Metallurgical modeling of intergranular HAZ liquation cracking during laser welding newly developed crystallography-dependent aerospace materials. Part II: Grain boundary alloying segregation of microstructure modification

Abstract

Metallurgical modeling of the nonequilibrium grain boundary segregation was coupled with a nonequilibrium weld pool solidification to numerically analyze the contributing factors to liquation cracking resistance. The effect of the welding conditions, material composition, and average grain size on the kinetics of the grain boundary segregation in the Inconel 718-type nickel-based superalloy during laser welding were optimized. The grain boundary segregation of boron near the fusion boundary is of particular interest, and is closely correlated to the intergranular heat-affected zone (HAZ) liquation cracking. It is indicated that the low heat input, fine-grained material and full penetration contribute to the reversible desegregation of boron and suppress the grain boundary segregation of boron to significantly minimize the segregation-induced grain boundary liquation and improve the weldablity. The subsolidus temperature due to nonequilibrium weld pool solidification is capable of ameliorating the segregation kinetics to reduce the HAZ grain boundary segregation. Intergranular HAZ liquation cracking is evitable with a crack-free weld without using filler wire. The grain boundary segregation decreases and the threshold tensile stress of solid-liquid interfacial decohesion across the intergranular liquid film simultaneously increases in the recrystallized HAZ during the weld shape change from partial penetration to full keyhole penetration. Furthermore, the numerical analysis results are verified indirectly by the experiment results of the total crack length. In addition, this metallurgical modeling is also applicable for the alloying segregation behavior prediction of other nickel-based superalloys with similar metallurgical properties.