Improving crack reduction by tailoring minor element content in γ/γ′ cobalt-based superalloys processed by directed energy deposition

Abstract

This study investigates the possibility of manufacturing γ/γ′ cobalt-based superalloys by Directed Energy Deposition (DED). Due to their complex chemical composition, these alloys are highly sensitive to liquid phase cracking mechanisms, particularly at high-angle grain boundaries. The cracking sensitivity of two alloys is investigated in relation to chemical modifications of minor elements and changes in process parameters. Four different parameter sets are employed, resulting in four distinct microstructures. Regardless of the process parameters, changes in minor elements such as boron, zirconium, and silicon significantly influence the cracking sensitivity of the alloys. Reducing their content leads directly to an improvement in the as-built material’s health due to an increase in the solidification temperature of the final liquid pocket. The role of grain boundary density and length is also emphasized in regard to the cracking response of the samples. Increasing the density of high-angle grain boundaries allows for a better distribution of solutes and stresses across multiple interfaces. However, the reduction in cracking is less pronounced with changes in process parameters than with reductions in minor element content. This work highlights the importance of tailoring minor element content for additive manufacturing and provides guidance for the design of new alloys.