To ensure heat exchanger tube–tubesheet joints tightness, industrial standards may recommend performing a combination of roller expansion and seal welding, using conventional fusion welding processes. Solid state friction stir welding (FSW) has several advantages over the conventional fusion welding but has not yet proven its usefulness in seal and strength welding of heat exchanger tube–tubesheet joints where the available space is very limited and weld pass is of a relatively complex contour. In this work, a newly designed tool and procedure have been developed to friction stir seal weld tube–tubesheet joints. The effects of process conditions such as welding speed and tool offset on dependent process parameters including welding loads and joint quality have been investigated on a 6xxx-series aluminum three-tube test cell. The results of the investigation revealed that the quality of the seal weld of tube–tubesheet joints is affected by the above parameters. Lower weld speeds increase the size of the heat-affected zone while higher speeds lead to larger weld defects. Better weld quality is obtained when the center of the pin tool is offset from the tube–tubesheet interface by an amount lower than 40% of the pin diameter.

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