Selective Laser Melting of Nickel Superalloys: solidification, microstructure and material response

The Selective Laser Melting (SLM) process generates large thermal gradients during rapid melting, and during solidification certain nickel superalloys suffer from thermally induced micro-cracking which cannot be eliminated by process optimisation. This investigation sought to investigate and understand the root cause of micro-cracking in nickel superalloys when processed by SLM, with the aim of ultimately being able to predict the crack susceptibility of an alloy from composition alone. Microstructural analysis as well as implementation of Rapid Solidification Processing (RSP) theory and solute redistribution theory was used to establish SLM as a rapid solidification process. As a consequence, secondary dendrite arm formation and solute redistribution is largely inhibited, resulting in a bulk material which is near to a super saturated solid solution. The establishment of SLM as an RSP along with morphological and chemical analysis of micro-cracks support Elevated Temperature Solid State (ETSS) cracking as the primary cracking mechanism in SLM processed nickel superalloys. The crack susceptibility of a nickel superalloy, χ, was defined as the ratio between the solid solution strengthening contribution from alloying elements and apparent thermal stress generated by the process. Minor increases in the wt% of solid solution strengthening elements in Hastelloy X, a high crack susceptibility alloy, resulted in average reductions of crack density of 65%. Thereby supporting solid solution strength as a key factor in the crack susceptibility of a nickel superalloy. The addition of the apparent thermal stress component, further supported the crack susceptibility model, with the modified Hastelloy X being predicted to have a lower crack susceptibility. Additional validation of the crack susceptibility predictor was determined by taking compositions and material properties from published SLM investigations and calculating the crack susceptibility of the respective alloy. The results were found to be in good agreement with the reported observations