Chapter 12 - Fundamentals of Metal Forming

Question 1

What are the differences between bulk deformation processes and sheet metal processes?

Answer 1

In bulk deformation, the shape changes are significant, and the workparts have a low area to volume ratio. In sheet metal processes, the area to volume ratio is high.

Question 2

Extrusion is a fundamental shaping process. Describe it.

Answer 2

Extrusion is a compression process in which the work material is forced to flow through a die orifice, thereby forcing its cross section to assume the profile of the orifice.

Question 3

Why is the term pressworking often used for sheet metal processes?

Answer 3

Because most sheet metal operations are performed on presses.

Question 4

What is the difference between deep drawing and bar drawing?

Answer 4

Deep drawing is a sheet metal forming process used to fabricated cup-shaped parts; bar drawing is a bulk deformation process used to reduce the diameter of a cylindrical workpart.

Question 5

Indicate the mathematical equation for the flow curve.

Answer 5

The flow curve is defined as:

Y = Ke^n

Question 6

How does increasing temperature affect the parameters in the flow curve equation?

Answer 6

Increases both K and n.

Question 7

Indicate some of the advantages of cold working relative to warm and hot working.

Answer 7

1) Better accuracy
2) Better surface finish
3) Increased strength due to work hardening
4) Possible directional properties due to grain flow
5) No heating of work required

Question 8

What is isothermal forming?

Answer 8

An isothermal forming operation is performed in such a way as to eliminate surface cooling and thermal gradients in the workpart. This is accomplished by preheating the forming tools.

Question 9

Describe the effect of strain rate in metal forming.

Answer 9

Increasing strain rate tends to increase the resistance to deformation. The tendency is especially prominent in hot forming operations.

Question 10

Why is friction generally undesirable in metal forming operations?

Answer 10

1) It inhibits metal flow during deformation, causing residual stresses and product defects.
2) It increases forces and power required
3) It increases wearing of the tools

Question 11

What is sticking friction in metalworking?

Answer 11

When the work surface adheres to the surface of the tool rather than slides against it; it occurs when the friction stress is greater than the shear flow stress of the metal.

Question 12

Which of the following are bulk deformation processes:

a) Bending
b) Deep drawing
c) Extrusion
d) Forging
e) Rolling
f) Shearing

Answer 12

c, d, e

Question 13

Which of the following is typical of the starting work geometry in sheet metal processes:

a) High volume to area ratio
b) Low volume to area ratio

Answer 13

b

Question 14

The flow curve expresses the behaviour of a metal in which of the following regions of the stress -strain curve:

a) Elastic region
b) Plastic region

Answer 14

b

Question 15

The average flow stress is the flow stress multiplied by what?

Answer 15

1/(1+n)

Question 16

Hot working of metals refers to which one of the following temperature regions relative to the melting point of the given metal on an absolute temperature scale:

a) Room temperature
b) 0.2Tm
c) 0.4Tm
d) 0.6Tm

Answer 16

d

Question 17

Which of the following are advantages and characteristics of hot working relative to cold working:

a) Fracture of workpart is less likely
b) Friction is reduced
c) Increased strength properties
d) Isotropic mechanical properties
e) Less overall energy is required
f) Lower deformation forces is required
g) More significant shape changes are possible
h) Strain rate sensitivity is reduced

Answer 17

a, d, f, g

Question 18

Increasing strain rate tends to have which one of the following effects on flow stress during hot forming of metal:

a) Decreases flow stress
b) Has no effect
c) Increases flow stress

Answer 18

c

Question 19

The coefficient of friction between the part and the tool in cold working tends to be 
a) higher
b) lower
c) no different
relative to its value in hot working?

Answer 19

b