Fatigue Performance and Strength Assessment of AA2024 Alloy Friction Stir Lap Welds
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JMEPEG https://doi.org/10.1007/s11665-020-05145-6
Fatigue Performance and Strength Assessment of AA2024 Alloy Friction Stir Lap Welds Ruijie Wang
, Hong-Tae Kang, and Xinghai Lei
(Submitted October 11, 2018; in revised form June 10, 2020) This work experimentally studied the weld-seam cross section characteristics and the fatigue performance of AA2024 aluminum alloy friction stir linear welded joints. The Vickers hardness values on the cross section of the weld-seam have a tendency of being lower than those of the upper sheet, whereas in the lower sheet there is a significant downward profile across the nugget, like a ‘‘V’’ shape. The grains in the welded nugget are finer than those of the base material. Within low-cycle regime, the fracture always occurred at the hook vertex on the loading-carrying upper sheet, while on the loading-carrying lower sheet within relatively high-cycle regime. The typical SEM fractographs of the upper and lower sheets are observed and compared accordingly. The fatigue lives of these specimens were assessed by fictitious notch radius approach and two local damage equations, MorrowÕs modified Manson–Coffin damage equation and the Smith–Watson–Topper damage equation, combined with stress and strain at the weld-seam periphery obtained from finite element analyses. Results showed that both the MorrowÕs modified Manson–Coffin damage equation and fictitious notch radius approach can give more reasonable results. Keywords
fatigue life prediction, fracture fractograph, friction stir lap welding, hardness distribution, hook defect
fatigue analysis data of friction stir joints are mostly concerned with spot welds (Ref 11). There also have been considerable List of symbols
1. Introduction Materials with high strength-to-weight ratios, such as aluminum and its alloys, are becoming more and more widely used in the automotive industry, under the increasing demand for energy savings and environmental protection. However, aluminum alloys are often considered very difficult to weld with traditional welding technique due to the formation of an oxide film Al2O3 on the surface of the edges of the pieces to be joined, which hinders the melting of the underlying material (Ref 1). And the formations of various defects would degrade the joint strength (Ref 2). As a novel solid-state joining technique, friction stir welding (FSW) has demonstrated a promising approach for joining such lightweight materials with low melting temperature. This process is energy saving and environmentally friendly, making it widely applicable. It has been used for the connection of aluminum and aluminum alloys (Ref 3), steel (Ref 4), magnesium alloys (Ref 5), titanium alloys (Ref 6, 7) and metal composites (Ref 8, 9). In the automotive and aerospace industries, it has been widely used to replace riveting in the manufacture of longitudinal beam and frame members (Ref 10). There are many forms of structural samples used in researches on the strength of friction stir welds, such as butt welds, spot welds, lap welds, and so on. Up to the prese
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