Tube expansion & hybrid friction diffusion bonding of CU-NI and ASTM A516 G70 tube-to-tubesheet joints

Hybrid Friction Diffusion Bonding (HFDB) is a solid-state welding process. The development of HFDB process in achieving quality joints in tube-to-tubesheet heat exchangers is constrained on tube thickness of 1 mm and similarity of tube and tubesheet materials. In nuclear and petrochemical industries, there is significant demand on steel, copper-nickel, and dissimilar shell and tube (tube-tubesheet) heat exchangers. The present study evaluates the feasibility of HFDB on copper-nickel tubes into steel tubesheets with a tube thickness of 1.26 mm. To avoid relative movement of tubes with respect to the tubesheet, tubes are expanded prior to HFDB using commercial roller expander. The effect of tool rotational speed on joint strength and developed macrostructure was investigated employing mechanical pull-out tests to validate safe operation of the joint, and optical microscopy, respectively. Moreover, a comparison between expanded and welded-and-expanded tube-to-tubesheet joints is carried out.