From recycled machining waste to useful powders: sustainable fabrication and utilization of feedstock powder for metal additive manufacturing
Date
2018
Authors
Fullenwider, Blake Patrick, author
Ma, Kaka, advisor
Weinberger, Chris, committee member
Neilson, James, committee member
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Abstract
Gas atomized (GA) powders are the most common feedstock for state-of-the-art metal additive manufacturing (AM) technologies because of their spherical morphology and controllable particle size distribution. However, significant resource consumption, e.g., energy and inert gas, are required to produce GA powders, leading to high costs and limited availability in alloy compositions. To fulfill the growing demand for alternative and sustainable feedstock production for metal AM, my research aimed to explore a mechanical milling strategy to fabricate 304L stainless steel powders from recycled machining waste chips. A theoretical analysis was performed to evaluate the impact force on powder and the consequent maximum deformation depth per impact during ball milling with different ball diameters. The modeling results suggest that 20-mm-diameter balls effectively reduce the powder particle size while 6-mm-diameter balls are favorable in terms of forming spherical morphology of the powder. Various ball milling procedures were implemented to experimentally investigate the effect of ball diameter on the powder morphology evolution and particle size refinement. It is found that a novel dual-stage ball milling strategy effectively converts machining chips to powder with desirable characteristics (near spherical morphology with particle sizes of 38-150 μm) for metal additive manufacturing. The ball milled powders created from the machining chips also exhibit a higher hardness than GA powder, based on nanoindentation testing. To verify the viability of using the ball milled powder created from machining chips in metal AM, single tracks (ST) have been successfully deposited via laser engineered net shaping (LENS®) and compared to the single tracks made from GA powder (ST-GA) using identical deposition conditions. The microstructures of these single tracks exhibited adequate adhesion to the substrate, a uniform melt pool geometry, continuity, and minimal splatter. Minimal differences in grain structure were observed between the single tracks made from ball milled powder (ST-BM) and ST-GA. However, the average nanoindentation hardness of ST-BM is approximately 21% higher than that of ST-GA. Although the chemical compositions of both types of single tracks are within the compositional range of a 304L stainless steel, the increase in hardness of ST-BM is attributed to a 1.7 wt.% decrease in Ni content, potentially leading to an increase in the amount of martensite. Therefore, my research has discovered a sustainable approach to fabricate powders from recycled machining chips and has proved it is feasible to utilize these powders as feedstock in metal AM. Future work on depositing bulk samples with more complex geometry using the ball milled powder is proposed.
Description
Rights Access
Subject
metal additive manufacturing
plastic deformation
sustainability
nanoindentation
ball milling
stainless steel