2. Welding is a process of permanent joining two materials
(usually metals) through localized coalescence resulting
from a suitable combination of temperature, pressure and
metallurgical conditions
Welding is commonly classified in following types
Pressure Welding: The piece of metal to be joined are
heated to plastic state and forced together by external
pressure
Fusion Welding or Non-Pressure Welding: The material
at the joint is heated to a molten state and allowed to
solidify
4. In hml we use two processes mainly for joining sheet
metal, these are
• MIG(METAL INERT GAS WELDING): for joining parts of
swing arm and main stand, sheet metal thickness varies
from 2-7 mm
• SPOT WELDING: for joining parts of chain case, metal
thickness varies from 0.50-0.75mm
5. Gas metal arc welding (GMAW), sometimes referred as metal
inert gas (MIG) welding, is a welding process in which an
electric arc is formed 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, a shielding gas is fed through
the welding gun, which shields the process from contaminants
in the air.
A constant voltage, direct current power source is most
commonly used with GMAW.
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.
6. MIG Circuit diagram. (1) Welding torch, (2) Workpiece, (3)
Power source, (4) Wire feed unit, (5) Electrode source, (6)
Shielding gas supply.
7.
8. The typical MIG welding gun has a number of key parts—
a control switch, a contact tip, a power cable, a gas
nozzle, an electrode conduit and liner, and a gas pipe.
The control switch, or trigger, when pressed by the
operator, initiates the wire feed, electric power, and the
shielding gas flow, causing an electric arc to be struck.
The gas nozzle is used to evenly direct the shielding gas
into the welding zone—if the flow is inconsistent, it may
not provide adequate protection of the weld area.
9. • MIG torch. (1) Torch handle, (2) Molded phenolic
dielectric (shown in white) and threaded metal nut insert
(yellow), (3) Shielding gas diffuser, (4) Contact tip, (5)
Nozzle output face
10. There are three basic forms of wire feeders: the ‘push’
system, the ‘pull’ system and the ‘push–pull’ system.
As the name suggests, in the push system, the wire is
pushed by the wire feed drive rolls along the conduit to
the welding torch.
The pull system utilizes a set of wire rolls in the torch
handle which pull the wire from the wire reel.
This arrangement increases the weight of the torch and
does not increase the distance over which the wire can
be fed, this still being limited to around 3.5m, although
the consistency of the wire feed is improved and wire
diameters down to 0.8 mm can be used.
11. The push–pull system is a combination of the above two
systems with a set of drive rolls at both the wire reel
feeder and in the torch.
Here welding is done by two types of machines
• Semiautomatic special purpose machine
• Robotic machines(automatic)
• We use wire feed range between
10-12M/min, and we use push type
system for this purpose
A push system
12. Metal transfer in MIG is done in four modes.
1. Dip/short circuit mode
2. Pulsed mode
3. Spray mode
4. Globular mode
13. It is used for low current operation with lower electrode
diameter.
The molten metal forming on the tip of the electrode wire
is transferred by the wire dipping into the
weld pool thus causing a momentary short circuit
Metal is transferred only during a period when the
electrode tip is in contact with the weld pool
14. Welding current switches automatically from a low level
to a higher level in a periodic manner
Lower level current also known as back ground current is
set below the transition point and the higher level is set
well above the transition point in Spray transfer range.
Spray type metal transfer is achieved by applying pulses
of higher level current , each pulse having sufficient force
to detach a droplet.
The power supply are specially designed to produce
continuous wave forms and frequencies that PULSE the
welding current
15.
16. Either pure Argon or Argon rich with 0.5 to 5% oxygen
shield gas is used. With such gas mixture a true spatter
free axial spray transfer becomes possible with current
above transition point
Spray transfer mode can used in welding any position.
The metals droplets being very small, short circuit does
not occur and spatter is virtually eliminated
A superimposed pulsing current higher than the transition
current is necessary for spray transfer
17. It is characterized by a drop size with a diameter greater
then a electrode
The droplet detach when there weight exceeds the
surface tension of the molten metal that holds the drop to
the electrode tip
It takes place with a positive electrode (DCRP/DCEP)
when the current is relatively low regardless of the type of
shielding gas
The molten drop grows in size with increasing current
from its lowest value where the arc can be barely
sustained
19. Welding current
Arc voltage
Polarity
Electrode
Gas flow rate
Length of stick out
Shielding gas composition
20. Welding current depends upon welded metal thickness
and metal transfer mode required according to the parent
metal properties
For metal thickness
T <6mm=100-200amp
T6-8mm=200-450amp
T>8mm=450-700amp
21. In MIG process we generally use constant voltage is
used
This produce self regulation of arc length
For current range 150-200 amp it is kept in between 25-
30V for mild steel workpiece
Voltage, V
Current, A
22. In MIG we use DCEP (Direct Current Electrode Positive)
or reverse polarity
Positive terminal to electrode wire, negative terminal to
weld fixture
DC ensure elimination of arc blow
Heating effect is produced on electrode wire mainly for
welding sheet metal
23. Dia of electrode is dependent on welding current
With higher current dia should be larger and vice versa
It ranges from 0.7mm to 2.4mm depending upon current
For current ranging from 100-200amps 0.8-1.2 mm dia is
used
Electrode is made of same metal as parent metal coated
with deoxidizing agents such as copper, it also prevents
impurities
24. For different applications different flow rate is chosen
The four primary variations of GMAW have differing
shielding gas flow requirements—for the small weld pools
of the short circuiting and pulsed spray modes, about 10
L/min is generally suitable, while for globular transfer,
around 15 L/min is preferred.
Here we use dip mode and require good shielding so rate
is kept between 20-25L/min
25. Length of stick out is generally kept between 10-12mm
For stable arc it should not be larger
It is controlled by self regulation characteristic of MIG
26. Shielding gases are necessary for gas metal arc welding
to protect the welding area from atmospheric gases such
as nitrogen and oxygen, which can cause fusion defects,
porosity, and weld metal embrittlement if they come in
contact with the electrode, the arc, or the welding metal.
The most commonly used gas is argon, it is generally
mixed with other gases such as CO2
Pure argon doesn't provide much penetration with ferrous
metals
Whereas pure CO2 causes oxide formation
As a result, argon and carbon dioxide are frequently
mixed in a 75%/25% to 90%/10% mixture.
27. Improper choice of a welding gas can lead to welding
defects hence mixture should be chosen accordingly
Here metal transfer occurs in DIP mode and MILD
STEEL is welded so 80% AR and 20% C02 is used
Ratio of gasses is controlled by a mixer through which
gas mixture is supplied to welding apparatus
28. Common mixes
Argon-carbon dioxide
• C-50 (50% argon/50% CO2) is used for short arc welding of
pipes,
• C-40 (60% argon/40% CO2) is used for some flux-cored arc
welding cases. Better weld penetration than C-25.
• C-25 (75% argon/25% CO2) is commonly used by hobbyists
and in small-scale production. Limited to short circuit and globular
transfer welding. Common for short-circuit gas metal arc welding of
low carbon steel.
• C-20 (80% argon/20% CO2) is used for short-circuiting and
spray transfer of carbon steel.
• C-15 (85% argon/15% CO2) is common in production
environment for carbon and low alloy steels. Has lower spatter and
good weld penetration, suitable for thicker plates and steel
significantly covered with mill scale. Suitable for short
circuit, globular, pulse and spray transfer welding.
• C-10 (90% argon/10% CO2) is common in production
environment. Has low spatter and good weld penetration, though
lower than C-15 one; suitable for many steels.
• C-5 (95% argon/5% CO2) is used for pulse spray transfer and
short-circuiting of low alloy steel.
30. Some common defects in MIG are
Cracks: due to low speed, over deposition
Lack of penetration: low current, oil film on parent metal
spatter: caused due to high current
Blow holes: due to rusting on wire, impurities in shielding
gases or non continuous gas flow
Porosity: impurities in wire, inclusion of nitrogen of
oxygen causes it
Under deposition: less torch speed
Over deposition: high torch speed
31. • Over penetration: when the value of current is very high
then the melting of parent metal occurs causing over
penetration
• Bead out: in this weld bead is displaced from its position,
caused due to human or machine error
32. PENITERATION is the most critical to quality parameter
in welding
It should be 20% of the thickness of parent metal
For mild steel work piece with 20% CO2 in shielding gas
it gives 20% penetration
It can be checked by destructive testing namely
PENETERETION TEST
In this cross-section of weld is cut, cleaned with abrasive
paper
After this nitric acid is poured on it, this acid darken the
region of weld and parent metal turns out white
Now the depth of penetration can be measured by scale
