6061 vs 6063 aluminium extrusion: which alloy should you choose?

The short answer

One alloying element explains most of the difference

Both alloys are Al-Mg-Si from the 6000 series, and on a datasheet they look almost identical. The difference that matters is copper. 6061 carries 0.15 to 0.4 percent copper, magnesium at 0.8 to 1.2 percent and silicon at 0.4 to 0.8 percent. 6063 keeps copper under 0.1 percent, with magnesium at 0.45 to 0.9 and silicon at 0.2 to 0.6. That small addition of copper raises the strength reached after artificial ageing, and it also changes how the metal flows through a die and how evenly it takes an anodic film. Almost every trade-off on this page, from press speed to anodising streaks, follows from those numbers rather than from anything the sales sheet says about "premium alloy".

The practical consequence is that the alloy decision should be made before the cross-section is drawn, not after. A die is cut for one alloy family and one flow behaviour; if the profile is designed around 6063 and the structural calculation later demands 6061, the section usually has to be redrawn thicker and simpler rather than simply re-quoted. Buyers who reverse that order end up paying for a die twice.

Strength: what the numbers do and do not buy you

In the T6 temper 6061 reaches a typical 290 to 310 MPa tensile and 240 to 276 MPa yield, with elongation around 8 to 10 percent. 6063 in T6 sits at roughly 186 to 240 MPa tensile and 145 to 214 MPa yield; in T5, the temper most architectural profile is actually bought in, the figures are lower again, near 205 MPa tensile and 180 MPa yield. The exact values depend on section thickness and on which edition of the product standard the mill works to, so treat any single number as indicative and check the mill test record for the heat you receive.

Two things follow. First, the strength gap is real but narrower than the headline 40 percent suggests once you compare like tempers and like sections. Second, and more important for design, both alloys have essentially the same elastic modulus, around 68 to 70 GPa. Stiffness therefore comes from geometry, not from alloy: a deeper section in 6063 will usually deflect less than a shallow one in 6061, at lower cost. Alloy choice buys yield strength, fatigue performance and the ability to survive a proof load. It does not buy rigidity, and specifying 6061 to fix a deflection problem is one of the most common and most expensive mistakes in extrusion design.

Extrudability decides the shape you can afford

Extrudability is not a vague virtue; it sets press speed, scrap rate and the thinnest wall the die can hold. 6063 is the reference alloy of the industry because it flows at low pressure: a profile that runs at 50 to 60 feet per minute in 6063 typically runs at 30 to 40 feet per minute in 6061, and the slower pass means more furnace time per kilogram, more die wear and a higher scrap rate on start-up. Where the geometry is demanding, the gap widens further. Complex hollow sections with internal webs, multi-cavity heatsink profiles and walls down to 1.0 mm are routine in 6063 and marginal or impossible in 6061.

The design rule that follows is blunt: if the profile needs walls below about 1.25 mm, deep narrow slots, or several internal cavities, 6063 is not a preference but a requirement. 6061 generally needs roughly 25 percent more wall thickness for the same geometry, which erodes much of the weight advantage a buyer hoped to gain by choosing the stronger alloy. Die cost also moves: a harder alloy needs more bearing length and more robust tooling, and 6082 tooling typically costs 10 to 20 percent more than the same section in 6063.

Surface, anodising and heat

For anything visible, 6063 is the safer specification. Its lower impurity content and finer grain produce a smoother as-extruded surface, and the anodic film grows more uniformly, so clear and coloured anodising comes out even across a batch. 6061 anodises perfectly well for protective purposes but the copper content tends to produce a slightly darker, less uniform appearance and occasional streaking, which is why decorative architectural work almost never specifies it. If the part is to be powder coated rather than anodised, this advantage largely disappears, because the coating hides the substrate.

Thermal behaviour runs the other way from strength: 6063 conducts about 200 watts per metre-kelvin against roughly 167 for 6061, which is why LED housings and heatsink profiles are almost always 6063. Where the extrusion is both a heatsink and a structural member, the usual answer is a 6063 body with 6061 or steel inserts at the load path, rather than compromising both functions in one alloy.

Cost is per metre, not per kilogram

6063 is normally quoted 10 to 15 percent below 6061 on metal, and the gap widens once press speed and scrap are included: a faster pass uses the press less, generates less butt scrap and wears the die more slowly. But the comparison that matters is cost per finished metre or per finished part, not per kilogram. A 6061 profile that must be 25 percent thicker to extrude cleanly can easily cost more per metre than the 6063 section it replaced, while weighing more, and it may then need a machining operation to hold the tolerance the thicker wall was supposed to protect. The reverse is also true: on a heavily machined bracket, 6061 machines faster and holds a better finish, so the higher metal price is recovered at the spindle.

A decision rule that holds up in a quote review

Ask four questions in order. Does the section carry a calculated load, or survive a proof or fatigue case? If yes, start at 6061-T6 and check whether 6082 is needed. If no, start at 6063. Is the surface visible and anodised? If yes, stay at 6063 unless strength forces you up, and expect a cosmetic penalty if it does. Are the walls below 1.25 mm or the section complex? If yes, 6063 only. Finally, set the temper honestly: T5 is adequate and cheaper for most architectural and enclosure work, while T6 is for strength and for parts that will be machined after extrusion.

Where the answer is genuinely mixed, split the assembly. Visible covers and cosmetic profiles in 6063, hidden structural members in 6061, fasteners and inserts in steel or stainless: that combination is common in machine guards, display fixtures and modular frames, and it usually beats forcing one alloy to do both jobs.

See CNC machining for the secondary operations that turn an extruded profile into a finished part, surface finishing for anodising and powder coating on extrusion, and sheet metal fabrication where a folded part competes with an extruded one.