7075 cannot be safely welded for structural purposes, and here's why.
What is 7075 Aluminum?
7075 aluminum is actually an aluminum alloy. It's mostly aluminum, but also contains zinc (typically 5-6%) and a few other metals like magnesium, copper, and silicon. It's sometimes sold under brand names like Zicral and Ergal.
This alloy is extremely expensive, which limits its use. However, it's also very light and strong (although its exact tensile strength depends on how it's tempered). A Japanese company originally developed this alloy as a secret material to use in warships during World War II, but now this material sees industrial applications around the globe.
It is technically possible to weld 7075 aluminum. If a welder applies heat, they can join two pieces together. But the result isn't what a layperson might think.
When a welder applies high heat to 7075 aluminum, it creates microcracks as it cools. These are too small to be seen by the naked eye, so a casual observer would think the 7075 aluminum welded just fine.
Unfortunately, these microcracks have a serious impact on the tensile strength of the metal, and makes the weld very brittle. It doesn't take a lot of pressure for the weld to snap.
It's true that it's still possible to weld 7075 aluminum for some industrial applications, such as repairing molds for injection or blow molding plastics. However, these are extremely limited, non-structural applications -- and structural applications are exactly where this strong, lightweight material excels.
Some researchers are developing methods for safely welding 7075 aluminum and other alloys. It's possible that down the line these techniques will see industrial application, but they take highly specialized materials and processes. Right now, it's impossible for the average welder to safely work with 7075 aluminum.
Just because a welder can do something doesn't mean they should. It is technically possible to weld 7075 aluminum and comparable alloys, but never for any structural applications. The result simply isn't safe.