Production of metal matrix composites

Abstract

 A method of producing a composite cast article having a metal matrix containing divided dispersible solid material, i.e. divided solid filler, such as reinforcing fibers or particles, wherein a melt of the metal matrix is placed in a die cavity, a preheated preform of a divided solid filler is placed on top of the melt and pressure is exerted on the preform in a direction parallel to the cavity axis for forcing the preform downwardly into the molten metal whereby the molten metal fully infiltrates the the preform filler, and the resultant cast article is solidified by controlling heat flow within the die cavity in such manner that a solidification from advances unidirectionally, along the cavity axis, entirely through the metal-infiltrated preform layer, while the metal ahead of this front within and at least immediately beyond the preform layer remains molten.

Description

[0001] This invention relates to the production of metal matrix composites, and more particularly to methods of producing cast metal matrix composite articles, as well as to the products of such methods.

[0002] Metal matrix composites are articles constituted of a metal matrix, e.g. aluminum or alloys thereof, having distributed therein a divided solid filler, viz. a fibrous or particulate material which is capable of being incorpo­rated in and distributed through such metal matrix and which at least substantially maintains its integrity as thus incorporated rather than losing its form or identity by dissolution in or chemical combination with the metal.

[0003] In Japanese patent publication No. 58-204139, there is described a procedure for forming composites in which alternating layers of reinforcing fibers and aluminum foil are set between male and female molds, heated to "melt" the foil, and subjected to relatively light pressure to form an integrated composite. This procedure, however, is not suitable for the production of ingots having substan­tial bulk or thickness, at least when discontinuous reinforcing fibers are employed. In such case, a large number of individual foil layers would be necessary, each having surface oxide films, and the presence of oxide would prevent attainment of desired product properties. More­over, in the production of bulky composite ingots, it is difficult to determine the proper amount of metal to be provided; too much or too little metal results in an unacceptably nonuniform product.

[0004] It has also been proposed, for example in U.S. patent No. 3,970,136, to arrange a stack of alternating sheets of fibrous reinforcement and matrix metal (in solid state), and then to heat the stack and melt the metal, applying pressure when the metal is molten to effect infiltration of the metal into the fiber layers. With procedures of this type as heretofore known, however, difficulty has been encountered in that (especially where the fiber layers are of appreciable thickness) the produced composites tend to have porous zones exhibiting poor mechanical properties. Also, these techniques have been limited as to the configu­rations of fiber-reinforced zones that can be produced.

[0005] The present invention provides a new and improved method of producing a composite cast article comprising a metal matrix and a divided solid filler. A melt of the metal matrix is provided in a die cavty and a preheated preform of a divided solid filler is placed on top of the melt in the die cavity. Pressure is exerted on the preform in a direction parallel to the cavity axis for pushing the preheated preform into the molten metal and forcing the molten metal to fully infiltrate the preform filler, after which the resultant cast article is solidified. As used herein, the term "preform" refers to an effectively integral, porous compacted body of divided solid filler fibers or particles which has been subjected to sufficient compacting pressure to develop green strength, viz. strength such that the compacted body is self-sustaining in shape and dimensions when handled.

[0006] According to the novel procedure, a melt of the metal matrix is provided in the die cavity. Then a preheated preform is placed on the surface of the melt and pressure is applied thereon by a ram to push the preform downwardly into the melt until it is fully impregnated with molten metal. Preferably, spacers are provided between the top of the preform and the ram to assist infiltration of the preform.

[0007] This procedure of pushing the preform through the melt has important advantages. Thus, the melt is quescient and when a preheated preform is pushed through it, metal flashing is avoided because hot metal is not hitting cold surfaces. Also, heat flow is more uniform as convection can be eliminated.

[0008] The solidifying step is performed by controlling heat flow within the die cavity in such manner that a solidification front advances unidirectionally, along the cavity axis, entirely through the metal-infiltrated preform layer, while the metal ahead of this front within and at least immediately beyond the preform layer remains molten; and the metal layer disposed beyond the preform layer (with respect to the direction of advance of the aforementioned solidification front) contains metal in excess of the amount which infiltrates the preform from that metal layer during the pressure-exerting step. It is found that this combination of features enables production of composite articles which are advantageously free of porous zones and which, in consequence, are characterized by fully adequate mechanical properties throughout.

[0009] In procedures as previously proposed for producing fiber-reinforced or like metal matrix composites by heating and pressing metal and fiber layers, convergent solidi­fication fronts tend to meet within a metal-impregnated fiber layer. At their junction, owing to concentration of the solidifying metal, a porous zone is created; and since this zone is bounded on both sides by solidification fronts, no continuing supply of molten metal is available to fill the pores. As a result, the porous zone remains unfilled and weakens the produced composite. With the method of the present invention, however, contraction-created pores cannot be enclosed between converging solidification fronts in a filler layer because there is only a single solidification front advancing unidirectionally through the entire preform layer; and ahead of this front there is a continuing supply of molten metal, which fills any contraction-vacated space, under the continuing applied infiltration pressure.

[0010] The requisite control of heat flow within the die cavity during the pressure-exerting step is preferably effected by establishing a temperature differential between opposite ends of the die cavity, as for example by selectively heating or cooling one end of the die; by selectively thermally insulating one end of the die, so that heat is preferentially transferred from the other end of the cavity; or by an appropriate combination of these techniques. With such a temperature differential, two opposed solidification fronts may nevertheless advance toward each other along the cavity axis from the opposite ends of the cavity, but with different initiation times and/or rates of advance, so that they meet within the zone or layer of excess metal beyond the preform layer rather than within the preform layer itself, and only one of the solidification fronts passes through the preform.

[0011] The die cavity may be heated by moving an external source of heat progressively and unidirectionally along the die from one end of the die to the other and parallel to the axis thereof as infiltrating pressure is exerted endwise on the metal-preform within the cavity. A unidirectionally advancing solidification front, in this case, follows the heat source along the cavity axis.

[0012] Further features and advantages of the invention will be apparent from the detailed description hereinbelow set forth, together with the accompanying drawings.

Fig. 1 is a schematic sectional elevational view of a die for carrying out the method of the invention;

Fig. 2 is a partial sectional elevational view of a preform,

Fig. 3 is a top plan view of a preform, and

Fig. 4 is a view similar to Fig. 1 showing the preform in infiltrated position.

[0013] For purposes of illustration, the invention will be described as embodied in methods of producing cast compo­site billets of aluminum (matrix metal) and discontinuous reinforcing fibers of a refractory material such as SiC or Al₂O₃. Such composites have utility for structural and other purposes, being characterized by light weight and high strength.

[0014] Referring now more particularly to Fig. 1, there is shown a die 10 fabricated of a suitably thermally conduc­tive material (e.g. steel), defining an axially vertical and upwardly opening cylindrical die cavity having its lower end closed by a metal or like die plate 11. The die plate is supported on a steel base plate 12 and is sepa­rated therefrom by a layer 14 of compressible thermal insulation, e.g. Fibrefrax^R.

[0015] Within the die cavity are disposed a quantity of matrix metal 16 and a disc-shaped preform 18 of a refractory fiber material which is to be impregnated with the matrix metal to produce a composite metal-fiber article. Examples of suitable fiber materials for the preform include articles of alumina, zirconia, silica, silicon carbide, silicon nitride or titanium diboride, in particular alumina in the form of chopped fibers, and silicon carbide or silicon nitride in the form of either whiskers or chopped fibers.

[0016] The preform 18 is preferably provided with spacer strips 30 across the top face thereof; and these spaces 30 may be fixed to the preform, e.g. by means of a bead 31 of sodium silicate around the lower peripheral edge of each spacer. The spacers preferably cover no more than about 10% of the area of the top of the preform and preferably the sodium silicate does not migrate into the horizontal interface between a spacer and the preform.

[0017] In the practice of the present method, the die is heated, e.g. by means of a conventional induction or resistance heater 24 surrounding the die and arranged to minimize heating of the base plate 12. Molten metal is added to the heated die, resting on a layer 20 of thermal insulaion which in turn rests on the die plate 11. The insulating layer is made of fibrous or like material (e.g., Fibrefrax^R aluminum silicate fiber). A vertically movable ram 26, positioned in register with the die cavity, is advanced downwardly to bear against preform 18. With the metal fully molten, the ram is operated to exert pressure in a downward vertical direction.

[0018] Fig. 1 illustrates that point in the performance of the method at which the matrix metal is entirely molten, heating has been discontinued, and the ram has just come into contact with the preform 18. Thereafter, pressure exerted by the ram forces the preform downwardly to the position shown in Figure 4 and forces the molten matrix metal into the insulation layer 20 and the preform 18, and also compresses the insulation layer 14. Preferably, the insulation layer 20 is of coarser weave than the preform 18, so that this layer becomes infiltrated in advance of the preform. Also preferably, the preform has a compressive strengh sufficient to withstand the pressure applied by the ram, and hence substantially retains its initial shape and dimensions.

[0019] When molten metal is poured into a preheated die, the amount of superheat in the metal varies with die temperature. For example, a die temperature of 500°C may be used with a T_melt = T_liquidus + 120°C or a die temperature of 300°C may be used with a T_melt = T_liquidus + 200°C. When the cooler die is used, a larger offset is necessary between the preform and the die wall to prevent modifying metal tacking to the die wall and the preform. This offset is typically from about 2.5 mm to about 12.5 mm with a 75 mm diameter die. The preform is preferably preheated to a high temperature, e.g. in the order of 700°C.

[0020] The spacers attached to the preform serve two func­tions. Firstly, they provide space for the infiltrating liquid metal, nominally hydrostatic infiltration. Secondly, they are used to concentrate residual gas from the preform. Thus, if the spacer has a higher fibre loading than the preform, its metal breakthrough pressure and pressure gradient will be higher than in the preform and will tend to infiltrate less readily. For instance, the preform may have a typical volume fraction of fibers (V_f) of about 0.15 while the spacers may have a V_f of about 0.25. Preferably the spacers infiltrate last.

[0021] As the application of pressure continues, with impregnation of the preform, the matrix metal cools and solidifies since heat is no longer being supplied to the system. Finally, the ram is withdrawn upwardly, and the formed composite product may be removed from the die.

[0022] When the application of pressure is initiated, a temperature differential is established between the upper and lower ends of the contents of the die cavity. Cooling, and resultant solidification, proceed inwardly in vertical directions from the top and bottom of the die cavity, but owing to the temperature differential, the amount of metal below the preform and the vertical dimension of the preform itself, the advancing upper and lower solidification fronts meet in the metal below the preform rather than within the preform itself. In order that this will occur, it is also necessary to apply and maintain a ram pressure sufficient to achieve complete infiltration of the preform with matrix metal before metal solidification is completed.

[0023] As a result of the aforementioned features, while the impregnating matrix metal is solidifying within the preform there is a single solidification front proceeding unidirectionally therethrough, so that molten metal is always available to occupy space vacated by contraction of solidifying metal in the preform, i.e. until impregnation and solidification of metal in the preform are complete. In this way, undesired porosity of the produced composite is avoided, and satisfactory mechanical properties are achieved throughout the composite body, even with a composite of substantial thickness.

[0024] To achieve the initial temperature differential, in the embodiment of Fig. 1, the ram 26 is preferably fabricated of a metal of high thermal conductivity such as die steel (which may also be the material of the base plate 12) or copper bronze and is "cold," i.e., unheated, as introduced to the die cavity; if desired for expedited matrix metal solidification, the ram may be internally cooled, but such positive cooling is not required in all instances. In addition, the die plate 11 is heated by the resistance or induction heater 24 before the ram is introduced, it being noted that the peripheral portion of this plate is in direct contact with the heated die and that the plate is initially well insulated by layer 14 from the relatively unheated base plate 12, so as to minimize heat losses. Thus, as the ram begins to press against the upper end of the die cavity contents, its relatively low temperature, and the relatively high temperature of the die plate 11 at the lower end of the cavity contents, cause the lower end of the cavity to be hotter than the upper end.

[0025] Initially, the cool ram abstracts heat from the top end of the die cavity, initiating a first solidification front that moves downwardly through the cavity. When pres­sure exerted by the ram compresses the insulation layer 14 between the die plate 11 and the base plate 12, reducing the insulating effect of the layer 14 and enhancing the thermal contact between the initially heated die plate and the initially cool base plate, heat is abstracted from the lower end of the die cavity. Thereupon, solidification of the contained metal commences at that locality, sealing any escape paths for molten metal at the bottom of the die and initiating a second, upwardly moving solidification front. The effect of the initial heat differential between the top and bottom of the die cavity, however, is to retard the upward advance of the second front relative to the downward advance of the first front; hence the fronts ultimately meet at a level, in the cavity, which is substantially below the midpoint between their respective starting levels.

[0026] The relative thicknesses of the preform itself and the metal below the preform are so chosen that the two solidification fronts thus respectively advancing from the upper and lower ends of the cavity meet below the preform. The preform preferably has a thickness of no more than 25 mm in order to assure rapid heating of the fibers.

[0027] The lower metal layer thickness as seen in Figure 4, is sufficient so that, when impregnation of the preform is complete, there will remain a layer of excess metal below the preform in which the solidification fronts can meet. The provision of excess molten metal in the pool below the preform is important to ensure a continuing supply of molten metal for infiltration of the preform at all times as the first solidification front advances downwardly through the preform.

[0028] By way of specific illustration, in an example of apparatus as shown in Fig. 1 the die 10 has a wall thickness of 25 mm. and an internal diameter of about 75 mm., this being also the approximate diameter of the ram 26, which however can enter the die cavity with slight clearance. The central portion of the die plate 11 has a vertical thickness of 6 mm., and its thinner edge portion has a vertical thickness of 3 mm., while the base plate 12 has a vertical thickness of 25 mm. Both the base plate and the ram are fabricated of die steel. The insulating layers are constituted of "Fibrefrax" refractory fibers; layer 14 has an uncompressed vertical thickness of 3 mm., and the vertical thickness of layer 20 is 1.5 mm. The heater 24 is a resistance heater.

[0029] In a typical procedure utilizing the above apparatus, a refractory fiber preform 18 having a diameter of 70 mm. and a vertical thickness of 30 mm. is infiltrated with molten metal. After infiltration, the vertical thickness of matrix metal below the preform is 30 mm. A ram pressure of 20-200 mPa is applied for 240 seconds.

[0030] With the foregoing dimensions and conditions, and with a preform having a volume fraction of fibers (V_f) equal to 0.15, satisfactory composites have been produced using an Al - 2% Si alloy as the matrix metal and heating to 660°C. and a preform preheated at 700°C. At these die temperatures, the base plate temperature is about 200°C.

[0031] It is to be understood that the invention is not limited to the procedures and embodiments hereinabove specifically set forth, but may be carried out in other ways without departure from its spirit.

Claims

1. A method of producing a composite cast article comprising a metal matrix and a divided solid filler incorporated in and distributed through the matrix, wherein a melt of the metal matrix (16) is placed in a die cavity, a preheated preform (18) of a divided solid filler is placed on top of the melt and pressure is exerted on the preform in a direction parallel to the cavity axis for forcing the preform downwardly into the molten metal whereby the molten metal fully infiltrates the the preform filler, and the resultant cast article is solidified by controlling heat flow within the die cavity in such manner that a solidification front advances unidirectionally, along the cavity axis, entirely through the metal-infiltrated preform layer, while the metal ahead of this front within and at least immediately beyond the preform layer remains molten.

2. A method according to claim 1, wherein the step of controlling heat flow comprises establishing a temperature differential between opposite ends of the die cavity such that one end region of the die cavity is at a higher temperature than the other, and wherein said excess-metal-containing layer is disposed between said preform layer (18) and the higher-temperature end of the die cavity.

3. A method according to claim 2, wherein the step of establishing a temperature differential comprises selectively heating one end of the die (10).

4. A method according to claim 2, wherein the step of establishing a temperature differential comprises selectively cooling one end of the die (10).

5. A method according to claim 2, wherein the step of establishing a temperature differential comprises selectively thermally insulating one end of the die (10) such that heat is preferentially abstracted from the die cavity at the other end of the die.

6. A method according to claim 5, wherein the pressure-exerting step is performed by a ram (26) introduced to the die cavity at said other end of the die (10), and wherein said ram (26) is at a low temperature relative to said other end of the die (10) and is fabricated of a highly thermally conductive material.

7. A method according to claim 2, wherein said preform (18) is disc-shaped and has spacers (30) fixed to the top face thereof to space the top face of the preform from the ram (26).

8. A method according to claim 1, wherein said matrix metal is aluminum and said filler comprises reinforcing refractory fibers.

Drawing