Hot Tearing during the Start-Up Phase of DC Cast Extrusion Billets

Shrinkage, imposed strain rate, and (lack of) feeding are considered the main factors that determine cavity formation or the formation of hot tears. A hot-tearing model is proposed that will combine a macroscopic description of the casting process and a microscopic model. The micromodel predicts whether porosity will form or a hot tear will develop. Results for an Al-4.5 pct Cu alloy are presented as a function of the constant strain rate and cooling rate. Also, incorporation of the model in a finite element method (FEM) simulation of the direct-chill (DC) casting process is reported. The model shows features well known from literature such as increasing hot-tearing sensitivity with increasing deformation rate, cooling rate, and grain size. Similar trends are found for the porosity formation as well. The model also predicts a beneficial effect of applying a ramping procedure during the start-up phase, which is an improvement in comparison with earlier findings obtained with alternative models. In principle, the model does not contain adjustable parameters, but several parameters are not well known. A full quanti-tative validation not only requires detailed casting trials but also independent determination of some thermophysical parameters of the semisolid mush. DOI: 10.1007/s11661-009-9941-y The Author(s) 2009. This article is published with open access at Springerlink.co

Section: B Observations With Constant Parameter Simulationsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Integrated Approach for Prediction of Hot Tearing

Suyitno

Kool

Katgerman

2009

“…[13][14][15] The pressure drop is highest in the center of the billet and increases with casting speed. [13] However, some reports [14,15] show that the pressure drop has little relation with the measured crack length measured for various start-up conditions.…”

Section: Introductionmentioning

confidence: 99%

Hot tearing criteria evaluation for direct-chill casting of an Al-4.5 pct Cu alloy

Suyitno

Kool

Katgerman

2005

117

Predicting the occurrence of hot tears in the direct-chill (DC) casting of aluminum alloys by numerical simulation is a crucial step for avoiding such defects. In this study, eight hot tearing criteria proposed in the literature have been implemented in a finite-element method simulation of the DC casting process and have been evaluated. These criteria were based on limitations of feeding, mechanical ductility, or both. It is concluded that six criteria give a higher cracking sensitivity for a higher casting velocity and that five criteria give a higher cracking sensitivity for the center location of the billet. This is considered in qualitative accordance with casting practice. Seven criteria indicate that use of a ramping procedure (lower casting speed during start-up phase) does not make a significant difference. However, in industrial practice, this is a common procedure, needed for avoiding hot cracking. Only one criterion is in qualitative accordance with casting practice, but it fails to quantitatively predict the hot tearing occurrence during DC casting.

“…This simplified constitutive description has been used previously for examining the mechanical behavior of Al-Cu alloys. [5,8,41,42] The isotropic elastic modulus E = 10 GPa and isotropic Poisson coefficient m s = 0.30 are taken from Reference 8. As no hardening is observed in the experimental results of the Al-Cu alloy at high temperature, [41,42] the flow stress behavior of each grain beyond the elastic limit is assumed to be perfectly plastic and based on the following Norton-Hoff law:…”

Section: Solid Elementsmentioning

confidence: 99%

“…[1][2][3][4][5][6][7] The formation of hot tears is similar to porosity formation in the sense that it is linked to a lack of liquid feeding in the mushy zone, but requires additionally shear or tensile deformation in order to separate, or pull apart, the solid network. These deformations occur because of the thermal gradients, solidification shrinkage, solid contraction, and mechanical constraints.…”

Section: Introductionmentioning

confidence: 99%

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

Sistaninia

Phillion

Drezet

et al. 2010

As a necessary step toward the quantitative prediction of hot tearing defects, a three-dimensional stress-strain simulation based on a combined finite element (FE)/discrete element method (DEM) has been developed that is capable of predicting the mechanical behavior of semisolid metallic alloys during solidification. The solidification model used for generating the initial solid-liquid structure is based on a Voronoi tessellation of randomly distributed nucleation centers and a solute diffusion model for each element of this tessellation. At a given fraction of solid, the deformation is then simulated with the solid grains being modeled using an elastoviscoplastic constitutive law, whereas the remaining liquid layers at grain boundaries are approximated by flexible connectors, each consisting of a spring element and a damper element acting in parallel. The model predictions have been validated against Al-Cu alloy experimental data from the literature. The results show that a combined FE/DEM approach is able to express the overall mechanical behavior of semisolid alloys at the macroscale based on the morphology of the grain structure. For the first time, the localization of strain in the intergranular regions is taken into account. Thus, this approach constitutes an indispensible step towards the development of a comprehensive model of hot tearing.