Microstructural aspects of the dissolution and melting of Al₂Cu phase in Al-Si alloys during solution heat treatment

Abstract

The dissolution and melting of Al₂Cu phase in solution heat-treated samples of unmodified Al-Si 319.2 alloy solidified at ≈10 °C were studied using optical microscopy, image analysis, electron probe microanalysis (EPMA), and differential scanning calorimetry (DSC). The solution heat treat-ment was carried out in the temperature range 480 °C to 545 °C for solution times of up to 24 hours. Of the two forms of Al₂Cu found to exist,i.e., blocky and eutectic-like, the latter type is more pronounced in the unmodified alloy (at ≈10 °C) and was observed either as separate eutectic pockets or precipitated on preexisting Si particles, β-iron phase needles, or the blocky Al₂Cu phase. Dissolution of the (Al + Al₂Cu) eutectic takes place at temperatures close to 480 °C through frag-mentation of the phase and its dissolution into the surrounding Al matrix. The dissolution is seen to accelerate with increasing solution temperature (505 °C to 515 °C). The ultimate tensile strength (UTS) and fracture elongation (EL) show a linear increase when plotted against the amount of dissolved copper in the matrix, whereas the yield strength (YS) is not affected by the dissolution of the Al₂Cu phase. Melting of the copper phase is observed at 540 °C solution temperature; the molten copper-phase particles transform to a shiny, structureless phase upon quenching. Coarsening of the copper eutectic can occur prior to melting and give rise to massive eutectic regions of (Al + Al₂Cu). Unlike the eutectic, fragments of the blocky Al₂Cu phase are still observed in the matrix, even after 24 hours at 540 °C.