Dissimilar Friction Stir Welding Joints Research Articles

Microstructure and cryogenic mechanical properties of dissimilar friction stir welding joints between nitrogen-alloyed CoCrFeMnNi high-entropy alloy and high-manganese austenite steel

The microstructures and cryogenic mechanical properties of dissimilar friction stir welding (FSW) joints between nitrogen-alloyed CoCrFeMnNi high-entropy alloys (HEAs) and Fe–32.1Mn–7.5Cr–0.6Mo–1.2 N steel were investigated. The results reveal that defect-free dissimilar joints can be achieved through FSW. Furthermore, the grains of nitrogen-alloyed CoCrFeMnNi HEAs in the stir zone of the dissimilar joint are significantly more refined than those of Fe–32.1Mn–7.5Cr–0.6Mo–1.2 N steel. Joint efficiency at room and low temperature both exceed 90% of the base metal. Moreover, the cryogenic yield and ultimate strength of the dissimilar joints are higher than those recorded at room temperature. The fracture position is at the heat-affected zone of HEAs under two temperature conditions.

Grain Structure and Texture Evolution in the Bottom Zone of Dissimilar Friction-Stir-Welded AA2024-T351 and AA7075-T651 Joints.

High-strength dissimilar aluminum alloys are difficult to connect by fusion welding, while they can be successfully joined by friction stir welding (FSW). However, the asymmetrical deformation and heat input that occur during FSW result in the formation of a heterogeneous microstructure in their welded zone. In this work, the grain structure and texture evolution in the bottom zones of dissimilar FSW AA2024-T351 and AA7075-T651 joints at different welding speeds (feeding speeds) were quantitatively investigated. The results indicated that dynamic recrystallization occurs in the bottom zones of dissimilar FSW joints, and equiaxed grains with low grain sizes are formed at the welding speed of 60-240 mm/min. A high fraction of the recrystallized grains were generated in the bottom zones of the joints at a low welding speed, while a high fraction of the substructured grains are produced at a high welding speed. Different types of shear textures are produced in the bottom zones of the joints; the number fraction of shear texture types depends on different welding speeds. This study helps to understand the mechanism of microstructure homogenization in dissimilar FSW joints and provides a basis for further improving the microstructure of the welded zone for engineering applications.

Multi-phase modelling of heat and mass transfer during Ti/Al dissimilar friction stir welding process

Friction stir welding (FSW) has the capacity to join the Al/Ti dissimilar structures with superior mechanical properties. The microstructures and mechanical characteristics of Al/Ti dissimilar FSW joints are determined by the heat and mass transfer during the welding process. However, a quantitative study of the Al/Ti dissimilar FSW process is lacking. Therefore, using the computational fluid dynamics (CFD) and volume of fluid (VOF) approach, a multi-phase model was constructed for quantitatively analyzing the heat and mass transfer behaviour in dissimilar FSW of TC4 titanium alloy and AA2024-T4 aluminium alloy. The mixed material was treated as a functionally graded material (FGM) to compute the thermophysical characteristics at the weld nugget zone (WNZ). Due to the vast disparity in the thermophysical characteristics of aluminium and titanium alloy, the temperature field in Al/Ti dissimilar FSW was severely asymmetric. The temperature of titanium alloy on the advancing side (AS) was higher than that of aluminium alloy on the retreating side (RS) at the same distance from the tool centre line near the tool shoulder, but it was lower than that of aluminium alloy on the RS without the influence of the shoulder. Due to the high flow stress of titanium alloy, plastic material flow mostly occurred on the RS of aluminium alloy in the Al/Ti dissimilar FSW, with its percentage exceeding 80 %. This model was validated by experiment results.

Fatigue life improvement of similar and dissimilar aluminum friction stir welds by deep rolling

At the moment, high production costs prevent the friction stir welding (FSW) process from further industrial applications even if comparable high fatigue strength of the joints can be reached. Higher welding speed may reduce the production costs but decrease the fatigue strength of the manufactured FSW joints. A potential solution is a mechanical post weld treatment directly after welding to increase the fatigue strength of the FSW joint again. In this study, hydrostatic deep rolling was applied for fatigue strength improvement of similar and dissimilar FSW joints made of EN AW 5083 and EN AW 6082 alloys manufactured with different welding speed. Additionally, the fatigue strength was directly compared to conventional joints manufactured by metal inert gas (MIG) welding and to base material specimen made of EN AW 5083. The surface state in as-welded condition was characterized by surface roughness and residual stress measurements. Fatigue tests were performed to quantify the fatigue strength of the FSW joints. Similar compressive residual stresses were determined after deep rolling for similar and dissimilar joints. No fatigue life improvement was determined for deep-rolled similar joints made of EN AW 5083. However, in this condition, the fatigue life of the specimen was within the range of the base material. Thus, significant lower fatigue life and a high fatigue life improvement by deep rolling were reached for dissimilar joints. An increase of the welding speed from 300 to 800 mm/min strongly decreased the fatigue strength of dissimilar welded joints in the investigated case.

Dissimilar Friction Stir Welding of AA2519 and AA5182.

In this study, the friction-stir welding (FSW) technique was successfully applied for joining of AA2519 to AA5181 alloy. Microstructure and mechanical properties of dissimilar FSW joints were investigated by optical microscopy, microhardness, and tensile testing. The deformation behaviour of the welded joints was elucidated via the digital image correlation technique. After welding, the ultimate tensile strength of joints was ~300 MPa and ductility was ~16%. The microhardness values observed at the stir zone were higher than those in the base material AA5182. The produced welds demonstrate nearly 100% (based on AA5182) joint efficiency.

Characterizing the anisotropic response of similar and dissimilar FSW joints by DIC-based simultaneous strain measurements on two orthogonal surfaces

In friction stir welding (FSW), various zones with microstructural heterogeneity are formed with a difference in local constitute properties. To define anisotropy ratio through strain measurement in thickness and width direction simultaneously, utilizing a standard contact-type extensometer is challenging, or assuming volume constancy to calculate strain in the thickness direction may not be applicable for FSW joints. The present work proposes an approach where strain is measured concurrently on two mutually perpendicular (width and thickness) surfaces of a deforming tensile specimen by digital image correlation. Strain localization and local anisotropy ratio are calculated over the specimen gauge length, and the average anisotropy ratio is specified for the similar and dissimilar FSW joints of AA6061 and AA7075. For AA6061 similar FSW joints, tensile specimens with longitudinal axis along (θ = 0°) or close to weld line that contains solely stir zone (SZ) material and/or minimal presence heat affected zone (HAZ) exhibit uniform strain accumulation and failure from SZ. However, beyond that, the presence and orientation of HAZ influence the strain localization, anisotropy and subsequent failure. For dissimilar AA6061-AA7075 FSW joints, the anisotropic nature is largely governed by the strain localization at the softer material rather than the orientation of various zones within the specimen gauge length.

Prediction of Filler Added Friction Stir Welding Parameters for Improving Corrosion Resistance of Dissimilar Aluminium Alloys 5052 and 6082 Joints

Abstract The aluminium alloys 5052 and 6082 are extensively used in manufacturing lighter structural members, having improved strength and resistance to corrosion. Magnesium (Mg) and Chromium (Cr) powder were the filler materials selected for enhanced corrosion protection properties in this investigation. Friction stir welding (FSW) process parameters viz., spindle speed, welding speed, shoulder penetration, the centre distance between the holes and filler ratio are used to forecast the minimum corrosion rate from different weld regions of AA5052-AA6082 dissimilar joints. Response surface methodology based on a central composite design was used to evolve the mathematical models and estimate dissimilar FSW joints’ corrosion rates. Response optimization shows that the minimum corrosion rate was achieved by the welding parameters of spindle speed 1000 rev/min, welding speed 125 mm/min, holes spacing of 2 mm and filler ratio 95% of Mg and 5% of Cr.

Impact behaviour of dissimilar AA2024-T351/7075-T651 FSWed butt-joints: effects of Al2O3-SiC particles addition

Dissimilar friction stir welding joints are widely employed in the industrial field due to the excellent microstructural and mechanical properties of the resulting joints. Nevertheless, to further enhance the weld properties, the addition of reinforcement particles on the joint-line during the process has been proven effective for increasing its mechanical performance. In the present investigation, the microstructure and the impact behaviour of FSWed joints between AA2024-T351 and AA7075-T651 aluminium plates were investigated, considering the effect of different process parameters selected through a full factorial 2k design of experiments: both the rotational and translational speed of the tool, as well as the addition of Al2O3-SiC microparticles, were considered as input parameters. Unnotched 10 x 5 x 55 mm impact specimens were tested through an instrumented 50 J Charpy pendulum: total impact energy, the two complementary initiation and propagation energies as well as the peak force were correlated to the adopted process parameters. From the performed analyses, it was found that joints with reinforcing particles are prone to form wormhole defects across the stir zone that not only affect the microstructural development, but also the impact behaviour since they require less energy at break in comparison with joints fabricated without particles addition.

Effect of Ni interlayer on microstructure and mechanical properties of Al/Mg dissimilar friction stir welding joints

In this work, friction stir welding (FSW) was carried out for the flat butt welding tests of Al/Mg dissimilar alloys by introducing Ni foil interlayer with different thickness. The effect of the thickness of interlayer on microstructure and mechanical properties of the FSW joints was explored. The results revealed that with the increase of the thickness of Ni, the effective bearing area of the weld nugget zone (WNZ) increased. It achieved the maximum value when the thickness of interlayer was 0.3 mm, and a dense ‘onion rings’ structure which was composed of the cross distributions trips of magnesium alloy, aluminium alloy and the intermetallic compounds (IMCs), as well as the Ni particles of varying sizes were formed. Compared to the joint without introducing Ni, the introduction of Ni made the joints generate Mg2Ni and the Al3Mg2 was partially replaced and led to the ultimate tensile strength (UTS) increased by 13.5%.

Microstructure and texture correlation of secondary heating assisted dissimilar friction stir welds of aluminum alloys

Present work aims to study the influence of secondary heating on material behavior during the friction stir welding (FSW) and examine it through the texture analysis. In the present context, the secondary heating pertains to the additional heat provided using a gas torch other than the heat primarily produced due to the process itself. Stop tool action technique is used to appraise the material flow around the pin. The material flow and evolution of textures during the process is studied by employing electron backscatter diffraction analysis. For analysis, the transverse sections of dissimilar FSW joints of AA6061-AA7075 and AA7075-AA2014 prepared with and without the application of secondary heating keeping the tool pin embedded in the joint are used. The basic assertions on strain rate based upon texture analysis and grain boundary engineering are used to explain the nature of material behavior during the process. The misorientation angle distribution is observed to shift towards random distribution for severe plastic deformation and additional heat supplied. Texture analysis reveals dominance of B/B‾ shear texture with sufficient C component in joints supplied with additional heating. For AA7075-AA2014 (without secondary heating) strong α-fiber is observed in the texture orientation distribution function plots. An equation is established to estimate the material volume flow around the pin in one revolution. The optical micrographs of welded joints prepared with and without use of secondary heating are compared to visualize the increase in material flow volume. The hardness of the joints obtained tend to decrease along the weld section on application of additional heat due to the dissolution of precipitates.

Review on dissimilar structures joints failure

Fuel economy has become, more than ever, one of the central keys around which the industrial development of transportation means (mainly aeronautics and aerospace) orbits around. In order to improve fuel consumption efficiency, lightweight and multipurpose dissimilar structures are pushing the boundaries of innovation through generative design, joining of dissimilar materials through innovative technologies and advanced manufacturing systems, such as addictive manufacturing. Friction Stir Welding (FSW) and Laser Beam Welding and their hybrid technologies represent some of the solid-state welding technologies that play significant roles in what concerns to join dissimilar materials such as Steels, Aluminum and Titanium alloys as well as high performance thermoplastics like Polyether Ether Ketone (PEEK) and Polyphenylene Sulphide (PPS). This article makes a review on the failure modes associated to dissimilar FSW joints, through a comprehensive analysis and enumeration of the different cases that are mentioned in the literature. Initially, it will be reviewed the state of the art related to the technological processes and materials categorization, as well as a summary of several papers and the respective reported failure modes, organized by static strength and defects, fatigue behavior and crack growth, and corrosion proneness and corrosion cracking.

Acoustic effect on the tensile properties and metallurgical structures of dissimilar friction stir welding joints of Al/Mg alloys

Abstract The joint strength of friction stir welded Al/Mg alloys is a critical factor for the wide applications of dissimilar joints. In this study, dissimilar Al/Mg alloys joints were made by friction stir welding (FSW) and its variant, ultrasonic vibration enhanced FSW (UVeFSW), respectively. The tensile strength, fracture toughness and fracture location of the whole joints and three (upper, middle, lower) parts of the joints were compared by characterizing the macro-, mesoscopic- and micro-structures of the welds. The influence of ultrasonic vibration on the tensile properties of the joints was discussed. The results showed that the lower part of the welds was the weakest part due to the poor materials mixing/interlocking and a large amount of intermetallic compounds, while the upper part of the weld had the best strength. Good material mixing/mechanical interlocking in the welds was the basis for obtaining higher joint strength, and the thinner intermetallic compound layer was the key to the better toughness of the joint. The application of ultrasonic vibration can improve the tensile strength of each part of the welds, but the improvement rate of tensile strength is greater for the middle and lower parts of the welds.

Improvement in tensile strength of Mg/Al alloy dissimilar friction stir welding joints by reducing intermetallic compounds

There has been an increasing demand for joining Al to Mg in transportation vehicles to reduce weight further. However, it is difficult to manufacture high performance Mg/Al dissimilar joints due to the formation of massive intermetallic compounds (IMCs) during welding. In the present study, AZ31B Mg and 6061 Al were friction stir welded with a high joint efficiency by reducing the IMCs formation. Microstructural observations were conducted by using optical microscopy (OM), scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS), and the results showed that the content of IMCs reduced from about 31.79% to 7.8%. Mechanical test results showed the tensile strength of joint increase from 92 MPa to 168 MPa. The findings provide an alternative to improve Mg/Al dissimilar joint strength.

Characterizations of microstructure, crystallographic texture and mechanical properties of dissimilar friction stir welding joints for AA2024 and AA7075 under different tool shoulder end profiles

In this investigation, the influence of tool shoulder end profiles (concentric circle shoulder-CCS and three-helix shoulder-THS) and rotational speed (ROSP, 800, 1100, 1400 rpm) on microstructure and mechanical performance of the dissimilar FSW AA2024/7075 joints was analyzed in detail. The results indicated that the width of thermos-mechanical affected zone and the average grain size of nugget zone (NZ) in the CCS joints are lower than those in the THS joints, and they all increase with increasing the ROSP. There is a difference on the shear texture types generated in the different positions of the NZ in the CCS and THS joints. Tensile properties of the joints fabricated by employing the CCS tool are excellent compared to the THS tool. The maximum welding efficiency about 90.8 % can be obtained in the CCS joint at 1000 rpm. The fracture mode of the CCS and THS joints is ductile and brittle failure, respectively.

Post corrosion tensile strength and failure of dissimilar friction stir welded aluminium alloys

The present study efforts towards appraising the effects of corrosion on the tensile and fracture behaviour of dissimilar friction stir welding (FSW) of aluminium alloys. Three different dissimilar FSW joints obtained between AA6061-T6 and AA7075-T651, AA6061-T6 and AA2014-T6, AA7075-T651 and AA2014-T6, using threaded pin profile with three flat faces (TIF) tool at rotational speed of 1200 rpm and welding speed of 98 mm/min. The maximum joint tensile strength was achieved for AA7075-AA2014 joints followed by AA6061-AA2014 and least recorded for AA6061-AA7075 for as obtained FSW joints (non-corroded). The joints are further immersed into a corrosive solution for 1, 2, 7 and 14 days duration. The corrosion occurred all over the joint but much accelerated rate of exfoliation corrosion exists away from stir zone near the confluence of heat affected zone and base material irrespective of the advancing or retreating side. With increase in corrosion time the location of tensile failure shifted towards corroded region (AA6061-T6) instead of stir zone in dissimilar weld joint AA6061-AA2014, whereas it remained unchanged for other two joints. The fractured surfaces of AA6061-AA2014 FSW joints reveals the articulated view of pits and fracture morphology advocating the loss in YS, UTS and % elongation with increases in immersion duration.

Effect of intermetallic compounds on the fracture behavior of dissimilar friction stir welding joints of Mg and Al alloys

Joining Mg to Al is challenging because of the deterioration of mechanical properties caused by the formation of intermetallic compounds (IMCs) at the Mg/Al interface. This study aims to improve the mechanical properties of welded samples by preventing the fracture location at the Mg/Al interface. Friction stir welding was performed to join Mg to Al at different rotational and travel speeds. The micro structure of the welded samples showed the IMCs layers containing Al12Mg17 (γ) and Al3Mg2 (β) at the welding zone with a thickness (> 3.5 <m). Mechanical properties were mainly affected by the thickness of the IMCs, which was governed by welding parameters. The highest tensile strength was obtained at 600 r/min and 40 mm/rnin with a welding efficiency of 80%. The specimens could fracture along the boundary at the thermo-mechanically affected zone in the Mg side of the welded joint.

Experimental Study on Tensile and Bending Behavior of Friction Stir Welded Butt Joints of Dissimilar Aluminum Alloys

Abstract This paper describes the feasibility of dissimilar friction stir welding (DFSW) of two different aluminum alloys; AA6101- T6 and AA6351-T6 of 6 mm plate thickness. The influence of process parameters like tool rotational speed, welding speed and axially downward force on the tensile and bending behavior of the welded plates has been studied. Weld joints were produced using EN32 tool steel with a cylindrical pin tool having 6 mm and 18 mm diameter of pin and shoulder respectively.For all the FSW joints, a constant welding speed 60mm/min was maintained andtool rotational speeds vary as of 900rpm, 1100rpm, 1300rpm and 1500 rpm respectively. From X-ray radiographs, it has been observed that joints welded at theselected process parameters gave good quality and no defects.XRD analyses were done to find out thepresence of mixing intermetallic compound in the FSWjoints. Transverse tensile test (as per ASTM E8 M) were performed to study the tensile strength of DFSW joints. The bending behaviors of weld joints have been conducted by three point bend test method experimentally using Universal Testing Machine.It is observed that good correlation exists between these two mechanical properties and rotational speeds. The defect free FSW butt joint produced at rotational speed 1300rpm gives the better mechanical propertiescompared to other joints.The ultimate tensile strength obtained at 1300rpm rotational speed is very close to the tensile strength of the base metals.

Optimization of Tensile and Corrosion Properties of Dissimilar Friction Stir Welded AA2024-7075 Joints

In this work, we performed some joining experiments of dissimilar aluminum alloys AA2024-T351 and AA7075-T651 using friction stir welding (FSW) technique with different process parameters (rotational speed, welding speed and plunge depth). Response surface methodology based on a central composite rotatable design was used to establish the mathematical models predicting the tensile properties and corrosion rate of dissimilar FSW joints with some high confidence level. Analysis of variance method was employed to verify the developed models. Besides, the influences of FSW process parameters on tensile and corrosion properties of joints were analyzed. In order to obtain a desirable combination property of the dissimilar FSW joints, the optimum process parameters after experimental verification were proposed as: rotational speed 1495 rev/min, welding speed 187 mm/min and plunge depth 0.03 mm. This verification illustrated that the developed models are appropriate for the modeling and optimization of process.

Joint strength and failure studies of dissimilar AA6061-AA7075 friction stir welds: Effects of tool pin, process parameters and preheating

Abstract The present study aims at bracketing the effect of tool pin geometry, process parameters e.g. tool rotational speed, feed rate, and use of additional heat source (preheating) during dissimilar friction stir welding (FSW) between 6061-T6 and 7075-T651 aluminum alloys. Experimental designs following Taguchi orthogonal array and subsequent statistical analysis are carried out to assimilate the influence on joint tensile strength and flexural load. Detailed micrographic observations of the weld cross-section, joint failure studies are performed in addition to the fractographic analysis. Tool pin profiles and tool rotational speed are observed most prevailing factors and when suitable tool pin profiles are used, the tool rotational speed and feed rate are observed most significant. For the dissimilar FSW joints where a sound weld is obtained using suitable tool pin and process parameter combinations, the tensile failure of the joints observed from the base metal of the advancing side (AA6061-T6). On the flipside, failure is observed from the side of the stir zone when material flow, mixing and joining is inadequate. During flexural loading, initiation of failure is mostly observed from the vicinity of the void/tunnel region of the weld nugget. Study of fracture surfaces using scanning electron microscope shows the presence of numerous dimples in the sound bonded joints ensuring ductile failure of the joints under tensile loading. However tearing, rupture and brittle failure is observed for poorly bonded joints. Energy dispersive spectroscopy of fracture surface also shows the presence of oxides in aluminum matrix for poorly bonded joints resulting in brittle failure of the joints.

Microstructural and Mechanical Characterization of a Dissimilar Friction Stir-Welded AA5083-AA7B04 Butt Joint

Friction stir welding (FSW) has been used for joining AA5083 and AA7B04 alloy sheets with the aim of studying the microstructure and the mechanical properties of dissimilar FSW joints obtained by varying the initial base metal state of AA7B04 alloy. The results show that the initial base metal state has a significant impact on the material flow during dissimilar FSW. As compared with the joints placing hard alloy (artificially aged AA7B04-AA or naturally aged AA7B04-NA) on the retreating side, it becomes easier transporting AA5083 from advancing side to retreating side when soft alloy (annealed AA7B04-O) is placed on the retreating side. The atomic diffusion does not occur at the interface between AA5083 and AA7B04, indicating that the mixing of the two materials is merely mechanical. Grain refinement is observed in the stir zone. The failure location during tensile tests is different depending on the initial base metal state. The joints (AA5083/AA7B04-AA and AA5083/AA7B04-O) fail in the base metal on the soft material side which corresponds to the minimum values in hardness profiles. Differently, the joints (AA5083/AA5083 and AA5083/AA7B04-O) fail in the stir zone due to the presence of defects including “zigzag line,” kissing bond and discontinuous voids.