Papers by Keyword: Equiaxed Zone

Abstract: The liquid film re-growth behaviour resulting from pulsed laser surface melting (LSM) has been investigated for typical 2xxx, and 7xxx aerospace alloys, both on parent plate and friction stir welded (FSW) joints. In Zr free alloys, as a result of the high growth rate and steep thermal gradient, the melted layer re-grew with a stable front, epitaxially, from the parent subsurface grains. This caused a thin coarse grained solidified layer to form over the parent material, thermomechanically affected zone (TMAZ) and heat affected zone (HAZ), and fine columnar grains to develop over the FSW nugget zone of the same order in width as the nugget grain size. In the case of the Zr containing alloys, a very fine columnar grain structure was found over the entire surface, independent of the subsurface grain structure. This has been shown to occur by growth selection from a band of nanoscale Al grains epitaxially nucleated on Al3Zr dispersoids, at the melt solid interface, that had not fully dissolved in the melt.

Abstract: The as-cast properties of components with a columnar grain structure are very different from those with an equiaxed one. Under certain solidification conditions, zones of both structures can occur in an alloy casting; the boundary between the zones is the columnar-to-equiaxed transition (CET). A front-tracking model of dendritic solidification has been developed, which can predict the nucleation and growth of solid in undercooled liquid during a casting process. The growth process is described by dendrite tip kinetics, and is fully coupled to a fixed-grid control volume model of heat transfer during solidification. Using the front-tracking model, two methods for predicting the likelihood of an equiaxed zone forming ahead of a columnar front have been formulated, namely, an indirect method and a direct method. The indirect method is based on modelling the growth of the columnar front in the absence of equiaxed nucleation. The bulk liquid undercooling is monitored and an equiaxed indicator is calculated at each time step based on the extent of such undercooling at that time. The equiaxed indicator is a measure of the relative likelihood of an equiaxed zone forming. In the direct method nucleation and growth of individual equiaxed grains is treated ahead of the advancing columnar front. In this case, if impingement of neighbouring fronts is treated, the simulation to complete solidification will yield the macrostructure and the CET. In this paper, details of both methods of equiaxed prediction are presented. Results from the indirect method are compared to experimental results found in literature and agreement is found.