DEVELOPMENT AND CHARACTERIZATION OF MICRO FRICTION STIR BLIND RIVETING FOR MULTILAYER THIN DISSIMILAR MATERIALS
Open Access
- Author:
- Khan, Haris Ali
- Graduate Program:
- Industrial Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 14, 2019
- Committee Members:
- Jingjing Li, Dissertation Advisor/Co-Advisor
Jingjing Li, Committee Chair/Co-Chair
Saurabh Basu, Committee Member
Robert Carl Voigt, Committee Member
Jing Du, Outside Member
Scott Miller, Special Member
Ling Rothrock, Program Head/Chair - Keywords:
- multilayer thin sheets joining
mechanical behavior
microstructure characterization
process-structure-property linkage
online quality monitoring
grain boundary engineering - Abstract:
- This research focuses on unearthing the underlying scientific principles in thermo-mechanical micro joining of multilayers of dissimilar materials, where micro friction stir blind riveting (μFSBR) process was developed and characterized for this purpose. This research was performed as a continual effort to improve the viability of multilayer thin/ultra-thin joining. Four key research topics were identified for improving the wider applicability of the process. These topics include a) comprehending the influence of process parameters through developing methods of correlating joint quality and process mechanics, b) understanding the fundamental physics of metallurgical bond formation process, particularly in the context of friction stirring, from multi-scale, multilayer materials behavior and process mechanics perspectives, c) formulating the relationship between process-structure-property(PSP), and d) adaptations in the process to attain desired tailored properties. For the first research topic, quality guidelines in the form of process physics-based quality criteria and an online monitoring algorithm were developed to establish a relationship between joint quality and process mechanics. For this purpose, different levels of process parameters (spindle speed, feed rate and stacking sequence of Cu layer) were employed while real-time force and torque signals were also recorded. The process physics-based quality criterion was developed by analyzing the variations in the recorded force and torque signals. The developed parameters were successfully applied to two joint integrity scenarios, namely "no initial load drop" and "initial load drop" which were identified through load-displacement curves. Further, penetration force signals were analyzed by using data processing technique – pattern recognition protocol, where two distinct behaviors, were distinguished. For the second research topic, bonding mechanisms were established for the μFSBR joint at multi-scale, i.e. micro and nano, and for multilayer level. For this purpose, three interfaces (i.e. Al rivet and Cu sheet interface, Cu sheet and Al sheet interface, and Al rivet and Al sheet interface) were investigated through different microscopy techniques (i.e. scanning electron microscopy, SEM and transmission electron microscopy, TEM) to unearth the intricate yet distinct bonding phenomenon for each interface. The observations revealed that both mechanical and metallurgical bonding occurred simultaneously in μFSBR. For the third research topic, PSP linkage was formulated by first performing the mechanical testing which was then linked to microstructure and process physics. To accomplish this goal, a regimen of mechanical testing schemes, i.e. quasi-static tensile testing and load-controlled fatigue testing was employed. The improved mechanical performance was more pronounced for the Cu layer than Al layers which was attributable to the metallurgical bonding. Similarly, tensile tests for the overall μFSBR joints exhibited better load carrying capability than μBR joints. Further, hysteresis loops obtained during load-controlled fatigue tests revealed ratcheting strain as the dominant damage mechanism for low cycle fatigue scenarios, while fatigue damage is the governing damage mechanism in high cycle fatigue paradigms. Three distinct stages were classified from the mean displacement profile curves and acoustic emission (AE) hit datasets, namely initial plastic deformation, cyclic hardening or softening under load control (depending on loading condition, i.e. low cycle or high cycle fatigue), and crack initiation and growth till fracture. Life fraction – load – AE amplitude plots were developed to quantify the damage in three stages. The fourth research topic involves the utilization of friction stirring for grain boundary engineering. It was established that different grain boundary engineering mechanisms were involved in different zones, i.e. stir zone and thermomechanical affected zone, of μFSBR Cu sheet which in turn found strain- and temperature-dependent. Moreover, no abnormal grain growth (AGG) was observed for different configurations of post-heat-treated samples.