| CLAIMS 1) Method for producing abrasive tools, characterized in that it comprises the following operations: - preparing a metallic material; - preparing an abrasive material; - preparing a foaming agent suited to release a gas when a preset decomposition temperature is exceeded; - making a homogeneous mixture comprising at least said metallic material, said foaming agent and said abrasive material; - heating said mixture in order to aggregate said metallic material and to cause said gas to be released by said foaming agent, with formation of bubbles in said aggregated mixture; - cooling said aggregated mixture in order to obtain its cohesion. 2) Method according to claim 1), characterized in that said cooling process is performed before said bubbles join one another. 3) Method according to claim 2), characterized in that said heating of said mixture comprises the following stages: - first heating stage to a first preset temperature lower than said decomposition temperature, intended to aggregate said metallic material; - second heating stage to a second preset temperature at least equal to said decomposition temperature, intended to cause said gas to be released. 4) Method according to claim 3), characterized in that said first temperature is equal to the melting temperature of said metallic material. 5) Method according to claim 4), characterized in that said decomposition temperature is higher than or equal to said melting temperature. 6) Method according to any one of the preceding claims, characterized in that said mixture has the following composition in weight: - metallic material: between 30% and 60%; - abrasive material: between 1 % and 30%; - foaming agent: between 0.5% and 3%. 7) Method according to any one of the preceding claims, characterized in that said metallic material is Al. 8) Method according to any one of the claims from 1) to 6), characterized in that said metallic material is an alloy comprising two or more metals selected among Zn, Cu, Ag1 Sn, Ti. 9) Method according to any one of the preceding claims, characterized in that said foaming agent is a metal hydride. 10) Method according to claim 9), characterized in that said metal hydride is TiH2. 11) Method according to any one of the preceding claims, characterized in that said metallic material, said abrasive material and said foaming agent are provided in the form of powder. 12) Method according to any one of the preceding claims, characterized in that it comprises an operation for placing said mixture into a shaped mould before said heating process. 13) Abrasive tool comprising a shaped matrix provided with a plurality of dispersed abrasive particles, characterized in that said matrix is a metallic body in which it is possible to identify a plurality of closed and hollow cavities. |
TO SAID METHOD.
DESCRIPTION
The present invention concerns a method for producing abrasive tools, particularly suited to make grinding wheels and other tools having equivalent size and application.
The invention also concerns an abrasive tool obtained according to said method.
Various types of abrasive tools are known, which comprise a matrix in which a plurality of abrasive particles is dispersed, said particles deriving from very hard materials like, for example, SiC (silicon carbide), WC (tungsten carbide), diamond or other hard materials.
The above mentioned tools include, for example, abrasive disks, conical grinding wheels, cup grinding wheels, shaped grinding wheels for special applications, cutting tools for building applications, natural stone, hard metal and other materials.
The above mentioned tools generally comprise a resin-based matrix or a matrix based on other equivalent materials that advantageously make it possible to produce large tools with relatively simple and inexpensive processes.
However, the above mentioned materials pose the drawback that they have limited mechanical and thermal properties.
Consequently, the tools with matrices in resin or other equivalent materials pose the drawback that they wear out or deteriorate quickly, especially in the case of demanding applications, such as those involving the machining of metal and stone.
In addition to the above mentioned tools with resin-based matrix, tools with vitrified ceramic matrix are also known, said ceramic matrix being provided with a plurality of closed internal cavities, which in jargon are called "cells". As is known, vitrified ceramic materials, which hereinafter will be referred to simply as "ceramic materials", feature high hardness and thermal resistance and therefore are suited to be used for making the matrices of the tools.
Furthermore, the closed cavities present in the matrix catch the chips that are removed from the material during machining, removing them from the machined surface to the benefit of machining quality. Furthermore, the closed cavities are filled with lubricant and therefore guarantee the continuous supply of the latter towards the machining area, in such a way as to ensure the cooling and lubrication of the contact surfaces.
The above mentioned ceramic tools, though making it possible to obtain high quality machining results, however pose some important drawbacks.
A first drawback is represented by the high fragility of the ceramic material of which the matrix is made, which makes said tools delicate to handle and subjects them to frequent breakages, for example following even slight impacts. A further drawback posed by the ceramic tools described above lies in that they have low heat conductivity, which makes them particularly sensitive to local heating, with the further inconvenience that they become even more delicate and difficult to handle and to use.
Due to the above mentioned drawbacks, ceramic tools are used only in automatic systems for very specific applications, under the supervision of highly specialised technicians.
Another drawback posed by ceramic materials lies in that they are not suitable for the production of very large and thick tools, for example like those used in the building industry or for machining natural stone. In fact, ceramic matrices are obtained through a sintering process under high specific pressure, carried out in presses whose size and costs increase as the size of the matrix increases and which therefore become anti economic when a given tool size is exceeded.
Furthermore, the low heat conductivity of the material hinders the diffusion of heat through the matrix, which is necessary during the production process to avoid breakages due to thermal shocks.
Another type of abrasive tools has been developed, based on the use of a metallic matrix, provided with dispersed abrasive particles and with cavities that have the same functions described above for ceramic tools. To advantage, the tools with metallic matrix feature higher toughness than tools with ceramic matrix and therefore do not have problems related to brittle fractures.
Furthermore, metallic materials have good heat conductivity and are therefore less sensitive to thermal shocks. A further advantage offered by metallic materials compared to vitrified ceramic materials is represented by their lower sintering temperature, which makes it possible to limit the tool production costs.
However, the tools with metallic matrix of known type are subject to surface phenomena like strain hardening and creep, caused by the contact between the tool and the material being machined.
These phenomena pose the drawback that they considerably reduce the tool's cutting capacity, thus limiting its field of application.
In order to limit or eliminate the above mentioned phenomena, a porous metallic matrix provided with a plurality of cavities is used. There are various methods for producing tools with metallic matrices provided with internal cavities.
According to a first method, the tool is produced through aggregation of a powdery mixture of a metal and an abrasive material, to which a disposable substance that can be dissolved in a solvent is added. The above mentioned mixture is aggregated by means of a suitable thermal cycle in such a way as to form a compact body, which is then immersed in the solvent in order to eliminate the disposable substance and generate the cavities.
The tools obtained with this method pose the drawback that, in order to allow the disposable material to be completely eliminated from the matrix, the cavities cannot be closed but on the contrary have to be interconnected.
However, since interconnected cavities tend to let the lubricant flow out instead of keeping it in, they are less suited to convey the lubricant to the machining area compared to the closed cavities that are typical of the tools with ceramic matrix, with the drawback that the tool has reduced lubricating capacity.
A further drawback posed by interconnected cavities lies in that they limit the mechanical resistance of the matrix, thus reducing the field of application of this type of tools.
Sometimes the disposable material, instead of being eliminated by previously immersing the tool in a solvent, is removed from the tool cooling fluid during operation.
In this case there is the drawback that the modalities of formation of the cavities depend on the conditions of use of the tool and therefore are quite uncertain. It is clear, therefore, that the use of a disposable material limits the choice regarding the distribution, size and morphology of the cavities in the matrix, with the inconvenience that tools with limited effectiveness are obtained.
According to a further known method, the metallic matrix is produced using abrasive particles coated with a metal layer, which are sintered in a mould and subjected to a pulsating current that causes them to cohere at the level of the respective contact points.
Although this second method allows a homogeneous matrix to be obtained, however it does not allow the formation of closed cavities, and therefore the tools obtained in this way pose drawbacks that are analogous to those posed by tools obtained with the previous method.
Furthermore, this method is expensive and requires sophisticated equipment that allows pressure, temperature and current to be accurately checked during the process.
Consequently, said method poses a further drawback represented by the fact that it is suitable mainly for the production of small tools, with the exclusion, therefore, of grinding wheels and other tools with similar size, which would be too expensive.
According to a further known production method, a plurality of hollow spheres, generally in glass, is incorporated in the metallic matrix. A first drawback posed by the tools obtained with this method is represented by the fact that their matrix is not homogeneous and therefore scarcely effective in terms of abrasion results.
The above mentioned drawback is mainly due to the fact that the spheres tend to float in the mixtures, gathering in preferential points and hindering the uniform distribution of the spheres during the mould filling stage.
This last aspect, in particular, becomes more evident as the size of the piece to be produced increases.
Furthermore, since the dimension of the hollow spheres is pre-established and they cannot be smaller than the minimum size available on the market, there is the drawback that this method is suitable for producing tools for some specific applications only.
The above mentioned method poses the further drawback that the discontinuities induced in the metallic matrix because of the presence of the spheres limit the mechanical properties of the matrix. A further drawback posed by this method lies in that the sintering pressure cannot exceed the resistance limit of the hollow spheres, beyond which the spheres would collapse and the tool obtained would not be effective.
A reduced sintering pressure limits the compactness of the matrix and therefore its resistance and toughness properties. As a whole, the known methods described above do not allow the production of large tools like, for example, grinding wheels and other analogous tools, having at the same time mechanical properties and machining capacity comparable to those of small tools with metallic or ceramic matrix for large- scale production. Furthermore, in the known methods for the production of tools with metallic matrix the control of the distribution, size and morphology of the cavities is complicated and therefore the tools obtained with said methods are expensive and/or have limited effectiveness.
Furthermore, the above mentioned methods are not suitable for producing tools in any shape and size, but only in the shapes and sizes allowing good diffusion of heat in the matrix and the formation of a uniform morphology in the matrix itself.
The present invention intends to overcome all the drawbacks of the known art as outlined above. In particular, it is a first object of the invention to develop a method for producing abrasive tools with metallic matrix that, compared to the known tools of analogous type, have better lubricating capacity.
It is also the object of the invention to provide tools with better mechanical properties than the tools with metallic matrix obtained with the known methods. It is a further object of the invention to develop a method that makes it possible to produce abrasive tools in any shape and larger dimensions than allowed by the known methods described above, while guaranteeing the same machining efficiency.
In particular, it is the object of the invention to allow the production of grinding wheels and other tools with analogous dimensions, whose matrices have better mechanical properties compared to tools for analogous applications with matrix in resin or equivalent materials.
It is another, yet not the least object of the invention to develop a method for producing tools that is less expensive and easier to control compared to the known methods for the production of equivalent tools. The above mentioned objects are achieved by a method for the production of abrasive tools implemented according to claim 1.
The same objects are also achieved by an abrasive tool produced according to claim 13. Further characteristics and details of the invention are indicated in the corresponding dependent claims.
Advantageously, the high lubricating capacity of the tool that is the subject of the invention ensures a more regular operation of the tool itself, which makes it possible to improve the quality of the machined surface. Still advantageously, the higher cooling capacity limits the risk of damage to the tool during its normal use and reduces its wear rate compared to the tools of known type.
As a further advantage, the method of the invention makes it possible to produce large tools, for example grinding wheels and tools with comparable dimensions, having higher mechanical resistance and duration than analogous tools with matrices of known type, for example resin-based.
Still advantageously, the variety of shapes that can be obtained with the method of the invention makes it possible to produce tools for specific applications like, for example, grinding wheels for machining corners, shaped grinding wheels and other analogous tools.
Furthermore, advantageously, the method of the invention makes it possible to dimension the cavities of the matrix with accuracy, so as to produce tools that are lighter than analogous tools of known type having equivalent potential.
Still advantageously, the simplicity of application of the method of the invention reduces the costs of the tool to values that are lower than those involved in the production of tools having equivalent potential with known methods.
In particular, the production method of the invention does not require the use of high process pressures that, especially in the case of large tools, make it necessary to use large and costly presses. The said objects and advantages, and others which will be highlighted in greater detail below, are illustrated in the description of a preferred embodiment of the invention which is provided by way of non-limiting example.
According to the method of the invention, the matrix is obtained starting from a mixture comprising a metallic material and an abrasive material, using also the metallurgical technique of the so-called "metal foams" in order to obtain a metallic matrix provided with cavities.
To obtain the metal foam, a foaming agent is added to the mixture, of the type suited to release a gas following heating above a preset decomposition temperature. The metallic material, the abrasive material and the foaming agent, provided in the form of powder, are mixed homogeneously and placed inside a mould, preferably shaped according to the form of the tool to be produced. The mixture is then heated, both to obtain the aggregation of the metallic powder and to cause the release of the gas by the foaming agent. The aggregation of the metallic powder ensures the cohesion of the matrix, while the release of the gas leads to the creation of a plurality of bubbles inside the matrix itself.
The bubbles cause the expansion of the matrix, which is preferably prolonged until the foam has filled the entire volume of the mould. Finally, according to the method of the invention, the mixture is cooled in order to obtain its cohesion and to stop any further expansion of the bubbles, in such a way as to produce a solid matrix provided with a plurality of cavities. During the above mentioned cohesion stage, the particles of abrasive material remain trapped in the matrix, so as to obtain an abrasive body that can be used as a tool.
The above mentioned abrasive body is preferably subjected to successive mechanical machining in order to give it its final geometry. Advantageously, the method described above makes it possible to obtain a metallic matrix provided with cavities, avoiding the use of high process pressures which pose the drawbacks mentioned above with reference to the known art.
The above mentioned metallic matrix is suited to produce very effective tools since, as already noted, the cavities catch the chips removed from the material being machined and transport them away from the machining area. Consequently, to advantage, the quality of the machined surfaces can be compared to that which can be obtained with the tools with ceramic matrix. The above mentioned mixture cooling process is preferably carried out before the bubbles grow and migrate in the matrix to the extent that they join each other, in order to obtain closed cavities separated from one another. The cooling stage is preferably followed by a rest stage at ambient temperature or at a slightly higher temperature, in order to allow the gas trapped into the cavities to flow out by diffusion through the metallic matrix. Advantageously, the closed cavities hold the lubricant and guarantee that it is constantly conveyed to the machining area more effectively than with interconnected cavities, thus achieving the object to provide a tool with metallic matrix having higher lubricating capacity than the tools with metallic matrix of known type.
The closed cavities also make it possible to increase the structural integrity of the metallic matrix and therefore the tools obtained have better mechanical properties than the tools with metallic matrix of known type provided with interconnected cavities, thus achieving another object of the invention. Still advantageously, since the closed cavities are obtained by creating empty areas in the matrix instead of incorporating foreign bodies therein, the matrix is more homogeneous and has better mechanical properties than the matrices incorporating hollow bodies of the known type described above.
Still advantageously, the closed cavities involve a reduction in the effective contact surface during the utilization of the tool, thus determining an increase in the specific pressure, which means greater effectiveness of the tool itself. Furthermore, the method that is the subject of the invention achieves the object to allow the production of tools having any shape and large size, in particular grinding wheels.
In fact, the metallic matrix ensures a quicker diffusion of heat compared to ceramic materials and therefore does not suffer from the same limitations. Furthermore, the formation of the bubbles takes place in a uniform way in the whole volume of the mixture and therefore does not pose the drawbacks connected to non-homogeneous distribution that are typical of the matrices incorporating hollow bodies.
As a consequence of the above, the geometry of the tool can be cylindrical in order to obtain cylindrical grinding wheels, or different in the case of particular tools like for example grinding wheels for machining corners, cup grinding wheels, shaped grinding wheels, cutting tools for stone materials and the like. It is evident, however, that the mould can have any shape, provided that it is compatible with the requirements concerning the moulding technology, which are well known to the persons skilled in the art. The method described above offers the further advantage of being simpler and more economic than other known methods for producing tools with metallic matrix having equivalent potential, and thus allows another object of the invention to be achieved. In fact, the method of the invention requires neither the pressing of the matrix under high pressure with large and costly presses, nor the use of electric currents and related control devices.
Consequently, to advantage, the method of the invention requires only the process temperature to be controlled, while it is not necessary to control the pressure, as required by the known methods based on sintering. On the contrary, the thermal cycle included in the method of the invention has been widely tested and can be made easy to control by adding suitable retarders to the mixture, according to techniques that are well known in the production of the metal foams mentioned above. It is also evident that the formation of the bubbles takes place in a uniform way in the whole volume of the material and therefore advantageously makes it possible, differently from the known methods described above, to obtain a matrix with homogeneous morphology, independently of its shape and thickness. According to a construction variant of the invention, the metallic matrix contains the abrasive material only in the part of the tool to be used for machining, while the remaining support part contains no abrasive material. This variant is obtained using different mixtures to fill the areas of the mould corresponding to the above mentioned parts and advantageously allows the quantity of abrasive material to be reduced, especially in the case of large tools.
According to a further construction variant, the support part is made of a material that is different from that used for the abrasive part, and the two parts are connected by means of glue, bolts or other known fixing means. The aggregation of the powder and the release of the gas are preferably obtained by means of two distinct heating stages.
In particular, the aggregation is obtained during a first heating stage, by heating the mixture to a first preset temperature, lower than the decomposition temperature of the foaming agent, and maintaining it at this first temperature for the time necessary to obtain the aggregation. The above mentioned first temperature is preferably equal to the melting temperature of the metallic material, so that aggregation is obtained through melting.
The first heating stage is prolonged for a time sufficient to obtain the complete or almost complete melting of the metallic material. Advantageously, aggregation by melting simplifies the production process compared to the known methods using the sintering technique, as it does not require the use of high pressures.
The above mentioned first stage is followed by a second mixture heating stage, which is carried out at a second preset temperature, equal at least to the foaming agent's decomposition temperature, in order to cause the gas to be released with consequent formation of bubbles in the liquefied mixture.
The quantity of bubbles that form, and therefore the distribution of the cavities obtained, mainly depends on the percentage of foaming agent present in the mixture, while their morphology and their size mainly depend on the type of foaming agent used, on the value of the second temperature and on the duration of the second heating stage.
The above mentioned parameters are preferably selected so that the bubbles form closed cavities, preventing them from getting large enough to create a network of interconnected cavities. The choice of the correct temperature and duration advantageously makes it possible to dimension with precision the gas bubbles that develop in the matrix or, in an equivalent way, to obtain bubbles with uniform size even in large matrices.
Still advantageously, the possibility to dimension the bubbles by properly choosing the foaming agent and/or the temperatures and the duration of the heating stage makes it possible to obtain matrices with cavities whose size and morphology are more suitable for the application for which the tool is intended.
The size that is most suitable for the cavities in the matrices for tools preferably varies from a few microns to some dozens microns, while the percentage of hollow volume in the matrix is preferably included between 1 % and 50% of the total volume of the matrix.
According to a variant of the method of the invention, the aggregation of the metallic material is obtained in the solid phase instead of by melting, performing a sintering operation at a preset temperature lower than the melting temperature of the metallic material itself. In this case the foaming agent is obviously selected among those having a decomposition temperature near the above mentioned sintering temperature. In this way, the formation of gas takes place during sintering and the bubbles develop exploting the plasticity of metal at high temperature. The composition of the mixture preferably features the following percentages in weight:
- metallic material: between 30% and 60%;
- abrasive material: between 1 % and 30%;
- foaming agent: between 0.5% and 3%. Preferably but not necessarily the metallic material is Al and the foaming agent is TΪH2 (titanium hydride).
Advantageously, Al has good mechanical characteristics and low specific weight, which makes it possible to obtain resistant and very light tools. TΪH2 has a decomposition temperature that is slightly higher than the melting temperature of aluminium and approximately equal to 690 0 C, that is, only 3O 0 C higher than aluminium's melting temperature that, as is known, is 66O 0 C. Advantageously, the slight difference in temperature makes the process easier to control, as it allows the aluminium melting stage to be separated from the gas release stage. Still advantageously, TiH2 offers the advantage of releasing hydrogen that, as is known, is diffused in aluminium very quickly in the solid state, thus preventing the phenomenon known as "hydrogen embrittlement". According to construction variants of the invention, the metallic material comprises an alloy obtained from the combination of Cu with a metal selected among Zn, Ag, Sn and Ti.
According to further variants, the above mentioned alloy can be obtained from the combination of more than two metals among the four mentioned above. Advantageously, Al and the metal alloys indicated above feature high heat conductivity, thus favouring the rapid diffusion of heat inside the matrix. Advantageously, the above mentioned heat conductivity facilitates the control of the tool production process, since the heating and cooling of the mixture take place rapidly and uniformly in the entire volume.
Still advantageously, the high heat conductivity of the matrix favours the cooling of the tool during use, making it more versatile than the tools with ceramic matrix. According to further construction variants, the above mentioned metallic material can be any metal alloy, eutectic and preferably with melting temperature below 700 0 C.
Advantageously, the use of Al or of a metal alloy with melting temperature below 700° makes it possible to limit the maximum temperature reached during the tool production process.
The lower temperature of the process makes it possible to use cheaper moulds and to limit the quantity of energy necessary for heating.
As far as the foaming agent is concerned, some construction variants of the invention may include any metal hydride different from TiH2, or even other substances, provided that they release a gas when a preset decomposition temperature near the melting temperature of the metallic material is exceeded.
In general, the decomposition temperature of the foaming agent should preferably exceed the melting temperature of the metallic material, the difference preferably being maximum 50 0 C.
This makes it possible to obtain the aggregation of the matrix through the simple melting of the metallic material and furthermore makes it possible to limit the decomposition temperature, obtaining the advantages mentioned above. As regards the abrasive material, it is preferably diamond, which is fairly compatible with Al for the production of effective tools.
Obviously, construction variants of the invention may comprise other very hard compounds, like for example SiC, WC, CBN (cubic boron nitride) or any other known abrasive material, which will be selected by the manufacturer according to the components of the mixture and to the application for which the tool is intended.
Other construction variants of the invention may include further substances in the mixture, which according to the known art give the matrix specific mechanical and/or thermal properties. The above clearly shows that the method of the invention for producing an abrasive tool and the tool obtained with said method achieve all the set objects.
In particular, the presence of closed cavities and their uniform distribution in the metallic matrix ensure constant supply of lubricant to the machining area, thus giving the tool higher lubricating capacity compared to the known tool with metallic matrix.
Furthermore, the closed cavities obtained with the method of the invention maintain the integrity and homogeneity of the matrix, giving the tools better mechanical properties compared to the tools with metallic matrix obtained with known methods, in which the matrix is not homogeneous and/or the cavities are not closed.
Furthermore, the method of the invention makes it possible to produce tools having any shape and larger dimensions than the tools obtained with the methods described above, but granting the same or even higher effectiveness. In particular, the invention makes it possible to produce grinding wheels and tools having similar size with better mechanical properties than the tools for analogous applications having a matrix in resin or other equivalent materials. The method of the invention, based on the melting of the metallic material, is also simpler and easier to control than the methods of known type, as the cavities are obtained with a simple thermal cycle and according to consolidated techniques in the field of metal foams.
On implementation, the method and the tool that are the subjects of the invention may undergo further changes that, though not described herein or illustrated in the drawings, shall nonetheless be covered by the present patent, provided that they come within the scope of the claims that follow.
In the cases where the technical characteristics illustrated in the claims are followed by references, these have been added only with the aim to facilitate the comprehension of the claims themselves and therefore said references do not have any limiting effect on the degree of protection to be granted to each element they identify only by way of example.
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