In this webinar we introduce Multiscale Assessment on the Quality of Metal Powder Feedstocks for Additive Manufacturing.
Additive manufacturing (AM) is attractive for producing parts with access to unprecedented geometry/configuration complexity and material composition gradient control, which are not attainable by traditional processes. However, the long-term success of this rapidly developing technology hinges, to a large degree, on the ability to make metal AM components reliably. Reducing defects is of primary importance to attain AM metal parts with mechanical strength and fatigue life approaching forged parts.
Defects in AM parts fall into two main categories: porosity and crack. These can be incurred by various mechanisms: lack of fusion, keyhole collapse, gas porosity, balling, solidification cracking, solid-state cracking, and surface-connected porosity. Defects can also result from entrapment of impurities. Defects arise from the interplay between feedstock and AM beam energy, which involves complex, transient thermophysical and chemical processes. As such, defects can be equipment-, process-, and feedstock-related. Consequently, defect reduction starts with the quality control of feedstocks.
Two mainstream AM technologies, i.e. laser powder bed fusion and blown powder direct energy deposition, use spherical powder feedstocks with the desired compositions and particle sizes, typically in the range of 10 to 50 microns. In this kind of matter in particulate form, the population of surface and sub-surface atoms represent four to five orders larger ratio as compared to bulk materials. While bulk chemistry still lays the foundation for ultimate mechanical properties, the surface chemistry of micro sized powders is becoming equally important, if not greater, for the quality of finished AM parts. Driven by the increased surface free energy (i.e., thermodynamic favorable) and the decrease in diffusion length (i.e., kinetic favorable), the particle-to-particle uniformity of AM metal powder is inherently susceptible to local events such as surface contamination, agglomeration, and composition variation, during powder manufacturing, packaging, storage, use and reuse.
As defects arise in multiple length scales, it calls for analytical solutions with multi-length scale sampling capability. Numerous institutes have engaged in standardizing testing methods for AM powder feedstocks. This webinar is not intended to repeat these efforts; rather, we focus on selecting a few techniques, each with certain length-scale sampling capability, for identifying and quantitatively assessing the quality of AM metal powder feedstocks, e.g. Ti6Al4V AM powder. Combination of these techniques (Scheme 1) enable multiscale sampling and uniformity assessment of AM metal powder feedstock: