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Welding Arc: Definition, Structure And Types | Metallurgy Welding Voltage Definition

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Welding Arc: Definition, Structure and Types | Metallurgy

After reading this article you will learn about:- 1. Definition of Welding Arc 2. Structure and Characteristics of Welding Arc 3. Types 4. Role of Electrode Polarity.

Definition of Welding Arc:

An arc is an electric discharge between two electrodes which takes place through an electrically conducting hot ionised gas known as plasma. An electric arc used for welding is called the welding arc and is usually between a thin rod (or wire) and a plate it is therefore bell shaped, as shown in Fig. 3.1 (a).

Arc Characteristics

Structure and Characteristics of Welding Arc:

A welding arc is a high current low voltage electric discharge operating generally in the range of 10 to 2000 amperes and at 10 to 50 volts. In a welding circuit the arc acts as a load resistor.

Broadly speaking the welding arc consists of a mechanism for emitting electrons from the cathode which after passing through ionized hot gas merge into anode. For analysis, the welding arc is usually divided into five parts viz. the cathode spot, the cathode drop zone, the arc column, the anode drop zones and the anode spot. The voltage drops across the cathode and the anode drop zones are quite steep while the voltage drop across the arc column is more gradual, as shown in Fig. 3.1(b). From the figure it is evident that the arc voltage (V) is a sum of the cathode drop (Vc),column drop (Vp)and the anode drop (Va).

It can thus be expressed as:

V = Vc + Vp + Va……. (3-1)

Though a welding arc is normally bell shaped but considerable fluctua­tion in its shape may take place in those welding processes where the rod electrode (called just the electrode in the rest of the text) is consumable, for example, in shielded metal arc welding and gas metal arc welding. To have a comprehensive knowledge about the behaviour of a welding arc it is essential to know the characteristics of its different zones.

The Cathode Spot:

It is that part of the negative electrode wherefrom the electrons are emitted. Three types of cathode spot modes have been observed.

These are:

(a) Mobile cathode spot mode,

(b) The thermionic cathode spot mode, and

(c) The normal mode.

In a mobile cathode, spot mode one or more very small cathode spots appear at the cathode surface and travel at a high speed of 5 to 10 m/sec, and usually leave behind a visible trace. The behaviour of a mobile cathode spot is dependant on the material on which it forms. For example, on aluminium multiple spots which generate complex series of branched tracks are observed while on copper the trace left behind is commonly single without any branches as shown in Fig. 3.2.

Mobile Cathode Spots

The oxide film on the surface of the metal is loosened by the movement of a mobile cathode spot and sometimes a layer of the metal is also lost. This characteristic makes a mobile cathode very important for use in industry par­ticularly for welding aluminium and magnesium. The current density in such a cathode spot is of the order of 102 to 103 A/mm2.

In the thermionic mode the cathode spot forms at the tip of a sharply pointed tungsten or thoriated tungsten rod used with argon shielding. The cath­ode spot remains fixed in position and has a current density of the order of 102 A/mm2. It is visible either as a bright spot or can be located by the convergence of the arc column to a point at the cathode surface.

In the normal mode the cathode spot does not form any well-defined spot. For example, with a low carbon coated steel electrode the cathode spot appears to envelop the entire molten tip of the electrode. A similar type of cathode spot is observed in gas tungsten arc welding with argon’ shielded rounded tip tungsten electrode, as shown in Fig. 3.3.

Normal Mode Cathode Spots

Argon shielded tungsten arc operates either with the well-defined cath­ode spot of the second type or ill-defined cathode spot of the third type and the volt-ampere characteristic in the two cases are different.

Electron Emission Mechanisms:

Electron emission from the cathode can be by anyone of the several mechanisms such as thermionic emission, auto-electronic or field emission, photo-electric emission, and secondary emission.

a. Thermionic Emission:

It involves the liberation of electrons from the heated electrodes. As the temperature of the electrode is raised, the kinetic energy of the free electrons increases to a point where they can escape from the surface of the negative electrode at the cathode spot into the field-free space outside in the face of attraction by the positive ions left behind on the cathode.

The emission of electrons from the carbon and tungsten cathodes is believed to be thermionic in character, but most other metals boil at tempera­tures well below that required for thermionic emission.

b. Auto-Electronic Emission:

This type of electron emission is produced by sufficiently strong electric field, that is when the voltage across the elec­trodes is so high (of the order of 104 volts) that the air between them is ionised under its influence and the electric discharge ensues with the emission of electrons from the cathode surface.

c. Photo-Electric Emission:

It occurs when energy in the form of a beam of light falls on the cathode surface and results in increased kinetic energy of electrons and thus results in their emission from the cathode into vacuum or another material. Such a mechanism of electron emission is utilised in generat­ing X-rays.

d. Secondary Emission:

It refers to the emission of electrons under the impact of rapidly moving ions. When the velocity of incident ions exceeds the orbital velocities of electrons in the atoms of the material of the cathode it results in expulsion (or emission) of electrons.

In welding processes the electron emission is either of the thermionic type for example in gas tungsten arc welding, plasma arc welding, and carbon arc welding or it is of auto-emission type in conjunction with auxiliary means of ionising the air gap between the electrode and the work-piece such as for shielded metal arc welding, submerged arc welding and gas metal arc welding.

The emission of electrons from a cathode spot is dependent upon the excitation energy or the work-function of a material which is defined as the energy required, in electron volts (eV) or Joules, to get one electron released from the surface of the material to the surrounding space. Ionisation potential, which is defined as the energy per unit charge in volts, required to remove an electron from an atom to an infinite distance, also plays an important role in sustaining an electric discharge. Both the parameters for most of the materials involved in welding are given in table 3.1.

Electron Work Function and Ionisation

The Cathode Drop Zone:

It is the gaseous region immediately adjacent to the cathode in which a sharp drop in voltage occurs. The combined size of the cathode drop zone and the anode drop zone is of the order of 10 2 mm which is nearly equal to the electron mean free path. The voltage drop in the cathode drop zone for the argon shielded tungsten electrode has been found to be about 8 volts at 100 amperes and it increases as the current decreases.

The Arc Column:

It is the bright visible portion of the arc and has a high temperature and a low potential gradient. The temperature of the arc column depends upon the gases present in it and the amount of welding current flowing in the circuit. Usually the column temperature varies from 6000°C for iron vapours to about 20,000°C for argon shielded tungsten arc. At such a high temperature all mo­lecular gases present in the column get split into atomic form and the atoms themselves are further dissociated into electrons and ions. However, the num­ber of electrons and ions in any given volume of the arc remains the same thus keeping the arc electrically neutral.

As the average ion is about one thousand times heavier than an electron therefore the electrons are far more mobile and hence carry most of the current across the arc column. The potential gradient in the column is lower than that across the cathode drop zone or the anode drop zone and it generally varies between 0-5 to 5 volt/mm for argon shielded tungsten arcs whereas for shielded metal arc welding it is normally around 1 volt/mm.

The welding arc is almost invariably between a rod or a wire electrode and a flat or wide work-piece. This, irrespective of the electrode polarity, results in a bell or a cone shaped arc with the apex of the cone at or near the tip of the rod electrode. Due to this constriction of the arc near the rod electrode, it has the highest energy density there but because of the cooling effect due to the proximity of the electrode the maximum temperature is at the core of the column.

The region wherein the constricted column meets the electrode is called the arc root. The temperature distribution in the arc column for a 200 ampere argon shielded tungsten arc is shown in Fig. 3.4.

Temperature Distribution in An Arc Column

Fig. 3.4 Temperature distribution in an arc column

The flow of current in the arc column results in the development of electromagnetic forces. Now, it is also well known that two parallel conductors carrying current in the same direction attract each other.

If the current is conducted by a gaseous cylinder it can be considered as consisting of a large number of annular cylindrical conductors hence there is mutual attraction between the different gaseous cylinders with all the forces acting inwards due to high current density at the core of the conductor.

These constricting forces are balanced by a static pressure gradient established in the gaseous conductor with zero pressure at the outer periphery and a maximum along the axis.

How­ever, in the present case, due to the cone shape of the arc the electromagnetic forces acting on it have two components with the static pressure having the two opposing components one of which is along the arc axis and is the cause of formation of plasma jet that flows with a velocity of about 104 cm/sec towards the workpiece. The axial plasma velocity decreases as the arc periphery is approached, as shown in Fig.3.5.

Plasma Jet Velocity Pattern at The Cross-Section of An Arc

In a steady state the plasma jet has a streamline flow with the flow velocity approximately proportional to the welding current. Fig. 3.6 shows the pattern of gas flow lines and velocity lines in a 200A carbon arc. A consider­able amount of heat energy is believed to be conveyed to the workpiece through convective currents of plasma jet.

Gas Flow Lines and Plasma Velocity Line

Fig. 3.6 Gas flow lines and plasma velocity line patterns in carbon arc welding

When the current flow in the arc is not symmetrical it results in the setting up of magnetic forces which deflect the arc column. If this occurs in a welding arc it is known as arc blow and often results in unseemly and mis­placed welds.

The Anode and the Anode Drop Zone:

On reaching the anode the electrons lose their heat of condensation. However, unlike cathode spot, it is rare to observe a well defined anode spot and the current density is also low, as is shown in Fig. 3.7 for a 200A argon shielded tungsten cathode and copper plate anode. The current carrying area of an anode is slightly smaller than the widest spread of the arc at the anode end, and the mean current density is also quite low.

Current Distribution at The Anode

The voltage drop in the anode drop zone of this type of arc appears to b6 between 1 to 3 volts. The depth of anode drop zone is of the order of 10-2 to 10-1 mm. When the rod electrode acts as the anode, then it occupies the lower hemi-sphere of the molten droplet at the tip of the electrode. However, for low pressure plasma jet the anode appears to envelop the molten droplet.

The total heat input at the anode is due to the condensation of the electrons as well as conduction and convection due to the plasma jet In dc arc with non-consumable electrode like that of tungsten or carbon, the anode heat is greater than the heat liberated at the cathode as shown in Fig. 3.8.

Effect of Welding Current on Cathode and Anode Heats

With the increase in welding arc length the arc voltage increases and, therefore, for current above about 100A the heat input increases with increase in arc column particu­larly for cathode spot mode as shown in Fig. 3.9. However, with the increase in column length the column width also increases and that results in a still lower current density at the anode and thus the anode becomes more diffused.

Volt-Ampere Characteristics for Argon-Shielded Tungsten Arc

Arc Efficiency:

From the description of the characteristics of different parts of a weld­ing arc, it is possible to determine arc efficiency, mathematical treatment of which follows:

Now, the total heat energy developed at the anode, qa is given by the sum of the energy received through the electrons and the energy gained by passing through the anode drop zone, i.e.,

Problem 1:

Find the arc efficiency for GTAW process if the welding current is 150 amperes and the arc voltage 20 volts. Assume a cathode drop of 8 volts and anode drop of 3 volts with 30% of the arc column energy being transferred to the anode. Take arc temperature as 15000K. Work function, ΙΈ0for tungsten = 4.5 eV and Boltzmann’s constant = 8.62 x 10-5 eV/K.

Solution:

Problem 2:

In argon shielded tungsten arc welding the cathode drop was found to be 10 volts for a welding current of 120 volts and an arc voltage of 18 volts. Determine (a) the arc length, if the arc efficiency be 55% with an arc temperature of10000 Kelvin.

Assume column voltage drop is 1.2 volt I mm and that 20% of the heat of the column is transferred to the anode.

(b) The arc efficiency if the same process parameters are applicable to GMAW process and the wire electrode is made the anode.

Take work function for tungsten at OK = 4.5 eV and Boltzmann ‘s constant. K’ = 8-60 x10-5 eVIK

Solution:

Types of Welding Arcs:

From the welding point of view the arcs are of two types viz., immobile or stationary or fixed arc and a mobile or moving or travelling arc. A fixed arc is formed between a non-consumable electrode arid a workpiece. The arc may be used with or without filler. In the former case a separate wire is introduced into the arc column and is thus melted to transfer into the weld pool under the combined action of gravity, electromagnetic forces and the mechanical force exerted the plasma jet, in a fixed arc most of the heat going to the non- consumable electrode remains un-utilised and in fact may have to be taken away by the cooling water or the shielding gas. Thus, the thermal efficiency of such an arc is low and may lie between 45 to 60%. This type of arc is observed in carbon arc, gas tungsten arc and plasma arc welding processes.

A mobile arc is formed between a consumable electrode and a work- piece. As the filler wire melts, the molten metal at the tip of the electrode is detached by the action of gravity, electromagnetic forces, force exerted by the plasma jet and the pinch effect. However, a retaining force due to the surface tension also acts on the droplet. As the electrode melts the arc goes on moving upwards along the electrode. The mobile arc is associated with processes like shielded metal arc welding, gas metal arc welding and submerged arc welding.

An arc in which the molten metal from the tip of the electrode is transported through it to become a part of the weld pool is called a ‘metal-arc’. A mobile arc is a metal arc.

Most of the heat going to the electrode in the mobile arc is utilised for melting the metal and thus used effectively. The thermal efficiency of the process, using a mobile arc, is therefore high and normally lies between 75 to 90%. The welding processes using mobile arc are, therefore, thermally more efficient than those using immobile or fixed arc.

Role of Electrode Polarity in Arc Welding:

Arc welding can be carried out either by ac or dc. If ac is employed there is no question of electrode polarity as it changes every half cycle. How­ever, if dc is used it is possible to make electrode either negative or positive.

More heat is produced at the anode therefore in all processes using non- consumable electrodes it is better to connect the electrode to the negative ter­minal to keep the heat losses to the minimum. However, it may not be always possible to do so because, at times, the cleaning action of the mobile cathode spot needs to be utilised to release the tenacious refractory oxide layer from the metal, for example, in welding aluminium and magnesium.

In such cases it is preferable to use ac so as to make a compromise between thermal efficiency and cleaning action. Thus, gas tungsten arc welding and carbon arc welding processes normally employ ac power sources when leaning action on the work-piece is necessarily needed. When such a compulsion is not there than dcen may be used.

However, for shielded metal arc welding ac welding transformer is quite popular and at the same time, for the same specifications, it is much cheaper than the dc welding motor-generator set or transformer cum rectifier set required for obtaining dc supply. Also with dc welding there is the changer of arc blow which can cause unseemly zig-zag weld of poor quality.

Due to regular interruption of an ac arc it is not recommended when bare wire is used, for example, in gas metal arc welding. However, for shielded metal arc welding appropriate electrode coatings have been developed which facilitate easy initiation and maintenance of the welding arc.

When consumable electrode is used, the metal transfer from the wire electrode to the work-piece is more uniform, frequent and better directed if the electrode is made the positive. DCEP or reverse polarity is, therefore, popular with GMAW which also provides necessary cleaning action on metals with tenacious oxide layer such as aluminium.

Welding Arc: Definition, Structure and Types | Metallurgy

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