Effect of the initial substrate temperature on heat transfer and related phenomena in austenitic stainless steel parts fabricated by additive manufacturing using direct energy deposition

In this study, the effect of preheating a AISI 304 steel substrate to 300 °C, compared to a room-temperature substrate, just before direct energy deposition (DED) additive manufacturing of a AISI 316 L steel deposit in terms of absorptivity of the laser beam, heat transfer, microstructure, hardness and residual stress. The samples were thermally isolated using an alumina cup. Using direct energy deposition for 20 and 40 layers of steel, it was found absorptivities of 18% and 16%, respectively, in room temperature (RT) experiments. At high temperatures (HT, 300 °C) the net absorptivity is almost zero because of convective and radiative cooling. The estimated heat transfer coefficients are approximately 1.0 and 0.4 Wm−2K−1, for RT and HT respectively. The microstructure and hardness obtained under each condition (RT or HT) for 20 or 40 layers, were quite similar. The microstructures were composed of austenitic grains with a Vickers hardness values ranging from 170 to 220 HV. The measured residual stresses were quite low for additive manufacturing of austenitic stainless steel, ranging from 11 to −22 MPa for RT and from 37 to −17 MPa for HT.