Manufacturing Technology
MEC205
Deformation processes
FUNDAMENTALS OF METAL FORMING
Overview of Metal Forming
•Large group of manufacturing processes in which plastic
deformation is used to change the shape of metal work pieces
•The tool, usually called a die, applies stresses that exceed the yield
strength of the metal
•The metal takes a shape determined by the geometry of the die.
Stresses in Metal Forming
•Stresses to plastically deform the metal are usually compressive
Examples: rolling, forging, extrusion
•However, some forming processes
1. Stretch the metal (tensile stresses)
2. Others bend the metal (tensile and compressive)
3. Still others apply shear stresses
Material Properties in Metal Forming
Desirable material properties:
1. Low yield strength
2. High ductility
These properties are affected by temperature:
•Ductility increases and yield strength decreases when work
temperature is raised
•Other factors:
Strain rate and friction
Forming Processes - Definition & Types
“Forming processes are those in which the shape of a metal piece is changed by
plastic deformation”
•Forming processes are commonly classified into
1. Hot-working
2. Cold-working operations.
Typical forming processes are:
1. Bulk deformation 2. Sheet metalworking
a. Rolling a. Bending
b. Forging b. Deep drawing
c. Extrusion c. Cutting
d. Wire and bar drawing. d. Miscellaneous processes
Applications for Cold Formed Parts
Automotive
brake parts
ball joints & steering parts
starter pinions
oxygen sensors
constant velocity joints
manifold bolts
engine valves
Appliance Industry
gears
fasteners for assembly
Applications for Cold Formed Parts
Construction, Off-road equipment
bolts, nuts
screws – tapping, window, roofing, deck
transmission gears
similar parts for automotive
Aerospace
rivets, fuselage
engine bolts
fasteners - landing gear, interior
Material Behavior in Metal Forming
•Plastic region of stress-strain curve is primary interest because
material is plastically deformed
•In plastic region, metal's behavior is expressed by the flow curve:
n
K
where K = strength coefficient; and n = strain hardening
exponent
Flow curve based on true stress and true strain
Flow stress = instantaneous value of stress required to continue
n
Yf K
deforming the material where Yf = flow stress, that
is, the yield strength as a
function of strain
Average flow stress
Determined by integrating the flow curve equation between zero and
the final strain value defining the range of interest , we get
n
_
K
Yf
1 n
_
where Yf = average flow stress; and = maximum strain during
deformation process
Bulk deformation processes
Four basic bulk deformation processes
Metal forming operations which cause significant shape change by
deforming metal parts whose initial form is bulk rather than sheet
1. ROLLING-Slab or plate is squeezed between opposing rolls
2. FORGING-Work is squeezed and shaped between opposing dies
3. EXTRUSION- Work is squeezed through a die opening, thereby
taking the shape of the opening
4. WIRE AND BAR DRAWING- Diameter of wire or bar is
reduced by pulling it through a die opening
Bulk deformation processes
1. ROLLING-Slab(width 250 mm and thickness 40mm) or plate is
squeezed between opposing rolls
“Deformation process in which work thickness is reduced by
compressive forces exerted by two opposing rolls ”
Rotating rolls perform two main functions:
1. Pull the work into the gap between them by friction between
workpart and rolls
2. Simultaneously squeeze the work to reduce its cross section
Q-1. Rolling is a forming process in which thickness of the metal plate
is decreased by increasing its length.
a) True
b) False
Bulk deformation processes
1. ROLLING-Slab or plate is squeezed between opposing rolls
Types of Rolling
Based on workpiece geometry :
2. Flat rolling - used to reduce thickness of a rectangular cross
section
3. Shape rolling - square cross section is formed into a shape such
as an I‑beam
Based on work temperature :
4. Hot Rolling – most common due to the large amount of
deformation required
5. Cold rolling – produces finished sheet and plate stock
Bulk deformation processes
1. ROLLING-Slab or plate is squeezed between opposing rolls
1. Bloom- is a square
cross section of
150*150mm
2. Slab- width 250 mm
and thickness 40mm
3. Billet-square
dimension of 40mm
Bulk deformation processes
1. ROLLING-
Analysis of flat rolling
ho= Initial thickness
hf= Final thickness
Vr-=velocity of roller
Lp= Contact length
R=radius of roller
Lo=Initial length of wp
Wo= width of wp
Vf-=exit velocity of wp
V0= Entering velocity of wp
N= Neutral point
= angle of bite
Bulk deformation processes
1. ROLLING-
Analysis of flat rolling
Volume flow rate remains
constant
Bulk deformation processes
1. ROLLING-
Analysis of flat rolling
1. Calculation of draft
2. Reduction (r)
r
3. Forward Slip
1. ROLLING-
Analysis of flat rolling
4. Contact length Lp-
Solving and ignoring higher
power of we get
5. True strain calculation
1. ROLLING-
Analysis of flat rolling
6. Average Flow stress
n
_
K
Yf
1 n
7. Maximum draft achieved
8. Roller force required
1. ROLLING-
Analysis of flat rolling
Where w is wp width to be rolled
Lp length of contact
9. Torque on each roll
10. Power required
N- Speed of roller
Rolling Mills
• Equipment is massive and expensive
• Rolling mill configurations:
– Two-high – two opposing rolls
– Three-high – work passes through rolls in both directions
– Four-high – backing rolls support smaller work rolls
– Cluster mill – multiple backing rolls on smaller rolls
– Tandem rolling mill – sequence of two-high mills
Two-High Rolling Mill
Various configurations of rolling mills: (a) 2‑high
rolling mill.
Three-High Rolling Mill
Various configurations of rolling mills: (b) 3‑high
rolling mill.
Four-High Rolling Mill
Various configurations of rolling mills: (c) four‑high
rolling mill.
Cluster Mill
Multiple backing rolls allow even smaller roll diameters
Various configurations of rolling mills: (d) cluster mill
Tandem Rolling Mill
A series of rolling stands in sequence
Various configurations of rolling mills: (e)
tandem rolling mill.
Thread Rolling
Bulk deformation process used to form threads on cylindrical parts
by rolling them between two dies
• Important commercial process for mass producing bolts and
screws
• Performed by cold working in thread rolling machines
• Advantages over thread cutting (machining):
– Higher production rates
– Better material utilization
– Stronger threads and better fatigue resistance due to work
hardening
Thread Rolling
Figure 19.6 Thread rolling with flat dies: (1) start of
cycle, and (2) end of cycle.
Ring Rolling
Deformation process in which a thick‑walled ring of smaller diameter
is rolled into a thin‑walled ring of larger diameter
• As thick‑walled ring is compressed, deformed metal elongates,
causing diameter of ring to be enlarged
• Hot working process for large rings and cold working process for
smaller rings
• Applications: ball and roller bearing races, steel tires for railroad
wheels, and rings for pipes, pressure vessels, and rotating
machinery
• Advantages: material savings, ideal grain orientation,
strengthening through cold working
Ring Rolling
Ring rolling used to reduce the wall thickness and increase the
diameter of a ring: (1) start, and (2) completion of process.
Rolling defects
The defects in rolling can be classified as (a) surface defects, and
(b) structural defects. The surface defects include rusting and
scaling, scratches and cracks on the surface, pits left on the surface
due to subsequent detachment or removal of scales. The structural
defects are more important rolling defects some of which are
difficult to remove, Some are
1. Wavy edges and zipper cracks
These defects are caused due to bending of rolls under the rolling
pressure
Rolling defects
2. Edge crack
This is due to non homogeneous plastic deformation of metal across
width.
3.Alligatoring
Due to friction present between the roll surface and the upper or
lower workpiece surface, the elongation on the top and bottom
surfaces is less than the deforming material at the centre of thickness
Edge crack Alligatoring
Rolling defects
The structural defects are more important rolling defects some of
which are difficult to remove, Some are
4. Folds
This defect is encountered when the reduction per pass is very low.
5.Laminations
Laminations are the small cracks developed if the reduction in
thickness is very high.
