The Efficiency and Fading Mechanism in Using Master Alloy of Al—Ti—B in Aluminum—Silicon Alloy Casting Grade 356 for Grain Refinement

Abstract

Grain refinement plays an important role in both aluminum and aluminum alloy casting in improving mechanical properties and decreasing some casting defects. Master alloy containing Aluminum—Titanium—Boron (Al—Ti—B) is the preferred choice for grain refiners. This research is to evaluate the efficiency of Al—Ti—B grain refiner in Aluminum—Silicon casting Grade 356 in terms of the amount needed and holding time. It was found that using greater amount of Al-Ti-B grain refiner could decrease the grain size to overcome the poisonous of silicon. The appropriate holding time to gain the highest efficiency, e.g. the critical time, in this study falls between 20 to 60 minutes. If holding time is too short, the efficiency of grain refiner may not reach the optimum point. In contrast, if the holding time is too long, the grain size becomes larger again. One of major fading mechanisms, particles settling, was demonstrated by holding the melt after adding master alloy for a long period of time before solidifying and cooling down to room temperature. It was found that the settlement of both TiAl₃ and TiB₂ particles should be the major reason for this fading mechanism. However, melt agitation can bring the particles to the melt and later acts as heterogeneous nucleation sites to some limited extents.

Keywords : Grain Refinement I Aluminum—Silicon Alloy Casting I Master Alloy /

Fading