Welding without stress relief can work for the lower frame of larger machines, if the welding is only at the ends of the main rail-mounting tubes or other triangulation, and the machine is heavy enough to plant it to the floor. Then adjustable feet can be used to level the X rail mounting surfaces.
Other options are:
1. Bolt & pin together using lap plates (like your blue plates)
2. Rivet & epoxy together using lap plates
3. Maybe brazing. I don't know if it distorts over time. Probably much less than welding, if at all.