Friction Stir Welded Aluminum Alloy Research Articles

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.

A review on the effects of shoulder diameter to pin diameter (D/d) ratio on friction stir welded aluminium alloys

Friction stir welding is a predominant procedure to bind numerous materials. An optimal parameter is vital to create a better and defect-free weld. The thermomechanical joining methods have a unique advantage, and it needs an appropriate key variable for better joining. Though many parameters control the process, the D/d ratio and rotational speed are the key factors. An aluminium alloy with a similar and dissimilar joint was considered. The association of the D/d ratio of a straight cylindrical pin with rotational speed and welding speed on an aluminium alloy butt joint was studied. In addition to various parameters, the D/d ratio immensely impacts mechanical strength and grain refinement. A shoulder diameter to pin diameter ratio of three significantly increases the tensile strength, joint efficiency, and elongation.

The effect of heat generated on mechanical properties of friction stir welded aluminum alloys

The aim of this work is to analyze the effect of the heat generated within the joints during the friction stir welding process. Four different aluminum alloys were considered, i.e., three precipitation-hardening alloys (AA2024-T3, AA6082-T6, and AA7075-T6) and a work-hardening alloy (AA5754-H111). In order to study the cooling effect, the welds were performed with and without a water-cooling system. The temperatures reached during the FSW process were measured by three type-K thermocouples positioned near the nugget, in the thermo-mechanically affected zone and in the heat-affected zone. The results suggest a dependence of the thermal gradient on both the process parameters used and the alloy considered. From the mechanical point of view, the Rockwell B hardness tests and tensile tests were performed. It was possible to evidence a relationship between the hardness distribution and the cooling systems. In fact, the water-cooled joints showed higher values of hardness reached in the thermo-mechanically affected zone and in the heat-affected zone for all the precipitation-hardening-tested alloys. The tensile tests executed orthogonally to the welding direction showed, for all the alloys, a reduction of the real elongation percentage in water-cooled joints; this can be related to the hardness increase that implies a decrease in the elongation of these welds.

Investigation of an early stage of the abnormal grain growth in the friction-stir welded 6061-T6 aluminum alloy

In this work, quasi in-situ electron backscatter diffraction (EBSD) was applied to study an early stage of the abnormal grain growth occurring in the friction-stir welded 6061 aluminium alloy during post-weld solution heat treatment. It was found that the catastrophic grain growth completed during the heating stage of the annealing treatment, and further static storage at the solutionsizing temperature provided almost no microstructural changes.

A Study on Friction Stir Welding of Extruded Aluminum Alloy with Fillet Joint

A Study on Friction Stir Welding of Extruded Aluminum Alloy with Fillet Joint

Effect of Cavitation Peening on Fatigue Properties in Friction Stir Welded Aluminum Alloy AA5754

Friction stir welding (FSW) is an attractive solid-state joining technique for lightweight metals; however, fatigue properties of FSWed metals are lower than those of bulk metals. A novel mechanical surface treatment using cavitation impact, i.e., cavitation peening, can improve fatigue life and strength by introducing compressive residual stress into the FSWed part. To demonstrate the enhancement of fatigue properties of FSWed metal sheet by cavitation peening, aluminum alloy AA5754 sheet jointed by FSW was treated by cavitation peening using cavitating jet in air and water and tested by a plane bending fatigue test. The surface residual stress of the FSWed part was also evaluated by an X-ray diffraction method. It was concluded that the fatigue life and strength of FSWed specimen were improved by cavitation peening. Whereas the fatigue life at σa = 150 MPa of FSWed specimen was about 1/20 of the bulk sheet, cavitation peening was able to extend the fatigue life of the non-peened FSW specimen by 3.6 times by introducing compressive residual stress into the FSWed part. This is the first paper to demonstrate the improvement of fatigue properties of FSWed metallic sheet by cavitation peening.

Mechanical Properties and Microstructural Characterization of Dissimilar Friction Stir Welded AA5083 and AA6061 Aluminium Alloys

Solidification is one of the major issues that was faced during the fusion welding of dissimilar non-heat treatable and heat treatable aluminium alloys. To overcome this issue Friction Stir Welding played a very vital role, since it is a solid state welding process. In the current study, dissimilar friction stir welding was carried out between non heat-treatable aluminium alloy AA5083-H111 and heat-treatable aluminium alloy AA6061-T6. The microstructural analysis and the mechanical properties of the dissimilar friction stir welded aluminium alloy AA5083-H111 and AA6061-T6 have been investigated. Both optical microscopy and scanning electron microscopy was used to evaluate the microstructural features. The elemental analysis was carried out using SEM-EDX. The tensile properties are studied using Universal Testing Machine. Hardness at various zones of the welded joints was measured using Vicker’s Hardness Testing Machine. The mechanical properties of the friction stir welded joints were correlated with the microstructure of the dissimilar welded joints.

Probing the Mechanism of Friction Stir Welding with ALE Based Finite Element Simulations and Its Application to Strength Prediction of Welded Aluminum

In this study, a simulation-based examination on the deformation mechanism in the friction stir welding (FSW) process is conducted, which may not be efficiently feasible by experiment due to severe deformation and rotation of material flow near a tool pin. To overcome the severity of distortion of plastically deforming finite element meshes in the Lagrange formulation, and an over-simplified elastic-plasticity constitutive law and contact assumption in the Eulerian formulation, the arbitrary Lagrangian–Eulerian (ALE) formulation is employed for the finite element simulations. Superior accuracy in predicting the temperature profiles and distributions of the friction stir welded aluminum alloy workpiece could be obtained compared to the results of Eulerian based simulations. In particular, the ALE based simulations could predict the sharper gradient of temperature decrease as the distance from the welding zone increases, while the Eulerian based model gives more uniform profiles. The second objective of the study is to investigate the coupling of simulation-based temperature histories into the strength prediction model, which is formulated on the basis of precipitation kinetics and precipitate-dislocation interaction. The calculated yield strength distribution is also in better agreement with experiment than that by the Eulerian based model. Finally, the mechanism of the FSW process is studied by thoroughly examining the frictional and material flow behavior of the aluminum alloy in the welded zone. It is suggested that the initially high rate of temperature increase is attributed to frictional heat due to slipping of material on the tool surface, and the subsequent saturated temperature is the result of sequential repetitive activations of the sticking and slipping modes of the softened material. The sticking mode is the main source of plastically dissipated heat by the large plastic deformation around the rotating tool pin. The present integrated finite element simulation and microstructure-based strength prediction model may provide an efficient tool for the design of the FSW process.

Optimization of friction stir welding parameters using response surface methodology

The aim of this article is to create a new technique for predicting discontinuity formation, its place and magnitude during aluminium alloy (AA6061) friction stir welding (FSW). The effectiveness of the technique was demonstrated using visual inspection, hardness and tensile test of the friction stir welded joints. The measured current was analysed through power calculations. In each of the FSW stages, the energy consumption is significantly varied, clearly distinguishing the penetration of the tool, its revolution, its traverse movement and its metal removal rate. The findings of tracking the energy consumption indicate that using power consumption means the significance of weld quality. FSW has been carried out based on two factors - two levels. Response surface methodology (RSM) is employed to develop a mathematical model. Analysis of variance (ANOVA) technique checks the adequacy of the developed mathematical model, which is used effectively at 95% confidence level. In contrast, tensile and hardness tests also showed that welds at high power usage failed continuously within the welding area, due to reduced welding temperature and absence of penetration in the welding zone.

Influence of Exposure to Environment on Degradation of a Friction Stir Welded Aluminum Alloy

We carried out a comparative study on both the stress corrosion response and corrosion damage characteristics of aluminum alloy 2219, both the base material and a friction stir welding (FSW) counterpart upon exposure to exfoliation corrosion (EXCO) solution. The results reveal that the test specimen containing an FSW joint reveals better electrochemical corrosion resistance than that taken from the base metal. When test specimens upon exposure to EXCO solution are concurrently subjected to a tensile stress, since the mechanical properties of the FSW joint are lower than the base metal, a test specimen containing an FSW joint is more easily prone to the early initiation of fine microscopic cracks. This makes the test specimen containing the FSW joint to be less resistant to stress corrosion damage than that taken from the base metal for the various levels of applied stress and exposure time to EXCO solution. The average corrosion depth of the test specimen containing the FSW joint is less than that of the base metal, while the maximum corrosion depth of it is greater than that of the base metal. This reveals that test specimen containing the FSW joint is more susceptible to damage and degradation than test specimen taken from the base metal.

Effect of welding speed on the mechanical properties of friction stir welded aluminium alloy 5083

Effect of welding speed on the mechanical properties of friction stir welded aluminium alloy 5083

Study on corrosion fatigue properties and fracture characteristics of 7075 aluminum alloy FSW joint

Taking 7075-T6 aluminum alloy friction stir welding joint as the research object, the microstructure, 3.5 wt % NaCl solution corrosion fatigue life and corrosion fatigue fracture characteristics were studied, and the corrosion fatigue performance and fracture process of 7075 aluminum alloy friction stir welding joint were analyzed. The results showed that: the S-N curve equation of corrosion fatigue of 7075-T6 aluminum alloy friction stir welded joint is lgN=5.845-0.014S, With the increasing of stress amplitude, the corrosion fatigue life decreased greatly. The corrosion fatigue crack originated in the thermal affected zone of the joint, gradually expanded and finally broke in the welded core zone of the joint. There were multiple crack sources in the corrosion fatigue fracture, and with the influence of stress concentration, the crack source originated in the corrosion pit. The high stress aggravated the corrosion damage of the corner part of the sample, led to more serious corrosion than that of the plane position. Obvious intergranular fracture and fatigue striation appeared in the crack growth zone. Under the combined action of corrosive medium and alternating load, the crack growth zone suffered the most serious corrosion, and the anodic dissolution occurred at the grain boundary, resulting in the morphology characteristics of “rock candy” and “ant nest”. Instantaneous fault zone was brittle fault,and there were a lot of cleavage steps and secondary cracks in this zone, the pore morphology appeared in the second phase particle distribution region.