Titanium can be explosively welded especially in tubejoint type configurations. It can be welded to almost any other metal with this process. It is interesting since it does not exibit the characteristic wavy interface usually found in explosive welding (bonding).
Explosivew welding will provide the greatest joint integrety possible with alloy 20 or almost any other tubesheet to tube material combination. It is somewhat restrictive in its' design and use to specialty companies but has no heat treatment requirements preheat.
While washers are usually not a Code issue, they can be part of the manufacurers requirements. Sometimes Belville style washers are used to keep gasket loaded after operation especially in cyclic duty. Some lock style washers are often a manufacturers requirement where vibration can loosen nuts...
