Altan T. Metal Forming Handbook

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In addition, hole punching, notching, bending or threadingprocesses can be

performed on the part in cases where this has not already been performed

as part of the hydroforming sequence itself (Fig.5.7.2).By calibrating the

part, it is possible to produce special contours at the ends of the tubes. A

separate process is required, generally by means of welding, in order to

assemble add-on componentsor to assemble the part with other components

produced using different production processes. In the case of a production

lot-related multiple pressing,components that are assembled together are

formed in a single pressing process and must be separated by cutting.

5.7.2Technical and economic considerations

The most important technical and economic aspects of hydroforming

are that:

–Components with complex geometries can be manufactured which

previously could not be produced in a single piece.

431

General considerations

Fig. 5.7.2 Hole punching, notching and bending during hydroforming

inward

hole punching

notching and

bending over

outward

hole punching

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

– The single-part construction eliminates the need for welding seams.

– The calibration process ensures a high degree of accuracy in the con-

tours and dimensions of hydroformed components.

– Strain hardening generally lends hydroformed components a high

degree of torsional rigidity. Their elastic recovery properties are less

pronounced than is the case with welded sheet metal shells. For this

reason, it is often possible for the wall thickness of hydroformed

components to be relatively low compared to cast parts or shell con-

structions. This effect can be used either to reduce weight or to

enhance strength or rigidity.

– Using the same die, it is possible to use and test tubes which, within

certain limits, have different wall thicknesses and starting diameters

and are made of different materials (provided the ultimate deforma-

tion is not exceeded). This allows the weight and strength properties

of the part to be optimized.

– Experience has shown that lower flow resistance and larger fatigue

strength are achieved in exhaust systems.

By selective component and process configuration, it is possible to save

both energy and material and to reduce development and production

times. Thus, generally it is possible to reduce manufacturing costs.

Bibliography

Leitloff, F.-U.: Innenhochdruckumformen mit dem Schäfer-ASE-Verfahren, Stahl (1995) 5.

Malle, K.: Aufweiten - Stauchen - Expandieren, VDI Journal 137 (1995) 9.

N. N.: ASE-Verfahren bringt Hohlkörper in Form, Stahlmarkt (1993) 4.

N. N. : Hochdruckumformen - Fertigungsverfahren mit Zukunft, Stahl (1993) 4.

N. N.: Mit Wasser immer in Form, Automobil-Industrie (1993) 4.

N. N.: Schäfer bringt Hohlkörper in Form, Technische Informationsbroschüre zur Schäfer-ASE-

Technologie, Wilhelm Schäfer Maschinenbau GmbH & Co.

Schäfer, A. W., Fröb, K., Bieselt, R., Hassanzadah, G.: Kaltgeformte T-Stücke für den Anlagenbau –

Anforderungen und technische Realisierbarkeit, 3R International 29 (1990) 9.

432

Hydroforming

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

6Solid forming (Forging)

6.1General

Processes and parts

The terms sheet metal forming and solid forming are based on practical

industrial application, and subdivide the field of forming technology

into two distinct areas. This distinction is made in addition to the defi-

nitions established by DIN 8582 “Forming” (cf. Fig. 2.1.3).Solid form-

ing entails the three-dimensional forming of “compact” slugs (for

example sheared billets), while the sheet metal forming method gener-

ally processes “flat” sheet metal blanks to create three-dimensional hol-

low structures with an approximately constant sheet metal thickness.

The forces exerted during solid forming are substantially higher than

those necessary for sheet metal forming. As a result, it is necessary to

use relatively rigid compact-design machines and dies. Solid forming

involves not only the processes of extrusion, indentation and drawing

which are dealt with in most detail here, but also for example rolling,

open die forging (cf. Fig. 2.1.4)or closed die forging (cf. Fig. 2.1.5).

The processes cold extrusion and drawinginvolve forming a slug or bil-

let placed in a die, most commonly for the production of discrete parts.

Hot extrusion uses a basically similar forming mechanism to create

semi-finished products such as rods, tubes and sections.The basic processes

involved in cold extrusion (Fig.6.1.1)are classified depending on their

forming direction as forward, backward and lateral extrusion and also

according to the final form of the workpiece as rod (solid), tube and cup

extrusion. When producing solid parts, the classical operations of cold

extrusion technology are supplemented by processes like upsetting and

ironing (Fig. 6.1.2). The shape variety of parts formed from slugs or bil-

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

434

Solid forming (Forging)

Fig. 6.1.1 Schematic representation of the cold extrusion (cold forging) processes:

I prior to forging;

II after forging (BDC);

a punch; b container;

c workpiece; d ejector;

e counterpunch; f mandrel

III

III III

III

d,e

d, e

d,e

rod extrusion

tube extrusion cup extrusion

forward extrusion

backward extrusion

lateral extrusion

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

lets that can be produced today is increased substantially by the use of

piercing, flow piercing, trimming and bending processes.

The range of parts includes not only fastening elements(screws, bolts,

nuts, rivets, etc.), but parts for the construction industry(L and T-fittings

for heating systems) and components used in mechanical engineering

applications (ball bearing races, balls, rollers and cages). Most solid

formed parts are used in the automotive and motor cycle industry. Engine

components (valves, camshafts, cams, connecting rods), gear parts (gear

shafts, gear wheels, synchronous rings, differential bevel gears), chassis

components (stud axles, ball hubs, stepped housing elements, shock

absorbers), steering components (steering column joints, tie rod ends),

brake components (brake pistons) and components for electrical motors

(starter motor pinions, drivers, pole rotors), can be economically pro-

duced by solid forming. In addition, solid formed parts are used in

household appliances, office equipment, clocks or medical equipment.

Figure 6.1.3offers an overview of different shapes which can be pro-

duced. The forming techniques described here are currently also used to

producegearsthat can be produced very economically. In many cases,

no machining is necessary or at most one final grinding or hardening

process may be required at the teeth. Components with straight teeth

(spur gears) – such as hub sleeves, bevel wheels and gears –, helical teeth

(e.g. planetary gears, small gears) and special teeth forms (e.g. ring

gears) can be produced. This type of gears can be formed with any

desired tooth that is often involute in shape.

Contours are formed on the surface using the indentation processes

(DIN 8583 sheet 5) coining and roll embossing. Here, the complete

435

General

Fig. 6.1.2 Upsetting, open die extrusion, ironing

upsetting

extrusion

ironing

punch

ironing die

container

workpiece

upsetting die

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

workpiece or the area to be embossed is enclosed in trapped (closed) dies.

During embossing, the part thickness is reduced, in some cases by a con-

siderable amount. Coining is a typical example of an embossing process

(cf. Fig. 2.1.6).Bimetal coins are a good example here, as they involve a

joining process (DIN 8593) combined with coining and roll embossing.

Embossing techniques are also used for example in the production of

road signs, and in the cutlery or hand tool industry (Fig. 6.1.4).

Cold coining of hot forged parts (sizing) (DIN 8583 sheet 4) is used to

improve dimensional, shape and positioning accuracy and/or to

improve surface quality. The objective is to press dimensions and shape

to the required tolerances, and thus avoid the need for subsequent met-

al cutting operations. In contrast to indentation, when using cold coin-

ing, the component already has almost its complete geometry, meaning

that the height reduction during cold coining is relatively small. Sizing

of the bosses in connecting rods (which are required to have a tighter

thickness tolerance compared to castings), and sizing of plier halves

436

Solid forming (Forging)

Fig. 6.1.3 Range of cold, warm and hot forged parts

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

(whose functional surfaces require particular positioning accuracy) are

typical examples of this type of application.

Hot, warm and cold forming and process combinations

It is generally true that almost all solid forming processes can theoreti-

cally be applied at different temperatures. It is only when different fac-

tors come together to create interrelated complexities that either hot,

cold or warm forming emerges as the ideal production method in any

particular case (cf. Sect. 2.2.1). Above the recrystallization temperature,

hot forming(forging)takes place. Forging is the oldest known meta form-

ing technique. If the billet or workpiece is not heated prior to pressing,

the method used is referred to as cold forming (cold forging). This method

did not become feasible for use with steel until the thirties, when a suit-

able separating layer between the die and the workpiece could be

developed. Warm forming (or forging),the last of the three techniques,

developed for the industrial practice, was introduced in the eighties.

For steels, for example, this method works at a temperature range of

between 750 and 950 °C. For many parts and special workpiece materi-

als with a high proportion of carbon, this method combines the tech-

nical advantages of hot forging with those of cold forging for optimum

economical conditions.

The flow stress (Fig.6.1.5)decreases with increasing temperature.

Depending on the material being processed, when using the warm forg-

ing method, the flow stress reaches only around one half to a third of

437

General

Fig. 6.1.4 Examples of coined parts, such as coins, medals, cutlery and various hand tools,

as well as cold-coined hot forged parts

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

the corresponding values for cold forging. When hot forging, even low-

er flow stress values can be achieved, corresponding to as little as one

fifth of the flow stress level during cold forging. Again depending on

the material used, in warm forging and hot forging respectively forge-

ability is two or six times higher than that in cold forging. In the high-

er temperature range from around 700 °C upwards, the increase in scale

formation which has a detrimental effect on dimensional accuracy and

surface quality of the parts must be taken into account. The tempera-

ture required for warm forging must be carefully selected as in its “low-

er range” it is limited by the size of the press force and the achievable

degree of deformation and in its “upper range” by scale formation.

Table6.1.1 provides a comparison of the advantages of all three

methods on the basis of processing engineering criteria. The weight

of the components produced using hot forging ranges between

0.05 and 1,500kg, in warm and cold forging between 0.001 and 50kg.

Dimensional accuracy and surface quality decrease with increasing

438

Solid forming (Forging)

Fig. 6.1.5 Flow stress, scale formation and achievable degree of deformation (forgeability) in

function of temperature

500

1000

1500

200

400

600

800 1000

1200

[°C]

forming temperature

flow stress k

fracture strain

scale layer

µm

X8CrNiTi1

C60

X10Cr13

ϕ=70s

ϕ=0,7

C15

16MnCr5

Ck45

5 min

C60

-1

= 90s

-1

min

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

temperature. This is due to thermal expansion, which cannot be pre-

dicted with accuracy, and also to increased die wear.

As an example, let us consider warm forging as the most economical

process for selected parts, subject to technical feasibility. If this process

is then assigned a cost factor of 100, the relative costs of hot forging can

be up to 13% and of cold forging up to 47% higher. Therefore, in warm

and especially hot forging the cost of removing additional material due

to larger machining allowances must be considered. In the case of cold

forging, in contrast, it is frequently possible to compensate completely

for the “highest forming costs” by extremely low or even entirely elim-

inated finishing costs. In extrusion, therefore, cold and warm forging

must be compared as the most economical process alternatives. With

an increasing degree of geometrical complexity (higher degree of defor-

mation, for example for steering knuckles), however, hot forging is still

an economical process.

In practice, the “ideal processes” of solid forming defined here are also

used as process combinations in three different respects. First, almost all

complex workpieces are manufactured today by using multiple-stage

combinations (process or processing step sequence) of individual opera-

tions. To produce a bushing, for example, a combination of the process-

es upsetting, centering, backward cup extrusion, piercing and ironing

are applied in sequence. Second, it is possible for two processes to be car-

ried out simultaneously in a forming operation. Using combined back-

439

General

Table 6.1.1: Comparison between hot, warm and cold forging

Forming

Hot Warm Cold

workpiece weight 0.05 – 1,500 kg 0.001 – 50 kg 0.001 – 30 kg

precision IT 13 – 16 IT 11 – 14 IT 8 – 11

surface quality R

. 50 – 100 mm

flow stress ~ 20 – 30 % ~ 30 – 50 % 100 %

formability

f (T, material)

w # 6 w # 4 w # 1.6

“Forming costs“

VDW study 1991,

Darmstadt

up to 113 % 100 % up to 147 %

machining required high low very low

f (T, material)

. 30 µm

. 10 µm

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

ward and forward cup extrusion, for example, tube-shaped workpieces

with a central web (Fig.6.1.1)can be produced. These and similar

process combinations reduce the necessary number of processing steps

and generally involve lower press forces than the sum of press forces

used for individual operations. Such process combinations are more eco-

nomical, but are only possible using multiple-action presses and dies.

Third, in forming processes where workpieces are initially hot or

warm forged and then cold forged, “process combination” has also

become an established term. Using this technology, it is possible to

benefit simultaneously, for example, from (a) the economic benefits of

warm forming to create “near-net shape” components and (b) the tech-

nical benefits of cold extrusion or cold forging, in particular its dimen-

sional accuracy and surface quality. In this process, the cold forming

stage is not by any means restricted simply to a sizing operation, but

can include a multiple-stage forging process.

440

Solid forming (Forging)

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998