Ultrasonic Vibration Enhanced Friction Stir Welding Research Articles

Ultrasonic Influence on Macrostructure and Mechanical Properties of Friction Stir Welded Joints of Al/Mg Sheets with 2 mm Thickness.

Friction stir welding (FSW) and ultrasonic vibration enhanced FSW (UVeFSW) experiments were conducted by using 6061-T6 Al alloy and AZ31B-H24 Mg alloy sheets of thickness 2 mm. The suitable process parameters windows were obtained for the butt joining of Al/Mg sheets. The effect of ultrasonic vibration on the macrostructure and mechanical properties of the dissimilar joints was studied. The results showed that the width of the weld nugget zone (WNZ) was enlarged to some extent and the hardness distribution in WNZ was more uniform in UVeFSW. In addition, the application of ultrasonic vibration effectively promoted the interpenetration degree of dissimilar materials in the WNZ so that the mechanical interlocking on the bonding interface of dissimilar Al/Mg materials was enhanced. The facture positions were changed from the bonding interface in FSW to the boundary between WNZ and the thermo-mechanical affected zone, and the ductile fracture zone was expanded. The highest ultimate tensile strength was 205 MPa at the process parameters set of 1200 rpm-50 mm/min in UVeFSW in this experiment. The average ultimate tensile strength of FSW/UVeFSW joints was 172.3 MPa and 184.4 MPa, respectively, and the average ultimate tensile strength was increased by 7.02% with the introduction of ultrasonic vibration.

Revealing the mechanism of superimposing ultrasonic vibration on microstructure evolution in friction stir welding by multi-physical multi-scale simulation

Ultrasonic vibration-enhanced friction stir welding (UVeFSW) is a promising joining method for aluminum‑lithium (Al-Li) alloy. However, directly observing the microstructural evolution in UVeFSW only by experimental methods is still a challenge. The mechanism of ultrasonic vibration (UV) on the microstructure evolution during FSW of Al-Li alloy remains unrevealed. In this study, a multi-physical multi-scale model was developed by coupling the finite element (FE) method and the Monte Carlo (MC) method to quantitatively study the impact of superimposing UV on the microstructure evolution in FSW. The preheating effect, acoustoplastic effect and residual hardening effect of UV were all considered in the model. Welding thermal processes, microstructure evolution of the material on the advancing side (AS) and in the center of the weld were quantitatively analyzed. And dislocation density and grain size in UVeFSW and FSW of Al-Li alloy were compared. Temperature field simulation results show that superimposing UV on FSW has a certain preheating effect in front of the tool. Microstructure simulation results show that the main effect of UV on microstructure evolution is the dynamic recrystallisation (DRX) process, where the maximum dislocation density in the weld nugget zone (WNZ) increases from 2.49 × 1013 m−2 to 3.49 × 1013 m−2 after superimposing UV, leading to an increase in the plastic deformation stored energy. Thereby, the driving force for nucleation and DRX increases, resulting in more nuclei in the WNZ, together with an increase in the width of the WNZ. Superimposing ultrasonic vibration in FSW can significantly refine the grain size by enhancing the activity and multiplication of dislocation in the UVeFSW of Al-Li alloy. The model is validated by the experimental results.

The effect of Sn interlayer on weld structure, intermetallic compounds, and joint properties of Al/Mg friction stir welding with ultrasonic assistance

ABSTRACT: The formation of intermetallic compounds (IMCs) is a primary factor that affects the welding joint between dissimilar materials. In this study, experiments were performed using Al/Mg friction stir welding (FSW) and ultrasonic vibration enhanced FSW (UVeFSW) with varying thicknesses of Sn interlayer. Detailed characterization and tests were conducted to explore the effect of Sn interlayer with or without ultrasonic vibration on weld surface morphology, macro-micro structure, generation of IMCs, and joint tensile strength. The study discusses the effect of ultrasonic vibration coupled with Sn interlayer and provides ideas for IMCs regulation when joining dissimilar materials. The study showed that adding a Sn interlayer cannot eliminate the generation of Al-Mg IMCs, but it can significantly reduce their thickness. Applying ultrasonic vibration with Sn interlayer can further reduce the thickness of Al-Mg IMCs and improve the interface microstructure from ‘IMC + matrix +IMC + matrix’ multilayer structures to simple IMCs structures. Furthermore, the highest joint strength was achieved when the Sn interlayer thickness was 0.1 mm during FSW. UVeFSW’s joint strength can be further improved with 0.05 mm and 0.1 mm Sn interlayer.

Numerical and experimental investigation on intermetallic compounds formation in ultrasonic vibration enhanced friction stir welding of dissimilar Al/Mg alloys

In friction stir welding (FSW) of dissimilar Al/Mg alloys, inevitable formation of intermetallic compounds (IMCs) has deleterious effect on the performance of joints. Although ultrasonic vibration enhanced FSW (UVeFSW) was invented to suppress the IMCs formation, the mitigating mechanism is not elucidated. In this work, the measured maximum temperature in weld nugget zone (WNZ) confirmed that in butt FSW of Al/Mg alloys the IMCs are generated by diffusion of solid atoms. The acoustic softening and antifriction effects were taken into account for computation of temperature, strain rate and dislocation density which are main factors affecting the inter-diffusion coefficients of Mg and Al atoms. An atom-diffusion based model was developed, and the formation and growth of the IMCs were quantitatively analyzed in FSW/UVeFSW of Al/Mg alloys. It was found that as the checking point flows toward to the tool, Al12Mg17 layer is first formed at the bonding interface, and then Al3Mg2 is formed at the Al side and grows rapidly. Under the conditions used in this study, the predicted values of IMCs thickness at the WNZ center in FSW and UVeFSW are 3.9 µm and 1.8 µm, respectively. The calculated IMCs thicknesses at the selected locations on the transverse cross-section and horizontal cross-section agree well with the measured ones. Ultrasonic induced dislocation-annihilation and associated decreasing of dislocation density in UVeFSW cause declining of the atomic diffusion coefficients, so that the formation and growth of IMCs are inhibited with ultrasonic action.

Research on the mechanical properties and fracture mechanism of ultrasonic vibration enhanced friction stir welded aluminum/steel joint

Friction stir welding (FSW) and ultrasonic vibration enhanced friction stir welding (UVeFSW) were used to connect 3 mm thick 2024-T3 Al alloy and Q235 carbon structural steel. The weld forming quality, mechanical properties, fracture location and interface behavior of the whole and layered joints were compared and analyzed, and the different mechanisms of the ultrasonic vibration to improve the tensile strength of the joint were summarized. As the results show, the ultrasonic vibration promoted the plastic flow ability of materials and improved the welding seam forming quality. Moreover, the ultrasonic vibration made the plastic deformation of the material in the nugget zone more uniform, which improved the strength of the nugget zone. In the middle layer of the joint, the ultrasonic vibration reduced the thickness of intermetallic compound layers (IMCLs), eliminated the laminated structure that harmed the connection quality of the joint, and formed a micromechanical interlocking structure in which IMCLs was inserted into the steel, which improved the interfacial bonding strength and improved the tensile strength by 11.1%. The ultrasonic vibration optimized the macro-mechanical interlocking structure in the joints of the lower layer, which enhanced its mechanical bonding ability with Al alloy.

Analysis on the performance of aluminum/steel UVeFSW joints by changing ultrasonic powers

The optimization mechanism of ultrasonic power on the performance of aluminum/steel dissimilar metal ultrasonic vibration enhanced friction stir welding (UVeFSW) joints was studied. The results indicated that ultrasonic vibration can significantly improve the surface forming quality of the weld seam, increase the width of the joint interface zone, adjust the height of the hook structure, suppress the generation of intermetallic compounds, and enhance the failure load of the joints. When the ultrasonic power was 500 W, the lap joints with the best mechanical properties were obtained, and its failure load was 5.14 kN, which was 51.2% higher than that of conventional FSW (0 W).

Achievement of high-quality joints and regulation of intermetallic compounds in ultrasonic vibration enhanced friction stir lap welding of aluminum/steel

The lap joints of 6061-T6 aluminum alloy/Q235 steel were obtained by conventional friction stir welding (FSW) and ultrasonic vibration enhanced friction stir welding (UVeFSW). The weld forming characteristics and microstructure differences of the joints with or without ultrasonic were compared and analyzed, and the effects of acoustic energy on the mechanical properties and intermetallic compounds (IMCs) layer of the joints were revealed. The results showed that ultrasonic vibrations could effectively improve the weld surface forming quality, increase the volume of plastic flow materials, significantly improve the welding speed of defect-free joints and expand the welding process window. The additional ultrasonic vibration can refine the grain structure of the weld nugget zone and the thermo-mechanically affected zone, improve the mechanical properties of the lap joints, and change the joint's fracture position and fracture mechanism. The assistance of acoustic can significantly enhance the mechanical interlocking ability of the center of the lap interface and change the type and thickness of the IMCs layer.

Acoustic effect on the joint quality and process of friction stir lap welding of aluminum to steel

To gain a high strength-to-weight ratio and economic viability for multiple industrial applications, the Al and steel need to be joined successfully. In the present work, we used conventional friction stir welding (FSW) and ultrasonic vibration enhanced friction stir welding (UVeFSW) techniques to obtain high-strength industrial-grade dissimilar joints of Al and steel. The effects of acoustic energy on the joint quality and welding process were investigated. Macrographs of the joints revealed that ultrasonic vibrations could effectively promote the material flow and increase the mixing degree and width of the Al/steel interface. Steel strips' fragmentation and intermixing with the Al substrate were enhanced under ultrasonic effects. The tool torque, axial force, and spindle power were significantly decreased with ultrasonic vibration. Acoustic assistance was found beneficial to increase the joints’ failure load and change the fracture mechanism. The microhardness of acoustically treated regions was found to be higher compared to its counterparts. The thinning of the brittle intermetallic compounds (IMCs) layer at the interface of the UVeFSW joint was caused by decreased peak temperature during welding. In UVeFSW, when the welding speed was 50 mm/min, the maximum failure load of the joint was obtained under the joint action of the macro interlocks (interface zone), the thinner IMCs layer, and the micro interlocks (laminated structure). When the welding speed was 100 mm/min, the failure load of the joint was improved by 25.8% compared with conventional FSW.

Evolution of microstructures and intermetallic compounds at bonding interface in friction stir welding of dissimilar Al/Mg alloys with/without ultrasonic assistance

Complete understanding of the evolution behaviors of the microstructures and intermetallic compounds (IMCs) along the interface materials flow path in friction stir welding (FSW) of dissimilar Al to Mg alloys is of great significance. In this study, conventional FSW and ultrasonic vibration enhanced FSW (UVeFSW) experiments of Al/Mg alloys were performed, and the instantaneous evolution features of the interface materials around the tool were "frozen" by using the "sudden stop" and simultaneous cooling techniques. The microstructures and IMCs formation at different locations around the exit hole were observed and characterized by scanning electron microscope, energy dispersive spectrometer and transmission electron microscope. It was found that before the materials started to deposit near the back of the tool, “IMC+Mg+IMC+Al” multilayer microstructure and simple IMC layer with (β+γ) sequentially emerged on the Al/Mg interface. With the application of ultrasonic vibration, the multi-layered interface structure only appeared at the middle stage of materials flow around the pin, and ultrasonic vibration just began to play a suppression role on the growth of two sub-layers IMC at a position where the materials deposit. With assistance of ultrasonic vibration in UVeFSW, the tool drove a larger volume of Mg alloy to move toward the retreating side, and the final IMCs thickness was thinner than that in FSW.

Influence of tool pin geometry on the thermal processes and plastic material flow in ultrasonic vibration–enhanced friction stir welding

Influence of tool pin geometry on the thermal processes and plastic material flow in ultrasonic vibration–enhanced friction stir welding

Enhancing the mechanical properties in the weld nugget zone of friction stir welded 2195 Al–Li alloy joint via superimposing ultrasonic vibration

Enhancing the joint properties of Al–Li alloys is of paramount importance to reduce the weight of aircraft. Third-generation 2195 Al–Li alloy was welded by the ultrasonic vibration enhanced friction stir welding (UVeFSW). And the effect of superimposing ultrasonic vibration on grain structure, precipitates and dislocations in the weld nugget zone was studied. It reveals that superimposing ultrasonic vibration can refine the grain size, increase the dislocation density and enhance the nucleation and growth of precipitates in the weld nugget zone of friction stir welded 2195 Al–Li alloy joint. Therefore, the mechanical properties of the weld nugget zone of the UVeFSW joint are enhanced, so the fracture was shifted to the heat-affected zone. It provides a promising method to enhance the joint properties of the third-generation Al–Li alloy.

Numerical simulation of ultrasonic vibration-enhanced friction stir welding process of dissimilar Al/Mg alloys

Numerical simulation of ultrasonic vibration-enhanced friction stir welding process of dissimilar Al/Mg alloys

Material flow velocity, strain and strain rate in ultrasonic vibration enhanced friction stir welding of dissimilar Al/Mg alloys

Friction stir welding (FSW) and ultrasonic vibration enhanced FSW (UVeFSW) of dissimilar Al/Mg alloys were conducted to elucidate the effect of ultrasonic vibration on the main process variables like material flow velocity, strain and strain rate. 1060 aluminum foils as the marker material were embedded respectively in the Mg on the advancing side and the Al on the retreating side, and the deformation of the aluminum foil at different positions was measured on the horizontal cross-section of metallographs after welding. The principles of mass conservation and plane strain were applied to obtain the flow velocity, strain and strain rate of the marker material around the tool in FSW and UVeFSW. The ultrasonic vibration induced increases in the material flow velocity, strain and strain rate in front of the tool were experimentally determined. Such measured results of the process variables lay solid foundation to reveal the UVeFSW process mechanism of joining dissimilar Al/Mg alloys.

Effect of ultrasonic field on microstructure evolution in friction stir welding of dissimilar Al/Mg alloys

Understanding of the microstructure evolution and the influence of ultrasonic vibration on friction stir welding (FSW) of dissimilar Al/Mg alloys is of great significance in optimizing the process parameters and improving the joint quality. In this study, the microstructure evolutions in the weld nugget zone (WNZ) of dissimilar Al/Mg joints during FSW and ultrasonic vibration enhanced FSW (UVeFSW) were characterized and compared. It was found that ultrasonic vibration (UV) has great effect on the grains structure of the WNZ in the Mg alloy side. Specially, the exerted UV changed the main dynamic recrystallization (DRX) mechanism of grains in the Mg alloy side from continuous DRX (CDRX) to discontinuous DRX (DDRX) under lower or higher welding speeds but constant tool rotation speed. Along the thickness direction, the average grain sizes near the bonding interface in the WNZ increased first and then decreased, and the maximum grain size was located at the mid-depth of the weld. Low strain shear texture appeared more in the WNZ of the Al alloy side during FSW, while in UVeFSW more locations in the WNZ were with high strain shear textures. The application of UV field improved the DRX degree in the whole weld through promoting the entanglement, aggregation and rearrangement of dislocations.

Analysis of heat and mass transfer in ultrasonic vibration-enhanced friction stir welding of 2195 Al–Li alloy

Ultrasonic vibration (UV)-enhanced friction stir welding (UVeFSW) is used for welding of 2195 Al–Li alloy plates. A model is developed to study the heat and mass transfer in UVeFSW of 2195 Al–Li alloy. It reveals that additional UV in FSW could reduce the heat flux and temperature at the contact interfaces when compared to those in conventional FSW. Although the temperature is slightly decreased with additional UV, the acoustoplastic effect of the UV could reduce the flow stress and viscosity in the shear zone, resulting in an enhanced fluidity of the plastic material with a higher flow velocity. It is demonstrated that the variations of viscosity, strain rate and flow stress around the tool at the same radial distance are more severe at low heat input condition, such as with a low rotation speed or a high welding speed. The model is validated by the experimental data.

Acoustic effect on the tensile properties and metallurgical structures of dissimilar friction stir welding joints of Al/Mg alloys

Abstract The joint strength of friction stir welded Al/Mg alloys is a critical factor for the wide applications of dissimilar joints. In this study, dissimilar Al/Mg alloys joints were made by friction stir welding (FSW) and its variant, ultrasonic vibration enhanced FSW (UVeFSW), respectively. The tensile strength, fracture toughness and fracture location of the whole joints and three (upper, middle, lower) parts of the joints were compared by characterizing the macro-, mesoscopic- and micro-structures of the welds. The influence of ultrasonic vibration on the tensile properties of the joints was discussed. The results showed that the lower part of the welds was the weakest part due to the poor materials mixing/interlocking and a large amount of intermetallic compounds, while the upper part of the weld had the best strength. Good material mixing/mechanical interlocking in the welds was the basis for obtaining higher joint strength, and the thinner intermetallic compound layer was the key to the better toughness of the joint. The application of ultrasonic vibration can improve the tensile strength of each part of the welds, but the improvement rate of tensile strength is greater for the middle and lower parts of the welds.

Effects of tool shoulder size on the thermal process and material flow behaviors in ultrasonic vibration enhanced friction stir welding

To improve the joint quality and lower the welding loads, ultrasonic vibration enhanced friction stir welding (UVeFSW) had been developed and had shown several benefits. The most important geometric parameter in UVeFSW tool design is the shoulder diameter, but its effect on the thermal process and plastic material flow behaviors has not been quantitatively analyzed. To elucidate these effects, an integrated computational fluid dynamics (CFD) model was adopted to quantitatively study the effects of tool shoulder diameter on the thermal process and plastic material flow behaviors in UVeFSW. It is found that UVeFSW process can have significant effects in enhancing plastic material flow and reducing the welding loads without increasing the process temperature when compared to that in FSW by using larger tool shoulder to enhance the plastic material flow. It revealed that the maximum width of the shear zone at the top surface of the workpiece was highly dependent on the shoulder diameter. However, the minimum width of the shear zone at the bottom surface of the workpiece was insensitive to the shoulder diameter. Based on the combination of the integrated CFD model with the theory of maximum traction of the tool torque on the plastic material, the optimum shoulder diameter at various welding parameters had been systematically studied. It was found that the optimum shoulder diameter decreases with an increase of the tool rotation speed, while it increases with an increase of the welding speed. This model provides a reasonable method for optimum the shoulder diameter in UVeFSW process. The integrated CFD model is validated by a comparison of the predicted and measured thermal cycles and welding loads.

Constitutive equation with residual hardening effect for modeling the ultrasonic vibration enhanced friction stir welding process

ABSTRACTA modified acoustic-plastic constitutive model characterising both acoustic softening and residual hardening effects on aluminium alloys is developed and applied into a computational fluid dynamic model for the ultrasonic vibration enhanced friction stir welding (UVeFSW) process. The flow stress and material flow field in UVeFSW are quantitatively analysed. The effect of acoustic residual hardening on the streamlines density at rear advancing side is examined by comparing with that based on the model considering just acoustic softening. The numerical simulation results are experimentally validated, and it is found that the modified constitutive equation improves the prediction accuracy of the flow stress and material flow field in UVeFSW.

Analysis of friction reduction effect due to ultrasonic vibration exerted in friction stir welding

A model is developed to characterize the ultrasonic induced friction reduction in ultrasonic vibration enhanced friction stir welding (UVeFSW). Two effects, i.e., acoustic softening and ultrasonic induced friction reduction (UiFR), are both considered in analyzing the heat generation, temperature profile and material flow in UVeFSW. It is found that at the tool/workpiece contact interfaces the friction coefficient shows a butterfly-like distribution if the ultrasonic induced friction reduction (UiFR) is taken into consideration. With UiFR, the predicted heat generation in UVeFSW is more reasonable, the predicted TMAZ (thermo-mechanically affected zone) boundary is in more agreement with the measured one, and the calculated thermal cycles match better with the experimentally measured ones.

Analysis of welding load reduction in ultrasonic vibration-enhanced friction stir welding

A novel process variant of friction stir welding (FSW), i.e., ultrasonic vibration-enhanced FSW (UVeFSW), has been developed to improve the joint quality and lower the welding loads. To elucidate the underlying mechanism of the UVeFSW process, an integrated 3D model is developed to analyze the effects of ultrasonic vibration on welding loads quantitatively. The predicted traverse force and tool torque agree well with the experimental measured ones. It is found that superimposing ultrasonic vibration in UVeFSW can reduce the traverse force and tool torque, and the reduction percentage of welding loads decreases with an increase of tool rotation speed. Analytic formulae are developed to calculate the traverse force and tool torque in UVeFSW under different levels of welding speed and rotation speed. The predicted welding loads from the analytic formulae fit well with both experimentally measured data and the numerically simulated data by using the validated integrated 3D model. The work lays the foundation for optimizing the process parameters in UVeFSW in order to efficiently use the ultrasonic vibration energy.