Friction Stir Welded Aluminum Alloy Research Articles

Investigations on the Effect of Cyclic Heat Treatment on the Mechanical Properties of Friction Stir Welded Aluminum Alloys (AA5052 & AA6061)

Investigations on the Effect of Cyclic Heat Treatment on the Mechanical Properties of Friction Stir Welded Aluminum Alloys (AA5052 & AA6061)

Effect of thermal exposure on microstructure and mechanical properties of friction stir welding 7B50-T7451 aluminium alloy thick plate joint

In the present work, thermal exposure was performed on the 7B50-T7451 aluminium alloy friction stir welding (FSW) joints at different temperatures (150 °C, 175 °C and 220 °C) for 200 h. The results show that the microstructure and mechanical properties of as-welded joints along the thickness direction of the plate are heterogeneous. The hardness and strength of FSW joints decrease while the elongation continuously increases with increasing thermal exposure temperature. Compared with the base material, the joints present much higher hardening capacity which is further increased with increasing temperature. The precipitates in the heat-affected zone (HAZ) exhibit continuous coarsening and the precipitate distribution becomes broader as the temperature rises. The transformation from η′ phase to η phase is obvious at 220 °C. The weld nugget zone (WNZ) re-precipitates a large number of nano-scale η particles subjected to 200 h thermal exposure at 150 °C and re-precipitated η phase in WNZ of the double-sided joint has reached micron-sized at 175 °C and 220 °C. When exposed at different temperatures, all FSW joints show typical ductile fracture features. Fracture positions at different thermal exposure temperatures are inconsistent and this variation is closely related to different microstructural characteristics.

Friction stir welding parameters and their influence on mechanical properties of welded AA6061 and AA5052 aluminium plates

Friction stir welding (FSW) process is the preferred technique for joining of dissimilar metals. This paper intends to provide comprehensive study on the mechanical and metallurgical properties of dissimilar friction stir welded aluminium alloys, AA6061 and AA5052. The objective of the study is to find the optimum welding parameters at which the maximum weld joint strength can be achieved and to identify the influence of those parameters on the weld strength and microstructure of the AA6061 with AA5052 welded joints. The FSW process parameters such as traverse speed, tool rotational speed, axial force and tilt angle were considered. The mechanical properties measured are yield strength, tensile strength and percentage of elongation. Scanning Electron Microscopy (SEM) and optical microscope were used to observe the microstructure of weld zone (WZ) and heat affected zone (HAZ) of welded samples. Energy Dispersive x-ray Analysis (EDS) was used to obtain the elemental composition at the weld zone. Visual inspection reveals that there is no existence of weld defects like voids and porosity developed on the surface of the joints. The welds produced by the dissimilar aluminium alloys exposed an equiaxed and fine-grained structure in the weld zone. Analysis of variance (ANOVA) Technique is used to check the adequacy of the developed mathematical model. The difference between calculated and adjusted R2 is 0.2 which indicates that the model is adequate. The percentage error is also less for the estimated and predicted values of the properties of welded joints.

Detection of tunnel defects in friction stir welded aluminum alloy joints based on the in-situ force signal

The demand for high quality and effective manufacturing has raised the need for defect identification in friction stir welding. This work aims to identify the tunnel defect inside the welded joint based on the feature of in-situ force signals. First, we measured the traverse force (Fx), lateral force (Fy) and plunge force (Fz) during the welding process and analyzed the variation characteristic of the force signals for the joints with and without tunnel defect. Then, fast Fourier transform was utilized to obtain the frequency components of the force signal for analyzing the cause of the waveform distortion of the welding force, which showed a direct relationship with the occurrence of the tunnel defect in the joint. The combination result of the frequency components indicated that the waveform distortion of Fx and Fy for the defective joint was mainly influenced by the first harmonic and third harmonic. Thus, a defect detection indicator was proposed by calculating the normalization ratio of the amplitudes of the third harmonic and first harmonic. Tshe best accuracy of the indicator from Fx for identifying the tunnel defect reached 96.42%. Furthermore, the simulation results of the FSW process provide a physical understanding of material flow and defect formation indicated by the first harmonic and third harmonic of Fx, which helps explain the high accuracy of the proposed indicator from Fx signal in the detection of tunnel defect.

Effects of pin morphology on the interface defects of the FSWed lap joints of 2A12 aluminum alloy

The current work aims to study the effects of stirring pin morphology on the FSWed lap joints defects and mechanical properties. The 2A12 aluminum alloy friction stir welding lap joints were obtained using two shapes and three thread lengths of stirring pins by experiments. Besides the interface defects forming mechanism were studied by joints plastic metal flow numerical simulation. The results show that the conical pin can lead to uniform and continuous plastic metal velocity field which forms a Hook defect with sharp end, smaller deflection angle (minimum 8°) and larger size (maximum 0.18mm). It also will lead to a larger failure load (maximum is 9.5kN), and the fracture position easily occurs at the Advancing side (AS) of lower plate. The three section pin has a periodic shearing effect causing velocity difference between the arc edge and cutting section of the pin which forms a Hook defect with thicker end, larger deflection angle (maximum 42°) and smaller size (minimum 0.06mm). It also causes discontinuous wavy debris forms in the AS. The three section pin leads to a lower failure load (maximum is just 7.6kN), and the fracture position turns to occur at the lap interface. The increase of the thread length can promote the exchange of plastic metal in the thickness direction and interface materials flow velocity. So the angle (from 20°to 150°) and size (from 0.1mm to 0.6mm) of Hook increases but the height of Cold Lap reduces (increasing the effective sheet thickness of retreating side from 0.69mm to 0.95mm). In addition, the increase of the thread length can make the periodic shear effect of three section pin gradually weak. It is also found that the fracture position is closely related to the shape and the thread length of pins. When using conical pin, the fracture occurs at AS of lower plate or Retreating Side of Nugget Zone (NZ-RS) of upper plate; When using three section pin, the fracture occurs at the bonding interface. And with the increase of the thread length, the fracture position changes from the bonding interface to the NZ-RS of upper plate, and finally to the AS of lower plate.

Flexural and crashworthiness studies of friction stir welded aluminum alloy pipes

Friction stir welding (FSW), a solid-state joining process, is commonly used to join Aluminum alloys in aerospace, marine, car industry, and numerous industrial applications. Lightweight materials are commonly used in the transport industry to minimize component weight, and thin-walled structures are increasingly being used as energy-absorbing components in the industry. Even though FSW of plates is well established, FSW of pipe is still at the laboratory level. In this work, FSW for 6063 aluminum alloy pipe has been experimented through a novel procedure and it’s flexural and crashworthiness studies are carried out. The attempt of FSW is performed with a unique welding fixture mounted on a traditional milling machine. The crashworthiness test is conducted to analyze the ability of the structure to nullify the life loss during impact and the crash force efficiency of the pipe is identified. It is further analyzed for the flexural test to identify the bending load of welded aluminum alloy pipes. This study also exposes the basic understanding of the FSW procedure and its metallurgical implications of a friction stir welded pipe.

New insight into the phenomenon of the abnormal grain growth in friction-stir welded aluminum

In this work, the annealing behavior of friction-stir welded aluminum alloy 6061 was studied. Due to essential grain refinement and dissolution of secondary particles, this material usually exhibits relatively low stability against abnormal grain growth during post-weld solutionizing treatment. In the present study, it was found that the abnormal microstructural coarsening actually develops during the heating stage of the tempering process, i.e., during material warming from the ambient condition to the solutionizing temperature.

Study on Quality Prediction of 2219 Aluminum Alloy Friction Stir Welding Based on Real-Time Temperature Signal.

The online prediction of friction stir welding quality is an important part of intelligent welding. In this paper, a new method for the online evaluation of weld quality is proposed, which takes the real-time temperature signal as the main research variable. We conducted a welding experiment with 2219 aluminum alloy of 6 mm thickness. The temperature signal is decomposed into components of different frequency bands by wavelet packet method and the energy of component signals is used as the characteristic parameter to evaluate the weld quality. A prediction model of weld performance based on least squares support vector machine and genetic algorithm was established. The experimental results showed that, when welding defects are caused by a sudden perturbation during welding, the amplitude of the temperature signal near the tool rotation frequency will change significantly. When improper process parameters are used, the frequency band component of the temperature signal in the range of 0~11 Hz increases significantly, and the statistical mean value of the temperature signal will also be different. The accuracy of the prediction model reached 90.6%, and the AUC value was 0.939, which reflects the good prediction ability of the model.

Friction stir welding of ultrafine grained aluminum alloys: a review

Severe plastic deformation (SPD) has been one of promising routes to fabricate ultrafine-grained (UFG) materials, especially aluminum alloys. However, the SPD products often suffer from their small size. This issue implies the necessity of welding of UFG aluminum alloys for making them usable in complex, large forms in industries. Among various welding processes, those based on solid state welding seem to be more consistent with UFG materials. This is associated with the instability of UFG materials upon intense heating cycle that is common in fusion state welding processes. Friction stir welding (FSW) as a well-known process in the category of solid state welding is widely used for welding of aluminum alloys. This review paper provides an overview of the state-of-the-art of FSW of UFG aluminum alloys. To do so, specific attention is given to microstructural and textural evolutions, effect of secondary particles, and cooling medium. Applying cryogenic cooling medium as well as secondary nanoparticles could inhibit excessive grain growth in the stir zone, which were beneficial to improve the strength of the stir zone without remarkable decrease in the ductility. These processing routes did not affect the main recrystallization mechanism of the stir zone.

Mechanical and Microstructural Characterization of Friction Stir Welded SiC and B4C Reinforced Aluminium Alloy AA6061 Metal Matrix Composites.

This study focuses on the properties and process parameters dictating behavioural aspects of friction stir welded Aluminium Alloy AA6061 metal matrix composites reinforced with varying percentages of SiC and B4C. The joint properties in terms of mechanical strength, microstructural integrity and quality were examined. The weld reveals grain refinement and uniform distribution of reinforced particles in the joint region leading to improved strength compared to other joints of varying base material compositions. The tensile properties of the friction stir welded Al-MMCs improved after reinforcement with SiC and B4C. The maximum ultimate tensile stress was around 172.8 ± 1.9 MPa for composite with 10% SiC and 3% B4C reinforcement. The percentage elongation decreased as the percentage of SiC decreases and B4C increases. The hardness of the Al-MMCs improved considerably by adding reinforcement and subsequent thermal action during the FSW process, indicating an optimal increase as it eliminates brittleness. It was seen that higher SiC content contributes to higher strength, improved wear properties and hardness. The wear rate was as high as 12 ± 0.9 g/s for 10% SiC reinforcement and 30 N load. The wear rate reduced for lower values of load and increased with B4C reinforcement. The microstructural examination at the joints reveals the flow of plasticized metal from advancing to the retreating side. The formation of onion rings in the weld zone was due to the cylindrical FSW rotating tool material impression during the stirring action. Alterations in chemical properties are negligible, thereby retaining the original characteristics of the materials post welding. No major cracks or pores were observed during the non-destructive testing process that established good quality of the weld. The results are indicated improvement in mechanical and microstructural properties of the weld.

Parametric Optimization of Mechanical Properties via FSW on AA5052 Using Taguchi Based Grey Relational Analysis

The Friction stir processing benefits of aluminium composites contain advanced exploration in the region of aluminium alloy Friction Stir Welding - FSW. The modern advancements in Friction Stir Welding are concentrated on the optimization of welding parameters for multi response attributes. The investigations were carried out with the tool pin profiles, tool rotational speed and traverse speed as predictable process parameters for multi response optimization in Friction Stir Welding of 5052 aluminium alloy. GRG (grey relational grade) was obtained by the grey relational analysis of the friction stir welding process through different qualities, particularly, UTS-ultimate tensile strength and micro hardness. The significant process variables on GRG and most substantial parameters traverse speed and tool pin profiles are examined by ANOVA. Excluding tool rotational speed, tool pin profiles and traverse speed were likewise observed to be significant. To approve the investigation, verification of tests was completed at optimal parameters arrangement and predicted outcomes were observed to be in great concurrence with test values.

Relationship between welding conditions, abnormal grain growth and mechanical performance in friction-stir welded 6061-T6 aluminum alloy

In this work, a relationship between welding conditions, annealing behavior, and mechanical performance of friction-stir welded (FSWed) 6061-T6 aluminum alloy was examined. In the entire studied FSW range, the welded material was found to be unstable against abnormal grain growth during post-weld solutionizing treatment. However, a lowering of the FSW heat input tended to inhibit this undesirable phenomenon. In addition to the stir zone, the abnormal microstructural coarsening was also observed in the heat-affected zone, whereas the thermo-mechanically affected zone experienced static recrystallization. In all cases, the abnormal grain growth was found to result in ~25-35°<110> rotation of the original crystallographic texture. Hence, this process was suggested to be governed by the orientation-growth mechanism. Due to the relatively high strain-hardening ability intrinsic to the coarse-grained materials, the abnormal grains suppressed tensile strain in the stir zone during transverse tensile tests thus degrading weld ductility.

Superficial residual stress, microstructure, and efficiency in similar joints of AA2024-T3 and AA7475-T761 aluminum alloys formed by friction stir welding

Friction stir welding (FSW) represents a conceptually simple technique that consists of joining either similar or dissimilar solid-state materials through higher plastic deformation rates. FSW is an important technique in the aeronautical and aerospace industries, and its development is vital because of the significant difficulty in joining higher resistance AA 2000 and AA 7000 aluminum alloys with conventional techniques, like fusion welding, due to porosity and mechanical property losses. Thin sheets with a 1.6-mm nominal thickness of AA2024, heat treated to condition T3, and thin sheets with a 1.6-mm nominal thickness of AA7475, heat treated to condition T761, were used to investigate the influence of welding parameters under superficial residual stress and the efficiency of joints by FSW of AA2024-T3 and AA7475-T761 aluminum alloys. A central composite design (CCD) was used as a statistical model in this study (23 factorial points, six stellar points, two central points, and two replicas). Micrographic analysis showed that in the nugget zone of the AA7475-T761 alloy, there was hardness recovery. The fractography images showed that failures occurred mainly due to the joint line remnant defect, evidenced by the presence of cracks. The superficial residual stresses show a maximum value of 81 MPa at the advancing side in run 27 (hot welding) of AA2024-T3, whereas in AA7475-T761, a value of 57 MPa was found in the same run. Finally, tensile strength represents an efficiency of ~92% of the AA2024-T3 base metal value, while for AA7475-T761, this value was ~85%. From a component design perspective, the parameter window of this study is identified as interesting for its evaluation in the possible application in component manufacturing, due to the low values of superficial residual stresses found compared to those in previous work.

A comprehensive review on the effect of precipitation on mechanical properties of friction stir welded aluminium alloys

Friction Stir Welding (FSW) is one of the excellent metals joining processes in terms of joint strength efficiency, quality of the weld and eco-friendliness. The strength of the joint and quality of the weld depend upon the FSW processing parameters. This paper aims to provide overview of the FSW process by considering the effect of alloying elements and its precipitates on mechanical properties of aluminium and its alloys. The importance of grain growth and its effect on mechanical properties after welding is also analysed and discussed in detail. The interesting observation is that the precipitates and its size, residual stress play an important role in changing the hardness, tensile strength and fatigue strength across the weld zones. In general, the hardness of the stir zone is lesser when compared to base metal and this is noticed for all series of aluminium and its alloys, irrespective of the precipitates and its structure. Grain size and its morphology also have significant role in affecting the mechanical properties of the welded joint and it is found that post welding process has noticeable effect on the mechanical properties closer to base aluminium and its alloys.

Examination of the Mechanical, Corrosion, and Tribological Behavior of Friction Stir Welded Aluminum Alloy AA8011

Aluminum alloy AA8011 is emerging as a promising material for modern engineering applications in which improved tensile strength, hardness, corrosion-resistance, and wear-resistance of materials are required. Typically, AA8011 alloys are utilized in air-conditioning ducts and heat exchanger fins in ships, leisure boats, luxury vessels, workboats, fishing vessels, and patrol boats. However, the conventional welding of AA8011 is a challenging procedure. In this context, this paper focuses on the development of an effective solid-state welding methodology for AA8011 alloy welding. The AA8011 alloy was friction stir welded by varying the tool rotation speed, traverse speed, and shoulder diameter. The microhardness, tensile strength, joint efficiency, elongation, corrosion rate, and wear rate of the friction stir welded specimens were compared with the base material. Fractography analysis was conducted after the tensile test and surface morphology analysis after corrosion and wear tests, using scanning electron microscopy. The compositional elements in the corroded and worn section of the specimens were analyzed using energy-dispersive X-ray spectroscopy. Based on the joint efficiency as a primary constraint, the optimum process parameters for friction stir welding of aluminum alloy AA8011 have been established as follows: tool rotation speed of 1200 rpm, tool traverse speed of 45 mm/min, and tool shoulder diameter of 21 mm.

Effect of tool axis offset and relative position of materials on the performance of dissimilar friction stir welded aluminium alloy joints–an overview

At present most of the manufacturing sectors are looking for high strength lightweight structures with high specific stiffness and good corrosion resistance. In this context, combined use of different types of aluminium alloys are required and warrants efficient joining of the dissimilar alloys. But, efficient joining of dissimilar aluminium alloys by fusion welding techniques is very difficult due to the poor weldability of dissimilar aluminium alloys. It is reported that FSW being a solid state welding technique can avoid all problems associated with fusion welding and is a potential candidate technique for joining difficult to weld materials including dissimilar aluminium alloys. With regard to FSW dissimilar aluminium alloys, the tool axis offset and relative positioning of alloys are also reported to be important. In this article, a comprehensive discussion on the influence of tool axis offset and relative positioning of the materials on the joint performance that reported in the literature is presented. The literature showed that there is uniqueness in the value and sign of the tool axis offset that can result defect free joint with good joint properties. With regard to relative positioning of the materials also, conflicting arguments are reported in the literature.

Avoiding abnormal grain growth in thick 7XXX aluminium alloy friction stir welds during T6 post heat treatments

Abstract Friction stir welding (FSW) is becoming an essential welding technique in the aerospace industry. However, friction stir welds are usually weaker than the parent material. The dissipated heat causes a softening of the 7XXX series aluminium alloys. Post-welding heat treatments are used to restore the highest strength after welding. The reference treatment is composed of two-steps heating: a solution heat treatment typically at 470 °C for 30 min, quench in water followed by artificial aging 120 °C for 24 h. These treatments are responsible for very significant grain coarsening in the weld nugget zone known as secondary recrystallization, sometimes leading to abnormal grain growth (AGG). This is particularly true for thick plates. This secondary recrystallization phenomenon is detrimental for tensile performances. A detailed analysis of the conditions leading to secondary recrystallization during post-welding heat treatments is provided. A procedure to avoid secondary recrystallization in FSWed joints is then highlighted. Thermal cycles during welding are controlled by welding on a low thermal conductivity backing plate (i.e. stainless steel) which avoids very fine grains in the bottom of the weld nugget. This new procedure opens the path to excellent welds presenting at least the strength and ductility of the T6 base material.

Mechanical Behavior of Friction Stir Welded AA-6061 Thick Plates in Different Heat Treatment Conditions

In this study, mechanical properties of friction stir welded Aluminum Alloy (AA) 6061 in three different heat treatment conditions i.e. Annealed (O), Artificially aged (T6) and Post Weld Heat Treated (PWHT) were compared. Plates were welded in a butt joint form. Parameters were optimized and joints were fabricated using tool rotational speed and travel speed of 500 rpm and 350 mm/min respectively. Two sets of plates were welded in O condition and out of which one was, later, subjected to post weld artificial aging treatment. Third set was welded in T6 condition. The welds were characterized by macro and microstructure analysis, microhardness measurement and mechanical testing. SEM fractography of the tensile fracture surfaces was also performed. Comparatively better mechanical properties were achieved in the plate with PWHT condition.

Friction stir welding and processing-a perspective review

The paper reviews the basic principles of friction stir welding, its methodology, and all other technical aspects. As reported by different researchers, the effect of varying welding variables such as tool geometry, tool-tilt angle and tool materials on the microstructure and mechanical properties of the welded materials, and weld quality are reported and discussed. The paper covers the various mechanical and metallurgical properties of similar/dissimilar aluminium alloys and other metal joints welded using friction stir welding (FSW). The attempt has been made to present a comprehensive review of some of the salient works carried out by various researchers pertaining to friction stir processing (FSP) /FSW of aluminium alloys. The paper also identifies the existing research gaps in the area of FSW, which should be explored in future.

Unified Principal S\u2013N Equation for Friction Stir Welding of 5083 and 6061 Aluminum Alloys

With the popularization of friction stir welding (FSW), 5083-H321 and 6061-T6 aluminum alloy materials are widely used during the FSW process. In this study, the fatigue life of friction stir welding with two materials, i.e., 5083-H321 and 6061-T6 aluminum alloy, are studied. Fatigue tests were carried out on the base metal of these two materials as well as on the butt joints and overlapping FSW samples. The principle of the equivalent structural stress method is used to analyze the FSW test data of these two materials. The fatigue resistances of these two materials were compared and a unified principal S–N curve equation was fitted. Two key parameters of the unified principal S–N curve obtained by fitting, Cd is 4222.5, and h is 0.2693. A new method for an FSW fatigue life assessment was developed in this study and can be used to calculate the fatigue life of different welding forms with a single S–N curve. Two main fatigue tests of bending and tension were used to verify the unified principal S–N curve equation. The results show that the fatigue life calculated by the unified mean 50% master S–N curve parameters are the closest to the fatigue test results. The reliability, practicability, and generality of the master S–N curve fitting parameters were verified using the test data. The unified principal S–N curve acquired in this study can not only be used in aluminum alloy materials but can also be applied to other materials.