When welding using a fusion process, the edges of a component are melted together to form weld metal.

Table: Heat source, mode of shielding, thickness range and metal deposition rates for various fusion processes

Process	Heat source	Shield	Parent  metal  thickness mm	Deposition  rate kg/hr
Arc
MMA	Arc	Gas/flux	1-100	1-4
MIG	Arc	Gas	0.5-100	1-8
TIG	Arc	Gas	0.1-100	1-4
SAW	Arc	Flux	5-250	5-20
ES/EG	Resistance/arc	Flux/gas	5-250	5-20
Stud	Arc	–	4-20	–
Gas
Oxyfuel	Flame	Gas	0.6-10	1-2
Power beam
Laser	Radiation	Gas	0.2-25	–
EB	Electrons	Vacuum	0.2-250	–
Resistance
Spot/Seam	Resistance	–	0.2-10	–
Thermit
Thermit	Chemical	Gas	10-100	–

Table 2 shows heat source, mode of shielding, thickness range and metal deposition rates for a range of fusion processes. Although fusion welding is one of the simplest joining techniques, problems likely to occur include porosity in the weld metal, and cracking in either the weld or heat affected zone (HAZ). Porosity is avoided by ensuring adequate shielding of the weld pool and, for materials such as aluminum, the addition of filler wire.

Consideration of the joint design and the chemistry of the weld metal will prevent weld metal cracking. HAZ cracking which might be caused by hydrogen, is avoided by using low hydrogen consumables and controlling the heat input and the rate of cooling of the parent metal.