Friction Stir Welded Lap Joints Research Articles

Effects of tool rotation and pin diameter on fatigue properties of friction stir welded lap joints

The objective of this study was to determine the effects of the tool pin diameter and tool rotation on the fatigue behaviour of friction stir welded (FSW) lap joints. FSW lap joints of AA 5754 aluminium alloy plates were produced by means of a conventional semiautomatic milling machine. Consequently, defect free FSW lap joints were produced on alloy plates at a constant traverse speed but with different tool pin diameter and tool rotation. Therefore, within this study, tool rotation and the tool pin diameter were accepted as variable parameters, while others held fixed. The results of the tests performed, indicate that an optimisation is required for the studied parameters, in order to obtain reasonable fatigue strength. An index, related to tool rotation, traverse speed, pin diameter, and pin height can be identified and used to select optimum parameters for FSW applications.

Fatigue behaviour of Al2024-T3 friction stir welded lap joints

The aim of this work is the characterization of the fatigue behaviour of AA2024-T3 friction stir welded overlap joints produced at German Aerospace Center (DLR) of Cologne (D). In these joints, two crack-like unwelded zones are present at overlap ends. The stress intensity factor at the crack tip and the fatigue crack path have been studied using the FE code Franc2d and the lifetime has been estimated by integrating the material propagation law with the software AFgrow, in which the stress intensity factor calculated with Franc2d was introduced. The numerical results predict lifetimes shorter than the experiments. This fact is attributed to an initial propagation in shear mode for a short distance, which was indeed observed with an optical microscope and it is predicted by mixed mode “ failure mechanism map”.

Fatigue properties of friction stir overlap welds

Friction stir welding (FSW) is currently used for many applications involving lap or T-joints, e.g. hermetically closed boxes such as cooling elements and heat exchangers. The frequent pressure changes in these make them susceptible to fatigue. The fatigue characterization of lap joints involves a combination of shear and bending. Forces applied to the ends of lap joints result in non-axial stresses in the connection area. FSW lap joints of Al–Mg–Si alloy 6082 in the artificially aged condition T6 were studied. A pin (probe) based on the Triflute™ concept was used with two modifications to the pin, the pin end being either convex or concave. Tool shoulders of 15 and 18 mm respectively were utilized, producing four different weld series. Fracture was initiated in the highly stressed area where the weld cuts through the interface between the two sheets. The cracks typically propagated through the weld in the upper sheet (tool side). The broadest tool shoulder with a concave end of pin design gave the best fatigue performance. This was due to an improved flow path provided by the hollowed out end of the pin; allowing material flow around the pin which resulted in minimal “hooking” of the sheet interface adjacent to the weld nugget. Additionally heat energy was supplied by the increased contact area. The stress intensity factor Δ K was determined. It was found that a simplified approach, developed to estimate Δ K for overlap spot welds, could be applied to friction stir overlap joints. The corresponding crack propagation rates were in fair accordance with the experimental results.

CJ 88-03. Strain energy model and life predictability of time-dependent fatigue : By He Jinrui, Dong Zhaoqin, Duan Zuoxiang and Ning Youlian (Beijing Institute of Aeronautical Materials, Beijing, China). Chinese J. Mech. Engng24, No. 1 (March 1988), pp. 30–36

Alclad 2024-T4 aluminum alloy was successfully joined by friction stir lap welding (FSLW) assisted by external stationary shoulder in the present study. Joint surface appearances, cross-sections, microstructure and lap shear failure loads of the joints were mainly studied. The assisted external stationary shoulder is beneficial to the joint formation, by which arc corrugation, flash and void can be eliminated or diminished. Compared with the conventional FSLW joints, FSLW joints assisted by external stationary shoulder own smaller effective sheet thickness of cold lap (ESTC), bigger effective lap width (ELW) and better lap shear failure strength. Lap shear failure loads of the FSLW joints assisted by external stationary shoulder do not agree with ESTC or ELW due to different fracture modes. Fracture morphologies of the FSLW joints assisted by external stationary shoulder indicate ductile fracture.