Characterization and porosity analysis of arc-based directed energy deposition of AZ31 magnesium alloys using X-ray computed tomography

This study used directed energy deposition-arc (DED-Arc) with the cold metal transfer (CMT) process to produce a thin magnesium wall. The microstructure of DED-Arc AZ31 reveals equiaxed grains, with the grain sizes increasing from the bottom (20.53 μm) to the middle (42 μm) and top layers (36.18 μm) due to heat conduction variation. The mechanical properties of DED-Arc AZ31 are anisotropic and superior to those of squeeze-cast counterparts. X-ray computed tomography (XCT) shows a non-uniform porosity distribution at the start of the deposition due to the high heat sink and height variation. The DED-Arc process outperforms the high-pressure die-casting (HPDC) process in preventing shrinkage porosity. No shrinkage porosity is observed in the middle of deposition for DED-Arc, occurring only at the edge. K-means clustering identifies a sphericity threshold of 0.4 to distinguish shrinkage and gas porosity. Two classification methods of porosity, the bounding box (BB) and shape morphology, were evaluated. It is recommended to use the BB method in combination with the porosity diameter for porosity classification, ensuring a more comprehensive approach. The BB classification reveals a dominance of cube-like pores, while the formation of rod-like and blade-like pores is minimal. Defect orientation analysis shows that plate-like pores result in a higher stress concentration factor (SCF) of 6 along the build direction compared to 3 in the longitudinal direction. A novel method based on point clouds measures geometric parameters and achieves errors of 18.68% for the surface waviness and 13.7% for the effective wall area.