Temporal modulation of the laser emission power for microstructural manipulation in powder bed fusion

Continuous Wave (CW) emission is typically employed in powder bed fusion by laser beam melting (PBF-LB/M) to additively manufacture components. Contemporary fiber laser sources also provide time-varying emission profiles which may be exploited to modify the microstructural characteristics of the depositions. In order to explore the wide range of possibilities enabled by temporal waveform modulation of the power, in the present research experimental data was coupled to a digital model of the PBF-LB/M process to develop a methodological framework to investigate the solidification mechanisms induced by different emission profiles. Detailed metallurgical analysis was conducted on single track deposition obtained with different emission modes corresponding to CW, Square Wave (SQW) and Ramp Up (RUP) temporal profiles. The process was observed in-situ by means of high speed imaging to characterise the melt pool geometry whilst the microstructure was examined along both the longitudinal and transverse direction of the depositions exposing grain texture and morphology. Microstructure could be tailored employing different emission profiles whereby band-like columnar grain growth at a 30° inclination with respect to the build direction was obtained with RUP with a strong (100) texture whereas CW emission promoted equiaxed fine grains. SQW emission showed a compromise between columnar and equiaxed grain growth avoiding significant pore formation which was on the other hand present in the case of RUP emission. A Computational Fluid Dynamics (CFD) model was validated by comparing the predicted melt region with the metallographic cross-sections, thus supporting the creation of a digital platform to explore emission profiles in the future. Moreover, the numerical model allowed the observation of the sub-surface melt pool geometry disclosing the dynamics of pore formation which could be associated to keyhole collapse due to abrupt power variations.