Effect of non-equilibrium heat treatments on microstructure and tensile properties of an Al-Si-Cu alloy

The influence of heat treatment on the morphology of ß-Al5FeSi inclusions in Fe-rich Al-Si-Cu was investigated and the tensile properties were improved. Samples were sand casted and subjected to various T6 heat treatments. Metallographic samples were examined by optical microscopy as well as scanning electron microscopy (SEM) and the average length and area fraction of the Fe-rich phases were quantified with image analysis software. It was found that a minimum solution temperature at 520-525 °C and solution time of 1 h is required for a significant fragmentation and dissolution of the detrimental long-needle-like ß-Al5FeSi inclusions. Solution time longer than 2 h leads to incipient melting of Cu-rich phases and coarsening of the eutectic Si. Tensile specimens were machined from the heat-treated samples and tensile tests performed. The highest ultimate tensile corresponds to the samples heat treated at 525 °C for 1.5 h. Fracture surface observations indicate a transgranular cleavage mechanism for the non-treated specimens, and massive ß-Al5FeSi platelets were identified as main crack initiation sites. After heat treatment, a mixed fracture mode and dissolved ß-Al5FeSi phases were observed.