Welding Difficult and Dissimilar Metals

Welding is a critical process in the production of reliable parts, especially in industries with demanding applications that subject parts to extreme stresses. In aerospace and defense applications, the failure of a weld could lead to the loss of a vehicle and perhaps the crew. In medical applications, a weld failure might likely have a catastrophic result. Energy applications, too, whether involving batteries or valves at a refinery, have dire consequences if a weld fails.
 
Often, parts in critical applications are made from metals or alloys that have enhanced properties, such as a strong dimensional stability under high temperatures, or extreme corrosion resistance in corrosive environments. The very properties that make these materials so useful in a particular application can also make them problematic to weld. The issues can be compounded when the part consists of different materials, each with metallurgical properties and problems, which must be welded together. 

There are many ways a weld can be sub-par. Here are the most common problems that are the result of a bad weld:

• Leaks: Bad welds can cause leaks in parts ranging from medical device implants to fuel / thruster valves, coolant systems, batteries, pipelines, etc. Leaks into parts situated in an adverse environment can damage or destroy the part. Leaks out of a part can cause contamination of a delicate or controlled environment.
• Reduced strength: Bad welds can reduce the strength of a part, making it more prone to failure, particularly under stress.
• Fatigue: Stress concentrations in a part can lead to fatigue and eventual failure of a weld, especially under cyclic loading.
• Corrosion: Low-quality welds can trap moisture and other contaminants that can cause corrosion, reducing the lifespan of a part and potentially leading to failure.
• Incomplete fusion: If a weld is not fully fused, it can result in a weak weld joint that is prone to failure, especially under load.
• Porosity: Bad welds can contain trapped gas and other impurities that create high levels of porosity in the weld, reducing its strength and making it more prone to failure.
• Warping or cracking: Poor welds can cause warping or cracking of the metal, reducing its strength and making it more prone to failure.

Many factors go into why a material or a combination of materials might be hard to weld. However, these are the culprits that most often crop up when dealing with problematic welding configurations.

 
• High melting point: Metals with high melting points, such as titanium and nickel-based alloys, can be difficult to fusion weld because they require more heat input to melt and join.
• Different thermal expansion coefficients: When metals with different thermal expansion coefficients are welded, they may expand and contract at different rates as they heat and cool, causing warping, cracking, gapping, deformation and other welding defects.
• Reactivity: Some metals, such as aluminum and titanium, can react with air during the welding process, forming a surface oxide that can prevent complete fusion and weaken the weld joint.
• Porosity: Welding certain metals, such as aluminum, can result in porosity (small voids) in the weld due to the release of gases during the welding process.
• Difficult to control heat input: Controlling the heat input during welding is critical for good fusion, but it can be difficult with some metals due to their high thermal conductivity, surface reflectivity, or other properties that affect heat transfer.

There are metals that on their own, due to their characteristics, can be hard to weld. 

• Aluminum: Aluminum is difficult to weld because it is highly reactive, meaning it reacts with air and forms a surface oxide that can interfere with the formation of a solid weld joint. Additionally, aluminum has a low melting point and a high thermal expansion coefficient, which can cause warping and cracking during welding.
• Magnesium: Like aluminum, magnesium is highly reactive and forms a surface oxide during welding. Additionally, magnesium has a low melting point, which can lead to porous, weak weld joints.
• Titanium: Titanium is difficult to weld because; A) it has a high melting point, which requires high heat input to melt the metal, and B) titanium can react with other elements during welding, such as nitrogen and oxygen, to form oxides that can contaminate the weld joint.
• Stainless Steel: Stainless steel is difficult to weld because some alloys are prone to cracking and warping during welding due to their high thermal expansion coefficient. Additionally, the different types and grades of stainless steel have different welding requirements, which can make welding dissimilar stainless steel alloys challenging.
• Copper: Copper is difficult to weld because it has a high thermal conductivity, which can cause it to cool quickly during welding, forming a brittle weld joint. Additionally, copper’s high thermal conductivity is also conducive to heat deformity and damage, making it very easy to warp or distort parts. Copper is also highly reflective, which can cause issues when it is laser welded.
• Inconel: Inconel is a nickel-chromium alloy that is highly resistant to corrosion and heat, and is difficult to weld because it has a high thermal expansion coefficient, which can cause warping and cracking during welding.

Welding different metals together can be challenging due to differences in their chemical composition, melting points, thermal expansion coefficients, etc. Some metal combinations that are particularly difficult to weld include:

• Aluminum and Steel: Different thermal expansion coefficients and reactivity, which can cause problems such as warping, cracking, and porosity, are these two materials a challenge to weld.
• Titanium and Steel: Again, thermal expansion coefficients and reactivity make these materials hard to join together. Further, titanium has a low thermal conductivity, which can make it difficult to control the heat during welding.
• Copper and Aluminum: Both of these materials are reactive, copper especially so, and have different thermal expansion coefficients, which can cause weld defects like cracking and porosity.

Please note that challenging doesn’t mean impossible: we have been welding these materials together for decades.