Dynamic laser welding hot cracking behavior and mechanism of new structural material Ni–28W–6Cr alloy for molten salt reactor

Abstract

The newly-developed Ni–28W–6Cr alloy is the structural material for the next-generation molten salt reactor (MSR). Due to the detrimental effects of welding hot cracks on the quality of welded structures, it is essential to study the hot cracking behavior and mechanism to achieve the qualified Ni–28W–6Cr alloy welded joint. This paper analyzes the characteristics of the dynamic cracking behavior of welding hot cracks using a high-speed camera. The microstructure, morphology, and cracking mechanism of hot cracks are studied. The results show that solidification cracks and liquation cracks occur in the welded joint. According to the propagation direction of the hot crack, there are longitudinal cracks and transverse cracks, respectively. The quantity and length of hot cracks increase with the laser power. The laser welded joint's hardness value fluctuates from 200 HV to 260 HV. The width of heat affected zone (HAZ) is approximately 0.5–1 mm, and there is a slight softening zone in the HAZ. Meanwhile, the hardness of the weld metal is the highest in the welded joint. The solidification cracks tend to initiate and crack along the solidification grain boundary, which is mainly related to the thermal stress caused by the welding process and the weak bonding strength of grain boundaries caused by the element segregation and precipitation at the grain boundary. Liquation cracking is mainly affected by welding thermal stress and element segregation at the grain boundary in the HAZ. However, the eutectic carbides play the role of inhibiting the propagation of liquation cracks.