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Aluminum Alloys – Fundamentals
This article provides a comprehensive overview of aluminum alloys based on SAE J451-2018, covering properties, alloying elements, processing techniques such as heat treatment and joining, as well as forming and finishing considerations. It is intended for engineers and designers working with aluminum materials.
Properties and Alloying of Aluminum
Commercially pure aluminum is a lightweight metal with the following key properties:
| Property | Value |
|---|---|
| Specific Gravity | 2.71 (0.098 lb/in³) |
| Melting Point | ~1215°F |
| Thermal Conductivity (25°C) | 0.52 cgs units |
| Coefficient of Thermal Expansion | 0.000013 per °F |
| Electrical Conductivity | 62% of IACS |
| Tensile Strength (cast) | 14,000 psi |
| Elongation (cast) | 30% in 2 in |
| Modulus of Elasticity | 10,000,000 psi |
Alloying elements such as copper, silicon, magnesium, manganese, and zinc are added to enhance strength, corrosion resistance, or to enable heat treatment. For example, copper increases strength but reduces corrosion resistance, which can be mitigated by cladding. Silicon and magnesium reduce density, while most additions lower thermal and electrical conductivity.
Processing: Heat Treatment, Joining, and Forming
Aluminum alloys are classified as heat treatable or non-heat treatable. Heat treatable alloys (e.g., 2xxx, 6xxx, 7xxx) gain strength through solution treatment at 900-1000°F, rapid quenching, and aging (natural or artificial). Non-heat treatable alloys (e.g., 1xxx, 3xxx, 5xxx) rely on cold work for strengthening. Annealing restores ductility but strength must be regained by cold work for non-heat treatable alloys.
Joining methods include MIG and TIG welding, brazing, soldering, and adhesive bonding. Oxyfuel and coated electrode welding require careful flux removal to avoid corrosion. Work-hardened alloys offer good as-welded strength, while heat-treated alloys may require post-weld heat treatment to restore pre-weld properties.
Forming depends on temper selection: for severe forming, use annealed (O) or as-quenched tempers. Intermediate tempers (H12, H14, T4) suit moderate forming. Full hard (H18, T6) tempers are best for applications with minimal forming. Spring-back increases with strength, so larger allowances are needed.
Design Insights and Frequently Asked Questions
🛠️ Design Insight: Aluminum’s coefficient of thermal expansion is about twice that of steel (0.000013 per °F). In composite structures, ensure thermal expansion mismatches are accommodated. Also, galvanic corrosion can occur when aluminum is coupled with more noble metals (e.g., copper) in the presence of an electrolyte. Use isolation or cladding as needed.
⚠️ Common Mistake: Neglecting to remove flux after oxyfuel or coated electrode welding can promote corrosion. Always thoroughly clean the joint after welding. Additionally, avoid using fully hardened tempers for severe forming operations; use a softer temper or annealed condition.
What are the typical properties of pure aluminum?
Pure aluminum has a specific gravity of 2.71, melting point around 1215°F, tensile strength of about 14,000 psi (cast) and excellent corrosion resistance in many environments. It is highly conductive both thermally and electrically.
How do alloying elements affect aluminum properties?
Alloying elements generally increase strength, decrease electrical and thermal conductivity, and can either improve or reduce corrosion resistance. Some combinations enable heat treatment, as with copper, magnesium, and silicon in various series.
What is the heat treatment process for aluminum?
It involves solution treatment at 900-1000°F, rapid quenching to retain alloying elements in solid solution, followed by natural or artificial aging to precipitate particles that strengthen the material. Non-heat treatable alloys rely on cold work for strength.
What joining methods are suitable for aluminum?
MIG and TIG welding are most common. Brazing (torch, dip, furnace) and adhesive bonding are also used. Flux-based processes require complete flux removal to avoid corrosion. Heat treatable alloys may need post-weld heat treatment.
Based on SAE J451-2018, Aluminum Alloys – Fundamentals.
