Influence of spattering on in-process layer surface roughness during laser powder bed fusion

Laser powder bed fusion (LPBF) based additive manufacturing (AM) holds great promise to efficiently produce high-performance metallic parts. However, LPBF processes tend to incur stochastic melt pool (MP) spattering, which would roughen workpiece in-process surface, thus weakening inter-layer bonding and causing issues like porosity, powder contamination, and recoater intervention. Understanding the consequential effect of MP spattering on layer surface is important for LPBF process control and part qualification. Yet it remains difficult due to the lack of process monitoring capability for concurrently tracking MP spatters and characterizing layer surfaces. In this work, using our lab-designed LPBF-specific fringe projection profilometry (FPP) along with an off-axis camera, we quantitatively evaluate the correlation between MP spattering and in-process layer surface roughness for the first time to reveal the potential influence of MP spatters on process anomaly and part defects. Specifically, a method of automatically and accurately extracting and registering MP spattering metrics is developed by machine learning of the in-situ off-axis camera imaging data. Each image is analyzed to obtain the MP's center location and the spatter count and ejection angle. These MP spatter signatures are registered for each monitored MP across each layer. Then, regression modeling is used to correlate each layer's registered MP spatter signature and its processing parameters with the layer's surface topography measured by the in-situ FPP. We find that the attained MP spatter feature profile can help predict the layer's surface roughness more accurately (> 50 % less error), in contrast to the conventional approaches that would only use nominal process setting without any insight of real process dynamics. This is because the spatter information can reflect key process changes including the deviations in actual laser scan parameters and their effects. The results also corroborate the importance of spatter monitoring and the distinct influence of spattering on layer surface roughness. Our work paves a foundation to thoroughly elucidate and effectively control the role of MP spattering in defect formation during LPBF.