Microstructures and fatigue properties of friction stir lap welds in aluminum alloy AA6061-T6

Abstract

Microstructural features and fatigue properties of friction stir lap-welded joints for AA6061-T6 alloy are investigated and the influences of single pass welding (SPW) or double pass welding (DPW), hooking defects and fatigue stress ratio on the fatigue properties are analyzed. It is found that the fatigue strengths of FSW lap-welded joints are obviously lower than that of the fusion lap-welded joints of IIW FAT22 and only approximately correspond and close to the IIW FAT12 design curve. The higher fatigue stress ratio R will lead to the decrease of the fatigue strength of lap-welded joints. The existence of hooking defects is the key factor to reduce the fatigue strengths. Fatigue cracks always initiate at the tip of hooking defect in the RS of SPW and AS of DPW joints respectively. The severity of hooking defects and the quality of lap-welds could not be improved by the DPW process as compared with the SPW process. The fatigue strength Δσ_m and Δσ_k (R = 0.1) of DPW joints will be 21.26% and 17.88% lower than that of SPW joints respectively. The fracture of lap-welded joints exhibits multiple crack initiations from the tips of the hooking locations, and the crack propagation shows some brittle fracture and is mainly characterized by the fatigue striations. The locations of crack initiation and the amount of secondary cracks in DPW joints are more than that of in the SPW joints which leading to the lower fatigue properties. The ductile fracture mode is obviously shown in the final fracture zone with deep-hole type dimples in the SPW joints but shallow-hole type dimples in the DPW joints.

Introduction

Friction stir welding (FSW) is an advanced solid-state joining process that was invented by The Welding Institute (TWI) of United Kingdom in 1991 [1], and it is mainly suitable for the joining of aluminum alloys that are always difficult to be welded by the traditional fusion methods without solidifying cracks, porosity or distortion, etc. Currently FSW technology has been highly developed and widely used in many structural manufacturing fields such as the aerospace and airplane, ship-construction, high-speed train and automotive industries for high-performance structural demanding applications [2], [3]. The aluminum alloys of 6xxx-series, containing magnesium and silicon as major alloying elements, have attractive combinations of properties such as high strength, formability, fatigue resistance and relatively low cost [4], [5] and are one of the widely used aluminum alloys in the above mentioned industrial fields. The investigations on the friction stir welded 6xxx-series aluminum alloy technologies are really necessary and important for the industrial application of FSW.

Up to now many researches on the FSW for aluminum alloys butt-welded joints have been reported such as the features of stirring zone microstructures, the distributions of micro-hardness, the formations of various defects and bonding strength have all been discussed in depth [6], [7]. In recent years, much attention has been focused on the FSW lap-welded joints in order to verify their bearing capabilities to replace riveted joints in aircraft structures. Actually, airplane panels, wing frames and floor decks are often strengthened with stringers and profiles are always welded to the outer skin with lap-welded joints [8], [9]. In addition, automotive engine frames, wheel rims, car back supports and hermetically closed boxes such as cooling elements and heat exchangers all inevitably involve lap-welded joints [10].

It has been shown that concerning static strength, FSW lap-welded joints can be comparable with resistance spot welded and riveted joints. But as for fatigue performances it was obviously lower than that of butt-welded joints [11], [12], [13]. The peculiarity of FSW lap-welded joints with respect to butt ones is that two crack-like weak-bonded regions are inevitably present at the ends of lap interfaces (the hooking defects). These should induce the severely stress concentrations and the net reduction of the cross section of sheets and lower the strength of the joint especially in fatigue [14]. How to eliminate or reduce the stress concentrations and understand the fatigue properties of FSW lap-welded joints should be the urgently solved problems for the industrial applications. Recently, some related research institutes are working on improving the fatigue properties of lap-welded joints. Much efforts have been made for developing specific shapes of tools (such as Flared-Triflute™, Skew-Stir™, Re-Stir™ and Trivex™) for FSW lap-welded joints [8], [15], which could allow to promote the material plastic flow around tool pin to minimize hooking degree of the sheet interface adjacent to the weld nugget. Besides the broader tool shoulder with a concave end of tool pin design could provide the better fatigue performance due to the increased contact area and the improved flow path provided by the hollowed out end of the pin [10].

It should be emphasized that the fatigue performances are known to be one of the crucial assessment qualities for the structural materials bearing the dynamical loading, and because of the effects of random and statistical factors on fatigue behaviors it is very important to accumulate the fatigue testing results under various conditions. However, currently the general fatigue assessment specification of FSW welded joints and components have not been established. Especially for FSW lap-welded joints the fatigue behaviors for aluminum alloys are really seldom in publications. Some influence factors such as the pass number that is the double pass welded (DPW) or single pass welded (SPW), the hooking defect and fatigue stress ratio (R) for FSW lap-welded joints on the fatigue properties are unclear and unspecific. This work concentrated on the understanding of microstructural features and fatigue properties of AA6061-T6 FSW lap-welded joints. Fatigue specimens made by SPW and DPW lap joints were tested under load control with different fatigue stress ratio (R) to do contrast analysis and the effects of hooking defects on the fatigue properties of FSW lap-welded joints were discussed. The fatigue life and fatigue strength characteristic values were calculated in accordance with the IIW recommendations [16] and the fracture surfaces were observed to examine the fatigue fracture features of FSW lap-welded joints.