Two-Phase Modeling of Hot Tearing in Aluminum Alloys: Applications of a Semicoupled Method

Abstract

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 hot tearing criterion may differ. It is foreseen that a fully predictive theoretical tool could be obtained by coupling such a model with a granular approach. These two techniques do, indeed, permit coverage of the range of the length scales and the physical phenomena involved in hot tearing.

Simulation of Semi-Solid Material Mechanical Behavior Using a Combined Discrete/Finite Element Method

“…The results from two other simulations, using strain rates of 10 -4 and 10 -2 s -1 , also are shown. Although these results could have been produced using an averaging constitutive model (e.g., The one by Mathier containing an internal variable related to the evolution of coherency) [10] containing an internal variable related to the evolution of coherency, such alternative approaches cannot give access to stress and strain inhomogeneities such as those shown in Figures 6 and 7. Figure 6 shows strain contours from cross sections within the RVE for three values of g s (g s = 0.92, 0.96, and 0.98), outlined in Figure 4, when the overall or bulk strain is 1 pct.…”

“…4-A comparison between the tensile experimental results of partially solidified Al-2 wt pct Cu alloys [8] (continuous curves) and simulation results (dashed line curves) for the following fractions of solid: 9 g s = 0.92 (T = 883 K (610°C)); h g s = 0.94; s g s = 0.96 (T = 858 K (585°C)); D g s = 0.98 (T = 824 K (551°C)). [10] (continuous curves) and simulation results (dashed line curves) for g s = 0.99 (T = 813 K (540°C)) and different strain rates.…”

“…[8][9][10][11] In these works, a representative volume element (RVE) is assumed to contain a mixture of solid and liquid with local volume fractions g s and g l , respectively, interacting through the averaged conservation equations. [12] Unfortunately, such approaches are unable to describe any strain inhomogeneity at the grain level, particularly at grain boundaries where hot tears form.…”

“…[8,10] The mechanical behavior of the solid grains is modeled using a simple viscoplastic law, while the intergranular liquid has been replaced by connector elements. First, the methodology for generating the granular grain structure based on a solidification model is briefly described.…”

“…[32] Third, the deformation results from the FE/DEM simulation are discussed and validated against experimental data from the literature. [8,10] II. MODEL DEVELOPMENT…”