PROCESSING OF BUTT JOINT USINGGAS METAL ARC WELDING

A Mini Project report submitted in partial fulfillment of the requirements for the award of degree of

BACHELOR OF TECHNOLOGYIN

MECHANICAL ENGINEERING

BY

MD.WASEEM QURESHI (11H11A0345)

ADNAN (11H11A0304)

ABDUL AMIR KHAN (11H11A0301)

AMIR SOHAIL (11H11A0307)

Under the Guidance of

Internal Guide External Guide

Mr. S. Rahamthulla Khan Mr. K. Velmyl(Asst. Professor) (Manager)

DEPARTMENT OF MECHANICAL ENGINEERING

AL-HABEEB COLLEGE OF ENGINEERING AND TECHNOLOGY

(Affiliated to J.N.T.U., Hyderabad)

2014-2015

AL-HABEEB COLLEGE OF ENGINEERING & TECHNOLOGY

Damergidda,Chevella,R.R District.

CERTIFICATE

This to certify that Project work entitled “PROCESSING OF BUTT

JOINT USING GAS METAL ARC WELDING” which is being

submitted by

MD.WASEEM QURESHI (11H11A0345)

ADNAN (11H11A0304)

ABDUL AMIR KHAN (11H11A0301)

AMIR SOHAIL (11H11A0307)

in partial fulfillment of the requirements for the award of the

degree of Bachelor of Technology in Mechanical Engineering is a

record of bonafide work carried out by them under our guidance

and super vision

HEAD OF DEPARTMENT PRINCIPAL

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ACKNOWLEDGEMENT

First and foremost, we express our sincere thanks to

PROFESSOR SHAIK MAHABOOB BASHA, Principal of AL-Habeeb

College of Engineering & Technology for providing us this

opportunity to carry out our project work.

We express our sincere gratitude to Mr. S.uday bhaskar sir,

HEAD OF THE DEPARTMENT (HOD), Mechanical engineering for

providing us this opportunity to carry out our project work.

During our project he has constantly provided required guidance

and constant encouragement without which we could not have

completed our project.

We would like to thank our department faculty, for their help and

coordination to bringing this project successfully through out the

way.

Finally, we would like to express our thanks to all the people who

have helped us directly or indirectly in successful completion of

our project at the earliest.

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ABSRTACT

Reproduction of any product is time taking and costly affair.

Welding place a vital role to rejoin any damaged or breaking

material. This project consists of the arc welding process generally

used in industries.

Arc welding uses a welding power supply to create an electric

current arc between an electrode and the base material to melt

the metals as the welding point. They can use either direct current

(DC) or alternating current (AC) and consumable or non-

consumable electrodes. The welding is sometimes protected by

some types of inert or semi-inert gas, known as shielding gas, and

or an evaporating filler material. The process of arc welding is

widely used because of its low capacity and running costs.

One of the most common type of arc welding is shielding metal arc

welding, which is also known as manual metal arc welding or stick

welding. An electric current is used to strike an arc between the

base material and consumable electrode rod or stick. The

electrode rod is made of 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 separate filler unnecessary.

Gas tungsten arc welding, or tungsten inert gas(TIG) welding, is a

manual welding process that uses a non-consumable electrodes

made of tungsten, an inert or semi inert gas mixture , and a

separate filler material. Especially useful for welding thin material,

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this method is characterized by stable arc an high quality

welds ,but it requires significant operator skill and can only be

accomplished at relatively low speeds. It can be used on nearly all

weldable metals, thought it is most often applied to stainless steel

and light metals. It is often used when quality welds are extremely

important, such as in bicycle, air craft and naval applications.

Compared to other welding process arc welding is less time

consuming process and accurate.

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CHAPTER 1

INTRODUCTION TO GMAW

In Gas Metal Arc Welding (GMAW), also known as Metal

Inert Gas (MIG) welding, an electric arc is established between the

work piece and a consumable bare wire electrode. The arc

continuously melts the wire as it is fed to the weld puddle. The

weld metal is shielded from the atmosphere by a flow of an inert

gas, or gas mixture. Figure 1-1 shows this process and a portion of

the welding torch. The MIG welding process operates on D.C.

(direct current) usually with the wire electrode positive. This is

known as ”reverse” polarity. ”Straight” polarity, is seldom used

because of poor transfer of molten metal from the wire electrode

to the work piece. Welding currents of from 50 amperes up to

more than 600 amperes are commonly used at welding voltages of

15V to 32V. A stable, self correcting arc is obtained by using the

constant potential (voltage) power system and a constant wire

feed speed. Continuing developments have made the MIG process

applicable to the welding of all commercially important metals

such as steel, aluminum, stainless steel, copper and several

others. Materials above .030 in. (.76 mm) thick can be welded in

all positions, including flat, vertical and overhead. It is simple to

choose the equipment, wire electrode, shielding gas, and welding

conditions capable of producing high-quality welds at a low cost.

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Fig 1.1 - Basic MIG Welding Process

The MIG welding process provides many advantages in manual

and automatic metal joining for both low and high production

applications. Its combined advantages when compared to covered

(stick) electrode, submerged arc, and TIG are:

1)   Welding can be done in all positions.

2)   No slag removal required.

3)   High weld metal deposition rate.

4)   Overall times for weld completion about 1/2 that of covered

electrode.

5)   High welding speeds. Less distortion of the work piece.

6)   High weld quality.

7)   Large gaps filled or bridged easily, making certain kinds of

repair welding more efficient.

8)   No stub loss as with covered electrode.

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1.1 PRINCIPLES OF GMAW

• GMAW stands for Gas Metal Arc Welding

• GMAW is commonly referred to as MIG or Metal Inert Gas welding

• During the GMAW process, a solid metal wire is fed through a

welding gun and becomes the filler material

• Instead of a flux, a shielding gas is used to protect the molten

puddle from the atmosphere which results in a weld without slag

Fig 1.2 - GMAW Welding Machine Components

• Three things happen when the GMAW gun trigger is pulled:

– The wire electrode begins to feed

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– The circuit becomes electrically ‘hot’

• Current flows from the power source through the gun cable, gun,

contact tip to the wire and across the arc. On the other side of the

arc, current flows through the base metal to the work cable and

back to the power source

– Shielding gas flows through the gun and out the nozzle

Fig 1.3 - Gun Trigger

• A GMAW electrode is:

– A metal wire

– Fed through the gun by the wire feeder

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– Measured by its diameter

Fig 1.4 - GMAW electrodes are commonly packaged on spools,

reels and coils ranging from 1lb to 1000lbs.

• An electric arc occurs in the gas filled space between the electrode

wire and the work piece.

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Fig 1.5 - Electric arcs can generate temperatures up to 10,000°F

• As the wire electrode and work piece heat up and melt, they form

a pool of molten material called a weld puddle

• This is what the welder watches and manipulates while welding

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Fig 1.6 .045” ER70S-6 at 400 ipm wire feed speed and 28.5 Volts

with a 90% Argon/ 10% CO2 shielding gas

• GMAW welding requires a shielding gas to protect the weld puddle

• Shielding gas is usually CO2, argon, or a mixture of both

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Fig 1.7- The gauges on the regulator show gas flow rate and bottle

pr.

• The welder “lays a bead” of molten metal that quickly solidifies

into a weld

• The resulting weld is slag free

Fig 1.8 - An aluminum weld done with the GMAW process

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CHAPTER 2

TYPES OF GAS METAL ARC WELDING

MIG or GMAW welding equipment can be used either manually or

automatically.

2.1 Manual Welding

A manual welding station is simple to install. Because arc travel is

performed by the welder, only three major elements are

necessary:

1) Welding torch and accessories

2) Welding control and wire feed motor

3) Power source

Figure 2.1 – Manual Welding Installation

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1. POWER CABLE (NEGATIVE)

2. WATER FROM TORCH - POWER CABLE

3. SHIELDING GAS

4. TORCH SWITCH

5. WATER TO TORCH

6. WIRE CONDUIT

7. SHIELDING GAS FROM CYLINDER

8. COOLING WATER OUT

9. COOLING WATER IN

10. 115 VAC IN - WELDING CONTACTOR CONTROL

11. POWER CABLE (POSITIVE)

12. TO PRIMARY POWER 230/460/575 V

2.2 WELDING TORCHES AND ACCESSORIES

The welding torch guides the wire and shielding gas into the weld

zone. It also brings welding power to the wire. Different types of

welding torches have been designed to provide maximum welding

utility for different types of applications. They range from heavy

duty torches for high current work to lightweight torches for low

current and out-of-position welding. In both types, water or air

cooling and curved or straight front ends are available. Figure 2.2

shows a cross-sectional view of a typical air cooled, curved front

end torch with these necessary accessories: a. contact tube (or

tip) b. shielding gas cup or nozzle c. wire conduit and liner d. one-

piece composite cable Figure 2.2 - Typical MIG Welding Torch

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Fig 2.2 Welding Torch

The wire guide tube, also called ”contact tube”, is made of

copper and is used to bring welding power to the wire as well as

direct the wire toward the work. The torch (and guide tube) is

connected to the welding power source by the power cable.

Because the wire must feed easily through the guide tube and also

make good electrical contact, the bore diameter of the tube is

important. The instruction booklet, supplied with every torch, lists

the correct size contact tube for each wire size. The tube, which is

a replaceable part, must be firmly locked to the torch and

centered in the shielding gas cup. The shielding gas cup directs a

protective mantle of gas to the welding zone. Large cups are used

for high- current work where the weld puddle is large. Smaller

cups are used for low-current welding. The wire conduit and its

liner are connected between the torch and wire drive (feed) rolls.

They direct the wire to the torch and into the contact tube.

Uniform wire feeding is necessary for arc stability.  When not

properly supported by the conduit and liner, the wire may jam.

The liner may be either an integral part of the conduit or supplied

separately. In either case, the inner diameter and material of the

liner are important. When using steel wire electrodes, a steel

spring liner is recommended. Nylon and other plastic liners should

be used for aluminum wire. The literature supplied with each torch

lists the recommended conduits and liners for each wire size and

material.

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2.3 WELDING CONTROL AND WIRE-FEED MOTOR

The welding control and wire-feed motor are often supplied

in one package (wire feeder) as shown in Fig 2.1. Their main

function is to pull the welding wire from the spool and feed it to

the arc. The control maintains pre-determined wire-feed speed at

a rate appropriate to the application. The control not only

maintains the set speed independent of load, but also regulates

starting and stopping of wire feed on signal from the torch switch.

Shielding gas, water, and welding power are usually delivered to

the torch through the control box. Through the use of solenoids,

gas and water flow are coordinated with flow of weld current. The

control determines the sequence of gas flow and energizing of the

power supply contactor. It also allows some gas to flow before and

after arc operation.

2.4 POWER SOURCE

Almost all mig welding is done with reverse polarity. The

positive (+) lead is connected to the torch while the negative (–)

lead is connected to the work piece. Since wire feed speed and,

hence, current, is regulated by the welding control, the basic

adjustment made through the power source is arc length. Arc

length is set by adjusting the power source voltage. Power source

may also have one or two additional adjustments for use with

other welding applications. Most power sources require either

230V or 460V AC input power. Except for the power cable, the only

other connection to the power source is a multi-connector cable

from the control, so as to have the power in sequence with other

control functions. Power sources will be discussed further in the

next section. SEQUENCE OF OPERATION As an example, consider

the operation of the welding installation pictured in Figure 2-1:

1)   Main line power to power source turned on.

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2)   Set power source switch to ”READY” to turn on power source

cooling fan motor and control circuit.

3)   Turn the welding control switch to ”ON” to energize the

control.

4)   Close torch switch to cause shielding gas and cooling water to

flow. Weld power goes to torch and wire feed begins at set speed.

The feeding wire electrode touches the work piece. Welding

begins.

5)   Release torch switch – No. 4 above reversed. Most welding

installations operate in a similar manner. However, the design and

construction of the equipment will differ. It is for this reason that

the equipment instruction booklet should be consulted. Complete

troubleshooting data is generally supplied with all equipment.

2.5 Mechanized Welding Station

A mechanized station is used when the work can more

easily be brought to the welding station or where a great deal of

repetitive welding justifies special fixtures. Arc travel is automatic

and controlled by the fixture travel speed. Weld speed is usually

increased and weld quality improved. As shown in Fig 2.3 the

welding equipment in a mechanized fixture is much the same as in

a manual station except:

1)   The welding torch is usually mounted directly under the wire

feed motor, eliminating the need for a wire conduit.

2)   The welding control is mounted away from the wire feed

motor. Remote control boxes can be used.

3)   In addition, other equipment is used to provide automatic

fixture travel. Examples of this equipment are side-beam carriages

and turning fixtures. The welding control also coordinates carriage

travel with the weld start and stop.

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Figure 2.3 - Automatic (Mechanized) Welding Installation

Chapter 3

MANUFACTURING PROCESS OF BUTT JOINT

A butt joint is a joinery technique in which two members are

joined by simply butting them together. A butt weld is made

between two pieces of metal usually in the same plane, the weld

metal maintaining continuity between the sections. Weld metal is

generally contained within the profiles of the welded elements.

The butt joint is the simplest joint to make since it merely involves

cutting the members to the appropriate length and butting them

together. The butt joint is a very simple joint to construct.

Members are simply docked at the required angle (usually 90°)

and required length. One member will be shorter than the finished

size by the thickness of the adjacent member. We have to prepare

a butt joint , we need two metal plates which should have groove

angle, bevel angle, root gap & root face as shown in the figure

below.

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Fig 3.1 Groove Angle

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This below figure shows us the bevel angle , groove angle , root

gap & root face that we have prepare for the butt welded joint .

Fig 3.2 Arrangement Of Butt Joint

Here we have many processes for welding as follows:

3.1 Fusion Welding

– melting base metals

– Arc Welding (AW) – heating with electric arc

– Resistance welding (RW) – heating with resistance to

an electrical current

– Oxy fuel Welding (OFW) – heating with a mixture of

oxygen and acetylene (oxyfuel gas)

– Other fusion welding – electron beam welding and

laser beam welding

3.2 Solid State Welding

– No melting, No fillers

– Diffusion welding (DFW) – solid-state fusion at an

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elevated temperature

– Friction welding (FRW) – heating by friction

– Ultrasonic welding (USW) – moderate pressure with

ultrasonic oscillating motion

Here we are going to use arc welding process to make a butt

welded joint. In arc welding we are using gas metal arc

welding(GMAW) or metal inert gas (MIG) or metal active gas

(MAG).

3.3 MIG Welding

In MIG the arc is formed between the end of a small diameter wire

electrode fed from a spool, and the work piece. The shielding gas,

Argon or CO2 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 weld

groove. Main equipment components are : power source Wire feed

system Conduit Gun.

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Fig 3.3 Wire Electrode In MIG welding, a shielding gas is fed into the welding

torch and exits around the filler wire. The arc and the weld pool

are protected from the atmosphere by this gas shield. This enables

bare wire to be used without a flux coating. However, the absence

of flux to 'mop up' surface oxide places greater demand on the

welder to ensure that the joint area is cleaned immediately before

welding. This can be done using either a wire brush for relatively

clean parts, or a hand grinder to remove rust and scale. The other

essential piece of equipment is a wire cutter to trim the end of the

electrode wire. In this process a filler metal is stored on a spool

and driven by rollers [ current is fed into the wire ] through a

tube into a 'torch'. The large amount of filler wire on the spool

means that the process can be considered to be continuous and

long, uninterrupted welds can easily be made. In this process they

key issues are selecting the correct shielding gas and flow rate

and the welding wire speed and current. MIG process can readily

be automated and MIG welding is now commonly carried out by

robots. This welding process is widely used on steels and on

aluminium. Although the inert gas shield keeps the weld clean,

depending upon the process settings, there may be spatter of

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metal globules adjacent to the weld which detracts from its

appearance unless they are removed.

CHAPTER 4

MANUFACTURING OF BUTT WELDED JOINT

BY USING GMAW

To manufacture a butt welded joint we need two metal

plates which should be kept parallel to each other in a same plane

and those plates must have bevel angle , included angle or groove

angle, root gap & root face in between them as shown below.

Fig 4.1 Groove Angle Of Butt Joint

Now to make a butt welded joint of these plates we need to

choose either manual welding station or mechanized welding

station , here we have chosen the manual welding station because

is simple to install & arc travel is performed by the welder where

as in mechanized welding station an arc travel is automatic and

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controlled by the fixture travel speed . The below figure shows the

manual welding installation .

Figure 4.2 – Manual Welding Installation

 From this we can prepare a butt welded joint as shown below.

Fig 4.3 Butt Welded Joint

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CHAPTER 5

DEFECTS , APPLICATIONS , LIMITATIONS &

ADVANTAGES OF GMAW

5.1 DEFECTS

Common defects in MIG welding are ;

Undercutting, Excessive melt-through, Incomplete fusion,

Incomplete joint penetration, Porosity, Weld metal cracks, Heat

affected zone cracks.

5.1.1 UNDERCUT

A groove melted into the base metal adjacent to the weld toe or

weld root and left unfilled by weld metal.

Fig 5.1 Undercuts

5.1.2 POROSITY

Porosity is the presence of cavities in the weld metal caused by

the freezing in of gas released from the weld pool as it solidifies.

The porosity can take several forms:

• distributed

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• surface breaking pores

• wormhole

• crater pipes

Porosity is caused by the absorption of nitrogen, oxygen and

hydrogen in the molten weld pool which is then released on

solidification to become trapped in the weld metal.

5.1.3 WELD METAL CRACKS

Cracks and planar discontinuities are the most dangerous,

especially if fatigue loading conditions (i.e. successively increasing

and decreasing) are present in service. Their shape extends

mainly in two dimensions and constitutes stress raisers. In visual

inspection only a linear indication may be visible.

Different types of cracks are described. Usually none are tolerated

(at the prescribed detection level), so that they must be removed

by careful grinding (if superficial) or repaired by welding. The most

insidious ones are those not open to the surface that may require

specialized techniques to be detected and evaluated.

Globular volumetric three dimensional discontinuities, porosity or

inclusions, are usually found deep inside the weld.

Fig 5.2 Weld Metal Cracks

5.1.4 HEAT AFFECTED ZONE

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When the weld pool is cooling and solidifying, the majority of the

heat flows through the parent metal alongside the joint. The steel

is thus subjected to heating and cooling cycles similar to those

experienced in heat treatment practice.

As shown in Figure 5.3 , the structure of the steel will be changed

in this region (called the heat affected zone, HAZ)

Fig 5.3 Heat Affected Zone

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Fig 5.4 HAZ Due To GMAW

5.1.5 OVERLAP

The protrusion of weld metal beyond the weld toe or weld root.

There may be fusion problem.

Fig 5.5 Overlap

5.1.6 LACK OF SIDE WALL FUSION

Lack of complete fusion or incomplete penetration are internal

planar discontinuities difficult to detect and evaluate but most

dangerous especially if low Impact Strength and elevated

Transition Temperature are determined for the material in cause

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and if cold weather may occur to promote low Toughness and

brittle fracture.

Fig 5.6 Lack Of Side Wall Fusion

5.2 APPLICATIONS

Constructions, Piping, pressure vessels.

Boilers and storage tanks, Shipbuilding, Aerospace.

Automobile and Railroad.

Automation - Machine, Automatic and Robotic welding.

5.3 LIMITATIONS

Less portable with shorter gun lengths (15 foot guns).

GMAW equipment is more expensive than SMAW equipment.

External shielding gas can be blown away by winds.

High radiated heat.

Difficult to use in out of position joints.

5.4 ADVANTAGES

High operating factor.

Easy to learn.

Limited cleanup.

Use on many different metals: stainless steel, mild (carbon) steel, aluminum and more.All position.

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Great for home use with 115V and 230V units.

Gas metal arc welding (GMAW), sometimes referred to by its subtypes metal

inert gas (MIG) welding or metal active gas (MAG) welding, is

a welding process in which an electric arc forms between a

consumable wire electrode and the workpiece metal(s), which heats the

workpiece metal(s), causing them to melt, and join. Along with the wire

electrode, ashielding gas feeds through the welding gun, which shields the

process from contaminants in the air. The process can be semi-automatic or

automatic. A constant voltage, direct current power source is most commonly

used with GMAW, but constant current systems, as well as alternating current,

can be used. There are four primary methods of metal transfer in GMAW, called

globular, short-circuiting, spray, and pulsed-spray, each of which has distinct

properties and corresponding advantages and limitations.

Originally developed for welding aluminum and other non-ferrous materials in

the 1940s, GMAW was soon applied to steelsbecause it provided faster welding

time compared to other welding processes. The cost of inert gas limited its use

in steels until several years later, when the use of semi-inert gases such as carb

dioxide became common. Further developments during the 1950s and 1960s

gave the process more versatility and as a result, it became a highly used

industrial process. Today, GMAW is the most common industrial welding

process, preferred for its versatility, speed and the relative ease of adapting the

process to robotic automation. Unlike welding processes that do not employ a

shielding gas, such as shielded metal arc welding, it is rarely used outdoors or

in other areas of air volatility. A related process, flux cored arc welding, often

does not use a shielding gas, but instead employs an electrode wire that is

hollow and filled with flux.

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Butt Joints and Welds

One of the most common types of weld joints in manufacturing is the butt joint.

This joint is formed when the two pieces to be welded are simply placed face to

face and the welding head run over it. In the case of GTAW and PAW this joint

can only be used on very thin pieces without extensive preparation and the use

of filler wire. Both the laser and the electron beam, on the other hand, can butt

weld very thick pieces, up to 30 cm for the electron beam. This is accomplished

by using the keyhole method, in which the beam is used to bore a path through

the piece for itself, allowing it to distribute energy evenly across the joint,

regardless of its depth. This makes the electron beam, and to a lesser extent

the laser, the ideal welding system for many jobs.

The butt joint has many advantages over other types of joint. The first of these

is that it results in a uniform surface, which allows them to be used in places

where fit or appearance is extremely important. A second advantage is

strength. Due to the fact that the area of the weld is nearly the same as that of

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the pieces being welded, the tensile strength can be comparable to that of the

base metal. The third advantage of the butt joint is simplicity to set up and

weld. Unlike some of the other joints, which require complicated geometry, such

as flanges, to work, the butt joint only requires a smooth interface.

There are also disadvantages to the butt joint. The first of these is that they are

especially sensitive to weld defects. Because all of the forces on the joint are

absorbed by the weld, defects such as porosity, inclusions, cracks, etc, can

cause easily cause the joint to fail catastrophically. A second disadvantage of

the butt weld is that it is usually not self-aligning. Whereas some other joint

types will hold together before welding, the butt joint will not. In many cases

this greatly increases the complexity of the fixturing necessary to hold the

pieces to be welded in place before and during the welding processes. A third

disadvantage of the butt weld is that it is nearly impossible to butt weld very

thin materials, due to the fact that aligning the faces properly is very difficult.

There are three major types of butt weld. The simplest of these involves simply

butting two smooth faces together and welding down the joint. This is quick and

easy and requires little in the way of joint preparation. However, it is susceptible

to all of the disadvantages mentioned above.

The first variation on the standard butt weld involves

matching notches in the two pieces. This is relatively easy to machine and has

two major advantages over the standard butt weld. First, the corresponding

cuts provide some self-alignment of the joint, reducing fixturing and potentially

increasingaccuracy. Second, the lip that is created prevents drop through,

where surface tension can no longer support the molten weld and it falls

through the bottom of the joint. There are also disadvantages to this method,

primarily that it reduces the weld area an therefore strength, and that it can

increase the residual stress due to the less even heat application.

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The second variation of the butt weld involves adding a

flange to the bottom of the parts, rather than machining it off as is the case

with the first alteration. This provides similar advantages to those of the first

alteration, but without decreasing the area of the weld and with less potential

for residual stress. It can, however, be harder and more expensive to make this

lip than the simple cuts required for the first alteration.

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Butt welded joint in T-DRILL Method

Butt welded joint is typically used in the process piping and heavier industrial

applications. It provides a better quality inside surface, which is needed, where

good flow characteristics and cleanliness are

important. The T-DRILL collaring process is a method of

producing outlets for branch connections directly from

the run material. The process from hole cutting to collar trimming is carried out

in a single workstation in one set-up from outside the pipe.

First an elliptical hole is milled in the pipe. Since more material is needed in the "stirrup" area to get a good collar height for butt welding, an elliptical hole is used as a pilot hole rather than a round hole, which is used in the lap joint. After the pilot hole is made, the forming pins of the collaring head are extended and the collar is formed. This is aided by automated lubrication and optimized forming. Then the collar is trimmed to the desired height and the branch pipe is connected to the run pipe by butt welding

ADVANTAGES OF T-DRILL METHOD

No costly inventories Improved flow characteristics Instead of three welded joints, only one simple weld joint is required Remarkable faster through-put times Minimized inspection cost Less chance of leakage or call-backs

Can be used in any malleable material

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Operation

GMAW weld area. (1) Direction of travel, (2) Contact tube, (3) Electrode, (4) Shielding gas, (5) Molten weld metal, (6) Solidified weld metal, (7) Workpiece.

For most of its applications gas metal arc welding is a fairly simple welding process to learn requiring no more than a week or two to master basic welding technique. Even when welding is performed by well-trained operators weld quality can fluctuate since it depends on a number of external factors. All GMAW is dangerous, though perhaps less so than some other welding methods, such as shielded metal arc welding.

Technique

The basic technique for GMAW is quite simple, since the electrode is fed automatically through the torch (head of tip). By contrast, in gas tungsten arc welding, the welder must handle a welding torch in one hand and a separate filler wire in the other, and in shielded metal arc welding, the operator must frequently chip off slag and change welding electrodes. GMAW requires only that the operator guide the welding gun with proper position and orientation along the area being welded. Keeping a consistent contact tip-to-work distance

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(the stick outdistance) is important, because a long stickout distance can cause the electrode to overheat and also wastes shielding gas.

Stickout distance varies for different GMAW weld processes and applications.

The orientation of the gun is also important—it should be held so as to bisect the angle between the workpieces; that is, at 45 degrees for a fillet weld and 90 degrees for welding a flat surface. The travel angle, or lead angle, is the angle of the torch with respect to the direction of travel, and it should generally remain approximately vertical. However, the desirable angle changes somewhat depending on the type of shielding gas used—with pure inert gases, the bottom of the torch is often slightly in front of the upper section, while the opposite is true when the welding atmosphere is carbon dioxide.

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Quality

Two of the most prevalent quality problems in GMAW are dross and porosity. If not controlled, they can lead to weaker, less ductile welds. Dross is an especially common problem in aluminum GMAW welds, normally coming from particles of aluminum oxide or aluminum nitride present in the electrode or base materials. Electrodes and workpieces must be brushed with a wire brush or chemically treated to remove oxides on the surface. Any oxygen in contact with the weld pool, whether from the atmosphere or the shielding gas, causes dross as well. As a result, sufficient flow of inert shielding gases is necessary, and welding in volatile air should be avoided.

In GMAW the primary cause of porosity is gas entrapment in the weld pool, which occurs when the metal solidifies before the gas escapes. The gas can come from impurities in the shielding gas or on the workpiece, as well as from an excessively long or violent arc. Generally, the amount of gas entrapped is directly related to the cooling rate of the weld pool. Because of its higher thermal conductivity, aluminum welds are especially susceptible to greater cooling rates and thus additional porosity. To reduce it, the workpiece and electrode should be clean, the welding speed diminished and the current set high enough to provide sufficient heat input and stable metal transfer but low enough that the arc remains steady. Preheating can also help reduce the cooling rate in some cases by reducing the temperature gradient between the weld area and the base material.

Safety

Gas metal arc welding can be dangerous if proper precautions are not taken. Since GMAW employs an electric arc, welders wear protective clothing, including heavy leather gloves and protective long sleeve jackets, to avoid exposure to extreme heat and flames. In addition, the brightness of the electric arc is a source of the condition known as arc eye, an inflammation of the cornea caused by ultraviolet light and, in prolonged exposure, possible burning of the retina in the eye. Conventional welding helmets contain dark face plates to prevent this exposure. Newer helmet designs feature a liquid crystal-type face plate that self-darken upon exposure to high amounts of UV light. Transparent welding curtains, made of a polyviny chloride plastic film, are often used to shield nearby workers and bystanders from exposure to the UV light from the electric arc.

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Welders are also often exposed to dangerous gases and particulate matter. GMAW produces smoke containing particles of various types of oxides, and the size of the particles in question tends to influence the toxicity of the fumes, with smaller particles presenting a greater danger. Additionally, carbon dioxide and ozone gases can prove dangerous if ventilation is inadequate. Furthermore, because the use of compressed gases in GMAW 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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THANK YOU

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