Understanding the effects of temporal waveform modulation of laser emission power in laser powder bed fusion: Part II - Experimental investigation

The laser powder bed fusion (LPBF) process has historically been operated with high-brilliance fibre laser sources with continuous wave (CW) emission. Nonetheless, temporal waveform modulation of the laser emission power at high-frequency levels can provide a means to enhance the deposition process by modifying the melt dynamics and solidification mechanisms. In order to disclose the effect of different waveform shapes and their parameters, an experimental study using an open LPBF system was conducted. This paper is the second part of an investigation on this topic, which aims to validate the analytical model proposed in the first part of the work. The LPBF system that was developed enabled the power emission profiles to be programmed during single-track depositions. Four different waveform shapes were tested (namely square wave, ramp up, ramp down and triangle wave) at different levels of waveform amplitude (ΔP= 200–400 W) and different frequencies (fw = 2–4–6–8 kHz) during the single-track deposition of stainless steel AISI316L. High-speed imaging acquisitions allowed the melt dynamics to be disclosed and the melt-oscillation frequency to be identified. Larger waveform amplitudes and waveforms with sudden variations of emission power generated melt ejections and process instabilities. Stable conditions could be identified when employing ramp up and triangle waveforms with ΔP = 200. Melt-surface oscillation frequency corresponded to the values imposed via the modulation of the laser emission power, thus validating the analytical model of Part I, which correlated the melt-surface temperature to the recoil pressure induced over the molten pool. Optical microscopy images and metallographic cross-sections confirmed the high-speed video observations. Three-dimensional reconstructions of the depositions via focus variation microscopy allowed the build rates and roughness of the single tracks to be determined. Build rates obtained in stable deposition conditions with waveform modulation are analogous to values obtainable with CW emission, and beneficial effects over the roughness were reported.