Cold and Hot Forging: Fundamentals and Applications / Edited by Taylan Altan, Gracious Ngaile, Gangshu Shen

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CHAPTER 23

Near-Net Shape

Forging and New Developments

Manas Shirgaokar

Gracious Ngaile

Fig. 23.1

Examples of net shape forged parts [Meidert et al.,

2000]

23.1 Introduction

Net and near-net shape forging companies

generally produce for the automotive industry.

These components are characterized by close di-

mensional tolerances, minimal draft, and the ab-

sence of ﬂash during forging, with minimal or

no postforging machining. This is a major ad-

vantage when machining costs are taken into

consideration. Net shape forging can be deﬁned

as the process of forging components to ﬁnal

dimensions with no postforging machining nec-

essary. Near-net shape forged components, on

the other hand, are forged as close as possible to

the ﬁnal dimensions of the desired part, with lit-

tle machining or only grinding after forging and

heat treatment.

The possibility of eliminating the high costs

of machining, combined with the inherent ad-

vantages of cold forging regarding material

strength and ﬁnish, make near-net forging very

attractive to the automotive industry.

Examples of typical near-net shape parts are

given in Fig. 23.1. These parts are characterized

by complex geometries and very close toler-

ances, in the range of 0.0004 to 0.002 in. (0.01

to 0.05 mm), at complicated and functionally

important surfaces. The steering spider seen in

the forefront in Fig. 23.1 is a typical net shape

part. The other forgings require minor machin-

ing or grinding. However, functional areas, such

as the gear tooth geometry of the bevel gear, are

ready for assembly [Meidert et al., 2000].

Some of the parameters affecting the quality

of near-net shape forged components are illus-

trated in Fig. 23.2.

23.2 Tolerances in Precision Forging

To reduce cost, it is often desirable to replace

conventional forging that requires several sub-

sequent machining and ﬁnishing operations with

precision forging, i.e., cold and warm forging,

which yields parts with good dimensional tol-

erances.

The current minimum dimensional error in

practical cold forging is Ⳳ20 to 50 lm, while

the error in machining has been reduced over the

years to less than Ⳳ1 lm. For cold forging pro-

cesses to compete with machining, the dimen-

Cold and Hot Forging Fundamentals and Applications

Taylan Altan, Gracious Ngaile, Gangshu Shen, editors, p319-335

DOI:10.1361/chff2005p319

www.asminternational.org

320 / Cold and Hot Forging: Fundamentals and Applications

Fig. 23.2 Factors inﬂuencing the quality of near-net shape forged components. FE, ﬁnite element [Meidert et al., 2000]

sional accuracy would have to be increased to

within Ⳳ10 lm. Some of the causes of dimen-

sional variation in precision forging are [Osak-

ada, 1999]:

●

Die manufacturing: The dimensional accu-

racy of the dies directly inﬂuences that of the

parts being produced with them. Hence, die

manufacturing is a very crucial part of the

manufacturing sequence of precision parts.

The use of electrical discharge machining

(EDM) and wire EDM machines in manu-

facturing cold forging dies has considerably

improved the accuracy of forging dies.

●

Elastic deﬂection of the press and tools:

When the forming load is applied, the press

and the tools undergo elastic deﬂection, thus

affecting the ﬁnal tolerances on the part be-

ing forged.

●

Variation of process conditions: In practical

situations, process variables such as forming

pressure, lubrication conditions, billet di-

mensions, and material properties do not re-

main constant. These ﬂuctuations affect the

dimensional accuracy of the forming pro-

cess. The billet dimensions and material

properties (ﬂow stress) can be controlled by

involving the suppliers in the design process

to ensure procurement of billets with consis-

tent dimensions and ﬂow stress. The varia-

tion in lubrication conditions can be reduced

by using lubricants with consistent friction

coefﬁcients.

23.2.1 Precision Die Manufacturing

Electrical Discharge Machining. In preci-

sion forging, cemented carbide is the die mate-

rial of choice, rather than high-speed steel, be-

cause of its high stiffness, low thermal effect,

and high wear resistance. A ﬂow chart for manu-

facturing precision dies is shown in Fig. 23.3

Near-Net Shape Forging and New Developments / 321

Fig. 23.3

Flow chart of die manufacturing by electrical discharge machining (EDM). CAM, computer-aided manufacturing [Yosh-

imura et al., 1997]

[Yoshimura et al., 1997]. The die insert impres-

sion is ﬁrst made by rough grinding, followed

by electrical discharge die sinking. The com-

pression ring is produced by machining and

milling, followed by hardening or quenching to

increase its stiffness. The die insert is then

shrink-ﬁtted into the compression ring, and the

assembled ring is machined for ﬁnishing. The

die insert impression is made by die sinking us-

ing a master electrode, followed by lapping to

obtain a mirrored ﬁnish.

Surface coating is done to increase surface

hardness on the working portion of the tools, so

as to reduce wear and damage in that area. Crack

initiation is delayed due to the hardness of the

coating, which prevents any lubricant or work-

piece material from getting wedged in these

cracks. Two commonly used methods of surface

coating are [Yoshimura et al., 1997]:

●

Chemical vapor deposition: TiC, TiCN, TiN,

●

Physical vapor deposition: TiN

23.2.2 Die Deﬂection in

Net Shape Forging

The stages of process planning, tool design,

and manufacture in net shape forming require

not only the use of a highly qualiﬁed engineer

and sophisticated computer-aided design/com-

puter-aided manufacturing (CAD/CAM) pack-

ages but also strict control on the tool and press

settings. Process design in precision forging usu-

ally requires extensive trials before the optimum

settings can be identiﬁed to yield the desired part

tolerances. Thus, it is possible to compensate for

the errors in the dimensions by evaluating the

various factors affecting the part tolerances.

Some of the phenomena affecting die deﬂection/

deformation are listed below in the order that

reﬂects the increasing dependence on production

run-time [Kocanda et al., 1996]:

●

Elastic deﬂection is a reversible change in

dimension brought about by the applied

load. It can be controlled by appropriate cal-

culations and design.

322 / Cold and Hot Forging: Fundamentals and Applications

Fig. 23.4 Various stages and inﬂuencing factors of the forging process [Ishinaga, 1996]

●

Plastic deformation is a permanent defor-

mation as a result of an unintended increase

in forming load or stress concentration. This

could be caused by improper tool alignment,

inadequate lubrication, change in initial

properties of the workpiece, etc.

●

Cyclic softening or hardening causes signiﬁ-

cant changes in the stress-strain response of

the tool steel. This phenomenon should not

be ignored when the die geometry has stress

concentration features.

●

Thermal expansion of the tooling is a result

of the heat generation during deformation.

●

Relaxation and creep phenomena consider-

ably decrease the prestresses in the die insert.

●

Change in the modulus of elasticity may be

caused by change in temperature of the tool

steel. Generally, the modulus of elasticity de-

creases with increasing temperature.

●

Cyclic plastic deformation, which occurs

near areas of stress concentration, may result

in initiation of microcracks.

●

Softening of tool steel by unwanted temper-

ing. Tempering is a function of both tem-

perature and time and can occur during cold

deformation processing at temperatures

lower than the tempering temperature chosen

for heat treatment of the tool.

23.2.3 Press Deﬂection in

Net Shape Forging

Major factors that inﬂuence the part toler-

ances are the tool and machine deﬂections and

the various process parameters, such as material

properties, lubrication, temperatures, etc. While

the process parameters and tool design have

been the subject of extensive research, the inﬂu-

ence of press deﬂection on the accuracy of the

formed part and methodologies to compensate

for these errors have yet to be standardized. Fi-

nite-element (FE) simulation has been employed

to calculate the stress and deﬂection behavior of

press components such as guide layout, press

frame, and the construction of the crosshead

[Doege et al., 1990]. However, the nonlinear

characteristics of the load-deﬂection behavior

between adjacent components, the clearances in

the guiding system, and asymmetry in press as-

sembly make it difﬁcult to analyze the elastic

behavior of the press. The horizontal offset and

tilting, which occur during multistage forging or

forging of complex parts, have a detrimental ef-

fect on the part tolerances. In order to quantify

the elastic characteristics of the press, it is nec-

essary to measure the load-deﬂection relation-

ship of the ram under center and off-center load-

ing conditions together with superimposed

horizontal forces.

In one particular study [Balendra et al., 1996],

a three-dimensional (3-D) FE simulation was

conducted to determine the elastic characteris-

tics of a 400 metric ton screw press. The simu-

lation considered the modeling of the press com-

ponents, appropriate deﬁnition of the joints, and

the prestressing requirements for assembling the

frame. The forming loads were obtained by con-

ducting a two-dimensional closed-die, plane-

strain, aerofoil forging simulation. This simula-

tion enabled the determination of the horizontal

offset and tilting of the forging dies for various

orientations of the die parting line, and methods

to minimize the component form errors due to

elastic behavior of the press and location and the

elastic behavior of the forging dies.

Near-Net Shape Forging and New Developments / 323

23.2.4 Process Variables in Forging

Figure 23.4 shows the various stages in forg-

ing, with the parameters affecting part attributes

and tolerances [Ishinaga, 1996]. The variable

factors (enclosed in square brackets) affect each

other and cause variations in the dimensions of

the forged part. The entire forging process is di-

vided into three stages:

●

The preprocessing stage addresses the vari-

ables related to the billet. One way to control

the part quality is to ensure minimum devi-

ation in each of the billet factors, such as

dimensions, ﬂow stress, formability, etc.

●

The forging process, which encompasses all

the variables related to the forming process

(temperature, lubrication, etc.), tooling, and

equipment. Variations in the forging pressure

would affect the thickness of the forging due

to elastic deﬂection of the press and tools.

Off-center loading not only affects the ac-

curacy of the process but is also detrimental

to tool life.

●

The postprocessing stage deals with machin-

ing, surface ﬁnishing, or heat treatment of

the forged parts and affects the material

properties, surface conditions, or chemical

composition of the part.

The growing demands for precision cold-

forged products in the automotive industry led

to the development of economical means to

manufacture these parts. Apart from improve-

ments in forging processes, there has been con-

siderable progress in tooling and equipment de-

sign, materials engineering, and lubrication

techniques in order to produce precise compo-

nents. Some of these developments and future

trends are discussed in the subsequent sections.

23.3 Advances in Tool Design

In order to produce precision-forged parts,

factors related to the tooling, namely, elastic de-

ﬂection, eccentricity, thermal expansion, etc.,

have to be considered during the process design

stage. This is especially true in the case of cold

forging, where precision die manufacturing is

necessary to ensure the production of parts to

close tolerances. The following factors have to

be considered in die manufacturing [Yoshimura

et al., 1997]:

●

Proper equipment and process to ensure high

accuracy

●

Proper design of die structure to achieve high

stiffness

●

Proper heat and surface treatments to elimi-

nate the heterogeneity of the die material

●

Proper control of heating conditions to main-

tain constant temperature at the die

Tool life is also a major factor to be consid-

ered while quoting a part. Since the cost of

manufacturing a die is considerable, simultane-

ous process design involving process planning

and die design are essential to ensure that con-

ditions such as loading pressure, stress concen-

tration, etc. remain within the allowable limits

of die performance. Thus, tool wear, tool frac-

ture, and process failure have to be considered

for cost reduction.

23.3.1 Enclosed- or Trapped-Die Forging

Enclosed-die forging uses multiple-action

tooling as punches press the material in a pre-

enclosed die to ﬁll in the die space (Fig. 23.5).

By controlling the motion of rams, metal ﬂow

can be controlled to obtain the optimum defor-

mation. The ram motions for upper and lower

punches can be set as synchronous, asynchro-

nous, or with back pressure to reduce forming

load or to improve the ﬁlling of material. Some

of the advantages of enclosed-die forging are:

●

The ability to form complex shapes in one

process

●

Elimination of ﬂash, thus resulting in mate-

rial savings. Since parts are formed to close

dimensional tolerances, subsequent machin-

ing operations are either reduced or, in some

cases, eliminated.

●

Low forming load is available where the area

penetrated by the punch is relatively small.

A Japanese tool supplier has recently devel-

oped a special die set for forging radially ex-

truded parts and a family of bevel gears (Fig.

23.6 and 23.7) [Yamanaka et al., 2002]. The ad-

vantages obtained from this die set were high

productivity, easy installation in nearly all

presses, the requirement of only compressed air,

and low initial cost.

A similar concept in die design, developed

earlier, incorporates a pantograph, as shown in

the schematic in Fig. 23.8 [Yoshimura et al.,

1997 and 2000]. Hydraulic pressure generated

by an external hydraulic unit and an accumulator

is used for closing the upper and lower die to

form the die cavity. The die-closing pressure is

adjustable. However, if this pressure is too low,

324 / Cold and Hot Forging: Fundamentals and Applications

Fig. 23.5

Enclosed-die forging. 1, billet; 2, upper die; 3, lower die; 4, upper punch; 5, lower punch; 1⬘, ﬁnished part [Oudot et al.,

2001]

Fig. 23.6 Enclosed forging die set developed by Yamanaka Engineering [Yamanaka et al., 2002]

Fig. 23.7 Some example parts forged in an enclosed forging die set [Yamanaka et al., 2002]

Near-Net Shape Forging and New Developments / 325

Fig. 23.8 Die set for enclosed-die forging developed by Nichidai Corp. [Yoshimura et al., 1997]

it could result in ﬂash formation in the die gap

and cause chipping in the die. The die cavity,

composed of the upper and lower dies, moves at

half the punch speed by the pantograph mecha-

nism in order to obtain uniform deformation of

the material in the axial direction [Yoshimura et

al., 2000]. Similar die designs have been devel-

oped at Institut fu¨r Unformtechnik (Institute for

Metal Forming Technology) at the University of

Stuttgart as well as by European die and press

makers [Siegert et al., 2003].

23.3.2 Innovations in Compressive

Prestressing of Die Inserts

An important factor in enhancing die life is

the design of compressive prestressing container

systems, which house the die inserts. The con-

ventional prestressing container normally con-

sists of single or double stress rings. Depending

on the complexity of the part and the required

tolerances, the compressive prestress generated

may be too low. STRECON Technology has de-

veloped strip-wound radially prestressed con-

tainers with strength that is 2 to 3 times that of

conventional stress rings [Groenbaek et al.,

1997]. The high strength makes it possible to

provide an optimum prestress for the die, lead-

ing to two- to tenfold improvement in the die

life (Fig. 23.9a). Sometimes, in cold forging

dies, fracture may occur because the conven-

tional radial prestressing does not have any ap-

preciable effect on the stress condition in the

axial direction. To counteract this effect,

STRECON has developed strip-wound contain-

ers with integrated axial prestressing (Fig.

23.9b) [Groenbaek et al., 1997].

The strip-wound containers are manufactured

by winding a thin strip of high-strength steel

around a core of tool steel or tungsten carbide.

During the winding process, the steel strip is pre-

loaded with a controlled winding tension. The

core material has a structure and a hardness that

can withstand high prestress and cyclic working

load. The strip steel is developed especially for

optimum combination of the physical and me-

chanical properties. Optimum stress distribution

is obtained by varying the winding tensions

from layer to layer. The prestressed condition in

the coiled strip is equal to that of a conventional

construction with ‘several hundred’ stress rings.

Consequently, the strip-wound containers can be

loaded with a higher internal pressure than a

conventional multiple stress ring set before the

material will deform plastically. Thus, it is pos-

sible to obtain a higher interference when ﬁtting

a die into a strip-wound container than into a

conventional multiple stress ring set.

326 / Cold and Hot Forging: Fundamentals and Applications

Fig. 23.9 Prestressed strip-wound containers developed by STRECON Technology [Groenbaek et al., 1997 and 2000]

23.3.3 Reduction of Forging

Pressure by Divided Flow Method

To reduce forging load and tool stresses,

Kondo et al. have developed the divided ﬂow

method and applied this concept in forging a va-

riety of parts, such as the gear parts shown in

Fig. 23.10. There are two principles of ﬂow re-

lief shown in Fig. 23.10(a) and (b), namely, ﬂow

relief hole and ﬂow relief axis. In the former, a

blank with a relief hole is compressed by ﬂat

tools, resulting in a centripetal ﬂow as an out-

come of the hole shrinkage, thus creating di-

vided ﬂow. In the relief axis principle, a reduc-

tion in working pressure occurs due to two

reasons: extrusion of the axis serves as ﬂow re-

lief and suppresses the increase in fractional re-

duction in area, and the formation of a friction

hill is prevented due to the divided ﬂow. Re-

search shows that the relief hole principle is

more suitable for working pressure reduction,

since the resistance to ﬂow increases gradually

during forming.

Manufacture of a helical gear utilizing ﬂow

relief axis with back-up pressure is shown in Fig.

23.11 [Kondo, 1999 and 2002]. The forming

process was completed in one step, because the

inner plate moved down with the die plate until

the preset backup load. The ﬁnal part with the

boss is also shown in Fig. 23.11.

23.4 Advances in Forging Machines

Net shape forging of complex parts, such as

helical gears, helical-tooth pinions, etc., requires

new concepts in press and tooling design, with

consideration of a multitude of interacting vari-

ables such as those shown in Fig. 23.12. In order

to increase the accuracy of the product and to

extend the service life of the tools, press builders

have developed multislide and multiaction hy-

draulic presses [Nakano, 1997, and Ishinaga,

1996].

23.4.1 Multislide Forging Press

Aida Engineering Co. has developed a mul-

tislide forging press for the purpose of minimiz-

ing the effect of off-center loading during mul-

tiprocess transfer forming. The features of this

press are [Nakano, 1997]:

●

Multislide construction: Each slide has its

own independent slide.

●

Time difference operation: Each slide

strokes with a phase difference.

In model MF-7500, three independent slides

operate with a phase difference of 30⬚ and a ca-

pacity per slide of 280 tonf (2500 kN). The total

capacity is 845 tonf (7500 kN) with different

working timing. A schematic of the press with

the slide and die area is shown in Fig. 23.13(a),

Near-Net Shape Forging and New Developments / 327

Fig. 23.10 Gear forging process utilizing divided ﬂow [Kondo, 1999]

Fig. 23.11 One-step divided ﬂow method with back-up pressure [Kondo, 1999 and 2002]

328 / Cold and Hot Forging: Fundamentals and Applications

Fig. 23.12 Characteristics of press and accuracy of formed products [Nakano, 1997, and Ishinaga, 1996]

whereas Fig. 23.13(b) shows the slide motion.

As a result of these features it was possible to:

●

Reduce the press capacity and reduce facility

cost due to reduction of total load and torque

of the press

●

Reduce slide tilting and thus improve the ac-

curacy of the formed parts, since the slides

perform independently without any mutual

interference

●

Reduce the vibration and noise during press

operation

Figure 23.14 shows a helical gear cup formed

using the MF-7500 press system [Nakano,

1997]. These cups require a higher forming load

in the ﬁrst and second stage and hence are ideal

candidates for production in using this press.

The cup, which traditionally required ﬁve form-

ing stages, is formed in just three steps. Addi-

tionally, two annealing processes and phosphate

coating were eliminated.

23.4.2 Multiaction Forging Press

Multiaction forging is an effective means of

net shape forging parts with complex features.

In multiaction forming, there is more than one

pressure source to operate the dies and the slide.

Also, the dies make several relative motions dur-

ing one stroke. A multiaction press for forming

helical gears is shown in Fig. 23.15(a). This hy-

draulically operated press has ﬁve cylinders: one

for driving the slide, two cylinders in the slide,

and two in the bed. Figure 23.15(b) shows the

construction of a die for forming the helical gear

seen in Fig. 23.15(c) [Nakano, 1997].

23.4.3 Servomotor Press

Figure 23.16 shows a servomotor press that

combines a newly developed large-sized, high-

torque servomotor drive with a crank mecha-

nism. The press uses computer numerical con-

trols for high functionality. It is thus possible to

program the forming motion and speed in order

to set the optimum parameters required to form

the part (Fig. 23.17). This can be used advan-

tageously to increase die life and produce hard-

to-form materials. The press achieves reduced

power usage by using a capacitor to store energy

(Fig. 23.16).

23.5 Innovative Forging Processes

Besides the conventional precision-forging

processes, which are commonly used in the in-

dustry, new processes such as microforming and

orbital forging are being further developed for

practical and economical applications.