It is possible to weld two different metals together, though the process is significantly more complex than joining two pieces of the same material. Traditional fusion welding often results in a weak or brittle joint. However, specialized techniques and materials have been developed to successfully combine metals with vastly different properties. Joining dissimilar metals is important in modern engineering, allowing structures to combine materials that offer complementary attributes like strength, corrosion resistance, or conductivity.
Why Dissimilar Metals Resist Fusion
The primary difficulty in welding dissimilar metals stems from fundamental differences in their physical and chemical properties. When two different metals are melted together, the resulting joint often lacks the strength and ductility of the parent materials.
One major challenge is the difference in melting points between the two materials. When a heat source is applied, the metal with the lower melting point will liquefy long before the other metal reaches its melting temperature. This disparity leads to uneven heating and cooling rates, which can cause the lower-melting metal to burn off or evaporate, resulting in a defective joint.
Another significant issue is the difference in the coefficient of thermal expansion (CTE) between the two metals. As the weld cools, materials with different CTEs will contract at different rates, creating internal stresses within the joint. These residual stresses can lead to warping, distortion, and cracking, especially in the heat-affected zone (HAZ).
The most serious metallurgical problem is the formation of intermetallic compounds (IMCs) in the weld zone. When certain elements mix in a molten state, they chemically react to form new, distinct compounds that are typically hard, brittle, and glass-like. These IMCs, such as iron aluminides formed when welding steel to aluminum, severely compromise the joint’s strength and ductility, making it highly susceptible to failure.
Specialized Techniques for Joining Different Metals
To overcome the inherent challenges of fusion welding, specialized techniques focus on either avoiding the formation of brittle intermetallic compounds or managing the thermal stresses. These methods often rely on introducing a third, compatible material to act as a buffer between the two base metals.
One common and accessible solution is brazing or soldering, which are non-fusion joining processes. These methods use a filler metal that has a melting point significantly lower than the two base metals, meaning the base metals are never melted. The filler metal flows into the joint via capillary action, creating a bond without the risk of forming brittle intermetallic compounds.
For arc welding processes like Gas Tungsten Arc Welding (TIG) or Gas Metal Arc Welding (MIG), the use of a transition filler metal is the standard approach. This filler metal is specifically chosen to be metallurgically compatible with both base metals, effectively creating a buffer zone. For example, when joining carbon steel to stainless steel, a high-alloy austenitic filler metal like Type 309 or 312 is often used because it can tolerate the dilution from both parent materials while maintaining crack resistance.
More advanced, industrial solutions utilize solid-state welding techniques, such as friction welding or friction stir welding (FSW). These processes generate heat through mechanical friction and pressure, raising the material temperature to a plastic state without reaching the melting point. By avoiding the liquid phase entirely, solid-state welding minimizes the formation of detrimental intermetallic compounds and significantly reduces the heat-affected zone, making it highly effective for difficult combinations like aluminum to steel.
Key Considerations for a Successful Weld
Achieving a strong, reliable dissimilar metal joint requires meticulous planning and execution, focusing heavily on material selection and preparation. The most important decision is the choice of the filler metal, which must be able to accommodate the chemical and physical differences between the two base materials. For instance, when welding copper to steel, nickel-based filler metals are often used because nickel is soluble with both copper and iron, creating a stable transition layer.
Thorough preparation of the joint surfaces is also necessary. All oxides, dirt, oil, and moisture must be completely removed, as contaminants can exacerbate the formation of brittle phases or lead to porosity in the final weld. Proper joint design, such as beveling the edges, helps control the dilution of the base metals into the weld pool, which manages the final chemical composition of the joint.
Certain metal pairings are commonly encountered and have established procedures for successful joining. Welding stainless steel to carbon steel is routine using the appropriate austenitic filler metal, provided the service temperature remains below the point where thermal expansion differences cause fatigue. Conversely, joining aluminum to steel remains one of the most challenging combinations due to the extreme difference in melting points and the rapid formation of brittle iron aluminides, often requiring specialized transition inserts or solid-state methods to achieve a durable bond.