Aluminium, stainless steel, steel, engineering plastics: choosing the right material for your parts

A material is chosen for its function, not out of habit. Five questions quickly shorten the list:

• What loads? Strength, stiffness, hardness, wear, fatigue.
• What environment? Corrosion, temperature, chemicals, food or medical contact.
• What mass? Aluminium weighs about a third as much as steel (2.7 g/cm³ against 7.85), and engineering plastics even less.
• What processing? Welding, heat treatment, surface treatment: not all grades suit them.
• What budget? The price of the material, but above all the machining time it requires.

Describe the function in your request: the CNC machining workshop can confirm your choice or suggest a more economical grade.

Some common trade-offs

• A light bracket, machined quickly: an aluminium alloy rather than a steel.
• A part exposed to sea spray: 316L stainless steel or 5083 rather than S235, even painted.
• A roller or a quiet slide: POM or PA6 rather than a metal, if noise and lubrication matter.

Aluminium machines quickly, takes surface treatments well and lightens parts. The choice comes down to the alloy:

• 6082 or 6061: the default choice. Versatile, weldable, anodisable. 6082 is more common in Europe, 6061 in North America; they are often interchangeable, hence the value of writing “or equivalent”.
• 2017A: stronger and very pleasant to machine, but less protected against corrosion and hardly weldable. Suited to mechanical parts machined from solid.
• 5083: for corrosion resistance, notably in marine environments, and for welding. It is found mainly as plate and gives long chips when machined.
• 7075: when strength comes first (aerospace, highly stressed parts). It is not weldable, costs more and resists corrosion less well: surface protection is often necessary.

The temper (for example T6 or T651) is part of the designation: state it on the drawing.

Steels

• S235: common structural steel, weldable and economical. Its strength is modest and it is not meant to be hardened: base plates, frames, brackets.
• C45: medium-carbon unalloyed steel, which can be hardened and tempered to gain hardness. The classic choice for shafts, pins and mechanical parts; welding it is tricky.
• 42CrMo4: chromium-molybdenum alloy steel, used after hardening and tempering for parts under heavy fatigue loads (shafts, gears). In the treated state it is harder to machine: it is often rough machined, treated, then finished or ground to hold dimensions despite distortion.

Stainless steel

• 304L: the standard stainless steel, low carbon, weldable. It work-hardens during machining, which wears tools and lengthens times compared with steel.
• 316L: molybdenum improves resistance to pitting corrosion, notably in the presence of chlorides: marine, chemical, pharmaceutical, medical. It costs more than 304L.
• 303: enriched with sulphur to ease machining, it suits turned parts in volume. In exchange, its corrosion resistance is lower and welding is not recommended.

Free-cutting brass (CuZn39Pb3) is one of the easiest metals to machine: short chips, a fine surface finish, good conductivity. It is common in fittings, plumbing hardware and contacts. It contains lead: for drinking water or food use, check the applicable requirements; lead-free grades exist.

Titanium (grade 2 or Ti-6Al-4V) combines lightness (about 4.5 g/cm³), high strength, corrosion resistance and biocompatibility. It is difficult to machine, as it dissipates heat poorly and wears tools: allow for a markedly higher material cost and machining time.

Pure copper, prized for its electrical and thermal conductivity (busbars, heat sinks), is soft and gummy: it machines less well than brass.

Engineering plastics are machined from bars, plates or tubes. They expand much more than metals, can creep under load and some absorb moisture: tight tolerances are harder to hold in them (see the guide to tolerances).

• POM: rigid, stable, low friction. The right compromise for precision mechanical parts.
• PA6: resistant to wear and impact, but it swells as it takes up moisture: avoid it for tight fits.
• PEEK: temperature resistance (around 250 °C in continuous use), chemical resistance, sterilisable. A very expensive material.
• PTFE: very low friction and chemical inertness, but soft and prone to creep: poorly suited to precise load-bearing parts.
• PMMA: transparent and rigid, but brittle; it crazes under stress or on contact with certain solvents.
• HDPE: economical, light, good chemical resistance, not very rigid and highly expandable.

Qualitative benchmarks for comparing materials. Relative cost refers to the raw material (€ economical, €€€€ very high); it says nothing about machining time.

The price of a machined part adds up material, machine time and tooling (see how a machining quote is calculated). The material that is cheapest per kilo is therefore not always the cheapest per part: an alloy that machines quickly can offset a higher price per kilo, whereas a stainless steel or titanium that is hard to machine increases machine time.

Two other levers weigh on cost. First, the blank size: a part machined from solid consumes a whole bar or plate, a large share of which ends up as chips. A section close to commercial dimensions reduces waste. Second, the material certificate (for example type 3.1 under NF EN 10204), useful for traceability but to be requested only when necessary.

Materials with high internal stresses (thick plates, plastics) can distort after machining: the workshop then plans a rough cut, stress relief and then finishing, which adds time.

As for availability, common grades (6082, S235, C45, 304L, 316L, POM, PA6) are held in stock by distributors. Special grades, such as titanium, PEEK or 7075 in certain dimensions, may require lead times or minimum quantities.

Use the standardised designation rather than the common name, with the temper or treatment: “EN AW-6082 T6”, “S235JR (1.0038)”, “42CrMo4 +QT (1.7225)”, “X2CrNiMo17-12-2 (1.4404)” for a 316L, “POM-C”. State whether an equivalent grade is accepted and whether a certificate is required. Avoid ambiguous wording such as “alu” or “stainless” alone: it forces the workshop to contact you again, or even to quote on an assumption different from yours.

Ready to ask for prices? Post your request on Meaflow: it is free and commission-free, you receive quotes from several suppliers and can compare them before choosing.