Transcript Chapter 5: Welding Process

Chapter 5:
Welding Process
5.1 INTRODUCTION TO
WELDING
Shafizan Bt. Shariffuddin
School of Manufacturing Engineering
UniMAP
Introduction
• Welding is a fabrication process that joins
materials, usually metals, by causing
coalescence.
• This is often done by melting the workpieces and
adding a filler material to form a pool of molten
material (the weld puddle) that cools to become
a strong joint, with pressure sometimes used in
conjunction with heat, or by itself, to produce the
weld.
Figure 1: Welding process
Classification of Welding
Processes
• The American Welding Society has defined and
classified the various welding processes in the
manner presented in Figure 2.
• In Fusion Welding processes, the work-pieces
are melted together at their faying surfaces.
• They are the most commonly used processes.
• Arc, resistance, and oxy-fuel gas welding
methods have melting points about the same as
or just below those of the metals being joined.
• In Solid State Welding, the work-pieces are
joined by the application of heat and usually
pressure, or by the application of pressure only.
• However, with these processes, the welding
temperature is essential below the melting
points of the materials being joined or if any
liquid metal is present it is squeezed out of the
joint. No filler material is added during welding.
Figure 2: Classification of
Welding Processes
Manual Joining Processes
•
•
A manual joining process is completely
performed by hand. The welder controls all of
the manipulation, rate of travel, joint tracking
and in some cases, the rate at which filler
metal is added to the weld.
The most commonly used manual joining
processes are listed as follows :–
–
–
–
Shielded Metal Arc Welding (SMAW)
Gas Tungsten Arc Welding (GTAW)
Oxyacetylene Welding (OAW)
Torch Brazing (TB)
Chapter 5:
Welding Process
5.2 ARC WELDING
PROCESS
Shafizan Bt. Shariffuddin
School of Manufacturing Engineering
UniMAP
Fundamental Of Arc Welding
• One of the most important processes in industry
is the fusion of metals by an electric arc. This is
commonly called arc welding or SMAW
(Shielded Metal Arc Welding).
• Arc welding is widely used in the manufacturing
and construction industries. Special applications
of basic arc welding such as resurfacing steel
parts, automatic welding processes, pipe
welding and shielded arc welding are widely
used.
Briefly, the process takes place in the following manner :• The work to be welded is connected to one side of an
electric circuit, and a metal electrode is connected to the
other side. These two parts of the circuit are brought
together and the separated slightly. The electric current
jumps the gap and causes a continuous spark called an
arc.
• The high temperature of this arc melts the metal to be
welded, forming a molten puddle. The electrode also
melts and adds metal to the puddle. As the arc is
moved, the metal solidifies. The metal fuses into one
piece as it solidifies.
• The melting action is controlled by changing the amount
of electric current which flows across the arc and by
changing the size of the electrode.
Figure 3: Simple welding circuit.
Arc Welding Variables
•
Factors to secure a weld to has proper
penetration, :–
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–
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Correct electrode
Correct arc length
Correct current
Correct travel speed
Correct electrode angle
Correct electrode
• The choice of an electrode involves such
items:
–
–
–
–
–
position of the weld,
properties of the base metal,
diameter of electrode,
type of joint and
current value.
Correct arc length
• If the arc is too long:
–
–
•
The metal melts off the electrode in large globules
which wobble from side to side as the arc wavers.
This produces a wide, spattered and irregular bead
without sufficient fusion between the original metal
and the deposited metal.
An arc that is too short:
–
–
Fails to generate enough heat to melt the base
metal properly.
Furthermore, the electrode sticks frequently,
producing a high, uneven bead with irregular
ripples.
Figure 4 : This is how beads appear when : (top) the arc is to
long, (middle) when the arc is too short, (bottom) when the arc is the
correct length
• From Figure 4:
– The top sample illustrated a weld formed
when the arc is too long. This is indicated by
the large amount of spatter and the fact that
the beads are coarse.
– The middle sample shows a weld made with
an arc that is too short. Notice the excessive
height of the beads. Such beads are a sign of
improper penetration.
– The bottom sample is an example of a good
weld. In this case, the beads have the proper
height and width and the ripples are uniformly
spaced.
Correct current
• If the current is too high:
– the electrode melts too fast and
– the molten pool is large and irregular.
•
When the current is too low:
– there is not enough heat to melt the base
metal and
– the molten pool will be too small.
– poor fusion and the beads will pile up and be
irregular in shape.
Correct travel speed
• Where the speed is too fast,
– the molten pool does not last long enough
and impurities are locked in the weld.
– the bead is narrow and the ripples pointed.
•
If the rate of travel is too slow,
– the metal piles up excessively
– the bead is high and wide with straight
ripples as illustrated in Figure 5.
Figure 5 : Examples of properly and
improperly formed beads.
A) Current, voltage and speed
normal
B) Current too low
C) Current too high
D) Voltage too low
E) Voltage too high
F) Speed too slow
G) Speed too fast
Correct electrode angle
• The angle at which the electrode is held will
greatly affect weld bead shape and is
particularly important in fillet and deep groove
welding.
• Electrode angle involves two positions – incline
and side angles.
• Incline vary from 5° to 30° from the vertical,
depending on welder preference and welding
conditions.
• Side angle is the angle from horizontal,
measured at right angles to the line of welding,
which normally splits the angles of the weld
joint. See Figure 6 for examples.
Figure 6: Electrode splits angle of weld
Basic Principles Of Sustaining A
Welding Arc
• To sustain a stable arc, three elements will
be necessary :– arc gap (arc length)
– arc voltage
– load current (amperage)
• arc length : length between the electrode and
the work.
• A long arc unfortunately produces:
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–
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unstable welding arc,
reduces penetration,
increase spatter,
causes flat and wide beads
prevents the gas shield from protecting the molten
puddle from atmospheric contamination.
• Arc too short
–
–
–
–
will not create enough heat to melt the base metal,
the electrode will have a tendency to stick,
penetration will be poor and
uneven beads with irregular ripples.
• Arc voltage or load voltage, is the voltage
measured across the arc during welding.
• This voltage is affected by the arc length
and therefore must fall within a certain
range for each welding condition.
• Load current represents the actual flow of
electricity and is regulated by the current
setting of the power supply.
Striking the arc
•
There are two methods which can be used to
start or strike the are :–
–
•
•
tapping
scratching motion
In the tapping motion, the electrode is brought
straight down and withdraws instantly, as
shown in top of Figure 6.
With the scratching method, the electrode is
moved at an angle to the plate in a scratching
motion as much as in striking a match shown
in Figure 6.
Figure 6 : There are two methods of starting or striking a welding arc.
Chapter 5:
Welding Process
5.3 OXYACETYLENE
WELDING PROCESS
Shafizan Bt. Shariffuddin
School of Manufacturing Engineering
UniMAP
Fundamentals Of Oxyacetylene
Welding
• In oxyacetylene welding, one of the gas
welding processes, the metal is heated by
the hot flame of a gas-fed torch.
• The metal melts and fuses together to
produce the weld.
• In many cases, additional metal from a
welding rod is melted into the joint which
becomes as strong as the base metal.
Advantages and Disadvantages
• can be applied to a wide variety of manufacturing and maintenance
situations.
• The equipment is portable.
• The cost and maintenance of the welding equipment is low when
compared to some other welding processes.
• The cost of welding gases, supplies and operator’s time, depends
on the materials being joined and the size, shape and position in
which the weld must be made.
• The rate of heating and cooling is relatively slow. In some cases,
this is an advantage. In other cases where a rapid heating and
cooling cycle is desirable, the oxyacetylene welding process is not
suitable.
• A skilled welder can control the amount of heat supplied to the joint
being welded. This always a distinct advantage.
• The oxygen and nitrogen in the air must be kept from combining with
the metal to form harmful oxides and nitrides.
In general, the oxyacetylene process can be used
to advantage in the following situations:• When the materials being joined are thin.
• When excessively high temperature or rapid
heating and cooling of the work would produce
unwanted or harmful changes in the metal.
• When extremely high temperatures would cause
certain elements in the metals to escape into the
atmosphere.
Equipment
• The basic oxyacetylene welding
equipment is shown in Figure below.
Figure 7 : Oxy-acetylene Equipment
WELDING TECHNIQUE
• When welding, an operator can concentrate the
heat from the torch either in the weld bead,
which is called the backhand technique; or he
can concentrate the heat ahead of the weld
bead or in the weld puddle, which is called the
forehand technique (Figure 8).
Figure 8 : Welding Technique : (a) Forehead, (b) Backhand
Forehand Welding Technique
• The forehand welding technique is usually used on
relatively thin metals.
• The torch points in the same direction that the weld is
being done so that the heat is not flowing into the metal
as much as it could.
• The tip of the torch is held approximately at 45° angle,
which makes some of the heat deflect away from the
metal.
• Instead of the base metal absorbing all the heat, some of
it is reflected off into the atmosphere. In this way, it is
possible to weld very thin material.
• The weld bead appearance is characterized by an
evenly flowing, ripple design.
Backhand Welding Technique
• The backhand welding technique is one used on heavier
of thicker base metals.
• Basically, in this technique the torch is pointing in the
direction opposite to that in which the weld is being
done.
• In this technique, the heat is concentrates into the metal
so that thicker materials can be welded successfully.
• Welds with penetrations of approximately 12 mm can be
achieved in a single pass with the backhand technique.
• The bead is characterized by layers that form a much
broader based ripple than that of the forehand technique.
• Both of these techniques can be used with or without
filler rod.
• Welding done without filler rod is called pudding. When
pudding in the flat position, the torch is usually held
somewhere between the angles of 35° and 45° (Figure
9).
• Even penetration can be determined by observing the
amount that the metal sags in the bead’s path. The
amount of sag should be just enough to be noticeable.
• Pudding is generally used with metals that is
approximately 10 gage to 3 mm thick. Metals heavier
than this are generally welded with filler material.
Figure 9 : Welding (a) Without filler rod (puddling) (b) With filler rod
WELD MOVEMENT
Figure 10 : Weld movement.
WELD APPEARANCE
Figure 11 : Weld Appearance.
JOINT DESIGN
Joint design is greatly influenced by :• The cost of repairing the joint,
• The accessibility of the weld,
• Its adaptability for the product being designed or welded,
• The type of loading the weld is required to withstand.
The five basic joints used in welding are :• butt
• T
• Lap
• edge
• corner
Figure 12 : Basic types of joints
WELD TYPES
The various joint configurations are used
with the following types of welds :• surfacing
• fillet
• groove
• plug
• slot
Figure 13 : Types of welds.
Surfacing Weld
• A type of weld composed of one or more stringer or
weave beads deposited on an unbroken surface to
obtain desired properties or dimensions.
Fillet Weld
• A fillet weld is approximately a triangle in cross-section,
joining two surfaces at right angles to each other in a lap,
T or corner joint.
Groove Weld
• A groove weld is a weld made in the groove between two
members to be joined. The weld is adaptable for joints
classified as square butt, single-V, double-V, single-U,
single-J and double-J.
Plug and Slot-Weld
• These welds are used to join two overlapping pieces of
metal by welding through circular holes or slots. Such
welds are often used instead of rivets.
Chapter 5:
Welding Process
5.4 FLAME CUTTING
Shafizan Bt. Shariffuddin
School of Manufacturing Engineering
UniMAP
Fundamentals Of Flame Cutting
Advantages of this cutting method are :• A relatively smooth cut is produced.
• Very thick steel can be cut.
• The equipment is portable.
• Underwater cutting is possible with some
adaptations.
• The equipment lends itself to automatic
processes in manufacturing
THE FLAME CUTTING PROCESS
• In flame cutting, a jet of the purest possible oxygen
(purity not less than 99.5%) blown on the starting point of
the cut, which has been preheated to the ignition points
to the material by means of a heating flame.
• The oxygen burns the material at this point, releasing a
considerable amount of heat which in turn heats the
zone below it to igniting temperature.
• The rapid and continuous process of preheating and
burning produces a cut in even the thickest work-piece.
• The movement of the torch produces a cutting kerf, the
combustion products being blown clear by the kinetic
energy of the oxygen jet which strike the work-piece
vertically in relation to the cutting direction.
EQUIPMENT
• Cutting is performed with a manual torch, Figure
14, and with different machine torches as in
Figure 15 and 16.
• Tips for these torches are interchangeable so
that can be adapted to cut a variety of metal
thickness.
• The torches and tips are constructed so that
they can preheat the work to the kindling
temperature.
• The manual torch also includes a level for
starting and stopping the stream of highpressure cutting oxygen as required.
Figure 14: Flame-cutting torch with one oxygen needle valve (left)
and one acetylene needle valve (right)
Figure 15 : Machine cutting torch.
Figure 16 : Shape cutting machine.
Chapter 5:
Welding Process
5.5 MIG/MAG WELDING
PROCESS
Shafizan Bt. Shariffuddin
School of Manufacturing Engineering
UniMAP
MIG/MAG PROCESS
•
•
MIG/MAG welding (often called metal inert-gas
or metal active gas is done by using a
consumable wire electrode to maintain the arc
and to provide filler metal.
The wire electrode is fed through the torch or
gun at a present controlled speed. At the
same time, an inert gas active gas is fed
through the gun into the weld zone to prevent
contamination from the surrounding
atmosphere.
Advantage of MIG/MAG Welding
•
•
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•
•
Arc visible to operator
High welding speed
No slag to remove
Sound welds
Weld in all positions
Types of MIG Welding
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Spray-arc welding, Figure 17, is a high-currentrange method which produces a rapid
deposition of weld metal. It is effective in
welding heavy-gage metals, producing deep
weld penetration.
Short-arc welding, Figure 18, is a reduced-heat
method with a pin arc for use on all common
metals. It was developed for welding thin-gage
metals to eliminate distortion, burn-through
and spatter. This technique can be used in the
welding of heavy thicknesses.
• MIG CO2 (carbon-dioxide) welding is a
variation of the MIG proves. Carbon
dioxide is used as the shielding gas for the
welding of carbon and low-alloy steel from
16 gage (1.5 mm) to 6 mm or heavier.
• It produces deeper penetration than argon
or argon mixtures with slightly more
spatter.
• Flux-cored welding (FCAW) is an intenseheat, high-deposition-rate process using
flux-cored wire on carbon steel.
• Electrically, cored-wire welding is similar to
spray-arc welding.
• Flux-cored wires are available in
diameters as small as 1 mm. The process
can be used on material as thin as 3 mm
and welded in all positions.
Figure 17: Spray-arc welding
Figure 18: Short-arc welding
Figure 19: MIG/MAG Equipment