Material is often chosen by habit — “make it stainless” — and paid for on every part. The right choice balances strength, weight, corrosion resistance, machining cost and the finish you need. Getting it wrong shows up as parts that are heavier, more expensive or slower to make than necessary, or that corrode in the field.
Quick comparison of common grades
| Grade | Typical strength | Corrosion | Machinability | Typical use |
|---|---|---|---|---|
| Aluminum 6061-T6 | Medium (yield ≈ 275 MPa) | Good; anodize for more | Excellent | General structural parts, housings, brackets |
| Aluminum 7075-T6 | High (yield ≈ 500 MPa) | Moderate | Good | Highly loaded light parts, robot links |
| Carbon steel 1045 / C45 | Medium–high; heat-treatable | Poor — needs coating | Good | Shafts, gears, wear parts |
| Alloy steel 4140 / 42CrMo4 | High; heat-treatable | Poor — needs coating | Fair | Highly loaded shafts, tooling |
| Stainless 303 | Medium | Good | Very good (free-machining) | Turned parts, fittings |
| Stainless 304 | Medium | Very good | Fair | Food, outdoor, general corrosion resistance |
| Stainless 316 | Medium | Excellent (chlorides, marine) | Fair–poor | Marine, chemical, medical environments |
Strength figures are typical handbook values for the stated temper and vary with product form and supplier; use the material certificate and your own design margins for engineering decisions. Density for reference: aluminum ≈ 2.7 g/cm³, steels and stainless ≈ 7.8–8.0 g/cm³ — roughly three times heavier.
Decision guide
- Weight matters (robots, drones, handheld devices) → aluminum; 7075 when stiffness and strength are critical.
- Wear, hardness or high loads in small sections (shafts, gears, pins) → heat-treated carbon or alloy steel, with a protective finish.
- Corrosive, wet, food or outdoor environments → stainless 304; 316 for salt spray, marine or chemicals.
- Many turned parts in stainless → 303 machines faster, if its slightly lower corrosion resistance and weldability are acceptable.
- Cosmetic colored parts → aluminum with anodizing (see aluminum finishes).
Cost: material price is only half the story
Machining time often costs more than the raw material. Aluminum cuts much faster than stainless, so an aluminum part can be cheaper even when it needs anodizing. Stainless 316 and hardened steels are slow to machine and wear tools faster. See CNC machining cost drivers.
Pitfalls buyers run into
- Galvanic corrosion — aluminum fastened directly to stainless in a wet environment can corrode; isolate with washers or coatings.
- Equivalent grades — Chinese GB grades are often equivalent to ASTM/EN grades but not identical; state the standard and ask for certificates.
- “Stainless” is not one material — 201 and 304 look the same but behave very differently outdoors. Specify the grade.
- Heat treatment and distortion — hardening after machining can distort parts; plan finishing passes or grinding for precise features.
Not sure which material fits your part? Send the drawing and the operating conditions — we will suggest options with their cost impact. More on CNC machining.