1WELDING PROCESS

Dr. N.K. SinghAssociate Professor (Workshop)

Department of ME&MMEIndian School of Mines

Dhanbad

Welding

Welding is the process of joining two metal pieces as a result of significant diffusion of the atoms of the welded pieces into the joint (weld) region.

Welding is carried out by heating the joined pieces to melting point and fusing them together (with or without filler material) or by applying pressure to the pieces in cold or heated state.

Advantages of welding:

Strong and tight joining; Cost effectiveness; Simplicity of welded structures design; Welding processes may be mechanized and

automated.

Disadvantages of welding:

Internal stresses, distortions and changes of micro-structure in the weld region;

Harmful effects: light, ultra violate radiation, fumes, high temperature.

Applications of welding:

Buildings and bridges structures; Automotive, ship and aircraft constructions; Pipe lines; Tanks and vessels; Machinery elements.

Classification of welding process

Arc Welding Gas Welding Resistance welding Solid State welding Unique Processes

Thermit welding LBW EBW ESW

Welding processes

Arc welding Carbon Arc Welding; Shielded Metal Arc Welding (SMAW); Submerged Arc Welding (SAW); Metal Inert Gas Welding (MIG, GMAW); Tungsten Inert Gas Arc Welding (TIG, GTAW); Plasma Arc Welding (PAW);

Resistance Welding (RW); Spot Welding (RSW); Flash Welding (FW); Resistance Butt Welding (UW) ; Seam Welding (RSEW);

Gas Welding (GW); Oxyacetylene Welding (OAW); Oxyhydrogen Welding (OHW); Pressure Gas Welding (PGW);

Solid State Welding (SSW); Forge Welding (FOW); Cold Welding (CW); Friction Welding (FRW); Explosive Welding (EXW); Diffusion Welding (DFW); Ultrasonic Welding (USW);

Thermit Welding (TW); Electron Beam Welding (EBW); Laser Welding (LW).

Gas Welding

Gas Welding is a welding process, utilizing heat of the flame from a welding torch. The torch mixes a fuel gas with oxygen in the proper ratio and flow rate, providing combustion process at a required temperature. The hot flame fuses the edges of the welded parts, which are joined together forming a weld after Solidification.

Gas Welding

The flame temperature is determined by a type of the fuel gas and proportion of oxygen in the combustion mixture: 4500°F - 6300°F (2500°C - 3500°C). Depending on the proportion of the fuel gas and oxygen in the combustion mixture, the flame may be

chemically neutral (equal ratio of the gases), oxidizing (excess of oxygen), carburizing (excess of fuel gas).

Gas Welding

Filler rod is used when an additional supply of metal to weld is required. Shielding flux may be used if protection of weld pool is necessary.

Gas Welding equipment:

Fuel gas cylinder with pressure regulator; Oxygen cylinder with pressure regulator; Welding torch; Trolley for transportation of the gas cylinders.

1. Oxyacetylene Welding (OAW)

Oxyacetylene Welding is a Gas Welding process using a combustion mixture of acetylene (C2H2) and oxygen (O2) for producing gas welding flame.

Temperature: 6000°F (3300°C). Combustion of acetylene proceeds in two

stages:1. Inner core of the flame. C2H2 + O2 = 2CO + H2

2. Outer envelope of the flame: CO + H2 + O2 = CO2 + H2O

Arc welding

Arc welding uses a Electric power supply to create an electric arc between an electrode and the base material to melt the metals at the welding point.

Electric arc between the electrode and work piece closes the electric circuit. The arc temperature may reach 10000°F (5500°C), which is sufficient for fusion of the work piece edges and joining them.

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Arc welding

They can use either direct (DC) or alternating (AC) current, and consumable or non-consumable electrodes.

The welding region is sometimes protected by some type of inert or semi-inert gas, known as a shielding gas, and/or an evaporating filler material.

The process of arc welding is widely used because of its low capital and running costs.

Arc Welding process

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ARC Welding Power Supply

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Power Supplies To supply the electrical energy necessary for arc

welding processes, a number of different power supplies can be used. constant current power supplies constant voltage power supplies.

In arc welding, the voltage is directly related to the length of the arc, and the current is related to the amount of heat input.

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Power Supplies

Constant current supply is more important because in manual welding, it can be difficult to hold the electrode perfectly steady, and as a result, the arc length and thus voltage tend to fluctuate.

Constant voltage power supplies hold the voltage constant and vary the current, and as a result, are most often used for automated welding processes such as gas metal arc welding, flux cored arc welding, and submerged arc welding.

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Consumable electrode methods One of the most common types of arc welding is

shielded metal arc welding (SMAW), which is also known as manual metal arc welding (MMA) or stick welding.

An electric current is used to strike an arc between the base material and a consumable electrode rod or 'stick'.

The electrode rod is made of a material that is compatible with the base material being welded and is covered with a flux that protects the weld area from oxidation and contamination by producing CO2 gas during the welding process.

The electrode core itself acts as filler material, making a separate filler unnecessary.

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Schematic representation of MIG Welding

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MIG Welding

Gas Metal Arc Welding (GMAW) is frequently referred to as MIG welding. 

MIG welding is a commonly used high deposition rate welding process. 

Wire is continuously fed from a spool.  MIG welding is therefore referred to as a

semiautomatic welding process. 

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MIG Welding Shielding Gas

The shielding gas, forms the arc plasma, stabilizes the arc on the metal being welded, shields the arc and molten weld pool, and allows smooth transfer of metal from the weld wire to the molten weld pool. 

There are three primary metal transfer modes:

- Spray transfer

-  Globular transfer

- Short circuiting transfer WELDING PROCESS 26

The primary shielding gases

 Argon  Argon - 1 to 5% Oxygen  Argon - 3 to 25% CO2  Argon/Helium CO2 is also used in its pure form in some MIG

welding processes.  However, in some applications the presence of

CO2 in the shielding gas may adversely affect the mechanical properties of the weld. 

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MIG Welding taking place

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Glow of MIG Welding Process

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Photographic view of MIG Weld

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Gas tungsten arc welding (GTAW, TIG)

GTAW or tungsten inert gas (TIG) welding, is a manual welding process that uses a non-consumable electrode made of tungsten , an inert or semi-inert gas mixture, and a separate filler material.

Especially useful for welding thin materials, this method is characterized by a stable arc and high quality welds, but it requires significant operator skill and can only be accomplished at relatively low speeds.

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Gas tungsten arc welding(GTAW)

It can be used on nearly all weldable metals, though it is most often applied to stainless steel and light metals.

It is often used when quality welds are extremely important, such as in aircraft and naval applications.

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GTAW Welding

Gas Tungsten Arc Welding (GTAW) is frequently referred to as TIG welding. 

TIG welding is a commonly used high quality welding process. 

TIG welding has become a popular choice of welding processes when high quality, precision welding is required. 

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Schematic View of the TIG Welding Process

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TIG Welding taking place

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Welded surface of TIG welding

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TIG Welder

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TIG Welding Benefits

 Superior quality welds  Welds can be made with or without filler

metal  Precise control of welding variables (heat)  Free of spatter  Low distortion

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Shielding Gases of TIG Welding

 Argon  Argon + Hydrogen  Argon/Helium Helium is generally added to increase heat input

(increase welding speed or weld penetration).  Hydrogen will result in cleaner looking welds and

also increase heat input, however, Hydrogen may promote porosity or hydrogen cracking.

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Plasma Arc Welding Process The plasma welding process was introduced

to the welding industry in 1964 as a method of bringing better control to the arc welding process in lower current ranges.

Today, plasma retains the original advantages it brought to industry by providing an advanced level of control and accuracy to produce high quality welds in miniature or precision applications and to provide long electrode life for high production requirements.

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The Plasma Arc Welding Process Plasma Arc Welding is a welding process

utilizing heat generated by a constricted arc struck between a tungsten non-consumable electrode and either the work piece (transferred arc process) or water cooled constricting nozzle (non-transferred arc process).

Plasma is a gaseous mixture of positive ions, electrons and neutral gas molecules.

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Transferred arc process produces plasma jet of high energy density and may be used for high speed welding and cutting of Ceramics, steels, Aluminum alloys, Copper alloys, Titanium alloys, Nickel alloys.

Non-transferred arc process produces plasma of relatively low energy density. It is used for welding of various metals. Since the work piece in non-transferred plasma arc welding is not a part of electric circuit, the plasma arc torch may move from one work piece to other without extinguishing the arc.

Plasma Arc Welding Process

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How Plasma Welding Works

A plasma is a gas which is heated to an extremely high temperature and ionized so that it becomes electrically conductive.

Similar to GTAW (TIG), the plasma arc welding process uses this plasma to transfer an electric arc to a work piece.

The metal to be welded is melted by the intense heat of the arc and fuses together.

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How Plasma Welding Works

In the plasma welding torch a Tungsten electrode is located within a copper nozzle having a small opening at the tip.

A pilot arc is initiated between the torch electrode and nozzle tip.

This arc is then transferred to the metal to be welded.

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How Plasma Welding Works

By forcing the plasma gas and arc through a constricted orifice, the torch delivers a high concentration of heat to a small area.

With high performance welding equipment, the plasma process produces exceptionally high quality welds.

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How Plasma Welding Works

Plasma gases are normally argon. The torch also uses a secondary gas, argon,

argon/hydrogen or helium which assists in shielding the molten weld puddle thus minimizing oxidation of the weld.

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Equipment List of Plasma Arc Welding

Power Supply Plasma Console (sometimes external,

sometimes built in) Water re-circulator (sometimes external,

sometimes built in) Plasma Welding Torch Torch Accessory Kit (Tips, ceramics, collets,

electrodes set-up gages)

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Resistance Welding

Resistance Welding is a welding process, in which work pieces are welded due to a combination of a pressure applied to them and a localized heat generated by a high electric current flowing through the contact area of the weld.

Resistance Welding

Heat produced by the current is sufficient for local melting of the work piece at the contact point and formation of small weld pool (”nugget”). The molten metal is then solidifies under a pressure and joins the pieces.

Process parameters: Time of the process Applied pressure flowing current resistance

Resistance Welding

AC electric current (up to 100 000 A) is supplied through copper electrodes connected to the secondary coil of a welding transformer.

The most popular methods of Resistance welding are:

Spot Welding (RSW); Flash Welding (FW); Resistance Butt Welding (UW) ; Seam Welding (RSEW).

Spot Welding (RSW) Spot Welding is a Resistance Welding (RW) process, in which

two or more overlapped metal sheets are joined by spot welds.

The method uses pointed copper electrodes providing passage of electric current. The electrodes also transmit pressure required for formation of strong weld.

Diameter of the weld spot is in the range 1/8” - 1/2” (3 - 12 mm).

Spot welding is widely used in automotive industry for joining vehicle body parts.

Flash Welding (FW)

Flash Welding is a Resistance Welding (RW) process, in which ends of rods (tubes, sheets) are heated and fused by an arc struck between them and then forged (brought into a contact under a pressure) producing a weld.

The welded parts are held in electrode clamps, one of which is stationary and the second is movable.

Resistance Butt Welding (UW)

Resistance Butt Welding is a Resistance Welding (RW) process, in which ends of wires or rods are held under a pressure and heated by an electric current passing through the contact area and producing a weld

Resistance Butt Welding (UW)

The process is similar to Flash Welding, however in Butt Welding pressure and electric current are applied simultaneously in contrast to Flash Welding where electric current is followed by forging pressure application.

Butt welding is used for welding small parts. The process is highly productive and clean. In contrast to Flash Welding, Butt Welding provides joining with no loss of the welded materials.

Seam Welding (RSEW)

Seam Welding is a Resistance Welding (RW) process of continuous joining of overlapping sheets by passing them between two rotating electrode wheels. Heat generated by the electric current flowing through the contact area and pressure provided by the wheels are sufficient to produce a leak-tight weld.

Advantages of Resistance Welding: High welding rates; Low fumes; Cost effectiveness; Easy automation; No filler materials are required; Low distortions.

Disadvantages of Resistance Welding: High equipment cost; Low strength of discontinuous welds; Thickness of welded sheets is limited - up to 1/4” (6 mm);

Safety issues Welding can be a dangerous and unhealthy practice

without the proper precautions; however, with the use of new technology and proper protection the risks of injury or death associated with welding can be greatly reduced.

Because many common welding procedures involve an open electric arc or flame, the risk of burns is significant.

To prevent them, welders wear protective clothing in the form of heavy leather gloves and protective long sleeve jackets to avoid exposure to extreme heat, flames, and sparks.

Additionally, the brightness of the weld area leads to a condition called arc eye in which ultraviolet light causes the inflammation of the cornea and can burn the retinas of the eyes.

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Safety issues

Goggles and helmets with dark face plates are worn to prevent this exposure and, in recent years, new helmet models have been produced featuring a face plate that self-darkens upon exposure to high amounts of UV light.

To protect bystanders, transparent welding curtains often surround the welding area.

These curtains, made of a polyvinyl chloride plastic film, shield nearby workers from exposure to the UV light from the electric arc, but should not be used to replace the filter glass used in helmets.

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Safety issues Welders are also often exposed to dangerous

gases and particulate matter. Processes like flux-cored arc welding and

shielded metal arc welding produce smoke containing particles of various types of oxides.

The size of the particles in question tends to influence the toxicity of the fumes, with smaller particles presenting a greater danger.

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Safety issues Additionally, many processes produce

various gases (most commonly carbon dioxide and ozone, but others as well) that can prove dangerous if ventilation is inadequate.

Furthermore, the use of compressed gases and flames in many welding processes pose an explosion and fire risk; some common precautions include limiting the amount of oxygen in the air and keeping combustible materials away from the workplace.

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