MICROSTRUCTURAL ASPECTS OF THE DISSOLUTION AND MELTING OF AL2CU PHASEIN AL-SI ALLOYS DURING SOLUTION HEAT-TREATMENT

The dissolution and melting of Al2Cu phase in solution heat-treated sa mples of unmodified Al-Si 319.2 alloy solidified at similar to 10 degr ees C/s were studied using optical microscopy, image analysis, electro n probe microanalysis (EPMA), and differential scanning calorimetry (D SC). The solution heat treatment was carried out in the temperature ra nge 480 degrees C to 545 degrees C for solution times of up to 24 hour s. 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 similar to 10 degrees C/s) and was observed either as separate eutecti c pockets or precipitated on preexisting Si particles, beta-iron phase needles, or the blocky Al2Cu phase. Dissolution of the (Al + Al2Cu) e utectic takes place at temperatures close to 480 degrees C through fra gmentation of the phase and its dissolution into the surrounding Al ma trix. The dissolution is seen to accelerate with increasing solution t emperature (505 degrees C to 515 degrees C). The ultimate tensile stre ngth (UTS) and fracture elongation (EL) show a linear increase when pl otted against the amount of dissolved copper in the matrix, whereas th e yield strength (YS) is not affected by the dissolution of the Al2Cu phase. Melting of the copper phase is observed at 540 degrees C soluti on temperature; the molten copper-phase particles transform to a shiny , structureless phase upon quenching. Coarsening of the copper eutecti c can occur prior to melting and give rise to massive eutectic regions of (Al + Al2Cu). Unlike the eutectic, fragments of the blocky Al2Cu p hase are still observed in the matrix, even after 24 hours at 540 degr ees C.