Advisor(s)

Teiichi Ando

Date of Award

2010

Date Accepted

8-2010

Degree Grantor

Northeastern University

Degree Level

M.S.

Degree Name

Master of Science

Department or Academic Unit

College of Engineering, Department of Mechanical and Industrial Engineering

Keywords

materials science, ultrasonic materials, Cu-Cr composites, surface hardening

Disciplines

Materials Science and Engineering

Abstract

Ultrasonic materials process (UMP) has been found to produce unusual effects on the thermodynamic state and kinetics of structural changes in metals and alloys. This research was conducted with the objective of applying UMP to the consolidation of Cu and Cr powder mixtures into Cu-Cr composites suitable for electric contact and welding electrode applications, and also to the surface hardening of aluminum by rapid diffusion of Cu into Al and in-situ age hardening in the diffusion zone.

The process variables involved in the ultrasonic powder consolidation (UPC) study were the temperature and time of consolidation, clamping pressure, punch-die geometry and the atmosphere of consolidation. Both air and argon gas atmospheres were used. The aluminum surface hardening study involved the same variables except only air atmosphere was used.

Simultaneous consolidation of Cu-10~30wt% Cr composites on a Cu substrate with near theoretical density was achieved without using a lubricant at clamping pressures as low as 17.5 MPa and moderate temperatures ≤ 500 °C (773 K) within a very short consolidation time of a few seconds or less. Metallographic observation by optical microscopy and scanning electron microscopy (SEM) indicated near full-densification and metallurgical bonding of the Cu powder particles into a Cu matrix in which nearly undeformed particles of Cr were distributed uniformly. In the composites consolidated under optimal conditions, only a small amount (< ~1%) of isolated porosity was detected at some of the Cu matrix-Cr particles interface. Full metallurgical bonding was achieved also at the interface between the composite and the Cu substrate. Energy-dispersive X-ray fluorescence spectroscopy (EDS) revealed no trace of oxygen in the Cu-Cr composites produced under optimal conditions, suggesting that oxygen gas was effectively purged out during UPC without leaving significant amount of oxide in the consolidates. Compacts made without ultrasonic vibration but under otherwise identical conditions had significantly lower fractional density than those processed with ultrasonic vibration. In addition, no metallurgical bonding occurred at the compact-substrate interface in the absence of ultrasonic vibration.

Rapid diffusion of Cu into Al took place when the surface of Al sheet covered with Cu foil was subjected to ultrasonic vibration parallel to the surface at temperatures as low as 160 °C (433 K). The interdiffusivity determined form EDS profiles was as high as 0.14 µm2/s at 250 °C (523 K), a value four orders of magnitude higher than the normal value of diffusivity of Cu in Al. The microhardness of the diffusion zone of the specimens measured significantly higher than that of the interior of the Al sheet where no Cu reached. The hardening peaked at a processing temperature of 160 °C (433 K), suggesting that the rapid diffusion of Cu promoted in-situ precipitation hardening. Much less hardening, due primarily to strain hardening and dynamic recovery, was noted with Al sheet processed without Cu foil.

The above results of both the Cu-Cr composite consolidation and Al surface hardening studies reflect the high concentration of lattice defects, particularly vacancies, introduced in the material by the high strain-rate deformation caused by UMP which is estimated to be of the order of 103 s-1.

Document Type

Master's Thesis

Rights Information

copyright 2010

Rights Holder

Faryar Tavakoli-Dastjerdi


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