Friction Stir Lap Welding Research Articles

The pin tool wear identification with vibration signal of friction stir lap welding based on a new pin tool wear division model

Aluminum alloy (Al) is a commonly lightweight material, which has wide applications in the development of modern railways and automobiles. Friction stir lap welding (FSLW) is a commonly lightweight material connection technology, which avoids the defects produced by traditional fusion welding methods. The pin tool of FSLW is not replaced in time after wear will influence welding quality and waste resources. To reduce the unnecessary losses, this paper proposes a FSLW pin tool wear division model based on the convolutional neural network (CNN), bidirectional long-short term memory (BiLSTM) and Attention module. Firstly, the wear of the pin tool is divided into four levels based on the macro morphologies of the pin tool, welds surface appearance, internal defects and tensile strength. Then, the time–frequency characteristics of vibration signals are extracted by power spectral density (PSD) and wavelet packet transformation (WPT). The wear characteristics datasets of the pin tool are built by these features. More detailed information can be provided for the wear prediction of the pin tool. Finally, the CNN-BiLSTM-Attention network is established to classify the pin tool wear levels. CNN and BiLSTM are used to extract effective information of wear characteristics in spatial and temporal directions, respectively. Attention mechanism is used to distribute different weights to choose critical features and then enhance the role of important features in the calculation process. The wear features datasets are normalized and input into the classification model. The classification result shows that the average accuracy of the CNN-BiLSTM-Attention model reaches 95.8 %, which is 5.7 % higher than that of the CNN-BiLSTM model. Compared with the mature classification model, it has higher accuracy and anti-noise ability. Therefore, a FSLW pin tool wear division model is established based on the CNN-BiLSTM-Attention neural network, which the pin tool wear information during FSLW could be predicted more detailed, and the pin tool is managed more efficiently.

Lap joining of Ti6Al4V titanium alloy by vortex flow-based friction stir welding

The current study uses a novel vortex flow-based friction stir lap welding (VFSLW) process to weld 1.4 mm thick Ti6Al4V sheets, trying to replace the diffusion bonding in the superplastic forming/diffusion bonding process. The mechanical properties and joining mechanism of the VFSLW joint were investigated. Good weld formation was obtained at 300–350 rpm and 80 mm/min. No hook defect was formed in the lap interface. The lap joint fractured across the stir zone (SZ) in the top plate under the optimal parameters. The cracks initiated from the oxidation defect, where the oxides on the workpiece surface were involved in the SZ by the vortex during the welding. It suggests that a good argon shield is very important for the VFSLW of titanium alloy. The highest tensile strength reaches ∼890 MPa, up to 95 % of the base material. The formation of an α + β lamellar structure in the SZ is due to the peak welding temperature exceeding the β-transus temperature. In the bonded zone (BZ), a very thin layer with ultrafine α grains was formed by dynamic recrystallization in the α phase field, although its two sides are α + β lamellar structures. This is because of the oxidation on the workpiece surface during the welding process and the O element is a strong α stabilizer.

Effect of Ultrasound on Microstructure and Properties of Aluminum–Copper Friction Stir Lap Welding

In this paper, the influence mechanism of ultrasound on plastic flow and microstructure features of the aluminum–copper friction stir lap welding (Al/Cu-FSLW) process is systematically investigated by adjusting the welding speed and improving the shear rheology in the plastic stirring zone. Through adjusting the ultrasonic vibration and welding speed, the directional control of mechanical properties is realized. It is found that increasing the welding speed properly is beneficial to enhance the mechanical shear between the tool and the workpiece, thus forming more staggered layered structures at the copper side and improving the tensile strength of the weld. The acoustic softening enhances the viscoplastic fluid mixing and strengthens the mechanical interlock of the Al/Cu lap interface. As the welding speeds increase or ultrasonic vibration is applied, the thickness of Al/Cu intermetallic compound (IMC) decreases, and the tensile strength and elongation of the Al/Cu joints are enhanced. Compared with adjusting the welding speed, the ultrasonic vibration can further refine the copper particles which are stirred into the plastic zone, and the thinning effect of ultrasound on IMC layers is better than that of increasing welding speed. At the welding speed of 60 mm/min, the IMC layer thickness is reduced by 42% under ultrasonic effect. In three welding speed conditions, the UV reduced the absolute value of the effective heat of formation (EHF) for Al2Cu and Al4Cu9 and suppressed the formation of AlCu phase. Meanwhile, only when the welding speed is increased from 60 mm/min to 100 mm/min can the formation of AlCu be suppressed. Under the ultrasonic optimization, the stable improvement of welding efficiency is ensured.

Achievement of superb-strength lap joint via opposite-directions flowing friction stir lap welding of 2024 aluminum alloys

It is common for the rotating pin largely plunging into lower sheet to break up lap interface of Friction Stir Lap Welding (FSLW) joint, but the unavoidable up-bending morphology of hook outside Nugget Zone (NZ) largely reduces the joint bearing ability. Based on the novel Opposite-directions Flowing FSLW (OF-FSLW) by the self-developed rotating tool with an X-shaped right-left thread pin, the 2024 aluminum alloys lap joint was successfully welded in this study. The migration law of lap interface during welding was investigated by the experimental and numerical methods, and then how the rotating pin and its rotating velocity affect the formation and strength of OF-FSLW joint was further analyzed. The results show that the Material Concentrated Zone (MCZ) which formed above the original lap interface made the hook bend downward, the NZ greatly enlarged and the beginning part of cold lap compressed and thickened, thereby heightening the joint bearing ability. For the OF-FSLW joint, its maximum tensile strength was 403 MPa, and the corresponding joint efficiency of 90.8% was an incredible and superb value for the 2000 series heat-treatment strengthened aluminum alloys friction stir welded joint. The OF-FSLW technology by the rotating tool with an X-shaped right-left thread pin is proven to be a greatly effective approach for manufacturing the aluminum alloys lap joint with superb strength.

Research on ultrasonic-stationary shoulder assisted friction stir lap welding of thermoplastic polymer and aluminum alloy

Friction stir lap welding (FSLW) has advantages on obtaining dissimilar upper polymer and lower aluminum materials joint (polymer/Al joint), and how to heighten the bearing capacity of polymer/Al joint as much as possible is greatly meaningful for the manufacturing industries. In this study, choosing dissimilar polyamide 6 (PA 6) polymer and 6061-T6 aluminum alloy materials as research object, traditional FSLW, stationary shoulder assisted FSLW (SSFSLW) and ultrasonic-stationary shoulder assisted FSLW (U-SSFSLW) processes were compared to understand the mechanism of ultrasonic enhancement on the polymer/Al joint. Firstly, the surface morphologies were studied. Then the analyses of micro/macro-structures, mechanical properties and fracture features were carried out by optical microscopy, scanning electron microscopy, differential scanning calorimetry, X-ray diffraction and tensile tests. Results showed that the lap shear failure load of polymer/Al joint by U-SSFSLW reached 1502 N from 814 N by traditional FSLW, because the addition of ultrasonic avoided the bulged-nodule structures on top surface and the cavity defect in stir zone (SZ), broke the Al anchor in upper plate, enlarge the width of SZ in lower plate, and made the deeper micro-pits and larger dents at the interface between SZ bottom and lower plate. The U-SSFSLW process provides an effective way to fabricate the high-strength hybrid joint of dissimilar upper polymer and lower Al materials.

Performance Evaluation of Laser-Deposited Functionally Graded Stellite 6-WC Tools in Friction Stir Lap Welding of CuCrZr-SS304

Performance Evaluation of Laser-Deposited Functionally Graded Stellite 6-WC Tools in Friction Stir Lap Welding of CuCrZr-SS304

In Situ Monitoring and Defect Diagnosis Method Based on Synchronous Compression Short-Time Fourier Transform and K-Singular Value Decomposition for Al-Carbon Fiber-Reinforced Thermoplastic Friction Stir Lap Welding

In Situ Monitoring and Defect Diagnosis Method Based on Synchronous Compression Short-Time Fourier Transform and K-Singular Value Decomposition for Al-Carbon Fiber-Reinforced Thermoplastic Friction Stir Lap Welding

Welding load, temperature, and material flow in ultrasonic vibration enhanced friction stir lap welding of aluminum alloy to steel

To elucidate the effect of ultrasonic vibration on the welding process (welding load, thermal cycle, and material flow) of aluminum/steel dissimilar metals in friction stir lap welding, conventional friction stir lap welding (FSLW) and ultrasonic vibration enhanced friction stir lap welding (U-FSLW) were used to weld aluminum/steel dissimilar metals. The results indicate that applying ultrasonic vibration to the FSLW of aluminum/steel dissimilar metals can effectively decrease welding loads (tool torque, axial force, and spindle power), and it was found that the reduction in welding load was inversely proportional to the welding speed. The effect of additional ultrasonic vibration on axial force was the largest. Ultrasonic vibration had a significant preheating effect, and its thermal effect in the width and depth directions of the workpiece gradually weakened. Additional ultrasound can reduce the peak temperature during the welding process. The instantaneous state of the weld was obtained using the "emergency stop+emergency cooling" technology, and the three-dimensional visualization characterization of material flow near the keyhole was achieved using X-ray based computer tomography (CT). It was found that ultrasonic vibration could increase the material's plastic flow, change the steel structure's flow path, and significantly increase the steel particles' flow range near the pin tool. Compared with conventional FSLW, the changes in the macro/microstructure of U-FSLW joints were closely related to the welding thermal cycle and material flow.

Interfacial reaction characteristics and mechanisms during dissimilar friction stir lap welding of pure copper and Al0.1CoCrFeNi alloy

In order to explore the weldability and widen the applicability of high entropy alloys (HEAs), dissimilar friction stir lap welding of pure copper and Al0.1CoCrFeNi alloy was investigated in this research. The results indicate that a sound dissimilar weld without tunnels and voids is produced, where a ∼20 μm thick interfacial layer is evident at the Cu-HEA interface. The lap joint is fractured at the upper Cu sheet rather than the lap surface during tensile shear test, suggesting an effective bonding of the dissimilar materials. The lap weld roughly consists of three layers along the plate thickness direction, i.e. welds of Cu, HEA and Cu-HEA interface, respectively. The interface of weld, which has experienced the most severe Cu-HEA interaction during welding, is characterized by uniformly distributed and highly recrystallized ultra-fine grains (∼0.79 μm). Meanwhile, intermetallic compounds particles (Al13Cr2 and Al13Co4) ranging in size of 150∼400 nm are generated at the fine grain boundaries. Fast diffusion of Al, Cr and Co elements and easy segregation of these elements during welding result in the formation of the particles. The sufficient DDRX and the pinning effect of particles are responsible for the formation of ultra-fine grains in the weld interface. The present study lays a theoretical foundation for the joining of conventional Cu and the multi-component HEA, which contributes to the improvement of HEA availability in the field of manufacturing industry.

Friction stir lap welding of AZ31B magnesium alloy to AISI 304 stainless steel

Abstract AZ31B magnesium alloy plates were lap-joined to AISI 304 stainless steel plates through the friction stir welding (FSW) method and utilizing various tool welding speeds. It has been found that the most important factor governing the weld strength is the hook formed on the advancing side of the welds. The weld tensile shear strength improved with an increase in the tool feed rate. Because, in general, height, length, and width of the hook taking place on the advancing side shrunk. Furthermore, the angle between the hook and interface of the plates increased, leading to reduced sharp corner formation. Apart from these, imperfections such as cavities, voids, and uncombined regions at the weld interface reduced and disappeared when increasing the welding speed. During the tensile shear test, all the welds fractured tensile mode and brittle type from the top AZ31B plate next to the hook on the advancing side. There was no breakage occurred in the weld interface, which is an indication of the strong joints. No intermetallic compounds between iron and magnesium were determined at the fracture region. At lower welding speeds, a higher amount of AISI 304 particles occurred at the weld stir zone resulting in a higher hardness.

Microstructure and Mechanical Properties of Mg-Li/Al Dissimilar Joints via Dynamic Support Friction Stir Lap Welding.

In this study, two-mm-thick dual-phase LA103Z Mg-Li and 6061 Al alloys, known for their application in lightweight structural designs, were joined using dynamic support friction stir lap welding (DSFSLW). The microstructural evolution and mechanical properties of dissimilar joints were investigated at different welding speeds. The analysis revealed two distinct interfaces: the diffusion interface and the mixed interface. The diffusion interface, characterized by a pronounced diffusion zone, is formed under slower welding speeds. The diffusion zone height, the effective lap width, and the interface layer thickness decrease with increasing welding speed due to low plastic deformation capacity and weak interfacial reactions. Conversely, the mixed interface, associated with higher welding speeds, contained large Al fragments. The extremely high microhardness values (130.5 HV) can be ascribed to the formation of intermetallic compounds (IMCs) and strain-hardened Al fragments. Notably, the maximum shear strength achieved was 175 N/mm at a welding speed of 20 mm/min. The fracture behavior varied significantly with the interface type; the diffusion interface showed enhanced mechanical strength due to better intermetallic reactions and interlocking structures, while the mixed interface displayed more linear crack propagation due to weaker IMCs and the absence of hook structures. Fracture surface analysis indicates that fractures are more likely to propagate through the Al matrix and interface layers.

Underwater dissimilar friction stir lap welding of aluminum alloy and CFRTP

With the rapid development of the transportation and electronic equipment industries, the demand for robust joining technology for metal/polymer has become imperative, especially in the pursuit of lightweight solutions. Underwater welding is one of the methods to reduce the formation of holes and other welding defects, which is conducive to improve the quality and strength of joints. In this study, the underwater friction stir lap welding (UFSLW) of 6061-T6 Al alloy and carbon fiber reinforced thermoplastic (CFRTP) joints is investigated. The welds are evaluated by optical microscope (OM) image, scanning electron microscope (SEM), and energy dispersive X-ray spectrometer (EDS) of the fracture, tensile, and hardness analysis. Research reveals that the presence of cracks and tunnel defects similar to those in air FSLW (AFSLW) is reduced in the underwater welding process. From the microscopic result analysis, the rapid dissipation of frictional heat and substrate viscosity in the cooling medium of circulating water in the stirring zone reduces the interaction between Al alloy and molten CFRTP, which leads to the rapid transformation of the plasticized material into a solid state and reduces the formation of thick stacked layer. In addition, finer Al alloy fragments are obtained in the welding zone due to the fast cooling rate. Ultimately, the maximum tensile strength of 32.12 MPa is achieved, which is 10.76% higher than that of the same welding parameters in air. Based on UFSLW technology, the joint strength of this study is 69% higher than that of Al alloy and polymer joints, which demonstrates enormous application potential and opens up new prospects for the joining technology of dissimilar materials.

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

The amount of heat input during welding impacts the weld's thermal and mechanical behavior and the joint's properties. The current study involved conducting AA 6061 and AZ31B Mg dissimilar welding, using friction stir lap welding (FSLW) and ultrasonic vibration-enhanced FSLW (UVeFSLW). The comparison and analysis of the welding load, the weld's macro-microstructure, intermetallic compounds (IMCs), and joint properties were conducted by adjusting the process parameters. The study also examined the effect of ultrasonic vibration (UV) variations on welding heat input. The study demonstrated that it is possible to reduce the welding load by employing UV. Moreover, this impact becomes more pronounced as the welding heat input decreases. Additionally, the material flow in the weld, the width of the weld nugget zone, and the continuous IMC layer are significantly influenced by ultrasonic vibration, irrespective of the heat input during welding. However, the impact on large areas of irregular IMCs or eutectic structures is relatively small. Furthermore, achieving better joint properties becomes more feasible when a higher welding speed is employed for the Al alloy placed on top. Specifically, the impact of UV becomes more evident at higher welding speeds (≥220 mm/min).

Monitoring of Weld Bead Profile in Friction Stir Lap Welding for Polycarbonate Sheets Using Wavelet Packets of Power Signal

Monitoring of Weld Bead Profile in Friction Stir Lap Welding for Polycarbonate Sheets Using Wavelet Packets of Power Signal

Enhanced formation quality and tensile shear load of 7B04 aluminum alloys lap joint by impacting flow friction stir lap welding

Avoiding the upward-bending hook is a highly pressing and challenging task in the field of similar aluminum alloys friction stir lap welding (FSLW). To enhance formation quality and then obtain the welded lap joint with high tensile shear load, a novel impacting flow FSLW (IF-FSLW) process based on the X-shape reverse-threaded pin was proposed in this study. Three welding tools with different tool geometry parameters were developed to analyze how the locations of the materials accumulated zone (MAZ) influenced the joint formation and the joint loading capacity, and 7B04 aluminum alloys were chosen as research subject. Results showed that the horizontally-impacting and vertically-impacting effects of MAZ on and above the original lap interface made the hook respectively present the forky structure at the end and the downward-bending morphology, and the horizontally-impacting effect of MAZ made the stir zone (SZ) have a protruding morphology near the MAZ. Under the tensile fracture mode, the IF-FSLW joint under configuration A had a greatly heightened tensile shear load under the combined actions of the downward-bending hook and the enlarged SZ. The tensile shear load of IF-FSLW joint reached 8.62 kN/mm under the reasonable combination of tool geometry parameters and welding process parameters. The IF-FSLW process by the X-shape reverse-threaded pin provides an effective way to obtain a superb-strength lap joint of aluminum alloys.

Extrinsic-Riveting Friction Stir Lap Welding of Al/Steel Dissimilar Materials.

To obtain high-quality joints of Al/steel dissimilar materials, a new extrinsic-riveting friction stir lap welding (ERFSLW) method was proposed combining the synthesis advantages of mechanical riveting and metallurgical bonding. SiC-reinforced Al matrix composite bars were placed in the prefabricated holes in Al sheets and steel sheets, arranged in a zigzag array. The bars were stirred and mixed with Al sheets under severe plastic deformation (SPD), forming composite rivets to strengthen the mechanical joining. SiC particles were uniformly dispersed in the lower part of the welding nugget zone (WNZ). The smooth transition between the SiC mixed zone and extrinsic-riveting zone (ERZ) ensured the metallurgical bonding. The maximum tensile shear load of the joints reached 7.8 kN and the maximum load of the weld per unit length was 497 N/mm. The fracture occurred at the interface between the rivets and steel sheets rather than the conventional Al/steel joining interface. Moreover, ERFSLW can prolong the service life of joints due to three fracture stages. This method can be further extended to the welding of other dissimilar materials that conform to the model of "soft/hard".

Evolution of interfacial phases between Al alloy and high entropy alloy during annealing

While the interfacial phases between Al and high-entropy alloy (HEA) share the same types as that between Al and steel, it has been found previously that the thermal stability of the Al-HEA interfacial phases is significantly higher than that of the Al-steel interfacial phases. To further elucidate the reason for the exceptional thermal stability of Al-HEA interfacial phases, this study investigated the evolution of the interfacial phases formed between Al alloy and FeCoCrNiMn HEA during annealing. An Al-FeCoCrNiMn thin film diffusion couple was extracted from a friction stir lap welding (FSLW) joint by focused ion beam and then annealed at 400 °C. An amorphous layer is formed initially at the interface due to the high strain rate deformation during FSLW. The amorphous layer remained even after prolonged annealing at 400 °C (∼0.72 Tm of Al alloy) for 60 min with very limited thickening (from 50 nm to 55 nm), showing remarkable thermal stability at the relatively high temperature. Al13Fe4-type intermetallic compound (IMC) with the Fe site occupied by Fe, Co, Cr, Ni and Mn first nucleates at the interface between Al alloy and amorphous layer. With the annealing time extended to 180 min, the amorphous layer is partially transformed into Al5Fe2-type IMC. The rate constant for the thickening of the reaction layer at the Al/FeCoCrNiMn interface is 0.03 μm∙min1/2, which is much smaller than (only 0.02 times) that at the Al/steel interface under similar annealing condition. The high thermal stability of Al-HEA interfacial layer is thus attributed to the slow kinetics in crystallization of the initially formed amorphous layer, the barrier effect of amorphous layer for interdiffusion, and the reduced diffusivity inside of IMCs caused by significant fluctuations in lattice potential energy due to high-density doping with HEA elements. This finding provides new insights into the fabrication of thermally stable Al-HEA hybrid structures suitable for service conditions involving high-temperature exposure.

Diagnosis of Al-CFRTP TA-FSLW defect using acoustic emission signal based on SPWVD and ResNet

As one of the important development directions in modern railway and automobile, lightweight requires reliable connection technology of dissimilar materials. Thermal-assisted friction stir lap welding (TA-FSLW) is an available welding technology for the joining of dissimilar materials, which can solve the problems of physical and chemical properties in the welding process of dissimilar materials. Unfortunately, defects may occur during the welding process. This paper proposes a defect diagnosis method based on smooth pseudo Wigner-Ville distribution (SPWVD) and ResNet18 for Al-CFRTP TA-FSLW. Firstly, the acoustic emission (AE) signal and the temperature of the weld nugget are acquired by the AE sensor and thermography camera in the FSLW process. Then, the time–frequency domain features of AE signal are extracted by SPWVD, and the maximum temperature of the weld nugget is extracted by exponential moving average (EMA) from thermography. Moreover, the dataset is established by SPWVD features, maximum temperature of welding nugget, and several time–frequency features. Finally, the ResNet18-attention network is employed to classify welding defects and stages. The Al-CFRTP TA-FSLW experiments are carried out based on the proposed method. Experimental result indicates that the SPWVD spectrum can effectively extract the feature frequency of deep and shallow hole defects in the welding process. In the section of classification, the accuracy of the dataset with SPWVD features is improved by 2% to 4% compared to other methods, which demonstrates that the SPWVD feature is superior in defect feature extraction.

Investigation on microstructural evolution and local mechanical performance of friction stir lap welded AA6061-T6/ AA7075-T6 joints

The current study uses the friction stir lap welding (FSLW) technique to successfully fabricate two layered dissimilar AA6061/AA7075 joints. A defect-free joint with superior interfacial properties was obtained at the optimized process parameters (tool rotational speed / transverse speed) of 850 RPM/ 55 mm/min. The detailed investigations of microstructure evolution, crystallographic texture, and strength correlation were performed using the field emission scanning electron microscope (FE-SEM), optical microscope (OM), electron backscattered diffraction (EBSD), Vickers microhardness, nano-indentation, and miniature tensile tests. EBSD analysis reveals the presence of a significant grain refinement at the bottom (3.65 µm), middle (4.58 µm), and top region (6.34 µm) of stir zone (SZ) compared to AA6061 (42.56 µm) and AA7075 (34 µm) base metals. An increasing trend in the fraction of high angle grain boundary (FHAGBs) and recrystallization fraction was noticed from top to bottom of the build within the SZ. Continuous dynamic recrystallization (CDRX) with the gradient in strain, strain rate, and temperature leads to these microstructural variations within the SZ. The microhardness study shows a decrease in microhardness in the SZ due to thermal softening and precipitate dissolution at higher temperatures. Appreciable interfacial material mixing leads to remarkable strength, ductility, and strain-hardening behaviour within the SZ, especially at the interface section. Pole figures (PF) reveal the presence of major shear texture components (B/B̅, and C) with some minor presence of A1*/A2*and A/A̅ that confirms the presence of higher strains within the SZ. The dominant presence of recrystallization textures components (like cube {001} 〈110〉, Goss {011} 〈100〉, and P {011} 〈122〉 ) within the SZ confirms the presence of the CDRX mechanism.

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

Achieving high-strength welding joint of aluminum to steel is a highly pressing and challenging task in the manufacturing industries, and friction stir lap welding (FSLW) has advantages for joining these two metals. To further heighten the strength of dissimilar aluminum and steel metals (Al/steel) FSLW joint, the ultrasonic-assisted FSLW (UAFSLW) process was used, and the upper 2024-T4 aluminum alloy and the lower 304 stainless steel were chosen as research object. The results show that the addition of ultrasound eliminates the micro pores, changes the aluminum-rich intermetallic compounds (IMCs) into the iron-rich IMCs and enhances the micro and macro mechanical interlocking structures along the Al/steel lap interface. Under the rational IMCs layer thickness lower than 1.5 μm, the UAFSLW joint has the failure load higher than the traditional FSLW joint. The maximum failure load of UAFSLW joint reaches 7.06 kN, and the loading capacity of this joint is higher than that of reported Al/steel traditional FSLW joint. The UAFSLW process is an effective way to fabricate the high-strength Al/steel lap joint.