There are many accepted ways of making a welded joint for grease exhaust ductwork. The decisions will count on the code in your jurisdiction but really should include things like flanged, telescoping, Bell kind or overlapping joints. Promat has made self-supporting systems with no metal liner, by way of example https://greasetightextractductwor91233.blogdomago.com/35652166/5-essential-elements-for-grease-tight-extract-ductwork