In this paper, fatigue behaviour of friction stir spot welded joints in Al-Mg-Si aluminium alloy was investigated. Fatigue tests were conducted using lap-shear specimens at a stress ratio R = 0.1. It was found that fatigue fracture morphology was dependent on load level. When high load was applied, debonding and opening deformation took place along the boundary between the upper and lower sheets around the weld nugget. The opening deformation caused stress concentration, which resulted in fatigue crack initiation at the edge of the nugget followed by crack growth through the mixed zone (MZ) until final fracture. When low load was applied, fatigue crack initiated at the edge of the nugget and propagated through the sheet thickness and grew to the width direction. Based on experimental observation of the weld zone, fatigue fracture mechanism of friction stir spot welded joint was discussed.
Al A5052-H14 Table 1 2 6mm WC-12Co (high velocity oxygen fuel, HVOF) WC-12Co 25 70 120μm 3 DLC WC-12Co #2000 15μm DLC/WC-12Co A5052 Thickness Effect of Interlayer on Fatigue Behavior and Fatigue Fracture A5052 Mechanisms in Aluminum Alloy with DLC/Thermally Sprayed WC-12Co Hybrid Coatings by Toshifumi KAKIUCHI , Yoshihiko UEMATSU , Makoto TAKEKAWA Takema TERATANI , Yoshifumi KOBAYASHI and Yoshio HARADA WC-12Co was thermally sprayed by a high velocity oxygen fuel (HVOF) method on A5052 aluminum alloy as an interlayer with the thicknesses of 25, 70 and 120μm and subsequently diamond-like carbon (DLC) film was deposited with the thickness of 15μm to fabricate DLC/WC-12Co hybrid coating. Rotary bending fatigue tests were conducted using the specimens with DLC/WC-12Co hybrid coating, DLC single coating, WC-12Co single coating and without coating, and the thickness effect of interlayer on the fatigue behavior was investigated. The fatigue strengths of the specimens with DLC single coating and WC-12Co single coating were higher than those of the substrate specimens. The fatigue strengths in the specimens with WC-12Co single coating increased with increasing the thickness of WC-12Co layer. The specimens with hybrid coating exhibited higher fatigue strengths than the specimens with WC-12Co single coating when the thickness of WC-12Co layer was the same. However the thicker WC-12Co interlayer led to the lower improvement of fatigue strength by hybrid coating. In the specimens with WC-12Co layer, the boundary between WC-12Co layer and substrate was rough and uneven due to the thermal spray process and a fatigue crack initiated at the concave site of substrate on the boundary. In the specimens with hybrid coating, cracking in DLC film and WC-12Co interlayer occurred simultaneously and suddenly near the end of fatigue life. A fatigue crack in the substrate had grown up to a large size and the specimen fractured soon after a crack appeared on the surface.
In this paper, the effect of post heat treatment on fatigue behaviour of friction stir spot welded Al-Mg-Si aluminium alloy was investigated. The microstructure of the weld zone was classified into two regions: stir zone (SZ) and mixed zone (MZ), where fine equiaxed grains due to dynamic recrystallization were observed. Two kinds of post heat treatment, namely aging and T6 treatment, were applied to the as-welded joints. The grains in the SZ and MZ were extremely enlarged only by T6 treatment, but some fine grains still remained near the boundary of MZ. Fatigue tests were conducted using lap-shear specimens at a stress ratio R = 0.1. Post heat treatments exhibited little influence on fatigue strength, but fatigue fracture morphology was dependent on both load level and post heat treatment. At high applied loads, fatigue fracture took place through the MZ in the as-welded and aged joints, while along the boundary of MZ in the T6 treated joint. At low applied loads, fatigue crack initiated at the edge of the nugget and then propagated through the upper sheet in the as-welded joint, while the lower sheet in the aged and T6 treated joints. The dependence of fracture morphology on post heat treatment was attributed to the change of microstructures and hardness distribution around the nugget by post heat treatment.
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