The potential adaptation of stationary shoulder friction stir welding technology to steel

Stationary shoulder friction stir welding is a newly developed technique currently used for joining plates of relatively soft metals at different angular planes. The process is not currently applicable to steel, hence the present study was developed to investigate the theoretical and technical viability of stationary shoulder technology in DH36 steel. Aluminium welds were produced using both conventional rotating shoulder and stationary shoulder friction stir welding techniques, and steel welds were produced using only conventional friction stir welding techniques. The effects of stationary shoulder technology on both the microstructural evolution and resultant mechanical properties of aluminium have been evaluated so that the likely effects on steel could be predicted. In the aluminium welds, the stationary shoulder technique results in a distinct transition between stirred and unstirred material, contrasting to the gradual change typically seen in conventional friction stir welds produced with a rotating shoulder. An investigation of weld properties produced in DH36 steel has demonstrated that if the stationary shoulder weld technique was used, the microstructure likely to be formed, would be dominated by a bainitic ferrite phase and so would exhibit hardness and tensile properties in excess of the parent material. It is predicted that if the same abrupt transition between unstirred and stirred material observed in aluminium occurred in steel, this would lead to crack initiation, followed by rapid propagation through the relatively brittle weld microstructure. Hence, these findings demonstrate that without further design and process improvements, stationary shoulder friction stir welding is unlikely to be applicable to steel.     

Fatigue performance of friction stir welded marine grade steel

An extensive study on the fatigue performance of friction stir welded DH36 steel was carried out. The main focus of this experimental testing programme was fatigue testing accompanied by tensile tests, geometry measurements, hardness and residual stress measurements, and fracture surface examination. The S–N curve for friction stir butt welded joints was generated and compared with the International Institute of Welding recommendations for conventional fusion butt welds. Friction stir welds of marine grade steel exceeded the relevant rules for fusion welding. This newly developed S–N curve is being proposed for use in the relevant fatigue assessment guidelines for friction stir welding of low alloy steel. Fracture surfaces were examined to investigate the fatigue failure mechanism, which was found to be affected by the processing features generated by the friction stir welding tool.     

Influence of tool geometries and process variables on friction stir butt welding of Al–4.5%Cu/TiC in situ metal matrix composites

Present work describes friction stir welding of in-house produced and hot rolled Al–4.5%Cu/TiC in situ metal matrix composites by using hardened bimetallic tool with varying shoulder surface geometries and other process variables. Joining of the said composite using friction stir welding process has been seen to provide beneficial effects such as grain refinement of the matrix and subsequent redistribution and refinement of reinforcements. A predictive model has also been developed to estimate the weld properties such as tensile strength and ductility with respect to the tool geometry used and input process variables. The X-ray diffraction analysis results of Al–4.5%Cu/TiC butt welds indicated formation of CuAl₂O₄ and CuAl₂ to some extent in the stir zone. Fractography of the weld samples revealed dimpled ductile nature of fracture. Through multi response optimization of the welding parameters and tool geometry, weld strength of 89% that of the base material was achieved.     

Comparative study of fatigue and fracture in friction stir and electron beam welds of 24 mm thick titanium alloy Ti‐6Al‐4 V

In this study, friction stir welding of Ti‐6Al‐4 V was demonstrated in 24 mm thickness material. The microstructure and mechanical properties, fatigue, fracture toughness and crack growth of these thick section friction stir welds were evaluated and compared with electron beam welds produced in the same thickness material. It was found that the friction stir welds possessed a relatively coarse lamellar alpha transformed beta microstructure because of slow cooling from above the transus temperature of the material. The electron beam welds had a fine acicular alpha structure as a result of rapid solidification. The friction stir welds possessed better ductility, fatigue life, fracture toughness and crack growth resistance than the base meal or electron beam welds. Thus, even though friction stir welding is a relatively new process, the performance benefits it offers for the fabrication of heavy gage primary structure make it a more attractive option than the more well‐established electron beam welding method.     

Fatigue of friction stir welds in aluminium alloys that contain root flaws

Although the vast majority of friction stir welds will be free of flaws, it is not always possible to assume that they are. The properties of welds with flaws are needed to enhance confidence in the design and application of friction stir welded joints. The monotonic strength and fatigue behaviour of single-sided butt welds in 6–7 mm thick AA5083-O, AA5083-H321 and AA6082-T6, both without and with root flaws, was investigated.

Examination of the root flaw faces showed that there was bonding between the flanks of the flaws but the bonding was of poor quality and incomplete. This meant that the strength and ductility of the flaws were lower than the surrounding material. However, the comparison of the mechanical test results suggests that root flaws up to a certain size are tolerable without a significant loss in performance when compared to nominally flaw-free welds. These data also suggest that even friction stir welds with root flaws exceed the design life for equivalent fusion welds set out in the draft Eurocode 9 and that a higher rating may be warranted. Limited test results produced for this work need to be supplemented with a wider range of tests.     

Development of a process envelope for friction stir welding of DH36 steel – A step change

Friction stir welding of steel presents an array of advantages across many industrial sectors compared to conventional fusion welding techniques. However, the fundamental knowledge of the friction stir welding process in relation to steel remains relatively limited. A microstructure and property evaluation of friction stir welded low alloy steel grade DH36 plate, commonly used in ship and marine applications has been undertaken. In this comprehensive study, plates of 2000 × 200 × 6 mm were butt welded together at varying rotational and traverse speeds. Samples were examined microscopically and by transverse tensile tests. In addition, the work was complemented by Charpy impact testing and micro-hardness testing in various regions of the weld. The study examined a wide range of process parameters; from this, a preliminary process parameter envelope has been developed and initial process parameter sets established that produce commercially attractive excellent quality welds through a substantial increase in the conventionally recognised weld traverse speed.     

Fracture toughness and fatigue crack growth in Ti‐6Al‐4V friction stir welds

Friction stir welding of titanium holds the promise for producing joints with microstructures and mechanical properties that are more comparable to wrought material than traditional fusion welding processes. Extensive data exist on the microstructure and static mechanical properties of titanium friction stir welds, but very little are available on the durability (fatigue) and even less on the damage tolerance (fracture toughness and fatigue crack growth). This paper presents the results of an investigation into the damage tolerance of friction stir welds made in 6 mm thick Ti‐6Al‐4V after a post‐weld heat treatment. It was found that the apparent fracture toughness was lower than the wrought base material, 7–25% depending on the crack orientation relative to the weld, but the crack growth performance (ΔK vs. da/dN) of the weld in the absence of weld‐induced residual stresses was identical to the base material.     

R‐curve behaviour of friction stir welds in aluminium‐lithium alloy 2195

ABSTRACT The fracture resistance of friction stir welds in 2195‐T8 is described in this paper. R‐curves were produced for several crack planes, parallel to the welding direction and situated at varying distances from the weld centreline. The friction stir weld was also characterized by hardness and tensile testing. Fracture resistance of the friction stir weld is compared to the base metal and to a variable polarity plasma arc weld. Results show that the material in and around the friction stir weld is tougher than the base metal and the variable polarity plasma arc weld. The friction stir weld fracture performance is discussed in the light of the observed hardness and fractographic data presented.     

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

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

Comparison of fatigue properties between friction stir welds and TIG welds

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

Effects of various tool plunge depths on microstructure evolution,mechanical properties and dome structure features of friction stir spot welded AA5052-H32 similar joints

The solid-state nature of friction stir spot welding process provides outstanding advantages for the sound joining of aluminum alloys. Within this study, 3 mm-thick AA5052-H32 sheets are successfully joined by friction stir spot welding using 2344 hot-worked steel pin to investigate the effects of various tool plunge depths on the microstructure, mechanical and metallurgical properties of similar welds. Therefore, the experiments are performed at different plunge depths in the range of 3 mm–4 mm. Accordingly, the relationships between the process parameter (tool plunge depth) and the responses (microstructure, dome structure, microhardness and lap shear tensile load) are established. Microstructure analyses demonstrate that the increase in the plunge depth leads to more grain refinement within the stir zone, which significantly affects the mechanical performance of the similar joints. This study also indicates that the tool plunge depth in friction stir spot welding process has a noteworthy influence on the characteristic features of the 5052 aluminum alloy joints, such as the dome structure. Moreover, an explicit increase in the microhardness towards the weld stir zone is observed in all specimens. It is found that the average maximum tensile-shear force enhances with the increment in the tool plunge depth from 3 mm to 4 mm.     

Process optimisation and mechanical properties of friction stir lap welds of 7075-T6 stringers on 2024-T3 skin

This paper focuses on the results of process optimisation and mechanical tests that were used to ascertain the feasibility of using friction stir welding (FSW) to join stringers to skin. The effects of process parameters on weld quality of 1.5-mm 7075-T6 stringers lap-joined on 2.3-mm 2024-T3 skins were investigated. Advancing and retreating side locations on the joint configuration were alternated to determine optimal design arrangements. The effects of travel and rotation speeds on weld quality and defect generation were also investigated. Weld quality was assessed by optical microscopy and bending tests. It was found that: (i) the increase of the welding speed or the decrease of the rotational speed resulted in a reduction of the hooking size and top plate thinning but did not eliminated them, (ii) double pass welds by overlapping the advancing sides improved significantly the weld quality by overriding the hooking defect, and (iii) change of the rotational direction for a counter clockwise with a left-threaded probe eliminated the top sheet thinning defect. Subsequently, FSW lap joints were produced using optimum conditions and underwent extensive mechanical testing program. Several assembly configurations including discontinuous and continuous welds as well as single and double pass welds were produced. The results obtained for cyclic fatigue performance of FSW panels are compared with riveted lap joints of identical geometry. S–N curves, bending behaviour, failure locations and defect characterisation are also discussed. It was found that: (i) the tensile strength of FSW joints approached that of the base material but with a significant reduction in the fatigue life, (ii) the probe plunge and removal locations served as the key crack nucleation sites in specimens with discontinuous welds, and (iii) double pass welds with overlapping advancing sides showed outstanding fatigue life and very good tensile properties. The present work provided some valuable insight into both the fabrication and application of FSW on stringer/skin lap joints.     

Thermal cycles and their effects during friction stir welding of AA7075 thicker plates with and without in-process cooling

Friction stir welding of AA 7075 plates in three different thicknesses such as 10, 16 and 25 mm at natural convection condition was carried out successfully without defects. Water cooled friction stir welds were also produced on 16 mm thick plates. The thermal cycles at different locations of the plate, during the friction stir welding process, were predicted using a three-dimensional thermal model. Mechanical properties of the welds were evaluated using tensile and hardness tests. Weld microstructures were also examined with optical and transmission electron microscopes. The weld hardness values and tensile properties were found to decrease with increase in plate thickness. The use of water cooling was found to improve the weld properties to some extent, although not to the level of base metal. The reasons for this behavior are discussed, correlating thermal cycles, mechanical properties, fracture locations and precipitate morphology.     

A comprehensive investigation on the effects of laser and shot peening on fatigue crack growth in friction stir welded AA 2195 joints

The effects of various surface treatment techniques on the fatigue crack growth performance of friction stir welded 2195 aluminum alloy were investigated. The objective was to reduce fatigue crack growth rates and enhance the fatigue life of welded joints. The crack growth rates were assessed and characterized for different peening conditions at a stress ratio (R) of 0.1, and 0.7. The surface and through-thickness residual stress distribution were also investigated and presented for the various regions in the weld. Tensile residual stresses introduced during the welding process were found to become significantly compressive, particularly after laser peening. The effect of the compressive stresses was deemed responsible for increasing the resistance to fatigue crack growth of the welds. The results indicate a significant reduction in fatigue crack growth rates using laser peening compared to shot peening and native welded specimens. This reduced fatigue crack growth rate was comparable to the base unwelded material.     

Friction stir welding for the transportation industries

This paper will focus on the relatively new joining technology—friction stir welding (FSW). Like all friction welding variants, the FSW process is carried out in the solid-phase. Generically solid-phase welding is one of the oldest forms of metallurgical joining processes known to man. Friction stir welding is a continuous hot shear autogenous process involving a non-consumable rotating probe of harder material than the substrate itself. In addition, FSW produces solid-phase, low distortion, good appearance welds at relatively low cost. Essentially, a portion of a specially shaped rotating tool is plunged between the abutting faces of the joint. Once entered into the weld, relative motion between the rotating tool and the substrate generates frictional heat that creates a plasticised region around the immersed portion of the tool. The contacting surface of the shouldered region of the tool and the workpiece top contacting surface also generates frictional heat. The shouldered region provides additional friction treatment to the weld region as well as preventing plasticised material being expelled. The tool is then translated with respect to the workpiece along the joint line, with the plasticised material coalescing behind the tool to form a solid-phase joint as the tool moves forward. Although the workpiece does heat up during FSW, the temperature does not reach the melting point. Friction stir welding can be used to join most aluminium alloys, and surface oxide presents no difficulty to the process. Trials undertaken up to the present time show that a number of light weight materials suitable for the automotive, rail, marine, and aerospace transportation industries can be fabricated by FSW.     

Interfacial bonding mechanism in Al/coated steel dissimilar refill friction stir spot welds

Defect-free dissimilar Al/zinc coated steel and Al/AlSi coated steel welds were successfully fabricated by refill friction stir spot welding. However, Al alloy and uncoated steel could not be welded under the same welding condition. Al-Zn eutectic layer formed at the Al/zinc coated steel interface showed non-uniformity in thickness and nanoscale intermetallic (IMC) produced was discontinuous. The bonding formation between the Al-Zn layer and the surrounding materials was attributed to a liquid/solid reaction mechanism. Bonding formation at Al alloy and AlSi coated steel interface was attributed to a solid/solid reaction mechanism, as the joining process did not involve with melting of base metals or AlSi coating materials. Kissing bond formed at the weld boundary acted as a crack initiation and propagation site, and the present study showed that weld strength of Al 5754/AlSi coated steel was greatly influenced by properties of original IMC layer.     

Fatigue behavior of friction plug welds in 2195 Al–Li alloy

The focus of this paper will be on the fatigue behavior of friction stir welded 2195 Al–Li plates that contain friction plug welds. Tensile tests were performed for specimens containing base metal, friction stir welded 2195-T8, and friction stir welded 2195-T8 containing a friction plug weld consisting of a 2195-T8 plug. The ultimate strength was determined for base metal, friction stir welded material, and friction plug welded material. Fatigue properties were determined for both the friction stir weld and friction plug welded specimens in the medium to high cycle regimes. Comparison of the results show that the friction plug weld reduced both the UTS and fatigue life as compared to specimens containing only friction stir weld. The reduction in fatigue life is most likely due to the complication of weld geometry, interacting heat affected zones, and strength mismatch between base metal, friction stir weld, and plug material.     

Friction stir welding of thick section Al‐Zn‐Mg‐Cu aluminum alloy

Friction stir butt welding of 25 mm thick AA7075–T651 plates has been investigated. Careful process parameter selection resulted in single pass, full‐penetration defect free welds. The weld nugget exhibits a significant grain refinement while facing the dissolution of strengthening precipitates. Microhardness survey gives a W‐shaped profile with lower hardness values recorded in the thermo‐mechanically‐affected zone. Tensile fractures occur, again, in the thermo‐mechanically‐affected zone, where minimum hardness occurred. The friction stir welds demonstrate an excellent root bend performance while falling behind base material in face bend test. The welds also displayed outstanding impact toughness compared to that of parent material. It is concluded that defect free single pass friction stir welds can be successfully made on 25 mm thick AA7075–T651 plates.     

Characterization of mechanical properties,fatigue-crack propagation,and residual stresses in a microalloyed pipeline-steel friction-stir weld

The influence of the friction-stir welding process on microstructure and mechanical properties of API 5L X80 skelp was investigated. Friction-stir welds were produced using welding parameters optimized to promote weld toughness. The solid-state welding process produced microstructures that significantly varied from those observed in the base metal, namely the redistribution and resizing of Martensite–Austenite constituent in the heat-affected zone and stir zone regions of the welds. Mechanical properties of the welds and base metal were evaluated with uniaxial tension testing and microhardness testing revealing overmatching welds and a hard zone within the weld stir zone. Residual stresses were determined in several directions with respect to the joint revealing that stress in the longitudinal direction is highest, yet well below material yield strength. Fatigue-crack propagation behavior was characterized in the different weld regions and base metal by testing with the compact tension specimen configuration showing that welds have impeded fatigue-crack growth compared to the base metal mostly due to welding-induced residual stress fields interacting with the crack.