thick and hot isostatic pressing can not make pores inside the sample completely disappear.