Light alloy composite production by liquid metal infiltration

Light alloy composite production by liquid metal infiltration Z ZHANG, S. LONG and H M FLOWER (Department of Matertals, Impertal College) Recetved 8 October 1993, revtsed 20 January 1994 L~quld metal infiltration of a ceramic fibre preform offers a cost-effective netshape production route for the preparation of hght alloy composite materials. However, incomplete infiltration and shrinkage on solidification can produce senous defects, degrading the mechamcal properties of the cast component. Additionally, chemical ~nteract~ons between the hqu=d metal and the ceramic can alter the matrix alloy chemistry and m~crostructure, and produce poor metal/ ceramic interface structures. Understanding the infiltration process, the heat and mass flow and chemical interactions, as functions of process conditions, is essential to the production of composites with low defect densities and minimized chemical interactions. A specially designed, highly ~nstrumented casting facihty has been constructed to carry out quantitatwe studies of liquid metal infiltration of ceramic preforms. The facdity is described and its application to the study of composite casting ~s d~scussed. Key w o r d s : metal-matrtx composttes; liqutd metal mfiltration; casting facility," non-mfiltratton defects; melt flow behawour," modelling

The need for materials with property mixes including low density, high strength, high suffness, increased wear resistance and ~mproved high temperature performance has focused much interest on the development of metalmatrix composites (MMCS) Compared with solid-state MMC fabrtcatton routes, such as powder metallurgy t or &ffus~on bonding of thin sheets interleaved with ceramic fibres -~, hquld-state methods--especially the liquid metal lnfiltraUon methods involving the infiltration of a preform by apphed posluve or negauve pressure--have outstanding advantages because the matrix metal ~s in the hquld state during manufacturing processes The advantages of hqmd metal mfiltrauon methods include. • • • • • •

producuon of near-net-shape composite parts of complex shape, ease of fabncauon of selectively reinforced components: similarity with conventional casting technology: smtabfl~ty for mass production, flexlblhty for MMC design by combination of various kinds of metal matrices and reinforcements, relauve simplicity, speed of fabncatton and low cost.

These features of the infiltration method have made It one of the most compeutlve processes for the production of reinforced hght metal components Some of the infiltration methods that promise to find mdustrml apph-

caUon include squeeze casting ~ ~, low-pressure casting v and vacuum casting s. After about 30 years of acuve research, both into processing technology and the underlying fundamental principles, metal-matrix composites produced by mfiltrauon methods are now beginning to make a true contribution to industrial and engineering pracuce. During the past decade, cast MMCS have been commercially produced in a number of pracucal applications. Table 1 shows the practical apphcatlon of cast MMCS for commercml products m Japan ~J°. However, MMCS generally have not yet achieved w~despread industrial applicauon. This SltUaUon is attributable to a number of factors including high cost, lack of commercially acceptable means of production, quahty assurance of MMC products and the mechamcal properUes not reaching the theoretically predicted values. The latter point ~s parucularly relevant to MMCS produced by hquid metal mfiltrauon routes. From the technical point of wew, fibre-reinforced cast MMCS as engineering matermls lack the comprehensive understanding of the process control of m~crostructure and properties, and lack manufacturing data on the production of commercml MMC components, which ~s essenUal for commercial apphcation Compared w~th traditional h~gh-pressure die casting ~ and low gas pressure casting ~-',the casting conditions and

0010-4361/94/05/0380-13 O 1994 Butterworth-Hememann Ltd 380

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Table 1. The practical application of cast M M C s for commercial products in Japan s

Product

MMC system

Method of manufacture

Charactenst=cs of apphed Year MMC (maker)

Ring groove reinforced p~ston

AI203/AI alloy

Squeeze casting (SC)

L~ghtweight, wear resistance at h~gh temperature

1983 (Toyota)

Golf goods Face of screwdrwer

S~Co/AI alloy

SC

Light weight, abrasion resistance

1 984 (Nippon Carbon)

Connection rod of gasohne engine

SUS flbre/AI alloy

SC

Specd~c strength

1985 (Honda)

M6 ~ 8 bolt

S~Cw/6061

SC, extrusion, tread rolhng

Neutron absorption, high 1986 temperature strength, (Toshiba) httle degassmg

Vane, pressure s~de plate of od pressure vane pump

AI203 S~02/ AC4C

SC

Wear resistance, noise damping

1987 (Hiroshlma Alummlum)

Joint of aerospace structure

S~C./7075

SC, rolhng

Specific strength, low thermal expansion

1988 (Mitsublshl Electromcs)

Rotary compressor vane

S~C./AI-17% S~-4% Cu alloy

SC

Specific strength, wear resistance, low thermal expansion

1989 (Sanyo)

Shock absorber cyhnder

S~C0/AI alloy

Compocastmg, SC, extrusion

Light weight, wear resistance, thermal diffusion

1989 (Mitsublshl Alumlmum)

D~esel engine p~ston

S~Cw/AI alloy

SC

Light weight, wear resistance

1 989 (NHgata)

Cyhnder hner

AI203, CF/AI alloy

Low pressure SC Light weight, wear reslsta ace

1991 (Honda)

Hub of damper pulley 1°

AI203 SIO2/AI alloy

SC

1991 (Toyota)

soh&ficatton processes during the fabncauon of MMCS are slgmficantly altered by the presence of remforcmg phases. These differences mclude the need for preparauon and precise location of reinforcement, hlgher infiltrauon resistance created, change of matrix melt flow mode. movement, deformauon or damage of the preforms, non-umform &stribuuon of reinforcement, nonmfiltrauon defects and residual porosity, chemical segregaUon, mterfacml bonding and chemical reacUon between the matrix and reinforcement, and mo&ficauon of matrix m~crostructure. Any of these aspects, ~f not properly controlled, can adversely affect the mechamcal properues of MMC materials. In recent years, some ~mportant work has been done allowing a very good understanding of some aspects of the squeeze casung process ~3 =s the gas pressure castmg process ~9 -'~ and the underlying principles deahng with the processes of mfiltraUon and sohdlficauon, fired flow, heat and mass transfer and processing modelhng -'2 ~-~ However. some areas are stdl unclear For example. hqmd metal mfiltraUon of a shaped fibre preform can be achieved by a variety of methods revolving the apphcation of h~gh or low pos~uve pressure and/or negauve pressure, all of which have been employed mdiwdually m numerous stu&es elsewhere However, experimental equipment is normally designed to study only one of

Light weight, reduction of vibration

these processes m ~solat~on and comparison of results from different studies is comphcated by lack of direct comparabdlty of the data generated: d~fferent alloy chemistnes, melting and pouring procedures, fibre preform varmUons, melt superheat and preform temperature, process pressure and process ume, etc, all make the fundamental assessment and comparison of casting methods impracucal As such, a proper comparison of the fabncauon methods cannot be made by reference to the existing hterature, and this kmd of compartson is essenttal for an engineer to choose the correct manufacturing route for a specific MMC product Furthermore, in pracuce, a s~gmficant number of processing defects--including non-mfiltrauon voids, undesirable interracial reaction, shrinkage pores and non-uniform d~stnbutlon of reinforcement--are repeatedly observed m cast MMCS These types of defect can cause h~gh stress concentrations and act as crack m m a tors or result m fadure to achieve effectwe load transfer from the matrtx to the reinforcement, thereby sertously degrading the mechamcal performance of the MMCS. Some previous research describes the relationship between threshold mfiltraUon pressure and fibre array geometry from the v~ewpomt of hydrostaUcs-'~-'7.-'L and analyses the statlsucal average mfiltraUon behavlour of

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metal melt in a unidirectional c o n t i n u o u s fibre a r r a y ~" ~t, c h o p p e d fibre p r e f o r m ~-' ,4 and ceramic partxculate ~ from the viewpoint o f h y d r o d y n a m i c s . M o s t o f the work has assumed that the infiltration front is p l a n a r and separates the sample into a region o f fully infiltrated c o m p o site and a region o f non-infiltrated p r e f o r m However, the ceramic f i b r e / a l u m m l u m c o m p o s i t e systems are nonwetting, and in a practical p r e f o r m the fibres are distributed n o n - u n i f o r m l y on the m i c r o s c o p i c scale In this c o n d m o n , the infiltration front in the samples is p l a n a r only on a m a c r o s c o p i c scale and IS microscopically irregular The melt must infiltrate the large pores first and then infiltrate the smaller ones It ts o b v m u s that f o r m a t i o n of some o f the processing defects, such as non-lnfiltratmn defects, porosity, movelnent o f the ternforcing phase and segregation, is directly related to the microscopic flow b e h a v , o u r o f the metal melt in the fibre reinforcement F u r t h e r m o r e , melt flow b e h a v t o u r is different with c o n t i n u o u s fibre and d i s c o n t i n u o u s fibre as the m a j o r remforcement However. little attention has been paid to microscopic flow behavlour. M o d e l s u n s u b s t a n t i a t e d by experimental work, while o f theoretical interest, are o f little practLcal utility It ts also a f u n d a m e n t a l weakness o f much previously published work on p r o p e r t y assessment in m e t a l - m a t r i x c o m p o s i t e s that the relatmnshlps between processing history and mlcrostructure have not been d e t e r m m e d Therefore, a clear picture o f the f o r m a t i o n mechanisms o f some processing defects is not yet available There is still a considerable gap between the research work on MM(S and their industrial a p p h c a t l o n In o r d e r to improve the current properties and to accelerate the wider a p p l i c a t i o n o f cast MMCS, further research t~ needed to o b t a i n a detailed u n d e r s t a n d i n g o f the physical nature o f mliltratlon and sohdlticatlon p h e n o m e n a , to quantitatively determine the relationship between process p a r a m e t e r s and mlcrostructure,.property develo p m e n t , and to establ,sh valid models to predict the c o n d i t i o n s for the p r o d u c t i o n o f quahtied MM(S To a p p r o a c h this target, a basic requirement ts that the MMC fabrication facility should be c a p a b l e o f bemg o p e r a t e d ,n widely xarylng conditions, while all the process p a r a meters should be precisely c o n t r o l l e d and m o n i t o r e d to assess and optimize the fabrication p a r a m e t e r s This p a p e r briefly describes o n g o i n g research on fabrlcatmn m e t h o d s and processes for fibre-reinforced cast alumlnlum alloy c o m p o s i t e s carried out by the authors, deahng with a specially designed, well-instrumented c o m b i n e d MIVlCcasting facdaty and its a p p h c a t l o n to the study o f the MMCS The studies include fabrication o f MM(_s by five infiltration process routes, investigation o f the melt flow b e h a v l o u r d u r i n g infiltration, investigation of non-infiltration defects, and m o d e l h n g o f the infiltration process

a c o m m o n die cavity. All o f the processing p a r a m e t e r s can be precisely c o n t r o l l e d and recorded The basra design o f the machine and ancillary e q u i p m e n t is reported elsewhere '~7 Since then further d e v e l o p m e n t and modifications have taken place, the recent position is detailed below

Elements of the facthty Fig 1 is a schematic d i a g r a m o f the a p p a r a t u s for fabricating cast MMCS The a p p a r a t u s consists o f an induction furnace and a muffle furnace, an i n d u c t i o n - h e a t e d die assembly, an ejector mechanism with a w a t e r - c o o h n g jacket, a 50 ton h y d r a u l i c press for squeeze casting, a c o m p r e s s e d gas system for low gas pressure casting, a v a c u u m system for venting/negative pressure casting, a c o m p u t e r m o n i t o r i n g system and a safety interlocked control system

Five M M C fabrication routes The basic principles o f the o p e r a t i n g processes in this facility are as follows A schematic o f the squeeze casting unit is given In Fig 2 In thc squee,'e casting process, after a p r e f o r m is put m the die, a melt is p o u r e d o n t o the p r e f o r m t h r o u g h a l a u n d e r The ram is m o v e d to close off the die cavity and squeeze the liquid metal into the p r e f o r m to form an MMC In thc low-pressure casting process, as shown in Fig 3, the gate valve and l a u n d e r are first moved a w a y and a p r e f o r m is put into the die cavity The gate valve, in the open position, is then m o u n t e d o n t o the die After molten alloy is p o u r e d o n t o the preform, the gate w i v e is closed i m m e d i a t e l y , and c o m p r e s s e d nitrogen gas is a d m i t t e d whmh forces the liquid metal into the p r e f o r m In the v a c u u m casting process, as shown m Figs 2 a n d 3, after liquid metal is p o u r e d o n t o the preform, sealing its surface, the v a c u u m force a p p h e d t h r o u g h the venting filter d r a w s the hquld rnetal into the p r e f o r m In the squeeze-assisted v a c u u m casting and gas pressure plus v a c u u m casting processes, just after molten metal is p o u r e d o n t o the preform, thus sealing the prelk-wm from the a t m o s p h e r e , the v a c u u m reservoir is o p e n e d to the die and the air m the p r e f o r m is removed This is followed by squeeze casting or gas pressure casting, respectively, as shown m Figs 2 and 3

Operational features of the facthty •

•

THE COMBINED M M C CASTING FACILITY F o r the research, a p u r p o s e - b u i l t l a b o r a t o r y - s c a l e combined MMC casting fac,hty was designed and constructed by the authors, m which five different hquld metal infiltration m e t h o d s - - s q u e e z e casting, low-pressure casting. vacuum casting, squeeze-assisted v a c u u m casting and gas pressure plus v a c u u m casting---can be carried out m

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•

•

This facility is well instrumented, all the processing p a r a m e t e r s , such as t e m p e r a t u r e , pressure and Infiltration distance d u r i n g the process, can be precisely c o n t r o l l e d and recorded The melt furnace has a heated launder and an associated inert gas p r o t e c t i o n system which, together with a degasslng o p e r a t m n , can p r o d u c e high quality AI- and M g - b a s e d alloys with precisely c o n t r o l l e d melt c o m p o s i t i o n and t e m p e r a t u r e , thereby smlulating a wide range o f possible industrial p r o d u c t i o n conditions Three vent c o n d i t i o n s are available' no vent, n o r m a l vent a n d vacuum-assisted vent p r o v i d i n g assessment o f air e n t r a p m e n t and air back pressure I n d u c t i o n - h e a t e d die and a d j u s t a b l e w a t e r - c o o l e d ejector allow control o f sohdlfiCatlon rate and direction and p r e f o r m temperature.

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F~g 3 Schematicof gas pressurecasting unit

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Die size can be changed, permlttlng variation of casting geometry A range of continuous and discontinuous fibre preforms can be prepared in-house and cast with this facility, permitting study of different fibre types and variation of volume fraction, binder and shape of preform METHOD FOR TEMPERA TURE MEASUREMENT DURING M M C FABRICATION

In order to understand the processes oflnhiltratlon, sohdification and microstructural development of MMCS, it is essential to measure the temperature variation at specified locations inside and outside the preform during fabrication Previous researchers have often used the measured mould temperature as the preform or MMC sample temperature, and used the temperature of the melt in the crucible to represent the temperature of the melt after pouring and before mhiltration. Temperatures obtained in this way are not sufficiently accurate for describing the heat flow in the infiltration process How.ever, there are several difficulties with more direct methods of monitoring the temperature field in MMCS during fabrication The lnhiltration of ceramic preforms takes place in a closed chamber, so that it is difficult to access and locate thermocoup]es at required positions in a preform The liquid metal infiltration tnne is only about 1 s for a preform of 15 mm thickness, therefore, the response of the thermocouples and data logging instrument must be sensitive, fast and accurate enough to accumulate the required data Since the mhiltration process is under pressure, the thermocouples used must be reliable in this situation After solidification of the MMC sample, the thermocouples are embedded in metal and have to be cut out, thereby damaging them Thus the thermocouples used should be inexpensive A method has been developed to monitor temperature at any position m an MMc sample during infiltration by the above-mentioned five fabrication routes, in which all the problems noted above have been solved for the proper measurement of the temperature fields. A purposedesigned accessible MMC die assembly, thin and flexible thermocouples and a dedicated computer data logging system are used The response time of the method is approximately 50 ms with sufficient accuracy The results generated have been successfully used to model heat flow through the fibrous preform TM Seven thermocouples are employed to monitor the temperature field during casting of an MlViC sample. The thermocouple locations are shown in Fig 2 Four thermocouples, labelled TC1 to TC4, are located in the preform, the other three thermocouples, labelled TC5 to TC7, are located at appropriate positions in the die, only 2 mm away from the inside surface, to measure temperatures in the immediate environment of the MMC The specially designed ejector (3), with the detachable thermocouple holder (6), makes it possible for thermocouples to be inserted at any position in the preform in the die chamber For example, as shown in Fig 2, the thermocouple TC4 is located at the top of the preform, to rneasure the actual melt temperature just before infiltration. TC3, TC2 and TC1 are located at selected

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overall sectton 1 mm xl 4 rnm sO

01 mm copper s h e a t h . _ _ ~

gla;02m '2SKtype ,hermocouple wl res 0 5 mm bare measuring junction _ Fig 4 Construcnonofthethermocouples positions in the preform to measure temperatures within tile sample. For gas pressure casting and vacuum casting thermocouples TC1 to TC4 are located in the preform through the gate valve, as shown in Fig 3 Fig 4 shows the construction of the thermocouples, which are made from 0 2 mm diameter nlckel-chromlum/mckel alumlniun~ type-K conductor thermocouple cable and copper foil The overall cross-section of the thermocouple is about 1 x 1.4 mm Accuracy, stability, high thermal electromotive force and low cost are ensured by the selection of K-type thermocouple wires The small bare measuring junction and the fine diameter of the thermocouple offer a fast response, which is about 50 ms It is flexible enough to be formed into the required shape The thin copper foil sheath prevents melt permeatlng through the glassfibre insulation under pressure which would cause short circuits, and its small mass has a negligible effect on the surrounding heat field. An IBM-compatible 486-33 computer, a Data Translation 2811-PGL analogue and digital l/O board, a DT707-T screw terminal panel equipped with a thermocouple cold-junction compensation circuit and a colour printer are employed for data collection and treatment A dedicated computer software package, written in the C programming language, has been developed for this system. During operation, this system enables up to seven processmg parameters--such as temperature (at tip to four positions in tin MMC sample), squeeze pressure, gas pressure, vacuum pressure and ram speed--to be simultaneously logged on a disk and displayed on the computer monitor The sampling rate is continuously adjustable from every 10 ms to every 6 s for each parameter as required. The maximum sampling speed is sufficiently high to study the infiltration process of MMC fabrication. Fig 5 compares the temperature records measured by different thermocouples and the computed temperature curve of the infiltration process at the location of TC2 in Fig 2 In the experiment, commerlclal purity aluminmm was used as the matrix, and a 100 mm diameter, 15 mm thick Saffil alumina fibre preform with 21% volume fraction l]bre and 3 lam fibre diameter was used as the reinforcement The ram speed was about 0 02 m s-~ and the squeeze pressure was 26 MPa The experimental plot (hne 2), which was measured by the specially designed thermocouple, is in agreement with the computer prediction curve (line 1), the difference of the response times between the computed curve and the measured plot being less than 0 05 s. Thus the time delay in line 2 from

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F~g 5 Comparison of temperature records measured by different thermocouples hne 1, computed prediction curve, hne 2, spectally designed thermocouple w~th qb0 5 mm bare bead, hne 3, thermocouple w~th q~l 2 mm bare bead coated w~th thin refractory slush, hne 4, commercml q~3 mm stainless steel sheathed thermocouple

the m~tml melt contacting with the bead of the thermocouple to achiewng the melt temperature ~s only about 100 ms. Line 4 was recorded by a commercml 3 mm dmmeter stainless steel sheathed thermocouple, and hne 3 was recorded by a thermocouple with a 1 2 mm dinmeter bare bead coated w~th a thin refractory slush. Both were recorded under simdar conditions to these of line 2, but both had unacceptably large errors Line 2 is sufficiently accurate to meet the requxrement of modelhng heat flow m the infiltration process, APPLICATION OF COMBINED M M C CASTING FA CIL I TY Several types of MMC casting experiment have been carried out w~th th~s facility For example, MMCS were fabricated by the five mfiltrauon process routes to enable characterizauon and comparison of the d~fferent methods. Fig. 6 shows a group of samples produced by the five processes All the samples were produced with the same alloy, the same melting and pouring procedures, the same melt superheat and preform temperature and the same preform parameters, except that the volume fracuon of the preform for vacuum casting was much lower than the others because the vacuum force was too weak to suck the melt into preforms w~th higher volume fract,ons of Saffil alumina fibres. Cross-sections of a set of MMC samples produced by squeeze casting with different melt temperatures and d~fferent configurations of the continuous fibre preform are shown m F~g. 7 In each sample m F~g. 7, two types of contmuous alumina fibre (LD and SD Safimax) with different volume fractxons were placed m 70 mm long, square cross-sectton, perforated steel frame holders with dtfferent hole sizes W~th this type of preform, mfiltraUon from all dlrecUons was possible (surrounding mfiltrauon mode) Preforms were also prepared from 40% LD Sailmax fibre contained m a circular cross-secuon, openended steel tube. Th~s tube preform restricted mfiltratton to the bldlrecuonal infiltration mode All the continuous fibre preforms were placed on a 100 mm diameter by 15 mm thick disc-shaped Saffil fibre preform with 21% fibre volume fracuon The preform temperature was 430°C when ,nfiltrated The a~m of this set of experiments was to study the relationship between melt superheat, infilt-

F~g 6 Companson of five M M C fabncatlon processes squeeze casting, gas pressure casting, vacuum casting, squeeze-assisted vacuum casting and gas pressure + vacuum casting In each case the disc-shaped preform (q5100 x 23 mm thick) was infiltrated from above and excess metal ts present in the upper part of each macrograph The fibre volume fraction was 21% m all cases, except for vacuum casting where it was 5% Despite the low fibre volume fraction, the vacuum force was insufficient to achmve complete infiltration and gross porosity Is observed In all other cases mfdtration Is complete

rauon modes (bidirectional vs surroundmg mfiltrauon) and process defects of the MMC samples Fig 7 shows that, on the macro scale, infiltration Is complete without deformauon of the preforms when the melt temperature is higher than 670°C (for a squeeze pressure of 50 MPa). In the sample fabricated with a melt temperature of 620°C, obvious deformauon of both the square secuon and disc-shaped preforms is observed. This deformauon Is caused by prematurely sohdlfied melt crushing the preforms especmlly m areas near the wall of the d~e, the heat-extracting surface Fig. 8 is an experimental record of squeeze-assisted vacuum casting, dlustratmg the relauonsh~p between the process parameters during the manufactunng history of a sample. The locaUons of the thermocouples used m th~s experiment are shown m F~g 2. The thermocouple traces m Fig 8 indicate that, although vacuum is apphed 12 s before squeeze pressure, complete infiltratxon of the preform ,s only achieved when the squeeze pressure exceeds a threshold value at about 22 s after vacuum apphcauon Fig. 9 gives an experimental record of squeeze casting during infiltration, Fig. 2 again indicating the thermocouple Iocauons The I 4 s record shows that the temperature d]stribuuon, pressure and speed during infiltration have been accurately determined by the m o m t o r m g instruments Th~s kind of detaded data ts essential for understanding the MMC fabncaUon process

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T~me (s) TCI tn l~eform (20,4), TC2 (20,12), TC3 (20,20),TC4 (20,24), cenlre of predorm bottom. (O,O)mm F~g 9 An expenmental record of squeeze casting dunng tobitration, thermocouple locations as s h o w n m F~g 2

Fig 7 A set of M M C samples produced by squeeze casting different fibre preforms at different melt temperatures In each bulk sample different continuous alumina fibre preforms were formed ~n square cross-section perforated steel frames and ctrcular cross-section open-ended steel tubes, these stood on a ~/~100 x 15 mm thick discshaped Saffd fibre preform Thts ptcture shows that, on a macro scale, under 50 MPa squeeze pressure the mflltratton ts complete w~thout deformatton of the preforms when the melt temperature is h g h e r than 670°C However, m the sample infiltrated with melt at 620°C, obwous deformation of both square-section and the lower disc preforms is observed The deformatton is caused by the prematurely sohdthed melt crushing the preforms, especLally ~n the areas near the d~e wall

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IN VESTIGA TION OF NON- INFIL TRA TION DEFECTS

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Non-lnfiltraUon votdage is one of the most detrimental mtcrostructural defects present in composites, because this type of pore can act as an imtlal crack to seriously degrade the MMC mechanical performance. Understandlng the formation mechanism of non-infiltration defects is an essentml step in designing a process in which they are eliminated

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Invesugatlon of non-infiltration defects, by scanmng electron microscopy (SEM) and transmission electron m~croscopy (TEM), in&cated that the mare reasons for their presence In fibre/alumlntum composite systems are" •

Fig 8 An expenmental record of squeeze-assisted vacuum casting, thermocouple locations as shown m Fig 2 The thermocouple traces s h o w that, although vacuum ~s apphed 12 s before squeeze pressure, complete mhltratton only occurs when the squeeze pressure exceeds a threshold value at about 22 s after vacuum apphcatlon

• •

Samples have also been produced with this facdlty to study the influence of various process parameters on the MMCS. Some of this work will be described in following sections

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locally high fibre volume fraction (Fig 10) and fibre clumping (Fig 11), leadmg to high infiltration resistance, caused by non-uniform geometry of the fibre preform; fibre contact (Fig 12), leading to increased capdlary pressure and greater resistance to infiltrauon; microscopic and macroscopic a~r entrapment resultlng from the unstable flow front and random (Fig 13) and bidirectional lnfiltrauon (Fig 14), occurrence of melt sohd~ficatlon and insufficient external mfiltrauon pressure applied (Fig. 15)

Fig 11 SEM fractograph of ~bl 5 p.m alumina fibre/AI-7% Si4) 3% Mg alloy composite showing a non-infiltration void surrounded by clumped fibres

Fag 14 SEM fractograph of ~15 I~m alumina fibre/AI-3% Mg0 3% Si alloy composite showing a non-infiltration votd between bidirectional flow fronts

Fig 15 SEM image of the sphttmg fracture of q~311malumma fibre AI-10% Mg alloy composite showing the stopped melt flow due to sohdff~cat~on Fig 12 TEM mtcrograph from a cross-sectton of ~3 pm alumina flbre/Al-10% Mg alloy composite showing that non-infiltration voids at Iocattons adjacent to fibre-fibre contact points

To solve this problem involves almost all aspects of the fabrlcauon process of MMCS preform and melt preparatlon, vent, control of heat and mass transfer, and optlm,zatlon of infiltration parameters

INVESTIGATION OF MELT FLOW BEHAVIOUR DURING INFIL TRA TION The flow behavlour of an AI-10% SI melt in a Saffil chopped fibre preform during unidirectional infiltration by squeeze casting has been stud~ed by observing the morphology of the solidified melt front in the preform after infiltration ~.

Fig 13 SEM Image of the sphttmg fracture of q~31amalumina fibre/ A1-10% Mg alloy composite showing that sideways fnflltratlon of the melt causes air entrapment and leads to the occurrence of voids

Partial infiltrauon by squeeze casting was carried out in the casting facility. The fibrous reinforcement used was a 28% volume fracuon, ~100 × 23 mm Saffil fibre preform with silica binder The average diameter of the chopped alumina fibres was 3 !am The preform with thermocoupies was pre-fixed in the die as shown m Fig. 2 The metal matrix was an AI-10% SI alloy. After fixing the preform in the die, the gap between the die wall and the preform was sealed by alumina paste to ensure that the preform was infiltrated unldlrectlonally Before infiltration, the

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dm and the preform were preheated to an average temperature of 400°C and the AI-10% SI melt was heated to 800°C To partially infiltrate the preform, the squeeze pressure was released at 1.6 MPa with 0 3 s total infiltration time as shown m Fig 16. The temperature recorded indicates that the melt had passed the thermocouple located at 3 mm below the preform surface, but had not passed the thermocouple located at 12 mm below the preform surface. A cross-section of the sample (Fig. 17) revealed that the preform had been partially infiltrated to a depth of 6 mm Fig 16 presents the relationship between externally apphed pressure and melt infiltration depth during the infiltrating period The pressure/infiltration depth relauon can be divided into two almost linear stages" in the first, stage I, the pressure increased sharply to overcome the capillary resistance and lnluate infiltration; in stage II the pressure increased slowly but linearly for the forward infiltration The hnear relationship between external pressure and mfiltrat,on depth ,n stage II means that the infiltration pressure gradient in the flowing melt varied linearly from the threshold value at the infiltration front

62 61 60

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57

04 56

55

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I

I

I

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02

03

04

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T 1me (S)

F~g 16 Vanatmn of infiltration pressure and ram displacement durmg partlal mflltratmn of the preform shown m Fig 17 Thejagged appearance of the graphs ~s an artefact o w i n g to the uncertainty m each experimental point measured at a rate of 1 O0 points per second for each parameter

°,

P

J

lOmm

F

Fig 17 Macroscopm conhguratmn of the melt mflltratmn front m the preform M, matnx alloy, I, mhltrated region of the preform, F, non-mfdtrated regmn of the preform, arrow mdmates mhltratmn direction

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to the full value of the externally applied pressure at the entry of the preform The macroscopic infiltration front of the melt during this unidirectional process was basically flat, as shown in Fig. 17. The flat infiltration front not only means that the resistance to melt flow m the fibre preform was macroscoplcally uniform, but also implies the macroscopm uniformity of fibre distribution In the preform The optical mlcrographs of Figs 18(a)-18(d), taken at infiltration depths of 0, 2, 4 and 6 mm, respectively, show the average degree of filling of the lnterspaces in the infiltration direction by the melt. The degree of filling gradually increased from a very low value at the Infiltration front to almost fully saturated at a position close to the entrance of the preform This observation means that saturation of the Interspaces between the fibres was gradually achieved as the infiltration pressure increased Since a hnear relationship between external pressure and infiltration depth is observed, it can be considered that the saturation variation of the interspaces was dictated by the pressure gradient, and that the degree of saturation was proportional to the local pressure value. Figs 18(c) and 18(d) also show that the melt ran into the lnterspaces of the preform not only along the infiltration direction but also along other off-infiltratmn directions, so-called sideways flow, resulting in the shape of the developing melt column resembling a coral tree The development of the sideways flow column was determined by the available infiltration pressure gradient in the infiltration direction. As a result of the mlcroscopm non-uniformity of the preform fibre distribution and the preferential penetration behavlour of the melt under pressure, air entrapment took place in areas of high fibre density (which were surrounded by loosely packed fibre regions), as well as at sites where small interspaces were separated by larger ones The shape of the flowing fronts in the downstream melt column is shown m Fig. 19, taken m the direction normal to the mfiltratmn front Fig 19 reveals that the flowing fronts have almost round heads surrounded by fibres and consist of fibre surfaces contacted by the out-curved hquld meniscus, determined by the local capillary pressure This meniscus has shrunk during solidification. The infiltration fronts in Fig. 19 possess different sizes and infiltration lengths, since the melt preferentially milltrates large lnterspaces first and then small ones, resulting in a microscopically uneven and unstable infiltration front. According to the above observauons and analysis, it can be concluded that the melt flow behavlour in the nonwetted preform was governed by capillarity. It is the micro non-uniformity of the preform fibre distribution that causes preferential penetration of the melt It is the micro non-uniformity of the flow conduit size in the preform that results In dlspersmn of the flow front on the microscopic level. When the external applied pressure increased to the threshold value, equal to the capillary pressure of large interspaces at the fibre preform entrance, the melt began

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Fig 18 Optical micrographs of the partially infiltrated sample (Fig 17) at different infiltration depths along infiltration direction Infiltration depth (mm) from the top of the preform (a) O, (b) 2, (c) 4, (d) 6

tion o f the interfibre gaps. This sideways flow continued during the whole infiltration process and resulted in gradual filling o f the interconnecting lnterspaces in a pore dimensional order with the local pressure increase As a consequence, the development of sideways flow brought about increased filling of the nnterfibre spaces in the preform along the infiltration direction

M O D E L L I N G OF THE INFIL TRA TION PROCESS

Fig 19 SEM mlcrograph showing the morphology of the flowing fronts

to preferentially penetrate into these large lnterspaces and infiltraUon commenced. The melt then preferentially ran ahead along the intricate conduits formed from interconnecting large lnterspaces in the preform The infiltration pressure in the melt column decreased linearly in the infiltration dnrectlon, from a value equal to the external pressure at the entry o f preform to that equal to the local capillary pressure at the flowing front. Sideways flow o f melt commenced when the increasing pressure in the melt column reached the capillary restrlc-

A mathematical model for liquid metal infiltration of a unidirectional continuous fibre array has been estabhshed 4" A schematic diagram of the forces exerted on the flowing melt in a capillary tube is shown in Fig. 20. In the form o f the model presented here it is assumed that infiltration speed is constant and no solidification takes place during the process: these conditions are duplicated m squeeze casting experiments 20-,, VfcosO 3217PV~ 7 P .... - (-[ : ~ + (1 - V~)-~R~- -

p g Z + Ph,¢~ (1)

where P .... is the externally applied pressure from the squeeze ram. Vr is_the preform fibre volume fraction. Rr is the fibre radius. V is the average flow velocity. Z is the infiltration depth, cq, is the h q m d - v a p o u r surface tenslon. Ois the contact angle, q is the coefficient o f viscosity o f the melt. p is the specific density of the melt. g is the

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&fference, A P .... for m m a t m g mfiltraUon between the largest interspace and the smallest one can be gwen as:

Pra m I

APma, =

Pthm,n-

Pthma,~ =

(,

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,)

Reqm~i )-

w h e r e R~q.... Is the eqmvalent hydrauhc radms of the

smallest interspace and R~qm,, that of the largest interspace Under this pressure &fference. before mfiltratmn commences m the smallest interspace, the flow length of the melt m the largest interspace. A Z .... can be approximately gwen as.

(

AZm,, = ~ 8 q V

re

+ Pg)-I AP.,,,

(3)

This infiltration length difference will be enlarged during the whole infiltration process owing to the internal frlctmn resistance of the flowing melt being reversely proportlonal to the eqmva]ent hydraulic dmmeter of the flow condmt. Th~s means that the non-uniform fibre dlstrtbutmn m a practmal fibre array not only results m non-uniform entry, but also causes an unstable and fluctuating flow front. As analysed above, there is a pressure &fference between two fluid columns m flow condmts with different hydrauhc dmmeters If this pressure difference reaches the value needed to overcome the capillary pressure between the s~de surfaces of these two adjacent fibres, I.e.,

P~.,p b Fig 20 Schematm diagram of the forces exerted on the flowing melt m a capdlary tube, where P,~mts the externally apphed pressure from the squeeze ram, Pc,p is the capillary pressure, f ts the internal wscous friction of the melt, G ts grawty and P~,,~~sthe back pressure due to the existence of mr m the flowing front

acceleratmn due to gravity and Ph,,~ Is the back pressure caused by air at the flowing front Thus mfiltraUon of the fibre array is determined by: (1) a statm term, the capillary resistance, represented by the first term on the right-hand side (RHS) of E q u a t m n (1), (2) a dynamm term, the internal wscous frmuon, represented by the second term on the RHS o f Equation (I) and (3) the effect of gravity and back pressure The key factors, however, are the capillary resistance and the internal wscous frlcuon F r o m E q u a u o n (1) ~t can be seen that the geometry o f the fibre array has a s~gmficant effect on these terms. When externally apphed pressure overcomes the threshold pressure originating from capillary resistance, the melt will be driven into the mterspaces between the fibres and mfiltratmn starts For a regular fibre array, the threshold pressure for all lnterspaces is the same and the melt will penetrate them simultaneously under the same pressure, l e., so-called umform entry For a practmal n o n - u m f o r m fibre array, the hydrauhc dmmeters and the threshold pressures (P,h) o f the Interspaces over the cross-sectmn are greatly different from each other This means that the increasing external pressure will mlUate mfiltrauon gradually at the entrances o f mterspaces w~th &fferent hydrauhc dmmeters, i.e., melt entry into a pracucal fibre array is not uniform. The threshold pressure

390

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o-t, COSb0

(4)

where b is the distance between the fibres, the melt m the larger c o n d m t will flow into the smaller one by s~deways flow through the mterfibre gap under the pressure difference This p h e n o m e n o n helps to restrict the mfiltratmn length d~fference between the two flows. But, ~f the two fibres are not smctly parallel to one another so that there ~s a wider gap at the far end o f the array, s~deways flow may take place m front o f the melt flow m the smaller c o n d m t In this case, melt from the large c o n d m t will flow into the small conduit from two d~rectlons, as shown tn Fig. 21, I.e., mxcro b~&rectmnal mfiltrauon occurs The a~r between the two opposing flow fronts m the smaller c o n d m t will become trapped, subsequently producing a mmro non-mfiltratmn defect. According to the above analysis the mfiltratmn process of a practical fibre array can now be described as follows In the presence of a large gap ( > > mterspaces m the fibre array) between the die wall and fibre preform, the melt is driven to occupy th~s surrounding gap prior to the start of mfiltratmn and compresses the array. As a result of th~s phenomenon, the real mfiltratmn o f an array can be dw~ded into two stages' first, o c c u p a t m n o f the fibre-free surrounding gap and then mfiltrauon of the compressed fibre array Under the same external pressure, the mfiltrattan depth m conduits w~th &fferent eqmvalent hydrauhc diameters is variable, as is the pressure distnbutton m the flowing melt columns The combination o f these effects will result m fibre &splacement and fibre contact taking place, and non-infiltration defects will occur owing to fibre contact and a~r entrapment caused by undesirable mmroscopm and m a c r o s c o D c mfiltratmn modes.

SUMMARY

Melt

Fibre

Metal-matrix composites produced by hquld metal infiltration methods are now beginning to become a true class of engineering materials. To improve the current properties and to broaden the industrial applications of cast MMCS, further research is needed to obtain a detailed understanding of the physical nature of infiltration and solidification phenomena, to quantitatively determine the relationship between process parameters and microstructure, property development, and to establish valid models to predict the conditions for the production of qualified MMCS This paper briefly reports ongoing research on fabrication methods and processes for fibre-reinforced cast alumlnlum alloy composites

Melt

Fig 21 Sideways flow causing bidirectional infiltration and mr entrapment

In order that the model can bc used to quantitatively describe the relationship betx~een variables and guide the optimization of process parameters, the unknown parameters in the model--for example, the hquld-vapour surface tension cry,, the contact angle 0and the coefficient of viscosity of the melt r/--need to be determined by rigorous experiments. For example, the pressure curve in Fig. 9 can be analysed in terms of Equation (I) From a to b, the squeeze pressure rises sharply to 0 5 MPa, this is the capillary pressure and infiltration commences. From b to c, the squeeze pressure increases linearly as the infiltration front moves down from the top surface of the preform to 4 mm away from the bottom surface; the pressure b to c equals to the capillary pressure plus the viscous pressure under normal ventmg conditions Then, from c to d, the squeeze pressure increases rapidly during infiltration of the final 4 mm, because the air trapped in the preform has been compressed at the final infiltration stage. Therefore, from the experimental pressure data, the capillary pressure, the viscous pressure and back pressure can all be determined Some other parameters in Equation (1) also can be experimentally determined, The radius of the fibres Rj and the preform fibre volume fraction l'j can be measured from the preform, and the contact angle 0 can be observed from the melt flow front in partially infiltrated samples Substituting values for R~, I/j, 0 and the capillary pressure in the first term In the RHS of the Equation (I), the true hquld vapour surface tension crl, under squeeze casting conditions can be determined. Similarly, the viscosity coefficient of the melt r/can be calculated from the second term in the RHS of Equation (1) Ongoing work will establish the quantitative relationship between the variables for real composite systems.

A specially designed combined MMC casting faclhty is reported, in which five different hquld metal infiltration methods--squeeze casting, low-pressure casting, vacuum casting, squeeze-assisted vacuum casting and gas pressure plus vacuum casting---can be carried out in a common die cavity This faclhty is well instrumented, all the processing parameters, such as temperature, pressure and infiltration distance, can be precisely controlled and recorded A method has been developed to monitor temperatures at any position in an MMC sample during infiltration by the above-mennoned five fabrication routes The response time of the method is approximately 50 ms with sufficient accuracy A nurnber of apphcatlons of this i2lcillty have been discussed fabrlcanon of MMCS by the five infiltration process routes to enable characterization and comparison of the processes, investigation of non-infiltration defects in MMC samples, investigation of the melt flow behavlour during partial infiltration experiments, and samples produced with this faclhty to study the influence of various process parameters. A nlathematlcal model for liquid metal infiltration of a unidirectional continuous fibre array has been developed The relationships between infiltration speed, fibre dmmeter, fibre volume fraction, physical properties of the metal melt and externally apphed squeeze pressure needed to maintain a constant infiltration speed have been estabhshed in the model. The theoretical results are applied to analyse the infiltration process of practical unidirectional continuous fibre arrays and the formation of the macro and micro non-infiltration defects commonly present in composites. Further work IS being carried out to determine the fluid flow, heat and mass transfer and chemical interactions in the fabrication process, as functions of process condltlOnS, to develop a mathemaucal model for predicting optimized conditions for production of composites with low defect densities and minimized chemical interactions

ACKNOWLEDGEMENTS Financial support for the construction of the MMC casting facility and provision of a research associateshlp for one of the authors (Z Zhang) by tile SERC is gratefully acknowledged Additional support and supply of fibre by ICl is also acknowledged.

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Lee, P.W., Kumar, P., Vedula, K. and Ritter, A. Pro~e~tng and Propertte~ /or Powder Metalllwgi Contpoute~ (The Metallurgical Socmty AIME, PA, USA. 1988) Smith, P.R. and Froes, F.H. "Titanium naeta[-matrr~ composites L S Pat 4 499 156 122 March 1983) Suganuma, T. and Tanaka, A. "Apphcatlon of metal matrix composites to diesel engine pistons" J Iron Steel In~t Jl m 75 No 9 (1989) pp 1790 1797 Clegg, A.J. 'Squeeze forming--forging integrity with casting complexity' Met hid A'e~ ~ 2 No 3 (September 1985) p 6 Chadwick, G.A. and Yue, T.M. "Principles and apphcatmns of squeeze casting" Met Mater 5 No 1 lJanuar 3 1989) pp 6 12 Ohizumi, S., Nishida, Y., Maki, H. and Yamada, T. 'Fabrication of potassium manate whisker-reinforced a l u m m m m allo.~s by squeeze casting and their properties' 41utopia (JpnJ 18 No I (January 1988) pp41 51 M)kura, N. "LNuld pressure lk~rmmg of engineered metal matrix composites' Cast Rein/raced Metal ('ompo~tte~ edited by S G l:lshman and A K Dfimgra IAmencan Society of Metals OH. 19881 pp 173 178 Folgar, F. 'Alumina fibre metal matrl\ composite connecting rods design, t~lbncatlon and performance" 4FS T~an~ 94 (1988) pp 395 402 Fukunaga, H. 'Exploratmn ol new apphcat~on of M M C s manufactured by squeeze casting process Ptoc Ninth hit Con/on Contpo~lte Matetmh edited b~¢ A Mira~ete (Woodficad Publishing Ltd Madrid Spain, Jul.', 1993)pp I 355-1 362 Nogachi, M. Present and future of composite materials I\~r automotive apphcahon in Japan' ~bld p l 119 Koch. P. 'Annual re~le~ pressure die casting' Gte~wtel 72 No 18 (September 1985) pp 521 524 Vrinssen, ~& "Lo~-pressure caster" 4llontntum ( .Vetherlamh ) 3 No 5 (September 19881 pp 22 23 Bader, M.G., Clyne, T.W., Cappleman, G.R. and Hubert, P. ~,. 'The Ihbrlcauon and properues of metal-naatrix composites based on a l u m m m m alloy infiltrated alumina fibre preform,," Contpmm'~ S~ t and TeHmol 23 No 4 (1985) pp 287 301 Pecbersk), M.J., Bhagat, R.B., Upd~ke, C.A. and Amateaa, M.F. "Control of d a m p m g charactcri,,t~cs of graphite fiber reinforced alummium composites Ptot ('on/ on Metal & Ceranlt¢ AIatrt~ Cotnpo~tte~ Ptott'~mg. Modelhng & Methant~al Behavlm (The Minerals, Metals & Materials Socmty, PA, USA, 1990) pp 19 22 Das, A.A., Yacoub, M.M., Zantout, B. and Clegg, A.J. 'Cast metalmatn,~ compo,,ites' Ca~t Met I No2 (1988) pp 69 78 CI)ne, T.W. and Withers, P.J. An httto~h, tum to ,th, tal Matrt~ Compmite~ (Cambridge Untver,,tt.,, Press. Cambridge, UK, 1993) Fukunaga, H. "Squeeze casting processes Ibr fiber reml'orccd metal,, and their mechanical properties" Cast Reinlot(ed Metal Conlpmtte~ op oil pp 101 107 Zhang, Z. "A literature survey on fabrication methods of cast reinforced metal composites' ,b,d pp 93 99 Mortensen, A., Corn(e, J.~t., and Flemings, M.C. 'Solidification processing of metal-matr*,~ composites" J O M 40 No 2 (February 1988) pp 12 19 Chioa, J.M. and Chnng, D.D.L. "Characterlzatmn of m e t a l - m a r e \ compos,tes fabricated by xacuum mfihratmn of a hqmd metal under an inert gas pressure' J ,~later S¢t 26 ( 1991 ) pp 2583 2589 Ponzi, C. "Metal matrix composite fabrltatmn proces,,es for high perl'omaancc aerospace structures' Compo~tte~ A[anitfa~ tio tn,~ 3 No I (1992) pp 32 42 Fukunaga, H. and Goda, K. 'Formation and role of the sohdlficd layer on a hber during the fabrication of fiber reinforced metal by the hqmd process" J Japan ht~t Metal~ 49 No I (1985) pp 78 83

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Mortensen, A., Michaud, V.J. and Flemings, M.C. "Pressure-infiltration processing of reinforced aluminum" J O M 45 No I (January 1993) pp 3 6 4 3 24 Mortensen, A. and Jim I. 'Sohdlfication processing of metal matrl\ composites' hu Mater Rev 37 No 3 (1992) pp 101-128 25 Mortensen, A. and Comte, J.A. "On the infiltration of metal matrix composites' Metall Tran~ 18A (1987) pp 1160-1163 26 Nourbakhsh, S., Liang, F.L. and Margolin, H. 'Calculation of mm~mum pressure for liquid metal infiltration of a fiber array' Metall Tran~ 20A (1989) pp 1861 1866 27 Mortensen, A. and-Wong, T. 'Infiltration of fibrous preforms by a pure metal part 3 Capillary phenomena" Metall Tratm 21A (1990) pp 2257 2263 28 Feest, E.A., Young, R.M.K., Yamada, S.I. and Towata, S.I. "Developments in the science and technology of composite matermls" Ptm ECCM-3 (Elsevier Applied Science, London, 1989) p 165 29 Young, R.M.K. "A hquid metal infiltration model of unidirectional fibre preforms in mert atmospheres' Muter S~1 and Engng AI35 (1991) pp 19 22 30 Xia, Z., Zhou, Y., Mao, Z. and Shang, B. "Fabrication of fiberreinforced metal-matrix composites by variable pressure mfiltr,mon" Metall Tmn~ 23B (1992) pp 295 302 31 Martins, G.W., Olson, D.L. and Edwards, G.R. "Modelling of mfiltratmn kinetics for hqutd metal processing of composites" Metall Tran~ 19B (1988) pp 95 101 32 Clyne, T.W. and Mason, J.F. 'The squeeze mliltratlon process for fabrication of metal-matrix composites" Metall Tram 18A (1987) pp 1519 1530 33 Mortensen, A., Masur, L.J., Corn(e, J.A. and Flemings, M.C. "Infiltratmn of fibrous preforms by a pure metal part I Theory" Metall Trans 20A (1989) pp 2535 2547 34 Mortensen, A. and Miehaud, ~. "Infiltration of fibrous preforms by a binary alloy part I Theory" Metall T~ans 21A (1990) pp 2059 2071 35 Alonso, A., Pamies, A., Narciso, J., Gareia-Cordo~illa, C. and Louis, E. "Evaluatmn of the wettabfllty of hqmd a l u m m m m with ceramic pamculatcs (SIC, TiC, AI,O,) by means of pressure mfiltrattan' Metal/ 7tan~ 24A (1993) pp 1423 1432 36 Zhang, Z., Fox, S. and Flower, H.M. "An maproved experimental casting apparatus for the fabrication of metal matrix composites" Pro~ Euromat 91. I ol 2 edited by T W Clyne and P J Withers (Inst Metals. London, 1992) pp 100- 104 37 Zhang, Z. and Flower, H.M. "A method of temperature measurement during M M C fabrtcatmn' Pto~ Ninth Int Con! on Composite Mate~ml~op tit pp 1 913 I918 38 You, H., Bader, M.G., Zhang, Z., Fo~, S. and Flower, H.M. 'Heat flow anal ss~s of the squeeze infiltration t.asting of metal m a r e \ composites' Compo~tt(,~ Manu/a~ taring (accepted for pubhcatlon) 39 Long, S., Zhang, Z. and Flower, H.M. 'Flow behavlour of metal melt m fibrous preforrn during mfiltratmn process by squeeze casting" submitted for pubhcatlon 40 Long, S., Zhang, Z. and Flower, H.M. "Hydrodynamic analysis of hqutd infiltratmn of umdircctmnal fibre arrays by squeeze casting' •l( ta Metall (accepted l\~r pubhcatmn) 23

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AUTHORS The authors are with the Department of Materials at Imperial College of Science, Technology and Medxcme, London SW7 2BP, UK Correspondence should be addressed to H.M. Flower