Effect of Lack of Fusion Formed during Electron Beam Powder Bed Fusion of Ti-6Al-4V Alloy on Impact Toughness

Abstract

Selective electron beam melting (EBM), a powder bed-based fusion process of Ti-6Al-4V alloy, is now being applied more and more widely particularly in biomedical and aerospace manufacturing industries. However, how Charpy impact energy (IE) of Ti-6Al-4V is affected by the build direction and lack of fusion (LOF) formed during the EBM process are not sufficiently understood. This information is generally very important in the applications of EBM. In this study, the build direction (BD) and LOF on IE have been studied and the effective factors have been found depending on the notch direction (ND) and thus the crack propagation direction in the microstructure. In this study, the influence of the BD and LOF on Charpy IE has been studied. Three effective factors have been found associated with the ND and thus crack propagation direction. The first factor is the BD which is the IE increases as the angle between the ND and the BD increases. This leads to the result of cracks propagating readily along with the α (hcp) grain boundaries, absorbing less energy. The second factor is LOF to assist crack propagation and this effect is maximized when the ND and the BD coincide. The last factor is the microstructural variances. In vertical samples, the crack has been found to propagate mostly through the α + β colonies and barely propagate in the prior β grain boundaries which leads to an increase in IE. While in the horizontal and 45° built directed samples, the crack propagates in both α + β colonies and the prior β grain boundaries which lead to lower the IE.