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Welding Procedures
SHIELDING GAS

Aviation Structural Mechanic (H&S) 3&2 - How airplanes are built and how to maintain them
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Figure 15-47.—GMA welding equipment. weld puddle are shielded from the atmosphere by a gas, or a gas and a flux. The shielding gas protects the molten weld metal from oxidation or contamination by the surrounding atmosphere. The consumable-wire electrode for GMA welding is fed through the torch to the welding arc at the same rate as the heat of the arc melts off the end of the electrode. The shielding gas flows through the torch to the arc area. The melting rate of the tiller wire depends on the level of the welding current, but must be the same as the feeding rate to maintain a constant arc length. This means that a constant balance must be maintained between the welding current and wire feeding rate. GMA Welding Equipment There arc numerous types and models of GMA welding equipment used in the Navy. Each must have a source of direct current reverse polarity (DCRP) welding current, a wire feed unit for feeding the wire tiller metal, a welding gun for directing the wire filler and shielding gas to the weld area, and a gas supply. Figure 15-47 shows GMA welding equipment. POWER SUPPLY.—The recommended machine for gas metal-arc welding is a rectifier or motor generator that supplies direct current with normal limits of 200 to 250 amperes. Direct current reverse polarity is most generally used because it provides maximum heat for better melting, cleaning action. deeper penetration, and excellent Two types of direct-current power sources are used for gas metal-arc welding–the constant-current type and the constant-voltage type. The constant-current power source is used if the controls and wire-driven mechanism control the arc length by varying the wire- drive speed. In this case, a change in the arc length causes a change in the arc voltage. The control circuit senses this change and varies the wire-feed speed to bring the arc length back to the desired value. When arc length is controlled through changes in welding current, constant-voltage power supplies are used. The wire-feed speed is constant. Any changes in arc length cause automatic changes in welding current, which compensate for the arc-length change. If the arc length becomes shorter, the welding current auto- matically increases. This causes the wire to melt faster and the arc length to increase. The reverse happens if the arc is lengthened during welding. WIRE FEEDING MECHANISM.—The wire feeding mechanism automatically drives the electrode wire from the wire spool to the welding gun and arc at a uniform rate. The speed of the wire feeding mechanism is adjustable, so that the wire-feed speed can be set to equal the melting rate. If the drive unit is designed to be used with a constant-voltage power source, the speed is set before welding starts, and 15-34







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