Analysis of Cracks in the Pulsed Nd:YAG Laser Welded Joint of Nickel-Based Superalloy

The weldability of GTD-111 nickel-based superalloy by pulsed Nd:YAG laser welding with an average power of 250 W was studied, and the microstructural evolution and cracking characteristics were also investigated. The solidification cracking of the fusion zone (FZ) and the intergranular liquation cracking in the heat affected zone (HAZ) were observed in the joint. Solidification cracking was caused by the residual liquid metal originated from the segregation of Ti, Nb and Al elements in the interdendritic region at the last stage of solidification. And the HAZ liquation cracking was associated with the constitutional liquation of 𝛾́, MC carbides, and the melting of Cr-rich boride. Ti was introduced as the most important factor in the formation of the liquation cracks in HAZ by reducing the start temperature of γ−γ́ eutectic reaction and increasing the 𝛾́ dissolution temperature. Chemical analysis of the crack edges at HAZ revealed the presence of high amounts of Ti and Al elements which can be attributed to 𝛾́ partial melting. Gleeble physical simulation revealed that in casting the sample, the liquation started at significantly lower temperatures than in the 1200℃ solution heat treated samples. This is attributed to the boride and intermetallic particles, which had dissolved by the 1200℃ heat treatment. The formation of fine grains due to the high cooling rate of the weld as well as the formation of dispersed carbides in the fusion and heat affected zones led to an increase in the microhardness by about 130 HV compared to the base metal.

Keywords:

GTD-111 superalloy , Nd:YAG laser , Solidification cracking , Liquation cracking , Grain boundary melting , Segregation coefficient , Gleeble simulation