Carbon Fiber vs Aluminum

Carbon fiber is lighter than aluminum, but the rest of the comparison is less one-sided: carbon is far ahead in some categories and behind in others. The figures here are typical values for common grades, not design allowables.

Weight

Aluminum alloys weigh about 2.7 g/cm³. A carbon fiber laminate is about 1.5 to 1.6 g/cm³. Same volume, the carbon part is roughly 40 percent lighter.

Stiffness

This is where it gets less obvious. 6061-T6 aluminum has a modulus of about 69 GPa, in every direction. A quasi-isotropic carbon laminate, with plies running in several directions, is often around 45 to 55 GPa in-plane. So a carbon panel of the same thickness can be slightly less stiff than aluminum. Per kilogram it's stiffer, but by less than is often assumed.

The big gains come from putting the fiber where the load is. A laminate with most of its fibers running along the load can be roughly twice as stiff as aluminum in that direction, at well under two thirds of the weight. That's why carbon shines in tubes, beams and panels with a clear load path, and why a carbon copy of a metal part, laid up any old way, often disappoints.

Strength

6061-T6 breaks at around 310 MPa, and 7075-T6 at around 570 MPa. A quasi-isotropic carbon laminate is commonly in the 500 to 800 MPa range in tension, and a unidirectional laminate along its fibers can go well past 1,500 MPa. Compression strength is lower, and strength through the thickness is low, so bolted joints, edges and impact zones need extra care.

Fatigue and damage

Aluminum has no true fatigue limit: given enough load cycles, it eventually cracks. Carbon laminates generally handle cyclic loads well along the fibers. On the other hand, aluminum dents where it's hit and shows you the damage. Carbon can take an impact, look fine, and be delaminated inside.

Heat

Aluminum grows about 23 µm per meter for every degree Celsius. A quasi-isotropic carbon laminate grows only about 2 to 4, which is why carbon is popular for precision fixtures and anything that has to hold its size as the temperature changes. Aluminum conducts heat very well. Carbon laminates conduct it poorly, especially through the thickness. And the resin in carbon softens near its glass transition temperature, which for some epoxies is not much above 100 °C.

Corrosion

Carbon doesn't corrode, but it conducts electricity and causes galvanic corrosion when it touches bare aluminum in wet or salty conditions. Mixing the two takes an insulating layer at the joint. More on that in when to use carbon fiber.

Cost and lead time

An aluminum part can be machined from bar stock the same week. A carbon part needs a mold, a layup and a cure, so it usually costs several times more per part at low volumes and takes longer. Tooled processes like compression molding close the gap as volumes grow.

Repair and recycling

Most aluminum alloys can be welded or straightened, and aluminum is easy to recycle. Carbon repairs are possible but specialized, and recycling options are still limited.

So which one?

Pick aluminum when cost, speed, heat conduction, impact tolerance or easy repair matter more than the last few hundred grams. Pick carbon fiber when weight or stiffness per weight is the point, the loads are well understood, and the volume justifies the tooling. A lot of good products use both, like our SSD Enclosure 1, which pairs a carbon fiber shell with a 6061-T6 aluminum backplate.

If you're deciding for a specific part, send it over: this comparison is the first thing we look at on any carbon fiber project.