Chapter 18 Metal Forming

Question 1

plastic region flow curve equation

Answer 1

[sigma = stress

K = strength coefficient

epsilon = strain

n = strain hardening exponent]

Question 2

flow stress (def.)

Answer 2

the instantaneous value of stress required to continue deforming the material— to keep the metal ‘‘flowing.’’

Question 3

flow stress (eq)

Answer 3

[Y_f = flow stress [MPa}

K = strength coefficient

epsilon = strain

n = strain hardening exponent]

Question 4

avg flow stress

[mean flow stress}

eq’n

Answer 4

K = strength coefficient

epsilon = maximum strain

n = strain hardening exponent

Question 5

benefits of cold working

Answer 5

(1) greater accuracy,
(2) better surface finish;
(3) higher strength and hardness of the part due to strain hardening;
(4) grain flow during deformation provides the opportunity for desirable directional properties
(5) no heating of the work is required,

Question 6

cons of cold working

Answer 6

(1) higher forces and power are required
(2) part must be clean from scale and dirt
(3) ductility and strain hardening of the work metal limit the amount of forming that can be done

in some operations, the metal must be annealed in order to allow further deformation to be accomplished. In other cases, the metal is simply not ductile enough to be cold worked.

Question 7

warm working temp

Answer 7

usually 0.3 Tm [melting Temp] is warm working

Question 8

warm working benefits

Answer 8

(1) lower forces and power,
(2) more intricate work geometries possible,
(3) need for annealing may be reduced or eliminated.

Question 9

hot working temp

Answer 9

The recrystallization temperature usually about 0.5Tm

Hot working takes place usually between 0.5Tm and 0.75Tm

Question 10

hot working benefits

Answer 10

(1) significantly alteration possible
(2) lower forces and power are required to deform the metal,
(3) metals that usually fracture in cold working can be hot formed,
(4) strength properties are generally isotropic because of the absence of the oriented grain structure typically created in cold working,
(5) no strengthening of the part occurs from work hardening.

Question 11

hot working drawbacks

Answer 11

(1) lower dimensional accuracy,
(2) higher total energy required (due to the thermal energy to heat the workpiece),
(3) work surface oxidation (scale),
(4) poorer surface finish,
(5) shorter tool life.

Question 12

isothermal forming

Answer 12

heating the tools and the part to the same temperature to avoid cooling of the workpiece

Question 13

true strain rate

eq’n

Answer 13

[epsilon dot = true strain rate [m/s/m or s^-1]

v = velocity

h = instantaneous height of workpiece being deformed]

Question 14

strain rate sensitivity

def

eq’n

Answer 14

The effect of strain rate on strength properties

[Yf = flow stress

C = strength coefficient in flow curve eq’n

m = strain rate sensitivity exponent

epsilon dot = true strain rate]

Note: if temperate goes up, m goes up and C goes down. It is a ballon tugging on the exponent.