Intelligent Virtual Design of Precision Forging Processes in Consideration of Microstructure Evolution

Prof. Dr.-Ing. E. Doege, Dr.-Ing. J. Dittmann and Dipl.-Ing. C. Silbernagel

Institute for Metal Forming and Metal Forming Machine Tools, University of Hanover,

Hanover, Germany Prof. Dr.-Ing. E. Doege is head of the Institute for Metal Forming and Metal

Forming Machine Tools (IFUM), University of Hanover, Welfengarten 1A, 30167 Hanover

Dr.-Ing. J. Dittmann Project Manager BMW Group Munich Dipl.-Ing. C. Silbernagel is a research assistant of the IFUM Summary This paper represents the opportunity to establish the precision forging

process as a suitable manufacturing method for helical gear wheels, using an innovative tool concept and an intelligent process design by FEM-simulation. Based on the simulation and implemented subroutines, this study shows a process design and optimisation as well as the prediction of the expected microstructure in the early stadium of process development. The represented precision forging process can be more than an alternative manufacturing method, especially in cost and time reducing respects, because of the use of lower-cost sheared billets, short cycle times and a reduced use of grinding operations.

1. Introduction

Precision forging of gears is a near-net-shape technology, which is a distinguished manufacturing method characterised by high tolerance accuracy and end contour quality. The advantages of precision forging become especially obvious in high productivity. The most important criterion for the successful establishment of precision forging of gears is the economical aspect. This present paper shows the advantages of using an innovative tool system coupled with an intelligent virtual process design performed by the Finite Element Method (FEM) simulation. 2. An innovative tool concept and a new strategy of precision forging processes

Nowadays gears for passenger cars are manufactured with less exceptions by machining. The process of precision forging of gears represents more than an alternative, if the existing problems of still high die wear can be solved. At the institute (IFUM) intensive investigations were conducted to forge complex part geometry within an single-stage precision forging process in the last few years [1, 2]. The extreme process parameters, for instance the high pressure (1200MPa), high work piece temperatures (1250°C) as well as the small cycle times, are responsible for significant die wear. In particular at the end of the precision forging process the thermal and the mechanical loading rise up to the maximum, because of the closed dies and high process temperatures. For this reason on this point the tribological loading and die wear is reached the maximum.

A promising strategy is to reduce the wear to a minimum especially at the final stadium of the forging process, where the teeth of the die are filled. Therefore the whole forging process is subdivided in three forging stages. To this an innovative tool concept was developed (figure 1). During the forging stage 1 (preforming) an axial material flow takes place. Due to the next stage the final top and bottom contours of the gear will be realised by an axial and radial material flow. As a result of the radial material flow during the last forging stage 3 the helical teeth of the gear will be shaped. A further advantage of the new tool system in respect of cost-saving is the possibility to use lower-cost sheared billets. Whereas in the case of the conventional one-stage precision forging processes only expensive precision tubes have been used.

1 3 2

armouringmandril

closing top

gear dieejectorbottom die

Figure 1: Three-stage tool system of the precision forging process for gears

A virtual process design should be performed by FEM-simulation, based on these requirements of the process.

3. Precision forging process design using FEM-simulation

FEM-simulation (Finite Elements Method) has established for the last few years as an effective tool for design and planing in the field of metal forming. In particular for the design of complex multiple-stage forging processes the use of FEM-simulation is indispensable. Therefore the focal points in this present study are:

• Simulation of planned 3 stage precision forging process • Parameter variation and optimisation of the tool geometry

• Prediction of material flow, tool loading, material state etc. • Simulation of the heating process and cooling process to predict the relevant

microstructure evolution (austenizing, phase transformation) For forging processes and especially for precision forging processes a series of

required working steps compose a complex process chain. Figure 2 shows the structure of the manufacturing chain of precision forged helical gear. Such a complex process chain starts usually with the heating processes (convective or inductive heating), than follow the multiple-stage forging process, the heat-treatment and some different grinding operations. The heating, forging and the cooling (heat-treatment) processes are considered in the FEM-simulation. The material properties are mostly defined within the first three working steps (heating, forging, cooling). The relevant microstructure mechanisms during these processes were described by user subroutines, which are based on macro-mechanical and empirical approaches. These subroutines are implemented in the used commercial FE-codes AUTOFORGE, ABAQUS and FORGE 3. The validation of the subroutines is published in [3, 4].

Figure 2: Manufacturing chain of precision forged helical gear wheel

3.1 Origin conceptual designed precision forging process

Initial conditions for all FEM-simulations, in particular for the optimisation of the forging process, were the planned geometry of the 3 forging stages (figure 3) and a maximum of forming force up to 800 - 1000 tons.

forging stage 1

upper die

lower diework piece

forged gearforging stage 2 forging stage 3

Figure 3: Planned geometry of the 3 forging stages of the precision forging process

3.2 Optimised precision forging process

With the aim to develop an optimised multistage precision forging process some contours of tool geometry were varied. The variations of tool geometry performed by the FEM-simulations show an improved material flow and a lower needed force level for the forging stages. The optimum of the different tool geometry of the first stage (figure 4) is variation 3.

variation 1a, b variation 2 variation 3

a b∆ zb∆ za

∆ zb∆ za <

Figure 4: Variations of tool geometry of the first forging stage

Using the variation 1 b wrinkles has been predicted by the numerical simulation (figure 5). In fact this variation (1 b) was not considered for further investigations. The best results for this are performed by the variation 3.

28 0 34 44 48 5640

way of upper die in mm

end of stage 1

Figure 5: Prediction of wrinkles as a result of tool variation 1 b

In order to design a multiple-stage precision forging process, where the needed forming force is below the allowed force limit of 800 - 1000 tons, the forming force for the different tool variations during stage 1 and their effects for the further forging stages were determined. Figure 6 represents the needed forming force depending on the needed forming distance of the upper die. A comparison between the optimum tool variant 3 and the planned origin variant resulted in a significantly lower needed maximum of force up to 32%. Consequently the tool loading are lower in the upper and lower dies than in the case of the planned origin variant. At the forging stages 2 and 3 appear contours which are close by those of final predetermined of the component. For this reason the tool geometry was modified only in the first forging stage.

distance in %

forging stage 3

forc

e in

MN

8

6

4

25 50 75

2

.

originvariation 1variation 2variation 3

forging stage 2

18 35 53 70

forc

e in

MN

forging stage 1

1,21,00,80,60,40,2

020 distance in %60

1 MN = 100 t.40

0

2

0

8

6

4forc

e in

MN legend

0123

3

0123

0

3

89 in %

Figure 6: Computed forming force versus the forming distance of the upper dies

An optimised material flow from the slug to the gear was predicted by the FEM-simulation for the case of tool variation 3 and here is only shown for the stage of teeth shaping in figure 7. The optimised tool geometry is responsible for a more homogeneous material flow and die filling during the forming stage 2. Most of the material was pushed

out of the piercing area during the forming stage 1. The final shape of upper and lower side of the gear was formed already after completion of the forging stage 2.

For this reason the forging stage 2 is already a kind of precision forging process. The teeth of the gear start to develop not until 90% of the distance of upper die 3 is reached (figure 7). Furthermore the material thickness of the piercing area was reduced to 3 mm. At the end of the forging stage 3 the material of the workpiece (steel 1.1191) completely fill the die.

x

y

z

FE-code: Forge 3

z3 = 0 %

z3 = 15 %

z3 = 30 %

z3 = 40 %

z3 =55 %

z3 = 70%z3 = 90 %

z3 = 2.5 mmdistance of upper die III:

z3 = 95 %

z3 = 100 %z3

z3

z3

Figure 7: Optimised material flow as a result of tool variation 3 of the final forging stage 3 The results of the FEM-simulations, which are represented in this present study, in respect of:

• an unwrinkled multiple-stage precision forging process, • the dimensional accuracy of the final part shape, • the needed forming force for the different tool geometry and the forging stages

and • the material flow for the different tool geometry and the forging stages,

are the main aspects for a virtual precision forging process design and the process optimisation.

The investigations and explanations mentioned before resulted finally in the following optimised sequence of forging steps (figure 8).

scale removing & forming of a defined

outside diameter

preforming gear tooth formingmassive bars

forging stage 1 forging stage 2 forging stage 3slug

Figure 8: Optimised sequence of forging stages

Further positive effects of the optimised forging process are significantly lower die loading in particular for the forging stage 1 and 2. The reasons are the more homogeneous material flow and die filling due to the tool variation 3. For tool steel (hot-work steel 1.2367) of the FEM-model a pure elastic material description was assumed. A maximum of loading capacity of tool material using quenching and temper treatment could be realised up to 2050 MPa. In the case of the origin conceptual designed tool geometry FEM-analyses considering meshed tools resulted in a maximum mechanical die loading of 3291 MPa of the lower die of stage 2. Wide areas of the dies have a stress state above the limit of yield stress. Plastic deformation would be the result. The simulation showed a significant improved situation of mechanical tool loading in the case of the optimised tool variant 3 of the stage 1. The maximum of the mechanical load (1660 MPa) is on the half of the value of origin tool variant.

4. Microstructure evolution during multistage precision forging process

In addition to the virtual process design the prediction of the microstructure during the process chain (figure 2 - considered processes in the FEM) and therewith the material properties performed by suitable FE-codes respectively subroutines have been more and more used in the last few years. Figure 9 shows the needed material mechanisms with the relevant mathematical, physical and empirical approaches for the numerical description and prediction of the microstructure evolution.

The following exposition of this chapter includes the most relevant studies performed by other researchers in the field numerical description of thermo-mechanical analysis in consideration of microstructure changes during forming.

During the first working step of the process chain (heating process) a phase transformation from Fe-α crystal lattice into the Fe-γ crystal lattice takes place. Depending on the heating parameter different states of austenizing and grain sizes could be developed. The state of the austenizing structure could be described by FEM-simulation with the aid of the implemented time temperature austenizing diagram within a subroutine. The numerical description of the grain growth during the heating and the further relevant working steps, transport steps and holding is based on the equations (3), where D is the grain size, t the time, QKW the starting energy, R the general gas coefficient and T the temperature [5].

The phase transformations from the austenizing phase (Fe-γ) into the different structure constituents, depending on the current process and cooling conditions, can be described by the equations (1) and (2) [6]. In equation (1) ζi is the volume fraction of the growing phase (ferrite, perlite, bainite) and t the time. The factor k represents the velocity of migration of the interface and certain time independent values to describe the nucleation. The factor n represents the kind of structural constituent growth. To consider the material dependent factors of a phase transformation within a FEM-simulation, it is necessary to identify the coefficients k and n in the equation (1) by experimental investigations. Results of these measurements performed using a dilatometer are continuous and isothermal TTT-diagrams. In equation (2) ζM is the volume fraction of martensite, MS the martensite-start-temperature, T the occuring temperature and k and κ coefficients [7]. Hougardy [8] verified this formula and determined values for the coefficients k = 0.0206 and κ = 0.93 for carbon-steels. These equations describe the phase growth of one structure constituent. The starting and endpoint of the phase growth are based upon the measurement data from the time temperature transformation diagram of the current steel alloy. Due to the forging process, in particular for fleshless precision forging processes, the material flow results in a typical continuous texture or fiber orientation. Main indicator for the texture are the flow lines or marking grids, which are described by the equation (4) [9]. In equation (4) )( ttx ∆+

ris a coordinate of a mesh point at the end of a time

increment and ))(( tru is the displacement vector. rr

structural constituent

grain size evolution

volume fraction

accuracy of dimensionand shape

distribution of structural constituent

degree of austenitizing

fiber orientation (texture)

diffusion phase transformation:

diffusionless phase transformation:

as a function of (x,y,z)

TTA-digrams

mathematical and physicalbackground

ζ i, M

needed material mechanisms and processes for prediction

FE-code / process design

)(00 )( RT

kWQ

ettADD nn −⋅−=−

))(()()( trutxttx rrrr+=∆+

(1)

(2)

(3)

(4)final

par

t with

des

ired

mat

eria

l pro

pert

ies

Figure 9: Theoretical description of relevant microstructure mechanisms

4.1 FEM-simulation of the heating process

Inductive heating processes are favorably applied for precision forging processes. Consequently the heating time is closed to the cycle time of the forging process. The

general FEM-model, the electric power profile (input parameter) as well as a result of the thermal computation are shown in figure 10.

coils

slugceramicsplate

ceramicstube

time in sec.6

15

50

18

elec

trica

lpo

wer

in k

W

electric power profile

time temperature profile

surface

core

tem

pera

ture

in °C 1200

900

0

600

300

6 1time in sec.0 8

12501240123012201210

°C

Figure 10: Electric-magnetic-thermal coupled FEM-simulation

On the basis of the assumed electric power profile as input parameter for the heating process a homogeneous temperature distribution in the small range of 1250°C and 1210°C was computed. Based on these thermal computations the effects on the state of austenizing and the grain growth were simulated (figure 11).

1100

1000

900

800

7000,1 1 10 102 103 104 105 106

time in sec.

tem

pera

ture

in °C

2400 300 100 10 1 °C / sec.

CFe3+αAC1

689

10

ASTM

AC3

inductive heating rateconvective heating rate

70 °C /sec.1.4 °C /sec.

ASTM 10 (>10µm)

ASTM 9 (>15µm)ASTM 4

time temperature austenitizing diagram austenizing state austenizing state

induction heating convection heating

≥(grain size 81µm)

material: carbon steel 1.1191

CFe3++γα

Figure 11: Simulation of state of austenizing and grain size due to the heating processes

The simulations of the inductive and convective heating processes show a significantly different result for the state of austenizing. The heated slugs have a pretty homogeneous microstructure. Due to the longer heating duration with an average heating rate of 1.4°C/sec. (convective heating) a stronger grain growth (ASTM 4) took place. In the case of the inductive heating a grain size was computed to ASTM 9 in the most areas of the cylinder and ASTM 10 for a small zone of the slug. Regarding the small process times of the forging (0.31 seconds) and the quenching step (some seconds), the final grain size and therefore the properties of strength are mostly defined due to the heating process. For this reason the inductive heated part has a more advantageous material behaviour. 4.2 FEM-simulation of the forging processes

One characteristic result due to the precision forging process is the continuous texture in particular near the surface of the forged part.

Figure 12: Start formations of the flow surfaces as indicator for the texture

The schematic formations of the flow surfaces at the beginning of the forging simulation are represented in figure 12. The changes of the surface positions within the FE-grid during the forging stages are shown in the figures 13 and 14.

horizontal flow markers

slug end of stage 1 end of stage 2 end of stage 3 Figure 13: Simulated horizontal fiber orientation (texture) at the forging stages

slug end of stage 1 end of stage 2

vertical flow markers

end of stage 3

different fibers

Figure 14: Simulated vertical fiber orientation (texture) at the forging stages

The compressed flow surfaces near the outer surface of the gear at the end of the forging process are very remarkable and typical for precision forging processes. This characteristic texture is the reason for the favorably material properties in particular in respect of the high fatigue strengths.

In addition to this, state variables like temperature, deformation degree and equivalent stress characterise the forging process. Hence it is possible to design and control a safe precision forging process within the practicable process limits. As one example for the state variables the figure 15 shows the computed temperature distribution at the three forging stages.

Figure 15: Computed temperature distribution after the forging stages 1, 2 and 3

4.3 FEM-simulation of the forging integrated cooling process

Due to the high current contact pressure and pressure dwell during the forging step the workpiece temperature sinks in the contact area until to 707°C. The time temperature transformation behaviour of the work piece depends on the process conditions of the forging and the cooling step [9]. The controlled cooling process takes place direct from forging temperature. It depends on the transport time, how strong a temperature equilibrium can occur. But a significant thermal gradient still exist before the cooling process occurs. The consequence of these aspects is a different microstructure evolution compared to single cooling processes without any precedent material history. Figure 16 represents the computed results of the phase transformation from Fe-γ into Fe-α of the integrated quenching process direct after the last forging stage.

Figure 16: FEM-simulation of the phase transformation process due to a quenching process

This computed microstructure (figure 16) in particular the high rate of martensite near the outer surface of the teeth is very suitable for the use of the gear in practice, because of the high resistance to wear of the martensite structure.

5. Acknowledgement

This present work has been developed within the project Do 190/161-3 of the German Research Group "Werkstoffbezogene numerische Simulation thermischer Prozesse in der Produktionstechnik" and the project Do 190/132-1 supported by the “Deutsche Forschungsgemeinschaft.

6. References

[1] Doege, E. et al. 1999, Precision Forging of helical Gear Wheel, Manufacturing, Heat-Treatment and Test, Final Report Do-190/92, University of Hanover, TP IV

[2] Bohnsack, R., 1999, Investigations of precision forging of running gears, Dissertation, University of Hanover

[3] Doege, E.; Dittmann, J., Neumaier, T.,

1999, FEM-Simulation of Phase Transformations for Steel in Metal Forming with integrated Heat-Treatment. Euromat’99, Munich, September of 1999, Proceeding, Vol. 7: 39-44

[4] Doege, E.; Dittmann, J.; Neumaier, T.

FEM-Simulation of Phase Transformations for Steel in Metal Forming with integrated Heat-treatment. Advanced Engineering Materials 2000, 2, No.7: 434-437

[5] Sellars, C.M. The physical metallurgy of hot working. Hot working and forming processes, Eds. C.M. Sellars and G.J. Davies, Met. Soc. London, 1980, 3-15

[6] Avrami, M. Kinetics of phase change. Journal of Chemical Physics, Volume 8, 1940, 1103-1112

[7] Koistinen, D.P.; Marburger, R.E.

A general equation prescribing the extend of the austenite-martensite transformation and temperature evolution during quenching of steels Acta Metallurgica 1950 No 7 pp 59 60

quenching of steels, Acta Metallurgica, 1950, No. 7, pp. 59-60

[8] Hougardy, H.P.; Yamazaki, R.

An improved calculation of the transformation of steels, Steel Research, 1986, No. 57, pp. 466-471

[9] Neubauer, I. Numerische Untersuchungen zur Auslegung von Präzisions-schmiedeprozessen am Beispiel schrägverzahnter Stirnräder. Dissertation, University of Hanover, 2002

[10] Doege, E.; Dittmann, J.

2001, Numerical Simulation of Microstructure Evolution During Forging Processes. 7th International Conference on Production Engineering Design and Control, PEDAC 2001, Alexandria, 13th -15th February of 2001, Proceeding, Vol. III: 1391-1401