welding process gmaw

8/10/2019 welding process gmaw

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Objective

To learn MIG (metal inert gas) welding theoretically and practically.

To learn about technique of MIG(metal inert gas) welding in fillet (t-joint)

To know about advantages, disadvantages and application of MIG (metal inert gas)welding

Theory

Power source

MIG welding is carried out on DC electrode (welding wire) positive polarity (DCEP).

However DCEN is used (for higher burn off rate) with certain self- shielding and gas

shield cored wires.DC output power sources are of a transformer-rectifier design, with a

flat characteristic (constant voltage power source). The most common type of power

source used for this process is the switched primary transformer rectifier with constant

voltage characteristics from both 3-phase 415V and 1-phase 240V input supplies. The

output of direct current after full wave rectification from a 3-phase machine is very

smooth. To obtain smooth output after full wave rectification with a 1- phase machine, a

large capacitor bank across the output is required. Because of the expense of this, many

low cost 1-phase machines omit this component and therefore provide a poorer weld

characteristic. The switches to the main transformer primary winding provide the output

voltage steps at the power source output terminals. Another method of producing

different voltages at the power source output terminals is to use a Thyristor or a

Transistor rectifier instead of a simple diode rectifier. This system offers continuously

variable output voltage, which can be particularly useful on robot installations and the

cost of this type of rectifier can be partly offset with no need for primary voltage switch

or switches and a single tapped main transformer primary winding. Most MIG powersources have a contactor or relay used to switch the output ON/OFF with operations of

the trigger on the MIG torch. The switch off operation of this contactor is normally

delayed to allow the welding wire to Burn back out of the molten weld pool. A

thermostat is fitted on the hottest point in the power source, in series with the contactor

coil to provide thermal protection to the machine. Power source performance is measured


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by its ability to provide acertain current for a percentage of a 10 minute period before

Thermal CutOut. This is the Duty Cycle.

The Wire-feed Unit

The wire-feed unit, or sub-assembly where this is mounted in the power source cabinet

(known as a composite MIG), provides the controlled supply of welding wire to the point

to be welded. According to the welding wire size and arc voltage provided by the power

source, a constant rate of wire speed is required, in MIG welding the power source

provides Arc voltage control and the wire feed unit provides welding wire speed control,

( in MIG this equates to welding current ).Most modern wire feed units control the wire

feed speed via a DC motor and thyristor control PCB to provide continuos control of

Armature volts and hence RPM of motor. The wire feed motor spindle has a feed roller

fitted and another pressure roll, adjustable spring mounted to lightly grip thewire and push it up the length of the MIG torch.


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Welding Process Variables

During a manual welding operation, the welder has to have control over the welding

variables, which affect the weld penetration, bead geometry and the overall weld quality.

A proper selection of welding variables will increase the chances of producing welds of a

satisfactory quality. However, these variables are not completely independent and

changing one variable generally requires the changing of some of the others in order to

achieve the desired result. When all these variables are in proper balance, the welder can

deposit higher quality weld metal and produce sound welds.The selection of the welding

variables should be made after the base metal, filler metal and joint design have been

determined.The welding process variables mainly affect the geometry of the weld bead

such as the penetration, bead reinforcement, bead width and the deposition rate, which is

the weight of the metal deposited per unit of time. These variables are as follows:

Welding Current

Travel speed

Welding voltage

Type of shielding gas

Electrode angle

Wire electrode size

Weld joint position

Electrode extension


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Metal Transfer Across The Arc

Short circuiting transfer

In short-circuit transfer, the electrode touches the work and short circuits, causing the metal

to transfer as a result of the short. This happens at a rate of 20 to more than 200 times persecond. The advantage of the short-circuit transfer is its low energy. This method is

normally used on thin material inch or less, and for root passes on pipe with no backing.

It can be used to weld in all positions.

Globular transfer

Globular transfer mode happens when a molten droplet is formed on the end of the electrode

then drops off due to gravity or from short circuiting. This droplet has to be at a larger

diameter than the electrodes diameter to be considered globular transfer.

Spray transfer

Spray transfer is named for the spray of tiny molten droplets across the arc. Spray transfer

usually is smaller than the diameter of the wire and uses relatively high voltage and wire

feed speeds or amperage. Unlike short-circuit transfer, once the arc is established, it is on at

all times. This method produces very little spatter and is most often used on thick metals in

the flat and horizontal positions

Pulsed spray transfer

A type of metal transfer in which as little as one droplet of metal forms on the end of the

electrode during periods of high current, but ceases during periods of low current. Pulsedspray welding reduces the heat input to the base metal, which makes it easier to weld thin

aluminum with GMAW.


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Advantages & Disadvantages

Advantages

This process is easy to learn. Within a short period of time, you can become pretty good at

quality welds. Cleanup from a welding session is minimal. The next advantage of MIGwelding is its speed. In MIG welding process, you need not change rods because a solid

continuous wire is used. As compared to other welding processes, quality welds can be

produced faster by this method. MIG welding enables you to achieve a higher output of work

compared to other types of welding. The elimination of flux in MIG welding helps to prevent

slag defects in the weld. You can easily weld thick and thin, both types of workpieces

effectively The shield gas protects the arc. So you need not replace the electrode frequently.

This also helps to put a stop to weld splatter. This process can be applied on different metals

including stainless steel, mild steel and aluminum. Your weld deposition rate will be higher

as compared to other alternative processes The MIG welding process can be either manual or

automated. This process enables you to produce joints with deep penetration. The process canbe easily mechanized. The welded surfaces produced by this method are smooth, clean and

spatter free.

Disadvantages

The MIG welding process does not work well in outdoor welding applications because air

drafts may disperse the shielding gas. In this process, the weld metal cooling rates are higher

as compared to processes that deposit slag over the weld metal. Furthermore, this process

requires a Wire Feeder which is difficult to move and can sometimes be a maintenance /

repair burden and welding gun is difficult to get into tight places as well.


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Tools & Equipment

Personal protective equipment

Welding - Personal Protective Equipment

Body Part Equipment Illustration Reason

Eyes and

face

Welding

helmet, hand

shield, or

goggles

Protects from:

radiation

hot slag, sparks

intense light

irritation and

chemical burns

Wear fire resistant head

coverings under the

helmet where

appropriate

Exposed

skin (otherthan feet,

hands, and

head)

Fire/Flame

resistantclothing and

aprons

Protects against:

heat, fires

burns

Notes: pants should not

have cuffs, shirts should

have flaps over pockets

or be taped closed

Feet and

hands

Boots,

gloves

Protects against:

electric shock

heat

burns

fires


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Welding equipment

Torch & Accessories

The welding torch guides the wire and shielding gas to the weld zone.

Brings welding power to the wire as well

Major components/parts of the torch are the contact tip, shielding gas nozzle, gas

diffuser, and the wire conduit

Welding Control &Wire Feed Motor

Welding control & Wire feed motor are combined into one unit

Main function is to pull the wire from the spool and feed it to the arc

Controls wire feed speed and regulates the starting and stopping of wire feed

Wire feed speed controls Amperage


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Power Source (welding machine)

Almost all GMAW is done with reverse polarity also known as DCEP

Positive (+) lead is connected to the torch

Negative (-) lead is connected to the work piece

Provides a relatively consistent voltage to the arc

Arc Voltage is the voltage between the end of the wire and the work piece

Wire Brush

Wire brush is used for cleaning rust

It is used toclean surfaces and to create a better conductive area

for attaching electrical connections.
http://en.wikipedia.org/wiki/Cleanhttp://en.wikipedia.org/wiki/Clean


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Plier

Hand tool used to hold objects firmly (hot object such as weldmetal)

Shielding Gases

Purpose of shielding gas is the protect the weld area from the contaminants in the

atmosphere Gas can be Inert, Reactive, or Mixtures of both

Gas flow rate is between 25-35 CFH

Argon, Helium, and Carbon Dioxide are the main three gases used in GMAW


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Procedure

Before Welding

Step 1Safetyprecaution

Welder ensured the area has a suitable safe work by keep it clean & free of grease, oils &

flammables. In addition, welding bay was closed properly to protect others from any UVflash exposure. Furthermore, area of welding bench was ensured that it was well ventilated

(with fume extraction). For PPE, long sleeve shirt, long pants, safety shoes, and gauntlet

gloves has to be wore before, during and after weld. The long sleeve shirt is required because

UV rays can cause minor skin burns identical to sun burns. Gauntlet Gloves are required

because they protect your hands from the immense heat created during the welding process.

Step 2Preparing work piece

Firstly, plate was prepared by cutting it followed by the dimension given. 6mm thicknessplate with 60mm width cut into 150mm length for 2 pieces. Next, by using wire brush and a

piece of remnant fabric, contaminant at the plate such as rust, paint or grease was removed to

ensured good electrical contact during weld.

Step 3Welding machine setup

For welding machine setup, the equipment of weld was checked to ensure all the cableconnection was fitted tightly and free of fraying or others damage. After that, for electrodepolarity, it was required for (DCEP) direct current electrode positive. Moreover, gas flow was

setup by valve turned into half. (15-20 psi)

Step 4Work piece setup

After welding machine set up completed, earth clamp was clipped to the working table for

making one completed circuit of electrical flow. And then, by using tack weld mode, 2 pieces

of plate 150mm x 60mm x 6mm was tacked to make fillet (t-joint)


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During welding

Step 5Begin welding

Fillet (T-joint) that has been tacked was positioned to 1F (900

degree work angle). By using 4step mode, the opposite side of fillet (t-joint) has been used as weld area. Drag (pull)

technique was used during weld the work piece and the gap of contact tip was 6 to 10 mm

from the weld metal. After the first layer finished, the surface of welded area has been

cleaned before it was continued for the second layer.

Welder

Name

Specimen

Length(mm)

Time During

Weld (sec)

Travel Speed

(mm/min)

Heat Input (kJ/sec)

Voltage CurrentTravelSpeed

HeatInput

Ariff150 100 90.00 18.3 133 90.00 1.6226

Fadzil 150 111 81.08 20 100 81.08 1.48

Step 6Surface finishing

Afterward, welded area was cleaned by using wire brush and then spatter has been removed

using chisels.


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Result & Discussion

Mohd Ariff Bin Zakaria (50249113541)

Discussion

Discontinuities Cause Prevention

1.Uneven cap profile

2.Overlap

3.poor welding stop

1.excessive weaving/ arc too

long/short

2.excessive weld

3. improper weld technique

1. hand-eye weld technique

required

2. control welding parameter

such as current and speed travel

during weld


required

21 3


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Welding parameter

Mohd Ariff Zakaria - 50249113541

Material Mild steel

Work piece size ( L x W x T) 150mm x 60mm x 6mm

Type of joint Fillet (tjoint)

Wire feed diameter size 1mm

Shielding Gas CO2

Welding current (before weld)

Welding current (during weld)

107 ampere

133 ampere

Voltage (before weld)

Voltage (during weld)

18.3 V

18.2-18.3 V

Mode (2 step/4step) 4 step

Position 1 F

Average time taken

(travel speed)

1min 40 sec

(150mm100 sec = 1.5mm/sec)

Heat input = voltage(V) x current(I) x 60

travel speed(mm/s) x 1000

=18.3 x 133 x 60

1.5(60) x 1000

=1.6226 kJ/mm


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Muhammad Fadzil bin Mohd Murad (50249113880)

Discussion

Discontinuities Cause Prevention

1.convexity

2.Overlap

3.poor welding stop

1.excessive weaving/ arc too

long/short

2.excessive weld

3. improper weld technique


required

2. control welding parameter

such as current and speed travel

during weld


required

1

23


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Welding parameter

Muhammad Fadzil bin Mohd Murad - 50249113880

Material Mild steel

Work piece size ( L x W x T) 150mm x 60mm x 6mm

Type of joint Fillet (tjoint)

Wire feed diameter size 1mm

Shielding Gas CO2

Welding current (before weld)

Welding current (during weld)

100 ampere

97 ampere

Voltage (before weld)

Voltage (during weld)

20 V

19.8-20.1 V

Mode (2 step/4step) 2 step

Position 1 F

Average time taken

(travel speed)

1min 51 sec

(150mm111 sec = 1.35mm/sec)

Heat input = voltage(V) x current(I) x 60

travel speed(mm/s) x 1000

=20 x 100 x 60

1.35(60) x 1000

=1.48 kJ/mm


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Conclusion

For safety precautions in gas metal arc welding, wear PPE during welding progress, it is

mandatory procedure for user to comply the safety basic which is can avoid bad scenario

from happened. Furthermore, by getting familiar with basic equipment/apparatus can also

help user from applying each of the equipment properly during weld. Next, user learnt

how to maintain the machine for example when user had to change the wire electrode

spool from 1.2mm diameter wire electrode to 1.0mm, the roller need to switched to the

other side according to the size of the track of roller. In technical side, user able to know

drag/pull and push gun technique during weld for example when using push/pull

technique, weld bead showed narrower compare to push technique that make wider bead

and less penetration. Moreover, user able to gain skills in fillet (t-joint) weld, such as the

position of metal (1F/2F) weld, the work angle during weld and the distance of wire feed

with weld pool. Meanwhile, user also learnt how to get good metal transfer by control thecurrent flow. Then, user will get to know advantages, disadvantages & limitations of

Gmaw. For instance, Gmaw can have high travel speeds and deposition rates if it is

compared to Shielded Metal Arc Welding (Stick/SMAW), which can save money on

down time however the welding machine itself is more complex then the SMAW power

supplies, which leads to an increase in equipment costs. In shorts, this this process is one

of the best process that may have great result of weld because it has minimal percentage

for contaminant to get into it.

welding process gmaw

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