Solid wire MIG welding
Solid wire MIG welding
Metal inert gas (MIG) welding was first patented in the USA in 1949 for welding aluminum. The arc and weld pool formed using a bare wire electrode was protected by helium gas, readily available at that time. From about 1952 the process became popular in the UK for welding aluminum using argon as the shielding gas and for carbon steels using CO2. C02and argon-CO2 mixtures are known as metal active gas (MAG) processes.
Heat for welding is produced by forming an arc between a metal electrode and the workpiece; the electrode melts to form the weld bead. The main difference is that the metal electrode is a small diameter wire fed from a spool. As the wire is continuously fed, the process is often referred to as semi-automatic welding.
The manner, or mode, in which the metal transfers from the electrode to the weld pool largely determines the operating features of the process. There are three principal metal transfer modes: short circuiting, droplet / spray, pulsed. Short-circuiting and pulsed metal transfer are used for low current operation while spray metal transfer is only used with high welding currents. In short-circuiting or dip transfer, the molten metal forming on the tip of the wire is transferred by the wire dipping into the weld pool. This is achieved by setting a low voltage; for a 1.2mm diameter wire, arc voltage varies from about 17V (100A) to 22V (200A). Care in setting the voltage and the inductance in relation to the wire feed speed is essential to minimize spatter.
For droplet or spray transfer, a much higher voltage is necessary to ensure that the wire does not make contact i.e. short-circuit, with the weld pool; for a 1.2mm diameter wire, the arc voltage varies from approximately 27V (250A) to 35V (400A). The molten metal at the tip of the wire transfers to the weld pool in the form of a spray of small droplets (about the diameter of the wire and smaller).
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4. Подчеркните главное слово: gas-shielded metal arc welding, metal arc inert gas a versatile welding technique, a DC power source, voltage/welding current relationship, open circuit voltage, power source voltage level, wire feed speed, wire burn-off rate, pre-set arc length.
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6. Подчеркните формы герундия и причастия: 1. The low arc forces would be insufficient to prevent large droplets forming at the tip of the wire. 2. The pulsed mode was developed as a means of stabilizing the open arc at low current levels. 3. Spray type metal transfer is achieved by applying pulses of current. 4. In additional to general shielding of the arc and the weld pool, the shielding gas performs a number of important functions. 5. The shielding gas will have a substantial effect on the stability of the arc. 6. While welding the special gas mixtures may contain helium when the shielding gases – a mixture of argon, oxygen and CO2. 7. Argon based gases containing oxygen or CO2 are normally employed. 8. Welders must have a good knowledge of equipment requirements and should also recognize fully the importance of setting up and maintaining component parts correctly. 9. The joint area is cleaned immediately before welding.
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