Improvement and Development of Powder Spreadability Testing

Sammanfattning: With rising interest in metal additive manufacturing and, specifically, in powder bed fusion processes, it is essential to understand relevant process parameters and the behavior of powders used in such processes. At the time of writing, the flow behavior of powders is in the spotlight whereas little research into the spreadability of a powder is being conducted. While the two characteristics are related, powders that are being spread are subject to different force loads which are not present during the simple flow of a powder. This work develops a testing system capable of qualitatively and quantitatively assessing spreadability in metal powders. Seven gas atomized powders of varying size distributions and four different chemistries supplied by Uddeholms AB were used to examine the efficacy and accuracy of the system. Image analysis of the spread layers was found to be effective in measuring the areas of the powder layers. It was also possible to assess the quality of powder coverage in the layer in terms of defects, which were sensitive to process parameters such as layer thickness and rake speed. From measurements of the mass of powder in each layer, a layer density was calculated and shows greater sensitivity than powder coverage to changes in layer thickness parameters. The spreadability data collected were compared to relevant existing flowability metrics, including some derived from powder rheometry. Two metrics were created to assess how well the rheometry data can predict spreading behavior. Firstly, the change in area coverage as a function of rake speed correlated to an increase in basic flowability energy, both of which became less sensitive to rake speed at higher speeds. Finally, an equation was formulated to assess the gap that forms between the true height of a spread layer and the nominal layer thickness. This gap showed great sensitivity to the cohesion values attained from shear cell tests: highly cohesive powders produced larger spread layer gaps. This work is expected to contribute to moving toward a standardized method to attain a powder characteristic for spreadability.With rising interest in metal additive manufacturing and, specifically, in powder bed fusion processes, it is essential to understand relevant process parameters and the behavior of powders used in such processes. At the time of writing, the flow behavior of powders is in the spotlight whereas little research into the spreadability of a powder is being conducted. While the two characteristics are related, powders that are being spread are subject to different force loads which are not present during the simple flow of a powder. This work develops a testing system capable of qualitatively and quantitatively assessing spreadability in metal powders. Seven gas atomized powders of varying size distributions and four different chemistries supplied by Uddeholms AB were used to examine the efficacy and accuracy of the system. Image analysis of the spread layers was found to be effective in measuring the areas of the powder layers. It was also possible to assess the quality of powder coverage in the layer in terms of defects, which were sensitive to process parameters such as layer thickness and rake speed. From measurements of the mass of powder in each layer, a layer density was calculated and shows greater sensitivity than powder coverage to changes in layer thickness parameters. The spreadability data collected were compared to relevant existing flowability metrics, including some derived from powder rheometry. Two metrics were created to assess how well the rheometry data can predict spreading behavior. Firstly, the change in area coverage as a function of rake speed correlated to an increase in basic flowability energy, both of which became less sensitive to rake speed at higher speeds. Finally, an equation was formulated to assess the gap that forms between the true height of a spread layer and the nominal layer thickness. This gap showed great sensitivity to the cohesion values attained from shear cell tests: highly cohesive powders produced larger spread layer gaps. This work is expected to contribute to moving toward a standardized method to attain a powder characteristic for spreadability.