Effect of trace yttrium addition on the microstructure and tensile properties of recycled Al-7Si-0.3Mg-1.0Fe casting alloys

In the present work, the effect of yttrium (Y) addition (0, 0.15, 0.3, 0.5 and 0.7 wt%) on the microstructure and tensile properties of recycled A356 cast alloys containing 1.0 wt% Fe has been studied systematically. With the addition of Y, despite no transformations of beta-Fe (Al5FeSi) phases into alpha-Fe phases, the acicular beta-Fe phases were refined remarkably, and the volume fraction of beta-Fe phases were decreased evidently. With addition of 0.3 wt% Y, the average length of beta-Fe phase decreased from similar to 78 mu m to similar to 20 mu m and the finest beta-Fe phases were obtained, in the meantime, the eutectic silicon particles were present in fully modified form, and the secondary dendrite arm spacing (SDAS) reached the lowest value. In addition, it was proposed that according to the microstructural analysis, the Al2Si2Y intermetallic phases might be responsible for the refinement of the beta-Fe phases. With addition of 0.3 wt% Y, the maximum improvement of quality index was achieved, approximately 32% in both as-cast and T6 heat treated alloys, and the corresponding Y-modified alloys in T6 heat treated condition obtained the best tensile properties compared with other experimental alloys, with the corresponding ultimate tensile strength (UTS) and elongation (EL) values being 383.86 MPa and 4.85%, respectively. Furthermore, the tensile properties of 0.3 wt% Y modified recycled AI-7Si-0.3Mg-1.0Fe alloys (T6) exceed the minimum properties standard for ZL101A alloy (JB, T6), approximately 30.12% for UTS value and 61.67% for EL value, suggesting it can be a good candidate for the commercial applications. (C) 2016 Elsevier B.V. All rights reserved.