Abstract:
A method for preparing metal-matrix composites including cold-process isostatic compaction of previously mixed powders and hot-process uniaxial pressing of the resulting compact is disclosed. The method enables metal-matrix composites with improved properties to be obtained. A device for implementing isostatic compaction comprising a latex sheath into which the mixture of powders is poured, a perforated cylindrical container in which the latex sheath is arranged, and means for sealed insulation of the mixture of powders contained in the sheath is also disclosed.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a process for preparation of metal-matrix composites (MMC). 
     The invention also relates to a device making possible the implementation of such a process. 
     CMMs can be aluminum alloys reinforced by particles such as, for example, particles of silicon carbide, boron carbide, alumina, or any other ceramic material. 
     CMMs are mainly used for manufacturing metallic parts in the field of aeronautics, such as rotor parts for helicopters. 
     The stamping of parts made of MMC is done using billets weighing several tens of kilos which are obtained by compaction of powders mixed beforehand. 
     In certain known processes, the main compaction step is done by uniaxial pressing leading to the formation of strata in the billets, which is disadvantageous for the mechanical properties of the metallic parts obtained from these billets. 
     In effect, it is necessary for each billet to have the most homogeneous possible distribution of the elements constituting it, and particularly of the reinforcing particles, so that the parts manufactured from these billets have the required mechanical properties. 
     Finally, simplicity of a process for manufacturing MMCs is necessary in order to limit the production costs of these MMCs. 
     SUMMARY OF THE INVENTION 
     The process of the invention makes it possible to ameliorate the aforementioned disadvantages and is essentially characterized by the fact that it includes at least the steps of: (a) cold isostatic compaction of pre-mixed powders  5 , and of (b) uniaxial hot pressing of compact  12  obtained in step (a). 
     These two steps make it possible to produce an MMC with improved mechanical properties at a lower cost. 
     Advantageously, the powders are dry mixed in a suitable mixer subjected to a gas under pressure containing a neutral gas and oxygen. 
     The dry mixing of the powders has the advantage of being more economical than a wet mixing process, and the presence of a neutral gas makes it possible to avoid the risks of explosion present during a dry mixing operation. 
     Preferably, the pressure in the mixer is between 15 and 25 mbar, the neutral gas is nitrogen, and the percentage of oxygen is regulated and between 5 and 10%. 
     Regulation of the percentage of oxygen makes it possible to limit the risks of explosion even further. 
     More preferably, the pressure in the mixer is 20 mbar, and the percentage of oxygen is 6%. 
     Preferably, powder mixture  5  is composed of an aluminum alloy reinforced by particles such as, for example, particles of silicon carbide, boron carbide, alumina, or any other ceramic material. 
     More preferably, powder mixture  5  contains 94.7 wt % aluminum, 4 wt % copper, 1.3 wt % magnesium and 15 vol % silicon carbide. 
     Furthermore, powder mixture  5  is subjected to a packing operation on a vibrating table before isostatic compaction step (a). 
     Also before isostatic compaction step (a), the gas possibly contained in the mixture of packed powders  5  can be evacuated by pumping in order to obtain a solid compact  12 . 
     During the compaction step, compaction fluid  15  advantageously contains water and lubricating additives. 
     Preferably, the pressure of compaction fluid  15  is between 1500 and 4000 bar, and more preferably, the pressure is 2000 bar. 
     It is also possible to provide that the compact obtained in step (a) be subjected to a degassing operation at a temperature between 100 and 450° C., preferably 440° C. 
     Preferably, uniaxial hot pressing step (b) is carried out at a temperature between 400 and 600° C., preferably at a temperature of 450° C., and with an applied pressure between 1000 and 3000 bar, preferably 1800 bar. 
     Advantageously, billet  22  obtained in step (b) is hot extruded. 
     Very advantageously, the aluminum matrix composites are reinforced by particles of silicon carbide or any other ceramic particles such as boron carbide or alumina. 
     The invention also relates to billet  22  obtained by the process described in the preceding. 
     The invention moreover relates to a device for implementating step (a) of the process described in the preceding, which includes: latex sheath  1  in which powder mixture  5  is poured, perforated cylindrical container  2  in which latex sheath  1  is arranged, and some means of hermetic isolation  7 ,  10 ,  11  of powder mixture  5  contained in sheath  1 , in which sheath  1 , perforated container  2  and hermetic isolation means  7 ,  10 ,  11  form isostatic compaction device  14  which can be placed in compaction liquid  15  of the isostatic press in order to undergo the isostatic compaction step (a). 
     Advantageously, hermetic isolation means  7 ,  10 ,  11  at least include plug  7 , made of an elastically deformable material, force fit into sheath  1 . 
     Very advantageously, hermetic isolation means  7 ,  10 ,  11  include upper edge  10  of sheath  1  which is folded in the direction of the bottom of sheath  1 , forming annular rim  11  which elastically rests against external surface  13   a  of lateral wall  13  of perforated container  2 . 
     Preferably, sheath  1  and perforated container  2  are arranged in a removable manner in cylindrical container  3  before isostatic compaction step (a). 
     In this case, upper edge  10  of sheath  1  is folded in the direction of the bottom of sheath  1  and elastically rests against external surface  12   a  of lateral wall  12  of cylindrical container  3 . 
     Furthermore, the device of the invention can have means  7   a  for producing a vacuum in sheath  1  in such a way that the gas contained in powder mixture  5  is evacuated before isostatic compaction step (a). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood, and other aims, advantages and characteristics of it will appear more clearly upon reading the following description, which is given in reference to the appended drawings that represent non-limiting embodiments of the device of the invention and in which: 
         FIG. 1  is an exploded perspective view of the device of the invention making it possible to evacuate the residual gases before isostatic compaction step (a); 
         FIG. 2  is a view in section along line II-II of  FIG. 1  of the assembled device of  FIG. 1 ; 
         FIG. 3  is an identical view of the device of  FIG. 2  without the container and arranged like this in the isostatic press; 
         FIG. 4  is a view of the device during the degassing step; and 
         FIG. 5  is a view in section of the uniaxial pressing device. 
     
    
    
     DETAILED DESCRIPTION 
     The embodiment presented hereafter is suitable in a non-limiting manner for the preparation of aluminum matrix composites reinforced by silicon carbide particles. 
     Powder mixture  5  combined beforehand, composed of 94.7 wt % aluminum, 4 wt % copper, 1.3 wt % magnesium and 15 vol % silicon carbide, is dry mixed in a ball mill or in a conventional powder mixer. 
     In order to avoid any risk of explosion during mixing of the powders, the surrounding atmosphere contains a neutral gas such as nitrogen at a pressure between 15 and 25 mbar, preferably 20 mbar, as well as oxygen in a percentage between 5 and 10%, preferably 6%. 
     In reference to  FIGS. 1 and 2 , latex sheath  1  is arranged in perforated container  2  in such a way as to leave free space between the bottom of sheath  1  and the bottom of perforated container  2 . 
     Latex sheath  1  and perforated container  2  are placed in container  3  which has nozzle  4  penetrated by channel  4   a  opening into container  3 , said channel  4   a  being intended for connection to a vacuum pump via a pipe, which is not represented. 
     After hermetically closing off the device by some suitable means which is not represented, a slight vacuum is created at the site of nozzle  4  such that latex sheath  1  becomes flattened against the walls of perforated container  2 , defining a volume with the largest possible capacity. 
     After application of the vacuum is stopped by closing channel  4   a , the aforementioned powder mixture  5  is poured into sheath  1  and simultaneously packed in said sheath by means of a vibrating table, which is not represented. 
     In order to obtain optimal sealing for the operations which follow, upper part  10  of sheath  1  is arranged in such a way as to project from container  3  by being folded in the direction of the bottom of sheath  1  in order to form annular edge  11  which bears elastically against external surface  12   a  of lateral wall  12  of container  3 . 
     Approximately cylindrical nitrile rubber plug  7  is force fit into sheath  1  while allowing annular edge  11  to project as described in the preceding. 
     The arrangement of nitrile rubber plug  7  and that of annular edge  11  of sheath  1  make it possible to obtain a completely sealed system. 
     Nitrile rubber plug  7  has central bore  7   a  intended for connection to a vacuum pump by means of a pipe, which is not represented. 
     A vacuum is effected until powder mixture  5  becomes solid compact  12 ; then vacuum application is stopped by closing off channel  7   a  by means of closure valve  7   b.    
     Filter  6 , attached on internal surface  9  of plug  7  and in contact with packed powder mixture  5 , makes it possible to prevent dust from powder mixture  5  from entering the system for applying a vacuum during the drawdown. 
     In reference to  FIG. 3 , the assembly that forms device  14  for isostatic compaction, consisting of compact  12 , sheath  1 , perforated container  2  and plug  7 , is extracted from container  3 , the seal being preserved by the elasticity of sheath  1 , making it possible, simultaneously with the extraction of this device  14  from container  3 , for annular edge  11  to flatten against external surface  13   a  of lateral wall  13  of perforated container  2 . 
     This device  14  is immersed in compaction liquid  15  of isostatic press  16  containing water and lubricating additives, and is thus subjected to the operation of cold isostatic compaction by application of a pressure between 1500 and 4000 bar, and preferably 2000 bar. 
     The speed of the pressure rise during this step is between 20 and 50 bar per minute, and the time for which the aforementioned maximum pressure is maintained is at least one minute. 
     In this way, the forces exerted on compact  12  are exerted over its whole surface, making it possible to obtain uniform compaction without forming strata or other discontinuities of the material. 
     Compact  12  obtained after the isostatic compaction operation has a density of approximately 85%. 
     After this operation, sheath  1  is extracted from perforated container  2 , and the outside of sheath  1  as well as plug  7  are thoroughly cleaned in order to avoid any contact between compaction liquid  15  and compact  12 . 
     Then, sheath  1  and plug  7  are removed, and the residues of filter  9  are removed by grinding or polishing the upper part of compact  12 , if necessary. 
     In reference to  FIG. 4 , compact  12  is then arranged in tubular container  17  made of aluminum which has bottom wall  18 . 
     Container  17  is closed by soldering opposite upper wall  19  made of aluminum, which has opening  20  in which tube  21 , intended for connection to a vacuum pump, is soldered. 
     A vacuum is created for approximately 30 min after having checked the sealing of aluminum container  17 , and while continuing the pumping, container  17  is placed in an oven at approximately 440° C. for approximately 12 h in order to undergo a degassing operation. 
     After this last operation, tube  21  is closed approximately 10-20 cm from upper wall  19 . 
     Aluminum container  17  containing compact  12  is then quickly placed in tool  23  pre-heated to a temperature higher than 300° C., preferably between 400 and 600° C., and advantageously 450° C., so that compact  12  does not cool down after the degassing step. 
     The aforementioned temperature is maintained for the duration of the uniaxial hot pressing operation. 
     Tool  23  has cylindrical central bore  24  whose diameter is approximately equal to the diameter of container  17  so that it is possible to insert container  17  in said bore  24 . 
     For reasons explained subsequently, container  17  rests on a piece forming matrix ejector  25  that is firmly attached in a removable manner to internal surface  26  of central bore  24 . 
     Punch  27  then applies a pressure between 1000 and 3000 bar, preferably 1800 bar, onto container  22  in the vertical direction indicated by arrow  28  until punch  27  no longer moves, the pressure which is reached then being maintained for approximately one minute. 
     The application of a vertical pressure allows the matrix to be centered relative to this pressure. 
     After the uniaxial pressing operation, punch  27  is withdrawn, and billet  22 , consisting of compact  12  in aluminum container  17  after the uniaxial pressing operation, is ejected from tool  23  by ejector  29  arranged on the side opposite punch  27 , by application of pressure in the direction of arrow  20 . 
     The ejection of billet  22  through the upper part of the tool is made possible by movable matrix ejector  25 , which slides in central bore  24 . 
     Mechanical peeling is then carried out in order to remove the layer of aluminum of the container around billet  22 . 
     After the uniaxial pressing operation, billet  22  with a density of 100% is obtained, 
     This billet  22  is hot extruded at a temperature of approximately 400° C. in order to give it better cohesion and optimal mechanical properties. 
     Billet  22  can then be machined in order to produce a metallic part of any shape by forging, machining or any other known technique. 
     By the process which has been implemented, the particles of silicon carbide are uniformly distributed in the billet obtained, which thus has improved mechanical properties. 
     The properties of the metallic-matrix composite thus obtained depend on the nature of the aluminum matrix, on the percentage of particle reinforcement and on the heat treatment carried out on the product. 
     The rapture strength is typically greater than 500 MPa, and the Young&#39;s modulus is between 95 and 130 GPa for a reinforcement percentage varying between 15 and 40 vol %. 
     The fatigue stress limit at 10 7  cycles is situated between 250 and 350 Mpa, having the consequence that the mechanical parts produced from this CCM prepared according to the process described in the preceding can have a service life multiplied by a factor of 10 compared to conventional materials.