Microstructural understanding for achieving high-quality and low-cost solid-state bonding
File(s)
Author(s)
Wang, Wei
Type
Thesis
Abstract
Solid-state diffusion bonding is of paramount importance for metals that are difficult to be joined by other processes. However, the current applications of solid-state bonding technologies are restricted by a high-vacuum environment and longtime heating. This brings a significantly high cost, limiting the applications of this technology. This work thus aims to improve the understanding of solid-state bonding mechanisms so as to propose newly designed processing methodologies to achieve low-cost and high reliable solid-state bonding.
Firstly, microstructure evolutions and their interactions during solid-state diffusion bonding were systematically investigated and correlated with the corresponding tensile strength. The results reveal that the recrystallization (Rx) and the following grain growth significantly promote grain boundary (GB) migration at the interface.
Secondly, we offer the unrevealed micro-mechanisms of individual microstructures on local deformation. The results demonstrated that a migrated GB does not alter the activated slip systems but precludes GB-multislip interaction around interface voids to alleviate stress concentrations.
Next, based on the above findings, we propose a fast interface elimination method. Results demonstrated that the fast interface elimination originates from the high dislocation density induced by cold work. A grain boundary migration ratio of 0.82 can be achieved within 1.5mins in the cold rolling sample with a 50% reduction, more than 10 times faster than that in the annealed sample.
Then, we proposed an open-air solid-state bonding strategy to achieve oxide-free bonding interface by using a fine-grain Ni coating. The results demonstrated that the 316L can be successfully bonded with the relative bonding strength of the as-bonded sample reaching 95%. In contrast to other existing techniques for bonding/joining metals, the fast bonding speed, simple environmental requirements and loading process of this work widen its applications and thus open a new avenue for economically and efficiently fabricating unweldable metals in the solid state.
Firstly, microstructure evolutions and their interactions during solid-state diffusion bonding were systematically investigated and correlated with the corresponding tensile strength. The results reveal that the recrystallization (Rx) and the following grain growth significantly promote grain boundary (GB) migration at the interface.
Secondly, we offer the unrevealed micro-mechanisms of individual microstructures on local deformation. The results demonstrated that a migrated GB does not alter the activated slip systems but precludes GB-multislip interaction around interface voids to alleviate stress concentrations.
Next, based on the above findings, we propose a fast interface elimination method. Results demonstrated that the fast interface elimination originates from the high dislocation density induced by cold work. A grain boundary migration ratio of 0.82 can be achieved within 1.5mins in the cold rolling sample with a 50% reduction, more than 10 times faster than that in the annealed sample.
Then, we proposed an open-air solid-state bonding strategy to achieve oxide-free bonding interface by using a fine-grain Ni coating. The results demonstrated that the 316L can be successfully bonded with the relative bonding strength of the as-bonded sample reaching 95%. In contrast to other existing techniques for bonding/joining metals, the fast bonding speed, simple environmental requirements and loading process of this work widen its applications and thus open a new avenue for economically and efficiently fabricating unweldable metals in the solid state.
Version
Open Access
Date Issued
2023-07-10
Date Awarded
01/01/2024
License URL
Advisor
Jiang, Jun
Shi, Zhusheng
Publisher Department
Mechanical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)