ASM Metals HandBook Vol. 14 - Forming and Forging

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Fig. 44

Top view of aluminum alloy 6061 disk (a) compressed between dies (b). Cracks form at the transition

region between rough and smooth areas of the die.

Grid marks placed on the die contact surface of the disks were used to measure the distribution of surface strains in the

radial direction. Figure 45 gives an example of such measurements. In the rough central region, the strains are zero, while

in the smooth outer region the strains are equal and constant. In the transition, however, the circumferential strain, θ,

jumps abruptly from zero to its constant value in the outer region, and the radial strain,

, overshoots to a very high value

before returning to its constant value in the smooth outer region.

Fig. 45 Radial variation of contact surface strains after 30% compression of the disk shown in Fig. 44.

The strains shown in Fig. 45 were the same regardless of the friction condition in the smooth outer region. Therefore,

fractures in the transition region occur because of the combination of large tensile surface strains and low hydrostatic

stress state.This explains the occurrence of cracks at low reduction when Teflon is used, and no occurrence of fracture

when no lubricant is used. The Teflon, having a near-zero friction coefficient, results in very low radial back pressure on

the transition region, while grease and no lubricant provide progressively larger back pressures.

By means of visioplasticity analysis, the stresses were determined at the contact surface in the vicinity of the transition

region. The resulting normal die pressure plus the surface radial and circumferential strains define the stress and strain

states in the transition region and can be illustrated on a forming limit diagram. Figure 46 shows the change in surface

strains and the increase in the fracture line due to increasing normal pressure during compression of a disk with grease

lubricant in the outer region (indicated by the increments of reduction to 45%). The fracture line increases at a slower rate

than the strains increase with increasing pressure, and at 45% reduction the strain path exceeds the fracture line and

cracks are observed. For Teflon lubricant, the crossover occurs at about 30% reduction, while in the case of no lubricant

the fracture line moves progressively away from the strain path.

Fig. 46

Progression of surface strains and fracture line at the transition region between rough and smooth

zones of the compressed disk shown in Fig. 45.

Points 1, 2, 3, and 4 represent 10, 20, 30, and 45% reduction,

respectively.

Example 4: Fir Tree Defect.

The criterion for contact surface fracture can be applied qualitatively for interpretation of the fir tree defect in extrusion.

Such defects occur on the surfaces of extruded bars as well as in localized areas of forgings containing ribs.

In a section of a rib-web forging from a preform of sintered aluminum alloy powder, small cracks formed with a regular

spacing on the rib surfaces (Fig. 47). Such defects occurred only when the thickness of the rib was greater than

approximately one-half the web length (that is, extrusion reduction less than one-half). Each crack formed by shear at the

corner as material flowed from the web into the vertical rib.

Fig. 47

Die contact surface cracking during forging extrusion of aluminum alloy powder compact. (a) Cross

section. (b) Normal to vertical rib surface. Note also the cracks at the top free surface.

Stress and strain states

are defined in Fig. 4(b).

The stress and strain state on a surface material element is shown in Fig. 48. As material in the web is compressed, the

surface element experiences tensile strain in the direction of flow, and the strain increases as the element approaches the

die corner. At the same time, there is a compressive stress from the die onto the surface element. This pressure

diminishes, however, as the element nears the die corner and almost disappears as the element moves around the corner.

Fig. 48 Increase in strain

and decrease in die contact pressure P

on a surface element as it moves from the

web into the rib section of the extrusion forging shown in Fig. 47. Stress and strain states are defined in

Fig.

5(a).

This can be illustrated schematically on the fracture strain diagram shown in Fig. 49. Because the deformation is in a state

of plane strain, the strain path is represented as a vector of increasing length along the vertical axis. Meanwhile, the

fracture line decreases in height because the pressure acting normal to the element is progressively decreasing. When the

strains cross the fracture line, fracture occurs.

Fig. 49 Progression of strain

and decline of fracture line due to decrease in pressure P

for an element

moving from the web to rib section during the forging extrusion shown in Fig. 48.

At stage 3, the strain exceeds

the fracture line.

This phenomenon does not take place when extruding ribs of small thickness, because the extrusion reduction, and

therefore the pressure, is larger, which maintains the fracture line at a high level. For thick ribs, two solutions were

considered. One approach is to increase friction along the rib walls by roughening the die surface or avoiding lubrication

of the die rib. This produces greater back pressure at the die corner, elevating the fracture line and preventing cracking.

Such an approach is difficult to implement and can be used only with segmented dies because the formed rib cannot be

removed from the die. The second approach is to use a draft angle on the rib, which has the same effect as increased

friction. An angle of 10° prevented fracture in the current case, but other alloys may require a smaller or larger angle. A

quantitative analysis combining the pressure effect on the fracture line and plasticity analysis would provide a method of

predicting he draft angle in order to prevent fracture.

References cited in this section

10.

B. Avitzur, "Stra

in Hardening and Strain Rate Effects in Plastic Flow Through Conical Converging Dies,"

Paper 66-Prod-17, American Society of Mechanical Engineers, 1966

13.

J.J. Shah and H.A. Kuhn, An Empirical Formula for Workability Limits in Cold Upsetting and Bolt

Heading, in Proceedings of 13th NAMRC, Society of Mechanical Engineers, 1985

14.

C.L. Downey and H.A. Kuhn, Application of a Forming Limit Criterion to Design of Preforms for Powder

Forging, J. Eng. Mater. Technol. (Trans. ASME), Vol 97H, 1975, p 121

15.

H.A. Kuhn, Deformation Processing of Sintered Powder Materials, in Powder Metallurgy Processing,

H.A.

Kuhn and A. Lawley, Ed., Academic Press, 1978, p 99

16.

Z. Zimerman, H. Darlington, and E.H. Kottcamp, Jr., Selection of Operating Parameters to Prevent Ce

ntral

Bursting during Cold Extrusion, in Mechanical Working and Steel Processing,

The Metallurgical Society,

1970, p 405

Workability Theory and Application in Bulk Forming Processes

Howard A. Kuhn, University of Pittsburgh

References

1. H.A. Kuhn, P.W. Lee, and T. Erturk, A Fracture Criterion for Cold Forging, J. Eng.

Mater. Technol.

(Trans. ASME), Vol 95, 1973, p 213-218

2. H. Ortiz and H.A. Kuhn, Physical Modeling of Bar Rolling for Workability Study, in

Physical Modeling of

Bulk Metalworking Processes, Conference Proceedings, ASM INTERNATIONAL, 1987

3. E. Erman and H.A. Kuhn, Novel Test Specimens for Workability Measurements, in

Compression Testing of

Homogeneous Materials and Composites,

STP 808, American Society for Testing and Materials, 1983, p

279-290

4. F.A. McClintock, J. Appl. Mech. (Trans. ASME), Vol 90, 1968, p 363

5. Z. Marciniak and K. Kuczynski, A Model of Localized Thinning in Sheet Metalforming, Int. J. Mech. Sci.,

Vol 9, 1967, p 609

6. P.W. Lee and H.A. Kuhn, Fracture in Cold Upset Forging--A Criterion and Model, Metall. Trans. A,

Vol

4A, 1973, p 969-974

7. M.G. Cockcroft and D.J. Latham, Ductility and the Workability of Metals, J. Inst. Met., Vol 96, 1968, p 33-

8. B. Avitzur, Metal Forming--Processes and Analysis, McGraw-Hill, 1968

9. W.M. Evans and B. Avitzur, "Die Design for Drawing Extrusion," Paper MF67-

582, Society of

Manufacturing Engineers, 1967

10.

B. Avitzur, "Strain Hardening and Strain Rate Effects in Plastic Flow Through Conical Converging Dies,"

Paper 66-Prod-17, American Society of Mechanical Engineers, 1966

11.

R. Hill, On the Inhomogeneous Deformation of a Plastic Lamina in a Compression Test, Philos. Mag.,

Vol

41, 1950, p 733

12.

J.F. Nye, Experiments on the Plastic Compression of a Block between Rough Plates, J. Appl.

Mech. (Trans.

ASME), Vol 19, 1952, p 337

13.

J.J. Shah and H.A. Kuhn, An Empirical Formula for Workability Limits in Cold Upsetting and Bolt

Heading, in Proceedings of 13th NAMRC, Society of Mechanical Engineers, 1985

14.

C.L. Downey and H.A. K

uhn, Application of a Forming Limit Criterion to Design of Preforms for Powder

Forging, J. Eng. Mater. Technol. (Trans. ASME), Vol 97H, 1975, p 121

15.

H.A. Kuhn, Deformation Processing of Sintered Powder Materials, in Powder Metallurgy Processing,

H.A.

Kuhn and A. Lawley, Ed., Academic Press, 1978, p 99

16.

Z. Zimerman, H. Darlington, and E.H. Kottcamp, Jr., Selection of Operating Parameters to Prevent Central

Bursting during Cold Extrusion, in Mechanical Working and Steel Processing, The Metallur

gical Society,

1970, p 405

Introduction to Process Design for Bulk Forming

H.L. Gegel and J.C. Malas, Air Force Wright Aeronautical Laboratories/Materials Laboratory; S.M. Doraivelu and V.A. Shende,

Universal Energy Systems, Knowledge Integration Center

Introduction

In designing material-working processes for components made of complex engineering materials, the most important task

is the selection of the controlling process parameters that will ensure part quality as well as specific mechanical and

physical characteristics. The controlling process parameters are the sequence and number of material flow operations, the

heat-treating conditions, and the associated quality assurance tests. When designing forging processes, special features

such as nonlinear irreversible finite-deformation flow must be considered. Simultaneously, the complex interdependence

of forging process parameters, as shown in Fig. 1, and their effect on the quality of the finished part, the reliability, and

the ability to inspect must be considered.

Fig. 1 Interdependence of forging process parameters.

Another important goal in forging is to determine the optimum means for producing defect-free parts on a repeatable

basis. The optimization criteria depend on the manufacturing goals and the product specifications; establishing the

appropriate criteria requires in-depth views--both global and local--of manufacturing processes and material behavior.

From an optimization viewpoint, manufacturing processes require the determination of material flow mechanics to

achieve proper process design and to develop a rational strategy for process control.

Acknowledgements

The authors wish to express their appreciation for the helpful suggestions and comments provided by the following

individuals who reviewed the articles in this Section: S. Gopinath, Knowledge Integration Center, Universal Energy

Systems; V.K. Jain, Department of Mechanical Engineering, University of Dayton; J.S. Gunasekera, Department of

Mechanical Engineering, Ohio University; T.E. Howson, Wyman-Gordon Company; T. Furman, A. Hayes, and C. White,

Ladish Company, Inc.; E. Raymond, J. Walters, and R. Wallies, Cameron Iron Works, Inc.; S.C. Jain and S.K. Srivatsa,

General Electric Company. Special acknowledgment and sincere thanks are also due to L.L. Jenney and D. Ashok,

Knowledge Integration Center, Universal Energy Systems, for their assistance in preparing this document.

Introduction to Process Design for Bulk Forming

H.L. Gegel and J.C. Malas, Air Force Wright Aeronautical Laboratories/Materials Laboratory; S.M. Doraivelu and V.A. Shende,

Universal Energy Systems, Knowledge Integration Center

Modeling

Modeling of the forging process involves both mechanics and thermodynamics. The process, as a rule, is inhomogeneous

and transient over a large volume of workpiece material, and the material flow process can be characterized as highly

irreversible and stochastic in nature. The mechanics of the forging process are well established, and different analytical

tools are available for analyzing most of the important steps of a total forging process. Three articles in this Volume

illustrate how the various analytical tools and material behavior models can be used in cooperation to design complex

forging processes. The first article, "Forging Process Design," summarizes:

• The controllable factors in the forging process

• The various tasks performed in the manufacture of forgings a

nd the software available to support task

automation

• The required data base of information required to support the software

• The methodology of forging process design

The next article, "Modeling Techniques Used in Forging Process Design," reviews:

• The concepts involved in material modeling and the approaches used to understand the fund

amentals of

flow, fracture, and workability

•

The methods available for the analytical modeling of the forging process and the application of these

methods for various forging operations

• Physical modeling through simulation using model materials (for examp

le, Plasticine, wax, and

polymers) or through laboratory tests

Finally, the third article, "Acquisition of Data for Forging Process Design," discusses the use of compression testing for:

• Generating flow curves

• Determining workability parameters

• Developing and interpreting processing maps

• Obtaining interfacial frictional data

In addition to the articles mentioned above in this Section, supplementary information on computer-aided forging process

design can be found in the articles "Open-Die Forging," "Closed-Die Forging," and "Introduction to Workability" in this

Volume.

Forging Process Design

H.L. Gegel and J.C. Malas, Air Force Wright Aeronautical Laboratories/Materials Laboratory; S.M. Doraivelu and V.A. Shende,

Universal Energy Systems, Knowledge Integration Center

Introduction

THE TRADITIONAL BUILD-AND-TEST METHODS of developing a complete manufacturing process were heavily

experience based, because the analytical approach to process design was inadequately developed. The part geometries are

generally quite complex, and the simpler analytical methods (as opposed to the computer-aided numerical methods) could

not handle the task of providing accurate solutions to engineering problems. The advent of numerical methods such as

general-purpose finite-element method (FEM) codes to handle large deformation plasticity has made accurate analysis

possible, but these techniques still have several limitations. FEM codes such as ALPID (Ref 1, 2) (developed under

programs funded by the U.S. Air Force) are analysis tools, not design tools, and are quite expensive to use in terms of

required computer time and facilities. Because the potentially viable processing routes are numerous, many FEM process

simulations would be necessary to identify optimal processing methods unless some means of restricting the solution

space are implemented.

The problem of large design solution spaces is not restricted to the design of the deformation process, but also applies to

the selection of correct combinations of processing temperatures and strain rates and to the identification of optimal heat

treatment techniques.

In the design and manufacture of forging dies, two methodologies can be used. In the first method, the design of the die is

obtained by making use of a computer-aided engineering (CAE) systems approach. This approach integrates dynamic

materials behavior modeling, geometric modeling, and analytical process modeling (Ref 3). Advanced process modeling

is the heart of this approach, and the understanding of dynamic materials behavior in a quantitative way for all materials--

composites, metals, alloys, polymers, and ceramics--makes the integration possible and generic.

In the second method, well-established empirical rules are used to arrive at an acceptable design alternative. Currently,

both of these procedures follow a trial-and-error approach to achieve the required goal. Either one used by itself is a time-

consuming and costly process. The CAE procedure lacks empirical aids during the selection of an initial design, and the

experience-based method lacks analytical aids for verification of the final design. The optimum die design procedure

would be to use both of these methods.

An initial design can be selected on the basis of empirical rules; such a design can be further refined by computation

involving analysis and simulation as followed in the CAE approach. This procedure is complex because it involves two

fundamental types of activities, information handling and problem solving. This can be made easier if an intelligent

system that makes use of the techniques is available from software engineering (SE), data base management systems

(DBMS), operating systems (OS), analytical modeling (AM), or artificial intelligence (AI).

To summarize, automation of engineering methodology in forging design and manufacture should address the following

basic issues:

•

Engineering disciplines of significant diversity are involved and need to be integrated in a uniform

fashion

• The potential design solution space is large and needs to be intelligently reduced so that the design-

analysis iterations remain practical

The global approach to automation is conceptually straightforward, and is suitable for incremental implementation as

newer software tools become available. The automation of the initial-guess design is done through the use of knowledge-

based expert systems (KBES); when possible, a rough analysis (for example, using slab analysis for plasticity) is provided

as a pre-screen to detail analysis (usually FEM), and the detail analysis is used primarily as a verification tool. It is useful

at this point to note that the solution space is reducible through two means:

• The use of KBES rough analysis-

detail analysis iterations can provide a local reduction of admissible

solutions, that is, in a specific domain such as plasticity, heat transfer, or material engineering

• The use of a knowledge-

based integration shell provides a more global reduction of the solution space

by implementing a methodology that controls design procedures across interdisciplinar

y domains; this

approach embeds procedural knowledge in the integration shell, and also provides a uniform user

interface

This article provides a list of controllable factors, various tasks performed in the manufacture of forgings, software tools

available to support task automation, and data bases of information required to support software tools. Forging process

design tasks are then outlined to illustrate how the various tools and data are used in the method.

References

S. Kobayashi, "Thermovisioplastic Analysis of Titanium Alloy Forging," AFWAL-TR-81-

4130, Air Force

Wright Aeronautical Laboratories, 1981

S.-I. Oh, "Finite Element Analysis of Metal Forming Processes with Arbitrarily-Shaped Dies,"

Int. J. Mech.

Sci., Vol 24, 1982, p 479

J.S. Gunasekera, H.L. Gegel, J.C. Malas, S.M. Doraivelu, and G. Griffin, Computer-

Aided Engineering

Approach to Metal Forming, American Society of Mechanical Engineering, 1982, p 1-6

Forging Process Design

H.L. Gegel and J.C. Malas, Air Force Wright Aeronautical Laboratories/Materials Laboratory; S.M. Doraivelu and V.A. Shende,

Universal Energy Systems, Knowledge Integration Center

Controllable Factors In Forging Process Development

As a first step towards identifying the procedures, it is useful to define the features of the product or process that are

controllable, and by whom. Table 1 shows applicable product specification, process design, and die design related factors.

This basic inspection can then be used to guide the development of the task hierarchy, or method.

Table 1 Applicable product specification, process design, and die design related factors

Controllable factor Controlled by

Product specification related factors

Final component geometry Customer/end-user

Sonic outline Customer/end-user