Effect of the addition of Sr modifier in different conditions on microstructure and mechanical properties of T6 treated Al-Mg2Si in-situ composite

The present work was carried out to investigate the effect of different sizes of Al4Sr phase in Al-10Sr alloy on the microstructure and mechanical properties of T6 treated Al-(10/15)Mg2Si in-situ composites. The finer Al4Sr phase could be obtained via rolling and hot-extruding the as-cast Al-10Sr master alloy, which thereby influenced its modification performance. The results showed that primary Mg2Si was transformed from imperfect octahedron to regular polygon morphology and eutectic Mg2Si was changed from Chinese script type to fibrous or fish-bone structure after the addition of Sr modifier in three conditions (as-cast, rolled and extruded). Moreover, the finer Al4Sr phase was beneficial to further refine the primary Mg2Si accompanied with a more uniform size distribution, and the spheroid transformation of eutectic Mg2Si was remarkably promoted during T6 heat treatment process. As a consequence, the superior ultimate tensile strength and elongation of T6 treated Al-10Mg2Si and Al-15Mg2Si were obtained (291 MPa and 4.45%, 289 MPa and 2.36%, respectively). The results of EDS analysis showed that Sr distributed at the interface of the primary Mg2Si, which lead to the morphology transformation of Mg2Si particles. Thus, this study offered a simple optimized method to enhance the modification efficiency of Sr on Al-Mg2Si composites.