Investigation of stationary shoulder friction stir welding of aluminum alloy 7075-T651

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Highlights

Abstract

Stationary shoulder friction stir welding (SSFSW) was successfully performed on 7075-T651 aluminum alloy. Defect-free joints with smooth weld surface were obtained. The redistribution of constituent particles and the dissolution of η′ precipitates resulted in the low crack initiation energy of the nugget zone. The low crack initiation energy in the heat affected zone (HAZ) can be attributed to both the presence of η precipitates and the redistribution of constituent particles. The presence of η precipitates in the HAZ was the major reason for the decreased tensile strength of SSFSW joint.

Introduction

Stationary shoulder friction stir welding (SSFSW) is a novel variant of conventional friction stir welding (FSW), employing a rotational probe and a stationary shoulder which slides on the weld surface during the welding process. It was initially reported by Russell and Blignault (2006) to weld low thermal conductivity titanium alloy Ti–6Al–4V. As stated by Davies et al. (2011), SSFSW creates focused heat input around the probe and eliminates unnecessary material flow under the shoulder, leading to a much symmetrical joint which consists of narrow heat affected zone (HAZ) and thermo-mechanical affected zone (TMAZ). Up to now, only a few reports have been published on SSFSW. Davies et al. (2011) studied the microstructure and crystallographic texture of Ti–6Al–4V SSFSW joints. Ji et al. (2014) reported the mechanical properties of SSFSWed 6000 series aluminum. However, the joint formation of the joint is rough. Moreover, Ahmed et al. (2011) investigated the crystallographic texture of SSFSWed 6000 series aluminum by electron backscatter diffraction.

High strength 7000 series aluminum alloys are widely used in critical condition in aircraft and aerospace structures. Along with 2000 series aluminum alloys, it is considered relatively difficult to weld using fusion welding techniques because of the crack sensitivity and severe mechanical property decrease. Nevertheless, FSW shows a great potential in welding 7000 series aluminum alloys according to Hwang and Chou (1997). However, limited work has been published on SSFSW joint of 7000 series high strength aluminum alloys.

Based on the reasons mentioned above, butt joints of high strength aluminum alloy 7075-T651 were fabricated by SSFSW using various welding parameters. Microstructure and tensile properties of SSFSW joints were investigated and the toughness was evaluated through Kahn tear tests. In addition, the effects of microstructures on the mechanical properties of SSFSW joint were discussed.

Section snippets

Experimental procedure

The base material used in the investigation was 5 mm thick 7075-T651 produced by Constellium. The nominal chemical composition and mechanical properties are listed in Table 1. SSFSW butt joints were manufactured perpendicular to the rolling direction, employing self-made stationary shoulder tool package (Li et al., 2014) and various welding parameters (see Table 2).

After the welding, microstructures of the joint made using 1500 rpm and 30 mm/min were investigated. Transverse section sample was cut

Microstructure

Fig. 3 shows the surface appearances of SSFSW joints made using various welding parameters. Except the joint made using 1000 rpm and 50 mm/min, all the other joints indicate smooth surface appearances. Only a little flash can be observed at the exiting hole. The joint made using 1000 rpm and 50 mm/min exhibits a groove defect on the surface, which was caused by insufficient material flow during the welding. However, the stationary shoulder helps to improve the weld surface appearance by preventing

Welding process

In classical FSW, the friction between the shoulder and the workpiece results in the biggest component of heating (Mishra and Ma, 2005). While in SSFSW, the elimination of the rotational shoulder makes the probe the only heat source. Thus, under the same rotational speed, a low welding speed is essential to produce adequate heat in SSFSW. In addition, in classical FSW, the welding speed for 7000 series aluminum alloy is relatively low compared to that for other aluminum alloys (such as 5000 and

Conclusions

The microstructure and mechanical properties of high strength aluminum 7075-T651 SSFSW joint are investigated in this paper. The following conclusions can be drawn:

  • (1)

    Defect-free joints of aluminum 7075-T651 with smooth surface can be manufactured by SSFSW. The narrow TMAZ and HAZ surrounding the NZ and are fairly symmetrical in the transverse section.

  • (2)

    The NZ and the TMAZ consist of fine equiaxed and bended grains respectively, while there is no evident difference in the grain structure in the HAZ

Acknowledgements

This work is financially supported by Research Program for Application Foundation and Advanced Technology of Tianjin (14JCZDJC38800).

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