Just a few posts ago, I shared some information on online and mobile apps that help take the guesswork out of material weldability. Since that post, I’ve gotten some feedback that leads me to believe a lot of people would like a bit more on the basic questions of “what electrodes should I use for spot welding?” and “can I spot weld (material A) to (material B)?”
Let’s start with materials. In general, materials fall into three categories: conductive (such as aluminum, copper, silver, and gold, etc.), resistive (such as steel, nickel, Inconel, titanium, etc.) and refractive (tungsten, molybdenum, etc.). Conductive materials require a lot of current to heat up during a weld, and the heat generated is rapidly conducted away, making it difficult to form a bond. Resistive materials, on the other hand, heat up quickly and thus require less current input to form a bond. Refractive materials have high melting points, are susceptible to cracking, and are generally harder than either conductive or resistive materials. For these reasons, welding refractive materials can be very challenging.
These material categories apply not only to the materials to be joined but also to the electrodes used to join them. Thus the “rule of opposites” applies to matching electrodes to the workpieces to be welded. The general rule is to utilize conductive electrodes against resistive parts and refractive electrodes against conductive parts. By extension, when welding dissimilar materials, the upper and lower (or anode and cathode) electrodes must be of different materials to each other in order to apply the “rule of opposites” (see diagram).
Then there is the question of “can I spot weld A to B?” Many materials can be joined, although with varying degrees of success. I recommend you check out the online material weldability tool I cited above for expected weldability, electrode material recommendations, and other handy notes which may assist you in your process. There is even a free mobile app, for both iPhone and Android platforms.
Of course resistivity and conductivity aren’t the ONLY variables to consider when choosing electrode materials and developing a spot welding process. You should also consider surface conditions/roughness, oxides, plating, finish, and part geometry. For more detailed information read our Welding Material Control Nugget
For more information, read our Fundamentals of Resistance Welding.