Friction Stir Spot Welding Research Articles

Modeling and optimization of aluminum–steel refill friction stir spot welding based on backpropagation neural network

Modeling and optimization of aluminum–steel refill friction stir spot welding based on backpropagation neural network

The influence of modification of the geometry of the front surface of the RFSSW tool inner sleeve on the fatigue life of joints during joining clad sheets made of aluminum alloy 2024-T3

AbstractRefill Friction Stir Spot Welding (RFSSW) has a number of advantages that make it a possible alternative to riveting and resistance welding in aerospace structures, the automotive industry and other applications. Adequate determination of technological parameters which ensure the desired properties of welds and their functioning in various operating conditions requires, among others, appropriate fatigue life of connections. The article presents the results of comparative tests of the mechanical properties of welds (load-bearing capacity and fatigue life at selected three load levels) made with a basic tool (G0) and a tool with a modified geometry (G4). The samples were made of 1.27 mm thick clad sheets of 2024-T3 aluminum alloy with an additional oxide anodic coating. It has been shown that the modified geometry of the working surface of the inner sleeve of the RFSSW tool improves the conditions and course of the plasticization and stirring process of the joined materials. The use of a G4 geometry tool allowed for approximately 30% higher joint load-bearing capacity and approximately twice as long fatigue life (at lower load levels) compared to welds made with the G0 tool.

Formation of the Interlock Morphology and Its Role in Refill Friction Stir Spot Welding of Aluminum Alloy to Steel

Considering energy conservation and emission reductions, lightweight automobiles have become a research focus in the automotive industry. Steel/aluminum joining is regarded as an ideal lightweight structure, which can not only reduce the energy consumption but also ensure safety and is already attracting extensive attention. In this study, aluminum alloy 6061 and B410LA steel sheets were successfully joined by refill friction stir spot welding. The tensile properties, microhardness distribution and interfacial microstructure characteristics of the steel/Al welded joints were investigated. The maximum tensile load of the steel/Al joint was 4.3 kN. The mechanical properties of the steel/Al refill friction stir spot welded joint were largely determined by the bonding quality of the sleeve-plunging zone. With the stirring of the sleeve and the pin during the refill friction stir spot welding, work hardening occurred in the stir zone (SZ). The microhardness of the SZ was significantly higher than that of the steel base metal (BM) and could be detected on the steel side. The Fe-Al intermetallic compound (IMC) layer was continuously distributed at the interface of the sleeve-plunging zone, revealing good uniformity in the thickness. In particular, a hook-and-vortex-like structure formed during the refill friction stir spot welding process in the sleeve-plunging zone, producing a mechanical interlock effect at the interface. The ideal mechanical properties of the welded joint could be attributed to the good quality of the metallurgical and mechanical bonding at the interface, especially the mechanical interlock effect, thereby depending on the hook-and-vortex-like structure.

Wear mechanisms and failure analysis of a tool used in refill friction stir spot welding of AA6061-T6

Investigating tool wear in refill friction stir spot welding (RFSSW) is essential for understanding its limitations and improving its efficiency. Increasing the tool's service life is important to push the technology's readiness level and transfer the technology from the laboratory to industrial applications. In this study, the quasi-static lap shear performance of AA6061-T6 similar welded spots was investigated and tool wear was continuously monitored until tool failure at 3450 welding cycles. Furthermore, the fundamentals of the wear mechanisms in RFSSW were further elucidated. The investigation shows that it is possible to achieve steady quasi-static lap shear performance of the spot welds over advancing tool wear by adjusting the heat input to compensate for losses in frictional heat generation efficiency (related to tool profile changes by abrasion). A subsequent tool failure case analysis showed the main causes for the continuous wear degradation of the shoulder. Tool wear was driven by plastic deformation of the hot-work tool steel and subsequent break-out of tool steel ridges, introducing big hard particles into the contact region between the moving and rotating tools. In addition, the formation and detachment of Fe-Al intermetallic compounds counteract with the rotating tools and increase tool wear.

Process optimization of refill friction spot stir of AA6061-T6 aluminum alloy for thick plate

To investigate the process parameters of refillable friction stir spot welding for AA6061-T6 aluminum alloy with a 3 mm plate thickness, and to extend the applicability of this technology, experiments were conducted using custom-made tools under varying rotational and penetration speeds. This study aimed to produce well-formed welds and analyze the metallurgical structure, tensile shear strength, and microhardness of the welded joints. The experiments were performed at rotational speeds of 1000, 1200, 1400, and 1600 rpm and penetration speeds of 20, 30, 40, and 50 mm/min. These conditions were found to facilitate effective refillable friction stir spot welding. The results indicated that at lower rotational speeds, the joints failed at the interface, while at higher speeds, failure occurred through the plug. Optimal tensile shear strength was achieved at a rotational speed of 1400 rpm and a penetration speed of 30 mm/min, with a peak force of 9.27 kN and a joint surface fracture mode. Microstructural examination revealed a significant refinement of grains in the weld zone compared to the base metal. Microhardness measurements showed a concentric ring pattern on the upper surface with higher hardness at the center, decreasing toward the edges. The cross-sectional hardness demonstrated a vertical gradient, being higher at the top and decreasing toward the bottom. Notably, the welded area exhibited signs of annealing with a reduced hardness compared to the base metal, where the maximum value was recorded as HV79.

Influence of heat input on pinless friction stir spot welding of aluminum‑copper dissimilar materials

The composite structures of aluminum/copper dissimilar materials hold significant practical value and potential in advanced technology and industrial applications. This study conducted lap welding experiments on 2 mm thick T2 copper and 1060 pure aluminum rods using the pinless friction stir spot welding process. By varying the dwelling time of the tool, the evolution behavior of the CuAl interface layer and the mechanical properties of the joints were analyzed. At low dwelling times, the heat input was relatively low, resulting in a thinner IMC layer and the best mechanical properties of the joints, with a pull-out force of 2238.2 N and a tensile strength of 28.49 MPa. The fracture modes of the joints under all parameters were brittle fractures. The connection of aluminum/copper dissimilar materials involves two processes: firstly, the diffusion bonding dominated by kinetics at the initial welding stage; and secondly, the solid-liquid instantaneous bonding dominated by thermodynamics. When the welding temperature exceeds the melting point of aluminum, the aluminum/copper interface continuously undergoes transformation-bonding-retransformation processes until the welding process is completed.

Optimization and Analysis of Refill Friction Stir Spot Welding (RFSSW) Parameters of Dissimilar Aluminum Alloy Joints by FE and ANN Methods.

The quality of the refill friction stir spot welding (RFSSW) process is heavily dependent on the selected welding parameters that influence the resultant joint characteristics. Thermomechanical phenomena integral to the process were investigated using finite element (FE) analysis on two dissimilar materials. This FE analysis was subsequently validated through controlled experiments to ensure reliability. An artificial neural network (ANN) was employed to create a neural model based on an experimental setup involving 120 different sets of welding parameters. The parameters adjusted in the experimental plan included pin penetration depth, rotational speed, retention time, and positioning relative to material hardness. To assess the neural model's accuracy, outputs such as maximum temperature and normal stress at the end of the welding process were analyzed and validated by six data sets selected for their uniform distribution across the training domain.

An efficient synergistic double-sided friction stir spot welding method: A case study on process optimization, interfacial characteristics and mechanical properties of 2198-T8 aluminum‑lithium alloy joints

Friction stir spot welding has important applications in the joining of thin metallic plates. However, the asymmetric thermomechanical action inherent in the single-sided friction stir spot welding inevitably generates the hook defect at the joint interface, and the defect deteriorates with the improvement of the interfacial metallurgical bonding, which causes the irreconcilable contradiction and significantly restricts the breakthrough of the process stability and welding efficiency. The synergistic double-sided probeless friction stir spot welding (SDP-FSSW) technology proposed in this study adopts a dual-axis synergistic control system, which introduces intense plastic deformation from both sides of the interface to form a wavy hook by accurate axial motion during the welding process. This innovative process solves the problem of hook warpage while realizing rapid metallurgical bonding and mechanical interlocking at the interface, thus obtaining AA2198-T8 aluminum‑lithium alloy spot joints with a tensile/shear strength exceeding 9kN in a short dwell time (0–3 s). Through response surface methodology, the optimal process parameters are identified, i.e. rotation speed of 603 rpm, dwell time of 3 s, and plunge depth of 0.26 mm, at which condition the average joint strength is 13.0 ± 0.5kN, a 47 % increase compared to an optimized joint of single-sided probeless friction stir spot welding (8.9kN). The optical microscope examination shows that the wavy hook can be adjusted by the rotation speed and the higher rotation speeds will make the wave disappear and produce an approximately straight interface. The fractographic analysis shows that the wavy hook can inhibit crack propagation and has good mechanical interlocking effect, but the flat hook at high rotation speeds leads to crack propagation directly along the interface, and the failure mode is changed from plug-pull fracture to interfacial fracture. Therefore, the joint strength at 1000 rpm is only 7.0kN, which decreases by about 47 % compared with the optimal strength. The electron backscattering diffraction analysis shows that continuous dynamic recrystallization occurs in the hook region and good metallurgical bonding is formed, and there are also some discontinuous dynamic recrystallized grains distributed between the large grains due to the larger strain rates and lower temperature at the interface during the SDP-FSSW process. It has been found that the wavy hook with good metallurgical bonding and mechanical interlocking can be introduced in spot welded joints by controlling the interface forming, which is a viable method to create high quality, efficient and reliable spot joints.

The effect of pin diameter, tool penetration depth and plate arrangement on mechanical and metallurgical properties of 6061-T6 and 5052-T32 aluminum dissimilar joints in friction stir spot welding (FSSW)

In solid-state welding, the basis of joining is the combination of materials in the weld area using the tool. The tool pin is a key parameter that can facilitate tool penetration into the parts and result in better material mixing. On the other hand, the presence of the end cavity of the process can be a factor in reducing the mechanical properties of the weld. To address these two conflicting issues, this study examined the effect of pin thickness on the mechanical and metallurgical properties of aluminum 6061 and 5052 plates. The variable parameters included pin diameters (4 mm, 8 mm, and no pin) and three tool penetration depths (0.5 mm, 1.5 mm, and 2.5 mm). The impact of changing the arrangement of the plates on each other was also investigated regarding the properties and quality. Experimental results showed that decreasing the pin diameter and increasing the tool penetration depth improved the weld breakage force. The failure modes of all welds were button pull-out and ductile failure. Using a pinless tool increased microhardness in the weld nugget area and led to more uniform microhardness variations. Placing the aluminum 6061 alloy on top of the arrangement improved shear tensile strength and microhardness of the joints. The microstructure in the heat-affected zone showed an increase in grain size, which was accompanied by a decrease in microhardness. The microstructure in the stirred zone was dense and compacted, and with increasing tool penetration depth and decreasing pin diameter, the grain size became finer.

Driving forces of solid-state Cu-to-Cu direct bonding suppressing the work-hardening loss by refill friction stir spot welding

In this study, the solid-state bonding of the Cu-to-Cu parallel connection with no fusion zone and keyhole formation was achieved using the refill friction stir spot welding (RFSSW) process. During the refilling process, significant plastic flow and thermo-compressive behavior promoted geometric and continuous dynamic recrystallization behaviors and highly oriented (111) microstructures, serving as key mechanisms to prevent work-hardening loss and reinforce the joint. Furthermore, the refinement of the grain size without work-hardening loss, increased by 50.8 % which was from 61 Hv in the base material to 92 Hv in the thermo-mechanically affected zone (TMAZ). Even, the hardness of the TMAZ/stir zone interface noticeably increased up to 138 Hv as the reduction in the stacking fault energy (SFE) due to the infiltration of Al element at the trace from the tool. The contribution of microstructural features to the mechanical property was quantitatively investigated by a modified Hall-Petch relationship in terms of the grain size, sub-grain size, and dislocation density. This study systematically investigated the driving forces for the implementation of Cu-to-Cu direct bonding, where work-hardening loss can be suppressed, through the RFSSW process from the perspective of dynamic recrystallization behavior and SFE due to partial alloying of Al elements.

Insights into the microstructural evolution and strengthening mechanisms of friction stir spot welded advanced high strength ultrafine bainitic steel

Unlocking the full potential of advanced high-strength bainitic steels in spot-welding is a pivotal endeavor for harnessing their extraordinary mechanical properties in the automotive industry. This study suggests a shift in paradigm to address the inferior fusion weldability of this type of steel because of the formation of coarse martensite within the welded zone of weldments obtained by liquid-state welding, through the strategy of refining the martensite within the weld zone using the solid-state friction stir spot welding (FSSW) process. This paper delves into an in-depth investigation of the microstructural evolution and load-bearing capacity of a Fe-0.25C-3Cr-1.63Mn-1.5Si (wt%) ultrafine bainitic steel, having a remarkable tensile strength of 1.7 GPa, during the intriguing process of FSSW at rotational speeds of 600 to 2000 rpm. The research uncovers that the stir zone (SZ) undergoes dynamic recrystallization, resulting in a significantly refined microstructure. Regardless of the rotational speed employed, the high hardenability of the steel and rapid cooling during the process inevitably results in martensite formation within this region. However, a comprehensive microstructural characterization conducted by the electron backscatter diffraction (EBSD) analysis proves that the different thermal cycles induced by different rotational speeds lead into various prior austenite grain sizes, martensitic/bainitic packets and blocks attributes, and density of high-angle grain boundaries (HAGBs). Through the evaluation of strengthening mechanisms in different weld zones, the study sheds light on the key factors influencing strength. Block boundaries emerge as the primary contributor to SZ hardening, surpassing the role of geometrically necessary dislocations (GNDs) and solid solution strengthening. The load-bearing capacity observed during the tensile-shear test is governed by a delicate interplay between bonding width and crystallographic features (block size and HAGBs density) of the martensite formed within the SZ. The highest peak load was achieved under optimal welding conditions (rotational speed of 1000 rpm), which enabled the production of a sufficiently large bonding width, providing ample bonding area for load-bearing, along with an abundance of blocks and HAGBs to effectively impeding crack propagation.

Friction stir spot welding of aluminum alloy/steel plate with position-controlled joining equipment

Friction stir spot welding of aluminum alloy/steel plate with position-controlled joining equipment

Numerical study of refill friction stir spot welding of dissimilar metallic materials using smoothed particle hydrodynamics (SPH)

Numerical study of refill friction stir spot welding of dissimilar metallic materials using smoothed particle hydrodynamics (SPH)

Enhancing the performance of honeycomb core sandwich panels through friction stir spot welding strategies and fracture of lap shear specimen

The manufacturing of honeycomb core sandwich structures using friction stir spot welding (FSSW) methods as alternatives for adhesive bonded (AB) structures is examined. AB, FSSW with disk insert (FSSW_D), FSSW_D with AB (FSSW_D_AB) are adopted for the purpose. The performance during the lap shear test and the cohesive zone modeling (CZM) in the finite element (FE) simulation served as the foundation for the comparison. A homogenized core and equivalent cohesive layer were used during the numerical simulations of lap-shearing of all joints. Maximum load-carrying capacity improved significantly when the FSSW_D and FSSW_D_AB were used; the improvement ranged from 592% to 878% over AB joints. Peak load was shown to be around 24% higher in FSSW_D_AB in comparison to FSSW_D joints. The peak load-to-weight ratio of the FSSW_D_AB joints was remarkable (542% to 747%). Stiffness measurements were approximately 0.43 kN/mm for FSSW_D and 0.63 kN/mm for FSSW_D_AB joints. Microstructural analysis revealed finer grains with increased rotation speed and distinct fracture modes, with SEM showing improved ductility. The good agreement between the numerical predictions and the experimental results for all types of joints showed the usefulness of the proposed numerical model. Moreover, FE simulations-based stress analyses of the three joints provided crucial results on normal and shear stress distributions enabling the sandwich structures to be tailor-made for their intended applications.

Characterization of thermoplastic composite joints fabricated by the combined refill friction stir spot welding and riveting process

Characterization of thermoplastic composite joints fabricated by the combined refill friction stir spot welding and riveting process

Considerations for Tungsten Carbide as Tooling in RFSSW.

Tool wear is a key issue for the manufacturing performance of refill friction stir spot welding in high-volume manufacturing environments. As such, the aim of this study is to examine conditions in which tungsten carbide with a cobalt binder can succeed as a tool material in the spot welding of 2029 aluminum for a sustained lifetime. Critical factors are shown herein to include cleanliness and thermal management. The life of a WC-Co toolset is demonstrated to be approximately 2998 welds, which is of the same scale as conventional steel tooling. With a WC-Co shoulder and probe, the H13 clamp showed the only significant wear.

Tensile-shear mechanical properties of friction stir spot weld bonding hybrid joint in welding prior to and after adhesive curing for vehicle using

This research proposed a temperature-controlled friction stir spot weld bonding process as a solution to the issues of adhesive carbonization and weld impurities in the traditional combination of friction stir spot welding and adhesive bonding. A comprehensive investigation was conducted, including the establishment of a simulation model using the DFLUX subroutine to analyze the welding temperature field and the implementation of an overflow groove to separate the weld from the adhesive. The microstructure and mechanical properties of the joint were examined in the joint of welding before and after the adhesive curing process. The results demonstrated significant improvements in the failure load of the hybrid joints, with increases of 57.4 % and 27.7 %, compared to the welding joint. Additionally, the failure mechanism of the hybrid joint was elucidated by analyzing the fracture morphology.

Revealing Deformation Heterogeneity of an Aluminum Alloy During Friction Stir Spot Welding: An EBSD Investigation

Revealing Deformation Heterogeneity of an Aluminum Alloy During Friction Stir Spot Welding: An EBSD Investigation

Influences of graphene nanoplatelet addition and pin lengths on the microstructure and mechanical properties of 7075 aluminum alloy under friction stir spot welding

This study investigated the impact of graphene nanoplatelets (GNPs) and pin lengths on the mechanical properties and microstructure of AA7075 Al alloy in friction stir spot welded (FSSW) joints. FSSW was conducted using varying volumes of GNPs (5, 10, 20, and 30 mm3) and different pin lengths (3.5 and 4 mm) with 1250 rpm of rotational speed and 10 s of dwell time. The results of this study indicated that the addition of GNPs led to significant grain refinement in the stir zone (SZ). An increase in GNPs volume from 5 to 30 mm3 resulted in substantial grain refinement during FSSW. This outcome was obtained from the pinning effect generated by nano-sized GNPs. This effect successfully hindered grain growth, followed by recrystallization during FSSW. Furthermore, the grain size grows as the pin length increases. The results of tensile lap shear tests revealed that 3.5 mm pin length and 10 mm3 volume of GNPs are the best welding conditions for the fabrication of the best-quality welds. The increase in the volume of GNPs of more than 10 mm3 induced a negative impact on the tensile shear strength due to the agglomeration of the GNPs. The tensile shear strength of the joints with 10 mm3 GNPs and pin lengths of 3.5 and 4 mm exhibited remarkable increases of 52.6% and 41.2%, respectively, compared with that of those without GNPs. Moreover, the microhardness of the SZ of the joints improved by increasing the addition of GNPs volume.

Hole inhibition mechanisms of Mg/steel lap joint by pinless friction stir spot welding

Hole inhibition mechanisms of Mg/steel lap joint by pinless friction stir spot welding