| This dissertation describes the results of a research program which was conducted to physically model, weld heat-affected zone (HAZ) liquation processes observed in Aluminum alloys during welding in the absence of an external strain. Three liquation processes were analyzed using a binary Al-3Cu alloy: (1) intergranular liquid infiltration from the fusion zone; (2) sub-solidus (below the equilibrium solidus temperature) grain boundary liquation due to non-equilibrium segregation during welding; and, (3) sub-solidus liquation (constitutional liquation) of undissolved precipitates in the matrix and along the grain boundary.; Silicon, from an Al-Si filler metal, was observed at the base metal, HAZ grain boundaries, of a solution treated Al-3Cu alloy after gas-tungsten arc welding. With no evidence for crack-backfilling, the mechanism of transport of fusion zone material into the matrix was tentatively identified as liquid metal penetration of grain boundaries governed by liquid diffusion kinetics.; Grain boundary regions in the heat-affected zone (HAZ) of arc welds, in a solution treated Al-3Cu alloy, were examined for liquation susceptibility. The welding experiments showed that certain grain boundaries, adjacent to the fusion line, were depleted in solute (copper), but were not liquated.; Investigation of the third mechanism, involved a comprehensive study of the solid-state, dissolution and liquation-dissolution of matrix and grain boundary precipitates. This represents the first complete study of its type, for any alloy system. The isothermal kinetics of solid-state and liquation-dissolution of &thetas; (Al2Cu) precipitates, were fully quantified during salt bath annealing.; The stability of the grain boundary precipitate dispersion was examined in order to determine the susceptibility of the grain boundary microstructure to liquation. These latter experiments were facilitated by a large, “static” grain structure, and this study may mark the first time that any grain boundary particle dispersion has been fully quantified as a function of time.; Previously derived analytical equations, for approximating the dissolution of metastable precipitates in the matrix, were utilized to: (1) analyze the present, and previous experimental isothermal dissolution kinetics, (2) reproduce isothermal dissolution data; (3) predict solid-state dissolution and liquation-dissolution, during continuous heating processes. (Abstract shortened by UMI.)... |
