The invention relates to a brake disc for a disc brake device for automobiles and similar vehicles comprising a mainly disc-shaped cast-iron body having a hub portion integral with a surrounding peripheral edge portion having opposite side faces intended for engagement with the wearing parts of brake blocks and to which an increased hardness and wearability has been imparted.

For the provision of asbestos-free disc brake devices International Patent Application PCT/DK89/ 00090 (published under No. O89/09889) discloses a disc brake device with a brake disc of the above- mentioned structure.

In this known brake disc the increased hardness and wearability of the opposite side faces of the edge portion of the disc have been achieved by casting the side faces together with wearing linings having the said properties. The wearing linings are designed as wearing plates composed of supporting parts of com- paratively thin sheet material which has a metallic surface layer of increased hardness and wearability provided by spray forming, applying the so-called "Osprey"-technology.

The casting together of the wearing plates at the edge portion of the disc is, according to the above publication, carried out using the plates as core elements in casting the discs in disposable moulds produced by an automatic mould forming machine. Although said known technology will allow manufacture of brake discs which to a large extent can meet both the operational requirements of automobile

disc braking devices and the above-mentioned environ¬ mental demands as concerns avoiding pollution of the atmosphere with asbestos fibres, the fact that the wearing linings are designed as separate wear-resist- ant plates which are cast together with the cast-iron body of the disc, but may otherwise be of a different material, e.g. steel, may cause problems insofar as for example the tolerance differences may cause an imbalance of the brake disc. The invention is based on the recognition that a sufficient hardness and wearability for practical applications can be achieved without the usage of separate wearing plates that are cast together with the brake disc. As a first aspect of the invention this recognition has been realized in a brake disc of the above-mentioned design, which according to the invent¬ ion is characterized in that the said increased hard¬ ness and wearability has been achieved through a change of the structure in the surface zones of the said opposite side faces.

In such a change of structure the said surface zones will typically assume a cementite, pearlite or martensite structure or a mixture of these structures with a considerably increased hardness and wearability as compared with the the rest of the disc, which norm¬ ally consists of ordinary grey cast iron.

Such a change of structure implies a binding of the carbon content of the cast iron, so that graphite does not occur in free form, and the said surface zones in themselves thus acquire improved frictional properties.

Such a structural change can be brought about by applying a technology known per se, e.g. by expos¬ ing the said surface zones to accelerated cooling, placing elements which have a good heat conductivity at the corresponding walls of the mould cavity or by injection hardening of the finished product, whereby one or more additives such as, e.g. hard metal com¬ ponents are diffused into the said surface zones.

In order to obtain optimum properties as con- cerns hardness and wearability it is, however, prefer¬ able to provide the brake discs with wearing linings, for instance of the same structure as described in the above- mentioned international publication.

According to another aspect the invention therefore relates to a brake disc for a disc brake device for automobiles and similar vehicles, compris¬ ing a mainly disc-shaped cast-iron body having a hub portion integral with a surrounding peripheral edge portion having opposite side faces intended for engagement with the wearing parts of brake blocks and to which wearing linings having a metallic surface layer with a considerably increased hardness and wear- ability have been applied. According to the invention such a brake disc is characterized in that said wear- ing linings have been applied to surface zones at said opposite side faces to which a change of structure has been imparted during manufacture of the disc.

Through the application of this method disc brakes can be produced having properties in respect of hardness and wearability which are on a par with the designs described in the above-mentioned international publication, but for which it has been avoided to use separate wearing plates in producing the wearing lin¬ ings.

The binding of the carbon content of the cast iron that occurs during the change of structure elim¬ inates the particular phenomenon, namely the existence of carbon in free form, which has made conventional grey cast iron unsuited for direct application of wearing linings.

The invention thus makes it possible to apply wearing linings directly on to the finished brake discs. This may be done by spray forming as described in the above-mentioned publication.

On a practical industrial scale another and presumable more suitable method consists in applying the wearing linings through friction welding, where as basis material is used a rod-shaped element prod- uced by spray forming, i.e. the above-mentioned "Osprey" technology. This means reduced manufacturing time and, thereby, a more rational manufacturing pro¬ cess.

For producing the above-mentioned changes of structure at the said opposite side faces of the edge portion of the brake disc the invention also relates to a method for manufacturing a brake disc by casting grey cast iron in a mould having a moulding cavity adapted to the shape of the brake disc. This method is characterized in that said opposite side faces of the edge portion of the brake disc is exposed to accelerated cooling to produce said changes of structure.

During the casting process the said accelerated cooling can be provided by arrangement of members hav¬ ing a high heat conductivity at the walls of the moulding cavity corresponding to said opposite side faces of the brake disc.

This method, which corresponds to what is known from chill casting per se, ensures a simple and rat¬ ional manufacturing process, whether the brake discs are used immediately after casting or are subsequently provided with wearing linings.

According to the invention brake discs can also be manufactured using a method which is characterized in that said opposite side faces of the edge portion of the brake disc of the finished product are exposed to injection hardening for producing the said change of structure by diffusion of an additive into said surface zones.

By further developing the above-mentioned methods brake discs can, according to the invention, be manufactured applying wearing linings direct on the brake disc, either by spray forming or friction weld¬ ing.

The latter method ensures more freedom in the choice of material as the basis material in friction welding may be a known hard-metal alloy such as, e.g. Stellite or a member which in itself has been produc¬ ed, e.g. rod-shaped, by spray forming ("Osprey" tech¬ nology) .

The invention is explained below in more detail with reference to the schematical drawings, in which

Fig.l is a perspective view of an embodiment of a brake disc according to the invention;

Fig.2 is an enlarged sectional view of a port¬ ion of the brake disc shown in Fig.l; Figs.3 and 4 are a sectional view and an en¬ larged section of a mould part for use in the manu¬ facture of a brake disc as shown in Figs.l and 2;

Fig.5 shows the application of a wearing lining on a brake disc by spray forming;

Fig.6 shows the application of a wearing lining by friction welding, and

Fig.7 is a section of Fig.2 of a brake disc with wearing lining. The brake disc 1 co-axially mounted on a vehicular wheel and co-rotating therewith in a con¬ ventional manner consists according to the invention of a mainly disc-shaped cast-iron body 2 with a hub portion 3 integral with a surrounding peripheral edge portion 4, the opposite side faces 5 of which according to the invention have a considerably in¬ creased hardness and wearability. Two uniform brake blocs are mounted in a caliper device known in the art at either side of the edge portion 4 of brake disc 2 so as to be displaceable towards each other, under the influence of the brake pressure power, in the axial direction of brake disc 1 for braking engage¬ ment with the opposite side faces 5 of the disc, whose radial width preferably is larger than the ex- tension of the brake blocs parallel to the plane of disc 1.

The central body 2 of disc 1 may be made from a conventional cast-iron alloy, e.g. GG20.

As shown in Fig.l according to the invention a brake disc may be made in a ventilated design in that from a central cavity, not shown in the Figure, in hub portion 3 a considerably number of radial ventilat¬ ion ducts 6 extend through the edge portion 4 of the cast-iron disc to the periphery of the disc. As shown in Fig.2 an increased hardness and wearability has been imparted to the opposite side faces 5 of the peripheral edge portion 4 of disc 1 through a change of structure in surface zones 7 at the side faces 5.

As a means to bring about a structural change, thereby obtaining a substantially increased hardness and wearability as compared with the adjacent material of the brake disc which may be conventional grey cast iron, especially cementite, pearlite or martensite structures or mixtures of same can be used. However, a change of structure to produce the desired surface properties can also be obtained by diffusion of one or more additives into the opposite side faces of the disc through a so-called injection hardening.

The surface zones 7 may for example be made with a depth of 1-1.5 mm by exposing the side faces 5 to accelerated cooling during the casting process, thus generating a cementite structure. As shown in Figs.3 and 4 such an accelerated cooling can be ob¬ tained by placing elements, e.g. steel-plate rings 10, having a high heat conductivity at the walls 8 of a moulding cavity 9 corresponding to the surfaces 5 of the brake disc. To avoid any action of the elt- ed cast iron during pouring into the moulding cavity through duct 11 the side of each of the plate rings 10 facing the cavity 9 is blackened in a manner known per se.

It is noted that Fig.3 shows only a single mould part 12 for use in an automatic plant of the type as described in the above-mentioned international publication in which casting is done in disposable moulds made of sand or similar workable material, in which moulds the pouring cavity is provided between the opposite end walls of a number of uniform mould parts as shown in Fig.3.

The casting process as described above, during which a change of structure in the surface zones 7 of the opposite side faces 5 of brake disc 1 is

accomplished, brake discs are produced which, even without the subsequent application of wearing linings, have a considerable hardness and wearability.

However, optimum results are achieved if wear- ing linings are applied as described in the following on the surface zones 7, in which the described change of structure implies a binding of the carbon content of the cast iron, so that graphite does not occur in free form. Fig.5 illustrates the method practised in the production of the wearing linings of the brake disc by spray forming, applying the so-called "Osprey" tech¬ nology. A furnace 20 contains a melt 21 of a met¬ allic material, in the actual case mainly aluminium oxide. The metallic melt is passed to an atomizer 22, in which by introduction of an inactive gas, e.g. nitrogen, at a high pressure, an atomization of liquid particles of the melt is generated. The atomized material in the protective atmosphere provided by the inactive gas is directed in a conical cloud 23 towards a substrate 24 passed through the otherwise closed atomization chamber on a belt conveyor 25. According to the invention, in the production of wear¬ ing linings for brake discs, the substrate 24 con- sists of cast brake discs having surface zones that have undergone a change of structure as described above.

The parts of a brake disc that are not to be provided with wearing linings, i.e. the hub portion 3 and possibly the peripheral edge portion, may be black¬ ened so as to be repellant to the atomized liquid particles. At the impact with the substrate 24 the liquid particles of the atomizing cloud 23 flatten out in flat blots and solidify instantaneously, where- by a homogeneous coherent surface coating 26 is pro¬ vided.

Due to the great hardness and wearability of the deposited material the thickness of the surface coating 26 may be quite small, e.g. in the range of 75-100 u. In the friction welding process as schematic¬ ally illustrated in Fig.6 a wearing lining made from hard metal or from a basis material produced by spray forming can be applied by pressing a rod-shaped body 30 of wearing lining material onto the side face 5 of the edge portion 4 of brake disc 1 at a high pressure P, e.g. 50 ton while the disc is rotating fast. The body 30 rotates on its own axis and a linear forwards and backwards movement in the radial direction of the brake disc is imparted over a length corresponding to the desired radial extension of the surface zone 7 onto which the wearing lining is to be applied.

The body 30 can as already mentioned be made from a hard-metal alloy know per se, e.g. Stellite, but it preferably consists of a rod-shaped body prod¬ uced by applying the "Osprey" technology, having, e.g. aluminium oxide as its main component.

Whether spray forming or friction welding is used, the wearing lining 26 as shown in Fig.7 is applied in a quite thin layer, e.g. as mentioned above, a thickness in the range of of 50-100 μ .