Less Loading Tube-Hydroforming Technology on Eccentric Shaft Partby Using Movable Die

Qi Zhang+, Chundong Wu and Shengdun Zhao

School of Mechanical Engineering, Xi’an Jiaotong University, Xi’an 710049, P. R. China

Eccentric shaft parts, such as crankshaft and camshaft, fabricated by thick-wall tube hydroforming are increasingly used in industries,especially air-conditioner and automotive industry. In hydroforming those thick-wall components, the very high internal pressure andincomplete filling of the die corner make the forming process become difficult. In this paper, a new less-loading hydroforming die with movablesleeve is introduced to form the tube-like eccentric shaft workpiece. The characteristic of less loading hydroforming is illustrated by comparingwith the conventional one by using FE simulation. Some typical loading paths and defects in the new hydroforming process are alsoinvestigated. The results obtained from this paper demonstrated the internal pressure, die closing force and die stress can be significantlydecreased, as well as the qualified tube-like eccentric shaft workpiece can be formed by using the new movable die hydroforming.[doi:10.2320/matertrans.MF201121]

(Received August 18, 2011; Accepted December 5, 2011; Published March 7, 2012)

Keywords: tube hydroforming, low internal pressure, tube-like eccentric shaft, less loading

1. Introduction

In the last decades, tube hydroforming has becomeestablished in industry as an innovative process formanufacturing the complex and elongate hollow metalcomponent geometries. Tube hydroforming technology,compared with conventional machining technology, candeform metal into the complex and changeable cross-sectionstubular with lightweight, high strength, and good resistanceto fatigue. Now this technology has recently obtained a wideapplication in automotive and aerospace industry.1,2) How-ever, this process is very complex and highly non-linear innature. The optimization of this process is still an importantgoal of application.3) It is known that the hydraulic pressure,required to deform a tube into a desired shape, mainlydepends on the minimum corner radius of the part, thethickness and the material properties of the tube.4,5) The highhydraulic forming pressure needs the high-pressure intensi-fier, which may reach to 400MPa, and the big capacityforming press, as well as the high-stiffness dies. Moreover,the high internal pressure also can induce the high contactpressure and friction in the tube feeding zone. The highinterface friction leads to large wall thickness reductionand corner thinning of workpiece.6,7) Therefore, to reducethe internal pressure and friction, some new hydroformingmethods were developed.

Yuan et al.8) introduced a pre-forming method to hydro-form the rectangular-sections with small radii by using petal-like section pre-forming shape part. It can be found thepressure for forming the transition corner can be greatlyreduced, and the components with small radii can be formedwith relatively lower pressure. A pulsating hydroformingprocess of the tube was examined by Mori et al.9) Theydiscussed the effects of the oscillation of internal pressure onthe formability of workpiece by using both finite elementsimulation and experiment. It can be found that the burst andthe wrinkles of workpiece are avoided due to the small

friction and low internal pressure when oscillation of pressureis applied. Elyasi et al.10) studied the deformation mechanismof cylindrical stepped tubes in a new hydroforming die withadditional bushes. They concluded that the material flowbecome easy and friction force is reduced by using the newdie-set. Thus, parts with sharp corners and uniform thicknessdistribution were obtained. Nikhare et al.11) investigated thelow pressure tube hydroforming for the square cross-sectiongeometry. They found that the internal pressure and dieclosing force required for low pressure tube hydroforming(LPTH) process is much less than that of the high pressuretube hydroforming (HPTH) process. And also the highpressure hydroforming is sensitive to friction, while in caseof low pressure hydroforming, friction is not an importantparameter.

At present, the tube hydroforming technology have beenmainly applied in the parts with thin-wall. Eccentric shaftparts, such as crankshaft, camshaft, have been widely usedin manufacturing industries, especially air-conditioner andautomotive industry. In order to reduce the weight of part,manufacture time and costs, thick-wall tube hydroformingprocess can be utilized to replace the conventional machiningprocess. Thus, the forming process of the thick-wall tube(outer diameter/inner diameter > 1.2) becomes a new chal-lenging goal of hydroforming technology.

In this paper, a new less loading die apparatus was putforward to improve the formability of the thick-wall tube.Compared with the traditional hydroforming apparatus, twoadditional movable sleeves were used. Numerical simulationwas carried out to compare the new die hydroforming andconventional hydroforming process, and also to investigatethe suitable load path for a kind of thick-wall eccentricshaft part by using the ABAQUS/nonlinear FE simulationsoftware. The generation reasons for defects, includingfold and wrinkles, were discussed. In addition, the stressdistribution and die closing force of hydroforming die setwere also analyzed. Finally, the formability and accuracy offormed thick-wall part were significantly improved underrelative low hydraulic pressure by using the new die.+Corresponding author, E-mail: henryzhang@mail.xjtu.edu.cn

Materials Transactions, Vol. 53, No. 5 (2012) pp. 820 to 825Special Issue on Advanced Tube Hydroforming Technology for Lightweight Components©2012 The Japan Institute of Metals

2. The New Die Apparatus Design

The designed shape and dimensions of the tube-likeeccentric part are shown in Fig. 1. The outer diameter andthickness of the tube billet are 40 and 3.5mm, respectively.The maximum diameter of the deformed part is 68mm, andhence the expansion ratio is ((68 ¹ 40)/40) © 100% = 70%,and the tube thickness ratio is 40/(40 ¹ 3.5 © 2) = 1.21 >1.2, which belongs to the thick-wall tube.

A conventional tube hydroforming die apparatus is shownin Fig. 2. For each side of the tube, the feeding punch, con-tacting with the end of tube, exerts axial feeding on the tube.After placing the tube in the lower die, the upper and lowerdies are closed, and the punches seal the tube. During thecontrollable axial feeding the tube is bulged by the control-lable internal pressure into the shape of the die cavity. Mean-while, the high friction force, generated between the tube anddie, makes tube material flow into the die cavity difficultly.

The principle of new hydroforming method is illustrated inFig. 3. Compared with the traditional hydroforming appara-tus, two additional sleeves are used. The appropriate initialclearance between the outer surface of the sleeve and dies isgiven. At the forming process, the sleeve stroke versus timecurve is additionally used. It will lead to a uniform-thicknesspart with small corner radii at the end of the process.

3. Calculations of the Initial Clearance between theSleeve and Die

In order to decrease the feeding force in the new die, we

can ensure that there is no a relative sliding motion betweenthe sleeve and die by setting a suitable initial clearancebetween them. In this section, the theory calculationprocedure of the initial clearance is given. We assume thatonly elastic deformation occurs for the sleeve, and thedeformation reaches the largest value when the highestinternal pressure is acting on the tube inner surface. Thus, theinitial clearance value between the sleeve and die should beequal with the radial deformation of the sleeve.

As it can be seen from Fig. 4, when internal pressure pincreases to the highest value, the sleeve is loaded by anequivalent internal pressure p�

i .p�i can be described as

p�i ¼ p� 2

ffiffiffi

3p ·s ln

rori

ð1Þ

where ·s is yield strength of the tube, ri and ro are the innerand outer diameter of the tube, respectively.

The radial elastic deformation (ur) of sleeve can becalculated as

ur ¼2p�

i R2iRo

EiðR2o � R2

i Þð2Þ

where Ei is the elastic modulus of sleeve. Ro is outer diameterand Ri is inner diameter of sleeve.

4. Simulation Modeling

The numerical simulations on the hydroforming processeswere carried out by using the FE code Abaqus/Explicit.

A

A

AA

BB

Fig. 1 Shape and dimensions of the formed eccentric shaft (dimension in mm).12)

Fig. 2 Schematic illustration of the conventional hydroforming die. Fig. 3 Schematic illustration of the new hydroforming die.

p p pi*

Tube

Sleeve Sleeve

Tube

Fig. 4 Pressure conditions of tube and sleeve.

Less Loading Tube-Hydroforming Technology on Eccentric Shaft Part by Using Movable Die 821

Simulation models of the two processes are shown in Fig. 5.All parts in the models were meshed by solid elementsC3D8R. A Coulomb friction law was assumed and frictioncoefficient between the tube and die was set as 0.06. DIN1.2344 steel was chosen as the material of forming die set.Young’s modulus and Poisson’s ratio of die material is207GPa and 0.28, respectively. St16 steel was chosen as thematerial of tube billet. The true stress-true plastic strain curveof the material is illustrated in Fig. 6. Due to the symmetricgeometry, only a half of FE model was utilized in simulation.

5. Simulation Results and Discussions

Similar to the conventional process, the forming factors,including internal pressure and axial feeding, can influencethe defects such as wrinkling, folding, buckling and bursting.Moreover, it can be found that the use of the sleeve inducesthe additional gap, which would gradually disappear with the

axial movement of the sleeve. If tube material flows into thegap during the whole hydroforming process, and then thefolding defect will appear in the gap region. Thus, in order toobtain a successful workpiece, a suitable load path isparticularly important.

5.1 Comparison between two hydroforming methodsThe load path of new die hydroforming method is quite

different to the conventional one. The load path for the newdie includes three stages, as shown in Fig. 7(a). In the firststage, the internal pressure increases until its value arrives to47MPa. In the second stage, to access more material feeding,the punch and sleeve moves under the pressure 47MPa.However, the moving displacement of punch is larger thanthat of the sleeve. In the third stage, in order to form a desireddie corner the stroke of sleeve and punch are the same, andthe forming pressure remains constant. The mainly functionof punch is to ensure a good sealing on end of the tube in thisstage.

The simulated load curve of the conventional die hydro-forming method can also be divided into three stages, asshown in Fig. 7(b). In the first stage, the internal pressureincrease until the tube is pre-bulged under forming pressure.Then in the second stage, by remaining the forming pressure47MPa, the tube formed to the desired shape with movingthe punch. In order to realize the small radii of part in the diecorner, the internal pressure was sharply increased in the thirdstage, which can reach to 180MPa.

According to the two load curves, we can find the thirdstage have a big difference. It is because the corner radii arebulged in the third stage of conventional method, whichneeds very high pressure.4,5) The maximum pressure 47MPa,used in the new hydroforming method, is quite smaller than180MPa in the conventional hydroforming method. Andalso, the total punch stroke in the new die is 32.5mm,whereas 23.5mm in the conventional die. Therefore, moretube material is pushed into the die cavity in the new die.Tube forming process by using new die and conventional dieis shown in Figs. 8 and 9, respectively.

From the simulation results, wall thickness and radiusdimension of the symmetry tube sections (see Fig. 10) arecompared. Figure 11 shows the thickness distribution of theupper and lower profiles. It can be seen that the tube, formedin the new die, has a more uniform thickness distribution with

(a)

Punch

Tube

Punch

Die

Die

(b)

Fig. 5 FE models of tube-like eccentric shaft hydroforming, (a) conven-tional die, (b) new die.

0.0 0.1 0.2 0.3 0.4 0.5100

150

200

250

300

350

400

450

500

True

str

ess

(MP

a)

True strain

Fig. 6 True stress-true plastic strain curve of the st16 steel tube.

(a)

0 2 4 6 8 100

10

20

30

40

50

Stage 1 Stage 3

Str

oke

(mm

)

Internal Pressure

Pre

ssur

e (M

Pa)

Time (10-3s)

Stage 2

0

5

10

15

20

25

30

35

Punch StrokeSleeve Stroke

(b)

0 2 4 6 8 100

20

40

60

80

100

120

140

160

180

200

Pun

ch S

trok

e (m

m)

Inte

rnal

Pre

ssur

e (M

Pa)

Time (10-3s)

Internal Pressure

0

2

4

6

8

10

12

14

16

18

20

22

24

Stage 1 Stage 3Stage 2

Punch Stroke

Fig. 7 Load path used in the simulation, (a) new die and (b) conventional die.

Q. Zhang, C. Wu and S. Zhao822

the maximum thinning ratio 21.4% than that in the conven-tional die with the maximum thinning ratio 35.1%. And also,it is obvious that the corner of the tube is well filled in thenew die (see Table 1). In the conventional die, the largestfilling radius can reach to 8.6mm. The corner filling radius ofthe camshaft part is very important, because it would offerenough contact surface to ensure operation stability ofcamshaft power transmission drive system.

5.2 Stress locus analysis on corner filling regionWe select a typical element A (see Fig. 12) to illustrate the

changing of the stress locus in the two processes, as shown inFig. 13. Here, it is assumed that the thickness stress is sosmall that it can be neglected. At the bulging stage of two

hydroforming processes, hoop tensile forces are mainlyapplied. The stress state of element A is the same in the twoprocesses as uniaxial tension. Then by keeping continuousaxial feeding with internal pressure, the tube was formed tothe desired shape. The material flow gradually becomedifficult in the corner filling region. The principal stress stateof the element is changed into the biaxial tensile stress state.At the end of both processes, the stress states of the elementare quite different. Due to the sharp increasing pressure in theconventional die, the material is expanded. Thus, theelements are still in the biaxial tensile stress state. For thenew hydroforming process, the continuous axial feeding ofmovable die makes the elements mostly being bent andcompressed. So, the stress state of the element A is the

(a) (b) (c)

Fig. 8 Tube forming process by using new die, (a) stage 1, (b) stage 2 and (c) stage 3.

(a) (b) (c)

Fig. 9 Tube forming process by using conventional die, (a) stage 1, (b) stage 2 and (c) stage 3.

Fig. 10 The symmetry tube section.

(a)

0 10 20 30 40 50 60 70 80 90 100 110 120 1302.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

Thi

ckne

ss (

mm

)

Distance along the up profile (mm)

new die conventional die

(b)

0 10 20 30 40 50 60 70 80 90 100 110 1202.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

Thi

ckne

ss (

mm

)

Distance along the down profile (mm)

new die conventional die

Fig. 11 Thickness distribution of two workpieces, (a) upper profile and (b) lower profile.

Table 1 Comparison of radius dimension obtained from the simulation ofnew die and conventional die.

Radius R1 R2 R3 R4

Radius dimension formed by new die (mm) 3.2 5.3 3.1 4.7

Radius dimension formed by conventional die (mm) 4.2 7.2 5.0 8.6

Less Loading Tube-Hydroforming Technology on Eccentric Shaft Part by Using Movable Die 823

tension-compression stress state which can greatly avoidcontinuous thickness thinning of tube workpiece and reducethe hydroforing internal pressure.

5.3 Effects of loading pathTo investigate the effects of loading path on the eccentric

shaft tube-like workpiece, 5 typical load paths (see Table 2)were simulated and studied. The loading path 1 [seeFig. 7(a)], the suitable one, is obtained by the trial-and-errormethod. According to the loading path, the qualifiedworkpiece is formed with the maximum thinning ratio21.4%. In others 4 loading paths, only one parameter waschanged compared with loading path 1. In the loadingpath 2, the sleeve stroke in the third stage increases from1.5mm (in loading path 1) to 2mm. Then, we can find thefolding defect appears in the gap between die and sleeve, asshown in Fig. 14. According to the loading path 4 and 5, ifthe maximum forming pressure value increases from 47MPa

(in loading path 1) to 50MPa or total punch stroke increasesfrom 32.5mm (in loading path 1) to 34.5mm, the foldingdefects can also be observed. It is because the redundantmaterial fills the gap between the die and sleeve. For theloading path 3, the maximum forming pressure valuedecreases from 47MPa (in loading path 1) to 44MPa. Then,the wrinkling appears in the middle of workpiece (seeFig. 15). The internal pressure 44MPa isn’t enough to flat thewrinkling, so the wrinkling exists in the workpiece.

5.4 Die closing force and die stress in hydroformingDie closing force curves, obtained from simulations of

the two hydroforming dies, are shown in Fig. 16. It canbe seen that the value of die closing force in the new dieis significantly lower than that in the conventional die,especially in the third forming stage. And the maximum

Fig. 12 Formed tube with the section I.

Fig. 13 Stress locus of element A of two hydroforming processes.

Table 2 Effect of typical load paths on formed workpiece.

Loadpath

MFPV(MPa)

SSTS(mm)

TPS(mm)

Result Descriptions

1 47 1.5 32.5Qualified formed workpiecewith thinning ratio 21.4%

2 ¼ +0.5 ¼ Folding

3 ¹3 ¼ ¼ Wrinkling

4 +3 ¼ ¼ Folding

5 ¼ ¼ +2 Folding

MFPV-maximum forming pressure value; SSTS-sleeve stroke in thethird stage; TPS-total punch stroke; “¼” means no change.

Folding

Fig. 14 Folding defect on the workpiece.

Wrinkling

Fig. 15 Wrinkling defect on the workpiece.

0 2 4 6 8 100

15

30

45

60

75

90

105

120

Stage 2 Stage 3Stage 1

Die

Clo

sing

For

ce (

104 N

)

Time(10-3s)

New DieConventional Die

Fig. 16 Comparison of two die closing forces.

Q. Zhang, C. Wu and S. Zhao824

closing force for the new and conventional die is 227 and1135KN, respectively. In the third stage of the conventionalprocess, a high pressure was applied to forming the small diecorner radius. Thus, a sharp increment of die force occurs atthe end of the conventional hydroforming process.

During the hydroforming simulation, the die set wasconsidered as a deformable body and meshed as the solidelement, so the die stress can also be illustrated. Figure 17shows the die stress distribution for the new die andconventional die at the end of hydroforming. It can be foundthat the maximum stress is 213MPa in new die and 588MPain conventional die, respectively.

6. Conclusion

In this paper, a new less loading hydroforming process fortube-like eccentric shaft workpiece was investigated by usingFE simulation. In the new die, two additional movablesleeves play an important role to improve formability of thethick-wall tube and the shape accuracy of formed workpiece.Some important conclusions can be drawn as following.(1) In the new die, the movable sleeve pushes the tube

material to fill into the small radius of the die corner. Sothe internal pressure can be significantly decreased from180 to 47MPa. In addition, it was illustrated that dieclosing force has a significant reduction from 1135 to227KN, when the new die hydroforming is used toreplace the conventional one.

(2) By using the new die hydroforming, the formed tube-like eccentric shaft workpiece has the relatively uniformthickness distribution and high shape accuracy. Themaximum thinning ratio is 21.4% and the radius ofinner die corner is 3.2mm.

(3) There is much difference in the stress state betweentwo hydroforming processes. At the end of new diehydroforming, the compressive stress appears in thecorner region of tube workpiece, which can greatlyimprove the formability and shape accuracy of theeccentric tube.

(4) The folding defect, as a special defect in hydroformingprocess, can be observed in the new die hydroforming,which is induced by the redundant tube materialflowing into the gap between the die and sleeve.However, the folding defect can be avoided bydecreasing in the internal pressure and total punchstroke.

Acknowledgements

This paper was financially supported by the Science andTechnology Research & Development Program of ShaanxiProvince (its number 2010K08-18), and also by the hydro-forming press program in High-end CNC Machine Toolsand Basic Manufacturing Equipment-National Science andTechnology Major Project (its number 2011ZX04001-011).The authors would like to express their sincere appreciationfor the continuing support.

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(a) (b)

Fig. 17 Stress distribution of the die obtained from simulation, (a) new dieand (b) conventional die.

Less Loading Tube-Hydroforming Technology on Eccentric Shaft Part by Using Movable Die 825