Aluminium and stainless steel account for most machined metal parts, and the choice between them is usually made on strength alone. That is the expensive way to decide: the material also sets cutting speed, tool consumption, the number of setups a thin wall survives and the finishing that follows the last cut.
This guide compares the two on values you can verify — specification minimums from ASTM, physical constants from published datasheets, and two worked calculations you can repeat with your own dimensions. It does not quote per-part prices, because a price given without a drawing, a quantity and a tolerance band is marketing rather than information.
Key takeaways
- The two families are not rated on the same machinability scale. MatWeb rates 6061-T6 at 50% against 2011-T3 = 100% for wrought aluminium, while steels are rated against AISI B1112 = 100%. Comparing those numbers directly, as most buyer guides do, is meaningless.
- Stainless takes longer to cut for three measurable reasons: thermal conductivity near 16 W/m·K versus 167 W/m·K for 6061-T6, austenitic work hardening, and a modulus around 193 GPa that changes how thin sections respond to tool pressure.
- Specification minimums are what a material certificate is tested against. ASTM A240/A240M guarantees 515 MPa tensile for 304 and 485 MPa for 316L — not the higher “typical” numbers on distributor datasheets.
- Aluminium expands about 36% more per degree than 304 (23.6 vs 17.3 µm/m·°C), so a 100 mm aluminium feature grows 0.024 mm across a 10 °C shop swing — wider than the whole ±0.01 mm ISO 2768-m band.
- Stainless is mandatory rather than preferable wherever ASTM F138 implant material or 21 CFR Part 177 food-contact compliance is written into the drawing. There, cost comparison does not apply.
- MW+ machines both families on the same floor across 60+ CNC machining centres and 70+ stocked materials, so a side-by-side quote reflects real cycle time rather than a shop preference.
- What do the published specifications actually say?
- Why does stainless steel take longer to machine?
- Which aluminium alloy, and which stainless grade?
- Worked example: stock mass and thermal growth
- How do tolerance and surface finish differ between the two?
- What finishing and documentation does each material need?
- When is aluminium the wrong call, and when is stainless?
- How do you brief a supplier so both quotes are comparable?
- Frequently asked questions
What do the published specifications actually say?
Buyers routinely miss one distinction. A datasheet “typical” value describes what a mill usually achieves; a specification minimum is what the material is contractually guaranteed to deliver, and it is what a mill certificate is tested against. Write 304 on a drawing and the certified values are the A240 or ASTM A276 minimums.
The table keeps the two apart. Aluminium figures are typical for the temper; the governing specifications are ASTM B221 for extruded bar and rod and ASTM B209 for sheet and plate, whose minimums vary with product form and section thickness.
| Property (units) | 6061-T6 aluminium | 7075-T6 aluminium | 304 stainless | 316L stainless |
|---|---|---|---|---|
| Density (g/cm³) | 2.70 | 2.81 | 8.00 | 8.00 |
| Tensile strength (MPa) | 310 typical | 572 typical | 515 minimum, A240 | 485 minimum, A240 |
| Yield strength, 0.2% (MPa) | 276 typical | 503 typical | 205 minimum, A240 | 170 minimum, A240 |
| Elastic modulus (GPa) | 68.9 | 71.7 | 193 | 193 |
| Thermal conductivity (W/m·K) | 167 | 130 | 16.2 | 16.3 |
| Thermal expansion (µm/m·°C) | 23.6 | 23.6 | 17.3 | 16.0 |
| Yield strength ÷ density (kN·m/kg) | 102 | 179 | 26 | 21 |
| Corrosion behaviour | Oxide film; anodise for service | Weaker than 6061; usually coated | Self-repairing passive film | Molybdenum-bearing, chloride resistant |
Two rows do most of the work. The specific-strength row explains why aerospace keeps choosing aluminium although stainless is stronger in absolute terms: per unit of mass carried, 7075-T6 delivers roughly seven times what 316L does. The thermal conductivity row explains almost everything about machining cost, and deserves its own section.
Why does stainless steel take longer to machine?
The heat has nowhere to go
Most of the energy spent shearing a chip becomes heat. At 167 W/m·K, aluminium and its chips carry that heat away quickly. Austenitic stainless sits near 16 W/m·K, about a tenth of that, so heat concentrates at the cutting edge where it does the most damage. That is the largest single driver of the cutting-speed difference, and why stainless work leans on through-tool coolant and conservative depths of cut.
Austenitic grades harden as you cut them
304 and 316L work-harden rapidly. A tool that rubs rather than cuts — a dull edge, too light a feed, a dwell at the bottom of a bore — leaves a hardened layer the next pass must cut through. The classic failure mode is therefore not a snapped tool but a finish that degrades and a dimension that drifts. Stainless punishes hesitant toolpaths in a way aluminium does not.
The chips behave differently
Aluminium produces long, ductile chips that weld to the edge as built-up edge and pack into deep pockets; stainless produces stringy chips that abrade the rake face. Neither is free. Free-machining grades exist on both sides — 2011 and 6262 in aluminium, 303 in stainless — precisely because chip control is worth paying for.
The machinability numbers are on different scales
This is where most published comparisons go wrong. Wrought aluminium is rated against 2011-T3 at 100%, which puts 6061-T6 at 50%. Steels are rated against free-machining AISI B1112 at 100%, which puts 304 in the mid-forties. The two figures look comparable and are not, because they are percentages of different reference materials.
The practical consequence: do not derive cycle time from a machinability index. Derive it from the toolpath, the removal rate your supplier’s CNC milling services actually run for that alloy, and the number of setups the geometry forces.
Which aluminium alloy, and which stainless grade?
“Aluminium” and “stainless” are families, not materials. Within each, grade choice often moves cost and performance more than the family choice does. The table covers the grades that account for most machined work.
| Grade | Family | Why you would specify it | Watch out for | Governing specification |
|---|---|---|---|---|
| 6061-T6 | Aluminium | Default structural grade: good strength, weldable, anodises cleanly | Softer than 7075 in bearing areas | ASTM B221 / B209 |
| 7075-T6 | Aluminium | Highest common strength-to-weight; airframe fittings and brackets | Poor weldability, lower corrosion resistance, stress-corrosion sensitivity | ASTM B221 / B209 |
| 2024-T351 | Aluminium | Good fatigue behaviour; damage-tolerant structure | Copper-bearing, so corrosion protection is mandatory | ASTM B209 |
| 5052-H32 | Aluminium | Formed and folded sheet parts, marine exposure | Low strength; a poor choice for heavy milling | ASTM B209 |
| 303 | Stainless | Free-machining; shafts, fittings, high-quantity turned parts | Sulphur addition reduces corrosion resistance and weldability | ASTM A276 |
| 304 / 304L | Stainless | General-purpose corrosion resistance; food and architectural work | Chloride pitting; work hardens aggressively | ASTM A240 / A276 |
| 316 / 316L | Stainless | Molybdenum gives chloride and acid resistance; marine and pharma | Slower to cut than 304; higher stock cost | ASTM A240 / A276 |
| 17-4 PH, H900 | Stainless | 1310 MPa minimum tensile with stainless corrosion behaviour | Heat-treat sequencing and distortion control add lead time | ASTM A564/A564M |
Two substitutions are worth knowing. If a turned stainless part is quoting painfully and the drawing demands neither weldability nor aggressive corrosion service, moving from 304 to 303 often shifts cost more than any design change. And if an aluminium part is failing on wear rather than strength, a hard anodic coating usually solves it for less than a jump to stainless would.
Worked example: stock mass and thermal growth
Two calculations show where the cost and the risk actually sit. Both use only the published constants in the table above, so you can repeat them with your own part.
Calculation 1 — how much more stock does stainless consume?
Take a bracket with a finished volume of 48 cm³, machined from a rectangular billet measuring 120 × 80 × 25 mm.
- Billet volume = 12.0 × 8.0 × 2.5 cm = 240 cm³
- In 6061-T6 at 2.70 g/cm³: billet mass = 240 × 2.70 = 648 g; finished part = 48 × 2.70 = 130 g; swarf = 518 g
- In 304 at 8.00 g/cm³: billet mass = 240 × 8.00 = 1,920 g; finished part = 48 × 8.00 = 384 g; swarf = 1,536 g
- Purchased mass ratio = 1,920 ÷ 648 = 2.96×
The buy-to-fly ratio is identical at 5:1, because that is a property of the geometry, not the alloy. But the mass you purchase, freight and sell back as swarf is almost three times greater in stainless before a minute of spindle time is counted. Shrinking the billet envelope, not switching alloy, is usually the first lever a design review pulls.
Calculation 2 — does shop temperature eat your tolerance?
Thermal growth of a feature is ΔL = L × α × ΔT. Take a 100 mm bore centre distance and a 10 °C swing between a cold morning and a warm afternoon on the shop floor.
- 6061-T6, α = 23.6 µm/m·°C: ΔL = 0.100 m × 23.6 × 10 = 23.6 µm, or 0.0236 mm
- 304 stainless, α = 17.3 µm/m·°C: ΔL = 0.100 m × 17.3 × 10 = 17.3 µm, or 0.0173 mm
MW+ holds general machining to ±0.01 mm against ISO 2768-m, and both results exceed that band. The part is not impossible; the inspection temperature simply has to be specified, and aluminium is about 36% more sensitive to getting it wrong. That is why metrology rooms are held at 20 °C. If you work to ±0.005 mm or tighter, state the reference temperature on the drawing.
How do tolerance and surface finish differ between the two?
Both materials reach the same tolerance bands; what differs is the effort required and the second-order effects. Aluminium gets there faster but moves more with temperature and clamping. Stainless is dimensionally steadier but demands sharper tooling and more patience to hold a fine finish without work-hardening the surface.
| Requirement | Aluminium: what it takes | Stainless: what it takes | MW+ capability |
|---|---|---|---|
| General tolerance | Straightforward; watch fixture-induced distortion in thin sections | Straightforward; rigid workholding essential | ±0.01 mm to ISO 2768-m |
| Precision band | Temperature control and light finishing passes | Sharp finishing tools, no dwell at depth | ±0.005 mm |
| Tightest achievable | Climate-controlled inspection required | Stress relief between roughing and finishing | ±0.001 mm |
| As-machined finish | Achieved readily; watch built-up edge | Achieved readily; chip control matters | Ra 3.2 µm |
| Fine-machined finish | Polished-flute tooling, high surface speed | Slower feeds, positive rake geometry | Ra 0.4 µm |
| Polished finish | Soft, so easy to polish and easy to scratch | Takes and holds a mirror finish well | Ra 0.1 µm |
| Geometric callouts | Flatness sensitive to residual stress in plate | Better form stability after finishing | Per ASME Y14.5 |
One note on residual stress: thick aluminium plate carries locked-in stress from rolling and quenching, so a part that removes material asymmetrically bows when it leaves the vice. Flag a flatness callout on a large, thin aluminium part at quotation — the answer is a rough, stress-relieve, finish sequence, and it changes the schedule. That conversation belongs in any serious review of CNC machining quality control requirements.
What finishing and documentation does each material need?
The comparison flips after the last cut. Stainless is usually finished by doing very little: passivation restores the chromium-oxide film that machining disturbs and the part is done. Aluminium almost always needs a coating, because bare aluminium in service scuffs, galls and stains.
| Requirement | Aluminium route | Stainless route | Reference document |
|---|---|---|---|
| Corrosion protection | Anodise Type II, or chemical conversion coating | Passivate; no coating needed in most service | ASTM A967 for passivation |
| Wear surfaces | Hard anodise Type III builds a thick, hard oxide | Nitride, or select a hardenable grade such as 17-4 PH | ASTM A564 for precipitation-hardening bar |
| Cosmetic appearance | Anodise in colour; bead blast then anodise | Bead blast, brush or electropolish | Surface texture to ISO 21920-2 (superseding ISO 4287) |
| Food contact | Rarely acceptable bare; usually coated or a different material | 304 or 316 with a defined, cleanable finish | 21 CFR Part 177 |
| Implantable or surgical | Not applicable | Implant-grade bar to a controlled composition | ASTM F138, UNS S31673 |
| Traceability | Mill certificate to B221 or B209 | Mill certificate to A240 or A276 | Supplied with every MW+ order |
Every MW+ order ships with a certificate of conformity, a CMM inspection report and material certificates, whichever material you chose; first article inspection to AS9102 and PPAP Level 3 are available on request, quoted per programme. Where a drawing calls a regulated composition, the material certificate is what proves it, so it belongs in the RFQ rather than in a post-delivery argument.
When is aluminium the wrong call, and when is stainless?
Comparison guides usually end by recommending aluminium on cost. That is often right and sometimes expensive. Here is the honest counter-case in both directions.
When aluminium is wrong despite being cheaper to cut
- Threaded holes that see repeated assembly. Aluminium threads gall and strip; thread inserts and the labour to fit them usually erase the material saving.
- Chloride environments. Anodising is a coating, and a coating has edges and scratches. In marine or de-icing-salt service, 316 does something anodised 6061 cannot.
- Sustained heat. 6061-T6 loses temper above roughly 150–200 °C, so near an exhaust or a motor the T6 condition is not durable.
- Stiffness-critical parts. At 68.9 GPa against 193 GPa, aluminium deflects roughly three times as much for the same section; adding section to compensate can cancel the weight advantage.
- Regulated compositions. If a standard names the material, the comparison is already over.
When stainless is wrong despite being stronger
- Anything where mass is the design constraint. Handheld, airborne and rotating parts pay for mass forever; the specific-strength row is the whole argument.
- Heat sinking. A stainless heat sink is close to a contradiction: 16 W/m·K means the part will not move heat.
- Large, mostly-air structures. High removal volume is where stainless cycle time bites hardest; a heavily pocketed frame is an aluminium part in almost every case.
- Fast iterative prototyping. With three revisions expected before freeze, proving geometry in aluminium and switching material at release is the cheaper learning path — see our guide to low-volume cost and speed trade-offs.
- Non-magnetic requirements after cold work. Austenitic stainless is nominally non-magnetic but turns slightly magnetic after heavy cold work. Aluminium never does.
How do you brief a supplier so both quotes are comparable?
For a genuine side-by-side comparison rather than two quotes built on different assumptions, the RFQ has to pin down the variables that move price. This is what a machining engineer needs before the numbers mean anything.
| What to state | Why it changes the quote | Common omission that forces a re-quote |
|---|---|---|
| Exact grade and temper or condition | 303 and 304, 6061-T6 and T651, H900 and H1150 all cut differently | Writing only “stainless” or “aluminium” |
| Quantity and expected repeat | Fixturing and programming amortise across the batch | Asking for one piece when 500 are planned |
| Tolerance scheme | A blanket tight tolerance on every feature multiplies inspection | Tight tolerances applied to non-functional features |
| Surface finish by surface | Ra 0.4 µm everywhere costs far more than Ra 0.4 µm on two faces | A single global finish note |
| Finishing and colour | Anodising adds a process step and an external lead-time dependency | Finish decided after the quote |
| Documentation required | FAI to AS9102 or PPAP Level 3 is a programme cost, not a line item | Requesting full PPAP after the order is placed |
| Reference temperature for inspection | Decides how tight tolerances are verified, especially in aluminium | Silence, which later becomes a dispute |
MW+ returns quotes within 24 hours from STEP, IGES, DXF, DWG, SolidWorks or PDF files. Standard prototypes run 3–5 business days and volume production 10–15 business days, with 48-hour express available and no minimum order quantity. Ask explicitly for both materials priced on the same geometry — it is the only way to see the real delta. The process envelope is set out under CNC machining services, and turned parts in 303 or 6262 fall under CNC turning services.
Frequently asked questions
Is aluminium always cheaper than stainless steel for a machined part?
No. Aluminium is faster to cut and lighter to buy, which usually wins on stock and machining cost. But if the aluminium version then needs hard anodising, thread inserts and corrosion protection the stainless version does not, finished cost can converge or invert. Compare finished, coated, inspected parts, not billets.
Why do two suppliers quote very different stainless prices from the same drawing?
Usually because one read the tolerance and finish callouts and the other did not. A blanket ±0.01 mm on a stainless part with twenty features is a different job from the same part with three toleranced features. Ask each supplier what cycle time and how many setups they assumed; the gap is nearly always there, not in the material.
Can I prototype in aluminium and produce in stainless?
Yes, and it is often the right sequence for validating geometry — but the aluminium prototype is not a dimensional proxy. Different expansion, springback and residual stress mean some features need re-tuning. Budget a first article inspection on the production material at that transition.
Does 316L genuinely cost more than 304, or is that a markup?
It is genuine. 316L carries roughly 2–3% molybdenum that 304 does not, and molybdenum is the expensive element in the recipe. Specify 316L when you need chloride or acid resistance; specify 304 when you do not, and keep most of the corrosion performance for less.
What tolerance can I actually hold in each material?
Both reach ±0.001 mm on suitable features in a controlled environment. For most parts the realistic answer is ±0.01 mm to ISO 2768-m generally, tightening selected features to ±0.005 mm where function demands it. At the tight end the limiting factor is temperature in aluminium, residual stress and tool condition in stainless.
Will an overseas supplier’s material certificate survive an audit?
It survives if it traces to a recognised specification and the supplier’s quality system is certified to a standard your auditor accepts. MW+ operates to ISO 9001:2015, AS9100D, ISO 13485, IATF 16949 and NADCAP. Ask for a sample certificate package before the first order rather than after it.
How do I reduce cost without changing material?
In rough order of impact: shrink the billet envelope, remove tolerances from features that do not need them, cut the number of setups, apply fine finishes only where they are functional, and consolidate small orders into one release. Our guide to tool wear by material type and the wider CNC materials reference cover the mechanisms.
Where does titanium sit against both?
Between them on weight, above both on cost: stainless-class strength at roughly 60% of the density, but slow to cut and hard on tooling. See titanium machining cost factors, or PEEK cost and performance for the non-metal route.
To have both materials priced on the same geometry with a DFM review attached, request a CNC machining quote; an engineer responds within 24 hours. Parts held to tight geometric callouts are covered under CNC precision parts.
