Microstructures and failure mechanisms of friction stir spot welds of aluminum 6061-T6 sheets

doi:10.1016/j.jmatprotec.2006.12.045

Journal of Materials Processing Technology

Volume 186, Issues 1–3, 7 May 2007, Pages 291-297

https://doi.org/10.1016/j.jmatprotec.2006.12.045 Get rights and content

Abstract

Microstructures and failure mechanisms of friction stir spot welds in aluminum 6061-T6 lap-shear specimens are investigated based on experimental observations. Optical micrographs of the cross-sections of friction stir spot welds in lap-shear specimens before and after failure are examined. These friction stir spot welds show the failure mode of nugget pullout under lap-shear loading conditions. The experimental observations suggest that under lap-shear loading conditions, the failure is initiated near the possible original notch tip in the stir zone (SZ) and the failure propagates along the circumference of the nugget to final fracture. Microindentation hardness data of base metal (BM), heat affected zone (HAZ), thermal–mechanical affected zone (TMAZ) and SZ are obtained. The interface between the HAZ and the TMAZ is the softest region, where the cracks of friction stir spot welds in the lap-shear specimens under the loadings initiate and lead to fracture of the specimens.

Introduction

A rapid development of applications of lightweight materials in the automotive industry is reflected in the increasing use of aluminum and magnesium alloys. Many components produced from these alloys, by stamping, casting, extrusion and forging, have to be joined as a part of manufacturing processes. Resistance spot welding (RSW) is the most commonly used joining technique for parts made of steel sheets. The main advantages of the resistance spot welding process are its relatively low capital cost, ease of maintenance, and high tolerance to poor part fit up compared with other fusion welding technologies [1]. However, resistance spot welding of aluminum sheets by contrast has several technological challenges. First, the electrode tip life is shorter than that for welding steel sheets. Resistance spot welding of aluminum sheets is also likely to produce such defects as porosity, as reported in Thornton et al. [2] and Gean et al. [3]. Recently, a new friction stir spot welding technology has been developed by Mazda Motor Corporation and Kawasaki Heavy Industry [4], [5] with much lower operating and investment cost.

A schematic illustration of the friction stir spot welding process is shown in Fig. 1 [4]. The process is applied to join two metal sheets. A rotating tool with a probe pin plunges into the upper sheet and a backing tool beneath the lower sheet supports the downward force. The downward force and the rotational speed are maintained for an appropriate time to generate frictional heat. Then, heated and softened material adjacent to the tool deforms plastically, and a solid-state bond is made between the surfaces of the upper and lower sheets.

One benefit of friction stir spot welding compared to the conventional fusion welding processes is that for aluminum-based alloys, it is possible to make joints where the strength of the weld is comparable to that of the base metal alloy. Aluminum alloys are difficult to be fusion-welded due to the requirements of (i) gas shielding of weld pool, and (ii) removal of oxide layers prior to or during the welding process [6], [7]. In addition, aluminum alloys are subject to voids and solidification cracking defects when they cool from a liquid [8], [9]. Therefore, friction stir spot welding offers significant performance advantages. Because melting is avoided, the energy input used for friction stir spot welding is considerably low. Consequently, the HAZs and residual stresses associated with the welds can be relatively small [1].

In this paper, microstructures and failure mechanisms of friction stir spot welds in aluminum 6061-T6 lap-shear specimens are investigated based on experimental observations. A tool with a flat tool shoulder and a cone-shaped probe pin was used. Micrographs of friction stir spot welds in lap-shear specimens before and after failure are obtained. Microstructures for friction stir spot welds are then presented. Finally, the failure mode and failure mechanism for these friction stir spot welds are discussed.

Section snippets

Experimental procedures

In this investigation, aluminum 6061-T6 sheets with a thickness of 1 mm were used. Table 1 lists the chemical compositions (wt.%) of the 6061-T6 aluminum sheets. Fig. 2 schematically shows a lap-shear specimen used to investigate the strength of friction stir spot welds under shear loading conditions. The weld nugget is idealized as a cone. The lap-shear specimen has a thickness of 1 mm, a width of 25 mm, an indentation diameter of approximately 12 mm, an overlap length of the upper and lower

Results and discussion

Fig. 6(a) shows a lap-shear friction stir spot weld specimen. Fig. 6(b) shows a close-up top view of the friction stir spot weld on the upper sheet. As shown in the top view, the top surface of the weld looks like a button with a central hole. The squeezed-out material is accumulated along the outer circumference of the shoulder indentation. Fig. 6(c) shows a close-up bottom view of the friction stir spot weld on the lower sheet. In the bottom view, the contact mark due to the backing tool can

Conclusions

Microstructures and failure mechanisms of friction stir spot welds in aluminum 6061-T6 lap-shear specimens were investigated based on experimental observations. For friction stir spot welds made in this investigation, the circumferential failure mode or the nugget pullout failure mode was observed. The experimental results suggest that under lap-shear loading conditions, the failure is initiated near the SZ in the middle part of the nugget and the failure propagates along the circumference of

Acknowledgements

This project was supported by the National Science Council of the Republic of China under grant no. NSC93-2212-E-212-019. The authors would like to express their appreciation to the staff of the metallurgy laboratory and the machining centers at National Chung Hsing University and Da Yeh University for their assistance.

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Microstructures and failure mechanisms of friction stir spot welds of aluminum 6061-T6 sheets

Abstract

Introduction

Section snippets

Experimental procedures

Results and discussion

Conclusions

Acknowledgements

J. Mater. Process. Technol.

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Int. J. Plast.

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Mater. Design

Acta Mater.

Aluminum spot weld

Weld. J.

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