Why is my aluminum weld much weaker than the parent material?
aluminiumsheet > 12-17-2019, 06:34 AM
In steels a weld can be made as strong as the parent material, but this is not the case with aluminum. In almost all instances, the weld will be weaker than the parent material.
To further understand why this occurs, let's look at the two classifications of aluminum alloys: heat treatable and non-heat 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.
But, when you weld an alloy that has been cold worked, you locally anneal the material around the weld so that it goes back to its 0 tempered (or annealed) condition and it becomes "soft". Therefore, the only time in the non-heat treatable alloys that you can make a weld as strong as the parent material is when you start with 0 tempered material.
With heat treatable aluminum alloys, the last heat treatment step heats the metal to approximately 400° F. But when welding, the material around the weld becomes much hotter than 400° F so the material tends to lose some of its mechanical properties. Therefore, if the operator doesn't perform post-weld heat treatments after welding, the area around the weld will become significantly weaker than the rest of the aluminum - by as much as 30 to 40 percent. If the operator does perform post weld heat treatments, the proprieties of a heat treatable aluminum alloy can be improved.
The following is a guide as to which series of aluminum alloys are heat treatable and which are not:
Heat treatable series: 2000, 6000, 7000.
Non-heat treatable: 1000, 3000, 4000, 5000.