Friction Stir Lap Welding Research Articles

Fretting Fatigue as a Limiting Factor on the Durability of Friction Stir Welded Lap Joints Using AA2099-T83 and AA2060-T8E30 Aluminium Alloys

Friction stir welding (FSW) has been proposed as an alternative modern joining technology and demonstrated important benefits for the manufacturing of efficient and lightweight aircraft structures using high-strength aluminium alloys. These structures are required to be corrosion- resistant and thus, it is necessary to use technologies such as surface treatments and sealants in their manufacturing and assembly. In this work, the feasibility of combining innovative Cr-free surface treatments, sealants and FSW technology was investigated with the focus on the durability of the joints in fatigue. FSW lap joints were produced using AA2099-T83 extrusions and AA2060-T8E30 sheets in the as-received or surface-treated condition. A sealant was also applied in some cases at the overlapping interface before the FSW process. Static tensile tests and fatigue tests were carried out applying hoop-stress loading conditions. Different fracture modes were identified depending on the stress levels applied in the fatigue tests: High stress levels resulted in fractures in the HAZ of the FSW joints, while the specimens tested at low stress levels showed fractures out of the FSW joint. In general, FSW joints produced using surface-treated aluminium components and sealant presented improved fatigue life and extended durability in comparison with non-treated aluminium joints. The surface treatments and sealant at the interface of AA2099-T83 extrusions and AA2060-T8E30 sheets reduced the friction and local damage produced due to the sliding movement during the fatigue tests, minimizing the fretting fatigue effect, which was found to be the main limiting factor on the durability and fatigue life of the FSW joints.

Influence of tool pin length and dissimilar material configuration on friction stir lap welding of Al and Mg alloys

Influence of tool pin length and dissimilar material configuration on friction stir lap welding of Al and Mg alloys

Numerical simulation of friction stir lap welding of Al-to-Mg alloys under different lap configurations and pin lengths

The lap configurations and pin lengths have important influences on the joint quality and performance in Al-to-Mg friction stir lap welding (FSLW). But how these two factors affect the thermo-mechanical process in Al-to-Mg FSLW remains unclear. In this study, a computational fluid dynamics model considering the effects of local liquation, tool adhesive layer, tool tilt angle and pin thread is developed to analyze the FSLW process of Al-to-Mg with sheet thickness 3 plus 3 mm. The numerical simulation results are experimentally verified by the measured interface temperature and thermal mechanical affected zone profile. For pin length 3.8 mm, the Al/Mg configuration results in higher temperature and stronger material flow, so that intermetallic compounds (IMCs) are easy to form and joint strength is poor. The Al–Mg mixed region in Mg/Al configuration is mainly from the pin bottom to the lap interface, while that in Al/Mg configuration extends from the pin bottom to the position close to the top surface on the advancing side. In Mg/Al configuration, when the pin length increases, the peak temperature increases, the high-temperature region around the tool obviously expands and the tool can drive more materials to flow, which results in a larger Al–Mg mixed region, and the IMCs are more likely to be generated.

Efficient Build-Up of High-Strength Aluminum Structures Using Friction Stir Additive Manufacturing

Friction Stir Additive Manufacturing (FSAM) is a novel process with which large-scale aluminum structures can be produced from high-strength alloys such as the 7xxx series. Due to the prevalence of these alloys in airplanes and rockets, the process offers high application potential, for example in fabricating stringers and stiffeners. The building process in FSAM is characterized by sequentially stacking and friction stir lap welding (FSLW) metal sheets. Before adding the next layer, the surface is machined (i.e., by milling). So far, this is a necessary step to enable gap-free welding of the layers, which results in increased costs and reduced layer heights. The investigations described in this paper were aimed at improving the weld surface quality to enable defect-free FSAM without the additional machining step. For this, FSLW was conducted using different welding tools. The resulting welds were evaluated based on superficial and internal characteristics as well as the mechanical properties (shear strength). With a welding tool in which both a rotating and a stationary shoulder were combined, defect-free weld seams with a mean underfill and a mean flash height of 0.07 mm were produced. In a subsequent study, it was proven that defect-free FSAM without surface machining is possible up to the fifth layer using the combined welding tool.

The experimental and numerical studies on load bearing capacity in lamellar tearing of friction stir lap weld

The load bearing capacity in lamellar tearing is one of the key mechanical properties for the friction stir lap welding which has not been investigated before. Both numerical model and experiments were used to study the load bearing capacity in lamellar tearing of friction stir lap weld. The constitutive model of the weld zone in the crack propagation model was obtained by the combination of the proposed precipitate evolution model and the moving heat source model, and the effectiveness of the model was verified by comparing the numerical data with the experimental data. Results indicate that the crack initiated from the keyhole and propagated on the centerline of weld along the weld direction. The rotation of the joint region in the tearing process is a key factor for the increase of the load capacity in the tearing test and the simulation. The joint region can be rotated in the tearing process and the rotation angle can be increased with the increasing of weld length. The load bearing capacity along the weld direction is linearly increased with the increasing of the weld length.

Coupling effect of axial ultrasonic vibration and tool thread on the microstructure and properties of the friction stir lap welding joint of Al/Mg dissimilar alloys

In this study, three tools with different thread characteristics were designed for the ultrasonic-assisted friction stir lap welding (UAFSLW) of a 3-mm-thick 6061-T6 aluminum alloy and an AZ31 magnesium alloy. The coupling effect of different thread characteristics and axial ultrasonic vibrations on the size, amount, and distribution of intermetallic compounds (IMCs) and the tensile-shear strength of welded joints was analyzed. Experimental results confirmed the formation of IMCs in the stir zone (SZ) and the bottom interface in the SZ in all joints, with Al3Mg2 predominating in the Al-Mg configuration (in which the 6061-T6 aluminum and AZ31 magnesium alloys were placed at the upper and lower plates, respectively) and Al12Mg17 predominating in the Mg-Al configuration (in which the alloys on the upper and lower plates were reversed). The bottom interfaces of the joints in the Mg-Al configuration were tortuous, forming a mechanical interlocking structure; consequently, the joints welded in the Mg-Al configuration exhibited higher tensile-shear strength than those welded in the Al-Mg configuration. Furthermore, the tool with a variable-pitch thread pin achieved welds with excellent tensile-shear strength in both the configurations (223.2 and 268.3 N/mm in the Al-Mg and Mg-Al configurations, respectively). This performance improvement was attributed to the strong coupling of the tool with axial ultrasonic vibrations, which enlarged the mixing zone and increased the uniformity of the IMCs distribution in the joints.

Investigation on tool positioning in friction stir lap welding of AA6061 to polycarbonate sheets using force-torque signals

The lightweight high-performance thermoplastics are often replaced the metallic parts in aircraft and automobile industries due to high corrosion resistance along with upgraded strength to weight ratio. However, the joining of dissimilar non-ferrous aluminium to amorphous polymers is highly difficult caused by a drastic change in thermal properties which indicates the necessity of solid state welding processes. Therefore, the current work reports the practicality of friction stirred welding unlike AA6061 aluminium alloy and polycarbonate sheets for overlapped conditions by a cylindrical tool having a taper pin of H13 steel. The prime objective was to analyse the influence of tool tilt angle at different plunging depths and tool rotational speeds on the dissimilar weldability using a load-elongation curve. The aluminium to polycarbonate weld interface has also been studied to correlate with mechanical behaviour due to tensile load. The axial thrust force with tool stirring torque signals has been acquired for these parametric conditions for further process monitoring by segregating consecutive stages in the welding cycle. The weld bead profile along welding direction has been monitored using time domain thrust-force signals in the welding phase whose extracted significant features were compared. Finally, a sensitivity analysis was also processed to investigate the degree of parametric impact on thrust-torque as well as weld quality features. The highest joint strength efficiency was found to be 39.4% at higher tool rotational speed (1400 rpm) with reduced plunge depth (0.1 mm) without any tool tilting whereas weld ductility was significantly improved at reversed parametric settings with tool inverted position. The tool plunging force is a better indicator whereas both plunge depth as well as tilt angle were almost equally important for the prediction of lap weld quality.

Prediction of residual stresses within dissimilar Al/steel friction stir lap welds using an Eulerian-based modeling approach

Friction stir welding (FSW) was applied to fabricate dissimilar welds between 5052 aluminum alloy and DP590 dual-phase steel with a lap configuration. The interfacial formation and microstructure were investigated experimentally, and in-process thermomechanical conditions with emphasis on resultant residual stress distribution were numerically studied through a fully coupled three-dimensional (3D) Eulerian-based finite element modeling. The Al/steel thermal-mechanical interaction driven by the rotational pin penetrating the steel produced the serrated bonding interface. The brittle intermetallic layer was formed in the interface, showing distribution discontinuity along the welding direction. The simulation results showed that increasing rotation velocity resulted in higher welding temperature and cooling rate, leading to the increment in the residual stress level of the weld zone. Near interfacial residual stresses exhibited an M-shape in the Al alloy while it showed W-shape in the steel, which originated from the thermomechanical processing condition and the effect of mismatch of thermal expansion coefficient. Increasing rotation velocity decreased the near interfacial compressive stress in the steel side. Numerically predicted residual stresses were in acceptable agreement with those measured using the X-ray diffusion measurement with the cosα method, validating prediction reasonability. Hopefully, the current simulation work will provide a new alternative modeling strategy for predicting residual stress fields within the same or dissimilar FSW.

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.

Optimization of aluminum-to-steel friction stir lap welding for the fabrication of high-integrity structural components

Friction stir lap welding is a solid-state joining technique used to effectively fabricate high-quality lightweight and energy-efficient hybrid structures by overlapping thin parts of Al and other similar or dissimilar alloys. In this study, friction stir lap welding between 6061-T651 Al and JAC270 45/45 galvanized steel sheets was investigated, considering both single-pass and double-pass protocols to optimize the process and quality of the resulting welds. The single-pass welding was first studied by conducting experiments at constant tool rotation and traverse speeds (1200 RPM and 1 mm/s) and various tool plunge depths (0.05, 0.2, and 0.4 mm). The microstructures and Al-steel interfaces of the welds were characterized using microscopy and energy-dispersive X-ray spectroscopy, and the weld strength was evaluated by lap shear tensile testing. The defects and intermetallic compounds that form at the Al-steel interfaces were identified as the limiting factors for the weld strength. The intermediate tool plunge depth of 0.2 mm resulted in the highest weld strength due to the least amount of interfacial defects and intermetallic compounds amongst the studied single-pass welds. A double-pass welding method was then developed to optimize the strength of the Al-steel lap welds. The selection of the processing conditions for the double-pass welding was guided by the behavior and properties of the single-pass welds. The double-pass welding successfully eliminated the defects and intermetallic compounds from critical locations, creating a robust Al-steel interfacial bond and significantly improving the weld strength. The knowledge and findings in this study can be applied to optimize the design, material integration, and mechanical properties of lap welds for high-integrity structural components.

Fatigue strength of friction stir lap welded AA2024 to Ti6Al4V dissimilar joints

Fatigue testing has been conducted on welds of AA2024 (Al4.5Cu1.4 Mg0.5Mn) alloy to Ti6Al4V alloy made using friction stir lap welding (FSLW). During FSLW, pin bottom aimed for touching the Ti6Al4V plate (dPin≈0), although it could readily penetrate (dPin > 0). It has been found that fatigue limit of the AA2024/Ti6Al4V welds was slightly higher than the fatigue limits of the FSL Al-to-Al alloy welds reported in literature. Examination has demonstrated that forming a very thin interface layer during FSLW was the major mechanism responsible for the good fatigue strength. The diffusion weld distance outside the pin width in dPin > 0 welds was significantly larger than that in dPin≈0 welds. Thus, the fatigue limit of dPin > 0 welds was comparable to the fatigue limit of dPin≈0 welds despite of the mix stir zone (MSZ) in dPin > 0 welds being a brittle one. The different thermomechanical conditions for the different diffusion-weld widths will be illustrated.

Effect of Microstructure and Tensile Shear Load Characteristics Evaluated by Process Parameters in Friction Stir Lap Welding of Aluminum-Steel with Pipe Shapes

In recent years, friction stir welding (FSW) of dissimilar materials has become an important issue in lightweight and eco-friendly bonding technology. Although weight reduction of low-rigidity parts has been achieved, the weight reduction has been minimal because high-rigidity parts such as chassis require the use of iron. Considering the difficulty of welding a pipe shape, it is necessary to understand the effect of process parameters on mechanical performance. As a result of the study by various process parameters affecting the joint between aluminum and steel in the shape of a pipe, it can be seen that the tool penetration depth (TPD) has the most important effect on the tensile shear load (TSL). However, the effect of TPD on intermetallic compound (IMC), which has the most important influence on fracture, has not been well established. In this study, the effect of process parameters on IMC thickness and TSL in FSW of A357 cast aluminum and FB590 high tensile steel was investigated to reduce the weight of the torsion beam shaft of an automobile chassis. After the FSWed experiment, measurements were performed using an optical microscope and scanning electron microscopy (SEM) to investigate the microstructure of the weld. The formation of an IMC layer was observed at the interlayer between aluminum and steel. TPD is a major factor in IMC thickness variation, and there is a direct relationship between IMC thickness reduction and TSL increase, except for certain sections where the welding speed (WS) effect is large. Therefore, in order to improve mechanical properties in friction stir lap welding of aluminum and steel for high-rigidity parts, it is necessary to deepen the TPD at a level where flow is dominant rather than heat input.

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.

Elucidation of intermetallic compounds and mechanical properties of dissimilar friction stir lap welded 5052 Al alloy and DP590 steel

The friction stir lap welding (FSLW) was employed to join the dissimilar 5052 Al alloy and DP590 steel. The intermetallic compound (IMC) formation area and thickness increased linearly with increasing rotation speeds at 300 mm/min welding speed. Microvoid and gap defects existed on low-rotational-speed-produced lap interfaces, disappearing with increasing rotation speeds. However, cavities defects neighbouring the large-sized steel fragment appeared in the stir zone of the Al matrix with rotation speeds higher than 750 rpm. The negative influences from the Al-rich IMCs layer exceeding 1.5 µm thick and the aggravating internal cavities defects significantly reduced the joint strength. Correspondingly, the fracture mode of the friction stir lap welded 5052 alloy and DP590 steel was converted from the semi-brittle fracture into the full-brittle fracture. The maximum tensile shear force of 4.52 kN reached 75.2% of the Al 5052 base metal, obtained at the rotation speed of 750 rpm. However, from the combined perspective of integral bearing load capacity and local shear strength, the rotation speed of 500 rpm was recommended to achieve sound FSLW joints of Al5052/DP590.

Effects of rotation tool-induced heat and material flow behaviour on friction stir lapped Al/steel joint formation and resultant microstructure

Reliable joining of Al/steel via friction stir lap welding (FSLW) is strongly dependent on interfacial macro-/micro-structures. However, how the underlying thermo-mechanical process affects them and how they can be controlled remain unclear. In this study, a 3D coupled Eulerian–Lagrangian finite element model was integrated with a tracer particle technique to simulate the FSLW of Al alloy 5052 and high-strength DP590 steel, to elucidate the role of tool-induced heat and material flow. The results showed that the material flow recirculated near the shoulder and pin, which mainly originated from the advancing side of the Al and the retreating side of the steel. Increasing the rotational velocity intensified the overall material flow, resulting in increased migration on the steel side. Insufficient steel migration around the pin at lower rotational velocities was mainly responsible for the micro-voids and non-bonding defects at the lap interface. The asynchronous convergence of Al and steel flowing from the advancing and retreating sides, respectively, refilled the instantaneous gap, thereby normally bonding the lapped interface behind the pin. The rotational pin affected the hook structure and steel fragment formation through the shearing and squeezing of the steel migration flow. Material inter-migration produced an intercalated structure at the lapped Al/steel interface. The extremely high temperature, approximately above Al 5052 solidus temperature, under the pin bottom caused the formation of a thicker intermetallic compound (IMC), thereby considerably reducing the interfacial strength. Multi-scale mechanical strength assessments indicated that an intercalated interfacial structure with a thickness of Al-rich IMC layer less than ∼1.0 μm was desirable because it exhibited a higher local interfacial strength than the Al matrix.

Diffusion Bonding of Al-Mg-Si Alloy and 301L Stainless Steel by Friction Stir Lap Welding Using a Zn Interlayer.

Friction stir lap welding (FSLW) is expected to join the hybrid structure of aluminum alloy and steel. In this study, the Al-Mg-Si aluminum alloy and 301L stainless steel were diffusion bonded by FSLDW with the addition of 0.1 mm thick pure Zn interlayer, when the tool pin did not penetrate the upper aluminum sheet. The characteristics of lap interface and mechanical properties of the joint were analyzed. Under the addition of Zn interlayer, the diffusion layer structure at lap interface changed from continuous to uneven and segmented. The components of the diffusion layer were more complex, including Fe-Al intermetallic compounds (IMCs), Fe-Zn IMCs and Al-Zn eutectic. The largely changed composition and thickness of uneven and segmented diffusion layer at the lap interface played a significant role in the joint strength. The tensile shear load of Zn-added joint was 6.26 kN, increasing by 41.3% than that of Zn-not-added joint. These two joints exhibited interfacial shear fracture, while the Zn interlayer enhanced the strength of diffusion bonding by extending the propagation path of cracks.

Microstructural characterization of Friction stir assisted laminated lap welding of AA6063 sheets

Friction stir welding is a pioneering new technique for solid-state joining of Aluminium alloys, which also further strengthens the mechanical and microstructural properties of the base metal. Laminated lap welding on AA6063 sheets was performed using a stainless-steel tool with a square cross-section pin. The sample thus obtained was examined for their microstructural and hardness properties, and was characterized accordingly. Equiaxed grain structures are obtained commonly in friction stir welded samples. A refined grain size of ∼ 15 μm was observed in the microstructural characterization. The hardness profile depends on the degree of grain refinement as explained by the Hall-Petch effect. The microhardness was observed to be 135 % of the base metal hardness. The study has hinted towards the improvement of properties after friction stir lap welding and serves as a base for multilayer solid-state joining as a future prospect.

Effects of interlayer metal on microstructures and mechanical properties of friction stir lap welded dissimilar joints of magnesium and aluminum alloys

The content of intermetallic compounds (IMCs) in friction stir lap welded (FSLW) dissimilar joints of Mg and Al alloys could be reduced by the addition of interlayer metal to improve the mechanical properties of the joints, while the underlying mechanism is still unclear. In this study, Cu and Ni powders are added as the interlayer materials into the FSLW joints of AZ31 Mg alloy and Al–Zn–Cu–Mg alloy, respectively. The types, sizes, and distributions of IMCs have been characterized, and the shear properties of the joints have been evaluated. Moreover, the mechanism underlying the influence of the interlayer metals on the evolution of IMCs in the joints and the effect of IMCs on the shear properties of the joints have been studied. The results showed that the order of phase formation in the joint without interlayer was Al12Mg17, Al3Mg2, MgZn2, CuMg2, Al2Cu, while the order of phase formation in the joint with added Cu interlayer was Al2Cu, CuMg2, AlCu, Al12Mg17, Al3Mg2, Cu5Zn8, and that in the joint with added Ni interlayer was Mg2Ni, Al3Ni, Al3Ni2, MgNi2, AlNi, Al12Mg17, Al3Mg2, MgZn2, Ni3Zn22, NiZn, Al2Cu. Due to the formed physical barrier and the mechanism of phase formation, the addition of interlayer metal effectively inhibited the formation of a Mg–Al phase. The bonding strengths between Mg–Cu, Al–Cu, Al–Ni, and Mg–Ni phases and matrix were higher than that between Mg–Al phase and matrix, which improved the shear properties of the joints with Cu and Ni interlayers.

Multi-Pass Friction Stir Lap Welding of AA 6061-T6: Implication of Tool Pin Overlapping on Microstructure and Mechanical Properties of Joints

The present study focuses on the influence of a 5-pass overlapped friction stir lap welding of AA6061-T6 on the mitigation of defects and its simultaneous effect on microstructure and mechanical properties. Tool pin overlapping percentage of 0%, 25%, 50% and 75% was employed, from which it was observed that defects significantly reduced at higher overlapping. Fine equiaxed grains were observed across all the nugget zones in both top, and bottom regions were developed due to simultaneous overlapped passes; the grain size of which varied from 8.5 µm to 3.5 µm. The grains became finer, which increased of overlapping percentage. The microhardness of the processed zone was significantly reduced during overlapping that varied from a range of 43 HV0.1 to 49 HV0.1 as compared to base metal which was interpreted due to changes in grain size and secondary precipitate distribution during overlapping. A maximum ultimate tensile strength of 230 MPa was achieved at 50% overlapping, whereas minimum ultimate tensile strength of 160 MPa was achieved at 0% overlapping. Fractography of fracture surface showed a ductile failure at 50% that showed a number of dimples of varying shapes and sizes.

Friction Stir Lap Welding of Alloy 625 and a High Strength Steel

Friction Stir Lap Welding of Alloy 625 and a High Strength Steel