arc welding basics and its definition
Arc welding process that uses the arc discharge for the heat source is widely used in the world. And the most popular welding process that has been used from the old time is shielded metal arc welding (SMAW). As shown in the above picture, the voltage applied across the core rod of the electrode and the base metal generate an arc. The arc temperature for SMAW is about 5,000 – 6,000 K. This arc heat generate molted droplets of electrode tip that transfer to the weld pool.
There is a voltage gap between the anode and the cathode. An arc voltage consists of the cathode (negative pole) voltage drop, the anode (positive pole) voltage drop, and the arc column voltage drop between cathode and anode. The arc column is composed of neutral particles, such as atoms and molecules, and charged particles such as ions and electrons that are generated by ionization of neutral particles. The arc column is characterized by “thermal equilibrium” and “electric neutrality.” The gas in this condition is called ionized gas (plasma).
Arc currents are carried mostly by electrons. That is, while particular numbers of electrons are emitted from the cathode into the arc column, the same numbers of electrons are absorbed by the anode. An arc is a high-temperature conductive gas which can carry considerable amounts of currents. The ionization degree of an arc is maintained by the electric power supplied to the arc. An arc column is maintained at a high temperature of between 5000 – 50,000K. In the case of shielded metal arc welding of mild steels, the temperature of the arc becomes lower (5,000 – 6,000K) because the arc contains a large number of metal vapors that are apt to be ionized.
In direct current (DC) arc welding, the covered electrode (or welding wire) may be connected to the
anode terminal and the base metal is connected to the cathode terminal or vice versa. The former polarity is called DC electrode positive (DCEP) or DC+, and the latter polarity is called DC electrode negative (ocan) or 130-.
Electric potential gradient and thermal and electromagnetic pinch effect
The voltage drop per unit arc column length is called the electric potential gradient. it is normally around 0.3 – 5 V/ mm. This value varies depending on the type of gases, current intensity. The high-temperature portion of an arc column becomes thinner to contract its cross section when its perimeter is cooled rapidly. This is called “thermal pinch effect.” When the arc becomes thinner, its electric resistance increases, and thereby the electric potential gradient is increased. This arc phenomenon is substantially the same as the phenomenon in which the arc with the carbon dioxide gas shielding becomes thinner than the arc with the argon gas shielding with the same arc current. Carbon dioxide gas, which is a molecular gas, needs dissociation energies to become an ionized gas; this is why the arc shielded with this gas is cooled much more.
When the same directional electric currents ﬂow in two parallel conductors, the attracting force is
generated between the conductors by an electromagnetic force. Since an arc column is a gaseous aggregate of parallel conductors, the cross section of an arc column is constricted by the attracting force induced between individual conductors. This is called “electromagnetic pinch effect,” and the force that constricts the cross section of an arc is called electromagnetic pinching force.
When electric currents ﬂow in the axially symmetrical molten metal at the tip of the welding wire, the cross-section of the molten metal is constricted and squeezed by the electromagnetic pinching force generated by the electric current. This is a very important phenomenon to understand the mechanism of molten metal transfer.
In arc welding, small particles of molten metals are emitted from the electrode tip and the weld pool, which are called spatter. Spatter not only degrades the welding performance remarkably but also requires considerable work to remove from the weld, thereby reducing the welding work efficiency markedly.
The melting rate of welding wire in consumable electrode welding is governed by the arc heat the wire tip undergoes and the resistance heat (Joule heat) generated in the wire extension. The former is almost proportional to the welding current, and the latter is proportional to the square of the welding current. The electrode melting rate can be varied depending on such factors as wire
diameter, welding current, the type of shielding gas, and polarity. In submerged arc and MAG/ MIG welding, the effect of the resistance heat of the wire extension on the melting rate cannot be negligible because the welding wire of large electric resistivity is used with high current density.
Wire melting rates with various welding processes
In the DCEP welding conditions, the contribution ratio of the electric resistance heat to the melting rate is a few percents or less for aluminum alloy wires, while as high as 40-70% for mild steel and stainless steel wires. The metal transfer from the tip of an electrode takes various modes depending on the welding process and parameters. The metal transfer modes can be classiﬁed according to the size and shape of a molten metal droplet and the transfer mechanism. When the size of a drop of molten metal transferring from the electrode tip is excessively big or irregular, the arc becomes unstable because of the arc length ﬂuctuates largely during the droplet transfer.
The globular transfer can be deﬁned as a metal transfer in which the molten metal drops with the
which can be seen in shielded metal arc welding with low—hydrogen electrodes, CO2 gas metal arc welding, MAG welding, and MIG welding with a low current. Typically, large amounts of spatter generate.
Short-circuiting and bridging transfer
In this transfer mode the molten metal at the tip of electrode bridges with the weld pool, and then the molten metal bridge is squeezed and.pinched off by the electromagnetic pinching force. and the surface tension to transfer the molten metal to the weld pool. In MlG/ MAG welding with low currents, this short-circuiting transfer mode takes place with low arc voltage (short arc length). This is called short circuit arc welding, which is characterized by shallow penetration and is suitable for sheet metal welding and out-of-position welding.
In spray transfer, the molten metal drops with the size smaller than the electrode diameter ﬂies through the arc to transfer. This transfer mode can be observed in relatively high current MAG welding (in the use of a shielding gas mixture with 80% or higher argon), MIG welding, and shielded metal arc welding with the high titanium oxide type covered electrodes. Typically, this transfer mode offers low spatter and stable arc. In MIG welding and MAG welding with a shielding gas mixture containing 80% or higher argon, the transfer mode becomes globular with low currents, but it changes to the spray transfer with high currents over the critical current relevant to the shielding gas composition.
The ratio of the thermal energy transferred to the base metal to the energy (power) supplied to generate the arc is called thermal efficiency. The energy that is not transferred to the base metal is radiated into the air an is conducted to the torch. the thermal efficiency in arc welding varies markedly from 21- 99% depending on the type of base metal and the welding process.
The weld penetration in arc welding can be affected by the shape of the arc, welding current, and the
convection of molten metal in the weld pool. It is known that the convection of molten metal is affected by plasma jet, surface tension, and the content of such microelements as sulfur, oxygen, and aluminum in the molten metal.
There are many factors that inﬂuence the shape of the penetration; particularly, the heat input into the base metal and its distribution, the characteristics of the base metal, and the groove conﬁguration are the essential factors. what is the effect of the welding position on the penetration? In uphill welding, the molten metal ﬂows backward from the weld pool by the gravity force and thus the bottom of the weld pool is exposed directly to the arc; consequently, the base metal is melted in the gouging condition, and thereby the penetration becomes deeper.
The effect of the shielding gas on the penetration shape cannot simply be expressed, because of the
penetration shape can also be affected by the molten metal transfer mode in such a way that a change in the molten metal transfer mode affects the shape of the arc and the intensity of the plasma jet, and thereby the distribution of heat input and the depth of the heating point from the surface can be changed. As the welding speed increases the weld pool becomes longer, thereby causing undercut. When the welding speed is increased excessively, the weld bead becomes a humping bead which exhibits an irregular appearance where the quantity of weld metal changes cyclically in the direction of the welding line.