Refill Friction Stir Spot Welding Research Articles

Evaluating the Influence of Tool Material on the Performance of Refill Friction Stir Spot Welds in AA2029

Joining high strength 2xxx series aluminum is known to be complex and difficult; these alloys are traditionally considered non-weldable for fusion welding. This paper describes details on welding AA2029-T8 for skin-stiffened structures using refill friction stir spot welding (RFSSW). RFSSW is a solid-state process invented in the early 2000s that produces spot welds that are strong, lightweight, flush, and hermetic. Cycle times between 1 and 3 s are discussed, and process forces within a range of 8 to 14 kN are demonstrated. Furthermore, lap-shear quasi-static tensile strengths are shown to be between 10 kN and 12 kN in 9 mm diameter spots. A comparison of the performance of RFSSW welds made with various tool materials—which include H13 tool steel, tungsten carbide, and MP159—is detailed. Comparisons of parameters, weld consolidation, and heat-affected zones are presented with discussion related to heat generation specific to each tool material.

Experimental study and numerical simulation of cellular I-beam manufactured using refill friction stir spot welding technology

The paper presents analyses of thin-walled I-beams constructed from 6061-T6 aluminum alloy using refill friction stir spot welding (RFSSW). A parametrized finite element model of the reference cellular beam was developed, and the results from numerical simulations and experiments were compared. Subsequently, the validated numerical model was utilized to conduct an in-depth analysis of three additional beams featuring distinct cross-sections. To assess the weld behavior, experiments were conducted on samples composed of two sheets connected by two spot welds. The outcomes of these investigations underscore the suitability of RFSSW as a method for constructing thin-walled structures in civil engineering applications.

Refill friction stir spot welding (RFSSW): a review of processing, similar/dissimilar materials joining, mechanical properties and fracture mechanism

Refill friction stir spot welding (RFSSW): a review of processing, similar/dissimilar materials joining, mechanical properties and fracture mechanism

Refill friction stir spot welding tool plunge depth effects on shear, peel, and fatigue properties of Alclad coated AA7075 aluminum joints

This study investigates the impact of refill friction stir spot welding (RFSSW) on Alclad-coated AA7075 aluminum alloy, specifically exploring the effects of two plunge depths (g = 1.55 mm and g = 1.9 mm) on the mechanical and fatigue properties. The influence of the Alclad layer on the final joint properties is assessed. Internal flow analysis reveals a concentration of the Alclad layer at the corners of the stir zone for an increased plunge depth. Notably, the sample using a plunge depth of 1.55 mm exhibits a 32 % increase in pure shear strength compared to the sample executed with a plunge depth of 1.9 mm, emphasizing the significance of the plunge depth for achieving superior mechanical performance. SEM analysis of the fracture surface highlights deformation and dimples in the g = 1.55 mm sample, while EDS confirms the Alclad layer's presence, indicating its positive influence on mechanical resilience. Additionally, peel tests demonstrate a 41 % strength increase for the g = 1.9 mm sample. Low and high cycle fatigue tests reveal superior fatigue performance in the g = 1.55 mm sample, supported by SEM analysis illustrating crack initiation and fracture mechanisms. These findings contribute to a nuanced understanding of the interaction between plunge depth, Alclad ldistribution and resulting mechanical and fatigue properties.

Analytical Approach for Forecasting the Load Capacity of the EN AW-7075-T6 Aluminum Alloy Joints Created Using RFSSW Technology.

To ensure the high reliability of aircraft structures, the Refill Friction Stir Spot Welding (RFSSW) process must be characterized by a high load capacity of the welds and a small standard deviation of the load capacity spread. This allows us to obtain uniform functional properties in the connections, ensuring the high quality of the process. This work aims to select the most favorable technological parameters for the welding process of EN AW-7075-T6 Alclad aluminum alloy sheets, which are used for the production of aircraft structures. The best networks were calculated using the Statistica 13.3 program. The obtained results were compared with the results of previous investigations. It has been shown that a model using neural networks allows for the determination of connection parameters with much greater accuracy than the classical model. The maximum error in estimating the load capacity of the connection for the mathematical model was 6.13%, and the standard deviation was 14.51%. In the case of neural networks, the maximum error value did not exceed 1.55%, and the standard deviation was 3.74%. It was shown that, based on the neural model, it is possible to determine the process parameters that ensure the required quality capacity of the process, ensuring a probability of obtaining the required load capacity of the connections amounting to P = 0.999935 with a defect rate of 0.0065%. This possibility is not provided by the classical model due to its large error in estimating the process spread and the high sensitivity of the process input parameters to the output parameters.

Exploring a novel chamfered tool design for short duration refill friction stir spot welds of high strength aluminium

This work investigates refill friction stir spot welded joints of AA2024-T3 aluminium alloy, produced with short welding times between 3 s and 0.75 s. A novel tool geometry that incorporates a chamfer on the inner edge of the shoulder tip is investigated as a means of improving joint quality at short welding times by easing material flow during the refill stage. The influence of shoulder design on weld microstructure, defect formation, material flow, and mechanical properties was assessed. When compared with a standard shoulder geometry, it was found that the introduction of a chamfer on the inner tip edge improved material flow during the refill stage and led to improved material mixing at the weld periphery. The formation of voids in the region of the weld periphery was eliminated and tensile lap-shear strength of the welded joints was increased by 19% to 7.2 kN, and 27% to 8.16 kN, for 0.75 s and 1.5 s duration welds, respectively.

Exploring the boundaries of refill friction stir spot welding: influence of short welding times on joint performance

AbstractRefill friction stir spot welding is a solid-state spot-welding technique suited to lap joining of thin aluminium sheets, including difficult-to-weld 2xxx series alloys that are prone to hot cracking during fusion welding processes. Long welding time is an ongoing challenge that hinders industrial adoption of the process. To address this, the present study explores much shorter welding times than those previously reported in the literature and assesses the impact on joint quality. Joints of 1.8 mm thick AA2024-T3 sheet were produced with welding times from 3 s, down to 0.75 s and rotational speeds of 1000 rpm to 2500 rpm. Defect formations within the welds were studied with the aid of optical microscopy. The mechanical properties were evaluated using tensile lap shear testing and microhardness mapping, and failure modes were characterised using scanning electron microscopy. Various weld defects were found at all welding times and rotational speeds, and the defects enlarged with decreasing welding time and increasing RS. The highest lap shear strength of 9.21 kN was achieved with a welding time of 3 s and rotational speed of 2000 rpm; lap shear strengths of 7.02 kN and 6.37 kN were achieved for 1.5 s and 0.75 s welds, respectively.

Experimental investigation of crack propagation mechanism in refill friction stir spot joints of AA6082-T6

Since many aluminum alloys preferred in structural engineering can be welded conventionally only with great effort and energy input interest in alternative joining techniques is growing, such as solid state joining processes. In this work, the effect of refill friction stir spot welding (refill FSSW) on the crack propagation behavior in AA6082-T6 is studied. To be able to identify the individual fracture mechanism, refill FSSW was performed as a blind weld, i.e. only in one sheet designed as C(T)100 specimens. The vertical distance between notch and spot weld was varied and tested in two phases. First, a cyclic pre-crack was induced and then the specimen was caused to fail in quasi-static conditions, resulting in two different fracture modes. The results showed that the cyclic crack is dominated by residual stresses but the microstructure mainly influences the quasi-static crack propagation. It was also found that a stress concentration occurs in the transition area even without a hook. Furthermore, it was found that the crack propagation is not exclusively driven by the local strength but also by the angle at which the crack hits the spot weld.

Research on the Surface-State Parameterization of a Refill Friction Stir Spot Welding Joint Made of Aluminum Alloy and Its Connection to the Fracture Mode.

Surface features are crucial for assessing welding quality because they serve as an intuitive depiction of the quality of the joint and have a major influence on welding strength. According to the characteristics of the refill friction stir spot welding (RFSSW) process and an analysis of the surface-state and internal morphology of RFSSW joints, a method of predicting the mechanical properties of RFSSW joints based on surface-state characteristics was proposed. In this paper, a laser-ranging sensor was used to characterize the surface state of RFSSW joints, and parametric characterization methods of the surface-state features of RFSSW joints were proposed. On this basis, a support vector machine was used to predict and analyze the fracture mode of RFSSW joints. The accuracy of the analysis of the test samples reached 95.8%. This paper provides a more efficient and convenient new method for the quality evaluation of RFSSW joints.

Microstructure and Mechanical Properties of Refill Friction Stir Spot-Welded Joints of 2A12Al and 7B04Al: Effects of Tool Size and Welding Parameters.

To solve problems in dissimilarly light metal joints, refilled friction stir spot welding (RFSSW) is proposed instead of resistance spot welding. However, rotation speed, dwell time, plunge depth, and the diameter of welding tools all have a great influence on joints, which brings great challenges in optimizing welding parameters to ensure their mechanical properties. In this study, the 1.5 mm thick 2A12Al and 2 mm thick 7B04Al lap joints were prepared by Taguchi orthogonal experiment design and RFSSW. The welding tool (shoulder) diameters were 5 mm and 7 mm, respectively. The macro/microstructures of the cross-section, the geometrical characteristics of the effective welding depth (EWD), the stir zone area (SZA), and the stir zone volume (SZV) were characterized. The shear strength and failure mode of the lap joint were analyzed using an optical microscope. It was found that EWD, SZA, and SZV had a good correlation with tensile-shear force. The optimal welding parameters of 5 mm diameter joints are 1500 rpm of rotation speed, 2.5 mm of plunge depth, and 0 s of dwell time, which for 7 mm joints are 1200 rpm, 1.5 mm, and 2 s. The tensile-shear force of 5 mm and 7 mm joints welded with these optical parameters was 4965 N and 5920 N, respectively. At the same time, the 5 mm diameter joints had better strength and strength stability.

Toward defect-less and minimized work-hardening loss implementation of Al alloy/high-purity Cu dissimilar lap joints by refill friction stir spot welding for battery tab-to-busbar applications

The refill-friction stir spot welding (RFSSW) process on the Al5052–H32 alloy joined with high-purity Cu film resulted in notable changes in the work-hardened Al alloy. The stirring action caused dynamic recrystallization in the weld zone, leading to the loss of work-hardening. Surprisingly, the plot of grain size-hardness in Al alloy after RFSSW did not follow the Hall-Petch relationship. Obtained through combined analysis with a transmission electron microscope, X-ray diffraction, and thermodynamic calculation, at the interface of Al and Cu, diffusion in nanoscale and microscale between the metals induced the formation of secondary phases (e.g., CuxAly intermetallic compounds, θ′ or θ'' phases), positively contributing to increased mechanical properties. The near-surface region in contact with the tool experienced significant compressive stresses, resulting in maximized compressive residual stresses. This, in turn, sustained or even enhanced the initial hardness of the material. Our study systematically investigates whether creating a lap joint with Cu in work-hardened Al alloy with RFSSW can yield a robust joint without defects in Al and without an overall loss of work-hardening.

Quality improvement of refill friction stir spot welds in 2A12-T42 aluminum alloy with alclad by adjusting sleeve design

Quality improvement of refill friction stir spot welds in 2A12-T42 aluminum alloy with alclad by adjusting sleeve design

Evaluation on Joint Performances of Advanced Multi-Material Dissimilar Lap Joints

As a result of R&D efforts about the advanced materials, an ultra-high strength steel (UHHS) and a carbon fiber reinforced thermos-plastic (CFRTP) have been developed. In this study, various advanced multi-material dissimilar lap joints using UHHS, CFRTP and an aluminum alloy of A5083P-O were fabricated by the resistance spot welding (RSW), refill friction stir spot welding (RFSSW), friction stir spot welding (FSSW) and laser irradiation assistance plastic welding (LIAPW). Where, the total heat input for each joining method was varied in order to examine the influence of welding condition on joint performances, which were the tensile shear strength, shear fatigue property and corrosion shear fatigue property. The tensile shear strength of UHHS and A5083P-O joined by RFSSW is almost two times of that produced by RSW, and this difference seems to be mainly caused by the difference of joint area. The difference of joint area also affects the tensile shear strength of UHHS & CFRTP or A5083P-O &CFRTP lap joints made by FSSW and LIAPW, and the joint strength of LIAPW is larger than that of FSSW. As for the shear fatigue tests, it is found that the fatigue strength seems to decrease almost monotonically with increasing the applied load regardless of the types of dissimilar materials. Moreover, through the corrosion shear fatigue tests for A5083P-O and CFRTP joined by FSSW or LIAPW, it is found that the corrosion fatigue property seems to be controlled by the crack propagation behavior.

Evaluation of quantitative ultrasonic C-scan testing for refill friction stir spot welding joints based on time-frequency analysis

This paper focuses on a comparative study of different ultrasonic feature-based C-scan testing imaging methods for characterizing refill friction stir spot welding (RFSSW) joints, and accurately identifying and measuring the nugget boundary. The aim is to explore a suitable method for nugget characterization and automatic size analysis. The research indicates that the frequency-domain C-scan imaging method outperforms the time-domain C-scan imaging method in accurately characterizing the nugget. Furthermore, the C-scan imaging method based on the feature value of the base material zone (BMZ) echo signal performs better than the method based on the nugget echo signal. The tested nugget sizes obtained by using the –6 dB drop-off method to identify the boundaries in the C-scan images are generally larger than the metallographic measurement values. A novel method is proposed in this paper, using the difference in main frequency amplitude values between the BMZ and the nugget as the feature value for the C-scan testing image, and employing the Hough circle transformation for an automatic extraction of joint size. This method achieves improved nugget characterization and higher accuracy in size analysis.

The influence of coefficient of friction in thermal prediction of refill friction stir spot welding process of AA7075‐T6

AbstractNumerical modelling of refill friction stir spot welding helps in the deep investigation of physics involved in the joining process. The current study investigates four variants of contact models and their influence on thermal prediction in the process. The contact models with constant shear and coulomb friction coefficients and temperature‐dependent shear and coulomb friction coefficients were used in the numerical models to investigate the influence of friction coefficients in the prediction of thermal cycles in refill friction stir spot welding. The results from the numerical model are compared and validated with the experimental results from a previous study. The contact model with temperature‐dependent coulomb friction coefficient was concluded to be more reliable when compared to the other three contact models. The temperature for this contact model at the centre of the weld is 505 °C, which differs from the temperature recorded in the experiment by 2 %. The peak temperatures of numerical models with constant coulomb and shear friction coefficients differ from the experimental temperature at the weld centre by 5.17 % and 16.8 % respectively.

Microstructural development of as-cast AM50 during Constrained Friction Processing: Grain refinement and influence of process parameters

Mg and its alloys have a wide range of structural applications despite the limitations regarding the workability and low ductility associated with its hexagonal closed-packed structure. The Constrained Friction Processing (CFP) is a novel processing technique, developed based on the refill friction stir spot welding process, that has been proposed as an interesting alternative that can help to overcome challenges associated with the processing of Mg and its alloys. This technique is shown to be able to produce homogenous fine-grained rods. Correlation between processing conditions and the evolved microstructure, i.e. texture and grain size, were established for AM50 rods. In the center, the produced rods present a strong B-fiber texture. As the distance from the center changes along the radial direction, there is a progressive outward tilt of the 〈0001〉 because of specific flow conditions during the processing. CFP is shown to be able to produce fine-grained rods with grain sizes comparable with other severe plastic deformation techniques, with advantages like no requirement of additional preheating and short processing times.

Texture evolution of refill friction stir spot welding in alclad 2A12-T42 aluminum alloy

Electron backscatter diffraction (EBSD) was used to investigate the microstructure and texture evolution of 2A12-T42 aluminum alloy during refill friction stir spot welding (refill FSSW). The stir zone (SZ) and the thermo-mechanically affected zone (TMAZ) occurred dynamic recrystallization (DRX), with typical shear texture components. The texture is dominated by B component in the SZ, and dominated by A∗ and B components in the TMAZ. As the occurrence of the DRX, the storage energy of the grains gradually decreases, and there is a tendency to form lower energy grain orientations and texture weakening. The welding parameters determine the composition of the texture in different regions by influencing the degree of recrystallization. In the test group with the highest degree of crystallization, the SZ and TMAZ are dominated almost entirely by the B component. The TMAZ has more grains with soft orientation due to the difference in shear pattern and degree of recrystallization, which makes the texture reduce the mechanical properties of the joints. However, this reduction is not significant, with <10% variation for different test groups and regions. Compared to other strengthening mechanisms, the effect of texture strengthening is limited.

Refill friction stir spot welding of thermoplastic composites: Case study on Carbon-fiber-reinforced polyphenylene sulfide

Refill Friction Stir Spot Welding (refill FSSW) is an innovative solid-state welding technique that has been successfully applied to various combinations of metallic materials. The objective of the present study is to investigate the feasibility of refill FSSW for polymer–polymer structures, with a specific emphasis on carbon-fiber-reinforced polyphenylene sulfide (CF-PPS). The influence of the key joining parameters, i.e. force, plunge depth, rotational speed, and tool diameter, has been analyzed in terms of the resulting joint microstructure, mechanical strength, and failure mechanisms. The lap shear tests revealed two primary failure modes: interfacial shear failure and nugget pull-out. Fracture surfaces exhibited broken fibers. The depth of the joint was found to play a crucial role in determining the failure mode, with interfacial shear failure resulting in higher lap shear strength. Thermal analyses conducted on the produced joints showed no evidence of thermal degradation, which aligns with the temperature measurements during the process, as they remained below the melting temperature of CF-PPS.

Hot-pressing treatment to improve the shear strength of refill friction stir spot-welded 1060 Al–carbon fiber–reinforced plastic joint

Refill friction stir spot welding (RFSSW) is applied in various industrial areas. Herein, we developed a hot-pressing post-treatment method for improving the shear strength of an Al 1060–carbon fiber–reinforced plastic joint by >40%. The strengthening effect of the hot-pressing treatment was based on the reinforced micro-mechanical interlocking mechanism and the infiltration of melted thermoplastic epoxy into the Al layer. Exploring such a high-strength joint made of dissimilar materials would open a novel route for RFSSW process design.

Identification of the influence of micro two-stage refilled friction stir spot welding with variation dwell time on dissimilar brass CuZn30-aluminum AA1100

Welding and joining of brass CuZn30-aluminum AA1100 were obtained using micro two-stage refilled friction stir spot welding (mTS-RFSSW), which was carried out to eliminate the holes formed by the micro friction stir spot welding (mFSSW) process. The mTS-RFSSW process begins with the mFSSW welding process using tools with a pin inner diameter of 2.69 mm, pin outer diameter 1.811 mm and shoulder diameter of 4.954 mm, followed by a second stage process called mTS-RFSSW, which is a hole closing process using a tool with a pinless tool with shoulder diameter 4.954 mm. This study aimed to determine the effect of the second stage of dwell time on the mechanical properties produced in the mTS-RFSSW welding technique using brass CuZn30 and aluminum AA1100 with a thickness of 0.42 mm. In this study, the variable parameter is the second stage dwell time which varies from 3 s, 4 s, 5 s, and 6 s, respectively. An optical microscope that aims to observe the macrostructure shows that an upward hook is formed in each joining process. Based on the scanning electron micrograph, the resulting formation of different intermetallic compounds (IMC) with varying thicknesses occurs in every variation of dwell time. The high dwell time indicates discontinued IMC, which affects the tensile force. The IMC formed at the interface of brass CuZn30 and aluminum AA1100 is dominated by more than 30 % Cu. The highest hardness value is found in the stir zone because the formation of intermetallic compounds influences refined grains. The highest maximum shear force and cross tensile force was obtained 371.35 N and 54.88 N, respectively, in the dwell time of 3 s. The result of fracture properties after the lap shear test shows the presence of small dimples with microcracks that indicate brittle failure