Direct chill and electromagnetic casting of aluminium alloys: Thermomechanical effects and solidification aspects
The tolerances of semi-continuously cast products of aluminium alloys are very critical if the scalping of the ingot faces is to be minimised before rolling. In the steady state regime of casting, the dimensions of the section of the solidified ingot are lower than those of the mould in the Direct Chill Casting (DCC) or of the inductor in the Electromagnetic Casting (EMC). The contraction of the section, several percents, is larger than the value associated with the thermal contraction of the solid and is also inhomogeneous: the short sides of the ingot contract less than the centre of the rolling faces. In order to study and to understand the mechanisms responsible for such deformation, insitu measurements and laboratory investigations have been performed whereas a thermomechanical model has been developed. The specific points of this study are as follows: measurements of the distortions undergone by the ingot during casting and after complete cooling, determination of the thermal boundary conditions corresponding to the primary cooling (contact in between the metal and the mould) and secondary cooling (water jet), using in-situ temperature measurements and inverse modelling, measurements of the thermal conductivity of two industrial alloys using inverse modelling and one dimensional casting experiments, measurements of the thermomechanical behaviour of two industrial alloys, specifically the elastic modulus by ultrasonic method, the thermal expansion coefficient by dilatometry and the creep behaviour in the solid and mushy state using tensile and indentation tests, study of the solidification path of the AA1201 alloy using a finite difference microsegregation model coupled with the Al-Fe-Si phase diagram data; the results have been validated against DTA measurements and extended to situations encountered in the DC/EM casting process, computation of the temperature, stress and strain fields in DC and EM-cast ingots with the help of the 2D and 3D thermomechanical finite element code Abaqus; the final ingot cross section for a given mould/inductor design has been calculated, comparison of the simulation results with the experiments. The mechanisms responsible for the main ingot distortions undergone during casting and subsequent cooling, notably the non-uniform contraction of the ingot cross section, have been identified. Finally, an inverse thermomechanical method for the optimisation of the mould/inductor design is proposed based upon a criterion of maximum flatness of the final ingot. Such a method should allow a reduction in the costs associated with the definition of the mould/inductor designs for new casting conditions. The present work will be extended to more complicated geometries in the BriteEuram project EMPACT (European Modelling Programme on Aluminium Casting Technology).