Influence of solidification history and particle size on defect evolution in Al2024 alloy additive manufacturing powder

Kurzfassung

This study investigates the impact of solidification history on defect evolution in Al2024 alloy additive manufacturing powder. A measurement utilizing synchrotron powder X-ray diffraction (XRD) was performed with a photon energy of 105.7 keV (0.1173 A). XRD measurements were completed for six distinct particle diameter ranges from 20 to 63 µm of both as-atomized and annealed Al2024 samples. The dislocation density was determined using X-ray line profile analysis and ranged between 3.1x10^14 and 7.4x10^14 m^-2 for as-atomized samples, and between 1.3x10^14 and 4.0x10^14 m^-2 for annealed samples. The results of this investigation suggest a trend of decreasing dislocation density in as-atomized samples as the particle size increases, demonstrating the influence of particle size on defect formation. The annealed samples show a weak size dependence on the dislocation density across all particle sizes, affirming the efficacy of the annealing process in healing microstructural defects. The reduction in dislocation density upon annealing confirms that the dislocations in the as-atomized samples were introduced during rapid solidification and are largely removed by heat treatment. Field-emission scanning electron microscopy and electron backscatter diffraction (EBSD) were used in conjunction with image analysis tools to quantify the grain interface areas of the as-atomized samples and estimate cooling rates during solidification. Cooling rates were found to vary with particle size within a range of roughly 22000 to 164000 K s^-1. Results from both XRD analysis and metallography were used to predict average misorientation angles between grains; the predicted angles range from roughly 0.68deg to 0.83deg. EBSD was utilized to quantify average misorientation angles and compare them with the predicted values. The EBSD measured angles ranged from roughly 0.74deg to 0.95deg and followed the same trend as the predictions, linking the dislocation density to the cooling rate and solidification history of the powder. Quantifying the dislocation density and average misorientation angle provides a deeper understanding of the microstructure produced during rapid solidification. These results establish a physical relationship between particle size, cooling rate, dislocation density and grain misorientation during rapid solidification of Al2024 powder. This relationship is relevant to the study of solidification and to applications where gas-atomized powder is utilized without necessarily being fully remelted.