Most metal parts sourced from China are made of one of two families, and the material line on the drawing decides more than the quote - it decides whether the part survives its service. Aluminum gives low cost, light weight, and fast machining. Stainless gives corrosion resistance, stiffness, and a metal that tolerates environments aluminum cannot. The question is not which family is better; it is which one the part actually lives in, and the answer is on the drawing if you read the service conditions rather than the habits of the last project.
The quick filter for most buyers: if the part must be light, cheap, or cosmetic and works indoors at moderate temperature, aluminum is the default and stainless is a premium without a payoff. If the part sees moisture, chemicals, heat, repeated assembly, or a safety function, stainless is the default and aluminum is a risk dressed up as a saving. The grades and the decision table below settle the details after that filter.
1. What does aluminum actually deliver on a machined part?
Aluminum earns its place on density and machinability. At about 2.7 g/cm³ it weighs a third as much as steel, yet the structural grades carry respectable strength, and strength per kilogram beats most steels outright. That is why aluminum owns the brackets, housings, heat sinks, and enclosures that move, fly, or are carried by hand - and why it dominates the prototype orders that every sourcing relationship starts with.
The machining story is equally good. Aluminum cuts fast with standard tooling, holds a fine surface, and does not work-harden as it is cut, so complex pockets, thin walls, and fine detail are comparatively easy to produce at precision bands. Tool wear is low and chips clear cleanly, and all of that shows up in the quote as a shorter cycle time and a lower price than the same part in any steel.
The finishing story completes the case. Aluminum anodizes - clear, dyed, or hard - which gives a colored, wear-resistant surface at a modest cost, which is why so many visible machined products are 6061 under an anodize. What aluminum does not do is shrug off salt, acids, or dissimilar-metal contact without protection, and its stiffness is about a third of steel. When the service environment is aggressive or the geometry is long and unsupported, aluminum is the wrong answer no matter how cheap it is to machine.
2. When does stainless steel earn its higher price?
Stainless earns its premium in exactly the environments aluminum cannot serve. Where a part meets moisture, chemicals, food, or marine air, the chromium oxide layer protects it without a coating that can wear or scratch. Where a part is cycled, cleaned, or sterilized, stainless survives the routine. Where a part carries pressure or a safety function, the stiffness and toughness of steel are the engineering default that no aluminum geometry fully replaces.
The numbers are straightforward. At about 7.9 g/cm³ stainless weighs nearly three times as much as aluminum, and its elastic modulus of roughly 200 GPa against aluminum's 69 GPa means a stainless part of the same shape is about three times stiffer. That stiffness is what stops brackets from flexing and shafts from whipping in service - and it is the same stiffness the machine shop must manage during cutting, because the workpiece pushes back harder and builds more heat.
The price of those properties is real. Stainless stock costs several times more than aluminum per kilogram, machining speeds fall to a fraction, and inserts wear faster, so an equivalent stainless part typically quotes at a meaningful multiple of the aluminum version. The buying discipline is to pay for those properties only where the service demands them, and never to save money on a material that will fail in the field.
3. Which aluminum grade belongs on a sourced-part drawing?
6061-T6 is the workhorse of machined aluminum and the default for sourced parts for a reason. It machines cleanly, anodizes well, welds acceptably, and costs less than the aerospace grades, with a yield strength around 276 MPa. For housings, brackets, panels, and prototypes, 6061-T6 satisfies the drawing at the lowest cost, and it should stay the default until a requirement forces a change.
7075-T6 exists for the requirements 6061 cannot meet. With a yield strength near 503 MPa it is roughly 80 percent stronger, and it is the grade of choice for highly loaded structural parts. The costs are real: the stock is about twice the price of 6061, it is harder on tooling, and it is more prone to stress-corrosion issues under sustained tension, so 7075 parts often go to anodize and are designed with an eye on grain direction and residual stress.
The decision rule is simple. If the part failed as 6061 because it bent or cracked under load, move to 7075. If it has not failed, 7075 is a cost carried for a specification sheet. For most sourced parts, and nearly all prototypes, the engineering answer and the budget answer are the same: 6061-T6.
4. Which stainless grade belongs on a sourced-part drawing?
The stainless family is less uniform than buyers assume, and picking "stainless" without a grade is how corrosion failures happen. 303 is the free-machining grade - added sulfur breaks the chips and speeds cutting - and it is the right call for machined fittings, shafts, and fasteners where corrosion demands are moderate. The same sulfur that helps machining slightly reduces corrosion resistance, so 303 is a shop-floor friend with a service-life limit.
304 is the general-purpose stainless: the default for parts that need corrosion resistance without exotic environments. 316 adds molybdenum and is the marine, chemical, and food-processing grade - costlier stock, slower cutting, but resistance to the chlorides and acids that pit 304. And 17-4PH is the precipitation-hardening grade for strength-plus-corrosion applications: it machines in the soft condition and age-hardens to roughly 1170 MPa yield, which is why it appears wherever a loaded part must also resist its environment.
The selection path is short. Moderate environment and machined-only part: 303 if turning dominates, 304 otherwise. Salt, chemical, or food service: 316L. High strength plus corrosion: 17-4PH with an aging step. Cost climbs along that path in the same order, which is exactly why the grade belongs on the drawing and not in a purchasing email.
5. How do the two families differ in the machining quote?
The machining difference is where the quote is won and lost. Aluminum cuts fast with light cutting pressure, predictable chips, and long tool life; a 6061 housing can be roughed and finished in a fraction of the time the same part takes in 316. Stainless is tough, work-hardens where the tool dwells, conducts heat poorly, and welds to the cutting edge if speeds and coolant are wrong. The shop runs stainless with lower speeds, controlled feeds, and a strict eye on tool wear.
Those differences land in the quote as cycle time and tooling. The same feature set that takes minutes in 6061 can take two to three times as long in 316, with more frequent insert changes and tighter control of chatter and deflection. That is the real source of the stainless premium - not the stock price alone, but the machine time the material demands and the tooling it consumes.
Deflection behaves differently too. Aluminum's low modulus cuts both ways: thin walls flex under the tool and must be supported, but the material does not harden as it flexes. Stainless is stiffer per section but transmits force into the workpiece and the fixture, and the heat it builds can grow dimensions that read out of tolerance at inspection. Both families hold precision bands - the bands the quality system verifies - ±0.003 mm in milling and turning, ±0.002 mm in grinding - but the discipline that achieves them is different for each.
6. How does the choice affect tolerances, finishes, and fasteners?
Tolerance is a promise the process must keep, and the material decides the cost of keeping it. In aluminum, a critical dimension is mostly a stable setup and a good toolpath, because the material cuts predictably. In stainless, the same dimension must survive thermal growth during cutting and the work-hardened layer from the previous pass, so dimensional stability is earned with feeds, coolant strategy, and sometimes a finishing pass with a fresh tool.
Surface finish follows the same pattern. Aluminum turns and mills to bright, consistent surfaces, so a roughness callout on a cosmetic part is met with routine passes. Stainless reaches the same value with more effort and more tool changes, and a very fine finish may need a secondary operation. If the drawing names a roughness, the material determines how much the finish costs - and a finish that is routine in one family can be a project in the other.
Fasteners add a third consideration. Threads in aluminum gall and strip under repeated assembly, which is why aluminum parts that get assembled often carry stainless threaded inserts. Stainless threads resist galling but need correct tapping practice because the work-hardened hole wrecks taps. If the assembly is serviced or reassembled, the material and the thread specification belong together on the drawing.
7. A decision table for the drawing
Decisions are faster when the numbers sit side by side. For machined parts sourced from China, the comparison that matters most is between the workhorse grades:
| Property | 6061-T6 (Al) | 7075-T6 (Al) | 303 (SS) | 304 (SS) | 316 (SS) | 17-4PH H900 |
|---|---|---|---|---|---|---|
| Density (g/cm³) | 2.70 | 2.81 | 8.00 | 8.00 | 8.00 | 7.78 |
| Typical yield (MPa) | ~276 | ~503 | ~240 | ~215 | ~290 | ~1170 |
| Modulus (GPa) | ~69 | ~72 | ~193 | ~193 | ~193 | ~196 |
| Corrosion resistance | Good with anodize | Moderate | Good | Good | Excellent | Good |
| Machining speed | Fast | Moderate | Moderate | Slow | Slow | Moderate; ages after |
| Relative machined cost | Baseline | Higher | ~2-3x | ~2-3x | ~2.5-3.5x | Highest |
Property figures are standard published values; treat the cost row as a planning relationship, not a quote.
Read the table against the part's real service. A bracket that lives indoors in a dry assembly is a 6061 part, and stainless there is money spent on a service the part never sees. A component that meets salt spray or washdown is a 316 decision regardless of the machining penalty. A loaded part that must also resist corrosion is a 7075 or 17-4PH decision, set by whether fatigue or the environment dominates the failure mode.
8. Making the call before the quote
Make the material decision before the quote and write the grade and condition on the drawing. A drawing that says "aluminum" invites one supplier to quote 6061 and another to quote 7075, and the comparison that follows is meaningless. The same discipline applies to stainless: the drawing should read 303, 304, 316L, or 17-4PH with the condition, because the grades differ in cost by factors, not percentages.
Then test the design assumptions rather than the brochure. If the part is weight-critical, calculate whether aluminum's stiffness passes the deflection check at the section you can afford. If it is corrosion-critical, name the environment - indoor air, salt spray, chemical washdown - because that sentence alone picks the grade. If the part is assembled and serviced repeatedly, plan for the insert or the material change that prevents galling.
None of this requires a metallurgist. It requires asking what the part does, where it lives, and what it costs to fail - and letting the answers pick the metal. A China machining supplier can cut either family to the same drawing at the same precision; the alloy decision, and the money and risk attached to it, belongs to the buyer who owns the design.
Frequently Asked Questions
How much more does a stainless part from China cost than an aluminum one?
For an equivalent part, plan on the order of two to three times the machined aluminum price before finishing. The gap comes from costlier stock, slower machining, and faster tool wear, and it widens with part size and complexity. At prototype quantity the multiple can look larger, because fixed costs dominate while the material difference stays modest in absolute terms.
Which is stronger - 6061-T6 aluminum or 304 stainless?
On yield strength they are closer than most people expect: 6061-T6 is roughly 276 MPa and annealed 304 around 215 MPa. Stainless wins on stiffness (about three times the modulus), on strength after work-hardening, and on temperature capability. But aluminum weighs about a third as much, so for a weight- or stiffness-limited design it often wins despite the lower modulus. Decide on strength per weight and on the environment.
Why does 303 stainless appear so often on sourced machined parts?
Because it was designed for the machine. Added sulfur makes the chips break short and keeps them from welding to the tool, so shops can run higher speeds with predictable results. The trade is a small loss of corrosion resistance and weldability compared with 304, which matters little for machined parts that are not welded and do not face aggressive chemistry.
Will a stainless part rust in service?
Stainless is corrosion-resistant, not corrosion-proof. 304 and 316 protect themselves with a chromium oxide layer, and 316 adds molybdenum for chlorides and marine exposure. 303 resists a little less than 304 because of the sulfur that makes it machine well. Clean, passivated machined surfaces behave better than as-received stock in every environment.
Can I prototype in aluminum and produce in stainless?
You can, but budget a design pass and a first-article cycle in the final material. Stainless is about three times denser, machines hotter, and behaves differently on thin walls and threaded features, so dimensions validated in aluminum can shift in steel. If production is stainless, qualify the geometry in stainless before the volume releases.
Which family gives the better machined surface finish?
Aluminum machines to a bright, consistent finish with less effort, which is why visible parts default to it. Stainless reaches the same roughness with more care and more insert changes, and very fine finishes may need grinding or polishing. If the drawing names a roughness value, confirm the shop can hold it in the material you selected.
What tolerances can each family hold in precision machining?
Both hold precision bands - milling and turning at ±0.003 mm, grinding at ±0.002 mm, with capability targets such as Ppk ≥ 1.67 at the initial study on key characteristics and Cpk ≥ 1.33 on general characteristics. Capability is reported per characteristic: every report names the feature, its tolerance band, the sample size, the sigma method, and the study window. - when the geometry suits the process. Aluminum reaches them more easily on thin-walled or complex geometry because it deflects less relative to its cutting forces; stainless needs tighter control of heat, feeds, and tool wear.
Can stainless parts be anodized?
No - anodizing is an aluminum process. Stainless is protected by its own oxide layer and receives passivation, electropolishing, or plating when a surface treatment is needed. If the design wants a colored, wear-resistant surface, that points to aluminum; if it wants a clean, passive surface in a harsh environment, stainless already provides it.
Unsure which grade fits the part?
Send the drawing to [email protected] with the service conditions - environment, load, temperature - and get a material recommendation alongside the quote.