If the two parts do not share nodes and you want to tie them together, please try *contact_tied_nodes_to_surf (or surf_to_surf). *contact_tied_..._to_surface are constraint-based contacts, and nodes from surface A will be projected to master surface (surf B) during initialization. It's best to use segment sets to define surf A and surf B of a TIED contact and to include ONLY the nodes or segments you wish to be tied. In this way, any warnings of untied nodes are actually helpful.
Note that constraint-based tied contacts should not be used in explicit analysis IF there are other constraints (SPCs, prescribed motion, rigid bodies, etc) associated with the slave or master surface. Otherwise, they should be fine. The contacts cards with the word "OFFSET" (unless it says constraints_offset) are all penalty-based contacts. Please see LS-DYNA user's manual for more information.Â