Effects of the Reversely Rotating Assisted Shoulder on Microstructures During the Reverse Dual-rotation Friction Stir Welding

The reverse dual-rotation friction stir welding(RDR-FSW) has the capability to adjust the heat generation because of the separately designed tool shoulder and tool pin.The welding torque exerted on the workpiece by the reversely rotating shoulder is opposite to that exerted by the rotating tool pin,so the total welding torque is reduced,which is beneficial to reducing the clamping requirement of workpieces.In the present paper,a RDR-FSW joint was welded in a condition similar to the optimal welding condition of conventional FSW,and microstructures in various zones were investigated by comparison,aiming to highlight effects of the reversely rotating assisted shoulder.Due to the heat conduction of the middle cylinder and the bottom end cover on which the assisted shoulder was machined,the thermal effect of RDR-FSW was smaller than that of the conventional FSW.Moreover,the effect of assisted shoulder on the plastic flow or deformation of material or was constrained in a thin layer near the weld top surface,and thus the flow of material especially along the thickness direction was clearly decreased in the RDR-FSW.In the heat-affected zone(HAZ),the precipitate coarsening was the main evolution and was completed through the dissolution of small precipitates and the continuous growth of large precipitates.By contrast,the dissolution degree of precipitates increased significantly in the thermomechanically affected zone(TMAZ),and a small amount of original meta-stable precipitates transformed to block-shaped stable precipitates.Precipitate evolutions in the shoulder affected zone(SAZ)and the weld nugget zone were similar,i.e.the majority of original meta-stable precipitates dissolved into the matrix and the remainder transformed to stable precipitates,though the dissolution degree was greater in the SAZ.Compared with the conventional FSW joint,the coarsening degrees of precipitates in the HAZ and TMAZ of RDR-FSW joint were much smaller,as well as the dissolution degrees of precipitates in all four specified zones.     

Effects of tool rotation speed on microstructures and mechanical properties of AA2219-T6 welded by the external non-rotational shoulder assisted friction stir welding

The external non-rotational shoulder assisted friction stir welding (NRSA-FSW) was applied to weld high strength aluminum alloy 2219-T6 successfully, and effects of the tool rotation speed on microstructures and mechanical properties were investigated in detail. Defect-free joints were obtained in a wide range of tool rotation speeds from 600 rpm to 900 rpm, but cavity defects appeared on the advancing side when the tool rotation speed increased to 1000 rpm. The microstructural deformation and heat generation were dominated by the rotating tool pin and sub-size concave shoulder, while the non-rotational shoulder helped to improve the weld formation. Microstructures and Vickers hardness distributions showed that the NRSA-FSW is beneficial to improving the asymmetry and inhomogeneity, especially in the weld nugget zone (WNZ). At the tool rotation speed of 800 rpm, both the tensile strength and the elongation reached the maximum, and the maximum tensile strength was up to 69.0% of the base material. All defect-free joints were fractured at the weakest region with minimum Vickers hardness in the WNZ, while for the joint with cavity defects the fracture occurred at the defect location.     

Effect of tool rotational speed and pin profile on microstructure and tensile strength of dissimilar friction stir welded AA5083-H111 and AA6351-T6 aluminum alloys

The relatively new welding process friction stir welding (FSW) was applied in this research work to join 6 mm thick dissimilar aluminum alloys AA5083-H111 and AA6351-T6. The effect of tool rotational speed and pin profile on the microstructure and tensile strength of the joints were studied. Dissimilar joints were made using three different tool rotational speeds of 600 rpm, 950 rpm and 1300 rpm and five different tool pin profiles of straight square (SS), straight hexagon (SH), straight octagon (SO), tapered square (TS), and tapered octagon (TO). Three different regions namely unmixed region, mechanically mixed region and mixed flow region were observed in the weld zone. The tool rotational speed and pin profile considerably influenced the microstructure and tensile strength of the joints. The joint which was fabricated using tool rotational speed of 950 rpm and straight square pin profile yielded highest tensile strength of 273 MPa. The two process parameters affected the joint strength due to variations in material flow behavior, loss of cold work in the HAZ of AA5083 side, dissolution and over aging of precipitates of AA6351 side and formation of macroscopic defects in the weld zone.     

Investigations on the effects of friction stir welding parameters on intermetallic and defect formation in joining aluminum alloy to mild steel

Joints of Al 5186 to mild steel were performed by using friction stir welding (FSW) technique. The effects of various FSW parameters such as tool traverse speed, plunge depth, tilt angle and tool pin geometry on the formation of intermetallic compounds (IMCs), tunnel formation and tensile strength of joints were investigated. At low welding speeds due to the formation of thick IMCs (which was characterized as Al₆Fe and Al₅Fe₂) in the weld zone the tensile strength of joints was very poor. Even at low welding speeds the tunnel defect was formed. As the welding speed increased, the IMCs decreased and the joint exhibited higher tensile strength. The tunnel defect could not be avoided by using cylindrical 4 mm and 3 mm pin diameter. By using a standard threaded M3 tool pin the tunnel was avoided and a bell shape nugget formed. Therefore tensile strength of the joint increased to 90% of aluminum base alloy strength. At higher welding speed and lower tool plunge depth, the joint strength decreased due to lack of bonding between aluminum and steel. Based on the findings, a FSW window has been developed and presented.     

Influence of tool geometry and rotational speed on mechanical properties and defect formation in friction stir lap welded 5456 aluminum alloy sheets

Friction stir welding of AA5456 aluminum alloy in lap joint configuration is with two different tempers, T321 and O, and different thicknesses, 5 mm and 2.5 mm was investigated. The influences of tool geometry and various rotational speeds on macrostructure, microstructure and joint strength are presented. Specifically, four different tool pin profiles (a conical thread pin, a cylindrical–conical thread pin, a stepped conical thread pin and Flared Triflute pin tool) and two rotational speeds, 600 and 800 rpm, were used. The results indicated that, tool geometry influences significantly material flow in the nugget zone and accordingly control the weld mechanical properties. Of particular interest is the stepped conical threaded pin, which is introduced for the first time in the present investigation. Scanning electron microscopy investigation of the fracture location of samples was carried out and the findings correlated with tool geometry features and their influences on material flow and tension test results. The optimum microstructure and mechanical properties were obtained for the joints produced with the stepped conical thread pin profile and rotational speed of 600 rpm. The characteristics of the nugget zone microstructure, hooking height, and fracture location of the weld joints were used as criteria to quantify the influence of processing conditions on joint performance and integrity. The results are interpreted in the framework of physical metallurgy properties and compared with published literature.     

Dissimilar friction stir lap welding of AA 5754-H22/AA 6082-T6 aluminium alloys: Influence of material properties and tool geometry on weld strength

Dissimilar friction stir welding (FSW) of heat (AA 6082-T6) and non-heat (AA 5754-H22) treatable aluminium alloys, in lap joint configuration, was performed in this work. The base material plates were 1 mm thick. Welds were performed combining different plates positioning, relative to the tool shoulder, in order to assess the influence of base materials properties on welds strength. Three different tools were tested, one cylindrical and two conical, with different taper angles. Welds strength was characterized by performing transverse and tensile–shear tests. Strain data acquisition by Digital Image Correlation (DIC) was used to determine local weld properties. The results obtained enabled to conclude that the dissimilar welds strength is strongly dependent on the presence of the well-known hooking defect and that the hooking characteristics are strongly conditioned by base materials properties/positioning. By placing the AA 6082-T6 alloy, as top plate, in contact with the tool shoulder, superior weld properties are achieved independently of the tool geometry. It is also concluded that the use of unthreaded conical pin tools, with a low shoulder/pin diameter relation, is the most suitable solution for the production of welds with similar strengths for advancing and retreating sides.     

Improving friction stir weldability of Al/Mg alloys via ultrasonically diminishing pin adhesion

Formation of intermetallic compounds (IMCs) during friction stir welding (FSW) of aluminum/magnesium (Al/Mg) alloys easily results in the pin adhesion and then deteriorates joint formation. The severe pin adhesion transformed the tapered-and-screwed pin into a tapered pin at a low welding speed of 30 mm/min. The pin adhesion problem was solved with the help of ultrasonic. The weldability of Al/Mg alloys was significantly improved due to the good material flow induced by mechanical vibration and the fragments of the IMCs on the surface of a rotating pin caused by acoustic streaming, respectively. A sound joint with ultrasonic contained long Al/Mg interface joining length and complex mixture of Al/Mg alloys in the stir zone, thereby achieving perfect metallurgical bonding and mechanical interlocking. The ultrasonic could broaden process window and then improve tensile properties. The tensile strength of the Al/Mg joint with ultrasonic reached 115 MPa.     

Formation and mechanical properties of stationary shoulder friction stir welded 6005A-T6 aluminum alloy

6005A-T6 aluminum alloy is welded by stationary shoulder friction stir welding (SSFSW). At a constant rotational velocity of 2000 rpm, the effect of welding speed on mechanical properties of SSFSW joint are investigated in detail. Defect-free joint with gloss surface and small flash is attained and no cracks appear at the bending angle of 180°. Compared with traditional friction stir welding (FSW), width of rotational shoulder affected zone is relatively small because of the smaller diameter of rotational shoulder. Increasing welding speed is benefit for reducing the width of softening region and the softening degree. The fracture position of welding joint locates in thermo-mechanically affected zone and the fracture surface morphology presents the typical ductile fracture. The maximum tensile strength of joint at the welding speed of 400 mm/min reaches 82% of base metal (BM).     

Microstructure and mechanical properties of dissimilar Al alloy/steel joints prepared by a flat spot friction stir welding technique

The 6061-T6 Al alloy and mild steel plate with a thickness of 1 mm were successfully welded by the flat spot friction stir welding technique, which contains two steps during the entire welding process. The rotating tools with different probe lengths of 1.0, 1.3 and 1.5 mm were used in the first step, during which a conventional spot FSW was conducted above a round dent previously made on the back plate. However, sound Al/Fe welds with similar microstructure and mechanical properties can still be obtained after the second step, during which a probe-less rotating tool was used to flatten the weld surface. The sound welds have smooth surface without keyholes and other internal welding defects. No intermetallic compound layer but some areas with amorphous atomic configuration was formed along the Al/Fe joint interface due to the lower heat input. The shear tensile failure load can reach a maximum value of 3607 N and fracture through plug mode. The probe length has little effect on the weld properties, which indicates that the tool life can be significantly extended by this new spot welding technique.     

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.     

High power fiber laser arc hybrid welding of AZ31B magnesium alloy

High power fiber laser–metal inert gas arc hybrid welding of AZ31B magnesium alloy was studied. The fusion zone consisted of hexagonal dendrites, where the secondary particle of Al₈Mn₅ was found at the center of dendrite as a nucleus. Within hybrid weld, the arc zone had coarser grain size and wider partial melted zone compared with the laser zone. The tensile results showed the maximum strength efficiency of 5 mm thick welds was up to 109%, while that of 8 mm thick welds was only 88%. The fracture surface represented a ductile–brittle mixed pattern characterized by dimples and quasi-cleavages. On the fracture surface some metallurgical defects of porosity and MgO inclusions around with secondary cracks were observed. Meanwhile, a strong link between the joint strength and weld porosity were demonstrated by experimental results, whose relevant mechanism was discussed by the laser–arc interaction during hybrid welding.     

Friction Stir Welding of Al Alloy Thin Plate by Rotational Tool without Pin

For friction stir welding (FSW), a new idea is put forward in this paper to weld the thin plate of Al alloy by using the rotational tool without pin. The experiments of FSW are carried out by using the tools with inner-concave-flute shoulder, concentric-circles-flute shoulder and three-spiral-flute shoulder, respectively. The experimental results show that the grain size in weld nugget zone attained by the tool with three-spiral-flute shoulder is nearly the same while the grain sizes decrease with the decrease of welding velocity. The displacement of material flow in the heat-mechanical affected zone by the tool with three-spiral-flute shoulder is much larger than that by the tool with inner-concave-flute shoulder or concentric-circles-flute shoulder. The above-mentioned results are verified by numerical simulation. For the tool with three-spiral-flute shoulder, the tensile strength of FSW joint increases with the decrease of welding velocity while the value of tensile strength attained by the welding velocity of 20 mm/min and the rotation speed of 1800 r/min is about 398 MPa, which is 80% more than that of parent mental tensile strength. Those verify that the tool with three-spiral-flute shoulder can be used to join the thin plate of Al alloy.     

Microstructure and tensile strength of friction stir welded joints between interstitial free steel and commercially pure aluminium

In the present study, the joining of interstitial free steel and commercial pure aluminium was carried out by friction stir welding (FSW) technique using tool rotational speeds of 600, 900, 1200 rpm and traverse speed of 100 mm/min. The microstructure and micro-hardness of the weld interface have been investigated. Optical microscopy was used to characterize the microstructures of different regions of friction stir welding joints. The scanning electron microscopy-back scattered electron (SEM-BSE) images show the existence of the different reaction layers in the welded zone. The Al₃Fe intermetallic compound has been observed in the weld interface and their thickness increase with the increase in tool rotational speed. Tensile strength was also evaluated and maximum tensile strength of ∼123.2 MPa along with ∼4.5% elongation at fracture of the joint have been obtained when processed at 600 rpm tool rotational speed.     

Effect of welding parameters on microstructure and mechanical properties of friction stir spot welded 5052 aluminum alloy

Friction stir spot welding (FSSW) is a newly-developed solid state joining technology. In this study, two types of FSSW, normal FSSW and walking FSSW, are applied to join the 5052-H112 aluminum alloy sheets with 1 mm thickness and then the effect of the rotational speed and dwell time on microstructure and mechanical properties is discussed. The lower sheet material underneath the hook didn’t flow into the upper sheet due to the concave surface in the shoulder and groove in the anvil. The hardness profile of the welds exhibited a W-shaped appearance and the minimum hardness was measured in the HAZ. The results of tensile/shear tests and cross-tension tests indicate that the joint strength decreases with increasing rotational speed, while it’s not affected significantly by dwell time. At the rotational speed of 1541 rpm, the tensile/shear strength and cross-tension strength reached the maximum of 2847.7 N and 902.1 N corresponding to the dwell time of 5 s and 15 s. Two different fracture modes were observed under both tensile/shear and cross-tension loadings: shear fracture and tensile/shear mixed fracture under tensile/shear loadings, and nugget debonding and pull-out under cross-tension loadings. The performance of the welds plays a predominant role in determining the type of fracture modes. In addition, the adoption of walking FSSW brings unremarkable improvements in weld strength.     

Comparative study on local and global mechanical properties of bobbin tool and conventional friction stir welded 7085-T7452 aluminum thick plate

7085-T7452 plates with a thickness of 12 mm were welded by conventional single side and bobbin tool friction stir welding (SS-FSW and BB-FSW, respectively) at different welding parameters. The temperature distribution, microstructure evolution and mechanical properties of joints along the thickness direction were investigated, and digital image correlation (DIC) was utilized to evaluate quantitatively the deformation of different zones during tensile tests. The results indicated that heat-affected zone (HAZ), the local softening region, was responsible for the early plastic deformation and also the fracture location for SS-FSW samples, while a rapid fracture was observed in weld nugget zone (WNZ) before yield behavior for all BB-FSW specimens. The ultimate tensile strength (UTS) of SS-FSW joints presented the highest value of 410 MPa, 82% of the base material, at a rotational speed of 300 rpm and welding speed of 60 mm/min, much higher than that of BB-FSW joints, with a joint efficiency of only 47%. This should be attributed to the Lazy S defect produced by a larger extent of heat input during the BB-FSW process. The whole joint exhibited a much higher elongation than the slices. Scanning electron microscopic (SEM) analysis of the fracture morphologies showed that joints failed through ductile fracture for SS-FSW and brittle fracture for BB-FSW.     

The optimisation of process parameters for friction stir spot-welded AA3003-H12 aluminium alloy using a Taguchi orthogonal array

The aim of the present work is to optimise the welding parameters for friction stir spot welded non-heat-treatable AA3003-H12 aluminium alloy sheets using a Taguchi orthogonal array. The welding parameters, such as the tool rotational speed, tool plunge depth and dwell time, were determined according to the Taguchi orthogonal table L9 using a randomised approach. The optimum welding parameters for the peak tensile shear load of the joints were predicted, and the individual importance of each parameter on the tensile shear load of the friction stir spot weld was evaluated by examining the signal-to-noise ratio and analysis of variance (ANOVA) results. The optimum levels of the plunge depth, dwell time and tool rotational speed were found to be 4.8 mm, 2 s and 1500 rpm, respectively. The ANOVA results indicated that the tool plunge depth has the higher statistical effect with 69.26% on the tensile shear load, followed by the dwell time and rotational speed. The tensile shear load of the friction stir spot welding (FSSW) joints increased with increasing plunge depth. Additionally, examination of the weld cross-sections, microhardness tests and fracture characterisation of the selected friction spot welded joints were conducted to understand the better performance of the joints. All the fractures of the joints during tensile testing occurred at stir zone (SZ), where the bonded section was minimum. The tensile shear load and tensile deformation of the FSSW joints increased linearly with increasing the bonded size. The finer grain size in the SZ led to the higher hardness, which resulted in higher fracture strength. When the tensile shear load of the joints increased approximately 3-fold, the failure energy absorption of the joints increased approximately 15-fold.     

Quenching microstructure and properties of 300M ultra-high strength steel electron beam welded joints

The 300M steel was welded by electron beam welding (EBW) with optimized welding parameters in the annealed state. As-welded, for comparison, and as-quenched (oil quenching at 870 °C × 1 h and tempering at 315 °C × 2 h) welded joints were investigated in this paper. The microstructure and fracture morphology were analyzed using scanning electron microscopy (SEM) and optical microscope. X-ray energy spectrum analysis was used to determine chemical composition of phases formed at the joint. The microhardness and tensile strength were evaluated. Results indicate that the weld metal microstructures of the as-welded joint are lower bainite, retained austenite and pro-eutectoid ferrite; the heat affected zone microstructure is sorbite with undissolved particles. The microstructure of as-quenched joint is tempered martensite. The tensile strength of the joints after quenching reached 1900 MPa.     

Intermediate layer characterization and fracture behavior of laser-welded copper/aluminum metal joints

Copper and aluminum were welded using a continuous Nd:YAG laser, and the influence of the processing parameters on the intermediate layer was investigated. The intermediate layer along the interface was characterized, and the failure mechanism was identified. Four distinct zones with various intermetallic compounds and structures formed in the intermediate layer and determined the corresponding joint strength. Utilizing gradually increasing heat input produced different thicknesses for these four zones. A laser beam power of 1650 W and a welding speed of 95 mm/s were the optimized parameters. The thickness of the intermetallic compound γ₂-Cu₉Al₄ and the shear–tensile strength of the joint decreased with the increase of welding speed in the weld. The shear–tensile load of the dissimilar metal joint reached 539.52 N with the optimized parameters. Fracture during shear–tensile testing occurred in the zone with 20.08–54.65% Cu. It was concluded that eutectic and hypoeutectic structures containing a significant amount of θ-CuAl₂ led to a weak joint. The relationship between the mechanical properties and thickness of the different intermediate zones is thoroughly illustrated.     

Microstructure evolution and mechanical properties of linear friction welded S31042 heat-resistant steel

S31042 heat-resistant steel was joined by linear friction welding(LFW) in this study. The microstructure and the mechanical properties of the LFWed joint were investigated by optical microscopy, scanning electronic microscopy, transmission electron microscopy, hardness test and tensile test. A defect-free joint was achieved by using LFW under reasonable welding parameters. The dynamic recrystallization of austenitic grains and the dispersed precipitation of NbCrN particles resulting from the high stress and high temperature in welding, would lead to a improvement of mechanical property of the welded joint.With increasing the distance from the weld zone to the parent metal, the austenitic grain size gradually increases from ~1μm to ~150μm, and the microhardness decreases from 301 HV to 225 HV. The tensile strength(about 731 MPa) of the welded joint is comparable to that of the S31042 in the solution-treated state.     

Experimental study of friction taper plug welding for low alloy structure steel: Welding process,defects, microstructures and mechanical properties

A research investigation has been undertaken to identify the various stages and variation of welding parameters in friction taper plug welding (FTPW) process and to explore their effects on the performance and properties of the welds. According to the variation of axial force, the overall FTPW process is divided into feeding phase, pressing phase, welding phase, and forging phase. The rotating speed, welding force, and burn-off rate remain nearly constant in welding phase. However, the torque peaks in welding phase when after few seconds of welding force setting is reached. Rising the welding force would increase the peak torque, welding torque, and burn-off rate, but decrease the welding time. When improper welding parameter is used lack of bonding and incomplete filling defects would form within the weld. The microstructure of the weld metal is consist of retained austenite, pearlite, and various Widmanstätten ferrite. In heat affect zone, it is mainly of lathy upper bainite. Defect free welds exhibit favorable tensile properties of which 548.3 MPa tensile strength and 27.5% elongation that equal to the base metal could be found.