Mig Mag Welding Process

MIG / MAG WELDING PROCESS Page no. 1

What is Welding


1a. Basic Information of Welding Process

Welding is a metal joining process by : Heating to temperature high enough to cause softening or melting

With or without application of pressure

With or without use of filler metal

Melting point same as metals being joined or melting point

BASE METAL

WELD

SOLIDIFIED SLAG

ARC POOL

WELDING ATMOSPHERE

CORE WIRE

ELECTRODE COATING

ARC STREAM

PENETRATION DEPTH

Elements in Welding Process


Electrical Model of Welding Process

(+)

(-)

(+)

(-)

How Welding Takes Place ?

(+)

(-)

Electrode

Torch

Work Piece

Stage - I Stage - II Stage - III

(+)

(-)

(+)

(-)

Stage – I :- Electric Circuit remains open & no current flows though the circuit.

Stage – II :- When the electrode strikes with work piece whether by operator or some mechanical means (i.e. SPM / Robot) , electric circuit gets closed and electrons flow from cathode to anode & Current flows from anode to cathode. Electrical energy converts into power / heat energy using simple equation :- Power / Heat = Current (I) x Voltage (V)

(+)

(-)

Stage - IV

Weld Pool Weldment


Stage – IV :- When the welding is completed , Electrode is taken backwards smoothly to terminate electric circuit.

Types of Polarity in DC

Reverse Polarity i.e. Direct Current Electrode Positive

(DCEP)

Straight Polarity i.e. Direct Current Electrode

Negative (DCEN)

Electrical Polarity : - Depending upon how the electrical circuit is completed in welding & Types of output current , Welding Process is classified into following

Output Current

Alternating Current (AC) - Polarity changes after half cycle

Direct Current (DC) -Polarity remains constant through out the welding -Widely used across the industry

Stage – III :- Once the electric circuit is established between cathode & anode, electrode is taken slightly backwards to establish the arc which allows the formation of weld puddle / pool of homogenous material of molten electrode/filler & molten work piece. Weld puddle is transferred on work piece by short circuiting effect of Current & surface tension effect of molten metal. Thus Welding continues.

Application : 1. GMAW 2. MIG / MAG Welding

Application : 1. TIG Welding


a) Current :- Electric current is the flow of electric charge i.e. electrons. It is measured in Amperes (Amp). Electric current is displayed on welding Power Source

b) Voltage :- It is the potential difference between cathode & Anode which Provides the electromotive force between cathode & anode for electrons to flow. It is measured in Volt (V). Welding power source has display for Welding Voltage

Typical Welding Set up of Arc Welding

Typical Set up :- 1. Constant Voltage (CV)

Welding Power Source 2. Power Cord 3. Weld Cable to Wire Feeder 4. Ground Cable to Work Piece 5. Work Piece 6. Welding Gun 7. Constant Speed Wire Feeder 8. Electrode Wire 9. Gas hose 10. Shielding Gas Cylinder

1. Welding Power Source :- Function of Welding Power Source is to establish welding circuit and provide electrical energy (i.e. Current & Voltage)

2. Power Cord :- Cable used to connect anode & cathode.

3. Weld Cable to Wire Feeder :- Cable used to provide drive to feeder.

4. Ground Cable to Work piece :- -Ve / +Ve Cable used to provide earthing to work piece.


6. Welding Gun :- Function of welding gun is to direct the welding electrode into weld pool. It works as anode or cathode of welding circuit.

5. Work piece :- Welding is done on work piece. Different weld joints used of work piece are described below:-

Diffuser


7. Constant speed wire feeder :- It provides drive to the welding electrode to feed into weld pool at constant set speed.

8. Electrode Wire :- A metal or alloy in rod or wire forms used in electric arc welding to maintain the arc and at the same time supply molten metal or alloy at the point where the weld is to be accomplished.

As per American Welding Society (AWS), the welding electrodes are designated as follows:-

E R X X X X

Solid / Cored

Strength Chemical Composition

E 70 S 6

Electrode Rod

Electrode

Tensile Strength 70,000 psi (480 Mpa)

Solid wire

Chemical Properties as per No.6

Example :-


Type of Welding Electrodes

Flux Cored Electrode

Solid Electrode

Metal Cored Electrode

9. Gas hose :-Tube through which the shielding gas flows at welding pool

10. Shielding Gas :-Shielding gas works as molten metal protection media so that molten metal does not react with atmosphere.

During fusion welding, the molten metal in the weld “puddle” is susceptible to oxidation. So it should be protected from atmosphere by use of :

Weld Fluxes (e.g. SiO2, TiO2, FeO, MgO, Al2O3) Inert Gases (e.g. Ar, He, N & CO2) Vacuum

Gas Supply System :

Pre-heater Flow meter


Welding Position

ISO / ASME Welding Positions for Different Weld Joints :-

Down hand: PA/1G

Horizontal-Vertical: PC/2G

Overhead: PE/4G

Vertical-up: PF/3G

Vertical-Down: PG/3G

Down hand: PA/1F

Horizontal: PB/2F

Overhead: PD/4F

Vertical-up: PF/3F

Vertical-Down: PG/3F

Plate to Plate Weld Joint :-


Pipe rotates with axis horizontal, Welding down hand: PB/2FR

Pipe fixed with axis vertical : PB/2F

Pipe fixed with Axis vertical, Welding overhead: PD/4F

Pipe Fixed with axis Horizontal, welding Upwards: PH/5F

Pipe fixed with axis Horizontal ,

Welding downwards:

PJ / 5F

Pipe rotates with axis horizontal, Welding down hand: PA/1G

Pipe fixed with axis vertical, Welding horizontal-vertical: PC/2G

Pipe fixed with axis horizontal, Welding upwards: PH/5G

Pipe Fixed with axis Horizontal, welding downwards: PJ/5G

Pipe fixed with axis under 45 deg angle ,

welding upwards: H-LO45 / 6G

Pipe to Pipe Weld Joint :-

Pipe to Plate Weld Joint :-


Welding Defects & Causes

A flaw or flaws that by nature or accumulated effect render a part or product unable to meet minimum applicable acceptance standards or specifications.

Common Welding Defects

Sr.

No. Defect Definition Image Causes

1 Lack of

Fusion

When fusion in any plate

occurs less than

specifications (min 25% of

min material thk)

-Low current

-High voltage

-Excessive speed

-Torch orientation not

correction

-Torch position not

correct

2 Shabby Weld

Bead

Weld bead which does not

look uniform throughout

the welding

-Unsteady hand

movement by

operator

- More stick out

Length

3 Excessive

Spatters

Spillage of molten metal at

the time of welding on the

work piece

-Excessive current,

voltage & Speed

-Unsteady Hand

movement

-More stick out length

4 Under Cut

A portion where a bead is

not filled, forming a

groove in the base metal

along the toe of a weld

-Excessive current

-Excessive stick out

-Slow Speed

5 Blow Holes

Spherical cavity in a bead -Insufficient gas flow

-Surface irregularities

e.g. oil, rust, moisture

-Inadequate torch

angle


Sr.

No. Defect Definition Image Causes

6 In sufficient

Leg Length

Leg Length : Distance

between the root of a joint

and the toe of a fillet weld

-Torch orientation not

correct

-Torch position not

correct

7 Weld Cracks

A discontinuity

characterized by a break

or gap in the surface of a

weld

-Raw material too

brittle

-Hydrogen

embrittlement

8 Silica Island

(Glass Bead)

Silicon Oxide formed on

welding bead surface

which looks glassy

-Insufficient gas flow

-Gas composition not

as per standard

-More silicon content

in welding wire

9 Weld Lump

Excessive welding done at

particular point where the

Leg length is more than the

specified

-Excessive wire feed

speed

-Travel speed too

slow


Welding Bead Geometry

Convex Weld Bead Concave Weld Bead

X

X

Section - XX

P1

P2 Plate 2

Plate 1

B

A

P1 : Penetration Depth on Plate 1

P2 : Penetration Depth on Plate 2

L1

L2

A : Throat Depth

L1 : Leg Length on Plate 1

L2 : Leg Length on Plate 2

B : Bead Width

Measuring Parameters for Welding Quality


1b. Various Types of Welding Process

Welding Process broadly classified into following category :-

B) Plastic Welding or Pressure Welding :- The piece of metal to be joined are heated to a plastic state & forced together by external pressure

A) Fusion Welding or Non-Pressure Welding :- The material at the joint is heated to a molten state & allowed to solidify.

Fusion / Arc Welding

Carbon Arc

Metal Arc

Metal Inert Gas

Tungsten Inert Gas

Plasma Arc

Submerge Arc

Electro Slag

Pressure / Resistance Welding

Flash Butt Spot Seam Projection Percussion


C) Thermit Welding

D) Solid State Welding

Solid State Welding

Friction Ultrasonic Diffusion Explosive

E) Latest Welding Processes

Electron - Beam Laser

F) Low Heat Input Welding Processes

Brazing Soldering


1a. Basic Information of MIG / MAG Welding Process

Definition MIG / MAG Welding is a type of Gas Metal Arc Welding (GMAW) Process which produces the coalescence of metal by heating them with an arc between a continuously fed filler metal electrode and the work piece. Depending upon the combination of gas & action it performs while welding, Process is divided in two category:-

Metal Inert Gas Welding (MIG) :- If the shielding gas used is an inert gas such as argon or helium then it is termed as metal inert gas (MIG) welding. Metal Active Gas Welding (MAG) :- if shielding gas is active gas such as CO2

or mixture of inert and active gases then process is termed as metal active gas (MAG) welding.

Principle of MIG/MAG welding

Heat generated by the electric arc melts the surfaces of the base metal to develop a weld joint. The arc is generated between the base metal and the electrode. Welding arc and weld pool are well protected by a jet of shielding gas coming out of the nozzle and forming a shroud around the arc and weld. Consumable electrode is feed automatically while the torch is controlled manually or automatically. Applicability of MIG / MAG Welding ranges widely for Carbon Steel, Stainless Steel, Aluminium, Magnesium, Copper, Nickel, silicon, bronze, and tubular metal cored surfacing alloys.


Limitations :-

1. Higher skilled welder is required

2. Shielding of weld pool 100% from external environment becomes difficult which leads to formation of silica island (Glass Bead) defect. 3. Part to Part matching accuracy requirement is stringent to get the specified weld

bead geometry by MIG / MAG Welding Process i.e. process capability less to bridge the gap.

Advantages :-

1. The ability to join a wide range of material types & Thicknesses. 2. Higher Deposition rate of welding electrode. 3. Excellent Weld Bead Appearance 4. Easily adopted for high-speed robotic & semi automatic applications 5. Feasibility in all position welding 6. Lower hydrogen weld deposit – generally less than 5 mL / 100 g of weld metal 7. No need for slag removal 8. Less welding fumes as compared to SMAW & FCAW process


Metal Transfer in MIG/MAG welding

Short Circuit Transfer

Short circuit transfer refers

to the welding wire actually

“short circuiting” the base

metal 90-200 times per

second. Wire feed speed,

voltages and deposition

rates are lower than other

types.

Short circuit metal transfer

is very versatile in allowing

the welder to weld on thin

or thick metal in any

position.

Limitations

• Low metal deposition

rate

• Lack of fusion on

thicker materials

• More spatters

Globular Transfer

Globular transfer is the

state of transfer between

short circuit and spray arc

transfer. Large globs of

wire are expelled and enter

the weld pool. Results

when welding parameters

are higher than the short

circuiting.

Limitations

• Presence of spatters

• Less desirable weld

appearance

• Limited to flat and

horizontal fillet weld

position..

• Limited to metals 3

mm or thicker.

Spray Arc Transfer

Spray arc transfer “sprays”

a stream of molten droplets

across the arc, from the

electrode wire to the base

metal. Used typically

higher voltage, wire feed

speed and amperage values

than short circuit transfer.

Advantages

• High deposition

• Good fusion and

penetration

• Good bead appearance

• Presence of very little

spatter.

Limitations

• Used only on metals 3

mm and thicker.

• Limited to flat and

horizontal fillet weld

position.

• Good fit-up is always

required.


1c. Input Material Condition

1. Input material must be free from oil & rust

2. Maximum permissible gap between two matting part is 80% of wire diameter

3. Input parts must qualify the PQS / CTQ parameters. - Pipes Qualify to Relation Gauge - Sheet Metal Parts / Sub Assembly Qualify to Relation Gauge

4. Particularly for Robotic Welding Application, the maximum allowable variation in

trim line is +/- 0.8 mm.


1f. Method

Stick Out :- Robotic torch configuration with respect to to work piece: Following figure shows typical configuration of welding torch with respect to work piece

Wire stick out shall be maintained in the range of 12-15 times wire diameter (e.g. for wire diameter 1.2 mm, wire stick out of about 18 mm to be maintained).

Fig : Robotic Torch configuration & Wire Stick Out

Weld Torch Angle/Orientation

• Weld torch angle for welding is very crucial to get required penetration and strength at weld joint. The angle / orientation (W, P & R values) should be recorded for reference.


• Travel angle refers to the angle in which welding takes place. This angle should be between 0-10 degrees.

• Travel angle may be either a push angle or a drag (Pull) angle, depending on the position of the torch. When the torch is ahead of the weld, it is known as pulling (or dragging) the weld. When the torch is trailing the weld, it is known as Push Weld.

• Work Angle is the angle of torch from vertical to the weld direction, which is to be kept as 45 +/- 5 Deg.

• Maximum allowable Torch position wrt to Weld Joint Center Point is 80% of Wire Diameter.

• Efforts should be made to weld all joints in down hand position so as to prolong / prevent chocking of nozzle with weld spatters.

• Push angle gives better weld finish but lower penetration as compared to pull angle. Ascent welding gives deeper penetration with narrower weld bead where as descent welding gives shallower penetration with flatter weld bead.


• Direction of travel changes slightly when welding in the vertical position to 0-10 deg.

Weld Parameters

a) Weld Current : Increased welding current gives more weld deposition and deeper penetration

b) Weld Voltage : Increased welding voltage gives flatter bead & reduced penetration & vice versa.


c) Welding Speed : As a higher speed is used, the bead width decreases, the depth of penetration decreases, and the reinforcement of weld decrease. Weld speed to be optimized to get the desired weld bead and penetration.

d) Gas Flow Rate : Specified gas flow rate range : 12-15 LPM Too Less gas flow rate causes blow holes Too excess gas flow rate causes arc to become unstable.


1i. Measurement

S

N Term

Explanatory

illustration Definition

BE IS P-001-00

Fillet Joint

1a Penetration Depth

Distance between

the top of a base

metal fused part and

the surface to be

welded

a) Lower Limit – Average 30

% of Minimum Thickness

b) Upper Limit – 100% of

Minimum Thickness

1b Penetration

Evaluation Range

Length of welding

after leaving the

start and end bead

shape

Except for Temporary welding,

penetration should be checked

for a length measured between

center of the welding start point

and the center of the welding end

point

1c No of Sections

Sections taken for

penetration

checking along the

Evaluation Range

a) 1 section to be checked at

center if weld length <=

150 mm

b) For weld length >150 mm,

1 section to be cut at center

and remaining sections at

75 mm distance away from

center

2 Minimum Bead

Thickness

Length from the

root of a weld cross-

section to a point on

the bead surface at

the shortest distance

Minimum Equal to Plate

Thickness

3 Minimum Bead

Width

Minimum width

between bead

surfaces

Lower Limit – 5 mm

Upper Limit – 7 mm


4 Leg Length

Distance between

the root of a joint

and the toe of a

fillet weld

Minimum Equal to Plate

Thickness

5 Under cut

A portion where a

bead is not filled,

forming a groove in

the base metal along

the toe of a weld

Depth : 0.5 mm or less

6 Overlap

A portion where a

bead is not fused

with the base metal

and overlaid at the

toe of a weld

The bead end shall be smooth

(Not Steep)

Width : approx. 1 mm or less

7 Blow Holes Spherical cavity in a

bead

Cavity with 1 mm or large

diameter is not acceptable. Five or

less cavities per cross-section

surface are acceptable.

(Cavity with 0.2 mm or smaller

diameter shall not be counted)

8 Bead Joint

Minimum required

lap length for bead

joint

Lap dimension : 5 mm or more

12 Weld tack position Position of tack along

the weld length

Tacking to be done 15 mm away

from start and end point of trim

line of component

13 Weld Tack Geometry Diameter and Height

of Tack

Diameter of tack should not

exceed 6 mm and height should be

4-6 mm

15 mm


16 Over Run

Extra Length of welding

before the start and after the

end point of trim line of

component

Critical Weld joints must be

completed with 10 mm over runs at

start and end points

18 Hardness of

HAZ

HAZ - The portion of the

base metal that has not

been melted, but its

mechanical properties have

been altered by the heat of

welding

Hardness of weld bead & HAZ shall

not exceed 400 HV1. HAZ hardness

shall be checked at 0.1 mm from

weld bead and average of three

readings to be taken.

Mig Mag Welding Process

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