die design for flashless forging of complex parts

Die design for ¯ashless forging of complex parts

Victor Vazquez, Taylan Altan*

ERC for Net Shape Manufacturing, The Ohio State University, Columbus, OH 43210 USA

Abstract

In conventional hot forging of connecting rods, the material wasted to the ¯ash accounts approximately 20±40% of the original

workpiece. In order to reduce the cost of forged products, the forging must be performed in a closed cavity to obtain near-net or net shape

parts. In ¯ashless forging, the volume distribution of the preform must be accurately controlled to avoid overloading the dies and to ®ll the

cavity. Additionally, the preform must be simple enough to be mass-produced.

This study deals with the preform design for ¯ashless forging of a connecting rod and introduces a new tooling concept for forging of

complex parts with a controlled amount of ¯ash. In both studies the use of process simulations has been helpful in the performance of

several iterations without requiring the construction of expensive tooling. # 2000 Elsevier Science S.A. All rights reserved.

Keywords: Die design; Flashless forging; Tool design

1. Introduction

If the weight of a connecting rod (see Fig. 1) can be

reduced while increasing its strength, an automobile's fuel

ef®ciency will be improved. Currently, steel connecting rods

are used in passenger cars. However, some manufacturers

have attempted to use alternative lighter materials. Recently,

various composite materials based on aluminum have been

considered, but not yet successfully adopted, for automotive

engines. The main reasons are that these materials are not

strong enough, or when strong enough, are too expensive.

Flashless forging offers the possibility of producing alu-

minum composite connecting rods at competitive costs. The

design of ¯ashless forging processes is more complex than

the design of conventional closed die forging with ¯ash.

Therefore, in order to accelerate the development of the

manufacturing process as well as to reduce the development

costs, a new design method must be developed and applied.

The ®nite element method (FEM) offers the possibility to

design the entire manufacturing process on a computer. This

leads to a reduction of the cost and time in process and tool

design, tool manufacturing, and die try-out. In addition, it is

possible to modify the process conditions in the simulation

to ®nd the best manufacturing conditions for a product.

1.1. Forging of connecting rods

In the forging of connecting rods, three main methods are

employed. The ®rst method consists of making a rough

preform from a non-porous billet and hitting it several times

in a press until the ®nal shape is obtained (left of Fig. 2).

This method results in 20±40% of the material to be wasted

as ¯ash. A major advantage of the closed-die forging with

¯ash is that the volume of the preform can vary within a

wider range than for ¯ashless forging. This makes it easier to

continuously manufacture products with the same quality.

However, a trimming process is necessary to remove the

existing ¯ash.

The second method is net shape ¯ashless forging (right of

Fig. 2). During this process the preform is totally enclosed in

the die cavity so that no ¯ash formation is allowed. There is

no material waste as in impression forging. However, tight

volume control of the preform is necessary to insure ®lling

of the cavity and to avoid overloading the tooling. The third

method, which is widely used, is hot forging of powder

metallurgy preforms. This method yields virtually no mate-

rial waste and produces near-net shape products. However,

the metal powder is expensive compared to conventional

materials.

In principle, forging operations are non-steady state pro-

cesses, in which the deformation of the material takes place

under three-dimensional stress and strain conditions. The

material ¯ow depends mainly on the following [1]:

Journal of Materials Processing Technology 98 (2000) 81±89

* Corresponding author. Tel.: +1-641-292-9267; fax: +1-614-292-7219.

E-mail address: [email protected] (T. Altan)

0924-0136/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved.

PII: S 0 9 2 4 - 0 1 3 6 ( 9 9 ) 0 0 3 0 8 - 8

1. geometry of the cavity;

2. geometry of the flash opening;

3. initial and intermediate billet geometry;

4. percentage of flash;

5. heat transfer between the tooling and the billet.

Thus, the requirements to perform a successful ¯ashless-

forging process are:

1. The volume of the initial preform and the volume of the

cavity at the end of the process must be the same.

2. There must be neither a local volume excess nor a

shortage, which means that the mass distribution and

positioning of the preform must be very exact.

3. If there is a compensation space in the dies, the real cavity

must be filled first.

This study deals with the preform design for ¯ashless

forging of a connecting rod and introduces a new tooling

concept for forging of complex parts with a controlled

amount of ¯ash. In both studies the use of process simula-

tions has been helpful in the performance of several iterations

without requiring the construction of expensive tooling.

2. Tool and process design for flashless forging of aconnecting rod

The design of ¯ashless forging processes is more complex

than the design of conventional closed die forging with ¯ash.

The ®nite element method (FEM) and the physical modeling

techniques offer the possibility to accelerate the develop-

ment of the manufacturing process as well as to reduce

the development costs associated with the design of the

entire manufacturing process. In addition, it is possible to

perform several iterations to modify the process parameters

to determine the best manufacturing conditions for a forged

product.

2.1. Physical modeling applied to preform design

Before 3D FEM simulations were practical, physical

modeling experiments and 2D FEM simulations were used

[2] to de®ne a preform for the ¯ashless forging of a con-

necting rod. 3D FEM simulations of the ¯ashless forging of a

connecting rod were attempted [3] but were unsuccessful

due to limitations in remeshing.

2.1.1. Physical modeling experiments

Physical modeling experiments were performed for the

¯ashless forging of a connecting rod using plasticine billets

and aluminum tooling (see Fig. 3) [2]. The experiments

were performed with the ERC/NSM ®ve-ton multi-action

press. The main objective of the plasticine experiments was

to ®nd a preform geometry that would result in complete

tilling of the die cavity.

The volume distribution in the connecting rod was

obtained by cutting several transverse sections and comput-

ing the area of each one. These values are plotted in Fig. 4 as

the cross-section area versus the length of the connecting

rod. The area under the curve represents the volume dis-

tribution of the ®nal shape of the connecting rod. Based on

these results an axisymmetric preform was designed. The

preform suggested in [2] is shown in Fig. 5. This preform

was modi®ed based on the physical modeling experiments.

The ®nal plasticine preform and connecting rod is shown in

Fig. 6 (Preform I).

Fig. 1. Connecting rod.

Fig. 3. Aluminum tooling for physical modeling of flashless forging of

connecting rod.

Fig. 2. Closed-die forging with and without flash.

82 V. Vazquez, T. Altan / Journal of Materials Processing Technology 98 (2000) 81±89

2.2. 3D FEM simulation of flashless forging process

In order to verify the applicability of the new FEM code

DEFORM 3D to the optimization of the preform design, a

simulation of the real connecting rod was performed using

Preform I as de®ned in previous studies [2,3].

An upsetting step of the initial preform had to be carried

out to start the whole forging process, because the pin-end of

the initial preform was too big to ®t into the die cavity. This

simulation is performed at hot forging temperature (see

Fig. 7). The relevant data for this simulation are shown in

Table 1.

The simulation of the ¯ashless forging was carried out

using one upper punch, one die, and the previously deformed

preform as shown in Fig. 8.

The material ¯ow of the connecting rod forging process is

shown in Fig. 9. Fig. 10 shows the contact condition

between the crank-end portion of the billet and the tooling

at the end of the forging. It can be seen from this ®gure that a

relatively large cavity remains at the upper surface of the

crank journal ring (marked as `*' in Fig. 10). The I-beam

section is formed from the ends, thus the metal ¯ow for this

section is not under plane strain.

It was concluded from these results that to optimize the

initial geometry of the preform the following problems have

to be solved [4]:

1. For the crank-end section, it is necessary to redesign the

preform so that it ®lls the cavity completely and

uniformly.

2. For the pin-end section, it is necessary to control the

initial volume distribution and transfer the excessive

volume to other features of the product.

3. For the I-beam section, it is required to determine the

diameter for which plane strain flow could be achieved.

Fig. 4. Axisymmetric plasticine preform.

Fig. 5. Sketch of Preform I of the connecting rod [4].

Fig. 6. Plasticine preform and connecting rod [2].

Fig. 7. Effective strain distribution after upsetting.

Table 1

Input data for the upsetting process

Simulation parameter

Billet material Al 2618

Punch velocity 20 mm/s

Stroke 4.5 mm

Simulation mode Isothermal

Simulation steps (NSTEP) 90

Fig. 8. FEM model for the forging of the connecting rod [4].

V. Vazquez, T. Altan / Journal of Materials Processing Technology 98 (2000) 81±89 83

2.3. Optimization of the preform geometry

It would be very dif®cult to optimize the whole geometry

of the preform at once, since the preform shape is relatively

complex and has a lot of shape parameters as shown in

Fig. 5. Hence, the workpiece was divided into three sections:

crank-end section, pin-end section, and I-beam section. Each

section was optimized independently. This optimization

procedure was adopted for the following reasons [4]:

� Since the I-beam section deforms nearly under plane

strain conditions, it is assumed that the deformation of

the large-end section and that of small-end section do not

strongly interfere with each other.

� The number of shape parameters is reduced and the

optimization process becomes easy to handle.

� The simulation time is reduced by working with a smaller

model.

The seven shape parameters for the crank-end were

reduced to three independent parameters and four dependent

parameters. Three preform designs were selected from the

combinations of parameters.

The top-view of the material ¯ow for the crank-end

preforms is shown in Fig. 11. The shaded area indicates

contact between the billet and the tools. It is clear that the

selection of the geometrical parameters affect signi®cantly

the metal ¯ow.

Similarly for the pin-end, three preforms were de®ned.

The top-view of the ®nal shape achieved for each preform is

shown in Fig. 12. The deformation pattern of the pin-end is

not sensitive to the initial geometry of the preform because it

is completely formed before the forging process for the

whole connecting rod is completed.

There are three parameters in the connecting I-beam

section: diameter d3 and segment lengths s4 and s5, as

shown in Fig. 5. The area of a cross-section of the I-beam

part calculated by I-DEAS was 42.637 mm2. Hence, assum-

ing that the material of this part is ¯ows under plane strain

conditions the initial diameter d3 of the connecting I-beam

section was set to 7.37 mm.

2.4. FE simulation with the optimized preform

Evaluating the results obtained from the optimization

method, a new preform design is proposed (Preform II),

shown in Fig. 13. A second 3D FEM simulation was per-

formed with this preform design, and was compared with the

earlier results. The dimensions of the new preform are

shown in Table 2.

The material ¯ow of the connecting rod forging process

with Preform II resulted as follows: the pin-end section is

formed completely, the I-beam section ¯ows nearly under

plane strain conditions, and the crank-end section is ®lled

Fig. 9. Material flow in forging of the connecting rod.

Fig. 10. Contact condition with the tools at the crank-end section: (�)

contact, (*) no contact.

Table 2

Shape parameters of Preform II, mm

s1 s2 s3 s4 s5 s6 s7 s8 d1 d2 d3 d4 d5

14.50 6.82 20.18 18.50 13.50 7.19 7.11 4.70 9.00 20.00 7.37 15.75 7.00


almost completely. It was concluded that fair results may be

obtained with the design for Preform II.

2.5. Manufacturing the preform

The preform suggested in the previous section could

be manufactured by cross rolling. However, variations in

the cross-rolling process may affect the required dimensions

of the initial preform for the ¯ashless forging process. In

order to verify these points, further investigations of the 3D

FEM simulation or physical modeling experiments are

needed.

3. Die design for forging of connecting rod withcontrolled amount of flash

As discussed above, in the ¯ashless forging of the con-

necting rod, the material savings could be signi®cant. How-

ever, the manufacturing of preforms with a tight controlled

volume may increase the manufacturing cost. An alternative

die design has been proposed at the ERC/NSM. This die

design consist in a closed die that would be able to produce

forgings with a controlled amount of ¯ash (5%).

3.1. Research objectives

1. Design a tooling concept that can save material by

allowing the formation of only a small amount of ¯ash.

Optimize the tooling design with the aid of ®nite

element simulations.

2. Establish guidelines and procedures to design blockers

and preforms in order to accelerate the development of

the production process of a forged part.

Fig. 11. Material flow for the preforms defined for the optimization of the crank-end.

Fig. 12. Final stage deformed billets of the pin-end section in top view

(XY plane).

Fig. 13. Comparison of the shape parameters of Preform II with that of

Preform I.


3.2. Tool design concept for forging of connecting rod with

a controlled amount of flash

Several researchers [3±5] conducted investigations at the

ERC/NSM on ¯ashless forging of connecting rods. These

investigations were performed with the objective of forging

an aluminum connecting rod without ¯ash. However, to

achieve ¯ashless forging it would be necessary to have very

tight tolerance control in the manufacturing of the preform.

That may not be economically feasible. It was concluded

that the formation of ¯ash should be allowed in order to ®ll

the cavity and produce connecting rods with an economical

process.

The locus of this investigation was to develop a hot

forging tooling that will allow the forging of a connecting

rod with a controlled amount of ¯ash. It has been established

that 5% of material waste may be reasonable under the

present production conditions. Thus, this tooling should be

simple enough to be used in mass production.

The proposed tooling concept consists of three different

parts (Fig. 14): (1) punch, (2) outer die, and (3) bottom die.

The tooling works as follows:

� Stage 1: The blocker is inserted in the bottom die.

� Stage 2: The ram moves down and closes the cavity. The

outer die is in contact with the bottom die. In this stage the

blocker does not come into contact with the outer die

walls. There is no workpiece deformation during this

stage.

� Stage 3: The punch keeps moving down with the ram and

the deformation starts. The workpiece fills the cavity

flowing in the direction of least resistance. When the

cavity is filled, or almost filled, the material starts to fill

the flash gap. The material flowing into the flash helps to

compensate for inaccuracies in the control of the volume

of the blocker.

Since the ¯ash is in a restricted area, the die/workpiece

contact stresses increase at the ¯ash-land entrance. This

allows ®lling of the cavity without overloading the tooling.

A 3D model was designed using I-DEAS (Fig. 15) from

the tooling concept described above. This model was based

on the tooling used in [3], for the simulation of ¯ashless

forging of a connecting rod. This tooling was modi®ed in

order to allow the formation of ¯ash. Some draft and ®llets

were added in order to permit the ejection of the part after

forging and to decrease the load on the tooling.

3.3. Two-dimensional simulations of the I-beam section

In order to validate the previously described tooling

concept two-dimensional simulations using DEFORM-2D

were performed for the I-beam section of the connecting rod.

Previous studies [4] showed that the three-dimensional

simulations are time consuming.

In order to save computation time isothermal plane strain

simulations were conducted. In this simulations the position

and thickness of the ¯ash gap were varied. These simulations

helped to de®ne an optimum tooling geometry. Then some

non-isothermal simulations were carried out.

In addition, isothermal simulations with a conventional

closed die tooling with ¯ash were performed. These simula-

tions were performed with a ¯ash amount of 18%, which is

common for this kind of forging. These simulations were

used to compare forging with controlled ¯ash and conven-

tional forging with ¯ash.

3.3.1. Simulation parameters

Three ¯ash locations were selected in the tooling

(Fig. 16):

1. Flash 2.5 mm above the web symmetry plane (Geometry

I).

2. Flash gap in the web symmetry plane (Geometry II).

3. Flash 2.5 mm below the web symmetry plane (Geometry

III).

Fig. 14. Tooling concept for forging with controlled flash.

Fig. 15. Exploded view of the tooling for controlled flash forging.


These three locations were selected because they are the

probable extreme ¯ash locations in the tooling. The ¯ash

thickness varied from 0.4 to 0.8 mm (Fig. 16). The para-

meters used to simulate the forgings with the controlled ¯ash

are given in Table 3.

The parameters used to simulate the conventional closed

die forging with ¯ash are the same as the ones used for the

previous simulations except that the stroke was 3.66 mm and

the ¯ash amount was 18%.

All the simulations started with 300 elements. When the

workpiece started ®lling the ¯ash gap, re-meshing was

performed. The re-meshing was done with 900 elements

with a high-density ¯ash entrance.

The load-stroke curve shown in Fig. 17 can be divided

into four zones where the workpiece behaves in different

way:

� Zone I: This zone covers roughly 70% of the stroke. In

this zone the workpiece is upset between the punch and

the bottom die.

� Zone II: This zone covers up to 75% of the stroke. In this

zone the workpiece sides touch the die walls. The load

increases since material movement is restrained.

� Zone Ill: This zone covers up to 95% Of the stroke. In this

zone the workpiece fills the die corners and the load

increases rapidly. The highest load is reached after the

workpiece fills completely the die cavity but before flash

starts to form.

� Zone IV: This zone covers up to 100% of the stroke. In

this zone flash is extruded through the flash land. In this

zone the load increases before the extrusion starts, then it

decreases when the material flows through the flash gap.

3.4. Determination of the optimum geometry of the tooling

The goals in the tooling optimization were to ®nd a

geometry that requires the smallest load to ®ll the die cavity

without causing defects. Load is an important factor in die

life.

As explained in Fig. 17, several simulations were per-

formed with different geometrical parameters (¯ash location

and thickness). The process parameters (punch speed, mate-

rial) were the same for all the simulations. The results of

these simulations are compared in Fig. 18.

The lowest loads are achieved with Geometry II. If the

¯ash gap is located in the same place as it was for Geometry I

and Ill, the load increases. There is a 27% difference

between the loads for Geometry I and II for a ¯ash thickness

of 0.8 mm (Fig. 18). This difference is 10% when the ¯ash

gap is 0.4 mm thick. The load difference is 22% between

Geometry II and Geometry III, with 0.8 mm of ¯ash thick-

ness. This difference drops to 6% when the ¯ash thickness

reaches 0.4 mm.

As in all hot forging operations, the load is a function of

the ¯ash thickness. When the ¯ash thickness decreases, the

load at the end of the forming process increases. In the ®rst

approach, the ¯ash thickness was estimated using the com-

Fig. 16. Parameters modified during the tooling optimization.

Table 3

Input data for the controlled flash forging simulations of the connecting

rod I-beam section

Parameter Value

Stroke 2.56 mm

Punch speed 20 mm/s

Material (characteristics taken from DEFORM database) Al 2618

Friction factor, m 0.2

Workpiece and die temperature 4008CFlash amount 5%

Fig. 17. Load-stroke curve for controlled flash forging.


mon rules for ¯ash design [6]. These rules give a ¯ash

thickness of 1.6 mm. This thickness is the connecting rod

web thickness. The controlled ¯ash tooling allows a very

small amount of ¯ash. The biggest ¯ash thickness was set to

0.8 mm. This ¯ash thickness gives the lowest loads on the

tooling, as shown in Fig. 18.

The tooling with the geometry described above was

compared to a conventional closed-die tooling with the

same geometry. The amount of ¯ash for the billet in the

conventional closed-die forging was 18%. This is a common

amount of ¯ash for a conventional closed-die forging. The

controlled ¯ash forging gives a load 5% higher than the one

in closed-die forging (Fig. 18). If the billet volume has a

¯ash amount of 5%, the die cavity is not ®lled at the end of

the stroke for the conventional closed-die forging with ¯ash

(Fig. 19). For the ®nal geometry the ¯ash is located in the

middle of the tooling and the ¯ash thickness is 0.8 mm.

3.5. Non-isothermal forging simulation of the I-beam

section

Once the tooling geometry was de®ned, two non-isother-

mal simulations were performed. Since the process is a hot-

forging process, chilling the workpiece is an important

factor in the forging process. The simulation was performed

with the tooling Geometry II. This geometry gives the lowest

loads. The ¯ash gap thickness was set to 0.6 and 0.8 mm.

Only two simulations were performed because non-isother-

mal simulations are more time consuming than isothermal

ones. As expected, the computed loads increased about 13%.

This is mainly due to the workpiece chilling. These simula-

tions are compared in Fig. 20.

In the following section the results of the non-isothermal

simulation of the I-beam section, with the ®nal tooling

geometry are presented (Fig. 20). The parameters are the

same as those used in the isothermal simulations, except for

those speci®c to heat transfer (Table 4).

The temperature was uniformly distributed in all of the

parts at the beginning of the forming. The material ¯ow is

shown in Fig. 21. The die cavity is completely ®lled at the

end of the stroke with a ¯ash amount of 5%. This simulation

was completed without a gap between the punch and the

outer die. Thus, there is no ¯ash allowed in this area.

4. Conclusions and future work

In the case studies presented in this paper it has been

shown that physical and numerical modeling are helpful in

Fig. 18. Optimization on controlled flash forging tooling.

Fig. 19. Under filling on conventional closed-die forging with 5% of flash.

Fig. 20. Comparison isothermal and non-isothermal forgings.

Table 4

Parameters for the non-isothermal simulations [7]

Aluminum

2618

Tool steel

(AISI 301)

Thermal conductivity (W/m28C) 182 28

Heat capacity (�106 J/m38C) 2.84 3.56

Emissivity 0.15 0.15

Workpiece temperature at the beginning

of the forming (8C)

400

Dies temperature at the beginning of

the forming (8C)

200

Heat-transfer coefficient (kW/m2 K) 16 16


the current design practices of forging processes due to the

following reasons:

� these techniques are generally cheaper than performing

tryout with actual dies and equipment,

� modifications to the tooling model (CAD design or soft

tooling) are cheaper and less time consuming than mod-

ifications of the actual production tooling and equipment,

� modeling provides more information about the process,

i.e.: load requirements and metal flow at different stages

of the process,

� for most applications the results may be obtained faster

from modeling than from actual tryouts,

� flashless forging results should be compared with results

obtained from forging with flash. This would help to

determine more clearly the advantages and disadvantages

of flashless forging,

� tool stress analysis of the tooling must be performed in

order to analyze the best way to achieve the longest tool-

life with the highest accuracy,

� flashless forging could result in significant material sav-

ings, however, a more strict control of the volume of the

preform is necessary,

� the new tooling concept presented allows a small amount

of flash (5%) compared to the conventional forging

processes that impose 20±40% of material waste.

� the tooling geometry was optimized using two-dimen-

sional simulations. The best results were obtained for a

tooling geometry with the flash gap in the middle of the

connecting rod and a flash thickness of 0.8 mm,

� the proposed tooling design allows to forge a near net

shape product as in flashless forging,

� unlike conventional and flashless forging tooling, the

proposed tooling will not be overloaded because the flash

is extruded radially without a normal load, i.e., the upper

die does not compress the flash.

References

[1] T. Altan, S.I. Oh, H. Gegel, Metal Forming- Fundamentals and

Applications, American Society of Metals (ASM), Cleveland, 1983.

[2] A. Barcellona, K. Long, T. Altan, Flashless Forging of a Connecting

Rod of an Aluminum Alloy and a Metal Matrix Composite (MMC)

Material, ERC/NSM-B-94-32, 1994.

[3] J. Mezger, K. Sweeney, T. Altan, Investigation of the 3D CODE:

Flashless Forging of a Connecting Rod, ERC/NSM-B-94-31, 1994.

[4] T. Takemasu, V. Vazquez, T. Altan, Investigation of metal flow and

preform optimization in flashless forging of a connecting rod, J.

Mater. Processing Technol. 59(1)(2) (1996) 95±105.

[5] K. Long, V. Vazquez, B. Painter, T. Altan, Flashless Forging of a

Metal Matrix Composite (MMC), ERC/NSM-B-95-31, The Ohio

State University, Columbus, OH, 1995.

[6] YT. Im, T. Altan, G. Shen, Investigation of the Effect of Flash

Dimensions and Billet Size in Closed Die Forging, ERC/NSM-B-88-

19, The Ohio State University, Columbus, OH, 1988.

[7] P. Burte, S. Semiatin, T. Altan, Measurement and Analysis of Heat

Transfer and Friction During Hot Forming (Final Report), ERC/

NSM-B-89-20, The Ohio State University, Columbus, OH, 1989.

Fig. 21. Flow pattern for non-isothermal forging, Geometry I, flash

thickness 0.8 mm.


die design for flashless forging of complex parts

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