What is the best grade of aluminum to weld?

The 1000, 3000 and 5000 series, which are not heat treated, are the easiest aluminium alloys to weld.
The 6000 series is also easily weldable. However, depending on the welding technique, the joining process may cause a decrease in the mechanical properties of the aluminium.
The high strength alloys of the 2000 and 4000 families are more complex to weld and require special precautions.
Finally, some high strength aluminium such as the 7000 series can only be welded using the innovative FSW process.