Friction Stir Welded Lap Joints Research Articles

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

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

Study on the interfacial microstructure evolution of aluminum and CFRTP on FSLW joints: influence of pin length

Polymer/metal hybrid lightweight structures are increasingly employed in automotive, aerospace and electronics industries. Friction stir welding (FSW) is applied for the effective welding of metals and thermoplastic materials. For the welding of dissimilar materials, due to the significant property differences, it is hard to achieve high strength welded joints with smooth appearance. In this article, the influence of pin length is studied by experiments for the friction stir lap welding (FSLW) of Al alloy and carbon-fiber-reinforced thermoplastic (CFRTP). Tensile tests, OM, SEM and EBSD are carried out to evaluate the welding performance. Results indicate that the joint with smooth surface topography is obtained at the pin length of 2.5 mm with a peak temperature of 250.3 °C and the CFRTP matrix PEI thermal decomposition temperature residence time of 47.6 s. From OM and SEM micro analysis, the width of the bonding area is 3.02 mm and a thin uniform interaction layer is formed in the joint. EBSD confirms that the Al alloy in the stirring zone is refined after recrystallization. It is difficult to form a stacked layered structure in the bottom of joint, so the property of the joint is improved. The tensile strength is 32.9 MPa, which is 63.1% of CFRTP.

Elucidating role of sheets mutual position and copper interlayer in FSW of dissimilar Ti6Al4V-SS316L lap joints: Metallurgical and mechanical characterizations

Technological advancements catering to the specific requirements of different industries have led to a rise in the use of multi-material components. Combining titanium and stainless steel can reduce the weight of components used for many industrial applications, making it a popular choice. The present investigation compares Ti6Al4V/SS 316 L friction stir welded (FSW) lap joints' metallurgical and mechanical properties as a function of velocity ratio, sheets' mutual position, and the presence of a Cu interlayer. The process mechanics and the impact of different strategies on intermetallic (IMC), grain morphology, intermittent phases, joint resistance, and microhardness are discussed. Successful FSW lap joints of SS 316 L/ Ti6Al4V, utilizing SS 316 L as the upper plate and a Cu interlayer, have been documented and deliberated for the first time in the present study. These specimens demonstrated immaculate joints for all the processing velocity ratios owing to superior mechanical interlocking offered by distinguished extruded hook accompanied by comparatively lean IMC layers affecting tensile shear strengths.

Comprehensive influence of silane layer and interfacial thermo-mechanical effect on joining mechanism of silane pretreated Al/CFRP FSLW joint: FEA and experiment

Al/CFRP composite structure has the advantage of lightweight and high specific strength, making it a promising material for aviation, railway, and automotive applications. Silane coupling pretreatment is a chemical surface pretreatment method that can improve the mechanical properties of the Al/CFRP friction stir lap welding (FSLW) joint. To further investigate the strengthening mechanism of silane-treated Al/CFRP FSLW joint, a 6061-T6 Al/CF-PA66 FSLW experiment was carried out and the finite element analysis (FEA) was used. The thermophysical properties of semi-crystalline resin during the welding process were also considered. The joining mechanism of silane surface pretreated Al/CFRP FSLW joints was further revealed by comparing the changes in interface morphology, temperature, stress, and mechanical properties. The results indicated that the silane coupling agent forms a silane film by an Al-O-Si chemical bond at the interface. The silane film can enhance the Al/CFRP interface's tightness and reduce interfacial defects. The FSLW process affected the joint morphology and mechanical properties through interfacial temperature and stress, which were decided by welding parameters and thermophysical properties of the resin. By adjusting the thermal-mechanical effect of the Al/CFRP interface, defects can be reduced and the joining force can be improved. According to the analysis of interfacial chemical bonds and mechanical properties, the introduction of silane film enhances the hydrogen bond between Al and resin and thus enhances the mechanical properties of the interface. This study provides a new method for revealing the strengthening mechanism of silane-pretreated FSLW Al/CFRP joint.

Formation and Evolution Mechanism of Intermetallic Compounds of Friction Stir Lap Welded Steel/Aluminum Joints.

Interfacial layers with brittle intermetallic compounds (IMC) greatly influence the performance of steel-aluminum friction stir lap welding (FSLW) joints. Thus, the formation and evolution of IMC between 7075-T6 aluminum alloy and galvanized DP590 steel in steel-aluminum FSLW joints were investigated. An FSLW numerical model was developed using the computational fluid dynamics method to analyze the interface temperature between the aluminum alloy and steel. Scanning electron microscopy (SEM) was conducted to observe the microstructure characterization and measure the IMC thickness. Phases among different joint zones were analyzed by X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS). IMC layer formation was predicted by the effective Gibbs free energy model presented in this paper according to thermodynamic principles. The Monte Carlo method was utilized to predict the thickness of IMC layers. It was found that the IMC layer at the interface of the welded joint is composed of Fe2Al5, FeAl3, and Al-Zn eutectic. The IMC thickness decreased from 4.3 μm to 0.8 μm with the increasing welding speed, which was consistent with the Monte Carlo simulation results.

Microstructure and Properties of Nonlinear Lap Joint of 6061 Aluminum Alloy by Friction Stir Welding

The microstructure and properties of non-linear friction stir welded lap joints of the AA6061-T6 aluminum alloy were investigated, with a particular focus on the influence of corner curvature on the formability and mechanical properties of the joints. The research results indicate that for the 6061-T6 aluminum alloy lap joint friction stir welding with a smaller radius (R < 7 mm), there is a more severe accumulation of welding material. When the radius exceeds 7 mm, good macroscopic joint formation can be achieved. Various regions at the joint corners are composed of α-Al and intermetallic precipitations β phases. The microstructure of the heat-affected zone (HAZ) appeared relatively coarse, the weld nugget zone (WNZ) had the finest grain, and partial dissolution of the β phase occurred. The grain size in the middle WNZ at the corner was larger than at the ends, and the grain size on the inner side of the corner was larger than on the outer side. The hardness distribution of the joint exhibited a “W” shape, with the lowest hardness in the inner HAZ. When R ≤ 7, with an increase in R, the shear strength of the friction stir welded joints increased, and then the change became relatively small. The maximum shear strength of the joint was 101.32 ± 6.89 MPa at R = 7, and the fracture mode was primarily a ductile mixed fracture.

Effects of Zr interlayer on microstructure, mechanical property and corrosion behavior of Al/Mg dissimilar friction stir welding lap joints

Effects of Zr interlayer on microstructure, mechanical property and corrosion behavior of Al/Mg dissimilar friction stir welding lap joints

Effect of tool pin length on interfacial behaviors and mechanical properties of friction stir-welded lap joints of 430/304 stainless steels

In this study, the effect of tool pin length on friction stir lap welding of 430 and 304 stainless steels has been investigated. The thermal history of interface was measured by thermocouple. The interfacial macro- and micro-structures under different tool pin lengths were analyzed via optical microscope and scanning electron microscope. Also, tensile shear tests were carried out to evaluate the strength of the lap joint. The results show that increasing the pin length from 0.5 to 1.3 mm increases the peak temperature of the interface, which has a significant effect on diffusion layer formation and hook geometry. The fracture mode of the joints shifts from shear fracture to tensile fracture with the increase of the pin insertion depth. When the pin length is 1.0 mm, i.e., a slight penetration into the 304 plate, the joint presents a maximum failure load of 2.86 kN. The findings reveal that the effective top plate thickness (Teff) used to describe the hook geometry is an important factor determining the joint strength and fracture mode.

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.

Effects of curvature direction on friction stir welding lap joint of aluminum alloy “S” curved surface

In this paper, the friction stir welding (FSW) of single curvature and double curvature “S” curved surface lap joints were realized using split-type pressing blocks fixture and robotic system with welding parameter 1400 rpm, 150 mm/min. By finite element method (FEM) and axial force analysis, the effects of curvature direction on friction stir welding lap joint of 5083 aluminum alloy “S” curved surface is explored. The results show that Z-axis curvature direction has important influence on joint forming. Z-axis positive curvature makes the actual axial compressive force smaller than the device input, which decreases the interface stress of the lap joint and increases the tendency of upper plate weld edges bending deformation and the probability of interface defects. So the joint mechanical properties are reduced, and the tensile load can be as low as 1.5 kN. Z-axis negative curvature makes the actual axial compressive force greater than the device input, which increases the interface stress of the lap joint. Meanwhile, it decreases the tendency of upper plate weld edges bending deformation and leads to the interface defects disappearing. So the joint mechanical properties are good, and the tensile load can be more than 2.4 kN. For Y-axis curvature direction, its impact is mainly shown in changing the direction of resultant force FR. The inclination of force FR is helpful to eliminate the interface defects of the joint. At the same time, it will cause inconsistent stress between the advancing side (AS) and the retreating side (RS) of the joint interface, which is the reason that the double-curvature lap joint welding process is unstable.

Friction stir lap joining techniques effects on microstructure and tensile properties of high-strength automotive steel top hat sections

Dual Phase (DP) steel, a type of Advanced High Strength Steel (AHSS) with a thickness of 1.7 mm, is used to fabricate single-hat components that are then joined to the base plate using two friction stir welding processes: friction stir lap welding (FSLW) and friction stir spot lap welding (FSSLW). It is difficult to join this assembly using fusion welding techniques. The welding variables for the FSLW process, tool rotation speed (TRS), tool traverse speed (TTS), and plunge depth (PD), were optimized using the design of an experiments-based response surface method by experimentally measured tensile shear failure load (TFL) of top hat assembly. For the FSSLW process, the welding variable TTS was replaced by dwell time (DT). Peak temperature, microstructure at different zones, microhardness mapping, and energy absorption capacity of both processes were evaluated under optimal welding conditions. For both processes, the stir zone and the heat-affected zone had the highest and lowest microhardness, which can be correlated with the level of martensite tempering, martensite lath spacing, polygonal ferrite volume, and precipitated carbides. Under optimum welding conditions, the TSL and energy absorption of FSLW joints were 14 kN and 170 J, respectively, which is 20% and 47 higher than the TSL and energy absorption of FSSLW joints.

Experimental based determination and analysis of mechanical properties of AA 3003 alloy welded with friction stir lap welding process

In recent years, friction stir welding (FSW) methods have been used to obtain good joints with mechanical and physical process properties. The development of FSW lap joint assembly will expand its use in a high number of industrial applications that can benefit from this technology. In this paper a study based on experimental methods were performed on FSW lap joints. The investigation includes the interface morphology and mechanical properties of the joint assembly. Experimental measurements were carried out on micro-hardness; tensile shear test, resulting material flow as well as the effect of flow variation on the obtained mechanical properties of FSW butt lap joints of aluminium alloy AA3003. The study also presents the effects of different welding parameters on the lap joints assembly structure. It was found that the hardness in the welded region is significantly lower with respect to the base material. The fracture analysis shows the characteristics of ductile-brittle mixed fracture.

Effects of the microstructure on the fatigue fracture of friction stir lap welded Al-clad Al and Al-clad steel sheets

The microstructural characteristics and mechanical properties of friction stir lap welding (FSLW) of thin Al-clad Al (1.5 mm) and ultra-thin Al-clad mild steel (0.7 mm) sheets are experimentally investigated. Optical microscopic observation confirms that the FSLW joints are successfully fabricated without noticeable joining defects, such as cracks and hooks. The material flow is correlated with the silicon (Si) element distribution in the stir zone (SZ). The distribution of Si indicates that material flow is asymmetrical between the advancing side (AS) and the retreating side (RS) of the SZ. The tensile test result shows that all the FSLW joints fracture from the Al-clad Al side of the joints. During the fatigue tests, two different crack propagation modes (vertical crack in the Al-clad Al loading side; horizontal crack in the Al-clad mild steel loading side) are observed simultaneously due to the combined effect of the asymmetric geometry of the FSLW joint and the different loading configurations. The results of fatigue tests suggest that the asymmetric microstructure of the joint retards the propagation of a vertical crack in the Al-clad Al loading side for the RS-loaded FSLW joint. As a result, the fatigue life of the RS-loaded FSLW joint becomes clearly higher than that of the AS-loaded FSLW joint.

Improving the tensile strength of non-keyhole friction stir lap welding joint of 2024-T4 Al alloy by radial basis function neural network and improved particle swarm optimization algorithm

The non-keyhole friction stir lap welding (N-KFSLW) technology assisted by the outer stationary shoulder and the inner upper half-thread rotating pin was proposed to obtain the welding joint without keyhole through one-time process. Choosing 2024 aluminum alloys as the research object, the formation, microhardness and tensile strength of N-KFSLW joint were investigated. The improved particle swarm optimization (IPSO) algorithm was newly developed and had the advantages of large convergence speed and strong search ability, by which the radial basis function (RBF) neural network was optimized to enhance its prediction accuracy. After that, the RBF and IPSO (IPSO-RBF) system was used to predict the joint strength and optimize the process parameters combination. The results showed that the lap joint had not only the SZ with the thickness almost equal to the thickness of upper sheet but also the cold lap with a very small height, thereby leading to the high tensile strength of joint. The optimized parameters of welding speed, rotating speed and pin type by the IPSO-RBF system were respectively 612 rpm, 80 mm/min, and upper half-thread pin, and the tensile strength of lap joint reached 11.88 kN/mm. The N-KFSLW technology assisted by upper half-thread pin provides an effective way to obtain the lap joint with high performance, and the IPSO-RBF system can be used to maximize the strength of welding joint.

Microstructure and texture characterization in friction stir lap welded TIMETAL 21S

The evolution of microstructure and texture during friction stir lap welding (FSLW) of TIMETAL 21S (β-type Ti-15Mo-3Nb-3Al-0.2Si, wt%) sheets were investigated through electron backscatter diffraction (EBSD). Excellent grain refinement is obtained through stir zone (SZ) thickness (1.2–1.8 μm). The microstructure of the thermomechanically affected zone (TMAZ) is characterized by elongated deformed grains surrounded by small recrystallized grains indicating the occurrence of discontinuous dynamic recrystallization (DDRX). The microstructure of heat affected zone (HAZ) is quite similar to the base metal (BM). The texture transformed from weak rolling-recrystallization texture in BM and HAZ to a typical shear texture with the domination of D1 or D2 components in the SZ and TMAZ area. A net shear texture gradient is formed across the SZ thickness which is connected with the heterogeneity of deformation. It is believed that the concomitant occurrence of grain size, dislocation and texture strengthening is responsible for the mechanical property distribution in different parts of FSLW joint.

Enhancing interfacial bonding in friction stir lap welding of light metal and carbon fiber reinforced polymer composite

The key strategic technology of automotive lightweight design advances the development of light metal/carbon fiber reinforced polymer composite hybrid joining. In this work, Al alloy (AA6061) and carbon fiber reinforced polyamide 66 (CF-PA66) were joined using friction stir lap welding (FSLW) technique with laser ablation pretreatment. The enhancing interfacial bonding mechanism and failure analysis of hybrid joints were investigated in detail. The maximum tensile-shear strength reached 24.48 MPa based on the bonding mechanism including macro/micro/nano-mechanical interlocking and chemical bonding, increasing by approximately 600 % compared with FSLW joints without surface pretreatment. Fracture located at weak interfaces between metal and composite without laser-textures and mixing region of resin and carbon fibers. This study provides novel insights into material-process-structure integrated lightweight design and benefits future research in the processes to enhance metal/composite hybrid structures.

Interface Characteristics and Mechanical Properties of 2024 Aluminum Alloy and 304 Stainless Steel Dissimilar Alloys FSLW Joint with Ni Interlayer

The composite structure of aluminum and steel (Al/steel) dissimilar metals has been applied in manufacturing industries for lightweight products, and friction stir lap welding (FSLW) has advantages for the welding of these two metals. To further enhance the strength of the Al/steel FSLW joint, a 0.02 mm thick nickel (Ni) foil was chosen as the interlayer, and slight plunging depth of a tool pin into the lower steel sheet was designed, which can respectively change the intermetallic compounds (IMCs) type and produce small welding heat. Choosing dissimilar 2024 aluminum alloy and 304 stainless steel materials as the research subject, the characteristics of the lap interface and the mechanical properties of the joint were mainly discussed. The results showed that the lap interface between the upper aluminum and lower steel sheets was made up of an AlNi3 IMCs layer, hook structure and mechanical occlusion. The AlNi3 IMCs layer with 2 μm thickness was in a reasonable range positive to the joint strength. When the rotating speed of the stirring tool increased from 800 to 1200 rpm and the welding speed of 75 mm/min was constant, the hook structure and the mechanical occlusion were both enhanced, and the tensile shear strength of the joint was decreased. A maximum joint tensile shear strength of 217 MPa was obtained at 800 rpm, and the strength value was 47.2 percent of that of the 2024 aluminum alloy base material. The Al/steel joint with shear fracture mode presented a brittle–ductile mixed fracture.

Strategized friction stir welded AA6061-T6/SiC composite lap joint suitable for sheet metal applications

Friction Stir Welding (FSW) is one of the best choices of joining light weight metal structures involving lap joints as conventional welding methods find difficult in getting sound weld joints. Aluminium alloys are lightweight materials relatively used in space frame structures, ship buildings and panels of automotive vehicles. Main objective of this research is to investigate Al–SiC FSW lap joints and to examine the suitable pre-fill nanoparticles in the configured grooves to augment the weld strength. Three different volume additions of SiC nanoparticles and tool rotational speed were considered to fabricate lap joints for the investigations. Tool rotational speed and volume of SiC reinforcement have a significant impact on the dissemination of SiC in the weldment and weld quality. The spread of SiC nanoparticles in the weld area strongly affects the weld microstructure and mechanical characteristics. Nano SiC filled lap joint has shown a significant improvement in mechanical properties as compared to a joint without nanoparticles. The best lap joint is achieved with 20 vol% of SiC and 1500 rpm of rotational speed. The investigations imply that the tensile strength is decreased for joints with 26 vol% of SiC, due to clustering of reinforcement particles. The supplement of SiC nanoparticles restricts the growth of grains and refines the stir zone (SZ) microstructure and hence this method of weld process is recommended for lap weld joints in metal structures.

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.

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.