A Comparative Investigation on Conventional and Stationary Shoulder Friction Stir Welding of Al-7075 Butt-Lap Structure

Yu Chen,Hua Ding,Huaying Li,Fenghe Zhang,Xiaoyu Wang

doi:10.3390/met9121264

Abstract

Both conventional friction stir welding (C-FSW) and stationary shoulder friction stir welding (S-FSW) were employed to join the Al-7075 butt-lap structure, then the microstructural evolution and mechanical characterization of all FSW joints were systematically studied. The C-FSW joint exhibited a rough surface with flashes and arc corrugations, while the surface of the S-FSW joint became smooth. Moreover, for the S-FSW joint, the shoulder-affected zone got eliminated and the material flow mode during FSW was changed owning to the application of stationary shoulder. Furthermore, in comparison to C-FSW, the lower welding heat input of S-FSW decreased the average grain size in the nugget zone and inhibited the coarsening of strengthening precipitates in the heat-affected zone, elevating the overall hardness for the S-FSW joint. In addition, the tensile strength of the S-FSW joint became higher compared to the C-FSW joint, and all the FSW joints failed inside the nugget zone attributing to the existence of hook defect. The sharp-angled hook defect deteriorated the plasticity of the C-FSW joint further, which was only 70% that of the S-FSW joint.

Highlights

As a new solid-state joining/welding technology, friction stir welding (FSW) has proved to be very suitable for joining aluminum alloys [1], after decades of development, some variantFSW techniques have been proposed and applied for the industry [2,3,4]
The top surface topographies of the conventional friction stir welding (C-FSW)/shoulder FSW (S-FSW) joints are shown in Figure 2; large flashes generate at retreating side (RS) and obvious arc corrugations can be detected on the top surface of nugget zone (NZ) (Figure 2b)
The Macrostructures of FSW Joints surface topographies of the C-FSW/S-FSW joints are shown in Figure 2; large flashes

Summary

Introduction

As a new solid-state joining/welding technology, friction stir welding (FSW) has proved to be very suitable for joining aluminum alloys [1], after decades of development, some variantFSW techniques (such as stationary shoulder FSW, pinless FSW, and bobbin tool FSW) have been proposed and applied for the industry [2,3,4]. It is believed that the rotational shoulder generates more frictional heat at the top of workpieces, resulting in both the thermal and microstructural gradients across the thickness of workpieces [6]. The rotational shoulder gives rise to the formation of arc corrugations on the surface of workpieces, influencing the fatigue property of the C-FSW joint [7]. The tool of S-FSW consists of a nonrotational shoulder (namely stationary shoulder) and a rotational pin, almost no frictional heat input is produced by the stationary shoulder, and the frictional heat input of rotational pin is nearly linear, as a result, the gradients in the temperature and microstructures across the thickness can be significantly reduced [9]. The smooth joint surface can be obtained due to the scraping effect of the stationary shoulder [10]

Methods

Results

Conclusion