Comparative study between CW and PW emissions in selective laser melting

The definition of process parameters depending on the geometry of the workpiece is one of the main challenges for selective laser melting (SLM). The possibility to use different emission modes is an essential feature of the contemporary fiber lasers, which still requires further attention for controlling the melt pool size. Most of the commercially available SLM systems operate with continuous wave (CW) emission lasers. As a consequence, a substantial effort has been directed toward obtaining full densification by considering the variation of process parameters in CW modality. Pulsed wave (PW) emission achieved by power modulation of a fiber laser is preferred by a smaller fraction of the industrial SLM systems. The differences between the two emission regimes, advantages and disadvantages in their use have not been fully understood. Accordingly, this work proposes a comparative study between the two emission regimes in SLM, namely PW and CW. For this purpose, a single-mode fiber laser is coupled to a prototype SLM system composed of an automated powder bed and a scanner head. The laser source is extensively characterized for pulsed wave emission characteristics in a power modulated regime. Conditions providing the same energy content over the single track are determined and their effect on single-track densification is studied. High-speed imaging is used to observe the differences in the melt pool formation in situ. The overall results confirm that CW emission provides a larger and more stable molten pool during the process, resulting in higher deposition rates. On the other hand, under stable conditions, PW emission provides relatively narrow tracks, which might be problematic for porosity formation and at the same time useful for the production of fine geometries.