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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 785–791

journa l homepage: www.e lsev ier .com/ locate / jmatprotec

aser-TIG hybrid welding of ultra-fine grained steel

ing Gao ∗, Xiaoyan Zeng, Qianwu Hu, Jun Yanivision of Laser Science and Technology, Wuhan National Laboratory for Optoelectronics, Huazhong University of Science andechnology, Wuhan, Hubei 430074, PR China

r t i c l e i n f o

rticle history:

eceived 31 August 2007

eceived in revised form

7 January 2008

ccepted 24 February 2008

eywords:

a b s t r a c t

For improving the weldability of ultra-fine grained (UFG) steel, detailed experiments of laser-

tungsten inert gas (TIG) hybrid welding were carried out on this material to investigate the

effects of welding parameters on weld shape, microstructure, grain growth in heat-affected

zone (HAZ) and mechanical performance. For the hybrid welds, increasing energy ratio of

laser to arc (ERLA) can narrow weld width, reduce the tendency of grain growth and ferrite

grain coarsening in HAZ and also make the microstructure of fusion zone gradually change

from coarse pearlite to fine martensite and bainite. The hybrid weld with low ERLA has

obvious softening zone in HAZ; while that with high ERLA has no softening zone because of

ltra-fine grained steel

ybrid welding

eld shape

icrostructure

the low line energy. Compared to laser weld, under appropriate welding parameters hybrid

weld with high ERLA can obtain higher welding speed, better weld shape and more sound

mechanical performance including similar tensile strength and higher toughness. These

results demonstrated that laser-TIG hybrid welding is an effective process for UFG steel.

the industrial applications of UFG steels. There have been very

echanical performance

. Introduction

ltra-fine grained (UFG) steel with refined microstructure isbtained by processing technique without adding alloying ele-ent. The ferrite grain size of UFG steel is in the order oficron and sub-micron, which greatly increases the strength

nd toughness of steel, as well as the ratio of performanceo cost. But after welding, the investigations showed therain coarsening in heat-affected zone (HAZ) may cause theerformance mismatch between HAZ and substrate and dete-iorate the mechanical performance of weld joint dramaticallyHrivnak, 1995; Qui et al., 2003). For this reason, considerablenterest had been paid in recent years to the problem on how tomprove the weldability of UFG steel. For example, Peng et al.2003a,b) had compared laser welding with plasma arc weldingnd MAG (metal inert gas) welding of 400 MPa grade UFG steel

S400. By doing researches on the microstructure and micro-ardness distribution of weld joints, they found laser weldingith high energy density could decrease the width of softening

∗ Corresponding author. Tel.: +86 27 87792404; fax: +86 27 87541423.E-mail address: mgao@mail.hust.edu.cn (M. Gao).

924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2008.02.062

© 2008 Elsevier B.V. All rights reserved.

zone and avoid the mechanical performance deterioration ofweld joint resulting from the coarsening grain. Consequently,by laser welding, UFG steel could get good weld joint more eas-ily. However, because of the high cooling speed, laser weldingusually obtains a much higher microhardness in fusion zone(FZ) and lower toughness of weld, which is also unfavorableduring service of UFG steel.

On the other hand, except for the advantages of laserwelding, laser–arc hybrid welding can obtain deeper weldpenetration, higher welding efficiency, fewer weld defect andstronger bridging capability to groove gap, etc. by the adding ofarc (Bagger and Olsen, 2005; Graf and Staufer, 2003; Kaierle etal., 2000), which has received considerable attention. For theseadvantages, laser–arc hybrid welding may solve the weldingproblems of UFG steel more efficiently, which is significant for

few researches on the laser–arc welding of UFG steel so far.Only Liu and Kutsuna (2005) investigated the microstructureof UFG steel welded by laser-metal inert gas hybrid welding.

786 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t

Fig. 1 – Schematic setup of CO2 laser-TIG hybrid welding.

From a literature survey, it appears that none of the previ-ous work is concerned with the laser-tungsten inert gas (TIG)hybrid welding of these materials. In this paper, the trails ofhybrid welding by combing CO2 laser with TIG arc were carriedout on 400 MPa grade UFG steel to study the microstructureand grain growth in hybrid weld and test the feasibility oflaser-TIG hybrid welding for UFG steel.

2. Experimental methods

During the hybrid welding trials, a 5 kW Rofin-sinar TR050CO2 laser was used together with a Miller 300 A conven-tional DCEN TIG welder. A combined welding head includinglaser beam and TIG torch was developed, as shown inFig. 1. The mode of laser beam was TEM01. The focallength of laser beam was 286 mm with a focused spot sizeof approximately 0.6 mm diameter. The diameter of tung-sten electrode was 2.4 mm. The material used was 400 MPagrade UFG steel similar to SS400 and the composition(wt%) was C0.172–Si0.224–Mn0.648–Fe98.928–P0.018–S0.010.The dimension of plate was 100 mm × 50 mm × 2.5 mm. Asshown in Fig. 1, the optimal spatial position of laserbeam, weld torch and workpiece were obtained by theprevious studies (Gao et al., 2007a,b), which were laserdefocused distance 0 mm, angle of welding torch to work-piece surface 55◦, the distance between tungsten electrode

tip to laser beam axis (DLA) 3 mm and the height oftungsten electrode tip to workpiece 3 mm. The gas flow-ing from the TIG torch was the mixture of argon–heliumat 15 l/min (argon 7.5 l/min and helium 7.5 l/min) and

Table 1 – Welding parameters used in this experiment, P is laseis welding speed and ERLA is energy ratio of laser to arc

No. P (kW) I/U (A/V) v (m

#0 5.0 – 1#1 5.0 150/12 2#2 4.0 150/12 1#3 3.0 150/12

e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 785–791

that flowing from the laser coaxial nozzle was argon at7.5 l/min.

In the laser–arc welding, the laser power is the main heatsources for the improving of welding efficiency because ofthe appearance of laser keyhole and deep penetration pro-cess (Gao et al., 2006, 2007a,b). So in this study, the arc currentwas kept constant and laser power was the only variable. Theexperiment was carried out in butt joints and four combina-tions of welding parameters were used as shown in Table 1.Because of the constant arc parameters, energy ratio of laserto arc (ERLA) varied with the change of laser power. It shouldbe pointed out when ERLA used in hybrid welding were con-stant, a too low welding speed will cause the breakout defectof weld and a too high speed cannot obtain full penetratedweld. Therefore, for different ERLA, the welding speeds usedwere the fastest speed to get full-penetrated weld withoutmacroscopical defect. For the full-penetrated weld with sameplate thickness, bigger ERLA can get higher welding speed andsmaller line energy defined as heat input per unit length alongwelding direction.

After welding, specimens were cut midway from the weld,prepared according to the standard procedures and etched by4% HNO3 + C2H5OH solution to reveal the bead shape and size.The weld bead were observed, measured and photographedby an optical microscope. The mechanical performances, suchas microhardness and tensile strength were also tested. TheVickers hardness was measured on polished surfaces acrossthe weld, using HV0.2 and a load time of 20 s. Tensile tests wereperformed on a 250 kN servo-hydraulic machine operated inram displacement control at room temperature and with anominal strain rate of 0.2 s−1.

3. Results and discussion

3.1. Weld shape

As shown in Fig. 2, the joints for hybrid weld were full but thatfor laser weld had obvious undercut defect. With the decreas-ing of ERLA, because of the increasing of line energy, HAZwidth of hybrid weld increased in the sequence of 0.55 mm,1.1 mm and 1.4 mm, all of which were larger than that oflaser weld 0.5 mm. This phenomenon demonstrated that highERLA or high laser power can obtain larger weld width andthus bigger depth–width ratio of weld. The weld shape com-parison of laser welding with hybrid welding indicated that

with the constant laser power, hybrid weld shape was betterand the HAZ was little wider than that of laser weld, whichcan also avoid the weld mechanical performance deteriora-tion resulting from the grain coarsening by the constrained

r power, I and U is arc current and voltage respectively, v

m/min) Line energy (J/mm) ERLA

500 200 –100 194 2.78200 290 2.22800 360 1.67

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 785–791 787

orp

rw

3

3FA

Fig. 2 – Cross-section m

einforcement like laser welding because of the narroweld.

.2. Microstructure

.2.1. Microstructure of fusion zoneig. 3 was the microstructure in the fusion zone of weld #0–#3.s shown in Fig. 3a, the fusion zone microstructure of single

Fig. 3 – Fusion zone microstructure of weld #0–#3; F is fe

hology of weld #0–#3.

laser welding was composed of martensite and a small quan-tity of ferrite, pearlite and bainite. For the hybrid weld, themicrostructure of weld #1 was mainly the bainite and somemartensite, ferrite and pearlite as shown in Fig. 3b, but had

more percent of ferrite than that of #0. Both weld #2 and #3,as shown in Fig. 3c and d, consisted of dendritic ferrite andpearlite between dendrites in the fusion zone, but the grainsize of #3 was larger. It was obvious that for the hybrid weld,

rrite, Pe is pearlite, B is bainite and M is martensite.

788 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 785–791

gion

density of heat source, the smaller the heat input to pene-trate the same thick plate and consequently the faster theheating and cooling speed of weld joint. So in the laser weld-ing, because of the high energy density, weld fusion zone and

Fig. 4 – HAZ microstructure of weld #3, (a) coarse-grained resubstrate.

with the decreasing of ERLA, the phases with harder micro-hardness, such as martensite and bainite was disappearing,the content of ferrite and pearlite was becoming larger andthe grain size was also growing bigger because of increasingheat input.

3.2.2. Microstructure of heat-affected zoneFig. 4 shows the HAZ microstructure of weld #3. It could beseen that, the HAZ of hybrid weld was also mainly com-posed of coarse- and fine-grained region. In the HAZ of hybridweld, the change of structure and grain size in coarse-grainedzone compared to substrate was greater. Also as shown inFig. 4a and c, the structure of coarse-grained region which wasobviously different with substrate presented evident “Wid-manStatten structure” and the grain size was far larger thanthat of substrate; while the structure of fine-grained regionwhich was composed of ferrite and a little pearlite was thesame as that of substrate, but the grain size of this regionwas only 1–3 �m, which was smaller than that of substrate(about 4.5 �m). In general, there existed an incomplete recrys-tallization region between fine-grained region and substrate,which had the irregular grain size and mechanical perfor-mance (Zhang, 1995). But for the weld obtained in this studyit was not obvious and almost similar with the fine-grainedregion, so it was not considered and discussed.

Furthermore, as shown in Fig. 4b, between coarse- andfine-grained region, there existed a transitional region withobviously coarsening ferrite grain. For the weld #3, the ferritegrain size of this region was about 10–20 �m. But as shownin Fig. 5, there existed no transitional region in weld #1, andin the scope from coarse-grained region to the substrate,even the biggest size of ferrite grain was smaller than thatof the substrate. This phenomenon demonstrated that withthe increasing of ERLA, in the hybrid welding of UFG steel, thetendency of ferrite grain coarsening in the HAZ was weakenedand this ferrite grain coarsening region became narrower andeven disappeared.

Fig. 6 presented the microstructure of coarse-grainedregion in the HAZ of weld #0–#3. From this figure, it could

be seen that the coarse-grained region of weld #0 consistedof martensite and little ferrite growth along the grain bound-ary, that of weld #1 was mainly composed of bainite and asmall quantity of ferrite; that of weld #2 and #3 were both

, (b) transitional region, (c) fine-grained region and (d)

composed of massive pearlite and proeutectoid ferrite withcoarse grain size. The measured grain size in coarse-grainedregion of weld #0, #1, #2 and #3 were in the range of 10–20 �m,15–30 �m, 25–50 �m, 40–80 �m, respectively. These changingrules indicated that during laser-TIG hybrid welding of UFGsteel the increasing ERLA accompanying with the decreasingof heat input could reduce the tendency of grain growth incoarse-grained region.

3.2.3. Microstructure change mechanism of hybrid weldFrom the microstructure change of hybrid weld describedabove, it could be concluded that for the lower ERLA, suchas weld #2 and #3, because of the lower welding speed andhigher heat input the microstructure of fusion zone was coarsepearlite and ferrite, and so was coarse-grained region of HAZ;while ERLA was higher, such as weld #1, fusion zone couldobtain the fine grain martensite and bainite and at the sametime the tendency of grain growth in coarse-grained regionand ferrite grain coarsening between coarse-grained regionand fine-grained region in HAZ could also be restrained. Thesephenomena could be explained by the following discussions.

For the welding process of mild steel, the higher the energy

Fig. 5 – HAZ microstructure of weld #1.

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 785–791 789

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Fig. 7 was the microhardness distribution of weld #0–#3, andthe tested points were under the surface of specimen 1 mm. Asshown in Fig. 7, the microhardness in the fusion zone of hybrid

Fig. 6 – Microstructure of coars

AZ can obtain fine grain austenite during the fast heatingnd more residual austenite can be retained and cooled toow temperature zones for structure change, such as marten-ite point (about 220 ◦C) during the fast cooling. Consequently,aser welding can easily obtain hard structure, such as marten-ite or bainite. However because of the relatively low energyensity, arc welding needed large heat input to achieve theenetration of same thick plate compared to laser welding,hich resulted slower heating and cooling speed. This caused

usion zone and HAZ to stay in high temperature zone forxcessively long time, resulting in bigger grain austenite withlarge quantity of which transited to ferrite before eutectoid

hange and the other transited to the pearlite.In laser–arc hybrid welding, the laser beam with high

nergy density and the arc with low energy density were cou-led into one process. Many previous studies (Kutsuna andhen, 2003; Gao et al., 2006, 2007a,b) had observed the interac-

ion between laser and arc mainly including two mechanisms.ne was observed that laser can stabilize and compress therc by the laser–arc plasma interaction, which can enhancehe center temperature of arc column and arc stability. Thisncreased the centralizing of arc energy and the welding effi-iency. The other was the preheating effect of arc to laseream because arc could preheat the workpiece and make laseream on the warmer workpiece, which improved the heatingfficiency of laser beam and had stronger “keyhole” effect ornergy centralizing ability. As a consequence, hybrid weldingbtained higher speed than single laser welding and made theelding process under relatively lower heat input to diminish

he effect by the adding of arc heat.

In general, the energy centralizing effect of laser beam is

tronger than the arc and the self energy centralizing extentf laser beam is also far bigger than that of the arc, so theigher the ERLA, the bigger is the enhancement of arc energy

in zone in HAZ of weld #0–#3.

centralizing. This demonstrated in laser–arc hybrid welding,the increasing of laser power could enhance the energy cen-tralizing of this process and increase the welding speed,and consequently improve the heating and cooling speed ofwelded metal. In other words, when the hybrid welding hadhigh ERLA, this process was more like laser welding, mightobtain the harder structure, such as martensite or bainite andfine grain in fusion zone and restrain the grain growth ofcoarse-grained region in HAZ. While the hybrid welding hadlow ERLA, this process was more like arc welding and onlyobtained the coarse grain pearlite and ferrite with relativelylow microhardness in fusion zone, as well as a wider ferritecoarsening region in HAZ, such as weld #3 as shown in Fig. 4b.

3.3. Mechanical performance

Fig. 7 – Microhardness of weld #0–#3.

790 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 785–791

Fig. 8 – Specimens for tensile strength experiment (a)

without arc notch for first test (b) with arc notch for secondtest.

welds was obviously lower than that of laser weld. For weld-ing joints of mild steel, lower microhardness correspondedto higher toughness (Peng et al., 2003a,b; Sevim, 2006), whichproved that UFG steel weld of laser-TIG hybrid welding couldpossess higher toughness than that of laser welding.

Form these results, it could be found that the microhard-ness matched well to the microstructure. With the decreasingof ERLA, the microhardness of hybrid weld reduced becauseof the change of microstructure and the growth of grain sizeresulting from the increasing of heat input. Moreover, in HAZof weld #2 and #3, there existed softening zone with lowermicrohardness compared to substrate, and the lower laserpower was corresponding to the wider and lower microhard-ness of softening zone. It was obvious that the softening zonewas resulted from the ferrite-coarsening region discussed inSection 3.2.2. For the weld #1, however, because of the low-est heat input, the fast heating and cooling speed restrainedthe dramatic grain coarsening of coarse-grained region andalso resulted in the more similar microstructure and grain sizein both FZ and HAZ. Consequently, weld #1 had the narrowHAZ and no obvious ferrite-coarsening region in HAZ, whichavoided the appearance of softening zone.

The tensile strength of weld was also tested. Fig. 8a was thespecimen dimension of tensile strength of first test. The testresults showed that all fracture were at the substrate. Thisphenomenon indicated that under the welding parametersused in this experiment, laser-TIG hybrid welded UFG steelhad the reliable strength. For evaluating the effects of weldingparameters on the tensile strength of hybrid weld, the speci-mens for second test were designed again and shown in Fig. 8b,and the results were presented in Fig. 9. From this figure, it

could be seen that the tensile strength of laser weld had thehighest value and was 250 MPa higher than that of substrate(515 MPa). However the tensile strength of hybrid weld withhigher ERLA, weld #1 was very close to that of substrate and

Fig. 9 – Tensile strength of weld #0–#3.

only 15 MPa lower than it. With the decreasing of ERLA, thoseof hybrid weld decreased but were also 100 MPa or so higherthan that of substrate.

The above results showed that laser-TIG hybrid welded UFGsteel could get higher strength compared to substrate andlower microhardness resulting in higher toughness comparedto laser welded UFG steel. Under appropriate welding param-eters, for example weld #1, the hybrid weld even obtained thesimilar tensile strength to laser weld. These results demon-strated that compared to laser weld, laser-TIG hybrid weldedUFG steel with high ERLA could obtain more sound mechani-cal performance including similar tensile strength and highertoughness. So laser-TIG hybrid welding may become an effec-tive process for UFG steel.

4. Conclusions

The following conclusions can be formed from this work.

(1) Laser-TIG hybrid welding is an effective process for ultra-fine grained steel. Compared to laser welding, the hybridwelding can obtain higher welding speed, better weldshape, and lower microhardness resulting in higher tough-ness. Compared to laser weld, hybrid weld with high ERLA

can obtain more sound mechanical performance includingsimilar tensile strength and higher toughness.

(2) For the constant arc current, with the increasing of energyratio of laser to arc, the hybrid weld width becomes nar-rower and the tendency of grain growth and ferrite graincoarsening reduces in HAZ; the microstructure of fusionzone gradually change from coarse pearlite with lowermicrohardness to fine martensite and bainite with highermicrohardness. In other words, the microstructure andgrain size of laser-TIG hybrid welded ultra-fine grainedsteel can be controlled by adjustment of energy ratio oflaser to arc.

(3) Under the low energy ratio of laser to arc, the hybrid weldof ultra-fine grained steel also has obvious softening zone

in HAZ; but under high-energy ratio of laser to arc, thereis no softening zone in HAZ because of the low heat inputand narrow HAZ.

t e c

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