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Effect of chill condition and grain refinement on the microstructure and mechanical properties of B319 aluminum alloy

University of British Columbia

The demand for lightweight vehicles continues to stimulate extensive research into the development of lightweight aluminum alloys for automotive castings and associated optimization of the processing parameters. However, the mechanical properties (hardness, tensile strength and thermal fatigue resistance) of many alloys still remain low for many applications in the automotive engine market. In order to improve the mechanical properties, a fine grain size of the as-cast alloy is preferred, which can be most commonly achieved in the foundry industry by: a) Higher cooling rate; or b) Addition of grain refiners. To provide higher cooling rates, chill is often placed in the casting cavity to act as a heat sink and consequently increase the local solidification rate. More aggressive cooling can be achieved by running water through the chill. However, high thermal gradients may develop residual strain in the as-cast alloy, which may be detrimental to the life-cycle of the automotive part and thus needs to be evaluated. The results of this thesis suggest that water cooled chill provides higher thermal gradient, resulting in a finer grain size and improved hardness with lower residual strain and texture as compared to no water cooled chill cast samples. Delaying water flow in the chill after 10 seconds introduces anisotropy in a casting by developing a fibre texture as compared to the samples prepared with continuously water cooled chill. Grain refinement is a well established foundry practice to inoculate the melt by the addition of grain refiner, which acts as heterogeneous nucleation site for solidifying α-Al to form a fine grained microstructure. Industrial grain refiners used for aluminum castings are generally master alloys of Al, Ti, B and C with specific stoichiometric ratio. In the present study, TiC powder was tested as grain refiner to effectively refine the microstructure of B319 alloy. The results showed that 0.03 wt% TiC addition refined the microstructure, lowered the SDAS and assisted in the segregation of secondary phases, which in turn resulted in increased hardness and yield strength.

Text

2015-06-02

Attribution-NonCommercial-NoDerivs 2.5 Canada

http://hdl.handle.net/2429/53677

Mechanical Engineering

University of British Columbia

UBCO

http://creativecommons.org/licenses/by-nc-nd/2.5/ca/