Ok, maybe some helpful back ground, simply:
1. Steel and Alum are really two different animals, in many ways, fatigue, and weldability
2. Some Alum alloys are stronger then steel, but don't weld well, are very expensive and only used in special applications. Some of these alloys are described as not heat treatable.
3. The alloys that are heat treatable and weld well are strong in their heat treated condition and affordable.
4. All alum eventually fatigues, lower stress levels means more cycles before failure, but the outcome is the same. Steel has an almost infinite fatigue life at low levels.
5. ANY welding on ALUm creates a HAZ (heat effected zone), since the part is never at the same temp evenly during the weld process at the weld area.
6. Since a lot of the strength, sought in the heat treatable alum alloys mentioned above, is gained by heat treating at the factory the raw material, the weld process reverts the HAZ back to a natural and less strong alloy in the weld area. You cannot avoid that. The problem is, the joint is always weaker then the surrounding metal in alum.
7. This weakness also causes an inherent stress riser.
8. Things don't like getting bent under load ( such as a k member), but they really hate getting bent both ways.
9. Heat treating a welded alum part is seldom and not easily done, distortion being a big issue.

All the above is easily found and backed up online.