The other day I cut apart a largish (2.5'x6') air tank (125psi max) made in 1960 and noted the construction. The stamp formed bowl shaped ends were mig welded on as were all the fittings. The tank body was made of a rolled sheet and was overlapped and copper? brazed along the seam. What is the reason for the brazing? Why not just mig the whole thing? Does the braze hold up better under the stress of flexing?