Because for GTA welding, the electrode is not consumed as part of the weld deposit. This means filler metal is added separately and can be manipulated by the welder. The solid wire can be matched to the base metal composition with no supplemental additives, as is with other welding processes that use flux.
Second point is that GTA welds are shielded by gas versus slag (molten flux). This eliminates potential inclusions and slag pockets resulting in high quality GTA weld deposits.
GTAW or TIG uses a small, sharp, non-consumable electrode and a shielding gas that keeps the arc very tight. This allows for less heat input into the weld and a smaller heat affected zone (HAZ). With the tighter arc you get good penetration into the base metal.
A GMAW arc in spray transfer will give you good penetration and uses basicly the same filler metal but has a larger arc, a larger heat input and a larger HAZ.
bholland,
I'm looking at a narrow groove, automatic orbital, GTAW deposited weld cross section and I can assure you that the HAZ is not smaller than similar weld deposits made with GMAW spray arc that I have examined. HAZ size is also be affected by the total heat input - travel speed being a primary variable and travel speed with GTAW, especially manual GTAW, is quite slow compared to other processes.
One aspect often ignored is energy efficiency of a process, and GTAW can be extremely low. This has a lot to do with the filler metal being independent of the arc. The HAZ is not necessarily smaller just because the weld bead is.
Distortion (shrinkage, angular) also increases with manual GTAW.
