The most common type of welding is done between similar metals. Whether you’re MIG, TIG or stick welding, you’re generally working with the same base materials. However, there are cases where two different metals need to be welded together, and that’s where it gets tricky.

There are a number of factors that will determine whether your base materials are even compatible, what filler you’ll need to join them, and the process involved to produce a good weld.

First things first, though: what counts as a dissimilar metal?  

What Are Dissimilar Metals?

When it comes to welding, dissimilar metals aren’t just steel, stainless steel and aluminium. It can also include working with different steels, including mild steels, high-carbon steels, and low-carbon steels. The metal properties of each of these differ enough that a filler material will match one more so than the other.

Dissimilar metals also include more uncommon metals like nickel and copper, which may need to be welded directly onto or can be used as ‘transition’ metals to help match other incompatible metals.

Why Would You Weld Dissimilar Metals?

Most welding is done between like metals. It’s much easier to weld similar-grade stainless metals together than it is to weld stainless to carbon steel, and it generally requires less prep and research to ensure the weld doesn’t fail later on.

That isn’t always the case, however. Some industries and applications require joining dissimilar metals due to material availability, cost-effectiveness or the need for specific properties in the finished weld.

A few examples of dissimilar metals include welding stainless balustrade posts to structural steel in the construction industry or stainless exhaust pipes to cast iron engine blocks in the automotive industry.

What You Want to Know Going Into It

What Materials You’re Welding

Where possible, know your base plate’s chemical makeup, or at least have an idea of what you’re trying to weld – it makes choosing a filler metal much easier if you know what you’re working with.

A couple of ways you can check your metal chemistry are: 

• Perform a spark test
• Get a material sample to be analysed 
• Consult with the original manufacturer or blueprints for the metal grades
• Use a magnet to determine if it is ferrous or non-ferrous (ferrous metals contain iron, non-ferrous metals don’t)

In some cases, however, it’s unlikely to know what exactly you’re working with, especially when it comes to repair jobs.

What Are the Requirements for the Finished Weld?

What is the welded part needed for? The final application of your welded component will impact how it is done, the filler material, and the post-weld prep of the joint.

If the weld is going to experience high stress or high temperatures, the filler metal properties need to be able to withstand those conditions.

If the joint needs to be corrosion resistant, your filler may need to provide that, or post-weld protection or paint may need to be applied afterwards to prevent future weld failure.

Is Preheating Required?

Do either of your metals need to be preheated? Do they each have different preheating requirements?

It’s recommended to use the preheat and interpass (multi-pass) temperature instructions for the higher tensile strength metal, as these are more critical and failure to do so can lead to cracking and loss of strength in some materials.

If the difference between the required preheats is too big (e.g. 300°C vs 100°C), it can potentially damage the material that needs less heat. An average between the two may be the best way to weld the metals together while minimising the risk of failure.

Things to Consider When Welding Dissimilar Metals

Thermal Conductivity & Expansion

How heat conductive a metal is and its coefficient of thermal expansion plays a role in how well two metals will weld. The coefficient of thermal expansion refers to how much a metal’s size or volume changes as its temperature changes.

For example, carbon steel has good heat conduction and a lower thermal expansion coefficient in comparison to stainless steel, which has poorer heat conduction and a higher thermal expansion coefficient.

When welding dissimilar metals, the closer the coefficient of thermal expansion, the better. If the mismatch in expansion rates is too big, the changes in temperature around the joint can lead to thermal stresses, warping, or even cracking during heating or cooling cycles.

Weldability of the Materials

The weldability of a metal refers to how easily it can be welded to create a strong, durable joint. A metal with high weldability can be easily welded with minimal defects. In contrast, a metal with low weldability may be challenging to weld or require special techniques to achieve a satisfactory joint.

Weldability is determined by Carbon Equivalency (CE), which takes into consideration a material’s chemical composition, mechanical properties and metallurgical compatibility with the welding process. The CE also helps determine the cracking susceptibility, necessary preheat, and interpass temperature control needed for each metal.

Solubility of the Materials

The solubility of a metal is its ability to dissolve in a solvent. When two metals are welded together, they melt and then solidify to form a joint. While using the fusion welding process, the metals should have similar solubility to be successful.

If the solubility of the two metals is similar, they’ll dissolve into each other and mix when they melt. However, if there is little or no solubility, the joint won’t weld, as the metals won’t dissolve into each other.  

In cases where the solubility isn’t a close enough match, a third metal that is soluble with the other two can be added to produce a successful weld.  

Intermetallic Compounds

When two different metals are successfully welded together, a ‘transition zone’ is formed, producing a single metal with intermetallic compounds. These compounds can change the overall structure and properties of the material, including its crack sensitivity, resistance to corrosion and ductility.

Where these compounds negatively impact the weld joint, a buttering layer (a third metal soluble with both) can provide compatible weld metal between the two dissimilar metals.

Corrosion Resistance

How susceptible is each metal to corrosion? Because welding dissimilar metals produces intermetallic compounds which can make the joint less resistant to corrosion, the original corrosion resistance of each base metal needs to be taken into consideration.

An electrochemical scale can be used to determine a metal’s susceptibility to corrosion. If the two metals being welded are too far apart on this scale, corrosion will likely be a serious problem in the final joint.

Melting Temperature

What is the melting temperature of each metal? This is more important when using a welding method involving heat, like fusion welding, as one metal may become fluid long before the other.

For example, carbon steel melts at roughly 1425-1540°C, whereas aluminium melts at 660°C.

How to Weld Dissimilar Metals

There are several ways that dissimilar metals can be welded together, though the preferred method will depend entirely on which metals you’re trying to join.

Unfortunately, there isn’t a one-size-fits-all, as all the above properties need to be considered to determine the appropriate filler material and weld technique, as well as whether the two can be joined directly together or if a third metal needs to be introduced.

Fusion Welding

The first way to weld dissimilar metals is by fusion welding. Fusion welding includes traditional methods such as TIG, MIG, and stick, which use a filler metal to create the weld.

As well as being compatible with both base metals, your filler metal choice will need to match the lower-strength metal’s tensile, or it won’t hold up under the strain after it’s welded.

‘Overmatching’ your filler metal (matching it to the higher-strength metal) can result in too much stress on the lower-strength material of the weld, which can potentially cause failure, usually at the toe of the weld. This is only a concern when matching steels, as other metals like aluminium and copper have wildly differing tensile strengths.

Other times, the metals may need a buttering layer or composite insert in order to be fusion welded, and your filler will need to match this metal instead.

Low-Dilution Welding

If standard fusion welding methods are not an option, you can try low-dilution welding instead. Low-dilution welding doesn’t melt the surface of each material or penetrate as far as traditional fusion welding does, keeping the mixing of the two metals to a minimum and reducing the intermetallic layer.

Low-dilution welding methods include laser welding, pulsed arc welding and electron beam welding, none of which need filler.

Laser welding uses a laser beam as the heat source to create a high-quality weld with minimal dilution between the base metals.

Low-dilution pulsed arc welding is a variation of the pulsed arc welding process, which minimises the dilution by carefully controlling the welding parameters, including the pulse frequency, peak amps, and base amps.

Electron beam welding uses a focused, high-energy electron beam as the heat source, which is generated by an electron gun in a vacuum chamber. When the beam strikes the material’s surface, it transfers kinetic energy to the material, creating heat that melts the metal with minimal dilution.

Electron beam welding is especially good for dissimilar metals as the high energy avoids issues that occur from different thermal conductivities. However, it does require specialised equipment and speed from the welder to be successful.

Non-Fusion Joining

If fusion and low-dilution welding methods aren’t viable for your chosen metals, the final option is non-fusion welding. Non-fusion welding, also known as solid-state welding, is the process of joining metals through pressure rather than melting them.

Non-fusion welding methods include explosion welding, friction welding and diffusion welding. Like low-dilution welding, a filler metal isn’t needed to join the metals.

Explosion welding works by accelerating one of the materials at high velocity with the help of chemical explosives. The weld is made in milliseconds, severely reducing the heat input and eliminating the intermetallic and heat-affected zone. However, it requires specialised equipment to be completed and is most commonly used for cladding.

Friction welding generates heat through mechanical friction, which softens the metals and creates a bond. Because the metals don’t reach their melting point, they don’t mix like they would when using fusion welding, minimising the intermetallic and heat-affected zone.

Diffusion welding, also known as diffusion bonding, joins materials by applying heat (roughly 50-75% of the melting temperate of the metals) and pressure to create a bond through atomic diffusion across the joint surface.  

Buttering Layer

Some welds are impossible without a ‘transition layer,’ so a third material that is soluble with both is used as an intermediary metal. This transition metal is layered on the surface (called ‘buttering’ or ‘surfacing’) to join the two incompatible metals.

For example, stainless steel and copper are incompatible metals; their solubility is too far apart to form a weld. In this case, a third metal, like nickel, can be added to the weld surface of both pieces of metal.

Nickel is soluble with both stainless steel and copper, so once each piece is buttered, a nickel filler material can be used to weld the dissimilar metals, as it technically becomes a weld between like metals.

Composite Inserts

Besides buttering, the second way you can add a transition layer to dissimilar metals is by adding a piece of metal between them at the weld joint, known as a composite insert. This third piece of metal can be made from a single material or a combination of materials and should be compatible with both base pieces. The composite insert is generally joined using a welding process that doesn’t involve heat (non-fusion welding).

Adding a composite insert to both pieces of base metal means the weld becomes a like-to-like weld rather than one between dissimilar metals.

Common Steel Combinations & the Filler Materials for Them

One of the most common cases of welding dissimilar metals is welding stainless steel to mild steel. Welding mild and stainless steel together is arguably one of the easier joinings of dissimilar metals.

It can be done with the MIG, TIG and stick processes, with a dedicated filler material designed explicitly for this application. 312L and 312Lsi filler metals (as a MIG wire, TIG rod or stick electrode) are made specifically for welding dissimilar metals and steels of unknown composition. You can also use 309L and 309Lsi as filler metals, as they also work well on dissimilar metals.

Other dissimilar steel combinations include:

A36 steel (low-carbon) to A514 steel (high-strength), which can be joined using E7018 low-hydrogen stick electrodes, ER70S-6 solid wire or E71T-1 flux-cored wire with the stick and MIG processes.

A36 steel (low-carbon) to AISI 4130 steel (low-alloy), which can be joined using E71T-1 flux-cored wire or E7018 low-hydrogen stick electrodes with the MIG and stick processes.  

A36 steel (low-carbon) to Abrasion Resistant (AR) steel plate (which has varying chemical and mechanical properties) can be filled with 70ksi filler metals like E7018 low-hydrogen electrodes.

However, determining the AR manufacturer will help with picking a suitable filler material.

Welding Other Dissimilar Metals

Copper to Other Metals

Copper and copper-base alloys can be welded to mild and low-alloy steels and stainless steels. 
For thin metals, TIG can be used with a high-copper-alloy filler rod, with the arc directed at the copper (to reduce the amount of iron picked up), and using a pulse to assist.

For thicker metals, buttering the steel with the same high-copper-alloy filler material and then welding the two together with minimal penetration to the steel will work.

You can also butter the copper piece with a nickel-base material instead (adding extra layers on thicker metal). MIG, TIG and stick welding can all be used to weld this mix. Your choice will depend on how thick the base metals are.  

Nickel-Base Alloys to Steels

Nickel-base alloys can be welded to mild and low-alloy steels, plus stainless steels, using any of the arc processes with a nickel filler.

What Does a Good Weld Look Like on Dissimilar Metals?

A good weld between dissimilar metals is one that is as strong as the weaker metal and should have adequate tensile strength and ductility with as small an intermetallic zone as possible to prevent joint failure. 

Welding dissimilar metals can be extremely challenging. While some combinations, like stainless steel to mild steel, can be done with just a bit of practice, other metal combinations can be incredibly specialised and require training, research and the necessary equipment before they can be achieved.