| This treatise describes the results of research conducted to determine the microstructural evolution governing the heat affected zone of Al-Cu and Al-Cu-Li alloys. It is based on detailed analyses of the heat affected zone of variable polarity plasma arc welds produced with alloys 2219-T87 and 2195-T8 and included thermal analyses, optical microscopy, neutron diffraction studies, transmission electron microscopy in conjunction with stereological analyses, and microstructural simulations.; Microstructural characterization indicated that positions within the heat affected zone that encounter low peak temperatures are governed by coarsening of strengthening precipitates through a reduction in surface energy. This involved coarsening of the metastable {dollar}rmthetaspprime (Alsb2Cu){dollar} for the binary alloy and {dollar}rm Tsb1 (Alsb2CuLi){dollar} for the Al-Cu-Li alloy. The microstructures within this region exhibit minor changes in the size distributions of these precipitates. In the vicinity of the fusion zone boundary, which is accompanied by relatively high peak temperatures, the chemical driving force results in complete dissolution of precipitates and nucleation and growth upon cooling. Concurrent nucleation and growth of multiple phases within the matrix and at high angle grain boundaries was observed. Grain boundary precipitation may also be accompanied by the formation of precipitate free zones.; The region that is bounded by these two extremes, and which typically represents the minimum strength within the heat affected zone, is dictated by a sequence of dissolution and growth. The complete dissolution of smaller precipitates during heating and growth of particle remnants during cooling has been referred to as anisothermal growth. This region has been shown to exhibit mean precipitate sizes within the upper range of the original microstructure but having significantly lower number densities. In the case of the binary alloy, anisothermal growth was found to occur at peak temperatures approaching the extended, metastable, solvus of the {dollar}thetaspprime{dollar} phase, i.e., approximately {dollar}rm 415spcirc C.{dollar}; A cursory analysis involving the simulation of microstructural evolution of an array of particles under nonisothermal conditions using a kinetic field model corroborated, to a large extent, the results of microstructural characterization. The simulations were found to be sensitive to the anisotropic gradient energy and appeared to closely resemble the dissolution process during heating. |
