Grain gradient in the heat-affected zone acquires an advancement in the ductility of flash-butt welded IN718 alloy

The microstructure evolution and mechanical properties are investigated systematically on Flash-butt welded IN718 alloy joint after annealing at 980°C and subsequent aging. The yield strength (YS) is increased from 667MPa to 1175MPa and the ultimate tensile strength (UTS) is increased from unprocessed 980MPa to 1412MPa. A grain gradient distribution is achieved in the heat-affected zone (HAZ) with the grain size gradually increasing from the weld seam (WS) to the matrix after heat treatment, and the ductility of the welded sample is higher than that of the matrix without any loss of strength. The contribution mechanism of grain gradient to ductility is investigated by varying the fraction of the HAZ. The results demonstrate that the ductility of the welded sample higher than that of the matrix is because of the co-deformation between the HAZ and the matrix, which results in the effectively dynamic accumulation of geometrically necessary dislocations (GNDs) from gradient transition zones into the matrix. Therefore, premature stress concentration is prevented and additional ductility is obtained for welded samples. Increasing the fraction of HAZ effectively enhances local strain within coordinated deformation zones, leading to improved ductility.