Dissimilar Friction Stir Lap Welding Research Articles

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.

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).

Extremely thin intermetallic layer in dissimilar AA6061-T6 and mild steel friction stir lap welding using a hemispherical tool

The dissimilar friction stir lap welding of AA6061-T6 and mild steel using the hemispherical tool tilted towards the retreating side is investigated. Critical defects such as hook features and internal voids are avoided by limiting the plunge depth in the lower plate to a tenth of a millimeter. The low heat generation guaranteed by the hemispherical tool produces a nanoscale intermetallic compound layer alternatively composed of an Al-rich and a ternary Al–Fe–Mg phases. The complex and extremely thin interlayers strengthen the Al–Fe mechanical bonding, guaranteeing high mechanical properties and rupture within the Al-stirred zone. Thermomechanical phenomena governing friction stir lap welding with the hemispherical tool drastically limit the growth of intermetallics, leading to the high mechanical strength of the lap joint.

Dissimilar Friction Stir Lap Welding of Aluminium to Steel: Influence of Alloy Type and Sheet Thickness on Strain Distribution and Failure Location

Dissimilar joining through solid-state welding is an important engineering tool to address the transportation industry’s sustainable goals. The dissimilar friction stir lap welding (FSLW) of two different aluminium alloys (AA5182 and AA5052 with two different thicknesses) to steels AISI1010 and DP1000 was performed in this work, in order to analyse the effect of the mismatch in base material properties and plate thickness on the joint strength and fracture location. The mechanical behaviour and the strength of the welds were assessed using transverse tensile–shear testing and hardness measurements. Strain data acquisition through Digital Image Correlation (DIC) was used. The differences in fracture location registered for the different joints are explained based on the alloy’s plastic properties and on the mismatch in thickness between the plates. Local stress–strain curves were plotted, using the strain data acquired through DIC, to highlight the mechanisms resulting in the differences in tensile behaviour among the joints. It is concluded that despite the differences in failure location and tensile behaviour, the strength of the joints was very similar, irrespective of the base material combinations.

Interfacial Microstructure Produced during Dissimilar AA6013/Ti-6Al-4V Friction Stir Lap Welding under Zero-Penetration Condition

The purpose of this study was to investigate the interfacial microstructure that was produced during dissimilar friction stir lap welding (FSW) of 6013 aluminum alloy and Ti-6Al-4V. FSW was conducted under a zero-penetration condition, i.e., the welding tool was plunged exclusively into the upper (aluminum) plate of the dissimilar lap joint. To facilitate the interpretation of microstructural processes, finite element modeling (FEM) was applied to evaluate the temperature field within the weld zone. The FEM simulation revealed a very sharp temperature gradient across the dissimilar interface. This effect was attributed to the generation of FSW heat exclusively within the aluminum part and a relatively low thermal conductivity of titanium. The abrupt temperature drop on the titanium side imposed a strict limitation on the diffusion penetration of aluminum and thus resulted in a relatively thin (~0.5 μm) and discontinuous intermetallic compound. Due to the complex chemical composition of the FSWed aluminum alloy, the diffusion processes also involved alloying elements. Consequently, the evolved intermetallic compound had a complicated chemical composition, with the principal elements being aluminum, titanium, silicon, manganese, and magnesium.

Strengthening characteristic and mechanism of AlCoCrFeNi high-entropy alloy particles for Al-Cu dissimilar friction stir lap welded joint

Regarding the light-weight welding application, the strategy of non-tool penetration into lower hard sheet is considered as a promising dissimilar friction stir lap welding (FSLW) process for restriction of tool wear and reduction of welding force. In order to further improve the mechanical properties of the lap joint for such a case, AlCoCrFeNi high-entropy alloy (HEA) particles were selected as reinforcement for the Al-Cu dissimilar FSLW. A novel approach to incorporating HEA particles into lap weld, where a HEA particle-reinforced Al matrix was pre-prepared by FSLW, was proposed to prevent the overflow of particles from weld effectively during Al-Cu FSLW. The strengthening characteristic and mechanism of the HEA particles were systematically explored in this paper. Microstructural analyses indicate that the spherical HEA particles with large size contribute to the grain refinement via particle-stimulated nucleation mechanism, furthermore, nanoscale particles (Al3Ni and Al13Fe4) are formed adjacent to the HEA particles in the Al matrix. The induced grain refinement strengthening and secondary phase strengthening by the HEA particles are responsible for the significant improvement in the mechanical properties of Al-Cu lap joint. This study provides a technological approach and a theoretical foundation for the high-quality dissimilar FSLW used in light-weight manufacturing field.

Effect of tool tilt angle on microstructure, mechanical properties and fracture behavior of dissimilar friction stir lap welding joint of SiCp/ZL101 and ZL101

In order to achieve the lightweight goal of urban rail train braking system, friction stir lap welding (FSLW) was used to weld SiCp/ZL101 and ZL101 sheets to prepare a new type of brake disc material. The effects of different tool tilt angles (TTAs) on microstructure and mechanical properties of the welding joints were systematically studied. The TTA had a significant effect on the microstructure and defect formation of the dissimilar FSLW joints. There were tunnel defects under the advancing side at 1° and 2°. Internal defect-free weld structures were formed at 3° and 3.5°. A large number of dislocations caused by plastic deformation remained in the microstructure of the weld region at 1° and 2°, and the transformation of the microstructure of the weld region at 3° and 3.5° was dominated by dynamic recovery and dynamic recrystallization. The distribution of SiC particles in the nugget area was uniform, and the grains were obviously refined. With the increase of the TTA, the average grain size of the nugget area increased, the average size of the SiC particles decreased, and the average hardness of the nugget area increased. The four samples all formed a transition zone with good metallurgical bonding at the welding interface. The shear strength of the welding joints increased first and then decreased with the increase of the TTA. In the absence of welding defects, the shear strength reached the best level of 130.71 MPa at 3°. Both shear and tensile fractures were ductile fractures.

Tailoring of Dissimilar Friction Stir Lap Welding of Aluminum and Titanium

An approach was proposed to optimize dissimilar friction stir lap welding of aluminum and titanium alloys. The basic concept of the new technique included (i) the plunging of the welding tool solely into the aluminum part (i.e., no direct contact with the titanium side) and (ii) the welding at a relatively high-heat input condition. It was shown that sound welds could be readily produced using an ordinary cost-effective tool, with no tool abrasion and no dispersion of harmful titanium fragments within the aluminum side. Moreover, the intermetallic layer was found to be as narrow as ~0.1 µm, thus giving rise to excellent bond strength between aluminum and titanium. On the other hand, several important shortcomings were also revealed. First of all, the high-heat input condition provided significant microstructural changes in the aluminum part, thereby resulting in essential material softening. Furthermore, the new approach was not feasible in the case of highly alloyed aluminum alloys due to the relatively low rate of self-diffusion in these materials. An essential issue was also a comparatively narrow processing window.

Microstructure and mechanical properties of dissimilar friction stir lap welding between AZ31 Mg alloy and DC01 steel

AZ31 magnesium alloy and DC01 steel were friction stir lap welded (FSLW) with Z -axis force controlled at 7.5 kN. The microstructural features and element distributions of the joint were studied in detail. Mg 17 (Al, Zn) 12 particles with significantly high concentration of Zn were found to precipitate in the stir zone on the AZ31 side. A bowl-shaped region formed with a maximum depth of ~770 μm on the DC01 side, with the grain size decreasing rapidly from the base material to the area at the interface (~120 nm) due to the operation of dynamic recrystallization. Significant amount of Mg and Al atoms was found in DC01 grains near interface, which can be attributed to local mechanical intermixing between AZ31 and DC01 and the enhanced solid solubility via severe deformation. A thin Al-rich layer of ~50 nm thick formed within DC01 grains upside at the interface with the Al atoms crossed the interface from AZ31, while abundant nanoscale Mg particles precipitated out at the boundaries of nano-sized DC01 grains due to low solid solubility of Mg in Fe. The tensile shear strength of the joint reached 295.6 N/mm, with the crack passing through the root of hook-like structure and the stir zone of AZ31 instead of the interface, indicating a high bonding strength of such an interface microstructure. • An Al-rich layer with a width of ~50 nm formed within DC01 grains at the interface. • Nanoscale Mg-rich particles precipitated at the grain boundaries on DC01 side. • Significant Zn enriched in Mg 17 (Al, Zn) 12 particles were precipitated on AZ31 side. • CDRX and DDRX operate simultaneously in DC01 leading to refined grain structure.

Effect of Ni Addition on the Interfacial Strength of Al/Cu Dissimilar Welds Produced by Friction Stir Lap Welding

Al/Cu dissimilar joining is a key technology for reducing the weight and cost of electrical components. In this study, the dissimilar friction stir lap welding (FSLW) of a Ni-containing Al alloy to pure Cu was performed, and the effects of the addition of Ni on the weld strength and interfacial microstructure were examined. A thin intermetallic compound (IMC) layer was observed at the Al/Cu weld interface produced by FSLW. The addition of 3 at.% Ni effectively improved the weld strength, although the thickness of the IMC layer increased. The IMC layer formed at the Al/Cu interface without Ni comprised CuAl2 and Cu9Al4 from the pure Al side. In contrast, the IMC layer formed with 3 at.% Ni consisted of (Ni,Cu)Al, CuAl, and Cu9Al4 from the Al side. The addition of Ni eliminated the weak CuAl2/Cu9Al4 interface, thereby improving the weld strength. The results of this study suggest that the strength of the Al/Cu weld can be effectively improved by the thinning of the IMC layer caused by FSLW and the change in interfacial microstructure caused by Ni addition.

Dissimilar friction stir lap welding of AA2198-AA6082: Process analysis and joint characterization

This paper deals with the dissimilar friction stir lap welding of AA2198 and AA6082 thin sheets. The influence of processing parameters, namely welding speed and tool rotational speed on joint features, microstructure, and mechanical properties were investigated by implementing a full factorial design of experiments. During the welding process, axial and transversal forces were continuously measured using a dedicated fixture equipped with sensors aiming at the correlation of this processing parameter with the quality of the achieved joints. The microstructural analysis provided a detailed view of the metallurgical features such as hook, and its relative influence exhibited on the welded joints. Microhardness distribution and lap shear behavior were assessed and correlated with the metallurgical analysis.

Dissimilar friction stir lap welding of AA2198 and AA7075 sheets: forces, microstructure and mechanical properties

This paper investigates the dissimilar friction stir lap welding of AA2198 and AA7075 sheets. The influence of processing parameters, namely welding speed and tool rotational speed on joint features, microstructure, and mechanical properties were studied by implementing a full factorial design of experiments. Axial and transversal forces were continuously measured during the welding process using a sensed fixture aiming at the correlation of processing parameters, forces, and quality of the achieved joints. Obtained outcomes showed hook formation for all the combination of parameters and the existence of a very narrow processing window in which it is possible to avoid the formation of internal defects, such as grooves and tunnels. The influence of the weld bead morphology on the lap shear strength was elucidated proving that the strength is ruled by the hook morphology. The microstructure of the joints was studied and discussed considering also the microhardness distribution.Graphical abstract

Dissimilar friction stir lap welding of AA7075 to AZ31B in the presence of Sn interlayer

The present study aims to investigate the effect of Sn interlayer on the quality, microstructure and mechanical properties of 7075 T6-AZ31H24 dissimilar joints fabricated by friction stir lap welding. In this regard, various joints were fabricated by changing the interlayer thickness and the number of FSW passes. The microstructure of the joints was studied by optical, scanning and transmission electron microscopes linked to EDS and EBSD detectors. Shear-tensile test was utilized for evaluation of the mechanical properties of the joints. The results showed that the use of 40 μm-thick Sn interlayer was very effective on the improvement of joint quality and joint strength. In this case, the tunneling defect was eliminated and the lap shear fracture load improved by 10% with respect to conventional friction stir lap welding. This was attributed to the facilitated displacement and more proper mixing of dissimilar materials in the nugget due to the lubricating effect of molten tin. Although increase of interlayer thickness showed positive effects on dissimilar material mixing, width of nugget and thickness of mixed region, however, joint strength decreased due to the formation of Mg2Sn intermetallic compound at the out part of the mixed region. Increasing the FSW passes from one to three led to spreading of Mg-Al mixed regions throughout the nugget, thereby the joint strength was deteriorated significantly by formation of various brittle intermetallic compounds (Mg17Al12, Mg2Al3 and Mg2Sn) within the nugget.

Modeling of material flow in dissimilar friction stir lap welding of aluminum and brass using coupled Eulerian and Lagrangian method

Dissimilar friction stir welding of metals is crucial in terms of inter-material mixing and the intermetallic compound formation which affects the achieving a sound joint with fine mechanical properties. In this investigation, the formation of intermetallic compounds along with the material flow during friction stir lap welding (FSLW) of aluminum and brass is studied. First, the experimental investigations were performed using SEM, EDS, and XRD to probe the chemical compositions and intermetallic formation in the stir zone (SZ). Experimental results show the presence of Al2Cu and Al4Cu9 intermetallics in the SZ while are dispersed in separated zones. Moreover, to study further the material mixing, the coupled Eulerian-Lagrangian (CEL) method is employed to simulate the SZ formation during the FSLW of aluminum and brass. The model successfully predicts the SZ formation and material mixing in the Al/CuZn34 interface with penetration of Al into brass at the center and penetration of earring shape brass into Al sheet at the periphery of the SZ. Results show that two distinct zones inside the SZ could be observed, in which one is filled mostly by Al2Cu intermetallics near the Al sheet and the other is filled mostly by Al4Cu9 near the brass sheet. Additionally, by increasing the tool traverse speed, the amount of inter-material mixing significantly decreases, but on the other hand, the formation of intermetallic phases, which reduces the mechanical properties of the joint, is restricted.

Dissimilar Friction Stir Lap Welding of Al to Mg: Characteristic Signal, Microstructure and Mechanical Properties

The spindle characteristic signal (forces and vibrations) at different friction stir lap welding (FSLW) parameters were studied. The result indicated that the spindle force and vibration have different trends with the change of welding parameters. For further study, the spindle dynamic performance evaluation model by means of the Technique for Order Preference by Similarity to the Ideal Solution (TOPSIS method) was established. The model was used to calculate the relative approach degree B under different welding process parameters. The correlation between the characteristic signal and the joint properties was obtained. The model was validated by mechanical performance testing and microscopic observation. The results showed that the model evaluations were consistent with the experimental results.

Intermetallic compounds: Formation mechanism and effects on the mechanical properties of friction stir lap welded dissimilar joints of magnesium and aluminum alloys

The formation mechanism of intermetallic compounds (IMCs) during dissimilar friction stir lap welding (FSLW) of Mg and Al alloys, as well as the effects of IMCs on the shearing properties of FSLW joints, are still ambiguous and require clarification. In this study, FSLW dissimilar joints of AZ31 Mg alloys and Al–Zn–Cu–Mg alloys are investigated to clarify the formation mechanism of IMCs and their effect on the joint strength. Microstructures and shearing properties of the joints are characterized. The results showed that new phases of Al12Mg17, Al3Mg2, MgZn2, and Al2Cu are generated at the welding interface, and the original Cu2Mg phase is dissolved into the Al matrix during FSLW. Furthermore, the formation sequence of IMCs in the same binary system is affected by the effective enthalpy of formation, while it is related to the element concentrations in different binary systems. The order of IMC formation is as follows: Al12Mg17, Al3Mg2, MgZn2, and Al2Cu. During shearing deformation of the FSLW joint, the crack is initiated at the hook zone in the retreading side and propagated along the welding interface, where IMCs aggregate and the strain concentrates. The small lattice mismatch between the IMCs and the matrix improves their interfacial bonding strength, which increases in the following order: Al12Mg17 and Al, Al12Mg17 and Mg, Al3Mg2 and Al, Al3Mg2 and Mg, MgZn2 and Al, MgZn2 and Mg, Al2Cu and Al, Al2Cu and Mg, Cu2Mg and Mg, and Cu2Mg and Al.

Elucidation of interfacial microstructure and properties in friction stir lap welding of aluminium alloy and mild steel

The rate of heat generation and plastic deformation of materials should be precisely controlled in order to get a strong and sound weld joint during dissimilar friction stir lap welding (FSLW) of aluminium and steel. The effect interfacial microstructure at the weld joint interface on mechanical properties is elucidated in the present investigation with the help of detailed microstructure evaluation in FSLW of AA6061 and mild steel (MS). The results indicated a substantial variation in interfacial structure from formation of a bi-metallic straight interface, intercalated structure to mechanically mixed zone (MMZ) with the increase in tool rotation speed from 300 rpm to 1700 rpm. In addition, significant differences in the microstructure evolution mechanisms were noticed from the AA6061 side to MS side through the interfacial microstructure. The variation in interfacial structure and corresponding change in the fracture location predominantly affected the mechanical properties of the joints at different tool rotation speeds. The fracture locations in MMZ and intercalated structure showed the highest and lowest lap shear strengths, respectively.

Dissimilar friction stir lap welding of aluminum to brass: Modeling of material mixing using coupled Eulerian–Lagrangian method with experimental verifications

The intermetallic compounds formation and inter-material mixing have a critical impact on achieving a sound joint with fine mechanical properties in the case of dissimilar metal friction stir welding. In this investigation formation of intermetallic compounds as well as material flow that results in inter-material mixing during friction stir lap welding of aluminum and brass are studied. First, the 2.5-mm-thick aluminum sheet was laid on the same-thickness brass sheet and then, friction stir lap welding was applied using a tool with 4-mm pin length and 6-mm pin diameter. Next, experimental investigations were performed using scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, microhardness, and tensile shear test to probe the chemical compositions and intermetallic formation in the stir zone. Moreover, the coupled Eulerian–Lagrangian method is employed to simulate the stir zone formation during the process to further study inter-material mixing. In this method, the workpiece is modeled using Eulerian formulation, while Lagrangian formulation is utilized to model the tool. The model successfully predicts the stir zone formation and inter-material mixing in the aluminum–brass interface. Results shows that by increasing tool rotational speed from 500 to 2000 r/min the amount of inter material mixing significantly increases. Therefore, to achieve a joint with the highest strength in friction stir lap welding of aluminum–brass, the rotational speed should be lowered as much as it can produce defect-free joint.

Fractal Dimension Characterization of Joint Surface Morphology on Dissimilar Friction Stir Lap Welding of Al/Mg.

The joint surface of the 1060 aluminum and AZ31 magnesium alloy was prepared through friction stir lap welding (FSLW) under different welding process parameters. The joint surface was characterized three-dimensionally using a three-dimensional (3D) optical profiler, and the coordinate data were obtained. The fractal dimension of the joint surface was calculated by the box-width transformation method using a MATLAB program. Furthermore, the influence of the welding process parameters on the fractal dimension of the joint surface was studied. The response surface model was established based on the principle of central composite design (CCD), and analysis of variance (ANOVA) was carried out to test the accuracy of the response surface. The results showed that the joint surface morphology had fractal characteristics, and the fractal dimension could be used as an index to characterize the quality of the joint surface. The change of the welding process parameters had a great impact on the fractal dimension of the joint surface, the interaction between the parameters was small, and the fitting accuracy of the response surface model was high. The fractal dimension of the joint surface decreased with the increase in the welding and rotational speeds and the effect of the rotational speed was more significant.