Presentation Notedited .Pptx

7/30/2019 Presentation Notedited .Pptx

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M.S.Ramaiah School of Advanced Studies 2

MIG WELDING


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M.S.Ramaiah School of Advanced Studies

INTRODUCTION

GMAW(Gas Metal Arc Welding) is commonly referredto as MIG (Metal Inert Gas) Welding .It is a widelyused process for both ferrous and non ferrous materials.A small diameter wire is fed from a spool continuously,

henceforth the process is also known as semiautomaticwelding. This process can produce quick and neat weldover wide joint ranges.

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Classification of joining processes

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TIG Welding

MIG Welding

Plasma ArcWelding

SMA Welding

FCA Welding


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MIGW process

MIG welding is an semiautomatic process, in which a wireconnected to a source of DC act as an electrode to join twopieces of metal as it is continuously passed through a weldinggun.

The power is supplied from source through the welding torch.While squeezing the trigger the current travels through the

contact tip into the wire and makes it conductive. A flow of an shielding gas (inert) is also passed through thewelding gun at the same time along the wire electrode. Thisinert gas act as a shield, keeping air borne contaminants awayfrom the welding zone.

The torch contact tip is the positive electrical connection and thenegative electrical connection in the circuit comes from thewelding machine transformer which has a direct contact with thework, also known as the earth clamp which completes thecircuit.

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When the electrode touches the work an arc is formed betweenelectrode and work piece. The arc heats the metal parts until bothmelt and join together.

The heat produced by the short circuit ,along with the inert gasmelts the metal and allows them to mix together. As the heat isremoved, the metal begins to cool and solidifies to form a newpiece of fused metal.


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Equipments for MIG welding

DC power source Wire Feed Unit Torch

Shielding gas supply Electrode

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DC Power Source

DC power sources are of transformer rectifier design, witha constant voltage power source.

MIG welding is carried out on aDC electrode (welding wire)which is polarized positively.

The DC parameters are 60-500A 16-40V 1-20kW

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Wire Feed Unit

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The wire feed unit provides controlled supply of welding wire to the areato be welded According to the welding wire size and arc voltage provided, a constantrate of wire speed is required In MIG power source provides arc voltage control and wire feeder unit

provides wire speed control The wire feed motor spindle has a feed roller fitted and another pressureroll, adjustable spring mounted to lightly grip the wire and push it up thelength of the MIG torch


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Welding Torch

The welding torch delivers the wire andcurrent from source to point at whichwelding is required.

The shielder gas flows around outside of the liner .

The torch can be either water cooled or

air cooled Types of welding torch

Heavy duty (500A) Medium duty (400A) Light duty (250A)

Torch Angle Angle between the torch and the work is very

important since the welder only can see what'shappening during the process.

Angle depends on the type of joints being done

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Techniques of holding torch Fore hand welding

Back hand welding

It is always advisable to good practice to weld from bottom to top toget good welding penetration.

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1.Torch handle2.Molded phenolicdielectric(white) and threadedmetal nut insert(yellow)3.Shielding gas nozzle4.Contact tip5.Nozzle output face


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SHIELDING GAS

The primary purpose of the shielding gas in MIG welding is toprotect the molten weld metal and heat affected zone from oxidationand other contamination by the atmosphere

Inert and Semi inert gases are used for MIG welding An inert gas is one whose atoms are very stable and will not react

easily with atoms of other elements

For nonferrous metals inert gases such as argon and helium are used For ferrous metals Carbon dioxide, argon +5-20% carbon dioxide,

or argon +1-5% oxygen are used.

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BASE METAL SHIELDING GAS ADVANTAGESCarbon steels 75% Ar, 25% CO 2

Ar +25-50% CO 2 CO2

Less than 1/8 in.(3.2mm) thick: high welding speed

without melt through; minimum distortion and spatter.More than 1/8 in. (3.2mm) thick: minimum spatter, cleanweld appearance, good puddle control in out of positionwelding.Deeper penetration and faster welding speeds.

Low-alloy steel 60-70% He

+25-35% Ar+4-5% CO2 75% Ar+25% CO 2

Minimum reactivity; excellent toughness, arc stability,wetting characteristics and bead contour; little spatter.

Fair toughness; excellent arc stability, wettingcharacteristics and bead contour; little spatter

Stainless steels 90%He + 7.5%Ar +

2.5%CO2

No effect on corrosion resistance; small HAZ; no

undercutting; minimum distortionAluminium,Copper, Magnesium,Nickel and theiralloys

Ar & Ar-Hemixtures

Argon satisfactory on thin sections; argon heliummixtures preferred on thicker materials.

Base metals and shielding gases


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ELECTRODE

Electrode selection is based on the combination of metal being welded, joint design and metal surface condition

The electrode itself acts as the filler material for the weld,the fillermaterial(electrode) is used to bond the two surfaces being weldedtogether

The selection greatly influence the mechanical property of weld and

weld quality Mainly used electrode for MIG are copper alloy, bronze alloy, carbon

steel, low alloy steels, nickel alloy

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Thicker metals and complicated joint designs usually require fillerwires that provide high ductility
http://www.airgas.com/Welding%20Products/Filler%20Metal/MIG%20Wire%20(GMAW%20&%20SAW)/MIG%20Wire%20-%20Copper%20Alloyhttp://www.airgas.com/Welding%20Products/Filler%20Metal/MIG%20Wire%20(GMAW%20&%20SAW)/MIG%20Wire%20-%20Stainless%20Steelhttp://www.airgas.com/Welding%20Products/Filler%20Metal/MIG%20Wire%20(GMAW%20&%20SAW)/MIG%20Wire%20-%20Bronze%20Alloyhttp://www.airgas.com/Welding%20Products/Filler%20Metal/MIG%20Wire%20(GMAW%20&%20SAW)/MIG%20Wire%20-%20Aluminum


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Advantages of MIG welding

MIG welding process has several advantages which account for itspopularity and increased use in the industries.

The continuous wire feed eliminates the need to change electrodes,increase the welding speed, quality of weld and overall control.

Higher deposition rate than SMAW. Less operator skill is required.

The gaseous shield protects the molten metal from the atmosphericgases eliminating the flux or slag, and keeping the spatter minimal.

Because of the fast travel speed at which MIG welding can be done,there is a smaller heat-affected zone,which results in less grain growth,less distortion, and less loss of temper in the base metal .

Both thick and thin metals can be welded successfully andeconomically.

Less time is needed to prepare weld joints since the MIG welds aredeep penetrating.

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Disadvantages of MIG welding

The MIG welding cannot be used in the vertical or overhead weldingpositions because of the high heat input and the fluidity of the weld

puddle. The equipment is complex and higher initial setup cost. Higher maintenance cost due to extra electronic components. The setting of plant variables requires high skill level.

Limited to draught free atmospheric condition.

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Type of weld joints

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BUTT JOINTEDGE JOINT

T JOINT

LAP JOINT

CORNER JOINT


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BUTT JOINT Is used when two piece of metal of the same thickness are joint along their length so

that each is the extension of the other.

EDGE JOINT The edges are welded together so that the two pieces remain parallel or nearly

parallel but do not exceed 45. Generally used on thinner metals(less than 1/4 )

LAP JOINT

Used to fuse different thickness of metal together by overlapping them. This type of joints are stronger since the thickness is increased. May be used in addition to a butt joint to increase the overall strength of the section.

CORNER JOINT When two materials come together at a right angle, a corner joint is applied to bond

the two. T JOINT

When one piece of metal to be welded is placed vertically on another piece lyinghorizontally, to form shape of an inverted T

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T JOINTBUTT

CORNER

LAPEDGE
http://www.mig-welding.co.uk/arc-fillet-joints/fillet-legs-large.jpg


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WELD SPATTER Small particles of materials that are expelled during the fusion of weld and the base

metal. BURN BACK

While welding a weld forms in the contact tip called burnback. It occurs when the wire is fed too slowly or the torch is held too close to the base

metal

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Incomplete penetration Incompletefusion

Undercut

Visual weld defects

Blow hole


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lumps formation (current variation )

not continuous (untrained welder)

Visual weld defects


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Weld Under cutWeld Spatter & Burn through

Visual weld defects


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S.No.

Defects Causes Remedies

1 Porosity

1.GAS CONTAMINATION2.ELECTRODECONTAMINATION3.WORK PIECECONTAMINATION4.ARC VOLTAGE TOOHIGH5.TOO LONG STICK OUT6.SHIELDING COVERAGE

LOW

1.USE WELDING GRADE SHIELDING GAS2.USE CLEAN DRY WIRE NOT RUSTED.

3.REMOVE,GREASE,OIL,MOISTURE,RUST, PAINT. USEDEOXIDISED WIRE4.USE OPTIMUM ARC VOLTAGE

5.USE OPTIMUM STICK OUT6.USES OPTIMUM GAS FLOW. CHECK LEAKS, DRAFTS,

FANS, HEATER. REDUCE TRAVEL SPEED, NOZZLE TO JOBDISTANCE RIGHT ANGLE OF THE TORCH

2 UNDERCUTTING

1.VERY HIGH TRAVELSPEED

2.VERY HIGH WELDINGVOLTAGE

3.VERY HIGH WELDINGCURRENT

4.LESS DWELL AT SIDEWALL

5.GUN ANGLE.

1.USE OPTIMUM TRAVEL SPEED

2.USE OPTIMUM VOLTAGE

3.USE CORRECT WIRE SPEED

4.DWELL LONGER AT EDGES

5.USE THE CORRECT GUN ANGLE

Welding defects: causes and remedies


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S.No. Defects Causes Remedies

3 INSUFFICIENTFUSION

1.IMPURITY IN WELD ZONE

OF JOB2.INSUFFICIENT HEATINPUT

3.VERY LARGE WELD POOL

4.INCORRECT WELDTECHNIQUE

5.INCORRECT JOINTDESIGN

6.VERY HIGH TRAVELSPEED

1.CLEAN WELD AREA PRIOR TO WELDING

2.INCREASE CURRENT (WIRE FEED SPEED). REDUCE STICK OUT

3.REDUCE WEAVING TO GET A CONTROLLABLE WELD POOL

4.FOLLOW CORRECT TORCH ANGLE & TECHNIQUE

5.HAVE THE OPTIMIUM JOINT DESIGN

6.USE OPTIMUM TRAVEL SPEED

4 Excess melt1.HIGH HEAT INPUT

2.JOINT PENETRATION NOTPROPER

1.REDUCE VOLTAGE AND CURRENT. INCREASE TRAVEL SPEED

2.INCREASE ROOT FACE. REDUCE ROOT OPENING

5Spatters

1. Excess current

1. Exccess voltage

1. Control current

2. Control voltage


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WELD TESTING


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Welding Symbols

Leg length

Weldlength

Weld Process No.(CO2)Direction

of theWeld

No.of Welds

WeldSpacing

Remark -1)Leg length should be 90%of the min.sheet thk


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Wear welding gloves, helmet, leather apron, welding chaps, leather shoes, andother personal protective equipment to help prevent weld burns. Never weld with presence of flammables (matches, butane lighters, fuel stick, etc.) Never leave hot metal where others may touch it and be burned. Welding fumes should be ventilated away from the welder, not across the welder'sface. Store inert gas cylinders in a cool, dry storage area. Do not lay the gun on the work or worktable. Wear a helmet with filter lens and cover plate while welding.

SAFETY MEASURES


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M S Ramaiah School of Advanced Studies

THANK YOU

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