Demonstrating a Grain Boundary Engineering Approach for Selective Laser Melting

ASRC Federal Astronautics (AFA) is pleased to propose this plan to perform a feasibility study of using Selective Laser Melting (SLM) Additive Manufacturing (AM) to produce grain boundary engineered (GBE) 316L austenitic stainless steel. Coincident site lattice (CSL) and low-angle grain boundaries, which result from GBE, improve metal alloys’ properties such as fatigue life and stress/corrosion cracking. Fatigue life is an area of concern for AM technologies, as they tend to produce materials that have lower fatigue properties than their traditionally manufactured counterparts. In addition, GBE grains are, by necessity, equiax grains, whereas AM’s rapid cooling encourages elongated columnar grains in the “as-built” condition because of limited nucleation cites for grain boundary formation. AFA proposes implementing two different approaches for obtaining equiax grains in the “as-built” 316L austenitic stainless steel with a focus on identifying the feasibility of using these for GBE. Method one involves using an ultrasonic transducer attached to the build plate to manipulate the microstructure during solidification of the meltpool. Method two uses titanium carbide (TiC) nanoparticles mixed into 316L austenitic stainless steel using a ball mill. Both methods increase the amount of nucleation cites, promoting equiax grain formation. These methods will be investigated independently and together to judge both their relative effectiveness at producing equiax and GBE grains. This approach has the potential to create three commercializable technologies. Ultrasonics can readily be employed across various machines and alloys while also being nearly non-invasive in that it does not requires customized SLM computer software or optics systems to further control laser parameters. Adding TiC nanoparticles to 316L leads to developing processes for nano-reinforcing other alloys as well. Finally, using one or both of these methods together perform GBE in 316L is a stepping stone to developing GBE processes in even more SLM AM alloys such as GRCop-84, Inconel alloys, aluminum alloys, etc.