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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.
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.
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.
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.
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.