29 FUNDAMENTALS OF WELDING
Chapter Contents
29.1 Overview of Welding Technology
29.1.1 Types of Welding Processes
29.1.2 Welding as a Commercial Operation
29.2 The Weld Joint
29.2.1 Types of Joints
29.2.2 Types of Welds
29.3 Physics of Welding
29.3.1 Power Density
29.3.2 Heat Balance in Fusion Welding
29.4 Features of a Fusion-Welded Joint
The term joining is generally used for welding, brazing, soldering, and adhesive bonding, which form a permanent joint between the parts—a joint that cannot easily be separated.
The term assembly usually refers to mechanical meth ods of fastening parts together. Some of these methods allow for easy disassembly, while others do not.
30 WELDING PROCESSES
Chapter Contents
30.1 Arc Welding
30.1.1 General Technology of Arc Welding
30.1.2 AW Processes—Consumable Electrodes
30.1.3 AW Processes—Nonconsumable Electrodes
30.2 Resistance Welding
30.2.1 Power Source in Resistance Welding
30.2.2 Resistance-Welding Processes
30.3 Oxyfuel Gas Welding
30.3.1 Oxyacetylene Welding
30.3.2 Alternative Gases for Oxyfuel Welding
30.4 Other Fusion-Welding Processes
30.5 Solid-State Welding
30.5.1 General Considerations in Solid-State Welding
30.5.2 Solid State-Welding Processes
30.6 Weld Quality
30.7 Weldability
30.8 Design Considerations in Welding
Introduction
Welding is a process for joining two similar or dissimilar metals by fusion with or without the application of pressure and with or without the use of filler metal. The heat for fusion may be generated either from combustion of gases, electric arc, electric resistance or by chemical reaction. Welding provides a permanent joint.
The welding is widely used as a fabrication and repairing process in industries. Some of the typical applications of welding include the fabrication of ships, pressure vessels, automobile bodies, off-shore platform, bridges, welded pipes, sealing of nuclear fuel and explosives, etc.
Weldability
The ease of welding metals is described by the term ‘weldability’. The weldability may be defined as property of a metal which indicates the ease with which it can be welded with other similar or dissimilar metals.
Weldability of a material depends upon various factors like the metallurgical changes that occur due to welding, changes in hardness in and around the weld, gas evolution and absorption, extent of oxidation, and the effect on cracking tendency of the joint. Plain low carbon steel (Carbon up to 0.12%) has the best weldability amongst metals. Generally it is seen that the materials with high castability usually have low weldability.
ELEMENTS OF WELDING PROCESS
Edge preparation
For welding the edges of joining surfaces of metals are prepared first. Different edge preparations
may be used for welding butt joints.
Welding joints
Welding joints are of generally of two major kinds namely lap joint and butt joint.
Lap weld joint
Single-Lap Joint
This joint, made by overlapping the edges of the plate, is not recommended for most work. The single lap has very little resistance to bending. It can be used satisfactorily for joining two cylinders that fit inside one another.
Double-Lap Joint
This is stronger than the single-lap joint but has the disadvantage that it requires twice as much welding.
Tee Fillet Weld
This type of joint, although widely used, should not be employed if an alternative design is possible.
Butt weld joint
Single-Vee Butt Weld
It is used for plates up to 15.8 mm thick. The angle of the vee depends upon the technique being used, the plates being spaced approximately 3.2 mm.
Double-Vee Butt Weld
It is used for plates over 13 mm thick when the welding can be performed on both sides of the plate. The top vee angle is either 60° or 80°, while the bottom angle is 80°, depending on the technique being used.
Welding Positions
There are four types of welding positions.
1. Flat or down hand position
2. Horizontal position
3. Vertical position
4. Overhead position
Flat or Downhand Welding Position
The flat position or down hand position is one in which the welding is performed from
the upper side of the joint and the face of the weld is approximately horizontal. This is the
simplest and the most convenient position for welding. Using this technique, excellent welded
joints at a fast speed with minimum risk of fatigue to the welders can be obtained.
Horizontal Welding Position
In horizontal position, the plane of the workpiece is vertical and the deposited weld head
is horizontal. The metal deposition rate in horizontal welding is next to that achieved in flat
or downhand welding position. This position of welding is most commonly used in welding
vessels and reservoirs.
Vertical Welding Position
In vertical position, the plane of the workpiece is vertical and the weld is deposited upon a vertical surface. It is difficult to produce satisfactory welds in this position due to the effect of the force of gravity on the molten metal. The welder must constantly control the metal so that it does not run or drop from the weld. Vertical welding may be of two types viz., vertical-up and vertical-down. Vertical-up welding is preferred when strength is the major consideration. The vertical-down welding is used for a sealing operation and for welding sheet metal.
Overhead Welding Position
The overhead position is probably even more difficult to weld than the vertical position. Here the pull of gravity against the molten metal is much greater. The force of the flame against the weld serves to counteract the pull of gravity. In overhead position, the plane of the work piece is horizontal. But the welding is carried out from the underside. The electrode is held with its welding end upward. It is a good practice to use very short arc and basic coated electrodes for overhead welding.
WELDING PROCESSES
Types of Welding Processes
The general classification of welding processes
1. Oxy-Fuel Gas Welding Processes
2. Arc Welding Processes
3. Resistance Welding
4. Solid-State Welding Processes
5. Thermit Welding Processes
6. Radiant Energy Welding Processes
GAS WELDING PROCESSES
A fusion welding process which joins metals, using the heat of combustion of an oxygen /air and fuel gas (i.e. acetylene, hydrogen propane or butane) mixture is usually referred as ‘gas welding’. The intense heat (flame) thus produced melts and fuses together the edges of the parts to be welded, generally with the addition of a filler metal. The fuel gas generally employed is acetylene; however gases other than acetylene can also be used though with lower flame temperature. Oxy-acetylene flame is the most versatile and hottest of all the flames produced by the combination of oxygen and other fuel gases. Other gases such as Hydrogen, Propane, Butane, Natural gas etc., may be used for some welding applications.
Oxy-Acetylene Welding
In this process, acetylene is mixed with oxygen in correct proportions in the welding torch and ignited. The flame resulting at the tip of the torch is sufficiently hot to melt and join the parent metal. The oxy-acetylene flame reaches a temperature of about 3300°C and thus can melt most of the ferrous and non-ferrous metals in common use. A filler metal rod or welding rod is generally added to the molten metal pool to build up the seam slightly for greater strength.
Types of Welding Flames
In oxy-acetylene welding, flame is the most important means to control the welding joint and the welding process. The correct type of flame is essential for the production of satisfactory welds. The flame must be of the proper size, shape and condition in order to operate with maximum efficiency. There are three basic types of oxy-acetylene flames.
1. Neutral welding flame (Acetylene and oxygen in equal proportions).
2. Carburizing welding flame or reducing (excess of acetylene).
3. Oxidizing welding flame (excess of oxygen).
Neutral Welding Flame
A neutral flame results when approximately equal volumes of oxygen and acetylene are mixed in the welding torch and burnt at the torch tip. The temperature of the neutral flame is of the order of about 5900°F (3260°C). It has a clear, well defined inner cone, indicating that the combustion is complete. A neutral flame affects no chemical change on the molten metal and, therefore will not oxidize or carburize the metal. The neutral flame is commonly used for the welding of mild steel, stainless steel, cast Iron, copper, and aluminium.
Carburising and Reducing Welding Flames
The carburizing and reducing flames have excess of acetylene and can be recognized by acetylene feather, which exists between the inner cone and the outer envelope. The outer flame envelope is longer than that of the neutral flame and is usually much brighter in color.
With iron and steel, carburizing flame produces very hard, brittle substance known as iron carbide. A carburizing-flame is used in the welding of lead and for carburizing (surface hardening) purpose.
A reducing flame may be distinguished from carburizing flame by the fact that a carburizing flame contains more acetylene than a reducing flame. A reducing flame has an approximate temperature of 3038°C. A reducing flame, does not carburize the metal; rather it ensures the absence of the oxidizing condition. It is used for welding with low alloy steel rods and for welding those metals, (e.g., non-ferrous) that do not tend to absorb carbon. This flame is very well used for welding high carbon steel.
Oxidising Welding flame
The oxidizing flame has an excess of oxygen over the acetylene. An oxidizing flame can be recognized by the small cone, which is shorter, much bluer in color and more pointed than that of the neutral flame. The outer flame envelope is much shorter and tends to fan out at the end. Such a flame makes a loud roaring sound.
It is the hottest flame (temperature as high as 6300°F) produced by any oxy-fuel gas source. The excess oxygen especially at high temperatures tends to combine with many metals to form hard, brittle, low strength oxides. Moreover, an excess of oxygen causes the weld bead and the surrounding area to have a scummy or dirty appearance. For these reasons, an oxidizing flame is of limited use in
welding. It is not used in the welding of steel. A slightly oxidizing flame is helpful when welding (i) Copper-base metals Hi) Zinc-base metals and (Hi) A few types of ferrous metals such as manganese steel and cast iron. The oxidizing atmosphere in these cases, create a base-metal oxide that protects the base metal.
Gas Welding Equipments
The basic tools and equipments used for oxy-acetylene welding are following:
Acetylene and Oxygen gas cylinders.
Acetylene and oxygen gas is stored in compressed gas cylinders. These gas cylinders differ widely in capacity, design and colour code. However, in most of the countries, the standard size of these cylinders is 6 to 7 cubic meters. It is painted black for oxygen and maroon for acetylene. An acetylene cylinder is filled with some absorptive material, which is saturated with a chemical solvent acetone. Acetone has the ability to absorb a large volume of acetylene and release it as the pressure falls. If large quantities of acetylene gas are being consumed, it is much cheaper to generate the gas at the place of use with the help of acetylene gas generators. Acetylene gas is generated by carbide-to-water method.
Oxygen gas cylinders are usually equipped with about 40 litres of oxygen at a pressure of about 154 Kgf/cm2 at 21°C. To provide against dangerously excessive pressure, such as could occur if the cylinders were exposed to fire, every valve has a safety device to release the oxygen before there is any danger of rupturing the cylinders. Fragile discs and fusible plugs are usually provided in the cylinders valves in case it is subjected to danger.
Gas pressure regulators
Gas pressure regulators are employed for regulating the supply of acetylene and oxygen gas from cylinders. A pressure regulator is connected between the cylinder and hose leading to welding torch. The cylinder and hose connections have left-handed threads on the acetylene regulator while these are right handed on the oxygen regulator. A pressure regulator is fitted with two pressure gauges, one for indication of the gas pressure in the cylinder and the other for indication of the reduced pressure at which the gas is going out.
Welding torch
Welding torch is a tool for mixing oxygen and acetylene in correct proportion and burning the mixture at the end of a tip. Gas flow to the torch is controlled with the help of two needle valves in the handle of the torch. There are two basic types of gas welding torches:
(1) Positive pressure (also known as medium or equal pressure), and
(2) Low pressure or injector type
The positive pressure type welding torch is the more common of the two types of oxy-acetylene torches.
Torch tips
It is the portion of the welding apparatus through which the gases pass just prior to their
ignition and burning. A great variety of interchangeable welding tips differing in size, shape
and construction are available commercially. The tip sizes are identified by the diameter of
the opening. The diameter of the tip opening used for welding depends upon the type of metal
to be welded.
Hose pipes
The hose pipes are used for the supply of gases from the pressure regulators. The most
common method of hose pipe fitting both oxygen and acetylene gas is the reinforced rubber
hose pipe. Green is the standard color for oxygen hose, red for acetylene, and black hose for
other industrially available welding gases.
Goggles: These are fitted with colored lenses and are used to protect the eyes from harmful heat and ultraviolet and infrared rays.
Gloves: These are required to protect the hands from any injury due to the heat of welding process.
Spark-lighter: It is used for frequent igniting the welding torch.
Filler rods
Gas welding can be done with or without using filler rod. When welding with the filler rod, it should be held at approximately 900 to the welding tip. Filler rods have the same or nearly the same chemical composition as the base metal. Metallurgical properties of the weld deposit can be controlled by the optimum choice of filler rod. Most of the filler rods for gas welding also contain deoxidizers to control the oxygen content of weld pool.
Fluxes
Fluxes are used in gas welding to remove the oxide film and to maintain a clean surface. These are usually employed for gas welding of aluminium, stainless steel, cast iron, brass and silicon bronze. They are available in the market in the form of dry powder, paste, or thick solutions.
ARC WELDING PROCESSES
The process, in which an electric arc between an electrode and a workpiece or between two
electrodes is utilized to weld base metals, is called an arc welding process. Most of these processes use some shielding gas while others employ coatings or fluxes to prevent the weld pool from the surrounding atmosphere.
Various arc welding processes
1. Carbon Arc Welding
2. Shielded Metal Arc Welding
3. Flux Cored Arc Welding
4. Gas Tungsten Arc Welding
5. Gas Metal Arc Welding
6. Plasma Arc Welding
7. Atomic Hydrogen Welding
8. Electroslag Welding
9. Stud Arc Welding
10. Electrogas Welding
Arc Welding Equipment
The important components of arc welding setup are:1 . Arc welding power source
Both direct current (DC) and alternating current (AC) are used for electric arc welding,
each having its particular applications. DC welding supply is usually obtained from generators
driven by electric motor or if no electricity is available by internal combustion engines. For
AC welding supply, transformers are predominantly used for almost all arc welding where
mains electricity supply is available. They have to step down the usual supply voltage (200-
400 volts) to the normal open circuit welding voltage (50-90 volts).
2. Welding cables
Welding cables are required for conduction of current from the power source through
the electrode holder, the arc, the work piece and back to the welding power source. These are
insulated copper or aluminium cables.
3. Electrode holder
Electrode holder is used for holding the electrode manually and conducting current to
it. These are usually matched to the size of the lead, which in turn matched to the amperage
output of the arc welder. Electrode holders are available in sizes that range from 150 to 500
Amps.
4. Welding Electrodes
An electrode is a piece of wire or a rod of a metal or alloy, with or without coatings. An
arc is set up between electrode and work piece. Welding electrodes are classified into following
types:
(1) Consumable Electrodes: (a) Bare Electrodes, ( b ) Coated Electrodes
(2) Non-consumable Electrodes: (a) Carbon or Graphite Electrodes, ( b ) Tungsten Electrodes
Consumable electrode is made of different metals and their alloys. The end of this
electrode starts melting when arc is struck between the electrode and work piece. Thus
consumable electrode itself acts as a filler metal. Bare electrodes consist of a metal or alloy
wire without any flux coating on them. Coated electrodes have flux coating which starts
melting as soon as an electric arc is struck. This coating on melting performs many functions
like prevention of joint from atmospheric contamination, arc stabilizers etc.
Non-consumable electrodes are made up of high melting point materials like carbon,
pure tungsten or alloy tungsten etc. These electrodes do not melt away during welding. But
practically, the electrode length goes on decreasing with the passage of time, because of
oxidation and vaporization of the electrode material during welding. The materials of non-
consumable electrodes are usually copper coated carbon or graphite, pure tungsten, thoriated
or zirconiated tungsten.
5. Hand Screen: Hand screen used for protection of eyes and supervision of weld bead.
6. Chipping hammer: Chipping Hammer is used to remove the slag by striking.
7. Wire brush: Wire brush is used to clean the surface to be weld.
8. Protective clothing: Operator wears the protective clothing such as apron to keep away the exposure of direct heat to the body.
1. Carbon Arc Welding
In this process, a pure graphite or baked carbon rod is used as a non-consumable electrode to create an electric arc between it and the work piece. The electric arc produces heat and weld can be made with or without the addition of filler material. Carbon arc welding may be classified as:
(1) Single electrode arc welding, and
(2) Twin carbon electrode arc welding
In single electrode arc welding, an electric arc is struck between a carbon electrode and
the workpiece. Welding may be carried out in air or in an inert atmosphere. Direct current
straight polarity (DCSP) is preferred to restrict electrode disintegration and the amount of
carbon going into the weld metal. This process is mainly used for providing heat source for
brazing, braze welding, soldering and heat treating as well as for repairing iron and steel
castings. It is also used for welding of galvanized steel and copper.
In twin carbon arc welding the arc struck between two carbon electrodes produces heat
and welds the joint. The arc produced between these two electrodes heats the metal to the
melting temperature and welds the joint after solidification. The power source used is AC
(Alternating Current) to keep the electrodes at the same temperature. Twin-electrode carbon
arc welding can be used for welding in any position. This process is mainly used for joining
copper alloys to each other or to ferrous metal. It can also be used for welding aluminium,
nickel, zinc and lead alloys.
2. Shielded Metal Arc Welding (SMAW) or Manual Metal Arc Welding (MMAW)
Flux coated electrode is used
Shielded metal arc welding (SMAW) is a commonly used arc welding process manually carried by welder. It is an arc welding process in which heat for welding is produced through an electric arc set up between a flux coated electrode and the workpiece. The flux coating of electrode decomposes due to arc heat and serves many functions, like weld metal protection, arc stability etc. Inner core of the electrode supply the filler material for making a weld. If the parent metal is thick it may be necessary to make two or three passes for completing the weld.
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. Due 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. 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
4. 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. 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 joining
Functions of Electrode Coating Ingredients
The covering coating on the core wire consists of many materials which perform a
number of functions as listed below:
1. Slag forming ingredients, like silicates of magnesium, aluminium, sodium, potassium,
iron oxide, china clay, mica etc., produce a slag which because of its light weight
forms a layer on the molten metal and protects the same from atmospheric
contamination.
2. Arc stabilizing constituents like calcium carbonate, potassium silicate, titanates,
magnesium silicates, etc.; add to arc stability and ease of striking the same.
3. Gas shielding ingredients, like cellulose, wood, wood flour, starch, calcium carbonate
etc. form a protective gas shield around the electrode end, arc and weld pool
4. Deoxidizing elements like ferro-manganese, and ferro-silicon, refine the molten
metal.
5. Alloying elements like ferro alloys of manganese, molybdenum etc., may be added
to impart suitable properties and strength to the weld metal and to make good the
loss of some of the elements, which vaporize while welding.
6. Iron powder in the coating improves arc behavior, bead appearance helps increase
metal deposition rate and arc travel speed.
7. Proper coating ingredients produce weld metals resistant to hot and cold cracking.
Suitable coating will improve metal deposition rates.
3. Submerged Arc Welding (flux feeder tube)
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.
4. Gas Tungusten Arc Welding (GTAW) or Tungusten Inert Gas Welding (TIG)
In this process a non-consumable tungsten electrode is used with an envelope of inert shielding
gas around it. The shielding gas protects the tungsten electrode and the molten metal weld
pool from the atmospheric contamination. The shielding gases generally used are argon,
helium or their mixtures.
Electrode materials
The electrode material may be tungsten, or tungsten alloy (thoriated tungsten or zirconiated tungsten). Alloy-tungsten electrodes possess higher current carrying capacity, produce a steadier arc as compared to pure tungsten electrodes and high resistance to contamination.
Electric power source
Both AC and DC power source can be used for TIG welding. DC is preferred for welding
of copper, copper alloys, nickel and stainless steel whereas DC reverse polarity (DCRP) or AC
is used for welding aluminium, magnesium or their alloys. DCRP removes oxide film on
magnesium and aluminium.
Inert gases
The following inert gases are generally used in TIG welding:
1. Argon
2. Helium
3. Argon-helium mixtures
4. Argon-hydrogen mixtures
The Nozzle
The nozzle or shield size (the diameter of the opening of the shroud around the electrode)
to be chosen depends on the shape of the groove to be welded as well as the required gas
flow rate. The gas flow rate depends on the position of the weld as well as its size. Too high
a gas consumption would give rise to turbulence of the weld metal pool and consequently
porous welds. Because of the use of shielding gases, no fluxes are required to be used in inert
gas shielded arc welding. However for thicker sections, it may be desirable to protect the root
side of the joint by providing a flux. The process is generally used for welding aluminium,
magnesium and stainless steel.
5. Gas Metal ARC Welding (GMAW) or Metal Inert Gas Welding (MIG)
Metal inert gas arc welding (MIG) or more appropriately called as gas metal arc welding (GMAW) utilizes a consumable electrode and inert gas supply. The term metal appears in the title indicating the additional metal deposition through consumable electrode.
There are other gas shielded arc welding processes utilizing the consumable electrodes, such as flux cored arc welding (FCAW) all of which can be termed under MIG. Though gas tungsten arc welding (GTAW) can be used to weld all types of metals, it is more suitable for thin sheets. When thicker sheets are to be welded, the filler metal requirement makes GTAW difficult to use. In this situation, the GMAW comes handy.
The consumable electrode is in the form of a wire reel which is fed at a constant rate, through the feed rollers. The welding torch is connected to the gas supply cylinder which provides the necessary inert gas. The electrode and the work-piece are connected to the welding power supply. The power supplies are always of the constant voltage type only. The current from the welding machine is
changed by the rate of feeding of the electrode wire. Normally DC arc welding machines
are used for GMAW with electrode positive (DCRP). The DCRP increases the metal deposition
rate and also provides for a stable arc and smooth electrode metal transfer. With DCSP, the
arc becomes highly unstable and also results in a large spatter.
But special electrodes having calcium and titanium oxide mixtures as coatings are found to be good for welding steel with DCSP. In the GMAW process, the filler metal is transferred from the electrode
to the joint. Depending on the current and voltage used for a given electrode, the metal transfer is done in different ways.
RESISTANCE WELDING
In resistance welding the metal parts to be joined are heated by their resistance to the flow
of an electrical current. Usually this is the only source of heat, but a few of the welding
operations combine resistance heating with arc heating, and possibly with combustion of
metal in the arc. The process applies to practically all metals and most combinations of pure
metals and those alloys, which have only a limited plastic range, are welded by heating the
parts to fusion (melting). Some alloys, however, may welded without fusion; instead, the parts
are heated to a plastic state at which the applied pressure causes their crystalline structures
to grow together. The welding of dissimilar metals may be accomplished by melting both
metals frequently only the metal with the lower melting point is melted, and an alloy bond
is formed at the surface of the unmelted metal.
In resistance welding processes no fluxes are employed, the filler metal is rarely used
and the joints are usually of the lap type.
Types of Resistance welding
The major types of resistance welding are given as under:
(1) Spot Welding
(2) Seam Welding
(3) Projection Welding
(4) Resistance Butt Welding
(5) Flash Butt Welding
(6) Percussion Welding
(7) High Frequency Resistance Welding
(8) High Frequency Induction Welding
Some of the above important welding processes are discussed as under,
1. Spot Welding
In this process overlapping sheets are joined by local fusion at one or more spots, by the
concentration of current flowing between two electrodes. This is the most widely used resistance
welding process. It essentially consists of two electrodes, out of which one is fixed. The other electrode is fixed to a rocker
arm (to provide mechanical advantage) for transmitting the mechanical force from a pneumatic
cylinder. This is the simplest type of arrangement. The other possibility is that of a pneumatic
or hydraulic cylinder being directly connected to the electrode without any rocker arm. For
welding large assemblies such as car bodies, portable spot welding machines are used. Here
the electrode holders and the pneumatic pressurizing system are present in the form of a
portable assembly which is taken to the place, where the spot is to be made. The electric
current, compressed air and the cooling water needed for the electrodes is supplied through
cables and hoses from the main welding machine to the portable unit. In spot welding, a
satisfactory weld is obtained when a proper current density is maintained. The current
density depends on the contact area between the electrode and the work-piece. With the
continuous use, if the tip becomes upset and- the contact area increases, the current density
will be lowered and consequently the weld is obtained over a large area. This would not be able
to melt the metal and hence there would be no proper fusion. A resistance welding schedule
is the sequence of events that normally take place in each of the welds. The events are:
1. The squeeze time is the time required for the electrodes to align and clamp the two
work-pieces together under them and provide the necessary electrical contact.
2. The weld time is the time of the current flow through the work-pieces till they are
heated to the melting temperature.
3. The hold time is the time when the pressure is to be maintained on the molten
metal without the electric current. During this time, the pieces are expected to be
forged welded.
4. The off time is time during which, the pressure on the electrode is taken off so that
the plates can be positioned for the next spot.
Before spot welding one must make sure that
(i) The job is clean, i.e., free from grease, dirt, paint, scale, oxide etc.
(ii) Electrode tip surface is clean, since it has to conduct the current into the work with
as little loss as possible. Very fine emery cloth may be used for routine cleaning.
(iii) Water is running through the electrodes in order to
(a) Avoid them from getting overheated and thus damaged,
( b ) Cool the weld.
(iv) Proper welding current has been set on the current selector switch.
(v) Proper time has been set on the weld-timer.
Spot welding electrodes
Spot welding electrodes are made of materials which have
(1) Higher electrical and thermal resistivities, and
(2) Sufficient strength to withstand high pressure at elevated temperatures.
Copper base alloys such as copper beryllium and copper tungsten are commonly used
materials for spot welding electrodes. For achieving the desired current density, It is important
to have proper electrode shape for which three main types of spot welding electrodes are used
which are pointed, domed and flat electrodes.
Applications of Spot Welding
(i) It has applications in automobile and aircraft industries
(ii) The attachment of braces, brackets, pads or clips to formed sheet-metal parts such
as cases, covers or trays is another application of spot welding.
(iii) Spot welding of two 12.5 mm thick steel plates has been done satisfactorily as a
replacement for riveting.
(iv) Many assemblies of two or more sheet metal stampings that do not require gas tight
or liquid tight joints can be more economically joined by spot welding than by
mechanical methods.
(v) Containers and boxes frequently are spot welded.
2. Resistance Seam Welding
It is a continuous type of spot welding wherein spot welds overlap each other to the desired extent. In this process coalescence at the faying surfaces is produced by the heat obtained from the resistance to electric current (flow) through the work pieces held together under pressure by circular electrodes. The resulting weld is a series of overlapping resistance-spots welds made progressively along a joint by rotating the circular electrodes. The seam welding is similar to spot welding, except that circular rolling electrodes are used to produce a continuous air-tight seam of overlapping welds. Overlapping continuous spot welds seams are produced by the rotating electrodes and a regularly interrupted current.
Applications
1. It is used for making leak proof joints in fuel tanks of automobiles.
2. Except for copper and high copper alloys, most other metals can be seam welded.
3. It is also used for making flange welds for use in watertight tanks.
3. Resistance Projection Welding
This process is a resistance welding process in which two or more than two spot welds are
made simultaneously by making raised portions or projections on predetermined locations on
one of the workpiece. These projections act to localize the heat of the welding circuit. The
pieces to be welded are held in position under pressure being maintained by electrodes. The
projected contact spot for welding should be approximately equal to the weld metal
thickness. The welding of a nut on the automotive chasis is an example of projection
welding.
4. Resistance Upset Butt and Flash Butt Welding
This welding is also used for joining metal pieces end to end but it has largely replaced
the butt-welding method for weld articles small cross-sections. It can be used for thick
sections also. Initially the current is switched on and then one end the moveable part to be
welded is brought gently closer to the fixed end of the other part to localize heat at the ends
and thus raises the temperature of the ends quickly to the welding heat. On acquiring contact
of fixed end and moveable end with each other, the moveable end is then pressed against one
another by applying mechanical pressure. Thus the molten metal and slag to be squeezed out
in the form of sparks enabling the pure metal to form the joint and disallowing the heat .to
spread back. In this resistance welding single phase A.C. machines are commonly employed.
The merits and demerit of flash welding over simple butt- welding are follows:
Merits
1. It is comparatively much faster than butt welding.
2. This method utilizes less current in comparison to butt welding as the small portion
of the metal is only being heated for getting a good weld
3. Created joint by this welding is much stronger than the butt welding joint. Also the
strength of the weld produced is high even more than that of the base metal. The
end of the metal pieces to be welded in this welding need not be squared as it is
the basic requirement in butt-welding.
4. A high degree of accuracy can be easily achieved in terms of length alignment of weld.
Demerits
1. The periodic maintenance of machine and replacement of insulation is needed as
flashing particles of molten metal are thrown out during welding which may enter
into the slide ways and insulation of the set up.
2. Welder has to take enough care against possible fire hazard due to flashing during
welding.
3. Additional stock has to be provided for compensating loss of metal during flashing
and upsetting. This increases to the cost of weld.
4. Cost of removal of flash weld metal by trimming, chipping, grinding, etc. will increase
to the welded product.
5. Surface of the jobs where they come in contact with the gripping surfaces, should
be clean otherwise they will restrict the flow of electric current.
6. The available power, opening between the jaws of the gripping clamps and upsetting
pressure of the welding set limit the size and cross sectional area of the jobs to be
welded.
Applications
All conducting forged metals can be easily be flash welded. A number of dissimilar metals
can also be welded by controlling the welding conditions carefully. Metals generally welded
metal by the process involves lead, tin, antimony, zinc, bismuth and their alloys, low carbon
steels, stainless steel, alloy steels, tool steels, copper alloys, aluminium alloys, magnesium
alloys nickel alloys, molybdenum alloys, and titanium alloys. This process is used in automobile
industry, welding of solid and tubular structural assemblies, etc. in air-craft industry, welding
of band saw blades, welding of tool steel drills, reamers and taps etc. to mild steel or alloy
steel shanks, welding of pipes and tubes.
Common Advantages of Resistance Welding
Some common advantages of resistance welding include:
(a) It is well suited for mass production.
( b ) It is economical in operation, since nothing is consumed except electrical power.
(c) Skilled welders are not required.
(d) Welds are quickly made.
(e) It is possible to weld dissimilar metals.
Some disadvantages of resistance welding include:
(a) High initial cost of the resistance welding equipment
( b ) Certain resistance welding processes are limited to lap joints.
(c) A lap joint has an inherent service between the two metal pieces, which causes stress concentrations in applications where fatigue is present. This service may also cause trouble when corrosion is present
SOLID STATE WELDING PROCESSES
In these processes, the base materials to be joined are heated to a temperature below or just
up to the solidus temperature and then continuous pressure is applied to form the welded
joint. No filler metal is used in solid-state welding processes. The various solid-state welding
processes are-
(1) Forge Welding
(2) Cold Pressure Welding
(3) Friction Welding
(4) Explosive Welding
(5) Diffusion Welding
(6) Thermo-compression Welding
1. Forge Welding
In this welding process, the work-pieces to be welded are heated to the plastic condition (above 1000°C), and then placed together and forged while hot by applying force. Force may be applied by hammering, rolling, drawing or squeezing to achieve the forging action. Forge welding was originally the first process of welding. In this process the two metal pieces to be joined are heated in a forge or furnace to a plait condition and then they are united by pressure. The ends to be joined are heated in a furnace to plastic condition and formed to the required shape by upsetting. Then they are brought together and hammered, so as to get the finished joint similarly, a butt joint can be prepared by forge welding. Before joining the two pieces, their ends are formed to the required shape according to the type of joint.
The forge welding is a manual process and is limited to light work because all forming and welding are done with a hand sledge. It is a slow process and there is considerable danger of an oxide scale forming on tile surfaces. The tendency to oxidize can be counteracted somewhat by using a thick fuel bed and by covering the surfaces with a fluxing material, which dissolves the oxides. Borax in combination with salt ammoniac is commonly used as flux. The forge welding is recommended to such metals, which have a large welding temperature range like low carbon steel and wrought iron. By the increase of carbon content, this range decreases rapidly. High carbon steels alloy steels require considerably more care in controlling temperature and producing the welds. Large work may be welded in hammer forges driven by steam. Welded steel pipe is made mechanically by running the preheated strips through rolls, which form the pipe to size and apply the necessary pressure for the weld.
2. Friction Welding
In this process, the heat for welding is obtained from mechanically induced sliding motion between rubbing surfaces of work-pieces. In friction welding, one part is firmly held while the other (usually cylindrical) is rotated under simultaneous application of axial pressure. As these parts are brought to rub against each other under pressure, they get heated due to friction. When the desired forging temperature is attained, the rotation is stopped and the axial pressure is increased to obtain forging action and hence welded joint. Most of the metals and their dissimilar combinations such as aluminium and titanium, copper and steel, aluminium and steel etc. can be welded using friction welding.
3. Explosive Welding
In explosive welding, strong metallurgical bonds can be produced between metal combinations which cannot be welded by other methods or processes. For example, tantalum can be explosively welded to steel although the welding point of tantalum is higher than the vaporization temperature of steel. It is carried out by bringing together properly paired metal surfaces with high relative velocity at a high pressure and a proper orientation to each other so that a large amount of plastic interaction occurs between the surfaces. The work piece, held fixed is called the target plate and the other called flyer plate.
While a variety of procedures have been successfully employed, the main techniques of explosive welding can be divided into contact techniques and impact techniques. In critical space and nuclear application, explosive welding permits fabrication of structures that cannot be made by any other means and, in some commercial applications, explosive joining is the least costly method. The main advantage of explosive welding includes the simplicity of the process, and the extremely large surface that can be welded. Incompatible materials can also be bonded, and thin foils can be bonded to heavier plates.
THERMIT WELDING
It may be of forge or fusion kind of welding. Fusion welding requires no pressure.
It is a process which uses a mixture of iron oxide and granular aluminium. This mixture in superheat liquid state is poured around the parts to be joined. The joint is equipped with the refractory mold structure all around. In case of thermit pressure welding, only the heat of thermit reaction is utilized to bring the surface of metal to be welded in plastic state and pressure is the applied to complete the weld. The temperature produced in the thermit reaction is of the order of 3000°C. Thermit welding is
used for welding pipes, cables, conductors, shafts, and broken machinery frames, rails and
repair of large gear tooth.
RADIANT ENERGY WELDING PROCESSES
In radiant energy welding processes, heat is produced at the point of welding when a stream of electrons or a beam of electro-magnetic radiations strikes on the workpiece. The heat is generated when the electron beam impinges on work piece. As the high velocity electron beam strikes the surfaces to be welded, their kinetic energy changes to thermal energy and hence causes the workpiece metal to melt and fuse.
This process employs an electron gun in which the cathode in form of hot filament of tungsten or tantalum is the source of a stream of electrons. The electrons emitted from filament by thermionic emission are accelerated to a high velocity to the anode because of the large potential difference that exists between them. The potential differences that are used are of the order of 30 kV to 175 kV. The current levels are low ranging between 50 mA to 1000 mA.
The electron beam is focused by a magnetic lens system on the work pieces to be welded. The depth of penetration of the weld depends on the electron speed which in turn is dependent upon the accelerating voltage. As these electrons penetrate the metal, the material that is directly in the path is
melted which when solidifies form the joint.
Electron beam welding has several advantages which may not be found in other welding processes. The penetration of the beam is high. The depth to width ratios lies between 10:1 to 30:1 can be easily realized. welding. It is also possible to closely control this penetration by controlling the accelerating
voltage, beam current, and beam focus. The process can be used at higher welding speeds typically between 125 and 200 mm/sec.
No filler metal or flux needs to be used in this process. The heat liberated is low and also is in a narrow zone, thus the heat affected zone is minimal as well as weld distortions are virtually eliminated.
It is possible to carry out the electron beam welding in open atmosphere. This welding can be carried out in vacuum or at low pressures.
Using Vacuum
For welding in vacuum, the work-piece is enclosed in a box in which the vacuum is created. When electron beam moves in the normal atmosphere, the electrons would be impinging with the gas molecules in the atmosphere and would thus be scattered. This scattering increases the spot size of the electron beam and consequently there is lower penetration. As the vacuum increases, the scattering effect of the electron beam decreases and hence, penetration increases. The other advantage of using vacuum is that the weld metal is not contaminated.
The EBW process is mainly used for welding of reactive metals (nuclear reactor components), titanium, zirconium, stainless steel, etc. for aero-space and automotive industries.
WELDING DEFECTS
Lack of penetration
Lack of Fusion
Incomplete fusion
Under cuts
Crater cracks
Poor Weld Bead Appearance
Distortion
1. Lack of Penetration
It is the failure of the filler metal to penetrate into the joint. It is due to
(a) Inadequate de-slagging
( b ) Incorrect edge penetration
(c) Incorrect welding technique.
2. Lack of Fusion
Lack of fusion is the failure of the filler metal to fuse with the parent metal. It is due to
(a) Too fast a travel
( b ) Incorrect welding technique
(c) Insufficient heat
3. Porosity
It is a group of small holes throughout the weld metal. It is caused by the trapping of gas
during the welding process, due to
(a) Chemicals in the metal
(b) Dampness
(c) Too rapid cooling of the weld.
4. Slag Inclusion
It is the entrapment of slag or other impurities in the weld. It is caused by
(a) Slag from previous runs not being cleaned away,
( b ) Insufficient cleaning and preparation of the base metal before welding commences.
5. Undercuts
These are grooves or slots along the edges of the weld caused by
(a) Too fast a travel
( b ) Bad welding technique
(c) Too great a heat build-up.
6. Cracking
It is the formation of cracks either in the weld metal or in the parent metal. It is due to
(a) Unsuitable parent metals used in the weld
( b ) Bad welding technique.
7. Poor Weld Bead Appearance
If the width of weld bead deposited is not uniform or straight, then the weld bead is termed
as poor. It is due to improper arc length, improper welding technique, damaged electrode
coating and poor electrode and earthing connections. It can be reduced by taking into
considerations the above factors.
8. Distortion
Distortion is due to high cooling rate, small diameter electrode, poor clamping and slow arc
travel speed
9. Overlays
These consist of metal that has flowed on to the parent metal without fusing with it. The
defect is due to
(a) Contamination of the surface of the parent metal
( b ) Insufficient heat
10. Blowholes
These are large holes in the weld caused by
(a) Gas being trapped, due to moisture.
( b ) Contamination of either the filler or parent metals.
11. Burn Through
It is the collapse of the weld pool due to
(a) Too great a heat concentration
( b ) Poor edge preparation.
12. Excessive Penetration
It is where the weld metal protrudes through the root of the weld. It is caused by
(a) Incorrect edge preparation
(b) Too big a heat concentration
(c) Too slow a travel.
No comments:
Post a Comment