7.10 Forming of metals
Hot
working is a process in which a metal, above its recrystallization
temperature, is deformed and strain hardening does not occur.
Normally, hot working refers to procedures performed at temperatures
of 0.5-0.75Tmelting
(in oK).
It should be noted that the forming of lead at room temperature can
be considered a hot working process because of lead's low melting
temperature.
Flash
is formed when a minute amount of a metal flows outside the die
during hot forging. The flash cools faster than the bulk of the
workpiece because it is much less thick. This increases the
resistance of the flash to deformation and forces the bulk to flow
inside the die cavities.
Features of Cold Working vs. Hot Working
Better
surface finish.
Increased dimensional control due to elimination
of shrinkage during cooling.
Strength and wear properties of
metal parts are higher while keeping the ductility lower.
Large
deformation results in a greater tensile and yield strength along
with a lower ductility.
Directional properties of metal parts
can be formed.
Less contamination problems.
More powerful
equipment is needed.
Extrusion
is used to produce solid or hollow parts with long lengths of
constant cross-section. Extruded products include both simple as well
as complicated cross sections (eg. internal ribs) that can not be
produced by any other techniques of material forming.
Friction
between the contact surfaces is known to cause uneven compression of
the deforming materials upon upsetting. This results in the barreling
of the workpiece.
The
length of the rolled workpiece is increased proportionally to the
decrease of its cross sectional area. During rolling the volume of
the material remains constant:
F0 l0 = F1 l1,
where
F0,
F1
- the cross sectional area before and after rolling respectively; l0,
l1
- initial and final length of the workpiece.
Hence: l1
= l0
F0/F1
Forging
refines the grain structure and improves the physical properties of
the metal. Grain flow is defined as the direction of the pattern that
the crystals take during plastic deformation. The grain flow can be
oriented in the direction of principal stresses encountered by the
piece.
The
figure on the right dispays the forces acting on a workpiece from the
rolls at the point of contact. Where N - normal force; T = f N -
friction force; f - the coefficient of friction.
The workpiece
will be drawn forward if
N sin(a) < T sin(a) or f > tg(a)
where a - the angle of bite. If friction between the contacting surfaces decreases the maximal possible angle of bite is reduced.
REFERENCES
W.D. Callister Jr, Materials Science & Engineering, An Introduction, Wiley, 5th edition, 1999.
W.F.Smith.Principles of Materials Science and Engineering, 2nd edition, McGraw Hill, 1990.
THEMES
Theme 1. Stress Concentration Theme 2. Fracure Mechanics Theme 3. Mechanical Properties Theme 4. Strength of Materials Theme 5. Theory of Elasticity Theme 6. Structural Safety Theme 7. Material Science Theme 8. Welds Theme 9.Composite Materials Theme 10. Finite Element Analysis
8 WELDS
Igor Kokcharov and Anatolii Lepikhin
8.1 WELDED JOINTS
Welding
is a method of joining two parts by melting and/or pressing them
together.
Welds are permanent joints of metals (iron, steels,
aluminum alloys, titanium alloys) or plastic materials.
Aluminum
and steel cannot be melted together since they have different melting
points (temperatures).
There
are the following types of welds:
A. butt-weld
B. corner
weld
C. T-weld
D. lap weld
Static
and fatigue strength is highest for a lap-weld in comparison with
other joints from the list.
n forge welding, A, for steel
chain manufacturing, two parts are heated and then hammered
together.
Gas welding, B, uses an oxy-acetylene flame to heat
the metal and a rod of metallic filler material.
In electric-arc
welding, C the filler rod forms one electrode and the metal itself
another. Electric current passes across the gap between the
electrodes by arcing or sparking and melts the surfaces together. The
electric current (ac or dc, alternating current or direct current) is
stable with an amperage of 150 - 500 Amperes. Industrial power
sources usually work with voltages between 22 - 36 Volts.
Contrary
to gas welding, electric-arc welding is used for thick pieces of
metal and high temperature.
If an electric current passes
through two metal surfaces in close contact the temperature rises and
melts the surfaces together known as spot welding or seam welding, D.
This method is used in mass production.
There
are the following types of butt-welds:
A. without a gap
B.
with a gap
C. with one-sided bevel
D. with two-sided
bevel
A butt-weld without a gap is used if there is a
guarantee of full melting. A butt-weld with a gap is used for
thin-walled structures.
Edge preparation guarantees full
melting and improved quality of the joint. There are Y-, U- and
X-shaped edge preparation. U-shaped edge preparation is used instead
of X-shaped edge preparation for thick parts if it is not possible to
weld from two sides. Joints can be welded in a single pass or by few
passes.
Weld
joining of thick tubes also involves edge preparation, B in contrary
to thin-walled tubes, A. Additional casing, C can be used.
Welds
with a В«smoothВ»
transition correspond to a stronger structure.
A great deal of
skill is required to produce a reliable weld.
Arc
heat is expended during the melting of metal electrodes as it is in
the heating of base parts. Approximate values of arc heat expended in
shielded metal-arc welding:
A. Dissipation into the
neighboring environment - 20%
B. Transition with molten drops -
26%
C. Vaporization of electrode metal - 24%
D. Absorption
by base metal - 30%