MIG Welding Machine
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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.
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|
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.
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.
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.
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.
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.
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.
In MIG Welding Machine semi-automatic versions requires the holding of the welding gun in hand so that the operator may switch on or off the system as desired. Before initiating the welding arc it is usual to set the open circuit voltage on the panel of the power source with the regulator provided on the unit itself.
Apart from setting the open circuit voltage it is also required to decide about the other welding machine variable so as to control the welding processes to obtain the desired results. These variables may include the wire feed rate, electrode stickout, nozzle-to-plate distance, electrode-to-work angle, and gas flow rate.
With a flat characteristics power source the arc voltage is controlled mainly by setting the open circuit voltage (O.C.V). A small difference in the actual value of the arc voltage and the set value of the O.C.V. is on account of the voltage drop in the cable and the slight drop in the V-I characteristic of the power source itself.
The change in arc voltage leads to change in arc length and that affects the bead width directly. The change in arc voltage not only affects the outer dimensions of the bead but also influences the micro structure and even the success and failure of the operation by affecting the mode of metal transfer.
Wire Feed Rate
For a flat characteristic power source the welding current varies with the change in wire feed rate and a generalized relationship between the two. The relationship is linear at lower feeding rate however as the wire speed is increased, particularly for small diameter wires, the melting rate curve becomes non-linear. This is normally attributed to increased resistance heating which itself is increased with the increase in wire feed rate.
Weld penetration is maximum at a particular welding speed and it decreases as the speed is varied either way. However, the decrease in speed is accompanied by increase in width while increase in speed results in narrower beads. The decrease in penetration with reduction in speed is caused due to excessive molten metal sliding into the weld pool resulting in shallower weld pool.
The distance from the lower tip of the contact tube to the tip of the protuding electrode wire. It is an important mig welding machine or welding machine parameter for controlling the deposition rate and the bead geometry.With the increase in stickout its electrical resistance increases and that results in preheating of wire which leads to lower requirement of current at any given wire feeder rate in mig welding machine.
Weld bead geometry is also affected by the position in which the work piece is held with respect to the welding gun. Down hand or flat welding position gives the most satisfactory bead shape and all modes of metal transfer can be effectively utilised. However, overhead and vertical welding position demand that metal transfer be either by sprays or short-circuit mode.
Each electrode wire size has a workable limit within which it can be effectively used. Welding current lower than the optimal range results in lack of fusion and higher current results in increased spatter, porosity and poor bead appearance.
Electrode size also affects the penetration and weld width in that for the same current lower diameter wire gives deeper penetration while welder beads with shallow penetration are obtained with bigger diameter wires.
Overall, however, there is a tendency to use smaller diameter wires because of the following reasons.
a) Rapid arc length adjustment,
b) Spray mode of metal transfer,
c) Easy to spool,
d) Higher deposition efficiency.
On the complex shapes, the surface curvatures may not allow good contact or coupling, the angles of surfaces may prevent back wall echoes with 00 probes and some forgings, simple or complex may be anisotropic in grain structure (different grain size in different directions).
When searching the defects in forgings you should have, as a minimum, the following information, which is usually written on a techniques or instruction sheet.
- The test component identification and area to test.
- Actions to be taken when defects are found.
- The purpose of the test (defects sought and acceptance criteria).
- Equipment required.
- What method and level of test sensitivity to use.
- The method of scanning.
The instruction sheet would also contain sections giving details of any relevant safety procedures such as the cleaning of the test area afterwards. It would also have the company name, a unique technical reference number, the originator’s name and signature and an authorising signature.
The test may involve testing the whole, of a component, or just parts, this must be specified.
Actions to be taken
When defects are found it may be required that the defects are reported, e.g. on a diagram as a written description, or the component, or material, may be accepted or rejected according to the defects found. If defects are to be reported then the defect information that needs reporting would be contained in this section, i.e. Defect type, size, lateral and longitudinal position in relation to datums, etc.
Purpose of the test
This sections tells us the accept/reject criteria for particular defects, i.e. what size and type of defects to report or which defects render the component rejectable.
The section should give information on; the type of flow detector, type, size, and the frequency of probes, type of couplant, calibration blocks and reference block to use.
Method of setting and level of sensitivity need to be quoted for each scan, e.g. Set the bwe from the DGS block to 80% fsh and note the gain setting. Still on the DGS block, maximise the signal from the flat bottom hole at target depth (test material thickness) and set that to 80% fsh and note the difference in dBs between the new gain setting and the previous one. Set the bwe from the test material to 80% fsh and add the difference noted in the first two gain settings to the present gain and scan at this level.
The method of scanning of the material is either a written, step by step, instruction or technique sheet, or involves following the step laid out in the relevant national standard. And example written step by step could be:
- Prepare the material surface by removing any loose scale, rust, dirt or other debris and visually inspect for surface defects or damage.
- Calibrate the screen on the flaw detector, using a 00 probe and A2 calibration block, for a range of 0 to 200 mm.
- Set the sensitivity (as quoted in the relevant section above) and apply couplant to the test area.
- Scan the designated test area, with a probe overlap between scans of at least 20% of the probe’s diameter and at a maximum probe movement rate of 150 mm/sec.
- When defects meeting the criteria in the “purpose of the test” section are found, record the relevant defect data as in the “Action to be taken” section.
- Prepare a neat concise report giving details of the component identification, test area, equipment used, sensitivity method and settings and a drawing with the defect detailsas recorded in section five above.
Post test Procedure
This would involve cleaning any remaining couplant and dirt from the test area and covering the surface with protective coatings according to client’s requirements.