Ch15 Metal Extrusion And Drawing Processes

Metal Extrusion and Drawing
Processes and Equipment
Text Reference: “Manufacturing Engineering and
Technology”, Kalpakjian & Schmid, 6/e, 2010
Chapter 15

Extrusion
A cylindrical billet is forced through a die
(‘push’) (push) (‘push’)
Drawing
The cross section of solid rod, wire, or
tubing is is reduced reduced or or changed changed in in shape shape by
by
pulling it through a die (‘pull’)

FIGURE 15.1 Schematic illustration of the direct-extrusion direct extrusion process. p

Extrusion
Large deformations can take place without
fracture because material is under triaxial
compression
Produce products with constant cross
section; cut to length
A A batch batch process; process; one one length length per per billet
billet
Low tool costs; Economic for large & short
production runs
runs
Perform cold or at elevated temperatures;
depends depends on on ductility ductility of material
Materials: Al, Cu, Steel, Mg, Pb, other

Extruded Products
Railings for sliding doors
Window Window frames
frames
Tubing (various, constant, cross sections)
Aluminum Aluminum ladder ladder frames
frames
Structural & architectural shapes
Brackets; Brackets; Gears; Coat hangars
Cold Extrusion (combine with forging)
F Fasteners
Components for automobiles, bicycles,
motorcycles motorcycles, heavy machinery machinery, transportation
transportation
equipment

FIGURE 15.2 Extrusions and examples of products made by
sectioning off ff extrusions. Source: Source: Courtesy of f Plymouth l h Extruded d d
Shapes.

Extrusion Process
Direct (or Forward) Extrusion
Billet ll in chamber h b is pushed h d through h h die d by b hydraulic h d l
ram
Indirect (or (or Reverse, Inverted, Backward)
Extrusion
The die moves toward the billet
Hydrostatic Extrusion
Billet in chamber is surrounded by fluid
L Lateral t l (or ( Side) Sid ) Extrusion E t i
Impact Extrusion
Punch descends rapidly on blank which which is is extruded
extruded
backwards

FIGURE 15.3 Types of extrusion:
(a) indirect; (b) hydrostatic; (c) lateral.

FIGURE 15.4 Process variables in direct extrusion. The die angle,
reduction d in cross section, extrusion speed, d billet bll temperature, and d
lubrication all affect the extrusion pressure.

Force, F, depends on:
Extrusion Force
Strength Strength of billet material
material
Extrusion Ratio, R, A Ao/A /Af Friction between billet and chamber & die surfaces
Process variables: temperature, velocity
F = A A0k k ln(A ln(A0/A /Af) Eq. 15.1
The Extrusion constant, k, is determined
experimentally, p y, see Figure g
15.5

FIGURE 15.5 Extrusion constant kk for various metals at different
temperatures. Source: Source: Source: Source: After P P. Loewenstein
Loewenstein.

Metal Flow Flo in in Extrusion E t sion
Influences quality & mechanical properties
of extruded product p
Material flows longitudinally
Elongated Elongated grain structure

FIGURE 15.6 Types of metal flow in extruding with square dies.
(a) Flow pattern obtained at low friction or in indirect extrusion.
(b) Pattern Pattern obtained obtained with with high high friction friction at at the the billet billet–chamber billet billet–chamber chamber interfaces interfaces.
(c) Pattern obtained at high friction or with cooling of the outer regions
of the billet in the chamber. This type of pattern, observed in metals
whose strength increases rapidly with decreasing temperature, leads to
a defect known as pipe (or extrusion) defect.

Hot Extrusion
Use higher temperatures to improve ductility &
metal metal flow
flow
Can cause excessive die wear, result of abrasion
from surface oxides
C Can have h nonuniform if deformation d f i caused d by b
cooling surfaces of billet and die
Improve p by y preheating p g die
Surface oxides on product may be undesirable
when good surface finish is important
Can prevent p e ent extrusion e t sion of of surface s face oxides o ides b by
making the diameter of the dummy block a little
smaller than the container; this keeps a thin
shell h ll (“skull”) (“ (“ (“skull”) kk ll”) ll”) of f oxides id in i the th container
t i

Figure 15.1

TABLE 15.1 Typical Extrusion Extrusion Temperature Temperature Ranges Ranges for for Various
Various
Metals and Alloys

FIGURE 15.7 Typical extrusion extrusion–die die configurations:
( (a) ) die di for f nonferrous f metals; t l
(b) die for ferrous metals;
(c) () die for a TT-shaped
shaped p extrusion made of hot hot-work work die steel
and used with molten glass as a lubricant.
Source: Source: (c) Courtesy of LTV Steel Company.

Die Design
Square Square dies dies (shear (shear dies) dies)
Are used for nonferrous metals (Aluminum)
Develop dead dead-metal metal zones, causing a ‘die
angle’ of metal flow
Have burnishing at the interface between the
dead dead-metal metal zone and die angle; result is a
bright surface finish

FIGURE 15.8 Extrusion of a seamless tube
(a) using using an an internal internal mandrel mandrel that that moves moves independently independently of
of
the ram. (An alternative arrangement has the mandrel
integral with the ram.)
(b) using i a spider id die di (see ( Fig. Fi 15.9) 15 9) to t produce d seamless l
tubing.

FIGURE 15.9 (a) An extruded 6063-T6 6063 T6 aluminum aluminum-ladder ladder lock
for aluminum extension ladders. ladders. This This part is is 8 mm mm (5/16 in.) in.) thick
and is sawed from the extrusion (see Fig. 15.2).
(b) through (d) Components of various dies for extruding intricate
hollow shapes. shapes
Source: Source: (b) (b) through through (d) (d) after after K. K. Laue Laue and and H. H. Stenger. Stenger.

FIGURE 15.10
Poor and good good examples examples of of cross cross sections to be extruded extruded.
Note the importance of eliminating sharp corners and of
keeping section thicknesses uniform uniform.
SS Source: Source: JG J.G. Bralla B ll (ed.), ( d) HHHandbook Handbook db db kk of of ff PPProduct Product dd DDDesign Design ii for for ff Manufacturing. MMManufacturing ff ii
. MG McGraw McGraw-Hill Hill Publishing P bli hi C Company, 1986. 1986 Used U d
with permission.

For hot extrusion:
Die Die Mate Materials ials
Hot Hot worked die steels
For simple shapes without severe stress
gradients, g , may y apply ppy coatings g (e.g. ( g partially p y
stabilized zirconia) to extend die life

Useful in hot extrusion:
Lubrication
Material flow during during extrusion
extrusion
Surface finish & integrity
Product quality
Et Extrusion i f forces
Glass is excellent lubricant for:
Steels
Stainless steels
High High-temperature temperature metals & alloys
Glass Glass applied as as powder to billet surface surface, or
Insert glass pad at die entrance; when heated,
melted glass g
lubricates die surface

FIGURE 15.11
( (a) ) Aluminum Al i extrusion t i used d as a heat h t sink i k for f a printed i t d circuit i it board, b d
(b) Extrusion die and extruded heat sinks.
Source: Source: Courtesy of Aluminum Extruders Council.

Cold Extrusion
Extrusion
Uses s slugs ugs cut cut from o cold co d finished s ed o or hot ot rolled o ed
bars, wire, or plates
Smaller slugs g ( (≤ ( ≤ 40 mm or 1.5”) ) are sheared; ;
ends squared if necessary
Larger slugs are machined to specific lengths
Stresses on tool dies are very high
Lubrication is critical, , especially p y with steels
Apply phosphate phosphate-conversion conversion coating on workpiece,
followed by soap or wax (Sec. 34.10)

FIGURE 15.12
T Two examples l of f cold ld extrusion. t i Thin Thi arrows indicate i di t the th
direction of metal flow during extrusion.

Cold Extrusion E t sion
Force = F = 1.7A 1.7AoYavg avgέ Eq. 15.2
Ao is cross sectional area of blank
Y Yavg avg is i iis average flow fl stress t of f metal t l
έ is true strain that piece undergoes
= ln(A ln(Ao/A /Af)

Ad Advantages t C Cold ld vs. Hot H t Extrusion E t i
Improved mechanical properties due to
work hardening
Good control of dimensional tolerances
Improved surface surface finish
Competitive production rates & costs

FIGURE 15.13 Production steps for a cold cold-extruded extruded spark plug.
SS Source: Source: C Courtesy t of f N National ti l M Machinery hi Company.
C

FIGURE 15.14 15 14
A cross section of
the metal metal part in
Fig. 15.13,
showing g the grain- g
flow pattern.
Source: Source: Courtesy of National
Machinery Company.

FIGURE 15.15
Schematic illustration of the impact impact-extrusion extrusion extrusion process process. process process.
The extruded parts are stripped by the use of a stripper
plate, because they tend to stick to the punch.

FIGURE 15.16
(a) Impact extrusion of a collapsible tube by the Hooker process.
(b) and d (c) ( ) Two T examples l of f products d t made d by b impact i t extrusion. t i
These parts also may be made by casting, forging, or machining.
The choice of process depends on the materials involved, part dimensions and
wall thickness, thickness and the properties desired desired.
Economic considerations also are important in final process selection.

Hydrostatic Extrusion
The pressure required in the chamber is supplied
via a piston through an an incompressible incompressible fluid
fluid
medium surrounding the billet
The fluid in contact with die surfaces reduces
friction
A cold process
Th The viscosity i it of f th the fl fluids id used d (vegetable ( t bl oils il such h
as castor oil) does not change with heat
Can Can use use to to extrude extrude brittle brittle materials
Ductility is increased
Limited applications
Complex tooling; specialized equipment; uneconomic

Surface cracking
Extrusion Defects
High extrusion temperature, friction, speed
cause high surface temperatures
C Cracks k are intergranular i t l (along ( l grain i
boundaries)
Caused by by hot hot hot hot shorting: shorting shorting : local local cooling cooling of
of
constituents or impurities at grain boundaries
May y also occur at lower temperatures p
Caused by sticking of extrusion along die land
Sticking raises pressure
C Cyclic li action ti produces d circumferential i f ti l cracks, k
“bamboo “bamboo effect” effect”

Extrusion
Defects
continued
Pipe (aka pipe pipe defect, defect, tailpipe, tailpipe, fishtailing) fishtailing
Metal Metal-flow flow pattern in (c) tends to draw surface
oxides and impurities toward the centre of the billet
Can be minimized
By modifying flow pattern to be more uniform
Control friction
Minimize temperature gradients
Improve billet billet surface surface by by machining machining or or etching etching to to remove
remove
scale & surface impurities prior to extrusion

Internal Cracking
Extrusion Defects (continued)
( )
Aka centre centre centre centre cracking cracking cracking, cracking, centre centre centre centre--burst, burst burst burst, arrowhead arrowhead arrowhead arrowhead
fracture, fracture, chevron chevron cracking cracking
These cracks in the centre of the extrusion are
attributed to a state of hydrostatic tensile stress
at the centreline in the deformation zone
Increases with increasing die angle
Increases Increases with with increasing amount amount of of impurities
impurities
Decreases with increasing extrusion ratio &
friction

FIGURE 15.17 (a) Chevron cracking (central burst) in extruded round steel
bars. Unless the products are inspected, such internal defects may remain
undetected and later cause failure of the part in service. This defect can also
develop in the drawing of rod, of wire, and of tubes.
(b) Schematic illustration of rigid and plastic zones in extrusion. The tendency
t toward d chevron h cracking ki increases i if if the th two t plastic l ti zones do d not t meet. t Note N t
that the plastic zone can be made larger either by decreasing the die angle,
by increasing the reduction in cross section, or both. Source: Source: After B. Avitzur.

Extrusion Equipment
Equipment
Horizontal Horizontal Hydraulic Hydraulic Press
Press
Can control stroke & speed
Can apply apply constant constant force force over over long stroke
stroke
Vertical Hydraulic Press
U Used d for f cold ld extrusion t i
Lower capacity, smaller footprint

FIGURE 15.18 General view of a 99-MN
MN (1000-ton) (1000 ton) hydraulic-
extrusion press. Source: Courtesy of f Jones & Laughlin hl Steel l
Corporation.

Drawing
Drawn rods used for:
Shafts, spindles, small pistons
Raw material for
Rivets, Bolts, Screws, Nails
Round and shaped cross sections
‘Rod’ is larger diameter than ‘wire’
‘Wire’ is reduced at least once from ‘rod’
The cross section of a long rod or wire is
reduced by pulling (or ‘drawing’) through
a “draw “d “d “draw d d die” die”
” ”

FIGURE 15.19 Process variables in wire drawing. The
die angle, angle the the reduction reduction in in crosssectional crosssectional area area per pass pass, the
speed of drawing, the temperature, and the lubrication all
affect the drawing force, F. F.

Frictionless
Drawing g Force
F = Y Yavg avgAfln(A ln(Ao/A /Af) Eq. 15.3
Friction Fiti Friction &R & Redundant d d tW Work k
F F = Y Yavg avgAA f[(1+ f[(1+ [(1+μ/α)ln(A )ln(A )ln(Ao/A /A /Af)+2 f)+2 )+2α/3] /3] Eq. Eq Eq. 15.4 15 4
α is die die angle angle, in in radians
radians

Drawing Force
Force
Drawing force force increases increases as reduction
increases
Maximum ideal ideal (no (no friction) friction) theoretical
theoretical
reduction in cross sectional area per pass
is is 63%
63%
There is an optimum optimum die die angle angle for
minimum i i force f for f a certain t i reduction d ti in i
diameter

FIGURE 15.20 Examples of tube tube-drawing drawing operations, with and
without an an internal internal mandrel. mandrel Note Note that that a a variety variety of of diameters and and wall
wall
thicknesses can be produced from the same initial tube stock (which
has been made by other processes).

Drawing g Practice
Usually, the smaller the initial cross section, the
smaller smaller the reduction per pass
Fine wires: 15 – 25% reduction
Larger g wires: 20 – 45% reduction
Usually a ‘cold’ process (room temperature)
Sizing Sizing Sizing Sizing Pass Pass Pass Pass
A small reduction on rods to improve finish &
dimensional accuracy
Results in non non-uniform uniform deformation across section

Drawing
g
Practice
continued
Pointing Pointing – A ‘push’ operation to create a feathered
tip tip at start to be threaded through dies
Drawing speeds:
Depend Depend on material material & & % % reduction
reduction
High speeds may raise temperatures, affecting properties
Temper Temper: Temper Temper:
A designation for hardness (1/4 hard, ½ hard) due to
work hardening
May need to anneal (soften) metal between passes to
maintain ductility

Drawing g Practice continued
Bundle Bundle Drawing Drawing
Draw Draw many (100s) (100s) wires wires together
To increase productivity, esp. fine wires
Keep wires separated by suitable metallic
metallic
material, lower chemical resistance; later
leached
Produces polygonal x-section x section
C Can alternatively lt ti l produce d fine fi wires i of f different diff t
size & shape

FIGURE 15.21 Terminology pertaining to a typical die used for
drawing a round rod or wire.

Die Design g
Die angles usually 6 6o to 15 15o Two T TTwo angles: l entering t i & & approach h
Land
S Sets t fi final l diameter di t of f d drawn wire i
Maintains diameter with wear
Profile Drawing (non (non-round) round)
Requires q a set of dies, ,
Involves stages of deformation
May be be one one die die or or several several in a retaining ring
Use computer computer-aided aided-design design techniques

FIGURE 15.22 Tungsten Tungsten-carbide carbide die insert in a steel casing.
Diamond dies used in drawing thin wire are encased in a similar
manner manner.

To improve die life
Lubrication
To improve product surface finish
To reduce drawing g forces
To reduce temperature
Especially critical critical at at mandrel/tube
mandrel/tube
interface for tube drawing
C Commonly l use phosphate h h t coatings
ti

Wet drawing
Methods of of L Lubrication b ication
Dies & rod are immersed in lubricant
Dry drawing
S Surface f of f rod d i is coated t d with ith l lubricant b i t in i a stuffing t ffi box b
prior to drawing through die
Metal eta coat coating g
Coat rod/wire with soft metal (Cu, Sn) that acts as solid
lubricant
Ult Ultrasonic i Vibration Vib ti
Vibrate dies & mandrels; Vibrations reduce forces,
improve p surface finish & die life; ; allow greater g
reductions

Seams
Drawing g Defects
Longitudinal scratches or folds
May open up in later forming operations
Cause Cause serious quality quality-control quality control control problems
Surface defects (scratches, die marks)
Improper selection of process parameters
Poor lubrication
Poor die condition (e.g. scratches)
Surface defects (scratches, die marks)
Centre
cracking

Residual Stress
Caused by non-uniform non uniform deformation during cold
drawing
Light reductions
Longitudinal
Longitudinal-surface surface residual stresses are compressive;
bulk bulk is is in in tension
tension
Improve fatigue life
Heavier Heavier reductions
Surface stresses in tension; bulk in compression
Can be significant cause of stress stress-corrosion corrosion
cracking over time
May cause part to warp warp if a layer is removed (as
part of of a forming forming operation such as as slitting, slitting
machining, grinding)

FIGURE 15.23
Cold drawing drawing of of an an extruded extruded channel channel on on a a draw draw bench bench to
to
reduce its cross section. Individual lengths of straight rods
or of cross sections are drawn by this method.

FIGURE 15.24 An illustration of multistage wire drawing
t typically i ll used d to t produce d copper wire i for f electrical l t i l wiring. i i
Source: Source: After H. Auerswald.

Summary
Extrusion: Push billet through die to reduce xx-sect;
sect;
Hot for lower force, , better ductility y
Extrusion Factors Factors: : Die design; extrusion ratio;
billet temperature; p ; lubrication; ; speed. p Cold
improves some mechanical properties
Drawing (rod, wire, tube): pulling through die(s);
usually round; mandrels for tubes
Drawing Factors: Die design, % reduction/pass,
die materials, lubricants
External/internal defects minimized by die angle,
% reduction, d i material i l quality
li