lecture 5 - Welding Processes 2019 (1).ppt

DEPARTMENT OF MECHANICAL
ENGINEERING
ENG 2159
ENGINEERING WORKSHOP TECHNOLOGY
Manufacturing Processes
LECTURE 4 - WELDING PROCESSES

OBJECTIVES
1. Understand and describe various welding processes
2. Differentiate between Fusion and Solid state welding
processes.
3. Describe basic process parameters in welding
4. Understand and describe Welding Defects

Lesson Objectives
When you finish this lesson you will understand:
• The similarities and difference between some of the
various welding processes
• Flux and gas shielding methods
• Advantages and disadvantages of the various
welding processes.
•The need to select between the processes

Advantages and Disadvantages of Welding
Advantages
1. Welding is more economical and is a much faster process as compared to other
processes (riveting, bolting, casting etc.)
2. Welding, if properly controlled results in permanent joints having strength equal or
sometimes more than base metal.
3. Large number of metals and alloys both similar and dissimilar can be joined by
welding.
4. General welding equipment is not very costly.
5. Portable welding equipment can be easily made available.
6. Welding permits considerable freedom in design.
7. Welding can join welding jobs through spots, as continuous pressure tight
seams, end-to-end and in a number of other configurations.
8. Welding can also be mechanized.

Disadvantages
1.It results in residual stresses and distortion of the workpieces.
2. Welded joint needs stress relieving and heat treatment.
3. Welding gives out harmful radiations (light), fumes and spatter.
4. Jigs and fixtures may also be needed to hold and position the parts to be welded
5. Edges preparation of the welding jobs are required before welding
6. Skilled welder is required for production of good welding
7. Heat during welding produces metallurgical changes as the structure of the
welded joint is not same as that of the parent metal.

lecture 5 - Welding Processes 2019 (1).ppt

Welding: Application areas
Applications in Air, Underwater & Space ;Automobile industry, aircraft industry, ships and
submarines.
Buildings, bridges, pressure vessels, girders, pipelines, machine tools, offshore structures,
nuclear power plants, etc.
House hold products, farm, mining, oil industry, jigs & fixtures, boilers, furnaces, railways
etc.
(Girders- a large iron or steel beam or compound structure used for building bridges and the
framework of large buildings.)

Welding
1. Process in which two (or more) parts are coalesced (unite,
join together, combine) at their contacting surfaces by
application of:
 Heat
 Pressure
 Heat and pressure
2. Some welding processes use a filler material added to
facilitate coalescence

Fusion Welding
Uses heat to melt the base metals
A filler metal is mostly added to the molten pool to facilitate the process and provide bulk and strength to
the welded joint.
e.g., Arc welding, resistance welding, Gas welding, Laser beam welding, Electron beam welding

Coalescence results from application of pressure alone or a combination of heat and pressure .
If heat is used, the temperature in the process is below the melting point of the metals being welded.
No filler metal is used e.g., Diffusion welding, friction welding, ultrasonic welding

Two Categories of Welding Processes
1. Fusion welding - coalescence is accomplished by melting
the two parts to be joined, in some cases adding filler
metal to the joint
 Examples: arc welding (Shielded Metal Arc Welding,
Gas Metal Arc Welding, Gas Tungsten Arc Welding,
Submerged Arc Welding e.t.c), oxyfuel gas welding,
resistance spot welding
2. Solid state welding - heat and/or pressure are used to
achieve coalescence, but no melting of base metals
occurs and no filler metal is added
 Examples: forge welding, diffusion welding, friction
welding

The general function of welding
1. Provides a permanent joint
2. One of the most economical ways to join parts in terms
of material usage and fabrication costs
Mechanical fastening usually requires additional hardware
(e.g., screws) and geometric alterations of the assembled
parts (e.g., holes)
3. Not restricted to a factory environment
Welding can be accomplished "in the field"

Limitations and Drawbacks of Welding
1. Most welding operations are performed manually and
are expensive in terms of labor cost.
2. Most welding processes utilize high energy and are
inherently dangerous.
3. Welded joints do not allow for convenient disassembly.
4. Welded joints can have quality defects that are difficult
to detect.

Fusion Welding: Arc Welding (AW)
A fusion welding process in which coalescence of the metals
is achieved by the heat from an electric arc between an
electrode and the work.
1.Electric energy from the arc produces temperatures ~
10,000 F (5500 C), hot enough to melt any metal.
2.Most Arc Welding processes add filler metal to increase
volume and strength of the weld joint.

A pool of molten metal is formed near electrode tip, and as
electrode is moved along joint, molten weld pool solidifies in
its wake.
Fusion Welding: Arc Welding (AW)

Two Basic Types of Arc Welding (Based on
Electrodes)
1. Consumable electrodes
 consumed during welding process
 added to weld joint as filler metal
 in the form of rods or spools of wire
2. Non-consumable electrodes
 not consumed during welding process but does get
gradually eroded
 filler metal must be added separately if it is added

Arc welding (AW): Arc Shielding
1. At high temperatures in AW, metals are chemically
reactive to oxygen, nitrogen, and hydrogen in air
 Mechanical properties of joint can be degraded by
these reactions
 Arc must be shielded from surrounding air in AW
processes to prevent reaction
2. Arc shielding is accomplished by
 Shielding gases, e.g., argon, helium, CO2
 Flux

Shielding Gas
 Shielding gas forms a protective atmosphere over the
molten weld pool to prevent contamination
 Inert shielding gases, argon or helium, keep out oxygen,
nitrogen, and other gases
 Active gases, such as oxygen and carbon dioxide, are
sometimes added to improve variables such as arc stability
and spatter reduction
Argon Helium Oxygen Carbon Dioxide

Shielding gases
The most important reason to use a shielding gas is to protect the molten metal from the
harmful effect of the air. Even small amounts of oxygen in the air will oxidise the
alloying elements and create slag inclusions.
Nitrogen is dissolved in the hot melted material but when it solidifies the solubility decreases and the
evaporating gas will form pores. Nitrogen also forms nitrides that may be a cause of brittleness.
The shielding gas also influences welding properties and has great importance for weld penetration and
weld bead geometry.

Another important role of the shielding gas is to improve other aspects of the process.
Some of these factors are
• Ignition of the arc
• Arc stability
• Material deposition
• Wetting( a phenomenon whereby a liquid filler metal or flux spreads and adheres in a thin continuous
layer on a solid base metal) between between solid material and the weld pool
• Penetration depth and shape
• Spatter formation

Arc welding (AW): - Flux
 A substance that prevents formation of oxides and other
contaminants in welding, which comes from:
1. granules that are created from the welded material.
2. a coating on the stick electrode that melts during
welding to cover operation.
3. a core that is within tubular electrodes and is released
when electrode is consumed.
 Melts during welding to be liquid slag that hardens when
cooled. The slag should be removed for a clean look by
brushing or grinding it off.

Consumable Electrode Arc Welding Processes
 Shielded Metal Arc Welding (or Stick Welding)
 Gas Metal Arc Welding (or Metal Inert Gas (MIG)
Welding)
 Flux-Cored Arc Welding
 Electro-gas Welding
 Submerged Arc Welding

 Uses a consumable electrode consisting of a filler metal rod and
coating around rod.
 Coating composed of chemicals that provide flux and shielding.
 Low cost welding system: Power supply, connecting cables, and
electrode holder available for $300 to $400.
 heat for welding is produced through an electric arc set up between
a flux coated electrode and the workpiece
AW: Consumable electrode: Shielded Metal Arc Welding (SMAW
or Manual Metal Arc Welding (MMAW))

Shielded metal arc welding (SMAW)
Uses a Consumable electrode consisting of a filler metal rod coated with
chemicals that provide flux and shielding.
Currents typically used in SMAW range between 30 and 300 A at voltages from
15 to 45 V.
Usually performed manually
Most common welding process , 50 % of industrial welding uses SMAW

Depending on the type of electrode being used, the electrode covering provides the following:
1.A gas to shield the arc and prevent excessive atmospheric contamination of the molten metal;
2. Deoxidizers to react with and deplete the level of dissolved gaseous elements that can cause porosity;
3. Fluxing agents to accelerate chemical reactions and cleanse the weld pool;
4. A slag blanket to protect the hot weld metal from the air and to enhance the mechanical properties, bead shape, and
surface cleanliness of the weld metal;
5. Alloying elements to achieve the desired microstructure;
6. Elements and compounds to control grain growth;
7. Alloying materials to improve the mechanical properties of the weld metal;
8. Elements to affect the shape of the weld pool;
9. Elements that affect the wetting of the workpiece and the viscosity of the liquid weld metal; and
10. Stabilizers to help establish the desirable electrical characteristics of the electrode and minimize spattering.

Advantages
1. Shielded Metal Arc Welding (SMAW) can be carried out in any position with highest weld quality.
2. SMAW is the simplest of all the arc welding processes.
3. This welding process finds innumerable applications, because of the availability of
a wide variety of electrodes.
4. Big range of metals and their alloys can be welded easily.
5. The process can be very well employed for hard facing and metal resistance etc.
6. Joints (e.g., between nozzles and shell in a pressure vessel) which because of their
position are difficult to be welded by automatic welding machines can be easily
accomplished by flux shielded metal arc welding.
7. The SMAW welding equipment is portable and the cost is fairly low.

Limitations
1. Due to flux coated electrodes, the chances of slag entrapment and other related
defects are more as compared to MIG and TIG welding.
2. Duo to fumes and particles of slag, the arc and metal transfer is not very clear and
thus welding control in this process is a bit difficult as compared to MIG welding.
3. Due to limited length of each electrode and brittle flux coating on it, mechanization
is difficult.
4. In welding long joints (e.g., in pressure vessels), as one electrode finishes, the weld
is to be progressed with the next electrode. Unless properly cared, a defect (like slag
inclusion or insufficient penetration) may occur at the place where welding is restarted
with the new electrode
5. The process uses stick electrodes and thus it is slower as compared to MIG welding.
Applications
1. Today, almost all the commonly employed metals and their alloys can be welded by
this process.

2. Shielded metal arc welding is used both as a fabrication process and for maintenance
and repair jobs.
3. The process finds applications in
(a) Building and Bridge construction
(b) Automotive and aircraft industry, etc.
(c) Air receiver, tank, boiler and pressure vessel fabrication
(d) Ship building
(e) Pipes and
(f) Penstock ( pipes or long channels that carry water down from the hydroelectric reservoir
to the turbines) joining.

SMAW Advantages
 Easily implemented
 Inexpensive
(initial investment in the process is low
in comparison to other welding
processes such as gas metal arc
welding)
 Flexible
(SMAW’s flexibility is unprecedented in
narrow access applications and, as the
above photograph shows, even in
underwater welding.)
 Not as sensitive to part fit-
up variances
Shielded Metal Arc Welding

Advantages
 Equipment relatively easy to use, inexpensive,
portable
 Filler metal and means for protecting the weld puddle
are provided by the covered electrode
 Less sensitive to drafts, dirty parts, poor fit-up
 Can be used on carbon steels, low alloy steels,
stainless steels, cast irons, copper, nickel, aluminum

Quality Issues
 Discontinuities associated
with manual welding
process that utilize flux
for pool shielding
 Slag inclusions
 Lack of fusion
 Other possible effects on
quality are porosity, and
hydrogen cracking
Shielded Metal Arc Welding

Limitations
 Low Deposition Rates
(This is because each welding rod contains
a finite amount of metal. As each
electrode is used, welding must be
stopped and a new rod inserted into the
holder)
 Low Productivity
The overall productivity of the process is impeded
by:
• Frequent changing of electrodes,
• Interpass cleaning (grinding, brushing, etc.),
• Grinding of arc initiation points and stopping
points,
• Slag inclusions which require removal of the
defect and rewelding of the defective area.
 Operator Dependent
Shileded Metal Arc Welding

Other Limitations
 Heat of welding too high for lead, tin, zinc, and their
alloys
 Inadequate weld pool shielding for reactive metals
such as titanium, zirconium, tantalum, columbium

SMAW Applications
 Used for steels, stainless steels, cast irons, and certain
nonferrous alloys.
 Not used or rarely used for aluminum and its alloys, copper
alloys, and titanium.
 Can be used in windy weather.
 Can be used on dirty metals (i.e. painted or rusted surfaces).
 Good for repair work.
 Makes thick welds.

Structural Steel Welding with the Shielded Metal Arc Process

Submerged Arc Welding
In this welding process, a consumable bare electrode is used in combination with a flux feeder
tube. The arc, end of the bare electrode and molten pool remain completely submerged under
blanket of granular flux.
The feed of electrode and tube is automatic and the welding is homogenous in structure. No
pressure is applied for welding purposes. This process is used for welding low carbon steel,
bronze, nickel and other non-ferrous materials.

Factors that should be considered when determining whether submerged arc welding can or should be used
for a given application include the following:
1. The chemical composition and mechanical properties required of the final weld deposit,
2. Thickness of base metal and alloy to be welded,
3. Joint accessibility,
4. Length of the joint,
5. Position in which the weld is to be made,
6. Frequency or volume of welding to be performed, and
7. The availability of capital for the submerged arc welding equipment expenditure.

ADVANTAGES AND LIMITATIONS
The main advantage of using the submerged arc welding process is high quality and productivity.
The process can be implemented in three different operational modes-semiautomated, mechanized, and
automated.
The main disadvantage of submerged arc welding is that it can be used only in the flat or horizontal
welding positions for plate and pipe welding.

Quality issues: weld defects
There are a range of weld defects that affect submerged arc welding. These include:
• Hydrogen embrittlement
• Solidification cracking
• Pores and pinholes
• Poor impact strength
• Undercutting
• Slag inclusions
• Uneven weld beads

AW: Consumable Electrodes: Gas Metal Arc Welding
(GMAW) or Metal Inert Gas (MIG) Welding
Uses a consumable bare metal wire as electrode with shielding by flooding arc with a
gas
1.Wire is fed continuously and automatically from a spool through the welding gun.
2.Shielding gases include argon and helium for aluminum welding, and CO2 for steel
welding.
3.Bare electrode wire (no flux) plus shielding gases eliminate slag on weld bead. No
need for manual grinding and cleaning of slag
4.Medium cost welding system: $1000 to $1200

GMAW Advantages over SMAW
1. Continuous welding because of continuous wire electrode. Sticks must be periodically
changed in SMAW.
2. Higher deposition rates.
3. Eliminates problem of slag removal.
4. Can be readily automated.
5. Has better control to make cleaner & narrower welds than SMAW.
6. Welding speeds are higher than those attained with shielded metal arc welding because of the
continuous electrode feed and higher filler metal deposition rates;
7. Because the electrode (wire feed) is continuous, long welds can be deposited without
intermediate stops and starts;

Limitations
 Equipment is more expensive and
complex than SMAW
 Restricted access
 GMAW gun is larger than
SMAW holder
 Gas metal arc welding is difficult to use in
hard-to-reach places because the welding
gun is larger than a shielded metal arc
electrode holder and the welding gun must
be close to the joint, (i.e., between 10
millimeters (mm) and 19 mm.) to ensure
that the weld metal is properly shielded.
Gas Metal Arc Welding

GMAW Applications
1. Used to weld ferrous and various non-ferrous metals.
2. Good for fabrications such as frames and farm equipment.
3. Can weld thicker metal (not as thick as SMAW).
4. Metal must be clean to start weld.

Shielding Gas
 Shielding gas can affect
 Weld bead shape
 Arc heat, stability
 Surface tension
 Drop size
 Puddle flow
 Spatter
Ar Ar-He He CO2
Gas Metal Arc Welding

Turn to the person sitting next to you and discuss (1 min.):
• When comparing processes that have spray and globular
metal transfer, which type of transfer mode do you thnk
results in more spatter? Why?

Non-consumable Electrode Welding Processes
 Gas Tungsten Arc Welding
 Plasma Arc Welding
 Carbon Arc Welding
 Laser Beam Welding
 Electron Beam Welding

1- Gas Tungsten Arc Welding (GTAW) or
Tungsten Inert Gas (TIG) Welding
Uses a non-consumable tungsten electrode and an inert gas
for arc shielding
1.Melting point of tungsten = 3410C (6170F).
2.Used with or without a filler metal. When filler metal used,
it is added to weld pool from separate rod or wire.
3.Applications: aluminum and stainless steel mostly.
4.High cost for welding system: $4000.

Gas tungsten arc welding (GTAW) is an arc welding process that uses an arc between a non-consumable
tungsten electrode and the workpiece to establish a weld pool. The process is used with shielding gas and
without the application of pressure, and may be used with or without the addition of filler metal.
Because of the high quality of welds that can be produced by gas tungsten arc welding, the process has
become an indispensable tool for many manufacturers, including those in the aerospace, nuclear,
marine, petrochemical and semiconductor industries.
NOTE
• Filler metal may or may not be used.
•It is also know as;
•TIG(Tungsten inert gas) in UK
•WIG(wolfram inert gas) in Germany
•GTAW in the USA

Gas Tungsten Arc Welding
Filler rod

Advantages and Disadvantages of GTAW
Advantages:
1. High quality welds for suitable applications
- Welds are cleaner and narrower than MIG
2. No spatter because no filler metal through the arc .Little or no post-weld cleaning because no
flux
4. Very little, if any, postweld cleaning is required
5. The arc and weld pool are clearly visible to the welder
Disadvantages:
1. More difficult to use than MIG welding
2. More costly than MIG welding

GTAW Applications
1. Used to weld ferrous and various non-ferrous metals.
2. Can weld various dissimilar metals together.
3. Good for fabrications such as aircraft or race car frames.
4. Used for welding thinner metal parts (not as thick as MIG).
5. Metal must be very clean to start weld.

2. Plasma arc welding (PAW): Introduction

􀂄PAW is an arc welding process that uses a constricted arc between a non consumable electrode and
the weld pool (transferred arc) or between the electrode and the constricted nozzle (non-
transferred arc).
􀂄 The process is used without the application of pressure. Filler metal may or may not be used.
􀂄 Shielding is obtained from the ionized gas issuing from the torch, which may be supplemented by an
auxiliary source of shielding gas.
􀂄 PAW is also used for metal cutting and for metal spraying.

Advantages of Plasma Arc Welding
Following are the advantages of Plasma Arc Welding:
➨Torch design allows better control of arc.
➨This method provides more freedom to observe and control the weld.
➨The higher heat concentration and plasma jet allows faster travel speeds.
➨The high temperature and high heat concentration of plasma allow keyhole effect. This
provides complete penetration with single pass welding of many joints.
➨Heat affected zone is smaller compare to GTAW.

Disadvantages of Plasma Arc Welding
Following are the disadvantages of Plasma Arc Welding:
➨It produces wider welds and heat affected zones compared to LBW and EBW.
➨Plasma welding equipments are very costly. Hence it will have higher start up costs.
➨It requires training and specialization to perform plasma welding.
➨It produces ultraviolet and infrared radiation.
➨The torch is bulky and hence manual welding is bit difficult and requires training.

PAW: Advantages and major uses
􀂄Advantages of PAWwhen compared to TIG stem from the fact that PAW has a higher energy concentration. Its higher
temperature, constricted cross-sectional area, and the velocity of the plasma jet create a higher heat content.
•The torch-to-work distance is less critical than for TIG , hence more freedom to observe and control the weld.
•The HAZ and the form of the weld are more desirable. The HAZ is smaller than with TIG, and the weld tends to
have more parallel sides, which reduces angular distortion.
•The higher heat concentration and the plasma jet allow for higher travel speeds. PAW has deeper penetration and
produces a narrower weld.
􀂄PAW is used the manufacturing of tubing, components made of thin metal, root-pass welds on pipe.
􀂄PAW is normally applied as a manual process. Automatic and mechanized app. (limited)
􀂄Join practically all of the commercial metals.
􀂄Filler rod is used for making welds in thicker materials.

PAW: Materials required
􀂄Filler metals is used except when welding the thinnest metal. The composition of the filler metal should
match the base metal. The size of the filler metal rod depends on the thickness of the base metal and the
welding current.
􀂄Plasma and shielding gas: An inert gas, either argon, helium, or a mixture, is used for shielding the weld
area from the atmosphere. Argon is more commonly used since it is heavier and provides better shielding
at lower rates.
Limitations:
Equipment and apparatus are delicate and complex.
The torch must be water cooled.
The tip of tungsten and orifice must be maintained within very close limits.

Electron beam welding (EBW)
■ It is a welding process that produces coalescence with a concentrated beam,
composed primarily of high-velocity electrons, inpinging on the joint.
■ The process is used without shielding gas and without the application of pressure.
■ It is a fusion welding process with the melting together of base metal, and possibly of
filler metal, to produce a weld.
■ Heat is generated in the workpiece as it is bombarded by a high velocity electron
beam. The kinetic energy (energy of motion) of the electrons is transferred to heat
upon impact.
■ The generated heat is a highly concentrated, high powered source and acts similar to
the arc of gas tungsten arc welding in making welds.
Welding
Technology

Advantages and major uses
■ Almost all metals can be welded with the electron beam welding process, i.e. Superalloys, refractory metals (tungsten), the
reactive metals and stainless steel.
■ EBW increases the range of alloys and thicknesses that can be welded to produce high integrity joints.
■ The manufacture of high specification components in military and commercial aircraft, space vehicles, satellites, and rockets
requires low heat input welds with minimum distortion.
■ EB systems have also been deployed for many other automotive applications, including gear, turbochargers, camshaft hardening,
etc.
■ EBW has tremendous penetration and has smaller HAZ.
■ The cooling rate is much higher.
■ Disadvantages:
□ high capital cost, expensive to operate due to need for vacuum pumps
□ May result porosity in welding of plain carbon steel

Laser beam welding (LBW)
■ LBW is a welding process that uses the heat generated when a focused laser beam
impinges on the joint.
■ LBW is used to join multiple pieces of metal through the use of a laser. The beam
provides a concentrated heat source, allowing for narrow, deep welds and high welding
rates. The process is frequently used in high volume applications, such as in the
automotive industry.
■ The process is used with or without a shielding gas and without the application of
pressure.

LBW: Main uses
■ Use of laser welding falls into two main categories:
□ precision processing applications
□ special, one-off tasks which draw on the unique capabilities of laser technology.
■ Materials that can be laser welded include:
□ low carbon and microalloyed steels
□ coated steels (including zinc coating)
□ stainless steels
□ nickel alloys
□ aluminium, titanium and magnesium and their alloys
□ some refractory metals
■ The sheet thicknesses which are suitable for laser welding are in the range 0.2-6mm
■ Current applications cover a wide range of industrial sectors including automotive, domestic
products, electronic and electrical, power generation, aerospace, shipbuilding and medical.

LBW: Principal characteristcs
■ The key features of the laser welding process are,
In terms of advantages:
■ high welding speed
■ continuous joints possible for improved stiffness, sealing and for corrosion resistance
■ low distortion
■ narrow weld profile
■ minimum finishing for visible panels, no mark welding possible on hem joints
■ high degree of control of heat input for welding of thinner gauge material
■ easy to automate

In terms of disadvantages are:
■ high capital and running costs
■fit-up/pressing tolerances must be accurate for beam/joint alignment and focus position
■ perceptions regarding safety issues

OTHER WELDING PROCESSES
Oxyfuel Gas Welding (OFW)
Group of fusion welding operations by a high temperature flame that burns various fuels
mixed with oxygen
Oxyfuel gas is also used in flame cutting torches to cut and separate metal plates and
other parts
Most important OFW process is oxyacetylene welding (has high temperatures – up to
3480C)
Filler metal is sometimes added
1. Composition must be similar to base metal
2. Filler rod often coated with flux to clean surfaces and prevent oxidation
Low cost for welding system: $400

1. Maximum temperature reached at tip of inner cone, while outer envelope spreads
out and shields work surface from atmosphere
2. Shown below is neutral flame of oxyacetylene torch indicating temperatures
achieved
Oxyacetylene Torch

Oxyacetylene Gas Welding Applications
 Suitable for low quantity production and repair jobs
 Used for welding thinner parts

Resistance Welding (RW)
Resistance welding includes a group of processes that produce coalescence of the faying surfaces with the
heat obtained from the resistance of the work pieces to the flow of the welding current in a circuit of which
the work pieces are a part, and by the application of pressure.
• The heat required for welding is produced by means of electrical
resistance across the two joining components
• Process does not require:
– Consumable electrodes
– Shielding gasses
– Flux
• Bond strength depends on surface roughness & cleanliness
• Requires specialized machinery (generally non-portable)
• Many facilities now automated
• Low operator skill level

The following factors affect the amount of heat generated in the weld joint by a given current for a
unit of weld time:
1.The electrical resistances within the workpieces and the electrodes,
2. The contact resistances between the workpieces and between the electrodes and the workpieces,
and
3. The heat lost to the workpieces and the electrodes.

Heat Balance
Heat balance occurs when the depth of fusion (penetration) in the two workpieces is approximately the
same. The majority of spot and seam welding applications involves the welding of similar thicknesses of the same metal,
with electrodes of the same alloy, shape, and size. In these cases, heat balance is automatic.
The heat generated in the workpiece is unbalanced, for applications in which welding is performed on
different gauges and grades of materials.
Heat balance may be affected by the following conditions:
1.Relative electrical and thermal conductivity of the workpieces,
2. Relative geometry of the workpieces at the joint,
3. Thermal and electrical conductivity of the electrodes, and
4. Geometry of the electrodes.

WELDING CYCLE
The welding cycle for spot and seam welding consists of the four basic phases: squeeze time, weld time,
hold time, and off time. Off time generally is used only for manually initiated repetitive welding cycles.
The phases of the welding cycle are described as follows:
1. Squeeze time—the time interval between initiating the timer and the first application of current; the
time interval added to ensure that the electrodes contact the workpieces and establish the desired
electrode force before welding current is applied;
2. Weld time—the time that welding current is applied to the workpieces in making a weld in single-
impulse welding;
3. Hold time—the time during which force is maintained on the workpieces after the last impulse of
current ends, allowing the weld nugget to solidify and cool until it has adequate strength; and
4. Off time—the time during which the electrodes are off the workpiece and the workpiece is moved to
the next weld location. The term is generally applied when the welding cycle is repetitive.

ADVANTAGES AND LIMITATIONS
The major advantages of resistance spot welding are its high speed and adaptability for automation in the
high-rate production of sheet metal assemblies. Spot welding is also economical in many job shop
operations because it is faster than arc welding or brazing and requires less skill to perform.
Some of the limitations of the process are the following:
1.Disassembly for maintenance or repair is very difficult;
2. The equipment costs generally are higher than the costs of most arc welding equipment;
3. The short time, high current, and high power requirements produce unfavorable line power demands,
particularly with single-phase machines; and
5. Heat produced by the spot weld tends to develop in the center of a stack of sheets; therefore, welding a
thin outside sheet to two thick sheets becomes difficult and is not recommended.

Solid State Welding Processes
It is a welding process, in which two work pieces are joined under a pressure providing
an intimate contact between them and at a temperature essentially below the melting
point of the parent material.
Bonding is a result of diffusion of the interface atoms

Solid State Welding Processes
• Diffusion welding
• Explosion welding
• Friction welding
• Friction- stir welding
• Forge welding
• Cold welding
• Roll welding
• Hot pressure welding
• Ultrasonic welding, etc.

Solid State Bonding Involves one or more of:
• Diffusion: the transfer of atoms across an interface
– Facilitated by heat
• Friction
• Electrical-resistance
• Pressure:
– The higher the pressure, the stronger the interface
– May combine pressure & resistance heating
• Relative interfacial movements
– Create clean surfaces
– Even small amplitudes improve bond strength

Diffusion Bonding
• Uses high pressure autoclaves for complex parts
• Suitable for joining
– Dissimilar metals (most common)
– Reactive metals (e.g. Titanium, Beryllium)
– Metal-matrix composite materials
• An important PM sintering mechanism
• Relatively slow process
– To allow time for diffusion
• Automation enables economic production in moderate volumes;
– Aerospace, nuclear, electronics
• Requires skilled operator

The process is particularly suitable for welding aluminium, e.g. for making longitudinal welds along aluminium
extrusions. It is also possible to use the method with certain other materials such as copper, titanium, lead, zinc and
magnesium. Trials of welding plastics have also been carried out.
The advantages of the method are as follows:
• The quality of the joint is consistently good. The root face can be so good that the weld is almost invisible, while the
top is essentially smooth, but with a puddled surface effect left by the rotating tool.
• The welded joint has excellent fatigue strength.
• With a low heat input, there is very little thermal stress or distortion.
• Mechanical properties are better preserved compared to arc welding.
• No joint preparation is necessary.
• FSW may be used also for alloys that are crack sensitive when they are welded with normal fusion welding processes.

Advantages
• There is no visible radiation, noise or fume generation.
• No filler materials are required.
• The production rate is comparable with that of other methods.
• The method shows good profitability due to very little need for preparation or
subsequent processing.
Limitations:
• The formation of a hole from the tool where it stops can be a disadvantage.
• Heavy and powerful fixtures are needed to keep the parts of the workpiece
together and pressed to the backing plate.

TYPES OF DEFECTS
Slag Inclusion
Undercut
Porosity
Incomplete fusion
Overlap
Underfill
Spatter
Excessive Convexity
Excessive Weld Reinforcement
Incomplete Penetration
Excessive Penetration

SLAG INCLUSION
Cause:- Low amperage, improper techniques, slow travel rate
Prevention:- Increase amperage, increase travel rate Repair:-
Remove by grinding or other mechanical process

UNDERCUT
Cause:- High amperage, wrong electrode angle, long arc length, rust
Prevention:- clean metal before welding
Repair:- Weld with smaller electrode, sometimes must be low hydrogen with
preheat.

Cavities
Two defect types, similar to defects found in castings:
1. Porosity - small voids in weld metal formed by
gases entrapped during solidification
 Caused by inclusion of atmospheric gases,
sulfur in weld metal, or surface contaminants
2. Shrinkage voids - cavities formed by shrinkage
during solidification

INCOMPLETE FUSION
Cause:- Low amperage, steep electrode angle, fast travel speed, short arc
gap, lack of preheat, electrode too small, unclean base metal, arc off seam
Prevention:- Eliminate the potential causes
Repair:- Remove & reweld, being careful to completely remove the defective
area.

OVERLAP
Cause:- Improper welding technique, steep electrode angle, fast travel speed
Prevention:- Overlap is a contour problem. Proper welding technique will
prevent this problem

UNDERFILL
Cause:- Improper welding techniques
Prevention:- Apply proper welding techniques for the weld type & position.
Use stripper beads before the cover pass.
Repair:- Simply weld to fill. May require preparation by grinding.

SPATTER
Cause:- High arc power, Damp electrodes
Prevention:- Reduce arc power, reduce arc length, use dry electrodes
Repair:- Remove by mechanical process

EXCESSIVE CONVEXITY
Cause:- Amperage & travel speed
Prevention:- Observe proper parameters & techniques
Repair:- Must blend smoothly into the base metal

EXCESSIVE CONCAVITY
Cause:- Amperage & travel speed
Prevention:- Observe proper parameters & techniques
Repair:- Must blend smoothly into the base metal

INCOMPLETE PENETRATION
Cause:- Low amperage, low preheat, tight root opening, fast travel speed,
short arc length
Prevention:- Correct the contributing factors. Repair:- Back gauge and back
weld

 Various forms of welding cracks
Welding Cracks

lecture 5 - Welding Processes 2019 (1).ppt

More Related Content

Similar to lecture 5 - Welding Processes 2019 (1).ppt (20)

Recently uploaded (20)

lecture 5 - Welding Processes 2019 (1).ppt

Editor's Notes