Chapter 17

Question 1

Plasticity?

Answer 1

The ability to flow as solids without deterioration of their properties.

Question 2

Deformation processes

Answer 2

All processing done in solid state. Required forces are often high.

Question 3

deformation may be

Answer 3

Bull flow, simple shearing, or compound bending

Question 4

Independent variables

Answer 4

Aspects of a process over which the engineer or operator has direct control

Question 5

Some independent variables

Answer 5

Starting material, starting geometry, tool or die geometry, lubrications, starting temp, speed of operation, amount of deformation

Question 6

Dependent variables

Answer 6

Consequences of the independent variable selection.

Question 7

Some dependent variables

Answer 7

Force or power requirements, material properties of the product, exit or final temp, surface finish and dimensional precision, nature of the material flow, anisotrope and residual stresses.

Question 8

Gaining information on the independent dependent relationship, using three ways:

Answer 8

1) experience: requires longtime exposure to a process. 2) experiment: can be time consuming and costly. Size and speed of deformation are often reduced. 3) Process modeling: high speed computing and mathematical models. Models are used to compute précipitons for dependent outcomes.

Question 9

Increase in temperature brings about

Answer 9

A decrease in the rate of strain hardening and strength, with an increase in ductility.

Question 10

Hot working

Answer 10

Recristallisation occurs simultaneously with the deformation

Question 11

Hot working is defined as

Answer 11

The plastic deformation of metals at a temperature above the recristallisation temperature

Question 12

Hot working positive characteristics

Answer 12

Does not require excessively high forces, elevated temps promote diffusion, pores can be welded shut, and the recrystallization improves the final properties.

Question 13

Hot working negative characteristics

Answer 13

High temps may promote undesirable reactions, tolerances are poorer distortion can occur from no uniform cooling.

Question 14

Minimum temperature for hot working

Answer 14

Recrystallization determines this

Question 15

If a cast metal is reheat with a sufficient amount of prior deformation it will produce strain free grains

Answer 15

This recrystallization is follow by 1) grain growth, 2) additional deformation and recrystallization, or 3) a drop in temperature that will terminate diffusion and feee the recristallises structure. By replacing the initial structure with a new one, it is possible to rodoirs and increase in strength and ductility.

Question 16

Stringers

Answer 16

Impurities that flow along with the base metal or fracture into rows or fragments that are aligned in the direction of working. These impurities do not recristallise and they retain their distorted shape

Question 17

Heat loss in hot working

Answer 17

Heat is lost through the workpiece surfaces, with a majority of the loss occurring where the workpiece is in contact with lower temperature tooling.

Question 18

Residual stress

Answer 18

Nonuniform cooling from hot working. Associated with this is warping distortion and cracking

Question 19

Cold working

Answer 19

Plastic deformation of metals below the recristallisation temp

Question 20

Advantages of cold working

Answer 20

1) no heating required. 2) better surface finish. 3) superior dimensional control because the tooling sets dimensions at room temp. 4)products possess better reproducibility and interchangeability. 5) strength, fatigue, and wear properties are all improved through statin hardening. 6) Directional properties can be imparted. 7) contamination problems are minimized.

Question 21

Disadvantages of cold working:

Answer 21

1) higher forces required. 2) heavier and more powerful equipment required. 3) less ductility is available. 4) metal surfaces must be clean and scale free. 5) intermediate anneals may be required to compensate for the loss of ductility. 6) imparted directional properties may be detrimental. 7) undesirable residual stress may be produced.

Question 22

Metal properties of cold working

Answer 22

Cold working alters the structure of the metal, thus changing its mechanical properties.

Question 23

Spring back

Answer 23

A cold working phenomenon. When a metal is deformed by the application of a load, part of the deformation is elastic. When the load is removed, it will return its original size and shape.

Question 24

Elastic spring back

Answer 24

The decrease in strain

Question 25

Cold working properties,

Answer 25

Material must be clean, and scale can be removed by pickling, a process in which the metal is dipped in acid and then washed. A light cold rolling may be used to ensure uniform starting thickness and a smooth surface.

Question 26

Yield point phenomenon

Answer 26

Can be removed by a light cold rolling pass

Question 27

Yield point runout

Answer 27

After loading to the upper yield point, the material exhibits this, wherein the material can statin up to several percent without any additional force being required.

Question 28

Annealing heat treatment

Answer 28

To maximize starting ductility, this treatment is applied to a metal prior to cold working.

Question 29

Intermediate anneals

Answer 29

If the required amount of deformation exceeds the fracture limit, this may be necessary to restore ductility.

Question 30

Anisotropy of cold working

Answer 30

Related to the distorted crystal structure

Question 31

Residual stresses

Answer 31

Associated with cold working. They can be quite harmful.