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In a recent article, Liu et al. described the formation of phases during the diffusion bonding of Al to Mg at temperatures in the range 450 °C to 490 °C and pressures in the range 0.08 to 0.10 MPa. They present micrographs of the phases in the weld zone resulting from welding at 480 °C and 0.08 MPa for 60 minutes and a concentration-distance plot resulting from welding at 470 °C and 0.08 MPa for 60 minutes. However, the mechanism they propose for the formation of the phases in the weld zone is incorrect. It is the purpose of the present discussion to provide a correct analysis of the formation of the phases in the weld zone. PENG LIU, YAJIANG LI, HAORAN GENG, JUAN WANG, HAIJUN MA, and GUOLIN GUO: Metall. Mater. Trans. B, 2006, vol. 37B, pp. 649–54. Discussion submitted September 21, 2006.
J.R. CAHOON (Professor)Email:
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The effect of preweld overaging heat treatment on the microstructural response in the heat-affected zone (HAZ) of a precipitation-hardened nickel-base superalloy INCONEL 738LC subjected to the welding thermal cycle (i.e., rapid) was investigated. The overaging heat treatment resulted in the formation of an interfacial microconstituent containing M23X6 particles and coarsening of primary and secondary γ′ precipitates. The HAZ microstructures around welds in the overaged alloy were simulated using the Gleeble thermomechanical simulation system. Microstructural examination of simulated HAZs and those present in tungsten inert gas (TIG) welded specimens showed the occurrence of extensive grain boundary liquation involving liquation reaction of the interfacial microconstituents containing M23X6 particles and MC-type carbides. In addition, the coarsened γ′ precipitate particles present in the overaged alloy persisted well above their solvus temperature to temperatures where they constitutionally liquated and contributed to considerable liquation of grain boundaries, during continuous rapid heating. Intergranular HAZ microfissuring, with resolidified product formed mostly on one side of the microfissures, was observed in welded specimens. This suggested that the HAZ microfissuring generally occurred by decohesion across one of the solid-liquid interfaces during the grain boundary liquation stage of the weld thermal cycle. Correlation of simulated HAZ microstructures with hot ductility properties of the alloy revealed that the temperature at which the alloy exhibited zero ductility during heating was within the temperature range at which grain boundary liquation was observed. The on-cooling ductility of the alloy was significantly damaged by the on-heating liquation reaction, as reflected by the considerably low ductility recovery temperature (DRT). Important characteristics of the intergranular liquid that could influence HAZ microfissuring of the alloy in overaged condition are also discussed.
O.A. Ojo (Assistant Professor)Email:
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The kind and amount of alloying elements strongly affects the formation of ultrafine-grained microstructures. Aluminum alloys with different amounts of the alloying element magnesium, and a commercially pure aluminum alloy, have been investigated in order to evaluate how the obtained microstructures will affect the mechanical properties. X-ray profile analysis has been used to determine grain size and dislocation density. With increasing amounts of alloying elements, a smaller grain size and a higher dislocation density after severe plastic deformation (SPD) are obtained, which lead to higher hardness and improved fatigue properties. This article is based on a presentation made in the symposium entitled “Ultrafine-Grained Materials: from Basics to Application”, which occurred September 25-27, 2006 in Kloster Irsee, Germany.
J. MayEmail:
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