Guide
Is 6061 or 7075 aluminum better for CNC parts?
6061 aluminum is the general-purpose heat-treatable wrought alloy of the aluminum-magnesium-silicon family, and 7075 aluminum is the high-strength alloy of the aluminum-zinc-magnesium-copper family. For most machined parts 6061 is the right default. 7075 earns its higher material price only when the part is limited by strength, not by stiffness, and when the design can live with its weaker corrosion and welding behavior.
By Saola Manufacturing engineering team · Published · Updated
This guide is for mechanical engineers and sourcing managers deciding which alloy to put in the material block of a machined-part drawing. It takes every chemistry figure from the Aluminum Association’s registration record and every strength figure from the military design handbook that published allowables for both alloys, then works through the questions that actually move the decision: strength, stiffness, distortion, stress corrosion, anodizing, welding and whether a die casting would beat both.
On this page
What is the real difference between 6061 and 7075?
The difference is the main alloying element: 6061 is hardened by magnesium silicide, while 7075 is hardened by zinc and magnesium with a copper addition. That zinc-copper chemistry is what gives 7075 its strength, and it is also what costs it corrosion resistance and weldability.
Both alloys were registered with the Aluminum Association in 1954 and both are listed in its “Teal Sheets”, the free registration record for wrought alloy designations (Aluminum Association, revised December 2025). The same record gives the international chemical symbols that European drawings sometimes use: AlMg1SiCu for 6061 and AlZn5.5MgCu for 7075. Mill certificates from Asian suppliers often print the grade with a national prefix, such as A7075 under the Japanese system, but the four-digit number refers to the same registered chemistry.
| Element or property | 6061 | 7075 |
|---|---|---|
| Zinc (Zn) | 0.25 | 5.1–6.1 |
| Magnesium (Mg) | 0.8–1.2 | 2.1–2.9 |
| Copper (Cu) | 0.15–0.40 | 1.2–2.0 |
| Silicon (Si) | 0.40–0.8 | 0.40 |
| Iron (Fe) | 0.7 | 0.50 |
| Chromium (Cr) | 0.04–0.35 | 0.18–0.28 |
| Manganese (Mn) | 0.15 | 0.30 |
| Titanium (Ti) | 0.15 | 0.20 |
| Others, each / total | 0.05 / 0.15 | 0.05 / 0.15 |
| Aluminum | Remainder | Remainder |
| Nominal density, lb/in³ (g/cm³) | 0.098 (2.70) | 0.101 (2.81) |
How much stronger is 7075 than 6061?
In plate around one inch thick, 7075-T651 has roughly twice the design yield strength of 6061-T651, and the more corrosion-resistant 7075-T7351 still has about one and a half times as much. The table gives the handbook values so you can check the ratio for your own section.
The figures come from MIL-HDBK-5J, the last public edition of the military handbook of design allowables for aerospace metals, dated 31 January 2003 and since succeeded by the MMPDS series. It is free to download and still the most accessible primary source that lists both alloys on the same basis. A-basis values are the level at least 99 percent of the material is expected to meet, which is a sensible way to compare alloys; typical catalog values run higher.
| Property | 6061-T651 | 7075-T651 | 7075-T7351 |
|---|---|---|---|
| Plate thickness band, in | 0.250–2.000 | 0.500–1.000 | 0.500–1.000 |
| Ultimate tensile strength, ksi | 42 | 78 | 69 |
| Tensile yield strength, ksi | 35 | 68 | 57 |
| Elastic modulus, 10³ ksi | 9.9 | 10.3 | 10.3 |
| Density, lb/in³ | 0.098 | 0.101 | 0.101 |
| Stress-corrosion rating, short transverse (plate) | A (highest) | D (lowest) | A (highest) |
| Arc weldability rating | A (readily weldable) | C (limited) | C (limited) |
Two things in the handbook matter more for machined parts than the headline numbers. First, strength falls with plate thickness, and faster for 7075: per MIL-HDBK-5J, the A-basis long-transverse yield of 7075-T651 drops from 68 ksi in the one-inch band to 54 ksi in the 3.501–4.000 inch band, while 6061-T651 holds 35 ksi out to 6 inches on an S-basis. Second, the plate tables are indexed by the thickness the plate was supplied in, which is the thickness it was quenched at (the handbook spells out the quench-thickness rule in so many words only for extrusions). In practice, a thin web cut from 4-inch plate still carries 4-inch plate properties, so size the stock, not just the finished part, when you check the margin.
Will switching to 7075 make a part stiffer?
Barely. The two alloys have almost the same elastic modulus, so a bracket, frame or fixture plate that deflects too much in 6061 will deflect almost as much in 7075.
Per MIL-HDBK-5J the tensile modulus is 9.9 thousand ksi for 6061 plate and 10.3 thousand ksi for 7075 plate, a gain of about 4 percent. Density rises by a similar amount, so stiffness per pound is a wash. Most machined aluminum parts in industrial equipment are sized by deflection, vibration or thread and bearing surfaces rather than by yield, and for those parts 7075 buys nothing that deeper sections or ribs would not buy more cheaply.
The cases where 7075 does pay are the ones where stress, not deflection, sets the section: highly loaded lugs and clevises, thin-walled structural parts where weight is at a premium, tooling and fixture components that see impact, and parts that must be small because space is fixed. If you cannot name the load case that 6061 fails, 6061 is probably the right call.
Which alloy machines better and holds its shape after machining?
Both machine well in their standard stress-relieved tempers, and 7075 typically gives shorter chips and a cleaner finish because it is harder. Distortion depends more on the temper and the amount of material removed than on the alloy, so order the stretched T651 or T7351 temper for plate and the equivalent T6511-type temper for extruded bar.
The last two digits in T651 and T7351 are the part that protects your flatness. Per MIL-HDBK-5J, the temper designation T_51 means the plate or bar was stretched after solution heat treatment, by 1½ to 3% permanent set for plate, to relieve the quench stresses, and received no further straightening. Plain T6 plate has not been through that step, and a part that removes most of its stock from one side can bow noticeably when those stresses are released. A drawing that just says “6061-T6” leaves the choice to the supplier; “6061-T651 per ASTM B209” does not.
Machining practice is otherwise similar. Softer tempers such as O and T4 are gummy and hard to finish in either alloy, so avoid them for machined parts unless the part is heat treated after machining. For flatness- or tolerance-critical work, the CNC milling page describes how prismatic parts are fixtured and inspected, and the tolerance guide covers how to write general tolerances so they do not over-constrain either alloy.
How do stress corrosion, welding and anodizing change the choice?
All three favor 6061. 7075-T6 and T651 carry the lowest stress-corrosion rating across the grain, 7075 is rated only of limited arc weldability, and 7075 is excluded from chromic-acid anodizing unless the drawing allows it.
Stress corrosion is the one that bites machined parts. The handbook rates 7075-T6 plate, rod, bar, extrusions and forgings D in the short-transverse direction, meaning cracking has occurred in service or would be expected under any sustained tensile stress (MIL-HDBK-5J, Table 3.1.2.3.1(a)). Section 3.1.3.1 of the same handbook says press or shrink fits, taper pins, clevis joints where bolt tightening bends the lugs, and straightening or assembly operations that leave sustained surface tensile stress should be avoided in 7075-T6 and T651. If a part will hold a pressed bearing, a dowel or a bushing, specify 7075-T7351 or 7075-T73, which the same table rates A in every direction, or stay with 6061-T6, which is also rated A throughout. The T73 temper gives up some strength for that resistance, as the table above shows.
Welding follows the same pattern: the handbook rates 6061 A for inert-gas arc welding and 7075 C, where C means crack sensitivity, loss of corrosion resistance or loss of properties may occur. If a machined part will later be welded into an assembly, 7075 is the wrong alloy.
Both alloys take sulfuric (Type II) and hard (Type III) anodize to MIL-A-8625F, which requires heat-treatable alloys to be in their heat-treated temper before anodizing. Chromic (Type I) coatings are not applied, unless the contract or drawing says otherwise, to alloys whose total nominal alloying content exceeds 7.5 percent, and 7075’s registered zinc, magnesium and copper minimums alone add up to more than that. As typical guidance, expect a slightly different shade when 6061 and 7075 parts are anodized side by side in one assembly; if color match matters, keep the visible parts in one alloy. The finishing comparison and the surface finishing page cover the coating choice itself.
When is 7075 worth the higher material cost?
7075 is worth it when a stress calculation, not habit, shows that 6061 lacks margin and the part cannot simply be made thicker. In every other case, 6061 is cheaper to buy, easier to source in every bar and plate size, and more forgiving in service.
7075 bar and plate generally sell at a premium to 6061, and the gap moves with the market, so check current mill or service-center pricing rather than trusting a ratio from an old article. The material premium is usually a small share of a machined part’s price, which is why the decision should rest on the engineering questions above, not on cost alone. A practical checklist for the drawing:
- State alloy, temper and product specification together, for example 7075-T7351 plate per ASTM B209 or AMS 4078, or 6061-T6511 extruded bar per ASTM B221.
- For turned parts from round bar, say whether rolled or cold-finished bar to ASTM B211 or extruded bar to ASTM B221 is acceptable.
- Ask for a mill test report with every shipment that names the alloy, temper and specification, and match it to the heat or lot on the parts.
- Treat any temper swap, such as T6 for T651 or T651 for T7351, as a deviation that needs written approval.
That last point matters most when sourcing from a new region. A supplier in Vietnam or elsewhere in Asia may buy bar from several mills, and the cheapest stock on the market is not always the temper on your drawing. The RFQ guide shows where these notes go in the package, and the CNC turning page covers bar-fed turned parts.
When does a die casting beat both wrought alloys?
A die casting beats both when the part is a housing, cover or bracket made in volume, its strength need is modest, and most of its shape can come out of the die with only bores, threads and datum faces machined afterward.
A machined 6061 or 7075 part starts as a block and pays for every cubic inch of chip removed. A die-cast part pays once for the tool and then comes out close to net shape, so the break-even depends on quantity, geometry and how much machining is left. The trade-offs are real: die-casting alloys such as ADC12 have far lower yield strength and ductility than either wrought alloy in its peak temper and are normally used as cast rather than heat treated. Their silicon and copper also make ADC12 not a good anodizing alloy, as the ADC12 vs A380 guide explains. If a part needs a bright anodized finish or 7075-level strength, stay wrought. If it is a stiffness-limited enclosure made in quantity, send the drawing for a die casting review alongside the machined version.
Frequently asked
Is 7075 harder to machine than 6061?
Not in practice. In the T6-type tempers both cut cleanly, and 7075 often gives the better surface finish. The bigger machining risk in either alloy is distortion from using an unstretched temper.
Can I substitute 7075 for 6061 without changing the drawing?
Not without checking three things: whether the part is welded, whether it holds press fits or pins under sustained load, and whether it is chromic-acid anodized. Any one of them can make 7075-T651 the worse choice even though it is stronger.
What is the difference between 7075-T651 and 7075-T7351?
T7351 is overaged to trade some strength for much better stress-corrosion and exfoliation resistance. Both are stretched after quenching, so both machine with similar stability.
Is 6061 good enough for structural brackets?
Usually, yes. Most brackets are limited by deflection, and the two alloys have nearly the same stiffness, so 6061 with a slightly deeper section is often the lighter and cheaper design overall.
Where do I send a drawing in either alloy?
Send it through the quote form or to rfq@saolamfg.com as drawn, with the temper and product specification stated, and say whether the alloy is fixed or open to a proposal.
Sources
- International Alloy Designations and Chemical Composition Limits for Wrought Aluminum and Wrought Aluminum Alloys (Teal Sheets), The Aluminum Association, revised December 2025
- MIL-HDBK-5J, Metallic Materials and Elements for Aerospace Vehicle Structures, Department of Defense, 31 January 2003 (sections 3.1.2, 3.1.2.3, 3.1.3, 3.6.2 and 3.7.6)
- MIL-A-8625F, Anodic Coatings for Aluminum and Aluminum Alloys, Department of Defense, September 1993
- ASTM B209/B209M-21a, Aluminum and Aluminum-Alloy Sheet and Plate, ASTM International, 2021 (scope)
- ASTM B211/B211M-19, Aluminum and Aluminum-Alloy Rolled or Cold Finished Bar, Rod, and Wire, ASTM International, 2019 (scope)
- ASTM B221-21, Aluminum and Aluminum-Alloy Extruded Bars, Rods, Wire, Profiles, and Tubes, ASTM International, 2021 (scope)