Dendrite coherency of Al-Si-Cu alloys

The dendrite coherency point of Al-Si-Cu alloys was determined by thermal a nalysis and rheological measurement methods by performing parallel measurem ents at two cooling rates for aluminum alloys across a wide range of silico n and copper contents. Contrary to previous findings, the two methods yield significantly different values for the fraction solid at the dendrite cohe rency point. This disparity is greatest for alloys of low solute concentrat ion. The results from this study also contradict previously reported tl end s in the effect of cooling rate on the dendritic coherency point. Considera tion of the results shows that thermal analysis is not a valid technique fo r the measurement of coherency. Analysis of the results from rheological te sting indicates that silicon concentration has a dominant effect on grain s ize and dendritic morphology, independent of cooling rate and copper conten t, and thus is the factor that determines the fraction solid at dendrite co herency for Al-Si-Cu alloys.