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Shielded Metal Arc Welding Machine (Arc welding Machine)
In India nearly 90% of the welded fabrication is done by this process and even in the most advanced countries like USA, USSR, Japan, and the west European countries it accounts for nearly 60% of the metal deposited by welding machine. Though its use is slowly decreasing but it is expected to remain indispensable for repairs and short-run jobs. One of its attractive features is the lowest initial cost for a workable installation. Welding power source for SMAW welding machine or arc welding machine are available which can be plugged-in, if required, in domestic single phase electric supply, hence its popularity even with small volume fabricators.
The major welding equipment for SMAW is the power source which may be a welding transformer, a dc rectifier or dc motor-generator set. The selection of equipment depends upon the provision for initial investment and the range of the materials to be handled. The size and type of electrodes that are used and the penetration and welding speeds desired determine the current supply requirements. The welding power sources employed for SMAW welding machine are almost invariably of the constant current type as they serve the purpose best in maintaining the arc current undisturbed even when the welder’s hand is inadvertently disturbed through temporarily.
Of the three types of welding power sources each one has its own definite advantages. The dc welding power source is very versatile in welding a variety of metals in any desired thickness. It permits portable operation and uses efficiently a large variety of coated electrodes. The welding transformer has the lowest initial cost as well as low operation is quite. The dc rectifier welding power source is simple in design and it combines the advantages of a welding transformer and a dc welding set.
Welding Equipment Accessories
The welding equipment accessories for the welding power source include the connecting cables or leads, an electrode holder, cable connectors and the ground clamp. The cables that carry the current in welding circuit are quite flexible and are generally made of copper or aluminum wires. These wires are very find (0.2 mm diameter) and number between 800 to 2500 depending upon the current carrying capacity of the cable. Aluminum cables are much lighter and weigh only one-third of copper cables but their current carrying capacities are also lower being about 60% that of copper cables.
Electrode holder: Electrode holder is generally matched to the welding cable and the cable size depends upon the current required to be carried in the welding circuit. Usually electrode holders are specified depending upon the current that they can carry; the normal range being 150 to 500A. The electrode holders of the popular design have grooves cut in the jaws which facilitate the holding of electrode at different angles for easy manipulation.
The ground clamp is used to connect the other terminal of the welding circuit. It sometimes resembles the electrode holder but often it is like a C clamp is fitted tightly to the work table to avoid sparking, however most often it is rather loosely attached to facilitate easy detachment.
Welding Machine Rods and Wires
Bare welding wires and rods are used in short lengths of about 1 meter or in coiled form in spools. Whereas short lengths are used for processes like tig welding machine and plasma arc welding machine wherein they are not part of the welding circuit, long wires are employed for processes like mig welding machine and saw welding machine where a part of the wire conducts current. When a welding wire forms a part of the electrical circuit is called a welding electrode otherwise it is referred to as a welding rod.
Most wires used for welding structural steel usually contain 0.10% carbon and 0.35 to 0.60% manganese content. Some other types have increased amounts of carbon, manganese and silicon.
Excess silicon in welding wire results in heavy spatter, gassing in the weld pool, and non-metallic materials in the weld metal. Maximum silicon content permitted, therefore, is upto 0.95%.
The contents of harmful impurities like sulphur and phosphorous should not exceed 0.04% each. In some wires, particularly those used for welding alloy steels the maximum amount of sulphur and phosphorous allowed is each 0.03% each.
The range of wire diameter extends from 0.5 to 2.5 mm with 0.5, 0.6, 0.8, 0.9, 1.0, 1.2, 1.6, 2.0, 2.4 and 2.5 mm diameter wires being normally available. Welding machines use continuous wires in coils. Depending on the wire diameter, a coil may weigh anywhere between 5 to 500 Kg and measure 150 to 1000 mm across.
The welding wires are usually copper coated to prevent rusting and to improve current pickup from the contact tube, it also helps during drawing of wires through dies. To avoid harmful effects and peeling of copper coating it is usually kept very thin and the maximum amount of copper is specified at 0.4% by weight of the wire.
Apart from low carbon steels, welding wires are also produced from stainless steels, aluminum and its alloys, nickel alloys, magnesium alloys, titanium alloys, and copper alloys.
The welding wires are available both in solid and tubular forms, the latter contains flux in it.
Specifications for Solid Wires and Rods
Several systems are in use to specify welding electrodes and rods. AWS specification is one of the well known systems of codification. It consists of a prefix letter or letter S and then a suffix which may be figure or a letter or both.
|E||a welding electrode|
|R||a welding rod|
|RB||a welding rod/brazing filler|
|ER||an electrode or a welding rod|
Comparison between AC and DC Arc Welding Machine
AC Arc Welding Machine
DC Arc Welding Machine
Striking of arc with electrode is relatively difficult maintenance of a short arc is also difficult except with iron powder electrode.
Developing an arc is easier maintenance of short arc is also easier.
No problem of arc blow in AC arc welding machine. Work piece do not get magnetized in DC.
Arc blow is a serve problem and minimised with the use of proper corrective measure. Work piece may get magnetized due to current flow in one direction.
Arc is never stable.
Arc is more stable.
No polarity change possible and hence no suitable for welding all metals. It is used for welding ferrous metals.
Polarity (DCSP or DCRP) can be changed and hence suitable for welding both ferrous and non-ferrous metal s quality efficient.
More suitable for higher current value. It is less suitable for use at low current value with small dia of electrode.
It is most suitable with lower current value is also, for example at low amperage with small diameter electrode.
Bare electrode cannot be used. Only flux coated electrode with arc stabilizing agent influx can be used.
Bare and coated electrode can be used.
Not suitable for thin sheets or sheet metal work due to difficulty in striking the arc.
It is suitable for welding of sheet metal as striking arc is easier and arc remains steady.
Distribution of heat in arc is equal at electrode and job.
Most of heat (upto 66.67%) is liberated in the positive side of arc i.e. DC RP.
Voltage drop in welding is less and hence welding is suitable for longer distance from welding plant using long welding lead.
Voltage drop is relatively higher and hence start cables are used to weld only close to the welding plant.
Welding transformer has no moving part and working is salient.
A DC generator set has several moving parts therefore operation is noisy.
AC transformer welding set is not costly, simpler in operation maintenance cost is also very low.
A dc generator set is costly, difficult to operate and very high maintenance cost.
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.
Welding Machine- D.C. Welding Power Sources
This type of welding power source usually consists a transformer and a bank of rectifying cells. A unit rectifying cell is called a diode and it allows the electrical current to pass only in one direction and thus helps in converting alternating current to direct current.
The welding industry uses solid-state devices like semi-conductors to make rectifying cells. Earlier selenium was mainly used to make these cells but because of the demand for higher economy, reliability, and efficiency most rectifying cells are now made of silicon.
In comparison with motor-generator welding set, a rectifier welding power source has the following advantages.
- No rotating parts, easy maintenance,
- Higher efficiency,
- Smaller weight, size and cost.
General theory of DC Rectifier Design
Due to easy control of heat balance as well as the ease of arc initiation and its maintenance, dc is used particularly for bare wire welding like mig welding machine. However, ac is more easily and commonly available. The best solution, therefore, appears to be to use ac to produce the required dc and that is done with the help of a dc generator, a convertor or a dc rectifier.
Basically an electric rectifier is a device which permits flow of current on one direction only and can thus convert ac into a fluctuating dc. In the case of sinusoidal supply voltage suppression of the negative half of the curve results in intermittent pulses of energy which show no reversal in polarity. Obviously, such a source of supply would not be suitable for welding as periods of interrupted supply of energy between consecutive pulses would make it impossible to maintain a stable arc.
This difficulty can, however, be overcome if the source of supply is a three-phase current, each phase displaced by 1200 with reference to preceding or succeeding phases. When the negative half cycles of all three phases are suppressed the resulting graph for the uni-directional current shows an approach to straight line with much smaller fluctuations than that in the case of single phase. On similar basis, systems with more than three phase ac would attain better closeness to true dc transient. However, it is common to come across systems with more than three phases, thus three phase system is the one most used. But there is another way of improving the shape of the rectified current, and that is referred to as full-wave rectification.
DC rectifier, as mentioned earlier, is a device which permits current flow in one direction only, or more correctly it suppresses most of the current flow in reverse direction. Although the relation between voltage and current in the first quadrant of the graph is not linear, it can be noticed that for the third quadrant even a very large increase in voltage results in transmission of very small amount of current.
As is evident from the different transients of rectified current, there is inherent fluctuation in such a system. One method of obtaining smooth dc from a rectifier unit is the use of capacitors. If a capacitor is connected in the circuit it stores energy and supplies the same at nearly constant output voltage, although the energy received by it from the rectifier is in variable pulses.
Initially mercury arc rectifiers were developed but they are very fragile and have now been completely replaced by solid-state rectifiers.
MIG Welding Machine Metal Transfer
For 1.2 mm diameter electrode wire the welding current above 200 A results in the formation of drops at the tip of a conical region. The cone attains a quasi-stationary state with the liquid metal flowing into the base of the cone and flowing out at its tip. It has been shown expediently that for 1.2 mm diameter electrode the pencil-point tip forms for current higher than at which the electrode tip is completely engulfed by the visible arc root.
The geometric form of the drop at the electrode tip depends on, amongst other factors, the electrode polarity. As a rule electrode positive is the polarity used for mig welding machine. With this polarity the anode spot forms almost symmetrically around the electrode tip and the form of the drop or molten region at the electrode tip is correspondingly axi-symmetric. However, certain commercial mig welding machine (GMAW Welding) steel wires are adequately treated and, therefore, may be used with electrode negative. At high currents the cathode spot wanders symmetrically over the lower part of the electrode melts and the metal transfer in drops.
Most of the above discussion is in connection with solid wire mig welding machine (GMAW Welding). However, high speed fils of metal transfer in flux-cored arc welding indicate that the character of the transfer varies according to the flux. For example, with rutile flux core a fine spray-like transfer occurs whereas with a basic flux core the transfer is in relatively large droplets that form asymmetrically. The flux appears partly to transfer as a solid material which presumably melts on transfer to the weld pool. Overall it appears that as with SMAW the dominant factor, both for metal transfer and droplet transfer frequency, is the composition of the flux.
The introduction of the pulsed mig welding machine in 1960’s offered the opportunity of obtaining spray transfer at lower mean currents by introducing current pulses to detach droplets at controlled intervals, against a lower background current which maintained the arc and allowed molten drops to form. This has made it possible to use spray transfer for thinner materials and also in various welding positions.
Like metal transfer in constant current MIG Welding Machine (GMAW Welding), in pulsed GMAW it can also be classified into projected or drop spray and streaming spray. All features of the two transfer processes are the same both for constant current and pulsed GMAW. The first droplet transferred in pulse current welding is in the drop spray mode but subsequent droplets transferred during the same current pulse will be in the streaming spray mode.
The time for the formation and detachment of a droplet is inversely proportional to the magnitude of the peak current but is independent of its duration. Once the necking process has initiated the droplet detaches after a specific time which is characteristic of the wire diameter and peak current, and is independent of the current level at the time of its detachment.
Voltage-Current Characteristics of Welding Machine
V-I Characteristics of Welding Machine
Means for different welding plant different V-I characteristics. It shows the relationship between the arc voltage and arc current. During the welding arc length between the electrode tip and the workpiece determines the arc resistance and consequently the potential drop across the arc. In other manner, the arc length find out the arc voltage longer the arc length higher the arc voltage and it is this voltage which allow certain flow of current according of the characteristics of welding plant (unit).
There are basically three type of characteristics:
- Drooping characteristics (or constant current)
- Flat (or constant voltage)
- Rising voltage type
Our all concentration will be on only drooping type characteristics as it is used mostly in arc welding machine plant, both ac and dc type.
1. Drooping Type (Constant Current): Drooping V-I characteristics is used on constant current type welding machine. When arc is struck in arc welding machine (GMAW Welding Machine) electrode is essentially in short-circuit which would immediately required a sudden of current otherwise machine is design to prevent this. A constant current machine is design to minimise theses sudden surges.
As we know that a manual metal arc welding plant consist a drooping V-I characteristics. Drooping means that the terminal voltage of welding machine decreases as the welding current increases. In arc welding machine (MMA Welding) , arc length (gap between the workpiece and electrode) length, from a shorter arc B to longer arc A, there is a marked variation(K) in the voltage but the corresponding variation (c) in the current is very small.
Drooping V-I characteristic is applicable for both AC and DC welding machine which is used for SMAW Welding Machine, TIG Welding Machine, and Submerged Arc Welding Machine (SAW Welding Machine) and Plasma Arc Welding Machine and the MMA (Manual Metal Arc) Welding Machine, voltage at the time of welding is approximate 30-40 V.
2. Flat or constant voltage type characteristics are used with semi-automatic MIG Welding Machine and other automatic welding machines.
3. Rising voltage type characteristics are used with fully automatic welding machine.
a) Open circuit voltage usually is in the range of 70 to 80 volt.
b) System to adjust welding current is usually in the AC section of the machine before the rectifiers control of current is based on the principle of variable inductance or impedance various method for varying impedance for current control are as:
a) Moving shunt
b) Tapped reactor
c) Moving coil
d) Saturable reactor
e) Moving reactor core
In welding circuit flow of current is controlled by inductor in line between the electrode and transformer current can be vary by varying the inductance. For current control during welding a means of varying this inductance is must.
a) Tapped type reactor
b) Moving core type reactor
c) Saturable type reactor
Flux Cored Arc Welding Machine (FCAW Welding Machine)
This process is growing in popularity. It is being used for more than 20% of arc welding machine. Some FCAW still uses CO2 shielding, but the use of flux cored wire alone is increasing. In many cases, the flux-cored wire alone produces welds equal to or better than the original metal and its uses eliminates the need for the gas shield equipment and cost of the gas.
Definition and Concept
The FCAW is a process in which coalescence is produced by heating with an electric arc between a continuous tubular consumable electrode and the work. The electrode is flux cored, i.e. the flux is contained within the electrode which is hollow. In addition to flux, mineral and ferro alloys in the core can provide additional protection and composition control.
The flux cored electrode is coiled and supplied to the arc as a continuous wire as in CO2 welding. The flux inside the wire provides the necessary shielding of the weld pool. Additional shielding may (or may not) be obtained from an externally supplied gas (e.g. CO2) or gas mixture.
Principle of Operation
As explained above, FCAW utilizes the heat of an arc between continuously fed consumable flux cored electrode and the work. The heat of the arc melts the surface of the base metal and the end of the electrode. The metal melted off the electrode is transferred through the arc to the workpiece where it becomes the deposited weld metal. Shielding is obtained from the disintegration of ingredients contained within the flux cored electrode. Additional shielding may be obtained from an envelope of gas supplied through a nozzle to the arc area. Ingredients within the electrode produce gas for shielding and also provide deoxidizers, ionizers, purifying agents and in some cases alloying elements (for composition control). These ingredients from a glasslike slag, which is lighter in weight than the deposited weld metal and which floats on the surface of the weld as a protective cover. The flux cored electrode is fed into the arc automatically from a coil. The arc is maintained automatically and arc travel can be manual or by machine.