Arc Welding Machine
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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.
Welding Equipment
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.
Comparison between AC and DC Arc Welding Machine
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AC Arc Welding Machine |
DC Arc Welding Machine |
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Striking of arc with electrode is relatively difficult maintenance of a short arc is also difficult except with iron powder electrode.
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Developing an arc is easier maintenance of short arc is also easier. |
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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. |
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Arc is never stable. |
Arc is more stable. |
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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. |
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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. |
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Bare electrode cannot be used. Only flux coated electrode with arc stabilizing agent influx can be used. |
Bare and coated electrode can be used. |
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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. |
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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. |
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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. |
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Welding transformer has no moving part and working is salient. |
A DC generator set has several moving parts therefore operation is noisy. |
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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. |
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.
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.
MIG Welding Machine
Introduction:
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.
Application
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 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.
ULTRASONIC TESTING OF FORGING
General
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).
Techniques:
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.
Test area
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.
Equipment
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.
Sensitivity
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.
Scanning Methods
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.
SETTING THE CURRENT IN MIG WELDING MACHINE
MIG Welding Machine, proper safety protection, and one piece of mild steel plate approximately 12 in. (305 mm) long x 1/4 in. (6 mm) thick, you will change current setting and observe the effect on MIG Welding.
On a scale of 0 to 10, set the wire feed speed control dial at 5, or halfway between the low and high setting of the unit. The voltage is also set at a point halfway between the low and high settings. The shielding gas can be CO2, argon, or a mixture. The gas flow should be adjusted to a rate of 35 chf 916 L/min).
Hold the welding gun at a comfortable angle, lower your welding hood, and pull the trigger. As the wire feeds and contacts the pate, the weld will begin. Move the gun slowly along the plate. Note the following welding conditions as the weld progresses: voltage, amperage, weld direction, metal transfer, spatter, molten weld pool size, and penetration.
Reduce the voltage somewhat and make another weld, keeping all other weld variables (travel speed, sickout, direction, amperage) the same. Observe the weld and upon stopping record the results. Repeat this procedure until the voltage has been lowered to the minimum value indicated on the machine. Near the lower end the wire may stick, jump, or simply no longer weld.
Return the voltage indicator to the original starting position and make a short test weld. Stop and compare the results to those first observed. Then slightly increase the voltage setting and make another weld. Repeat the procedure of observing and recording the results as the voltage is increased in steps until the maximum machine capability is obtained. Near the maximum setting the spatter may become excessive if CO2 shielding gas is used. Care must be taken to prevent the wire from fusing to the current tube.
Return the voltage indicator again to the original starting position and make a short test weld. Compare the results observed with those previously obtained.
Lower the wire feed speed setting slightly and uses the same procedure as before. First lower and then raise the voltage through a complete range and record your observations. After a complete set of test results is obtained from this amperage setting, again lower the wire feed speed for a new series of tests. Repeat this procedure until the amperage is at the minimum setting shown o the machine. At low-amperage and high-voltage setting, the wire may tend to pop violently as a result of uncontrolled arc.
Experienced welders will follow a much shorter version of this type of procedure any time they are starting to work on a new machine or testing for a new job. This experiment can be repeated using different types of wire, wire sizes, shielding gas, and weld directions. Turnoff the welding machine and shielding gas and clean up your work area when you are finished welding.
