New apparatus for characterising tensile strength development and hot cracking in the mushy zone

A novel apparatus for solidification research has been developed which utilises a modified tensile testing machine and customised mould to control thermal and mechanical parameters during solidification. The test provides constraint during solidification and generates information about strength development, strain accommodation and hot cracking behaviour of the mushy zone material. The equipment has been used to determine the effect of grain refinement and composition variations on these parameters. This paper describes the test apparatus and the type of information it generates, It includes a comparison of hot cracking produced in the rig with that obtained from direct chill cast product, The development of strength of a relatively pure material, alloy AA194, and a highly alloyed, extended freezing range alloy, AA7075 is presented, In the case of alloy AA7075 strength development began at a solid fraction of approximately 0.7 and continued to increase as solidification progressed. The development of strength in alloy AA194 did not occur until a fraction solid of 0.9, Under tensile loading conditions, the development of strength appears to occur at higher solid fractions than for rheological test methods.