Chapter 7: Metal Forming

1
Chapter 7 Metal Forming

• Learning objectives
• Understand the basic metal forming processes,
  including forging and sheet metal stamping
• Understand how the metal forming process changes
  the shape and the material properties of the
  metal
• Recognize different metal forming machines
  (presses), including mechanical press and
  hydraulic press
• Recognize the dies for sheet metal forming, their
  design and constructions

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• The aspects of metal forming
• The material
• The machine (press)
• The tool (dies)
• The operation (process)

Material
Process
Product
Press
Die
3

• Types of metal forming processes
• Bulk deforming
• Cold forging
• Hot forging
• Rolling
• Sheet metal forming
• Extrusion
• Drawing

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Forging

• Types of forging
• Open die forging and closed die forging
• Hot forging and cold forging
• Materials
• Commonly used materials include
• Ferrous materials low carbon steels
• Nonferrous materials copper, aluminum and their
  alloys

5

• Hot forming
• Hot forging is used to for large bulk steels
• The temperature effect is significant
• Finding the right temperature range using phase
  diagram

oC
6

• The forming process affects the grain size
• Finer grain have higher yield strength and
  toughness and lower internal strains and stresses
• The process of forging and grain size

punching
Heating
7

• Cold forming
• Cold forming is used for near shape manufacturing
• The micro-process of cold forming
• Reorientation (deformation ratio 3)
• Single crystal slip
• Polycrystalline deformation
• Crystal elongation (deformation ratio 8 10)
• Crystallographic fibrous structure

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• Open-die forging
• Multi-step open-die forging
• The stress varies continuously

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• Open-die forging (continue)
• Friction causes the barreling effect
• The directional flow of the material in forging

Folding
10

• The facture
• Under excessive force, the barreling will become
  fracturing
• Improved lubrication can reduce fracture

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• Closed-die forging
• The workpiece resemble the shape of the die
• Anatomy of a die set gutter and flash
• Parting line

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• Closed-die forging (continue)
• An application example

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• The Presses
• There are many kinds of machines
• Hydraulic presses
• Mechanical presses
• C frame
• Straight sided
• Others

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• Mechanical presses (continue)

Straight sided mechanical press
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• A simple example (simple crank slider
  mechanism)
• The structure
• The mathematical model

u
f
r
l-r
l
lr
x
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• The basic kinematics equation
• x lcosa rcosf (l r)
• u rsinf lsina
• Hence,

u
f
r
l-r
l
a
Set f wt, we can then get the trajectory of the
punch (also called the ram) The velocity
lr
x
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• A simulation result
• Do it yourself example For r 1, l 3, w 1,
  find the travel and the velocity of the ram

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• Another example the crank-and-toggle
• The configuration and the simulation result
• A comparison to the crank-slide mechanism

Crank-slider
Crank-toggle
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• A study of the forming energy of the press
• The sources and sinks of energy

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• Computing the energy source
• Energy arising from a hydraulic press
• The volume and pressure relation inside the
  cylinder
• pVk C, k 1.4
• The driving energy
• An example l0 0.3 m, l1 1.2 m
• d1 0.25 m, d2 0.18 m, h 0.06 m
• p0 7 x 105 N/m2
• V0 p x 0.252 x 0.3 0.0589 m3
• V1 p x 0.252 x 1.2 0.2945 m3
• Since
• p1 p0(V0/V1)k 7.345 x 105 N/m2

d1
Volume
Pressure
Initial p0
Initial l0
Stroke l1
m
l1
h
d2
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• Hence,
• Position energy of the hammer
• The formula
• Ep mgl1
• The example
• Ep 2500 x 9.81 x 1.2 29430 J
• The resultant energy
• E Ea Ep 78360 J
• The striking velocity from E mv2/2
• v 7.92 m / sec.
• Note that the actual striking force (and the
  striking energy) is dependent on the work and
  will be discussed later

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• Computing the energy source (continue)
• Kinetic energy of the flywheel
• The instantaneous force could be very large.
  However, the force is acted on the punch only for
  small amount of time. Hence, we can use flywheel
  to smooth the demand on the energy supply.
• The computation is rather tedious. So, we only
  show that the flywheel slows down after the punch
• The moment of inertia of a flywheel
• dJ 2prdr?br2 2p?br3dr
• The speed variation
• An example W 560 J, J 14.54 kgm2
• At w1 41.9, w2 40.9 0.98w1

dr
r
R
The flywheel slows down and give rise the energy
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• Computing the energy sinks
• The energy sink owing to the motion of the anvil
  (i.e. the frame)

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• Let
• m1 mass of the hammer
• m2 mass of the anvil
• The work balance before and after the shot
• m1v1 (m1 m2)v2
• Hence,
• The kinetic energy
• Therefore
• An example

For r 25, 4 energy lost
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• The dies
• Open die forging (making a train axel)

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• Closed die forging (making an aircraft landing
  gear)

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• The mechanics - how the parts are formed?
• The metal forming process is essentially a
  controlled plastic deformation process
• The fundamental questions to be answered include
• Determine total forces and energies to assist the
  design of dies and the selection of the required
  forming machines
• Assess whether workpiece deformation without
  failure is feasible
• Determine the pressure, frictions and
  temperatures as they effect the strength and life
  of the parts and the forming tools

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• An example forging of a solid cylindrical
  workpiece
• Assuming material 204 stainless steel, diameter
  150 mm, high 100 mm, reduction 50, ? 0.2
• Then, the final radius can be found from
• ?(75)2(100) ?(r)2(50) ? r 106 mm
• The true strain is
• From the material property data, the
  corresponding stress is 1000 MPa.
• Therefore,