A two-phase continuum model for an isotropic mushy zone is presented. The model is based upon the general volume-averaged conservation equations, and quantities associated with hot tearing are included, i.e., after-feeding of the liquid melt due to solidification shrinkage is taken into account as well as thermally induced deformation of the solid phase. The model is implemented numerically for a one-dimensional model problem with some similarities to the aluminium direct chill (DC) casting process. The variation of some key parameters that are known to influence the hot-tearing tendency is then studied. The results indicate that both liquid pressure drop due to feeding difficulties and tensile stress caused by thermal contraction of the solid phase are necessary for the formation of hot tears. Based upon results from the one-dimensional model, it is furthermore concluded that none of the hot-tearing criteria suggested in the literature are able to predict the variation in hot-tearing susceptibility resulting from a variation in all of the following parameters: solidification interval, cooling contraction of the solid phase, casting speed, and liquid fraction at coherency.

Two-phase modeling of mushy zone parameters associated with hot tearing

The heat-transfer coefficient was measured during the unidirectional solidification of Al-7 wt pct Si alloy castings against a water-cooled Cu chill. Heat-transfer coefficients in the range of 2.5 to 9 kW m−2 K−1 were obtained with solidification vertically upward associated with higher values than solidification vertically downward. Horizontal solidification was associated with intermediate values. Profiles taken across the diameters of the casting surfaces at the interface with the chill showed them to be convex toward the chill by an amount which would produce a mean gap between the casting and the chill that would account for a significant proportion of the measured heat-transfer coefficient. The convex casting surfaces were attributed to the deformation of the initial casting skin by the thermal stress produced during solidification. Heat transfer during casting solidification is shown to be a complex mechanism controlled by the microscale surface roughness of the respective surfaces, mesoscale deformation of the casting skin by thermal stress, and macroscale movements of the casting and the chill due to their relative thermal expansion and contraction.

The Heat-Transfer Coefficient during the Unidirectional Solidification of an Al-Si Alloy Casting

Numerical simulation of filling and solidification of permanent mold castings

An indirect squeeze casting process applied to a wrought 5083 Al alloy (Al–4.7Mg–0.7Mn) was investigated experimentally and numerically. A two-dimensional finite element computer code for fully coupled heat transfer and deformation analysis, abaqus , was used to simulate the cooling curves obtained from the experiments. Thermal contraction of the material during solidification creates an air gap between the mold and the cooling material. The formation of this air gap is explained using the calculated results. The experimental and predicted results are discussed in conjunction with the relationships between the cooling rate, microstructure, die geometry and applied pressure. The effect of applied pressure and cooling rate on the macrosegregation is also discussed.

Effect of the gap distance on the cooling behavior and the microstructure of indirect squeeze cast and gravity die cast 5083 wrought Al alloy

ALE method for solidification modelling

A thermomechanical three-dimensional (3-D) finite element analysis of solidification is presented. The heat transfer model is based on a multidomain analysis accounting for noncoincident meshes for the cast part and the different mold components. In each subdomain, a preconditioned conjugate gradient solver is used. The mechanical analysis assumes the mold is rigid. A thermoelastic-viscoplastic rheological model is used to compute the constrained shrinkage of the part, resulting in an effective local air gap width computation. At each time increment, a weak coupling of the heat transfer and mechanical analyses is performed. Comparisons of experimental measurements and model predictions are given in the case of a hollow cylindrical aluminum alloy part, showing a good quantitative agreement. An application to an industrial aluminum casting is presented, illustrating the practical interest of thermomechanical computations in solidification analysis.

Thermomechanics of the cooling stage in casting processes : three-dimensional finite element analysis and experimental validation

The heat transfer coefficient, h, during die filling has been measured for various die surfaces. The heat transfer coefficient has been found to vary strongly with the porosity of the surface, giving rise to an insulating air film at the interface. Values vary from 400 W m−2 K−1 for a ceramic coated surface up to 10 000 W m−2 K−1 for a finished die surface. Parameters such as velocity of flow, die temperature and alloy composition was found to have a limited influence on the h values.

Heat transfer during filling in die casting processes

The literature survey about the thermomechanical behavior of the mushy state first demonstrates the lack of data about the rheological parameters for viscoplastic component. Then it shows discrepancies between experimental and modelling studies about the elastic component. In order to fill the gap underlined in the first point, we develop a new methodology which consists in associating a simple test geometry (needle indentation test), able to prduce the suitable strains and strain rates, with an FEM analysis of the induced flow. The thermal dependance of the power creep law (Norton-Hoff) can than be identified. The results obtained for the Al-4.5%Cu-Mg-Ti alloy are presented and applied to the thermomechanical prediction of air gap in permanent mold casting by comparisons of measurements and calculations.

Mushy state behavior : rheological characterization and influence on air gap formation

On the correlation between microstructural parameters and the thermo-mechanical fatigue performance of cast iron

Grey iron alloyed with molybdenum and niobium in seven different compositions has been casted using three, in industrial components viable, solidification times which resulted in 21 different sampl ...

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Pål Schmidt

Papers

Thermomechanics of the cooling stage in casting processes : three-dimensional finite element analysis and experimental validation

Heat transfer during filling in die casting processes

Mushy state behavior : rheological characterization and influence on air gap formation

On the correlation between microstructural parameters and the thermo-mechanical fatigue performance of cast iron

Thermo-mechanical fatigue of grey cast iron for cylinder heads - Effect of niobium, molybdenum and solidification time