Metallurgy Interview Question:

Will aluminum alloy like 380 or 356 settle? Meaning while in "Liquid form" above melting point will the heavier elements like copper, Fe, Silicon, zinc etc. settle to the bottom of the liquid/molten aluminum after a period.

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Yes settling, due to gravity, of heavier elements takes place in the molten state in the absence of any convection. As you know, severity of settling depends on the density difference among alloying elements. For instance, in the case of A356 alloy, the main elements Al and Si have rather similar densities and settling of Al is too sluggish. Nevertheless settling of heavy elements like Cu may lead to their accumulation in the bottom of container after a rather short time, provided there is NO convection. In practice, settling in the molten state is not too likely and will not be problematic.

In contrast, during soaking in the semi-solid state, settling would be considerable since presence of a network of primary dendrites interferes with convection in the liquid phase. My own experiments have evidenced considerable settling of Zn in the case of ZA27 alloy (Zn-28.5wt%Al-2.5wt%Cu) after a soaking time of 55 minutes in the semi-solid state (452C). Normally for this alloy primary dendrites are Al-base (refer to Al-Zn binary phase diagram) but due to Zn settling, primary phase in the bottom of container had changed to a Zn-base phase which will only form when zinc content is more than 98wt% or so. Primary dendrites at the upper part of container were Al-base
Submitted by: Administrator