Download Preparation of porous Al2O3-Ti-C perform by combustion synthesis

ARCHIVES
of
ISSN (1897-3310)
Volume 9
Issue 2/2009
69-72
FOUNDRY ENGINEERING
16/2
Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences
Preparation of porous Al2O3-Ti-C
perform by combustion synthesis
K.Naplocha a,*, K.Granat a
a
Instytut Technologii Maszyn i Automatyzacji Politechniki Wrocáawskiej,
Politechnika Wrocáawska, 50-371 Wrocáaw, ul.àukasiewicza 3/5, Polska
* e-mail : [email protected]
Received 26.02.2009; accepted in revised form: 30.03.2009
Abstract
Using combustion synthesis porous ceramic preforms for composite reinforcing were produced. Prepared mixture of alumina Saffil
fibres, Ti powder and graphite flakes, after drying were placed in waveguide of microwave reactor. Supplied with constant power of 540W
magnetron ignited and maintained reaction in flowing stream of CO2 gas. Al2O3 fibres should improve preliminary endurance of perform,
whereas Ti powder processed to hard titanium carbides and oxides. During microwave heating ignited plasma additionally improve
process and partly fused metallic Ti.
Recorded temperature curves were similar for various samples. The highest synthesis temperature revealed samples containing 10% of
Al2O3 , 10% of Ti and 5% of graphite, all percentages atomic.
Microscopic observation showed considerable microstructure inhomogeneity of some samples. Both irregular component ordering and
partly processed Ti particles inside preform exclude them for subsequent infiltration. Chemical analyze EDS of Ti based compounds partly
confirmed work purpose, evidencing presence of Ti oxides and carbides. Independently of graphite content these compounds formed
folded strips around solid or empty volume. Depends on CO2 availability, reaction could be slowed down resulting in more compacted Ti
compounds. Created as a result of combustion synthesis Ti compound after infiltration with liquid metal properly bounded with the matrix.
It could be assumed that redox reaction proceeded and on surface of Ti compound alumina and Al-Ti compounds were created.
The preforms of proper strength and homogeneous structure were infiltrated with AlSi7Mg by squeeze casting method. In relation to
typical composite reinforced only with fibres no significant increase of defects quantity was observed. Preliminary examination of
mechanical properties confirmed that assumed work purpose is reasonable.
Key words: preform, combustion synthesis, composite, infiltration
1. Introduction
Manufacturing of composite materials by infiltration of
porous ceramic preform allows to locally reinforce casting in the
most loaded places. Properties of prefom play fundamental role to
achieve desired improve of composite quality. Presented in this
work production method of preform utilizes combustion synthesis
extensively developed also in the casting and metallurgy areas
[1-4].
Natural brittleness of intermetallic compounds, often
manufactured in these processes, can be reduced by means of
additional components or creating laminates [5-9]. Assumed in
this work conception consists in infiltration with relatively plastic
aluminium alloy of preform pores built from hard and brittle
phases. Used microwave reactor for igniting of plasma and
combustion synthesis allows not only to improve the process but
also create the structure and control reaction degree of initial
mixture.
A R C H I V E S O F F O U N D R Y E N G I N E E R I N G Vo l u m e 9 , I s s u e 2 / 2 0 0 9 , 6 9 - 7 2
69
Benefits of microwave heating or plasma application have
been already confirmed [10-12]. In the experiment a special
reactor allowing for concentration of field and heating of metallic
materials was built. Some of research results requires further
additional analyses and therefore sometimes there were used
general statements.
Magnetron was supplied with 240 W power supporting plasma
and synthesis development.
Preforms were infiltrated with EN-AC AlSi7Mg aluminium
alloy by direct squeeze casting. Preheated to about 500°C
preforms were put into die, then liquid metal was poured and
immediately pressurised with 40 MPa. Microscopic observation
were performed with an optical microscope and a scanning
microscope Hitachi S-3400N equipped with a microanalyser EDS
(4 nm, BSE detector). Phase identification was carried out using
an X-ray diffractometer Rigaku Ultima IV with Cu KĮ radiation
at 40 kV and 40 mA and using ICDD (2008) cards.
2. Experimental procedure
Saffil fibres and powders provided by Alfa Aesar company
are used for production of preform. Granularity of Ti, C amounts
44 Pm (-325 mesh) while Al2O3 +4% SiO2 fiber diameter ranges
between 4-6Pm. A proper portions of powder and fiber were
mixed in inorganic binder solution and next dried and pressed
into perpendicular 1x4x6cm samples. Volume content of fiber
was constant and amounted 10%, Ti powder ranged between 1012% and graphite 5-15%. Simplified symbols informing about
volume content of Al2O3 fiber-A, titanium -B and graphite -C
were used. Exemplarily, sample marked with A10T10C6 symbol
contains 10% of Al2O3, 10% of Ti and 6% of graphite.
Prepared samples were heated in microwave field to ignite
combustion synthesis and process preform component with
flowing CO2 gas. Designed and built microwave furnace consists
of rectangular waveguide, chamber with quartz tube and
moveable short circuit at the and of the line. More detailed
description was presented in [13]. Temperature was measured
with a pyrometer Raytek Marathon MM with 500-3000°C
temperature range and the measuring spot dia. 0.6 mm.
a)
3. Combustion synthesis
Comparing examined samples, course of combustion
synthesis, temperatures changes and process dynamics were
analyzed. The maximum temperature of the synthesis for samples
without graphite usually ranged between 1560-1670°C, with 6%
of C between 1500-1660°C and 1560-1670°C for samples
containing 10% of C. Temperature curves of all samples were
fairly similar (Fig.1), though material susceptibility to microwave
heating was different. Presented curves do not reflect often
observed fluctuation from discharges and unstable plasma.
Operating microwaves resulted in slower cooling, and in 800600°C range temperature was almost constant. Heated the
A10T12C6 samples revealed sometimes second temperature
increase presumably as a result of reaction of substrate remains.
b)
1800
1800
1600
1400
Temperature [C]
Temperature [C]
1600
1200
1000
800
1400
1200
1000
800
600
600
400
400
20
40
60
Time [s]
80
100
120
0
20
40
60
80
100
Time [s]
Rys.1. Synthesis temperature curve for the A10T10C10 sample a) and for the A10T10C5 sample b)
Combustion synthesis with flowing CO2 gas may include
complex reactions with all system substrates. Probably proceeded
intermediate stages related to adsorption and diffusion gas
elements into solid particles. Taking into account EDS, XRD
analyze results and literature investigation in Fig.2 is presented
proposal of synthesis course.
CO2 molecules are first of all oxygen source and by graphite
or on Ti particle surface forms carbon monoxide. As a reaction
result strongly increases temperature and molten Ti vigorously
reacted creating spread and grained structure of Ti-O-C
70
compounds. Component transport can be significantly disturbed
by microwave field and induced plasma. Though Ti particles
reflect microwave then on their surface, especially at sharp
corners are induced currents resulting in discharges. This
increases temperature and also disorders magnetron work.
Analyze of microwave influence based on observation and
comparison of samples produced in microwave field periodically
disconnected have not clear explained this problem yet.
A R C H I V E S O F F O U N D R Y E N G I N E E R I N G Vo l u m e 9 , I s s u e 2 / 2 0 0 9 , 6 9 - 7 2
CO2
C
C
desorption
reaction
with C
CO
Ti
CO
adsorption
TiO2
TiC
diffusion
and reaction
Flaky structure of graphite despite of high temperature was
maintained during synthesis (Fig.5b). No reaction products at the
boundary between graphite and Ti compound or aluminum alloy
were observed. It is possible that they were displaced or only
intermediate gas phase (CO) reacted.
a)
Rys.2. Proposal of combustion synthesis course with flowing CO2
gas
Microscopic observation showed poor homogeneity of
samples and considerable complexity of its structure. Light areas
in Fig.3 present matrix of AlSi7Mg aluminium alloy. Irregular
spreading phase are composed mainly from titanium oxide
(Fig.4). Properly embedded in the matrix, without reaction
products or porosities at the phase boundary can efficiently
enhance composite hardness [14]. Unfortunately, at limited gas
availability inside perform or insufficient processed components
there were formed impossible to infiltrate pores (Fig.3).
b)
Fig.4 Ti based compound a) EDS analyze b)
Fig. .3. Microstructure of the A10T10 sample, visible pores
inside Ti compounds
Slightly dark oval shapes are alumina fibres in cross section.
Scanning observation showed some extent of its degradation due
to high synthesis temperature. Probably GAl2O3 allotrope type
decaying resulted in volume change and cracking still before
infiltration with aluminum alloy. Synthesis of subsequent samples
containing additionally graphite from 5 to 15% was more stable,
though in the case of 15% its ignition was difficult. Moreover
sometimes large gas pressure inside perform and occurred high
stresses induced delamination.
Nevertheless preform components were uniformly ordered
(Fig.5a), without conglomerates or porosities as a result of local
micro ignition.
5.Conclusions
Production method of porous preform based on Al2O3 fibres,
Ti powder and flaky graphite was presented. Prepared mixture of
initial materials was heated in microwave field which ignited and
supported combustion synthesis. Flowing CO2 gas reacted with
Ti at presence of plasma and scattered discharges. Independently
of preform composition temperature curves were almost in all
cases similar. The highest synthesis temperature of 1860°C was
recorded for the A10T10C5 sample.
Microscopic observations showed in the most cases
inhomogeneous preform structure, layer texture and different
stage of unprocessed substrates. There were often observed
damaged Al2O3 fibres and frayed Ti compound. EDS
microanalyze and X-ray phase investigations indicate that Ti
formed mainly carbides and oxidizes. Selected samples with
A R C H I V E S O F F O U N D R Y E N G I N E E R I N G Vo l u m e 9 , I s s u e 2 / 2 0 0 9 , 6 9 - 7 2
71
uniform structure were used to locally reinforce aluminum
casting. Presence of very hard compounds based on Ti and
graphite as a lubricant could significantly improve wear
resistance.
a)
b)
Fig.5. Microstructure of the A10T10C10 sample a), visible
graphite flakes with Ti compounds embedded in aluminum alloy
matrix
Acknowledgements
This work was subsidized by the Polish Ministry of Education and
Science under the research project No. N508 054 31/2810.
72
References
[1] E. FraĞ, A. Janas, A. Kolbus: Ccast in situ aluminium matrix
composite reinforced with titanium boride particles,
COMPOSITES, 1(2001)1, 23-27
[2] Yaodong Liu, Wei Liu: Mechanical alloying and spark plasma
sintering of the intermetallic compound Ti50Al50, Journal of
Alloys and Compounds, 440, Issues 1-2, (2007), 154-157
[3] F. Zhang, L. Lu, M. O. Lai: Study of thermal stability of
mechanically alloyed Ti–75% Al powders, Journal of Alloys
and Compounds, 297, Issues 1-2, (2000), 211-218
[4] S. Djanarthany, J. -C. Viala, J. Bouix: An overview of
monolithic titanium aluminides based on Ti3Al and TiAl,
Materials Chemistry and Physics, 72, Issue 3, (2001), 301-319
[5]M.Konieczny: Processing and structure of laminated ironintermetallics composites, Archives of foundry Engineering,
vol.8, issue 4, 2008, 71-76
[6] M. Kostecki, A. Olszyna: Modification of mechanical
properties of Ti-Al intermetallic matrix by introducing TiB2
ceramic, Composites 6(2006), 65-71
[7] L.M. Peng , J.H. Wang, H. Li, J.H. Zhao, L.H. He: Synthesis
and microstructural characterization of Ti–Al3Ti metal–
intermetallic laminate (MIL) composites, Scripta Materialia 52
(2005) 243–248
[8] Jinkeun Oh, Sung Gyu Pyo, Sunghak Lee, Nack J. Kim:
Fabrication of multilayered titanium aluminide sheets by selfpropagating high-temperature synthesis reaction using hot
rolling and heat treatment, Journa; of Materials Science 38
(2003) 3647 – 3651
[9] Jian-Guo Luo, Viola L. Acoff: Using cold roll bonding and
annealing to process Ti/Al multi-layered composites from
elemental foils, Materials Science and Engineering A 379
(2004) 164–172
[10] Yaodong Liu, Wei Liu: Mechanical alloying and spark
plasma sintering of the intermetallic compound Ti50Al50,
Journal of Alloys and Compounds, 440, Issues 1-2, (2007), 154157
[11] A.Dudek, Z.Nitkiewicz: Characteristics of microstructure in
composite surface layers, Archives of foundry Engineering,
vol.8, issue 1, 2008, 75-78
[12] Jokisaari J.R., Bhaduri S., Bhaduri S.B.: Microwave
activated combustion synthesis of titanium aluminides,
Materials Science & Engineering, A394, 2005, s. 385-392.
[13] K.Naplocha, K.Granat: Reaction synthesis and
microstructure of Al-Ti perform for composite reinforcing,
Archives of foundry Engineering, vol.8, issue 1, 2008, 227-232
[14] J. ĝleziona, M. Dyzia, B. Formanek, A. Dolata-Grosz, J.
Wieczorek: Application of reactive powders containing Fe and
Ti compounds for manufacturing aluminium matrix composites,
Composites 2(2002)5, 302-304
A R C H I V E S O F F O U N D R Y E N G I N E E R I N G Vo l u m e 9 , I s s u e 2 / 2 0 0 9 , 6 9 - 7 2