Improving the Quality of Dissimilar Al/Steel Butt-Lap Joint via Ultrasonic-Assisted Friction Stir Welding

Abstract

A dissimilar AA7075/Q235 butt-lap joint was fabricated via ultrasonic-assisted friction stir welding (UaFSW), and the characteristics of the UaFSW joint were investigated systematically. The acoustoplastic effect of the ultrasonic vibration led to the softening of the materials and enhanced the material flow during welding, decreasing the volume of welding defects in the nugget zone of the UaFSW joint. With the help of ultrasonic vibration, a smooth and thin intermetallic compounds (IMCs) layer could generate along the Al/steel interface at the top of nugget zone, which possibly consisted of Al5Fe2 and Al13Fe4 phases. However, the positive effects of the ultrasonic vibration were weakened at low temperatures; consequently, the IMCs layer became discontinuous at the bottom of the nugget zone and the welding defects also formed. The ultrasonic vibration accelerated the dynamic recrystallization and refined the microstructures in the nugget zone due to the increased strain rate and stored energy. As a result, the UaFSW joint exhibited a better mechanical performance in comparison to the FSW joint, and the increment in the peak tensile load/elongation was more than twice. In addition, the UaFSW joint failed in the nugget zone along with the Al/steel interface, and the fracture mode was a mixture of ductile and brittle.

Improved Interface Morphology and Failure Load of Ultrasonic-Assisted Friction Stir Lap Welding Joint of 2024 Aluminum Alloy to 304 Stainless Steel

“…In the experiment, the upper and lower plates were 2024-T4 aluminum alloy and 304 stainless steel respectively, which were joined by the FSW-3LM-4012 friction stir welder (FSW, Fort Myers, FL, USA). As is known, for the butt joint by FSW, the steel plate is always put at the advancing side (AS) based on the difference in the thermal flow behaviors of these two materials [24][25][26]. For the lap joint, the aluminum alloy plate is often served as the upper plate in order to greatly decrease and even avoid the tool wear, and can be placed at the AS [30,33] or the retreating side (RS) [16,19].…”

“…In recent years, to heighten the loading capacity of Al/steel FSW joints, the assisted welding process has gradually become a research hotspot in addition to the optimization of process parameters such as welding speed, rotating velocity, and so on [21][22][23]. The ultrasonic-assisted process has been successfully used in friction stir butt welding [24][25][26] and friction stir spot welding [27][28][29], and some researchers have also used ultrasonicassisted FSLW (UAFSLW) to join aluminum to steel [5,[29][30][31]. Hong et al [29] studied the UAFSLW joint of 6061 aluminum alloy and 301L steel and showed that the excessive growth of the Al-Fe IMC layer in the weld core region of the joint was suppressed by the stirring action of the welding tool and the ultrasonic vibration; the average thickness of the IMCs was reduced from 10 µm in the FSLW joint to 6 µm in the UAFSLW joint, and the 16.5 kN tensile load of UAFSLW joint was 27.9% higher than the traditional FSLW joint.…”

“…Their results showed that the assistance of ultrasound improved the mechanical interlocking capacity of the interface center, changed the IMC type from highly brittle FeAl 3 to highly ductile FeAl, reduced the thickness of the IMC layer from 5.01 µm to 0.71 µm, and then fabricated the joint with the failure load 21% larger than the traditional FSLW joint. In fact, similar to the UAFSLW process, ultrasonic-assisted FSW (UAFSW) of Al/steel butt joints has an enhanced joint loading capacity [5,[24][25][26]30]. Wu et al [5] reported that the addition of ultrasound during FSW improved the micro and macro mechanical interlocking structures which were positive on the joint loading capacity.…”

Effect of Welding Parameters on Al/Mg Dissimilar Friction Stir Lap Welding with and without Ultrasonic Vibration

“…To prevent the formation of IMCs and enhance the overall quality of the joint, certain researchers have suggested incorporating ultrasonic energy into the FSW process as a supplementary form of energy. Consequently, they have developed multiple techniques for ultrasonic vibration-assisted FSW [15][16][17][18][19]. Several research studies show that ultrasonic vibration can enhance material flow in the weld nugget zone (WNZ), prevent the growth of intermetallic compounds (IMCs), and enhance joint properties.…”

Characteristics of the Dissimilar AA7075 and Q235 Steel Joints Fabricated by Friction Stir Welding

“…Therefore, various welding techniques, such as the diffusion bonding, ultrasonic joining, and friction stir welding (FSW), have been proposed to improve welding quality [6,7]. In particular, as a solid-state joining technique, FSW generates lower welding heat input and can restrict the excessive growth of the IMC layers [8], showing great potential for fabricating high-quality Al/steel joints.…”