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SLOVENIAN TOOL AND DIEDEVELOPMENT CENTRE

1. Introduction

The benefits of cold deformation processes are gener-ally lost when it comes to the production of workpieceswith flanges. Poor deformability due to excessive workhardening limits the possible flange diameters. Low strainsat one deformation step are the reason for lots of interme-diate annealing operations or the choice of a warm defor-mation process with its known disadvantages.

2. New process idea

For reducing the amount of deformation steps a combi-nation of cold and hot forming has been proposed by [1].Their suggestion was to start with a hot forging process withhigh strains followed by cold forging for the final deforma-tion steps. New processing concepts describe how to inte-grate a partial heating for the following upsetting into aprocessing line [2]. But the cold parts of the workpiece re-main undeformed. In contrast to this the new process, de-scribed in the following, combines both forging types (coldand hot forging) in a way that they occur simultaneously inone deformation step [3]. It benefits from the advantages ofcold and hot forging whilst it reduces the disadvantages to aminimum (Table. 1). This kind of deformation process canbe realised in an industrial process in a way as indicated inFig. 1. An industrial robot is equipped with an induction coilintegrated in the gripping device. During the positioning ofthe workpiece the heating of desired area takes place. After

this heating and positioning the deformation process begins.A second industrial robot with another modified gripping de-vice removes the workpiece afterwards. During this opera-tion a controlled cooling of the heated area can be realised byintegrating water cooling jets into the gripper of the robot tohave desired mechanical properties for the flange.

Table 1. Advantages and disadvantages of cold and warmforging.

3. Process parameters

The main process parameters were identified and theirinfluences investigated with several finite element

SIMULTANEOUS COLD AND HOT FORGING IN A SINGLE FORMINGSTEP � PRINCIPLE, POSSIBILITIES AND LIMITATIONS

K. Kayatürk b, A. Kurt b, U. Weidiga, K. Steinhoff a, A. E. Tekkaya b

a Research & Development / QM, SM AG, Altdorf, Switzerlandb Dep. of Mechanical Engineering, Middle East Technical University, Ankara, Turkey

Abstract

The production of workpieces with flanges by hot forging is suffering from the poor surface quality and accuracy, whereas theproduction by cold forging is limited by local excessive work hardening and corresponding low deformability. The proposed newproduction approach is based on the idea is to increase the deformability with the help of dynamic recrystallization only in theseareas where it is needed. During the positioning an induction coil integrated into a gripping device of an industrial robot heatsonly a small part of the workpiece to forging temperature. The heated area bulges during the following deformation and simul-taneously the ends of the part are formed cold. The main process parameters are heating zone length, temperature, total length,friction and relative position of the heated zone. Failure can occur by buckling, incomplete filling of the die and folding defects.The process offers the possibility to locally influence the mechanical properties with defined heat treatments and to producehollow workpieces as well as workpieces with several flanges.

Keywords: Simultaneous Hot and Cold Forging, Cold Forging, Hot Forging, Forward Extrusion, Finite Element Analysis

Cold Forging Hot Forging surface quality high deformability

high tolerances high strains

near net shape few deformation steps

Hardening low forces Adv

anta

ges

material saving low deformability poor surface

a lot of deformation steps

scaling

high forming forces low tolerances D

isadv

anta

ges

low strains machining

SIMULTANEOUS COLD AND HOT FORGING IN A SINGLE FORMINGSTEP � PRINCIPLE, POSSIBILITIES AND LIMITATIONSK. Kayatürk, A. Kurt, U. Weidig, K. Steinhoff, A. E. Tekkaya

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simulations. Table. 2 shows an overview over these pa-rameters and their contribution to the forming process.

Table 2. Overview and influence of main process parameters.

Fig. 1. Industrial realisation of simultaneous cold andhot forging.

The underlying criterion for these parameter investi-gations was to obtain a maximum flange diameter. Thetemperature reduces the flow stress and with increasingtemperature the bulging of the heated area is enhanced.The heating zone length determines the material volumethat bulges and forms the flange. Both parameters influ-ences the bulge volume and finally the flange volume. Ina different way acts the relative position of the heatingzone. It controls the way the contact between bulge and

die surface takes place. Depending on the position thefirst contact between bulge and die surface occurs on bothsides at the same time or only one-sided. A one-sidedcontact can lead to a bending of the bulge and defects arepossible.

The role of the friction in this combined process hasto be explained thoroughly. The process consists of a par-tial free forming and a forming, that depends on the die.The free forming is the bulging of the heated area. Withincreasing friction the free bulging is favoured with theresult of a larger bulge volume. As a result the flangediameter increases (Fig. 2).

Fig. 2. Temperature vs. Flange Diameter.

On the other hand the flange diameter can also de-crease with increasing friction. The reason for this in-consistent influence is the change of the forming typeof the heated area from free forming to a forming de-pending on a die. As long as there is no contact be-tween bulge and tool increasing friction favours thebulging process and raises the material volume of theflange. After the first contact between bulge and toolthe friction impedes the radial material flow. As a re-sult the final flange diameter slightly decreases withincreasing friction.

4. Process optimization

Fig. 3 shows the two types of investigatedworkpieces: solid and hollow shafts. To achieve themaximum flange diameter, the parameters rod length,heating zone length and position of heating zone wereoptimised. The heated material volume is directly con-nected to the volume of the final flange (Fig. 4). Thereis a linear trendline, but the flange volume exceeds theheated zone volume. Due to heat conduction more ma-terial volume exhibit lower flow stresses. This addition-ally heated material and also parts of cold material flowsinto the flange.

Parameter influence temperature decrease of flow stress, favouring

radial material flow friction hinders material flow, favours free

forming of bulge heating zone length determines the material volume

which flows in radial direction relative position of heating zone

controls the contact of bulged area and tool

total specimen length impelling buckling

temperature, °C

flang

e di

amet

er, m

m

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220

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240

press force: max. 3600 t

m=0 ( 70 mm)l =hz

m=0 ( 90 mm)l =hz

m=0.1 ( 90 mm)l =hz

m=0.2 (l =90 mm)hz

increasing friction

increasing heating zone length

positioning of workpiece withintegrated inductive heating

deformation controlled cooling

SIMULTANEOUS COLD AND HOT FORGING IN A SINGLE FORMINGSTEP � PRINCIPLE, POSSIBILITIES AND LIMITATIONSK. Kayatürk, A. Kurt, U. Weidig, K. Steinhoff, A. E. Tekkaya

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a) Solid Parts b) Hollow PartsFig. 3. Different workpiece geometries.

Fig. 4. Linear dependency between final volume and ini-tial heated zone volume.

5. Failure modes

The investigation of the process by numerical simula-tion revealed several process conditions which lead to afailure of the forming process (Tab. 3).

Table 3. Conditions and consequences of failure modes.

Near the end of the forming process, after the contactbetween bulge and die, the increasing diameter in theflange zone lead to a rapid increase of the punch force(Fig. 5). If the maximum force of the press is too low theforming process stops before the final position of the

punch is reached. The design of the initial geometry incombination with a defined heating zone length is of im-portance for the moment of the contact between bulgeand die. If the contact occurs too early the cold formingprocess will not be completed and as a result the die willnot be filled. The relative position of the heating zonecontrols the first contact between bulge and die in a dif-ferent way. If the contact occurs mainly on one side abending of the bulge is observed during the followingdeformation. After the contact with the other side of thedie material foldings are possible. These foldings can notbe removed in the course of the deformation. Conse-quently they remain in the flange (Fig. 6).

Another failure of this process can occur at the verybeginning of the process. Elastic and plastic buckling dueto a too short supported length or inconvenient aspectratios is well known in free forming. If the rod is onlypartially heated along its length an additional influenceto this problem can be observed. Fig. 7 shows an exam-ple of the simulation results. For these results the cross-section of the rod was fully heated. With identical ratioof heating zone length to diameter the deflection of therod is lower if the rod is fully heated.

Additional simulation revealed the enhanced bucklingby an insufficient heating of the desired zone. This meansthe cross-section of the rod is heated to forming tempera-ture only in the outer areas. With only a partially heatedcross-section the percentage of buckling is about twiceas high as with a fully heated cross-section.

Fig. 5. Punch displacement versus punch force.

6. Discussion

The failure modes reveal the central role of the bulge.It is decisive for the forming process to control the for-mation of the bulge. Its position relatively to the die hasto be optimized in the way, that the contact between bulgeand die occurs symmetrically. Under this condition nobending of the bulge is possible and the necking areas ofthe bulge are removed during the radial flow (Fig. 6).This necking is quite pronounced and can cause some

300

300

250

250

200

200

150

150

100

100

50

500

0

flang

e vo

lum

e, 1

0³ m

m³

heated zone volume, 10³ mm³

trendline:y = 1.1423 * xR² = 0.9255

condition result incomplete forming of flange

insufficient punch load incomplete filling of the die incorrect relative position of heating zone

folding defects in the flange

excessive necking of bulge

folding defects in the flange

incorrect length of free forming zone

buckling before bulging, failure of forming process

insufficient heating buckling before bulging

1200

300

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250

800

200

600

150

400

100

200

5000

punc

h fo

rce,

tons

punch displacement, mm

final variant 1TOL = 0.1 (rel. force)m = 0.1element size: 7.1 mm

100

SIMULTANEOUS COLD AND HOT FORGING IN A SINGLE FORMINGSTEP � PRINCIPLE, POSSIBILITIES AND LIMITATIONSK. Kayatürk, A. Kurt, U. Weidig, K. Steinhoff, A. E. Tekkaya

problems for the real forming process. But the tempera-ture gradients between heated zone and cold zone are verysharp. This is not exactly the case in the real process.Thus simple forming tests have to verify these simula-tion results. For achieving maximum flange diametershigh material volumes of the bulge have to be realized.But if the bulge volume exceeds a critical value the re-sulting diameter leads to a too high punch force. Incom-plete forming of the flange is the consequence. Too bigbulges can also cause an early contact with the die. Thisearly contact lead to an incomplete cold forming not bethe same in the real process. Thus simple forming testshave to verify these simulation results too.

Fig. 6. Folding defects in the flange.

The combination of this forming process with a con-trolled cooling, as implied in Fig. 1, offers the possibilityfor various in-situ thermal treatments only in the flangezone. Slow cooling rates can be chosen for achieving asoftening effect in the flange for further cold formingsteps, such as rolling of gears.

Different workpiece geometries with local high de-formations are suitable for this deformation process. Be-sides also thick-walled tubes for the production of hol-low shafts are possible. Up to now these shafts are con-

ventionally produced with drilling operations after theforming process.

Several heating zones can be used for the productionof workpieces with several flanges. But the material flowand the additionally influencing process parameters haveto be investigated for this application.

Fig. 7.Buckling of a partially heated part due to insuffi-cient supported length.

7. Conclusions

The simultaneous cold and hot forging is a solution forworkpieces with local high deformations. It combines mostof the advantages of cold and hot forging. The most impor-tant parameter of this process is the forming of the bulgewhich depends on the heating zone length and tempera-ture. The bulge determines the final flange. The combina-tion of this simultaneous process with a following control-led cooling offers various possibilities of influencing me-chanical properties with the help of heat treatments.

References

[1] Körner, E.; Knödler, R.: �Possibilities of warm extrusionin combination with cold extrusion� Journal of MaterialsProcessing Technology, 35(1992) 3/4, S. 451 - 465

[2] Wagener, H.- W.: �Das Online-Rekristallisations-glühenbeim Kaltfliesspressen von Stahl�, Disserta-tion, THDDarmstadt, Darmstadt 1996.

[3] Weidig, U.; Kayatürk, K.; Kurt, A.; Steinhoff, K.;Tekkaya, A. E.: Combination of cold and hot forging ina single forming step � production technique, workpiecegeometries, material characteristics. In: Proceedings ofthe 14th Int. Forgemasters Meeting, Sept. 3-8, 2000,Wiesbaden, Germany, pp.199-206.