Advisor(s)

Teiichi Ando

Contributor(s)

Yung Joon Jung, Peter Wong

Date of Award

2010

Date Accepted

12-2010

Degree Grantor

Northeastern University

Degree Level

Ph.D.

Degree Name

Doctor of Philosophy

Department or Academic Unit

College of Engineering. Department of Mechanical and Industrial Engineering.

Keywords

aluminum, carbon nanotube, electrical resistivity, ultrasonic powder consolidation, ultrasonic welding, vacancy concentration

Disciplines

Mechanical Engineering

Abstract

Ultrasonic welding (USW) of metals is a widely used commercial process for bonding electrical connections in the solar, automobile and semiconductor industries. USW utilizes high strain rate plastic deformation to create a metallurgical bond but there is not a complete understanding of the bonding mechanisms. In the present work, the measured increase of electrical resistivity during ultrasonic deformation of aluminum indicated the generation of excess vacancy concentrations (Xv) orders of magnitude larger than equilibrium values. TEM observation of ultrasonically deformed aluminum revealed dislocation structures that reflected concurrent strain hardening and dynamic recovery. Dislocation loops were also observed indicating the generation of a large excess Xv during ultrasonic deformation. The very large amount of excess vacancies generated during ultrasonic deformation increases diffusion by orders of magnitude and as such may be an important bonding mechanism of USW. Gaining a better understanding of USW bonding mechanisms including the resulting effects of enhanced diffusion will help to determine the temperature-pressure parameters required to create full-density, metal powder composites using the ultrasonic powder consolidation (UPC) technique.

UPC is a recently developed method capable of consolidating full-density metal powder composites within a few seconds or less without the need for subsequent sintering. The present work utilized two different UPC techniques, the punch and packet methods, to create full density, pure aluminum and aluminum-matrix CNT composites. X-ray diffraction (XRD) results showed that no aluminum carbide (Al4C3) was formed during consolidation of UPC aluminum-CNT composites. Mixing of aluminum powder with 0.5, 1.0 and 2.0 wt.% CNTs showed that homogeneous dispersion of CNTs became more difficult with greater mass fraction of CNTs. As a result, CNT agglomerates within aluminum-1.0 and 2.0 wt.% CNTs composites created porosity in the microstructure that decreased microhardness and ductility. Aluminum-0.5 wt.% CNT composites contained lower amounts of CNT agglomerates and were cold rolled to create 90 µm thick foils that had twice the yield strength and a 10% increase in microhardness compared to pure aluminum rolled foils. The CNTs produced an increased strain gradient within the aluminum matrix resulting in greater work hardening during the rolling process.

Document Type

Dissertation

Rights Holder

David Scott Colanto


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