Improvement of Microstructure and Mechanical Properties of Rapid Cooling Friction Stir-welded A1050 Pure Aluminum

Two-mm thick A1050 pure aluminum plates were successfully joined by conventional and rapid cooling friction stir welding(FSW), respectively. The microstructure and mechanical properties of the welded joints were investigated by electron backscatter diffraction characterization, Vickers hardness measurements, and tensile testing. The results showed that liquid CO_(2) coolant significantly reduced the peak temperature and increased the cooling rate, so the rapidly cooled FSW joint exhibited fine grains with a large number of dislocations. The grain refinement mechanism of the FSW A1050 pure aluminum joint was primarily attributed to the combined effects of continuous dynamic recrystallization, grain subdivision, and geometric dynamic recrystallization. Compared with conventional FSW, the yield strength, ultimate tensile strength, and fracture elongation of rapidly cooled FSW joint were significantly enhanced, and the welding efficiency was increased from 80% to 93%. The enhanced mechanical properties and improved synergy of strength and ductility were obtained due to the increased dislocation density and remarkable grain refinement. The wear of the tool can produce several WC particles retained in the joint, and the contribution of second phase strengthening to the enhanced strength should not be ignored.

Beam oscillating parameters on pore inhibition,recrystallization and grain boundary characteristics of laser-arc hybrid welded AZ31 magnesium alloy

Oscillating laser-arc hybrid welding of AZ31B magnesium alloy was carried out,the effects of beam oscillation parameters on pore inhibition,microstructure,grain boundary characteristics and tensile properties were investigated.The results showed that the pore formation can be inhibited with oscillating frequency higher than 75 Hz and radius smaller than 0.5 mm.The columnar grains neighboring the fusion line can be broken by the beam oscillation behavior,while the grain growth was promoted with the increase of frequency or radius.It should be noted that the coincidence site lattice(CSL)boundaries were mainlyΣ13b andΣ29 boundaries,which were contributed by{10■2}tensile twins and{11■2}compression twins,respectively.The total fraction of CSL boundaries reached maximum at radius of 0.25 mm and frequency of 75 Hz,which was also confirmed as the optimized parameters.In this case,the elongation rate increased up to 13.2%,12.8%higher than that of the weld without beam oscillation.Finally,the pore formation and inhibition mechanisms were illustrated according to the state of melt flow and keyhole formation,the abnormal growth was discussed basing on secondary recrystallization,and the relationship among the pore formation,grain size,boundary characteristics and weld toughness were finally established.

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 homogeneity and mechanical properties of laser-arc hybrid welded AZ31B magnesium alloy

Laser-arc hybrid welding of AZ31B magnesium alloy was carried out,the effects of welding parameters on weld formation,microstructure homogeneity and mechanical properties were investigated.The results showed that laser-arc hybrid welding was beneficial to improve the weld formation of magnesium alloy by inhibiting the defect of undercut and pores.The weld microstructure was mainly columnar grains neighboring the fusion line and equiaxed grains at the weld center.It was interesting that the grain size at the upper arc zone was smaller than that at the lower laser zone,with the difference mainly affected by laser power rather than welding current and welding speed.The welding parameters were optimized as laser power of 3.5 kW,welding current of 100 A and welding speed of 1.5 m/min.In this case,the weld was free of undercut and pores,and the tensile strength and elongation rate reached 252 MPa and 11.2%,respectively.Finally,the microstructure homogeneity was illustrated according to the heat distribution,and the evolution law of tensile properties was discussed basing on the weld formation and microstructure characteristics.

Effect of long-term thermal exposure on microstructure of laser-welded UNS N10003 alloy

The evolution of the microstructure and tensile rupture mechanism of laser welds in UNS N10003 alloy exposed to 700℃are investigated.Fine M_(6)C carbides precipitate around the primary eutectic M_(6)C-carbides in the fusion zone after 100 h of exposure.During long-term thermal exposure,the size of the fine M_(6)C carbides increased.The eutectic M_(6)C-carbides in the as-welded fusion zone transformed into spherical M_(6)C carbides as the exposure time extends to 10000 h.Additionally,the spherical M_(6)C particles exhibit size coarsening with increasing exposure time.The tensile properties of the welded joints are not adversely affected by the evolution of eutectic M_(6)C-carbides and the coarsening of M_(6)C carbides.

Numerical Simulation Analysis of Welded Joints in Arch Ribs of Large Span Steel Pipe Arch Bridges

In order to study the residual stress distribution law of welded joints of arch ribs of large-span steel pipe concrete arch bridges,numerical simulation of temperature,stress and strain fields based on ABAQUS for welded joints of arch-ribbed steel tubes using 7-,8-and 9-layer welds is carried out and its accuracy is demonstrated.The steel pipe welding temperature changes,residual stress distribution,different processes residual stress changes in the law,the prediction of post-weld residual stress distribution and deformation are studied in this paper.The results show that the temperature field values and test results are more consistent with the accuracy of numerical simulation of welding,the welding process is mainly in the form of heat transfer;Residual high stresses are predominantly distributed in the Fusion zone(FZ)and Heat-affected zone(HAZ),with residual stress levels tending to decrease from the center of the weld along the axial path,the maximum stress appears in the FZ and HAZ junction;The number of welding layers has an effect on the residual stress distribution,the number of welding layers increases,the residual stress tends to decrease,while the FZ and HAZ high stress area range shrinks;Increasing the number of plies will increase the amount of residual distortion.

Research on microstructure and properties of boron/Q235 steel laser welded dissimilar joints under synchronous thermal field

The mechanical mismatch effect frequently occurs in the dissimilar materials welded joints, thus leading to plastic gradient at the interface between the weld and heat-affected zone(HAZ). In this work, the boron steel and Q235 steel were selected for laser tailor welding,which obtained boron/Q235 steel tailor-welded blanks(TWBs). The method of welding with synchronous thermal field(WSTF) was utilized to eliminate the mismatch effects in TWBs. The WSTF was employed to adjust cooling rates of welded joints, thereby intervening in the solidification behaviors and phase transition of the molten pool. Boron/Q235 steel was welded by laser under conventional and WSTF(300-600 ℃) conditions, respectively. The results show that the microstructure of weld and HAZ(boron) was adequately transitioned to ferrites and pearlites instead of abundant martensite by WSTF. Meanwhile, the discrepancy of microhardness and yield strength between various regions of welded joints was greatly reduced, and the overall plasticity of welded joints was enhanced by WSTF. It is indicated that WSTF can effectively contribute to reducing plastic gradient and achieving mechanical congruity in welded joints by restraining the generation of hardbrittle phase, which could significantly improve the formability of TWBs in subsequent hot stamping.

Mechanical deformation properties of Continuous Welded Rail on kilometer-span suspension bridge for high-speed railway

The complex bridge-track interaction between kilometer-span bridges and continuous Welded Rail(CWR)brings great challenges to CWR designing.Taking a suspension bridge with laying CWR as a case,the mechanical properties of CWR on the bridge are analyzed to reveal the sensitive areas of the track,and the design method of CWR and track structures on the beam ends are proposed.The results show that the unidirectional Rail Expansion Joints(REJ)need to be installed on the beam end of the kilometer-span bridge to reduce rail longitudinal force.Due to the bridge characteristics,there is no CWR fixed area on the kilometer-span bridge,and rail longitudinal force on the main span caused by bending loads needs to be concerned.The deformation of track on the beam end is complex,which is the weak area on the kilometer bridge,the large relative displacement between the stock rail of REJ and the main beam can cause poor stability of ballast bed on beam end,small resistance fasteners need to be laid on the sides of stock rail on the main beam to increase the stability of ballast and fasteners on the beam end.To improve the driving safety and comfort of beam end,the Sleeper-Supporting Apparatus(SSA)should be specially designed to ensure the uniform transition of track on beam ends.Temperature and wind loads have a significant impact on track regularity on the kilometer span bridge,the dynamic response of trains and bridges under those loads needs to be attended to.

An innovative joint interface design for reducing intermetallic compounds and improving joint strength of thick plate friction stir welded Al/Mg joints

Friction stir welding of dissimilar Al/Mg thick plates still faces severe challenges, such as poor formability, formation of thick intermetallic compounds, and low joint strength. In this work, two joint configurations, namely inclined butt(conventional butt) and serrated interlocking(innovative butt), are proposed for improving weld formation and joint quality. The results show that a continuous and straight intermetallic compound layer appears at the Mg side interface in conventional butt joint, and the maximum average thickness reaches about 60.1 μm.Additionally, the Mg side interface also partially melts, forming a eutectic structure composed of Mg solid solution and Al_(12)Mg_(17) phase.For the innovative butt joint, the Mg side interface presents the curved interlocking feature, and intermetallic compounds can be reduced to less than 10 μm. The joint strength of innovative butt joint is more than three times that of conventional butt joint. This is due to the interlocking effect and thin intermetallic compounds in the innovative joint.

A Review of the Advances Made in Improving the Durability of Welded Wood against Water in Light of the Results of African Tropical Woods Welding

Wood plays a major role in the production of furniture and wooden structures.Nevertheless,in this process,the massive use of adhesives and plural connectors remains a definite problem for health and the environment.Therefore,wood welding is a breakthrough in this respect.This paper reviews the applications of wood welding in furniture and construction and then examines advances in improving the durability of welded wood against water.Our contribution also highlights the need to join African tropical woods using the rotational friction welding technique.According to our results,these woods present interesting chemical singularities,which could provide solutions to the water vulnerability of the welded wood.Moreover,the use of such a joining method would first free the Cameroonian furniture industry from the chemical industry,secondly position it at the forefront of new eco-design trends and thirdly make it competitive with other countries in the Central African sub-region.These works enrich the long and rich bibliography on the technique of wood welding,which has long been conspicuous by its absence of tropical woods.

Repairing of exit-hole in friction-stir-spot welded joints for 2024-T4 aluminum alloy by resistance welding

The exit-hole in friction stir spot welded(FSSWed) 2024-T4 aluminum alloy joints was successfully repaired by using a three-phase secondary rectification resistance spot welding machine, which is termed as filling exit-hole based on resistance welding(FEBRW). The filling dynamic behavior of force was recorded by a device monitoring. Optical microscope(OM), electron backscatter diffraction(EBSD), and tensile shear tests and finite element modelling were conducted to investigate the repairing stages and bonding mechanisms of the repaired joints in detail. Results showed that exit-hole was completely filled and repaired experiencing three stages. Metallurgical bonding was achieved between plug and exit-hole wall in two forms, including melting bonding in the middle of the joints and partial diffusion bonding on both the upper and bottom of the joints. The highest tensile shear strength of the repaired joints was 7.43 kN, which was 36.3% higher than that of the as welded joints. Resistance welding paves an efficient way to repair the exit-hole in FSSWed joints.

Experimental and numerical studies of titanium foil/steel explosively welded clad plate

Ti/Fe clad plate had attracted extensive attention because of its important application. In order to reduce the titanium layer thickness, the explosive welding of TA1 titanium foil to Q235 steel plate was carried out. The interfacial bonding performance was analyzed by micromorphology analysis and mechanical property test, and the formation process of interfacial wave and molten block in the vortex was simulated by smoothed particle hydrodynamics(SPH) method. The results showed that salt as pressure transfer layer used in explosive welding could play a good buffer effect on the collision between flyer and base layers. Regular waveforms were formed on the bonding interface, and the titanium foil/steel clad plate exhibited good welding quality and bonding property. The crest of the observed interfacial wave moved 200 μm from the beginning to the final formation, and it was important of jet on the formation of interfacial waveform. The interface was mainly bonded in the form of molten layer, and the grains near the interface were streamlined. Molten block containing intermetallic compounds and metal oxides appeared in the vortex of wave crest.

The MFL Testing Methods for Welded Pipes

Steel pipes are categorized into seamless pipes and welded pipes,and particularly the welded pipes’ NDT(Non-Destructive Testing)has been a challenging problem.In the case,on the basis of the presentation of welded pipes,the analysis of its relevant testing key is carried out.Afterwards,the MFL(Magnetic Flux Leakage)methods for longitudinally welded line-pipes and for helically welded pipes are respectively proposed.Meanwhile,their relevant experiments are conducted,and finally the two technologies for the two types of welded pipes are verified well.

High-Velocity Projectile Impact Behaviour of Friction Stir Welded AA7075 Thick Plates

In this study, high velocity impact behaviour of friction stir welded AA7075-T651 25 mm thick plates were investigated using a 7.62 mm × 51 mm lead core and 7.62 mm × 39 mm steel core projectiles. Prior to ballistic trails, mechanical and metallurgical properties of friction stir welded AA 7075-T651 25 mm thick plates were studied. Microstructural and hardness studies revealed that friction stir welds constituted three distinct regions namely Weld Nugget(WN), Thermo-Mechanically Affected Zone(TMAZ) and Heat Affected Zone(HAZ). Base Material(BM) and all three weld regions were ballistically tested as per military standard NIJ.0108.01 using lead and steel core bullets at maximum permissible velocities of 830 ± 20 and 700 ± 30 m/s, respectively. It has been found that base material(AA7075-T651)and all three weld regions of 25 mm thick plates were able to resist perforation by both types of projectiles used. However depth of penetration has been found to increase from BM to WN, HAZ and TMAZ for both types of projectiles. In all cases steel core projectiles caused higher depth of penetration compared to those caused by lead core projectiles. TMAZs of the friction stir welds were found to be the weakest zone. The fracture that occurred in the base material was spall fragmentation indicating brittle failure, whereas all zones of friction stir welded AA7075-T651 targets with a front petalling, indicating ductile failure. The post-ballistic tested samples showed no significant change in the microstructure of the BM and WN. On the other hand, TMAZ and HAZ showed severe grain deformation in the direction of projectile penetration, and the formation of adiabatic shear bands(ASB). This work showed that 25 mm thick friction stir welded AA7075-T651 joints responded well to ballistic impact loads, making them a good choice for light combat vehicles.

Comparison of FSW and TIG welded joints in Al-Mg-Mn-Sc-Zr alloy plates

In order to study the welding process,microstructure and properties of Al-Mg-Mn-Sc-Zr alloy,comparative methods of friction stir welding（FSW） and tungsten inert gas（TIG） were applied to the two conditions of this alloy,namely hot rolled plate and cold rolled-annealed plate.The relationships between microstructures and properties of the welded joints were investigated by means of optical microscopy and transmission electron microscopy.Compared with the base metal,the strength of FSW and TIG welded joints decreased,and the FSW welding coefficients were higher than the TIG welding coefficients.The loss of substructure strengthening and a very little loss of precipitation strengthening of Al3（Sc,Zr） cause the decreased strength of FSW welded joint.But for the TIG welded joint,the disappearance of both the strain hardening and most precipitation strengthening effect of Al3（Sc,Zr） particles contributed to its softening.At the same time,the grains in weld nugget zone of FSW welded joints were finer than those in the molten zone of TIG welded joints.

Comprehensive study of microstructure and mechanical properties of friction stir welded 5182-O/HC260YD+Z lap joint

The welding of aluminum(Al)and steel has attracted more and more interest due to the weight reduction trend in vehicle and aerospace manufacturing industries.5182-O/HC260YD+Z lap joint was produced by friction stir welding(FSW),and the microstructure and mechanical property of the joint were systemically characterized.The microstructure in horizontal direction of the Al and steel near interface was similar to their corresponding conventional friction stir welded joint.The joint was divided into stir zone of Al(ST-Al),stir zone of interface(ST-I),thermal-mechanically affected zone of steel(TMAZ-Fe)and base material of steel(BM-Fe)according to their distinct microstructure vertically.Three kinds of intermetallic compounds(IMCs)of FeAl_(3),FeAl and Fe_(3)Al were formed at the interface.The horizontal micro hardness distribution exhibited a hat shape and“M”shape in Al and steel,respectively.The hardest region of the joint was located at the ST-I,with a hardness of 175 HV−210 HV.The joint was fractured along the hook structure,with an average shear strength of 73.9 MPa.Fractural morphology of Al and steel indicted a cleavage fracture mode.

Fatigue behavior and life prediction of A7N01 aluminium alloy welded joint

Fatigue characteristics of A7N01 aluminium alloy welded joint were investigated and a fatigue crack initiation life-based model was proposed. The difference of fatigue crack initiation life among base metal, weld metal and heat affected zone (HAZ) is slight. Furthermore, the ratio of fatigue crack initiation life (Ni) to fatigue life to failure(Nf) is a material dependent parameter, 26.32%, 40.21% and 60.67% for base metal, HAZ and weld metal, respectively. Total fatigue life predicted using the presented model is in good agreement with the experimental data and that using Basquin’s model. The observation results of fatigue fracture surfaces, using scanning electron microscope (SEM), demonstrate that fatigue crack initiates from smooth surface due to welding process for weld metal, blowhole in HAZ causes fatigue crack initiation, and the crushed second phase particles play an important part in fatigue crack initiation in base metal.

Fatigue behavior of AZ31B magnesium alloy electron beam welded joint based on infrared thermography

Fatigue behavior of AZ31B magnesium alloy electron beam welded joint undergoing cyclic loading was investigated by infrared thermography. Temperature evolution throughout a fatigue process was presented and the mechanism of heat generationwas discussed. Fatigue limit of the welded joint was predicted and the fatigue damage was also assessed based ontheevolution of the temperatureand hotspot zone on the specimen surfaceduring fatigue tests. The presented results show that infrared thermography can not onlyquicklypredict the fatigue behavior of the welded joint, but also qualitatively identify the evolution of fatigue damage in real time. It is found that the predicted fatigue limit agrees well with the conventionalS-Nexperimental results. The evolution of the temperatureand hotspot zone on the specimen surface can be an effectivefatigue damage indicatorfor effectiveevaluationof magnesium alloy electron beam welded joint.

Residual Stress and Fracture Toughness Study in A516 Gr70 Steel Joints Welded and Repaired by Arc Processes

Structural components made of steel are used in several areas and require welding for assembly. In some situations, repair of the weld bead, also performed by electric arc welding, can be used to correct, and eliminate any discontinuities. However, electric arc welding causes the presence of residual stresses in the joint, which can impair its performance and not meet specific design requirements. In this paper, welded joints made of ASTM A 516 GR 70 steel plates, with a thickness of 30.5 mm, welded by the MAG—Metal Active Gas process (20% CO2) and using a “K” groove were analysed. The joints were manufactured with seven welding passes on each side of the groove. After welding, one batch underwent repair of the bead by TIG welding (Tungsten Insert Gas) and another batch underwent two repairs by TIG welding. Were presented results of the behaviour of the residual stress profile measured by X-ray diffraction and the Vickers microhardness profile in the joints as well the fracture toughness in the conditions only welded and submitted to repairs. The results indicated that the greater number of repair passes reduced the residual compressive stress values obtained in the material manufacturing process and caused a stabilization on the Vickers hardness values. It was concluded that compressive residual stresses did not play a major role in the R-curve results. The presence of discontinuities in the welded joint caused greater influence on the behaviour of the R curve.

AN IMPROVED FOSM METHOD FOR CALCULATING FAILURE PROBABILITY OF WELDED PIPES WITH FLAWS

The R F first order second moment method will produce more error for calculating the reliability of welded engineering pipe structures when the failure function is seriously nonlinear and the random variables don′t serve as normal distribution. In order to increase the computing accuracy of reliability, an improved FOSM method is used for calculating the failure probability of welded pipes with flaws in this paper. Because of solving the problems of the linear expansion of failure function at the failure point and constructing equivalent normal variables, the new algorithm can greatly improve the calculating accuracy of probability of the welded pipes with cracks. The examples show that this method is simple, efficient and accurate for reliability safety assessment of the welded pipes with cracks. It can save more time than the Monte Carlo method does, so that the improved FOSM method is recommended for engineering reliability safety assessment of the welded pipes with flaws.