Principle of Operation
Welding current, inert gas supply, water is turned on. Electric arc is struck between the non-consumable electrode and the work piece by touching the electrode with work piece or using a high frequency until. In 1st method arc is initially struck on a scrap metal piece and then broken by increasing arc length.
In 2nd method a high frequency current is supper-imposed on the welding current. Welding torch is brought near to the job when electrode tip reaches within a distance of 3 to 2 mm from job. A spark jumps across the air gap between the electrode and job. Air path gets ionized and arc is established.
TIG welding is also known as Gas Tungsten Arc Welding Machine (GTAW) both the AC and DC power source can be used for GTAW electrode employed varies in dia from 0.5 to 6.5 mm carrying current from 5A to 650A; welding torch used for carrying current higher than 1000A is normally water cooled. GTAW is all position (1G, 2G, 3G, 4G.6G) welding and gives the highest quantity weld amongst commonly used arc welding process.
Equipment used in TIG Welding Machine
a) Welding torch, tungsten electrode and filler rod.
b) Welding power source, high frequency unit, DC suppressor unit and cables.
c) Inert gas cylinder for shielding purpose, pressure regulator, flow meter.
d) Cooling water supply.
e) Gas and water solenoid valve.
Advantage of TIG Welding Machine
a) TIG welding is very suitable for high quality welding of thin material.
b) Deeper penetration can be achieved through this process.
c) No flux is used so there is no danger of flux entrapment.
d) As this process can be seen with the help of goggle therefore operator can exercise a better control on the welding process.
e) TIG is very suitable for welding of non-ferrous metals and stainless steel.
f) TIG welding is suitable for welding in all positions (1G, 2G, 3G, 4G, 6G).
a) MIG welding is much faster process as compared to TIG welding under similar application.
b) Tungsten if transfer to molten weld pool can contaminate the same.
c) Tungsten inclusion is hard and brittle.
d) TIG welding equipment is more costly as compared to other welding equipment.
e) Skill or trained operator is required for welding.
Application of TIG Welding Machines
a) Basically welding of aluminium, magnesium, copper, nickel and their alloys, carbon, alloys or stainless steel, high temperature and hard surfacing alloys such as zirconium, titanium etc.
b) Welding sheet metal and thinner section.
c) Precision welding in automatic energy aircraft chemical and instrument industries.
The process involves fusing two pieces of sheet metal together by penetrating entirely through one piece into the other. No joint preparation is required except proper cleaning of the overlap areas. The main operation in arc spot welding is to strike and hold an arc without travel at a point where the two parts to be joined are held tightly together.
A vented metal nozzle of a shape to suit the application is fitted to the MIG gun and is pressed against the workpiece at the desired area. The operation is carried out for a period of 1-5 seconds and a slug is melted between the parts to be joined. Timing is usually controlled automatically with the help of a timer. Thus, the process time can be varied to achieve welds of different sizes depending upon the thickness of the sheets. Arc initiation is a critical part of the process and therefore must be reliable and consistent. This is easy to achieve by a flat characteristics power source and clean surface of the work.
GMAW spot welding is a highly adaptable process which requires very little manipulative skill; does not require the use of a welding helmet. It is an extremely fast process and can be fully automated. Due to addition of extra metal the weld slug is free from piping defects. A wire composition different from the base metal may be used to control cracking, porosity, or strength. Argon and CO2 are shielding gases commonly used for GMAW spot welding.
GMAW arc spot process can be used more efficiently for downhand welding position. It can be successfully employed for horizontal position but fails for overhead welding position.
This process does not require a hole to be made in either member, thus in differs from plug welding in that respect. As the upper member is required to be melted through and through, its thickness is normally restricted to 3 mm. The thickness of the second member is not important. Through lap joints are the most often used type of joint for arc spot welding but fillet joints can also be successfully made by this process.
In this process the metal from the electrode wire scours deeply into the weld crater. This breaks up the oxide films at the faying surfaces so that the process can be used as successfully on aluminum as on mild, low alloys, and stainless steels.
The arc which is struck by direct contact between the wire electrode and the workpiece, is maintained at a constant length by the interaction of electrical parameters. The power source used is invariably of the rectified dc type. Both, constant voltage and constant current type power sources are in use.
Depending upon the work material, the shielding gas may be argon, helium, nitrogen, carbon dioxide, hydrogen, and their mixtures. When inert shielding gas is used the process is more popularly known as MIG (metal inert gas) welding and when CO2 is used as the shielding gas it is referred to as CO2 welding or MAG (metal active gas) welding.
GMAW is an all-position semi-automatic welding process through its automatic versions is also available. GMAW is a very versatile process and can be used for welding all metals for which compatible filler wires have been developed. However, its typical applications include medium-gauge fabrication such as structurals, earth moving equipment, plate and box girders, and automobile bodies. This process has great potentials for use with robotic welding systems.
Plasma-MIG Welding Machine
This process, as the name implies, has been developed by combining the features of plasma arc welding and MIG welding processes. It has two variants; one with separate non-consumable tungsten electrode and the other uses the torch nozzle as non-consumable electrode.
Essentially plasma-MIG welding process differs from the existing GMAW process in that the electrode wire is enveloped in a plasma sheath which controls heat and droplet transfer in such a way that much higher speeds and deposition rates are attained that possible with GMAW process.
This process can be used both for welding and surfacing. Most of the materials that can be welded by GMAW can as well be welded by this process and at much faster rates.
AC Welding Power Sources
Requirements of a Welding Transformer
A welding transformer should satisfy the following requirements.
- It should have a drooping static volt-ampere characteristic.
- To avoid spatter, the surge of the welding current during a short circuit should be limited to the least possible above the normal arc current.
- The open circuit voltage should not normally exceed 80 volts and in no case 100 volts.
- The output current should be controllable continuously over the full available range.
- The open circuit voltage should be just sufficiently high for ready initiation of arc and not too high to impair the economics of welding.
Basic Types of Welding Transformer
- The high reactance type
- The external reactor type
- The integral reactor type
- The saturable reactor type
The High Reactance Type Welding Transformer
When a transformer supplies current, magnetic fluxes are produced around its windings. The lines of the resultant magnetic flux traverse the magnetic circuit and cut the primary and secondary windings. Some of magnetic flux due to primary current do not cut the secondary turns and vice-versa, since both have their paths in the air. In the other words, they are responsible for the reactance of the coils and the respective reactive voltage drops across them. As the current increases, the leakage fluxes also increase and so does the e.m.f. o self-induction. This is why an increase in the primary or secondary current results in increase in the reactive voltage drop across the respective windings.
External Reactor Type Welding Transformer
This type of welding transformer consists of a normal reactance, single phase, step down transformer and a separate reactor or choke.
The inductive reactances and resistances of the windings in such a welding transformer are low, so that its secondary voltage varies but a little with the welding current. The required drooping or negative volt-ampere characteristic is ensured by the reactor placed in the secondary of the welding circuit.
Integral Reactor Type Welding Transformer
The welding transformer of the integral reactor type has a primary winding I, a secondary winding II, and a reactor winding III. Apart from the main limbs, the core has additional limbs carrying the reactor winding. The current is adjusted by means of moving core C placed between the additional limbs.
Saturable Reactor Type Welding Transformer
In this welding transformer an isolated low voltage, low amperage dc circuit is employed to change the effective magnetic characteristics of the magnetic core. Thus, a large amount of ac is controlled by using a relatively small amount of dc, hence making it possible to adjust the output volt-ampere characteristics curve from minimum to maximum. For example, when there is no dc flowing in the reactor coil, it has its minimum impedance and thus maximum output of the welding transformer .
Parallel Operation of Welding Transformer
In welding operation sometimes there is a need for current exceeding the maximum welding current obtainable from one transformer. In such a case the desired welding current can be obtained by parallel operation of two or more welding transformers. The precaution needed for such a parallel operation is that the no-load or open circuit voltages of the transformer should be the same.
Multi-Operator Welding Transformers
A multi-arc or multi-operator welding transformer system utilises a high current constant voltage power source for providing a number of welding circuits at the same time. Such a system is used when there is a large concentration of welding points in a relatively small operating area, for example, in ship-building, construction sites for power stations, refineries, and chemical plants.
Use of the welding in today’s technology is extensive. Welding is used in every branch of science and technology. Such as computer industries, electronic industries, mechanical industries, petrochemical industries, etc.
History of Arc Welding
Arc welding machine 1st described by Davy in England in late 1809, but beginning of arc welding machine could become possible only the improvement in generator/dynamometer. Welding of metals by carbon arc was 1st suggested by a French person in 1881. In 1889, Zerner proposed an idea for twin carbon arc welding. In 1907 in America Strohmenger obtained another patent on flux coated electrode and 1st good welding joint was produced, he used asbestors coating with sodium silicate binder. Now a lot of changes and development have occurred for constituents of flux coating and core wire composition.
In 1928 shielding of welding (CO2) possible by Alexender in USA. 1st TIG (Tungsten Inert Gas) welding machine and MIG (Metal Inert Gas) welding machine develop consequently in 1946 and 1948 respectively.
Joining Process: On the basics of composition of the joint, joining process are three types:
- Homogeneous Process: In the joining process filler rod (electrode) is used and two same metals are joining no pressure is applying during the welding process.
- Heterogeneous Process: In this joining process filler metal is soluble in both the parent metal which themselves are soluble in each other.
- Autogeneous Process: In this type of welding process no filler rod is used but pressure is extensively used. Resistance welding, forge welding, friction welding, diffusion welding are the examples of autogeneous welding.
Metals that can be welded
- Ferrous metals such as low carbon steel, high carbon steel, stainless steel, cast iron, etc. (ferrous metals are those metals which are richer in carbon means having iron as a main constituents).
- Non-ferrous metals are those materials which are richer other than carbon such as Al,Cu, brass, nickel magnesium, zinc and its alloys.
Advantages of Welding
- Welding joints are stronger as base metal.
- Machining are possible on welding joints such as (grinding, shapering etc.).
- Welding joints efficiency up to 100%.
- Appearance of welding joint is very good.
- Alteration and amendment is possible in existing structure use by welding.
- A large numbers of similar and dissimilar metals can be welded easily.
- Weld joints are light in weight.
- Welding is a permanent joint.
Limitations of Welding
- Skill person is needed for welding purpose.
- Ultra-violet rays and infrared rays generate during the welding process which harm full for operator’s skin as well as eyes.
- Distortion due to high temperature is possible in workpiece.
- Thermal stresses developed in workpiece heat treatment process are required to relieve the thermal stresses.
- Weld heat produces metallurgical changes in workpiece.
- Edge preparation is required before welding which is time and labor consuming process.
- Special jigs and fixtures are required for welding purpose.
Comparison of Welding with Other Joining Processes
- Welding joints are lighter in weight as compared to riveting and casting (process).
- Welding is more economical and much faster process as compared to casting and riveting.
- Welding joint have more tensile strength and rigid as compared to riveting and casting.
- Less cost involvement in welding as compared to riveting and casting.
- Welding produces 100% efficient joint which is not possible by any other joining process.
- Jig-fixture not so much required as compared in reverting and casting.
- Welding is a permanent joint while riveting and bolting are not permanent joint (temporary joint).
Applications of Welding
- Automobile industries
- Building construction
- Railroad industries
- Pressure vessels and tank (cylinders)
- Aircraft industries
- Storage tank
- Pipeline industries
- Petrochemical industries
- Earth moving equipment and machinery
- Shipbuilding industries
Spot Welding Process
All resistance welding operations are automatic and therefore all process variables are pre-set and maintained constant. Once a welding operation has been initiated there is no way in which its progress can be controlled and, thus, the weld cycle is completed as per the pre-set times.
The welding cycle for spot welding machine, seam welding machine and projection welding machine consist basically of four elements viz., squeeze time, weld time, hold time, and off time. These timing are pre-set for a particular metal and a thickness range and the shop operator normally cannot change them on his own. Each one of these four time phases has its own role to play in achieving a sound weld of the required size.
The time interval between the application of electrode pressure to the work and switching on the welding currents called the squeeze time. This time interval is provided to assure the contact between the electrode and the work and to initiate the application of force on it.
It is the time for which the welding current actually flows to melt the metal at the interface.
It is the time for which the electrodes are kept in position, after the welding current is switched off, to assure the application of pressure so as to consolidate the molten metal into a nugget which is then cooled by the dissipation of heat to the surrounding work material. If the applied force is excessive it may result in expulsion of molten metal from in-between the sheets.
The time allowed to shift the work to the next location before the cycle is repeated is referred to as the off time. The electrodes are kept off the work during this time interval.
Welding current, time of current flow and the electrode pressure are recognized as the fundamental variables of resistance spot welding machine. For achieving quality welds in most metals, these variables are required to be kept within very close limit.
The size of the weld nugget and in fact whether it will form or not depends upon the heat being generated faster than it is dissipated by conduction. Welding current is, thus , the most critical variable.
Both a.c. and d.c. are used to produce spot, seam, and projection welding machine. Most applications use single phase a.c. of mains frequency i.e. 50 hertz. However, d.c. is used for applications that need heavy current and the load for which can be balanced on a 3-phase power line. Also, with direct current machines the rate of current rise and fall can be programmed as per requirements. The current rise period or upslope and current decay period or down slope can be programmed with electronic control systems.
Control of upslope helps to avoid overheating and expulsion of molten metal at the beginning of the weld time as the interface resistance at that time is high. Down slope helps to control weld nugget solidification to avoid cracks in weldments particularly in metals that are prone to quench-hardening and hot tearing.
MIG Welding machine is defined as metal inert gas welding. It is also one of the types of arc welding machine. In this process no pressure is applied for welding. In this process of welding wherein coalescence is produced by heat the work piece with an electric arc establish between a continues feed of metal electrode (copper coated) and the work piece. No flux is used as used in submerged arc welding (SAW Welding) but a shielding gas (Ar, He, Co2) is used. It is also known as gas metal arc welding (GMAW).
Principle of Operation
Before welding set the current, wire feed speed and electrical connections. Now arc is struck by one of the two methods.
1st method current and shielding gas flow is switched on and electrode is scratched against the job as usual practice.
For striking the arc by 2nd method-electrode is made to touch the job is restricted and moved forward to carry out welding but before striking the arc shielding gas, water and current is switched on during the welding. Torch should be 10 – 12 mm. Away from the work pieces and arc length is kept between 1.5 to 4.0 mm. Arc are basically two types.
I. Self adjusted arc
II. Self controlled arc
In self adjusted arc, with decreases in arc length (from L2 to L1) voltage decreases and current increases from l2 to l1 melting the electrode at faster rate resulting into making the arc length normal for self adjusting arc, welding source with flat characteristics is required for self-controlled arc, when arc length decreases, arc voltage also decreases with reduces speed of electric motor and hence the feed rate of electrode this brings arc length to a set value for self-controlled arc, a welding source with dropping characteristics is preferred.
Equipment Required for MIG Welding Machine
I. Welding power source with cables;
II. Welding gun filler wire on a coiled spool;
III. Shielding gas cylinder, pressure regulator and flow meter;
IV. Control switch.
Different Types of Material can be welded by MIG Welding Machine
I. Carbon and low alloy steel
II. Heat resistant alloys
III. Copper and its alloys
IV. High strength low alloy steel (HSLA)
V. Stainless Steel
VI. Magnesium alloys
VII. Aluminum and its alloys
Advantage of MIG Welding Machine
I. Less number of spatters as compared with MMA welding;
II. MIG is very faster process as compared with TIG Welding Machine;
III. Deep penetration can be achieved through this process;
IV. No use of flux during welding process;
V. Process can be easily mechanized;
VI. MIG produces a high quality, weld bead with minimum defeats;
VII. Large metal deposition rate are achieved by MIG welding process.
Limitations of MIG Welding Machine
I. Welding equipment is more costly and complex as compared to ARC Welding Machine;
II. Trained operator is required to perform the operation;
III. Process is not economically for job shop production;
IV. All types of material cannot be welded.
I. For welding of Al, Cu, Mg, Ni and their alloys;
II. For welding of aircraft, pressure vessels and shipbuilding industry;
III. For manufacturing of refrigerator parts etc;
IV. Rail road industries;
V. Transport Industries.
Arc Welding is a welding process where in coalescence is produced by heating with an electric arc. Mostly arc welding is done by without pressure and with or without use of filler metal depending upon the plate (object) thickness.
In arc welding machine arc is formed when an electric current passes between two electrodes separated by a short distance from each other. In arc welding machine one electrode is the welding rod or wire while other is the metal to be welded (work piece), electrode and plate. Arc connected to the supply one of the positive pole and other to negative terminal. Arc is started by moment rally touching the electrode on the plate and the withdrawing it to about 3 to 4 mm from the plate. When the electrode touches the plates, a current flows and as it is withdrawn from the plate the current continues to flow in the form of a spark across the very small gap first formed, this cause the air gap to become ionized or made conducting and as a result the current is able to flow across the gap even when it is very wide, in the form of an electrode must always be touched on to the plate before the arc can be started.
Arc is generated be electrons flowing from negative (-ve) to positive (+ve) terminal and electrical energy is changed in the arc into heat and light approximately 2/3 of the heat is generated near the positive (+ve) terminal which burns into the form of a creater. Temperature range from 2700oC-5500oC. While remaining 1/3 is generated near negative (-ve) terminal as electrode connected with positive (+ve) terminal they will burn away 50% faster than if connected to negative (-ve) terminal. Therefore medium coated electrodes and bare electrodes are used.
Types of Arc Welding Machines
1. Consumable Electrode Process
i. Shielded Metal Arc Welding (SMAW) or Arc Welding Machine
ii. SAW Welding Machine
iii. MIG Welding Machine
iv. FCAW Welding Machine
v. Electrogas Welding (ECW)
vi. Electroslag Welding (ESW)
vii. Carbon Arc Welding (CAW)
2. Non-consumable Electrode Processes
i. TIG Welding Machine
ii. Atomic Hydrogen Welding (AHW)
iii. Plasma Arc Welding (PAW)
Different welding parameters and the forces acting on the molten droplet play characteristic roles with specific welding processes. The case of the coated electrodes, MIG Welding Machine-both with solid and flux cored wires and SAW Welding Machine are of special interest due to the important and extensive use of these processes in the welded fabrications.
Metal Transfer in Arc Welding Machine (SMAW)
The drop transfer is a good way of characterising the mode of the metal transfer for any particular process and as it is relatively easy to measure, experimental data are easily available.
The possible explanation for this is that the transfer may be of the explosive type when insufficient amounts of silicon and manganese are added to the electrode coating and this generates small droplets with high rate of metal transfer. On the other hand with fully deoxidised electrode, the droplets are relatively large, of the order of 1 mm diameter, and the metal transfer rate is low at about 10 droplets per second.
Due to low current densities employed is Arc welding machine, the metal transfer takes place mainly by three modes viz, short-circuit, globular, and projected spray. However, for any given current density transfer from coated electrodes is at a higher rate than that for MIG Welding Machine or SAW Welding Machine which is consistent with the fact that the general characteristics of transfer with coated electrode differs from that with bare wire processes.
In welding machine with coated electrodes it has also been observed the weld penetration is equal to the cavity formed in the weld pool due to the arc forces. In this process the current density is too low to produce an electromagnetic jet, and the gas flow takes place mainly as a consequence of the decomposition of the electrode coatings and to a limited extent due to the chemical reactions of the core wire material at the high temperature of the arc. Also, if the electrodes are baked at a temperature high enough to drive off all volatile material, it renders them unusual which points to the fact that in normal operation the metal droplets are carried across the arc in the gas flow generated by the decomposition of the coating. The intensity of the gas stream in Arc Welding Machine (SMAW) increases with coating thickness such that it becomes quite strong with heavily coated electrodes making them to fit for use as cutting electrodes for metals.
In Arc Welding Machine (SMAW) it is possible to make satisfactory welds with 3 mm diameter electrode at 50 to 120 A while in MIG Welding Machine the same sized wire needs 200 to 250A for its successful operation. The only possible explanation for this anomaly is that the gas flow and hence the arc flow is provided in Arc Welding Machine (SMAW) by the decomposition of the coating whereas in MIG Welding Machine , it is dependent on the electro-magnetically included jet which becomes effective only at relatively higher currents.