Aluminum vs Stainless Steel for CNC Machined Parts: How to Choose

Most material decisions for machined parts get made on one property. Usually that is corrosion resistance or price, and the rest gets discovered when the first batch arrives. That is how a bracket ends up three times heavier than it needed to be, or an outdoor enclosure starts to pit. Aluminum vs stainless steel for CNC machined parts is a decision with several variables, and it is worth settling before the drawing is released.

Short answer: Choose aluminum, usually 6061-T6, when weight, cost and lead time drive the part and it works indoors or in mild conditions. Choose stainless steel, usually 304 or 316L, when the part faces chlorides, sterilization, sustained heat, sliding wear or high loads in a small envelope. Aluminum machines faster and costs less. Stainless survives harsher service.

This guide compares the two on the properties that change the part and the quote. It then covers what most comparisons skip: what switching material does to tolerances, threads, finishing, and the move from prototype to production.

Aluminum vs Stainless Steel at a Glance

Here is how aluminum vs stainless steel compares on the numbers that most often decide a CNC part.

プロパティ	6061-T6 aluminum	304 stainless (annealed)
Density	2.70 g/cm³ (0.098 lb/in³)	8.00 g/cm³ (0.289 lb/in³)
降伏強度	276 MPa (40 ksi), typical	205 MPa (30 ksi), minimum
Tensile strength	310 MPa (45 ksi), typical	515 MPa (75 ksi), minimum
Elastic modulus	69 GPa (10 Msi)	193 GPa (28 Msi)
熱伝導率	167 W/m·K	16.2 W/m·K
Thermal expansion	23.6 µm/m·°C	17.3 µm/m·°C
Electrical conductivity	about 43% IACS	about 2.5% IACS
Machinability	素晴らしい	Fair; work hardens
耐食性	Good indoors; anodize for harsh service	Very good; 316L for chlorides
部品単価	下へ	より高い

These are typical handbook values for these tempers. Temper, stock form and supplier all move the numbers, so confirm critical values against the mill certificate for your material.

Which Is Stronger, Aluminum or Stainless Steel?

It depends on which strength you mean, and the answer surprises people.

Yield strength is the stress at which a part starts to deform permanently. On that measure, annealed 304 is actually lower than 6061-T6. Stainless pulls ahead on ultimate tensile strength, hardness and wear resistance. It also wins decisively on stiffness. Its elastic modulus, the resistance to bending under load, is about 2.8 times higher, so a stainless plate deflects far less than an aluminum plate of the same thickness. When you need raw strength, 17-4 PH in the H900 condition reaches a minimum yield of about 1,170 MPa (170 ksi).

Aluminum’s advantage is strength per unit of weight. 7075-T6 reaches roughly 572 MPa (83 ksi) tensile at under 2.9 g/cm³, which is why it shows up in robot arms, drone frames and anything with a motor that has to accelerate the part.

One detail is easy to miss. Divide stiffness by density and the two materials come out almost level. So on “stiff and light”, aluminum versus stainless is close to a draw. Stainless helps when the part must stay stiff within a fixed size, and aluminum helps when mass is the constraint.

Is Aluminum Easier to Machine Than Stainless Steel?

Yes, by a wide margin. That margin explains most of the price gap between aluminum and stainless steel parts.

Aluminum is soft, breaks into short chips and carries heat away from the cutting edge about ten times faster than stainless. Shops can run it at high spindle speeds and heavy feeds with tools that last. Machining stainless steel is a different job:

• Heat stays at the tool. Low thermal conductivity concentrates heat at the cutting edge, which shortens tool life.
• Austenitic grades work harden. In 304 and 316, a dull tool or a rubbing pass hardens the surface, and the next pass has to cut through that hardened layer.
• Chips are long and stringy. They wrap around tools and can mark finished surfaces.
• Built-up edge. Material welds to the cutting edge unless speeds and coatings are right.

Shops respond with sharp coated carbide, lower surface speeds, a steady chip load, high-pressure coolant and rigid fixturing. All of that adds machine time. On most quotes, machine time drives CNC machining cost far more than the bar stock does.

There is one exception worth knowing. 303 stainless contains at least 0.15% sulfur, which breaks chips and makes it noticeably easier to machine than 304. It is common for high-volume CNC旋盤加工 of fittings and shafts. The trade-off is that 303 welds poorly and handles chlorides worse than 304. For a broader ranking of metals and plastics, see our guide to CNC machining materials by machinability.

Corrosion, Heat and Conductivity

<!– IN-CONTENT IMAGE: corrosion-anodized-vs-passivated.webp | ALT: Anodized 6061 aluminum part beside a passivated 316L stainless steel part after salt exposure –>

Stainless steel resists corrosion because it contains at least 10.5% chromium. The chromium forms a thin passive film that repairs itself when scratched. 304 handles most indoor, food and atmospheric service. Chlorides from seawater, road salt or cleaning chemicals cause pitting in 304, and 316 stainless steel adds 2 to 3% molybdenum to resist it.

Machining can embed free iron from tooling into the stainless surface, which leaves rust spots later. Passivation per ASTM A967/A967M removes that contamination and should be called out on the drawing for any stainless part going into wet service.

Aluminum forms its own oxide layer, which is adequate indoors. For outdoor, wash-down or marine exposure, it needs anodizing or a conversion coating.

Mixing the two needs care. When aluminum and stainless steel parts touch in the presence of moisture, the aluminum corrodes preferentially. NASA’s corrosion protection standard for flight hardware, NASA-STD-6012A, requires dissimilar-metal assemblies like this to be evaluated for galvanic corrosion and protected. The usual fixes are nylon washers, sealant or coated fasteners.

On heat, aluminum alloys lose a large share of their strength at temperatures that are routine for stainless. If a part runs hot for long periods, design from elevated-temperature data rather than room-temperature data. On conductivity, the table settles it: for heat sinks, bus bars and EMI enclosures, aluminum is the practical choice.

Which Grades Get Specified Most?

Settling the aluminum or stainless steel question is only half the decision. “Stainless steel” alone on a drawing invites the shop to pick whatever grade suits its stock, so always specify the alloy and condition.

グレード	Best for	Watch for
6061-T6 aluminum	Brackets, housings, fixtures, heat sinks	Needs a finish for harsh environments
7075-T6 aluminum	Light, high-load structural parts	Poor weldability; less corrosion-resistant than 6061
303 stainless	High-volume turned fittings and shafts	Poor for welding; weaker in chlorides than 304
304 stainless	Food equipment, general enclosures	Pits in chlorides; work hardens
316L stainless	Marine, chemical, medical parts	Slower to machine than 304
17-4 PH stainless	Shafts, pins, high-load parts	Heat treatment adds a step and lead time

6061 aluminum is the default for most jobs and anodizes evenly. Extruded bar is commonly supplied to ASTM B221, so cite the spec with the alloy and temper. See our aluminum CNC machining page for stock and finish options. If you are deciding between the two common austenitic grades, our 304 vs 316 stainless steel comparison goes deeper.

What Changes on the Drawing When You Switch Materials

Changing the material on a drawing without reviewing the drawing is one of the most common sources of first-article rejections.

Tolerances and temperature

Aluminum expands about 36% more per degree than 304. A 150 mm aluminum part that warms by 5 °C between the machine and the inspection room grows about 0.018 mm. The same stainless part grows about 0.013 mm. On a ±0.01 mm feature, that growth can use up most of the tolerance band. For tight work, say so on the drawing: inspect at 20 °C.

Threads and galling

Stainless fasteners in stainless tapped holes can gall, meaning the threads cold-weld and seize during assembly. Anti-seize, coated fasteners or a mixed fastener grade prevent it. Aluminum has the opposite problem: tapped threads wear out when a screw goes in and out repeatedly, so specify helical thread inserts wherever that happens in service or maintenance.

Finishes that change size

Hard anodizing (Type III) typically builds about 0.05 mm (0.002 in), and roughly half of that grows outward from the original surface. Bores, press fits and threads need masking or a pre-finish machining size. Passivation removes almost no material, so stainless dimensions stay essentially where they were machined.

Prototyping in Aluminum Before Producing in Stainless

Prototyping in 6061 aluminum and then moving to stainless for production is common and often sensible. The aluminum parts are faster and cheaper to make, and the aluminum vs stainless steel differences only show up later, in ways a prototype can hide.

It helps to be clear about what that prototype proves. Geometry, fit and assembly sequence transfer well. Mass does not: the stainless part weighs nearly three times as much. Stiffness does not either, since the stainless part is about 2.8 times stiffer, which shifts natural frequencies in moving assemblies. Cost does not transfer at all. Deep narrow pockets and thin walls that were cheap in 6061 can multiply cycle time in 316L.

Our position is to run at least one iteration in the production material before releasing the design, or to get a DFM review in stainless before the prototype order. Our ラピッドプロトタイピングサービス can quote both materials side by side from one drawing.

Aluminum vs Stainless Steel CNC Machining FAQs

Is aluminum cheaper to CNC machine than stainless steel? Usually, yes. Aluminum bar stock generally costs less, but the bigger lever on CNC machining cost is machine time. Aluminum runs at much higher speeds and feeds with longer tool life, so the same geometry spends less time on the machine. The gap narrows for simple turned parts in 303.

Can stainless steel be CNC machined to tight tolerances? Yes. Stainless holds tight tolerances well because it is stiff and dimensionally stable. Machining stainless steel to tight limits is about process control. Sharp tooling, steady chip load and good coolant prevent work hardening and heat distortion. Expect longer cycle times than aluminum for the same tolerance.

Can aluminum and stainless steel parts be used in the same assembly? Yes, with isolation. Where the two metals touch and moisture is present, galvanic corrosion attacks the aluminum. Anodize or conversion-coat the aluminum, and separate the metals with nylon washers, sealant or coated fasteners. Dry indoor assemblies carry much lower risk.

Which is better for medical device parts, aluminum or stainless steel? For anything that touches patients or goes through autoclave sterilization, stainless steel is the usual choice, most often 316L or 17-4 PH. Aluminum works for housings, carts and fixtures that never see sterilization. Biocompatibility requirements depend on the device, so confirm them with your regulatory team.

Should I choose 304 or 316 stainless for a machined part? Choose 304 for indoor, food-processing and general-purpose parts, where it costs less and machines slightly easier. Choose 316 or 316L when the part meets seawater, road salt, chlorinated cleaners or body fluids. Its molybdenum content resists the pitting that attacks 304 in those environments.

Choosing Between Aluminum and Stainless Steel for Your CNC Parts

For most indoor, weight-sensitive or cost-driven parts, 6061-T6 aluminum is the better default, because it machines quickly, quotes low and finishes easily. When the part faces chlorides, sterilization, sustained heat or heavy sliding wear, go straight to stainless steel, and in most of those cases 316L rather than 304.

Either way, the final choice for aluminum vs stainless steel for CNC machined parts should come from the service conditions, not the property that first comes to mind. Specify the exact grade, the finish and the inspection temperature on the drawing.

If you want a second opinion, send your drawing to エリート・モールド・テック. Our engineers review material, tolerances and finishing as part of every CNC machining service quote, from single prototypes through production runs.