Fea of Tailor Welded Blanksssss

Finite Elements in Analysis and Design 37 (2001) 117}130

Finite element analysis of tailor-welded blanks

K.M. Zhao*, B.K. Chun, J.K. Lee

The Ohio State University, 155 W Woodruw Avenue, Columbus, OH 43210, USA

Abstract

A typical tailor-welded blank (TWB) is composed of several base metals, which might have di!erentmechanical properties as well as thickness. Moreover, a TWB contains a heat-a!ected zone (HAZ) which hasquite di!erent mechanical properties from base materials. The forming behaviors of TWBs have been widelystudied since they were "rst developed as a way of using collectible o!al. They are often modeled by shellelements assuming the midsurfaces are in the same plane. Base materials are tied together along the weld linesso that the HAZ is neglected. In this paper, various "nite element models for TWB including HAZ arepresented. An appropriate model based on the considerations of accuracy and computing e$ciency issuggested. Free-bend test (three-point bend test), stretch-bend test (OSU formability test) and limit domeheight (LDH) test are performed to verify the proposed numerical modeling technique for TWBs. ( 2001Elsevier Science B.V. All rights reserved.

Keywords: Sheet metal forming; Tailor-welded blanks; Stamping; Springback; Numerical modeling

1. Introduction

A tailor-welded blank consists of two or more sheets that have been welded together in a singleplane prior to forming. The sheets can be identical, or they can have di!erent thickness, mechanicalproperties or surface coatings. Various welding processes, i.e. laser welding, mash welding, elec-tron-beam welding or induction welding, can join them. Radlymayr and Szinyur [1] measured themechanical properties of the weld bead after wire eroding away the base material. In order todetermine the stress}strain relationship of the much narrower laser weld bead, Saunders [2]developed an analytical procedure to extract the properties of weld bead from the combination ofthe base and weld bead materials. By assuming iso-strain in both weld and base materials,

*Corresponding author. Present address: General Motors Corp., 6600 E. 12 Mile Road, MC: 480-400-111, Warren,MI 48092, USA.

E-mail address: [email protected] (K.M. Zhao).

0168-874X/01/$ - see front matter ( 2001 Elsevier Science B.V. All rights reserved.PII: S 0 1 6 8 - 8 7 4 X ( 0 0 ) 0 0 0 2 6 - 3

Table 1Chemical compositions of SPCEN and SPRC

Steel grade Chemical compositions (wt.%)

C Si Mn P S T}Al S}Al Ti Nb

SPCEN (0.005 0.013 0.130 0.009 0.012 0.035 0.032 0.050 (0.005SPRC35 0.022 0.022 0.140 0.070 * 0.019 0.017 Tr (0.005

a portion of the load supported by weld bead can be calculated from the known base materialhardening laws and known total load of TWB. However, the iso-strain assumption in tensile testcannot be achieved easily for the combination of two base materials with di!erent load carryingcapacities. As can be seen later, the iso-strain assumption can be available only when eccentric loadis applied. Saunders [2] employed laboratory sheet forming tests to compare the formability oflaser and mash seam welded blanks of steel sheets with di!erent strength and thickness andconducted a stretch draw experimental analysis of a scaled down automotive fender to investigatethe forming behavior of a tailored blank under realistic conditions. They pointed out that "niteelement modeling is an accurate predictor of deformation pattern and weld line displacement whendraw restraining forces and material properties are known accurately. Mustafa et al. [3] controlledthe weld line movement by adjusting the blank holder forces during deep drawing of a round cup.These studies presented a wide range of information about the formability and failure patterns ofwelded metal blanks. A wide range of information about the formability and failure patterns oftailor-welded blanks and information about the springback of non-welded sheet metal partshave been presented. However, published results on the springback of tailor-welded blanks havehardly been found. In this paper, tensile tests with the eccentric load are employed for thedetermination of the mechanical properties of base materials and HAZ. Di!erent "nite elementmodels for TWB, i.e. 3-D shell element without HAZ, 3-D shell element with HAZ and 3-D solidelement with HAZ, are compared in terms of accuracy and cost e!ectiveness. Based on thesecomparisons, the suggested modeling for TWB is used to simulate two typical sheet metal formingprocesses, namely, free bend and stretch bend. The corresponding tests are performed to verify themodeling technique.

2. Base materials and welding process

The chemical compositions of sheet steels SPCEN (0.8 mm thick) and SPRC (1.4 mm thick)reported by the materials supplier (Hwashin Automotive Company) are listed in Table 1. High-power CO

2laser welds are made at 3 kW with shielding gas of 75% Ar and 25% He, lens focal

length of 190.5 mm, and traveling speed of 2.54 m/min.A schematic TWB tensile test specimen is shown in Fig. 1, where t

1, t

2and t

ware the thickness of

base materials and HAZ, respectively. The total width of the specimen is b and the width of theHAZ is b

w. In this study, b

wis found to be 1.2 mm, approximately, by measuring microhardness

across the HAZ, with typical results presented in Fig. 2. For testing various thickness TWB, thethinner part is shimmed by using a (t

2!t

1) thickness steel sheet.

118 K.M. Zhao et al. / Finite Elements in Analysis and Design 37 (2001) 117}130

Fig. 1. Tailor-welded blank for tension test.

Fig. 2. Micro-hardness plots traversing the weld lines.

Fig. 3. Free body diagrams of tailored blank in tension test with eccentric load.

Table 2Specimen characteristics and hardening law coe$cients

SPCEN SPRC TWB/10 TWB/5 TWB/2.5 Weld bead0.8 mm 1.4 mm R

8"10 R

8"5 R

8"2.5

Type Standard Standard Standard Subsize Miniature N/Ab 12 mm 12 mm 12 mm 6 mm 3 mm N/Abw

N/A N/A 1.2 mm 1.2 mm 1.2 mm 1.2 mmK 51.68 60.858 59.87 63.791 72.144 98.859n 0.2161 0.1966 0.1737 0.1497 0.1155 0.0629

The mechanical properties of HAZ (see Table 2) can be extracted from the tensile test byassuming iso-strain in both of weld and base materials. Zhao et al. [4] pointed out that an eccentricload is needed in order to meet the iso-strain requirement and the eccentric distance b

e(as shown in

K.M. Zhao et al. / Finite Elements in Analysis and Design 37 (2001) 117}130 119

Fig. 4. FEA models for specimen with weld line perpendicular to bending moment: (a) 3D shell element w/o weld; (b) 3Dshell element w/ weld; (c) 3D solid element w/ weld.

Fig. 3) can be approximated by

be+A

p1t1!p

2t2

p1t1#p

2t2BA

b#bw

4 B. (1)

3. Finite element modeling for TWB

As the accuracy of numerical simulation concerned, only 3-D solid "nite element model is able todescribe exactly the geometry of typical TWBs, namely, the middle layers of base metals are not inthe same plane after welding. On the other hand, 3-D shell element model is popular in simulationof sheet metal forming due to its cost e!ectiveness. In the current numerical experiments three "niteelement models are employed to simulate the forming and springback of TWBs with weldorientations perpendicular and parallel to bending moment, as shown in Figs. 4 and 5, respectively.In the simplest model, Figs. 4(a) and 5(a), the base metals are tied together along the weld line andthe HAZ is neglected. The HAZ is taken into account and modeled as shell elements like the base


Fig. 5. FEA models for specimen with weld line parallel to bending moment: (a) 3D shell element w/o weld; (b) 3D shellelement w/ weld; (c) 3D solid element w/ weld.

materials in Figs. 4(b) and 5(b). The exact geometry of TWBs is modeled by 3-D solid elements asshown in Figs. 4(c) and 5(c), where the #at surfaces face down.

The accuracy of plastic bending problem with shell element depends on the number of integra-tion points through thickness. In this study, 31 integration points through thickness are used forshell element and 15 layers through thickness are used for solid element. As boundary conditionsconcerned, the left edges are "xed and the right ones are displacement controlled in verticaldirection. They move upward 10 mm and return to their original positions. All simulations in thispaper are performed by ABAQUS/Standard 5.7 on a personal computer with processor ofPentium, clock speed of 333 MHz, memory of 130 Mb, hard drive of 9 Gb and operating system ofWindows NT 4.0.

Simulation results are plotted as load versus displacement curves in Fig. 6. In the case of weldline parallel to bending moment, the weaker metal experiences large deformation while the strongermetal deforms elastically. More detailed results are presented in Table 3.

3-D solid element model increases the computing time and problem size dramatically. The CPUtime for solid elements is almost 15 times as that for shell elements. Compared to shell modelwithout HAZ, solid model with HAZ and shell model with HAZ increase the reaction forces by25% and 7%, respectively, when the bending moment is applied perpendicular to the weld line,while 10% and 11% when bending moment parallel to the weld line. However, these three di!erent


Fig. 6. Simulation results for specimens with di!erent orientation of weld line: (a) Weld line perpendicular to bendingmoment; (b) Weld line parallel to bending moment.

element models predict similar springback. HAZ has little in#uence on the springback because thatincreasing yield strength will increase springback and on the contrary increasing elastic moduluswill decrease springback. These numerical experiments show that 3-D shell element model hasadvantages of cost e!ectiveness and accuracy in simulation of TWBs. HAZ can be neglected safelyin real applications when blank size is large enough compared to the size of HAZ.

4. Free bend test and simulation

The principle of free bending, also called three-point bending, is illustrated in Fig. 7. The punch issimply made of two rollers. Each bearing of the die consists of four rollers. The upper rollers in both


Table 3Comparison of simulation results with di!erent element models

Weld lineorientation

FEM simulationelement models

DOF (no.) CPU (h) Mem (Mb) Size (Mb) Max load(N)

Springback(mm)

Perpendicular tobending moment

3D shell w/o weld 10086 0.615 10 38.39 143.86 1.63808

3D shell w/ weld 10086 0.601 10 38.39 153.76 1.688853D solid w/ weld 55473 10.27 45 424.3 179.438 1.66843

Parallel to bendingmoment

3D shell w/o weld 10086 1.262 10 38.39 109.96 1.22494

3D shell w/ weld 10086 1.308 10 38.39 121.03 1.217483D solid w/ weld 55473 16.26 45 424.3 120.64 1.28814

Fig. 7. Illustration of free-bend test (three-point bend test).

punch and bearings are "xed in their position and the positions of the lower ones can be adjusted sothat various specimens with di!erent thickness are clamped appropriately. During the tests, thebending axis is perpendicular to the weld line. The specimen is clamped tightly between the tworollers of the punch in order to get stable and continuous unbending and reverse bending. Eachbearing rotates about a "xed center as a rigid body and each roller rotates about its own centerfreely. The major advantages of this type of bending are small capacity requirement and ease ofcontrol.

As can be seen in Fig. 8, Bauschinger e!ects of base materials and TWB are observed in thebend/reverse-bend test. The HAZ in TWB increases the springback to a small extend in this test.This e!ect can be neglected on numerical simulation when large welded blanks are concerned inreal applications. Fig. 9 shows the simulation results of one base material with di!erent hardeningmodels. In order to simulate Bauschinger e!ect accurately, combined kinematic/isotropic harden-ing model should be used [5,6].

In this study, a 3-D shell model without HAZ is used to simulate the free bend of TWB.A combined hardening law is used together with the isotropic yield criterion. The measured andsimulated punch loads of one cycle bend/reverse bend are plotted in Fig. 10. The springback values


Fig. 8. Measured punch loads and displacements for free-bend tests of base materials and TWB.

Fig. 9. Free-bend simulation results for base material (SPRC) with various hardening models.

are presented in Table 4. The good agreement between simulation and measurement proves thevalidity of this modeling technique.

5. Stretch-bend test and simulation

Stretch-bend tests are performed on OSU Formability Test hydraulic simulator to investigatethe springback under stretch and bend. The geometry of the stretch-bend test and the dimension of


Fig. 10. Free-bend simulation results for TWB with combined hardening model.

Table 4Simulation and test results for free-bend test

Simulation Free-bend test

TWB SPRC TWB SPCEN

Springback (mm) 3.2018 3.5923 3.5967 3.8438Max stroke (mm) 12.167 12.167 12.167 12.167

the tailor-welded blanks are shown in Figs. 11 and 12. The 178 mm dimension is parallel tomajor stretch axis and is perpendicular to the weld line. High-elongation large three-elementrectangular planar rosette is installed on the weaker metal (SPCEN), and small three-elementrectangular stacked rosette on the stronger one (SPRC). The maximum punch stroke for weldedblank is less than that for non-welded one because the formability of tailor-welded blanks isdegraded by the altered deformation patterns introduced by dissimilar material strength andthickness.

The same model for TWB, i.e. shell elements without HAZ, is used for the simulation of thistest. Instead of using real drawbead geometry, #at binder with su$cient normal pressure isused to represent the appropriate restraining. The coe$cient of friction between blank andtools is assumed to be 0.14 in the simulation. Measured and simulated punch loads are plotted inFig. 13. Strain distributions along centerline are shown in Fig. 14. Strain histories at two locationsare shown in Fig. 15. Springback values for stretch-bend test are presented in Table 5. Resultsshow that the weaker metal dominates the plastic #ow. Punch load, strain distribution andstrain histories are predicted correctly by using isotropic hardening model because of no reverse


Fig. 11. Illustration of stretch-bend test (OSU formability test).

Fig. 12. Contour of thickness in simulation of stretch-bend test.

loading. Neglecting the e!ect of real drawbead shape causes the apparent discrepancy betweensimulation and test results for the stronger metal (Fig. 15a). Stretching is found helpful to reducespringback.

6. LDH test and simulation

Fig. 16 illustrates the geometry of limit dome height (LDH) test and the dimension of the testblanks with length of 178 mm perpendicular to weld and varying width from 25.4 to 178 mm. One


Fig. 13. Comparison of punch loads in stretch-bend test and numerical simulation.

Fig. 14. Strain distribution along symmetric line perpendicular to the weld in OSU formability test.

Table 5Simulation and test results for stretch-bend test

Simulation Stretch-bend test

TWB SPRC TWB SPCEN

Springback (mm) 0.4671 0.6139 0.3163 0.2977Max stroke (mm) 15.051 20.092 15.051 20.092


Fig. 15. Strains histories of TWB in stretch-bend test: (a) SPRC; (b) SPCEN.

way to compensate the thickness dissimilarity is to shim the die contact in the thinner material.Better clamping condition can be obtained by using split female die. Fig. 17 shows the simulateddeformation pattern. Failure occurs away from HAZ and close to plane strain. Weaker base metalof the small size specimen buckles near the HAZ. Weld line movement is a signi"cant indicator ofoverall deformation pattern and determined primarily by the properties of the base metals and theboundary restraints.

7. Concluding remarks

Six "nite element models are analyzed to simulate TWB. 3-D shell element model without HAZis recommended in simulation of TWB. This modeling technique is implemented into three typical


Fig. 16. Illustration of limit dome height (LDH) test.

Fig. 17. Simulation results of LDH tests: (a) 178]178; (b) 178]102.


sheet metal forming processes, namely, free-bend test, stretch-bend test and LDH test. Correspond-ing test are performed to verify the simulation results. Some conclusions are as follows:

(1) Mechanical properties of HAZ are obtained from hardness test and tensile test with eccentricload.

(2) Shell element modeling of TWB has the advantage of much less computing time and fairlygood accuracy compared to 3-D solid element.

(3) Inclusion of HAZ (modeled by shell element as well) improves the forming results (load), buthas little e!ect on springback. The reason is that higher yield strength and higher Young'smodulus of weld zone have opposite e!ect on springback.

(4) Proposed FEM model correctly predicts the failure pattern, buckling, and springback oftailor-welded blanks.

Acknowledgements

Authors are grateful for "nancial supports provided by OSU CAMMAC and Hwashin Automo-tive Co. Generous computing services provided by the Ohio Supercomputer Center are alsoacknowledged.

References

[1] K.M. Radlmayr, J. Szinyur, IDDRG Working Groups Meeting, Associazione Italiana Di Metallurgia, I-20121MILANO, Piazzale Rodolfo Morandi, 2, Italy.

[2] F.I. Saunders, Forming of tailor-welded blanks, Ph.D. Dissertation, Ohio State University, Columbus, OH, 1995.[3] A.A. Mustafa, D. Brouwers, L. Shulkin, et al., Deep drawing of round cups from tailor-welded blanks. J. Mater.

Process. Technol. 53 (1995) 684}694.[4] K.M. Zhao, B.K. Chun, J.K. Lee, Numerical Modeling Technique for Tailor Welded Blanks, SAE Congress and

Exposition, Detroit, 2000, Paper d 2000-01-0410.[5] K.M. Zhao, Cyclic stress}strain curve and springback simulation, Ph.D. Dissertation, The Ohio State University,

Columbus, OH, 1999.[6] K.M. Zhao, J.K. Lee, On simulation of bending/reverse bending of sheet metals, Proceedings of International

Mechanical Engineering Congress & Exposition, MED-10, Tennessee, USA, 1999, pp. 926}931.


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Fea of Tailor Welded Blanksssss