An Article Deciphering Aluminum Alloys for CNC Machining

The first digit groups the alloy by its main alloying element. A 1xxx grade is nearly pure aluminum, 2xxx is copper-based, 3xxx manganese, 5xxx magnesium, 6xxx magnesium and silicon, and 7xxx zinc. The last two digits are only a serial number within that family. That number tells you nothing about strength on its own.

The temper suffix is where most design errors start. Annealed material is O. F is as-fabricated, with no thermal treatment. T4 is solution heat treated and naturally aged. T6 is solution heat treated and artificially aged, which is why 6061-T6 is the default structural choice. T651 means the same plus a controlled stretch to relieve internal stress before machining.

Numbers matter more than names here. A part called out as 6061-T6 at 276 MPa yield behaves very differently from 6061-T4 at roughly 145 MPa. Two drawings can both say 6061 and still not describe the same material. Always write the full designation on the print, including the temper.

• 1
  OAnnealed, softest, best formability, worst machined finish
• 2
  T4Natural aging, moderate strength, bends well
• 3
  T6Artificially aged, high strength, standard for machined parts
• 4
  T651Stretched after aging to reduce distortion during milling

6061-T6 covers most of what a machine shop sees. It machines cleanly at 300–600 m/min surface speed with carbide, welds, anodizes evenly, and costs less than the 7xxx grades. Housings, brackets, fixture plates, manifolds and prototype frames all land here by default. Its weakness is wear resistance; a bare 6061 sliding surface will gall.

7075-T6 is the high-strength option at roughly 503 MPa yield, close to some mild steels. Aerospace ribs, bicycle components and high-load tooling use it. Machining is less forgiving: it is notch sensitive, chips break harder, and the material is more prone to residual stress movement when you remove a lot of stock. Rough, stress-relieve, then finish.

2024-T4 machines to a better finish than 7075 and has good fatigue resistance, but it is a copper-bearing alloy and corrodes in a marine or humid environment unless it is anodized or primed. It is also difficult to weld. Aircraft skins and fuselage parts use it for a reason, but a wet industrial enclosure usually should not.

5052 and 5083 are the marine and sheet-metal grades. Strength is modest, corrosion resistance is excellent, and formability is the best of the group. Sheet metal enclosures, fuel tanks, chassis panels and low-pressure covers fit here. Do not expect a 5052 bracket to hold a 500 kg static load at 6061 wall thickness.

ADC12 is a die casting alloy, not a wrought one. Silicon content near 11 percent gives it excellent castability and die life, so it suits high-volume housings and gearbox covers. It does not machine as cleanly as 6061, and porosity can appear under thin walls. For 10,000+ unit runs, ADC12 die casting usually beats milling from plate on cost.

Aluminum cuts fast, which is what makes it attractive and also what causes most scrap. Thermal expansion is roughly 23 × 10⁻⁶ per °C, so a 500 mm plate that warms 10 °C grows about 0.12 mm. On a ±0.005 mm callout, that is the whole tolerance. Rough in the morning, let the part stabilize, then finish.

Chip evacuation decides finish quality more than spindle speed does. Aluminum chips are soft and stick to flutes. Deep pockets and narrow slots need high-pressure coolant or through-tool air blast, plus a ramp or helical entry instead of a straight plunge. A plunging cutter in a blind pocket will pack chips and rub.

Cutting tools should be uncoated or use a polished carbide designed for non-ferrous material. Two or three flutes with high helix angles clear chips better in pockets. For finishing walls, a single-flute cutter reduces contact and chatter. Tool runout above 0.01 mm shows up immediately as a stair-step on a vertical face.

Thin walls are the classic failure mode. A 1.5 mm web in 6061 at 80 mm depth will sing and deflect. Support it with tabs or leave sacrificial stock, take light finishing passes at 0.1–0.2 mm radial depth, and use a cutter with a small nose radius. Removing material equally from both sides of a wall keeps the part balanced.

Anodizing converts the surface into aluminum oxide, so the alloy chemistry matters. 6061 and 6063 take clear, color and hardcoat anodizing evenly. 7075 darkens and can turn bronze or gray in clear anodize because of its copper and zinc content. If the color has to match between two parts, use the same alloy and the same anodizer batch.

Hardcoat anodizing builds a 25–50 μm layer with high surface hardness and is a good choice for sliding wear surfaces. It does reduce fatigue strength slightly and it changes dimensions, so the print must state whether the coating is included in the tolerance. Threads and press-fit bores are often masked.

Electroless nickel, zinc and silver plating work on aluminum but need a zincate pretreatment step, or adhesion will fail. For 2024 and 7075, chemical conversion coating (chromate or a non-chromate alternative) is common as a corrosion base under primer. ADC12 castings can be plated, but casting porosity traps chemicals and causes blistering; seal the surface first.

Bead blasting, tumbling, brushing and polishing are mechanical and mostly alloy-agnostic. 6061 blasts to a uniform matte; 7075 shows more variation because of its grain structure. Laser marking works on anodized surfaces and needs a minimum character height of 1.5 mm to stay legible after handling.