6063 aluminum alloy is the extrusion workhorse of the industry: architectural profiles, curtain-wall frames, handrails and structural members. It welds more easily than stronger heat-treatable alloys such as 6061 - less heat-affected-zone (HAZ) distortion - but like all age-hardening alloys it loses strength in the weld zone, and that loss must be engineered around. This article sets out the composition, welding procedure, and heat-treatment sequence with the numbers you need.
1. Material Baseline of 6063 Aluminum
6063 is an Al-Mg-Si alloy: magnesium 0.45–0.9%, silicon 0.2–0.6%, remainder aluminium. It is strengthened by precipitation of Mg₂Si. In the T6 condition it delivers roughly 240 MPa tensile strength and about 215 MPa yield with excellent surface finish and anodizing response - which is why it dominates extruded architectural sections.
2. Weldability: What the HAZ Really Does
6063 produces smooth weld seams and low distortion, but the mechanical strength in the weld and HAZ drops by 20–50%, with an average reduction of about 30%. The actual loss depends on the base temper (T5, T6), the welding process, heat input and travel speed. For structural loads, either post-weld re-aging or strength-compensated design (thicker section at the joint) is required.
3. Welding Methods
TIG (GTAW): the choice when weld quality and appearance matter - clean, visually appealing beads, low heat input, minimal distortion; ideal for architectural and decorative work.
MIG (GMAW): the choice for productivity - thick-walled extrusions, large cross-section profiles, automated or high-volume manufacturing.
4. Filler Selection: 4043 vs 5356
| Filler | Strengths | Trade-offs |
| ER4043 (Al-Si5) | Excellent flow, smooth bead, good hot-crack resistance, matches anodized colour well | Lower weld strength than 5356; bead may darken after anodizing |
| ER5356 (Al-Mg5) | Higher tensile strength; tougher welds where mechanical performance matters | Contains magnesium - anodizes to a different shade; lower ductility; higher crack sensitivity on 6063 in some configurations |
For most 6063 architectural work, ER4043 is the safe default; choose ER5356 when the joint must carry higher load and colour matching after anodizing is not critical.
5. Welding Procedure
- Surface preparation: the Al₂O₃ oxide film melts far above the base metal, so it must be removed - brush with a dedicated stainless-steel wire brush (used only on aluminum) or clean chemically until the surface is bright and metallic.
- Shielding gas: high-purity argon ≥99.99%, flow 8–12 L/min; proper shielding prevents porosity and oxidation.
- Preheating: for wall thickness above 8 mm, preheat to 100–150 °C to improve penetration and reduce the chance of cold cracking; keep preheat low - excessive heat defeats the purpose.
- Parameters: use relatively low welding current and a fast travel speed to limit total heat input; this narrows the HAZ and limits strength degradation.
- Post-weld: allow welded components to cool naturally (no forced quench); where higher strength is required, apply post-weld re-aging - particularly for T6 material.
6. Heat Treatment Principles
The objective of 6063 heat treatment is to control Mg₂Si precipitation, balancing strength, ductility and extrusion response. There are two heat-treatment stages plus a production shortcut:
Solution treatment: heat to 530–540 °C (holding 1–2 hours depending on section thickness) to dissolve the Mg₂Si into solid solution, then water quench with a cooling rate of at least 150 °C/min to freeze the supersaturated solid solution. Rapid quenching prevents premature precipitation and gives uniform properties through the section.
Artificial aging - T6: age at 170–180 °C for 8–12 hours, air cool. This delivers the highest achievable strength for 6063.
On-line aging - T5: performed directly after extrusion (quench at the press, then age at 150–170 °C for 4–6 hours). T5 offers high production efficiency with well-balanced mechanical properties and is the most common temper for architectural extrusions.
Natural aging: 7–14 days at room temperature reaches approximately 70–80% of T6 strength - suitable where maximum strength is not essential and lead time permits.
7. Design Guidance
- For welded structural assemblies, size the joint for the ~30% reduced HAZ strength, or specify post-weld re-aging.
- If the part will be anodized after welding, confirm filler choice with the anodizer - 4043 and 5356 develop visibly different colours.
- Document heat-treatment parameters and furnace calibration; verify final properties by hardness testing (e.g., 6063-T6 ≈ 73–80 HB).
