30-07-2012, 02:52 PM
STUDY OF GAS METAL ARC WELDING PROCESS (GMAW)
GMAW EXPERIMENT FINAL.docx (Size: 744.54 KB / Downloads: 55)
PROCESS DEFINITION
1 Gas Metal Arc Welding (GMAW), by definition, is an arc welding process which produces the coalescence of metals by heating them with an arc between a continuously fed filler metal electrode and the work. The process uses shielding from an externally supplied gas to protect the molten weld pool. The application of GMAW generally requires DC+ (reverse) polarity to the electrode.
2 In non-standard terminology, GMAW is commonly known as MIG (Metal Inert Gas) welding and it is less commonly known as MAG (Metal Active Gas) welding. In either case, the GMAW process lends itself to weld a wide range of both solid carbon steel and tubular metal-cored electrodes. The alloy material range for GMAW includes: carbon steel, stainless steel, aluminum, magnesium, copper, nickel, silicon bronze and tubular metal-cored surfacing alloys. The GMAW process lends itself to semi-automatic, robotic automation and hard automation welding applications.
TYPES OF GMAW
Gas Metal Arc Welding can be done in basically three different ways. Semiautomatic welding means that the equipment controls only the electrode wire feeding. The movement of the welding gun is controlled by hand. Thus, semiautomatic welding is sometimes called handheld welding. Machine welding uses a gun that is connected to a manipulator of some kind (not hand held). An operator has to constantly set and adjust controls that move the manipulator. Automatic welding uses equipment which welds without the constant adjusting of controls by a welder or operator. On some equipment, automatic sensing devices control the correct gun alignment in a weld joint.
PRINCIPLE OF OPERATION (SEMI- AUTOMATIC PROCESS)
1 Before igniting the arc, gas and water flow is checked. Proper current and wire feed is set and the electrical connections are ensured. The arc is struck by any one of the two methods.
2 In the first method current and shielding gas flow is switched on and the electrode is scratched against the job as usual practice for striking the arc.
3 In the second method, electrode is made to touch the job, is retracted and then moved forward to carry out welding; but before striking the arc, shielding gas, water and current is switched on.
4 About 15mm length of the electrode is projected from the torch before striking the arc.
5 During welding, torch remains about 10-12mm away from the job and arc length is kept between 1.5 to 4mm.
6 Arc length is maintained constant by using the principles of self-adjusted arc, self-controlled arc in semi-automatic and automatic sets respectively.
GMAW Welding Gun (Example of an un-cooled semiautomatic gun)
As compared to a SMAW electrode holder, the GMAW gun is more expensive and complex. This is because there is more required of it than just carrying electrical power to the electrode wire:
• A weld power cable connection brings electrical energy to the gun´s contact tube area.
• The gun guides shielding gas into the weld zone.
• The gun has also a connection for getting the electrode wire from a drive rolls, to the contact tube inside the gas nozzle.
• A trigger switch connection allows to control weld starting and stopping
JOINT DESIGN
A weld joint is the term used for the location where two or more pieces of metal are to be, or have been, welded together. In order to obtain a good weld and economical use of filler metal (welding wire for GMAW), joint design must be considered in any type of weld-ment. This will depend upon several factors including material type, thickness, joint configuration, strength required, etc.
It is quite possible that a welder would have little to do with how a particular joint is designed.
However, a good welder should be familiar enough with these joint designs to carry out a welding job.
A proper joint design will provide the required strength that codes and specifications designate. The correct joint design will then need the highest quality weld made at the most economical cost. The joint design selected will, of course, dictate what type of weld is to be made.