Investigation of metal flow and preform optimization in flashless forging of a connecting rod

ii optimize the preform design in each region independently, that is, large end section, connecting section, and small end section.. 2.2 3D F E M Process Simulation of a Connecting Rod

ELSEVIER Journal of Materials Processing Technology 59 (1996) 95-105

of

Materials Processing Technology

Investigation of metal flow and preform optimization

in flashless forging of a connecting rod

T e r u i e T a k e m a s u a, V i c t o r V a z q u e z b'', B r e t t P a i n t e r b, T a y l a n A l t a n b

aMechanical Engineering for Intelligent Machinery and Systems, Kyushu University, Japan

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

Industrial Summary

In conventional hot forging of connecting rods, the material wasted to the flash accounts approximately 20

to 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 flashless forging, the volume distribution of t h e preform must be accurately controlled to avoid overloading the dies and to fill the cavity Additionally, t h e preform must be simple enough to be mass produced

This study deals with the design of the optimum preform to forge a connecting rod without flash The initial preform design was obtained from physical modeling experiments The optimization of this preform was found through 3D FEM process simulations The advantage of performing simulations is that no tooling has to be built and the number of experimental tryouts can be significantly reduced A preform optimization methodology was derived for this investigation

1 Introduction

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

be reduced while increasing its strength, an

automobile's fuel efficiency will be improved

Currently, steel connecting rods are used in passenger

cars However, some manufacturers have a t t e m p t e d

to use alternative lighter materials Recently,

various composite materials based cn aluminum

have been considered, but not yet successfully

adopted, for automotive engines The main reasons

are that these materials are not strong enough, or

w h e n strong enough, are too expensive

Flashless forging offers the possibility of

producing aluminum composite connecting rods a t

competitive costs The design of flashless forging

processes is more complex than the design of

conventional closed die forging with flash

Therefore, in order to accelerate the development of

the manufacturing process as well as to reduce t h e

development costs, a new design method must be

developed and applied The Finite Element Method

(FEM) offers the possibility to design the entire

manufacturing process on a computer This leads to a

"Corresponding author

0924-0136/96/$15.00 © 1996 Elsevier Science S.A All rights reserved

(96) 02290-X

reduction of the cost and time in process and tool design, tool manufacturing, and die try-out In addition, it is possible to iteratively modify t h e process conditions in the simulation to find the best manufacturing conditions for a product

Fig 1: Connecting rod

1.1 Forging of Connecting Rods

The closed die forging process is often used to manufacture high quality mass production parts like connecting rods, crankshafts, etc., at moderate costs

In principle, forging operations are non-steady state

96 7q Takemasu et al / Journal of Materials Processing Technology 59 (1996) 95-105

processes, in which the deformation of the m a t e r i a l

takes place under three-dimensional stress and

strain conditions The material flow depends

mainly on the following [1]:

a) Geometry of the cavity

b) Geometry of the flash opening

c) Initial and intermediate billet geometry

d) Percentage of flash

e) Heat transfer between the tooling and t h e

b i l l e t

A sketch of a closed-die forging process w i t h

flash is shown c~a the left in Fig 2 The upper and

lower dies form the closed cavity; the f l a s h

originates in the gap between the dies A m a j o r

advantage of the closed-die forging with flash is

that the volume of the preform can vary within a

specific range, which makes it easier to continuously

manufacture products with the same quality

However, a trimming process is necessary to remove

the existing flash

As shown on the right in Fig 2 flashless forging

does not allow the material to leave the cavity and

therefore no flash is generated One of the most

important advantages of this process is that a

significant amount of material can be saved in

comparison to forging with flash Furthermore, a

trimming operation is not required

There are some requirements to get a successful

flashless-forging process:

a ) T h e volume of the initial preform and t h e

volume of the cavity at the end of the process

must be the same

b) 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

c) If there is a compensation space in the dies,

the real cavity must be filled first

1.2 Research Objectives

So far, most FE codes that simulate billet forming

processes consider only plane-strain or axisymmetric

deformations Since many industrial parts such as

connecting rods have very complex geometries, t h e

metal flow is three-dimensional and cannot be

properly modeled with a two-dimensional

approximation This means that a t h r e e -

dimensional simulation of the manufacturing process

must be performed to get adequate results The

commercial package DEFORM 3D v.2.015] offers t h e

possibility of simulating three-dimensional

material flow of complex geometries

The 3-D FEM simulation of the flashless forging

of a connecting rod has already been performed a t

the ERC/NSM with DEFORM-3D v.l.0 [2]

However, it was not possible to simulate the w h o l e forging process due to the limited remeshing capabilities of DEFORM 3D v.l.0 Therefore, it was not possible to verify that the preform designed from physical modeling experiments [3] was indeed the optimum preform Due to the improved remeshing capabilities of the recently released DEFORM 3D v.2.0, it is now practical to optimize the preform shape through the analysis of the FEM simulation results

Forging with Flashless FOrs~ng with Flashless

Up Die

L~~'l

;

' Lower Punch Start of Stroke End of Stroke

Forging

Fig 2: Closed-die forging with and without flash The objectives of this study are:

i) perform the 3-D FEM analysis of an actual connecting rod with DEFORM 3D v.2.0 using the preform defined in the previous studies and find out the problems in the design of t h e preform

ii) optimize the preform design in each region independently, that is, large end section, connecting section, and small end section iii) define a new preform design based cn t h e optimization results and verify t h e applicability of this optimization method

2 Previous S t u d i e s in Preform O p t i m i z a t i o n

Before 3D FEM simulations were practical, physical modeling experiments and 2D FEM simulations were used [3] to define a preform for t h e flashless forging of a connecting rod 3D FEM simulations of the flashless forging of a connecting rod were attempted [2] but were unsuccessful due to limitations in remeshing

2 ] Physical Modeling Experiments

In metal forming operations, in order to predict metal flow, die filling, defect occurrence, and

T Takeraasu et al / Journal o f Materials Processing Technology 59 (1996) 95-105

forming loads, the use of highly deformable model

materials represents a valid and powerful tool

In the tool design phase, a soft material, e i t h e r

metallic or non-metallic, can be used to carry out

several tests by changing the tooling geometry The

aim is to optimize material flow and die filling

using machinable tools and low cost die m a t e r i a l s

(i.e acrylic, glass or aluminum) Furthermore,

compared to hot forging processes, lower

temperatures are typically used in modeling tests

Physical modeling experiments were performed

for the flashless forging of a connecting rod using

plasticine billets and an aluminum tooling [3] The

experiments were performed c~ the ERC five ton

multi-action press The main objective of t h e

plasticine experiments was to find a preform

geometry that w o u l d result in complete filling of the

die cavity

The volume distribution in the connecting rod was

obtained by cutting several transverse sections and

computing the area of each These values were

plotted in Fig 3 as the height versus the length of

the connecting rod The area under the curve,

indicated by the arrow "A', represents the volume

distribution of the piece Based c~ these results an

axisymmetric preform was designed The preform

suggested in [3] is shown in Fig 4 This preform was

modified based cn the physical modeling

experiments The final plasticine preform and

connecting rod are shown in Figure 5

2.2 3D F E M Process Simulation of a Connecting Rod

3D FEM analysis of the flashless forging of

connecting rods was performed with two different

types of preform geometries [2], which were called

(1) real geometry and (2) simplified geometry

A

I

i

Fig 3: Connecting rod volume distribution versus

length

Fig 4: Axisymmetric plasticine preform

Fig 5: Plasticine preform and connecting rod [3]

The shape of the real connecting rod is a modification of a Nissan connecting rod Isometric views of the real and a simplified connecting rod are shown in Fig 6 The simplified geometry of t h e connecting rod was used to verify h o w simplifications, made in order to accelerate t h e simulation process, affect the simulation results These 3D FEM simulations used brick elements for the billet and rigid surfaces to model the tooling The software used was DEFORM 3D vl.0 The simulations were run isothermally The main limitation of the code used was that it did not h a v e automatic remeshing capabilities Therefore, t h e remeshing had to be performed s e m i - a u t o m a t i c a l l y

by approximating the deformed shape of the b i l l e t with surfaces and remeshing the enclosed volume with tetrahedrals which were latter broken into bricks Since this is a a time consuming operation i t was only possible to achieve 75% of the stroke for both geometries The effective strain in t h e deformed connecting rod is shown in Fig 7

3 Preform Optimization by 3D FEM Simulation

In order to verify the applicability of the new FEM code DEFORM 3D (v2.0), a simulation of t h e

98 T Takemasu et al / Journal of Materials Processing Technology 39 (1996) 95-105

real connecting rod was performed using preform-I as

defined in previous studies [2,3] A sketch of

preform-I is shown in Fig 8

3.1 F E Simulation o f the Forging Process

An upsetting step of the initial preform had to be

carried out to start the whole forging process,

because the smaller end of the initial preform was

too big to fit into the die cavity This operation is

performed at hot forging temperature

form half of the workpiece is almost 1.9 metric tons, this means that 3.8 metric tons are required for t h i s operation

The simulation of the flashless forging was carried out using one upper punch and one die w i t h the upsetted preform in between as shown in Fig 11 Before the upset preform was imported into t h e forging simulation, a remeshing was executed to make meshes finer especially at the smaller end section

1.240

1.13o

1.010

0.896 0.781

~ 0.665

O 550 0.435

0.319

~ 0.204 0.089

Fig 6: a) real and b)simplified connecting rod

geometry

The upsetting process is simulated using one f l a t

die (constructed by square shell elements) t h a t

moves in the negative Y (down) direction to compress

the small end of the connecting rod T e t r a h e d r a l

elements were used for the billet in this simulation

The simulation was stopped at a stroke of about 1.5

mm The relevant data for this simulation are

shown in Table 1

Fig 9 shows the final shape of the preform after

upsetting Very little deformation is present in t h e

connecting section and the large end of the preform

The necessary punch load curve calculated for this

operation is shown in Fig 10 The force required to

Fig 7: Effective Strain Distribution S1 $2 $3 $4 $5 $6 $7 $8

Fig 8:

Fig 9:

Sketch of preform-I of the connecting rod

m ,lt~

*000

Effective strain distribution after upsetting

1.864

i(10 4

1.553

1.243

Y

0 932

a

d

(N) ~1

.311

.000

IT Takemasu et al /Journal o f MaterJals Processing Technology 59 (1996) 95-105 99

#~0 t7~ im ~ 5 2e2~ 3eO~ 3¢7~ 4t 5~(

Y - S t r o k e (ram)

Fig 10: Punch force of preform upsetting process

Punch m o v e m e n t

Fig 12: Material flow in forging of the connecting rod

Fig 11: Simulation model for the forging of the

connecting rod

Table1: Input data for the upsetting process

Simulation Parameter

The material flow of the connecting rod forging

process is shown in Fig 12 Fig 13 shows the contact

condition between the large end portion of the b i l l e t

and the tooling at the end of the forging It can be

seen from this figure that a relatively large c a v i t y

remains at the upper surface of the bigger ring p a r t

in the large end section (marked as * in Fig 13) In

the connecting or I-beam section, the side wall was

initially formed from both ends, g r a d u a l l y

proceeded to the middle and combined together

finally So the deformation pattern of this section

m a y not be completely in plane strain and does not

have enough height at the center even at the end of

the stroke

+: contact

*: no contact Fig 13:Contact condition with the tools in the large end section

T Takeraasu et al / Journal o f Materials Processing Technology 59 (1996) 95-105

It was concluded from these results that to

optimize the initial geometry of the preform we t h e

following problems had to solved:

a) For the large end section, we need to optimize

the preform geometry to fill the c a v i t y

completely and uniformly

b) For the small end section, control the i n i t i a l

volume distribution of this part and transfer

the excessive volume to other features of t h e

product

c) For the connecting I-beam section, we need to

optimize the diameter of the preform in order

to deform the side wall nearly in plane strain

conditions

3.2 Optimization of the Preform Geometry

It seems very difficult to optimize the w h o l e

geometry of the preform at once, since the preform

shape is relatively complex and has a lot of s h a p e

parameters as shown in Fig 8 Hence, the workpiece

was divided into three sections: large end section,

small end section, and connecting section Each

section was optimized independently This

optimization procedure was adopted for t h e

following reasons:

i) Since the connecting section is deformed n e a r l y

in plane strain conditions, it is assumed t h a t

the deformation of the large end section and

that of small end section do not strongly

interfere with each other

ii) The number of shape parameters is reduced

and the optimization process becomes easy to

h a n d l e

iii) Simulation time is reduced by working w i t h

a smaller model

There are seven shape parameters in the large

end section (see Fig 8) In order to reduce the number

of parameters, the diameters dl and d3, the t o t a l

volume, and the total length were set to be constant

The diameters d2, the segment length sl, and t h e

length pl=sl+s2+s3 are selected as independent

parameters Then the other parameters, that are

segment length s2, s3, and s4, were chosen based on

these constraints (see Fig 8)

Three preform designs for the large end section

were selected for the FE simulation model from t h e

various combinations of parameters, and are shown

in Fig 14 They are named BT0, BT1, and BT2

respectively BT0 is the original preform and is

represented by the dotted line in those figures The

diameter d2 and the segment length sl of BT1 are

both larger than that of BT0 The length of pl of

BT2 is shorter than that of BT0

BT1 d2=20 s1=15

=41.5

,:,T2 d1=22 s1=14 s3=11

Fig 14 : Sketch of some preforms for BT1 and BT2

The top views of the material flow of the large end section for each preform are shown in Fig 15 Fully 100% of the stroke was achieved in each simulation The shaded area in the top views shows the contacting area between the surface of the b i l l e t and the tools

The outer wall of the bigger ring of BT1 and BT2

is deformed almost radially throughout the forging process and has sufficient height at the end of t h e stroke This is compared to BT0, which is sinked in

at the beginning of the forging process and remains a little concave even after deformation As for t h e side wall next to the connecting section, the cavity is filled completely in cases BT0 and BT1, while t h a t

of case BT2 is not filled at all Comparing m a t e r i a l flow in top views, as the diameter d2 is increased and the length p l is decreased, the die cavity of t h e outer wall of the bigger ring is filled more r a p i d l y and smoothly This is because the material is prevented from flowing to the bigger ring part a f t e r the material of the section s3 contacts the die

It is seen from these results that in order to deform this large end section successfully, t h e diameter d2, the segment length sl, and the length

p l have to be selected correctly

Since the small end section was completely formed before the end of the stroke in the FE simulation of preform-I, the volume distribution was

T, Takemasu et al / Journal o f Materials Processing Technology 59 (1996) 9.5-105 101

Fig 15: Material flow of the BT0 - top view (XY plane)

BT2

varied to optimize the preform geometry There are

also seven shape parameters in this section(see Fig

8) The diameters d3 and d5, the segment length s8,

and the length p2=s6+s7+s8 are fixed The i n i t i a l

volume of this section was controlled by changing

the diameter d4 and the segment length s6

Three preforms for the small end section were

modeled as shown in Fig 16 They are called TP1,

TP2, and TP3 respectively The geometry of t h e

small end section of the preform-I is represented by

the dotted line The shape parameters of these

preforms are compared in Table 2 The volume of TP1

is the smallest TP3 has the same volume as TP2, but

the diameter d4 of TP3 is a little larger than that of

TP2 Thus the volume distribution of TP3 is g a t h e r e d

to the top end relative to TP2

Table 2: Shape parameters of the small end section

The top view of the deformed billets at the end of

the stroke are shown in Fig 17 The small end

sections of TP2 and TP3 are completely deformed,

while in the case of TP1 a cavity remains at the side

wall From these results, it can be concluded that the

deformation pattern of this part is not sensitive to

(a) T P 1

(b) T P 2

(c) T P 3

Fig 16: Sketch of FE models of the small end section

102 T Takemasu et al / Journal o f Materials Processing Technology 59 ('1996) 95-105

the initial geometry of the preform, since this is

formed b y the upsetting process before the complete

forging process

3.2.3 Connecting I-Beam Section Optimization

There are three parameters in the connecting I-

beam section: diameter d3 and segment lengths s4

and s5, as shown in Fig 8 The area of a cross section

of the I-beam part calculated by I-DEAS was 42.637

m m z Hence, assuming that the material of this p a r t

is deformed under plane strain conditions the i n i t i a l

diameter d3 of the connecting I-beam section was set

to 7.37 ram

j U n d e r f i l l i n g

(a) TP1

(b) T P 2

(c) TP3

Fig 17: Final stage deformed billets of the small

end section in top view (XY plane)

z

Fig.18: Sketch of preform-II

3.3 FE Simulation with New Preform

Evaluating the results obtained from t h e

optimization method, we proposed a new w h o l e

preform design (preform-II), shown in Fig.18 A

second 3D FEM simulation was performed with t h i s

preform design, and was compared with the earlier results The dimensions of the new preform are shown in Table 3

The material flow of the connecting rod forging process with preform-II is shown in Fig 19 As one can see from this figure, the small end section is formed completely The connecting I-beam section flows nearly under plane strain conditions and t h e side wall has enough height after deformation The large end section is deformed almost completely, although a very slight cavity remains between t h e billet and the upper punch at the upper surface of the bigger ring

Fig 19: Material flow of the forging simulation with preform-II

7~ Takemasu et al / Journal o f Materials Processing Technology 59 (1996) 95-105 103

(b) Preform-II

Fig 20: Deformation condition of the outer wall of

the bigger ring at the final stage

Table.3: Shape parameters of preform-II, m m

14.50 6.82 20.18 18.50 13.50 7.19 7.11

9.00 20.00 7.37 1 5 7 5 7.00

s8

4.70

The deformation condition of the outer wall of

the bigger ring of preform-II at the final stage is

compared with that of preform-I in Fig 20 The area

pointed to by arrow 1 in preform-II has enough

height and contacts the upper punch, while t h a t

area in preform-I is sinked in and does not contact

the upper punch at all The areas pointed to by

arrows 2 and 3 in preform-I! are concave because

they reflect the geometrical pattern of the upper

punch, while such geometrical properties are not

observed in similar areas of preform-I The edge

areas pointed to b y arrows 4 and 5 in preform-II look

sharper than that of preform-I This is because t h e

die cavity of these areas of preform-II is f i l l e d

almost completely, while a small cavity can be

observed in the same areas of preform-I It was

concluded that fair results m a y be obtained with the

design for preform-II

Fig 21 shows the effective strain of t h e

connecting rod in the final stage The strain values

range between 0.26 and 2.29 at the end of the stroke The strain distribution of an i n t e r m e d i a t e simulation step for this model is comparable to Mezger's final results [2]

The load predicted for the forging of preform-II

is 30% higher than that of preform-I This is because the stroke for preform-II is longer than t h a t

of preform-I and a relatively large cavity is observed at the upper surface of the bigger ring p a r t

in the large end section of preform-I a f t e r deformation

.2~5E,01

| ::::

Fig 21: Effective strain of preform-II final stage, a)intermediate step and b) final step

4 M a n u f a c t u r i n g the P r e f o r m

In section 3, a new preform design was suggested and good simulation results were obtained But t h e r e

is still another important problem: that is, how to manufacture the preform Each manufacturing method has its own advantages and d i s a d v a n t a g e s concerning tooling cost, lead time, finished s h a p e , and product tolerances In this section, the design and dimensions of the new preform are compared with those of the old preform and the feasibility of producing the new preform by cross rolling is discussed Furthermore, the flashless forging of a connecting rod is also compared with the forging of a connecting rod with flash

104 1~ Takemasu et al / Journal o f Materials Processing Technology 59 (1996) 95-105

The principle of operation of cross rolling

machines is shown in Fig 22 [4] In this process, a

round billet is inserted transversely between two or

three rolls, which rotate in the same direction and

drive the billet The rolls, which hold replaceable

die segments with appropriate impressions, make

one revolution while the workpiece rotates several

times in the opposite direction Thus, the cross

rolling method can form axially symmetrical s h a f t s

with complex geometries in one operation

t.EAD

~'dGLE

KNIFE

GE

REDUCTI ON AT POS* TI O~ 2

~ RECTION OF METAL FLOW

ITION 3

Fig 22: Principle of operation of cross rolling

machine [4]

Fig 23 compares the dimensions and t h e reduction ratio of the n e w preform with those of t h e old preform The reduction ratio of preform-II is 63.15 % and that of preform-I 58.17 % These are both small enough for the allowable limitation of the reduction ratio for cross rolling The minimum diameters of preform-I and preform-II are much larger than the desired minimum diameter, and t h e total lengths of preform-I and preform-II are smaller than 400 mm So it seems that the cross rolling is applicable for making both i n i t i a l preforms

In the flashless forging process the volume of t h e preform must be exactly the same as the finished part and the mass distribution of the preform must

be exact to fill up the die cavity correctly Variations in the cross rolling process m a y affect the required dimensions of the initial preform for t h e flashless forging process In order to verify these points, further investigations of the 3D FEM simulation or physical modeling experiments are needed Especially for the forging process of t h e connecting rods with flash, because this process is not as strict as the flashless forging process in terms

of the volume distribution of the initial preform

5 C o n c l u s i o n s a n d Future W o r k

The cross rolling machines are suitable for

automatic production, using bar stock a u t o m a t i c a l l y

fed to the rolls through an induction heating unit

Therefore, cross rolling takes advantage of h i g h

productivity (about 900 parts per hour) The design

variables of this process are the lead angle of t h e

wedge, the flank angle, and the amount of reduction

The disadvantages of cross rolling are that self-

contained machines are expensive and the forming

rolls are difficult to design This process is also

limited to external surfaces The most important

problem in using cross rolling for this purpose is t h e

desired shape limitations in the preform design

At present, cross rolling machines will accept

only bars having a maximum diameter of 35 mm The

minimum diameter of the product is about 12.5 mm

The maximum reduction ratio is 75%, and rolled

length of product can be up to 400 mm, depending

upon the reduction required

The reduction ratio is defined as :

R e d u c t i o n R a t i o - Dmax - Dmin x 100 (1)

O m a x

w i t h : Dma x = maximum diameter of the preform

Dmi n = minimum diameter of the preform

The preform design was optimized by dividing the preform into three parts: small end section, connecting I-beam section, and large end section Simulations were performed independently for e a c h section

For the large end section, both the i n i t i a l geometry and the volume distribution of the preform was optimized at the same time by changing t h e position and the diameter of the hill section under a condition of constant volume It is concluded from the simulation results that to optimize the geometry

of this part is more difficult than the small end section or the connecting I-beam section due to t h e fact that the product shape of this part is v e r y complex and the material flow is strongly influenced

b y the initial geometry of the preform

For the small end section, the initial volume distribution was mainly controlled to fill the die cavity at the end of the punch stroke It is also clear that the deformation pattern of this part is not as sensitive to the initial geometry of the preform as the large end section

The deformation pattern of the connecting I-beam section approached plane strain conditions b y optimizing the initial diameter of this part The ribs of the I-beam were almost filled at the end of the simulation