Hot tear characterization of AlCu5MgTi and AlSi9 casting alloys using an instrumented constrained six rods casting method

The aim of this paper is to introduce an improved method for determining hot tear initiation combining qualitative (rapid and industrial) and quantitative (comprehensive and laboratory) approaches. The quantitative method is based upon generating a contraction load during solidification and plotting cooling curves for aluminum casting alloys (AlCu5MgTi and AlSi9 as case study) poured in a permanent steel mold. The cavity of the mold is formed of six constrained rods (a geometry of six different length I beams). The qualitative method consists of counting visually the number of cracked rods that appeared in casting and giving decision rapidly on hot tear susceptibility. The instrumented mold enabled real-time measurements of the contraction load developed in the casting and the temperature variations during solidification as a function of time. Characteristic solidification temperatures such as liquidus, dendrite coherency point (DCP), rigidity and solidus temperature were extracted from cooling curves and have been recognized as important parameters of solidified aluminum alloys because they mark transitions between several types of feeding mechanisms. The experiments conducted have elucidated the prevalence of rigidity temperature as a crucial parameter in hot tear initiation. For AlCu5MgTi alloy, the contraction load increased significantly at the vicinity of rigidity temperature causing clearly hot tearing initiation phenomenon when AlSi9 contraction load dropped to very weak values inhibiting hot cracking. The predictions were consistent with visual observations (qualitative method) and crack susceptibility models (hot tear criteria).