Influences of Al2. 5Cu0. 5Ti master alloy on microstructure and mechanical properties of Al- 7Si alloy

The influences of adding Al2.5Cu0.5Ti master alloy (0. 3%,0. 5%,0. 7%,0. 9% and 1. 3%, mass fraction,the same below) prepared by vacuum arc melting furnace with a water-cooled copper mold on the microstructure and mechanical properties of Al-7Si alloy were investigated by optical microscopy (OM),scanning electron microscopy(SEM),electron probe micro-analysis(EPMA),X-ray diffractometer (XRD),differential scanning calorimeter (DSC) and tensile testing machine. The results show that the primary a -Al and eutectic Si structure of Al-7Si alloy with Al2.5Cu0.5Ti master alloy addition are significantly refined. Compared with the unadded Al2.5Cu0.5Ti master alloy,when the addition concentration of Al2.5Cu0.5Ti master alloy is up to 0. 9%,the primary a- Al is refined from coarse dendrites to fine equiaxed dendrites and the size reduces from 218 mu m to 80 mu m,the second dendrite arm spacing (SDAS) significantly decreases from 25 mu m to 9. 3 mu m. The morphology of eutectic Si is transformed from coarse needle/flake to short flake and rod. The length of eutectic Si structure reduces from 34. 8 mu m to 11. 6 mu m, and the width decreases from 4 mu m to 1. 7 mu m. The tensile test indicates that when the addition content of Al2.5Cu0.5Ti master alloy is up to 0. 9%,the ultimate tensile strength(UTS)is increased by 13. 6% from 162 MPa to 184 MPa,and the elongation (El) is enhanced by 210% from 3% to 9. 3%. The fracture mode of the alloy changes from ductile-brittle mixed fracture to ductile fracture mode. However,when the addition content of Al2.5Cu0.5Ti alloy increases to 1. 3%, excessive modification occurs, resulting in coarsening of the microstructure and decrease in mechanical properties.