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.

Application of novel constrained friction processing method to produce fine grained biomedical Mg-Zn-Ca alloy

In order to obtain Mg alloys with fine microstructures and high mechanical performances,a novel friction-based processing method,name as“constrained friction processing(CFP)”,was investigated.Via CFP,defect-free Mg-Zn-Ca rods with greatly refined grains and high mechanical properties were produced.Compared to the previous as-cast microstructure,the grain size was reduced from more than 1 mm to around 4μm within 3 s by a single process cycle.The compressive yield strength was increased by 350%while the ultimate compressive strength by 53%.According to the established material flow behaviors by“tracer material”,the plastic material was transported by shear deformation.From the base material to the rod,the material experienced three stages,i.e.deformation by the tool,upward flow with additional tilt,followed by upward transportation.The microstructural evolution was revealed by“stop-action”technique.The microstructural development at regions adjacent to the rod is mainly controlled by twinning,dynamic recrystallization(DRX)as well as particle stimulated nucleation,while that within the rod is related to DRX combined with grain growth.

Effect of laser remelting on microstructure and mechanical properties of CrMnFeCoNi high entropy alloy

Equiatomic CrMnFeCoNi high entropy alloy prepared by powder metallurgy was remelted by laser.The relative density and microstructure of fusion zone are evaluated.The nanoindentation tests are conducted to reveal the hardness difference of dendrite arms and interdendritic areas.Tensile tests are conducted to assess the mechanical properties of remelted HEA.After laser remelting,the number and morphology of voids changed significantly.Dendritic structure with face-centered cubic phase form in the fusion zone.Fe,Cr and Co are enriched in dendrite arm,while Mn and Ni are enriched in interdendritic area.Elements segregation led to a nanohardness difference between dendrite arm and interdendritic area.Local deformation occurs in interdendritic area during tensile tests and results in a fracture with directionality.

Liquid vortexes and flows induced by femtosecond laser ablation in liquid governing formation of circular and crisscross LIPSS

Orientations of laser induced periodic surface structures(LIPSS)are usually considered to be governed by the laser polarization state.In this work,we unveil that fluid dynamics induced by femtosecond(fs)laser ablation in liquid(fs-LAL)can easily break this polarization restriction to produce irregular circular-LIPSS(CLIPPS)and crisscross-LIPSS(CCLIPSS).Fs laser ablation of silicon in water shows formation of diverse LIPSS depending on ablation conditions.At a high power of 700 mW(repetition rate of 100 kHz,pulse duration of 457 fs and wavelength of 1045 nm),single/twin CLIPSS are produced at the bottom of macropores of several microns in diameter due to the formation of strong liquid vortexes and occurrence of the vortex shedding effect.Theoretical simulations validate our speculation about the formation of liquid vortex with an ultrahigh static pressure,which can induce the microstructure trenches and cracks at the sidewalls for fs-LAL of Si and tungsten(W)in water,respectively.At a low power of 50 mW,weak liquid vortexes are produced,which only give birth to curved LIPSS in the valleys of grooves.Consequently,it is deduced that liquid vortex plays a crucial role in the formation of macropores.Mountain-like microstructures induce complex fluid dynamics which can cause the formation of CCLIPSS on them.It is believed that liquid vortexes and fluid dynamics presented in this work open up new possibilities to diversify the morphologies of LIPSS formed by fs-LAL.

Microstructures and properties of Fe-Co-B-Si-Nb coating prepared by laser cladding and remelting

High power laser cladding of [ （ Fe0. 5 Co0. 5 ） 0. 75 B0. 2 Si0.05 ] 95. 7 Nb4. 3 powder mixture afier-remelting was performed to fabricate Fe-based metallic glass coating on the surface of steel of China Classification Society： Grade B （CCS-B）. Scanning electron microscopy （SEM）, X-ray diffraction （XRD）, transmission electron microscopy （TEM） with energy dispersive spectrometer （EDS）, Vickers hardness tester and corrosion resistance tester were employed to characterize microstructures and evaluate properties of this coating. According to the results of SEM, XRD and TEM, the cladding coating consisted of nanocrystalline embedded in amorphous phase. EDS data indicated that Nb segregated in the amorphous matrix. The results of hardness test revealed that the hardness of the top layer was higher than that of the inner layer of the coating. The coating exhibited excellent corrosion resistance in a 3.5% NaCl solution.

Multipass welding of thick section steels using autogenous CO2 laser welding and CO2 laser-MIG hybrid welding

Laser multipass welding techniques for thick section steels have been developed using a new type of UV combined narrow groove. The shape and sizes at the bottom of groove are determined by analyzing the plasma behavior using high speed photographic equipment. A stable autogenous CO2 laser welding process and greater penetration are generated at the root pass because of strong reduction of the plasma volume. According to the waveforms of welding current and arc voltage, and the interaction between the arc and the laser induced plasma, a suitable groove angle is obtained. Laser-double MIG hybrid welding process is studied and the optimum distances between the laser and two arcs are determined. By using autogenous CO2 laser welding, CO2 laser-MIG hybrid welding and laser-double MIG hybrid welding, 28 mm thick steel plates are welded with four passes. The welds produced are assessed by X-ray. No crack is found and there is only a small amount of pores. The experimental results show that the multipuss welding procedures proposed can realize the joining of thick section steels with high efficiency and good quality.

Fabrication of Ni34.1Fe27.9B18Si18Nb2 amorphous matrix coating on mild steel by laser processing

Ni34.1 Fe27.9B18 Si18 Nb2 coating was deposited on mild steel substrate using high power laser cladding followed by laser remelting process. The laser processing was conducted by the powder feeding method using low purity materials without shielding box. To learn the surface amorphous matrix coating forming mechanism, the coating without remelting process was also studied. The phases and microstructures were analyzed by X-ray diffraction （XRD）, scanning- and transmission-electron microscopy（ SEM, TEM）. The microhardness and corrosion resistance property of the coating were also measured. The results of SEM, XRD and TEM analysis show that the remelted coating has an amorphous matrix layer embedded with some crystals due to high cooling rate during remelting process. The crystals phases are identified as Fe2 B phase, γ （Fe, Ni ） phase and α- Fe phase. No oxidation phases are found in the coating surface. Hardness profiles reveal microhardness more than 1 100 HVo.5 over the full depth of the amorphous matrix layer, while the unremtled coating and the substrate show relatively lower hardness than the remelted layer. Corrosion resistance tests exhibit that the remelted coating is nobler than the unremelted coating and the substrate material.

Analysis of melt ejection during long pulsed laser drilling

In pulsed laser drilling, melt ejection greatly influences the keyhole shape and its quality as well, but its mechanism has not been well understood. In this paper, numerical simulation and experimental investigations based on 304 stainless steel and aluminum targets are performed to study the effects of material parameters on melt ejection. The numerical method is employed to predict the temperatures, velocity fields in the solid, liquid, and vapour front, and melt pool dynamics of targets as well. The experimental methods include the shadow-graphic technique, weight method, and optical microscope imaging, which are applied to real-time observations of melt ejection phenomena, measurements of collected melt and changes of target mass, observations of surface morphology and the cross-section of the keyhole, respectively. Numerical and experimental results show that the metallic material with high thermal diffusivity like aluminum is prone to have a thick liquid zone and a large quantity of melt ejection. Additionally, to the best of our knowledge, the liquid zone is used to illustrate the relations between melt ejection and material thermal diffusivity for the first time. The research result in this paper is useful for manufacturing optimization and quality control in laser-material interaction.

Microstructure and Wear Behavior of FeBSiNbCr Metallic Glass Coatings

In this paper, FeBSiNbCr metallic glass coatings were prepared onto AISI 1045 steel substrate by using wire arc spraying process. The phase and structure of the coating were characterized by transmission electron microscopy （TEM）, X-ray diffraction （XRD） and scanning election microscopy （SEM） equipped with energy dispersive X-ray analysis （EDX）. The microstructure of the coating consists of full amorphous phase. The coating has high hardness and low porosity. Full density and little oxides are detected in the coating. The mechanical properties, especially wear resistance, were investigated. The relationship between wear behavior and structure of the coatings were analyzed in detail. The main failure mechanism of the metallic glass coating is brittle breaking and fracture. The results indicate that FeBSiNbCr metallic glass coating has excellent resistance to abrasive wear.

Effects of Active Gases on Droplet Transfer and Weld Morphology in Pulsed-Current NG-GMAW of Mild Steel

The current research of narrow-gap gas metal arc welding(NG-GMAW)primarily focuses on improving the sidewall fusion and avoiding the lack-of-fusion defect.However,the high cost and operation difficulty of the methods limit the industrial application.In this study,small amount of active gases CO_(2) and O_(2) were added into pure argon inert shielding gas to improve the weld formation of pulsed-current narrow-gap gas metal arc welding(NG-GMAW)of mild steel.Their effects on droplet transfer and arc behavior were investigated.A high-speed visual sensing system was utilized to observe the metal transfer process and arc morphology.When the proportion of CO_(2),being added into the pure argon shielding gas,changes from 5%to 25%,the metal transfer mode changes from pulsed spray streaming transfer to pulsed projected spray transfer,while it remains the pulsed spray streaming transfer when 2%to 10%O_(2) is added.Both CO2 and O_(2) are favorable to stabilizing arc and welding process.O_(2) is even more effective than CO_(2).However,O_(2) is more likely to cause slags on the weld surface,while CO_(2) can improve the weld appearance in some sense.The weld surface concavity in NG-GMAW is greatly influenced by the addition of active gas,but the weld width and weld penetration almost keep constant.This study proposes a new method which is beneficial to improving the weld bead formation and welding process stability.

Mathematical analysis of the role of recoil pressure in weld pool dynamics during laser-MIG hybrid welding

A mathematical model was established to simulate the weld pool development and dynamic process in stationary iaser-MlG hybrid welding. Surface tension and buoyancy were considered to calculate liquid metal flow patter, moreover, typical phenomena of MIG welding, such as filler droplets impinging weld pool, electromagnetic force in the weld pool, and typical phenomena of laser beam welding, such as recoil pressure, Inverse Bremsstrahlung absorption, Fresnel absorption were all considered in the model. The laser beam and arc couple effect were introduced into this model by the plasma width during hybrid welding. The role of recoil pressure in the weld formation was discussed. Transient weld pool shape and complicated liquid metal velocity distribution from two kinds weld pool to an unified weld pool were calculated. The simulated weld bead geometry with consideration recoil pressure was in good agreement with experimental measurement.

Joint performance of CO2 laser-MIG hybrid welding 5083-H116 alloy

Laser-MIG hybrid welding process was dealt with 6 mm thick 5083Hl16 Al-Mg alloy plate in butt-joint configuration. Weld formation principle during hybrid welding was explained. The joint properties and microstructure characteristics of welded joints were analyzed by tensile tests, fractographs observed by optical microscopy and scanning electron microscopy （SEM）. Higher heat input could obtain better mechanical properties, and tensile strength and elongation reached 97.2%, 81% of the base metal, respectively. Fracture position traasited from fusion line to weld center in the higher heat input, and fracture location were only in the center of welded joints for the heat input relatively small.

Low temperature impact toughness of laser hybrid welded joint of high strength low alloy steel

High strength low alloy steel with 16 mm thickness was welded by using high power laser hybrid welding. Microstrueture was characterized by using optical microscopy, scanning electron microscopy （ SEM ） , transmission electron microscopy （TEM） and selected area electron diffraction （SAED）. Low temperature impact toughness was estimated by using Charpy V-notch impact samples selected from the upper part and the lower part at the same heterogeneous joint. Results show that the low temperature impact absorbed energies of weld metal are （202,180,165 J） of upper samples and （178,145,160 J） of lower samples, respectively. All of them increase compared to base metal. The embrittlement of HAZ does not occur. Weld metal primarily consists of refined carbide free bainite and a little granular bainite since laser hybrid welding owns the character of low heat input. Retained austenite constituent film ＂locates among the lath structure of bainitie ferrite. Refined bainitic ferrite lath and retained austenite constituent film provide better low temperature impact toughness compared to base metal.

Research on weldability of ZL101/TiB2 MMCs by laser-TIG hybrid welding

The weldability of in situ TiB2 reinforced ZL101 metal matrix composites （ MMCs） was examined using laser-TIG hybrid welding technology. High speed camera was employed to observe the coupled results between laser plume and electric arc. Optical microscope （OM） , scanning electron microscope （SEM） , tensile machine and wearing equipment were used to evaluate the quality of welding joint. The effective hybrid welding was realized and defect-free seam was obtained when the laser power （8 kW）, TIG current （ 100 A） and welding speed （3 nv/min ） were combined. TiB2 distribution became much more homogeneous than the pre-welded, and microstructure of seam was .finer obviously. Tensile and wear resistance test showed that the weld seam presented higher strength and better wear-reslstance properties than the base metal. The results indicated that TiB2 reinforced aluminum matrix composites were successfully welded using the laser-T1G hybrid welding method.

Effect of Post-weld Heat Treatment on the Microstructure and Mechanical Properties of 2.25Cr-1Mo-0.25V Ultra-Thick Steel Plate

Ensuring the homogeneous and excellent mechanical properties of 2.25Cr-1Mo-0.25V ultra-thick steel plate is the key to the production of hydrogenation reactor equipment.Thus,it is required to understand the heterogeneity of microstructures and properties of ultra-thick plate after heat treatment.In this work,the effect of post-weld heat treatment(PWHT)on the strength,plasticity,toughness and microstructures of the 193-mm-thick steel plate was investigated,and the formation mechanism of heterogeneity was elucidated.The PWHT decreased the room-and high-temperature yield strength(YS)and ultimate tensile strength(UTS)of the steel plate after normalizing and tempering(NT),while the room-and high-temperature YS and UTS decreased from the surface to the center of 193-mm-thick steel plate.It was attributed to the enhanced decomposition of martensite-austenite(M-A)constituents and coarsening of grains and precipitated carbides.

Effects of post annealing on the microstructure,precipitation behavior,and mechanical property of a(CoCrNi)_(94)Al_(3)Ti_(3)medium-entropy alloy fabricated by laser powder bed fusion

The additive manufacturing of multi-principal element alloys has remarkable potential for industrial ap-plications.In this study,a(CoCrNi)_(94)Al_(3)Ti_(3)medium-entropy alloy(MEA)with adequate strength-ductility synergy was prepared via laser powder bed fusion.The microstructural evolution,mechanical property,and deformation mechanisms of the MEA were investigated after post annealing for a short period(0.5 h)at a temperature range of 773-1373 K using various microstructural characterization techniques and quantitative analysis.The static recrystallization temperature of the(CoCrNi)_(94)Al_(3)Ti_(3)MEA ranged from 973 to 1073 K.The average grain size first decreased and then increased,while the dislocation den-sity persistently decreased and texture gradually weakened with increasing annealing temperature.Cr-richσ-phase precipitates formed after 1073 K and then gradually dissolved at 1373 K,while Ni,Al,and Ti elements were aggregated to form a small amount of fine L1_(2)coherent precipitates with an aver-age diameter of approximately 70 nm at 1373 K.The evolution of the dislocation density,grain size,and precipitates significantly influenced the propensity of deformation twins and stacking faults,which consequently affected the strain hardening behavior and mechanical properties.The quantitative calcu-lation of strengthening mechanisms showed that dislocation strengthening played a dominant role at annealing temperatures below 1073 K,and it significantly weakened at 1373 K.Precipitation and grain boundary strengthening both markedly increased owing to the formation of precipitation particles and recrystallization-induced grain refinement after annealing at 1073 K.

Irregular LIPSS produced on metals by single linearly polarized femtosecond laser

Currently,supra-wavelength periodic surface structures(SWPSS)are only achievable on silica dielectrics and silicon by femtosecond(fs)laser ablation,while triangular and rhombic laser induced periodic surface structures(LIPSS)are achievable by circularly polarized or linear cross-polarized femtosecond laser.This is the first work to demonstrate the possibility of generating SWPSS on Sn and triangular and rhombic LIPSS on W,Mo,Ta,and Nb using a single linearly polarized femtosecond laser.We discovered,for the first time,SWPSS patches with each possessing its own orientation,which are completely independent of the light polarization direction,thus,breaking the traditional rules.Increasing the laser power enlarges SWPSS periods from 4–6μm to 15–25μm.We report a maximal period of 25μm,which is the largest period ever reported for SWPSS,~10 and~4 times the maximal periods(2.4μm/6.5μm)of SWPSS ever achieved by fs and ns laser ablation,respectively.The formation of triangular and rhombic LIPSS does not depend on the laser(power)or processing(scan interval and scan methodology)parameters but strongly depends on the material composition and is unachievable on other metals,such as Sn,Al,Ti,Zn,and Zr.This paper proposes and discusses possible mechanisms for molten droplet generation/spread/solidification,Marangoni convection flow for SWPSS formation,and linear-to-circular polarization transition for triangular and rhombic LIPSS formation.Reflectance and iridescence of as-prepared SWPSS and LIPSS are characterized.It was found that besides insufficient ablation on W,the iridescence density of Ta-,Mo-,Nb-LIPSS follows the sequence of melting temperatures:Ta>Mo>Nb,which indicates that the melting temperature of metals may affect the regularity of LIPSS.This work may inspire significant interest in further enriching the diversity of LIPSS and SWPSS.

Numerical Investigation on Fracture Initiation Properties of Interface Crack in Dissimilar Steel Welded Joints

Fracture toughness property is of significant importance when evaluating structural safety.The current research of fracture toughness mainly focused on crack in homogeneous material and experimental results.When the crack is located in a welded joint with high-gradient microstructure and mechanical property distribution,it becomes difficult to evaluate the fracture toughness behavior since the stress distribution may be affected by various factors.In recent years,numerical method has become an ideal approach to reveal the essence and mechanism of fracture toughness behavior.This study focuses on the crack initiation behavior and driving force at different interfaces in dissimilar steel welded joints.The stress and strain fields around the crack tip lying at the interfaces of ductile-ductile,ductile-brittle and brittle-brittle materials are analyzed by the numerical simulation.For the interface of ductile-ductile materials,the strain concentration on the softer material side is responsible for ductile fracture initiation.For the ductile-brittle interface,the shielding effect of the ductile material plays an important role in decreasing the fracture driving force on the brittle material side.In the case of brittle-brittle interface,a careful matching is required,because the strength mismatch decreases the fracture driving force in one side,whereas the driving force in another side is increased.The results are deemed to offer support for the safety assessment of welded structures.

Analysis on the stability of the single weld pool formed during the triple-electrode high speed CO2 welding

The pinch instability theory with free surface and wave superposition principle was applied to understanding the critical disturbance wavelength and the theoretical bead width for the single weld pool formed during the triple-electrode high speed CO2 welding. In addition, the reason for higher stability in the single weld pool was analyzed. Then single-, twin-, and triple-electrode overlaying experiments were conducted. It was found that triple-electrode CO2 welding can improve welding efficiency greatly without reducing the weld quality. The results indicated that the oscillation frequcncy of the weld pool increased, oscillation wavelength decreased, and bead width increased with the increase of welding electrodes under the same disturbance circumstances.

Effect of Grain Size and Compression Direction on the Hot Deformation Characteristics of High-Cr Ultra-Super-Critical Rotor Steel with Columnar Grains

The columnar grains with a duplex grain size distribution in the ingot increase the difficulty of the hot forging of the as-cast high Cr ultra-super-critical rotor steel.The hot deformation behaviors of the high Cr steel with different initial grain sizes under various compression directions were investigated.The results show that the hot deformation characteristic is strongly grain size and compression direction dependent.The finer grain size and compression direction perpendicular to the columnar grains increase the flow stress and activation energies of hot deformation,comparing with the large grain and deformation direction parallel to the columnar grains.The relationships between flow stress and deformation parameters for the different initial structures conform to the established constitutive equations.The former enhances the critical stress of the dynamic recrystallization(DRX),inhibiting the occurrence of DRX and reducing the dimensions of DRX grains.