As with other arc welding procedures, a good MIG/CO2 welding procedure starts with correct edge preparation and joint fit-up. The joint surfaces must be free from rust, scale grease, oil, paint and other foreign materials. For making full penetration joints by welding with spray transfer technique from both sides, it is necessary to gouge out the root from the second side before starting to weld that side. When welding is done only from one side, suitable weld backing must be provided. Sometimes weld backing can be avoided by making the root pass with the short-circuiting technique to obtain uniform penetration and depositing the fill-up passes by high current spray transfer technique.
The welding equipment must be assembled and the welding parameters set according to the manufacturer’s instructions. All gas and water connections must be absolutely leak - proof. If the shielding gas gets contaminated with air or water, the arc becomes erratic and pores appear on the weld.
The gun nozzle size and the shielding gas flow rate must be correctly set according to the material being, welded and its joints design. Some joint designs demand longer nozzle-to - work distance than normal; in such cases one must use higher gas flow rates than those recommended by the equipment manufacturer or as specified in standard procedures, and a gas nozzle of adequate size to cover the welding area. On the other hand, smaller nozzle sizes may be used for welding in confined areas or in the root of a thick joint. The electrode-feed rolls and the contact tube must be compatible with the size and composition of the electrode, as recommended by the manufacturer. If the contact tube is worn in usage, it must be replaced before the gun starts getting heated due to bad electrical contact between it and the electrode.
Electrode extension is the distance between the end of the contact tube and the gas nozzle opening, which is between 6.4 and 9.5 mm for normal spray-type welding. In special applications, the contact tube may be flush with or protruding from the gas nozzle. For example, when using the short-circuiting arc, the contact tube may extend 3 mm beyond the end of nozzle. Further guidance on procedures using contant-voltage power source is given in Table 8.6.
Table 8.6. Guidance on MIG/CO2 welding procedure
The wire-feed-speed determines the welding current.
Following the setting of Table 8.6, trial bead welds should be deposited to arrive at correct arc voltage and the electrode-feed rate (current). In the short-circuiting procedure, the choke should be finally adjusted to obtain good arc start and a stable arc with minimum spatter.