TIG Welding Filler Rod Guide: Types, Sizing & Selection by Base Metal

Match the filler rod to the base metal composition, size the rod diameter to the joint thickness, and keep everything clean. That’s filler selection in three rules. ER70S-2 covers mild steel. ER308L covers 304 stainless. ER4043 covers general aluminum. ER5356 covers structural aluminum. These four fillers handle about 85% of all TIG work.

The filler rod is the second most important consumable after the tungsten electrode. Wrong filler causes cracking, porosity, reduced corrosion resistance, or mechanical failure. Right filler matched to clean base metal produces welds that are as strong as or stronger than the parent material.

Steel Filler Rods

ER70S-2

The preferred TIG filler for mild steel and low-carbon steel. Contains triple deoxidizers: silicon, manganese, aluminum, titanium, and zirconium. This deoxidizer package produces clean, porosity-free welds even on base metal with light mill scale or surface rust.

• Tensile strength: 70,000 psi
• Base metals: A36, 1018, 1020, A500, and other mild/carbon steels
• Common diameters: 1/16", 3/32", 1/8"
• Why it’s better than ER70S-6 for TIG: Lower silicon content produces a less glassy bead, and the extra deoxidizers compensate for the TIG process’s lower dilution rate

ER70S-6

The standard MIG wire that also works as TIG filler. Higher silicon content (0.8-1.15%) improves puddle fluidity and wetting, especially on dirty base metal. Produces a slightly darker, glassier bead than ER70S-2.

• Tensile strength: 70,000 psi
• Base metals: Same as ER70S-2
• Why TIG welders use it: Widely available, often cheaper than ER70S-2, works adequately for non-critical work

ER70S-6 cut from MIG wire spools works in a pinch, but the wire’s cast (curl from the spool) makes it harder to feed smoothly. Purpose-cut TIG rod in 36" straight lengths feeds much better.

ER70S-3

Minimal deoxidizers. Requires perfectly clean base metal with no mill scale, rust, or contamination. Rarely used for TIG except in controlled laboratory or cleanroom welding. Not recommended for general shop work.

ER80S-D2

A low-alloy steel filler with 0.4-0.65% molybdenum. Tensile strength of 80,000 psi. Used for welding 4130 chromoly steel, especially when post-weld heat treatment is planned.

• Base metals: 4130, 4140, 4340 chromoly steels
• Key consideration: Higher crack sensitivity than ER70S-2. Requires preheat on material over 1/8" thick.
• When to use over ER70S-2: When you need matched strength to the 4130 base metal and will stress-relieve after welding

For a detailed comparison of ER70S-2 vs. ER80S-D2 on chromoly, see TIG welding chromoly.

Stainless Steel Filler Rods

ER308L

The standard filler for 304 and 304L stainless steel. The “L” means low carbon (0.03% max), which prevents carbide precipitation in the heat-affected zone.

• Base metals: 304, 304L, 301, 302
• Tensile strength: 75,000 psi
• Always use the L variant. No performance or cost penalty.

ER309L

Higher chromium (23-25%) and nickel (12-14%) than 308L. An overalloyed filler used for dissimilar metal joints.

• Base metals: 304 to carbon steel, dissimilar stainless grades, first layer of stainless overlay
• Not for: 304-to-304 joints (use 308L instead)

ER316L

Contains 2-3% molybdenum, matching 316/316L base metal composition.

• Base metals: 316, 316L
• Advantage over 308L: Maintains chloride corrosion resistance in the weld zone

ER347

Niobium-stabilized filler for welding 347 stainless steel. Prevents intergranular corrosion in high-temperature service.

For detailed stainless filler selection and heat control techniques, see TIG welding stainless steel.

Aluminum Filler Rods

ER4043

Silicon-based aluminum filler (5% Si). The most widely used aluminum TIG filler.

• Base metals: 6061, 6063, 3003, 4xxx casting alloys
• Characteristics: Flows smoothly, low crack sensitivity, bright shiny beads
• Tensile strength: 28,000 psi (as-welded)
• Not for: Parts that will be anodized (turns blotchy), 5xxx-series base metals, sustained service above 150°F

ER5356

Magnesium-based aluminum filler (5% Mg). Higher strength than 4043, preferred for structural applications.

• Base metals: 5052, 5083, 5086, 6061 (when strength or anodizing is needed)
• Characteristics: Stiffer rod, less fluid puddle, duller bead appearance
• Tensile strength: 38,000 psi (as-welded)
• Not for: Aluminum castings (crack-sensitive with casting alloys)

ER4047

High-silicon aluminum filler (12% Si). Very fluid, excellent crack resistance. Used for brazing and for welding thick aluminum castings where crack sensitivity is a concern.

ER1100

Pure aluminum filler for welding 1100 and 3003 base metals. Soft, ductile welds. Used in food-processing equipment and chemical tanks.

For the complete 4043 vs. 5356 decision matrix, see TIG welding aluminum settings.

Specialty Filler Rods

ERTi-2 (Titanium)

Commercially pure titanium filler for CP titanium base metals (Grades 1-4). Must be kept absolutely clean and used under full argon shielding.

ERTi-5 (Titanium Alloy)

Ti-6Al-4V filler matching the most common titanium alloy. Same extreme cleanliness and shielding requirements as ERTi-2.

ERCu (Deoxidized Copper)

For copper-to-copper joints. Contains deoxidizers (tin, silicon, manganese) to prevent porosity. Don’t substitute bare copper wire.

ERCuSi-A (Silicon Bronze)

Copper-silicon alloy for copper, brass, and decorative/art welding on steel. Produces golden-bronze colored welds. Lower melting point than ERCu.

ERNi-1 (Nickel)

For welding nickel 200/201 and as a butter layer when joining dissimilar metals. Also used for cast iron repair.

ERNiCr-3 (Inconel)

For nickel-chromium alloys (Inconel 600, 625) and as a transition filler between dissimilar materials.

For more on joining dissimilar metals, see TIG welding dissimilar metals.

Filler Rod Diameter Selection

The right filler diameter depends on base metal thickness, tungsten size, and joint type. Too thick wastes heat melting extra filler. Too thin melts before it reaches the puddle and gives you no control over deposition.

Rule of thumb: The filler rod diameter should be equal to or slightly smaller than the tungsten diameter. If the filler is thicker than the tungsten, you need excessive heat to melt it and the bead becomes humped and inconsistent.

When to Weld Autogenous (No Filler)

Autogenous TIG welding fuses the base metal edges together without adding filler. It works when:

• Material is thin (generally under 16 gauge)
• The joint is a butt joint with tight fit-up (no gap) or flanged edges
• The application is non-structural or the joint design compensates for lack of reinforcement
• You need to minimize heat input

Advantages of autogenous welding:

• Faster travel speed (no dipping rhythm)
• Lower heat input (not melting filler rod)
• No filler contamination issues
• Clean, flush bead surface

Disadvantages:

• No weld reinforcement (the bead may be concave)
• Requires perfect fit-up with zero gap
• Any gap becomes a weak point with no filler to bridge it
• Not suitable for code work requiring reinforcement

Autogenous welding is common for thin stainless tubing in food/pharmaceutical applications, edge welds on sheet metal, and cosmetic seams. On flanged edge joints, the flange provides extra material that acts as built-in filler.

Filler Rod Handling and Storage

Cleanliness

Contaminated filler rod is one of the top three causes of TIG weld defects (along with base metal contamination and gas issues). Follow these rules:

1. Wipe rods with acetone before each welding session. Don’t use shop rags that touched oil. Use clean paper towels or lint-free wipes.
2. Don’t touch the business end with bare hands. Oils from skin cause porosity, especially on aluminum filler.
3. Keep rods off the welding table. Ground clamp current can arc through rods touching the table surface, creating tiny burn marks that contaminate the next weld.
4. Don’t lay rods on concrete floors. Concrete dust, moisture, and contaminants transfer to the rod surface.

Storage

• Steel and stainless rods: Store in original tubes at room temperature. These are relatively tolerant of humidity but should stay dry.
• Aluminum rods: Most sensitive to moisture. Store sealed in a dry environment. Once opened, use within a few months. Extended exposure to humid air forms a hydrated oxide layer that causes porosity.
• Titanium rods: Store sealed. Clean immediately before use. Handle with nitrile gloves.
• Copper rods: Store dry. Tarnished copper filler works if wiped clean before use.

Cut Lengths

TIG filler rod comes in 36-inch straight lengths. Some suppliers offer 18-inch lengths for tight spaces. You can cut 36-inch rods in half with a cut-off wheel for bench work where full-length rods are unwieldy.

Don’t use filler wire cut from MIG spools as TIG rod for extended work. The curl (cast) from the spool makes it constantly curve away from the puddle. It works in a pinch but feeds poorly compared to straight rod.

Quick Reference: Filler to Base Metal

Common Filler Selection Mistakes

Using MIG Wire Classifications for TIG

MIG and TIG filler wire have the same AWS classification numbers (ER70S-2, ER308L, etc.) because the chemistry is identical. The difference is packaging. TIG rod comes in 36-inch straight lengths. MIG wire comes on spools. The metal itself is interchangeable, but the form factor matters for feeding.

Undermatching on Purpose Without Understanding Why

ER70S-2 on 4130 chromoly is technically an undermatch (70 ksi filler on 95 ksi base metal). This is acceptable and standard on thin-wall normalized tubing because the weld reinforcement compensates for lower filler strength. But on thick sections or heavily loaded joints, undermatching can cause the weld to fail before the base metal. Know why undermatching is acceptable for your specific application before choosing it.

Using the Wrong “L” Variant on Stainless

ER308 (not L) has higher carbon content (0.08% max vs. 0.03% max). Higher carbon improves high-temperature creep strength but causes carbide precipitation during welding, leading to intergranular corrosion. Unless the application specifically requires high-temperature creep resistance, always use the L variant: ER308L, ER309L, ER316L.

Neglecting Filler Rod Cleanliness

The best filler selection in the world doesn’t help if the rod is covered in shop grime. Wipe rods before use. Every time.