Influence of secondary cooling strategies on thermal gradients in the direct chill casting of magnesium alloys

Direct chill (DC) casting is the widely used technology for the production of ingots or billets of nonferrous alloys. Non-uniform thermal gradients generated during the process play a predominant role in generation of cracks within the billet. In the present work, a temperature-based finite element technique is used to estimate thermal gradients developed in different regions of billet during DC casting of magnesium alloys. Shrinkage-dependent realistic heat transfer coefficient and temperature-dependent nonlinear boiling curves are used as boundary conditions in the mould and secondary cooling zones, respectively. The simulated temperature profiles during DC casting of AZ31 alloy are validated with real-time plant measurements. Secondary cooling strategies such as pulsed water flow and water removal with wiper are widely used in industry for reducing thermal gradients in DC casting. In this work, the influence of these two techniques on the reduction of thermal gradients is investigated for the DC casting of wrought AZ31 and cast AZ91 alloys. Usage of wiper reduces thermal gradients by 30% in case of AZ31 alloy casting. Surface reheating is observed with the use of wiper in AZ91 alloy. The use of pulsed water is found to reduce thermal gradients at the bottom of the sump in AZ31 casting, whereas no thermal gradient reduction is observed in AZ91 casting for the investigated conditions.