Increased warping associated with lack of fusion porosity in additively manufactured PBFLB/Ti6Al4V cantilevers

Abstract

Powder Bed Fusion using a Laser Beam on Metals (PBF-LB/M) is one of the most widely used methods in the additive manufacturing of metals. This study used recommended process parameters on a commercial PBF-LB/M system to manufacture a series of cantilever specimens using the Ti6Al4V (ELI) alloy powder. During the manufacturing process, the material experienced residual stress (RS) causing cracking in some of the struts and resulting in warping, as expected for the cantilever specimens. The degree of warping indicated the amount of RS in the material and is a challenge for PBF-LB/M as-built parts. It is therefore important to study and minimize them as far as possible. Recently, post-processes, such as heat treatment and shot peening, are used to reduce RS. This paper reports on X-ray Computed Tomography (XCT) results acquired following the manufacturing process, showing an interesting correlation between lack of fusion (LOF) porosity and warping. The specimens were found to contain a range of different amounts of LOF porosity from 0.05 to 7.83%. It was observed that the increased LOF correlated with the increased warping in the series of cantilever specimens. It is reported that the previously contaminated protective window in the PBF-LB/M machine reduced the laser energy reaching the powder bed of specific parts, leading to the LOF porosity. With the support of numerical simulations, we propose that extensive LOF porosity is partly the cause for higher RS above the LOF pores due to reduced thermal dissipation, leading to stronger thermal gradients and more local RS and subsequent increased warping in the parts with higher amounts of LOF. This is the first reported observation of this interesting correlation between porosity and warping in additively manufactured metals.