Enhancement of mechanical properties in high silicon gravity cast AlSi9Mg alloy refined by Al3Ti3B master alloy

The microstructure and mechanical properties of high silicon Al9Si0.45Mg alloys without grain refinement and refined by Al5Ti1B and Al3Ti3B master alloys were investigated. The results showed that the primary α-Al grain sizes were 1288 ± 361 µm and 645 ± 146 µm in the non-refined and Al5Ti1B refined alloys respectively, and that was decreased significantly to 326 ± 116 µm in Al3Ti3B refined alloy. After T6 heat treatment, the yield strength, ultimate tensile strength and elongation of the non-refined alloy were 298 ± 2 MPa, 350 ± 2 MPa and 4.5 ± 0.7% separately. When the grain refiner was changed from Al5Ti1B to Al3Ti3B, the yield strength was increased from 304 ± 2 MPa to 311 ± 1 MPa, the ultimate tensile strength was increased from 357 ± 3 MPa to 366 ± 1 MPa, and the elongation was increased significantly by 38.8% from 8.0 ± 1.0% to 11.1 ± 1.2%. The Hall-Petch relation between the yield strength and grain size of the heat-treated alloys was determined as σy = 285.1 + 470.2·d−1/2. It was found that β'' phase was precipitated from the α-Al matrix for the peak strengthening of the heat-treated alloys. TiB2 and TiAl3 particles were found coexisting in Al5Ti1B master alloy, while TiB2 and AlB2 particles were verified coexisting in Al3Ti3B master alloy. The inoculation of AlB2 particles results in the significant grain refinement under Al3Ti3B. The increase of strength in the Al3Ti3B refined alloy is attributed to the refinement of the primary α-Al grains, while the increase of ductility in the Al3Ti3B refined alloy resulted from the reduced oxides and porosity defects as well as decreased grain size.