Arc Welding - the Basics
Filed Under MIG &TIG Welder Basics |
One method of welding used by both Miller welders and Lincoln welders is arc welding.
Arc welding is a process for joining metals by fusion. Intense heat is applied to two pieces of metal causing either the metal itself, or a filler metal, to melt and hence fuse the two parts together, Once cooled the the metals solidify, causing a bond. The bond is strong as it is a mixing of metals and has the same metallic properties as the metals of which it is part. The bond is much stronger then a soldered or brazed join would be as that type of bond is only as strong as the material used for the joint.
The electrodes
In order for the metals to melt an intense heat is needed and in arc welding this is supplied by an electric arc. An electrode is guided along a joint, either by hand or by a machine, and an arc forms between this electrode and the material being joined. The electrode may be of two sorts. A rod to only carry the current or a wire or rod that carries the current and supplies metal for the joint by melting as it is passed along the joint. In the manufacture of steel th e melting electrode type is the most often used.
The Electrode Circuit
To produce the circuit AC or DC power may be used. The power source is connected to the metal being worked by a cable and the circuit is completed by a cable to an electrode holder which is in contact with the working electrode. When the electrode is in close contact with the metal being worked an arc is created across the gap. This produces a temperature of about 6000F at the electrode tip which melts both the electrode tip and the working metal to produce a ‘crater’ of molten metal. When the electrode is moved along the joint the ‘crater’ solidifies forming the bond.
Shielding
When the metals are melted at this temperature reactions with oxygen and nitrogen in the air forming oxides and nitrides of the metals. This will considerably lower the strength of the newly formed bond. measures must be taken to prevent this occurring by covering the molten metals with a shield of some kind. This may consist of vapours, gases or slag and is called ‘arc shielding’ The shielding prevents oxygen and nitrogen contacting the molten metals thus improving the strength of the weld. One method involves adding deoxidisers to the weld using a granular flux.
The Arc
A cathode (negatively charged) and an anode (positively charged) are used and the arc is produced between them through a column of ionised gas. negative and positive ions produced collide in the column at a high rate. The arc provides the heat to melt both the working metal and the electrode tip but may also be a mechanism to transfer the molten metal at the tip to the surface of the working piece. There are many methods to accomplish this but the most common are spray arcing and surface tension transfer. In spray arcing an electric pinch ejects the molten metal drop onto the molten metal pool on the work. In the surface tension method the drop is pulled into the metal pool by surface tension when the drop touches the pool.
In a metal arc system the tip of the electrode melts and drops are transferred to the work through the arc column. When a carbon or tungsten (TIG) electrode (as in Miller TIG welders or LIncoln TIG welders) is used the electrode does not melt but a filler material is used from a wire or a rod. In metal arc systems more heat is transferred to the molten metal pool leading to higher efficiency and a smaller zone of the metal that is affected by the heat. MIG welding refers to the use of a consumable wire feed to produce a filler material. Both Miller MIG welders and Lincoln MIG welders use this method.
The arc must be ignited by either supplying a high voltage initially to cause a discharge or by the electrode and the work coming into contact and then withdrawing the electrode as the area is heated.A direct (DC) may be used with the work being either positively or negatively charged or an alternating current (AC) can be used.
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