DEVELOPMENT OF STRENGTH IN SOLIDIFYING ALUMINUM-ALLOYS

Quantitative measurements of the strength development in the mushy-zon e during solidification have been obtained in two commercial Al-Si all oys, AlSi7Mg and AlSi11Mg, an AlCu4 alloy and an AlMg5 alloy with a va ne method. The results show that the material does not have any streng th prior to dendrite coherency, confirming that the dendrites are free -floating and independent. Grain refining the AlSi7/Mg and AlCu4 alloy postpones strength development, and strength increases slowly in the grain refined AlCu4 alloy. Adding Sr to the AlSi7Mg alloy postpones de ndrite coherency and strength develops more slowly. Fading is observed to change the strength back towards that of the unmodified alloy with time. Adding 1% Si to the AlMg5 alloy postpones dendrite coherency, b ut the strength increases rapidly and makes the strength of the AlMg5S il alloy converge towards that of the AlMg5 alloy al increasing solid fractions. Strength develops very slowly in the AlSi11Mg alloy. The re sults are shown to be related to the size and morphology of the growin g dendrites. Copyright (C) 1997 Acta Metallurgica Inc.