The effect of reheating to the semisolid state (soaking treatment) on the microstructure
evolution of the A356 aluminum alloy prepared by ultrasonic melt treatment was studied in this
paper. The results showed that in general the longer the soaking process the larger and the more
round the grains obtained. Higher roundness occurs at shorter soaking times in the fine-grained ascast
samples, and at longer times in the inhomogeneous or the coarser-grained as-cast structures.
The optimum thixotropic condition (high roundness, 0.72, and small globule sizes < 90 μm) are
achieved after 5 min. soaking in the samples treated by UST at 623 and 620oC, which is the typical
soaking time dictated by the industrial practice in SSM. The amount of entrapped eutectic as
observed after soaking treatments is uniquely very small, suggesting that the UST-treated ingots will
have better formability in the semisolid state. The growth rate constants are substantially low: in the
order of 479-748 μm3/s. These growth rate constants are much lower than those reported for MHDcast
A356 ingots. The growth rates of the samples produced by UST in the liquid state (i.e., 626,
623 and 620oC. Note that liquidus temperature is 619oC) are lower than those of the samples treated
in the semi-solid temperatures, i.e., 617 and 614oC. The Ostwald ripening is most likely the
dominant growth mechanism in the UST-treated samples during the soaking treatments. These
results reveal the feasibility and competence of UST as a potential route for thixotropic feedstock
production.
Improving mechanical properties of AlSi10Mg aluminum alloy using ultrasonic melt treatment combined with T6 heat treatment
