Microstructure Analysis of Quenched Semi-Solid A356 Aluminum Alloy Slurry by Using Weck’s Reagent

Abstract

The semi-solid slurry of A356 alloy of various solid fractions was fabricated and water quenched to freeze the morphology of the Al grains in the semi-solid state. Two kinds of specimens with different volume (large and small) were prepared to change the cooling rate during the quenching. The difference in the amount of solid growth around the original spherical Al grains during the quenching was investigated by the color etching method with Weck’s reagent, which is sensitive to the solute segregation. It was found that the amount of solid growth during the quenching increased with a decreasing cooling rate. The amount of solid growth of each spherical Al grain also increased for a decreasing local solid fraction, or the amount of residual liquid and the mutual distance between the solid grains. The solid fraction obtained by the color etching method showed a good agreement with the estimated solid fraction from the phase diagram. It was confirmed that the present color metallography was effective to evaluate the correct solid fraction at the semi-solid state.

Fig. 5 Microstructures of the large specimen hold at and quenched from 588°C (a) etched by NaOH, A: the area of solid, (b) etched by Weck’s reagent, (c) the original solid phase eliminating solid growth during water quenching, A consists of A₀ (original solid area) and A_q (solid growth during water quenching).