Ultrasonic Welding of Nickel with Coarse and Ultrafine Grained Structures

Abstract

Solid state joints of samples of coarse-grained (CG) and ultrafine-grained (UFG) nickel have been obtained for the first time using spot ultrasonic welding (USW). The UFG structure in disk-shaped samples was processed by means of high-pressure torsion (HPT). On the basis of lap shear tests, the optimal values of the clamping force resulting in the highest values of the joint strength are determined. The microstructures in the weld joints obtained at optimal parameters of USW are characterized by scanning electron microscopy. It is shown that during ultrasonic welding of coarse-grained nickel, a thin layer with an UFG microstructure is formed near the weld surfaces. The bulks of sheets retain the CG microstructure, but a significant dislocation activity is observed in these regions. During USW of samples having an UFG initial microstructure, significant grain growth occurs. Fine grains are observed only along the welding interface. An average lap shear strength of 97 MPa was obtained by welding the UFG samples, which was approximately 40% higher than the strength of samples processed by welding coarse-grained sheets (70 MPa). It is concluded that higher strength weld joints can be obtained by using sheets with the UFG structure as compared to the CG sheets.

Microstructure of Joints Processed by Ultrasonic Consolidation of Nickel Sheets

“…The absence of structural changes in well-bonded regions of nickel tapes after ultrasonic consolidation does not agree with both the results of the structure examination of aluminum and copper samples made by USC and with the results of Ref. [42], where the ultrafine-grained structure was formed in the joint zone of coarse-grained nickel sheets after USW.…”

“…The surfaces to be joined were ground with abrasive papers, then washed with isopropyl alcohol and dried before consolidation. USC was carried out on an experimental home-made equipment for spot ultrasonic welding described in [42,44] and schematically presented in Figure 2. A sonotrode tip with dimensions of 4.5 mm × 6 mm having a serrated surface with seven parallel teeth oriented perpendicular to the vibration direction was used.…”

“…A sonotrode tip with dimensions of 4.5 mm × 6 mm having a serrated surface with seven parallel teeth oriented perpendicular to the vibration direction was used. The USC was carried out with a vibration frequency of 20 kHz, displacement amplitude of the welding tip of 15 μm, clamping force (P) of 6 kN, and welding time of 2 s. These values of welding parameters were selected based on the data of our earlier studies on ultrasonic welding of nickel sheets according to which allowed the formation of sound joints exhibiting high lap shear strength (about 70 MPa) [42]. Six sheets of nickel with a thickness of 0.2 mm were successively welded to a substrate of the same material with a thickness of 0.5 mm.…”