Microstructural characterization of primary and recycled aluminum AlSi7Mg alloy processed by the semi-solid thixocasting method

In recent years, the increasing need to use recycled aluminum alloys has driven the exploration of advanced processes. Among these, thixocasting is attractive due to its potential to produce high-performance components. Alloy composition and manufacturing routes influence semisolid Al-Si-Mg alloys' microstructure and mechanical properties. In order to exploit the potentiality of aluminum alloys that comes from metal scraps recycling, it is helpful to evaluate the influence of iron on the microstructure of recycled alloys by measuring the Rheocast Quality Index (RQI). The study focused on the production and testing of thixo-cast samples in two recycled AlSi7Mg aluminum alloys, with high (≈ 0.40 wt.% Fe) and low (≈ 0.15 wt.% Fe) iron content both featuring a high recycling rate (exceeding 70 % of the total alloy), and in a primary (i.e., from ore) AlSi7Mg alloy (≈ 0.10 wt.% Fe) serving as a benchmark. The microstructural characterization, carried out by optical and scanning electron microscope equipped with both electron backscatter diffraction and energy dispersive X-ray spectroscopy, focused on grain size, α-Al globules size and shape, and intermetallic amount. The high Fe content recycled alloy exhibited a higher area fraction of iron intermetallic compounds, coarser grains, and larger and not well spheroidized α-Al globules than the primary one with an RQI of about 0.30. Instead, the low-iron recycled and the primary alloy showed similar microstructures with a comparable RQI of about 0.40–0.45.