EAGER: Reduction of Melt Pool Balling in Metal Additive Manufacturing

Carnegie Mellon University, Pittsburgh PA

Investigators

Abstract

This EArly-concept Grant for Exploratory Research (EAGER) grant will explore the feasibility of using alloy metal powder for additive manufacturing. Additive manufacturing is changing how metal parts are built. The work in this research focuses on a particular additive manufacturing process where a laser beam traces a part shape over a layer of metal powder. The powder under the beam melts and solidifies as the laser moves away. When the trace is finished, new powder is spread and the trace repeated. Eventually a solid part is built. To build usable parts, the traces should be flat, continuous layers. This is determined by the power of the laser beam and the speed at which it travels. At the high values of power and speed, the melted material solidifies as a series of droplets instead of a continuous layer. This is called balling and it leads to unusable parts that are not completely solid. Balling limits how fast usable parts can be built because building is faster at high power and speed. Surprisingly, most metal powder compositions for additive manufacturing are based on those commonly used in subtractive manufacturing processes (e.g. cutting or milling) and are not optimized for additive applications. The team will tackle this problem by exploring the feasibility of controlling balling by adding alloying elements to the powders that change the behavior of the molten metal, and by demonstrating a new, cost effective method to evaluate different alloy compositions. Both research components of this EAGER award have the potential to make significant improvements in the speed and cost effectiveness of powder customization for additive manufacturing and component fabrication. The research work will investigate the balling defect in selective laser melting additive manufacturing and the effect of surface active elements. The reduction in surface energy with addition of surface active elements is expected to lead to reduce balling, since it has been associated with Plateau-Rayleigh instabilities and poor wettability. As sulfur lowers surface tension and strongly influences melt pool shape in laser welding, this research will investigate the effect of sulfur alloying content on stainless steel alloy melt pools. To overcome the prohibitively high costs associated with the fabrication of small batches of customized alloy metal powders for research purposes, this EAGER award will evaluate the suitability of testing on solid test samples. Solid test samples will be fabricated by arc-melting and machining processes. Initially the bulk material composition of the solid samples will be the same as commercial metal additive powders. Tracks of laser re-melted material will be made on these samples over a range of beam power and travel speeds with a focus on power and speed combinations where the transition to balling is expected. Tests will also be conducted with different powder layer thicknesses on the samples, and then on samples with varying sulfur content. Melt pool shapes and microstructures will be characterized and compared. The potential contributions of this work are validation of a cost-effective test method to evaluate alloy composition for selective laser melting additive manufacturing using solid samples, and a demonstration of process-related defect reduction through adjustments to alloy composition. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.