"ME THOD FOR MANUFACTURING A BRAKE D I SC AND BRAKE D I SC FOR

D I SC BRAKE "

FIELD OF APPLICATION

[0001] The present invention relates to a method for manfacturing a brake disc and a brake disc made with such a method.

PRIOR ART

[0002] A brake disc of a disc brake system of a vehicle comprises an annular structure, or braking band, and a central fastening element, known as a bell, through which the disc is fixed to the rotating part of a suspension of a vehicle, for example a hub. The braking band is provided with opposed braking surfaces adapted to cooperate with friction elements (brake pads), housed in at least one gripper body placed astride of said braking band and integral with a non-rotating component of the vehicle suspension. The controlled interaction between the opposing brake pads and the opposed braking surfaces of the braking band causes due to friction a braking action which allows the vehicle to be decelerated or stopped .

[0003] Generally, the brake disc is made of grey cast iron or steel. These materials allow, in fact, obtaining good braking performance (especially in terms of limiting wear) at relatively low costs. Discs made of carbon or carbon-ceramic materials offer much higher performance, but at much higher costs.

[0004] As an alternative to grey cast iron or steel discs, discs made of aluminium have been proposed in order to reduce the weight of the disc. Aluminium discs are provided with protective coatings. The protective coating serves on the one hand to reduce the wear of the disc and thus guarantee performances similar to cast iron discs, and on the other to protect the aluminium base from the temperatures generated during braking, well above the softening temperatures of aluminium (200 - 400 °C) .

[0005] The protective coatings available today and applied on aluminium discs, although offering resistance to wear, are however often subject to flaking that causes the detachment thereof from the disc itself. This complicates the production process of the disc. The disc must in fact be subjected to surface finishing treatments and must also be prepared for connection to the bell.

[0006] From the foregoing it is evident that aluminium or aluminium alloy discs with protective coatings are not currently capable of completely replacing steel or grey cast iron discs.

[0007] The lower density of aluminium compared to both steel and grey cast iron, however, maintains a very high interest on aluminium by the operators in the braking system field as an excellent substitute for steel and grey cast iron.

[0008] In the reference sector, therefore, there is the need to have aluminium-based brake discs which on the one hand allow to exploit the operating peculiarities deriving from aluminium (firstly, due to its lower density) and, on the other hand, have mechanical and wear resistance features comparable to steel or cast iron discs. There is also the need to make such discs with production processes as simple and cost-effective as possible .

DISCLOSURE OF THE INVENTION

[0009] The need to have aluminium-based brake discs that on one hand allow to exploit the operating peculiarities deriving from aluminium (first of all from the lower density) and, on the other hand, exhibit mechanical and wear resistance features comparable to discs in steel or grey cast iron, and that at the same time are obtainable with production processes as simple and cost-effective as possible, is met by a method for producing a brake disc according to claim 1 and by a brake disc for disc brakes according to claim 15.

[0010] The method according to the invention for making a brake disc (comprising a braking band and a bell) comprises the following operating steps:

[0011] a) providing a mould having an internal cavity which comprises a first portion of a shape corresponding to the braking band of the brake disc to be made, and a second portion of a shape corresponding to the bell of the brake disc to be made, wherein the first and the second portion of the internal cavity communicate with each other;

[0012] b) providing a preform made of porous ceramic material having the shape of the braking band of the brake disc to be made;

[0013] c) placing said preform inside the mould at the first portion of the inner cavity; and

[0014] d) introducing a liquid or semi-solid aluminium alloy inside the whole internal cavity of the mould so as to :

[0015] - infiltrate with said aluminium alloy the preform made of porous ceramic material, obtaining at the first portion an aluminium-based metal matrix composite reinforced by said ceramic preform which defines the braking band of the brake disc to be made, and

[0016] - fill with said aluminium alloy the second portion obtaining a fusion of aluminium alloy which is connected in one piece with the braking band made of a metal matrix composite and defines the bell of the brake disc to be made . [0017] The step b) of introducing the aluminium alloy inside the mould may be carried out according to:

[0018] - a technique of infiltration in the liquid state;

[0019] - a squeeze casting technique;

[0020] - a technique of infiltration in the semi-solid state; or

[0021] - a technique of infiltration by gravity.

[0022] Preferably, said preform made of porous ceramic material is obtained by subjecting in sequence to moulding, debonding and sintering, a mass of granules of ceramic material coated on the surface with a polymeric binder composition.

[0023] Preferably, said ceramic material is selected from the group consisting of silicon carbide, alumina, boron carbide, tungsten carbide, boron nitride and aluminium nitride or mixtures thereof.

[0024] Advantageously, the moulding is carried out in a uniaxial or isostatic manner.

[0025] Preferably, the debonding is conducted in air flow condition at a temperature lower than 700°C until the complete elimination of the organic phase present in the mass of granules of ceramic material after the moulding.

[0026] Preferably, the sintering is conducted in an inert atmosphere at a temperature not lower than 1,600 °C.

[0027] Advantageously, the preform made of porous ceramic material has a homogeneous density and a homogeneous porosity .

[0028] Advantageously, the aluminium-based metal matrix composite reinforced by said ceramic preform , which defines the braking band of the brake disc to be made , has an aluminium alloy matrix having a structure evenly distributed inside said ceramic preform, corresponding to the porosity of the ceramic preform.

[0029] In particular, the aluminium-based metal matrix composite reinforced by said ceramic preform which defines the braking band of the brake disc to be made is constituted of 35% to 60% by weight of ceramic reinforcement material deriving from said preform and of 40% to 65% by weight of aluminium alloy metal matrix.

[0030] Preferably, said aluminium alloy is selected from the group consisting of alloys with manganese and alloys with silicon.

[0031] The brake disc for disc brake according to the invention comprises a braking band and a bell connected to said braking band. The braking band consists of an aluminium-based metal matrix composite reinforced with ceramic material.

[0032] The aforementioned composite is obtained by infiltrating with an aluminium alloy a preform made of porous ceramic material having a shape corresponding to the braking band.

[0033] Said bell is connected in one piece with the braking band and is constituted of a co-fusion of aluminium alloy with the metal matrix composite constituting the braking band.

[0034] Advantageously, the matrix of aluminium alloy has a structure evenly distributed within said composite.

[0035] In particular, said composite is constituted of 35% to 60% by weight of ceramic reinforcement material and of 40% to 65% by weight of aluminium alloy metal matrix.

[0036] Preferably, the aluminium alloy is selected from the group consisting of alloys with manganese and alloys with silicon .

[0037] Preferably, said ceramic material is selected from the group consisting of silicon carbide, alumina, boron carbide, tungsten carbide, boron nitride and aluminium nitride or mixtures thereof.

[0038] In particular, the composite which said braking band is constituted of has a density of between 2.8 and 3.1 g/cm3, and an elastic modulus between 140 and 175 GPa. DESCRIPTION OF THE DRAWINGS

[0039] Further features and advantages of the present invention will appear more clearly from the following description of preferred non-limiting embodiments thereof, in which: [0040] - Figure 1 shows a schematised view of a mould for making a brake disc according to the invention with an aluminium-based MMC braking band and an aluminium alloy bell, co-fused with the braking band;

[0041] - Figure 2 shows a perspective view of a ceramic preform which defines the reinforcing structure of the final braking band according to the invention, made of MMC; and

[0042] - Figure 3 shows an orthogonal sectional view made on a diametrical plane of a brake disc according to the invention with an aluminium-based MMC braking band and an aluminium alloy bell, co-fused with the braking band.

[0043] Elements or parts of elements in common to the embodiments described below are referred to with the same reference numerals.

DETAILED DESCRIPTION

[0044] With reference to the above figures, reference numeral 1 globally denotes a brake disc according to the present invention.

[0045] According to a general embodiment of the invention, illustrated in the accompanying Figures, the brake disc 1 comprises a braking band 2, provided with two opposed braking surfaces 2a and 2b, each of which at least partially defines one of the two main faces of the disc.

[0046] The brake disc 1 further comprises a bell 3 connected to the braking band 2.

[0047] According to a first aspect of the invention, the braking band consists of an aluminium-based metal matrix composite reinforced with ceramic material.

[0048] In general, the aforementioned composite falls into the category of composites known in the art as MMC (Metal Matrix Composite) .

[0049] The use of this aluminium-based MMC composite in the braking band 2 allows to maintain mechanical and chemical-physical features in line with those of aluminium (in particular density and therefore lightness), and to add at the same time (compared to a simple casting in aluminium or an alloy thereof) features functional to a heavy application such as that required in a braking system without the need for protective coatings on the braking surfaces.

[0050] More in detail, compared to aluminium or to an alloy thereof, the presence of the reinforcement of ceramic material allows in fact to obtain greater hardness, greater rigidity, higher coefficient of friction and greater resistance to wear. All of these features make the aluminium-based aluminium matrix composite with ceramic reinforcement suitable for use as a brake disc.

[0051] In this way, it is possible to make a braking band with the advantageous features of aluminium (see in particular the lower density compared to steel and cast iron) , while avoiding at the same time the need to provide the braking surfaces with protective coatings, with the relative limits and drawbacks, both productive and operational.

[0052] Preferably, said ceramic material of which the reinforcement is made is selected from the group consisting of silicon carbide, alumina, boron carbide, tungsten carbide, boron nitride and aluminium nitride or mixtures thereof. Even more preferably, the above ceramic material is silicon carbide or alumina.

[0053] As will be resumed in the following description, the MMC composite which forms the braking band 2 of the brake disc 1 is obtained by infiltrating a porous ceramic preform with an aluminium alloy. Advantageously, the ceramic materials listed above are able to withstand the step of infiltration by the molten metal without altering their physical chemical structure and without being damaged macroscopically and microscopically in any way. Also for this reason they are particularly suitable for making the above composite.

[0054] Preferably, the aluminium alloy is selected from the group consisting of alloys with manganese and alloys with silicon .

[0055] In particular, the aluminium alloy is selected from the group 3xx.x and 4xx.x (nomenclature according to the IADS - International Alloy Designation System) , or from the group 4xxxx and 5xxxx (nomenclature according to the European designation EN-1780-1) .

[0056] More in detail, in the alloys of the group 3xx.x

(IADS) or 4xxxx (EN-1780-1), the main alloying element is Manganese. In general, the advantage imparted by Manganese is to increase the mechanical strength of the processed alloys and to reduce the sensitivity to intergranular corrosion and stress corrosion.

[0057] In the alloys of the group 4xx.x (IADS) or 5xxxx (EN-1780-1), the main alloying element is Silicon. The importance of silicon is due to the increase in fluidity and to the reduction of the thermal expansion coefficient imparted by the addition of small quantities of this alloyer, a very useful property in casting technology.

[0058] In a particularly preferred manner, the aluminium alloy is selected from the group consisting of silicon alloys, in particular alloys of the group 4xx.x (IADS) or 5xxxx (EN-1780-1) .

[0059] Advantageously, the matrix of aluminium alloy has a structure evenly distributed within the composite. As will be resumed below, this can be achieved by infiltrating a porous ceramic material preform with an aluminium alloy having a homogeneous porosity throughout its entire volume. The aluminium alloy - due to the infiltration process - permeates the porosity of the ceramic material creating a homogeneous structure.

[0060] Preferably, said composite is constituted of 35% to 60% by weight of ceramic reinforcement material and of 40% to 65% by weight of aluminium alloy metal matrix.

[0061] Advantageously, the MMC composite of which the braking band 2 is constituted has:

[0062] - a density of between 2.8 and 3.1 g/cm3;

[0063] - a residual porosity of between 0% and 5%, and

[0064] - an elastic module of between 140 and 175 GPa.

[0065] According to another aspect of the present invention, said bell 3 is connected in one piece with the braking band 2 and is constituted of a co-fusion of aluminium alloy with the metal matrix composite constituting the braking band 2.

[0066] As will be resumed in the following description, the bell 3 is obtained in the same mould in which the aluminium alloy infiltration of the preform of ceramic material is carried out, using the same aluminium alloy. In this way, in the same operative step the composite material is formed and the bell is casted, obtaining a complete union of the two materials.

[0067] The manufacture of the bell in co-fusion with the braking band allows significantly simplifying the production process. In fact, having to prepare both a production line dedicated to the production of the bell, and an assembly line of the bell on the band is prevented .

[0068] The combination of the above two essential aspects of the invention allows having aluminium-based brake discs which on the one hand allow exploiting the operating peculiarities deriving from aluminium (first of all from the lower density) and on the other hand have mechanical and wear resistance features comparable to steel or grey cast iron discs, and are at the same time obtainable with simple and cost-effective production processes .

k k k

[0069] For simplicity of treatment, the brake disc 1 will now be described together with the method according to the present invention. The brake disc 1 is preferably, but not necessarily, made with the method according to the invention which will now be described.

[0070] According to a general embodiment of the method according to the invention, the method comprises a first operating step a) of providing a mould 10 having an internal cavity 11 which comprises a first portion 11a having a shape corresponding to the braking band 2 of the disc brake 1 to be made and a second portion lib having a shape corresponding to the bell 3 of the brake disc 1 to be produced.

[0071] The first portion 11a and the second portion lib of such an internal cavity 11 are communicating with each other, as shown in Figure 1, which schematically illustrates an example of a mould that can be used in the method according to the invention.

[0072] Advantageously, as shown in Figure 1, the mould comprises a plurality of first openings 12 for injecting the aluminium alloy directly into the first portion of the inner cavity 11 of the mould 10. These first openings 12 radially develop around the circumferential extension of the first portion 11a having a shape corresponding to the braking band 2 of the brake disc. The mould comprises a second opening 13 for injecting the aluminium alloy directly into the second portion of the inner cavity 11 of the mould 10. Said second opening 13 develops coaxially with the circumferential extension of the second portion lib having a shape corresponding to the bell 3 of the brake disc 1 to be produced. Operationally, the injection of the aluminium alloy can therefore be made from several points, preventing the introduction of the alloy into the first portion 11a from affecting the introduction of the alloy into the second portion lib of the mould. [0073] The method comprises a second operating step b) of providing a preform 20 made of porous ceramic material having the shape of the braking band 2 of the brake disc 1 to be made .

[0074] The method further comprises the following operating steps :

[0075] - c) placing said preform 20 inside the mould at the first portion 11a of said inner cavity 11; and

[0076] - d) introducing a liquid or semi-solid aluminium alloy inside the whole internal cavity of the mould 10.

[0077] The introduction of the aluminium alloy is carried out so as to infiltrate with said aluminium alloy the preform 20 made of porous ceramic material, obtaining at the first portion 11a an aluminium-based metal matrix composite reinforced by said ceramic preform which defines the braking band 2 of the brake disc to be made, and so as to fill with said aluminium alloy the second portion lib obtaining a fusion of aluminium alloy which is connected in one piece with the braking band 2 made of a metal matrix composite and defines the bell 3 of the brake disc 1 to be made.

[0078] Advantageously, the step b) of introducing the aluminium alloy inside the mould may be carried out according to any technique suitable for the purpose.

[0079] In particular, step b) may be carried out: [0080] - according to a technique of infiltration in the liquid state;

[0081] - according to a squeeze casting technique;

[0082] - according to a technique of infiltration by gravity, or

[0083] - according to a technique of infiltration in the semi-solid state.

[0084] The aforementioned infiltration techniques are well known to a man skilled in the art and will therefore not be described herein.

[0085] Preferably, the step b) of introducing the aluminium alloy inside the mould is carried out according to a technique of infiltration in the semi-solid state. In fact, it has been verified that this technique is more suitable for infiltrating ceramics preforms so that, at the end of the process, the resulting disc of MMC material has homogeneous features throughout its structure. At the same time, this technique is suitable to form the bell within the same process.

[0086] More in detail, the infiltration at the semi-solid stage takes place at a temperature between the liquidus line and the solidus line of the aluminium alloy used, with the alloy being in a semi-solid state. Due to the low viscosity of the semi-solid mass, the process of injection in the mould and infiltration takes place easily and with low turbulence. The resulting microstructure that is created includes roundish grains (globules) formed by the mixing of the molten material before solidification. Due to the fact that the alloy which starts solidification in the mould and within the porosity of the preform of porous ceramic material is already partially solid, shrinkage is reduced and therefore also the porosity thereof.

k k k

[0087] According to a preferred embodiment of the method, the aforesaid preform 20 of porous ceramic material is obtained by subjecting a mass of granules of ceramic material, coated on the surface with a binding polymeric composition, to the following operating steps in sequence: moulding, debonding and sintering.

[0088] Advantageously, the aforesaid granules of ceramic material are granules of powders known as "ready-to- press" . This type of powders, available on the market, allows obtaining, after moulding, "net shape moulded" products, without the need of other components or additives in addition to the powders themselves.

[0089] Preferably, said ceramic material of which the granules are made is selected from the group consisting of silicon carbide, alumina, boron carbide, tungsten carbide, boron nitride and aluminium nitride or mixtures thereof. Even more preferably, the above ceramic material is silicon carbide or alumina.

[0090] Advantageously, as already highlighted above, the ceramic materials listed above are able to withstand the step of infiltration with aluminium without altering their physical chemical structure and without being damaged macroscopically and microscopically in any way. These materials therefore allow the ceramic preform 20 to not alter during the infiltration step.

[0091] Preferably, the binding polymeric composition which coats the granules of ceramic material is selected from the group consisting of thermoplastic and thermosetting polymers .

[0092] Preferably, the moulding of the mass of granules of ceramic material takes place by uniaxial or isostatic route, or any other technique which allows obtaining a preform of such size and shape.

[0093] At the end of the moulding, an aggregate of the aforesaid granules of ceramic material is obtained, connected to each other by ceramic connecting microstructures generated by the respective coating of the binding polymeric composition. This aggregate contains organic residues deriving from granule coatings. These organic residues are removed in the debonding step.

[0094] Advantageously, the debonding is conducted in air flow condition at a temperature lower than 700°C until the complete elimination of the organic phase present in the mass of granules of ceramic material after the moulding .

[0095] At the end of the debonding step, a green body is obtained consisting essentially of ceramic material. This green body is then subjected to the sintering step which transforms the green body into a continuous structure obtained from the formation of bridges [can we use the expression "connecting structures/bridges" instead of "bridges" or the expression "bridges" has a particular value?] between the individual ceramic particles. The result is a body that has homogeneous properties throughout the structure.

[0096] Preferably, the sintering is conducted in an inert atmosphere at a temperature not lower than 1,600 °C.

[0097] Advantageously, the preform 20 made of porous ceramic material thus obtained has a homogeneous density and a homogeneous porosity throughout its entire volume. These features make the preform suitable for producing a homogeneously distributed aluminium alloy matrix as a result of its infiltration with said alloy.

[0098] Preferably, the preform 20 has an average porosity of between 35% and 60%. [0099] Advantageously, the aluminium-based metal matrix composite (reinforced by the ceramic preform 20) which defines the braking band 2 obtained at the end of the infiltration step d) has an aluminium alloy matrix having a homogeneously distributed structure within the ceramic preform 20, corresponding to the porosity of the ceramic preform 20.

[00100] Preferably, the aluminium-based metal matrix composite reinforced by the aforementioned ceramic preform comprises:

[00101] - 35% to 60% by weight of reinforcing ceramic material deriving from the preform 20; and

[00102] 40% to 65% by weight of an aluminium alloy metal matrix.

[00103] Preferably, the aluminium alloy is selected from the group consisting of alloys with manganese and alloys with silicon.

[00104] In particular, the aluminium alloy is selected from the group 3xx.x and 4xx.x (nomenclature according to the IADS - International Alloy Designation System) , or from the group 4xxxx and 5xxxx (nomenclature according to the European designation EN-1780-1) .

[00105] In a particularly preferred manner, the aluminium alloy is selected from the group consisting of silicon alloys, in particular alloys of the group 4xx.x (IADS) or 5xxxx (EN-1780-1) .

[00106] For the sake of brevity, the information relating to the aforementioned alloys is not reported again, meaning that the information already provided when describing the brake disc 1 according to the invention is valid .

k k k

[00107] As can be seen from the above description, the brake disc and the method for making such a brake disc according to the invention allow the drawbacks of the prior art to be overcome.

[00108] A man skilled in the art may make several changes and adjustments to the disc and to the disc brake described above in order to meet specific and incidental needs, all falling within the scope of protection defined in the following claims.