On the solidification behaviour and cracking origin of a nickel-based superalloy during selective laser melting

Inconel 738LC samples were fabricated by selective laser melting (SLM) using different laser powers and scanning speeds and investigated using a range of characterisation techniques. High melting point Al-, Si- and W-based oxide particles and gamma cellular structure were observed in the as-fabricated samples, implying that the solidification of the alloy melt during SLM may have started with preferential nucleation of oxide particles, followed by the widespread nucleation and growth of gamma cellular structure. Cracks in the as-fabricated samples are associated with pores, or Al-, Si- and W-based oxide particles and small grains along some large grain boundaries (GBs). Those pores with sharp corners or rugged inner surfaces are particularly prone to cracking development during SLM. Most of the cracks were developed at gamma grain-oxide interfaces or along the central line of oxide clusters, which is attributed to solidification contraction and thermal shrinkage of the grains that are associated with oxides. Nano-indentation tests show that Si/W/O-rich GB regions demonstrate a significantly higher hardness as compared with gamma matrix, indicating that the segregation of Si, W and O has caused embrittlement to the GBs. Large misorientation was observed between grains (large or small) that are either associated with cracks or not, suggesting that large grain misorientation does not inevitably lead to cracking development in the current material. Small grains present along large grain boundaries were found to act as crack initiation sites or affect crack propagation path.