Solid-state welding processes join materials without melting them and include friction welding, ultrasonic welding, roll bonding, and cold welding. Friction welding generates heat through friction between rotating and stationary parts in contact under pressure. Ultrasonic welding uses oscillating shear stresses to break up oxides and contaminants at the interface. Roll bonding applies pressure through rolls to join sheets. Resistance welding generates heat through electrical resistance and is used for spot welding sheets in automotive and other fabrication.