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Precipitation strengthened nickel-base superalloys, such as IN 738, are very difficult to weld by fusion welding techniques due to their high susceptibility to heat-affected zone (HAZ) intergranular liquation cracking. An improvement in weldability could be realized by the deployment of innovative welding processes and/or the modification of the materials’ microstructural characteristics. Laser-arc hybrid welding is a relatively new welding process that appears to possess great potentials for joining the difficult-to-weld nickel-base superalloys. The research described in this Ph.D. dissertation was initiated to perform a systematic and comprehensive study of the cracking susceptibility of nickel-base IN 738 superalloy welds made by laser-arc hybrid welding process, and how to minimize it by using a combination of pre-weld microstructural modification and the application of various welding filler alloys. Laser-arc hybrid welding produced a desirable weld geometry in IN 738 Superalloy. Cracking did not occur exclusively in the fusion zone. Analysis of the fusion zone material using EPMA, SEM, TEM and EBSD revealed elemental partitioning pattern, the presence of secondary solidification reaction constituents and the grain structure of the fusion zone. Non-equilibrium liquation of various second phases that were present in the alloy prior to welding contributed to intergranular liquation in the HAZ that consequently resulted in extensive HAZ intergranular cracking. A very significant reduction in HAZ intergranular liquation cracking was achieved by the use of an industrially deployable and effective pre-weld thermal processing procedure developed during this research work. This novel procedure, designated as FUMT, was developed on the basis of the control of both boride formation and intergranular boron segregation in the pre-weld material. Propensity for HAZ intergranular liquation cracking in the weldments was also observed to vary depending on the Al+Ti+Nb+Ta concentration of the weld metal produced by different filler alloys, which can be attributed to variation in the extent of precipitation hardening in the weld metals. The newly developed FUMT treatment procedure, coupled with the selection of an appropriate type of filler alloy, is effective in reducing HAZ intergranular cracking both during laser-arc hybrid welding and during post-weld heat treatment (PWHT) of the laser-arc hybrid welded IN 738 superalloy. |
