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A fairly simple experimental setup has been designed for testing the resistance of the mushy zone of alloys during solidification under tensile conditions. It has been used to study the effect of coalescence among the solid grains at a late stage of solidification. The experimental approach involves both tensile-strength measurements and scanning electron microscope (SEM) observations of fracture surfaces. Complementary information can be obtained by numerical modeling of this solidification process. The latter takes into account heat flow in the sample, rheology of the mushy alloy, liquid feeding, and porosity formation. All of the available information indicates that the transition from a granular mushy alloy to a coalesced solid skeleton behavior starts for a solid fraction of approximately 92 pct.
Hot tearing formation in both a classical tensile test and during direct chill (DC) casting of aluminum alloys has been modeled using a semicoupled, two-phase approach. Following a thermal calculation, the deformation of the mushy solid is computed using a compressive rheological model that neglects the pressure of the intergranular liquid. The nonzero expansion/compression of the solid and the solidification shrinkage are then introduced as source terms for the calculation of the pressure drop and pore formation in the liquid phase. A comparison between the simulation results and experimental data permits a detailed understanding of the specific conditions under which hot tears form under given conditions. It is shown that the failure modes can be quite different for these two experiments and that, as a consequence, the appropriate ho...
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