Si poisoning and promotion on the microstructure and mechanical properties of Al–Si–Mg cast alloys

Hypoeutectic Al–xSi–0.45Mg (x = 6.5, 7.5, 8.5, 9.5) alloys were refined by Al5Ti1B containing TiB2 and Al3Ti3B containing TiB2 and AlB2, respectively. With increasing Si, Si poisoning on TiB2 results in the obvious coarsening of primary α-Al in Al5Ti1B-refined alloys from 350 ± 40 to 400 ± 50, 475 ± 50 and 560 ± 80 μm, and the competition between Si promotion on AlB2 and Si poisoning on TiB2 leads to the slight coarsening of primary α-Al in Al3Ti3B-refined alloys from 215 ± 30 to 265 ± 35, 265 ± 30 and 315 ± 25 μm. After T6 heat treatment, with increasing Si, the yield strength (YS) of Al5Ti1B-refined alloys increases from 294 ± 2 to 299 ± 2, 304 ± 1 and 309 ± 2 MPa, and the elongation first increases from 3.5 ± 0.8 to 4.5 ± 1.0 and 7.8 ± 1.4%, after decreases to 5.5 ± 1.2%, while the YS of the Al3Ti3B-refined alloys increases from 300 ± 1 to 305 ± 2, 312 ± 1 and 317 ± 2 MPa, and the elongation increases from 6.1 ± 1.1 to 8.5 ± 1.2, 11.8 ± 1.5 and 12.1 ± 1.6%. The increase in the secondary phase and precipitation strengthening results in the increase in strength with increasing Si. With increasing Si, the decrease in porosity formation by decreasing solidification interval and increasing fluidity is superior to the increase in porosity formation by slightly coarsening grain size, which leads to the increase in ductility in the Al3Ti3B-refined alloys, while the competition between porosity decreasing and increasing factors leads to the inverted ‘V’-shaped evolution of ductility in the Al5Ti1B-refined alloys.