AU2008315429A1

AU2008315429A1 - Metal powder mixture and the use thereof

Info

Publication number: AU2008315429A1
Application number: AU2008315429A
Authority: AU
Inventors: Roland Scholl; Ulf Waag
Original assignee: HC Starck GmbH
Current assignee: HC Starck GmbH
Priority date: 2007-10-26
Filing date: 2008-09-11
Publication date: 2009-04-30
Also published as: KR20100085019A; CA2695764A1; EP2205381A2; WO2009053156A2; RU2010120780A; WO2009053156A3; JP2011501783A

Description

WO 2009/053156 PCT/EP20081062060 Metal powder mixture and the use of the same The invention relates to a metal powder mixture and particularly advantageous uses of such a metal powder mixture. It is common to use such metal powder mixtures made of metal and metal alloy powders in order to be able to produce materials with a certain alloy composition. In doing so, metal alloy powder mixtures containing a selection of alloy elements and at least one metal corresponding to the metal powder used are also used for such metal powder mixtures. These are heterogeneous metal powder mixtures consisting of at least two chemically and/or morphologically different components constituting only an intermediate phase along the way to the formation of a desired metal alloy respectively of a requirement-compliant material to be produced during heat treatment. Metal powder mixtures consisting of a fine fraction (e.g. 10% < 10pm) occurring naturally during production and a residual gross fraction (e.g. 90% < 45pm and 10% > 10pm). Such mixtures have the advantage of allowing for an improved filling density on the basis of the favourable space utilisation with adapted particle size distribution. On the basis of metallurgical reasons metal powders and metal alloy powders normally are characterised by different fusing and phase conversion behaviour. Thus, there are temperatures respectively temperature ranges where there are solid and liquid (e.g. eutectic) phase fractions at the same time respectively a modification/increase of the mass transfer, e.g. on the basis of an increased diffusion coefficient of the element, can be observed on the basis of phase conversions in the solid phase. However, as the mentioned metal powder mixtures normally are not used for the metallurgical melting production of alloyed materials, the aforementioned plays a significant role for the final material production.

WO 2009/053156 PCT/EP2008/062060 Thus, during the heat treatment that has to be implemented for the formation of the desired metal alloy or a desired metal, the atomic and microscopic transfer properties (e.g. diffusion, relocation, grain growth) efficient in the contact area of the powder particles form the basis for the compression and homogenisation of the initially heterogeneous mixture of metal and metal alloy powder or a metal powder. The term heterogeneous within this meaning also comprises the difference as regards to the particle size distribution (coarse, fine fraction) causing the mass transfer on the basis of the high levels of activity of the fine particle fraction, as opposed to a (mostly) monomodal distribution of the particle size. In this, the effects to be demonstrated by means of caloric methods for the individual powders or the powder mixture (phase conversions, melting and solidification ranges) and the effects to be utilised via the chemical composition of the alloy, e.g. chemically graded between adjacent powder particles, play a central role during alloy or metal formation and representation of the material The formation of the desired metal alloy in form of a fine powder (d50 < 10pm) for the powder metallurgical production of corresponding materials/components is difficult from a technical and economic point of view and only has a low commercial importance when processing such powders on the basis of the occurring issues. During the required heat treatment (sintering) and on the basis of the high required temperatures, there are losses of essential components on the basis of their high steam pressures during powder metallurgical processing, at which the same result in modifications to the alloy composition aimed at and cause technical problems within the thermo processing systems on the basis of material accumulation. The corresponding powder mixtures are known from DE 10331785 Al and DE 10 2005 001198 Al. These metal powder mixtures are produced from at least two or even three different powders. In this, the individual powders are to be formed from different metal alloys and are to be characterised by a narrow particle size distribution.

WO 2009/053156 PCT/EP2008/062060 However, the metal powder mixtures described within the framework of this state-of-the art have to be produced in a very costly manner, in order to be able to reach the desired very low particle size in particular, which is difficult with some metal alloys, e.g. ductile metal alloys. Problems also arise on the basis of the fact that a homogeneous distribution of the individual metals that are to be used to form the desired metal alloy can be achieved to a very limited extent only within the finally produced material. Thus, it is hardly possible to form fine structures as the same are required for cellular materials for example. Applying this concept it remains difficult to achieve a homogeneous distribution of the alloy components. This, in turn, results in the fact that the material properties will deteriorate, particularly the tendency for corrosion and the mechanical strength. Producing metallic micro-powders (d50 < 10pm) from metal compounds to be reduced easily is possible without any efforts in accordance with the state-of-the-art. Metal alloy powders designed to form a part of a desired metal powder mixture can be produced in a cost-efficient manner by atomising (gas or water atomisation) an alloy melt or conventionally by means of melting and crushing, if particle sizes d50 < 45pm are aimed at. Producing metal powders that are coarser is also possible by atomisation. Thus, it is the assignment of the invention to provide a metal powder mixture that can be used to obtain a material composed of a metal alloy in a cost-efficient manner after the implementation of a heat treatment, at which the material has to be characterised by a more homogeneous distribution of the individual alloy- and metal-forming components (alloy and element powders). In a second aspect it has to be able to reduce the maximum temperature required within the framework of the heat treatment for the production of the material. In accordance with the invention this assignment can be solved by means of a metal powder mixture characterised by the features of claim 1. Suitable applications of such a metal powder mixture can be found in claim 16.

WO 2009/053156 PCT/EP2008/062060 Advantageous embodiments and further development of the invention can be obtained with the features mentioned in inferior claims. In this, the metal powder mixture in accordance with the invention is composed of at least two different powder fractions. For the first powder fraction, a metal powder is used that is composed of a metal alloy, which contains a first metal, as regards to which in connection with the other alloy components of the first powder fraction contained in the metal alloy the beginning of a phase conversion takes place at a temperature that is at least 200 K lower than the beginning of the melting of a material to be formed from the metal powder mixture by means of heat treatment. The first powder fraction has an average particle size d 50 < 45pm. The second powder fraction contained within the metal powder mixture in accordance with the invention is preferably composed of a single second metal that is part of the metal alloy of the first powder fraction. However, it may also be composed of a mixture of at least two metals. This powder fraction has an average particle size d 50 < 10pm. In this, for the second powder fraction the term "single metal" means a metal that consists nearly completely of the single metal or the mixture and at which only very small alloy additions or contaminations are admissible, up to a maximum of 3 weight %. In case of a second powder fraction that is composed of a mixture of at least two metals, one of the metals should be present with a significantly higher share than a further metal contained therein. In this, the share should be at least 75 weight % for a metal. This metal is called second metal in the following. The average particle size of the first powder fraction should be at least three times higher than the average particle size of the second powder fraction. The second powder fraction should be contained with a share of at least 1 weight %. As regards to the first powder fraction, a binary metal alloy can be used, i.e. a metal alloy consisting of two components. However, it is more cost-efficient to use a metal alloy WO 2009/053156 PCT/EP2008/062060 composed of at least three different metals for the first powder fraction. In this, at least one metal is to be contained in the first powder fraction in the corresponding metal alloy with a share corresponding to the double value of the share that is supposed to be contained in a material formed with the metal powder mixture upon implementation of a heat treatment. The share of the second metal in the metal alloy of the material produced after the heat treatment should be at least 10 weight %. A metal, the phase conversion of which is implemented at the lower temperature already mentioned above, can be used within the metal alloy of the first powder fraction, at which the same is selected from aluminium, magnesium, zinc, tin, and copper. In connection with other alloy components of the first powder fraction, these metals have the property of lowering melting temperatures of the metal alloy or reaching partial volume phase conversions, including molten conditions. Powdery iron, nickel, cobalt, and copper can be used as metal for the second powder fraction. In this, one of these metals can be contained alone within the second powder fraction. However, the second powder fraction can also be composed of at least two of these metals as powder mixture. One option of producing a metal powder mixture in accordance with the invention is to use a metal alloy for the first powder fraction that is characterised by a general composition M1M2CrR. In this, metal M1 is selected from aluminium, magnesium, tin, zinc, and copper. The metal M2 is selected from iron, nickel, and cobalt. R is selected from yttrium, molybdenum, tungsten, vanadium, manganese, a rare earth metal, a lanthanide, rhenium, hafnium, tantalum, niobium, carbon, boron, phosphor, and silicon. In such a metal alloy, metal M1 can be contained with a share of 1-70 weight %, metal M2 can be contained with a share of 1-60 weight %, Cr can be contained with a share of 0-80 weight %, and R can be contained with a share of 0-70 weight %.

WO 2009/053156 PCTIEP20081062060 Furthermore, it is advantageous to design aluminium with a share of at least 15 weight % in the metal alloy for the first powder fraction. This way, a share of aluminium of at least 3 weight %, preferably at least 15 weight %, can be contained in a material obtained after a heat treatment to be implemented, at which the material has been produced from the metal powder mixture in accordance with the invention. If, within a metal powder mixture in accordance with the invention, a powder is used in a first powder fraction that is formed with an alloy containing iron, chrome, and aluminium, together with a second powder fraction composed of powdery iron, for example, a material can be produced upon implementation of the heat treatment characterised by a share of chrome of 15 to 30 weight % and a share of aluminium of 5 to 20 weight %, along with predominantly iron. In a metal powder mixture in accordance with the invention, a second powder fraction should comprise at least 10 weight %, preferably at least 30 weight %, and particularly preferably at least 50 weight % of the entire mass. Within the powder the first powder fraction is composed of, the metal achieving a phase conversion temperature in connection with the other alloy components that is at least 200 K lower than the temperature of the beginning of melting of the material to be produced (transition temperature) should be contained with a share of at least 10 weight %. Using the metal powder mixture in accordance with the invention, materials can produced upon implementation of a heat treatment, in which all metal components are present in a significantly more homogeneous distribution within the material volume than is the case with metal powder mixtures used traditionally. In this, the heat treatment can be implemented at temperatures that are at least 10 K lower than the temperature required in accordance with the state-of-the-art. In this, the combination of the correspondingly selected two powder fractions in accordance with the invention and moreover the use of a significantly finer powder for WO 2009/053156 PCT/EP2008/062060 a second powder fraction with the substantially smaller particle sizes than is the case for the first powder fraction mentioned above is advantageous. During heat treatment, it is possible to achieve a significantly increased mass transfer on the basis of diffusion, relocation, and grain growth using the invention. This way, it is also possible to achieve a reduced maximum temperature required for the heat treatment during the production of the material from the metal powder mixture, along with the homogeneous distribution of the individual metal alloy components. The heat treatment can be implemented by means of using known sintering technologies. However, the same should be suitable for the desired sintering atmospheres and temperatures. Using a metal powder mixture in accordance with the invention, an improved sintering behaviour of a formed body obtained on the basis of slurry or by pressing. On the basis of an improved shrinking behaviour, the metal powder mixture in accordance with the invention can be used to improve the properties of a component produced thereof or of a corresponding protective layer applied to a component. Furthermore, the corrosion resistance may be improved as well. This way, the corrosion protection can be improved on the basis of the targeted formation of a corresponding oxide layer on the surface, at which the same is normally characterised by a layer thickness of 0.1 - 10pm, if the metal powder mixture in accordance with the invention contains aluminium. A material produced by using the metal powder mixture in accordance with the invention may be characterised by an improved pitting corrosion potential when compared to high alloy corrosion resistant steels. Within the framework of a further alternative embodiment of the invention, the metal powder mixture may also contain a further fraction composed of a metal. This preferably can be iron, at which the same should contain contaminations and trace elements with WO 2009/053156 PCT/EP2008/062060 a share of less than 3 weight %, if containing the same at all. In this, the further fraction can also be powdery and significantly more coarse-grained than the two powder fractions described above. This way, the average particle size may be higher than 150pm and significantly above the aforementioned value. However, the further fraction can also be composed exclusively of fibres or contain fibres along with particles. The fibres may be characterised by diameters in the range of around 1mm and length values of several millimetres. If the metal powder mixture in accordance with the invention contains a further fraction, the share of the second powder fraction may be very low, i.e. below 5 weight %. The application of layers onto surfaces of components can be implemented with the technologies known per se, such as thermal injection or deposition welding. Components or parts thereof may also be produced using the so-called rapid prototyping procedure. In this, it is recommendable to implement a heat treatment in addition upon completion of the production, in order to be able to achieve an even higher density and homogeneity. As regards to a material produced by using a metal powder mixture in accordance with the invention, at which the same is composed of a metal alloy NiCrAI, chrome evaporation can be avoided or at least reduced significantly as opposed to the solutions known in accordance with the state-of-the-art, because the temperatures during the heat treatment are lower. This is particularly applicable to the comparison to metal powder mixtures already characterised by the corresponding alloy composition of the material to be produced before the heat treatment is implemented. In the following, the invention is to be explained in more detail in an exemplary manner. In this: WO 2009/053156 PCT/EP2008/062060 Figure 1 shows a part of a shell of a hollow ball consisting of an FeCrAl alloy with five positions where the chemical analysis of the elements Fe, Cr, and Al has been implemented; Figure 2 shows a part of a hollow ball consisting of an FeCrAI alloy with a lower porosity of the shell; Figure 3 shows a graph demonstrating the mass increase during the removal of FeCrAI hollow balls; and Figure 4 shows a cross-section polish with chemical point analysis of an FeCrAI fibre. Example 1 In this, a first powder fraction having an average particle size d 50 of 25pm and a composition of the alloy Ni-50Cr-25Al-0, 125Hf, as well as a second powder fraction having an average particle size d 50 of 5pm mainly consisting of nickel (99.9 weight %) are to be used within the framework of the production of metallic hollow balls of a material Ni 20Cr-1 0Al-0.05Hf. The share of the first powder fraction is 40 weight % and the share of the second powder fraction is 60 weight %. 100g of the metal powder mixture are dispersed with 100g of water, 3g of polyvinylalcohol, and 0.5g Dolapix in a disperser for a period of 1h at a speed of 3000RPM, in order to obtain a homogeneous distribution of the particles in a suspension. The suspension obtained by means of the aforementioned does not sediment when stirring intensively. The obtained low-viscosity metal powder suspension is applied to spherical particles made of polystyrene as coating and dried. After the coating on the polystyrene particles has reached a layer thickness of 100pm, the heat treatment may be implemented.

WO 2009/053156 PCT/EP2008/062060 In this, the works are implemented in an atmosphere with flowing hydrogen (30L/min). Initially, the organic components are decomposed thermally, at which the heating procedure takes place at a heating rate of 1K/min until a temperature of 6000C is reached. Afterwards, the temperature is increased to up to 12800C, at which a heating rate of 5K/min is maintained. Upon expiration of the holding time of 2h at maximum temperature, a cool-down procedure to room temperature has been implemented at 5K/min. The metallic hollow balls produced by means of the aforementioned were characterised by external diameters of approx. 2mm and a wall thickness of approx. 70pm. The bulk density is 450g/L. The shell material of the metallic hollow balls that has been produced with this example of a metal powder mixture in accordance with the invention is composed of 20 weight % chrome, 10 weight % aluminium, and 0.05 weight % hafnium, along with nickel. Within the framework of a parallel test example, an alloy of Ni-20Cr-10-A-0.05Hf is processed to a fine alloy powder with a particle size d 5 o of 10pm by means of inert gas atomising a metal alloy. At an analogue approach as with the example in accordance with the invention, this powder is subjected to the steps suspension production, coating, drying of polystyrene particles, and heat treatment. The metallic hollow balls produced by means of the aforementioned achieved significantly higher ultimate strength values than the balls produced with the metal powder mixture in accordance with the invention, measured on the basis of the deformation until rupture. Example 2 A metal powder mixture containing a first powder fraction characterised by an average particle size d 5 0 of 15pm and a second powder fraction characterised by an average particle size d 50 of 3pm was used. For the first powder fraction an Fe-49Cr-23AL alloy was selected and the second powder fraction was mainly composed of iron (99.5 weight In analogy to example 1, 43.5% of the first and 56.5% of the second powder fraction have been processed. Within the framework of figures 1 and 2, the homogeneity of the shell material becomes obvious by means of a cross-section through the shell of a hollow ball WO 2009/053156 PCT/EP2008/062060 produced by means of the aforementioned. The shell material produced with the metal powder mixture in accordance with this example was an Fe-23Cr-1 0Al alloy. The sintering procedure in a hydrogen atmosphere was implemented at 12400C during a period of 2 hours. Upon completion of the sintering procedure, an apparent density (FD) in accordance with ASTM D212/417 of approx. 0.4 g/cm 3 , a carbon content of 70ppm, and a share of oxygen of 0.24% could be achieved with the hollow balls produced by means of the aforementioned. Using table 1 shown in figure 3, the oxidation properties of the material obtained by means of the aforementioned can be demonstrated on the basis of the mass increase after 700 hours at holding temperatures between 9000C and 10000C without and with previous removal to air at 1100 C for 2 hours. Example 3 Metallic fibres of the composition Fe-20-Cr-9A[ were produced from a metal powder mixture containing a first powder fraction having an average particle size d 50 of 8pm of an Fe-49Cr-23Al alloy with a weight of 2kg, a second powder fraction having an average particle size d 50 of 3pm and a weight of 0.1kg consisting of pure iron, and 3kg of a further fraction consisting of metallic fibres, the average outer diameter of which was 150pm and the average length of which was 5mm. In this, the metallic fibres that have been provided by milling from a block of the purest iron (99.9% iron) were circulated in a 5 litres Eurich mixer at a speed of 20RPM. The temperature of the mixing container was maintained at 5000 ± 10K by blowing at the same with heated air. A dispersion was made from 2kg of the first and 0.1kg of the second powder fraction, as well as 2kg acetone and 0.2kg polyvinylalcohol (PVA). This dispersion was sprayed into the mixer in a centralised manner during the circulation procedure until the entire powder mass had been applied to the surface of the fibres. In doing so, the required safety regulations on the basis of the organic flammable components have been complied with.

WO 2009/053156 PCT/EP2008/062060 The fibres coated by means of the aforementioned were placed into a pan consisting of A1 2 0 3 and subjected to a heat treatment during a period of 2 hours at 1240'C in a hydrogen atmosphere. In this, PVA was expelled, as described in example 1. Figure 4 shows a cross-section polish through a fibre produced by means of the aforementioned. On the basis of chemical point analyses, the composition of the fibre material upon completion of the heat treatment could be determined as homogeneous Fe-1 9Cr-9Al alloy. Example 4 In this, a metal powder mixture with a first powder fraction characterised by an average particle size d 50 of 4.4pm and a second powder fraction characterised by an average particle size d 50 of 3.Opm was used. For the first powder fraction an Fe-49Cr-23Al alloy was selected and the second powder fraction mainly consisted of iron (99.5 weight %). 1 00g of this metal powder mixture (45g of the first powder fraction and 55g of the second powder fraction) are dispersed with 100g of water, 3g of polyvinylalcohol, and 0.5g Dolapix in a disperser for a period of 2h at a speed of 3000RPM, in order to obtain a homogeneous distribution of the particles in the suspension. In accordance with the so-called Schwartzwalder procedure, as described amongst others in US 3,090,094 for example, an open-cell porous metal foam is produced. In this, a reticulated polyurethane foam cut into individual pieces and having a porosity of 80ppi and dimensions of 40*40*10mm of the pieces is coated with the metal powder binder suspension. In this, the polymer foam structure is to be coated with the suspension as completely as possible. The coated pieces then were dried for a period of 2h at a temperature of 60*C. Afterwards, a heat treatment in a hydrogen atmosphere was implemented. In this, a heating rate of 1 K/min was used to increase the temperature to a value of 600'C, in order WO 2009/053156 PCT/EP2008/062060 to remove the organic components. Afterwards, the temperature was increased further to 12800C maintaining a heating rate of 5K/min, at which this temperature was maintained for 2 hours. Within the framework of the cool-down procedure to room temperature, a rate of 5K/min was maintained as well. Upon completion of the heat treatment and cool-down an open-cell foam structure with a physical density of 0.8g/cm 3 was obtained, at which the webs of the porous structure upon heat treatment were composed of Fe23Cr1 OAI alloy.

Claims

1. Metal powder mixture composed of at least two different powder fractions, characterised by the fact that a first powder fraction is composed of a metal alloy containing a first metal, as regards to which in connection with the other alloy components of the first powder fraction contained in the metal alloy the beginning of a phase conversion takes place at a temperature that is at least 200 K lower than the beginning of the melting of a material to be formed from the metal powder mixture by means of heat treatment and the first powder fraction is characterised by an average particle size smaller than 45pm and the second powder fraction is composed of a second metal or a mixture of at least two metals and is characterised by an average particle size smaller than 10pm, at which the second metal or the mixture is contained within the second powder fraction with at least 97 weight %.

2. Metal powder mixture in accordance with claim 1, characterised by the fact that a metal alloy is used for the first powder fraction consisting of at least three metals.

3. Metal powder mixture in accordance with claim 1 or 2, characterised by the fact that within the metal alloy of the first powder fraction at least one metal is contained with a share corresponding at least to the 1.5-fold of the share that is to be contained in a material formed with the metal powder mixture upon completion of a heat treatment.

4. Metal powder mixture in accordance with one of the previous claims, characterised by the fact that one or several metal(s) are used for the second powder fraction WO 2009/053156 PCT/EP2008/062060 selected from iron, nickel, and cobalt.

5. Metal powder mixture in accordance with one of the previous claims, characterised by the fact that a first metal selected from aluminium, magnesium , zinc, tin, and copper is used for the first powder fraction.

6. Metal powder mixture in accordance with one of the previous claims, characterised by the fact that an additional element selected from boron, silicon, and carbon is contained within the first powder fraction.

7. Metal powder mixture in accordance with one of the previous claims, characterised by the fact that the first powder fraction is formed with a binary alloy consisting of the first and second metal, at which the first metal is selected from aluminium, magnesium , zinc, tin, and copper and the second metal is selected from iron, nickel, and cobalt.

8. Metal powder mixture in accordance with one of the previous claims, characterised by the fact that the first powder fraction contains at least one further element as further alloy element, at which the same is selected from chrome, yttrium, a rare earth metal, a lanthanide, molybdenum, tungsten, rhenium, hafnium, tantalum, niobium, vanadium, manganese, carbon, boron, phosphor, and silicon.

9. Metal powder mixture in accordance with claim 7 or 8, characterised by the fact that the first metal is contained with a share of 1 to 70 weight % and that the second metal is contained with a share of 1 to 60 weight %.

10. Metal powder mixture in accordance with claim 9, characterised by the fact that chrome is contained with a share of 0 to 80 weight % and that a further alloy element is contained with a share of 0 to 70 weight %.

11. Metal powder mixture in accordance with one of the previous claims, characterised by the fact that the metal alloy forming the first powder fraction contains aluminium as the first metal with a share of at least 15 weight %.

12. Metal powder mixture in accordance with one of the previous claims, characterised by the fact that the average particle size of the first powder fraction is three times as large as the average particle size of the second powder fraction.

13. Metal powder mixture in accordance with one of the previous claims, characterised by the fact that the second powder fraction is contained with a share of at least 1 weight %.

14. Metal powder mixture in accordance with one of the previous claims, characterised by the fact that a further fraction is contained composed of particles with an average particle size larger than 150pm and/or fibres of a metal.

15. Metal powder mixture in accordance with claim 14, characterised by the fact that the further fraction is composed of iron as metal.

16. Use of a metal powder mixture in accordance with one of the claims 1 to 15 for the production of sintered components, as material for being applied to component surfaces or open-cell porous structures.