Abstract
The energy evolution, fatigue life and failure behaviour of dissimilar Al/steel keyhole-free Friction stir spot welding (FSSW) joints were studied under different fatigue loads. The absorption energy of fatigue fracture, the fracture mechanism and the sensitivity of the fatigue limits to the fatigue load parameters were analysed. It was found that the stress ratio R determines the fatigue limit Ff, while the fatigue limit Ff is not sensitive to the loading frequency. The high-frequency fatigue load will increase the displacement deformation μ and fatigue fracture absorption energy Ea of the spot-welded joint, which are larger under asymmetric fatigue loading than those under symmetrical fatigue loading. At the same time, the symmetrical fatigue load can form the steady-state hysteresis loop, while asymmetric fatigue loading cannot, but asymmetric fatigue loading exhibits the displacement increment of fatigue softening. The fracture failure of spot-welded joints is a multiple crack source and the mixed-mode of ductile and brittle fracture mechanism, which exhibits typical fatigue striations in the fatigue fractures.
Highlights
With the proposed concept of energy conservation and emission reduction, lightweight technology is considered to be the most direct and effective means to achieve this goal
Retracted pin technology is used in dissimilar Al/steel keyhole-free Friction stir spot welding (FSSW) to eliminate the keyholes of joints
The conclusions conclusions are are summarized summarized as as follows: follows: (1) When the loading frequency is constant, the fatigue limit F−1 ≈ 1.47 kN of the spot-welded joint under the symmetric load is less than the fatigue limit Ff ≈ 2.24 kN under the asymmetric load
Summary
With the proposed concept of energy conservation and emission reduction, lightweight technology is considered to be the most direct and effective means to achieve this goal. Mg alloys and other light metal materials have been widely considered to replace steel, especially in the automotive field. The problems of connecting dissimilar metals, such as Al/Mg [1], Al/Cu [2], Al/steel [3] and Mg/steel [4], are discussed. It is easy to produce welding defects in the joint, such as porosity, inclusion, brittle phases, and welding hot cracks, especially in fusion welding. This seriously limits the application of Al, Mg and other light metals in the structural integration of dissimilar metals [1,2,3,4]
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