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.

Effects of shoulder on interfacial bonding during friction stir lap welding of aluminum thin sheets using tool without pin

To separately investigate the potential effects of shoulder on increasing interfacial bonded area and its mechanism,friction stir lap welding(FSLW)of 1.8 mm thick Al sheets without and with insert(copper foil or Al-12Si powders)was conducted using a special tool without pin,respectively.All the FSLW joints(without insert)fractured within top sheet but not along faying surface,suggesting that the shoulder plays an important role comparable or superior to pin in FSLW of thin sheets.Using several specially designed experimental techniques,the presence of forging and torsion actions of shoulder was demonstrated.The fracture surface of the joints with inserts indicates that interfacial wear occurs,which results in the oxide film disruption and vertically interfacial mixing over the area forged by shoulder with a larger diameter than a general pin,especially at the boundary region of weld.The boundary effect can be induced and enhanced by forging effect and torsion effect.

Tensile resistance,microstructures of intermetallic compounds,and fracture modes of welded steel/aluminum joints produced using laser lap welding

The joining of DP780 steel to Al5052 was conducted by laser lap welding,in which the metal vapor and spatters were monitored by a high-speed camera.A universal testing machine was used to test the mechanical properties of the welded joints,and the changing law of lap tensile resistance with the laser welding parameters was analyzed.Optical microscope and scanning electron microscope were used to observe the macro-structure and micro-structure,respectively.Three different intermetallic compounds(IMCs)phases,i.e.banded Fe2Al5,FeAl2 and needle-like FeAl3 were generated at the steel/Al interface on microscopic observation.The aim of this research is to investigate the relationship among the lap tensile resistance,the welding parameters and the failure mode under different energy densities.Experimental results showed that the steel/Al joints have two different fracture modes at low heat input and high heat input.The failures happened along the heat-affected zone of the weld and along the steel/Al joint interface,respectively.And both of the two failure modes are brittle fractures.Additionally,cracks appeared at the fracture interface,and needle-like particle clusters were found in the fracture microstructure.

Influence of rotational speed on mechanical properties of friction stir lap welded 6061-T6 Al alloy

The effect of rotational speed on macro and microstructures, hardness, lap shear performance and failure mode of friction stir lap welding on AA6061-T6 Al alloy with 5 mm in thickness was studied by field-emission scanning electron microscopy （FE-SEM）. The results represent much closer hardness distribution in the upper and lower plates at the lowest rotational speed. It indicates the Fe-compounds in the fracture surface of the nugget zone by EDX.

Effect of laser power on microstructure and mechanical properties of laser heat conduction lap welded joint between AZ31B magnesium alloy and DP780 galvanized steel

In this work, laser heat conduction lap welding(LHCLW) of AZ31B magnesium alloy sheet and DP780galvanized steel sheet was carried out by the defocused laser beam. The effects of laser power on the microstructure and mechanical properties of the joint were studied. The pros and cons of the joint were identified and evaluated by measuring the tensile shear strength, microhardness and microstructure observation. The formation mechanism of various phases at the Mg/steel interface was analyzed. The results indicated that the galvanized layer could promote the metallurgical bonding between magnesium alloy and steel by improving the diffusion ability of molten magnesium alloy at the steel interface and reacting with Mg, so as to enhance the strength of the joint. A continuous dense layered eutectic structure(α-Mg+MgZn) was formed at the interface of the joint, while MgZn_(2)and MgZn phase was formed at the weld edge zone and heat affective zone(HAZ), whereas no reaction layer was generated between the uncoated steel and magnesium alloy. A sound joint could be obtained at 2.5 kW, and the corresponding tensile shear strength reached the maximum value of 42.9 N/mm. The strength was slightly reduced at 2.6 kW due to the existence of microcracks in the eutectic reaction layer.

Effect of process parameters on interfacial microstructure and mechanical properties of Al/Cu friction stir lap welding joints

In this study,friction stir lap welding(FSLW)was performed for the welding test of 6061 aluminium alloy and T2 pure copper.The effect of process parameters containing rotation rate and travel speed on interfacial microstructure evolution and mechanical properties of Al/Cu dissimilar joints were explored.The experiments were carried out under the rotation rates of 600,900 and 1200 r/min and with the travel speeds of 30,70 and 100 mm/min.The characteristic of interface transition zones(ITZs)and the species of intermetallic compounds(IMCs)were investigated.The Al/Cu interface showed a layered structure composed of Al-Cu IMCs,which will affect the mechanical property.The layer consisting of Al2Cu was formed at lower heat input,and as heat input increased the Al4Cu9 phase started to form.Excessive heat input will increase the thickness of the interface and raise the brittleness of the joints.The thickness of the IMCs layers changed from0.89μm to 3.96μm as the heat input increased.The maximum value of tensile shear loading of 4.65 kN was obtained at the rotation rate of900 r/min and travel speed of 100 mm/min with the interface thickness of 2.89μm.The fracture mode of the joints was a mix of ductile and brittle fracture.

Effect of pin rotating speed on lap shear strength of stationary shoulder friction stir lap welded 6005A-T6 aluminum alloy

Stationary shoulder friction stir lap welding （SSFSLW） was successfully used to weld 6005A-T6 aluminum alloy in this paper. Effect of pin rotating speed on cross section morphologies and lap shear strength of the SSFSLW joints were mainly discussed. Results show that joints without flash and shoulder marks can be obtained by the stationary shoulder. Cross section of the SSFSLW joint presents a basin-like morphology and little material loss. By increasing the rotating speed from 1 000 rpm to 1 600 rpm, both effective sheet thickness and lap width increase, while lap shear failure load firstly decreases and then increases. The maximum failure load of 14. 05 kN /s attained when 1 000 rpm is used. All SSFSLW joints present shear fracture mode.

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.

Monitoring and phenomena observation during YAG laser lap welding of Zn-coated steel sheets

A study was performed with the objectives of understanding lap welding phenomena of Zn-coated steels with a Nd:YAG laser as well as obtaining a fundamental knowledge of monitoring signals for the formation judgment of sound or bad weld beads. The behavior of a molten pool and a reflected beam was simultaneously observed through a high-speed video together with the monitoring of reflected beam intensity.The effect of a gap between sheets on porosity formation and bead appearances was confirmed,and characteristic monitoring signals were obtained according to the gaps.In the case of no gap,spatters were frequently generated,and a reflected beam was fluctuated intensively at low frequencies.On the other hand,in welding sheets with a wide gap,lap welds were not produced and the high frequency signals of a reflected beam were detected.Moreover,sound welds were produced in the sheets with a proper gap,and a moderate reflected beam was monitored.From these results,it was found that monitoring of a reflected beam was beneficial to the judgment of sound,under-filled or incomplete lap welds.

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.

A novel seal-flow multi-vortex friction stir lap welding of metal to polymer matrix composites

The friction stir lap welding(FSLW)of metal to polymer is a challenging work due to the unavoidable polymer overflowing.Facing this problem,a novel seal-flow multi-vortex friction stir lap welding(SM-FSLW)technology based on the subversively-designed multi-step pin was put forward.Choosing 7075 aluminum alloy and short glass fiber-reinforced polyether ether ketone(PEEK)as research subjects,the welding temperature,material flow,formation and tensile shear strength of dissimilar materials lap joint under the SM-FSLW were studied and compared with those under traditional FSLW based on the conical pin.The multi-step pin rather than the conical pin effectively hindered the polymer overflowing due to the formation of vortexes by the step,thereby attaining a joint with a smooth surface.Compared with traditional FSLW,the SMFSLW obtained the higher welding temperature,the more violent material flow and the larger area with high flow velocity,thereby producing the macro-mechanical and micro-mechanical interlockings and then heightening the joint loading capacity.The tensile shear strength of lap joint under SM-FSLW was 27.8% higher than that under traditional FSLW.The SM-FSLW technology using the multi-step pin provides an effective way on obtaining a heterogeneous lap joint of metal to polymer with the excellent formation and high strength.

Bulging Distortion of Austenitic Stainless Steel Sheet on the Partially Penetrated Side of Non-Penetration Lap Laser Welding Joint

Non-penetration laser welding of lap joints in austenitic stainless steel sheets is commonly preferred in fields where the surface quality is of utmost importance.However,the application of non-penetration welded austenitic stainless steel parts is limited owing to the micro bulging distortion that occurs on the back surface of the partial penetration side.In this paper,non-penetration lap laser welding experiments,were conducted on galvanized and SUS304 austenitic stainless steel plates using a fiber laser,to investigate the mechanism of bulging distortion.A comparative experiment of DC01 galvanized steel-Q235 carbon steel lap laser welding was carried out,and the deflection and distortion profile of partially penetrated side of the sheets were measured using a noncontact laser interferometer.In addition,the cold-rolled SUS304 was subjected to heat holding at different temperatures and water quenching after bending to characterize its microstructure under tensile and compressive stress.The results show that,during the heating stage of the thermal cycle of laser lap welding,the partial penetration side of the SUS304 steel sheet generates compressive stress,which extrudes the material in the heat-affected zone to the outside of the back of the SUS304 steel sheet,thereby forming a bulge.The findings of these experiments can be of great value for controlling the distortion of the partial penetrated side of austenitic stainless steel sheet during laser non-penetration lap welding.

Improving tensile-shear properties of friction stir lap welded dissimilar Al/Mg joints by eliminating hook defect and controlling interfacial reaction

To improve tensile-shear properties of fiction stir lap welded(FSLW) dissimilar Al/Mg joints, pin-tip profiles were innovatively designed and welding speed was optimized, and effects of them on formation, interface microstructure and mechanical properties of different FSLW joints were investigated. With increasing the welding speed, the tensile-shear load of FSLW joints produced by three pins presents an increasing firstly and then decreasing trend. Compared with Rpin, the hook and hole defect in the joints made by S-pin and T-pin are eliminated owing to additional eccentric force. Moreover, the joints obtained by T-pin at 75 mm/min have the highest tensile-shear load, and a maximum value of 3.425 kN is produced, which increases by 96.8%.Meanwhile, the pin-tip profile improves significantly the interface reaction depending on the welding temperature. For R-pin, thick brittle intermetallic compounds of about 6.9 μm Al3Mg2and 13.3 μm Al12Mg17layers at the welding interface derived from diffusion reaction are formed, resulting in continuous cracks. However, using T-pin can raise the interface temperature, and which makes the interface liquefy locally to generate only 2.2 μm Al3Mg2layer and dispersive(Al12-Mg17+Mg) eutectic structure. This can release high residual stress and remove welding crack, consequently enhancing the interface properties of T-pin joints.

Effect of tool tilt angle on strength and microstructural characteristics of friction stir welded lap joints of AA2014-T6 aluminum alloy

Friction stir welding(FSW)has been extensively adopted to fabricate aluminium alloy joints by incorporating various welding parameters that include welding speed,rotational speed,diameters of shoulder and pin and tool tilt angle.FSW parameters significantly affect the weld strength.Tool tilt angle is one of the significant process parameters among the weld parameters.The present study focused on the effect of tool tilt angle on strength of friction stir lap welding of AA2014-T6 aluminium alloy.The tool tilt angle was varied between 0°and 4°with an equal increment of 1°.Other process parameters were kept constant.Macrostructure and microstructure analysis,microhardness measurement,scanning electron micrograph,transmission electron micrograph and energy dispersive spectroscopy analysis were performed to evaluate the lap shear strength of friction stir lap welded joint.Results proved that,defect-free weld joint was obtained while using a tool tilt angle of 1°to 3°.However,sound joints were welded using a tool tilt angle of 2°,which had the maximum lap shear strength of 14.42 kN and microhardness of HV 132.The joints welded using tool tilt angles of 1°and 3°yielded inferior lap shear strength due to unbalanced material flow in the weld region during FSW.

Dissimilar Al/Mg alloys friction stir lap welding with Zn foil assisted by ultrasonic

The Zn-added ultrasonic assisted friction stir lap welding(UaFSLW) was carried out to improve the quality of dissimilar Al/Mg alloys joint. The effects of ultrasonic power on the joint quality were also investigated.The results indicated that the larger effective lap width and mixing region between Mg and Al(Mg/Al MR) were attained by Zn foil addition and external ultrasonic assistance. Compared with the conventional joint, the finer and better-distributed Mg-Zn IMCs placing the continuous Al-Mg IMCs were formed in the Mg/Al MR of the Zn-added UaFSLW joint. The Zn foil addition and external ultrasonic assistance significantly improved the tensile shear load of the joint, and the load was increased with the increase of the ultrasonic power. The maximum tensile shear load of 7.95 kN was attained, which was 52.6% larger than that of the conventional joint.

Improving Joint Morphologies and Tensile Strength of Al/Mg Dissimilar Alloys Friction Stir Lap Welding by Changing Zn Interlayer Thickness

The pure Zn foils with different thicknesses(0.02, 0.05, 0.1, 0.2 and 0.3 mm) were selected as interlayers to improve the quality of friction stir lap welding joint of 7075-T6 Al and AZ31 B Mg dissimilar alloys. The effects of the interlayer thickness on joint formation, microstructure and tensile strength were analyzed. The results displayed that the maximum length of the boundary between stir zone(SZ) and thermo-mechanically affected zone in lower plate was obtained by the addition of the Zn interlayer with 0.05 mm thickness. The Mg–Zn intermetallic compounds(IMCs) were discontinuously distributed in the SZ, replacing the continuous Al–Mg IMCs. The size of Mg–Zn IMCs increased with the increase in the thickness of the Zn interlayer. The maximum tensile shear strength of 276 N mm-1 was obtained by the addition of 0.05 mm Zn foil, which increased by 45.6% of that of the joint without the Zn foil addition.

Experimental and numerical investigations of bonding interface behavior in stationary shoulder friction stir lap welding

Stationary shoulder friction stir lap welding(SSFSLW) was employed to weld 2024 aluminum alloy. A coupled Eulerian-Lagrangian(CEL) model was developed to investigate the lap interface behavior during SSFSLW. Numerical results of material movement and equivalent plastic strain were in good agreement with the experimental work. With increasing welding speed, the distances from the hook tip to the top surface of the upper workpiece on the retreating side(RS) and the advancing side(AS) increase, while the distance between two wave-shaped alclads decreases. A symmetric interface bending is observed on the AS and the RS during plunging, while the interface bending on the AS is bigger than that on the RS during welding. The peak temperature of the interface on the AS is higher than that on the RS. The equivalent plastic strain gradually increases as the distance to the weld center decreases, and its peak value is obtained near the bottom of the weld.

Effects of process parameters on microstructure and mechanical properties of friction stir lap linear welded 6061 aluminum alloy to NZ30K magnesium alloy

The microstructures and lap-shear behaviors of friction stir lap linear welded as-extruded 6061 Al alloy to as-cast Mg–3.0Nd–0.2Zn–0.7Zr(wt.%)(NZ30K)alloy joints were examined.Various tool rotation and travel speeds were adopted to prepare the joints.The analysis of temperature field indicates that the peak temperature for each sample can reach 450℃,which exceeds the eutectic reaction temperatures of 437℃ and 450℃ according to the binary phase diagram of Al–Mg system.The fierce intermixing can be found at the interface between Al and Mg alloys,forming the intermetallic of Al_(3)Mg_(2).Welds with the rotation speed of 900 rpm and travel speed of 120 mm/min display the highest tensile shear failure load of about 2.24 kN.The value was increased by 13%after the sample was heat treated at 400℃ for 0.5 h.

Microstructure and Mechanical Properties of Friction Stir Lap Welded Aluminum Alloy AA2014

Friction stir lap welds were produced in 3 mm thick Alclad sheets of Al alloy 2014-T4 using two different tools （with triangular and threaded taper cylindrical pins）. The effects of tool geometry on weld microstructure, lap-shear performance and failure mode were investigated. The pin profile was found to significantly influence the hook geometry, which in turn strongly influenced the joint strength and the failure mode. Welds produced in alloy 2014-T4 Alclad sheets by using triangular and threaded taper cylindrical tools exhibited an average lap-shear failure load of 16.5 and 19.5 kN, respectively, while the average failure load for standard riveted joints was only 3.4 kN. Welds produced in alloy 2014-T6 Alclad sheets and in alloy 2014-T4 bare sheets （i.e., no Alclad） were comparatively evaluated with those produced in alloy 2014-T4 Alclad sheets. While the welds made （with threaded taper cylindrical tool） in T6 and T4 conditions showed very similar lap-shear failure loads, the joint efficiency of the welds made in T6 condition （43%） was considerably lower （because of the higher base material strength） than those made in T4 condition （51%）. The Alclad layers were found to present no special problems in friction stir lap welding. Welds made with triangular tool in alloy 2014-T4 Alclad and bare sheets showed very similar lap-shear failure loads. The present work provides some useful insights into the use of friction stir welding for joining Al alloys in lap configuration.

Friction assisted solid state lap seam welding and additive manufacturing method

This paper describes results of seam welding of relatively high temperature melting materials, AISI 304, C-Mn steels, Ni-based alloys, CP Cu, CP Ni, Ti6Al4V and relatively low temperature melting material, AA6061. It describes the seam welding of multi-layered similar and dissimilar metallic sheets. The method described and involved advancing a rotating non-consumable rod(CP Mo or AISI 304) toward the upper sheet of a metallic stack clamped under pressure. As soon as the distal end of the rod touched the top portion of the upper metallic sheet, an axial force was applied. After an initial dwell time, the metallic stack moved horizontally relative to the stationery non-consumable rod by a desired length, thereby forming a metallurgical bond between the metallic sheets. Multi-track and multi-metal seam welds of high temperature metallic sheets, AISI 304, C-Mn steel,Nickel-based alloys, Cp Cu, Ti6Al4V and low temperature metallic sheets, AA6061 were obtained. Optical and scanning electron microscopy examination and 180 degree U-bend test indicated that defect free seam welds could be obtained with this method. Tensile- shear testing showed that the seam welds of AISI 304, C-Mn steel, Nickel-based alloy were stronger than the starting base metal counterparts while AA6061 was weaker due to softening. The metallurgical bonding at the interface between the metallic sheets was attributed to localized stick and slip at the interface, dynamic recrystallization and diffusion. The method developed can be used as a means of welding, cladding and additive manufacturing.