Effect of metallurgical parameters on the microstructure, hardness impact properties, and fractography of Al-(6.5–11.5) wt% Si based alloys

• Emphasizing the importance of the amounts of Sr-modifier and TiB2 grain refiner additions
• Highlighting the role of melt cleanliness and hydrogen content on the alloy performance
• Highlighting the significance of proper modification of the eutectic Si phase in improving the alloy toughness
• Significance of characteristics of precipitates (morphology and hardness) in relation to precipitation hardening

The present study was performed on 356 and 413 alloys containing various amounts of hydrogen (H2), titanium diboride (TiB2), and strontium (Sr). The results show that the eutectic silicon particles in non-modified alloys undergo coarsening during the solutionizing treatment. High energy levels are obtained when the alloy is degassed, Sr-modified, and grain refined. Precipitation of Mg2Si phase particles in 356 alloys is more effective as a hardening agent than the Al2Cu phase precipitated in the 413 alloy which contributes to the alloy strength/hardness. The increased Si content in non-modified 413 alloys accelerates crack initiation and propagation, leading to low toughness values. Modifying the 413 alloys with 200 ppm Sr results in a more even distribution of the spheroidized eutectic Si particles and hence better the alloy toughness. Exceeding the required amount of Ti-containing grain refiner may result in the introduction of a large amount of brittle Al3Ti platelets that would reduce the alloy toughness.

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