Nanomechanical properties of friction stir welded AA6082-T6 aluminum alloy

Lightweight alloys are of major concern, due to their functionality and applications in transport and industry applications. Friction stir welding (FSW) is a solid-state welding process for joining aluminum and other metallic alloys and has been employed in aerospace, rail, automotive and marine industries. Compared to the conventional welding techniques, FSW produces joints which do not exhibit defects caused by melting. The objective of the present study is to investigate the surface hardness (H) and elastic modulus (E) in friction stir welded aluminum alloy AA6082-T6. The findings of the present study reveal that the welding process softens the material, since the weld nugget is the region where the most deformations are recorded (dynamic recrystallization, production of an extremely fine, equiaxial structure), confirmed by optical microscopy and reduced nanomechanical properties in the welding zone. A yield-type pop-in occurs upon low loading and represents the start of phase transformation, which is monitored through a gradual slope change of the load-displacement curve. Significant pile-up is recorded during nanoindentation of the alloy through SPM imaging.     

Characterization of friction stir welded boron carbide particulate reinforced AA6061 aluminum alloy stir cast composite

Development of welding procedures to join aluminum matrix composite (AMCs) holds the key to replace conventional aluminum alloys in many applications. In this research work, AA6061/B₄C AMC was produced using stir casting route with the aid of K₂TiF₆ flux. Plates of 6 mm thickness were prepared from the castings and successfully butt joined using friction stir welding (FSW). The FSW was carried out using a tool rotational speed of 1000 rpm, welding speed of 80 mm/min and axial force of 10 kN. A tool made of high carbon high chromium steel with square pin profile was used. The microstructure of the welded joint was characterized using optical and scanning electron microscopy. The welded joint showed the presence of four zones typically observed in FSW of aluminum alloys. The weld zone showed fine grains and homogeneous distribution of B₄C particles. A joint efficiency of 93.4% was realized under the experimental conditions. But, FSW reduced the ductility of the composite.     

Microstructure and mechanical properties of friction spot welded 6061-T4 aluminum alloy

Friction spot welding (FSpW) is a relatively new solid state joining technology developed by GKSS. In the present study, FSpW was applied to join the 6061-T4 aluminum alloy sheet with 2 mm thickness. The microstructure of the weld can be classified into four regions, which are stir zone (SZ), thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ) and the base material (BM), respectively. Meanwhile, defects such as bonding ligament, hook and voids are found in the weld, which are associated to the material flow. The hardness profile of the weld exhibits a W-shaped appearance and the minimum hardness is measured at the boundary of TMAZ and SZ. Both the tensile/shear strength and cross-tension strength reach the maximum of 7117.0 N and 4555.4 N at the welding condition of the rotational speed of 1500 rpm and duration time of 4 s. Compared to cross-tension strength, the tensile/shear strength were stable with the variation of processing parameters. Three different fracture modes are observed under tensile/shear loading, which are plug type fracture, shear fracture and plug-shear fracture. There are also there different fracture modes under cross-tension loading, which are plug type fracture (on the upper sheet), nugget debonding and plug type fracture (on the lower sheet).     

Natural aging in friction stir welded 7136-T76 aluminum alloy

The long term natural aging behavior of friction stir welded aluminum 7136-T76 extrusions was investigated. The microstructural characteristics and mechanical properties in the as-welded, three years naturally aged and six years naturally aged conditions were studied and correlated to a coupled thermal/material flow model of the joining process. Hardness profiles taken along the mid-plane thickness of the workpiece displayed the characteristic W-shape typical to friction stir welded aluminum alloys. In the as-welded condition, however, the profile was skewed to the advancing side, such that the advancing side hardness was lower than that on the retreating side. With natural aging, hardness recovery occurred on both sides of the weld, but the position of the hardness minima, particularly on the advancing side, shifted away from the weld centerline. The numerical simulation demonstrated that the temperature profile is also skewed to the advancing side with greater processing temperatures occurring on this side of the weld. When compared to the dissolution temperature of the equilibrium phases, the extent of dissolution was greater on the advancing side and occurred to a greater distance from the centerline than on the retreating side. The hardness behavior upon natural aging, therefore, correlated to the temperature profile developed during welding and the degree to which phase dissolution occurred in the regions adjacent to the stir zone.     

Microstructure and failure mechanisms of refill friction stir spot welded 7075-T6 aluminum alloy joints

In this paper, the microstructure and mechanical properties of 7075-T6 aluminum alloy joints joined by refill friction stir spot welding (RFSSW) were investigated. The keyhole was refilled successfully, and the microstructure of the weld exhibited variations in the grain sizes in the width and the thickness directions. There existed defects (hook, voids, bonding ligament, etc.) associated to the material flow in the weld. Mechanical properties of the joint have been investigated in terms of hardness and tensile/shear and cross-tension test, and the fracture mechanisms were observed by SEM (scanning electron microscope). The hardness profile of the weld exhibited a W-shaped appearance in the macroscopic level, which reached the minimum at the boundary of the sleeve and the clamping ring. The variation laws between tensile/shear and cross-tension strength and processing parameters were rather complicated. The void in the weld played an important role in determining the strength of the joint. On the whole, the preferable strength can be obtained at lower rotational speed. Shear fracture mode was observed under tensile–shear loadings, and nugget debonding, plug type fracture (on the upper sheet) and plug type fracture (on the lower sheet) modes were observed under cross-tension loadings. It was also observed that the main feature affecting the mechanical properties of the joint is the alclad between the upper and lower sheets and the connecting qualities between the stir zone and thermo-mechanically affected zone.     

Tensile behavior of dissimilar friction stir welded joints of aluminium alloys

The heat treatable aluminium alloy AA2024 is used extensively in the aircraft industry because of its high strength to weight ratio and good ductility. The non-heat treatable aluminium alloy AA5083 possesses medium strength and high ductility and used typically in structural applications, marine, and automotive industries. When compared to fusion welding processes, friction stir welding (FSW) process is an emerging solid state joining process which is best suitable for joining these alloys. The friction stir welding parameters such as tool pin profile, tool rotational speed, welding speed, and tool axial force influence the mechanical properties of the FS welded joints significantly. Dissimilar FS welded joints are fabricated using five different tool pin profiles. Central composite design with four parameters, five levels, and 31 runs is used to conduct the experiments and response surface method (RSM) is employed to develop the model. Mathematical regression models are developed to predict the ultimate tensile strength (UTS) and tensile elongation (TE) of the dissimilar friction stir welded joints of aluminium alloys 2024-T6 and 5083-H321, and they are validated. The effects of the above process parameters and tool pin profile on tensile strength and tensile elongation of dissimilar friction stir welded joints are analysed in detail. Joints fabricated using Tapered Hexagon tool pin profile have the highest tensile strength and tensile elongation, whereas the Straight Cylinder tool pin profile have the lowest tensile strength and tensile elongation. The results are useful to have a better understanding of the effects of process parameters, to fabricate the joints with desired tensile properties, and to automate the FS welding process.     

Microstructure and fatigue performance of friction stir welded joints of 2A12-T4 and 7075-T6 dissimilar aluminum alloy

The objective of this work is to investigate the microstructure and fatigue performance of friction stir welding joint of 2A12-T4 and 7075-T6 dissimilar aluminum alloy. Microstructure shows that the grain growth of heat-affected zone in both sides of joint is noticeable; the grain size of thermo-mechanically affected zone in both sides of joint is deformed and distorted in various degrees; nugget zone is fine equiaxed grain with uniform distribution. Microhardness of joint presents an approximate “W” types distribution on the whole, and the highest hardness value of the joint appears near the center of nugget zone, up to 146.5 HV0.2. Since the joint has the higher the fatigue strength ratio, and the S−N curve of the joint shows a smaller downward trend, the joint has the best fatigue performance and 7075-T6 base metal has the worst fatigue performance. Linear fitting of 7075-T6 base metal is ; linear fitting of 2A12-T4 base metal is ; linear fitting of joint is . The fracture analysis presents that the low-cycle fatigue fracture mechanism of welding joint is quasi-cleavage fracture, and the main mode of high cycle fatigue fracture is the inter crystalline fracture, mixed with ductile fracture.     

Influences of joint geometry on defects and mechanical properties of friction stir welded AA6061-T4 T-joints

In this paper, AA6061-T4 T-joints with three different joint geometries of T-lap/T-butt-lap/T-butt were fabricated successfully by friction stir welding. The distributions and formation mechanisms of defects in friction stir welded (FSWed) T-joints were discussed through macro and micro-observations, respectively. Hardness profiles of the as-welded samples were also measured to evaluate the softening effect during the process. What’s more, influences of joint geometry and the traverse speed on the tensile properties of FSWed T-joints were investigated. All the experimental results indicate that tunnel defects and kissing bond are easily formed and vary significantly in T-joints of the three joint geometries. Defects are moderated to a large extent with decreasing the traverse speed, but the specific relationship to tensile properties is complicated. T-lap joints present the superior tensile properties along the skin direction among the three geometries, the same as T-butt joints along the stringer direction. All the as-welded samples almost fractured in the locations of softening zones and bonding surfaces.     

Ductile damage development in friction stir welded aluminum (AA2024) joints

Ductile damage development in a friction stir welded aluminum joint subjected to tension is analyzed numerically by FE-analysis, based on a total Lagrangian formulation. An elastic-viscoplastic constitutive relation that accounts for nucleation and growth of microvoids is applied. Main focus in the paper is on the interaction between changes in the material parameters in different regions of the weld, the damage development and the position of the final fracture. Especially changes in the yield stress profile transverse to the weldline are examined, since some process parameters have been shown experimentally to affect this. It is found that damage development is highly influenced by changes in the yield stress profile and a shift in final failure is shown for comparable yield stress in the thermo-mechanically affected zone (TMAZ) and the nugget zone (NG).     

Process parameter influence on defects and tensile properties of friction stir welded T-joints on AA6061-T4 sheets

In this study, AA6061-T4 T-joints characterized by combination modes of T-lap and T-butt were fabricated by friction stir welding with different processing parameters. Defects distributed in the obtained specimens were examined by using a stereo microscope and an optical microscope. Process parameter influence on the distribution and the size of original joint line with severe deformation (OJLwSD) defect was investigated. The microstructures and hardness profiles in the T-joints were studied. Influence factors on T-joints’ tensile properties were discussed. In addition, the fracture mode and the fracture surface of the failure samples were observed and discussed as well.     

Microstructure and tensile properties of friction welded aluminum alloy AA7075-T6

Solid-state welding processes like friction welding and friction stir welding are now being actively considered for welding aluminum alloy AA7075. In this work, friction welding of AA7075-T6 rods of 13 mm diameter was investigated with an aim to understand the effects of process parameters on weld microstructure and tensile properties. Welds made with various process parameter combinations (incorporating Taguchi methods) were subjected to tensile tests. Microstructural studies and hardness tests were also conducted. The results show that sound joints in AA7075-T6 can be achieved using friction welding, with a joint efficiency of 89% in as-welded condition with careful selection of process parameters. The effects of process parameters are discussed in detail based on microstructural observations.     

Effect of friction stir welding process parameters on the tensile strength of dissimilar aluminum alloy AA2024-T3 and AA7075-T6 joints

This work investigates the influence of friction stir welding parameters on the mechanical properties of the dissimilar joint between AA2024-T3 and AA7075-T6. Experiments are conducted consistent with the three-level face-centered composite design. Response surface methodology is used to develop the regression model for predicting the tensile strength of the joints. The analysis of variance technique is used to access the adequacy of the developed model. The model is used to study the effect of key operating process parameters namely, tool rotation speed, welding speed and shoulder diameter on the tensile strength of the joints. The results indicate that friction stir welding of aluminum alloys at a tool rotation speed of 1050 min⁻¹, welding speed of 40 mm/min and a shoulder diameter of 17.5 mm would produce defect less joint with high tensile strength.     

EBSD study on microstructure and texture of friction stir welded AA5052-O and AA6061-T6 dissimilar joint

Friction stir welded AA5052-O and AA6061-T6 dissimilar joint has a more obvious impact on microstructure and texture evolution compared to single material welding due to differences in physical and chemical parameters between two aluminum alloys. Microstructure, texture evolution and grain structure of AA5052-O and AA6061-T6 dissimilar joint were investigated by means of OM,EBSD and TEM measurements. Experimental results showed that FS weld was generalized in four regions–nugget zone (NZ),thermomechanically affected zone (TMAZ),heat affected zone (HAZ) and base metals (BM), using standard nomenclatures. NZ exhibited the complex structure of the two materials with flowing shape and mainly composed of the advancing side material Subgrain boundaries in weld nugget zone gradually transformed into high angle grain boundaries by absorbing dislocation and accumulating misorientations. Grain refinement of weld nugget zone was achieved by dynamic recrystallization. In the friction stir welding process, the presence of the shear deformation in weld made {001} < 100 > C cube texture, {123} < 634 > S texture in BM gradually transformed into {111} < 1(−)12(−) > A1¹ shear texture. HABs distribution were most significant in nugget followed by RS and then by AS. In TMAZ and NZ, numerous precipitates and lots of dislocations were observed.     

Effect of welding parameters on microstructure and mechanical properties of friction stir welded joints of AA7039 aluminum alloy

A high strength Al–Zn–Mg alloy AA7039 was friction stir welded by varying welding and rotary speed of the tool in order to investigate the effect of varying welding parameters on microstructure and mechanical properties. The friction stir welding (FSW) process parameters have great influence on heat input per unit length of weld, hence on temperature profile which in turn governs the microstructure and mechanical properties of welded joints. There exits an optimum combination of welding and rotary speed to produce a sound and defect free joint with microstructure that yields maximum mechanical properties. The mechanical properties increase with decreasing welding speed/ increasing rotary speed i.e. with increasing heat input per unit length of welded joint. The high heat input joints fractured from heat affected zone (HAZ) adjacent to thermo-mechanically affected zone (TMAZ) on advancing side while low heat input joints fractured from weld nugget along zigzag line on advancing side.     

Numerical simulation and experimental investigation on friction stir welding of 6061-T6 aluminum alloy

A modified three-dimensional model was established to simulate the friction stir welding of the 6061-T6 aluminum alloy. A detailed calculating method of the heat generation was proposed by taking account of the contact conditions between the tool and the work-piece. The results show that the heat mainly generated within the region close to the shoulder, the high temperature exists within the upper portion of the weld and decreases along the thickness direction. The strong material flow mainly occurs within the region around the tool and the material ahead of the tool sweeps toward the RS and finally deposits behind the tool. During this procedure the material is extruded to experience different shear orientations, and a defect-prone region exists in the region where material flow is weak. The temperature field and material flow behaviors predicted by the simulation method are in good agreement with the results obtained by the experiments.     

Microstructural evolution in friction self-piercing riveted aluminum alloy AA7075-T6 joints

Friction self-piercing riveting(F-SPR)is an emerging technique for low ductility materials joining,which creates a mechanical and solid-state hybrid joint with a semi-hollow rivet.The severe plastic deforma-tion of work materials and localized elevated temperatures during the F-SPR process yield complex and heterogeneous microstructures.The cut-off action of the work materials by the rivet further compli-cates the material flow during joint formation.This study employed the F-SPR process to join AA7075-T6 aluminum alloy sheets and systematically investigated the microstructural evolutions using electron backscatter diffraction(EBSD)techniques.The results suggested that as the base material approached the rivet,grains were deformed and recrystallized,forming two distinct fine grain zones(FGZs)surround-ing the rivet and in the rivet cavity,respectively.Solid-state bonding of aluminum sheets occurred in the FGZs.The formation of FGZ outside the rivet is due to dynamic recrystallization(DRX)triggered by the sliding-to-sticking transition at the rivet/sheet interface.The FGZ in the rivet cavity was caused by the rotation of the trapped aluminum,which created a sticking affected zone at the trapped aluminum/lower sheet interface and led to DRX.Strain rate gradient in the trapped aluminum drove the further expansion of the sticking affected zone and resulted in grain refinement in a larger span.     

Effect of welding parameters on microstructure and mechanical properties of AA6061-T6 butt welded joints by stationary shoulder friction stir welding

Stationary shoulder friction stir welding (SSFSW) butt welded joints were fabricated successfully for AA6061-T6 sheets with 5.0 mm thickness. The welding experiments were performed using 750–1500 rpm tool rotation speeds and 100–300 mm/min welding speeds. The effects of welding parameters on microstructure and mechanical properties for the obtained welds were discussed and analyzed in detail. It is verified that the defect-free SSFSW welds with fine and smooth surface were obtained for all the selected welding parameters, and the weld transverse sections are obviously different from that of conventional FSW joint. The SSFSW nugget zone (NZ) has “bowl-like” shapes with fairly narrow thermal mechanically affected zone (TMAZ) and heat affected zone (HAZ) and the microstructures of weld region are rather symmetrical and homogeneous. The 750–1500 rpm rotation speeds apparently increase the widths of NZ, TMAZ and HAZ, while the influences of 100–300 mm/min welding speeds on their widths are weak. The softening regions with the average hardness equivalent 60% of the base metal are produced on both advancing side and retreating side. The tensile properties of AA6061-T6 SSFSW joints are almost unaffected by the 750–1500 rpm rotation speeds for given 100 mm/min, while the changing of welding speed from 100–300 mm/min for given 1500 rpm obviously increased the tensile strength of the joint and the maximum value for welding parameter 1500 rpm and 300 mm/min reached 77.3% of the base metal strength. The tensile fracture sites always locate in HAZ either on the advancing side or retreating side of the joints.     

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.     

Analysis and modelling of fatigue failure of friction stir welded aluminum alloy single-lap joints

The fatigue strength of 2024-T3 aluminum alloy friction stir welded single-lap joints is determined by the presence of two crack-like unwelded zones at the overlap ends. Besides, mixed mode I/II conditions are present at the crack tip as the joint is prevalently loaded in shear. In this work, a finite element analysis is performed to predict the crack path and the stress intensity factor at the crack tip, then the lifetime is estimated by examining the crack propagation behaviour of the base material using the AFgrow software. The effect of an initial overload is also considered. The crack path is investigated optically and using the “fatigue failure mechanism map”.