ISIJ International, Vol. 51 (2011), No. 3, pp. 429–434

1. Introduction

Transformation Induced Plasticity (TRIP) steel is consid-ered one of the newest and most exciting materials beingdeveloped by the steel industry. In relation to other ad-vanced high-strength steels, TRIP steels exhibit better duc-tility at a given strength level. This enhanced formabilitycomes from the transformation of retained austenite (duc-tile, high temperature phase of iron) to martensite (tough,non-equilibrium phase) during plastic deformation. Be-cause of this increased formability, TRIP steels can be usedto produce more complicated parts than other high strengthsteels allowing the automotive engineer more freedom inpart design to optimize weight and structural perform-ance.1,2)

Steel automobile bodies and other structural componentsare assembled almost entirely by welding. Resistance spotwelding (RSW) is considered one of the two dominantmethods of auto body assembly. The characteristics of re-sistance spot welding of TRIP steels were studied.3,4) Otherwelding processes such as GMAW,5,6) friction stir weld-ing7,8) and brazing9) are also applied for TRIP steels weld-ing. In the recent years, laser welding is growing in impor-tance as a potentially more productive alternative for auto-mobile assembly due to the fast welding speed, excellent re-producibility of the joints, less distortion, reduced need forrefinishing and high joint rigidity. The effects of laser weld-ing on the microstructures and mechanical properties of theTRIP steels had been evaluated and discussed in previousworks.10–13)

The global mechanical behavior of the welds depends onthe mismatch in mechanical properties between the differ-ent welded zones (WM, HAZ and BM), their dimensions

and loading mode.14) The mechanical properties of thosezones may be drastically different depending on the heatingand cooling conditions imposed by the welding process andon the structure and chemical composition of the baseplate.14,15) In this context, it is important to study the influ-ence of the welding process on the overall mechanical behavior of the joint. However, the experimental analysis ofthe stress–strain distribution in a welded joint is a very dif-ficult task because of the non-linearities involved in theprocess, such as the different elastoplastic behavior of thevarious welded zones, its geometries, the non-homogeneousstrain and stress distribution makes all possible experimen-tal analysis very complicated. It is possible to analyze indi-vidually the various phenomena that occur in a tensile testof a welded joint using numerical simulation.14)

Numerical simulation is an effective tool for the clarifica-tion and confirmation of experimental observations. The ac-curacy of the simulation, however, depends on the availabil-ity of accurate mechanical properties for both the basemetal and the weld region. Methods for determining fusionzone and HAZ properties by comparing experimental ten-sile tests with simulation results have been proposed. Thefinite element simulations were used to evaluate the effectsof a soft zone on the strength and ductility of welds in highstrength steels subject to tensile testing. It was showed thatthe mechanical properties for the hardened and softenedzones are difficult to determine in practice. Many attemptswere carried out to numerically simulate the tensile behav-ior of the welds.14,16)

The evaluation of elastoplastic behavior of laser beamwelded TRIP700 steel under uniaxial tensile testing by experimental procedure and numerical simulation was themain goal of this study.

Numerical and Experimental Investigation of Tensile Behavior ofLaser Beam Welded TRIP700 Steel

Uwe REISGEN, Markus SCHLESER, Oleg MOKROV and Essam AHMED

RWTH Aachen University, Welding and Joining Institute, Germany. E-mail: essam.ahmed@rwth-aachen.de

(Received on August 31, 2010; accepted on November 15, 2010)

This paper aims to evaluate and simulate the mechanical behavior of laser beam welded TRIP700 steelsheet under uniaxial tensile loading. In this work TRIP700 steel sheets with thickness of 1.2 mm were buttwelded by CO2 laser beam welding using 4.5 kW as a laser beam power and 0 mm as a focus position. Thewelding speed was ranged from 2.1 to 3.9 m/min. Microhardness measurements and transverse tensiletesting were carried out to characterize the welds. Numerical simulation with the finite element analysiscode ABAQUS/CAE v6.9-1 was used to describe the elastoplastic behavior of the welded sheets. In a per-pendicular tensile test to the weld line, all specimens were fractured at the base metal and the strengthswere somewhat higher than those of base metal. The numerical results of the tensile testing had a goodagreement with the experimental results.

KEY WORDS: TRIP steels; tensile test; laser beam welding; finite element; Abaqus.

429 © 2011 ISIJ

2. Experimental Work

2.1. Material Selection

A cold rolled TRIP700 steel sheet with 1.2 mm thicknesswas used in this work. The chemical composition and car-bon equivalent are shown in Table 1. The tensile tests in 0°,45° and 90° angles with the rolling direction using DIN EN10002-1:2001 using a crosshead speed of 10 mm/min wereused to evaluate the tensile properties of the base metal.TRIP steel composed of ferrite as matrix and dispersed sec-ondary phase, namely martensite/bainite and retainedaustenite. The microstructure of studied steel sheets wascharacterized using an optical microscopy. A quantitativemeasurement of retained austenite present in the TRIP steelwas carried out by X-ray diffraction measurements usingCo Ka radiation.

2.2. Laser Beam Welding

Experimental welds were carried out with 6 kW CO2

laser under welding speed ranged from 2.1 to 3.9 m/min.The laser beam power and focal position were 4.5 kW and0 mm respectively. The mode of the laser was TEM10

(Transversal Electronic-Magnetic) and the laser beam waspassed through four reflectors before a 200 mm focal lengthfocusing mirror providing a focused spot size of approxi-mately 0.3 mm diameter. Butt joint configuration with theweld line oriented parallel to the rolling direction and fullpenetrations were obtained in all welds. Helium was em-ployed as a shielding gas from the top surface at a flow rateof 20 L/min.

2.3. Welds Characterization

Transverse samples were cut from representative weldsfor metallographic observations, microhardness measure-ments and tensile properties evaluations. The characteristicsof fusion and heat affected zones microstructures were car-ried out by the optical microscopy. Tensile testing usingDIN EN 895: 1995 was used to evaluate the tensile proper-ties of the welds. The tensile properties of the weld metalwere characterized using sub-size specimen tensile testingshown in Fig. 1.

3. Finite Element Simulation

3.1. Theoretical Background

The transverse weld specimen had three different zonesas a result of the heterogeneous heating and cooling ratesduring the welding process. These zones are different in ini-tial lengths, microstructures, material properties and plasticbehavior models. R1, R2 and R3 referred to these zones andrepresented the maximum hardness zone, transition hard-ness zone and the base metal (lower hardness zone) respec-tively (see Fig. 2).

l 01, l 0

2 and l 03 are the initial lengths of R1, R2 and R3 re-

spectively. The final length of each region will be differentnot only due to the different initial length of each region but

also due to differences in strain in the individual weld re-gions during deformation. The final lengths of the above re-gions after plastic deformation were defined as l1, l2 and l3

respectively. The total true strain is given as:

...................(1)

where l 0 and l are the total initial and final lengths of thewelded sample.

The flow stress is assumed uniform along the sample andeach zone is governed by its own constitutive equation:

.......................(2)

where k1 and n1, k2 and n2, and k3 and n3 are the strength co-efficients and strain hardening exponents of R1, R2, and R3

respectively. As the cross-sectional area is initially the samein all regions, the stress will initially be uniform throughoutthe sample. However, due to differences in mechanicalproperties (k- and n-values) each region will undergo differ-ent deformations which will lead to non-uniform strain. Foreach zone the strain will be as follows:

.....................(3)

and can be formulate as follows:

...........................................(4)

From this equation, it can be understood that the displace-ment and force developed in each region is governed by itsstrength coefficient, strain hardening exponent and initiallength of each region in the welded sample.16)

εσ σ

it

k kl

l

e l e l en n

� �� �

ln ln( / ) ( / )/ /

010

201

1 12

1 2 (( / ) /

( )

σ k n

l

l l l

31 3

30

10

20

30� �

⎛

⎝⎜⎜

⎞

⎠⎟⎟

εit i

i

l

li� �ln –

01 3where

σ ε ε ε� � �k k kn n n1 1 2 2 3 3

1 2 3

ε t l

l

l l l

l l l� �

� �

� �ln ln

( )

( )01 2 3

10

20

30

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Table 1. Chemical composition and C-equivalent of TRIP steel used in the present study.

Fig. 1. Sub-size tensile test specimen.

Fig. 2. The different welding zones.

3.2. Finite Element Modeling

A 3D numerical model, using FE analysis ABAQUS/CAE v6.9-1 software package, was created to simulate thetransverse tensile test of the welded steel sheets. Isotropic,homogeneous and linear elastic behavior of material prop-erty was assumed. A symmetric about the longitudinal andtransverse directions of the tensile sample was also consid-ered to develop this model. The model was considered as adeformable body with appropriate yield criterion andstress–strain relations during nonlinear plastic. The elasticmodel used the linear relationship between stress and strain.For the elastic analysis of simulation, the elastic materialproperties were defined by elastic modulus (E) and Pois-son’s ratio (n). In this simulation, the elastic material prop-erties were defined by elastic modulus (E) of 210 GPa andPoisson’s ratio (n) of 0.3. The von Mises yield criterion wasconsidered to describe the yielding behavior. It suggeststhat the yielding of materials begins when the second stressinvariant J2 reaches a critical value k. For this reason, it issometimes called the J2-plasticity or J2 flow theory. Thevon Mises criterion can be expressed as: s 2

YP�(3/2)SijSij

where sYP is the yield stress of the material in uniaxial ten-sile testing and Sij are the components of the stress deviatortensor. This quadratic yield condition in the Cartesian prin-cipal stress (s1, s2, s3) space is represented by the follow-ing equation in terms of principles stresses:

...........................................(5)

where s1, s2 and s3 are the principal stresses in three di-rections and sYP is the yield strength of the material. In theuniaxial tensile test s1≠0, s2�s3�0, and the previousequation can be reduced to s1�sYP.

Several empirical equations (such as Hollomon, Lud-wick, Swift, modification of Mecking–Kocks, etc.) havebeen used to describe experimental stress–strain curves.These equations can be used to describe stress–strain rela-tions for the estimation of the uniform strain (i.e. formabil-ity) of metals that were examined.17,18)

..........................(6)

......................(7)

......................(8)

...........................................(9)

where s and e are the true stress and true strain respec-tively.

Figure 3 shows the finite element meshes used in thesimulations detailing the different material regions andwidths. The finite element discretization used 3D eight-node hexahedron isoparametric elements. Only 1/4 of thesample was simulated considering geometrical and materialsymmetry relative to the axes Ox and Oy. The model con-sists of 7 644 nodes and 5 400 elements. A refined mesh

was used in the R1 and R2 regions to account for the changein stresses in these areas. The boundary conditions used inthe simulations are showed in Fig. 4. For simplicity thesample is represented in the Oxy plane. The tensile test issimulated by imposing positive displacements on the Ox direction.

4. Results and Discussion

4.1. Experimental Results

The microstructure of base metal was shown in Fig 5. Itis composed of polygonal ferrite, bainite and retainedaustenite. The volume fractions of retained austenite (byXRD method) and bainite are about 11 and 16% respec-tively.

The results of tensile tests of base metal were shown inFig. 6. It was shown that the yield stress and ultimate ten-sile strength in 90° angle with rolling direction are higherthan those in 45° and 0° angles. The fracture appearancewill be discussed later.

Figure 7 shows the characteristic weld hardness distribu-tions with welding speeds of 2.1, 3.0 and 3.9 m/min meas-ured at 0.1 mm intervals along virtual line at a half of thethickness of welded sample. The hardness reached maxi-mum value not only in the weld metal but also in the HAZnear the weld metal and decreased when approaching thebase metal along the virtual line. In the weld fusion zoneand the region of HAZ near to the fusion zone, the hardnesswas reached to maximum value of 490–516 HV approxi-mately. Outside the fusion zone, hardness decreased gradu-ally to the base metal level. The base metal hardness valuewas 260 HV approximately. The fusion zone is comprisedof mainly martensitic and some traces of ferrite. This factincludes that the martensite structure allows the weld metaland HAZ near the weld metal to have the maximum hard-ness and the decrease in the hardness of HAZ near the basemetal results from relatively soft ferrite having a low hard-ness. The macrograph TRIP/TRIP steel weldmnt at 3 m/minas a welding speed is shown in Fig. 8.

The mechanical properties of base metal (DIN EN10002-1: 2001), weld metal (sub-size specimen) and

σ σ ε ε� � � �0 1 2 31c c cp p Meckin–Kocks modif[ exp( )] iied

σ ε ε� �k n( )02

p Swift

σ σ ε� �01k n

p Ludwick

σ ε� k np Hollmon0

σ σ σ σ σ σ σYP� � � � � � �31

22 1 22

2 32

1 32J {( ) ( ) ( ) }

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Fig. 3. FE mesh used in the simulation detailing the differentmaterial regions and widths.

Fig. 4. The boundary conditions used in the simulation.

welded samples (DIN EN 895: 1995) were shown in Fig. 9.It was found that in all perpendicular tensile tests to theweld line, all welded specimens were fractured at the basemetal and the strengths were somewhat higher than those ofbase metal. The fracture appearance was shown in Fig. 10.This form of ductile fracture occurs in stages that initiateafter necking begins, where small microvoids form in theinterior of the material, and enlarge to form a crack. The

crack continues to grow and spreads laterally towards theedges of the specimen. Finally, crack propagation is rapidalong a surface that makes about a 45 degree angle with thetensile stress axis. The SEM of fracture surface is shown inFig. 11. The SEM shows a mixture of ductile and brittlefractures where a lot of dimples which characterize the duc-tile fracture and a little of smooth surfaces which indicatebrittle fracture are found.

The product of tensile strength and total elongation isoften used as a measure to evaluate the stretch formabilityof steels.10) It was found that the base metal shows a bettercombination of strength and total elongation. After weld-ing, fusion zones show no marked decrease ofstrength–ductility balance. This means the welding processhas no detrimental influence on the steels formability be-havior as shown in Fig. 12.

4.2. Numerical Results

The true stress–true strain of both base-metal and weld-metal were calculated by experimental engineeringstress–engineering strain data using the following relations:

..............................(10)

..............................(11)

where s, e, s and e are eng. stress, eng. strain, true stressand true strain respectively. The curves are shown in Fig.13.

Many attempts according to Hollomon, Ludwick andSwift equations were done to describe the plastic behaviorof base-metal (R3) and the maximum hardness zone (R1).

ε � �ln( )1 e

σ � �s e( )1

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Fig. 5. Base metal microstructure.

Fig. 6. Eng. stress–eng. strain curves of base metal in 0, 45 and90° with rolling direction.

Fig. 7. Hardness distributions with welding speeds of 2.1, 3.0and 3.9 m/min.

Fig. 8. Macrograph of the weldment at welding speed of3 m/min.

Fig. 9. Eng. stress–eng. strain curves of base metal, weld metaland welded samples.

Fig. 10. Fracture appearance of 2.1, 3.0 and 3.9 mm/min aswelding speed.

The results showed that the modified equation of Meck-ing–Kocks gave an optimum description for yielding behavior of base metal, while the plastic behavior of R1 wasoptimally described by the following relationship:s f�aep�k(e elastic�ep)

n. These fitting curves were shown inFig. 14.

To describe the plastic behavior of region R2 (transitionhardness zone), different models also examined dependingon the previous models and exported to Abaqus/CAE pro-gram. It was found that the following model gave a gooddescription to the plastic behavior in this region:s f�aep�k(e elastic�ep)

n.A comparison of the FEM simulation results with the

experimental engineering stress–engineering strain curve isshown in Fig. 15. The predicted and measured curvesmatched reasonably well.

Figure 16 shows the plastic strain contours for differentregions (R1, R2 and R3) at different time periods through thewidth of the tensile test welded sample. It was found thatthe R1 and R2 didn’t experience significant plastic strain,while the maximum plastic strain occurred in the R3 (basemetal). The major plastic strain is less than 5% in both R1

and R2 regions, while in the case of the BM (R3) it is above38%. This led to localized deformation and fracture of thesample at R3 (base metal). Figure 17 shows 3D plasticstrain distribution in the three regions at strain of 80% fromthe uniform strain of the welded sample. It showed that themaximum plastic strain was localized in the base metal (R3)and this region was coincident with the fracture region inthe experimental tests.

5. Conclusions

The basic characteristics of CO2 laser welded TRIP 700steel such as microhardness and tensile properties with dif-ferent welding speeds were investigated. The deformationbehavior of the welds was numerically simulated using FE-ABAQUS/CAE software package. The following resultswere obtained:

(1) In a tensile test perpendicular to the weld axis, allspecimens were fractured at the base metal and both yieldstrength and tensile strength in all studied welding speeds

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Fig. 11. SEM of fracture: a) Base metal, b) welded sheet.

Fig. 12. Formability behavior of base metal and welded sheets attested welding speed range.

Fig. 13. Eng. and true stress–strain curves for base and weldmetal.

Fig. 14. Fitting of plastic behavior of weld metal and base metal.

Fig. 15. Comparison of experimental and numerical results foreng. stress eng. strain.

were larger than those of raw metal but elongation waslower than that of raw metal.

(2) The plastic behaviors (yielding) of both base-metaland weld-metal didn’t obey Hollomon or Ludwick or Swiftmodel.

(3) There are good agreements between the experimen-tal- and FE-results when plastic behavior of the transitionhardness zone (R2) was described by the following models f�aep�k(e elastic�ep)

n.

Acknowledgements

This article was conducted within the framework of thePh. D. programme ‘Integration simulation and experimentalinvestigation of the laser beam welding of the advancedhigh strength steels’. The financial support of Welding and

Joining Institute of the RWTH Aachen (Germany) and theHigher Education Ministry (Egypt) is gratefully acknowl-edged.

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Fig. 16. Plastic strain propagation contours in different regionsat different time periods during tensile test.

Fig. 17. 3D-plastic strain contours in different region.