J. Phys. IV France 09 (1999) Pr9-53-Pr9-62
Validation of a new hot tearing criterion using the ring mould testJ.-M. Drezet
Laboratoire de Métallurgie Physique, École Polytechnique Fédérale de Lausanne, MX-G, 1015 Lausanne, Switzerland
Hot tear is one of the most serious defect which a casting can suffer. It represents a major limitation to the production of foundry cast parts and to the productivity of continuous casting processes such as the direct chill casting of aluminium alloys. As an exemple, the starting phase of the direct chill casting process remains particularly critical for some aluminium alloys because of their high propensity to develop either hot tears which initiate at non zero liquid fraction, or cold cracks which nucleate and grow exclusively in the solid metal. In order to validate a new hot tearing criterion recently proposed by Rappaz and Drezet , instrumented ring mould tests were carried out with aluminium alloys of different composition and further on analysed from a thermal, mechanical and microstructural point of view. The thermal field obtained in the test was determined with the help of an inverse method using five temperature histories measured at different locations and the stress build-up was computed with a transient thermomechanical model implemented in the finite element package Abaqus. Deformation in the solid was assumed to obey a viscoplastic law and the cooling conditions were those deduced by the inverse method. The new hot tearing criterion  based on the ability of the interdendritic flow of liquid to compensate for the thermally induced deformation of the roots of columnar dendrites, allowed the calculation of the maximum strain rate that the roots of the dendrites can undergo without initiation and/or propagation of hot tears. After implementation in the numerical FEM model of the ring mould test, the hot tearing criterion predicted the occurrence of tears precisely in the region where hot cracks were observed after the test. More generally, when implemented in thermomechanical models of casting processes, the present hot tearing criterion would be very helpful in diminishing the cracking tendency.
© EDP Sciences 1999