TIG Welding Dissimilar Metals: Filler Selection & Joint Strategies

TIG welding dissimilar metals requires matching the filler rod to the more demanding of the two base metals, managing the thermal expansion mismatch between them, and often directing more heat toward the higher-melting-point material. The most common dissimilar combination in fabrication is carbon steel to stainless steel, welded with ER309L filler on DCEN polarity.

Not every metal combination is weldable. Some pairs form brittle intermetallic compounds that crack immediately. Others require nickel-based intermediary layers to bridge the metallurgical gap. Knowing which combinations work, which need special procedures, and which to avoid entirely saves failed joints and wasted material.

Weldable Dissimilar Combinations

Steel to Stainless Steel

This is the most frequently encountered dissimilar weld in general fabrication. Structural transitions, piping connections, brackets mounted on stainless equipment, and repair work regularly involve joining carbon steel to austenitic stainless.

Why ER309L

ER309L has higher chromium (23-25%) and nickel (12-14%) than ER308L. The extra alloying compensates for dilution from the carbon steel side. When the arc melts both base metals and the filler, the carbon steel dilutes the chromium and nickel in the weld pool. ER309L starts with enough surplus that the final weld chemistry stays austenitic (non-magnetic, ductile) instead of forming brittle martensite.

Using ER308L on a steel-to-stainless joint doesn’t provide enough chromium and nickel to overcome the dilution. The weld may form martensite at the fusion line with the carbon steel, creating a hard, crack-prone zone.

Procedure

1. Clean both base metals. Remove mill scale from the carbon steel and ensure the stainless is free of iron contamination.
2. Set up on DCEN with 100% argon at 15-20 CFH.
3. Direct the arc slightly more toward the carbon steel. Carbon steel has a higher melting point and thermal conductivity than austenitic stainless, so it needs more heat to achieve fusion.
4. Feed ER309L filler into the leading edge of the puddle, same as any other TIG joint.
5. Watch for consistent wetting on both sides of the joint.

Settings

Use the amperage for the thinner of the two materials. If you’re joining 1/8" mild steel to 1/8" stainless, use the stainless settings (90-130A) because stainless retains more heat. If the carbon steel side is thicker, you may need to bump amperage 5-10% to achieve fusion on that side.

Considerations

The joint area on the carbon steel side will show a heat-affected zone that may develop light surface rust over time. The stainless side retains most of its corrosion resistance. The weld itself has intermediate corrosion properties. For outdoor applications, consider painting or coating the carbon steel side and the weld.

Copper to Steel

Joining copper to carbon steel is challenging because of extreme differences in thermal properties:

• Copper melting point: 1,981°F
• Steel melting point: 2,500°F
• Copper thermal conductivity: 226 BTU/hr-ft-°F (8x higher than steel)

The steel side reaches welding temperature long before the copper. By the time the copper is hot enough to melt, the steel can be overheated and distorted.

Methods

ERCuSi-A (silicon bronze): The easiest approach. Silicon bronze melts at about 1,800°F, below both base metals. It brazes onto the copper surface and fuses to the steel. The joint is technically a braze-weld, not a full fusion weld. Strength is lower than a fusion weld but adequate for many structural applications.

ERNi-1 (nickel): For a true fusion joint, use nickel as an intermediary. Butter the copper surface with a layer of nickel filler first, then weld the nickel-buttered copper to the steel using ERNi-1 or ERNiCr-3. Nickel bonds well to both copper and steel.

Procedure for ERCuSi-A method:

1. Preheat the copper side to 300-500°F depending on thickness. The steel side doesn’t need preheat.
2. Direct 60-70% of the arc toward the copper. It needs more heat to form a bond.
3. Feed ERCuSi-A filler into the puddle. The bronze flows well and wets both surfaces.
4. Travel speed should be moderate. Too slow overheats the steel. Too fast doesn’t heat the copper enough.

For detailed copper TIG settings, see TIG welding copper.

Nickel Alloys as Intermediaries

Nickel-based filler alloys are the universal problem-solvers for dissimilar metal welding. Nickel is metallurgically compatible with steel, stainless, copper alloys, and many other metals. It forms ductile welds that absorb thermal expansion mismatch without cracking.

ERNiCr-3 (Inconel 82)

The most widely used dissimilar metal filler. Contains approximately 72% nickel, 20% chromium, and 3% niobium. Compatible with:

• Stainless to carbon steel (alternative to ER309L when higher ductility is needed)
• Stainless to nickel alloys
• Nickel alloys to carbon steel
• Dissimilar stainless grades (e.g., 304 to 310)

ERNi-1 (Pure Nickel)

99% nickel filler. Used for:

• Cast iron repair and joining
• Nickel 200/201 to carbon steel
• Butter layers on copper for subsequent welding to steel
• Any joint where maximum ductility is needed in the weld metal

ERNiCrMo-3 (Inconel 625)

Contains molybdenum for corrosion resistance. Used for:

• Inconel 625 to any steel or stainless
• High-temperature dissimilar joints
• Chemical processing equipment where corrosion resistance at the weld is critical

Butter Layers

A butter layer is a preliminary weld deposit on one base metal that creates a compatible surface for welding to the other base metal. Buttering is used when direct welding between two metals is problematic.

Why Butter

Some metal combinations form brittle intermetallic compounds at the fusion line. Buttering one or both surfaces with a compatible intermediary (usually nickel) creates ductile fusion lines on both sides of the joint.

Butter Layer Procedure

1. Clean the base metal surface.
2. Deposit one or two layers of the butter filler (e.g., ERNi-1) on the surface using low heat input.
3. Allow the butter layer to cool completely.
4. Grind the butter layer smooth to create a flat joint preparation surface.
5. Weld the buttered surface to the other base metal using a filler compatible with the butter layer and the second base metal.

Common buttering applications:

• Nickel butter on carbon steel before welding to stainless (reduces dilution and martensite risk)
• Nickel butter on copper before welding to steel (nickel bonds to both metals)
• Nickel butter on cast iron before welding to steel (nickel absorbs carbon migration)

Thermal Expansion Mismatch

Different metals expand at different rates when heated. When two dissimilar metals are welded together and the assembly heats up in service, the joint experiences stress from differential expansion.

The larger the CTE mismatch between two metals, the more stress the joint experiences during thermal cycling. Carbon steel (6.5) welded to 304 stainless (9.6) has a significant mismatch. This stress concentrates at the weld interface, especially at the root and toes.

Managing Expansion Mismatch

• Use ductile filler. Nickel-based fillers deform plastically to accommodate some mismatch instead of cracking.
• Design for flexibility. Where possible, use joint designs that allow slight movement (expansion joints, bellows, slip connections).
• Minimize joint restraint. Heavily restrained dissimilar joints concentrate stress. Allow the assembly some freedom to move.
• Consider operating temperature range. A joint that works fine at room temperature may crack during thermal cycling between -20°F and 500°F if the CTE mismatch is large.

Combinations to Avoid

Some metal pairs are not practically weldable because they form brittle intermetallic compounds, have extreme property mismatches, or react chemically during welding.

Aluminum to Steel

Aluminum and iron form extremely brittle intermetallic compounds (Fe2Al5, FeAl3) at the fusion line. These compounds crack under any load. Direct TIG welding of aluminum to steel does not produce a serviceable joint. Use mechanical fastening (bolts, rivets) or transition joints (bimetallic strips manufactured by explosion welding or roll bonding).

Aluminum to Copper

Similar to aluminum-to-steel: brittle intermetallic compounds form at the interface. Not TIG weldable. Use mechanical connections or explosion-welded transition pieces.

Titanium to Steel

Titanium forms brittle intermetallics with iron. Direct welding fails immediately. Vanadium or tantalum intermediary layers can bridge the gap, but these are specialized procedures beyond typical shop capability. Use mechanical fastening for titanium-to-steel connections.

Titanium to Aluminum

Titanium and aluminum form intermetallics. Not directly weldable with TIG. Specialized friction stir welding or explosion welding processes can create transition joints, but these are industrial processes.

Common Problems in Dissimilar Welding

Cracking at the Fusion Line

The most common failure in dissimilar welds. The fusion line (where the weld meets the base metal) experiences the highest stress from thermal expansion mismatch and the most dilution-related metallurgical changes.

Prevention: Use a more ductile filler (nickel-based instead of steel-based). Butter the more crack-sensitive surface. Reduce restraint. Preheat if applicable.

Galvanic Corrosion

When two dissimilar metals are in contact with an electrolyte (water, process fluids), the more active metal corrodes preferentially. This isn’t a welding defect per se, but the weld creates the electrical connection between the two metals that enables galvanic corrosion.

The farther apart two metals are on the galvanic series, the more aggressive the corrosion. Carbon steel welded to stainless steel in a wet environment corrodes the carbon steel rapidly. Paint, coatings, or cathodic protection can mitigate this.

Uneven Melting

If one base metal melts at a significantly lower temperature than the other, the low-melting metal overheats while the high-melting metal is still solid. Direct the arc preferentially toward the higher-melting metal. The splash of heat from the arc is enough to fuse the lower-melting metal. This is especially important for copper-to-steel and aluminum-to-anything joints.

Carbon Migration

When carbon steel is welded to stainless or nickel alloys and the assembly is heated in service (above 900°F), carbon migrates from the steel into the stainless or nickel. This creates a decarburized (soft, weak) zone in the steel and a carburized (hard, brittle) zone in the stainless. Nickel-based buttering layers help block carbon migration.

Best Practices Summary

1. Always use the filler recommended for the specific dissimilar combination. Don’t guess.
2. Direct more heat toward the higher-melting or higher-conductivity base metal.
3. Use the amperage appropriate for the thinner or more heat-sensitive material.
4. Keep heat input as low as practical to minimize the size of the metallurgical interaction zone.
5. Consider nickel-based fillers whenever the standard filler for a dissimilar pair is questionable.
6. Test a sample joint before committing to a production weld. Bend test and visual inspection of the fusion line catch most problems.
7. Don’t assume two metals can be welded just because they can each be welded individually. Check the combination first.

For filler rod selection details across all base metals, see the TIG filler rod guide.