Friction stir processing (FSP) was applied to produce aluminum based in situ
composites from powder mixtures of Al-Fe, Al-Mo, and Al-Fe2O3. Billet of powder
mixtures was prepared by the use of conventional pressing and sintering route. The
sintered billet was then subjected to multiple passages of FSP. During FSP, the material
has experienced both high temperature and very large plastic strain. The basic idea for
fabricating the composites is to combine the hot working nature of friction stir
processing (FSP) and the exothermic reaction between aluminum and transition metals
(Al-Fe, Al-Mo) or metal oxides (Al-Fe2O3).
In the Al-Fe alloy, in situ Al¡VFe reaction can be induced during FSP and form
Al-Al13Fe4 composite. The size of reinforcing particles formed by the in-situ reaction is
~100 nm. In Al-Mo alloys, fine Al-Mo intermetallic particles with an average size of
~200 nm were formed and uniformly dispersed in the aluminum matrix by FSP. The
Al-Mo intermetallic particles were identified mainly as Al12Mo with minor amount of
Al5Mo. The exothermic reaction could result in local melting of Al at the Al/TM
interface, and the liquid Al may accelerate the reaction. In addition, it is suggested that
the critical mechanism responsible for the rapid reaction and the formation of nanometer
sized particles in FSP is the effective removal of the Al-TM intermetallic phase from
the Al-TM interface, maintaining an intimate contact between TM and Al.
In the Al-Fe2O3 system, the reactions taking place during FSP includes the thermite
reaction (2Al +Fe2O3 ¡÷ Al2O3 + 2Fe), and the reaction between the reduced Fe and Al
to form Al13Fe4. In the FSPed Al-Fe2O3 specimens, there are two types of second phase
particles, Al13Fe4 and Al2O3. The Al2O3 particles (about 10 nm in size) usually appear
as a cluster of 100-200 nm in diameter. There are two types of Al2O3 phases existed in
the Al matrix after FSP passes, depending on the content of Fe2O3. One is £^-Al2O3 in Al-2Fe2O3 specimens, and the other is £\-Al2O3 in Al-4Fe2O3 specimens. It is suggested
that the formation of different type of Al2O3 particles in the Al-Fe2O3 composites may
be attributed to different heat release in each system. The lower heat release in
Al-2Fe2O3 sample favors the formation of the while the higher heat release in
Al-4Fe2O3 sample results in the £\-Al2O3.
The Al-Al13Fe4/Al2O3 composite produced by FSP exhibits both high strength and
good tensile ductility. The higher strength in Al-Fe2O3 specimen may be due to the
presence of fine Al2O3 particles. The flow stress of the Al-4Fe2O3 composite can
maintain at 100 MPa even at 773 K. The good thermal stability and high temperature
strength of Al-Al13Fe4/Al2O3 composites could be attributed to the fine dispersion of
second phase particles in the aluminum matrix, especially the nanometric Al2O3
particles. These Al2O3 particles are very stable at elevated temperatures, even after long
time exposure at 873 K.
The temperature excursion in FSP is determined by both the FSP parameters and
the exothermic reaction involved. The peak temperature in Al-Fe or Al-Fe2O3
system during FSP was calculated as a function of the fraction of Fe or Fe2O3 reacted.
Based on calculated results, it is noted that with the in situ reaction, the value of
can easily reach the melting point of Al, especially for the Al-Fe2O3 system. The
reaction mechanism and microstructure evolution during FSP are discussed.
| Identifer | oai:union.ndltd.org:NSYSU/oai:NSYSU:etd-0326111-012021 |
| Date | 26 March 2011 |
| Creators | Lee, I-Shan |
| Contributors | Ker-Chang Hsieh, Pei-Ling Sun, Hsing-Lu Huang, Po-wei Kao, ¢Üew-Jin Ho |
| Publisher | NSYSU |
| Source Sets | NSYSU Electronic Thesis and Dissertation Archive |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.lib.nsysu.edu.tw/ETD-db/ETD-search/view_etd?URN=etd-0326111-012021 |
| Rights | not_available, Copyright information available at source archive |
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