TYPES OF GMA WELDING
When using the GMA welding process, metal is transferred by one of three methods: spray transfer, globular transfer, or short-circuiting transfer. The type of metal transfer depends on the arc voltage, current setting, electrode wire size, and shielding gas.
Spray-arc transfer is a high-current range method that produces a rapid disposition of weld metal. This type of transfer is effective for welding heavy-gauge metals because it produces deep weld penetration. The use of argon or a mixture of argon and oxygen are necessary for spray transfer. Argon produces a pinching effect on the molten tip of the electrode, permitting only small droplets to form and transfer during the welding process. Spray transfer is useful when welding aluminum; however, it is not practical for welding light-gauge metal.
Globular transfer occurs when the welding current is low. Because of the low current, only a few drops are transferred per second, whereas many small drops are transferred with a higher current setting. In this type of transfer, the ball at the tip of the electrode grows in size before it is transferred to the workpiece. This globule tends to reconnect with the electrode and the workpiece, causing the arc to go out periodically. This results in poor are stability, poor penetration, and excessive spatter.
Globular transfer is not effective for GMA welding. When it is used, it is generally restricted to thin materials where low heat input is desired.
Short-Circuiting Arc Transfer
Short-circuiting arc transfer is also known as short arc. Short arc was developed to eliminate distortion, burn-through, and spatter when welding thin-gauge metals. It can be used for welding in all positions, especially vertical and overhead where puddle control is more difficult. In most cases, it is used with current levels below 200 amperes and wire of 0.045 of an inch or less in diameter. Small wire produces weld puddles that are small and easily manageable.
The shielding gas mixture for short-arc welding is 75% carbon dioxide and 25% argon. The carbon dioxide provides for increased heat and higher speeds, while the argon controls the spatter. Straight CO,is now being used for short-arc welding; however, it does not produce the excellent bead contour that the argon mixture does.
GMA WELDING PREPARATION
Preparation is the key to producing quality weldments with the gas metal-arc welding process. As in GTA welding, the equipment is expensive; therefore, you should make every effort to follow the manufacturer's instruction manuals when preparing to use GMA welding equipment.
For the most part, the same joint designs recommended for other arc welding processes can be used for gas metal-arc welding (refer to chapter 3). There are some minor modifications that should be considered due to the welding characteristics of the GMA process. Since the arc in GMA welding is more penetrating and narrower than the arc for shielded metal-arc welding, groove joints can have smaller root faces and root openings. Also, since the nozzle does not have to be placed within the groove, less beveling of the plates is required.
GMA welding can actually lower material costs, since you use less weld metal in the joint.
The following suggestions are general and can be applied to any GMA welding operation:
Check all hose and cable connections to make sure they are in good condition and are properly connected.
Check to see that the nozzle is clean and the correct size for the particular wire diameter used.
Make sure that the guide tube is clean and that the wire is properly threaded through the gun.
Determine the correct wire-feed speed and adjust the feeder control accordingly. During welding, the wire-speed rate may have to be varied to correct for too little or too much heat input.
Make sure the shielding gas and water coolant sources are on and adjusted properly.
Check the wire stick-out.