As welding progresses along the joint, the weld pool solidifies fusing the parent and weld metal together. Several passes or runs may be required to fill the joint or to build up the weld to the design size. Image courtesy of William Haley Engineering Ltd. Welding is a complex interaction of physical and chemical science. Correct prescription of metallurgical requirements and sound practical application is a prerequisite for successful fusion welds. The metal arc welding process uses an electric arc to generate heat to melt the parent material in the joint. The weld pool is susceptible to atmospheric contamination and therefore needs protecting during the critical liquid to solid freezing phase.
Protection is achieved either by using a shielding gas, by covering the pool with an inert slag or a combination of both actions. Gas shielded processes receive gas from a remote source which is delivered to the welding arc through the gun or torch. The gas surrounds the arc and effectively excludes the atmosphere. Precise control is needed to maintain the gas supply at the appropriate flow rate as too much can produce turbulence and suck in air and can be as detrimental as too little. Some processes use a flux, which melts in the arc to produce a slag covering which, in turn, envelops the weld pool and protects it during freezing. The slag also solidifies and self releases or is easily removed by light chipping.