Effect of ultrasonic micro-forging treatment on microstructure and mechanical properties of GH3039 superalloy processed by directed energy deposition

Abstract In this work, ultrasonic micro-forging treatment (UMFT) was introduced to achieve homogeneous microstructure, reduce defects and improve mechanical properties of GH3039 superalloy cladding layer processed by directed energy deposition (DED). The microstructure, defects and mechanical properties of the cladding layers treated by UMFT with different ultrasonic powers (UIPs) were investigated. Results revealed a gradient structure as equiaxed grains distributed at the top, a columnar-to-equiaxed transition (CET) region that mixed of columnar dendrites and equiaxed grains distributed at the middle and columnar dendrites at the bottom of the cladding layer was formed. After UMFT, the proportion of equiaxed grains was increased, the average size of equiaxed grains was refined to 10 μm from 16 μm, the orientation of grains was more uniform and the phases enriched of Al, Ti, C, Nb and Mo were precipitated. The grain refinement can be attributed to the fracture of columnar dendrites induced by the ultrasonic vibration during solidification. Besides, the porosity of the cladding layer was reduced after UMFT. The microhardness of the cladding layers exhibited a depth-dependent gradient at the top region. The microhardness of the top surface was the highest and showed an increasing trend with the increase of UIP. The microhardness of different grain morphologies exhibited no substantial difference. However, due to grain refinement and precipitation of strengthening phase induced by UMFT, the microhadness of some local locations were improved. These results indicated UMFT has a significant effect on improving the microstructure, defects and mechanical properties of the deposited cladding layer.