I can understand why you might think that you should weld 5052 sheet with 5052 filler alloy. However, the fact is that 5052 is not available for GTAW or GMAW for the simple reason that it would be a terrible filler metal. Welds made using it would likely crack. Hopefully you remember that weld cracking is hot cracking and that the most important factor in controlling cracking is controlling the chemistry of the solidifying weld. For aluminum alloys that contain magnesium (e.g., 5XXX), you must keep the weld chemistry above approximately 3 percent magnesium to avoid weld cracking. This is relatively easy for most of the 5XXX alloys, which have a minimum of 3 percent magnesium. You can weld alloys like 5454, 5154, 5754, 5083, and 5086 autogenously (without adding filler metal) without experiencing cracking. Of course, in most cases, a filler metal is added. The correct filler metal for the 5XXX alloys is one of the 5XXX fillers, such as 5356, 5554, 5183, or 5556. However, 5052 is the only 5XXX alloy that cannot be welded autogenously. If you try it the weld oftentimes will crack. Why? The nominal chemistry of 5052 is only 2.5 percent magnesium. If you look at crack sensitivity graphs, you will find that alloys that contain around 2.5 percent magnesium have a pretty high crack sensitivity – not as high as 6061 (which you can’t weld autogenously either), but high nevertheless. So you can’t weld 5052 autogenously. If you think about it a bit, you will realize that welding 5052 autogenously is the same as welding it using 5052 filler. Neither is a good idea. So what filler should you use? Not surprisingly, 5356 is a very good choice. More surprisingly, 4043 also is a very good choice. In all other base/filler combinations, it is never recommended to weld a 5XXX alloy with 4043. However, the magnesium content of 5052 is low enough that 4043 works just fine.