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Fundamentals of Metal Forming. Aluminum extrudate – a simple angled shape. The die shown is more complex. This group of machines - which are over a half mile long - roll the slab into sheet steel using tremendous pressure. Fundamentals of Metal Forming

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slide1

Fundamentals

of

Metal Forming

slide2

Aluminum extrudate – a simple angled shape. The die shown is more complex.

This group of machines - which are over a half mile long - roll the slab into sheet steel using tremendous pressure.

Fundamentals of Metal Forming

Bulk deformation vs Sheetmetal working

Bulk deformation * Low surface area to volume ratios * Operations increase surface area to volume ratios

Operations are: rolling extrusion forging drawing

slide3

Fundamentals of Metal FormingBulk deformation

  • Rolling characteristics
  • * compressive deformation * 2 or more rolls * strong frictional relationship
  • Extrusion characteristics
  • * compressive deformation * force metal through die opening * strong frictional relationship * hot and cold working process
slide4

Fundamentals of Metal FormingSheet metalworking

Characterized by: * large surface area to volume ratios * little ratio change after process * machines called presses * parts called stampings * tools are punch and die

Operations:

bending drawing shearing

slide5

Material Behavior – Flow Stress

  • = K en

Concept offlow stress,Yf – the instantaneous stress required to deform the material

Yf = K en

Process force considerations:

* during compression, determine instantaneous force from Yf * max force is often all that is required, typically at end of stroke * force analysis may be based on average stresses and strains

Averageflow stress = Yf = K en /(1 + n)

slide6

Fundamentals of Metal FormingTemperature and Metal Forming

K and n depend on working temperature, characterized by 3 ranges:

Cold working

Warm working

Hot working

slide7

Fundamentals of Metal FormingTemperature and Metal Forming –cold working

Advantages:

* good accuracy and tolerances

* better surface finish

* strain hardening increases strength and hardness

* directional properties

* energy economy

Disadvantages:

* higher forces/power

* strain hardening limits deformation

* may need to anneal

slide8

Fundamentals of Metal FormingTemperature and Metal Forming –warm working

Advantages:

* above room temp., but below re-crystallization temp. (0.3 Tm)

* lower forces and power

* reduced strain hardening

* more difficult geometry

* no need for annealing

Disadvantages:

* more energy

* limited geometry

slide9

Fundamentals of Metal FormingTemperature and Metal Forming –hot working

Advantages:

* above re-crystallization temp. (0.5 Tm < T < 0.75 Tm)

* lower forces and power

* no strain hardening

* difficult geometry

* isotropic properties

Disadvantages:

* more energy

* poorer surface finish

* shorter tool life

slide10

Room temp.

Flow stress

400º C

800º C

1200º C

0.1 1.0 10 102 103 104

Strain rate

Yf = C em

Fundamentals of Metal Forming - Strain Rate

Note: Strain rate a strong function of working temperature

(high values -> 1000 s-1)

where

v = speed of testing head

h = instantaneous height of part being worked

Strain rate equation:

C = strength constant

m = strain-rate coefficient

We will mostly assume that strain rate is negligible at room temperature!

slide11

Friction and lubrication

Friction:

* undesirable

* retards metal flow (residual stresses & defects)

* increases force and power requirements

* wears tooling

* high temperature stiction

Lubricants:

* reduce frictional effects

* remove heat and material

* lengthen tool life

* may react chemically with tool or work

* concerns about toxicity, flammability….(mineral oils, emulsions, oils, graphite, glass..)

Lubricants are usually oil-based, and when used in extreme pressure situations, sulfur, chlorine and phosphorus in the lubricant may chemically react with the metal surfaces (tool, chip). The reactions form a surface boundary layer that is more effective than the lubricant itself in reducing friction. Lubricants are most effective at cutting speeds < 400 ft/min.

slide12

Example 20.6

For a metal with K = 50,000 lb/in2 and n = 0.27, determine the average flow stress that the metal experiences if it is subjected to a stress that is equal to its strength coefficient K.

Solution:

Yf = K = 50,000 = 50,000 en

Thus e = 1

Yf = 50,000 (1) 0.27/1.27 = 39,370 lb/in2