Laser remelting driven synergy of ordered precipitation strengthening and sub-grain structure for enhanced strength-toughness in K447A superalloy

Abstract

This study investigated the laser surface remelting (LSR) of non-weldable K447A superalloy castings to repair damaged surfaces and enhance the mechanical properties of the remelted layers. The results indicated a significant improvement in hardness, room temperature, and 760 °C high-temperature tensile properties of the LSR layer. The LSR process reduced element segregation, allowing for increased solid solution of elements such as Al, Ti, and Ta. This led to an increase in the volume fraction of γ′ phase and the anti-phase boundary (APB) energy, resulting in a 37.7 % improvement in the yield strength of the LSR layer. During tensile deformation, the remelted structure more readily formed dislocation forest, causing local differences in strain hardening rates and inducing sub-grain formation. The sub-grain structure provided an additional mechanism for strain coordination, alleviating local stress concentration, thus improving toughness under 25 °C tensile conditions. At 760 °C, sub-grain boundaries underwent dynamic recrystallization driven by the combined effects of thermal activation and tensile stress, which reduced stress concentration by eliminating deformation energy, resulting in stress relaxation and thus enhancing the high-temperature tensile toughness of the LSR layer. During room temperature tensile, the interaction between dislocations and γ′ precipitates in the base material (BM) and LSR samples followed the APB strongly coupled dislocation shear mechanism. At 760 °C high-temperature tensile, the activation of more slip systems and the thermal activation of dislocations led to dislocation climb, cross-slip, Orowan bypass, and stacking fault shear mechanism, which dominated 760 °C high-temperature plastic deformation.