Melt pool microstructure and morphology from single exposures in laser powder bed fusion of 316L stainless steel

Abstract

Single depositions of 316L stainless steel in pulsed laser powder bed fusion were performed to understand the influence of energy density input strategy on melt pool morphology, microstructure and temperature fields. Depositions performed at equal energy inputs, but variable laser powers, produced keyhole mode depositions for laser powers ≥150 W and conduction mode depositions for laser powers below 150 W. The laser power combined to the laser exposure time was found to control the penetration depth, increasing from 50 μm to 300 μm as the laser exposure increases. An increase in laser exposure time was found to decrease the isotherm liquidus velocity, from 1.2 mm s−1 to 0.2 mm s−1. EBSD maps of deposition cross-sections highlighted the change in temperature gradients as the melt pool increased in depth during keyhole and conduction mode depositions. Strong sub-surface temperature gradients were found for keyhole mode with weak gradients above the surface. In conduction mode variation in the temperature gradient throughout the melt pool was less pronounced. The decrease in cooling rate as the exposure time is increased was highlighted in coarsening of the cell spacing produced by the solute redistribution during solidification, diverging between bottom and dome of the deposition melt pool.