Process parameters/material location affecting hooking in friction stir lap welding: Dissimilar aluminum alloys

Influence of spindle and weld speeds, metal location, direction of spindle rotation, and tool pin length on hooking in lap FSW of dissimilar aluminum alloys and the effect of hook on tensile and fatigue weld strength was studied. Optical images of the cross-section of the specimen welded at different process parameters were analyzed. The results indicate that increased spindle speed, reduced weld speed, higher tool pin length, clockwise spindle rotation, and locating the stronger material at the bottom of the joint increased the size of the hooking defect. Higher weld speeds and very high spindle speeds resulted in lower hook size on the advancing side (AS) compared to the retreating side (RS) of the joint. Welding with low weld speed would result in higher advancing side hook size compared to the retreating side. Friction stir weld joints fabricated with anti-clockwise spindle rotation has been found to have extremely low hook both on the AS and the RS of the joint. The tensile and fatigue strengths of the weld joints and plates are degraded by the hook. The fatigue strength of welded alloys could be improved by a double pass weld, the second pass welded immediately adjacent to the first pass.     

Dissimilar friction stir lap welding of AA 5754-H22/AA 6082-T6 aluminium alloys: Influence of material properties and tool geometry on weld strength

Dissimilar friction stir welding (FSW) of heat (AA 6082-T6) and non-heat (AA 5754-H22) treatable aluminium alloys, in lap joint configuration, was performed in this work. The base material plates were 1 mm thick. Welds were performed combining different plates positioning, relative to the tool shoulder, in order to assess the influence of base materials properties on welds strength. Three different tools were tested, one cylindrical and two conical, with different taper angles. Welds strength was characterized by performing transverse and tensile–shear tests. Strain data acquisition by Digital Image Correlation (DIC) was used to determine local weld properties. The results obtained enabled to conclude that the dissimilar welds strength is strongly dependent on the presence of the well-known hooking defect and that the hooking characteristics are strongly conditioned by base materials properties/positioning. By placing the AA 6082-T6 alloy, as top plate, in contact with the tool shoulder, superior weld properties are achieved independently of the tool geometry. It is also concluded that the use of unthreaded conical pin tools, with a low shoulder/pin diameter relation, is the most suitable solution for the production of welds with similar strengths for advancing and retreating sides.     

Mixed mode I/II fracture of 2024-T3 friction stir welds

For the first time, a series of mixed mode I/II fracture experiments have been performed on both base material and three families of friction stir welds (FSWs) in 6.4 mm thick, 2024-T351 aluminum plate; the FSW joints are designated hot, medium and cold due to the level of nominal weld energy input per unit weld length (specific weld energy) during the joining process.Results from the fracture tests indicate that the measured critical crack opening displacement (COD) at a fixed distance behind the crack tip properly correlates both load-crack extension response and microstructural fracture surface features for both the base metal and all FSWs, providing measure of a quantitative fracture toughness. The COD values also indicate that transition from mode I to mode II dominant crack growth occurs at lower loading angles for FSW joints having higher specific weld energy input, with a truly mixed mode I/II COD measured during crack growth in the medium FSW joint. Using results from recent detailed FSW metallographic studies, specific features in the fracture process are correlated to the FSW microstructure. Finally, the observed ductile crack growth path in all three welds tends to exit the under-matched FSW weld region as the far-field applied shear loading is increased, with the medium FSW being the only case where the flaw remained within the FSW region for all combinations of shear and tensile loading.     

Effects of microstructural heterogeneity on very high cycle fatigue properties of 7050-T7451 aluminum alloy friction stir butt welds

In this study, the very high cycle fatigue (VHCF) properties of 7050-T7451 aluminum alloy and its friction stir welding (FSW) butt welds have been investigated. The results show that the failure of FSW joints still occurs at 7.0 × 10⁸ cycles. The fatigue properties of the FSW joints are superior to those of the base material, especially in the super long life regime. Most fatigue cracks initiate at the thermo-mechanically affected zone and heat affected zone on the advancing side of the FSW joints, and the susceptibility of these zones to fatigue is attributed to the metallurgical heterogeneity.     

Comparison of fatigue properties between friction stir welds and TIG welds

对比分析了搅拌摩擦和氩弧焊两种工艺方法对铝合金焊接接头疲劳性能的影响,建立了焊接接头的S-N曲线,结果表明：在相同的载荷条件下,搅拌磨擦焊接接头的疲劳性能优于氩弧焊接头。搅拌摩擦焊接头疲劳寿命N=106次的疲劳强度值约为59~65MPa之间。对焊接接头显微组织的分析表明：搅拌摩擦焊接接头具有比氩弧焊接头更为细小的晶粒和狭窄的焊接热影响区,阻碍了滑移带的形成和裂纹的扩展,从而提高了接头的疲劳性能。TIG焊接接头疲劳端口分析显示,焊接缺陷是主要的疲劳裂纹源。     

Friction stir welding characteristics of different heat-treated-state 2219 aluminum alloy plates

Friction stir welding (FSW) of 2219-O and 2219-T6 aluminum alloys was performed to investigate the effects of the base material conditions on the FSW characteristics. The experimental results indicated that the base material condition has a significant effect on weld morphologies, weld defects, and mechanical properties of joints. In the 2219-O welds, no discernible interface exists between the stir zone (SZ) and the thermal-mechanically affected zone (TMAZ), and weld defects are liable to form in the lower part of the weld. In the 2219-T6 welds, there is visible interface between the SZ and the TMAZ, and a weld nugget with an “onion ring”-like morphology clearly exists. The defects are liable to form in the upper part of the weld. The strength efficiency of 2219-O joints is 100%, while that of 2219-T6 joints is only up to 82%. In addition, the two types of joints have different fracture location characteristics.     

Process optimization in the self-reacting friction stir welding of aluminum 6061-T6

Self-reacting friction stir welding (SR-FSW), also called bobbin-tool friction stir welding (BT-FSW), is a solid state welding process similar to friction stir welding (FSW) except that the tool has two opposing shoulders instead of the shoulder and a backing plate found in FSW. The tool configuration results in greater heat input and a symmetrical weld macrostructure. A significant amount of information has been published in the literature concerning traditional FSW while little has been published about SR-FSW. An optimization experiment was performed using a factorial design to evaluate the effect of process parameters on the weld temperature, surface and internal quality, and mechanical properties of self-reacting friction stir welded aluminum alloy 6061-T6 butt joints. The parameters evaluated were tool rotational speed, traverse speed, and tool plunge force. A correlation between weld temperature, defect formation (specifically galling and void formation), and mechanical properties was found. Optimum parameters were determined for the welding of 8-mm-thick 6061-T6 plate.     

Effect of cold expansion on the fatigue life of Al 2024-T3 in double shear lap joints: Experimental and numerical investigations

In this paper the effect of cold expansion on fatigue life improvement of aluminum alloy 2024-T3 plates used in double shear lap joints is investigated experimentally by conducting fatigue tests and numerically by implementing finite element simulations. In the experimental part, fatigue tests were carried out on the plates with cold expansion levels of 0%, 1.5% and 4.7% which were used in double shear lap joints. In the numerical study, three-dimensional finite element models were employed to predict stress distributions in the cold expanded plates used in the double shear lap joint. The results obtained from finite element simulation, have been employed to explain the trends which were observed in the experimentally attained S–N data and the fatigue crack initiation location. The experimental and numerical results showed that cold expansion improves fatigue life at low load levels and the life enhancement is more for the bigger cold expansion size. However, the fatigue life improvement is smaller in double shear lap joints compared to a single cold expanded plate.     

氧化膜对6082铝合金搅拌摩擦焊接头疲劳性能的影响

对6 mm厚的6082-T6铝合金进行两种表面处理然后实施搅拌摩擦焊接,研究了对接面氧化膜对接头组织和疲劳性能的影响。结果表明,进行速度为1000 mm/min的高速焊接时,对接面未打磨和打磨的接头焊接质量都良好,接头强度系数达到81%;两种接头的疲劳性能基本相同,疲劳强度均为100 MPa;少数样品在焊核区外断裂,大部分样品在热影响区断裂。与接头相比,两种接头焊核区的疲劳性能有所提高,均为110 MPa,在疲劳测试中裂纹并未沿“S”线萌生和扩展。     

Effect of heat treatment on tensile properties of friction stir welded joints of 2219-T6 aluminium alloy

Abstract

The effect of post-weld heat treatment (PWHT) on the tensile properties of friction stir welded (FSW) joints of 2219-T6 aluminium alloy was investigated. The PWHT was carried out at aging temperature of 165°C for 18 h. The mechanical properties of the joints were evaluated using tensile tests. The experimental results indicate that the PWHT significantly influences the tensile properties of the FSW joints. After the heat treatment, the tensile strength of the joints increases and the elongation at fracture of the joints decreases. The maximum tensile strength of the joints is equivalent to 89% of that of the base material. The fracture location characteristics of the heat treated joints are similar to those of the as welded joints. The defect free joints fracture in the heat affected zone on the retreating side and the joints with a void defect fracture in the weld zone on the advancing side. All of the experimental results can be explained by the hardness profiles and welding defects in the joints.     

Mechanical,fatigue and microstructural properties of friction stir welded 5083-H111 and 6082-T651 aluminum alloys

In this study, 5083-H111 and 6082-T651 aluminum alloy plates in 6 mm thickness that are used particularly for shipbuilding industry were welded using Friction Stir Welding (FSW) method as similar and dissimilar joints with one side pass at PA position with the parameters of 1250 rpm tool rotation, 64 mm/min welding speed and 2° tool tilt angle. Tensile tests results showed sufficient joint efficiencies and surprisingly high yield stress values. Bending fatigue test results of all joint types showed fatigue strength close to each other. Fatigue strength order of the joints were respectively FSWed 5083-5083, and 6082-6082 similar joints and 5083-6082 dissimilar joint. Cross sections of the weld zones have been analyzed with light optical microscopy (LOM) and fracture surface of fatigue test specimens were examined by scanning electron microscopy (SEM). Although there were no voids in radiographic and microscopic analyzes, 5083-6082 joint showed rarely encountered voiding effect under fatigue load. Microhardness measurements revealed rare result for FSWed AW5083 and novel result for FSWed 6082 aluminum alloy.     

Effects of ultrasonic assisted friction stir welding on flow behavior,microstructure and mechanical properties of 7N01-T4 aluminum alloy joints

Conventional friction stir welding(FSW) and ultrasonic assisted friction stir welding(UAFSW) were employed to weld 6-mm thick 7 N01-T4 aluminum alloy plates. Weld forming characteristics and material flow behavior in these two different welding processes were studied and compared. Ultrasonic vibration was applied directly on the weld in axial direction through the welding tool. Metal flow behavior,microstructure characteristics in the nugget zone(NZ) and evolution of the mechanical properties of naturally aged joints were studied. Results show that the ultrasonic vibration can significantly increase the welding speed of defect-free welded joint. At the rotation speed of 1200 rpm, the UAFSW can produce defect-free welded joints at a welding speed that is 50% higher than that of the conventional FSW.Ultrasonic vibrations can also improve surface quality of the joints and reduce axial force by 9%. Moreover, ultrasonic vibrations significantly increase the volume of the pin-driven zone(PDZ) and decrease the thickness of the transition zone(TZ). The number of subgrains and deformed grains resulting from the UAFSW is higher than that from the FSW. By increase the strain level and strain gradient in the NZ,the ultrasonic vibrations can refine the grains. Ultrasonic energy is the most at the top of the NZ, and gradually reduces along the thickness of the plate. The difference in strengths between the FSW and the UAFSW joints after post-weld natural aging(PWNA) is small. However, the elongation of the UAFSW is8.8% higher than that of the FSW(PWNA for 4320 h). Fracture surface observation demonstrates that all the specimens fail by ductile fracture, and the fracture position of the UAFSW joint changes from HAZ(PWNA for 120 h) to NZ(PWNA for 720 and 4320 h).     

Mechanical behaviour of AA 7475 friction stir welds with the kissing bond defect

Friction stir welding (FSW) is restricted to non-safety–critical aerospace components because there is no reliable method for detecting kissing bonds (KB), which may have a significant effect on fatigue life. The effects of KB defects on the tensile and fatigue properties of 7475-T7351 friction stir welds were quantitatively evaluated with respect to a reference weld without any flaws and a base material. Various KB defects were investigated with the aim of determining the defect size that does not have a significant influence on the fatigue life of joined 6.35-mm-thick plates. A critical value for the KB geometry appears to be a depth of 0.3 mm considering the influence on fatigue life for the investigated configurations. This paper also presents results from micromorphological analyses of the fatigue crack initiation from the KB and from the analysis of the weld cross-section microstructure.     

An experimental investigation of the bolt clamping force and friction effect on the fatigue behavior of aluminum alloy 2024-T3 double shear lap joint

In this article, the effect of bolt clamping force on the fatigue life of bolted double shear lap joints was investigated. To do so, fatigue tests were carried out on the bolt clamped double shear lap joint specimens made of aluminum alloy 2024-T3. These fatigue tests were conducted with applied torques of 0.25, 2 and 4 N m at different cyclic longitudinal load levels in un-lubricated and lubricated states. From these tests the stress–life (S–N) data for different clamping forces for un-lubricated and lubricated states were obtained. The results show that clamping force increases fatigue life compared to clearance fit specimens. In general, at higher tightening torque higher fatigue lives were achieved, however, below a certain load level the life improvement was discontinued because of fretting phenomenon. Also lubricating the parts of the specimens reduces the advantage of clamping force or torque tightening.     

Investigation on dissimilar underwater friction stir lap welding of 6061-T6 aluminum alloy to pure copper

Friction stir welding (classical FSW) is considered to offer advantages over the traditional fusion welding techniques in terms of dissimilar welding. However, some challenges still exist in the dissimilar friction stir lap welding of the aluminum/copper (Al/Cu) metallic couple, among which the formation of the Al–Cu intermetallic compounds is the major problem. In the present research, due to the fact that the formation and growth of the intermetallic are significantly controlled by the thermal history, the underwater friction stir welding (underwater FSW) was employed for fabricating the weld, and the weld obtained by underwater FSW (underwater weld) was analyzed via comparing with the weld obtained under same parameters by classical FSW (classical weld). In order to investigate the effect of the external water on the thermal history, the K-type thermocouple was utilized to measure the weld temperature, and it is found that the water could decrease the peak temperature and shorten the thermal cycle time. The XRD results illustrate that the interface of the welds mainly consist of the Al–Cu intermetallic compounds such as CuAl₂ and Cu₉Al₄ together with some amounts of Al and Cu, and it is also found that the amount of the intermetallic in the underwater weld is obvious less than in the classical weld. The SEM images and the EDS line scan results also illustrate that the Al–Cu diffusion interlayer at the Al–Cu interface of the underwater weld was obviously thinner than that of the classical weld.     

Joint properties of dissimilar Al6061-T6 aluminum alloy/Ti–6%Al–4%V titanium alloy by gas tungsten arc welding assisted hybrid friction stir welding

Hybrid friction stir butt welding of Al6061-T6 aluminum alloy plate to Ti–6%Al–4%V titanium alloy plate with satisfactory acceptable joint strength was successfully achieved using preceding gas tungsten arc welding (GTAW) preheating heat source of the Ti alloy plate surface. Hybrid friction stir welding (HFSW) joints were welded completely without any unwelded zone resulting from smooth material flow by equally distributed temperature both in Al alloy side and Ti alloy side using GTAW assistance for preheating the Ti alloy plate unlike friction stir welding (FSW) joints. The ultimate tensile strength was approximately 91% in HFSW welds by that of the Al alloy base metal, which was 24% higher than that of FSW welds without GTAW under same welding condition. Notably, it was found that elongation in HFSW welds increased significantly compared with that of FSW welds, which resulted in improved joint strength. The ductile fracture was the main fracture mode in tensile test of HFSW welds.     

Microstructural aspects and mechanical properties of friction stir welded AA2024-T3 aluminium alloy sheet

In the present work, aluminium alloy AA2024-T3 thin sheets were joined by the Friction Stir Welding – FSW – process. Butt joints were obtained in 1.6 mm sheets, using an advancing speed of 700 mm/min. These joints were characterised by optical, scanning electron microscopy, tensile and fatigue mechanical tests. The results showed that the resulting microstructure is free of defects and the tensile strength of the welded joints is up to 98% of the base-metal strength. Fatigue tests result indicates an equivalent stress intensity factor (kt) of approximately 2.0 for the welded samples. Consequently, the FSW process can be advantageous compared to conventional riveting for airframe applications.     

Relationship between base metal properties and friction stir welding process parameters

Friction stir welding (FSW) is a solid state welding process for joining aluminum alloys and has been employed in aerospace, rail, automotive and marine industries for joining aluminium, magnesium, zinc and copper alloys. In FSW, the base metal properties such as yield strength, ductility and hardness control the plastic flow of the material under the action of rotating non-consumable tool. The FSW process parameters such as tool rotational speed, welding speed, axial force, etc. play a major role in deciding the weld quality. In this investigation, an attempt has been made to establish relationship between the base material properties and FSW process parameters. FSW joints have been made using five different grades of aluminium alloys (AA1050, AA6061, AA2024, AA7039 and AA7075) using different combinations of process parameters. Macrostructural analysis has been done to check the weld quality (defective or defect free). Empirical relationships have been established between base metal properties and tool rotational speed and welding speed, respectively. The developed empirical relationships can be effectively used to predict the FSW process parameters to fabricate defect free welds.     

搅拌摩擦与氩弧焊铝合金接头疲劳性能的比较

建立了铝合金焊接接头的S-N曲线,对比分析了搅拌摩擦和氩弧焊两种工艺对其焊接接头疲劳性能的影响,结果表明:在载荷相同的条件下,铝合金搅拌磨擦焊接接头的疲劳性能优于氩弧焊接头,搅拌摩擦焊接头的疲劳寿命N=106次的疲劳强度为59-65 MPa,搅拌摩擦焊接接头具有比氩弧焊接头更为细小的晶粒和狭窄的焊接热影响区,阻碍了滑移带的形成和裂纹的扩展,从而提高了接头的疲劳性能,铝合金焊接接头的缺陷是主要的疲劳裂纹源.     

Fatigue properties of combined friction stir and adhesively bonded AA6082‐T6 overlap joints

Even though friction stir welding (FSW) has been shown to produce high performing butt joints, stress concentration at the weld edges in overlap FSW significantly reduces the performance of these joints. By combining FSW and adhesive bonding into a friction stir (FS) weld bonding, joint mechanical performance is greatly improved. Quasistatic and fatigue strength of the proposed FS weld‐bonding joints was assessed and benchmarked against overlap FSW and adhesive bonding. The characterization of the structural adhesive is also presented, including differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), as well as mechanical characterization with curing temperature. A small process parameter study was made to select proper FSW parameters for AA6082‐T6 overlap FSW and FS weld‐bonded joints. FS weld bonding achieved a significant increase in quasistatic and fatigue strength when compared with overlap FSW, with 79.9% of the fatigue strength of adhesive‐bonded joints at 10⁶ cycles, whereas FSW had 41.6%.