Finite Element Modeling of Friction Diffusion Welding of Tube-Tubesheet Joint

Tube-tubesheet joints are considered critical element in heat transfer equipment, where joint tightness; in certain application, cannot be tolerated, were leakage may result in a catastrophic pollution or stop of production. Therefore, tube-tubesheet joints are directly related to safety and productivity of the equipment. To ensure joint tightness, standards like ASME and TEMA recommend performing combination of tube on tubesheet expansion or rolling, and seal welding. Up today, seal welding is performed using fusion-welding methods, which have number of problems/defects resulting from excessive heat input, melting and solidification. These problems or defects may include cracking, porosity formation, distortion, tensile residual stresses, filler chemical compatibility and rapid corrosion. Such defects may take a recently commissioned heat exchanger from service within few weeks. A solution for the aforementioned problems can be achieved by utilizing solid-state welding technologies. Recently, new solid-state welding processes of tube-tubesheet joint based on friction stir welding (FSW) and hybrid friction diffusion bonding (HFDB) were developed. The developed processes were found to be cost effective for aluminum tube-tubesheet joints and for tube thickness in the range of 1 mm for the HFDB process. However, for steel tube-tubesheet joints, FSW was found feasible and capable of producing hermetic seal weld but the weld cycle for one tube takes longer time (about 10 times) compared to the one for aluminum tube-tubesheet. Using HFDB for tube-tubesheet seal welding is expected to result in reducing the welding time for steel tube-tubesheet, but as mentioned previously, it is limited to thin tubes. Therefore, the proposed research project is to study the feasibility of using HFDB process for steel tube-tubesheet and for dissimilar copper nickel tube in tubesheet seal welding of tubes with thickness of 2 mm, through numerical and experimental work. In this work, a finite element model of HFDB of tube-tubesheet seal welding will be developed to understand the material flow during the welding process, also to study the feasibility of using the proposed technique for welding thicker steel tube on tubesheet. For validating purposes, an experimental setup for HFDB of tube-tubesheet will be developed to fabricate tube-tubesheet seal welds. During the welding process, number of parameters will be measured to be compared with model results. The developed model will be used also, to study the effect of process parameters on the integrity of the tube-tubesheet joints by analyzing the developed temperatures and residual stresses in the formed joint. This will help in optimizing the process conditions to ensure production of quality seal joints.