GRAIN REFINEMENT OF ALUMINUM CASTING ALLOYS

An overview is given of grain refinement in aluminum casting alloys. The mechanisms involved and the benefits of refinement are described. The review shows that current practices were developed long before modern Al-Ti-B refiners became available, and are employed now largely for historical reasons. The results of tests in Al-Si, Al-Si-Cu, Al-Cu, Al-Mg and Al-Zn-Mg alloys are presented. The grain refining response is different for each alloy system. It is important to understand that titanium can be present in two forms. One dissolves in aluminum; the other is nearly insoluble. Each must be controlled separately. With today's powerful Al-Ti-B refiners, there is no reason for large additions of soluble titanium in most alloys. In fact, it is better to say we grain refine with boron, not titanium. The recommended addition is 10-20 ppm of boron, preferably in the form of Al-5Ti-1B or Al-3Ti-1B rod. Lower dissolved titanium levels provide better grain refinement and an improved resistance to hot cracking in some alloys. Al-Si casting alloys which contain copper are an exception. In alloys such as 319 or 355, it is best to have a minimum of about 0.1 % Ti.