Multiscale modeling and experimental study of ultrasonic metal welding

• Multiskalenmodellierung und experimentelle Untersuchung des Ultraschallschweißens von Metallen

Mostafavi, Shimaalsadat; Markert, Bernd (Thesis advisor); Mahnken, Rolf (Thesis advisor)

Aachen : RWTH Aachen University (2022, 2023)
Book, Dissertation / PhD Thesis

In: Report. IAM, Institute of General Mechanics 18
Page(s)/Article-Nr.: 1 Online-Ressource : Illustrationen, Diagramme

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022

Abstract

The formation of a reliable joint between similar and dissimilar metals is of crucial importance for the automotive industry, specially for electric powered vehicles. The technology of the ultrasonic welding is used across a broad range of industrial applications and serves as a rapid, energy efficient and reliable joining method. Up to now, despite extensive researches, the lack of a comprehensive understanding of the underlying physics of this complex process, makes it difficult to be monitored and predicted effectively. Therefore, the aim of this thesis is to understand, which structural properties stipulate the joint characteristics, and therefore, manipulate the effect of the process parameters. It is shown via thermocouple sensing that the interface temperature increases with the vibrations amplitude rather than with the welding pressure. Whereas the welding pressure defines the potential positions for the weld points, the vibrations amplitude determines the degree of bonding due to the effective heat generation as a result of sliding friction. A finite element model displays the same trend, considering the coupled thermomechanical nature of the ultrasonic welding process. By using piezoelectric transducers, oscillation pattern of one mating part during the ultrasonic welding is captured, which exhibits a complex vibration scheme, containing the excitation frequency and its harmonics. Furthermore, subsequent analysis of the joint strength reveals that a larger welding pressure results in a larger maximum T-peel force, owing to the high density and the low portion of the air gaps inside the weld structure from micro computer tomography observations. However, large magnitudes of the pressure and the amplitude at the same time decrease the energy to failure of the weld and result in a weak joint. By using diverse microscopy techniques, it is shown that increasing the welding pressure results in excessive plastic deformations and softening of aluminum. However, it prevents the sliding motion and the generation of the sufficient friction energy for the formation of a good bond. Molecular dynamics simulations are used to investigate the structural-related diffusion pattern of the mating interface by using two aluminum crystallites of different orientations. The atomic scale simulations reveal that the orientations of the crystallites govern the interface diffusion and the tensile strength of the joint significantly. Furthermore, interface atom diffusion, quantified by the diffusion coefficient, increases with increasing the sliding velocity of the upper crystallite. The significant influence of increasing the sliding velocity on the disorder of the atomic structure is quantified by a probabilistic analysis through radial distribution function. Additionally, it is observed that a higher sliding velocity enhances the friction heat generation and significantly increases the interface temperature, as it is resulted from the macroscale simulations and experiments. The atomic scale simulations can be replicated at the microscale, leading to a bottom-up approach of the bond formation in ultrasonic metal welding. Therefore, the results of this thesis provide valuable insights into the joint formation and help to improve the process at the macroscale.