Finite element method simulation of mushy zone behavior during direct-chill casting of an Al-4.5 pct Cu alloy
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Finite Element Method Simulation of Mushy Zone Behavior during Direct-Chill Casting of an Al-4.5 Pct Cu Alloy SUYITNO, W.H. KOOL, and L. KATGERMAN In this article, the stresses, strains, sump depth, mushy zone length, and temperature fields are calculated through the simulation of the direct-chill (DC) casting process for a round billet by using a finite-element method (FEM). Focus is put on the mushy zone and solid region close to it. In the center of the billet, circumferential stresses and strains (which play a main role in hot cracking) are tensile close to the solidus temperature, whereas they are compressive near the surface of the billet. The stresses, strains, depth of sump, and length of mushy zone increase with increasing casting speed. They are maximum in the start-up phase and are reduced by applying a ramping procedure in the start-up phase. Stresses, strains, depth of sump, and length of mushy zone are highest in the center of the billet for all casting conditions considered.
I. INTRODUCTION
DIRECT-CHILL (DC) casting is a semicontinuous process for producing extrusion billets and rolling slabs. In this casting process, liquid metal is poured onto a moving bottom block inside a mold. The mold is cooled by water flow that is called primary cooling. At the exit of the mold, water flow directly impinges on the billet or slab, which is called secondary cooling. During the casting process, metal will pass through a mushy state, which is critical for the occurrence of some defects such as hot tearing and microporosity. The defects are related to the mechanical behavior of the mush, in combination with the feeding possibilities. Understanding of the behavior of the mush during DC casting is not an easy task because of the complex phenomena occurring during solidification. Modeling of stresses, strains, and temperatures during DC casting is generally done by using a finite-element method (FEM).[1–8] In the past, most researchers worked on the simulation of the thermomechanical behavior of a DC cast slab, and most attention was devoted to the thermomechanical behavior at temperatures lower than the solidus temperature.[1–4] The stresses and strains in the center of a billet at temperatures lower than the solidus temperature are tensile except for the axial strain, which is compressive.[1,2] Also, the stresses and strains in the center of a slab are found to be tensile.[3,4] Recently, simulation of the thermomechanical behavior of a billet or slab at temperatures above the solidus temperature has gained significant attention.[5–8] Several constitutive models of the mushy state are used for simulation of the thermomechanical behavior such as an elastoviscoplastic law[5,6] and Garofalo’s law.[7] All of these models have been fitted to experimental data. In another study,[8] solid data are simply extrapolated to temperatures corresponding to the mushy zone up to the coherency point.
By applying an elastoviscoplastic law, a tensile circumferential stress in the center of the billet an
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