Method for the diffusion alloying of metal powders

Description

[0001] The present invention relates to a method for the diffusion alloying of powder and/or granulate made of a metal or a metal alloy with a diffusion alloying metal powder, in which the powder and/or granulate to be diffusion alloyed and the diffusion alloying metal powder are heat treated at a temperature below the melting point of the diffusion alloyed powder and/or granulate to be formed, usually at a temperature between 200°C and 900°C.

[0002] Metal powders are used in a plurality of applications in the field of powder metallurgy and surface technology, where they serve as a basis for the die compacting and sintering as well as a base material for paints and corrosion protection.

[0003] Conventionally, these metal powders are produced by means of atomisation processes, grinding processes, electrolytic processes or chemical deposition processes. While atomisation processes and grinding processes allow the production of alloyed powders, electrolytic processes and chemical deposition processes generally result in fine and purest powders as described in the ASM Handbook Committee: Metals Handbook, Powder Metallurgy, volume 7, 1998.

[0004] For the production of alloyed powders, such as powders of a copper-tin alloy, from pure metal powders, such as from fine copper powder, usually a process is used in which a powder mixture is calcined at a temperature above the melting points of the metals of the single powders under a reducing atmosphere, before the resulting sinter cake is broken and grinded.

[0005] All aforementioned methods, however, have the disadvantage that the technical effort for the powder generation is generally considerable and that they are configured for mass production. Due to this, these processes are not well applicable for the production of different alloyed powders, because the melt-metallurgical methods must be costly and elaborately converted for different alloyed powders. Moreover, all aforementioned processes change the morphology of the starting powder. Methods based on grinding processes provide the further disadvantage that contaminants from previous grinding processes may be introduced during the grinding process.

[0006] Moreover, none of the aforementioned methods allows the possibility of alloy coating powder, i.e. of encasing powder with an alloy phase.

[0007] The object underlying the present invention is therefore to provide a method for the diffusion alloying and/or alloy coating of powder and/or granulate made of a metal or a metal alloy with a diffusion alloying metal powder, which is flexible and which allows to alloy an inexpensively produced powder of a first metal, such as copper, with a second metal, such as tin, zinc or the like, either over the whole width of the powder or in the surface region of the powder of the first metal without affecting the morphology of the powder grains.

[0008] According to the present invention this object is satisfied by providing a method for the diffusion alloying of powder and/or granulate made of a metal or a metal alloy with a diffusion alloying metal powder comprising tin and/or zinc, in which the powder and/or granulate to be diffusion alloyed and the diffusion alloying metal powder are heat treated at a temperature below the melting point of the diffusion alloyed powder and/or granulate to be formed, which is usually between 200°C and 900°C, wherein before the beginning of the heat treatment the oxygen content of the atmosphere being present in the reaction chamber, in which the mixture of powder and/or granulate to be diffusion alloyed and of the diffusion alloying metal powder is heat treated, is adjusted to 10 % by volume or less, whereafter the heat treatment is started and conducted in the so adjusted atmosphere, and wherein the mixture of powder and/or granulate to be diffusion alloyed and of the diffusion alloying metal powder is agitated during the heat treatment.

[0009] As used herein, the term diffusion alloying of powder and/or granulate does not only comprise the alloying of a powder and/or granulate over its whole width, but does also comprise the alloy coating of powder, i.e. the encapsulation of powder with an alloy phase.

[0010] This solution is based on the finding that by heat treating a mixture of a powder and/or granulate made of a metal or a metal alloy and of a diffusion alloying metal powder comprising tin and/or zinc at a temperature below the melting point of the diffusion alloyed powder and/or granulate to be formed (i.e. below the melting point of the desired alloyed powder and/or granulate, which is formed with the method), wherein before the beginning of the heat treatment the oxygen content of the atmosphere being present in the reaction chamber, in which the mixture of powder and/or granulate to be diffusion alloyed and of the diffusion alloying metal powder is heat treated, is adjusted to 10 % by volume or less, whereafter the heat treatment is started and conducted in the so adjusted atmosphere, and wherein the mixture of powder and/or granulate to be diffusion alloyed and of the diffusion alloying metal powder is agitated during the heat treatment, an alloyed powder or granulate is obtained which has the morphology of the starting powder or granulate, respectively. In other words, the method according to the present invention allows to diffusion alloy a powder and/or granulate made of a metal or a metal alloy with a diffusion alloying metal powder, which comprises tin and/or zinc, without changing the morphology of the starting powder or granulate to be diffusion alloyed, respectively. Due to this, the method according to the present invention allows the production of finely dispersed powder without caking or sintering the powder particles. Depending on the particle size and the amount of the powder or granulate particles, respectively, as well as on the amount of diffusion alloying powder applied in the method, the powder or granulate particles may be alloyed completely, i.e. over the whole width of the single particles, or may be alloyed exclusively in their surface region. A further particular important advantage of the method according to the present invention is that it allows the consumption of all starting diffusion alloy powder so that only those amounts of diffusion alloy powder have to be applied in the method which are necessary to achieve the desired composition of the alloyed powder or granulate, respectively.

[0011] As a consequence, the method according to the present invention allows to alloy for instance dendritic powders, which can be - due to their manufacturing processes - only produced in pure form. Furthermore, powders produced in industrial scale can be inexpensively and individually alloyed or coated in small amounts. Instead of alloying the starting particles or granulates over their whole width, the method according to the present invention also allows to diffusion alloy exclusively the surface area of the starting particles, which allows to exploit the sinter activity of the layers in subsequent powder metallurgical applications and which allows the cost-saving application of expensive alloying material. Accordingly, the term diffusion alloying as used herein means both, the alloying of the starting powder or granulate over the whole width of the particles, i.e. the complete alloying of the starting material, as well as the encapsulation of the particles of the starting powder or granulate with an alloy from the metal of the starting powder or granulate and from the metal of the diffusion alloying metal powder.

[0012] The term agitate as used herein means that the powder and/or granulate to be diffusion alloyed and the diffusion alloying metal powder are agitated, moved or mixed, respectively, during the heat treatment so that the particles of the powder and/or granulate to be diffusion alloyed and the particles of the diffusion alloying metal powder are intermittently or continuously mixed with each other.

[0013] The agitation of the powder and/or granulate to be diffusion alloyed and of the diffusion alloying metal powder may be effected by any means known for the mixing of powders with each other. Good results are particularly obtained, if the agitation during the heat treatment is effected by conducting the heat treatment in a rotated rotary furnace with a sealed retortroated retort. Furthermore, the heat treatment may be conducted in a fluidized bed, by tumbling, by vibrating or by stirring the mixture of powder and/or granulate to be diffusion alloyed and of the diffusion alloying metal powder during the heat treatment. In the latter case, the stirring may be effected by means of a static stirrer. In the case of using a rotary furnace, the furnace may include one or more baffles which facilitates the mixing of the powders during the agitation. The agitation is conducted so that the morphology or particle form, respectively, of the powder or granulate to be diffusion alloyed is maintained during the heat treatment, i.e. in a manner that the morphology of the alloyed particles after the heat treatment is at least essentially the same as the morphology of the particles of the starting powder or granulate. The term essentially the same morphology as used herein means that the dimensions of the resulting particles do not differ more than 50 %, preferably not more than 25 %, even more preferably not more than 10 %, even more preferably not more than 5 % and most preferably not at all from those of the starting particles.

[0014] Preferably, the reaction chamber of the device is vacuum-tight and/or gas-tight sealed during the heat treatment.

[0015] Basically, the powder and/or granulate to be diffusion alloyed may comprise or consist of any metal or metal alloy. Good results are particularly obtained if the powder and/or granulate to be diffusion alloyed comprises copper, iron, aluminum, a copper alloy, an iron alloy or an aluminum alloy. More preferably, the powder and/or granulate to be diffusion alloyed consists of copper, iron, aluminum, a copper alloy, an iron alloy or an aluminum alloy. Particular good results are obtained, if the powder and/or granulate to be diffusion alloyed consists of dendritic copper.

[0016] The method according to the present invention is not particularly limited with regard to the size of the powder or granulate particles to be diffusion alloyed. Accordingly, as powder to be diffusion alloyed a powder made of a metal or a metal alloy having an average particle size between 1 µm and less than 0.5 mm may be applied.

[0017] According to a further preferred embodiment of the present invention, the powder to be diffusion alloyed consists of dendritic copper having an average particle diameter between 1 and 10 µm and having an average particle length between 10 and 100 µm.

[0018] As granulate to be diffusion alloyed, granulate made of a metal or a metal alloy may be applied, which has an average particle size between 0.5 mm and 10 mm.

[0019] According to the present invention the diffusion alloying metal powder comprises tin and/or zinc, either in the form of metal or in the form of a metal alloy. Preferably, the diffusion alloying metal powder comprises between 90 and 100 % by weight of zinc and/or tin, respectively, and even more preferably between 99 and 100 % by weight of zinc and/or tin, respectively. Most preferably, the diffusion alloying metal powder consists of tin and/or zinc.

[0020] When tin powder is used as diffusion alloying metal powder and coarse copper powder or granulate is used as powder or granulate to be diffusion alloyed, the method according to the present invention allows to encase the copper particles with stable copper-tin phases having a comparable low melting point of about 415°C, such as with tin-rich copper phases, like Cu₆Sn₅ or Cu₃Sn. These encased copper particles may be used for example as sinter material, because the low melting point of the casing phases increases the sinter activity of the particles.

[0021] According to a further preferred embodiment of the present invention, the diffusion alloying metal powder has an average particle size between 3 and 12 µm. In this embodiment, the maximum particle size of the diffusion alloying metal powder is preferably at most 70 µm.

[0022] As set out above, it is an important feature of the method according to the present invention that the powder and/or granulate to be diffusion alloyed and the diffusion alloying metal powder are agitated during the heat treatment, in order to avoid or to at least significantly reduce a sintering of the powders during the heat treatment. Accordingly, it is preferred that the powder and/or granulate to be diffusion alloyed and the diffusion alloying metal are mixed before the beginning of the heat treatment, so that a homogenous mixture of these two starting materials is present at the beginning of the heat treatment. If a great amount of the diffusion alloying metal powder is used for example for producing highly alloyed powders, it is preferred to add the diffusion alloying metal powder during the heat treatment to avoid sintering effects.

[0023] In order to obtain with the method according to the present invention particular fine-grained powders, which are completely alloyed or alloy coated, preferably a fluxing agent is added before the beginning of the heat treatment or during the heat treatment to the powder and/or granulate to be diffusion alloyed and to the diffusion alloying metal powder. These fluxing agents are liquid at the process temperatures applied during the heat treatment so that they are able to wet the powder particles. Thus, powder oxides are disordered by the fluxing agents and dissolved so that possible diffusion barriers are locally removed. Furthermore, they form a further diffusion route, because ignoble metal ions are dissolved in the melt and may uniformly accumulate on more noble metal grains due to electrochemical transport and may diffuse into the lattice, which leads to a more uniform alloying of the particles. The fluxing agents may be removed after the diffusion alloying from the powder for instance by means of washing.

[0024] Good results are particularly achieved, if the fluxing agent is selected from the group consisting of chlorine, chlorine containing compounds, fluorine, fluorine containing compounds and any mixtures of two or more of the aforementioned compounds. Examples for suitable chlorine containing compounds and fluorine containing compounds are compounds which are selected from the group consisting of aluminum chloride, zinc chloride, tin chloride, ammonium chloride, sodium chloride, potassium chloride, calcium chloride, hydrogen chloride, hydrogen fluoride and any mixtures of two or more of the aforementioned compounds.

[0025] Apart from the agitation of the mixture of powder and/or granulate to be diffusion alloyed and of the diffusion alloying metal powder, a second important feature of the method according to the present invention is to adjust the oxygen content of the atmosphere being present in the reaction chamber, in which the mixture of powder and/or granulate to be diffusion alloyed and of the diffusion alloying metal powder is heat treated, before the beginning of the heat treatment to 10 % by volume or less. In order to efficiently avoid an oxidisation or reaction of the starting materials with oxygen, it is preferred that the oxygen content of the atmosphere being present in the reaction chamber, in which the mixture of powder and/or granulate to be diffusion alloyed and of the diffusion alloying metal powder is heat treated, is adjusted before the beginning of the heat treatment to 1 % by volume or less, more preferably to 0.5 % by volume or less, even more preferably to 0.1 % by volume or less, even more preferably to 0.05 % by volume or less and most preferably to 0.01 % by volume or less.

[0026] This adjustment of the oxygen content in the reaction chamber before the beginning of the heat treatment may be for example achieved by reducing the pressure in the reaction chamber before the beginning of the heat treatment to less than 0.8 bar, preferably to less than 0.4 bar, more preferably to less than 0.1 bar, even more preferably to less than 0.05 bar and most preferably to less than 0.01 bar, before the heat treatment is conducted in this atmosphere or before the reaction chamber is filled with an inert gas or a reducing gas and the heat treatment is conducted in the inert gas atmosphere or reducing gas, respectively.

[0027] Accordingly, it is preferred that during the heat treatment no gas is supplied to the reaction chamber or only gas is supplied to the reaction chamber, which has been so pretreated that its oxygen content is at most 100 ppm, preferably at most 10 ppm, more preferably at most 1 ppm and most preferably at most 0.1 ppm.

[0028] Examples for suitable inert gases supplied before and/or during the heat treatment to the reaction chamber are gases which are selected from the group consisting of noble gases, nitrogen, methane, C₁-C₄ alkanes, C₁-C₄ alkenes, C₁-C₄ alkynes, silanes, hydrogen, ammonia and any mixtures of two or more of the aforementioned compounds.

[0029] The temperature at which the heat treatment is conducted depends among others on the composition of the metal powder or granulate to be diffusion alloyed and on the composition of the diffusion alloying metal powder. According to the present invention, the heat treatment is conducted at a temperature below the melting point of the desired diffusion alloyed powder and/or granulate to be formed with the method, which is usually between 200 and 900°C. As a general rule, particular good results are obtained if the heat treatment is conducted at a temperature between 300°C and 550°C and more preferably between 340°C and 400°C.

[0030] If zinc powder is applied as diffusion alloying metal powder, the heat treatment is preferably conducted at a temperature between 300°C and 600°C and most preferably at a temperature between 300°C and 418°C. Because zinc has a melting point of approximately 420°C, no melt is formed during the heat treatment in the latter case. Due to the high vapour pressure of zinc in this temperature range, however, enough zinc vapour for the diffusion alloying is formed.

[0031] If tin powder is applied as diffusion alloying metal powder, the heat treatment is preferably conducted at a temperature between 200°C and 450°C and more preferably at a temperature between 250°C and 400°C. When tin powder is used as diffusion alloying metal powder, it is particularly preferred to add a fluxing agent to the powder mixture.

[0032] The heat treatment may be conducted under low pressure, under atmospheric pressure and under high pressure. Particular good results are obtained, if the heat treatment is conducted under a pressure in the reaction chamber between 1 and 1.5 bar and more preferably between 1.02 and 1.2 bar. Alternatively, the heat treatment may be conducted under a pressure in the reaction chamber between 1 and 990 mbar and more preferably between 1 and 10 mbar.

[0033] According to a further preferred embodiment of the present invention, no filler or, based on the volume of the reaction chamber, less than 60 % by weight, preferably less than 10 % by weight and more preferably less than 1 % by weight of filler is present during the heat treatment. The abandonment of filler is preferred, because the presence of a filler leads due to the high heat capacity of the filler to an increased energy consumption.

[0034] The heat treatment is typically conducted so long that the diffusion alloying metal powder is completely consumed. In order to achieve this, the amount of the diffusion alloying metal powder introduced before the beginning of the heat treatment or during the heat treatment into the reaction chamber is so adjusted that after the heat treatment the desired composition of the diffusion alloyed powder and/or granulate in the surface layer thereof or over the whole width thereof is achieved.

[0035] It is a matter of course that the required duration of the heat treatment depends on the chemical nature of the metal powders applied, on the precise size of the powder particles and on the extent of the desired alloying of the powder particles, i.e. on whether the particles shall be alloyed over their whole width or only in their surface region.

[0036] If desired, the heat treatment may be conducted so long and the amount of the diffusion alloying metal powder introduced before the beginning of the heat treatment or during the heat treatment into the reaction chamber may be so adjusted that the diffusion alloyed powder and/or granulate obtained after the heat treatment is alloyed over the whole width of the powder or granulate particles.

[0037] According to a further preferred embodiment of the present invention, the obtained alloyed powder is cooled down after the heat treatment, while it is still agitated, in order to avoid during the cooling phase a sintering of the alloyed powder material.

[0038] Subsequently, the present invention is further described by means of non limiting examples.

Example 1

[0039] 98 parts by weight of dentritic copper powder having a particle diameter between 5 and 15 µm and 2 parts by weight of fine zinc powder having a particle diameter between 5 and 20 µm were introduced into a rotary furnace and mixed. Afterwards the reaction chamber of the rotary furnace was evacuated and flushed with nitrogen having an oxygen content of less than 10 ppm, before the reaction chamber was heated for 2 hours to a temperature of 380°C. During the whole heat treatment, the rotary furnace was rotated so that the powder mixture was agitated and mixed. During the rotation of rotary furnace, the powder mixture behaved like in a drum mixer, i.e. an avalanche-like powder area was formed which completely mixed the powder during each rotation.

[0040] After the heat treatment, the alloyed powder was cooled to room temperature, while the furnace was still rotated. All of the zinc powder was consumed during the heat treatment.

[0041] A dentritic brass powder containing 98 % by weight of copper and 2 % by weight of zinc was obtained, wherein the powder particles showed the same morphology as the particles of the dentritic copper starting powder.

Example 2

[0042] 89.5 parts by weight of dentritic copper powder having a particle diameter between 5 and 15 µm, 10 parts by weight of fine tin powder and 0.5 parts by weight of zinc chloride were introduced into a rotary furnace and mixed. Afterwards the reaction chamber of the rotary furnace was evacuated and flushed with nitrogen having an oxygen content of less than 10 ppm, before the reaction chamber was heated for 4 hours to a temperature of 390°C. During the whole heat treatment, the rotary furnace was rotated so that the powder mixture was agitated and mixed.

[0043] After the heat treatment, the alloyed powder was cooled to room temperature, while the furnace was still rotated. All of the tin powder was consumed during the heat treatment.

[0044] A dentritic bronze powder containing 90 % by weight of copper and 10 % by weight of tin was obtained, wherein the powder particles showed the same morphology as the particles of the dentritic copper starting powder.

Claims

1. Method for the diffusion alloying of powder and/or granulate made of a metal or a metal alloy with a diffusion alloying metal powder comprising tin and/or zinc, in which the powder and/or granulate to be diffusion alloyed and the diffusion alloying metal powder are heat treated at a temperature below the melting point of the diffusion alloyed powder and/or granulate to be formed, wherein before the beginning of the heat treatment the oxygen content of the atmosphere being present in the reaction chamber, in which the mixture of powder and/or granulate to be diffusion alloyed and of the diffusion alloying metal powder is heat treated, is adjusted to 10 % by volume or less, whereafter the heat treatment is started and conducted in the so adjusted atmosphere, and wherein the mixture of powder and/or granulate to be diffusion alloyed and of the diffusion alloying metal powder is agitated during the heat treatment.

2. Method according to claim 1,
characterized in that
the agitation during the heat treatment is effected by conducting the heat treatment in a gas-tight rotated retort furnace, by conducting the heat treatment in a fluidized bed, by conducting the heat treatment in a tumbler, by conducting the heat treatment in a vibrator or by stirring the mixture of powder and/or granulate to be diffusion alloyed and the diffusion alloying metal powder during the heat treatment by means of a static stirrer.

3. Method according to claim 1 or 2,
characterized in that
the powder and/or granulate to be diffusion alloyed comprises copper, iron, aluminum, a copper alloy, an iron alloy or an aluminum alloy and preferably consists of copper, iron, aluminum, a copper alloy, an iron alloy or an aluminum alloy and preferably consists of dendritic copper.

4. Method according to any of the preceding claims,
characterized in that
as powder to be diffusion alloyed a powder made of a metal or a metal alloy is applied, which has an average particle size between 1 µm and less than 0.5 mm, wherein the powder preferably consists of dendritic copper having an average particle diameter between 1 and 10 µm and having an average particle length between 10 and 100 µm.

5. Method according to any of claims 1 to 3,
characterized in that
as granulate to be diffusion alloyed a granulate made of a metal or a metal alloy is applied, which has an average particle size between 0.5 mm and 10 mm.

6. Method according to any of the preceding claims,
characterized in that
the diffusion alloying metal powder comprises between 90 and 100 % by weight of zinc and/or tin and preferably between 99 and 100 % by weight of zinc and/or tin, wherein the diffusion alloying metal powder preferably has an average particle size between 3 and 12 µm, wherein the maximum particle size is preferably at most 70 µm.

7. Method according to any of the preceding claims,
characterized in that
the powder and/or granulate to be diffusion alloyed and the diffusion alloying metal are mixed before the beginning of the heat treatment or the diffusion alloying metal powder is added during the heat treatment.

8. Method according to any of the preceding claims,
characterized in that
a fluxing agent is added before the beginning of the heat treatment or during the heat treatment to the powder and/or granulate to be diffusion alloyed and to the diffusion alloying metal powder, wherein the fluxing agent is preferably selected from the group consisting of chlorine, chlorine containing compounds, fluorine, fluorine containing compounds and any mixtures of two or more of the aforementioned compounds and is more preferably selected from the group consisting of chlorine, aluminum chloride, zinc chloride, tin chloride, ammonium chloride, sodium chloride, potassium chloride, calcium chloride, hydrogen chloride, fluorine, hydrogen fluoride and any mixtures of two or more of the aforementioned compounds.

9. Method according to claim 8,
characterized in that
the fluxing agent is removed from the diffusion alloyed powder after the heat treatment.

10. Method according to any of the preceding claims,
characterized in that
before the beginning of the heat treatment the oxygen content of the atmosphere being present in the reaction chamber, in which the mixture of powder and/or granulate to be diffusion alloyed and of the diffusion alloying metal powder is heat treated, is adjusted to 1 % by volume or less, preferably to 0.5 % by volume or less, more preferably to 0.1 % by volume or less, even more preferably to 0.05 % by volume or less and most preferably to 0.01 % by volume or less.

11. Method according to any of the preceding claims,
characterized in that
before the beginning of the heat treatment in the reaction chamber a pressure of less than 0.8 bar, preferably of less than 0.4 bar, more preferably of less than 0.1 bar, even more preferably of less than 0.05 bar and most preferably of less than 0.01 bar is adjusted.

12. Method according to any of the preceding claims,
characterized in that
during the heat treatment no gas is supplied to the reaction chamber or only gas, which has been so pretreated that its oxygen content is at most 100 ppm, preferably at most 10 ppm, more preferably at most 1 ppm and even more preferably at most 0.1 ppm.

13. Method according to claim 12,
characterized in that
during the heat treatment gas is supplied to the reaction chamber, which is selected from the group consisting of noble gases, nitrogen, methane, C₁-C₄ alkanes, C₁-C₄ alkenes, C₁-C₄ alkynes, silanes, hydrogen, ammonia and any mixtures of two or more of the aforementioned compounds.

14. Method according to any of the preceding claims,
characterized in that
the heat treatment is conducted at a temperature between 200 and 900°C, preferably between 300°C and 550°C and more preferably between 340°C and 400°C, wherein, if zinc powder is applied as diffusion alloying metal powder, the heat treatment is preferably conducted at a temperature between 300°C and 600°C and more preferably between 300°C and 418°C, and wherein, if tin powder is applied as diffusion alloying metal powder, the heat treatment is preferably conducted at a temperature between 200°C and 450°C and more preferably between 250°C and 400°C.

15. Method according to any of the preceding claims,
characterized in that
during the heat treatment no filler or, based on the volume of the reaction chamber, less than 60 % by weight, preferably less than 10 % by weight and more preferably less than 1 % by weight of filler is present.