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The effects of microstructural features and process parameters on the hottearing in direct chill cast aluminum alloys

University of British Columbia

Hot tearing is an irreversible failure that occurs above the the solidus temperature of an alloy during casting, in the presence of a liquid phase. These cracks possess serious quality implications in industrial direct chill (DC) casting process. During solidification, thermal stresses are induced due to the heterogeneous temperature distribution, which causes variations in thermal strain and may result in cracks if these stresses are large enough. In order to investigate the occurrence of hot tears in DC casting, a DC casting finite element process model for round billets was incorporated with (1) a new semi-solid constitutive law for aluminum alloy AA5182 that takes into account cooling rate, grain size and porosity, and (2) a model for cooling rate induced grain size variation. A hot tearing index was calculated from the semi-solid strain predictions from the model. This hot tearing index, along with semi-solid stress predictions from the model, was used to link hot tearing with microstructural features (i.e. grain size and coalescence temperature) as well as process parameters (e.g. casting speed). It was found that grain refinement plays an important role in the formation of hot cracks. In addition, lower assumed coalescence temperature and slow casting speeds were found to improve hot tearing resistance. In addition to simulation of DC casting, experimental studies on an as cast AA5182 ingot (DC cast) were made in terms of grain size, chemical composition and solidification kinetics. Samples for these experiments were collected from the steady state region of the ingot. The results of these experimental investigations show that, (1) grain size increases from surface to centre of the ingot, (2) there is considerable macrosegregation of the alloying elements along the cross section of the ingot, and (3) the solidification kinetics vary as a function of both position and cooling rates. These experimental observations influence the hot tearing susceptibility of the DC cast product. Thus, for the process model to be more accurate in predicting hot tears, inclusion of these factors, along with an improved model for grain size variation is suggested.

Text

2012-10-02

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/43316

Mechanical Engineering

University of British Columbia

UBCO

http://creativecommons.org/licenses/by-nc-nd/4.0/