Fundamental Advances for High Quality/Low Cost Feedstock Powders

Fundamental Advances for High Quality/Low Cost Feedstock Powders

Comparison of simulated liquid nickel breakup by argon atomization gas (a) and experimental liquid nickel gas atomization spray high speed video frame (b).

Conventional gas atomized powders, even those produced by aerospace-qualified powder makers, are far from ideal feedstock powders for either laser or e-beam melted (EBM) methods of additive manufacturing (AM), even after size classification to either 15-45µm or 45-106µm, respectively, with off-sizes going to waste or inventory. Also, EBM powders tend to have “satellite” projections and poor flowability for build layer spreading and high internal porosity (20-30%) that causes harmful porosity in AM builds. Atomization process modeling, verified by experiments, demonstrated efficiency and effectiveness at solving these problems.

Adaptation of compressible flow modeling to incorporate high density liquid metals permitted ground-breaking simulation of supersonic gas atomization of molten metal in full 2-D to analyze melt break-up into droplets that solidify into spherical powders for use in AM. The modeling uses realistic melt (Ni) feeding and atomization gas (Ar) flows and shows droplet break-up, melt filming/gas velocity effects, and clues to avoiding trapped internal porosity in resulting powders. 2-D break-up simulations compared well with high speed video imaging of comparable gas atomization experiments (see Figure), permitting selection of modest gas velocities for producing EBM powder feedstock with extremely low interior porosity (4%), compared to 20-30% for typical commercial powder. While further 3-D simulation of droplet breakup will permit powder size predictions, empirical data allowed a greatly enhanced (3X) powder yield (50-60%) to be achieved in experiments performed with Ames Laboratory’s gas atomizer, compared to commercial practice. Development of 3-D incompressible macro-flow modeling produced realistic atomization spray chamber recirculation simulations for the Ames Laboratory gas atomizer, which explain the observed satellite suppression effects and suggest other methods to maintain smooth spherical powder shapes with excellent flowability.

With sufficient industry adoption of the methods and controls for precision gas atomization from this project, Ames Laboratory Senior Metallurgist Iver Anderson says the cost for highest quality powder (500-1000/kg), from the plasma rotating electrode process (PREP) can be reduced by about 10X. Also, the availability of trial powder batches of developmental alloys should improve, since much less waste and excess inventory must be absorbed by the powder makers.