Is the part made by a single part or different part?
if it's a different part kindly give local contact at that point.
Hope this helps.
Micheal JOhnson S
Despite defining a Global Contact Bonded for your frequency study as well as modeling your bodies with coincident contact you have a body that is disjoint from the rest. The results show that flange as freely rotating with relation to the rest of the structure. From here you would want to define an explicit Bonded Contact Set between the faces of the flange and the faces of the corresponding pipe to ensure a bond at that location. Then rerun the analysis. You may find that this flange then connects to the structure but other bodies are revealed to be disjoint. If you want to find all of the disjoint bodies at once you may want to employ the stability test shown in this video (GoEngineer - Simulation: Diagnosing an Unstable Study).