Role of Friction in Ultrasonic Welding

Abstract

The ultrasonic welding of metals has not been adequately investigated, and the mechanism behind formation of the bond has been given different explanations. In Refs. 1 and 2 ultrasonic welding is distinguished as a unique way of bonding metals, unlike the more familiar techniques, and yet the positive contribution of heating of the parts during welding due to friction is noted. The authors feel that the main factor causing breakdown of the continuity of the surface films and their elimination is the relative displacement of the parts and the plastic flow of the metal in the welding zone. In the opinion of the author of Ref. 3, the principal factors promoting formation of the bond are the increased atomic energy level of the crystal lattice of the metal near the welded surfaces, activation of diffusion processes in the ultrasonic field, and the appearance of considerable shear stresses at the microcontacts. The combined action of these factors causes the formation of crystals at the interface of the components. In Refs. 4 and 5 the similarity between the gripping action in ultrasonic welding and dry friction is pointed out. This postulate, however, is confirmed by calculation and experiment only for the case of contact between the tip of the ultrasonic tool and a plate. In Ref. 6 ultrasonic welding is regarded as a special case of pressure welding at high temperatures. The action of the ultrasonic oscillations amounts to removal of the surface film and heating of the contact zone to a temperature at which the resistance of the parts to deformation becomes lowered several times. Investigations conducted at the Metallurgy Institute, Academy of Sciences of the USSR, on formation of the bond in ultrasonic welding show that one important effect taking place in welding is heating of the parts at their point of contact. In the following article we present some data from a study of the distribution of the temperature field in welded parts during the process of ultrasonic welding.