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Weld crackling problem using 6063-T6 sheet material

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QUESTION: I am experiencing a weld cracking problem on our TIG (GTAW) production line where we weld thinner sections of 6063-T6 sheet material. We are often required to perform outside corner welds where we sometimes use little or no R4043 filler material, dependent on the joint fit up. Why do you think my welds are cracking? And why is it that not all of my welds, but only some of them are cracking?

ANSWER: We should start by considering the crack sensitivity of the 6xxx series base material. The aluminum/magnesium/silicon base alloys (6xxx series) are highly crack sensitive because they contain approximately 1.0% Magnesium Silicide (Mg2Si), which falls close to the peak of the solidification crack sensitivity curve (Fig 1 at Al-Mg2Si curve).

Fig 1

The Mg2Si content of these materials is the primary reason that there are no 6xxx series filler alloys made. The cracking tendency of these alloys is lowered to acceptable levels during arc welding by the dilution of the weld pool with excess magnesium (by use of the 5xxx series Al-Mg filler alloys) or excess silicon (by use of the 4xxx series Al-Si filler alloys).

When we TIG (GTAW) weld on thin material, it is often possible to produce a weld, particularly on corner joints, by melting both edges of the base material together without adding filler material. In the majority of arc welding applications with this base material, we must add filler material if we want to have consistently crack free welds. A possible exception would be counteracting the cracking mechanism by maintaining a compressive force on the parts during the welding operation, which requires specialized fabrication techniques and considerations. This method is seldom used.

I suspect that the welds in question that are not cracking are those that have had filler material added during welding. My advice would be to ensure that filler alloy is added to all welds during welding in order to reduce crack sensitivity. Consideration should also be given when evaluating the cause of cracking to any differences in welds associated with weld size, and variations in tensile stresses introduced by shrinkage, joint expansion, or externally applied loads.