The fundamental reasons for the poor mechanical properties of the Al–Cu conventional friction stir welding (CFSW) joints are serious material mixing and excessively thick intermetallic compounds (IMCs). In this study, stationary shoulder friction stir welding (SSFSW) was innovatively applied to join the Al–Cu dissimilar materials. The results indicated that the material mixing and the IMCs growth were successfully suppressed. Compared with the CFSW, the grains in the SZ of the SSFSW joint were refined and the plastic deformation in the SZ was enhanced. This was attributed to the frictional driving force of SSFSW was higher than that of CFSW. The mutual diffusion reaction of Al and Cu elements was inhibited owing to the lower heat input of the SSFSW. The mechanical properties of the joints were significantly improved by the SSFSW. The tensile strength of the optimal SSFSW joint was found to be 211.4 MPa which is 15.1% higher than that of the optimal CFSW joint. Moreover, the fracture occurred in the Al matrix. Reducing the stress concentration in the SZ caused by materials mixing was shown to be the main mechanism for SSFSW to improve the tensile performance of the joint. 

Stationary shoulder friction stir welding of Al–Cu dissimilar materials and its mechanism for improving the microstructures and mechanical properties of joint

The eventual material degradation of steel components in bio-implant, marine, and high-temperature applications is a critical issue that can have widespread negative ramifications from a safety and economic point of view. Stemming from their tribological, corrosion, and erosion-based properties, there is an increasing need to address these issues effectively. As one solution, surface processing techniques have been proposed to improve these properties. However, common techniques tend to suffer from issues spanning from their practicality to their high costs and negative environmental impacts. To address these issues, friction-stir-processing (FSP) has been one technique that has been increasingly utilized due to its cost effective, non-polluting nature. By inducing large amounts of strain and plastic deformation, dynamic recrystallization occurs which can largely influence the tribological, corrosion, and erosion properties via surface hardening, grain refinement, and improvement to passive layer formation. This review aims to accumulate the current knowledge of steel FSP and to breakdown the key factors which enable its metallurgical improvement. Having this understanding, a thorough analysis of these processing variables in relation to their tribological, corrosion, and erosion properties is presented. We finally then prospect future directions for this research with suggestions on how this research can continue to expand.

Friction Stir Processing on the Tribological, Corrosion, and Erosion Properties of Steel: A Review

Improving the microstructure and mechanical properties of Al-Cu dissimilar joints by ultrasonic dynamic-stationary shoulder friction stir welding

Three types of stainless steel (304 SS, 410 SS, and 2205 SS) were evaluated for their corrosion behaviors in microbiologically influenced corrosion (MIC) by Desulfovibrio ferrophilus strain IS5, a relatively new and very corrosive sulfate-reducing bacteria (SRB) strain. The incubation lasted for 7 days in enriched artificial seawater at 28°C and the results showed that 410 SS had a rather large weight loss (6.2 mg/cm2) and a maximum pit depth (118 µm), but 2205 SS and 304 SS did not suffer from significant weight loss or pitting. Electrochemical tests indicated that 2205 SS was slightly more resistant to SRB MIC than 304 SS, while 410 SS was far less resistant.

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Comparison of 304 SS, 2205 SS, and 410 SS Corrosion by Sulfate-Reducing Desulfovibrio ferrophilus

In this work, plates of dissimilar Al/Cu alloys were butt jointed by friction stir welding (FSW) with and without ultrasonic vibration. It was found that the assistance of ultrasonic vibration produced slight variations of both mechanical interlocking and material intermixing of dissimilar material, as well as fracture location during tensile tests, but it particularly reduced the thickness of intermetallic compounds (IMCs) layer in the middle part of the Al/Cu joint. Therefore, by homogenizing the IMCs distribution along the bonding interface within a thickness of 1.0 µm, the tensile strength of the Al/Cu FSW joint was improved from 195.5 to 215.0 MPa with the assistance of ultrasonic vibration. 

Homogenizing the intermetallic compounds distribution in Al/Cu dissimilar friction stir welding joint with the assistance of ultrasonic vibration

Microstructural evolution and mechanical properties of the Al–Cu dissimilar joint enhanced by stationary-dynamic shoulder friction stir welding

In robotic friction stir welding, high rotational speed is constantly employed to reduce both robot torque and the welding load. In this paper, a three-dimensional coupled thermo-mechanical model is developed. This model analyzes the multiphysics field in both friction stir welding and stationary shoulder friction stir welding at relatively high rotational speeds. Both experimental and numerical results indicate that the stationary shoulder friction stir welding joint has homogenous microstructure and temperature gradient distribution. Further research finds the peak temperature in the nugget zone of stationary shoulder friction stir welding and friction stir welding is 490° and 530°, respectively. Meanwhile, the effective strain of the stationary shoulder friction stir welding joint is lower when compared to the friction stir welding. However, the strain rate is higher. While being reduced along the thickness direction, the strongest material flow velocity is observed in the top surface of the workpiece. Stationary shoulder friction stir welding is shown as a favorable process for obtaining the homogeneity of the microstructure and reducing joint softening.

Numerical Modeling of Multiphysics Field in Conventional and Stationary Shoulder Friction Stir Welding of Al-Cu Alloy

In this study, 2205 duplex stainless steel was friction-stir-welded at different welding speeds. The microstructural characteristics such as grain sizes, grain boundary misorientation angles, and phase contents in the stir zones of the joints were detected. The potentiodynamic polarization and electrochemical impedance spectroscopy were also measured to evaluate the corrosion property of the stir zones. The effects of the microstructures on the corrosion property for friction-stir-welded 2205 duplex stainless steel were analyzed. The results indicated that the FSW process refined the grains and increased the ferrite contents in the stir zones. With increasing the welding speed from 30 mm/min to 50 mm/min, the grain size in the stir zone decreased from 1.64 μm to 0.96 μm, the ferrite content decreased from 59% to 54.4%, and the fractions of low angle grain boundaries for austenite and ferrite phases decreased from 63.9% and 92.6% to 18.0% and 41.1%, respectively. In this study, the effect of grain size on the corrosion resistance was the most significant and it was followed by the phase composition and the grain boundary misorientation angle. The stir zone obtained at 50 mm/min has the highest corrosion resistance, and it is followed by the base material and the stir zone obtained at 30 mm/min.

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Jiaqing You

Papers

Microstructural evolution and mechanical properties of the Al–Cu dissimilar joint enhanced by stationary-dynamic shoulder friction stir welding

Numerical Modeling of Multiphysics Field in Conventional and Stationary Shoulder Friction Stir Welding of Al-Cu Alloy

Microstructure Characteristics and Corrosion Resistance of Friction Stir Welded 2205 Duplex Stainless Steel