Fatigue behaviour of dissimilar friction stir spot welds between A6061-T6 and low carbon steel sheets welded by a scroll grooved tool without probe

Abstract

A B S T R A C T A6061 and low carbon steel sheets, whose thicknesses were 2 mm, were welded by a friction stir spot welding (FSSW) technique using a scroll grooved tool without probe (scroll tool). Tensile-shear fatigue tests were performed using lap-shear specimens at a stress ratio R = 0.1, and the fatigue behaviour of dissimilar welds was discussed. Tensile-shear force of the dissimilar welds was higher than that of the A6061 similar ones. Furthermore, the dissimilar welds exhibited nearly the same fatigue strengths as the A6061 similar ones, indicating FSSW by a scroll tool was effective technique for joining aluminium to steel sheet. Fatigue fracture modes of the dissimilar welds were dependent on load levels, where shear fracture through the interface between A6061 and steel occurred at high load levels, while crack grew through A6061 sheet at low load level.A = nugget size (mm 2 ) HV = vickers hardness N f = number of cycles to failure P max = maximum load (N) R = stress ratio I N T R O D U C T I O NThe application of lightweight metals such as aluminium alloys in transportation industries, especially in automotive industry, is rapidly developed in order to reduce CO 2 exhaust gas and fuel consumption. However, steels are still widely used for structural components because of the lower absolute strength and higher material cost of aluminium alloys. Therefore, assembling both aluminium alloys and steels is necessary to develop hybrid body structures. It is known that fusion welding between aluminium alloy and steel are not reliable because brittle intermetallic compounds are formed along the interface, 1 and results in the lower strength of welds. Recently, joining aluminium alloys to steels are tried by a friction stir welding (FSW) technique, which is a solid state welding process, and successfully achieved high strength welds. 2-6 A spot welding process using FSW technique has been newly developed, which is called friction stir spot weldCorrespondence: Yoshihiko Uematsu.

Two-sided friction stir riveting by extrusion: A process for joining dissimilar materials

“…Other samples fractured only along one edge of the steel, leaving the aluminum rivet attached to the opposite aluminum sheet. The failure in the aluminum/steel region is a shear fracture between the interface of the materials and was similar to results found by Uematsu in their study of dissimilar FSSW of A6061 and low carbon steel [20]. The lighter colored region in Fig.…”

Friction Stir Spot Welding: A Review on Joint Macro- and Microstructure, Property, and Process Modelling

“…The fractography analysis suggested that the effective top sheet thickness, interfacial hooking, and microstructure significantly affected the fatigue behavior of the friction stir spot welds in magnesium alloys [54]. In the similar welds, fracture mode was irrelevant to load levels for lap-shear specimens, and crack took place in lower sheet under high or low load levels [55,56]. However, the fatigue modes of FSSW in AZ31 depended on the cyclic load amplitude [53,57].…”

“…For A6061 and low carbon steel sheets dissimilar lapshear welds, fatigue fracture modes were dependent on fatigue load level. Shear fracture through the interface occurred at high load levels, and a fatigue crack grew through the upper sheet at low load levels [55,56]. Tran et al [59] investigated that failure mode of the 5754-7075 welds in lapshear specimens, under quasistatic loading conditions, crack propagated along the nugget circumference, under cyclic loading conditions, crack propagated in the width direction of the specimen, and the left part of the lower sheet was eventually separated (Figure 10 [61].…”

Failure mechanisms of friction stir spot welds of AA6061‐T6/DP590 steel during tensile‐shear testing

“…Shear fracture along the interface of AA6061‐T6/low carbon steel sheets was reported by Uematsu et al in FSSW using a pinless tool. It should be mentioned that the interface was free from IMC.…”