Porosity formation mitigation in laser powder bed fusion process using a control approach

This study deals with quality control of the laser powder bed fusion (L-PBF) process using a temperature measurement approach rather than the commonly used energy density criterion. Temperature domains corresponding to the most common porosities, namely; lack of fusion (LOF), lack of penetration (LOP), and keyhole, were determined in a range of process parameters using a thermal imaging system. A safe zone was introduced by defining a lower and an upper limit based on the critical temperatures causing transitions from LOP to defect-free and from defect-free to keyhole zones, respectively. A proportional-integral-derivative (PID) controller was used to maintain the melt pool temperature within the safe zone during the L-PBF process for Inconel 625 and avoid the formation of porosities, regardless of the initial condition selected and the scanning speed employed. In all cases, a short settling time in the order of the printing time for a few layers was required to reach the steady-state condition at which defect-free parts could be obtained. The knowledge gained from this study can pave the way for the development of new temperature-based criteria considering all process variables contributing a role in the L-PBF process.