Description

The present invention refers, in general, to a process for obtaining a diamond tool for the working of ceramic. More particularly, the present invention refers to a process for making a grinding wheel for the squaring of ceramic. The grinding wheel in question is made of an aluminum-based material loaded with super abrasives.

In the field of ceramic processing, in recent years the need has arisen to bring down the percentage of polluting metals such as copper, cobalt, nickel and others, from the sludge and powders deriving from the processes and, consequently, in the tools used in the processes.

In order to obtain tools that are more eco-friendly, the present diamond tools are produced by using metal alloys based on iron so as to ensure a percentage of polluting metals less than 5%.

The construction technique of said iron-based tools has allowed to produce grinding wheels with machining performance - in terms of processing quality, removal capacity and tool life - comparable to that of the conventional tools produced by using cobalt-based alloys or nickel-based alloys, when operating on wet processing lines, namely with use of water.

With the advent of ceramic dry processing lines, the iron-based metal tools are no more performing, especially in terms of processing quality, removal capacity of the tool and ability to dissipate the heat generated during the processing.

Accordingly, in the dry processes, it is necessary to use copper-based alloys or bronze-based alloys so as to obtain satisfying performances again. However doing so, with the use of copper-based alloys or bronze-based alloys it is not possible to provide a tool that meets ecological standards.

In order to obtain environmentally friendly tools, diamond tools have been even produced by using aluminum-based alloys for the processing of ceramic. However, to obtain a satisfactory performance in terms of machining quality and tool life, with the current tool making techniques it is necessary that the aluminum is loaded with high percentages of nickel and/or cobalt and/or iron.

For that reason, the so-obtained tools can not be used in any case on ceramic processing lines meeting the ecological standards required today.

Another problem with the tools used for processing ceramic, such as grinding wheels, is their heavy weight causing stress on the mechanics of the machines used for moving the tools,

An object of the invention is to overcome the aforementioned drawbacks and others by providing a diamond tool, in particular a grinding wheel for working ceramic, which is environmentally friendly and allows to obtain non-polluting processing waste, such as sludges and powders.

Another object of the invention is to provide a diamond tool so light to reduce the stress on the machine on which it is mounted and facilitate the transport and assembly operations.

The above-mentioned objects and others are reached according to the invention through a process for making a diamond tool, typically a grinding wheel, for working ceramic starting from a mixture of aluminum-based powders.

Hereinafter, the term "grinding wheel" or "grinding wheels" is to be considered as a substitute for "tool" or "tools".

In particular, the process according to the invention comprises the following steps:

- pressing of a mixture of powders inside a mold so as to obtain a semifinished product;

- pressing-sintering of the semi-finished product;

- finishing of the semi-finished product by performing also a further thickening of the semi-finished product so as to obtain a finished product.

The process according to the invention is characterized by the fact that the mixture of powders (I) includes the following components (percentages by weight):

(i) at least 70%, preferably at least 75%, more preferably at least 80% of aluminum,

(ii) from 0 to 2.5% of a lubricant, and

(iii) at least one powder component chosen among the following substances: copper, magnesium, silicon and zinc or mixtures thereof.

Said component (iii) is in the elementary state.

The aluminum and the other metals are preferably in the form of powders.

The aluminum powders typically have a size between 35 and 300 micrometers, with a maximum 3% of powders larger than 200 micrometers and 5% to 15% of powders smaller than 45 micrometers.

The powers have preferably a size between 45 and 200 micrometers, more preferably between 60 and 150 micrometers.

Besides, the process is characterized also by the fact that the copper is included in a quantity of 0.1% to 10%, preferably from 0.1 % to 6%, more preferably from 0.1% to 3% by weight in the powder mixture (I).

In this way, it is possible to obtain an eco-friendly diamond tool allowing to obtain processing waste, such as sludges and powders, non-polluting. Advantageously, in the process according to the invention, the mixture of powders (I) includes (percentages by weight):

- from 0 to 5%, more preferably from 0.1 % to 4% of magnesium;

- from 0 to 20%, more preferably from 0.1 % to 17% of silicon; and

- from 0 to 10%, more preferably from 0.1 % to 8% of zinc.

The presence of said elements has improved the various steps of the process as well as the physical characteristics of the so-obtained grinding wheel.

In fact, magnesium partly reduces alumina and activates sintering; in addition, magnesium improves the mechanical characteristics of the piece to be obtained.

Zinc is very soluble in aluminum and is used as alloying element for high strength alloys. Zinc contributes to hardening by precipitation.

Silicon improves wear resistance and hardness, as well as decreasing the thermal expansion coefficient.

Advantageously, the step of pressing-sintering is performed by heating the semi-finished product at a temperature comprised between 530°C and 650°C for a time comprised between 15 min. and 60 min.

Said temperatures make it possible not to affect the properties of the elements used in the process.

Besides, to this end, the pressing-sintering step can be performed in pure nitrogen atmosphere.

Advantageously, after the pressing step, the semi-finished product is delubricated by heating it at a temperature comprised between 380°C and 420°C for a time comprised between 15 min. and 25 min.

Advantageously, the finished piece may be subjected to at least one heat treatment in order to improve the mechanical characteristics.

In particular, the heat treatments that may be performed on the finished piece are the following: sintering and cooling at ambient temperature; heating in air, subsequent rapid cooling in water and natural or artificial aging; hardening, cooling in water and artificial aging.

In order to reduce the weight of the finished article, the pressing-sintering of the semi-finished product may be performed directly on an aluminum support previously made.

The process according to the invention for obtaining a tool for the working of ceramic envisages, therefore, the use of sintering.

According to prior art, sintering can take place using different production processes among which the most used is the pressing-sintering.

In this case, after mixing the powders and superabrasives and pressing the so-obtained mixture in molds, usually made of steel, the same mixture is heated in ovens or sintering machines. The final shape of the article is given by the steel mold.

In the case in which the element to be obtained is a sector grinding wheel, the same sectors are pressed-sintered in suitable graphite mold and then, they are welded to the base body.

Alternatively to the classical method of pressing-sintering, it is possible to use the so-called "free-sintering" which provides that after the mixing of the powders with the superabrasives, the mixture is molded in suitable molds. Then, the molded piece is sintered in ovens without further pressures as provided instead in the classical method.

During sintering, the molded piece retreats in percentage in all its dimensions. Thus, the sintered piece will be in percentage smaller than the molded piece. Hence, the molding step is very important which determines the dimensional goodness of the piece after sintering.

Another technique for obtaining a tool by sintering is the so-called "DIM", acronym of "Diamond Injection Molding". In this case, after mixing the powders and the superabrasives together with a powder of plastic materials, the mixture is molded by means of an injection molding machine. Then, the so-obtained piece must be suitably dewaxed before proceeding with the actual sintering phase. Also for this technology, the molding step is very important which determines the dimensional goodness of the piece.

Finally, another technique is the direct metal laser sintering, also known as "DMLS" or "SLM". In this case, after mixing the powders and the superabrasives, the piece is formed by increasing layers through laser sintering. This methodology is used mainly for small lots because the production of a mold would have high costs.

The process according to the invention for obtaining a tool for working ceramic provides mainly to subject the powder mixture to pressing and, in case, to a subsequent de-lubrication. Subsequently, the sintering of the semi-finished product is carried out and finally, the finish of the product.

In case, thermal treatments can be carried out on the finished product.

It is preferable that in the powder mixture (I), the aluminum powder is formed by air-atomized aluminum powder, irregularly shaped.

During atomization, the particles are covered by a very thin layer of oxide which can not be reduced in the classic sintering atmosphere.

In order to obtain a constant sintering result of high quality, it is necessary to take careful process control.

Also a constant granulometric distribution is fundamental and therefore, it is necessary a suitable control of the phases of separation and sieving.

Typically, the powders have a size between 35 and 300 micrometers, with a maximum 3% of powders of a size greater than 200 micrometers, and from 5% to 15% of powders of a size lower than 45 micrometers.

Preferably, the powders have a size between 45 and 200 micrometers, and preferably between 60 and 150 micrometers.

The powder mixtures, ready for the use in the process according to the present invention, include not only aluminum but, in a minimum part, also other elements.

In fact, the main problem connected to the non-reducibility of the superficial oxides is solved by adding other alloy elements in elementary powder or pre- bound powder, said elements performing various functions.

The used elements may be magnesium, copper, zinc and silicon.

Magnesium reduces partially alumina and activates the sintering; in addition, magnesium improves the mechanical characteristics of the piece to be obtained.

Copper improves the washability of the liquid phase with respect to the aluminum and contributes to the hardening by precipitation.

Zinc is very soluble in aluminum and is used as alloying element of high strength alloys. Zinc contributes to hardening by precipitation.

Silicon improves wear resistance and hardness, as well as decreasing the thermal expansion coefficient.

Said elements are soluble in aluminum so that during the sintering, said elements create liquid phases that are able to penetrate the oxide layer at the points where it has been broken during pressing and to spread and bind with aluminum.

The choice and combination of said alloy elements, together with their status and the process conditions, determine the behavior of the mixture and allow to obtain the best performance in relation to needs in a wide range of applications.

After the choice of the suitable mixture to be utilized, also according to the properties of the tool to be obtained, the pressing of the mixture is carried out.

To overcome the tendency of aluminum powder to weld to the mold walls and punches it is necessary to use suitable lubricants. To this end, stearamide waxes are used in amounts usually between 0.5% and 2.5% by weight. These waxes do not leave residues and are effective.

The compressibility of the aluminum-based mixtures is very good, much higher than that of iron-based mixtures and bronze-based mixtures; in fact, already at 200 MPa, 90% of the theoretical density is reached with resistances greater than 7 N/mm ² .

This makes it possible to use presses of limited power also for the production of articles having one or more large surface faces.

Concerning the molds, the usable material is high strength steel; in the case of large volumes, however, the use of hard metal is preferable, also for a better precision and a reduction of the necessary gaps.

During pressing at interlocking points between particles, the oxide layer is broken and metal contacts are created which allow the subsequent sintering processes.

Before proceeding with the actual sintering phase, it is preferable to carry out the delubrification.

The removal of the lubricant must be carried out at temperatures such as not to affect activation and diffusion in the liquid phase which in some mixtures takes place already at temperatures of 430°.

As described below, some mixtures can be delubricated directly by subjecting them to sintering temperatures, without any negative effect.

Then, the actual sintering takes place. The piece is heated in suitable molds. Precisely, the sintering temperatures vary typically between 530 and 650° C according to the type of alloy and mixture while the sintering time can vary between 15 and 60 min.

The pressure is applied also in a second phase on an oleodynamic press after the mold has reached the desired temperatures.

The high affinity of aluminum with oxygen is the main obstacle to be overcome in the sintering phase. It is also for this reason that the mixture has activating elements such as magnesium and the other previously described elements.

During the sintering phase, the diffusion and allegation processes need accuracy in the uniformity of the powders used and in the control of sintering temperature and time.

The best results are obtained by sintering in atmosphere of pure nitrogen.

Alternatively, hydrogen can be used. However, hydrogen can provoke a slight deterioration of the mechanical characteristics.

Once the sintering phase has been completed, the piece is subjected to the finishing phase. In fact, considering the excellent plasticity of the so-obtained piece, it is possible to carry out a further thickening so as to improve the mechanical properties.

Finally, the characteristics of sintered aluminum can be modified through heat treatments by natural or artificial aging.

The possible workings to be carried out include, in particular, one or more of the following thermal treatments, reported with the symbols provided by the International Classification:

T1 : sintering and cooling at ambient temperature without other processes;

T4: air heating at 500°C for 30 minutes followed by rapid cooling and natural aging at ambient temperature for at least 30 days;

T6: air heating at 500°C for 30 minutes followed by rapid cooling in water and artificial aging at 160°C for 18 hours;

T76: quench hardening at 470°C, cooling in water and artificial aging at 130°C for 24 hours.

Through the previously defined process it is possible to obtain tools for working ceramic, in particular grinding wheels for the squaring of ceramic, with optimal mechanical properties.

In fact, the properties of tensile strength of the sintered aluminum are comparable or better than those of bronze, brass and carbon steels of low and middle density. The properties of elongation are similar to those of bronze or stainless steel.

The following examples are provided to explain the present invention but they are not limitative of the present invention.

Example 1

A mixture of powders (I) is subjected to a process for obtaining a mold and consists of (percentages by weight): - 92.9% of aluminum,

- 4.5% of copper,

- 0.5% of magnesium

- 0.6% of silicon, and

- 1.5% of lubricant.

The mixture is put into a mold and the de-lubrication phase is performed at a temperature between 380°C and 420°C, preferably at 400°C, for a time of 20 minutes.

Then, the sintering phase is performed at a temperature between 590°C and 600°C for a time of 20 minutes.

The properties of the so-obtained article are the following: the density of the sintered aluminum is 2.52 g/cm ³ , the dimensional variation is -0.4%, the tensile strength is 190 N/mm ² for T1 , 260 N/mm ² for T4 and 320 N/mm ² for T6, wherein the hardness is 60 HB, 75HB and 100 HB, and the elongation A5 is 5% for T1 , 3% for T4 and 1 % for T6, respectively.

The so-obtained tool has a high mechanical resistance, a good dimensional stability and is suitable for the treatment of quench hardening and aging.

Example 2

The procedure of Example 1 is repeated but the mixture of powders (I) consists of (percentages by weight):

- 96.8% of aluminum,

- 0.2% of copper,

- 1 .0% of magnesium ,

- 0.5% of silicon, and

- 1.5% of a lubricant. The de-lubrication phase is performed at a temperature between 380°C and 410°C, preferably at 395°C, for a time of 20 minutes.

The subsequent sintering phase is performed at a temperature between 630°C and 635°C for a time of 30 minutes.

The properties of the so-obtained article are the following: the density of the sintered aluminum is 2.47 g/cm ³ , the dimensional variation is -0.5%, the tensile strength is 140 N/mm ² for T1 and 230 N/mm ² for T6, wherein the hardness is 40 HB and 75 HB, and the elongation A5 is 5% for T1 and 3% for T6, respectively.

The so-obtained tool has an excellent corrosion resistance, a good mechanical resistance and ductility, and is suitable for anodizing.

Example 3

The procedure of Example 1 is repeated but the mixture of powders (I) consists of (percentages by weight):

- 88.3% of aluminum,

- 1.7% of copper,

- 2.5% of magnesium,

- 6.0% of zinc, and

- 1.5% of a lubricant.

Besides, the de-lubrication phase is not performed because the sintering phase is performed at a temperature between 580°C and 590°C for a time of 60 minutes.

The properties of the so-obtained article are the following: the density of the sintered aluminum is 2.78 g/cm ³ , the dimensional variation is -1.5%, the tensile strength is 300 N/mm ² for T1 and 450 N/mm ² for T76, wherein the hardness is 100 HB and 150 HB, and the elongation A5 is 5% for T1 and 2% for T76, respectively. The so-obtained tool has a high mechanical resistance.

Example 4

The procedure of Example 3 is repeated but the mixture of powders (I) consists of (percentages by weight):

- 80.3% of aluminum,

- 2.6% of copper,

- 0.6% of magnesium,

- 15.0% of silicon, and

- 1.5% of a lubricant.

Also in this case, the de-lubrication phase is not performed because sintering occurs at a temperature between 580°C and 590°C for a time of 60 minutes.

The properties of the so-obtained article are the following: the density of the sintered aluminum is 2.67 g/cm ³ , the dimensional variation is -2.0%, the tensile strength is 200 N/mm ² for T1 and 280 N/mm ² for T6, wherein the hardness is 100 HB and 130 HB, and the elongation A5 is 1 % for T1 and 0.5% for T6, respectively.

The so-obtained tool has a good wear resistance and good mechanical characteristics up to 200°C, as well as a low coefficient of thermal expansion.

According to the previous explanations, the excellent properties of aluminum-based alloys, the aluminum of which is suitably sintered and reprocessed with an appropriate thermal cycle or through a forging process (cold pressing of the sintered piece), enable to produce diamond tools of new concept which are ecological, light and performing.

With the aluminum-based formulation it is possible to avoid the problems of pollution caused by copper and bronze present in the waste from processing. This facilitates the waste disposal and provides the possibility of re-use the powders resulting from the processing.

Besides, the grinding wheel obtained with the process according to the invention weighs about half in comparison to a conventional metal grinding wheel, with significant benefits. One of the main benefits is the lower stress on the mechanics of the machine that supports and moves the grinding wheel.

According to a variant of the invention, it is possible to perform the sintering of aluminum directly on a body of the grinding wheel, which body acts as a support.

In fact, according to the prior art, in order to make the grinding wheel lighter, the metal band is sintered on an iron ring and then, it is coupled to an aluminum body. In the aluminum-based grinding wheel according to the invention, the cutting part can be sintered directly on the aluminum body so as to avoid some steps of the working with the result of being much lighter.

A technician of the sector can conceive modifications or variants which are to be considered as included in the scope of protection of the present invention.