Dissertation Defence: Stress Characterization for Friction-Stir-Welded Hybrid Electric Battery Trays with Application of Neutron Diffraction

Nicholas Sabry, supervised by Dr. Dimitry Sediako, will defend their dissertation titled “Stress Characterization for Friction-Stir-Welded Hybrid Electric Battery Trays with Application of Neutron Diffraction” in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Mechanical Engineering.

An abstract for Nicholas Sabry’s dissertation is included below.

Examinations are open to all members of the campus community as well as the general public. This examination will be offered in a hybrid format. Registration is not required for in-person defences. Please email dimitry.sediako@ubc.ca to receive the Zoom link for this defence.

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Abstract

With the growing demand for increased battery capacity and performance, a corresponding requirement for adequate cooling of the battery cells has developed. This situation requires an efficient design of coolant channels in battery trays. The battery tray houses the cells and regulates their temperature, optimizing the distance per battery kilogram. The coolant channels in the battery tray require high-pressure die casting (HPDC) to create. However, the HPDC process is limited, and enclosed geometries for coolant flow cannot be created. Therefore, the coolant channels must be sealed with thin aluminum plates post-casting. A liquid seal is created by joining the aluminum plates to the HPDC component utilizing friction stir welding (FSW). FSW is an industry-emerging solid-state joining process by which its inherent nature overcomes many problems associated with fusion welding aluminum. These problems are due to aluminum’s high-melting point oxide layer, high thermal conductivity, solidification shrinkage, and high solubility of hydrogen and other gases in the molten state, which are not an issue with FSW as the material being welded does not melt, and the brittle oxide layer breaks with the torsion of the tool intrinsically overcoming many problems related to fusion welding of aluminum. Nevertheless, the FSW welding operation leads to the development of residual stress, which compounds into a significant amount of distortion, specifically in multi-welded components, leading to expensive downstream straightening processes to restore the pre-weld geometry, which may or may not further increase residual stress. Residual stress, exceeding the stress limit of the battery tray during operation, can compromise the battery-cell housing and risk the vehicle’s safety. Hence, understanding residual stress formation is vital for design, safety, service life longevity, and distortion control. Experiments utilizing neutron diffraction investigate and measure residual stress inside the battery tray, comparing the stress redistribution before and after straightening operations. The goal of the dissertation is to investigate and potentially optimize FSW in multi-welded structures to mitigate or eliminate straightening operations, enabling significant increases in production efficiencies, more robust final products, and, ultimately, a safe and environmentally friendly means of transportation.