< TUNGSTEN ARC GAS SHIELDED WELDING (TAGS & TIG) 2
MANUAL METAL ARC WELDING (MMA/STICK)
MMA (STICK) is an electric arc welding process powered by a step down transformer (or an inverter), similar in characteristics to those used for TIG processes. The MMA transformer may be cooled by a self contained oil reservoir, ambient air flow or fan assisted air flow. Welding can be carried out using either AC or DC output settings, though many basic MMA sets are either one or the other only. The upper output of small portable welding sets is in the region of about 130-160A (this being the maximum effective for use on single phase domestic 240v/13A ring main circuit or 110v/13A construction industry supply). Larger workshop welding sets are capable of outputting at a level in excess of 600A on 415v/60A three phase circuits. Small 130A sets are adequate for welding 6mm (1/4”) mild steel using up to 3.25mm general purpose RUTILE coated electrodes.
Adjustment of the welding current is via a dial or lever(s) mounted on the welding set’s control panel, this mechanism regulates the delivery of a welding current to a CONSUMABLE FLUX COATED ELECTRODE which is clamped in an insulated torch. The welding circuit is closed when the electrode is struck on the workpiece, returning the current flow back to the set via a secured return lead. Once an arc is struck, the electrode is lifted slightly off the workpiece to maintain an ARC GAP. The welder maintains a consistant arc gap, travel speed and electrode angle to the weld joint as the length of the electrode arcs into the molten WELD POOL. The direction of travel along a flat joint in MMA welding is normally from a left to right direction (the reverse of oxy-gas and TIG processes), this depositing technique is referred to as RIGHTWARDS WELDING. Vertical welding is usually done by starting the weld from the base base of the joint, working up towards the top with a weaving motion to prevent slag from contaminating the deposited metal.
The welding electrode itself consists of a length of alloy specific metal rod, available in various diameters according to the thickness of weld preparation to be filled. The current level used for MMA welding is in turn determined by the diameter of the electrode, with the recommended current range usually printed on the electrode's carton information label. Electrode diameters, which used to be designated by WIRE GAUGE (ie 10 SWG), are now expressed in millimetres (the metric equivalent of a standard 10 SWG electrode being 3.25 mm). The electrode diameter quoted on the carton does not include the additional thickness of FLUX COATING.
The electrode flux coating has a number of different functions, and may be composed of various compounds according to type. However, in all cases the flux melts into the weld pool together with the electrode’s metal core, the flux rises and then hardens over the top of the solidifying weld to form a SLAG protection to the cooling metal. The slag protection prevents atmospheric contamination and oxidisation of the underlying weld. Later, after the weld has cooled, the slag deposit can be removed by chipping.
MMA electrodes are available for many of the metal alloys used in art founding, including variations of cast irons, all steels, bronzes and aluminium alloys. For the most part, the use of MMA stick welding is restricted to the joining of stainless and carbon steels (mild or high tensile/low alloy versions). This is principally due to the comparatively poor performance offered by copper and aluminium alloy MMA electrodes, especially for positional welding (overhead and vertical). Electrodes designed to cut through metals are also available, though these too give generally poor results and their use is to be avoided if at all possible. Some electrodes require specialist storage conditions to function correctly, though this varies according to the type of flux coating over the electrode. The common RUTILE type flux coatings need simply to be stored in a warm dry environment.
The principal use of MMA welding in art foundries is in the construction of armatures and other steel support fabrications. Strong, sound joint welds can be created by this process with relative speed and economy. For the reasons given above (typically poor electrode performance), the use of the MMA process for welding non-ferrous art casts is not recommended.
MIG WELDING >
|
