< TUNGSTEN ARC GAS SHIELDED WELDING (TAGS & TIG) 1
TUNGSTEN ARC GAS SHIELDED WELDING (TAGS & TIG) 2
To start a TIG weld, inert gas is projected from the torch's ceramic shroud and an arc is initiated across the AIR GAP between the tungsten electrode and weld joint. Heat from the arc leads the formation of a molten WELD POOL, this can be built up with a FILLER ROD in much the same manner as described for GAS WELDING. Because the melting point of the cast alloy is much lower than the melting point of the tungsten electrode, the cast alloy melts and the tungsten (melting point 3,387°C) remains intact. This is referred to as NON-CONSUMABLE electrode welding. The tungsten needle and torch body is cooled by copper chills and circulating water. The flow of inert gas which prevents the electrode from oxidising and burning away and protects the newly deposited weld, also helps cool the torch head.
When using less sophisticated TIG welding equipment, the welder will often have to initiate the arc by briefly touching the electrode down on the work surface (this is known as SCRATCH START). The shielding gas supply with this type of equipment is normally controlled by a simple on/off switch mounted on the torch body.
With the gas supply turned on, the scratched electrode closes the welding circuit resulting in a current flow from the transformer to the torch, where it arcs onto the weld joint. The current then flows back to the set via a RETURN LEAD (more often, but incorrectly referred to as an EARTH LEAD). The clamped end of the return lead is normally secured to a convenient point on the cast or else a copper topped conductive work bench.
Better quality TIG sets allow the arc to be initiated by depressing a control switch mounted either on the torch body itself, or within a separate foot pedal housing which can also vary the supply of welding current. Depressing either control switch generates a HIGH FREQUENCY (HF) spark, which arcs across an ionised gap between the tungsten electrode and weld joint to start the weld. A high frequency start has an advantage over scratch start in that it prevents any direct contact between the electrode and the cast (direct contact can result in tungsten contamination of the weld). The on/off gas flow to the torch is automatically controlled by the same solenoid switch gear control that initiates the welding current. In addition, a timer mechanism often allows the shielding gas to flow briefly before and after arc initiation/shutdown, further protecting the weld from contamination and improving joint quality.
Once the arc is initiated, the welder maintains a short arc gap between the tungsten electrode and workpiece (of about 3/16 “ or 3 mm), carefully avoiding any physical contact which could cause contamination of the weld pool and require the re-grinding of the electrode. A matching filler rod alloy is introduced into the weldpool to build up the joint, this is done with a LEFTWARDS welding technique (as described earlier for oxy-gas welding).
The TIG welding process can be used for joining all the common metals and alloys used for making cast sculptures. Also, like gas brazing, TIG offers a means of joining dissimilar metals – provided an appropriate filler rod composition is used. Unlike the OXY-GAS welding of non-ferrous casts, the use of a flux protection is not necessary when TIG welding, the inert gas shield alone prevents any atmospheric contamination of the weld pool and surrounding area.
The vast majority of both cast and wrought metals, including copper alloys (bronzes), steels (low carbon, low alloy and stainless), nickel and silver alloys are TIG welded with a direct current in the ELECTRODE NEGATIVE mode (DCEN). DCEN means that when using a DC output welding set, the torch lead is connected into the negative (-) terminal on the welding set and the return lead is connected to the positive (+) terminal. When welding in the DCEN mode, the electrode concentrates the arc in the weldpool, carrying up to eight times the current of the alternative option, direct current ELECTRODE POSITIVE (DCEP) mode which concentrates heat in the tungsten electrode, which can overheat. If a manually operated HIGH FREQUENCY control switch is available on the welding panel, this should be set to the START ONLY position when welding in the DCEN mode.
Alloys that are based on aluminium and magnesium parent metals are TIG welded in the AC mode. The high frequency facility on the welding set (a requirement for AC welding), is manually switched to the CONTINUOUS position if the equipment does not automatically do this for the operator when the AC/DC SHUNT is set at AC. When in AC mode, the welding current fluctuates regularly between the electrode negative and electrode positive poles, concentrating the heat output into the weldpool for half a cycle, and into the tungsten electrode for the other half of the cycle. This current characteristic, in combination with the continuous high frequency background current, cleans and disrupts the REFRACTORY OXIDE that typically forms on the surface of light metals like aluminium. Tungsten electrodes for welding aluminium contain small additions of ZIRCONIUM OXIDE (up to about 2%), and are normally larger in diameter than the THORIATED OXIDE electrodes used for DCEN welding the same thickness of other metals such as steels or bronzes. This difference is principally due to the electrode’s requirement to transmit higher AC welding currents (necessary to initiate a weld pool in these light, conductive alloys).
Primary disadvantages of using a TIG welding process include the high initial cost of purchasing suitable equipment and the relatively slow speed at which a weld can be deposited when compared to other electric arc processes. To weld a full range of cast alloys, a set capable of both AC and DC outputs, complete with a high frequency facility, is essential. If necessary, an optional AC high frequency (HF) unit can often be added onto a suitable DC transformer as an accessory to enable aluminium welding.
A good electrical mains power supply is required for TIG welding heavy castings, and most professional foundries have a 415v/three phase supply to power a welding set capable of not less than 200 ampere (A) output (with 300A+ being preferred). A water/antifreeze coolant mix is circulated either direct from the water mains, or else recycled from a self contained reservoir. Nearly all TIG sets can be switched to MMA (STICK welding) mode, with the simple substitution of the TIG welding lead and torch for a MMA lead and electrode holder. Some TIG welding equipment is built around a modular multi-process system which is run off a common transformer/generator, this equipment is usually capable of supporting an added on MIG set.
MMA STICK WELDING >
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