Dave, I will quote from some Lincoln Electric literature, " ..7075 is one of the strongest aluminum alloys available, used principally in airframe structures and highly stressed parts. Though not easily welded by GTAW or GMAW processes, it responds well to electrical resistance welding." The major alloying element is Zinc, with magnesium the other principal addition. I am not an engineer but I have a decent library and sometimes I look this stuff up just to educate myself. Good luck.
Respectfully,
Mike Sherman
Shermans Welding
Lets stay away from opinions and rest safely in the embrace of factual data.
7075 aluminum (regardless of heat treatment) is not to be considered weldable with any ARC or Gas welding process. There are no exceptions, not for ornamental applications, not for "G" jobs, not at home, not in a box, and not with a fox. This one is not up for debate! Yes resistance welds are possible, but even these must be proven through special trials before production is considered.
When anybody, it doesn't matter how impressive their resume', posts advice which could end in catastrophic failure of what has obviously been described as a load bearing joint, a clear response is required. Many folks use these postings for decision making purposes and bad advice occurs all too often.
The ASM Handbook Vol. 6 presents for us on pages 731-735 a table, (14). This 5 page table provides Relative ratings of selected aluminum filler alloys used for fillet and butt welds of aluminum alloys. 7075 is not even mentioned! Why? Because it can't be successfully welded. This fact is stated with even greater authority on Page 723 Table 2 "Composition, physical properties, and weldability of heat-treatable wrought aluminum alloys.
You're in a tough spot Dave. The AWS in its objective literature regarding CWI's and Qualifications for "Entry Level", "Advanced" and "Expert" welders (Level 1,2, & 3) say a lot about Integrity, Attitude and Professionalism, going as far as to provide a code of conduct. Its in situations such as yours that the rubber meets the road.
My considered advice is that you respectfully propose another alloy for your project. You have the collective data of an entire industry behind you. Stand firm on this one. Arc welding 7075, under any circumstance, especially load bearing joints, for which YOU WILL be held responsible, must be a dealbreaker.
Lawrence Bower
Welding Instructor
United Airlines