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How To Weld Dissimilar Metals
If not the best, welding certainly is the strongest and most straightforward metal joining process, which makes it widely used in industries across the globe. However, welding shows the best results and highest strengths when welding same-grade metals.
The challenge arises once projects and repairs include dissimilar metal welding. To understand how to weld dissimilar metals, you will have to dig deep into the chemical and physical properties of each welded metal.
Only by understanding their differences, you can adapt the entire welding procedure to achieve a successful weld. Therefore, we saved you hours of grunt work and compiled this detailed guide on dissimilar metal welding.
Is It Possible To Weld Dissimilar Metals?
This is the question many beginners ask, and the most straightforward answer is yes, but with certain precautions. Physical properties such as thermal conductivity and expansion, magnetic properties, metallurgical structure, and corrosion resistance of two metals are solely responsible for different results in welding.
These make welding certain groups, such as aluminum to carbon steel, copper, and stainless steel, practically unweldable with typical fusion welding processes. Nonetheless, welding today is significantly advanced, and there are specific processes that will allow welding even the most different metals.
As a hobbyist, you are most likely to encounter joining carbon steel with stainless steel. Therefore, the main focus of the article will be carbon steel to stainless steel welding. However, we will also briefly explain how joining dissimilar metals works in specific situations.
Crucial Factors To Consider Before Welding Dissimilar Metals
When it comes to dissimilar welding, many inexperienced welders ask themselves where to start.
The first thing you should do before welding two dissimilar pieces is to identify them. Most metals have their physical, chemical, and mechanical properties standardized by ASTM/ASME. For example, each austenitic stainless steel, such as 304 or 316, will have the same properties regardless of the manufacturer.
Source: https://www.permanentsteel.com/newsshow/difference-between-astm-and-asme-standard.html
The issues occur in specific applications, such as repair welding, where you cannot precisely identify two pieces. In that case, you would have to perform a spark test, send a material sample to be analyzed, or consult with the original manufacturer or blueprints.
Once you understand the basic chemistry of dissimilar materials, you can make the strongest welds according to service conditions. The crucial factors to pay attention to are the physical properties, mechanical properties, and corrosion resistance of both pieces and the filler used to join them.
- 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. In certain applications, the American Society of Mechanical Engineers (ASME) code allows a weld strength of 95%. 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.
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.
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.
Source: https://www.electronicshub.org/laser-welding/
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
After understanding what can go wrong with your dissimilar welding, it is time for more direct hints and tips. While there is a wide variety of specific metals that can be joined using one of the welding methods we described, we will focus on dissimilar materials welding that you are likely to encounter as an everyday welder. These are common carbon steel to stainless steel welding, aluminum, copper, and nickel-alloys with different metals, and low alloy steel to high and medium carbon steel.
Welding Steel To Stainless Steel
If you browse the internet, in the vast majority of topics, dissimilar welding refers to joining mild steel to stainless steel. Most commonly, these are austenitic stainless steels such as 304 and 316. Mild and stainless steels are different in many properties, but the biggest issue is that carbon can affect the biggest advantage of stainless steel - corrosion resistance. In addition, there are vast differences in the physical and mechanical properties of stainless and regular steel.
To weld steel to stainless steel, you can use one of the common fusion welding methods, but typical choices are Metal inert gas (MIG) Welding and tungsten inert gas (TIG) welding. Carbon steels with less than 0.2% carbon can generally be welded with austenitic fillers without preheating. However, if the carbon content is greater than 0.3%, temperature control is crucial.
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 300°F will help both pieces heat evenly. The temperature of 400°F 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 galvanized 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 800°F. 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 800°F, 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.
Source: https://www.ansys.com/blog/thermal-cycling-failure-in-electronics
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.
Battling Thermal Expansion Differences
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.
Source: https://www.youtube.com/watch?v=4WaNtRuUteA
In addition, stainless steel takes more time to heat compared to steel, but it also needs more time to cool down. With thermal expansion differences, thermal stresses increase, leading to distortion.
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 YesWelder YWM-211P MIG Welder or YesWelder TIG-250P TIG welder.
Post Weld Heat Treatment And Cleaning
Postweld heat treatments are often beneficial in stress relief and improving the properties of the heat-affected zone in ferritic steels. However, post-heating to 1,100-1,300°F 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 Aluminum To Dissimilar Metals
Welding aluminum to steel and other dissimilar metals is quite challenging and often avoided due to unfavorable thermal characteristics. With a 1220 degrees F melting point, aluminum melts two times faster than mild steel with 2462o F-2786o F.
Trying to weld them with any fusion method will cause molten metal to flow away before fusing with steel. That's why welding aluminum to steel is commonly done with non-fusion welding. However, if you don't have any choice, you can use one of the typical aluminum welding processes, but with certain preparation.
Welding aluminum to steel with MIG or TIG welding will require aluminum-steel transition materials. These will have the same properties as aluminum 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 aluminum's properties, and for regular steel, it can be made of zinc. High-silicon aluminum 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 aluminum to stainless steel will require a pure aluminum coating. A piece of stainless can be dipped into molten aluminum or tinned by high-silicon aluminum alloy. Like with mild steel, it doesn't have to result in a successful weld.
Welding Copper To Different Metals
Compared to aluminum, 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. The pulse feature will help you avoid defects, and our recommendation is the YesWelder TIG-200P ACDC Aluminum welder with a pulse.
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 1000°F.
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. Therefore, logical choices are E71TGS flux cored wire, ER70S-6 MIG wire, or a 7018 low-hydrogen stick electrode.
Since preheat and interpass temperatures play a more crucial role in higher-strength metal, you will have to control them with A514 steel. That way, you prevent cracking and loss of strength when welding different-strength steels.
Final Thoughts
Dissimilar material welding is a possible but challenging process that might not yield the strongest results. To successfully do it, you will need to accredit the physical and mechanical properties of both welded pieces, as well as filler material as third metal.
Once you understand the welding materials and their characteristics, you can dedicate the entire process, including preparation, equipment and filler selection, technique, and post-weld treatment, to successfully join dissimilar materials.
🧐How To Weld Dissimilar Metals - FAQ
1. How to weld steel to stainless steel?
To weld steel to stainless steel, you can use one of the common fusion welding methods, but typical choices are Metal inert gas (MIG) Welding and tungsten inert gas (TIG) welding.
2. How to select the filler metal to weld mild steel to stainless steel?
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 800°F. 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.
3. How to welding aluminum to dissimilar metals?
Welding aluminum to steel with MIG or TIG welding will require aluminum-steel transition materials. These will have the same properties as aluminum 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 aluminum's properties, and for regular steel, it can be made of zinc. High-silicon aluminum filler is required.
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