Porosity Analysis in Metal Additive Manufacturing by Micro-CT

Abstract

This study aims at analyzing process-induced pores in selective laser melting (SLM), a laser powder-bed fusion additive manufacturing (AM) process. Porosity is one of the most problematic defects in SLM parts; it impairs the part performance, and yet, is sharply sensitive to the parameters of the SLM process itself. Detailed analysis of SLM pore formations using a computed tomography (CT) technique is desired in order to understand the porosity level under different process conditions. In this study, an SLM system was used to fabricate samples, using Ti-6Al-4V powder, with single tracks formed, at 60 μm layer thickness, with different laser powers and scanning speeds to vary the energy density. A micro-CT (μ-CT) scanner was used to measure the internal features of the SLM specimens without any post-build treatments and to analyze the porosity inside single tracks formed with different energy densities. There are different mechanisms of pore formation in SLM, in particular, this study first focuses on the pore formation due to the keyhole phenomenon, caused by a high energy density. μ-CT scanning at a 6 μm resolution is able to clearly reveal the pores in the SLM samples. From the CT scan and analysis results, it is observed that increasing the energy density increases the volume of pores. For example, with 195 W and 200 mm/s, the number of pores is 93 and the total pore volume is 0.014 mm3 for a scanning length of 12 mm. On the other hand, if the energy density is less than 0.24 J/mm, few or no pores were observed, because possibly the melting process changes from the keyhole mode to the conduction mode.