Weld controllers can be designed to provide several types of secondary currents. Until recently AC alternating current controllers were the dominant product. With the advent of robots and the need for smaller/lighter transformers mid frequency welding has become very popular. It has also replaced many applications for DC direct current welding. Capacitive discharge CD welding continues to have some applications.
Controls are manufactured for: AC, DC, MFDC and CD secondary currents.
Reference: RWMA - Resistance Welding Manual 4th Edition