Arc Welding Machine
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Methods of Arc Initiation for Welding Machines
It is not possible to create an arc between the electrode and the work piece just by connecting them in a welding circuit. This is because the current needs an ionized passage for flowing across the gap. Thus, a welding arc needs to be initiated. The method of initiating a welding arc depends upon the process used. However, in general these methods can be grouped into two categories. In one category the ionization of the gases between the electrodes to work gap achieved by the application of high voltage across it and in the other category the electrode and the work piece are short-circuited momentarily by touching each other. The former is used for immobile or fixed arcs and the latter for mobile or traveling arcs.
For immobile arcs the electrode and work piece are bought close to each other without touching and a high voltage of the order of 104 volts is applied. As a high voltage at normal mains frequency of 50 hertz will be lethal thus high frequency high voltage is applied, for arc initiation, with the help of the spark gap oscillator. Thus helps in ionizing the gases in the gap between the electrode and the work piece and the arc are, thus initiated in a few milli-seconds. As soon as the arc is stabilized, the auxiliary high frequency high voltage supply is switched off automatically. This method of arc initiation is utilized in the gas tungsten arc welding machine and carbon arc welding machine processes so as to avoid the contamination of tungsten electrode or to eliminate the chance of pickup of carbon from the carbon electrode if touch method is used to initiate the arc.
The touch method of initiating the arc is normally used for processes in which the mobile arc is employed. However, it has two variants depending upon the size i.e. diameter of the electrode. For thick electrodes, the arc initiation is done by touching the electrode to the work piece and then withdrawing it. Upon touching, a heavy short circuit current flows in the circuit causing melting of minute points of contact. When the electrode is withdrawn it results in sparking and ionization of the gap between the electrode and the work piece. If the arc is not initiated at the first attempt, the process is can be repeated till a stable arc is established. This method of initiating the welding arc is known as ‘touch’ method and the arc so initiated is called ‘drawn’ arc. The method is used for arc initiation in arc welding machine process or SMAW process.
For welding with wires i.e. thin electrodes an electrode is fed to the work at a pre-set rate. As soon as it touches the work piece, heavy short circuit current flows through it and the electrode melts resulting in ionization is adopted for gas metal arc welding or MIG welding machine and SAW welding machine and their processes, both in the semi-automatic and automatic modes.
In some limited cases the welding arc is also initiated by placing a ball of steel wool between the electrode and the work piece. When a heavy current flows through steel wool it melts and in the process provides an ionized and metal vapour path for the flow of current and a stable arc is established.
Stud Welding
This is a process of welding stud (a headless threaded bolt) or stud-like pieces (e.g. bolts, screws, rivets, rods, etc.) to flat workpieces like plates. Its a unique process which combines arc and forge welding processes and results in tremendous cost saving when compared to the conventional methods like drilling and tapping.
Stud welding was first used by British Navy in 1918 but its regular and extensive use started from 1938. There are four variations of the process viz., capacitor discharge stud welding, the drawn arc capacitor discharge stud welding, the consumable ferrule stud welding and the drawn arc stud welding. The last variation of the process is the most popular and the following description pertains to that only.
The main equipment for stud welding consists of a stud welding gun, a time control unit, a dc power source of 300 to 600 amperes current capacity, studs, and ceramic ferrules.
A stud is held in the welding gun and a ferrule is slipped on it. The stud is then made to touch the cleaned spot (shot blasted, ground, or wire brushed) where it is to be welded and the switch in the form of gun trigger is pressed and the process is completed in a couple of seconds. This necessitates the use of ultra-high speed power source to supply the desired welding current. A stud about 40 mm diameter requires about 5000 ampere current at 65 to 70 volts for 2 seconds. Therefore, motor-generator sets with their higher overload capacities are preferred over the rectifier welding sets.
For efficient results the plate on which the stud is to be welded must have the minimum thickness at least 20% that of the diameter of the stud, however for developing full strength it should not be less than 50% of the diameter of the stud base.
Current and Power Source Rating for Different Stud Sizes
| Stud Diameter (mm) | Current Required (amp) | Power Rating (amp) | Source Number |
| 5 | 325 | 300 | 1 |
| 8 | 500 | 400 | 1 |
| 10 | 625 | 600 | 1 |
| 13 | 925 | 1000 | 1 |
| 15 | 1100 | 1000 | 1 |
| 19 | 1600 | 1000 | 2 |
| 22 | 1750 | 1000 | 2 |
| 25 | 2000 | 1000 | 2 |
Stud welding is used mainly for mild steel, low alloy steels, and austenitic stainless steels. Drawn arc stud welding is not used for non-ferrous metals but other variants of the process can be utilized for welding lead free brass, bronze, chrome plated metals and aluminum. However heat-treatable aluminum alloys are not recommended for stud welding.
Typical applications of stud welding include steel decks of ships, for attaching brackets, hangers, cover plates, conduits, piping, etc. to metal workpieces. The process also finds wide use in automotive rail road machinery manufacturing and construction industries.
AC Welding Power Sources
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.
ARC WELDING MACHINE AND PROCESS
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)
ARC WELDING MACHINE AND METAL TRANSFER
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.
DIFFERENT ARC WELDING MACHINE POWER SOURCE
Pulsed Arc Welding Power Sources
Pulsed current finds increased use in TIG welding machine and MIG welding machine processes. Whereas in TIG welding machine it serves the purpose of controlling the weld pool size and cooling rate of the weld metal without any arc manipulation, in MIG welding machine it provides spray and controlled mode of metal transfer at lower welding current for a specific type and diameter of electrode used.
A typical pulsed arc welding machine power source normally consists of a 3-phase welding transformer cum rectifier unit provides background current and the single phase unit supplies the peak current. Both the transformer and rectifier units are mounted in a single housing with appropriate controls for individual adjustments background and peak currents.
Electrode size and feed rate are accounted for by the peak current setting. The peak current is set just above that provides spray mode of metal transfer for that electrode diameter and feed rate. The spray transfer occurs during the peak current duration while globular transfer does not take place due to the lack of time at the background current level. Thus, it provides the deposition rate between those for continuous spray transfer and globular transfer.
Transistorised Welding Power Source
Like a rectifier cell, a transistor is another solid-state device that is used in welding machine power sources. However, presently transistors are used only for such power sources which require accurate control of a number of variables.
A transistor is different from a SCR in that conduction through it is proportional to the control signal applied. Thus, when a small signal is applied there is a small conduction and for a large signal there is a large conduction. Also, a transistor can be turned off through a signal which is unlike a SCR wherein potential of the anode has to drop to a level lower than that of the cathode or the current flow has to stop for the SCR to stop functioning.
Transistors are used in welding machine power sources at a level between ‘off’ and ‘full on’ wherein they act as electronically controlled series resistance. Transistors can work satisfactorily only at low operating temperature which may necessitate cooling water supply to keep them within the desired temperature range.
Transistorised welding power sources have been developed for accurate control of welding parameters. The speed of operation and response of transistors are very high therefore such power sources are best suited to TIG welding machine and MIG welding machine processes. The latest power supply source is the outcome of developments in transistorised welding power source only. Such a power source can be adjusted to give any desired volt-ampere characteristic between constant current to constant voltage type. It is also possible to programme the control system to give the predetermined variable current and voltage during the actual welding operation. This feature makes it particularly attractive for pipe welding wherein the heat build-up demands higher welding speed as work progress wherein the heat build-up demands higher welding speed as work progress. Normally, such systems are of pulse current type for achieving maximum control over the mode of metal transfer and hence the quality of the weld.
Plasma Welding Machine
Plasma Arc Welding Machine
Plasma is defined as a flow of ionized gas. It is obtained by passing the gas through a high temperature arc which results in splitting the molecules of gas to atoms and then to ions and electrons. Through plasma flow takes place in most of the arc welding processes but in the process called plasma arc welding whole of the gas is converted into plasma by making it pass through a very narrow passage of high temperature arc.
The plasma welding machine was developed in 1925 but its industrial use for welding is reported to be form 1953. For welding, the plasma is also provided an outer envelope of a shielding gas.
In plasma welding machine the arc is created between a tungsten electrode and the work piece, as in gas tungsten arc welding. However, the plasma arc welding machine is constricted by making it pass through a narrow passage in a water cooled copper nozzle tip which is itself surrounded by an outer nozzle tip which is itself surrounded by an outer nozzle through which the shielding gas flows.
Energy of plasma welding machine is obtained invariably from a dc power source of the constant current type having an open circuit voltage of 70-80 volts and a duty cycle of 60%. The welding current employed range between 100-300 amperes.
There are two variations of the plasma arc welding process called non-transferred type and transferred type. In the former, the tungsten electrode is the cathode and the nozzle tip the anode. Such a torch is very similar to ox-acetylene torch regarding its maneuverability as work piece is outside the electrical circuit. However, such a plasma arc is less intense compared with the transferred arc wherein the work piece is anode. But, the maneuverability of the transferred arc is restricted. Such an arc, however, is very intense and the process results in higher thermal efficiency.
The temperature in a plasma welding machine can go up to 55,0000C but for welding it is restricted to about 20,0000C. This high temperature arc when it impinges upon the work piece results in reuniting of electrons and ions to form automatic and the molecular gas, releasing heat in the process which is thus utilized for welding.
Any gas that does not attack the tungsten electrode or the copper nozzle tip can be used in plasma welding. However, argon and argon-hydrogen mixture are more commonly used.
Compared with GTAW process, plasma welding machine, due to its high heat concentration, results in higher welding speeds to the extent of 40-80% Plasma arc welding is, however, comparatively a new process and not very popular, as yet. The actual process of welding with the plasma jet is by ‘keyhole’ process in which the plasma jet impinges upon the work piece and melts through and through and then torch is moved in the desired direction.
A variation of the process called micro-plasma welding uses current in the range of 0.1 to 10 amperes and can weld metal thinner than 1 mm while the range for the normal plasma welding 3-15 mm.
Through the plasma welding machine has high potential for the future use but it has certain serious drawbacks e.g. the intense arc results in excessive ultra-violet and infra-red radiation which can harm the skin even through the clothes necessitating special protective clothing for the operator. Also, the noise level in the process is around 100 db (decibel) which is far in excess of the safe working limit of 80 db for human ears.
Commercially the major users of the plasma users of the plasma welding process are the aeronautical industry, precision instrument industry and the jet engine manufactures. Typically the process is used for making piping and tubing made of stainless steels and titanium.
Shielded Metal Arc Welding (SMAW)
It is ‘the Arc Welding Process’ known to even a layman and can be considered a ‘roadside welding process’ in india. When invented in 1880’s it used bare electrodes, however the subsequent developments led to the use of coated electrodes. This process is also known as stick electrode welding or coated electrode welding or manual metal arc welding. It uses coated electrodes of 2.5 to 6.35 mm diameter and 300-450 mm length held in an electrode holder. The power source used is of the constant current type and both ac and dc supplies can be employed with equal ease and effectiveness in most of the cases.
In arc welding machine when an arc is struck between an electrode and the work piece, the electrode core wire and its coating melt, the latter provides a gas shield to protect the molten weld pool and the tip of the electrode from the ill effects of the atmospheric gases. The temperature in the core of the arc ranges between 6000-70000C. The radiations originating from the welding arc can damage the eyes thus necessitating the use of a protective shield.
In all types of welding machines, arc welding machine process is very versatile and is used for welding in all positions and all metals for which electrodes have been developed. The coated electrodes are presently available for welding low carbon steels, low alloy steels, quenched and tempered (Q&T) steels, high alloy steels, corrosion resistance steels and stainless steel as well as for cast iron and malleable iron. It is also used for welding nickel and nickel alloys and to a lesser extent for welding copper and copper alloys. It finds a limited use in welding aluminium alloys. Typical applications of the process include its extensive use by the industry for fabrication of ships, bridges, pressure vessels and structurals. However, as the process can be used in its manual mode only. It is slowly getting replaced by other welding processes for heavy fabrication where large quantity of metal need be deposited.
Arc Welding Machine and Portable Welding Machine
We are well recognized to manufacturing welding machines, especially manufacturer and exporter of Arc Welding Machines. Our range starts from 150 Ampere to 650 Ampere in different designs equipped with latest technology.
Our products are efficient in all welding positions like flat position, horizontal position, vertical position and overhead position. Model CW-ARC150,CW-ARC200,CW-ARC250,CW-ARC320 are portable type arc welding machines, these arc welding machines are easy to carry, light weight and specially designed for movable job purposes.
Basic advantage of portablewelding machine is use in two phase as well as in single phase connection. So it can be in small scale industries, home base industries, where three phase connection is rarely available. We recommend our arc welding machine for offshore sites and onshore sites both.
