Aluminium

Aluminium (or aluminum for most Americans) is a lightweight, silvery-white metal, symbolized by “Al” and atomic number 13. It is the most abundant metal in Earth’s crust, making up about 8% of its weight. Known for its corrosion resistance and high strength-to-weight ratio, it is widely used in industries such as construction, transportation (like aircraft and cars), packaging (e.g., cans and foils), and electronics.

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It is also highly recyclable, making it an eco-friendly material. And it naturally forms a thin oxide layer, protecting it from further oxidation, and is extracted primarily from bauxite ore through the Bayer and Hall-Héroult processes.

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Aluminium is not as strong as stainless steel or titanium, but it has a high strength-to-weight ratio, which makes it valuable for many applications. Pure it has a tensile strength of about 90 MPa (megapascals), but its strength can be significantly increased by alloying it with elements like copper, magnesium, or silicon.

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For example, aluminium alloys can reach strengths up to 400–500 MPa, making them suitable for structural uses, such as in aerospace and automotive industries. It is also strong enough to resist corrosion and impact in most environments, especially when treated or coated.

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For aluminium alloys, the Vickers hardness can vary significantly based on the alloy composition and treatment:

• Aluminium 2024-T351 (Al-Cu alloy): ~120-160 HV
• Aluminium 6061-T6 (Al-Mg-Si alloy): ~95-110 HV
• Aluminium 7075-T6 (Al-Zn alloy): ~175-200 HV

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Compared to 316L stainless steel: this has a Vickers hardness of about 150-200 HV. Pure aluminium (99.9% Al) has a Vickers hardness of around 25–35 HV but is not used for watches.

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Compare case materials on the Hardness Vickers scale (from hard to hardest):

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Watches from aluminium