Why impact windows use 6063-T6 aluminum.

What 6063-T6 actually means.

Read the spec sheet for almost any aluminum impact window and you will find the same short code: 6063-T6. It describes the metal the frame is made from, and it has two parts. The four digits, 6063, name the alloy, the recipe of aluminum and other elements. The letter and number after the dash, T6, name the temper, the heat treatment the extrusion received after it was formed. Together they set how strong the frame is, how cleanly it can be shaped, and how well it takes a finish. This guide explains both parts in plain words, and why the code matters on a hurricane-rated product.

The alloy: what the four digits tell you.

The Aluminum Association created the four-digit system for wrought aluminum alloys, adopted in the US in 1954 and later accepted internationally. The first digit tells you the main alloying elements. Window and door frames come from the 6xxx series, which uses magnesium and silicon. Those two elements combine to form magnesium silicide inside the metal, and that is what lets the alloy be strengthened by heat treatment. The 6xxx alloys are heat treatable, highly formable and weldable, with moderately high strength and excellent corrosion resistance. Extrusions from this series are the first choice for architectural and structural work.

• 1xxx: at least 99% aluminum.
• 2xxx: copper.
• 3xxx: manganese.
• 5xxx: magnesium.
• 6xxx: magnesium and silicon. This is where 6063 and 6061 sit.
• 7xxx: zinc.

The temper: how the metal was heat treated.

A frame starts as a heated aluminum billet pushed through a shaped steel die, which is called extrusion. What happens next sets the temper. In the T5 temper, the profile is cooled from the extrusion process and then artificially aged, meaning it is held in a furnace for several hours. In the T6 temper, the profile is solution heat-treated, which means it is brought to a high, uniform temperature and then quenched, before the same artificial aging. That extra step gives up some formability in exchange for meaningfully higher strength. Other common letters are F (as extruded) and O (annealed).

What those strength numbers mean.

Strength is given in ksi, thousands of pounds per square inch. Yield strength is the stress at which the metal starts to bend permanently instead of springing back. Ultimate tensile strength is the stress at which it would break in tension. The Aluminum Design Manual, the Aluminum Association's specification for aluminum structures, gives 6063-T6 a minimum yield strength of 25 ksi and a minimum ultimate tensile strength of 30 ksi. Engineers design with those minimums, not with typical values from a single test. For an impact window, yield strength matters most: a frame that bends permanently under hurricane wind pressure is no longer the shape that was tested.

Why 6063 and not a stronger alloy.

Stronger 6xxx alloys exist; 6061 is the most widely used alloy in the series. So why do window frames use 6063? Because an impact window frame is a complex shape. It has glazing pockets, screw bosses, weatherstrip channels, drainage paths and, on some lines, a slot for a thermal break. 6063 is softer and easier to form than 6061, so it can be pushed through intricate dies with thin walls and crisp detail. Engineers make up the strength difference with wall thickness and profile shape. Mullions, the bars that join windows side by side, are usually 6063 extrusions in the T5 or T6 temper. Choosing the T6 temper adds strength without giving up the alloy's extrusion advantages.

Corrosion resistance and finishing.

South Florida is hard on metal, and the 6xxx series has excellent corrosion resistance to begin with. 6063 also extrudes with a smooth surface that is well suited to finishing and anodizing. That matters because the finish is the frame's first defense against salt air and sun. Anodizing grows a protective oxide layer from the aluminum itself; paint and powder coat add a bonded layer on top. A clean, even extrusion surface helps either kind of finish look uniform and adhere properly. Finish choice still matters near the water, and warranty terms often depend on it, so treat the alloy as the foundation and the finish as the protection.

Why approved frames name the alloy and temper.

An impact window earns its Florida Product Approval or Miami-Dade Notice of Acceptance (NOA) by passing tests on real samples: missile impact, cyclic wind pressure, and structural load. The approval then describes the product as tested. That is why the approval drawings carry a bill of materials listing the alloy and temper of each extrusion. It is normal to see both 6063-T6 and 6063-T5 parts in one product's drawings, each assigned to a specific piece. A frame made from a softer temper than the one listed is not the product that passed the tests, even if it looks identical. The drawings also set minimum alloy, temper and thickness for any aluminum the window is anchored into.

How it shows up in our brands.

Several of the manufacturers we install state the frame alloy and temper directly, and the answer is the same across very different products: single-hung windows, fixed picture windows and a thermally broken door. That consistency is expected, since 6xxx extrusions are the first choice for architectural work. What separates one line from another is everything built around the metal: profile design, wall thickness, glass and anchors. These are the exact entries from each brand's product data.

• GIG: White, bronze or clear anodized aluminum (6063-T6).
• EMC builds from 6063-T6 aluminum.
• Eurocraft EC-100 frame: Aluminum 6063-T6.
• Mr. Glass MG-2500T: Thermally broken: aluminum 6063-T6 with glass-fiber-reinforced polyamide (PA66-GF25) thermal strips.

The approval drawing governs.

The alloy and temper are one line in a much longer specification. Two windows can both be 6063-T6 and still carry very different ratings, because wall thickness, profile shape, glass, reinforcement and anchors all change the result. Do not compare products on the alloy code alone, and do not treat a brochure's material note as a substitute for the approval. The approval for the exact product, size and glass governs, including the materials, anchors and substrate it lists. True Impact confirms the current approval for every opening before we order, and we can walk you through what the drawing shows.