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Available online at w.sciencedirect.com- ScienceDirectActa Metall.Sin. (Engl. Lett.) Vol.20 No.5 pp380-384 Oct. 2007

ACTAMETALLURGICA SINICA

(ENGLISH LETTERS)www.arns,org.cn

SIMULATION O F 3-D DEFORMATION AND MATERIALFLOW DURING ROLL FORGING PRO CESS USINGSYSTEM O F OVAL-ROUND GROOVEG . H . Liu1S2)*,G . S . R e d ) , and C.G. Xu')1) Beijing Research Institute of Mechanical & Electrical Te chno logy, Beijing 100083,China2) Huazhong University of Science and Technology, Wuhan 430074, ChinaManuscript received 8 June 2007

Basing on the analysis of the traits of the roll forging process, a system-model of computersimulation has been established. Three-dimensional rigid-plastic FEM has been used for thesimulation of the deformation process in the oval and round pass rolling, including the entering,rolling, and separating stages. The analysis was conducted using the Deform-3D ver.5.0 code.The important information concerned with the deformution area characteristic,material flow, andvelocity field has been presented. Otherwise,the location of the neutral plane in the deformationarea was shown clearly.KEY WORDS numerical simulation; 3-0 deformation; roll forging

1. IntroductionRoll forging is a process for reducing the cross-sectional area of heated bars or billets by passing

them between two driven rolls that rotate in opposite directions and have one or more matching groovesin each roll['].This process reduces the cross-sectional area of the bar while ch anging its shape. It is a verycost effective and property effective process tha t can be applied as a final forming operation or a prelimi-nary forming operation, followed by other forging processes in comp onents m anufacture, such as handshovels, spades, various agricultural tools, cranksh afts connecting rods, and other autom otive parts.

Nowadays, products for various fields in practice have to meet higher quality demands and need tobe developed in shorter times. The design and optimization of the forming processes are often done by em-ployees based on their operational experience. Therefore, it is often necessary to carry out experimentsthat are very expensive because of high machinery costs and loss of production[21. o prevent such empir-ic procedure, numerical simulation of the forming processes is applied increasingly and is becoming a veryimportant tool for the design and developm ent of new products. En th e 2 I" century, computer simulationhas become an important trend of plastic forming[31.t can be used not only to prove the feasibility of the'Corresponding autho r. T el .: +86 10 82415013; ax : +86 10 62933753.E-mail address : huamary20OO@yahoo.corn.cn (G.H. L iu )

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production process, but also to show the deformation behavior inside the workpiece. Thus, several re-searchers have engaged in the numerical description of the forming procesP". Recently, with the devel-opment of the available software, various hot-forming processes can be studied by FEM (finite elementmethod) s im~la t ion[~-~] .esearchers have already built the m odels and completed the simulation duringthe hot Roll forging process using the FEM program M SC Marc[''] and other FEM software["].

Although the principle involved in reducing the cross-sectional area of the work metal in roll forg-ing is essentially the same as that employed in rolling mills to reduce billets to bars, the forming processin roll forging with its own characteristic is complicated; on ly using the principle emp loyed in rolling millsis inadequate an d further research is necessary. In this study, the simulation of the deforma tion du ring theroll forging process using the oval-round groove-system is show n and the results are discussed.2. Conditionsof the Simulation

The subject of the simulation is forming a shaft with 30m m diameter using a roll forging machine inwhich the center distance is 460mm. The material of the billet is No.45 steel; its initial cross-section di-ameter is 40mm, and its length is 160mm. To shape such a shaft, tw o rolling passes using the oval-roundgroove system are selected, and then forging dies are designed based on the total coefficient of elongation.The cross-section shape a nd dimensions of the grooves are shown in Fig.1.

Along the length, the die is combined with three arc-stages called entering, steady-rolling, and sepa-rating, a nd the center angle of each is 5 " , 20, nd 5", respectively. The workpiece is deformed from around bar to an elliptical one in the first pass, and then enters the second pass after rotating 90" aroundits axis.

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Fig. 1 Cross-section dimensions of grooves for roll forging: (a) oval groove; (b) round groove (unit: mm).

3. FEMModelUsing a rigid-plastic ma terial law , the isothermal 3-D simulation o f the roll forging process was per-

formed using the comm ercial software Deform-3D V er 5.0.The flow stress of the material ( a steel with ap-proximately 0.45wt% C) is the function of the accumulated strain, the instantaneous strain rate, and thecurrent temperature, which can be read directly from the materials in the software. For the calculation ofthe friction stress, the law of constant friction (the friction factor model), which assumes a proportionali-ty between the friction stress and the shear yield stress, was used.

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The geom etry of the objects (twodies, a work-piece) is imported from the Solidworks-interface.The dies are defined as rigid objects while theworkpiece is discretized and defined as a plasticone. In the simulation, the dies rotate around the ax-is of each roller in the opposition direction with65rev/min. The dies and the workpiece can be seenin Fig.2 during the simulation of the second rollingpass.

During the simulation, the mesh of the work-piece was regenerated by the automatic tetrahedronremesher of Deform-3D, which is necessary ow-ing to the high deformation of the elements. Theremeshing parameters must be appropriately chosen. The calculation was accomplished with a total num-be r of elements of about 11000.

Fig.2 Simulation model of the second rolling pass.

4. Analysis and Discussionof the SimulationResults4.1 Simulation of the 3D-defomzation

Acco rding to the sim ulation results in Fig.3, the deformation of the workpiece during the roll forgingprocess either in oval groove or in round groove includes three stages called entering, steady-state rolling,and sep arating, which a re usually divided into steady-state and unsteady-state rolling processes. A t the be -ginning, the workpiece contacts the dies only on its top and bottom, and is then driven into the grooveby the friction between the rollers and the billet; this stage is not steady while the d eformation is slowand gradual. After a short transition, the rolling process is in the steady-state stage in which the main

Fig.3 Deformation shape of the workpiece in oval (a-c) and round (d-f) pass rolling at several stages: (a, d) enter-ing; (b , e) steady-state rolling; (c, f ) separating.

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deforma tion is comp leted. A t the end , with the workpiece separating from the groove in the dies, the pro-cess is in unsteady-state stage again. The reason for this is that the contact area between the dies and theworkpiece in the steady state is considerably larger than that in the beginning and the en d.

Unlike mill rolling, the deformation along the length direction of the workpiece is confined duringroll forging process. The workpiece does not enter the groove on its head; there is a certain distance be-tween the head and the contact location. At the tail of the workpiece, the condition is similar. Thus, nei-ther the section of the head nor that of the tail is concave, a s shown in Figs. 3a and c.4.2 Simulation of the velocity field

Fig.4 shows the simulation of the velocity field in th e deformation zone along the rolling direction.It is clear that the v elocity of each element is not equ al either in oval groove or in roun d g roove. Line G inFig.4a and line H in Fig.4b are regarded as the interfaces; on the right of them , the speed of the elementsnear the sides is slower than those near the m iddle; on the left of them, the result is jus t the opposite. Thus,lines G and H represent the neutral planes, which divide the deformation area into forward slip and back-ward slip. Since the max reduction of the workpiece in round groove (about 18mm ) is slightly heavierthan that in oval groove (about 16mm), the distribution of the elements' speed in the cross section is notconsiderably different in the two kinds groo ve.

Fig.4 Velocity field in oval and round pass rolling at steady-state during roll forging: (a) in oval groove;(b) in round groove.

6. ConclusionsUsing the finite element method numerical simulation, 3-D dynamic deformation simulation in oval

and round grooves during the roll forging process including entering, rolling, and separating stages hasbeen com pleted. The complete information of the workpiece deformation shape, the velocity field, and thematerial flow trait are obtained. The location of the neutral plane in the deformation area has been predict-ed correctly and clearly.

Acknowledgem ents-This work was supported by the National Natural Science Foundation of China (No.50675014) .

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