7.3.1 Shielding gases
The shielding gases, as with TIG welding, are the inert gases argon and helium or combinations of these two. Other, active, gases such as oxygen or nitrogen even in small amounts will give porosity and smutting problems. The most commonly used gas is argon which is used for both manual and some automatic welding. It is substantially cheaper than helium and produces a smooth, quiet and stable arc, giving a wide, smooth weld bead with a finger-like penetration to give a mushroom-shaped weld cross-section. Argon, however, gives the lowest heat input and therefore the slowest welding speeds. There is therefore a risk of lack of fusion defects and porosity on thick sections. Argon may also give a black sooty deposit on the surface of the weld. This can be easily removed by wire brushing. Sections of 3 mm thick butt welds using conventional and pulsed current are illustrated in Fig. 7.13. Thicker section butt and fillet welds are illustrated in Fig. 7.14. In these thicker section welds the characteristic finger penetration of an argon gas shield can be seen.
Helium increases the arc voltage by as much as 20% compared with argon, resulting in a far hotter arc, increased penetration and wider weld
7.13 (a) MIG, argon shielded 0.8mm wire, 3mm thick unbacked plate butt, flat position. (b) Pulsed MIG, argon shielded, 0.8mm diameter wire, 3mm thick unbacked plate butt, flat position.
7.14 (a) MIG, argon shielded, two pass, double sided, 12mm thick, flat position. (b) MIG, argon shielded, 15mm leg length fillet, 12mm thick plate, horizontal-vertical.
bead. The wider bead requires less critical positioning of the arc and assists in avoiding missed edge and lack of penetration-type defects. The hotter, slower cooling weld pool also allows hydrogen to diffuse from the molten weld metal, making this a method that may be used to reduce the amount of porosity. The increased heat also enables faster welding speeds to be achieved, as much as three times that of a similar joint made using argon as a shielding gas.
Helium, however, is expensive and gives a less stable arc than argon. Pure helium therefore finds its greatest use in mechanised or automatic welding applications. Helium shielded manual welds are illustrated in Fig. 7.15.
7.15 (a) MIG, helium shielded, two pass, double sided, 12mm thick, flat position. (b) MIG, helium shielded, 15mm leg length fillet, 12mm thick plate, horizontal-vertical.
For manual welding and some mechanised applications mixtures of argon and helium give good results with characteristics intermediate between the two gases. These mixtures are useful on thicker materials because they increase the heat input and provide a wider tolerance box of acceptable welding parameters than pure argon. They will also improve productivity by enabling faster travel speeds to be used. The most popular combinations are 50% and 75% of helium in argon. Typical welds using 50% helium/50% argon are illustrated in Fig. 7.16. These show weld bead shapes intermediate between the pure argon and pure helium welds in Figs. 7.14 and 7.15.
The last point to be made concerning gases is purity, already covered in Chapter 3, but worth re-emphasising because of the major effect that this has on weld quality. Shielding gases must have a minimum purity of 99.998% and low moisture levels, ideally with a dew point less than -50°C (less than 39 ppm H2O) - do not forget that this is at the torch, not at the outlet of the cylinder regulator!
7.3.2 Welding filler wire
The wire acts as both the filler metal and the anode in the welding arc. In order to do this the wire picks up the welding current by a rubbing contact between the wire and the bore of the contact tip. Filler wire diameters vary from 0.8 mm to 3.2 mm which results in a high surface area to volume ratio. This relatively large surface area requires the wire to be kept scrupulously clean since surface contamination will give rise to porosity. Wires should be stored in clean, dry conditions in their unopened packaging where possible. Wires that have been in store for a substantial period of time, e. g. 6 months or more, even when stored in their original packaging can deteriorate and give rise to porosity. If left on the welding machine overnight or over weekends they should be protected from contamination by covering the reel with a plastic bag. In critical applications it may be necessary to remove the reel from the machine and store it in a steel can between periods of use.
Condensation can form on the wire if it is brought into a warm fabrication shop from a cold store, and in conditions of high humidity moisture may once again form on the wire. Some power sources incorporate heaters in the wire feeder to prevent this from happening. If condensation is troublesome and this facility is not available, a 40 watt light bulb installed in the wire feeder cabinet provides sufficient heat to maintain the wire in a dry state.
It is possible to obtain wire cleaning devices that clip on the wire at the point where it enters the wire feed cable. These devices consist of a felt pad carrying a cleaning fluid which removes contaminants as the wire passes
7.16 (a) MIG, helium-argon shielded, two pass, double sided, 12mm plate. (b) MIG, helium-argon shielded, 15mm leg length fillet, 12mm thick plate, horizontal-vertical.
into the cable. They can be very effective at removing traces of grease and oils, dust, etc. on the surface of the wire. Better still is shaving the wire. This not only removes surface contaminants and oxides but hardens the wire, making it easier to feed and less likely to tangle.