Quick answer: pick by cross-section and load, not by price
For most fenestration and architectural trim — windows, doors, curtain-wall mullions, handrails, lighting housings — 6063 is the right alloy: it extrudes cleanly into thin, complex profiles and takes anodizing to a bright, consistent finish. For load-bearing structural parts — machine frames, brackets, transport chassis, marine and tooling components — 6061 is the safer choice because its higher strength (and copper content) carry the load. The two alloys are close in base-metal price, so the real cost driver is not the alloy but whether you picked the one your profile geometry and application actually need.
What separates 6063 from 6061
Both are 6000-series Al-Mg-Si alloys, heat-treatable and weldable, but they are tuned for different jobs:
| Property | 6063 (UNS A96063) | 6061 (UNS A96061) |
|---|---|---|
| Magnesium | 0.7% | 1.0% |
| Silicon | 0.4% | 0.6% |
| Copper | — | 0.25% |
| Chromium | — | 0.20% |
| UTS, T6 temper | ~240 MPa | ~310 MPa |
| Yield, T6 temper | ~215 MPa | ~276 MPa |
| Extrudability | Excellent (complex thin walls) | Good (thicker walls) |
| Anodizing response | Bright, clear finish | Grayer, satin finish |
| Typical use | Fenestration, trim, hollow profiles | Structural frames, machined parts |
The copper and chromium in 6061 are what give it roughly 30% higher strength than 6063-T6 — and also why it extrudes harder, finishes grayer, and is less ideal for fine, decorative cross-sections. For a broader look at how 6061 fits the aluminum family, see our 6061 vs 5052 comparison and the temper detail in 6061-T6 vs 6061-T651.
Extrudability and surface finish: why 6063 owns the window market
6063 is nicknamed the “architectural alloy” for a reason. Its low alloy content lets the metal flow through complex hollow dies at high speed, producing thin walls, sharp corners, and tight dimensional tolerances that 6061 simply will not hold at the same press speed. It also anodizes to a clear, specular finish that 6061 — with its copper — cannot match.
What that means for buyers:
- If the profile has thin walls (under ~1.5 mm), multiple hollows, or a visible decorative surface, 6063 is almost always the better choice and usually the cheaper one.
- 6063-T5 (air-cooled from the press) is enough for most non-structural fenestration; 6063-T6 (water-quenched and aged) adds strength where a mullion carries wind load.
- For anodized architectural systems, specify the anodizing layer (AA10 / AA15 / AA20 per AAM, in microns) so finish life is defined, not assumed.
Strength and structural use: where 6061 pulls ahead
When the profile is a beam, bracket, gusset, or machined component carrying real load, 6061-T6’s ~276 MPa yield is the difference between a part that holds and one that deflects. 6061 also machines better than 6063 (cleaner chips, tighter tolerance) and is the default for CNC-machined aluminum parts.
Notes for structural buyers:
- 6061-T6 loses strength in the weld heat-affected zone (HAZ), dropping toward the T4 condition. Design the weld zone with that strength reduction in mind, or specify re-heat-treatment after welding for critical parts.
- For marine structural aluminum, 5000-series alloys (non-heat-treatable, higher magnesium) often outperform 6061 in seawater. See our 5083 vs 5052 and 5052 marine buyers guide.
- For sheet rather than extrusion, our 3003 vs 5052 sheet guide and 5052-H32 vs H34 temper guide cover the non-extrusion alloys.
Cost and availability
6063 and 6061 billet are both commodity-priced and widely available, so the alloy choice rarely moves the price more than a few percent. What does move price is the die: a custom 6063 hollow die can be cheap because the alloy extrudes fast and the die lasts; the same profile in 6061 may need a slower press speed, more die wear, and a higher scrap rate. For standard architectural shapes (angles, channels, tubes, standard window sections), 6063 inventory is deep and lead times are short. For load-bearing tread or floor plate in aluminum, our aluminum checker plate buying guide covers the relevant alloys and patterns.
What to specify on your purchase order
- Alloy and temper: 6063-T5 / 6063-T6 for fenestration; 6061-T6 for structural. Do not leave temper open.
- Profile drawing with dimensional tolerances — reference EN 755-9 or ASTM B221 so the press knows the tolerance class.
- Wall-thickness minimums and any critical-to-function dimensions.
- Surface finish: mill, anodized, or powder-coated, with anodizing thickness (AA10/AA15/AA20) or paint system (polyester / PVDF) and gloss.
- Cut length and tolerance; squareness of cut ends.
- Mill certificate to confirm chemistry — see our MTC reading guide.
- For structural work: mechanical property certificate (UTS/YS/elongation) per heat, and whether re-heat-treatment after fabrication is required.
- Packaging: interleaving paper and nesting to protect anodized surfaces in transit.
Common mistakes buyers make
- Specifying 6061 for a decorative hollow profile “because it is stronger.” The result is a slower extrusion, grayer anodizing, and a higher price for strength the part does not need.
- Specifying 6063-T5 for a wind-loaded mullion when the design needs T6. T5 is cheaper but ~25% weaker than T6.
- Leaving temper blank. “6063” alone on a PO lets the supplier ship whatever is in stock.
- Not defining anodizing thickness. A “natural anodized” finish can mean 5 µm or 20 µm — very different outdoor life.
- Ignoring the HAZ in welded 6061 structures. A weld at mid-span on a 6061-T6 beam does not carry T6 strength locally.
Bottom line
Match the alloy to the job, not the price tag. 6063 for appearance-driven, thin-wall, complex fenestration and trim — usually cheaper and better-looking. 6061 for load-bearing, machined, or structural parts where strength and machinability matter more than a bright finish. Lock the temper, the tolerance standard, and the anodizing thickness on the PO, and verify chemistry on the mill certificate. Get those right and the right profile shows up on time, on spec, and ready to install.