|Title of Invention||
A METHOD OF MAKING AL-RICH AND ALN-RICH MATRIX COMPOSITES BY PRESSURELESS INFILTRATION OF MOLTEN AL-ALLOYS USING AN EXTERNAL GETTER
|Abstract||A method of making AI-rich and AIN-rich matrix composite comprising pressure less infiltration of molten Al alloy on ceramic preform using external getter as herein described by heating the crucible assembly containing matrix metal and preform as herein described along with the sad external getter in reactor tube of a gravimetric analyzer at 660Û_-11 OOºC on nitrogen atmosphere.|
The present invention relates to a method of making Al-rich and AIN rich matrix composites by pressureless infiltration of molten Al-AUoys using an external getter.
Al-alloys reinforced with ceramic reinforcements such as AI2O3, SiC, Si3N4, AIN etc., have attractive properties from the point of mechanical and functional applications. A variety of methods have been developed to process aluminum matrices reinforced with a range of volume fractions of ceramic reinforcements up to 90% by volume. More widespread usage of these materials is deterred by cost considerations though they offer superior performance in many respects when compared to monolithic metals or ceramics. Many techniques such as stir casting, powder metallurgy, spray deposition and in-situ techniques are available for processing of metal matrix composites with low volume fraction (' for the metal to penetrate into these preforms. These high volume fraction metal matrix composites have potential applications as wear resistant materials, in electronic packaging and in the automotive industry. The infiltration of molten metals into ceramic preforms can be driven in three ways; by (1) pressure, (2) vacuum and (3) capillarity.
Pressure infiltration can be applied to any matrix/reinforcement system and the process is quite fast. The major draw back of pressure infiltration is that the cost involved in making a component is quite high, sometimes prohibitively expensive.
hi the case of vacuum driven infiltration, there are limitations on the pressure gradient that can be applied to pull the molten metal into the preform. Capillarity induced or prcssureless infiltration is attractive due to the simplicity involved in processing, which manifests as low manufacturing costs and near net shape pressure is generated which is a function of the extent of wetting between the ceramic reinforcement and the molten metal
The criteria for the molten metal to wet a ceramic is that the contact angle between them should be less than 90^, otherwise negative capillary pressure will develop which needs to be overcome by imposed pressure if the molten metal has to be infiltrated into the porous ceramic preform.
Aluminium does not wet the ceramics such as AI2O3, SiC, Si3N4, B4C, AIN, Tie etc., readily. The reasons are two folds; (1) interference of the ubiquitous alumina layer present on the surface of Al and (2) poor wettability between many ceramics and Al. One of the most successful ways of overcoming the interference of the oxide layer is to alloy the Al with an element such as Mg which reduces the alumia and also changes the morphology of its oxide layer such that it allows the molten metal to come in contact with the ceramic reinforcement of the preform. In the early part of the 1990's, a technique of infiltration of Al into porous preforms made of ceramic reinforcements such as AI2O3, SiC, AIN etc. in N2 atmosphere was
invented by the Lanxide Corporation of USA and it was called PRIMEX^^^'^ Although the technique of capillarity induced or pressureless infiltration was used to infiltrate Cu into porous compacts of Fe as early as 1958, the special feature of pressureless infiltration of Al is that it fulfils both the requirements of removing the interference of oxide layer on Al melt as well as improving the wetting between the Al and the ceramic reinforcement. This was achieved by an alloying element such as Mg in combination with nitrogen atmosphere, which was believed to bring about the phenomenon of pressureless infiltration of Al-alloys into porous ceramic preforms. However, it was demonstrated by our pending patent application No,2359/Mas/97, "A method of making aluminium matrix composites by the infiltration of liquid-metal", that it is possible to infiltrate Al-alloys into porous ceramic preforms of Al203,SiC etc, without the need for a nitrogen atmosphere, U.S. patent No. 5,298, 283 granted on March 9, 1994 to Michael A Rocazella et al and "related patents on the pressureless infiltration of Al-alloys in nitrogen", describes a method of forming metal matrix composite bodies by spontaneously infiltrating a rigidised filler material The method described therein requires a temperature greater than 700^ C, Mg in the alloy and nitrogen in the fiimace atmosphere. It was also indicated that the amount of Mg in the starting alloy can be reduced by keeping free Mg powder in the spontaneous infiltration system, i.e. infiltrating metal/filler material/atmosphere.
An Indian patent No. 1,76,990 granted in the names of Marc S. Netwkirk et al. of Lanxide Corporation, and related patents on the pressureless infiltration of Al-alloys in nitrogen, describes a method for forming macro-composite useful as electronic package containers. They combined the steps
involved in forming metal matrix composite body and attaching or joining it to another macro-body, of same composition as that of the matrix metal or of different composition, into one step. The main requirements of the process are a minimum amount of Mg and a minimum process temperature and a nitrogenous atmosphere. They specified a minimum amount of Mg in the range of l-3wt% and a temperature of at least 750*^0 for spontaneous infiltration of Al-alloys into ceramic preforms. However, the amount of Mg required for the infiltration varies with the other process parameters employed to achieve spontaneous infiltration. Though they reported that Mg content of 1-3 wt% in the Al-alloy was enough to bring about spontaneous infiltration, they recommended at least a Mg content of 3 wt% or above if no Mg is added to the filter material or to the preform. According to their patent, in any case, the Mg content of the alloy should be at least 3 wt% so that sufficient infiltration of the filler or preforms of ceramic materials is possible. In their invention, the emphasis was more on the formation of Al-rich matrix composites. It was also mentioned that it is possible to form AIN-rich matrix composites, however, no indication of the thickness of such composites that can be grown. The thickness of AIN-rich composites is a very important issue since, as we show later, it is not possible to grow such composites beyond a certain thickness, of a few ( The shortcomings of their patent from the commercialization point are as follows.
first, Mg content more than 3wt.% may lead to the formation of interfacial reaction products which may limit the properties that can be attained in the absence of those reaction products.
secondly, AIN-rich composites of useful thickness can not be grown by their method.
SUMMARY OF THE INVENTION
The object of the present invention is to overcome the afore-said shortcomings.
Yet another object of the present invention is to obtain AIN-rich composites of thickness of >1 cm and also to obtain a AIN-rich matrix of uniform whole thickness.
Another object of this invention is to provide for an improved process for preparing Al-rich and AIN rich matrix composites, which is a significant improvement of the existing method of spontaneous or pressureless infiltration of Al-alloys.
To achieve the said object, the present invention provides a method of making Al-rich and AIN rich matrix composites by pressureless infiltration of molten Al-Alloys using an external getter comprising:
heating a crucible assembly containing the matrix metal and preform to at least above the melting point of the matrix metal along with the external getter.
Said external getter is Al-Mg alloy or Al- Li or Al-Sr alloy, preferably Al-Mg alloy. Al-5% Mg alloy is used as an external getter.
Said preform is a ceramic material such as Al203,SiC, AIN Si3N4
Said matrix metal is aluminium. The temperature of processing of said Al is at least above 660X. Said matrix metal with getter alloy is heated to 800 ^C, preferably 1000*^0 to form Al-N rich matrix composition.
Said crucible assembly is made of a material which does not decompose in N2 and does not inhibit pressureless infiltration.
Molten Al-alloys containing Mg can be infiltrated into the preforms of Al203,SiC, AIN Si3N4 etc. without the application of pressure in nitrogen atmosphere above 700^C by keeping an Al-Mg alloy as an external getter in the vicinity of a crucible containing the matrix metal and a ceramic preform or filler material. Presence of the external getter makes it possible to form Al-rich matrix composites of thickness of >1 cm fi^om lean Al-Mg based alloys, which is not feasible without an external getter.
The most important feature of this process is that a uniform ATN-rich matrix can be formed across the whole thickness. When there is no external getter, a gradient composite was formed with a large variation in AIN content of the matrix from one side to the other. The portion near the metal reservoir formed Al-rich matrix whereas the portion away from the metal reservoir formed an AIN-rich matrix.
In our earlier patent application no.: 2359/Mas/97 we have demonstrated the effectiveness of a getter in bringing about the phenomenon of pressureless infiltration of AJ-alloys in air. In the above said invention, we have shown that how Mg placed at the matrix metal/preform interface not only getters the oxygen coming into the preform before the metal melts and seals-off the preform from the furnace atmosphere but also getters the air entrapped in the preform after melting thereby initiating the infiltration of Al-alloys into the porous ceramic preforms.
The present invention may be seen as a further modification of the above principle to enable the fabrication of metal rich composites in a flowing gas atmosphere and of thick sections of AIN-rich composites.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Figure 1: describes a schematic diagram of the crucible assembly with the Getter.
Figure 2(a) shows the configuration of the crucible assembly with the getter
Figure 2(b) shows a cross-sectional view of the infiltrated sample showing AI/AI2O3 composite and the left out material.
Figure 3 illustrates AIN-AI/AI2O3 composite plate fabricated at 1 lOOX.
Detailed description of the invention
Al-Mg based alloys infiltrate spontaneously into porous ceramic preforms or filler materials in the presence of an external getter such as Al-Mg alloy.
The procedure for making Al-rich and AIN-rich matrix composites is described below.
Al-alloys may be prepared by melting the commercial purity metals or alloys and casting into the required shape. The required sizes and shapes of the billets were machined out from the cast ingots. Ceramic preforms are prepared by cold pressing ceramic powder to a predetermined porosity and sintering at a temperature which does not decrease the porosity of the preform but at the same time gives some strength. Ceramic materials such a AI2O3, SiC,ArN,Si3N4 etc. can be used as a reinforcement either in the form of particulate, platelet, whiskers or fibres. Sintering temperature varies for each ceramic and also with particle size. For example, alumina preform of 40% porosity containing alumina particles of - 50 jim size needs to be sintered for 1 hour at 1600'^ C. Preforms can be prepared by any of the conventional ceramic processing techniques such as slip casting, tape casting, extrusion etc. Of course, loose power can also be placed in place of preform if the volume fraction of the matrix in the final composite needs to be higher. Crucibles for housing the matrix metal and the preform can be any material which does not decompose in nitrogen, should not inhibit the pressureless infiltration of Al-alloys and at the same fime be cheap. Alumina and graphite meet the above requirements and serve the purpose as
crucibles. However, graphite can not be used above 1000 ^C due to severe reaction with Al. It can be used with coatings such as alumina.
The crucible containing the matrix metal and the preform is heated to at least above the melting point of the matrix metal along with an external getter such as Al-Mg alloy. In the case of Al as a matrix metal, the temperature of processing should be at least above 660X. The schematic of the crucible assembly along with the getter is shown in Fig. I. For Sufficient infiltration the crucible assembly should be heated preferably to 800X, Similarly, the crucible assembly should be heated along with the getter alloy to above lOOOX to form AIN-rich matrix composites. The words "AIN-rich" are used to indicate qualitatively a significant amount of AIN interspersed in the alloy. In fact, the proportion of AIN.Al is continuously variable from 0 to about 5 by appropriate choice of alloy and temperature. Unlike at lower temperatures, the getter alloy need not be placed in the same temperature zone where the crucible assembly is kept. It can as well be placed in a temperature zone lower than that of the crucible assembly thereby reducing unnecessary loss of Mg from the getter alloy. The function of getter alloy is to reduce the residual oxygen concentration in the incoming nitrogen gas. This brought about by the Mg in the getter alloy which getters the residual oxygen by vapour phase reaction with O2. Not bound by any theories, it is observed that the presence of external getter in the pressureless infiltration system ensures that the infiltration of Al-alloys continues for longer time compared to the case when there is no external getter. On similar lines, it also facilitates the formation of sufficiently thick AIN-rich matrix composites. Mg content of the getter alloy can be more than or less than the Mg in the matrix metal. It depends on the amount of Mg required in the
matrix for a particular application. For example, if the matrix Mg content needs to be more than 8% , the getter alloy can be of 5%. It depends on the severity of the conditions employed to bring about the pressureless infiltration and also the thickness of the composite one intends to make. More the thickness of the composite higher should be the Mg content of the getter alloy for a given cross-section so that it can act as a getter for longer time and consequently lead to a thicker composite. Severity of the conditions, here, means that if the nitrogeneous atmosphere contains more residual oxygen, which may be enough to suppress the infiltration completely, then the getter alloy should contain more Mg to reduce the oxygen concentration so that infiltration can take place.
The phenomenon of pressureless infiltration of Al-alloys, no doubt, requires presence of Mg in the alloy, whether in the form of alloy or in the form of Mg powder at the billet/preform interface, and nitrogen in the furnace atmosphere. But the process is self limiting in the sense that if sufficient Mg is not available at the infiltration front, the process terminates and does not start again at all on its own. It is in that sense that the present invention is a significant improvement with respect to the usefijl thickness that can be achieved by this invention.
Other alloying elements such as Li and Sr can be used in place of Mg to bring about the pressureless infiltration.
The infiltration can be carried out against or with gravity as well as laterally. The advantage of infiltrating in the downward direction is that gravity aids the infiltration of molten metal into the prefonn. Formation of Al-rich as
well as AIN-rich matrix composites by this method is illustrated with the following examples. In all of the following examples Al-5% Mg alloy was used as an external getter.
Al-lVoMg alloy/AI2O3 preform: Al-rich matrix composite
Al-2wt% Mg alloy was used as the infiltrant. Alumina crucibles were used to house the preform and the alloy billet as well as the getter alloy. The matrix metal was kept on top of the alumina preform in an alumina crucible. The alumina crucible containing the getter was kept on top of the crucible housing the matrix metal and the alumina preform. This whole assembly was kept in a reactor tube of a thermo-gravimetric analyser and the tube was evacuated to 0,07 mbar and back filled with N2-2%H2 gas and the gas was
flown at a rate of lOOMl/min. The whole assembly was heated to 900X at a rate of 20^C/min. and held there for two hours and cooled down to the room temperature at a rate of lOX/min. full 1 cm height of 40% porosity AI2O3 preform was infiltrated by the Al-2%Mg alloy in two hours. The cross-section of the sample is shown in fig. 2 AI/AI2O3 composite integrally bonded to the lefl:-out metal can be seen. In the absence of the getter, only 2 mm of the alumina preform was infiltrated in 2 hours and the infiltration tenninated afterwards.
Al-2%Mg alloy/AllO3 preform: AIN-rich matrix composite
AI2O3 prefonn of 4 cm x 4 cm x 0.5 cm dimensions and containing 40% porosity was kept on top of the alloy billet of 4 cm x 4 cm x 1.8 cm dimensions in an alumina crucible. The getter was also kept in anotlier crucible of same dimensions. The crucible assembly as well as the getter was kept in a tube furnace and evacuated to 0.05 mbar and back filled with N2-2%H2gas.
The gas was flown at a rate of 250 ml/min. The furnace was heated at a rate of 4'^C/min. to llOOX and held there for 3 hours and cooled down to the room temperature at a rate of 4X/min, In three hours, the whole preform was infiltrated and formed a lot of AIN in the matrix. In fact, the metal came out of the preform and formed AIN/Al composite on thetop surface of the preform, which shows that the actual infiltration of the preform must have taken less time than the total time kept at the set point temperature. The excess metal sticking to the Ain-AI/ AI2O3 composite was removed by cutting it off with a diamond cut-off wheel. The AIN-Al/ AI2O3 composite plate is shown in fig. 3.The hardness achievable in the matrix is -- 11 Gpa which is many times that of the maximum achievable in commercial aluminum alloys. In the absence of getter alloy, the alloy infiltrated on 2 mm of the preform and formed a gradient composite. Out of 2 mm, 1mm was Al-rich matrix which formed near the metal reservoir and the remaining 1 mm was AIN-rich matrix which formed on top of theAl-rich composite.
Terminology used in the present invention
Matrix metal: The metal which fills the pore space of the preform by infiltration and gives integrity to the whole structure. Here, metal and alloy are used interchangeably.
Preform/filler material: A preform is made ceramic material(s) either in the form of particulate, platelets, whiskers or fibres. If it is compacted and sintered to get a predetermined porosity, it is called as preform. If it is placed as a free powder, it is called as filler.
Useful thickness: The thickness of the composite is such that it can be used in engineering and other applications.
Getter: The function of a getter is to reduce the residual oxygen concentration of the nitrogenous atmosphere thereby facilitating the infiltration for longer time and also formation of significant amount of AIN in the matrix.
Example; Al-5%Mg alloy
1. M.K. Aghajanian, M.A. Rocazella, J.T. Burke and S.D. Keck, "The fabrication of metal matrix composites by a pressureless infihration technique", J. Mater. Sci., 26(1991)447-454.
2. K.A. Semlak and F.N. Rhines, "The rate of infiltration of metals". Trans ofAIME,212 (1958), 325-331.
3. Boddapati Srinivasa Rao and Vikram Jayaram " A method making of aluminium matrix composites by pressureless infiltration of liquid-metal", Indian patent application, 1997 (Application no.: 2359/Mas/97).
4. M.A. Rocazella, K.J. Becker and M.K. Aghajanian, "method for forming metal matrix composite bodies by spontaneously infiltrating a rigidised filler matenal", U.S. patent 52,98,283.
5. M.S. Newkirk, D.R. White, C.R. Kennedy, A.S, Negelberg, M.K. Aghajanian, R.J. Wiener, S.D. Keck and J.T. Burke, " IVfethod for forming macrocomposite useftil as electronic package container", Indian patent 1,76,990
1. A method of making Al-rich and ALN rich matrix composites by
pressureless infiltration of molten AI-Alloys using an external getter
heating the crucible assembly containing the matrix metal and preform to at least above the melting point of the matrix metal along with the external getter.
2. A method as claimed in claim 1 wherein said external getter is Al-Mg alloy or Al- Li or Al-Sr alloy
3. A method as claimed in claim 2 wherein said external getter is AI-Mg alloy,
4. A method as claimed in claim 3 wherein A1-5% Mg alloy is used as an external getter.
5. A method as claimed in claim 1 wherein Said preform is a ceramic material such as Al203,SiC, AIN Si3N4
6. A method as claimed in claim 1 wherein said matrix metal is aluminium.
7. A method as claimed in claim 6 wherein the temperature of processing of said Al is at least above 660^C.
8 . A method as claimed in claim 7 wherein said matrix metal with getter ' alloy is heated to 800 X, preferably lOOO to form Al-N rich matrix composition.
9. A method as claimed in claim 1 wherein said crucible assembly is
made of a material which does not decompose in N2 and does not inhibit
10. A method of making Al-rich and ALN rich matrix composites
substantially as herein described with reference to accompanying drawings
and foregoing examples.
|Indian Patent Application Number||365/MAS/2000|
|PG Journal Number||13/2008|
|Date of Filing||10-May-2000|
|Name of Patentee||THE DEPARTMENT OF METALLURGY, INDIAN INSTITUTE OF SCIENCE|
|Applicant Address||BANGALORE - 560 012,|
|PCT International Classification Number||C22 C 21/16|
|PCT International Application Number||N/A|
|PCT International Filing date|