ASM Metals HandBook Vol. 14 - Forming and Forging

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fracture locus is a straight line of slope -

. Some metals have a bilinear failure locus. The workability diagrams are used

during process design by plotting calculated or estimated surface strain paths that are to be imposed during forming on the

fracture locus diagram ( Fig. 7). If the final strains lie above the locus, part failure is likely, and changes are necessary in

preform design, lubrication, and/or material. The fracture locus concept has been used to prevent free surface cracking in

forging and to prevent edge cracking in rolling. With modifications, the fracture locus approach has also provided insight

into such failure modes as center bursting in extrusion and forging and die-workpiece contact fractures in forging.

Fig. 7 Schematic workability diagrams for bulk forming processes. Strain path (a) would

lead to failure for

material A. Both strain paths (a and b) can be used for the successful forming of material B. Source: Ref 8.

A related concept is the forming limit diagram used to quantify sheet metal formability. An example is shown in Fig. 8.

As for the bulk workability fracture locus, the sheet metal forming limit diagram is the locus of normal surface strains that

give rise to failure. The magnitudes of the failure strains are usually controlled by one of two processes: localized

through-thickness thinning or fracture prior to localized thinning. In either case, the forming limit diagram is most easily

determined by stretching experiments using a hemispherical punch. Strain path and failure strains (in terms of the so-

called major and minor strains) are varied by changes in lubrication and test blank width. Experimentally determined

forming limit diagrams are then compared to the strains that are to be developed during forming to determine the

possibility for failure. Additional information on forming limit diagrams can be found in the article "Formability Testing

of Sheet Metals" in this Volume.

Fig. 8 Typical forming limit curves for -brass (70Cu-30Zn), -brass (61Cu-39Zn), X5020-

T4 aluminum,

2036-T4 aluminum, and 6151-T4 aluminum. Source: Ref 9.

Process Simulation

The development of powerful computer-based simulation techniques, such as those based on the finite-element method,

has provided a vital link between advances in tooling and equipment design, on the one hand, and an improved

understanding of materials behavior on the other. Inputs to finite-element codes include the characteristics of the

workpiece material (flow stress and thermal properties) and the tool/workpiece interface (friction and heat transfer

properties), as well as workpiece and tooling geometries. Typical outputs include predictions of forming load; strain,

strain rate, and temperature contour plots; and tooling deflections. This information can serve a number of design

functions, such as selection of press capacity, determination of success or failure with regard to material workability or

formability, and estimation of likely sources of tooling failure (abrasive wear, thermal fatigue, and so on).

Process simulation techniques also provide a method for preform and die design through the ability to determine metal

flow patterns without constructing tooling or conducting expensive in-plant trials. In addition, the output from process

simulations can be helpful in selecting variables that are useful in process control (for example, ram speed or load

monitoring) or finished-product quality control. The advent of these computer-based technologies will help in eliminating

the hidden costs of trial-and-error design and in increasing productivity in the metalworking industries. Detailed

information on finite-element method process simulation for bulk and sheet forming operations can be found in the

articles "Modeling Techniques Used in Forging Process Design" and "Process Modeling and Simulation for Sheet

Forming" in this Volume.

References cited in this section

"Precision Forging Machines," Technical Literature, GFM Corporation

R. Shivpuri, "Past Developments and Future Trends in Rotary and Orbital Forging," Report ERC/NSM-87-

Engineering Research Center for Net Shape Manufacturing, Ohio State University, March 1987

R.L. Athey and J.B. Moore, "Progress Report on the Gatorizing Forging Process," Paper 751047, Society of

Automotive Engineers, 1975

C.H. Hamilton, Forming of Superplastic Metals, in Formability: Analysis, Modeling, and Experimentation,

S.S Hecker, A.K. Ghosh, and H.L. Gegel, Ed., The Metallurgical Society, 1978, p 232

S.V. Radcliffe and E.B. Kula, Deformation, Transformation, and Strength, in

Fundamentals of Deformation

Processing, W.A. Backofen et al., Ed., Syracuse University Press, 1964, p 321

G.E. Dieter, Ed., Workability Testing Techniques, American Society for Metals, 1984

P.W. Lee and H.A. Kuhn, Cold Upset Testing, in Workability Testing Techniques,

G.E. Dieter, Ed.,

American Society for Metals, 1984, p 37

S.S. Hecker, Experimental Studies of Sheet Stretchability, in

Formability: Analysis, Modeling, and

Experimentation, S.S. Hecker, A.K. Ghosh, and H.L. Gegel, Ed., The Metallurgical Society, 1978, p 150

Introduction to Forming and Forging Processes

S.L. Semiatin, Battelle Columbus Division

Future Trends

The metalworking industry is likely to see changes in the major areas of materials, processes, and process design. Some

of these changes will include the following.

Materials. Developments in materials will greatly affect the metals that are formed. These will range from aluminum-

and titanium-base alloys to alloy steels and superalloys. New classes of aluminum alloys that will be processed include

aluminum-lithium alloys, SiC whisker-reinforced aluminum metal-matrix composites, and high-strength high-temperature

powder metallurgy alloys. More use will be made of β-titanium alloys, which combine good strength and toughness, and

there will be increased use of thermal-mechanical processing for titanium alloys and superalloys. In the ferrous area,

microalloyed steels, which permit elimination of final heat treatment by controlled cooling afer hot working, are

becoming increasingly popular for a variety of automotive applications.

Processes. In the forming process area, metalworking methods that give rise to net or near-net shape will be

increasingly used to conserve materials and to reduce machining costs. These processes include precision forging,

isothermal and hot-die forging, and superplastic forming of sheet materials. There will also be increased use of automatic

tool change systems as lot sizes and delivery times decrease.

Process Design. With the development of user-friendly programs and the decreasing cost of computer hardware, there

will be significant growth in computer-aided techniques in tooling design and process control. In particular, there will be

more interaction between parts users and parts vendors during the design stage. Process simulation will streamline the

design process, and this will decrease delivery times as well as the overall cost of fabricated components.

Introduction to Forming and Forging Processes

S.L. Semiatin, Battelle Columbus Division

References

T. Altan, S.I. Oh, and H.L. Gegel, Metal Forming: Fundamentals and Applications,

American Society for

Metals, 1983

"Precision Forging Machines," Technical Literature, GFM Corporation

R. Shivpuri, "Past Developments and Future Trends in Rotary and Orbital Forging," Report ERC/NSM-87-

Engineering Research Center for Net Shape Manufacturing, Ohio State University, March 1987

R.L. Athey and J.B. Moore, "Progress Report on the Gato

rizing Forging Process," Paper 751047, Society of

Automotive Engineers, 1975

C.H. Hamilton, Forming of Superplastic Metals, in Formability: Analysis, Modeling, and Experimentation,

S.S Hecker, A.K. Ghosh, and H.L. Gegel, Ed., The Metallurgical Society, 1978, p 232

S.V. Radcliffe and E.B. Kula, Deformation, Transformation, and Strength, in

Fundamentals of Deformation

Processing, W.A. Backofen et al., Ed., Syracuse University Press, 1964, p 321

G.E. Dieter, Ed., Workability Testing Techniques, American Society for Metals, 1984

P.W. Lee and H.A. Kuhn, Cold Upset Testing, in Workability Testing Techniques,

G.E. Dieter, Ed.,

American Society for Metals, 1984, p 37

S.S. Hecker, Experimental Studies of Sheet Stretchability, in Formability: Analy

sis, Modeling, and

Experimentation, S.S. Hecker, A.K. Ghosh, and H.L. Gegel, Ed., The Metallurgical Society, 1978, p 150

Hammers and Presses for Forging

Revised by Taylan Altan, The Ohio State University

Introduction

FORGING MACHINES can be classified according to their principle of operation. Hammers and high-energy-rate

forging machines deform the workpiece by the kinetic energy of the hammer ram; they are therefore classed as energy-

restricted machines. The ability of mechanical presses to deform the work material is determined by the length of the

press stroke and the available force at various stroke positions. Mechanical presses are therefore classified as stroke-

restricted machines. Hydraulic presses are termed force-restricted machines because their ability to deform the material

depends on the maximum force rating of the press. Although they are similar in construction to mechanical and hydraulic

presses, screw-type presses are classified as energy-restricted machines.

Hammers and Presses for Forging

Revised by Taylan Altan, The Ohio State University

Hammers

Historically, hammers have been the most widely used type of equipment for forging. They are the least expensive and

most flexible type of forging equipment in the variety of forging operations they can perform. Hammers are capable of

developing large forces and have short die contact times. The main components of a hammer are a ram, frame assembly,

anvil, and anvil cap. The anvil is directly connected to the frame assembly, the upper die is attached to the ram, and lower

die is attached to the anvil cap.

In operation, the workpiece is placed on the lower die. The ram moves downward, exerting a force on the anvil and

causing the workpiece to deform. Forging hammers can be classified according to the method used to drive the ram

downward. Various types of hammers are described in the following sections; Table 1 compares the capacities of some of

these types.

Table 1 Capacities of various types of forging hammers

Ram weight Maximum blow

energy

Impact speed Type of hammer

kg lb kJ ft·lb m/s ft/s

Number of

blows per minute

Board drop 45-3400 100-7500 47.5 35,000 3-4.5 10-15

45-60

Air or steam lift 225-7250 500-16,000 122 90,000 3.7-4.9

12-16

Electrohydraulic drop

450-9980 1000-22,000

108.5

80,000 3-4.5 10-15

50-75

Power drop 680-31,750

1500-70,000

1153 850,000

4.5-9 15-30

60-100

Gravity-Drop Hammers

Gravity-drop hammers consist of an anvil or base, supporting columns that contain the ram guides, and a device that

returns the ram to its starting position. The energy that deforms the workpiece is derived from the downward drop of the

ram; the height of the fall and the weight of the ram determine the force of the blow.

Board-drop hammers (Fig. 1) are widely used, especially for producing forgings weighing no more than a few

kilograms. In the board-drop hammer, the ram is lifted by one or more boards keyed to it and passing between two

friction rolls at the top of the hammer. The boards are rolled upward and are then mechanically released, permitting the

ram to drop from the desired height. Power for lifting the ram is supplied by one or more motors. The hammers have a

falling weight, or rated size, of 180 to 4500 kg (400 to 10,000 lb); standard sizes range from 450 to 2250 kg (1000 to 5000

lb) in increments of 225 and 450 kg (500 and 1000 lb). The height of fall of the ram varies with hammer size, ranging

from about 900 mm (35 in.) for a 180 kg (400 lb) hammer to about 2 m (75 in.) for a 3400 kg (7500 lb) hammer. The

height of fall, and therefore the striking force, of the hammer is approximately constant for a given setting and cannot be

altered without stopping the machine and adjusting the length of stroke. Anvils on board-drop hammers are 20 to 25 times

as heavy as the rams.

Fig. 1 Principal components of a board-drop hammer

The air-lift gravity-drop hammer is similar to the board-drop hammer in that the forging force is derived from the

weight of the falling ram assembly and upper die. It differs in that the ram in the air-lift hammer is raised by air or steam

power. Stroke-control dogs, preset on a rocker and actuated by the ram, control power to the ram cylinder. With the

hammer shut down, the dogs can be reset on the rocker to adjust stroke length. A device is available that allows both a

long stroke and a short stroke in a variable sequence.

The ram is held in the raised position by a piston-rod clamp, which is operated by its own cylinder using a separate

compressed-air supply. When the clamp is oblique, the piston rod is clamped. When the operator's treadle is depressed, air

enters the cylinder and raises the clamp horizontally, and the ram cycles. Cycling will continue until the treadle is

released, causing the rod clamp to drop obliquely and grip the rod. The treadle should not be released on the downstroke

of the ram, because this will produce excessive strain in the rod and clamp parts.

The range of sizes generally available in air-lift hammers is 225 to 4500 kg (500 to 10,000 lb). The weight of forging that

can be produced in an air-lift hammer of a given size is about the same as that which can be produced in its board-drop

hammer counterpart.

Electrohydraulic Gravity-Drop Hammers. In recent years, two significant innovations have been introduced in

hammer design. The first is the electrohydraulic gravity-drop hammer. In this type of hammer, the ram is lifted with oil

pressure against an air cushion. The compressed air slows the upstroke of the ram and contributes to its acceleration

during the downstroke blow. Therefore, the electrohydraulic drop hammer also has a minor power hammer action.

The second innovation in hammer design is the use of electronic blow-energy control. Such control allows the user to

program the drop height of the ram for each individual blow. As a result, the operator can set automatically the number of

blows desired in forging in each die cavity and the intensity of each individual blow. The electronic blow control

increases the efficiency of the hammer operations and decreases the noise and vibration associated with unnecessarily

strong hammer blows.

Power-Drop Hammers

In a power-drop hammer, the ram is accelerated during the downstroke by air, steam, or hydraulic pressure. The

components of a steam- or air-actuated power-drop hammer are shown in Fig. 2. This equipment is used almost

exclusively for closed-die (impression-die) forging.

Fig. 2 Principal components of a power-drop hammer with foot control to regulate the force of the blow

The steam- or air-powered drop hammer is the most powerful machine in general use for the production of forgings by

impact pressure. In a power-drop hammer, a heavy anvil block supports two frame members that accurately guide a

vertically moving ram; the frame also supports a cylinder that, through a piston and piston rod, drives the ram. In its lower

face, the ram carries an upper die, which contains one part of the impression that shapes the forging. The lower die, which

contains the remainder of the impression, is keyed into an anvil cap that is firmly wedged in place on the anvil. The

motion of the piston is controlled by a valve, which admits steam, air, or hydraulic oil to the upper or lower side of the

piston. The valve, in turn, is usually controlled electronically. Most modern power-drop hammers are equipped with

programmable electronic blow control that permits adjustment of the intensity of each individual blow.

Power-drop hammers are rated by the weight of the striking mass, not including the upper die. Hammer ratings range

from 450 to 31,750 kg (1000 to 70,000 lb). The large mass of a power-drop hammer is not apparent, because a great deal

of it is beneath the floor. A hammer rated at 22,700 kg (50,000 lb) will have a sectional steel anvil block weighing

453,600 kg (1,000,000 lb) or more. The ram, piston, and piston rod will have an aggregate weight of approximately

20,400 kg (45,000 lb). The striking velocity obtained by the downward pressure on the piston sometimes exceeds 7.6 m/s

(25 ft/s).

Rating hammers by the weight of the striking mass is not correct, although it has been the common practice. The more

realistic method of rating hammers is by the maximum energy, in joules or foot-pounds, that the ram can impart to the hot

metal during a single blow at the maximum energy setting of the hammer controls. The useful energy supplied to the

forged metal by the hammer ram depends on the hammer design (weight of the ram and the pressure on the top of the

piston), the ratio of the anvil weight versus the ram weight, and the hammer foundation design.

Apart from the size of power-drop hammers and the force they make available for the production of large forgings

(forgings commonly produced in power-drop hammers range in weight from 23 kg, or 50 lb, to several megagrams),

another important advantage is that the striking intensity is entirely under the control of the operator or is preset by the

electronic blow-control system. Consequently, effective use can be made of auxiliary impressions in the dies to preform

the billet to a shape that will best fill the finishing impressions in the dies and result in proper grain flow, soundness, and

metal economy, with minimum die wear. When adequate preliminary impressions cannot be incorporated into the same

set of die blocks, two or more hammers are used to produce adequate shaping or blocking before the final die is used.

Although there are many advantages associated with the use of power-drop hammers, the greater striking forces they

develop give rise to several disadvantages. As much as 15 to 25% (and, in hard finishing blows, up to 80%) of the kinetic

energy of the ram is dissipated in the anvil block and foundation, and therefore does not contribute to deformation of the

workpiece. This loss of energy is most critical when finishing blows are struck and the actual deformation per stroke is

relatively slight. The transmitted energy imposes a high stress on the anvil block and may even break it. The transmitted

energy also develops violent, and potentially damaging, shocks in the surrounding floor area. This necessitates the use of

shock-absorbing materials, such as timber or iron felt, in anvil-block foundations and adds appreciably to the cost of the

foundation.

Die Forger Hammers

Die forger hammers are similar in operation to power-drop hammers, but have shorter strokes and more rapid striking

rates. The ram is held at the top of the stroke by a constant source of pressurized air, which is admitted to and exhausted

from the cylinder to energize the blow. The die forger hammers from one manufacturer are capable of delivering 5.5 to

89.5 kJ (4000 to 66,000 ft · lb) of energy per blow. Blow energy and the forging program (that is, the number of die

stations and the number and intensity of blows at each station) are preprogrammed by the operator.

Counterblow Hammers

The counterblow hammer, another variation of the power-drop hammer, is widely used in Europe. These hammers

develop striking force by the movement of two rams, simultaneously approaching from opposite directions and meeting at

a midway point. Some hammers are pneumatically or hydraulically actuated; others incorporate a mechanical-hydraulic or

a mechanical-pneumatic system.

A vertical counterblow hammer with a steam-hydraulic actuating system is illustrated in Fig. 3 (air-hydraulic systems are

also available). In this hammer, steam is admitted to the upper cylinder and drives the upper ram downward. At the same

time, pistons connected to the upper ram act through a hydraulic linkage in forcing the lower ram upward. Retraction

speed is increased by steam (or air) pressure acting upward on the piston. Through proper design relative to weights

(including tooling and workpiece) and hydraulics (slower lower-assembly velocities), the kinetic energy of the upper and

lower assemblies can be balanced at impact.

Fig. 3 Principal components of a vertical counter-blow hammer with a steam-hydraulic actuating system

The rams of a counterblow hammer are capable of striking repeated blows; they develop combined velocities of 5 to 6

m/s (6 to 20 ft/s). Compared to single-action hammers, the vibration of impact is reduced, and approximately the full

energy of each blow is delivered to the workpiece, without loss to an anvil. As a result, the wear of moving hammer parts

is minimized, contributing to longer operating life. At the time of impact, forces are canceled out, and no energy is lost to

foundations. In fact, counterblow hammers do not require the large inertia blocks and foundations needed for

conventional power-drop hammers.

Horizontal counterblow hammers have two opposing, die-carrying rams that are moved horizontally by compressed air.

Heated stock is positioned automatically at each die impression by a preset pattern of accurately timed movements of a

stock handling device. A 90° rotation of stock can be programmed between blows.

Open-Die Forging Hammers

Open-die forging hammers are made with either a single frame (often termed C-frame or single-arch hammers) or a

double frame (often called double-arch hammers) (Fig. 4). Open-die forging hammers are used to make a large percentage

of open-die forgings. The rated sizes of double-frame open-die forging hammers range from about 2720 to 10,900 kg

(6000 to 24,000 lb), although larger hammers have been built.