Comparison of Tensile Properties and Fracture Mechanisms Between Primary and Secondary Rheocast AlSi7Mg Aluminum Alloys

Abstract

Abstract To enhance the sustainability of aluminum castings, the use of recycled alloys is increasing due to their lower energy demand and reduced carbon footprint. At the same time, semisolid metal (SSM) rheocasting is emerging as a viable alternative to High Pressure Die Casting (HPDC), enabling the processing of low-silicon aluminum alloys. This reduces the need for critical raw materials, such as primary silicon, and allows casting at lower temperatures, further improving efficiency. However, contaminants typically present in recycled alloys, particularly iron, influence castability, defect formation, and mechanical performance, requiring further investigation. This study analyzes the tensile properties and cracking mechanisms of semi-solid cast AlSi7Mg alloys with varying iron content: primary (Fe = 0.08%), recycled (Fe = 0.19%), and recycled high iron (Fe = 0.42%), produced via the Rheometal™ method. Tensile testing was performed on as-cast and T6 samples. Statistical analysis was applied to evaluate variability and statistical significance of the results. The results show that recycled alloys have a higher defect density at an increased iron content, leading to lower elongation, reduced strength, and greater variability than in the primary alloy (recycled alloy Fe=0.19%: as-cast Rp 0.2 =113±8 MPa, R m =208±15 MPa, e f =4.13±0.95 %; T6 Rp 0.2 =240±17 MPa, R m =279±27 MPa, e f =1.40±0.20 %). The T6 treatment improved strength but reduced ductility due to defect enlargement and surface blistering caused by solubilization step, particularly pronounced in the recycled alloy with high-iron content. Scanning electron microscopy (SEM) fractography indicated that oxide films act as nucleation sites for β-Al 5 FeSi intermetallics, which promote shrinkage defects and thereby limit elongation.

Affiliated Institutions

• University of Bologna IT
• Jönköping University SE

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