Gas-shielded arc welding (GMAW), also known as arc welding, is an arc welding process in which the heat required to melt metal is generated by an arc between a consumable electrode and the metal (Figure 8.5). The electrode is a solid wire that is continuously fed through the arc in the weld pool and eventually becomes the filler metal.
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Wire type is selected to match the strength of the weld metal to that of the base metal.A mixture of inert gasses such as helium and argon is introduced into the weld zone to protect the arc, weld puddle, electrode and base metal adjacent to the weld from exposure to the atmosphere. The welding parameters controlled to produce acceptable welds are arc current, arc voltage, wire feed speed, electrode feed speed, current density, and preheat temperature. Preheating heats the base metal around the joint prior to welding.
is commonly used to reduce residual shrinkage stress and increase crack resistance in the weld area.
GMAW can be used to join dissimilar metals with similar melting points and metallurgical compatibility.A narrow melting point is required to achieve controlled fusion on both sides of the weld. Metallurgical compatibility is required to prevent cracking in the HAZ or base metals and to create a microstructure in the weld zone that ensures adequate joint performance and corrosion resistance. For example, when low carbon steels are welded to high alloy steels, the melting point can contain unacceptable levels of a very hard and brittle martensitic phase that reduces the strength of the joint.
With some metallurgically incompatible metals it may be possible to obtain a satisfactory weld with the appropriate filler material.
Steel and aluminum alloys are not suitable for arc welding because (1) there is a large difference between their melting points (see Table 8).3), (2) Iron is almost insoluble in aluminum and (3) brittle intermetallic compounds such as Fe2Al5 and FeAl3 are formed during welding. In addition, large differences in thermal properties such as coefficient of thermal expansion and thermal fullformcollection conductivity lead to internal shrinkage stresses after welding. For these reasons, fusion welds between steel and aluminum tend to crack and fracture in service.
Gas Tungsten Arc Welding
Gas Tungsten Arc Welding (GTAW), also known as inert gas arc welding, creates an arc between a non-consumable tungsten electrode and the parts to be assembled. As with the GMAW process, an inert gas shield is used around the arc to shield the weld pool. Tungsten is a good electrode material due to its high melting point of 3410°C. In the case of aluminum, the electrode itself serves to break up the oxide layer on the surface of the aluminum sheet. In some cases some filler may be gyanhindiweb required.When filler material is used, it is fed into the weld from a separate rod or wire rather than being passed through the electrode. The filler metal is melted by the arc and added to the weld pool. GTAW is slower than GMAW, but welds made from GTAW have a much better surface finish and require little or no post-processing because GTAW does not spatter.
