1-s2.0-S0924013699001995-main

Precision forging of aluminum and steel

Hyoji Yoshimura*, Katsuhisa Tanaka

Nichidai Corporation, Takigi, Kyotanabe, Kyoto 610-0341, Japan

Abstract

Outline of an enclosed die forging equipment is introduced ®rst. Then some net shape forming examples of steel and aluminum alloys are

enumerated. The possibility of enclosed die forging is discussed ®nally. # 2000 Published by Elsevier Science S.A. All rights reserved.

Keywords: Enclosed die forging; Net shape forging; Air compressor scrolls

1. Introduction

Development of the technology of plasticity, including

those of cold forging, has made the Japanese automotive

industry a most competitive one. The annual production of

automobiles increased drastically in 1960s in Japan. Mass

production of cold forging steel products were realized by

Bolt Makers and Maypres accordingly from then. At the

beginning, reduction of costs for relatively simple products

like ball studs was achieved to large extent by replacing

conventional machining process with cold forging. Cold

forging of larger sized products like cup-shaped products,

spline shafts, rear axle shafts, etc. were realized afterwards.

In 1970s, because of the energy crisis, researches to

improve the yields of material usage, to reduce forging

energy, to eliminate machining processes with high preci-

sion forging were carried out actively. Furthermore, as

problems such as environment pollution caused by forging

and aging of skilled workers became more severe, needs for

lower noise, less vibration and more automated forging

machines became stronger.

Since 1971, the author's group has begun various

researches about the optimum process from the billet to

the ®nal product [1]. One of technologies which has being

focused on is `̀ enclosed die forging''. Fundamental

researches like process analysis of enclosed die forging with

plasticine were carried out. A 450 ton enclosed die forging

multi-ram hydraulic press was developed in 1974. Actual

forging tests and development of mass production technol-

ogies were carried out with the same press [2].

The eventual target of enclosed die forging technology is

to improve yields of material usage with the optimal process

without ¯ashes, to lower the forming load by performing

necessary deformation from necessary directions to save

energy, and to realize automation of forging to reduce the

demand for skilled workers. Recently, enclosed die forging

has become a key technology for precision forging of

products such as constant velocity joints(CVJ), bevel gears,

etc.

In this paper, outline of an enclosed die forging equipment

and some net shape forming examples of steel and aluminum

alloys are introduced.

2. Enclosed die forging

Enclosed die forging uses multi-ram as punches to press

the material in a pre-enclosed die to ®ll in the die space

(Fig. 1). By controlling the motion of rams, metal ¯ow can

be controlled to obtain the optimum deformation. The ram

motions for upper and lower punches can be set as synchro-

nous, asynchronous (Fig. 2) or with back pressure (Fig. 3) to

reduce forming load to large extent or to improve the ®lling

of material.

3. A computer-controlled three cylinder hydraulic pressfor enclosed die forging

Although the optimum forming condition can be obtained

through fundamental experiment with plasticine, actual test

with the same material is required at last. For this purpose, a

computer-controlled three cylinder hydraulic press (Fig. 4)

Journal of Materials Processing Technology 98 (2000) 196±204

* Corresponding author.

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

PII: S 0 9 2 4 - 0 1 3 6 ( 9 9 ) 0 0 1 9 9 - 5

for enclosed die forging was developed by Nichidai with

help of a press maker [3]. The press is capable of doing

actual test. Position, speed and pressure of the upper and

lower punches of the press can be set individually. The data

can also be saved into memory and the motion simulation

can be done with the same data.

3.1. Structure

The press consists of a main slide, an inner slide and a bed

slide, each of which is controlled by an independent hydrau-

lic cylinder (Fig. 5). As a mother machine of enclosed die

forging, through which the optimum forming condition of

die and punch can be found, (easy data input and modi®ca-

tion). The following items can be input through an inter-

active data input system:

1. selection of axis,

2. start position,

3. finish position,

4. approach speed at various positions before having

contact with material,

5. pressure and speed at various positions,

6. start timing of inner ram,

7. start timing of bed ram,

8. cycle time.

3.2. Specifications

Main

slide

Inner

slide

Bed

slide

Capacity (kN) 80 40 40

Stroke (mm) 500 200 200

Daylight between bolster

and slide (mm)

900 ± ±

Slide (mm) 900 � 900 f 300 f 300

Bed (mm) 900 � 900 ± ±

Rapid approaching speed �300 �20 �20

Working speed (mm/s) �20 �20 �20

Return speed (mm/s) �150 �150 �150

Reaction capacity (kN) 6 4 4

4. Development of enclosed die forging die-sets

4.1. Special die-sets for enclosed forging

After it became known that enclosed die forging is

an effective technology, special presses for enclosed die

forging have been used in many companies to perform

mass production forging. However due to the low produc-

tivity of hydraulic presses, needs for using high speed

mechanical presses in enclosed die forging became stronger

and stronger.

In Japan, Aida and Komatsu developed simpli®ed

enclosed forging die-sets for using in mechanical presses.

The die-set by Aida uses a cam mechanism and that by

Komatsu uses a rack and pinion mechanism. These devel-

opment accelerated the application of enclosed die forging.

In 1988, Nichidai also developed a new type die-set for

enclosed die forging which uses a pantograph mechanism

(Fig. 6) and expand the application of enclosed die forging

technology to net shape forging of bevel gears, etc. In

Europe, forging production using such die-sets for enclosed

die forging is increasing rapidly.

Fig. 1. Enclosed die forging: (a) before forming; (b) after forming.

Fig. 2. Synchronous and asynchronous motion: (a) synchronous forming;

(b) asynchronous forming.

Fig. 3. Enclosed die forging with back pressure.

H. Yoshimura, K. Tanaka / Journal of Materials Processing Technology 98 (2000) 196±204 197

4.2. Die closing pressure of enclosed forging die-sets

Die closing pressure is generated through a hydraulic unit,

an accumulator and cylinders embedded in the die-set in the

same cycle time of mechanical press. Burrs or chipping may

occur in the parting line of upper and lower die surface if the

pressure is too small.

4.3. Synchronous and asynchronous motion of rams

When a billet in die cavity is pressed with the upper and

lower rams, deformation of the material may vary according

to the speed ratio of upper and lower rams as shown in Figs.

2 and 7.

Upper±lower symmetric products like cross journals are

formed in synchronous motion. Upper±lower asymmetric

products like bevel gears are formed in asynchronous

motion.

Fig. 8 shows how synchronous and asynchronous motions

are realized by a die-set with a pantograph mechanism.

Fig. 9 shows an example of press motion for bevel gears.

5. Examples of steel parts by enclosed die forging

5.1. Parts by orthogonal extrusion

5.1.1. Cross journals

Cross journals (Fig. 10) are the representative products of

the ®rst generation enclosed die forging products. Since theFig. 5. Structure of the hydraulic press.

Fig. 4. Three cylinder hydraulic press.

Fig. 6. Nichidai special enclosed forging die-set.

198 H. Yoshimura, K. Tanaka / Journal of Materials Processing Technology 98 (2000) 196±204

forming methods of cross journals of propeller shafts were

changed from conventional hot forging processes to

enclosed die forging ones from 1975, great saving of

material, automation of forging and release of after-machin-

ing processes have been realized.

5.1.2. CVJ tripods

Mass production of tripods with enclosed die forging

technology incubated from cross journals was realized

instantly when Japanese automotive makers began to study

the production of constant velocity joints for FF model

cars. When enclosed die forging of cross journals was

developed, experiment with plasticine was used. But appli-

cation of enclosed die forging technology to tripods was

studied by carrying out computer simulation of the forming

process due to the recent development of CAE techniques

(Fig. 11).

5.2. Parts by bulging forming

5.2.1. Inner races of CVJ

Like tripods, inner races are another kind of CVJ products

(Fig. 12) which are produced to near net shape in large

quantity by use of enclosed die forging method.

5.2.2. Bevel gears

Bevel gears (Fig. 13) used to be made by the combined

process of hot forging and cold coining from long time ago.

They were used in farming machines and motorcycles. Mass

production of bevel gears with enclosed die forging method

began from 1980 and the quantity of those used in trucks and

cars has increased rapidly since a few years before. This is in

part because of the advance of forming technologies as well

as the development of die-sets for enclosed die forging.

However, the largest reason may be due to the dies with high

precision and long life were able to be manufactured to

produce bevel gears with precision required by automotive

manufacturers satis®ed in large quantity.

Fig. 14 shows an example of the structure of die for

bevel gear production. The optimum speed ratio of upper

and lower punches was obtained after a number of test

(Table 1). In this case, the optimum speed ratio turned

out to be 1.5 : 1.

5.3. Parts by forming with aid of back pressure

By providing optimum auxiliary pressure on the metal

¯ow direction or the contrary direction, products which were

considered impossible for forging become forgeable. The

author has been working with car makers jointly on such

forming methods. The following are few examples.

Fig. 7. Speed ratio of upper and lower rams.

Fig. 8. Die-set with a pantograph mechanism. Fig. 9. An example of press motion.


1. Connecting rod cap.

Fig. 15 shows a forging process of connecting rod cap

used by automobiles. Trials were carried out for the one

stage forming process from round bar billet to the ®nal

product. Although mass production is not being done

yet, it is possible to carry out mass production on

mechanical presses by making improvement on the

structure of die-set. Net shape forming of parts with

even more complicated shape is possible by performing

improvements on press motion and die structures.

2. Tulip shafts of CVJ.

Enclosed die forging of tulip shafts (Fig. 16) of CVJ

was also challenged [4]. While bar billet is extruded into

triple branches in semi-hot forging, back pressure is

added to obtain larger-than-die-entrance section of the

part. The process was not adopted for mass production

due to die life problem using special enclosed die

forging hydraulic presses. However, mass production is

possible by using the simplified die-set of enclosed die

forging mentioned before.

6. Features of enclosed die forging

As shown above, by controlling the motion of upper

and lower punches, the optimum condition of enclosed

die forging can be obtained. Mass production of net or

near-net shape products with high precision in low costs

can be achieved by enclosed die forging methods. Char-

acteristics of enclosed die forging can be summarized as

follows:

Fig. 10. Cross journals.

Fig. 11. Metal flow analysis by FEM.

Fig. 12. Inner races by enclosed die forging.

Fig. 13. Bevel gears by enclosed die forging.

Table 1

Effect of speed ratio on enclosed die forging of bevel gears

Speed

ratio

Load Metal

flow

Die

fill

Fractures General

evaluation

3 : 1 oa �b �c � �2 : 1 � � � � �

1.5 : 1 o o � � o

1 : 1 o � � � �1 : 2 � � o � �0 : 1 � � o � �

aGood.bFair.cBad.


1. Flashless forming.

� Improving yield of material usage, therefore reduce

material cost.

� Reducing forming load, therefore downsize forging

machines and save energy.

� Making die fill easy, therefore increase die life.

2. Orthogonal extrusion and bulging forming.

� Complex shapes: cross journals, bevel gears.

� High precision forming: little displacement between

upper and lower die surfaces.

� Metal flow control: increasing strength of products.

3. Punch motion selection.

� Synchronous, asynchronous move to make left to right

and upper to lower deformation.

� Back pressure forming for complicated shape, difficult

to be formed material.

� Extrusion with stepwise tension-added main ram.

Up to now interests on enclosed die forging technologies

have being concentrated on upper and lower punch motions.

However more study on other factors is required for those

products with more complicated shape.

7. Technology of aluminum forging

7.1. General aluminum forging examples

Aluminum alloys have being used in automobiles and

airplanes for a long time due to their special features. Fig. 17

shows a number of aluminum forging products the author

has involved in their development until now.

1. Connection rods for general purpose enginesConnection

rods for small general purpose engines have been made

by aluminum forging products. To increase the yield of

Fig. 14. Structure of dies for enclosed forging of bevel gears.

Fig. 15. Connecting rod cap.

Fig. 16. Forming process of a tulip shaft.

Fig. 17. Various aluminum forging products.


material, blanks are made using forging rolls at ®rst.

After checking facial cracks, the blanks are heated again

and forged with dies. The trimming of inner and outer

shape is usually done in room temperature.

2. General aluminum forging products used in airplane-

s.Aluminum forging products for using in airplanes

require high quality of aluminum materials and

specified forging metal flow line. Careful study of

forging processes is necessary. Requirements for heat

treatment conditions and inspection method of facial

cracks are also strict. Aluminum forging requires the

most advanced technologies in the world. It is important

to invest in high level equipment for the small quantity

production from long term view.

3. No-draft aluminum forging products used in airplane-

s.Among those aluminum products used in airplanes,

no-draft aluminum forging products where special high

level technology is needed having produced in Japan

from some 20 years before.

4. Cold forging aluminum products used in air conditio-

ners.Receiver parts (Fig. 18) of air conditioners were

tested and put into mass production 25 years ago. The

receiver body with a deep cup was made by simple

impact extrusion. Wrinkle cracks occurred at the lower

side of outer surface of the product due to rough crystal

grain size near the surface of material from the same

charge.

On the other hand, receiver heads were cold forged

together with the screw section which was ®nish machined

later. The production costs were reduced signi®cantly with

this design change. The same idea is still used in a number of

companies now.

7.2. Reduction of cold forging costs by using irregular

billets

In aluminum forging, total cost can be reduced by using

irregular billets obtained from irregular bar materials

by sawing. It is effective specially for products with

complicated shape requiring multi-stage forming processes

(Fig. 19).

7.3. Forming of locker arms by combination of casting and

forging

Hot forging products are used for locker arms (Fig. 20)

used in engines of passenger cars. There were reports that

hot forging was applied to gravity casting parts (i.e., com-

bination of casting and forging) to reduce costs. Nichidai

was involved in die manufacturing and forming method

development. It is worth to keep an eye on the development

of the same technology.

7.4. Trials of bicycle parts with split dies

When producing complicated aluminum forging pro-

ducts, split die structure is sometimes used. Fig. 21 shows

an example. Necking section in the center is forged with split

dies. Both ends are formed by enclosed die forging simul-

taneously from upper and lower direction or can be formed

one by one separately.

7.5. Enclosed die forging of scrolls with back pressure

Scrolls used in scroller-type air compressors of air con-

ditioners are being changed from casting aluminum alloy

Fig. 18. Receiver body and head: (a) receiver body; (b) receiver head.

Fig. 19. Cold forging aluminum products using irregular billets.

Fig. 20. Locker rams by forging.


products to enclosed die forging ones which is cheaper in

terms of total costs. Nichidai has succeeded in developing

and mass production of enclosed die forging of scrolls

making use of back pressure forming technology. As shown

in Fig. 22, spiral wraps of scrolls are formed from round

plate billets in one stage where back pressure is applied to

the extruded surface of the spiral wraps. Productivity is so

high that cycle time to produce one part is only about 6 s.

The so forged scrolls have no draft angle and the extruded

wrap surface is uniform and ¯at where only small machining

allowance is remained. Recently, even from the view point

of adoption of new type of freon to protect the global

environment, enclosed die forging products with excellent

fatigue strength and fatigue crack propagation characteris-

tics are becoming more and more attractive.

7.5.1. Process of enclosed die forging of scrolls

Al±Si bar raw material ! cutting ! pre-heating !lubrication ! heating ! enclosed die forging ! heat

treatment ! shot blasting.

7.5.2. Mechanism of back pressure functioning

The structure of dies for scroll forging developed by

Nichidai is shown in Fig. 23. Back pressure which is about

40±80% of deformation resistance of spiral wrap is added

during the forming process to prevent under ®ll of the front

end, as well as to prevent wrinkles in the outer surface of

spiral wraps. As shown in Fig. 24, thickness of the product

after forming is thicker than that of the blank.

7.5.3. Effects of back pressure

Adding back pressure to the material during enclosed die

forging process of scrolls helps achieve proper metal ¯ow

without causing improper ¯ow line in the root section of

spiral wraps. Forming load does not increase due to the back

pressure, in contrary, forming load decreases and the die life

is improved to as long as about 50 000±200 000.

Fig. 25 shows the difference with and without back

pressure during the forming process.

7.5.4. Characteristics of enclosed die forging scrolls

The characteristics of enclosed forging scrolls can be

summarized as follows:

� Uniform and flat wrap surface Ð results in reduction of

machining and material costs.

Fig. 21. Aluminum bicycle part.

Fig. 22. Compressor scrolls.

Fig. 23. Structure of dies for scroll forming.

Fig. 24. Thickness of blank before and after forming.


� No draft angle Ð results in reduction of machining and

material costs.

� No inner cavity Ð results in improvement of productivity.

� Increase offatigue strength Ð applicable to new type freon.

Application of the forming method will be surely broa-

dened due to its advantages.

8. Summary

The state-of-the-art of the enclosed die forging methods is

reviewed from both steel and aluminum forging products.

Net shape forging with enclosed die forging technology

which has come into practical use recently has large poten-

tiality. It has been applied in cold and warm forging

fields. Application to semi-hot forging with simplified

enclosed forging die-sets is also increasing. The products

by enclosed die forging are becoming larger. Application to

general industries other than automotive ones is increasing

as well.

Researches on die structure, forming condition, die

manufacturing technology, etc. will no doubt be continued

from now on. Product design and die design engineers

should work together to challenge products with much

more merits making use of enclosed die forging technolo-

gies.

References

[1] Y. Iwasaki, H. Yoshimura, MHI Technical Report 12-5, 1975.

[2] H. Yoshimura, S. Shimazaki, J. JSTP 24 (1983).

[3] M. Nakamura, T. Koga, Kurimoto Technical Report, 1994.

[4] A. Ishii, H. Koshimaru, J. JSTP 22 (1981) 241.

Fig. 25. Effects of back pressure: (a) without back pressure; (b) with back

pressure.


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