Evaluation of Corrosion and Its Impact on the Mechanical Performance of Al-Steel Joints

Al-steel joints are increasingly used to create lightweight automobile structures to meet the strong demand on energy saving and emission reduction. Among various joining technologies proposed to join aluminum alloy to steel, self-pierce riveting (SPR) and resistance spot welding (RSW) are two promising technologies to provide cost effective solutions due to the fast operation speed and the ease to be automated in mass production. Recently, a newly invented method, resistance element welding (REW), is attracting interests from researchers and automobile manufacturers since it combines the merits of SPR and RSW in joining dissimilar materials. When a dissimilar material joint is exposed to the field environment, corrosion damage, especially caused by galvanic corrosion, will be a critical issue as it degrades the mechanical performance and the reliability of the automobile structure. Therefore, it is of great significance to develop a comprehensive understanding of the corrosion evolution in these Al-steel joints, including the initiation, propagation, and mechanism. Besides, the intrinsic relationship between corrosion and joint performance needs to be revealed to provide reliable guidance to mitigate corrosion damage. In this thesis, the corrosion evolution Al-steel SPR and RSW joints, as two mature joining technologies, was thoroughly investigated under microscopes. The most affected sites and corrosion mechanisms in SPR and RSW joints were revealed. In SPR joint, mechanical interlocking was damaged as the steel under the rivet leg was completely corroded and the rivet leg get exposed and corroded. While in RSW joint, stress corrosion cracking (SCC) occurs in the intermetallic compounds (IMCs) layer. By analyzing the performance parameters of corroded joints and relating it to the corrosion evolution, the impact of corrosion on the mechanical performance was revealed. During the early stage of corrosion, the strength and energy to failure of a SPR joint did not degrade because of the limited damage occurred in the mechanical interlocking and the tightening effect of corrosion products at the crevice, while the RSW joint failed much faster since stress corrosion cracking propagated through the whole layer of IMCs. Under long term corrosion, significant material loss in aluminum and steel in both joints results in the decrease of the stiffness to a large extent. Overall, SPR joints demonstrated higher corrosion resistance and performed better in retaining the mechanical performance than the RSW joints did. Meanwhile, to further investigate the impact of material loss on the material properties and predict the performance of corroded joints, a multi-step modeling framework was proposed: (1) First a validated joint performance model before corrosion is established; (2) then the corroded material properties, including the effective Young’s modulus and effective yield stress, are estimated; (3) the material properties of the corroded specimen are applied to the validated performance model to predict force vs. displacement (F-D) curve. The model agrees well with the experimental results and the methodology was proved to be feasible in predicting the performance of corroded joints. Finally, the corrosion behavior learned from the SRP and RSW joint was applied to the newly invented REW joints. As a combination of SPR and RSW processes, the corrosion mechanism of a REW joint is similar to a SPR joint that the corrosion damages the mechanical interlock between aluminum and rivet.PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/174558/1/wlwen_1.pd