Joining Tube to Tube Sheet for Coil Wound Heat Exchangers by Hybrid Friction Diffusion Bonding

Coil-wound heat exchangers (CWHE) for low temperature applications such as the liquefaction of natural gas (LNG) are often made of aluminium alloys. The fabrication of these aluminium coil-wound heat exchangers holds several challenges, one of which is joining the tubes to the tube sheet. For this specific task, conventional joining technologies such as laser beam welding (LBW) or tungsten inert gas (TIG) welding cannot be easily performed in fully-mechanised mode or are not cost-effective. A joint project between the Helmholtz-Zentrum Geesthacht (HZG) and LINDE Engineering aims at the development of a new solid state joining process, Hybrid Friction Diffusion Bonding (HFDB), to fabricate tube-to-tube-sheet connections for aluminium coil-wound heat exchangers. In the present study, the HFDB process has been developed to industrial maturity and the quality of the joints has been demonstrated by gas leak tightness tests and tensile pull-out tests. The joints meet the requirements for industrial application. Furthermore, the thermal field development in the weld area and the applied process forces have been monitored and correlated to process parameters. The microstructure of the joint has been investigated, and dynamic recrystallization is assumed to be the primary grain refinement mechanism in the thermomechanically affected zone.