TIG Rod Finder

Both are mild steel TIG rods. A15 (ER70S-2) is triple deoxidised with aluminium, titanium and zirconium, so it tolerates light rust and mill scale and gives sound root runs on pipe. A18 (ER70S-6) carries more silicon and manganese, which makes the pool more fluid and leaves a smooth, bright finish on clean steel. If you are unsure of the surface condition, A15 is the safer pick; for tidy work on clean, ground steel, A18 looks better.

Match the filler to the steel. 308L suits 304 and 304L, the most common stainless. 316L adds 2 to 3% molybdenum for better resistance to salt and chemicals, so use it on 316; it is also fine on 304. The reverse is not true: 308L on 316 leaves a weak spot for corrosion. 309L is over-alloyed for joining stainless to mild steel, where the weld mixes with the carbon steel side. The "L" means low carbon, which stops the weld area rusting at the grain boundaries.

5356 (AlMg5) is the all-rounder for 5xxx and 6xxx sheet and extrusion: strong, stiff enough to feed well, and it stays a good colour match after anodising. 5183 (AlMg4.5Mn) is stronger again and is the choice for 5083 marine plate. 4043 (AlSi5) flows more easily and resists hot cracking, which suits castings and some 6xxx work, but it turns dark grey when anodised. Never use 4043 on 5xxx alloys with more than 2.5% magnesium: the weld can become brittle.

TIG rods usually come as 1m lengths in sealed tubes sold by weight. A 1kg pack suits occasional repairs and lets you try a grade. 2 to 2.5kg is the common aluminium pack, because aluminium is light and the same weight gives far more rods. 5kg is the workshop size for steel and stainless and is the best value per kg. Keep rods in their tube with the cap on: dust, grease and moisture on the rod end up as porosity in the weld.