Why Aluminum Alloy Selection Matters
Aluminum is prized for its combination of light weight, corrosion resistance, and workability — but "aluminum" is really a broad umbrella. There are dozens of alloys in common commercial use, each with different compositions and properties. Picking the wrong alloy can mean parts that crack under load, corrode in unexpected environments, or can't be welded at all.
This guide walks through the major alloy series and focuses on the two most widely used grades: 6061 and 7075.
The 4-Digit Aluminum Alloy Numbering System
Aluminum alloys are classified by a 4-digit system where the first digit identifies the primary alloying element:
- 1xxx: Pure aluminum (99%+). Excellent corrosion resistance, very soft. Used in wiring and foil.
- 2xxx: Copper alloys. High strength, but poor corrosion resistance and difficult to weld. Aerospace use.
- 3xxx: Manganese alloys. Moderate strength, good formability. Used in cookware and roofing.
- 4xxx: Silicon alloys. Low melting point. Used as filler wire and in casting.
- 5xxx: Magnesium alloys. Excellent corrosion resistance (especially marine). Good weldability.
- 6xxx: Magnesium and silicon. Versatile, heat-treatable, weldable. The most common structural series.
- 7xxx: Zinc alloys. Highest strength of any aluminum series. Used in aerospace and high-performance applications.
Temper Designations
After the alloy number you'll often see a letter and number, such as -T6 or -T651. These indicate the temper — the heat treatment and work-hardening condition of the material. The most common:
- -O: Annealed (softened). Fully soft, maximum ductility.
- -T4: Solution heat treated and naturally aged. Good formability.
- -T6: Solution heat treated and artificially aged. Maximum strength for the alloy.
- -T651: T6 plus stress-relieved by stretching. Improved flatness for plate.
6061-T6: The Workhorse
6061 is the most widely used structural aluminum alloy in the world. It belongs to the 6xxx (magnesium-silicon) series and in T6 temper offers an excellent balance of properties:
- Tensile strength: ~45,000 PSI (310 MPa)
- Yield strength: ~40,000 PSI (276 MPa)
- Weldability: Excellent (use 4043 or 5356 filler)
- Machinability: Good
- Corrosion resistance: Very good
- Anodizing: Excellent response
- Cost: Moderate
Use 6061 for: structural frames, bicycle frames, marine components, automotive parts, machined parts, enclosures, and general fabrication.
7075-T6: The High-Performance Option
7075 belongs to the zinc-alloyed 7xxx series and is one of the strongest aluminum alloys available commercially. It's the go-to choice when maximum strength is the priority and weight savings are critical:
- Tensile strength: ~83,000 PSI (572 MPa) — nearly twice 6061
- Yield strength: ~73,000 PSI (503 MPa)
- Weldability: Poor — prone to cracking; generally not welded
- Machinability: Very good
- Corrosion resistance: Moderate (requires anodizing or plating)
- Cost: Significantly higher than 6061
Use 7075 for: aircraft structures, high-stress machined components, competitive bike and motorsport parts, tooling, and anything where maximum strength-to-weight is paramount and welding is not required.
Head-to-Head Comparison
| Property | 6061-T6 | 7075-T6 |
|---|---|---|
| Tensile Strength | ~45 ksi | ~83 ksi |
| Yield Strength | ~40 ksi | ~73 ksi |
| Weldability | Excellent | Poor |
| Corrosion Resistance | Very Good | Moderate |
| Machinability | Good | Very Good |
| Cost (relative) | Lower | Higher |
| Best Use | General fabrication | High-stress machined parts |
Other Notable Grades
- 5052-H32: Excellent formability and marine corrosion resistance. Great for sheet metal work, tanks, and marine panels.
- 2024-T3: High strength with good fatigue resistance. Used in aircraft fuselage skins. Not weldable.
- 6063-T5: Excellent extrudability and surface finish. The standard for architectural extrusions (window frames, door frames).
Which Should You Choose?
For the vast majority of metalworking projects — frames, brackets, enclosures, machined parts, marine components — 6061-T6 is the right answer. It's versatile, weldable, readily available, and cost-effective.
Reach for 7075-T6 only when you specifically need maximum strength in a machined component and won't be welding it. The higher cost and poor weldability make it impractical for general fabrication.