Industrial Engineering Chapter 6 Thus Low Die Wear Rates Can Achieved The The Flash Encourages Filling

Chapter 6

Bulk Deformation Processes

Questions

Forging

6.1 How can you tell whether a certain part is

forged or cast? Describe the features that you

than forged parts. Surface characteristics and

portant in closed-die forging?

6.3 What are the advantages and limitations of a

cogging operation? Of die inserts in forging?

6.4 Explain why there are so many different kinds

of forging machines available.

volved in equipment selection may be summa-

This material is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited

reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or

likewise. For information regarding permission(s), write to:

Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

6.5 Devise an experimental method whereby you

can measure the force required for forging

only the flash in impression-die forging. (See

Fig. 6.15a.)

An experimental method to determine the

forces required to forge only the flash (for an

part and the flash, respectively, can be mea-

6.6 A manufacturer is successfully hot forging a cer-

tain part, using material supplied by Company

A. A new supply of material is obtained from

Company B, with the same nominal composi-

tion of the major alloying elements as that of

the material from Company A. However, it is

found that the new forgings are cracking even

though the same procedure is followed as be-

regarding minor elements or impurity levels.

6.7 Explain why there might be a change in the

density of a forged product as compared to that

of the cast blank.

decrease after forging due to void formation.

6.8 Since glass is a good lubricant for hot extrusion,

would you use glass for impression-die forging

as well? Explain.

Glass, in various forms, is used for hot forging

operations. However, in impression-die forging,

6.9 Describe and explain the factors that influence

workpiece interface in cogging indicates that

the factors that influence spreading are:

(a) Friction: the lower the friction, the more

the spreading because of reduced lateral

resistance to material flow.

(b) Width-to-thickness ratio of the workpiece:

the higher this ratio, the lower the spread-

6.10 Why are end grains generally undesirable in

forged products? Give examples of such prod-

ucts.

As discussed in Section 6.2.5 starting on

This material is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited

reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or

likewise. For information regarding permission(s), write to:

Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

6.11 Explain why one cannot produce a finished

forging in one press stroke, starting with a

blank.

Forgings are typically produced through a series

termediate annealing, thus allowing less

ductile materials to be forged to compli-

cated shapes.

it can be seen that each operation will

6.12 List the advantages and disadvantages of using

a lubricant in forging operations.

The advantages include:

from the environment, especially in hot

forging, and also act as a parting agent.

(d) the lubricant must subsequently be re-

moved from the part surface, an additional

and difficult operation;

(e) disposal of the lubricant can present envi-

cool dies. Consequently, the flash has higher

strength than the hotter workpiece in the die

6.14 By inspecting some forged products (such as

a pipe wrench or coins), you can see that the

By the student. It is much easier and economi-

cal to machine cavities in a die (thus producing

6.15 It was stated that three factors that influ-

ence spreading in rolling are (a) the width-to-

This material is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited

reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or

likewise. For information regarding permission(s), write to:

Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

6.16 Explain how you would go about applying

front and back tensions to sheet metals during

rolling.

the braking torque on the pay-off reel or reduc-

forces. Which property(ies) of the roll material

can be increased to reduce flattening? Why?

6.18 Describe the methods by which roll flattening

can be reduced.

Roll flattening can be reduced by:

6.19 Explain the technical and economic reasons for

taking larger rather than smaller reductions per

pass in flat rolling.

6.20 List and explain the methods that can be used

to reduce the roll force.

(a) using smaller-diameter rolls,

(b) taking lower reduction per pass,

6.21 Explain the advantages and limitations of using

in flat rolling are the following:

(e) the smaller diameter rolls are less costly

and easier to replace and maintain.

The disadvantages include:

cessfully for the production of bearing races.

However, when the bearing race diameter is

supply is not reduced with a larger race size.

This material is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited

reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or

likewise. For information regarding permission(s), write to:

Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

6.23 Describe the importance of controlling roll

Control of tandem rolling is especially impor-

tant because the conditions at a particular

Consider that it is possible to apply a large

6.25 In addition to rolling, the thickness of plates

and sheets can also be reduced by simply

stretching. Would this process be feasible for

high-volume production? Explain.

feasible process, there are several significant

limitations associated with it, as compared to

rolling:

strain-hardening exponent, n.

(c) As the sheet is stretched, the surface finish

(e) There would be major difficulties in-

6.26 In Fig. 6.33, explain why the neutral point

moves towards the roll-gap entry as friction in-

uct in flat rolling is not crowned?

deformation of the work rolls.

(c) Apply a corrective moment to the shafts

of the work rolls.

6.28 List the possible consequences of rolling at (a)

too high of a speed and (b) too low of a speed.

lead to a condition where the rolls slip

against the workpiece. This can lead to a

This material is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited

reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or

likewise. For information regarding permission(s), write to:

Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

ish.

6.29 Rolling may be described as a continuous forg-

ing operation. Is this description appropriate?

6.30 Referring to appropriate equations, explain why

titanium carbide is used as the work roll in

Sendzimir mills, but not generally in other

rolling mill configurations.

The main reason that titanium carbide is used

6.31 It was stated that the extrusion ratio, die ge-

6.32 How would you go about preventing centerburst

defects in extrusion? Explain why your meth-

die angle and extrusion ratio. These defects can

6.33 How would you go about making a stepped

extrusion that has increasingly larger cross-

sections along its length? Is it possible? Would

your process be economical and suitable for

high production runs? Explain.

For shorter pieces, it is possible to make a die

This material is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited

reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or

likewise. For information regarding permission(s), write to:

Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

6.34 Note from Eq. (6.54) that, for low values of the

extrusion ratio, such as R= 2, the ideal ex-

principle and is correct. Note that the extru-

sion pressure, p, acts on the (undeformed) billet

area. Consequently, it is not necessary that its

6.35 In hydrostatic extrusion, complex seals are used

between the ram and the container, but not be-

tween the extrusion and the die. Explain why.

occur in (a) extrusion and (b) drawing.

tion 6.4.4 starting on p. 318. Examples include

poor surface finish or surface cracking (such

6.37 What is a land in a die? What is its function?

What are the advantages and disadvantages to

having no land?

not affect dimensions, since die wear mainly oc-

curs on the inlet side of the die. The disadvan-

6.38 Under what circumstances is backwards extru-

sion preferable to direct extrusion? When is

hydrostatic extrusion preferable to direct extru-

it is obvious that the main difference is that

in backward extrusion the billet is stationary,

and in direct extrusion it is moving relative to

the container walls. The main advantage be-

rect extrusion (see Fig. 6.47a)? Why is there no

container liner used in hydrostatic extrusion?

sive to replace a liner than to replace the entire

container. In hydrostatic extrusion, the billet

6.40 We have seen that in rod and wire drawing, the

maximum die pressure is at the die entry. Why?

The reason is that at the die entry, the state

This material is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited

reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or

likewise. For information regarding permission(s), write to:

Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

6.41 Describe the conditions under which wet draw-

of wire that can be dipped fully in the lubri-

6.42 Name the important process variables in draw-

p. 320. The important variables include:

•Yield stress, Y; it directly affects the draw

for encouraging lubricant entrainment.

•Friction coefficient, µ. The friction coef-

figuration. Also note that in a tandem opera-

tion, the front tension of one segment becomes

6.44 Refer to Fig. 6.60 and assume that reduction in

lowing situations: (a) frictionless, (b) with fric-

for drawing and extrusion predict the draw

stress or extrusion pressure, but do not show

p. 321. However, a qualitative sketch of the die

pressure can be generated based on the physi-

cal understanding of the friction hill and asso-

This material is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited

reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or

likewise. For information regarding permission(s), write to:

Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

6.46 Why does the die pressure in drawing decreases

toward the die exit?

6.47 What is the magnitude of the die pressure at

the die exit for a drawing operation that is be-

ing carried out at the maximum reduction per

pass?

pressure in a normal drawing operation, as de-

6.48 Explain why the maximum reduction per pass

in drawing should increase as the strain-

hardening exponent, n, increases.

dergo higher reductions per pass, as can also be

seen in Example 6.8.

6.49 If, in deriving Eq. (6.74), we include friction,

will the maximum reduction per pass be the

same (that is, 63%), higher, or lower? Explain.

The effect of back pressure is similar to that of

back tension in rolling (see Figs. 6.35 on p. 295

and 6.62 on p. 322), namely, the pressure drops.

By observing Figs. 6.12 on p. 276 and 6.13b

on p. 277, we note that the higher the h/L ra-

tio, the more nonuniform the deformation of

the material. For example, keeping hconstant

6.52 Describe the reasons for the development of the

swaging process.

(b) capacity to produce internal profiles on

long workpieces,

(c) compact equipment,

place within a sheath of a soft metal, such as

copper or lead. Why would this procedure be

effective?

The main reason that steel wire drawing takes

place in a sheath of a softer metal is to reduce

This material is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited

reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or

likewise. For information regarding permission(s), write to:

Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

6.54 Recognizing that it is very difficult to manufac-

ture a die with a submillimeter diameter, how

would you produce a 10 µm-diameter wire?

otherwise, the wires will weld together during

6.55 What changes would you expect in the strength,

dies? Why?

6.56 With respect to the topics covered in this chap-

The student is encouraged to provide several

specific examples. For example, friction is de-

6.57 Choose any three topics from Chapter 2 and

with a specific example for each, show their rel-

evance to the topics covered in this chapter.

6.58 Same as Question 6.57 but for Chapter 3.

and apply it to a discussion of cold versus hot

6.59 List and explain the reasons that there are so

The student should elaborate further with spe-

6.61 Make a summary of the types of defects found

in the processes described in this chapter. For

This material is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited

reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or

likewise. For information regarding permission(s), write to:

Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

p. 269. The equivalent version of Fig. 6.4 on

p. 269 is shown below.

dx

x

y

y

Using the stresses as shown in part (b), we have,

from equilibrium and assuming unit width,

the text it is compressive. Therefore, Eq. (6.11)

becomes

σy+σy=2

√3Y=Y′

σy=Ce−2µx/h

Using the boundary conditions that σx= 0

value of Cas

eter, up to a reduction of 70%, for the cases

of (a) no friction between the flat dies and the

specimen, (b) µ= 0.25, and (c) µ= 0.5. Ignore

barreling and use average-pressure formulas.

For annealed copper we have, from Table 2.3 on

h

From volume constancy, we have

π

oho=π

p. 272 as:

F=Yf1 + 2µr

3hπr2

Some of the points on the curves are the follow-

10 11.9 12.4 13.1

30 29.6 31.7 33.8

50 59.3 66.3 73.6

60 86.1 100. 114.

This material is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited

reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or

likewise. For information regarding permission(s), write to:

Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

6.63.

The force required for forging is the product

of the average pressure and the instantaneous

cross-sectional area. The average pressure is

obtained from Fig. 6.9b on p. 272. Note that

1.6 or so by 2r/h = 3. This is the basis for the

numbers below.

50 1.41 1.17 1.4

70 3.04 1.25 1.6

70 0.913 166. 213.

The results are plotted below. For comparison

purposes, the results from Problem 6.63 are also

included as dashed lines. As can be seen, the

results are fairly close, even with the rough in-

lem 6.63.

where

r=sr2

oho

This material is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited

reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or

likewise. For information regarding permission(s), write to:

Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

0.25 71,065

6.66 Determine the temperature rise in the specimen

sticking friction.

a Tresca friction model is used with m= 1

This material is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited

reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or

likewise. For information regarding permission(s), write to:

Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

σy−mY ′

Fig. 6.10 on p. 273 for 0 ≤x≤a. Since the

relationship is linear, then we can note that

¯σy=Y′1 + a

6.69 What is the magnitude of µwhen, for plane-

strain compression, the forging load with slid-

ing friction is equal to the load with sticking

friction? Use average-pressure formulas.

Problem 6.68 using m= 1. Equating these two

average pressures, we obtain

6.70 Note that in cylindrical upsetting, the frictional

stress cannot be greater than the shear yield

which, for the cylindrical state of stress, is Y

2.

Thus, in the limit, we have the condition

h= ln 1

2µ

Hence,

6.71 Assume that the workpiece shown in the accom-

flat dies. (a) Make a sketch of the die-pressure

distribution for the condition for which Fis not

large enough to slide the workpiece to the right.

(b) Make a similar sketch, except that Fis now

F

Applying a compressive force to the left bound-

This material is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited

reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or

likewise. For information regarding permission(s), write to:

Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

Y’

F

p

6.72 For the sticking example given in Fig. 6.10, de-

rive an expression for the lateral force Fre-

quired to slide the workpiece to the right while

the workpiece is being compressed between flat

6.73 Two solid cylindrical specimens, Aand B, both

made of a perfectly-plastic material, are being

forged with friction and isothermally at room

temperature to a reduction in height of 25%.

Originally, specimen Ahas a height of 2 in. and

a cross-sectional area of 1 in2, and specimen B

has a height of is 1 in. and a cross-sectional area

of 2 in2. Will the work done be the same for

the two specimens? Explain.

quire higher work because it has a larger die-

workpiece surface area, hence a higher fric-

tional resistance as compared to specimen A.

We can prove this analytically by the following

and height as

r=rV

πh

Hence,

pave =Y1 + 2µ

3V

π 1

h3/2

Because it consists of constants, let c=

(2µ/3)(V/π), which results in the following ex-

pression:

F=Y1 + c

WA=Y V −0.288 −c1

23/2(0.809)

=Y V (−0.288 −0.286c)

WB=Y V −0.288 −c1

13/2(0.809)

=Y V (−0.288 −0.809c)

6.74 In Fig. 6.6, does the pressure distribution along

the four edges of the workpiece depend on the

particular yield criterion used? Explain.

This material is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited

reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or

likewise. For information regarding permission(s), write to:

Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

6.75 Under what conditions would you have a nor-

mal pressure distribution in forging a solid

cylindrical workpiece as shown in the accom-

panying figure? Explain.

The pressure distribution is similar to the fric-

tion hill shown in Fig. 6.5 on p. 270, with the

6.76 Derive the average die-pressure formula given

by Eq. (6.15). (Hint: Obtain the volume under

the friction hill over the surface by integration,

expansion of the exponent term,

em= 1 + m+m2

2! +m3

3! +. . .

6.77 Take two solid cylindrical specimens of equal

diameter but different heights, and compress

hto

hso

and from volume constancy,

This material is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited

reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or

likewise. For information regarding permission(s), write to:

Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

6.78 A rectangular workpiece has the following orig-

inal dimensions: 2a= 100 mm, h= 30 mm and

average-pressure formulas.

h= (1 −0.2)(30) = 24 mm = 0.024 m

Since volume constancy has to be maintained

Thus, a= 62.5 mm = 0.0625 m. The absolute

value of the true strain is

(1.15)(255) = 293 MPa. Thus, from Eq. (6.13)

on p. 270 the pressure as a function of distance

= (293 MPa)e1.042−16.7x

x= 0.0625, which gives the force per unit width

given by Eq. (6.16) on p. 271, the answer will

be

= 1.11 MN

The discrepancy is due to the fact that in de-

6.79 Assume that in upsetting a solid cylindrical

specimen between two flat dies with friction,

the dies are rotated at opposite directions to

each other. How, if at all, will the forging force

change from that for nonrotating dies? (Hint:

Fig. 6.8b on p. 272, we first note that the fric-

tional stresses at the die-specimen interfaces

additional work has to be done in supplying

6.80 A solid cylindrical specimen, made of a per-

hammer is at a maximum when it first con-

This material is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited

reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or

likewise. For information regarding permission(s), write to:

Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.