Intrinsic dependence of welding quality and recrystallization on the surface-contacted micro-asperity scale during ultrasonic welding of Cu-Cu joints

Substantial shear deformation induced by high-frequency friction at the interface is inseparable from recrystallization and welding quality during ultrasonic welding. In this study, we prepared welding surfaces with various micro-asperity scales to analyze the comprehensive effects of deformation heat, shear strain and strain rate on the recrystal-lization and welding quality. Smaller surface-contacted micro-asperities with a size of 16.03 nm led to a surge in the low angle grain boundaries and recrystallization grains before the formation of a complete weld, which drastically increased the load carry ca-pacity by 139% for joints welded at 1500 J comparing with larger micro-asperities. Molec-ular dynamics simulation indicated that weld formation involved a dynamic process, including micro-connection, tear, fracture, flattening and weld spread, and the reduction in the micro-asperity scale accelerated the flattening process and aggravated shear defor-mation at the intimate contact positions. According to the calculation model of recrys-tallization induced by the shear deformation, smaller micro-asperities promoted the coupling between high shear strain and low strain rate, and maximum shear strain of 1.57 and minimum average strain rate of 346/s were obtained simultaneously, which decreased critical recrystallization strain and enlarged the dynamic recrystallization regions during ultrasonic welding.(c) 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Automated summary

This study investigated the effects of micro-asperity scales on recrystallization and welding quality during ultrasonic welding. Smaller micro-asperities led to increased low angle grain boundaries and recrystallization grains, enhancing the load carry capacity of the welded joints. The reduction in micro-asperity scale accelerated the flattening process and aggravated shear deformation. The coupling between high shear strain and low strain rate facilitated recrystallization and enlarged the dynamic recrystallization regions.