As mentioned above, pure aluminium is a relatively soft metal. But when combined with alloying elements it can produce a wide range of mechanical properties. These alloys are categorized into families according to the principal alloying elements by a four-digit identification system. Here is an overview of the of common families of aluminum alloys and their weldability characteristics/common filler metals:
Pure (99%) aluminum. Used to carry electrical current or for corrosion resistance in particular environments. 1000 series aluminum alloys are readily weldable with 1100 filler metal.
High-strength sheet or plate aerospace alloys. They are extremely sensitive to hot cracking, making them the most unweldable aluminum alloys (specifically 2024). But there are a couple exceptions. Specifically 2219 and 2519, which can be readily welded with 2319 or 4043 filler metal.
Medium-strength aluminum alloys. They are very formable and are often are used for heat exchangers and air conditioners. 3000 series aluminum alloys are readily weldable with either 4043 or 5356 filler metal.
Typically used as welding or brazing filler alloys than as base materials. However if being used as base materials, 4000 series aluminum alloys are readily welded with 4043 filler metal.
High-strength sheet and plate alloys. 5000 series aluminum alloys are readily welded with 5356 filler metal. However with stronger alloys, such as 5083, 5183 or 5556 filler metals should be used.
Primarily the extrusion alloys, but available in sheet and plate as well. 6000 series aluminum alloys are tricky to weld because they are prone to cracking. However, with the proper techniques, they can be readily welded using 4043 or 5356 filler metals.
High-strength aerospace alloys. These alloys are mostly unweldable due to their susceptibility to hot-cracking and stress-corrosion (specially 7075). The exceptions are 7003 and 7005 extrusion alloys and 7039 plate alloy, which are readily weldable with 5356 filler. The alloys can be further classified according to being non-heat-treatable or heat-treatable alloys.