Key takeaway: Most machined parts can be made from one of three aluminum alloys. Pick 6061 when you want a safe default, 7075 when strength per gram decides the design, and 5052 when the part lives in salt water. Everything else — cost, finish, tolerance, lead time — follows from that first choice.
Choosing between aluminum CNC machining materials is the first real decision on a custom part. It is made before the first tool path, and it is harder to undo afterwards. The alloy sets how fast the metal cuts, how much it moves during machining, whether it can be hardened, how it looks after anodizing, what it costs per kilogram, and whether the bar is on a shelf this week or eleven weeks away.
This guide is written for engineers, buyers and product designers who need to specify aluminum CNC machining materials and defend that choice in a design review. It explains the numbering system, walks through the alloy families, compares the grades people actually machine, and finishes with a decision framework you can apply to a drawing.
Two limits on scope. Numbers given here are typical published ranges for the alloy, not a promise about what one shop will achieve on your part. And no supplier capability claim is made — ask any shop for its own data before you design around it.
How aluminum alloys and tempers are numbered
Aluminum is not one material. The industry uses a four-digit number to say what the metal is mostly alloyed with, and a letter-number temper to say what has been done to it afterwards. Both matter to a machined part.
The first digit defines the family
The first digit names the main alloying element. Pure aluminum is the 1xxx series. Copper makes the 2xxx series strong but vulnerable to corrosion. Manganese defines 3xxx. Silicon defines 4xxx and is common in cast alloys. Magnesium alone gives 5xxx. Magnesium plus silicon gives 6xxx, the workhorse series. Zinc gives 7xxx, the strongest group.
The temper code matters as much as the alloy
The temper says how the metal has been treated. The codes you will meet on a quotation are these:
- O — annealed, soft, maximum formability, poor rigidity.
- H (H32, H34) — hardened by working the metal, not by heat. Common in sheet and plate of the 5xxx series.
- T5 / T6 — solution heat treated and artificially aged. Strong, but internal stress remains.
- T651 / T7351 — the same treatment plus stress relief by stretching (the “5” after the T). This is what you want for plate that will be milled into a pocketed part.
Here is the practical consequence. Order 6061-T6 plate and mill a deep pocket pattern into it, and the metal can move as the clamps come off, because cutting released the stress that was locked in the plate. Order 6061-T651 and much of that stress has already been removed at the mill. Same alloy, same strength, very different behaviour on the machine.
Why this is worth ten minutes of reading: alloy and temper are the two words that let a machinist quote your part correctly. A drawing that says “aluminum” gets you a guess. A drawing that says “6061-T651” gets you a plan.
The alloy families, one by one
This table covers the grades a machine shop is likely to stock. Read the last column first — it is the honest answer to “should I specify this?”
| Series / alloy | Main alloying element | Typical temper | What it is good at | Watch out for | Verdict for machined parts |
|---|---|---|---|---|---|
| 1100 | Nearly pure aluminum | O, H14 | Conductivity, corrosion resistance, lowest cost | Soft and gummy; edges smear, holds no shape under load | Rarely. Electrical or chemical use only |
| 3003 | Manganese | H14, H32 | Forming, corrosion resistance, cheap | Low strength; not heat treatable | Sheet brackets and enclosures, not structural parts |
| 5052 | Magnesium | H32, H34 | Salt-water corrosion resistance, weldability | Gummy machining, poor chip break, work hardens | Yes for marine and fluid-handling parts |
| 6061 | Magnesium + silicon | T6, T651 | Balanced strength, corrosion resistance, weldability, availability | Not the strongest; duller anodized colour than 6063 | The default. Start here |
| 6063 | Magnesium + silicon | T5, T6 | Best appearance, common in extrusions | Lower strength than 6061 | Yes when looks matter more than load |
| 7075 | Zinc | T6, T651, T7351 | Highest practical strength-to-weight | Poor corrosion resistance without protection; harder on tooling; dull grey anodize | Only when the load case demands it |
| 2024 | Copper | T3, T351 | High strength, good fatigue behaviour | Corrodes easily unless clad or coated; hazardous dust | Aerospace structures, with protection |
| C355 / A356 | Cast Al-Si-Mg | T6 (cast) | Complex near-net shapes | Porosity at machined surfaces; as-cast material machines badly | Yes when a casting is the right blank |
Copper-bearing alloys: strong, and hungry for protection
The 2xxx series trades corrosion resistance for strength. Copper improves the achievable hardness, but it also makes the metal electrochemically active. Parts are usually clad, painted or otherwise protected, and the chip dust is treated as a hazardous material because of the copper content. Specify 2024 only when a design genuinely needs its fatigue behaviour and the environment is controlled.
Cast alloys: a different conversation
A machined part often starts life as a casting that is then finish-machined where seals or bearings sit. Sand-cast and permanent-mould alloys such as C355 and A356 machine well after solution treatment; as-cast material is soft and gummy. The risk with castings is porosity, which appears as small holes in a machined face and can fail a leak test. Ask for the casting to be X-ray or section-checked if a machined surface seals fluid.
The three grades you will actually use
Roughly most custom machined aluminum work falls into three alloys. Learn these and you can specify the right material on the first pass.
6061: the default
6061 is the starting point for aluminum CNC machining materials because it is a compromise that rarely fails. It is moderately strong, weldable, corrosion resistant, machinable, cheap relative to specials, and available everywhere as plate, bar, round and extrusion. When a designer says “make it in aluminum,” this is usually what the shop will buy.
Use T651 for anything milled from plate. Use T6 round bar for pins and shafts. Weld it with 4043 or 5356 filler depending on whether you will heat-treat afterwards.
7075: when grams are the constraint
7075 is significantly stronger than 6061, and it is chosen when the part must carry a load at minimum mass. Racing hardware, aircraft fittings and high-load structural brackets live here. The costs come with it: the material is dearer, it wears tooling faster, its corrosion resistance is worse without treatment, and the anodized colour is grey and dull because of the zinc.
Do not specify 7075 just because the number looks impressive. If 6061 is stiff enough at the same wall thickness, you have bought yourself cost and lead time for nothing.
5052: the marine and sheet choice
5052 resists salt water better than the structural alloys and welds well. It arrives as sheet and plate in H tempers. Its weakness in the machine is that it is soft and gummy: chips want to weld to the tool, and edges can roll instead of cutting clean. It rewards a sharp tool, a higher feed and generous coolant.
Two others worth knowing
6063 is the appearance alloy. It anodizes brighter and cleaner than 6061 and is the usual choice for extruded housings and decorative frames. 2024 appears in aerospace structures where fatigue performance matters more than corrosion resistance, and it should not be specified without a protection plan.
Strength, weight and stiffness: what the numbers mean
Buyers compare alloys by strength. Designers should compare them by stiffness and by section. This is where intuition fails.
All aluminum alloys have the same stiffness
Young’s modulus for aluminum sits near 69 GPa regardless of alloy. Copper, zinc and magnesium change yield strength a great deal, and stiffness not at all. A 7075 part is not stiffer than a 6061 part of identical shape. It is stronger, meaning it will take more load before it yields, but it deflects just as much under the same load.
The consequence for design is important. If the part is deflecting, changing the alloy will not fix it. Add a rib, increase the section, change the geometry, or accept a heavier part in a stiffer metal.
Typical property ranges
| Alloy and temper | Yield strength (typical) | Density | Corrosion resistance | Machinability feel |
|---|---|---|---|---|
| 6061-T6 / T651 | mid-hundreds of MPa class | 2.70 g/cm³ | Good | Clean, free cutting |
| 7075-T6 / T651 | Clearly higher than 6061 | 2.81 g/cm³ | Poor without protection | Firmer, more tool wear |
| 2024-T3 | High, good fatigue behaviour | 2.78 g/cm³ | Poor without cladding | Gummy, copper smear |
| 5052-H32 / H34 | Modest, work hardens | 2.68 g/cm³ | Excellent in seawater | Soft, stringy chips |
| 6063-T5 / T6 | Lower than 6061 | 2.70 g/cm³ | Good | Very clean, best appearance |
| 1100-O | Low | 2.71 g/cm³ | Excellent | Very gummy |
The table deliberately avoids quoting exact figures. Published values move with plate thickness, quench rate and temper, and a design should be checked against the mill certificate for the actual lot, not against a rounded number copied from a website.
Weight saving is real, and it is not free
Aluminum is about one third the density of steel. To carry the same load in a stiff panel you generally need more material thickness, and the mass still ends up lower. What you give up is stiffness per unit thickness, which is why aluminum parts are ribbed more often than steel ones.
Machinability: how each alloy behaves at the cutter
Machinability is where aluminum alloy grades stop being a materials question and become a cost question. A free-cutting alloy can run twice as fast as a gummy one with a better surface, and the difference shows up on your invoice.
Built-up edge is the main enemy
Aluminum welds to the cutting edge at the heat of the cut. A ridge of work material builds on the edge and then breaks off, taking part of the tool with it. The visible symptoms are a rough surface, oversized holes and a tool that looks worn far too early. The countermeasures are sharp positive-rake geometry, polished flutes, adequate feed so the chip carries heat away, and coolant or air applied at the cutting edge.
Chip control differs by alloy
6061 and 6063 break chips reasonably and machine cleanly. 5052, 1100 and the copper-bearing 2xxx alloys are gummy: they produce long stringy chips that wrap the tool and pack into pockets. In a deep pocket, a packed chip re-cuts itself, ruins the floor and can snap the tool. Design the pocket radius generously so the shop can use a larger cutter that clears chips.
Thin walls move
Aluminum parts are usually thin, and thin walls deflect under cutting force. The wall is machined true while it is held, then springs back out of tolerance when the clamp is released. Supporting the wall, taking light finishing cuts and keeping the part rigid until the last operation are planning decisions, not machine limits.
Heat moves the part
Aluminum expands at roughly twice the rate of steel. A part measured warm reads differently from the same part cold, and a machine that has run all morning holds a different size from one just started. For tight dimensions, allow soak time and keep the shop temperature stable. This is also why a tolerance claim should always come with a statement of how the part was measured and at what temperature.
Corrosion and finishing
The alloy choice constrains what you can do cosmetically and protectively afterwards. Two finishes cover most machined aluminum parts.
Anodizing
Anodizing grows a hard integral oxide from the surface. Type II is the standard decorative and protective grade; Type III, hard coat, is thicker and much harder and is used on wear surfaces. Three consequences reach back into your drawing:
- Roughly half the total coating growth sits above the original surface, so holes shrink and shafts grow unless features are masked or machined afterwards.
- Colour is a batch property. Alloy lot, mill direction, preparation and bath age all shift it, and 7075 anodizes greyer than 6061 because of its zinc.
- It hides nothing. Scratches, tool lines and extrusion seams become more visible, not less.
Powder coating and conversion coating
Powder coating lays a thick plastic film, typically far thicker than an anodic layer. It covers large flat geometry well, comes in any colour, and hides minor surface history — at the cost of rounded edges and a risk of chipping. Chemical conversion coating is thin, cheap and keeps some surface conductivity, which matters when a part must ground to a chassis. The trade-off between the two thick-finish options is covered in our piece on anodizing versus powder coating.
Where the alloy decides the finish
If the part will be bright anodized and admired, 6063 or a well-prepared 6061 is the answer and 7075 is not. If the part must be electrically bonded at a machined pad, that pad must be masked or left bare. Decide the finish before the machining plan, not after.
Cost, availability and lead time
Ask a machinist what makes a quote move and the answer is rarely the alloy’s price per kilogram. It is what happens around the metal.
What actually drives the number
- Stock form. Plate, bar and round in 6061 are on the shelf almost everywhere. A 7075 thick plate or a special temper can mean a longer wait and a higher price.
- Setups. Every time the part is re-fixtured, a new locating error enters the stack and the shop must prove it. Fewer setups is the cheapest design change available to you.
- Tolerance and inspection. Tight geometric callouts force CMM time and often a fixture. Inspect what the function needs.
- Thin walls and deep pockets. Slow feeds, support, multiple passes, higher scrap risk.
- Surface callouts and finishing. Hand work, masking, colour lots and shipping between vendors.
- Documentation. Material certificates and first article reports are labour, and they are billed as such.
Availability is a design input
The most common avoidable delay is a drawing that specifies an exotic temper in a size nobody stocks. Before you lock the alloy, ask the shop what it buys routinely. Swapping an uncommon grade for 6061-T651 can be worth weeks, and it is almost never a functional loss.
Quantity changes the answer
Aluminum is usually machined, not cast, at prototype and low volumes because there is no tooling to amortise. As quantity climbs, a casting or an extrusion with machined features can become the cheaper route. That crossover point is specific to your geometry and your volumes, so get it quoted both ways rather than assuming.
A decision framework you can apply to a drawing
Work down the list and stop at the first line that applies. This is the shortest reliable path through aluminum CNC machining materials selection.
- Does the part carry a structural load where mass is the binding constraint? Then 7075-T651, and plan for corrosion protection.
- Is it an aerospace structure with fatigue as the driver? Then 2024, with a coating or cladding plan.
- Does it live in salt water or handle fluids? Then 5052, and expect a gummy cut.
- Is appearance the main requirement, or is it an extruded shape? Then 6063.
- Is the blank a complex near-net shape? Then a cast alloy such as C355 or A356, machined only where it seals or bears.
- None of the above? Then 6061-T651. Specify the temper, not just the alloy.
Now check the drawing against four questions: is every tight tolerance functional, is each pocket radius generous enough for a real tool, is the datum scheme stated, and is the finish decided with its dimensional allowance written down. Those four checks remove most of the arguments that happen after parts are made.
Glossary
- Temper — the condition produced by heat treatment or work hardening, written after the alloy number.
- Solution treated and aged — heated so alloying elements dissolve, quenched, then aged so they precipitate and strengthen the metal.
- Stress relief (the “51” in T651) — stretching the plate after treatment to release internal stress before machining.
- Young’s modulus — stiffness. Nearly the same for all aluminum alloys.
- Built-up edge — work material welded to the cutting edge, which degrades finish and tool life.
- Yield strength — the stress at which the metal begins to deform permanently.
- Cladding — a corrosion-resistant surface layer bonded to a stronger core alloy.
- Mill certificate — the document reporting the tested properties of the delivered lot.
References
- Aluminum Association. Aluminum Standards and Data — alloy composition, tempers and mechanical properties. (aluminum.org)
- ASTM B209 / current successor specification. Standard Specification for Aluminum and Aluminum-Alloy Sheet and Plate. (astm.org)
- SAE AMS4027 and related specifications. Aluminum Alloy Sheets, Plates and Forgings.
- ISO 2768. General tolerances for linear and angular dimensions. (iso.org)
- ASME Y14.5. Dimensioning and Tolerancing. — datum reference frames and geometric controls.
- ISO 7599. Aluminum and aluminum alloys — Anodizing — Specifications for anodic coatings. (iso.org)
- ASTM B580. Standard Practice for Anodic Coatings on Aluminum. (astm.org)
Frequently asked questions
Is 6061 or 7075 better for CNC machined parts?
Neither is better overall. 6061 is cheaper, more available, easier to machine, more corrosion resistant and anodizes nicer. 7075 gives markedly higher strength for a small weight penalty. If the design fails in 6061 on strength, move to 7075; if it fails on stiffness, neither alloy helps, because their modulus is the same.
Can I machine 5052 the same way I machine 6061?
The part will come out, but the parameters should not be identical. 5052 is softer and gummier, so it needs a sharper, more polished tool, a feed high enough to keep the chip moving, and good clearance in pockets. Expect more attention to built-up edge and to edge definition than with 6061.
Does anodizing change the dimensions of a machined part?
Yes. Roughly half of the total coating growth sits on the surface, so bores get smaller and shafts get larger. Hard coat moves considerably more. Specify which dimensions apply before coating, mask the features that must stay bare, or plan a finishing cut after anodizing.
Why does my aluminum part move after machining?
Usually residual stress and clamping, rarely the machine. Plate carries locked-in stress from rolling and quenching; cutting removes material that was holding it in balance, and the part relaxes. Specify a stress-relieved temper such as T651, rough and finish as separate operations, and clamp where the part is supported.
Which aluminum alloy should I choose for a marine enclosure?
5052 is the common answer for salt-water exposure and welded fabrication. If the enclosure also carries structural load, 6061 with proper protection is often the better compromise. Decide whether corrosion or strength is the binding requirement before selecting.
Is cast aluminum cheaper than machined aluminum?
At low volumes, usually not, because a casting needs tooling and a machining operation anyway. At higher volumes a casting that is machined only on sealing and bearing surfaces can beat a fully machined block by removing most of the cutting time. Get both routes quoted at your real quantity rather than assuming.
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