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Experimental and numerical analysis of semi-solid constitutive behaviour of B206 and A356 foundry alloys

University of British Columbia

Aluminum foundry alloys are used for a wide range of industrial applications. Hot tearing is often a challenging casting defect in aluminum alloys, occurring in the semi-solid state, which has a substantial impact on the quality of casting products. The constitutive response of the semi-solid material to deformation is crucial for controlling hot tear formation, and thus it is necessary to have a means of assessing a semi-solid’s constitutive behaviour, and the role of microstructure in deforming this two-phase medium. The semi-solid tensile behaviour of two commercially used foundry aluminum alloys was experimentally characterized using a Gleeble thermo-mechanical test apparatus and numerically characterized using a multi-physics numerical model. First, thermo-mechanical testing was carried out on samples prepared by chill wedge-shaped casting. The test results indicated that at relatively high fraction solid (fs=0.95-1), B206 has higher yield stress than A356. However, at lower fraction solids (fs<0.95), A356 has been shown to have higher yield stress. The minimum fraction solid when the yield stress reaches very low values (almost zero), known as rigidity point, are shown to be 0.93 and 0.80 for B206 and A356, respectively. Second, the combined effects of deformation, fluid flow, and microstructure were studied using a multi-physics granular microstructure model. Using feedable and unfeedable microstructure models, the effect of feeding of the semi-solid pressure drop was studied. Unfeedable domains with different microstructures including equiaxed globular, equiaxed dendritic, and combined dendritic and eutectic were developed. Considering that lack of liquid feeding and semi-solid deformation are the well-known contributors to the hot tearing formation, the effect of feeding and mechanical deformation were studied and discussed. The model demonstrated that the microstructure type and the eutectic formation also have a considerable effect on the liquid channels pressure drop and semi-solid bulk stress and strain, and consequently on hot tearing. Then, the obtained experimental and numerical results were used to assess the hot tearing susceptibility of A356 and B206. Both experimental and numerical results showed that B206 is more susceptible to hot tearing. However, hot tearing could occur in A356 as well if feeding is limited.

Text

2018-02-06

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/64585

Materials Engineering

University of British Columbia

UBCV

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