EXPERIMENTAL OBSERVATIONS OF DENDRITE COARSENING & Al-SI EUTECTIC GROWTH IN PROGRESSIVELY QUENCHED STRUCTURES OF Al-SI-CU CASTING ALLOYS

Computer controlled thermal heating and cooling curve cycles of the W319 (Al-7.5Si-3.5Cu-0.3Fe) and the WA328 (Al-9Si-1Cu-0.3Fe) alloy samples were studied in this work. Both alloys contain residual levels of Sr (similar to 20 ppm max) and are processed with no grain refiner From the cooling curve portion of the thermal cycle the characteristic temperature points that represent phase nucleation and growth are established. Once characteristic temperatures in the alloy samples are known, strategic quench points can be chosen to reveal the evolving mushy state using metallographic examination. In particular the growth kinetics of primary alpha-Al dendrites in quenched and unquenched alloys are studied and discussed in this work. In addition, growth modes for the Al-Si eutectic lamellae and polyhedral Si particles were also observed from quenched test samples during and post the Al-Si nucleation temperature event. For each microstructure examined (quenched and unquenched test samples) the Secondary Dendrite Arm Spacing (lambda(2)), Dendrite Cell Size (DCSz), and Microhardness (25g/15s mu HV) of coarse dendrites were measured. For the test samples quenched at the beginning of the Al-Si eutectic nucleation event Image Analysis techniques were used to assess the area fraction of dendrites. This result, found for both alloys, was compared to fraction solid determined from conventional thermal analysis methods which have software capabilities for fraction solid calculation. The purpose for the analysis in this article stems from previous research on cast component testing using the WA328 and W319 alloys. The WA328 alloy has lower porosity but contains almost the same lambda(2) values as the W319 alloy. The lower porosity enhanced fatigue performance.