A New Hot Tearing Assessment by Using Stepped Ring Core Mold and the Effect of Strontium on the Hot-Tearing Resistance of Al�6�wt% Zn Based Alloy

Abstract

Automobile and aerospace industries use thin wall aluminium alloy castings which provide lighter structures with excellent mechanical properties. Production of thin wall castings is more challenging due to hot tear formation. Lack of fluidity in molten alloy causes hot tears and must be addressed in thin wall castings of Al-alloys. The present study is focused on a new technique known as stepped ring mould casting. It is possible to assess the hot tear susceptibility of Al�6Zn alloys by varying ring thickness to find out the critical thickness for occurrence of hot tears. The alloy was cast using different strontium (Sr) concentrations (0.2, 0.4, 0.6%). Effects of strontium concentrations were studied in terms of fluidity, porosity content, microstructure and tensile properties of Al�6Zn alloy. In the present work, unmodified and Sr modified alloy casts were characterized by SEM, EDS and XRD respectively. Al�6Zn ingots were procured by master alloy route. Repetition of stepped ring test on the critical thickness showed that hot tear were successfully eliminated significantly due to the addition of Sr. On the other hand, 0.6% Sr also exhibited a good amount of porosity and decrease in elongation. Shorter fluidity length was observed in 0.2% Sr modified alloy. Mechanical and metallographic tests revealed that the alloy castings modified with 0.4% Sr offered better results in yield strength, less porosity and an improved hot tear resistance at micro and macro levels. � 2017, The Indian Institute of Metals - IIM.