Abstract

The formation of sub-grain cellular structures generated during the rapid solidification associated with selective laser melting (SLM) typically yields enhanced mechanical properties in terms of yield stress without considerable loss in ductility when compared with those of wrought material. The extent to which the sub-grain structure appears under standard metallographic preparation shows dependence on multiple systematic conditions. This study identifies the effects of solidification and cooling rate on the grain and sub-grain structure in stainless steel through varying the processing parameters (laser power, scan velocity and spot size) of single tracks on both as-received, small grain and annealed, giant grain substrates. The process parameters, in conjunction with the initial substrate microstructure, are key components in understanding the resulting microstructure. Process parameters, particularly scan velocity, dictate the solidification rate and primary regrowth directions while the initial microstructure and its thermomechanical history dictate the propensity for stored strain energy density. Modeling the thermal process allows for experimental analysis within the context of predicted location within processing space as it pertains to local interface velocity and temperature gradient. Furthermore, it highlights the fact that this specific material system behaves in a manner that is inconsistent with classical solidification theory.

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