Enhancement of multi-pass weld-crack resistance of Co-based superalloys by B-doping

Enhancing weld-crack resistance of Co-based suerpalloys is of both scientific and practical significance. In this work, the fusion zone weld-crack resistance of Co-9.2Al-9 W-xB (at.%, x = 0, 0.05, 0.20) superalloys during multi-pass fusion welding is investigated. It is found that weld cracks form in fusion zone without B-doping and with B-doping of 0.20%. However, no cracks form in the welded superalloy with 0.05% B alloys, suggesting the weld-crack resistance of Co-based superalloys can be enhanced by a small amount of B-doping. The mechanisms underlying the different weldability of the three Co-based superalloys are clarified. With B-doping of 0.05 at.%, the precipitation of μ-Co7W6 phase on grain boundaries can be fully suppressed, and the internal residual stress is largely reduced. The suppression of precipitated phases and low internal stress in the superalloy with 0.05% B-doping account for its much enhanced weld-crack resistance. However, with further B-doping of 0.20%, the M3B2 phase precipitates on the grain boundaries and the internal stress inside grains also increases, which account for the further deterioration of weldabilty. These findings are believed to promote the development of Co-based superalloys with high weld-crack resistance.