Q: Why is aluminum alloy 7075 not listed in AWS D1.2, Structural Welding Code - Aluminum?
A: Most aluminum alloys are weldable, but a fair number of them are not
, including 7075 aluminum. When designers and welders look for an aluminum alloy to use, many will start by reviewing a table that lists all of the aluminum alloys and their strengths. Alloy 7075 is often selected because it is one of the highest strength aluminum alloys. But, few of the higher strength aluminum alloys are weldable, especially those in the 7000 and 2000 series, and they should not be used.
The one exception to the rule of never using 7075 for welded applications is in the injection molding industry, which uses 7075 dies and will repair them with welding. However, 7075 should not be used for structural work.
When you need to design something of high-strength aluminum, look to a 5000 series high-magnesium alloy instead of a 2000 or 7000 series. The 5000 series alloys are weldable and will produce the best results.
Q: Why is my aluminum welded connection so much weaker than the base material?
A: In steel weldments, a welded connection can be made as strong as the base material, but this is typically not the case with aluminum. In almost all instances, the welded connection will be weaker than the base material.
To understand why this occurs, consider the two classifications of aluminum alloys: heat treatable and nonheat treatable.
The latter category is hardened only by cold working, which causes physical changes in the metal. The more the alloy is cold worked, the stronger it gets. When you weld an alloy that has been cold worked, you locally anneal the material around the weld so it goes back to its zero-tempered (or annealed) condition and it becomes "soft." Therefore, the only time you can make a weld as strong as the base material with a nonheat-treatable alloy is when you start with zero-tempered material.
With heat-treatable aluminum alloys, the last heat treatment step heats the metal to approximately 400?F (200?C). When welding, the material around the weld (the heat-affected zone) becomes much hotter than 400?F so the material tends to lose some of its strength. Unless a postweld heat treatment is applied, the area around the weld will become significantly weaker than the rest of the aluminum - by as much as 30 to 40%.
Post-weld heat treatment can restore this loss in strength if a heat-treatable aluminum is used.
Table 1 is a guide as to which series of aluminum alloys are heat treatable and which are not.
Table 1 - Guide to Heat-Treatable Aluminum Alloys
Heat Treatable Nonheat Treatable