The Effect of Water Ejection and Rewetting Phenomena on Hot Cracking Sensitivity during Direct Chill Casting of Magnesium Alloy

The production of industrial-scale ingots and billets with minimum defects through direct chill (DC) casting process is highly sensitive to casting parameters such as cooling water flow rate and casting speed. For given casting conditions, if the billet attains higher surface temperature, water ejection occurs in secondary cooling zone of DC casting. This affects the temperature distribution during the casting process and associated defects. In the present study, the influence of water ejection phenomenon on thermal stresses and occurrence of hot cracks in DC casting of magnesium alloy was studied using a thermo-mechanical mathematical model. The effect of casting speed and water flow rate on thermal profiles was investigated, and critical casting speed beyond which water ejection phenomenon occurs was established for AZ31 magnesium alloy. Detailed studies were carried out for two different cases of complete and delayed wetting (ejection followed by wetting). Results indicated that tensile stresses were high in the impingement region for complete wetting. For delayed wetting, peak tensile stresses were observed at the bottom of the billet immediately after rewetting. Hot tearing studies based on (Rappaz–Drezet–Gremaud) RDG criterion indicated that center of the billet near bottom block was more sensitive to hot cracking in both complete and delayed wetting cases. However, delayed wetting further increased the risk of hot tearing.