Direct answer: For most general CNC aluminum parts, stay on 6061-T6 or 6061-T651. Move to 7075-T6 / 7075-T651 when the design is strength-limited — yield, ultimate, bearing, or a mandated aerospace pedigree — not when the part is floppy under load, needs matching dyed Type II cosmetics, or when you simply want the stronger alloy on the print. If extreme flatness after heavy pocketing matters more than structural strength, look at MIC-6 or an equivalent cast tooling plate before forcing 7075.
This guide goes past the first-filter comparison: strength is not stiffness, what the temper digits mean, Aluminum Association typicals and why they are not for design, Type II and Type III implications, a proportionate stress-corrosion note, and the exits to MIC-6 or 2024.
Quick decision snapshot
| Situation | Prefer |
|---|---|
| Brackets, housings, general structural CNC with moderate loads | 6061-T6 / T651 |
| High-stress fittings, strength-limited weight reduction | 7075-T6 / T651 (confirm finish and environment) |
| Deflection or stiffness problem | Geometry change — not a 7075 swap |
| Branded dyed Type II color match | 6061 |
| Heavy plate machining with tight flatness, low structural need | MIC-6 or cast tooling plate |
| 7075 under sustained tension in humid or chloride service | Consider 7075-T73 / T7351, or redesign stress and coating |
Tempers you will actually see on prints
6061 (UNS A96061) is the workhorse Al-Mg-Si CNC alloy; 7075 (UNS A97075) is the classic high-strength Al-Zn-Mg-Cu aerospace alloy. For use-case lists, see the 6061 and 7075 pages.
| Temper | Meaning | CNC relevance |
|---|---|---|
| T6 | Solution heat-treated and artificially aged | Common peak-strength callout for bar and general parts |
| T651 | Solution heat-treated, stress-relieved by controlled stretching, then artificially aged | Preferred for flatness-critical plate machining |
| T6511 / T6510 | Extrusion and drawn-tube stretch-relieved family; T6511 allows minor corrective straightening after stretch | Match to extruded stock |
| T73 / T7351 | Overaged for improved stress-corrosion resistance; T7351 adds stretch relief | For 7075 where SCC environment and sustained tension matter |
The Aluminum Association treats each temper designation as a unique processing state. Do not assume free substitution of T6, T651, or T6511 on the certificate unless your quality system explicitly allows it.
Strength is not stiffness, and that distinction saves money
Typical moduli are about 68.9 GPa for 6061-T6/T651 and 71.7 GPa for 7075-T6/T651 — close enough that swapping to 7075 rarely fixes a deflection-limited design. Densities are also close, roughly 2.70 against 2.81 g/cm³. What changes meaningfully is strength and hardness: typical 7075-T6 yield is on the order of 1.8 times that of 6061-T6.
| Effect | 6061-T6/T651 | 7075-T6/T651 |
|---|---|---|
| Load before yield | Baseline | Much higher typical yield |
| Deflection under load | Nearly the same | Nearly the same |
| Thread and bearing capacity | Adequate for many brackets | Higher shear and bearing capacity |
| Bare-metal surface hardness | Softer, around HB 95 typical | Harder, around HB 150 typical |
| Welded assemblies | Far more practicable | Generally avoid fusion welding for structural use |
| Cosmetic dyed anodize | Preferred | Often compromised color and uniformity |
If the part is oversized because of yield, 7075 can shrink the section or raise the margin. The 7075 robotic arm bracket we machine is exactly that case: the web is thinned for moving mass while the bearing bores stay accurate, so the alloy is doing work a geometry change could not. If it is oversized because of deflection, change thickness, ribs, or span — not the alloy family alone.
Property comparison
These are typical values, not certified minima. Minima depend on form, thickness, temper, and the governing ASTM or AMS specification.
| Property | 6061-T6 / T651 (typical) | 7075-T6 / T651 (typical) |
|---|---|---|
| Density | 2.70 g/cm³ (0.0975 lb/in³) | 2.81 g/cm³ (0.102 lb/in³) |
| Ultimate tensile strength | 310 MPa (45 ksi) | 572 MPa (83 ksi) |
| Tensile yield (0.2%) | 276 MPa (40 ksi) | 503 MPa (73 ksi) |
| Elongation | 12% at 1.6 mm thick; 17% at 12.7 mm diameter | 11% at listed conditions |
| Brinell hardness (500 g / 10 mm) | 95 | 150 |
| Modulus of elasticity | 68.9 GPa (10,000 ksi) | 71.7 GPa (10,400 ksi) |
| Shear strength | 207 MPa (30 ksi) | 331 MPa (48 ksi) |
| Fatigue strength (R.R. Moore, 5x10⁸ cycles) | 96.5 MPa (14 ksi) | 159 MPa (23 ksi) |
| Thermal conductivity (about 25 °C) | 167 W/m·K | 130 W/m·K |
Thickness-dependent minimum properties on a mill sheet are not these typicals. Do not use this table as FEA allowables.
Machinability, and why the percentages disagree
Both alloys machine well with aluminum tooling. 7075 is substantially harder — around HB 150 against HB 95 typical — but it is not exotic. Published machinability percentage scales conflict: some rank 7075 higher, while steel-normalized charts often rank the 6061 series higher. Do not treat any single percentage as proof of faster or cheaper machining. Cycle time usually tracks chip evacuation, spindle limits, and engagement more than the alloy label.
What actually moves a quote: stock premium and availability, hardness-driven parameters, distortion scrap risk, cosmetic Type II rejects on 7075, and secondary operations such as masking and post-hardcoat fits.
Anodizing implications
Alloy chemistry affects color, uniformity, and dye response, so a Type, Class, and thickness callout without alloy context is incomplete DFM. For detail, see the anodize checklist and the hardcoat guide.
| Factor | 6061 Type II | 7075 Type II | 6061 Type III | 7075 Type III |
|---|---|---|---|---|
| Intent | Corrosion and appearance | Corrosion; appearance limited | Wear and corrosion | Wear; appearance limited |
| Cosmetic and dye | Excellent clear and dyed | Functional; yellowish-gray or less uniform dye is common | Undyed medium to deep gray; dyed black practical | Undyed bronze-gray or olive; non-black poorly decorative |
| Dimensional growth | About half the coating thickness outward | Same | Critical for fits | Same |
| Sealing | Usually sealed, especially Class 2 | Same | Often unsealed for wear | Same |
| Guidance | Default for cosmetic housings | Avoid where branded color match is critical | Good wear on 6061 | Functional hardcoat is fine; expect a darker cast |
Coating growth is a real dimensional problem, not a footnote — see how it is handled on a hardcoat anodized slide plate, where the wear face is machined with the coating allowance planned in.
7075 can be anodized — the cosmetics differ. Sample Class 2 coupons before production, and do not expect identical black across mixed 6061 and 7075 parts. Chem film against anodize is a separate finish decision.
Corrosion and stress-corrosion cracking
6061 is generally more favorable for general CNC corrosion service; 7075 needs a deliberate coating, cladding, or sealing plan. Stress-corrosion cracking on 7075 is temper- and environment-dependent: peak-aged T6/T651 is the temper most associated with susceptibility in chloride or humid service under sustained tensile stress. Overaged T73/T7351 improves SCC resistance with a documented strength trade-off, commonly discussed at 10 to 15 percent — confirm allowables for your case.
Indoor dry service with low sustained tension is not an automatic failure. Mitigate with coatings, temper choice, and stress-aware design.
Drawing callouts and RFQ essentials
The examples below are illustrative; match the ordered ASTM or AMS specification and product form. Typical plate blocks read 6061-T651 / ASTM B209 / PLATE or 7075-T651 / ASTM B209 / PLATE. For SCC-resistant overaging, call 7075-T7351 with an explicit temper note.
Include in the RFQ: alloy, temper, form, and material specification; finish Type, Class, thickness, seal, color, and masking; datums, flatness, and whether dimensions apply after coating; approved alternates and welding notes; and the service environment if SCC is a concern.
Before approving 7075, confirm that the failure mode is strength rather than deflection, that the finish and cosmetics are accepted, that the environment has driven an explicit temper choice, that stock form matches the temper digits, that a flatness plan exists, and that the total cost includes finish and stock risk.
Decision framework
| Driver | Favor 6061 | Favor 7075 | Favor an alternative |
|---|---|---|---|
| Strength | Moderate loads | Strength-limited weight saving | 2024, or a mandated 7xxx |
| Stiffness | Geometry fix; 6061 is fine | Not for stiffness alone | Ribs, thickness, inserts |
| Cosmetics | Strong preference | Weak | One alloy family for visible parts |
| Corrosion and SCC | Prefer 6061 plus finish | Coating or cladding; T73/T7351 if needed | Per application engineering |
| Flatness | 6061-T651 | 7075-T651 if strength is needed | MIC-6 where strength can be low |
| Welding and cost | Prefer 6061; lower total-cost default | Avoid welding; justify the strength need | Bolted design; do not buy margin just in case |
When the 7075 premium pays off
- Is the part strength-limited rather than deflection-limited? If not, stay on 6061 or redesign the geometry.
- Does cosmetic Type II color dominate? Prefer 6061.
- Are environment and sustained tension a 7075-T6 SCC concern? Consider T73/T7351, or stay on 6061.
- Is flatness after heavy machining critical while structural need is low? Consider MIC-6 before forcing 7075.
- Only if the strength need is real and the finish and corrosion plan is acceptable, move to 7075-T6/T651 or the aerospace temper on the drawing.
On cost and lead time: 6061 usually has broader common-size availability, while odd 7075 sections or tempers such as T7351 can extend the materials path. The finishing stack often dominates the calendar. Validate days and cost for your specific size and temper rather than quoting universals.
Alternatives: MIC-6 and 2024
MIC-6 and equivalents suit flatness-critical fixtures and heavily pocketed plates where strength can be low. Brochure typicals, historically around 24 ksi tensile and 15 ksi yield, are not a structural substitute for 6061-T6. See flatness control for the machining side, and a MIC-6 precision fixture plate for a plate that stays flat through heavy pocketing.
2024, often in T351, suits work where fatigue pedigree is required and corrosion protection is defined. It is not a cosmetic default — see the 2024 temper, distortion and finish guide before committing a temper. Where an aerospace drawing mandates a specific 7xxx alloy, follow the drawing rather than defaulting to 7075-T6.
Common mistakes
- Assuming 7075 is meaningfully stiffer. The moduli are nearly the same.
- Treating T6, T651, and T6511 as freely interchangeable. Each temper is a unique state.
- Using a stronger alloy to fix a floppy part. Fix the geometry if the part is deflection-limited.
- Expecting identical black anodize on 6061 and 7075.
- Ignoring hardcoat growth on fits and threads.
- Framing all 7075 as an SCC risk. Risk is environment, tension, and temper together.
- Treating MIC-6 as structurally equivalent to 6061-T6.
- Using typical values as FEA allowables, or buying 7075 just in case.
FAQ
Will switching to 7075 stop my part from bending?
Usually no. The moduli are nearly the same, around 69 to 72 GPa. Fix thickness, ribs, or spans instead.
Why do machinability percentages disagree?
They come from different scales. Some rank 7075 higher; steel-normalized charts often rank the 6061 series higher. Treat cycle time as process-specific.
T6 or T651 for plate, and can I substitute T6511 for T6?
For flatness-sensitive plate, specify T651. Accept T6511 against a T6 print only if your quality system explicitly allows it, because each temper is treated as a unique state.
When is the 7075 premium wasted, and when do MIC-6 or 2024 win?
The premium is wasted on deflection-limited or cosmetic parts, and on unused just-in-case margin. MIC-6 wins where flatness dominates and strength can be low; 2024 wins where fatigue pedigree is mandated and corrosion protection is defined.
Conclusion
6061-T6/T651 remains the default. 7075-T6/T651 earns its keep when the part is strength-limited and the finish and corrosion plan is honest — not for stiffness, not to match dyed black, and not for unused margin. Lock alloy, temper, form, and finish on the print; use MIC-6 where flatness dominates; and follow 2024 or a mandated 7xxx where aerospace pedigree requires it.
Send your drawing for an alloy, temper, and finish sanity check before the quote locks in an over-specified material.