Microstructure of Joints Processed by Ultrasonic Consolidation of Nickel Sheets

Ultrasonic consolidation is an advanced process of sequential solid-state joining of metal foils or sheets by ultrasonic welding. This process was used for joining six sheets of nickel with a thickness of 0.2 mm. Ultrasonic consolidation was accompanied by the formation of wear particles between the sheets. The appearance of microbonds between the sheet surface and the wear particles led to the formation of parallel rows of voids and swirl-like patterns near the interfaces. It was shown that ultrasonic consolidation of nickel sheets led to the formation of fine recrystallized grains near contact surfaces and a subgrain structure in the bulk of the consolidated layers. The microstructural changes were accompanied by an increase in the microhardness of nickel from 1567 MPa in the initial sheet to 2065 and 2400 MPa, respectively, in the bulk and joint zones of the consolidated sample. However, significant differences in the microstructure and microhardness of the layers were not revealed, despite the fact that the accumulated plastic deformation and thermal effects were different from layer to layer. This unexpected result was explained by an inhomogeneity of the microstructure of the nickel samples obtained by ultrasonic consolidation and by a possible interplay between ultrasonic residual hardening and softening.