Mastering the Art of Welding Dissimilar Metals

Welding is a powerful method for joining metals, but the process can become intricate when dealing with dissimilar metals. While the challenge is real, it’s certainly possible to weld dissimilar metals with careful consideration of their unique properties. This guide aims to simplify the complexities involved in welding different metals, with a focus on common scenarios like welding carbon steel to stainless steel.

Is Welding Dissimilar Metals Possible?

Yes, welding dissimilar metals is possible with the right precautions. The success of the weld depends on many factors. These include thermal conductivity, expansion rates, magnetic properties, metallurgical structure, and corrosion resistance of the metals involved. Although some combinations pose challenges, advancements in welding technology have introduced specific processes to overcome these obstacles.

Crucial Factors to Consider Before Welding Dissimilar Metals:

When it comes to dissimilar welding, many inexperienced welders ask themselves where to start. Most metals have their physical, chemical, and mechanical properties.

• The physical characteristics of the weld metal should be matched to the base metal. For example, different metals have different melting points and coefficients of thermal expansion. As a result, one can reach its melting point faster, then melt and flow away without fusing with the second piece. In addition, thermal expansion differences can make one piece extend more due to heat, leading to thermal cracking.

• When matching mechanical properties, the strength of the weld metal should be equal to or stronger than the weaker material. It is also desirable to match the ductility of the welded pieces, but it is not always possible.
• Retaining good corrosion resistance of the weld is crucial in pipe welding or any application where welds might be subjected to corrosion. The corrosion resistance should be matched to the least resistant base metal in the weld. If the weld is subjected to salt water, it should be more corrosion-resistant than both pieces to prevent galvanic corrosion.

Common Dissimilar Metal Welding Processes:

Welding dissimilar metals can be successfully done with one of the fusion welding methods, non-fusion welding or low-dilution welds.

1. Fusion welding includes the most common arc welding methods, such as gas metal arc welding (GMAW or MIG), gas tungsten arc welding (GTAW or TIG), Stick welding, Submerged Arc Welding (SAW), or Flux core welding (FCAW). As a hobbyist, your go-to choices for dissimilar metals are MIG or TIG welding method. The MIG welding process is more straightforward, but with TIG, you can control the filler metal deposition perfectly, which is crucial in dissimilar metal welding.

2. Low-dilution methods are the electron beam welding process, laser welding, and pulsed arc welding. These are commonly used to join delicate and thin dissimilar metals with no added filler metal.

3. Non-fusion welding includes friction welding, explosion welding, diffusion bonding along with brazing and soldering. Non-fusion and low-dilution methods are commonly used in heavy production and specific industrial applications. On the other hand, you will be more than fine with fusion welding in your everyday DIY, home shop, or repair welding projects.

Guide on How to Weld Dissimilar Metals:

Steel to Stainless Steel:

Weld Preparation:

Welding mild steel directly to stainless steel is not recommended since an exceptionally hard martensite phase of steel can occur. Martensite steel is prone to cracking, so welders usually bevel the pieces. Due to the nickel content in stainless steel, the root gap is larger, but the root face is reduced to promote wetting.

Stainless steels require a clean weld joint, so you must thoroughly remove any oil or grease. Any contaminants can introduce carbon to stainless steel, making it lose its corrosion-resistant properties. Meanwhile, carbon steel is susceptible to hydrogen cracking, so the base materials and fillers must be dry prior to welding.

If the carbon content surpasses 0.3%, preheating at 149°C will help both pieces heat evenly. The temperature of 205°C is used in severe conditions. Upon completion, the weld should be slowly cooled to allow hydrogen to diffuse from the HAZ, to reduce risks of porosity and cracking.

Specific pipeline applications may require joining galvanised steel and stainless steel. In that case, you must thoroughly remove the zinc coating. Burning zinc can cause liquid embrittlement, cracking, and toxic welding fumes, so cleaning it is crucial.

Filler Metal Selection: 

The filler metal selection in steel to stainless steel joining will differ depending on applications and service condition temperatures. As an everyday welder, you are likely to weld mild steel to 304(L) stainless steel at service condition temperatures below 427°C. In that case, common choices are higher alloy filler metal, such as type 309, with a ferrite number (FN) over 10, or type 312, with an FN over 25. Using a common 308 filler material for 304 stainless steels can cause quality problems due to iron dilution.

Once the service temperatures rise over 427°C, you will need a different approach. 309 and 312 filler are subjected to high-stress concentration at the steel-side fusion, which causes thermal fatigue failures. Therefore, more suitable filler material choices are AWS ERNiCr- 3 bare wire or AWS ENiCrFe-2 or ENiCrFe-3 electrodes. Nickel alloy fillers have a coefficient of thermal expansion (COE) between ordinary steel and austenitic stainless, which helps them battle thermal fatigue failures that are more likely with 309 or 312 fillers.

Shielding Gas Choice:

When joining mild steel to stainless, you want to exclude reactive gases such as oxygen from the mixture. Oxygen can react with the atmosphere causing imperfections and defects in carbon steel, but you can replace it with small amounts of CO2, which is semi-reactive.

Nitrogen in the mixture can decrease the ferrite content of the weld metal, which results in hot cracking. Therefore, you want to keep levels as low as possible.

Thermal Expansion: 

Both Thermal conductivity and thermal expansion of stainless steel and mild steel are significantly different, and these differences are what make welding dissimilar metals challenging.

Thermal expansion is defined as the change in length per degree temperature to length. Mild steel has a lower coefficient of 5.9 (10-6 in/(in oF)) compared to common stainless steel with 9.4. As a result, stainless steel will change its length more than mild steel during the welding, which can lead to residual stress.

The best way to battle the differences between thermal properties is to tack the ends, center, 1/4 points, and possibly 1/8 points on pieces and use shorter welds. In addition, you will need to limit the heat, and that’s where advanced features such as pulse can help you. You can find pulse within TIG325X AC/DC TIG Welder With Foot Pedal Welder or PRIMEWELD TIG225X TIG welder.

Post Weld Heat Treatment and Cleaning:

Post-weld heat treatments are often beneficial in stress relief and improving the properties of the heat-affected zone in ferritic steels. However, post-heating to 594-705°C can reduce the corrosion resistance of many standard grades of stainless steel.

Cleaning the weld joint is also an essential part of post-weld treatment. You must clean slag and heat tint to examine the weld integrity. To successfully clean the joint, you should protect the stainless steel from carbon steel grinding debris and smearing caused by sliding contact between these two.

Welding Aluminium to Dissimilar Metals:

Welding aluminium to steel and other dissimilar metals is quite challenging and often avoided due to unfavourable thermal characteristics. With a 660°C melting point, aluminium melts two times faster than mild steel with 1350-1530°C.

Trying to weld them with any fusion method will cause molten metal to flow away before fusing with steel. That’s why welding aluminium to steel is commonly done with non-fusion welding. However, if you don’t have any choice, you can use one of the typical aluminium welding processes, but with certain preparation.

Welding aluminium to steel with MIG or TIG welding will require aluminium-steel transition materials. These will have the same properties as aluminium or steel, and you can simply weld them to the required base metals.

Another solution is to layer a coating on top of steel or stainless steel. The coating must be compatible with aluminium’s properties, and for regular steel, it can be made of zinc. High-silicon aluminium filler is required. However, the final results can depend on the thickness of the coating, the bond between the coating and base metal, and the welding technique.

Welding aluminium to stainless steel will require a pure aluminium coating. A piece of stainless steel can be dipped into molten aluminium or tinned by high-silicon aluminium alloy. Like with mild steel, it doesn’t have to result in a successful weld.

Welding Copper to Different Metals:

Compared to aluminium, welding copper to different steels is more straightforward. Using a high-copper-alloy filler rod, you can weld thin copper to steel with a TIG welder. 

When welding thick sections of copper to steel, you will need to overlay or butter the steel with the high-copper-alloy filler and then weld it to the copper while avoiding excessive penetration. High heat can cause iron pickup in copper, resulting in a brittle material.

Welding thicker pieces can also be done with Stick (SMAW) welding process. However, you will need an overlay with a nickel-base electrode, and copper must be preheated to approximately 538°C.

Using similar approaches, you can join copper with stainless steel and brass with mild and low-alloy steel.

Welding Nickel-based Alloys to Steel:

Common Nickel-base alloys, such as Monel and Inconel, can be successfully joined with low-alloy steel by different arc welding processes. You should use the Inconel base electrode when welding Inconel to mild or low-alloy steel. Likewise, welding Inconel or Monel to stainless steels will require a proper Inconel or Monel-type electrode.

Welding Low-Carbon Steel to High-Strength Steel:

Repairing or welding structural or heavy equipment will often require welding low-carbon steel to high-strength steel. The high-strength steel, such as A514, typically offers a yield strength of 100,000 psi, while standard low-carbon steels have a yield strength of 70,000 psi.

If you remember the first part of the text, to successfully weld these two, you will need a filler material that matches the strength of the weaker metal.

Final Thoughts:

Welding dissimilar metals demands a comprehensive understanding of the materials involved. By carefully considering physical and mechanical properties, selecting the right equipment and filler materials, and employing proper techniques, you can successfully join dissimilar metals for various applications. Mastering the art of welding dissimilar metals opens up possibilities for tackling diverse projects with confidence.

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