The shear behaviour of partially solidified Al–Si–Cu alloys

A direct shear cell was used to measure the shear behaviour of partially solidified Al–Si–Cu alloys with compositions of AlSi4Cu1, AlSi7Cu1, AlSi4Cu4 and AlSi7Cu4. Determination of rigidity and shear strength with respect to solid fraction for each alloy revealed two transition points, identified in the literature as coherency and maximum packing solid fraction, where changes in shearing mechanism occurred. Coherency occurred between 0.18 and 0.21 solid fraction for the 7%Si alloys and between 0.24 and 0.27 for the 4%Si alloys. Maximum packing occurred between 0.33 and 0.35 solid fraction for the 7%Si alloys and between 0.44 and 0.47 for the 4%Si alloys. For a given solid fraction after coherency, the shear strength increased when the silicon content was increased. This was due to the effect of silicon where the 4%Si alloys have a globular, rosette-like microstructure, while the 7%Si alloys have a larger grain size and a more irregular dendritic morphology. After the maximum packing solid fraction, an increase in copper content resulted in increased strength for a given solid fraction. This is hypothesised to be caused by a correspondingly lower temperature for the same solid fraction with increasing copper content, leading to a more rigid dendritic network. The deformation mechanisms occurring during shear are also discussed.