| 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 treatments. Microstructures of the base alloy, heat affected zone (HAZ) and fusion zone of welded and post-weld heat treated coupons were examined by the use of standard metallographic techniques involving optical microscopy, analytical scanning electron microscopy (SEM) and analytical transmission electron microscopy. Moreover, non-equilibrium segregation behavior of boron atoms to the grain boundaries of the alloy during pre-weld heat treatments was evaluated using secondary ion mass spectroscopic system.;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. Therefore, such a technique is not usually recommended in order to ensure integrity of weld structures. The use of low heat input welding, which will minimize the microstructural damage is desirable. Thus, it is necessary that another approach be employed to minimize or eliminate HAZ cracking of the alloy which occurred during welding using low heat input. The use of suitable pre-weld heat treatment to engineer pre-weld microstructure capable of reducing or eliminating cracking during welding has been widely reported for other nickel-base superalloys. Therefore, during the second phase of this research, the effects of pre-weld thermal processing on the cracking response of 718Plus were investigated. The alloy was thermally treated at different temperatures and cooling rates before welding. Results from the quantification of the cracking response of the alloy showed that HAZ cracking during welding 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 recommended by the manufacturer of 718Plus. This was meant to relieve welding stresses as well as homogenize the microstructure of the welds. However, a significant solid state cracking of the alloy occurred during the post-weld heat treatment. This appears to constitute a major difference between the weldability of gamma' strengthened 718Plus and that of its parent alloy, 718, which is strengthened by gamma' precipitate that has a sluggish precipitation kinetics. It is hypothesized that post-weld heat treatment cracking of 718Plus occurred due to the combined presence of aging contraction stresses, due to fast precipitation of gamma' in HAZ, at a period when the grain boundaries in the zone have been embrittled. SEM and TEM analyses revealed a significant precipitation of gamma' phase during post-weld heat treatment of the welds. Also, the grain boundaries that mostly cracked during post-weld heat treatment were outlined by Laves phase and extensive needle-like precipitates of delta phase. The Laves phase formed as a resolidification product on liquated grain boundaries in weld HAZ during welding, while the needle-like delta phase precipitated around the Laves phase during PWHT. It has been reported in previous studies on 718-type alloy that Laves phase is very brittle and has a weak interface with the gamma matrix; therefore, it may provide preferential sites for crack initiation and propagation. The extensive formation of delta needles, as observed in the present work, has been also reported to impair the strength of 718-type alloys. Thus, during post-weld heat treatment of 718Plus, the presence of the embrittling phases on HAZ grain boundaries, coupled with aging contraction stresses that are generated during aging, would result in cracking of the grain boundaries at a period when the matrix has been hardened by a considerable precipitation of gamma' phase.;In the first phase of the research, 718Plus was welded using a low and high heat input CO2 laser to assess the response of the alloy to welding. Detailed examination of the welds by analytical electron microscopic technique revealed the occurrence of cracking in the heat affected zone (HAZ) of low heat input welds, while their fusion zone (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. |
