Abstract

Aluminum-copper (Al-Cu) alloy B206 is a high strength and ductile alloy showing promise for use in automotive suspension components. Incorporation of lightweight B206 alloy in automotive suspension components may significantly reduce overall vehicle weight and increase the vehicle’s fuel efficiency. However, one of the major factors inhibiting the use of B206 is its high susceptibility to hot tearing during casting. Hot tearing is a complex phenomenon attributed to alloy solidification, microstructure and stress/strain development within a casting. Numerous methods (e.g. preheating of mold, grain refinement, elimination of sharp corners in a component) help to reduce the occurrence of hot tears in castings, but the underlying mechanisms responsible for hot tearing remain ambiguous. This research aims to advance the understanding of the mechanisms responsible for hot tearing in B206 Al alloy. In this research, the conditions associated with the formation of hot tears in B206 were investigated via ex situ and in situ methods. Titanium was added in three levels (i.e. unrefined, 0.02 and 0.05 wt%) to investigate the effect of grain refinement on hot tearing. Ex situ neutron diffraction strain mapping was carried out on the three B206 castings to determine casting strain and stress. Further, in situ techniques were used to establish the onset temperature and solid fraction of hot tearing in B206 and to improve the understanding of microstructure development in B206. The results indicate that titanium additions had a significant impact on the hot tearing susceptibility of B206, by effectively reducing grain size and transforming grain morphology from coarse dendrites to fine globular grains. Further, thermal analysis suggested that grain refinement delayed the onset of dendrite coherency in B206 and therefore enhanced the duration of bulk liquid metal feeding for the refined casting conditions. As a result, the interactive effects of such factors resulted in a more uniform distribution of strain, and subsequent higher resistance to hot tearing for the grain refined castings. Finally, in situ analysis determined the onset solid fraction of hot tearing in B206 and provided an understanding of the role of microstructure on hot tearing in B206.

Highlights

  • Aluminum-copper (Al-Cu) alloy B206 is a high strength and ductile alloy showing promise for use in automotive suspension components

  • Titanium was added in three levels to investigate the effect of grain refinement on hot tearing

  • The results indicate that titanium additions had a significant impact on the hot tearing susceptibility of B206, by effectively reducing grain size and transforming grain morphology from coarse dendrites to fine globular grains

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Summary

Chapter 1 – Introduction

There is a growing interest in aluminum-copper (Al-Cu) alloys for use in automotive and aerospace industries. The ex situ portion of the research consisted of permanent mold casting, microstructure and thermal analysis and ex situ measurement of residual strain in the castings Such aspects were directly related to hot tearing severity in B206. A similar methodology was carried out for the in situ portion of the dissertation In this case, the permanent mold enabled the determination of the onset temperature of hot tearing in B206, while neutron diffraction was used to establish the onset solid fraction of hot tearing and to gain an understanding of the in situ development of microstructure during solidification of B206

Chapter 2 – Literature Review
Alloy Solidification
Hot Tearing
Theories of Hot Tearing
Effect of Alloy Composition
Effect of Casting Process Parameters
Effect of Melt Superheat
Effect of Mold Temperature
Ring Mold Test
Cold Finger Test
In Situ Methods
Neutron Diffraction
Analysis of Hot Tearing by Neutron Diffraction
Chapter 3 – Experimental Procedure
Ex Situ Casting Experiments
Permanent Mold Design
Mold Mounting
Mold Coating
Control of Casting Temperature
Alloy Melting and Casting
Neutron Diffraction Strain Mapping Experiments
Neutron Beam Sampling Volume The beam of “white” neutrons (see
Selection of Samples for Strain Analysis
Stress-Free Samples
In Situ Casting Experiments
Hot Tearing Apparatus
Determination of Onset Temperature of Hot Tearing
In Situ Neutron Diffraction Solidification Analysis
Preparation of Samples
FactSage Simulation
Neutron Diffraction Analysis The C2 neutron powder diffractometer at CNBC in
Casting Microstructure Analysis
Review of Prior Research by Candidate
Al-Cu Phase Diagram
Grain Size and Morphology
General Alloy Microstructure
Analysis of Hot Tear Regions
Unrefined Alloy
Casting Porosity
Thermal Analysis
Unrefined B206
Refined Alloys
Solidification Characteristics of B206
Neutron Diffraction Residual Strain Mapping
Residual Strain in the x-Direction
Strain At the Sprue-Bar Junction
Strain Along the Horizontal Bar
Residual Strain in the y-Direction
Residual Strain Along the Downsprue
Residual Strain in the z-Direction
Residual Stress in B206 Castings
Residual Stress in the x-Direction
Residual Stress in the y and z-Direction
Observation of Hot Tears on Casting Surfaces
Force-Temperature-Time Curves
Primary Aluminum Peak Evolution
Primary Aluminum Phase Evolution
Intermetallic Al2Cu Peak Evolution
Intermetallic Al2Cu Phase Evolution
Onset Solid Fraction of Hot Tearing in B206
Development of Microstructure in B206
Chapter 6 – Conclusions
Chapter 7 – Recommendations for Future Work
Unrefined Alloy a)
Cooling Curves
Uncertainty in Residual Strain Calculations
Peer-Reviewed Journal Paper Publications
Conference Proceedings
Findings
Fellowships and Awards
Full Text
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