A Comparative Study on Fatigue Response of Aluminum Alloy Friction Stir Welded Joints at Various Post-Processing and Treatments

Soran Hassanifard,Ahmad Varvani-Farahani

doi:10.3390/jmmp5030093

Soran Hassanifard, Ahmad Varvani-Farahani

Open Access

https://doi.org/10.3390/jmmp5030093

Copy DOI

Abstract

The present study examines the fatigue of friction stir welded (FSW) aluminum 6061, 7075, 1060 joints followed by (i) in situ and sequential rolling (SR) processes, (ii) plastic burnishing (iii) solution-treatment artificial aging (STA), (iv) local alloying through depositing thin copper foils, and (v) inserting alumina powder in the weld nugget zone (NZ). The microstructural features and fatigue life of post-processed joints were compared with those of as-welded joints. The in situ rolling technique offered simultaneous rolling and welding operations of aluminum joints, while through the sequential rolling process, the top surface of FSW joints was rolled after the welding process. The fatigue life of in situ rolled samples was increased as the ball diameter of welding tool increased. The fatigue life of friction stir welded joints after a low-plasticity burnishing process was noticeably promoted. The addition of 1 wt.% alumina in the NZ of joints resulted in a significant elevation on fatigue life of friction stir spot welded joints, while an increase in alumina powder to 2.5 wt.% adversely affected fatigue strength. Weld NZ was alloyed through the insertion of copper foils between the faying surfaces of joints. This localized alloy slightly improved the fatigue life of joints; however, its effects on fatigue life were not as influential as STA heat-treated or in situ rolled joints. The microstructure of weld joints was highly affected through post-processing and treatments, resulting in a substantial influence on the fatigue response of FSW aluminum joints.

Highlights

Friction stir welding (FSW), which is a prevalent permanent method of joining thin metal sheets, is widely used in different industrial sectors, such as automotive, aerospace, and marine
The impacFtSoWf i/nFSseSrWtinjogin1s0,0qμumas-i-tshtiactkicctoepnspieler tfeositlsbwetewreeceonntdhuecftaeydinong jsouinrftascaems polfetsh. eFigure 6 p FSW joints on tesennstilsetshteresntgattihc wstaressgsr–esatrtaerinthcaunrvtehsatfoorf 2b0a0seμ-mme-tthalicakscwopelplearsfopiol.stD-pureoctoestsheed alumin solubility limitjoofinctospapmepr liens.the aluminum matrix, the brittle intermetallic compound was generated in the stirred zone (SZ), resulting in a slight decrease in the tensile strength of joints with the
Adding large amounts of particles in the nugget zone (NZ) of friction stir spot welding (FSSW) joints caused the agglomeration of particles in the stirred zone, and reduced the joint area, resulting in a reduction in strength and ductility

Summary

Introduction

Friction stir welding (FSW), which is a prevalent permanent method of joining thin metal sheets, is widely used in different industrial sectors, such as automotive, aerospace, and marine. In the FSW technique, (i) a non-consumable rotating tool with interfacing surfaces is used to generate adequate frictional heat required to join metal sheets, and (ii) the stirring process by means of pin tool plays a crucial role in generating a plastically deformed zone around the pin. This technique is employed to fabricate spot welds on overlapped metal sheets and is referred as friction stir spot welding (FSSW). Lower possibility of distortion, less hot cracking, low residual stresses, and short welding time have classified FSW as an unprecedented technique over other traditional welding methods [1]. In an attempt to improve the fatigue resistance of friction stir welded joints, researchers have proposed several techniques including adding nano/micro particles to the weld nugget to create a local metal matrix composites (MMCs) [4,5,6,7], optimizing welding process parameters [8,9,10,11,12] through tool shape and operation rate, introducing novel welding tools with different geometrical features [13,14,15,16,17], inducing compressive residual stresses by means of different methods of cold working [18,19,20,21,22,23], and weld joint post-heat treatment [24,25,26]

Methods

Results

Conclusion