Effects of Mg-Addition, Sr-Modification and Solidification Rate on the Tensile Properties and Quality Index of 359 Type Al-Si-Mg Casting Alloys

The current study investigates the influence of Mg-level, Sr-content and solidification rate on the tensile properties and quality index values of 359 type Al-Si-Mg casting alloys. The addition of up to 0.6% Mg to 359 alloys has a significant influence on their strength without affecting their quality, whereas Mg-levels above 0.6% result in a slight increase in the alloy strength with a noticeable reduction in the ductility and quality index value. This reduction is attributed to the formation of a large volume fraction of pi-Al8Mg3FeSi6 phase, the particles of which have a propensity for crack initiation, thus accelerating the fracture of the sample. Sr-modification affects the response of 359 alloys to solution heat treatment in that the solution time needed to attain specific tensile properties and quality index values is shortened compared to the case of the unmodified alloys. Reducing the solidification time enhances the tensile properties and quality index values of 359 alloys and is related to various microstructural features which accompany the high solidification rate, namely: refinement of the SDAS; modification of eutectic silicon particles; reduced size and levels of porosity; and reduced size and volume fractions of the intermetallic phases.