The effect of grain refining on macrosegregation and dendrite arm spacing of direct chill cast AA5182
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6/10/03
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The Effect of Grain Refining on Macrosegregation and Dendrite Arm Spacing of Direct Chill Cast AA5182 A.M. GLENN, S.P. RUSSO, and P.J.K. PATERSON The effect of titanium and titanium diboride inoculation on the spatial variation of local solidification time for direct chill (DC) cast ingots of aluminum alloy 5182 (AA5182) was studied. The results have been compared to those of an ingot cast without grain refining. To accomplish this, the effect of grain refining on a number of ingot characteristics such as grain size, macrosegregation, spatial variation of dendrite arm spacing, and thermal conductivity was investigated. Furthermore, the effect of grain refining on the well-known relationship between dendrite arm spacing and local solidification time had to be established for AA5182. The results indicated that the spatial variation of dendrite arm spacing in the industrial ingots was independent of grain refining, although the nonrefined ingot produced significantly finer dendrite arm spacings in its center. This was attributed to the influence of showering crystals in the nonrefined ingot. The relationship between dendrite arm spacing and local solidification time was also found to be independent of grain refining.
I. INTRODUCTION
THE direct chill (DC) casting process has been the most important method for the production of aluminum rolling ingots for many years. Depending on the alloy being cast, grain refiners are frequently added to suppress the formation of columnar grains and promote smaller equiaxed growth. The resulting fine grain structure produces slightly higher strength.[1] Furthermore, grain refining is known to delay the onset of dendrite coherency,[2] improving the melt’s ability to accommodate the large thermal strains induced by solidification. This reduces the propensity for hot cracking.[3] Plant experience has shown, however, that certain products such as aluminum alloy 5182 (AA5182) are notoriously difficult to cast when inoculants are used, and higher ingot cracking rates can result. The reasons for this anomaly remain uncertain. Although grain refining is known to affect macrosegregation,[4] its influence on other important ingot characteristics such as spatial variation of thermal conductivity (resulting from macrosegregation), dendrite arm spacing, and local solidification time has not been published for the DC production of AA5182 rolling ingots. Therefore, this study has three objectives. First, to characterize the effect of different inoculants on important DC ingot characteristics such as grain size, macrosegregation, and the spatial variation of dendrite arm spacing in AA5182 rolling ingots; second, to assess the effect of grain refining on the well-known relationship between dendrite arm spacing and local solidification time; and finally, to determine the spatial variation of local solidification time with distance from the edge for a non-grain-refined (NGR) ingot, a titanium-grain-refined ingot and a titanium diboride (TiB2)– grain-refined ingot. A.M. GL
