Grain refinement of aluminum casting alloys

Website Only Article: Grain Refinement of Aluminum Casting Alloys

G. Sigworth
Alcoa Primary Metals, Rockdale, Texas

and T. Kuhn
Alcoa Primary Metals, Frederick, Maryland


The mechanisms and benefits of grain refinement are described. Downloadable from AFS site

Abstract

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