Abstract

Lightweighting has been regarded as a key strategy in the automotive industry to improve fuel efficiency and reduce anthropogenic environment-damaging, climate-changing, and costly emissions. Magnesium (Mg) alloys and Aluminum (Al) alloys are progressively more used in the transportation industries to reduce the weight of vehicles due to their high strength-to-weight ratio. Similarly, high strength low alloy (HSLA) steel is widely used to reduce gauge thickness and still maintain the same strength, and thereby reduce vehicle weight as well. A multi-material design of automotive structures and parts inevitably involve similar Mg-to-Mg and dissimilar Mg-to-Al, Al-to-steel, and Mg-to-Cu joints. Ultrasonic spot welding (USW) – a solid-state joining technique has recently received significant attention due to its higher efficiency in comparison with conventional fusion welding techniques. In this study, USW was used to generate similar joints of low rare-earth containing ZEK100 Mg alloy sheets and dissimilar ZEK100-to-Al5754, Al6111-to-HSLA steel, and Mg-to-Cu joints at different levels of welding energy or welding time. To optimize welding process and identify key factors affecting the weld strength, microstructural evolution, microhardness test, tensile lap shear test, fatigue test, and fracture analysis were performed on similar and dissimilar ultrasonic spot welded (USWed) joints. Dynamic recrystallization and grain coarsening were observed during Mg-to-Mg similar welding while rapid formation and growth of interface diffusion layer were observed in all dissimilar joints in the present study. It was due to significantly high strain rate (~103 s-1) and high temperature generated via frictional heating during USW. The interface diffusion layer was analyzed by SEM, EDS and XRD phase identification techniques which showed the presence of eutectic structure containing intermetallic compounds (IMCs). As a result, brittleness at the interface increased. The Zn coating in dissimilar USWed Al-to-steel joints eliminated the formation of brittle IMCs of Al-F, which were replaced by relatively ductile AlZn eutectic. The optimum welding energy or welding time during similar and dissimilar USW of lightweight alloys with a sheet thickness of 1-2 mm was in the range of ~500 J to 2000 J (~0.25 s to 1 s).

Highlights

  • Lightweighting has been regarded as a key strategy in the automotive industry to improve fuel efficiency and reduce anthropogenic environment-damaging, climate-changing, and costly emissions

  • It can be reasoned that as the heat input increased with increasing welding energy, the increase in temperature resulted in grain growth, causing a decrease in hardness based on the Hall-Petch type relationship which was observed in an AZ31B-H24 Mg alloy after friction stir welding or processing [121,122]

  • Much of the focus of this work was on evaluating the feasibility of joining a new highperformance rare earth containing ZEK100 Mg alloy in similar (ZEK100-to-ZEK100) and dissimilar configurations with automotive grade Al5754 alloy sheet (Chapter 4 and Chapter 5)

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Summary

Chapter 1. Introduction and Motivation

The mounting environmental concern to diminish anthropogenic climate-changing, environment-damaging, costly and human death-causing* emissions along with unstable energy prices has driven the transportation industry to improve fuel efficiency [1–7]. Laser welding is contactless, high precision, and high speed joining technique, disadvantages such as the need for shielding gas, high initial cost and higher energy consumption limit their applications [32] Since both are a fusion joining processes, high rate of intermetallic reaction occurs in the liquid phase, which degrades the joints quality [36,39]. USW is an emerging joining process with low energy consumption, shorter weld time, no shrinkage and distortion problems [9,38,44], and higher efficiency, since heat generation is at the weld interface [9,38,44] rather than on the top surface as in FSSW [27,42,43] It was reported during welding of aluminum alloys; the USW process uses only about 0.3 kWh energy for 1000 joints [45,46] compared to 2 kWh with FSSW and 20 kWh with RSW [46]. Joining takes place by the formation and growth of microwelds at the surfaces due to nascent metal-to-metal contact, creating metallurgical adhesion and interdiffusion across the interface [27,38,44,49]

Objective and scope of the dissertation
Structure of the dissertation
Chapter 2. Literature Review
Overview of USW
History of USW
USW principle
USW configurations
Lateral-drive USW
USW equipment
Process factors in the USW
Advantages and disadvantages of USW system
Recent studies in the area of USW
Welding of Mg alloys
Microstructure analysis
Mechanical properties
Welding of dissimilar alloys
Dissimilar Al-to-steel joints
Dissimilar Mg-to-Cu joints
Summary of the literature review
Materials
USW process parameters
Quantitative image analysis
Phase identification by X-ray diffraction
Microhardness tests
Tensile lap shear tests
Microstructure characterization
Shear strain rate during USW
Weld interface temperature
Zener-Hollomon parameter
Microhardness
Tensile lap shear strength
Fractography
Effect of test temperature on the tensile lap shear strength
X-ray diffraction phase identification
Fatigue behavior and failure mode
Summary
Chapter 6. Ultrasonic Spot Welding of Aluminum Alloy-to-HSLA Steel
Energy-dispersive X-ray spectroscopy analysis (a) Zn clusters
Tensile fractography
Vickers’s microhardness
Fatigue fractography
Chapter 7. Ultrasonic Spot Welding of Magnesium Alloy-to-Copper**
Energy-dispersive X-ray spectroscopy analysis
Diffusion pattern
Vickers microhardness
Conclusions
Major contributions
Findings
Recommendations for future work
Full Text
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