Heat Affected Zone Cracking of Allvac 718Plus Superalloy during High Power Beam Welding and Post-weld Heat Treatment
Idowu, Oluwaseun Ayodeji
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The present dissertation reports the findings of a study of cracking behavior of a newly developed superalloy, Allvac 718Plus during high power beam welding and post-weld heat treatment. Microstructures of the base alloy, heat affected zone (HAZ) and fusion zone (FZ) of welded and post-weld heat treated (PWHT) coupons were examined by the use of standard metallographic techniques involving optical microscopy, analytical scanning electron microscopy (SEM) and analytical transmission electron microscopy. Moreover, grain boundary segregation behavior of boron atoms during pre-weld heat treatments was evaluated using secondary ion mass spectroscopic system. In the first phase of the research, 718Plus was welded using a low and high heat input CO2 laser to assess its weld cracking response. Detailed examination of the welds by analytical electron microscopic technique revealed the occurrence of cracking in the HAZ of low heat input welds, while their FZ was crack free. However, both the FZ and HAZ of high heat input welds were crack-free. Resolidified constituents were observed along the cracked grain boundaries of the lower heat input welds, which indicated that HAZ cracking in this newly developed superalloy was associated with grain boundary liquation. However, despite a more extensive liquation of grain boundaries and grain interior in the HAZ of high heat input welds, no cracking occurred. This was attributed to the combination of lower welding stresses generated during cooling, and relaxation of these stresses by thick intergranular liquid. Although HAZ cracking was prevented by welding with a high heat input laser, it resulted in a significant damage to the parent microstructure through its extensive liquation. Thus, the use of low heat input welding is desirable. However, this resulted in HAZ cracking which needs to be minimized or eliminated. Therefore, during the second phase of this research, the effects of pre-weld thermal processing on the cracking response of 718Plus were investigated. Results from the quantification of the cracking of the alloy showed that HAZ cracking may be significantly reduced or eliminated through an adequate selection of pre-weld thermal cycle. In the third stage of this research, crack-free welds of 718Plus were post-weld heat treated using standard thermal schedules. A significant solid state cracking of the alloy occurred during the PWHT. The cracking was attributed to the presence of embrittling phases on HAZ grain boundaries, coupled with aging contraction stresses that are generated by a considerable precipitation of gamma prime phase during aging.