Thermal design considerations for a L-PBF built metal component: effects of Inter-Layer Cooling Time, Preheating Temperature and Gas Flow

Abstract

The paper aims to investigate some important thermal effects that could affect the Additive Manufacturing (AM) process of Laser Powder Bed Fusion. This analysis starts with investigating the variation of the material substrate temperature due to a variation of the Interlayer Cooling-Time (ILCT); then, the paper analyzes the effect of Preheating temperature on the material microstructure of the first building layers. Finally, we assess the effect of variation in gas flow speed as a function of part position on the building platform. In addition, in this work, the previously mentioned thermal aspects are evaluated in detail under particular geometrical and printing conditions considered the most critical for the L-PBF process. All cases studied are performed on IN718 superalloy specimens. In particular, for ILCT investigation, 60 microns layered specimens are printed for Preheating temperature analysis 40 and 60 layered specimens and for gas flow speed evaluation 40 microns one. All the results are evaluated through a porosity and melt pool analysis. The results obtained in this work highlight a critical range for low ILCT, 2-6 seconds, for part integrity that could be affected by overheating effects. To avoid this criticality, inserting ghost parts during the printing or reducing the laser power value is suggested. Concerning the preheating temperature effect, the first 1.2 mm of printed layers are found to be critical and affected by melt pool instability. In this case, a sacrificial substrate used in the first layers could save the quality of a few layers height part. The gas flow analysis highlights how some areas of the building platform are affected by particular thermal conditions negatively influencing material printability. To minimize this issue as much as possible, modify the job layout to avoid printing parts in the critical zones.