Aluminum Workshop: Weldable and unweldable aluminum alloys

Q: I want to weld on some 7075 aluminum, but I can't find a recommended filler metal for it. Can you tell me the correct filler metal to use?

A: Most aluminum alloys are readily weldable using GTAW or GMAW. However, some are not. Let's take a quick look at the common families of aluminum alloys and their weldability characteristics:

• 1XXX alloys. Essentially pure aluminum (99 percent pure) and used to carry electrical current or for corrosion resistance in specific environments, these alloys are all readily weldable. The most common filler metal is 1100.
  
  
• 3XXX alloys. This family comprises medium-strength alloys that are very formable. They often are used for heat exchangers and air conditioners. All are readily weldable using either 4043 or 5356 filler metal.
  
  
• 4XXX alloys. These usually are used as welding or brazing filler alloys. However, they are sometimes used as base materials. If that is the case, they are readily welded with 4043 filler metal.
  
  
• 5XXX alloys. This is a family of high-strength sheet and plate alloys. All of them are easily welded using 5356 filler metal, although 5183 or 5556 should be used for the stronger alloys, such as 5083.
  
  
• 6XXX alloys. These are primarily the extrusion alloys, although they are available in sheet and plate as well. They are prone to be crack-sensitive. However, with the proper techniques, they can all be readily welded using 4043 or 5356.

So why haven't I mentioned the 2XXX and 7XXX alloys yet?

• 2XXX alloys. These are high-strength aerospace alloys in sheet or plate form. Their chemistry makes most of them unweldable using GTAW or GMAW because of hot cracking. The exceptions are 2219 and 2519, which are both readily welded using 2319 or 4043 filler metal. In any case, you should never weld 2024. It is very common and very high in strength, but it is extremely crack-sensitive.
  
  
• 7XXX alloys. This too is a family of high-strength aerospace alloys. Like the 2XXX alloys, most of them are unweldable using GTAW or GMAW because of hot-cracking and stress-corrosion concerns. The exceptions are 7003 and 7005 extrusion alloys and 7039 plate alloy. All three of these are readily weldable using 5356 filler. Never weld 7075.