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WELDING

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Jespher Javier Joseph

on 2 April 2015

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Transcript of WELDING

DIRECT CURRENT ELECTRODE POSITIVE (DCEP)
WELDING

GAS METAL ARC WELDING (GMAW)
WHAT ARE GAS METAL ARC WELDING (GMAW)?
Gas metal arc welding (GMAW), sometimes referred to by its
subtypes metal inert gas (MIG) welding
or
metal active gas (MAG) welding
.
Its a
welding process
that uses an
arc
between a
continuous wire electrode
and the
weld pool
.
A shielding 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.
GMAW METHOD
GMAW MACHINE
SHIELDING GAS & ELECTRODES (GMAW)
TYPES OF CURRENT & POLARITY (GMAW)
EFFECTS OF POLARITY ON THE WELDING PROCESS
GAS TUNGSTEN ARC WELDING (GTAW)
DJJ1032 WORKSHOP TECHNOLOGY
(EOC 1)
LECTURER: AZIZEE SUKOR
GROUP MEMBER: CONTE DIAZ ALBERT (20DKM14F2027)
GEORGE GIBSON (20MKM14F2001)
CHELEVENS AZIZ (20DKM14F2033)
JESPHER JAVIER JOSEPH (20DKM14f2037)
ALBERT LAINK (20DKM14F2052)
DISTINGUISH
SHIELDED METAL ARC WELDING (SMAW)
GAS METAL ARC WELDING (GMAW)
GAS TUNGSTEN ARC WELDING (GTAW)
Inexpensive.
Has specialized electrode for hardening, high-strength, stainless, mixed, cast iron and other metal iron.
Uses a flux coating to shield weld.
Work on AC or DC.
Portable ( no need to carry gas or wire feeder.
GAS MIG
GASLESS MIG
Does not need shielding gas, saves cost of bottle.
Wire is more expensive.
Direct-current, electrode negative polarity (DCEN).
Not easy to use on thin metal.
More portable, no gas to transport.
Very clean welds with no slay or sputter.
Uses CO2 as a shieding gas.
Wire less expensive.
Direct-current, Electrode Positive Polarity (DCEP).
Expensive.
TIG uses a torch with a tungsten electrode.
Argon is uses as shielding gas to protect weld and tungsten.
DC is used on terrous metal (Steel)
AC is used on aluminium.
GTAW WELDING RESPECT TO THE PRINCIPLES
Metal are used together by heating them by an electric arc.
A filler metal may not be used depending on the design of the joint.
The moltent metal, tungsten electrode and the welding zone are shielded from the atmosphere, by inert gases through the welding torch.
Can be done in any position and in manual
The method used depend on the available and the application.
Process is obtained by maintaining the arc between tungsten alloy electrode and the workpiece.
Weld pool temperature can approach 2500 degree celcius (4530 degree farenheit).
The inert gases is normally argon, helium, or a mixture of helium and argon.
Method Of Welding (GTAW)
SHIELDING GAS
&
ELECTRODE
Pure argon is normally used as a shielding gas in TIG.
It works well with DC and AC.
The nozzle used are designed to match the flow characteristic of argon.
Helium is an inert gas provides an alternative to argon.
Arc voltages are higher when a tungsten arc is shielded with helium.
Increased heat input, giving improved welding on metals.
Helium is significantly more expensive than argon.
Pure helium can be justified is the welding of thick aluminium plate.
Stainless steel or nickels alloy can be increased by adding 1% to 5% hydrogen to the argon shielding gas.
Pure electrode can give satisfactory result.
Contain addition of Thoria or Zirconia to improve arc striking and stability.
Tungstun electrode diameters are sized by decimal (.010", .020", . 040")
Small-diameter use with low current.
Large-diameter use with high current
TYPE OF CURRENT TO WELD AND POLARITY (GTAW)
ALTERNATING CURRENT
To weld aluminium and magnesium alloys.
Positive charged gas ion attracted to the negative charged workpiece.
Gas ions, as they strike the workpiece.
GTAW do not have to be derated
ALTERNATE CURRENT HIGH- FREQUENCY
AC provides a combination of the penetrating qualities of DCEN and the DCEP.
Alternating current high-frequency uses a rapid alternation of electrode negative and electrode positive.
DIRECT CURRENT
Current flow in one direction only.
DC must be electrode negative (DCEN) or electrode positive (DCEP).
Electron flow from the electrode to the workpiece.
DIRECT CURRENT ELECTRODE NEGATIVE (DCEN)
Used for welding most metal because it produce deep penetration into metal.
Electron flow from the electrode to the workpiece.
The weld pool is deeper and narrowed than with DCEP.
Smaller diameter electrode can be used.
Current flow from workpiece to the electrode.
Greater concentration of heat at the electrode.
DCEP required a larger diameter electrode.
DCEP use with 125 A.
ADVANTAGES & DISADVANTAGES
GTAW
GMAW
SMAW
GTAW ADVANTAGES
Clean, high-quality welds
welds a wide range of metals
No spatters or slay, spark or smoke
Allow for welding in all position
GTAW DISADVANTAGES
Lower deposition rates
Requires high level of operator skill
Higher level of uv ray
Requires good eyes and hands coordination to achieve quality welds
WELDING DEFECTS AND ITS PREVENTION
• Welding defect may be defined as discontinuities or dis continuities that failed to meet the minimum acceptance per standard or specifications.
• The prevention is use appropriate welding parameters, attention to welding parameters and set appropriate parameters and instructs the welder to pay more attention.
• Reduce the excess, weld to complete the hole or remove the affected area and re-weld.
DISTORTION IN WELDING AND HOW TO MINIMIZE DISTORTION
1. Avoid over welding
• The bigger the weld, the greater the shrinkage. Correctly sizing a weld not only minimizes distortion, but also saves weld metal and time,
2. Thermal stress relieving.
• Another method for removing shrinkage forces is thermal stress relieving to control heating of the weldment to an elevated temperature, followed by controlled cooling.
3. Intermittent welding
• To minimize the amount of weld metal, use intermittent welds instead of continuous welds where possible.
4. Fewer weld passes
• A fewer number of big passes result in less distortion than a greater number of small passes with small electrodes and shrinkage accumulates from each weld pass.
5. Balance welds around the neutral axis
• Welding on both sides of the plate offsets one shrinkage force with another, to minimize distortion.
6. Presetting the parts
• Presetting parts before welding can make shrinkage work for you.
• The required amount of preset can be determined from a few trial welds.
7. Weld joint design
.
• Will effect the amount of distortion in a weldment.
• Distortion is easier to minimize in butt joints.
8. Part fit up
• Should be consistent to fabricate foreseeable and uniform shrinkage.
• Weld joints should be adequately and consistently tacked to minimize movement between the parts being joined by welding.

HOT WIRE WELDING
Is a process in which the filler metal is preheated as it enters the weld pool.
The filler metal is automatically fed from a wire feeder.
Filler metal is melted by an AC current.
PULSED GTAW (GTAW-P)
Process in which the welding current is pulsed.
The high and low level pulsating current produces overlapping spot welds.
Can be manual or automatic.
Can be use with or with-out filler metal.
The process can be usefor welding very thin metal.
SHORT CICUITING TRANSFER
A metal transfer mode in which molten metal from consumable welding wire is deposited during repeated short circuit.
It can be ed in all condition ( vertical, horizontal and overhead).
Produce shallow weld penetration.
As the molten welding wire is transferred to the weld, the drop touches the weld pool and the circuit is shorted, extinguishing the arc.
SPRAY TRANSFER
Metal transfer mode in which molten welding wire is propelled axially across the arc in small droplets.
Requires a high current density. (250 A, 300A, 400A)
With high input, thick welding wire melts readily and deep weld penetration becomes possible.
GLOBULAR TRANSFER
Is the transfer of molten metal in large droplets from the welding wire to the workpiece across an arc.
Occurs when the welding current is low or is below the transition current.
The molten ball at the tip of the welding wire grows until the diameter of the welding wire is two or three times of the diameter of the welding wire before it seperates and transfers across the arc to the workpiece.
PULSED SPRAY TRANSFER
A spray transfer mode in which current is cycled from low to high, at which point spray transfer occurs.
High level welding current ensures penetration of the metal and low level welding current permits pulses of high current.
Metal transfer is produced by directional force that is stronger than gravity.
Suitable for all-position welding.
Is an extension of spray transfer welding and allows current and voltage level much lower than those required for continuous spray transfer.
Shielding gases are necessary for gas metal arc welding to protect the welding area from atmospheric gases such as nitrogen and oxygen.
Shielding gas depends on the type of material being welded and the process variation being used.
Rate of shielding-gas flow depends primarily on weld geometry, speed, current, the type of gas, and the metal transfer mode.
Faster welding speeds, need more gas to be supplied to provide adequate coverage.
Higher current requires greater flow, and generally, more helium is required to provide adequate coverage than if argon is used.
Electrode selection is based primarily on the composition of the metal being welded, the process variation being used, joint design and the material surface conditions.
Electrodes contain deoxidizing metals such as silicon, manganese, titanium and aluminum in small percentages to help prevent oxygen porosity.
Some contain denitriding metals such as titanium and zirconium to avoid nitrogen porosity.
diameters of the electrodes used in GMAW typically range from 0.7 to 2.4 mm (0.028 – 0.095 in) but can be as large as 4 mm (0.16 in).
The smallest electrodes, generally up to 1.14 mm (0.045 in)
Any change in arc length (which is directly related to voltage) results in a large change in heat input and current.
A shorter arc length causes a much greater heat input, which makes the wire electrode melt more quickly and thereby restore the original arc length.
Sometimes a constant current power source is used in combination with an arc voltage-controlled wire feed unit.
Alternating current is rarely used with GMAW; instead, direct current is employed and the electrode is generally positively charged.
Requires significant skill to perform successfully.
The polarity can be reversed only when special emissive-coated electrode wires are used.
Helps operators keep the arc length consistent even when manually welding with hand-held welding guns
WELDING PROCESS (GMAW)
1. Set the voltage, wire feed, and shielding gas flow to the standard conditions for the required type of welding.
2. Adjust the welding wire to the proper stickout.
3. Start the arc and move the welding gun at uniform speed, maintaining the proper work angle.
4. Move along the gun along the joint using the pushing or pulling technique.
5. Release the trigger at the end of the weld to stop the wire feed and interrupt the welding current.
6. Properly shut down the welding machine when welding is completed :
a) Turn OFF wire speed control.
b) Shut OFF shielding gas at cylinders.
c) Squeeze welding gun trigger to bleed the lines.
d) Shut OFF welding machine.
e) Hang up welding gun.
GMAW PRINCIPLE
WELDING GUN
WIRE FEED UNIT
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 hose.
Normally made of copper
Connected to the welding power source through the power cable
Supplies the electrode to the work
Can reach feed rates as high as 30.5 m/min (1200 in/min)
Feed rates for semiautomatic GMAW typically range from 2 to 10 m/min (75 – 400 in/min)
WELDING ELECTRODE WIRE
Contain deoxidizing metals such as silicon, manganese, titanium and aluminum in small percentages to help prevent oxygen porositY.
Base material being welded the diameters of the electrodes used in GMAW typically range from 0.7 to 2.4 mm (0.028 – 0.095 in) but can be as large as 4 mm (0.16 in
SHIELDING GAS SUPPLY
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 desirable rate of shielding-gas flow depends primarily on weld geometry, speed, current, the type of gas, and the metal transfer mode.
WHAT IS GAS METAL ARC WELDING?
Gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding, is an arc welding process that uses a non-consumable tungsten electrode to produce the weld.
DISTINGUISH
SHIELDED METAL ARC WELDING (SMAW)
GAS METAL ARC WELDING (GMAW)
GAS TUNGSTEN ARC WELDING (GTAW)
Inexpensive.
Has specialized electrode for hardening, high-strength, stainless, mixed, cast iron and other metal iron.
Uses a flux coating to shield weld.
Work on AC or DC.
Portable ( no need to carry gas or wire feeder.
GAS MIG
GASLESS MIG
Does not need shielding gas, saves cost of bottle.
Wire is more expensive.
Direct-current, electrode negative polarity (DCEN).
Not easy to use on thin metal.
More portable, no gas to transport.
Very clean welds with no slay or sputter.
Uses CO2 as a shieding gas.
Wire less expensive.
Direct-current, Electrode Positive Polarity (DCEP).
Expensive.
TIG uses a torch with a tungsten electrode.
Argon is uses as shielding gas to protect weld and tungsten.
DC is used on terrous metal (Steel)
AC is used on aluminium.
GTAW MACHINE
WELDING TORCH
The torches are connected with cables to the power supply and with hoses to the shielding gas source and where used, the water supply.
The internal metal parts of a torch are made of hard alloys of copper or brass.
The size of the welding torch nozzle depends on the amount of shielded area desired.
POWER SUPPLY
ELECTRODE
EFFECTS OF POLARITY ON THE WELDING PROCESS
The electrode tends to melt off; therefore, direct-current reverse polarity (DCRP) requires a larger diameter elec-trode than direct-current straight polarity (DCSP).
DCSP produces a narrow, deep weld.
Since the heat is concentrated on the work, the welding process is more rapid and there is less distortion of the base metal.
Overall, straight polarity is preferredover reverse polarity because you can achieve betterwelds.
DIRECT CURRENT
ALTERNATING CURRENT
Partial or complete stoppage of current flow(rectification) causes the arc to be unstable and some-times go out.
Ac welding machines were developed witha high-frequency current flow unit to prevent this recti-fication.
The high-frequency current pierces the oxide film and forms a path for the welding current to follow.

WELDING PROCESS
(GTAW)
Lift-Arc Start
1 TIG Electrode
2 Workpiece
Touch tungsten electrode to workpiece at weld start point and enable output and shielding gas with torch trigger, foot control, or hand control.
Hold electrode to workpiece for 1-2 seconds, and slowly lift electrode.
Arc is formed when electrode is lifted.
Normal open-circuit voltage is not present before tungsten electrode touches workpiece; only a low sensing voltage is present between electrode and workpiece.
The solid-state output contactor does not energize until after electrode is touching workpiece.
This allows electrode to touch workpiece without overheating, sticking, or getting contaminated.
ADVANTAGES
DISADVANTAGES
1.
High deposition efficiency when used in certain transfer modes.
2.
All electrode weight is deposited into the weld whereas with other processes you must account for stub loss, flux/slag etc.
3.
No Slag to chip as compared to SMAW and FCAW
4.
The process can be used on thin materials with relative ease if properly set.
5.
GTAW can also be used on thin materials but in many cases such as Auto Body, GMAW wins hands down.
6.
Low Hydrogen weld deposit with all electrodes
7.
High production factor since no slag is required to be removed and uses a continuous electrode.
8.
With the parameters properly set for the application, anyone can weld after a very short amount of practice.
9.
One given electrode size can be used on various thicknesses of materials productively as compared to SMAW and GTAW
1.
Requires a Wire Feeder which is difficult to move and can sometimes be a maintanence/repair burden.
2.
Needs Shielding Gas so welding in windy conditions can be difficult.
3.
No slag system so out of position welds are sometimes more difficult.
4.
Increased chance of lack of fusion if parameters and welding technique is not controlled.
5.
The gun is difficult to get into tight places.
6.
Is not suitable for windy conditions.
ADVANTAGES
1.
Lower equipment cost than GTAW, FCAW and GMAW. (No bottle, gas hose, flowmeter, and tig rig/Wire feeder needed.
2.
Quick Change from one material to another.
3.
The process lends itself to welding in confined spaces and various positions with few problems.
4.
Deposition Rates faster than GTAW Manual
5.
Easy to move from one location to another. No Wire Feeder and Bottle.
6.
Some special electrodes are made for cutting/gouging
7.
Requires no outside shielding gas and can be used outdoors in light to medium wind.
8
. The ability to bend the electrode and the small space the electrode takes allows the process to be used in comparitively tight spaces. However keep in mind that for some jobs one of the other processes may also work or even work better. The FCAW-Self Shielded process can weld with a very long electrode stickout.
DISADVANTAGES
1.
Low deposition rate compared to GMAW/FCAW.
2.
Filler metal cost per weld can be greater due to a low deposition efficiency that can vary greatly with stub length.
3.
Production factor is typically lower (Unless welding on various materials) due to rod changes and chipping slag.
4.
Needs more hand eye coordination than GMAW/FCAW.
5.
Slag must be removed as compared to GTAW/GMAW
The electrode used in GTAW is made of tungsten or a tungsten alloy.
The electrode is not consumed during welding, though some erosion (called burn-off) can occur.
Can have either a clean finish or a ground finish—clean finish electrodes have been chemically cleaned, while ground finish electrodes have been ground to a uniform size and have a polished surface, making them optimal for heat conduction.
The diameter of the electrode can vary between 0.5 and 6.4 millimetres (0.02 and 0.25 in), and their length can range from 75 to 610 millimetres (3.0 to 24.0 in).
SHIELDING GAS
Shielding gases are necessary in GTAW 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 gas also transfers heat from the tungsten electrode to the metal, and it helps start and maintain a stable arc.
Argon is the most commonly used shielding gas for GTAW, since it helps prevent defects due to a varying arc length.
THE END...
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