Interface characteristic of friction stir welding lap joints of Ti/Al dissimilar alloys

Lap joints of TC1 Ti alloy and LF6 A1 alloy dissimilar materials were fabricated by friction stir welding and corresponding interface characteristics were investigated. Using the selected welding parameters, excellent surface appearance forms, but the interface macrograph for each lap joint cross-section is different. With the increase of welding speed or the decrease of tool rotation rate, the amount of Ti alloy particles stirred into the stir zone by the force of tool pin decreases continuously. Moreover, the failure loads of the lap joints also decrease with increasing welding speed and the largest value is achieved at welding speed of 60 mm/min and tool rotation rate of 1500 r/min, where the interracial zone can be divided into 3 kinds of layers. The microhardness of the lap joint shows an uneven distribution and the maximum hardness of HV 502 is found in the middle of the stir zone.

Microstructure and mechanical properties of dissimilar pure copper/1350 aluminum alloy butt joints by friction stir welding

The dissimilar friction stir welding of pure copper/1350 aluminum alloy sheet with a thickness of 3 mm was investigated. Most of the rotating pin was inserted into the aluminum alloy side through a pin-off technique, and sound welds were obtained at a rotation speed of 1000 r/min and a welding speed of 80 mm/min. Complicated microstructure was formed in the nugget, in which vortex-like pattern and lamella structure could be found. No intermetallic compounds were found in the nugget. The hardness distribution indicates that the hardness at the copper side of the nugget is higher than that at the aluminum alloy side, and the hardness at the bottom of the nugget is generally higher than that in other regions. The ultimate tensile strength and elongation of the dissimilar welds are 152 MPa and 6.3%, respectively. The fracture surface observation shows that the dissimilar joints fail with a ductile-brittle mixed fracture mode durin~ tensile test.

Strengthening strategy for high-performance friction stir lap welded joints based on 5083 Al alloy

During aircraft,ship,and automobile manufacturing,lap structures are frequently produced among Al alloy skins,wall panels,and stiffeners.The occurrence of welding defects severely decreases mechanical properties during friction stir lap welding(FSLW).This study focuses on investigating the effects of rotation rate,multipass welding,and cooling methods on lap defect formation,microstructural evolution,and mechanical properties.Hook defects were eliminated by decreasing welding speed,applying two-pass FLSW with a small welding tool,and introducing additional water cooling,thus leading to a remarkable increase in effective sheet thickness and lap width.This above strategy yielded defect-free joints with an ultrafine-grained microstructure and increased tensile shear force from 298 to 551 N/mm.The fracture behavior of FSLW joints was systematically studied,and a fracture factor of lap joints was proposed to predict their fracture mode.By reducing the rotation rate,using two-pass welding,and employing additional water cooling strategies,an enlarged,strengthened,and defect-free lap zone with refined ultrafine grains was achieved with a quality comparable to that of lap welds based on 7xxx Al alloys.Importantly,this study provides a valuable FSLW method for eliminating hook defects and improving joint performance.

Microstructure and mechanical properties of stationary shoulder friction stir welding joint of 2A14-T62 aluminum alloy

2A14-T62 butt joint was successfully welded by stationary shoulder friction stir welding(SSFSW)method.The results showed that using a pin with small shoulder could broaden the process window,and under a rotation speed of 2000 r/min and welding speed of 30 mm/min,joint with smooth surface,small reduction in thickness and little inner defects was obtained.The weld nugget zone was approx-imately circular,which was a unique morphology for SSFSW.The heat-affected zone(HAZ)and thermo-mechanically affected zone(TMAZ)were both quite narrow due to the lower heat input and slight mechanical action of the stationary shoulder.The fraction of high angle grain boundaries(HAGBs)exhibited a“W”shape along horizontal direction(from advancing side to retreating side),and the minim-um value located at HAZ.The average ultimate tensile strength and elongation of the joint were 325 MPa and 4.5%,respectively,with the joint efficiency of 68.3%.The joint was ductile fractured and the fracture surface contained two types of dimples morphology in different re-gions of the joint.Microhardness distribution in the joint exhibited a“W”shape,and the difference along the thickness direction was negli-gible.The joint had strong stress corrosion cracking susceptibility,and the slow stain rate tensile strength was 139 MPa.Microcrack and Al2O3 particulates were observed at the fracture surface.

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 microstructure of the welded samples showed the IMCs layers containing Al12Mg17(γ) and Al3Mg2(β) at the welding zone with a thickness(< 3.5 μm). 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/min 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.

Preparation of bimetallic nano-composite by dissimilar friction stir welding of copper to aluminum alloy

Butt friction stir welding between pure copper and AA5754 alloy was carried out.Reinforcing SiC nanoparticles were utilized in friction stir welded(FSW)joints to decline the harmful effects of intermetallic compounds.Tensile tests,micro-hardness experiments,scanning electron microscopy and X-ray diffraction analysis were applied to studying the properties of welded joints.The joints with a travel speed of 50 mm/min and a rotation speed of 1000 r/min showed the best results.The presence of nano-sized SiC particles reduced the grain size of aluminum and copper in the stir zone(SZ)from 38.3 and 12.4μm to 12.9 and 5.1μm,respectively.The tensile strength of the joint in the presence of reinforcing SiC nano-particles was~240 MPa,which is~90%of that for the aluminum base.Furthermore,the highest microhardness of the weld zone was significantly increased from HV 160 to HV 320 upon the addition of SiC nano-particles.The results also showed that raising the heat generation in FSW joints increased the amount of Al_(4)Cu_(9) and Al_(2)Cu intermetallic compounds.

Microstructure and mechanical characterization of Incoloy 825 Ni-based alloy welded to 2507 super duplex stainless steel through dissimilar friction stir welding

The feasibility of dissimilar friction stir welding(FSW)between the SAF 2507 super duplex stainless steel and the Incoloy825 Ni-based superalloy was evaluated.The microstructure and mechanical behavior of the weldments were examined too.The results showed that the alloys were successfully welded together by positioning the SAF 2507 on the advancing side.The nuggets displayed higher hardness than the base metals,due to the occurrence of dynamic recrystallization and the subsequent refinement of the microstructures.The welded sample obtained the similar strength to the Incoloy 825 parent metal,showing the ductile fracture mode after the tensile tests by SEM.Moreover,the weld zone(31 J)exhibited higher and lower toughness than the Incoloy 825(23 J)and SAF 2507(42 J)parent metals,respectively.Based on the obtained results,the FSW method could be recommended to weld the super duplex stainless steel/Ni-based superalloy joints.

Dissimilar friction stir welding between 5052 aluminum alloy and AZ31 magnesium alloy

Dissimilar friction stir welding between 5052 Al alloy and AZ31 Mg alloy with the plate thickness of 6 mm was investigated.Sound weld was obtained at rotation speed of 600 r/min and welding speed of 40 mm/min.Compared with the base materials,the microstructure of the stir zone is greatly refined.Complex flow pattern characterized by intercalation lamellae is formed in the stir zone.Microhardness measurement of the dissimilar welds presents an uneven distribution due to the complicated microstructure of the weld,and the maximum value of microhardness in the stir zone is twice higher than that of the base materials. The tensile fracture position locates at the advancing side(aluminum side),where the hardness distribution of weld shows a sharp decrease from the stir zone to 5052 base material.

Influence of tool plunge depth and welding distance on friction stir lap welding of AA5454-O aluminum alloy plates with different thicknesses

AA5454-O aluminum alloy plates with thicknesses of 1.4 and 1.0 mm were friction-stir-lap-welded (FSLWed).The influences of the tool plunge depth and welding distance on surface appearance,macrostructure and mechanical properties of the FSLWed plates were experimentally investigated.The tensile shear load of the FSLWed plates was compared with that of the adhesive-bonded plates.Defect-free FSLWed zones were successfully obtained in all the tool plunge depths and the welding distances.The FSLWed zones exhibited the relatively smooth surface morphologies.Under all the FSLWed conditions,the FSLWed zone exhibited higher average hardness than the base metal.In addition,the upper plate exhibited a higher average hardness than the lower plate,although there was no special tendency in spite of the change in the tool plunge depth and the welding distance.The maximum tensile shear load of the FSLWed plates was much higher than that of the adhesive-bonded aluminum alloy plate.Especially,under the FSLW condition of the plunge depth of 1.8 mm and the welding distance of 40 mm,the tensile shear load of the FSLWed plate reached a level about 41% greater than that of the adhesive-bonded aluminum alloy plate.In addition,the maximum tensile shear load of the FSLWed plate was increased with the increase of the welding distance.

Corrosion behavior of dissimilar copper/brass joints welded by friction stir lap welding in alkaline solution

This study was done to evaluate the nugget zone(NZ)corrosion behavior of dissimilar copper/brass joints welded by friction stir lap welding(FSLW)in a solution of 0.015 mol/L borax(pH 9.3).To this end,dissimilar copper/brass plates were welded with two dissimilar heat inputs(low and high)during the welding procedure.The high and low heat inputs were conducted with 710 r/min,16 mm/min and 450 r/min,25 mm/min,respectively.Using open circuit potential(OCP)measurements,electrochemical impedance spectroscopy(EIS)and Tafel polarization tests,the electrochemical behavior of the specimens in borate buffer solution was assessed.With the help of scanning electron microscope(SEM),the morphology of welded specimen surfaces was examined after immersion in the test solution.According to the results,the NZ grain size and resistance improvement reduced due to the nugget zone corrosion with a decreased heat input.The results obtained from Tafel polarization and EIS indicated the improved corrosion behavior of the welded specimen NZ with a decrease in the heat input during the welding process unlike the copper and brass metals.Furthermore,an increased heat input during the welding process shows a reduction in the conditions for forming the passive films with higher protection behavior.

Optimum traveling and rotation speeds in friction stir welding for dissimilar Al alloys

Aluminum alloys are subjected to large deformation and decreased strength due to the high expansion modulus caused by heat effects during friction stir welding （FSW）.The optimum conditions for friction stir welding of 5052-O and 6061-T6 Al alloys were determined.The optimum traveling and rotation speeds were identified to be 61mm/min and 1600r/min using various mechanical characteristic evaluation methods.

Friction Stir Welding of Dissimilar Aluminum Alloys

Dissimilar aluminum alloys AA2024-T365 and AA5083-H111 were welded by friction stir process. Welding parameters such as tool rotational speed (900, 1120 and 1400 rpm), weld speeds (16, 40 and 80 mm/min) and tool pin profiles (square, triangular and stepped) were used to weld many joints to study their effect on the mechanical properties of the joint. Also, different locations of the material were studied as other parameter. The mechanical properties were evaluated using tensile and hardness tests. The microstructure characterization of the processed alloys was carried out using optical microscopy. Macro and microstructures of parent and welded specimens indicated that the weld parameters have a significant effect on mechanical and microstructural properties of the welds. However, defect-free as well as higher strength was obtained at higher speed of 80 mm/min.

Visualization and simulation of plastic material flow in friction stir welding of 2024 aluminium alloy plates

Thin copper sheets as marker material were embedded into weld path of 2024 aluminium alloy plates and their final position after friction stir welding was examined by metallographic techniques. Referring to the visualized material flow patterns, a three-dimensional model was developed to conduct the numerical simulation of the temperature profile and plastic material flow in friction stir welding. The calculated velocity contour of plastic flow in close proximity of the tool is generally consistent with the visualized results. As the tool rotation speed increases at a constant tool travel speed, the material flow near the pin gets stronger. The predicted shape and size of the weld nugget zone match with the experimentally measured ones.

Microstructure and mechanical properties of dissimilar Al-Cu joints by friction stir welding

Dissimilar friction stir welding between 1060 aluminum alloy and annealed pure copper sheet with a thickness of 3 mm was investigated. Sound weld was obtained at a rotational speed of 1050 r/min and a welding speed of 30 mm/min. Intercalation structure formed at the crown and Cu/weld nugget （WN） area promotes interracial diffusion and metallurgical bonding of aluminum and copper. However, corrosion morphology reveals the weak bonding mechanism of internal interface, which causes the joint failing across the interface with a brittle-ductile mixed fracture mode. The tensile strength of the joint is 148 MPa, which is higher than that of the aluminum matrix. Crystal defects and grain refinement by severely plastic deformation during friction stir welding facilitate short circuit diffusion and thus accelerate the formation of A14Cu9 and A12Cu intermetallic compounds （IMCs）. XRD results show that A14Cu9 is mainly in Cu/WN transition zone. The high dislocation density and formation of dislocation loops are the major reasons of hardness increase in the WN.

Friction stir keyholeless spot welding of AZ31 Mg alloy-mild steel

Friction stir keyholeless spot welding（FSKSW） using a retractable pin for 1.0 mm thick galvanized mild steel and 3 mm thick AZ31 B magnesium alloy in a lap configuration was investigated.The process variables were optimized in terms of the joint strength.The effects of the stacking sequence on joint formation and the joining mechanism of FSKSW AZ31B-to-mild steel joints were also analyzed.It shows that the process window and joint strength are strongly influenced by the stacking sequence of the workpieces.While the process window is narrow and unstable for FSKSW of a magnesium-to-steel stack-up,a desirable process was established for the steel-to-magnesium stacking sequence,a desirable process and higher strength joint can be got when the steel-to-magnesium stacking sequence.XRD phase and EPMA analyses of the FSKSW joint showed that the intermetallic compounds are formed at the steel-to-magnesium interface,and the element diffusion between the mild steel and AZ31 B magnesium alloy revealed that the joining methods for FSKSW joints is the main mechanical joining along with certain metallurgical bonding.

Review on friction stir welding of dissimilar magnesium and aluminum alloys: Scientometric analysis and strategies for achieving high-quality joints

Magnesium and aluminum alloys continually attract interest as lightweight structural materials for transport applications. However, joining these dissimilar alloys is very challenging. The main obstacle that hinders progress in dissimilar Mg-Al joining is the formation of brittle intermetallic compounds(IMCs). As a solid-state joining technique, FSW is an excellent candidate to attenuate the deleterious IMC effects in dissimilar Al-Mg joining due to the inherent low heat inputs involved in the process. However, the IMCs, namely Al_(3)Mg_(2) and Al_(12)Mg_(17) phases, have also been reported to form during Al-Mg dissimilar FSW;their amount and thickness depend on the heat input involved;thus,the weld parameters used. Since the heat dissipated in the material during the welding process significantly affects the amount of IMCs,the heat input during FSW should be kept as low as possible to control and reduce the amount of IMCs. This review aims to critically discuss and evaluate the studies conducted in the dissimilar Al/Mg FSW through a scientometric analysis and also with a focus on the strategies recently applied to enhance joint quality. The scientometric analysis showed that the main research directions in Mg/Al FSW are the technological weldability of aluminum and magnesium during FSW, structural morphology, and mechanical properties of dissimilar welded joints. Considering the scope of application of the aforementioned joints, the low share of articles dealing with environmental degradation and operational cracking is surprising. This might be attributed to the need for well-developed strategies for obtaining high-quality and sustainable joints for applications. Thus, the second part of this review is conventional, focusing mainly on the new strategies for obtaining high-quality Mg/Al joints. It can be concluded that in addition to the necessity to optimum welding parameters to suppress the excessive heat to limit the amount and thickness of IMC formed and improve the overall joint quality, strategies such as using Zn interlayer, electric current assisted FSW(EAFSW), ultrasonic vibration FSW(UVa FSW), are considered effective in the elimination, reduction, and fragmentation of the brittle IMCs.

Effect of welding heat input and post-welded heat treatment on hardness of stir zone for friction stir-welded 2024-T3 aluminum alloy

The microstructure and hardness of the stir zone （SZ） with different welding heat inputs were investigated for friction stir-welded 2024-T3 aluminum by transmission electron microscopy, differential scanning calorimeter and Vickers micro-hardness test. The results show that welding heat input has a significant effect on the hardness of the SZ. Under high welding heat input condition, a higher welding speed is beneficial for improving the hardness of the SZ. However, when the welding heat input is low, the hardness of the SZ elevates with increasing the rotation speed. The hardness of the SZ decreases after post-welded heat treatment due to overaging. The joints welded at 500 r/min and 100 mm/min show a high resistance to overaging. The reduction of hardness in the SZ is only 3.8%, while in other joints, the reduction is more than 10%. The morphology of strengthening precipitates plays important roles for the improvement of hardness.

Thermal modeling of underwater friction stir welding of high strength aluminum alloy

The thermal modeling of underwater friction stir welding （FSW） was conddcted with a three-dimensional heat transfer model. The vaporizing characteristics of water were analyzed to illuminate the boundary conditions of underwater FSW. Temperature dependent properties of the material were considered for the modeling. FSW experiments were carried out to validate the calculated results, and the calculated results showed good agreement with the experimental results. The results indicate that the maximum peak temperature of underwater joint is significantly lower than that of normal joint, although the surface heat flux of shoulder during the underwater FSW is higher than that during normal FSW. For underwater joint, the high-temperature distributing area is dramatically narrowed and the welding thermal cycles in different zones are effectively controlled in contrast to the normal joint.

Numerical studies of effect of tool sizes and pin shapes on friction stir welding of AA2024-T3 alloy

Coupled thermo-mechanical model was used to investigate the effects of the pin diameter, the shoulder diameter and the in conical angle on the heat generations, the material deformations and the energy histories in friction stir welding（FSW） of AA2024-T3 alloy. Results indicate that the shoulder-plate contact area takes more important contribution to the heat generation than the pin-plate contact area. The increase of the shoulder diameter or the decrease of the pin diameter can lead to the increase of the welding temperature in FSW, but the change of shoulder size is more important. Compared to the cases in FSW of AA6061-T6, the input power is obviously increased in FSW of AA2024-T3 and the ratio of the plastic dissipation to the friction dissipation becomes decreased.

Optimization of Welding Parameters for Friction Stir Lap Welding of AA6061-T6 Alloy

Friction Stir Welding (FSW) is currently used in many aircraft and aerospace sheet metal structures involving lap joints and there has been growing interest in recent years in utilizing this process for joining aluminum alloys. In this paper, Friction Stir Lap Welding (FSLW) of the 6061-T6 aluminum alloy was carried out to obtain the optimum welding condition for maximum shear strength where the rotational speed, axial load, and welding speed were taken as process parameters. An L-9 orthogonal array, a Taguchi Method with consideration of three levels and three factors was designed and executed for conducting trials. Analysis of variance (ANOVA) and Signal to Noise (S/N) ratio were employed to investigate the influence of different welding parameters on the shear strength and obtain the optimum parameters. The Fisher-Test was also implemented to find the design parameter which had the most important effect on the characteristic of quality. The results indicated that the tool rotational speed had the maximum percentage contribution (51%) on the response (shear strength) followed by the welding speed (38%) and the axial load (8%) while the percentage of error was 3%. However, to confirm the main effects for the means and S/N ratios of the experiment, theoretical shear strength values were computed to predict the tensile strength. The maximum shear strength of 60 MPa was achieved and the effectiveness of the method was confirmed. The optimum parameter combinations that provided higher shear strength were: rotational speed of 1200 rpm, welding speed of 45 mm/min and the axial load of 11.5 kN.