Because the filler does not carry current, TIG rod comes in straight 36 in lengths rather than spools.
Alloy selection matches base metal. ER70S-2 and ER70S-6 for mild steel; ER80S-B2 for chrome-moly; ER308L, ER316L and ER347 for stainless; ER4043 and ER5356 for aluminium; ERCuSi-A for silicon bronze on sheet metal and dissimilar joints. The classification number is the specification, and using the wrong one changes both strength and corrosion behaviour.
Diameter is chosen for the joint, not for the machine. 1/16 in handles most thin to medium work; 3/32 in is general purpose; 1/8 in and above are for heavy sections where a large puddle is needed. Too large a rod for the heat available causes cold lap; too small a rod cannot fill the joint fast enough and leads to excessive heat input.
Rod surface condition matters. Oxides, oil and dirt on the rod go straight into the weld. Stainless and aluminium rod should be kept clean and stored dry.
