Aircraft have long been built from aluminum. In contrast, SpaceX’s Starship rocket has a stainless steel outer skin. It’s heavier but stronger, so which is the right choice? This stainless versus aluminum debate is interesting for anyone designing or buying fabricated metal assemblies or structures. In this blog, we’ll compare and contrast the two alloys and review the main considerations when choosing between them for your next metal fabrication project.
Alloy Considerations in Metal Fabrication
Tool cabinets, storage vessels, wire cages, and mezzanine floors and staircases are examples of things produced by fabrication. For these and more, the main processes are:
- Cutting from plate or sheet
- Cutting tube, angle, and section to length
Material properties are key to how these operations are performed, or whether they can be performed at all. Alloys suited for metal fabrication combine strength with ductility so they can be formed without cracking. If they will be welded, it’s important to consider how they behave as they melt and solidify.
Properties of Stainless Steel
“Stainless” refers to a family of ferrous alloys containing more than 10.5% chromium. This element is primarily what gives stainless its famous corrosion resistance, although some grades kick it up a notch by adding nickel too.
Stainless steel is produced in multiple forms, the most widely used being austenitic. (This refers to the atomic structure.) Austentic stainless steel contains 16% chromium and 6% nickel. Austenitic grades are identified by a three-digit code starting with a 3.
Austenitic stainless steels are both weldable and ductile, making them readily fabricated. From an application perspective, the yield strength of 304 stainless, the grade most widely used in metal fabrication, is 31,200 psi. Density is 0.289 lb/in³.
An important reason for choosing stainless steel is its strength at high and low temperatures. For 304 grade, yield strength at 400°F is only around 20% lower than at ambient, which is markedly better than aluminum.
Properties of Aluminum
Like stainless, aluminum alloy is naturally corrosion-resistant and is produced in a wide range of compositions. These are grouped into seven series, ranging from almost pure aluminum 1xxx series to the aerospace-grade 7xxx series. Of these, the 3xxx, 5xxx, and 6xxx series are considered the most suitable for fabrication. Their ductility is good and they are weldable.
The 6xxx series of aluminum alloys, and especially the “workhorse” grade 6061, can be heat treated to raise hardness. This elevates the strength of 6061 to 40,000 psi but significantly reduces ductility. With a density of 0.0975 lb/in³, it’s fair to say this alloy is light but strong.
A major limitation of aluminum is that yield strength drops more than 50% as the temperature rises above 400°F.
One of the justifications for using stainless steel in the SpaceX Starship was that it cost around $1.50/lb. In contrast, heat heat-treated 6061 plate is close to $10/lb.
Material cost is not the whole story though. The cost of cutting and forming should also be considered, along with the value placed on scrap material. Heat-treated 6061 aside, aluminum can generally be cut and bent faster and with lower forces. This means aluminum fabrication jobs can often run on smaller, less expensive machinery.
An additional consideration is the strong market for scrap aluminum. Scrap aluminum is always in demand because recycling takes less energy than smelting new alloys from bauxite. As a result, it’s always some cost recovery is always possible.
Applications and Use Cases
Stainless steel and aluminum are both used when corrosion resistance is sought and a bare metal appearance is desired. Unlike other grades of steel, these don’t need finishing operations like painting or powder coating, which helps offset the higher raw material cost. Used in food preparation equipment, this eliminates the risk of flaking paint particles.
If desired, both stainless and aluminum can be polished to a highly reflective finish or brushed for a matte directional appearance.
Aluminum is a lightweight material, which makes it attractive in motion and transportation applications where increased inertia could affect performance, increase energy costs, or reduce payload. Stainless steel is significantly more dense and therefore fabrications using it tend to be heavier. It’s also stronger than most grades of aluminum, so it’s widely used in structural applications.
An important differentiator is strength at high temperatures. Here stainless is clearly superior, making it the preferred choice in oven and furnace applications.
Environmental and Sustainability Factors
Ease of recycling means aluminum is seen as a “green” material that plays a part in the “circular economy”. Stainless can be recycled but a higher melting point and complex compositions make this less attractive economically.
As Your Metal Fabrication Shop
In choosing an alloy for the Starship, one of the deciding factors was high-temperature strength. This is what made stainless the clear winner. For other fabrication projects, the choice is not so clear-cut. Most grades of stainless are stronger and harder, but when mass is a consideration aluminum is often the better choice.
Complicating matters, both alloys are produced in multiple grades and compositions, and price, formability, and weldability vary widely. Given the number of points to consider, the best approach is usually to get advice from your preferred fabricator.
Tap Into Wiley Metal’s Expertise
We’ve been in the metal fabrication business a long time and there’s not much we haven’t seen. We’re very familiar with both stainless steel and aluminum and will be happy to discuss the merits and demerits of each. Contact us and let’s talk.