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

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

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

It is difficult to achieve Al/Cu dissimilar welds with good mechanical properties for medium-thick plates due to the inherent high heat generation rate at the shoulder-workpiece contact interface in conventional friction stir welding. Thus, double-side friction stir welding is innovatively applied to join 12-mm medium-thick 6061-T6 aluminum alloy and pure copper dissimilar plates, and the effect of welding speeds on the joint microstructure and mechanical properties of Al/Cu welds is systematically analyzed. It reveals that a sound Al/Cu joint without macroscopic defects can be achieved when the welding speed is lower than 180 mm/min, while a nonuniform relatively thick intermetallic compound (IMC) layer is formed at the Al/Cu interface, resulting in lots of local microcracks within the first-pass weld under the plunging force of the tool during friction stir welding of the second-pass, and seriously deteriorates the mechanical properties of the joint. With the increase of welding speed to more than 300 mm/min void defects appear in the joint, but the joint properties are still better than the welds performed at low welding speed conditions since a continuous uniform thin IMCs layer is formed at the Al/Cu interface. The maximum tensile strength and elongation of Al/Cu weld are, respectively, 135.11 MPa and 6.06%, which is achieved at the welding speed of 400 mm/min. In addition, due to the influence of welding distortion of the first-pass weld, the second-pass weld is more prone to form void defects than the first-pass weld when the same plunge depth is applied on both sides. The double-side friction stir welding is proved to be a good method for dissimilar welding of medium-thick Al/Cu plates. 

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Microstructure and Mechanical Properties of Dissimilar Double-Side Friction Stir Welds Between Medium-Thick 6061-T6 Aluminum and Pure Copper Plates

The medium-thick Al/Mg dissimilar friction stir welding (FSW) joint has serious groove and cavity defects due to uneven thermal distribution in the thickness direction. The submerged friction stir welding (SFSW) was employed to decrease the peak temperature of the joint and control the thermal gradient of the thickness direction, which were beneficial in suppressing the coarsening of the intermetallic compounds (IMCs) layer and improving the weld formation. According to the SEM results, the thickness value of the IMC layer in the nugget zone and shoulder affect zone decreased from 0.78 μm and 1.31 μm in FSW process to 0.59 μm and 1.21 μm in SFSW process at the same parameter, respectively. Compared with the FSW process, SFSW improves the thermal accumulation during the process, which inhibits the formation of the IMCs and facilitates the material flow to form a mechanical interlocking structure. This firm interface formation elevates the effective contact area of the whole joint interface and provides a strong connection between the dissimilar metals. Thus, the ultimate strength of the 6 mm thick Al/Mg dissimilar SFSW joints was enhanced to 171 MPa, equivalent to 71.3% of AZ31B Mg alloys strength.

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Li Liu

Papers

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