Grain refining aluminium foundry alloys

Grain refinement of aluminium alloys avoids hot tearing and porosity, allows a marked increase in casting speed and imparts to a casting high toughness, high yield strength, excellent formability, good surface finish and improved machinability. Master alloys are added to molten aluminum in order to achieve fine, equiaxed grains after solidification, which otherwise tend to be coarse and columnar. At present, there is a variety of grain refiners from the Al-Ti-B ternary system, AlTi5B1 being the most popular. Al-Ti-B grain refiners, which are indispensable for wrought alloys, fail to perform as well with aluminium foundry alloys. The latter contain substantial levels of Si to improve castability. Si, however, reacts with Ti to form Ti-Si binary phases and impairs the grain refinement efficiency. The grains of the Ti-free aluminium foundry alloys are very small and nearly globular for the entire range of holding times when inoculated with B additions, implying not only a remarkable grain refining capacity but also a strong resistance to fading of the grain refinement effect. Aluminium castings can enjoy grain sizes well below 200 m, with an addition of 0.02 wt% B, provided that their Ti content is controlled below 50 ppm.

Keywords:

Casting , Aluminium Foundry Alloys , Grain Refinement