A Study On Grain Refinement And Hot Tearing In Permanent Mold Cast Aluminum Alloys

Francesco D'Elia, Ryerson University

Date of Award

2009

Degree Type

Thesis

Degree Name

Master of Applied Science (MASc)

Department

Mechanical Engineering

First Advisor

C. Ravindran

Abstract

There is an enhanced interest in aluminum casting alloys for automotive and aerospace applications. The interest in these alloys is spurred by the need to produce lighter and more fuel efficient vehicles. The aluminum-copper (Al-Cu) and aluminum-silicon-copper (Al-Si-Cu) alloys are among the most common aluminum casting alloys. Aluminum alloy B206 is a relatively new Al-Cu alloy with high strength and ductility at room and elevated temperatures, while A319 is an Al-Si-Cu alloy with good strength and excellent wear resistance. However, despite their advantages, when these alloys are cast via the permanent mold casting (PMC) process, they show a high susceptibility to hot tearing. Grain refinement has shown promise as a means to reducing hot tears in aluminum alloys, but the underlying mechanisms remain unclear.

This research investigated the influence of Ti-B grain refiner on hot tearing in B206 and A319. Varying levels of Ti were added to each alloy prior to casting in a custom designed permanent mold. Casting solidification, alloy grain size measurements and microstructure were characterized and related to the hot tearing susceptibility of each alloy.

The results suggest that Ti-B grain refiner had a pronounced effect on the grain morphology and hot tearing severity of B206. The alloy's dendritic morphology transformed to a more globular grain structure, which in turn reduced hot tearing severity. On the contrary, Ti and Si formed a compound in A319 which poisoned the grain refining efficiency of the Ti-B grain refiner. As a result, the grain morphology of A319 remained dendritic despite the addition of grain refiner, thereby resulting in no elimination in hot tearing severity.