Controlling Microstructure Through Nucleation - The Key to Designing the Next Generation of High-Performance Alloys for Additive Manufacturing

Additive manufacturing (AM) of metals is a relatively recent advancement with proven benefits for production of many complex critical components and great potential for performance, readiness, and cost improvements with growing use. Most of todays known alloys have been developed for other processes like casting or forging and only a relatively small number of alloys have been adapted for AM processes. Fusion based AM processes introduce extreme thermal gradients and cooling rates and many alloys designed for wrought processing suffer from cracking and other defects when printed. Even for alloys that have proven to be printable, anisotropic properties typically result from elongated grains in the growth direction and properties may not match those of wrought parts. In this project, we propose to develop, validate, and use integrated computational materials engineering (ICME) framework to create new designed-for-AM alloys that offer improved printability, performance, and reliability compared to existing options. The key to development of improved AM alloys is understanding and control of nucleation and solidification to produce the targeted microstructure. Strengthening mechanisms will also be included in computational modeling and experimental development. In Phase I, modeling efforts demonstrated that addition of nucleation inoculants would be more effective at controlling grain growth and eliminating cracking than in-specification composition modifications, and we demonstrated this experimentally in crack-prone Haynes 230. Multiple inoculant formulations resulted in crack-free microstructures with high temperature yield strength improved by 60% compared to the wrought equivalent. In this Phase II effort, we will extend the Phase I work to improve models for predicting as-printed grain size, post- heat treatment microstructure development, and creep behavior. We will also investigate model-guided formulations for eliminating solidification cracking in gamma-prime containing alloys IN738 and Mar-M-247. After validation in laser powder bed fusion AM systems and formulation refinement, expanded specimen and component level testing will be done to evaluate the performance of these alloys in an actual gas turbine.  The improved alloys developed in this project will be offered commercially for use by manufacturers and end-users including the U.S. Navy, defense industrial base, and private sector customers.