6061 and 7075 turn up on nearly every machined prototype drawing in aluminium, and they are often treated as interchangeable grades of the same thing. They are not. One is a balanced structural alloy you can weld, anodise and machine all day; the other is a high-strength alloy that punishes the wrong choice in corrosion, cracking and cost. If the part may not need aluminium at all, the plastic-versus-metal decision comes first. If it does, this comparison covers the axes that decide prototype outcomes: strength, stiffness, corrosion, weldability, finishing, machining and cost.
They are different alloy families
6061 is a 6xxx-series alloy: magnesium and silicon form the strengthening phase, with a small copper addition and a touch of chromium for grain control. 7075 is a 7xxx-series alloy: zinc and magnesium with copper, which is what makes it respond so strongly to precipitation hardening. That composition difference is the whole story — every property below follows from it.
| Element | 6061 | 7075 |
|---|---|---|
| Zinc | ≤ 0.25 | 5.1 – 6.1 |
| Magnesium | 0.8 – 1.2 | 2.1 – 2.9 |
| Copper | 0.15 – 0.40 | 1.2 – 2.0 |
| Silicon | 0.40 – 0.80 | ≤ 0.40 |
| Chromium | 0.04 – 0.35 | 0.18 – 0.28 |
Both are supplied for machining stock in the T6 temper — solution heat-treated and artificially aged. T651 is the stress-relieved plate variant that stays flatter after heavy roughing, and it is the better starting point for prototype work in either alloy unless the drawing says otherwise. Specify the temper on the drawing: T6 bar and T651 plate behave differently once you start taking material off.
The numbers that actually decide it
These are published typical properties for the T6 temper. Use them to choose between alloys, not as design allowables — a drawing that carries load needs the specification values for the temper and direction you actually buy.
| Property | 6061-T6 | 7075-T6 | What it decides |
|---|---|---|---|
| Yield strength | ≈ 276 MPa | ≈ 503 MPa | Strength-limited parts; roughly a 1.8× difference |
| Tensile strength | ≈ 310 MPa | ≈ 572 MPa | Ultimate load before fracture |
| Elongation | ≈ 12% | ≈ 11% | Both ductile enough to deform before breaking |
| Elastic modulus | ≈ 68.9 GPa | ≈ 71.7 GPa | Within ~4% — stiffness buys almost nothing |
| Density | 2.70 g/cm³ | 2.81 g/cm³ | 7075 is ~4% denser; the net mass saving is still large |
| Thermal conductivity | ≈ 167 W/m·K | ≈ 130 W/m·K | Heat sinks run measurably warmer in 7075 |
| Fusion welding | Routine (TIG/MIG) | Generally avoided | Decides welded assemblies outright |
| Machinability | Excellent | Good | Tool life, cycle time, scrap risk |
Two consequences follow. First, a strength-limited part can be up to about 40 per cent lighter in 7075 for the same section strength. Second, a stiffness-limited part gains almost nothing, because the moduli differ by roughly four per cent — swapping alloys to stiffen a bracket is a category error; the geometry has to change instead.
Strength is not one property
The yield headline hides the rest of the mechanical picture, and the rest is where prototype failures come from:
- Fracture toughness and fatigue crack growth favour 6061. 7075 is strong but notch-sensitive: a scratch, a sharp fillet or a poorly broken edge is where it starts.
- Stress corrosion cracking is the known weakness of 7075-T6, especially in the short-transverse (through-thickness) direction under sustained tensile stress in humid or marine exposure. The T73 temper largely removes it, at roughly 10–15 per cent lower strength.
- Fatigue strength at high cycle counts is higher in 7075 — but only where stress concentrations are controlled. Round the fillets before relying on it.
- 6061 forgives. A prototype that gets dropped, over-torqued or mis-loaded usually dents; the same abuse in 7075 can crack.
Corrosion: the difference that bites
6061 has good general corrosion resistance and forms a stable oxide; bare parts weather acceptably and anodising improves it further. The copper in 7075 makes it markedly less resistant — it pits and stains in salt air, and it is more prone to galvanic attack when fastened to dissimilar metals. It also sits further from carbon fibre in the galvanic series than 6061 does, so in a bonded or fastened assembly with CFRP, 6061 is the more compatible partner. For any part that will live outdoors, 7075 needs a real coating — a conversion coat or an anodise — and that is a process and cost line, not a cosmetic one.
Weldability: usually a one-line answer
6061 is routinely welded by TIG and MIG. The heat-affected zone softens toward the as-welded condition, and a post-weld solution treatment and artificial age restores most of the strength — a standard, quotable operation. Fusion welding of 7075 is generally avoided: the heat-affected zone is crack-sensitive, and an arc-welded 7075 joint is a specialist procedure with real scrap risk. If the prototype is a welded frame, the alloy decision has already been made for you.
Anodising behaves differently
Both alloys anodise, but not identically. 6061 takes a clear, even colour — including deep blacks — because the layer is chemically uniform. The copper and zinc in 7075 darken and mottle the finish, so a decorative anodise on 7075 needs a trial run against an approved sample before the batch goes to the bath. Where anodising genuinely earns its cost on 7075 is hard coating: the thick, dense layer adds wear resistance to an alloy that already has the strength. Decide which of the two jobs you need — colour or wear — before specifying the process; the anodising overview covers the process types and their trade-offs.
Machining: 7075 costs more per minute
6061 cuts freely: chips break cleanly, tools last and feeds can be pushed. 7075 is harder and work-hardens at the shear zone — at aggressive parameters it smears rather than shears, builds up on the cutting edge and shortens tool life. Expect more conservative speeds and feeds, more tool changes and a cycle-time penalty on identical geometry. The setup structure — how many orientations the part needs — still dominates the quote, as the CNC cost breakdown sets out, but the per-minute rate moves against you in 7075. Stock price compounds it: 7075 bar and plate commonly cost two to three times 6061 per kilogram, and the difference shows up in the quote line before the part reaches the machine.
One shared point regardless of alloy: both hold the same tolerance classes when machined properly, and the tolerance callout on the drawing is a bigger driver of price than the alloy choice. The ISO 2768 tolerance guide covers how to title-block the general case and call out only the features that carry function.
A decision order for a specific part
Work down this list and stop at the first line that fires:
- Will the part be welded? Use 6061. Fusion welding of 7075 is not a prototype-friendly process.
- Is the part stiffness-limited rather than strength-limited? Use 6061 — the moduli are within about four per cent.
- Does the part live outdoors, near salt, or under sustained tensile load in humidity? Use 6061, or 7075 in the T73 temper with a coating. 7075-T6 in through-thickness tension outdoors is a stress corrosion risk.
- Is the visible surface anodised for colour? Use 6061 for a uniform finish; run a trial if it must be 7075.
- Does a 6061 section that still fits the envelope carry the load? Use 6061 and bank the saving.
- Only when the part is genuinely strength-limited and mass is the budgeted constraint — a load-bearing arm, a bracket where every gram counts — does 7075 earn its premium. Add hard anodising if the surface sees wear.
When 7075 is not worth it
In practice, most prototype brackets, housings, fixtures and enclosures never clear the bar. A 6 mm 6061 wall where a 5 mm 7075 wall would do is usually the cheaper and tougher part: more material, better corrosion resistance, easier welding, faster machining and less scrap risk. The premium buys strength the design may not be able to use, and it takes away corrosion resistance and machinability that were free. If a drawing says 7075 out of habit rather than calculation, ask which feature is load-limited. Often the honest answer is none, and the decision collapses back to 6061.
The hybrid that mixed-duty parts need
A common resolution on assemblies with mixed duty: the load path in 7075, everything else in 6061. Machine both in the same run — setup and programming overhead is shared, and the prototype tests the real stiffness and strength distribution instead of a uniform approximation. The 6061 material page and the 7075 material page give the machining and finishing specifics for each alloy, and the DFM design guide covers the geometry rules — fillet radii, wall thickness, datum choice — that matter in either.
Alloy selection is a calculation, not a preference. If the load path is unclear, send the STEP file and the drawing to the instant quote page: an engineer reviews the features that actually carry load, says where 6061 is sufficient and where 7075 earns its cost, and returns a firm quote with DFM notes within 24 hours. Parts are machined at ISO-certified partner factories in Dongguan and Shenzhen; the quality and NDA page describes the inspection records you can ask to see before ordering.