Aluminum 6061-T6: General-Purpose Structural Al-Mg-Si Alloy — Weldable, Corrosion-Resistant

ER4043 (Al-5%Si) is the standard choice: lower melting point, better fluidity, less hot-cracking sensitivity, and provides a cosmetically appealing weld. ER5356 (Al-5%Mg) is preferred when: (1) the weld will be anodized (ER4043 welds turn dark gray/black during anodizing due to silicon content — unacceptable for architectural applications; ER5356 anodizes to match the base metal); (2) higher weld strength is needed (ER5356 deposits are approximately 15-20% stronger than ER4043); (3) service temperature exceeds 65°C continuously (ER4043 over-ages faster at elevated temperature). The tradeoff: ER5356 is slightly more difficult to weld — higher melting point, slightly higher hot-cracking sensitivity in highly restrained joints.

T6 temper (fully solution-treated and artificially aged) has limited formability — minimum bend radius approximately 3t for sheet (3× thickness) with risk of orange-peel and cracking at tighter radii. For forming operations, specify 6061-O (annealed — fully recrystallized, excellent formability), form the part, then solution-treat and age to T6 after forming. If post-forming heat treatment is not possible, 6061-T4 (solution-treated and naturally aged — intermediate strength with improved formability) can be bent to approximately 1.5-2t radii, and will naturally age toward T6 properties over several weeks at room temperature.