Abstract:
A method for manufacturing an aluminum-based composite plate is disclosed. The method comprises the step of producing an aluminum-based composite billet. The billet production step includes reducing, by magnesium nitride, an oxide-based ceramic as a porous molded body. The reduced oxide-based ceramic has improved wettability. An aluminum alloy is then caused to infiltrate into porous sections of the reduced oxide-based ceramic to thereby provide the aluminum-based composite billet. The billet is extrusion molded into a flat plate form by using an extrusion press. Plates of desired shapes are punched from the molded flat plate by using a press.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an improved method for manufacturing an aluminum-based composite plate. 
     2. Description of the Related Art 
     Automobile disk brakes have disks which are disposed parallel to tires and sandwiched on both sides by pads to thereby halt the disks by friction. The pads are housed in a caliper along with a piston for operating the pads. The pads are frictional plates each prepared by bonding a friction material (produced by thermoforming and polishing a mixture of fibers, a filler material, a friction adjuster and a binder) to a back plate (back metal). The back plates must be of high strength and lightweight because of heat and the compressive force imposed on them through the pads. Recently, it has become common to use aluminum in automobiles and motorcycles for weight reduction. In particular, use of metal-based composite materials (fiber-reinforced metal-based composite materials (FRMs) or metal matrix composites (MMCs) with aluminum as the base metal (matrix phase) has been increasing. 
     One known process for manufacturing products by extrusion molding of aluminum-based composite materials is “CYLINDER MANUFACTURING METHOD” disclosed in Japanese Patent Laid-Open Publication No. SHO-59-206154. The steps involved in the disclosed method are as summarized below: 
     (a) SiC chips are stirred and dispersed in molten aluminum, and the mixture is allowed to solidify. 
     (b) the solidified product is drawn while heated to about 250° C., to fabricate a pipe. 
     (c) the pipe is cut into a sleeve shape, fitted into a die casting metal mold and then insertion-cast with an aluminum alloy (JIS-ADC12) to thereby provide a cylinder. 
     The process described in Japanese Patent Laid-Open Publication No. SHO-59-206154 can be utilized to manufacture back plates used on pads of such disk brakes as described above. 
     However, since the composite materials manufactured by the disclosed method are obtained by combining SiC chips in molten aluminum, they have high resistance to plastic deformation so that it is not easy to work the composite materials into tubes or plates by extrusion molding. In addition, the interface between the aluminum and SIC is in a simple mechanically bonded state. Therefore, such materials exhibit low elongation and have poor workability, similarly to ordinary composite materials. Consequently, it has been a problem that these composite materials have been difficult to mold when it is attempted to obtain desired shapes by extrusion molding or the like, and that production efficiency has therefore been difficult to increase. 
     Furthermore, when attempts are made to cut composite materials into predetermined shapes, the composite materials manufactured by the above-described method of manufacture which include a ceramic (SiC) and hence have poor workability by machining such as cutting or polishing, thereby increasing production costs. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a method for manufacturing an aluminum-based composite plate, which is easy to mold and inexpensive, as well as a back plate and a method for manufacturing the latter. 
     According to one aspect of the present invention, there is provided a method for manufacturing an aluminum-based composite plate, which method comprises the steps of: introducing an aluminum alloy and magnesium or a magnesium-generating source into a furnace together with a porous molded body composed of an oxide-based ceramic; reducing the oxide-based ceramic by magnesium nitride to increase wettability of the oxide-based ceramic; causing a molten aluminum alloy to infiltrate into the reduced oxide-based ceramic to provide an aluminum-based composite billet; pressing the aluminum-based composite billet into a sheet form by using an extrusion press; and punching a plate of predetermined shape out from the sheet by using a press. 
     Reduction of the oxide-based ceramic with magnesium nitride metallizes a porous surface and increases wettability between the oxide-based ceramic and the molten aluminum alloy. The aluminum-based composite material obtained in this manner is an aluminum-based composite material with excellent mold-ability, wherein the aluminum alloy and the reduced oxide-based ceramic as reinforcing materials are bonded with strong chemical bonds. This type of composite material facilitates extrusion molding in the subsequent press-extruding step and allows a higher extrusion ratio. As a result, it is possible to eliminate internal defects in the molded plate and achieve greater densification, thereby increasing the product quality. 
     Preferably, an extrusion ratio in the pressing step is set to fall in a range of 10-100, where the extrusion ratio is a value resulted from dividing a cross-sectional area of the billet before the pressing step divided by a cross-sectional area of the sheet after the pressing step. An extrusion ratio of  10  or greater will provide an aluminum-based composite material with roughly constant tensile strength and resistance. Because a larger extrusion ratio results in increased plate productivity, a larger extrusion ratio is preferred. However, if the extrusion ratio exceeds 100, the extrusion force becomes too great, thereby requiring new large-sized equipment. By setting the extrusion ratio to be within the range of 10-100, it is possible to increase the tensile strength and resistance of the aluminum-based composite material and reduce production costs by using existing equipment. 
     In a specific form, the composite plate may be a back plate as a constituent part of a disk brake, in which instance the pressing step may comprise placing an aluminum alloy billet closely to dies of the extrusion press, followed by positioning the aluminum-based composite billet immediately behind the aluminum alloy billet and continuously press-extruding the aluminum-based composite billet such that aluminum alloy is bonded to opposite sides or surfaces of the aluminum-based composite billet, to thereby provide a clad material of flat sheet form. The punching may comprise punching a back plate of predetermined shape out from the extruded clad material. 
     Upon extrusion molding, the aluminum alloy is positioned proximately to dies of the extrusion press while the aluminum-based composite billet is positioned behind the aluminum alloy. When extrusion is performed in this state, the aluminum alloy covers the aluminum-based composite material as it passes through the die. This results in continuous molding of a sheet-like form wherein the aluminum alloy is attached to both sides of the aluminum-based composite material, thereby facilitating molding of the clad material. Since both sides of the clad material are covered with an aluminum alloy of low hardness, less friction is applied to the dies during extrusion molding, thereby decreasing wear of the dies. 
     It is preferred that the back plate manufacturing method further comprises surface-processing the back plate to impart a desired degree of surface roughness to opposite surfaces of the back plate. In the surface-processing, the surfaces of the back plate can be ground easily and imparted with a desired level of flatness, because they are surfaced with workable aluminum alloy. 
     According to a second aspect of the invention, there is provided a back plate for use as a constituent part of a disk brake. The back plate is comprised of a clad material which comprises a flat sheet of aluminum-based composite material and thin plates of aluminum alloy attached to opposite sides of the flat sheet. 
     Preferably, the flat sheet of aluminum-based composite material comprises a porous molded body composed of oxide-based ceramics reduced by magnesium nitride, with a molten aluminum alloy infiltrated thereinto. 
     Since it employs an aluminum-based composite material as a core material, the back plate has increased strength and reduced thickness compared to one consisting solely of an aluminum alloy. Further, since the surfaces of the back plate are covered with an aluminum alloy of low hardness, it becomes easy to obtain desired surface roughness. Bonding a friction material to the aluminum alloy provides increased bonding strength compared to bonding the friction material used as the disk brake pad to the aluminum-based composite material. 
    
    
     BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
     Certain preferred embodiments of the present invention will hereinafter be explained in detail, by way of example only, with reference to the accompanying drawings, in which: 
     FIG. 1 is a schematic diagram illustrating a manufacturing apparatus for carrying out the process for the manufacture of an aluminum-based composite material according to the invention; 
     FIG.  2 A and FIG. 2B are operation diagrams for the manufacture of an aluminum-based composite billet using the manufacturing apparatus shown in FIG. 1; 
     FIG. 2C is a perspective view showing a cross-section of part of a manufactured billet; 
     FIG. 2D is a perspective view showing the billet cut to a predetermined length by a lathe; 
     FIG. 3 is a schematic diagram illustrating the extrusion step for extrusion molding of the aforementioned billet cut to the prescribed length into a flat sheet using an extrusion press; 
     FIG. 4 is a graph showing a relationship between tensile strength and resistance of a flat sheet with respect to an extrusion ratio in the extrusion step illustrated in FIG. 3; 
     FIG. 5A is a schematic diagram illustrating the step of punching a plate into a predetermined shape from a flat sheet obtained by the extrusion step of FIG. 3; 
     FIG. 5B is a perspective view showing an example of a disk brake pad back plate obtained by punching; 
     FIG. 6 is an exploded perspective view of a disk brake employing such a back plate; 
     FIGS. 7A-7C are diagrams illustrating the extrusion step for extrusion molding a flat sheet-like clad material using an aluminum alloy and an aluminum-based composite material; 
     FIG. 8 is a schematic diagram illustrating a step for punching a disk brake pad back plate from the aforementioned clad material; 
     FIG. 9 is a diagram illustrating a surface-processing step in which the surface of the aforementioned back plate is worked by a grinding wheel; 
     FIG. 10A is a perspective view showing the production of a pad to be used for a disk brake by bonding a friction material to one side of the aforementioned back plate; 
     FIG. 10B is an enlarged partial cross-sectional view of the pad; and 
     FIG. 11 is an exploded perspective view of a disk brake employing the pad, corresponding to FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description is merely exemplary in nature and is in no way intended to limit the invention or its application or uses. 
     Referring initially to FIG. 1, an aluminum-based composite material manufacturing apparatus  10  comprises an atmospheric furnace  11 , a heating apparatus  12  for heating the atmospheric furnace  11 , a gas supply apparatus  13  for supplying an inert gas to the atmospheric furnace  11 , and a vacuum pump  14  for lowering the pressure inside the atmospheric furnace  11 . The atmospheric furnace  11  has a first crucible  15  and a second crucible  16 . 
     The heating apparatus  12  has a control device  21 , a temperature sensor  22  and a heating coil  23 . 
     The gas supply apparatus  13  is provided with a first tank  25  filled with argon gas (Ar)  24 , a second tank  27  filled with nitrogen gas (N 2 )  26 , a conduit  28  for allowing passage of gases supplied from the tanks  25  and  27  to the atmospheric furnace  11 , and an argon gas pressure gauge  29 a and a nitrogen gas pressure gauge  29   b  mounted on the conduit  28 . 
     The first crucible  15  is a vessel for holding porous alumina (Al 2 O 3 )  31  serving as the oxide-based ceramic and an aluminum alloy  41 . The second crucible  16  is a vessel for holding magnesium (Mg)  42 . The aluminum alloy  41  used may be, for example, Alloy No. A6061 according to Japanese Industrial Standard (JIS) H-4000. A magnesium alloy may also be used instead of the magnesium (Mg)  42 . 
     FIGS. 2A-2D illustrate a manner of manufacture of an aluminum-based composite billet according to the present invention. 
     First, as shown in FIG. 2A, the oxide-based ceramic, alumina (Al 2 O 3 )  31 , is placed in the furnace  11  together with the aluminum alloy  41  and magnesium (Mg)  42 . That is, after placing the alumina  31  in the first crucible  15 , the aluminum alloy  41  is positioned on top of the alumina  31 . The magnesium  42  is placed inside the second crucible  16 . 
     Next, the inside of the atmospheric furnace  11  is evacuated with a vacuum pump  14  to remove oxygen in the atmospheric furnace  11 . When a prescribed degree of vacuum has been attained, the vacuum pump  14  is deactivated. Argon gas (Ar)  24  is then supplied to the atmospheric furnace  11  from the first tank  25 . The inside of the atmospheric furnace  11  is then heated with the heating coil  23 . 
     The temperature elevation is automatically controlled by the control device  21  while detecting the temperature in the atmospheric furnace  11  with the temperature sensor  22 . The aluminum alloy  41  melts during the course of reaching the prescribed temperature (for example, about 750° C. to about 900° C.). At the same time, the magnesium (Mg)  42  also melts and then vaporizes. Since the inside of the atmospheric furnace  11  is under an atmosphere of argon (Ar)  24 , there is no oxidation of the aluminum alloy  41  or magnesium (Mg)  42 . 
     Next, as shown in FIG. 2B, the inside of the atmospheric furnace  11  is pressurized by introducing nitrogen gas  26  from the second tank  27  into the atmospheric furnace  11 . Specifically, nitrogen gas (N 2 )  26  is supplied from the second tank  27  into the atmospheric furnace  11  to pressurize the inside of the atmospheric furnace  11  (for example, to atmospheric pressure+approximately 0.5 Kg/cm 2 ), so that the atmosphere of the atmospheric furnace  11  is exchanged with nitrogen gas (N 2 )  26 . 
     Once the atmosphere of the atmospheric furnace  11  is exchanged with the nitrogen gas (N 2 )  26 , the nitrogen gas  26  reacts with the magnesium (Mg)  42  to produce magnesium nitride (Mg 3 N 2 )  44 . Reduction of the alumina (Al 2 O 3 )  31  with the magnesium nitride  44  improves the wettability of the alumina  31 , and the molten aluminum alloy  41  infiltrates into the pores of the alumina  31 . The aluminum alloy  41  then solidifies to complete the aluminum based-composite billet  45 . 
     By pressurizing the atmosphere inside the atmospheric furnace  11  during the course of infiltration of the aluminum alloy  41  into the pores of the alumina  31 , it is possible to accelerate infiltration and thereby manufacture an aluminum-based composite billet  45  in a shorter time than at an atmospheric pressure. The infiltration can also be accomplished in a shorter time than at an atmospheric pressure even when the pressure inside of the atmospheric furnace  11  is lowered by the vacuum pump  14  for a reduced pressure nitrogen atmosphere. 
     The aluminum-based composite billet  45  (hereinafter referred to simply as “billet  45 ”) manufactured in this manner is illustrated in FIG.  2 C. The billet  45  consists of alumina  31 , oxide-based ceramics, with an aluminum alloy  41  infiltrated thereinto and hence has excellent moldability and can be plastically deformed easily. 
     Finally, as shown in FIG. 2D, the billet  45  is cut into prescribed dimensions by means of an NC (numerically controlled) lathe  46 . The dimensions are determined to match the extrusion press in the following step. 
     FIG. 3 illustrates the extrusion step for a billet in the plate manufacturing method of the present invention. The aforementioned billet  45  is inserted into the container  51  of the extrusion press  50  and extruded with a ram  52  to pass it through the die  53  in order to mold the billet  45  into a flat sheet  54 . Since the billet  45  is a composite material wherein the interface between the aluminum and the reinforcing material is firmly jointed by chemical contact, the moldability is satisfactory and extrusion molding into a flat sheet  54  shape is facilitated. 
     Where the cross-sectional area of the billet  45  before extrusion is A 0  and the cross-sectional area of the extruded flat sheet  54  is A 1 , an extrusion ratio R is represented by A 0 /A 1 , which is the ratio of the cross-sectional area A 0  of the billet  45  before extrusion to the cross-sectional area A 1  of the flat sheet  54  after extrusion. Consequently, when extrusion molding is carried out with a high extrusion ratio R, it is possible to eliminate defects in the interior of the flat sheet  54  after extrusion and to thus increase the product quality. 
     FIG. 4 is a graph showing the relationship between the tensile strength and resistance of the flat sheet  54  as the extrusion molded product, with respect to the extrusion ratio, wherein the extrusion ratio R is plotted on the horizontal axis and the tensile strength σ B  and the resistance σ 0.2  are plotted on the vertical axis. σ 0.2  indicates 0.2% resistance. 
     When the extrusion ratio R is less than 10, the tensile strength σ B  is proportional to the extrusion ratio R. Thus, increasing the extrusion ratio R can increase the tensile strength σ B . Likewise, it can also increase the resistance σ 02 . 
     When the extrusion ratio is 10 or greater, the tensile strength σ B  is almost constant, increasing only very slightly as the extrusion ratio R increases. The resistance σ 0.2  is also almost constant. 
     Since a large extrusion ratio R results in increased productivity, a larger extrusion ratio R is preferred. However, when the extrusion ratio R exceeds 100, the extrusion force becomes too great, thus requiring new large-scale equipment. Consequently, from the standpoint of the mechanical properties of the aluminum-based composite material, the lower limit is preferably 10, while from the standpoint of equipment performance (extrusion press output) the upper limit is preferably 100. 
     FIG. 5A is an illustration of the punching step for punching of a flat sheet  54  into a prescribed shape. The flat sheet  54  is set in a punching press  60  and the punch  61  is lowered so that the punch  61  and the die  62  cut out from the flat sheet  54  for a plate  63  of prescribed shape conforming to the shape of the punch  61 , as shown in FIG.  5 B. The punched plate  63  in this embodiment represents a back plate to be used for a disk brake. 
     FIG. 6 is an exploded perspective view of a disk brake in which back plates obtained by the plate manufacturing method described above are employed. 
     The disk brake  70  has a disk  71 , friction materials  72  and  72  that are in contact with both sides of the disk  71  to brake its rotation by resistance, and back plates  63  and  63  for mounting the friction materials  72  and  72 . The back plates  63  are made of an aluminum-based composite material and are therefore lightweight with high tensile strength, rendering them suitable as automobile parts. In this embodiment, the aluminum-based composite plate was used as a back plate, but it may also be used for other automobile or motorcycle parts. It may also be used for other industrial mechanical parts in addition to automobile parts. 
     FIG. 7A, FIG.  7 B and FIG. 7C illustrate an embodiment of an extrusion step wherein a clad material to be used for a back plate is molded. 
     Referring to FIG. 7A, an aluminum alloy billet  152  is introduced into the container  151  of an extrusion press  150  and positioned against the side of the die  153 , and an aluminum-based composite billet  45  is then introduced into the container  151  at a position behind the aluminum alloy billet  152 . The aluminum alloy billet  152  is preferably a corrosion resistant aluminum alloy such as, for example, an aluminum alloy of the Alloy No. A3000 Series or A5000 Series according to Japan Industrial Standards (JIS) H4000. 
     As shown in FIG. 7B, extrusion of the billet  45  by a ram  154  causes the aluminum alloy billet  152  to pass through the die  153  first, thereby molding a thick plate  155  of only the aluminum alloy. When the billet  45  is extruded by the ram  154 , the center section of the inner surface of the aluminum alloy billet  152  is depressed into a cup shape corresponding to the exit opening  153   a.  The billet  45  of the aluminum-based composite material fills in the depressed section. 
     As shown in FIG. 7C, press extrusion of the billet  45  by the ram  154  causes the billet to pass through the die  153 , thereby molding an aluminum-based composite flat sheet  156 . At this time, an aluminum alloy thin plate  157  is attached on both sides of the flat sheet  156 , thereby providing a flat sheet-like clad material  158 . 
     The thickness of the thin plate  157  is designated by “t”. The extrusion molding is accomplished in such a manner that the thickness t exceeds 0.2 mm. The thickness t is preferably not 0.2 mm or smaller because the aluminum alloy will tend to peel off from the aluminum-based composite material. 
     Because extrusion is accomplished in this manner with the aluminum alloy bonded to both sides of the aluminum-based composite material, the aluminum-based composite material does not contact directly with the die  153  and therefore no friction resistance is generated by the aluminum-based composite material, thus facilitating extrusion molding. Since it is the low-hardness aluminum alloy thin plate  157  that contacts with the die  153  during extrusion molding, the degree of friction on the die  153  is reduced so that the life of the die  153  is extended. 
     When the cross-sectional area of the billet  45  before extrusion is designated as A 0  as in the extrusion molding illustrated in FIG.  3  and the cross-sectional area of the clad material  158  after extrusion is designated as A 1  as the cross-sectional area of the flat sheet  54  shown in FIG. 3, the extrusion ratio R is represented by A 0 /A 1  as explained in relation to FIG.  3 . Since the thin plates  157  and  157  attached on both sides of the aluminum-based composite flat sheet  156  are made of an aluminum alloy with low hardness, the tensile strength and resistance of the clad material  158  are determined by the aluminum-based composite material of the flat sheet  156  which constitutes a major portion of the clad material  158 . Thus, the graph showing the relationship between the extrusion ratio R and the tensile strength and resistance of the clad material  158  becomes substantially the same as the graph of FIG. 4 showing the relationship between the extrusion ratio R and the tensile strength σ B  and resistance σ 0.2  of the flat sheet  54 . Consequently, the extrusion ratio R for extrusion molding of the clad material  158  is preferably between 10 as the lower limit and 100 as the upper limit, as explained in FIG.  4 . 
     FIG. 8 is an explanatory diagram illustrating a punching step for manufacture of a back plate as a product from a clad material obtained by the extrusion step in FIGS. 7A-7C. 
     The clad material  158  is set in a press  160  and a punch  161  is lowered so that the clad material  158  is cut out by the punch  161  and the die  162  to obtain a back plate  163  with the prescribed shape from the clad material  158 . 
     The back plate  163  is formed of the clad material  158  wherein aluminum alloy thin plates  157  and  157  are attached to both sides  164  and  164  of an aluminum-based composite flat sheet  156 . 
     FIG. 9 is a diagram illustrating a surface-processing step which accomplishes leveling of the surface of the back plate  163  obtained by the punching illustrated in FIG.  8 . The back plate  163  is set on the table  171  of a grinding machine  170 , and surface-grinding one side  164  of the back plate  163  by the grinding wheel  172  is followed by surface-grinding the other side  164 . 
     The prescribed flat sides are thus obtained on both sides of the back plate  163 . The flat sides have a desired surface roughness with the bonding strength for bonding a friction material described later, taken into consideration. Since both surfaces  164  and  164  of the back plate  163  are aluminum alloy surfaces, they can be easily worked and facilitate surface-grinding of the back plate  163 . 
     In FIG. 10A, after the back plate  163  has been washed, the friction material  175  is bonded to one side of the back plate  163  along the dotted lines to obtain a pad  177 . Since bonding of the friction material  175  to the back plate  163  is accomplished by bonding of the aluminum alloy thin plate  157  of the prescribed surface roughness using an adhesive  176 , as shown in FIG. 10B, the bonding strength is improved compared to direct bonding to the aluminum-based composite flat sheet  156 . 
     The surface treatment step for the aluminum alloy in FIG. 9 is optional. 
     FIG. 11 is a schematic illustration of a disk brake of the present invention, including a back plate as a constituent part thereof. 
     The disk brake  180  has a disk  181  and a pair of pads  177  and  177  which brake its rotation by contact with both sides of the disk  181 . Each pad  177  has a structure wherein a friction material  175  for contacting with the surface of the disk  181  is bonded to a back plate  163 . The back plate  163  is the clad material described above wherein an aluminum alloy is attached to an aluminum-based composite material (base metal), and it is therefore lightweight with high tensile strength as well as excellent in bonding strength with the friction material  175 . It is therefore suitable as a disk brake part, which must be able to withstand poor environments such as muddy water while exhibiting high tensile strength and resistance to shear force. 
     In this embodiment, magnesium (Mg) was placed in the crucible for production of the magnesium nitride (Mg 3 N 2 )  44  as shown in FIG. 2B, but this is only an exemplary case and is not intended to restrict the scope of the invention. For example, magnesium may be already included in the porous molded body for production of the magnesium nitride. 
     Obviously, various minor changes and modifications of the present invention are possible in the light of the above teaching. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.