Grain refinement and crack inhibition of hard-to-weld Inconel 738 alloy by altering the scanning strategy during selective laser melting

Abstract

Additive manufacturing of hard-to-weld nickel-based superalloys leads to the formation of elongated columnar grain structures and hot cracks along the build direction. Changing the scanning strategy during selective laser melting (SLM) without changing alloy composition is a promising method for the inhibition of hot cracking by grain refinement. In this work, the effect of grain structure on the cracking inhibition behaviour of hard-to-weld Inconel 738 alloy fabricated by SLM with different scanning strategies (rotation of 0°, 90° and 67° between layers, respectively) was investigated. Nearly crack-free, local equiaxed microstructures and an excellent combination of strength and ductility were obtained by rotation of 67° for the SLM-built Inconel 738 alloy due to bimodal grain structure with local equiaxed grains and columnar grains. The local equiaxed grains are attributed to dynamic recrystallization and give rise to thermal stress relaxation. Meanwhile, the aspect ratio of columnar grains is related to thermal gradient variation. In addition, the inhibition of initiation and propagation of cracks is attributed to grain refinement and reduced residual stress. This work demonstrates the feasibility of grain refinement by changing the scanning strategy and improved suppression of cracks in a hard-to-weld superalloy without changing the alloy composition.