Electric arc welding is a fusion welding process. Welding heat is obtained from an electric arc between the work (or base metal) and electrode.
An electric arc is produced when two conductors in an electric circuit are touched together and separated by a small distance. There is sufficient voltage in the circuit to maintain the current flow through the gaseous medium. The temperature produced is between 6000°C to 7000°C.
The depression created on the base metal due to the arc is called a crater.
DC or AC current can be used for arc welding. DC voltage required is 60 to 80 V for striking the arc and 15 to 25 V
for maintaining the arc. They are 80 to 100 V for AC current and 30 to 40 V, respectively.
An arc is a sustained electric discharge through the ionized gas column, called plasma, between the electrodes.
When electrons hit the anode at high velocity, kinetic energy is converted, producing large heat. Similarly, positively charged ions hitting the cathode also produce heat. 65 to 75% of the total heat generated is at the anode. So work is connected as an anode if it is required to generate more heat at the anode. This is called straight polarity or DCEN (direct current electrode negative) DCEN is required for thicker plates and materials of higher thermal conductivity. For thinner plates, reverse polarity or DCEP (Direct current electrode positive) is used. Weld penetration is more in DCEN. Weld penetration for AC is between DCEP and DCEN.
Arc Welding Equipment:
- AC Machines
(ii) Alternator engine driven by motor or engines
- DC Machines
(i) Transformer with rectifier
(ii) DC generator is driven by motor or engine
Transformer sets are more commonly used in AC welding. As there are no moving parts, power consumed and
noise is less. Also, maintenance cost is low and efficiency is more.
Specification of Arc Welding Machines:
- Maximum rated open-circuit voltage
- Rated current in amperes
- Duty cycle
American welding society (AWS) defines the duty cycle as the percentage time in ten minutes that a welding
machine can be used at its rated output without overloading. Usually, a 40% duty cycle is suggested (Indian standard specifies 5 minutes as the cycle time.)
Types of Welding Electrodes:
Non-consumable types of electrodes are made of carbon, graphite or tungsten. Carbon and graphite electrodes are used in DC welding. Only tungsten can be used for both AC and DC welding. As this electrode is not consumed, arc length is constant, stable, and easy to maintain. Separate filler rods are used in this case.
There are three types of consumable electrodes
- Bare electrodes
- Fluxed or lightly coated electrodes
- Coated or extruded/shielded electrodes
Bare electrodes may be used for mild welding steel and wrought iron. The primary function of a light coating is to increase arc stability. These are also called ionizing coatings. Coated electrodes contain arc stabilizing ingredients, slag forming ingredients, binding materials, alloying constituents etc. Sometimes iron powder is added to improve the deposition rate.
Types of Electric Arc Welding Process:
- Carbon arc welding
- Shielded metal arc welding (SMAW)
- Flux-cored arc welding (FCAW)
- Gas metal arc welding (GMAW)
- Gas tungsten arc welding (GTAW)
- Submerged arc welding (SAW)
- Atomic hydrogen welding
- Plasma arc welding (PAW)
- Stud welding
- Electro slag welding (ESW)