Aluminium or Stainless Steel? Choosing the Right Material for a Machined Part
Strength-to-weight, corrosion, thermal behaviour, machinability and cost compared across 6061, 7075, 304, 316L and 17-4PH — with the trade-offs that actually decide the choice.
Written by Engineering Team
"Aluminium or stainless?" is the most common material question we get, and it is almost always the wrong question. The right question is: which property drives the design — weight, strength, corrosion resistance, thermal conductivity, or cost?
Answer that and the material usually picks itself.
The short version
- Weight matters most → 6061-T6, or 7075-T651 if you also need strength
- Strength matters most and weight is free → 4140 or 17-4PH
- Corrosion matters most → 316L, or titanium Gr2 in chlorides
- Thermal conductivity matters most → 6061, or copper C101 if you can afford the mass
- Cost matters most and the part is not exposed → 1018 or 6061
- Hardness and wear matter most → 440C or D2, hardened after machining
Key properties side by side
| Material | Density | Tensile | Machinability | Corrosion | Relative cost |
|---|---|---|---|---|---|
| 6061-T6 | 2.70 g/cm³ | 310 MPa | Excellent | Good (anodised: very good) | 1.0 |
| 7075-T651 | 2.81 g/cm³ | 572 MPa | Very good | Fair — needs coating | 1.8 |
| 304 | 8.00 g/cm³ | 515 MPa | Fair | Very good | 1.6 |
| 316L | 8.00 g/cm³ | 485 MPa | Fair | Excellent | 2.2 |
| 17-4PH H900 | 7.75 g/cm³ | 1310 MPa | Poor | Very good | 3.5 |
| Ti-6Al-4V | 4.43 g/cm³ | 950 MPa | Poor | Excellent | 8 – 12 |
Cost is the delivered cost of a typical machined part, including material and cutting time — not the price per kilo. This matters: 316L costs roughly twice as much per kilo as 6061, but a 316L part often costs three times as much because it cuts more slowly and eats tooling.
Strength-to-weight: why 7075 wins and when it does not
7075-T651 has nearly twice the tensile strength of 6061-T6 at almost the same density, which is why it dominates aerospace structures and motorsport.
The catch: 7075 is far less corrosion resistant, cannot be welded reliably, and is more prone to stress-corrosion cracking. It also moves during machining. A 7075 plate with material removed from one side will bow, sometimes visibly, because the residual stress from rolling is no longer balanced.
We handle this by roughing, stress-relieving, then finishing — which adds a day to the lead time and a few percent to the cost. Budget for it rather than being surprised by it.
Corrosion: 304 is not the answer to everything
304 is the default stainless, and it is the wrong choice more often than people realise.
In chloride environments — marine, food processing with brine, coastal installations, swimming pools — 304 pits. 316L, with 2 – 3 % molybdenum, resists pitting far better, which is why it is the standard for medical, marine and pharmaceutical work.
For real chloride exposure at temperature, neither is sufficient: duplex 2205 or super duplex 2507 is the honest answer, at roughly double the cost again.
One more point: stainless steel is not corrosion-proof once machined. Cutting drags free iron across the surface, and that iron rusts. Passivation to ASTM A967 removes it and restores the chromium oxide layer. If your part is stainless and will see moisture, specify passivation — it costs very little and makes the difference between a part that stays bright and one that shows rust spots in six weeks.
Thermal conductivity: the property engineers forget
6061 conducts heat about eight times better than 316L (167 vs 16 W/m·K). For a heat sink, cold plate or laser housing, that difference dominates every other consideration.
Copper C101 is about twice as conductive as aluminium again (391 W/m·K), which is why busbars and high-power cold plates are copper despite being three times heavier and considerably harder to machine cleanly.
Machinability: what it costs in cycle time
Machinability affects price through cycle time and tool life:
- 6061 — the benchmark. Cuts fast, chips break well, tool life is long.
- 7075 — similar speeds, slightly more care needed on thin walls.
- 303 stainless — free-machining sulphur additions make it cut almost like brass; use it for turned parts where weldability is not required.
- 304 / 316L — work-harden aggressively. Feeds must stay high and constant; dwelling in the cut creates a hardened layer that destroys the next pass and the tool with it.
- 17-4PH — machine in condition A (solution annealed), then age-harden. Machining after hardening to H900 roughly doubles cycle time.
- Titanium — low thermal conductivity concentrates heat in the cutting edge. Requires high-pressure through-tool coolant and conservative speeds.
Plastics: when metal is the wrong answer entirely
- POM (Delrin) — gears, bushings, sliding parts. Machines beautifully, dimensionally stable, low friction.
- PEEK — sterilisable, chemically inert, good to 250 °C. Expensive but often the only option for medical and semiconductor.
- PTFE — seals and low-friction surfaces. Machines poorly and creeps under load; design accordingly.
- PC and PMMA — transparency. PC is tough; PMMA polishes to optical clarity but cracks.
A decision path that works
- List the loads, the temperature range and the chemical environment.
- Eliminate every material that fails any one of them.
- Among the survivors, pick the one that machines fastest.
- If weight is not a specification, pick steel — it is almost always cheaper.
- If corrosion is a specification, add passivation, anodising or plating and re-check the cost.
Not sure? Send the part
Tell us the application, the environment and the loads, and our engineering team will come back with two or three viable materials and the cost difference between them. It is part of the quotation, not an extra service.