Understanding the effects of temporal waveform modulation of the laser emission power in laser powder bed fusion: Part I - Analytical modelling

The architecture of contemporary fiber laser sources enables users a wide choice in terms of spatial and temporal profiles during the laser powder bed fusion (LPBF) process. Given the range of possibilities, the need for analytical modelling approaches to predict the consequences of waveform modulation in terms of both thermal and fluid-dynamic aspects over the powder bed, process dynamics and resulting part quality is of great interest. Within the present investigation a moving point source analytical model was developed to study the effect of temporally modulated laser beams over the temperature distribution and recoil pressure induced over the molten region during single track LPBF depositions. This study configures as the first part of an investigation on the topic presented with the aim of developing the modeling framework to predict the effects of temporal waveform modulation in the LPBF process. The model developed was implemented numerically to simulate the single track LPBF deposition of stainless steel AISI316L with different waveform shapes ranging from the conventional Square Wave emission to Ramp Up, Ramp Down and Triangle waveforms. Modulation at different amplitude levels and different waveform frequencies were also investigated. Results show that temperature variations followed the temporal profile of the power exposed over the material. Consequently, recoil pressure oscillations over the melt region exhibited a periodic profiles correlated to the waveform modulation of the laser power indicating that melt flow may be controlled by means of such techniques. Peak values of recoil pressure, which might be symptomatic of melt pool instabilities, could be reduced employing higher levels of modulation frequency or lower oscillation amplitudes between non-zero values of the emission power.