June, 2013
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ARC WELDING MACHINE AND METAL TRANSFER
Different welding parameters and the forces acting on the molten droplet play characteristic roles with specific welding processes. The case of the coated electrodes, MIG Welding Machine-both with solid and flux cored wires and SAW Welding Machine are of special interest due to the important and extensive use of these processes in the welded fabrications.
Metal Transfer in Arc Welding Machine (SMAW)
The drop transfer is a good way of characterising the mode of the metal transfer for any particular process and as it is relatively easy to measure, experimental data are easily available.
The possible explanation for this is that the transfer may be of the explosive type when insufficient amounts of silicon and manganese are added to the electrode coating and this generates small droplets with high rate of metal transfer. On the other hand with fully deoxidised electrode, the droplets are relatively large, of the order of 1 mm diameter, and the metal transfer rate is low at about 10 droplets per second.
Due to low current densities employed is Arc welding machine, the metal transfer takes place mainly by three modes viz, short-circuit, globular, and projected spray. However, for any given current density transfer from coated electrodes is at a higher rate than that for MIG Welding Machine or SAW Welding Machine which is consistent with the fact that the general characteristics of transfer with coated electrode differs from that with bare wire processes.
In welding machine with coated electrodes it has also been observed the weld penetration is equal to the cavity formed in the weld pool due to the arc forces. In this process the current density is too low to produce an electromagnetic jet, and the gas flow takes place mainly as a consequence of the decomposition of the electrode coatings and to a limited extent due to the chemical reactions of the core wire material at the high temperature of the arc. Also, if the electrodes are baked at a temperature high enough to drive off all volatile material, it renders them unusual which points to the fact that in normal operation the metal droplets are carried across the arc in the gas flow generated by the decomposition of the coating. The intensity of the gas stream in Arc Welding Machine (SMAW) increases with coating thickness such that it becomes quite strong with heavily coated electrodes making them to fit for use as cutting electrodes for metals.
In Arc Welding Machine (SMAW) it is possible to make satisfactory welds with 3 mm diameter electrode at 50 to 120 A while in MIG Welding Machine the same sized wire needs 200 to 250A for its successful operation. The only possible explanation for this anomaly is that the gas flow and hence the arc flow is provided in Arc Welding Machine (SMAW) by the decomposition of the coating whereas in MIG Welding Machine , it is dependent on the electro-magnetically included jet which becomes effective only at relatively higher currents.
DIFFERENT ARC WELDING MACHINE POWER SOURCE
Pulsed Arc Welding Power Sources
Pulsed current finds increased use in TIG welding machine and MIG welding machine processes. Whereas in TIG welding machine it serves the purpose of controlling the weld pool size and cooling rate of the weld metal without any arc manipulation, in MIG welding machine it provides spray and controlled mode of metal transfer at lower welding current for a specific type and diameter of electrode used.
A typical pulsed arc welding machine power source normally consists of a 3-phase welding transformer cum rectifier unit provides background current and the single phase unit supplies the peak current. Both the transformer and rectifier units are mounted in a single housing with appropriate controls for individual adjustments background and peak currents.
Electrode size and feed rate are accounted for by the peak current setting. The peak current is set just above that provides spray mode of metal transfer for that electrode diameter and feed rate. The spray transfer occurs during the peak current duration while globular transfer does not take place due to the lack of time at the background current level. Thus, it provides the deposition rate between those for continuous spray transfer and globular transfer.
Transistorised Welding Power Source
Like a rectifier cell, a transistor is another solid-state device that is used in welding machine power sources. However, presently transistors are used only for such power sources which require accurate control of a number of variables.
A transistor is different from a SCR in that conduction through it is proportional to the control signal applied. Thus, when a small signal is applied there is a small conduction and for a large signal there is a large conduction. Also, a transistor can be turned off through a signal which is unlike a SCR wherein potential of the anode has to drop to a level lower than that of the cathode or the current flow has to stop for the SCR to stop functioning.
Transistors are used in welding machine power sources at a level between ‘off’ and ‘full on’ wherein they act as electronically controlled series resistance. Transistors can work satisfactorily only at low operating temperature which may necessitate cooling water supply to keep them within the desired temperature range.
Transistorised welding power sources have been developed for accurate control of welding parameters. The speed of operation and response of transistors are very high therefore such power sources are best suited to TIG welding machine and MIG welding machine processes. The latest power supply source is the outcome of developments in transistorised welding power source only. Such a power source can be adjusted to give any desired volt-ampere characteristic between constant current to constant voltage type. It is also possible to programme the control system to give the predetermined variable current and voltage during the actual welding operation. This feature makes it particularly attractive for pipe welding wherein the heat build-up demands higher welding speed as work progress wherein the heat build-up demands higher welding speed as work progress. Normally, such systems are of pulse current type for achieving maximum control over the mode of metal transfer and hence the quality of the weld.
Plasma Welding Machine
Plasma Arc Welding Machine
Plasma is defined as a flow of ionized gas. It is obtained by passing the gas through a high temperature arc which results in splitting the molecules of gas to atoms and then to ions and electrons. Through plasma flow takes place in most of the arc welding processes but in the process called plasma arc welding whole of the gas is converted into plasma by making it pass through a very narrow passage of high temperature arc.
The plasma welding machine was developed in 1925 but its industrial use for welding is reported to be form 1953. For welding, the plasma is also provided an outer envelope of a shielding gas.
In plasma welding machine the arc is created between a tungsten electrode and the work piece, as in gas tungsten arc welding. However, the plasma arc welding machine is constricted by making it pass through a narrow passage in a water cooled copper nozzle tip which is itself surrounded by an outer nozzle tip which is itself surrounded by an outer nozzle through which the shielding gas flows.
Energy of plasma welding machine is obtained invariably from a dc power source of the constant current type having an open circuit voltage of 70-80 volts and a duty cycle of 60%. The welding current employed range between 100-300 amperes.
There are two variations of the plasma arc welding process called non-transferred type and transferred type. In the former, the tungsten electrode is the cathode and the nozzle tip the anode. Such a torch is very similar to ox-acetylene torch regarding its maneuverability as work piece is outside the electrical circuit. However, such a plasma arc is less intense compared with the transferred arc wherein the work piece is anode. But, the maneuverability of the transferred arc is restricted. Such an arc, however, is very intense and the process results in higher thermal efficiency.
The temperature in a plasma welding machine can go up to 55,0000C but for welding it is restricted to about 20,0000C. This high temperature arc when it impinges upon the work piece results in reuniting of electrons and ions to form automatic and the molecular gas, releasing heat in the process which is thus utilized for welding.
Any gas that does not attack the tungsten electrode or the copper nozzle tip can be used in plasma welding. However, argon and argon-hydrogen mixture are more commonly used.
Compared with GTAW process, plasma welding machine, due to its high heat concentration, results in higher welding speeds to the extent of 40-80% Plasma arc welding is, however, comparatively a new process and not very popular, as yet. The actual process of welding with the plasma jet is by ‘keyhole’ process in which the plasma jet impinges upon the work piece and melts through and through and then torch is moved in the desired direction.
A variation of the process called micro-plasma welding uses current in the range of 0.1 to 10 amperes and can weld metal thinner than 1 mm while the range for the normal plasma welding 3-15 mm.
Through the plasma welding machine has high potential for the future use but it has certain serious drawbacks e.g. the intense arc results in excessive ultra-violet and infra-red radiation which can harm the skin even through the clothes necessitating special protective clothing for the operator. Also, the noise level in the process is around 100 db (decibel) which is far in excess of the safe working limit of 80 db for human ears.
Commercially the major users of the plasma users of the plasma welding process are the aeronautical industry, precision instrument industry and the jet engine manufactures. Typically the process is used for making piping and tubing made of stainless steels and titanium.
MIG WELDING MACHINE DIFFERENT VARIABLES
MIG/GMAW Variables
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.
Arc Voltage
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.
Travel Speed
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.
Electrode Stickout
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.
Welding Position
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.
Electrode Size
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.
