Arc welding is a method of joining metals with heat produced by an electrical arc. In this process the heat necessary to melt the edges of the metal to be joined is obtained from an electric are struck between the electrode (filler rod) and the work, producing a temperature of 40000C, in the welding zone. The heat of the arc melts the base metal or edges of the parts fusing them together. Filler metal, usually added melts and mixes with molten base metal to form the weld metal. The weld metal cools and solidifies to form the weld. In most cases, the composition of the filler material, known as welding rod, needed to provide extra metal to the weld, is same as that of the material being welded.
- An arc welding circuit consists of a power supply to furnish electric power.
- An electrode to conduct the electricity to the arc.
- Cables which connect the power supply to the electrode and workpiece to complete the welding circuit.
- The arc itself provides the heat for welding.
- The workpiece to welded is kept on a metallic table.
The arc must be shielded because; as it hardens the molten metal combines with oxygen and nitrogen to form impurities that weaken the weld. Shielding can be obtained by adding a paste, powder or fibrous flux to the arc. The electrodes are usually coated with a flux. This coating forms a gaseous cloud that shields the molten metal from the atmosphere. The coating also forms a protective slag. The slag floats on the molten pool and hardens as the weld cools. This keeps impurities out of the weld.
Advantages:
- As a manual process it is applicable to an infinite variety of work and can be executed in any position.
- There is less buckling and warping of the work.
- It produces strong sound and ductile welds.
- Satisfactory welds can be produced in heavy as well as in light sections.
- Low cost process.
- Excellent joint properties can be obtained in mild, low alloy and stainless steels, nickel and copper-base alloys.
- Low accuracy in setting up required.
Disadvantages:
- Basically a manual process requiring adequate operator skill for good results.
- Electrodes require frequent changing.
- Multi run welds necessary on thick plate-slag chipping necessary after each run.
- The principal disadvantage has been the high heat of the metal arc which makes it unsuitable for use on materials less than 1.55 mm thick.
- High initial cost of welding equipment.
Arc welding is the most widely used fabrication process at present. This pre-eminence will be maintained in initial fabrication, repair work and maintenance.
