Doctoral Dissertations

Keywords and Phrases

Friction stir processing; Thermal stability

Abstract

"Friction stir processing (FSP) is an adapted version of friction stir welding (FSW), which was invented at The Welding Institute (TWI), 1991. It is a promising solid state processing tool for microstructural modification at localized scale. Dynamic recrystallization occurs during FSP resulting in fine grained microstructure. The main goal of this research was to establish microstructure/superplasticity relationships in FSP aluminum alloys.

Different aluminum alloys (5083 Al, 2024 Al, and Al-8.9Zn-2.6Mg-0.09Sc) were friction stir processed for investigating the effect of alloy chemistry on resulting superplasticity. Tool rotation rate and traverse speeds were controlled as the prime FSP parameters to produce different microstructures. In another study, lap-joints of 7475 Al plates were also studied to explore the possibility of developing FSW/superplastic forming route. Microstructures were evaluated using optical, scanning and transmission electron microscopy, orientation imaging microscopy and differential scanning calorimetry. Mechanical properties were evaluated using tensile testing.

FSP 2024 Al (3.9 µm grain size) exhibited an optimum ductility of 525% at a strain rate of 10-2 s-1 and 430°C. Grain boundary sliding mechanism was found to be the dominant mode of deformation in this alloy. In 5083 Al alloy, it was found that changing the process parameters, grain sizes in the range of 3.5-8.5 µm grain size could be obtained. Material processed with colder processing parameters showed a decrease in ductility due to microstructural instability, and followed solute drag dislocation glide mechanism. On the other hand, materials processed with hotter parameter combinations showed mode of deformation related to grain boundary sliding mechanism. FSP of as-cast Al-Zn-Mg-Sc alloy resulted in ultrafine grains (0.68 µm) with attractive combination of high strain rate and low temperature superplasticity. This also demonstrated that superplastic microstructures could be obtained directly from as-cast alloys via a single step of FSP.

All these showed that FSP has the potential of becoming an important grain refinement technique for producing fine grained aluminum alloys with superplastic properties. Microstructural instability of FSP aluminum alloys at higher temperatures can be limited or eliminated with proper control of process parameters and improved alloy design"--Synopsis, pages iv-v.

Advisor(s)

Mishra, Rajiv S.

Committee Member(s)

Van Aken, David C.
Newkirk, Joseph William
Peaslee, Kent D., 1956-2013
Dharani, Lokeswarappa R.

Department(s)

Materials Science and Engineering

Degree Name

Ph. D. in Metallurgical Engineering

Comments

The author gratefully acknowledges the financial support provided by the National Science Foundation (NSF) of the United States under grant no. DMI-0085044. He also acknowledges the NSF grant DMR-0076433 and University of Missouri Research Board for funding the acquisition of the friction stir welding machine at the University of Missouri--Rolla (UMR).

Publisher

University of Missouri--Rolla

Publication Date

Spring 2004

Journal article titles appearing in thesis/dissertation

  • Multi-sheet structures in 7475 aluminum by friction stir welding in concert with post-weld superplastic forming
  • High strain rate superplasticity in a commercial 2024 Al alloy via friction stir processing
  • Evaluation of microstructure and superplasticity in friction stir processed 5083 Al alloy
  • Low temperature superplasticity in a friction-stir-processed ultrafine grained Al-Zn-Mg-Sc alloy

Pagination

xvii, 143 pages

Note about bibliography

Includes bibliographical references.

Rights

© 2004 Indrajit Charit, All rights reserved.

Document Type

Dissertation - Restricted Access

File Type

text

Language

English

Subject Headings

Aluminum alloysAluminum alloys -- MicrostructureSuperplasticity

Thesis Number

T 8472

Print OCLC #

56726467

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