MANUAL METAL ARC WELDING (MMA/STICK)
METAL ARC GAS SHIELDED (MAGS & MIG)
In common with the TIG process, the METAL ARC GAS SHIELDED or MAGS welding process, projects a protective gas shield over the weld pool via a torch shroud, thus preventing atmospheric contamination of the molten weld pool. When the gas used to form the shield is of an inert composition (ARGON for example), the MAGS process is normally referred to as MIG or METAL INERT GAS welding. Alternative shielding gases such as CARBON DIOXIDE may also be used, as well as inert/active gas mixes which are popular for welding mild steel alloys (suitable gas mixtures here include argon, carbon dioxide and oxygen combined in varying proportions). The precise choice of gas for shielding, be it inert or active, is normally determined by the alloy composition of the parent metal, and the desired quality of weld finish. Casts formed in bronzes, gunmetals and silicon copper alloys are normally shielded with a HIGH PURITY ARGON gas, the same as for TIG welding..
Despite the common use of a gas shield, MIG welding is quite distinct from TIG welding, not least because MIG welding is essentially a semi-automated process. The metal alloy for filling a MIG joint is in the form of a spool mounted wire, this in turn is fed mechanically to the torch head. The mechanism which controls the wire feed speed also determines the welding current, the faster the feed rate, the higher the current flow. The current is conducted to the filler wire via a copper contact tip, which is also mounted inside the torch’s head. The voltage level, which is normally set automatically for TIG or MMA welding, is variably controlled in MIG sets via ‘fine’ and ‘coarse’ switches situated on the set’s control panels. The voltage controls can be adjusted manually in combination with the wire feed/current supply to vary welding characteristics, including the manner of the filler wire’s transfer to the weld pool (see below).
Weld is deposited by depressing a trigger on the torch body, this both activates the wire feed/power supply mechanism and initiates the flow of the shielding gas from the shroud. In common with the other electrical arc processes, a return lead secured by clamping to the workpiece enables a circuit to be completed back to the transformer, thus enabling the welding current to flow continuously.
The transfer of filler wire across the arc gap to the joint preparation is by either a DIPPING or SPRAYING action. The method of transfer is determined according to the voltage and wire feed settings selected on the set’s control panel. In most cases a dip transfer action is selected, this means the wire enters the weldpool melts and breaks off. The alternative ‘spray transfer’ projects already molten filler globules into the joint from the torch. In either case, the release of the torch trigger shuts down the gas supply valve and opens the electrical circuit, thus stopping the wire feed, current flow and weld deposit. A MIG weld is usually deposited from a right to left direction (LEFTWARDS WELDING) along the joint.
Spool mounted filler wires are selected to match the parent alloy of the cast or fabrication. Some filler wires may also contain a FLUX CORE, though unlike MMA flux coated electrodes, fluxed MIG filler wires are usually used in conjunction with a gas shielding rather than instead of. Many of the standard foundry alloys including most copper and aluminium alloys have a suitable matching filler wire available for MIG welding 0.8mm is a good general purpose wire diameter, though 1mm+ is available for heavier work.
MIG sets have something of an advantage over TIG sets, in that aluminium alloys can be welded using the same current characteristics used for steel or copper alloy welding – provided of course there is sufficient power available and a correct filler wire/gas shield are used.
Other advantages of a MIG welding process include the rapid speed in depositing a lengthy weld run and the relative ease of positional welding (vertical and overhead – though always using DIP TRANSFER settings). Heat distortion from welding, especially in constructions made from thin gauge material, is often much reduced when using a MIG welding process – which is why this equipment is often found in car body shops and sheet metal workshops. The skill level required to form a sound joint in thin material is rather less than that needed for other arc and gas welding processes and this makes MIG an ideal process for those artists and designers who have little welding experience on light gauge material.
On the downside, spools of filler wire can be expensive to buy – especially if only small quantities of a special alloy are required for a ‘one off’ job. Spools take time to change and this may slow down progress, as does the un-jamming of kinked wire in a feed mechanism – an occasional frustration of MIG welding, especially when using budget equipment. A great deal of dispersed weld SPATTER may also be generated by this process and although anti-spatter sprays are available to discourage metal globules from adhering to the artwork, the solvents contained in these sprays can adversely affect the cast’s finish, especially if the artwork is to be PATINATED later.
One other problem with the MIG process is the relative ease with which deceptively substantial looking welds can be made without complete FUSION of the joint and filler having actually occurred. This is especially an issue when working with heavier material, or in metals that readily conduct heat – both these factors are commonly encountered during the construction of art and design casts. Poor WELD PENETRATION into the cast’s parent material is potentially unsafe, and could lead to the eventual collapse of a sculpture, especially if subjected to stress and strain (if lifted awkwardly for example). For this reason many professional welders prefer to use of an alternative, more predictable process such as MMA or TIG for joining critical structural elements. If MIG welding is being used for joining a structural work, it is recommended that samples are first tested to ensure integrity in the finished design (see SUPPLEMENT) .
MIG welding equipment is found in some art foundries, though it is unlikely that this method of welding would be used to the exclusion of the TIG process. A number of equipment manufacturers produce small scale domestic mains powered MIG sets to varying degrees of quality. Even the best of these small sets, which are primarily designed for welding light gauge sheet, are woefully inadequate for joining the heavy casts produced in art foundries. Most foundries that retain MIG equipment have heavy duty three phase sets capable of high outputs, this enables satisfactory welding of heavier wall sectioned casts.
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