Effect of pulsed and continuous wave emission on the densification behaviour in Selective Laser Melting

The majority of commercially available Selective Laser Melting (SLM) systems operates with high brilliance fiber laser sources. These sources are most commonly operated in continuous wave (CW). On the other hand, a few employ pulsed wave (PW) emission by fast power switching, resulting in pulses with s level durations, kHz level repetition rates and low peak powers. No clear consensus is present in the academic and industrial communities over the choice of the emission regime. Clearly, the laser temporal emission mode can have an impact over key quality aspects, namely part density, geometrical errors, and roughness. The purpose of this paper is to investigate the effect of laser emission regime on the densification of AISI 316L stainless steel in the SLM process. In particular, single track formation was investigated by varying temporal overlapping of laser pulses from pulsed wave until continuous wave. A single mode fiber laser installed on a prototype SLM system constituted the experimental setup. The open hardware allowed for varying with high flexibility the laser emission. The CW and PW emission strategies were compared at fixed fluence levels per single track melting. The effect of duty cycle was evaluated starting from CW (i.e. 100% duty) moving towards PW. Furthermore, the densification behaviour was analysed at single and multiple layers, depicting the molten track stability in terms of volume. Results show that at fixed fluence, analogous operating conditions can be determined in the CW and PW processing of single tracks. The amount of deposited material is significantly higher when single tracks are produced with CW emission, although a lower variability may be identified when exploiting power modulation. Therefore, industrial systems could be more flexible enabling both CW and PW regime to exploit their peculiar advantages, where required