Ahh, the old pulse welding trick! ;)
In general, many strengthened aluminum alloys (T4, T5, T6, or H conditions), whether they are heat treatable or stain hardened, lose strength and ductility when they are welded. Some alloys lose strength and ductility more readily than others. From a practical stand point, only the "as wrought" or "in the annealed condition" can be welded without experiencing a degradation of mechanical properties.
Best regards - Al