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ORIGINAL ARTICLE Study of laser butt welding of SUS301L stainless steel and welding joint analysis Long Chen & Longzao Zhou & Chao Tang & Wei Huang & Chunming Wang & Xiyuan Hu & Jun Wang & Fei Yan & Xuefang Wang & Zhengguang Jiang & Xinyu Shao Received: 28 November 2013 /Accepted: 6 May 2014 # Springer-Verlag London 2014 Abstract This paper describes a study on laser butt welding of 4 and 2 mm SUS301L stainless steel and a detailed analysis of welding joints. The gap tolerance of butt joint was also studied with optimized process pa- rameters. The electrolytic etching in 10 % oxalate solu- tion was used to test the intergranular corrosion of the 4 mm SUS301L welded joint. Fatigue property of the 2 mm SUS301L welded joint was tested under the conditional cycle times of 1×10 7 . Using optical micros- copy, the changes of metallurgical microstructure in the weld zone of 4 mm SUS301L were also studied. It has been found that laser butt welding of 4 mm SUS301L is able to achieve sound metallurgical morphology and high strength weld joint when the butt gap is within certain tolerance. The weld joint also has good resis- tance to intergranular corrosion and has a fatigue limit of 310 MPa. Keywords Stainless steel . Laser welding . Intergranular corrosion . Fatigue property 1 Introduction With the demand for corrosion resistance, high temperature resistance, and fireproof, automotive body parts made by stainless steel are gaining more applications in rail transporta- tion. As a traditional welding technique of stainless steel, arc welding has a very low welding efficiency and it has a signif- icant impact on the appearance of automotive body parts, which is not desirable for better quality control. On the other hand, as an alternative, laser welding of stainless steel has many advantages over arc welding such as high welding speed, deep weld penetration, narrow heat-affected zone (HAZ), and uniform weld appearance [1]. Meanwhile, the toughness of laser-welded joint is favorable and the hardness of the welded joint is equivalent to base metal [2]. Therefore, laser welding technologies have being widely used for indus- trial applications. In order to understand various complex laser welding pro- cesses on different materials, many studies were carried out over the past decades. Among different studies, laser butt welding of super austenitic stainless steel is a hot topic. Sathiya et al. optimized the input parameters by considering multiple output variables simultaneously [3]. CO 2 laser butt welding of dissimilar stainless steels was studied by Daurelio et al. [4]. In his study, it was noted that those butt-welded joints exhibit sound weld beads even when they are structur- ally asymmetric with respect to the centerline of the butt gap. Manonmani et al. predicted the geometry of weld bead in butt joint of austenitic stainless steel 304 sheet of 2.5-mm thick- ness by using 3-factor 5-level factorial method [5]. They found that depth of penetration, bead width, and area of penetration decreases with an increase of welding speed, and due to the keyhole, depth of penetration and area of penetra- tion increases slightly at higher welding speed. A very com- prehensive review on corrosion behavior of butt-welded stain- less steel was conducted by Gooch [6]. In this study, it L. Chen : L. Zhou : C. Tang : C. Wang (*) : X. Hu : J. Wang : F. Yan School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China e-mail: [email protected] C. Wang e-mail: [email protected] W. Huang Technical Center, Alcoa, PA, USA X. Wang : Z. Jiang Zhuzhou Electric Locomotive CO., LTD, Zhuzhou 412000, China X. Shao School of Mechanical Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China Int J Adv Manuf Technol DOI 10.1007/s00170-014-5928-y

Study of laser butt welding of SUS301L stainless steel and welding joint analysis

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ORIGINAL ARTICLE

Study of laser butt welding of SUS301L stainless steel and weldingjoint analysis

Long Chen & Longzao Zhou & Chao Tang & Wei Huang &

Chunming Wang & Xiyuan Hu & Jun Wang & Fei Yan &

Xuefang Wang & Zhengguang Jiang & Xinyu Shao

Received: 28 November 2013 /Accepted: 6 May 2014# Springer-Verlag London 2014

Abstract This paper describes a study on laser buttwelding of 4 and 2 mm SUS301L stainless steel and adetailed analysis of welding joints. The gap tolerance ofbutt joint was also studied with optimized process pa-rameters. The electrolytic etching in 10 % oxalate solu-tion was used to test the intergranular corrosion of the4 mm SUS301L welded joint. Fatigue property of the2 mm SUS301L welded joint was tested under theconditional cycle times of 1×107. Using optical micros-copy, the changes of metallurgical microstructure in theweld zone of 4 mm SUS301L were also studied. It hasbeen found that laser butt welding of 4 mm SUS301L isable to achieve sound metallurgical morphology andhigh strength weld joint when the butt gap is withincertain tolerance. The weld joint also has good resis-tance to intergranular corrosion and has a fatigue limitof 310 MPa.

Keywords Stainless steel . Laser welding . Intergranularcorrosion . Fatigue property

1 Introduction

With the demand for corrosion resistance, high temperatureresistance, and fireproof, automotive body parts made bystainless steel are gaining more applications in rail transporta-tion. As a traditional welding technique of stainless steel, arcwelding has a very low welding efficiency and it has a signif-icant impact on the appearance of automotive body parts,which is not desirable for better quality control. On the otherhand, as an alternative, laser welding of stainless steel hasmany advantages over arc welding such as high weldingspeed, deep weld penetration, narrow heat-affected zone(HAZ), and uniform weld appearance [1]. Meanwhile, thetoughness of laser-welded joint is favorable and the hardnessof the welded joint is equivalent to base metal [2]. Therefore,laser welding technologies have being widely used for indus-trial applications.

In order to understand various complex laser welding pro-cesses on different materials, many studies were carried outover the past decades. Among different studies, laser buttwelding of super austenitic stainless steel is a hot topic.Sathiya et al. optimized the input parameters by consideringmultiple output variables simultaneously [3]. CO2 laser buttwelding of dissimilar stainless steels was studied by Daurelioet al. [4]. In his study, it was noted that those butt-weldedjoints exhibit sound weld beads even when they are structur-ally asymmetric with respect to the centerline of the butt gap.Manonmani et al. predicted the geometry of weld bead in buttjoint of austenitic stainless steel 304 sheet of 2.5-mm thick-ness by using 3-factor 5-level factorial method [5]. Theyfound that depth of penetration, bead width, and area ofpenetration decreases with an increase of welding speed, anddue to the keyhole, depth of penetration and area of penetra-tion increases slightly at higher welding speed. A very com-prehensive review on corrosion behavior of butt-welded stain-less steel was conducted by Gooch [6]. In this study, it

L. Chen : L. Zhou : C. Tang : C. Wang (*) :X. Hu : J. Wang :F. YanSchool of Materials Science and Engineering, Huazhong Universityof Science and Technology, Wuhan 430074, Chinae-mail: [email protected]

C. Wange-mail: [email protected]

W. HuangTechnical Center, Alcoa, PA, USA

X. Wang : Z. JiangZhuzhou Electric Locomotive CO., LTD, Zhuzhou 412000, China

X. ShaoSchool of Mechanical Science and Engineering, HuazhongUniversity of Science and Technology, Wuhan 430074, China

Int J Adv Manuf TechnolDOI 10.1007/s00170-014-5928-y

indicates that the corrosion behavior of different types ofstainless steel are mainly influenced by the stability of passivefilm, segregation during solidification, partitioning duringphase change, precipitation of second-phase particles, metal-lurgical microstructure, and the degree of residual oxide pres-ent on the joint. Aquino et al. [7] studied the intergranularcorrosion susceptibility of super martensitic stainless steelwelds. It was showed that redissolution of chromium carbideprecipitate is likely to occur due to the attained temperature atcertain regions of the HAZ during the electron beam welding.Intergranular corrosion of 316 L stainless steel diffusion-bonded joint (DBJ) was carried out by Shuxin Li et al. [8].They found that the degree of sensitization of DBJ was muchsmaller than that of the base material. Moreover, DBJ has betterintergranular corrosion resistance than the base metal. Theprecipitation behavior of the austenitic stainless steel and itseffect on the intergranular corrosion resistance were examinedby Terada et al. [9]. They demonstrated the preferential forma-tion of (Ti, Mo)C at higher aging temperatures compared toM23C6, which retained the chromium in solid solutionpreventing steel sensitization and intergranular corrosion. Pre-cipitation reactions in the coarse-grained HAZ of mediumgrade (Ti-free) 13 % Cr super martensitic stainless steels werestudied by Ladanova et al. [10]. They found that chromium-depleted zones are along prior austenite grain boundaries of themedium grade steel, and these zones are assumed to be thereason for the well-known sensitivity to intergranular corrosionof these steels. The applicability of eddy current testing tech-nique to assess and quantify sensitivity and intergranular cor-rosion in austenitic stainless steels was investigated by Shaikh

et al. [11]. In this study, it was indicated that the eddy currentamplitude can be a good indicator of the propensity for inter-granular corrosion attacks because of its reliable response tochanges in chromium depletion. The effects of welding meth-od, peak temperature, and cooling rate on the susceptibility tointergranular corrosion of alloy 690 weld were carried out byLee et al. [12]. They showed that intergranular corrosion oflaser welding process is less serious than that of gas tungstenarc welding process. This is because the very rapid cooling rateduring laser welding leads to an insufficient exposure timewithin the Cr-carbide precipitation temperature range, whichsuppresses Cr-carbide precipitation and Cr-depletion alonggrain boundaries in the weld decay region of the HAZ. Byusing modified Strauss test, the effect of welding processes onsensitization behavior of ferritic stainless steel joints was stud-ied by Lakshminarayanan et al. [13]. They found that laserbeam and electron beam-welded joints exhibit lowest corrosionrate compared to gas tungsten arc and friction stir-weldedjoints. Welding of two steel grades with different austenitepotentials under various heat inputs and welding speeds werecarried out by Greeff et al. [14]. They found that sensitizationemerges when lower heat inputs and faster cooling rates sup-press austenite nucleation during cooling.

Effect of gas tungsten arc welding (GTAW) repairs on theaxial fatigue strength of AISI4130 steel-welded joints wasinvestigated by Nascimento et al. [15]. They showed that thefatigue strength decreases with the number of GTAW repairs,and this is related to the microstructure and microhardnesschanges of the weld, the residual stress field, and the weldprofile geometry. Fatigue life and fatigue crack growth behav-ior of friction stir-welded AISI 409 M grade ferritic stainlesssteel joint were studied by Lakshminarayanan et al. [16]. Theyfound that the welded joint shows superior fatigue life andfatigue crack growth resistance compared to that of the basemetal. This is mainly due to the synergetic effect of dual phaseferritic–martensitic microstructure, superior tensile properties,and favorable residual stress. Fatigue behavior and fracturemechanism of carburized 316 stainless steel were examinedby Tokaji et al. [17]. They indicated that fatigue cracks alwaysinitiate underneath the carburized case at a very early stage of

Fig. 1 Schematic ofexperimental setup

Table 1 Process variables of laser welding

Equipment Variables Units Levels

YLR-4000 Laser power kW 3, 4

IBR-4400 Welding speed m/min 3.0, 3.5

Defocus distance mm 0

Shielding gas flow l/min 1.5

Butt gap mm 0, 0.1, 0.15, 0.2, 0.25

Int J Adv Manuf Technol

fatigue life and subsequently grow predominantly into thecore material. The influence of the weld on the fatigue prop-erties of the base material was studied by Zaletelj et al. [18].They showed that the influence of weld on fatigue of AISI 409is pronounced, and it can lower the fatigue of AISI 409 atlower load levels. Additionally, different aging conditions canimprove or deteriorate the fatigue properties of AISI 409,which is mainly owning to the transitions between differentmicrostructure phases and weld residual stresses. The effect offiber laser welding (FLW) on fatigue performance was inves-tigated by Xu et al. [19]. They indicated that fatigue life of theFLW-welded joints were equivalent to that of the base metal ata high stress amplitude above 300 MPa. Fatigue property andfailure characterization of spot-welded high strength steelsheet were carried out by Long et al. [20]. They indicated thatthe materials show very similar fatigue strength under low andhigh load cycles. When the intergranular corrosion occurs instainless steel, its exterior surface still looks bright, but itsinternal structure has already been damaged [21].

As the intergranular corrosion and fatigue failure of thejoint are the main concerns of the butt-welded austeniticstainless steel in automotive body parts, their evaluation arenecessary to avoid catastrophic failure particularly in weldedstructures. By considering the above facts in mind, it wasaimed to study these two properties of laser-welded austeniticstainless steel joint. Meanwhile, the mechanical properties ofthe joint, the influence of butt gap, and the changes of metal-lurgical microstructure in the weld zone are also discussed.

2 Experimental setup

In this study, a continuous wave fiber laser (YLR-4000) wasused for laser butt welding of SUS301L stainless steel. Thelaser has a maximum power of 4 kW with a 0.3-mm focusingspot. An ABBwelding robot (IRB4400) was used to carry outautomatic welding in the experiment. The schematic of thesetup is shown in Fig. 1, and the process variables are de-scribed in Table 1.

Austenitic stainless steel is used as the base metal in thisstudy. It has a good weldability, but its heat dissipation is quiteslow due to its low thermal conductivity which is one third ofregular carbon steel. Its corrosion resistance decreases as aresult of its instability under high temperature when carbide ofCr presents. In addition, its deflection is more pronouncedthan that of regular carbon steel under the same heat inputbecause of its higher thermal expansion coefficient (1.5 timeshigher than that of regular carbon steel). Therefore, laserwelding with a low heat input is more appropriate for austen-itic stainless steel [22]. The chemical compositions of the usedSUS301L austenitic stainless steel are listed in Table 2.

3 Experimental result and discussion

3.1 Mechanical properties of the welding joint

Table 3 shows the tensile strength and elongation of the basemetal. They are tested first in order to compare with that of theweld joint.

During the experiment, it is noted that the 4-mm plate is notfully penetrated when the laser power is 4 kW, the weldingspeed is 3.5 m/min, and the defocus distance is 0 mm. There-fore, the welding speed is reduced to 3.0 m/min while the laserpower and defocus distance are kept the same. With theseprocess variables the plate is penetrated and it has a good weldappearance shown in Fig. 2.

As the results shown in Tables 4 and 5, tensile and bendingtest of the weld are carried out. The average tensile strength ofthe weld is 843.77 MPa (99.8 % of the base metal), which ismainly owning to the small deformation, small HAZ, andgood weld microstructure of the joint. Figure 3a, b show thebending process of the test sample and the bending test samplewith spring-back when the load is off, respectively. Figure 4shows the top and bottom views of the bending test sample. Itis noted that there is no visual defect in the weld after beingbent, which indicates that the weld joint has a good anti-bendperformance.

3.2 Influence of butt gap

The influence of butt gap on the appearance and strength ofthe weld was studied when the laser power is 4 kW, thewelding speed is 3.0 m/min, and the defocus distance is0 mm. The butt gap is kept smaller than the diameter of the

Table 2 Chemical composition of SUS301L (wt%)

Composition C Si Mn P S Cr Ni N Fe

Percentage ≤0.03 ≤1.0 ≤2.0 ≤0.045 ≤0.03 18~20 8~10.5 ≤0.02 Remain

Table 3 Tensile strength and elongation of the base metal

1 2 3 Average

Tensile strength (MPa) 846.70 850.58 839.10 845.46

Elongation (%) 52.52 51.58 50.32 51.47

Int J Adv Manuf Technol

Fig. 2 The weld appearance atop view of weld and b bottomview of weld

Table 4 Tensile strength of weldjoint Fracture path Tensile strength (MPa) Elongation (%)

Top 843.77 45

Bottom

Table 5 Bend test results of the weld joint

Angle of bend Head diameter (mm) Result

Transverse top bend ≥120° 30 Eligible

Transverse bottom bend ≥120° 30 Eligible

a b

Fig. 3 a Bending test process and b the test sample after being bent

Int J Adv Manuf Technol

focusing spot which is 0.3 mm. The appearance of the weldsunder different butt gaps is shown in Table 6.

It is noted that the welds under different butt gaps do notshow big variance in weld appearance. However, small spat-ters are present at the bottom of the weld as the gap increases.

This is because when the gap increases, the molten pool iseasier to collapse and the metal in the molten pool spatters atthe bottom of weld under the pressure of metallic vapor.Meanwhile, it is also noted that at the top of the weld, theundercut is present as the gap increases, which is also causedby the collapse of the molten pool (The extra gap space can befilled by a shinkage).

Figure 5 shows the test results of tensile strength of theweld joints under different butt gaps. With the gap varyingfrom 0.1 to 0.25 mm, the minimum tensile strength of thejoints is 94 % of the base metal.

3.3 Intergranular corrosion test

Figure 6 shows the device for electrolytic etching. Theoxalic acid electrolytic etching can be used to quantify theintergranular corrosion. During the test with a currentdensity of 1 A/min, the anode is the test sample fromthe weld of laser butt welded 4 mm SUS301L which wasgotten under butt gap of 0 mm, laser power of 4 kW,welding speed of 3.0 m/min, and defocus distance of0 mm and the cathode is a stainless steel plate. Power isturned on after 10 % oxalate solution is poured into thecontainer. The etching time is 90 s and the temperature ofthe 10 % oxalate solution is kept between 20 to 50 °C.After being etched, the sample is washed up and dried.Figure 7 shows the microstructure of the etched sample.There are many grooves spreading over the grain bound-aries and many grains are surrounded by the grooves.

SUS301L contains less than 0.03 % of carbon content anda certain amount of nitrogen. More nitrogen in austeniticstainless steel results in more difficulties for the carbide ofCr to separate out under the same temperature and etchingcondition. At the same time, the lattice parameter and thegrowing dynamics of the M23C6 are decreased, its nucleationand growth are blocked, and the dislocation in the grainboundary is increased [23, 24]. This is mainly because thediffusion rate of nitrogen is higher than that of carbon, whichresults in preferential precipitation of nitrogen nearby thegrain boundary and then restrains the precipitation of thecarbide of Cr. It is just one of the reasons for corrosionresistance of the welding joint summarized by Gooch [6].Consequently, the joint has good resistance to intergranularcorrosion.

a b

Fig. 4 a Top view and b bottomview of the test sample after beingbended

Table 6 The appearance of welds under different butt gaps

Gap(mm) PositionAppearance of the

weld

0.1

Top

Bottom

0.15

Top

Bottom

0.2

Top

Bottom

0.25

Top

Bottom

Int J Adv Manuf Technol

Fig. 5 The tensile strength of thejoint under different butt gaps

Fig. 6 Schematic of device for electrolytic etching

Fig. 7 Microstructure of the etched sample

Table 7 Static tensile strength of the test samples

Type of sample Tensile strength(MPa)

Comparing with basemetal

Base metal 845.52

Laser butt weldedjoint

817.75 96.7 % of base metal

MIG welded joint 754.2 89.2 % of base metal

Fig. 8 Sketch map of cyclic stress

Int J Adv Manuf Technol

S-N curves of SUS301L base metal.Fig. 9 S-N curves of SUS301L base metal

S-N curves of laser butt weldedjoint.Fig. 10 S-N curves of laser butt-welded joint

S-N curves of MIG welded joint.Fig. 11 S-N curves of MIG-welded joint

Comparison of fatigue S-N curves of the base metal and

the laser butt welded joint. Fig. 12 Comparison of fatigue S–N curves of the base metal and the laserbutt-welded joint

Fig. 13 Fatigue failure of the base metal

Fig. 14 Fatigue failure of the laser butt-welded joint

Int J Adv Manuf Technol

3.4 Fatigue property test

Fatigue limit is defined as the amplitude of cyclic stress thatcan be applied to the material without causing fatigue failure.However, infinite number of cyclic stress is difficult to realizein the actual experimental setup. Therefore, it is assumed tohave no fatigue failure under cyclic stress, if there is nobreakage emerges in the steel material when the cyclic stressindex N>107. In this study, the cyclic index is also 107. Themaximum stress Smax corresponding to the cyclic index 10

7 isdefined as fatigue limit. For general engineering materials, thefatigue strength σ−1 is 35–50 % of the static tensile strengthand Mr JinGe formula shows the relationship:

σ−1 ¼ 0:25� 0:06ð Þ σ0:2 þ σbð Þ þ 5 ð1Þ

where σ0.2 is the conditional tensile strength and σb is the statictensile strength.

Fatigue property of the base metal, laser butt-welded joint,and metal inert gas (MIG)-welded joint are tested and com-pared under each stress level.

Table 7 shows the static tensile strength of weld joints oflaser butt welding and MIG welding of 2 mm SUS301L. Forlaser butt welding, the laser power is 3 kW, the welding speedis 3.0 m/min, and the defocus distance is 0 mm. For MIG

welding, the welding current is 135 A and the arc voltage is20 V.

Figure 8 shows the sketch map of the cyclic stress:

Maximum stress : Smax ¼ K � σb ð2Þ

Minimum stress : Smin ¼ R � Smax ð3Þ

Average stress : Sm ¼ Smax þ Sminð Þ.2 ð4Þ

Stress amplitude : Sa ¼ Smax−Sminð Þ.2 ð5Þ

where σb is the static tensile strength listed in Table 7, andstress ratio R is the ratio of the maximum stress Smax andminimum stress Smin. K is the stress level coefficient.

Three experiments are carried out for each set of parame-ters and then the stress levels under different cyclic index areobtained. Based on the fatigue test result, the stress is adjustedaccordingly to approach fatigue limit. The obtained points andS–N curves are recorded in the Cartesian coordinate. In the S–N curves, S is the stress amplitude and N is the cyclic indexcorresponding to fracture.

The fatigue S–N curves of SUS301L basemetal is shown inFig. 9. The fatigue strength is 330 MPa under the cyclic indexof 107. It is 39 % of the static tensile strength of SUS301L.Figure 13 shows the fatigue failure of the base metal.

Figure 10 shows the fatigue S–N curves of the laser butt-welded joint. Fatigue strength of the laser butt-welded joint is310 MPa under the cycle index of 107. It is 38 % of the statictensile strength of the weld and it fits the Mr JinGe formula.Figure 11 shows the fatigue S–N curves of MIG-welded jointwith a fatigue strength of 180 MPa (much lower than that of

Fig. 15 Fatigue failure of the MIG-welded joint

a b

Fig. 16 Microstructure of a thebase metal and b the laser butt-welded joint

Int J Adv Manuf Technol

the base metal which is 330 MPa) under a low cycle index of105 (much lower than 107).

Figure 12 compares the fatigue S–N curves of the basemetal and the laser butt-welded joint. The fatigue property ofthe laser butt-welded joint is slightly lower than that of thebase metal. This demonstrates that laser welding has a verylimited influence on the fatigue property of the base metalbecause the low heat input and high cooling rate of laserwelding results in a sound weld and a narrow HAZ (Fig. 13).

Figures 14 and 15 show the fatigue failures of the laserbutt-welded joint and the MIG-welded joint, respectively. Thefatigue crack of the MIG-welded joint occurs in the fusionzone between the base metal and the weld. This result agreeswith the fact that the fusion zone is usually more subjective tofatigue crack because of localized melting of grain boundaryand transgranular, heterogeneous distribution of compositionand organization, and serious overheating and bad plasticity.

3.5 Changes of metallurgical microstructure in the weld zone

In order to study the changes of metallurgical microstructurein the weld zone, microscopic images are taken for the basemetal and the laser butt-welded 4 mm SUS301L joint (whichwas gotten under butt gap of 0 mm, laser power of 4 kW,welding speed of 3.0 m/min, and defocus distance of 0 mm)are shown in Fig. 16. Small molten pool is obtained due to thesmall focusing spot of laser beam. The molten pool usuallysolidifies in tens of milliseconds and the non-equilibriummicrostructure can easily form. The HAZ of laser-welded jointis very narrow and for some cases there are almost no HAZ.Sheet austenite is observed in the optical photograph of thebase metal. Because of the high cooling rate, all the moltenmetal in the molten pool cools almost simultaneously; thecrystal growth direction of the grain is perpendicular to thefusion line in the optical photograph of weld seam. It is alsoobserved that the austenite and lath ferrite are prone to growalong the weld seam. Columnar crystal is present near theedge of the weld, where crystallization rate is quite high at thebeginning and slows down later. In general, the microstructureof the weld metal is homogeneous, very few weld defects arepresent, and the mechanical property of the weld is excellent.

4 Conclusion

This paper describes the study of laser butt welding of 4 and2 mm SUS301L stainless steel for application of automotivebody parts in railway transportation. The experimental setupand procedures are designed and carried out, and the resultsare analyzed. Following conclusions are obtained from thisstudy:

(1) Laser butt welding has certain tolerance to butt gap underlaser power of 4 kW, welding speed of 3 m/min, anddefocus distance of 0 mm. The average tensile strengthof the joints is 843.77MPa, which is 99.8% of that of thebase metal. The weld joint has no visual defect afterbeing bended 180°.

(2) The microstructures of the test samples show good resis-tance to intergranular corrosion.

(3) Fatigue limit of the laser welded joint is 310 MPa, slight-ly less than that of the base metal. It is far better than thatof MIG-welded joint at the same condition.

(4) Sheet austenite is observed in the microstructure of thebase metal and austenite and lath ferrite are observed inthe weld seam. Homogeneous microstructure of the weldmetal and very few weld defect are observed.

Acknowledgments We would like to express our deep gratitude toAnalysis and Test Center of HUST (Huazhong University of Scienceand Technology), for their friendly cooperation. This work was supportedby Zhuzhou Electric Loco-motive CO., LTD of China Southern RailGroup, the Fund Program of China (Grant No. 51318050107) the Na-tional Natural Science Foundation of China (Grant No. 51375191) andthe National Program on Key Basic Research Project (973 Program) ofChina (Grant No. 2014CB046703).

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