Effect of Grain Refiner on Intermetallic Phase Formation in Directional Solidification of 6xxx Series Wrought Al Alloys

摘要

The effect of a grain refiner on the formation of intermetallic phases in a directionally solidified (Bridgman grown) model 6xxx series wrought Al alloy has been investigated using X-ray diffractometry (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). A base alloy with and without Al-Ti-B grain refiner was directionally solidified in a Bridgman furnace at growth velocities in the range of 5-120 mm/min. In both cases, the Fe-containing intermetallic phases present were found to be mainly alpha-AlFeSi and beta-AlFeSi. However, in the alloy with grain refiner solidified at 5mm/min, Al_(13)Fe_4 was also observed. Quantitative XRD results indicated that the addition of Al-Ti-B grain refiner has a strong influence on the relative quantities of intermetallic phases forming during solidification at different growth velocities, which was also confirmed by TEM observations. TEM observations also show that depending on where the beta-AlFeSi particles solidified e.g. grain boundaries or triple grain junctions, the size and morphology of the particles may change dramatically. TiB_2 particles were observed to nucleate beta-AlFeSi at low and high growth velocities in the 6xxx series Al alloys.