Abstract:
A sinter metal part with a homogeneous distribution of non-homogeneously melting components, essentially comprising non-homogeneously melting non ferrous metal powder mixtures, produced in the following manner: continuous isostatic pressure sintering of the starting material to obtain densities which substantially correspond to the density of a high-temperature isostatically pressed solid having the same composition, using die in conditions avoiding the occurrence of a liquid phase in powder at temperatures of up to 70% of the metal melting point, preferably up to 60% of the metal melting point, forming a sinter profile substantially possessing a final contour.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention relates to sinter metal parts with homogeneous distribution of inhomogeneously melting components, essentially from inhomogeneously melting non-ferrous metal powder mixtures as well as processes for their manufacture.  
           [0002]    The term non-ferrous as used here is also understood to mean metal mixtures which contain small quantities of iron, up to approx. 8 wt. %—lower quantities of iron should be possible as alloying additives.  
           [0003]    The manufacture of sintered parts made of inhomogeneously melting metal powder mixtures is prior art. Usually, hot isostatic pressing (HIP), cold isostatic pressing (CIP) or also hot sintering are obvious solutions. All of these processes avoid a complete melting of the metal powder mixture, as otherwise segregation phenomena would occur. With HIP and CIP, the parts must be discontinuously and individually sintered in pressure chambers, which for series-type production is only acceptable in exceptional cases.  
           [0004]    Pressureless hot sintering requires somewhat less effort. With this method, the powder, granules, grit or the like to be sintered are as a rule uniaxially pressed together with compression aiding materials into a form close to the final contour, and the green compact thus produced is sintered in a sintering furnace at temperatures of between ⅔ and ⅘ of the melting or solidus temperature of the metal powder, optionally in a protective atmosphere.  
           [0005]    A further known technique for compacting metal powder is powder forging. This is also a discontinuous process, in which individual parts are produced in swages.  
           [0006]    In general, multi-phase powders and powder mixtures are sintered in the vicinity of the melting or solidus temperature of the lowest melting component of the mixture. With easily oxidising materials, sintering frequently takes place in a protective atmosphere on account of the increased oxidation rate and the prolonged soaking time at sintering temperature. This actual compaction of the green compact was followed by heat treatment which improved the structure thus formed. This is then frequently followed by finishing or calibration, in the course of which the sintered parts are given their final form. With hot sintered parts that tend to warp during cooling, these additional operations are cost-intensive and difficult to integrate in a production line.  
           [0007]    Generally, sintered parts produced using known methods with inhomogeneously melting metal mixtures, in particular if inhomogeneously melting powder mixtures were sintered, frequently had porous or segregated areas which were caused by the differing diffusion rates of the elements and in which individual phases of the non-miscible components crystallise out inhomogeneously. This causes long-term disruption of the structure of the sintered part and its mechanical technological properties. The properties of the powder-sintered parts produced in this way consequently suffered adverse effects from these disrupted areas, whose occurrence was unforeseeable, and great differences arose between individually produced sintered parts, which became apparent, for example, in an increased tendency for the material to break along the non-homogeneous areas, and in a consequently reduced elasticity. This is particularly unfavourable when the sintered parts are put into operation with alternating loads—for example as gear wheels, pump wheels etc.—and the conventional processes lead to high costs on account of the high waste rate. Finally, conventional hot sintering is also very energy-intensive, since high heating costs are incurred for the various stages of heat treatment and the sintering itself.  
           [0008]    In view of this, it is the object of the invention to produce sintered parts that have a more homogeneous, more uniform metal structure and which can be produced with less effort and at less cost.  
         SUMMARY OF THE INVENTION  
         [0009]    The object is solved by a sinter metal part with a homogeneous distribution of inhomogeneously melting components, essentially from inhomogeneously melting non-ferrous metal powder mixtures, which can be produced by: continuous isostatic pressure sintering at temperatures of up to about 70% of the melting point of the main component of the initial metal powder mixture through a mould under conditions preventing the occurrence of a fluid phase in the powder, with a sintered profile close to the final contour being formed.  
           [0010]    Furthermore, the invention relates to processes for the manufacture of a sinter metal part close to the final contour, characterised by: the production of a powder compact not close to the final contour, continuous isostatic pressure sintering at a pressure according to the formula  
           P   min     =         2                     σ   γ          (   T   )         3          ln        [     1     1   -   D       ]           ,                         
 
           [0011]    with σ y (T) representing the yield stress of the material at the projected pressure sintering temperature, and D specifying the relative density of the cold isostatically precompressed initial product. The relative density is the quotient of the absolute density of the precompressed initial product ρ 0  and the density of the solid alloy ρ B .  
           [0012]    The pressure sintering operation is effected through a two-sided open mould to form a sintered profile; optionally, cutting of the sintered profile into sintered products, heat treatment of the sintered products or of the sintered profile and optionally finishing of the same.  
           [0013]    Typical continuous isostatic pressure sintering at temperatures up to about 70% of the melting point of the main component of the initial metal powder mixture is preferably carried out on a hydraulic extruding press adapted to the process parameters—whereby conventional extrusion moulding, which is employed for homogeneously melting materials at temperatures near the melting point, leads to extrudates with low dimensional accuracy on account of an excessive heating of the extrudate, and these products then have to be finished, for example by pressing, forging etc. The continuous isostatic pressure sintering at very high pressures used here, according to the above formula, also makes it possible to select a temperature at which the extrudate in the extrudate caused by friction is also taken into account. The actual preheating temperature T v  of the initial CIP material is determined according to the following formula:  
         T   v     =     T   -         Q   B     -     Q   λ           π   4          ρ   0   2        l                   ρ   B          C   B                                 
 
           [0014]    Here, T stands for the temperature for the selected yield point at elevated temperature of the material, Q B  the quantity of heat fed to the initial material, Q λ  the quantity of heat given off to the tool surface, l the active tool length and c B  the compression velocity.  
           [0015]    The process is characterised in that the powder material is heated only a little and briefly on account of the possible reduction of the preheating temperature—unlike conventional extruders, where a uniform and higher temperature rise in the pressed material is desirable.  
           [0016]    On account of the fact that, according to the invention, the powder is pressed at high pressures when it is relatively cold, the fluid phase being substantially avoided, it was surprisingly possible to sinter metal powders, continuously and at relatively low temperatures, into a profile, powders of which it was previously assumed that this was only possible in processes that sintered the green compacts for a long time at temperatures in the order of 80-90% of the melting point of the main component. It was completely unexpected that this isostatically continuous pressure sintering could take place at these relatively low temperatures, whereby it was possible to use additional compression mechanisms known from hot isostatic presses, for example the Nabarro-Herring offset creep, using fine initial powders with mean grain sizes of 50 to 150μ.  
           [0017]    It was shown that this process produced sintered parts having superior grain structure which is characterised by an extremely homogeneous distribution of the non-miscible components, and thus have improved elastic properties and ductility compared with sintered parts produced according to conventional processes and to powder forging processes. This was achieved by almost completely suppressing the very temperature-dependent volume diffusion which causes coarsening and segregation of individual phases.  
           [0018]    According to the invention, the preferred initial material used for continuous isostatic pressing is a powder press compact—compressed using known pressing methods—without slip additive, lubricant or sintering aid. This powder press compact may already have a non-homogeneous material distribution—in particular if non-homogeneous sintered parts—that is, composite parts—are to be produced.  
           [0019]    A typical feature is for example an external material layer of a different, chemically or physically more resistant material—if for example a certain corrosion stability is required in an external or internal layer—as in powder metallurgical pipes or discs. The powder press compact is then isostatically relatively cold pressed/sintered in a press with a die plate, and undergoes a bonding reaction at the grain boundaries of the components through the shearing forces during pressing, without the occurrence of a fluid phase which could lead to segregation. In this way, a homogeneous sintered product can be produced with superior material properties.  
           [0020]    In another embodiment this continuously produced sintered product, formed by the die plate, can be cooled down by controlled cooling, for example by spraying with water, in such a way that a fine grained state is achieved by quenching or that a defined heat treatment, e.g. a T4 heat treatment for aluminium alloys, may take place. The cooled extrudate can then be mechanically aftertreated. For example, the continuously produced profile-type sintered product is usually cut at product height—by sawing, water cutting, laser cutting or other methods with which the expert is familiar. These sections of defined length of the continuously produced sintered product can then be used as such or after finishing—for example surface treatment or calibration. The sintered product cut to product length in this way can also optionally be subjected to heat treatment in order to change or temper the material structure. In this case, the heat treatment must be such that no fluid phases can occur.  
           [0021]    A typical form of finishing for the sintered product according to the invention is calibration on a press in order to obtain the dimensionally very close tolerances of the final form of the product. Machining is usually not required.  
           [0022]    In fact, the product, which has been sintered at temperatures of up to 70% of the melting point of the main component, already has final contour—this means that the finishing steps only require little effort.  
           [0023]    Because of the fact that the process is continuous, the production rate of the sintered parts exhibiting the superior properties is significantly higher than was possible so far using the discontinuous uniaxial or isostatic pressing method for the production of sintered parts. Because of the fact that the products are now isostatically continuously pressure sintered, segregation phenomena that can occur at higher sintering temperatures through the diffusion of the powder components in fluid phases can be avoided. A finer and more homogeneous structure of the inhomogeneously melting mixtures is obtained than in the classical hot isostatic pressing methods, in which considerable diffusion processes, particularly volume diffusion in the final stage of compression can lead to much coarsening, and crystallisation phenomena with segregation can occur. The sintered parts made of inhomogeneously melting metal mixtures produced according to the invention show improved workability and higher ductility and higher elongation than those that were produced using methods according to the state of the art. In further production stages, this is shown by improved workability. For the use of such components, the mechanical technological parameters of the sintered parts, elasticity, tensile strength and elongation are influenced very favourably. For example, a component sintered without pressure, made of an aluminium alloy with 13% silicon by weight has a breaking elongation of less than 0.5%, while a component made of the same alloy produced by a method according to the invention, has typical breaking elongation values of 7 to 12%. This is achieved by the suppression of the formation of the melting phase, so that the structure cannot part-crystallise inhomogeneously.  
           [0024]    In terms of process technology it has been shown that the scatter of the material parameters of sintered parts produced according to the invention is very much lower than that of hot sintered parts of the same composition—that is, they have closer material parameter limits than is the case with conventional HIP or hot sintering. Through the prevention of the occurrence of melting phases, non-miscible components such as hard phases can be incorporated so that they are homogeneously distributed, as well as materials which are hardly accessible or not accessible at all according to classical sintering methods. Typically, the material powder used is a powder mixture of metals or their alloys, and further materials such as hard parts, fibres, and carriers of wear and tear such as boron carbide, BN.  
           [0025]    In this way it is possible to produce metal matrix composites, whereby the second component may be fibrous or particle-like.  
           [0026]    As fibres, short or long fibres may be added, or particles in proportions of 5-30% by volume. Short fibres or whiskers have a length that is considerably shorter than 100 times the fibre diameter. Long, endless or continuous fibres are those whose fibre length is greater than 100 times their diameter. Fibres can serve to improve the strength of the sintered parts.  
           [0027]    Particle reinforced materials can also be produced in this way, that is, those with silicon carbide, boron carbide etc.  
           [0028]    On account of the high level of homogeneity guaranteed by this method, other, typically sintered alloys can also be processed, such as Ti alloys, in particular Ti/Nb alloys, TiAl and TiAl Nb as well as Co—Ti—B Mg+SiC, boron carbide, Al2O3 or AlPb alloys with high heat storage capacities which cannot be produced by melting metallurgy—that is, sintered composite material parts can be continuously produced, or beryllium parts, magnesium parts etc. Examples of typical compounds are aluminium with Si, Mg, Cu, Zn and optionally Fe, for example with 10-40% Si, Mg 0-3%, Cu 0-5%, Zn and Fe 0-7%, as well as other light metal alloys, for example those of magnesium, calcium, beryllium etc. With sintered aluminium parts, AlSi, AlSiCu, among others, are available, aluminium sintered materials: AlCuMg, with—Al Cu 3.8-4.4, Mg 0.5-1.0: AlMgSi with AlSi 0.4-0.8, Mg 0.5-1.0, AlZnMgCu 0.05-0.6, Cu 0.25-1.6, Mg 0.1-1.5, Zn 1.5-8.0, AlSi with more than approx. 7% Si are available. In particular, the invention relates to sintered light metal parts made of light metal alloys that are difficult to machine. Hypereutectoid alloys can also be produced, and the expert will be able to see further advantages on account of his expert knowledge.  
           [0029]    The following must be listed as advantages, among others, of the sintered parts produced according to the invention: very fine and uniform grain size of carriers of wear and tear, significantly finer distribution of carriers of wear and tear, no liquation or segregation, completely homogeneous structure as well as extremely high dimensional accuracy—the warping that occurs with hot sintering does not occur with the method according to the invention, leading to the production of dimensionally very accurate parts. In the method according to the invention, the former powder particles can no longer be detected under the microscope, while the very fine grain constitution of the structure also has an elongated deformation, which also improves the mechanical strength of the sintered composite produced in this way. Finally, the sintered parts according to the invention are characterised by an elongation that is at least 150% higher than an elongation of the same material composition produced by powder forging, sintering or casting.  
           [0030]    The incorporation of carriers of wear and tear is typical. A very fine and uniform grain size of the carriers of wear and tear can be achieved, as well as a significantly finer distribution of the same, compared to other methods. No liquation or segregation occurs, giving rise to a homogeneous structure. This is an extremely simple process for the manufacture of even very wear resistant sintered metal components with extremely high dimensional accuracy. Hypereutectoid alloys can also be produced.  
           [0031]    Through the incorporation by sintering of fibres such as ceramic fibres, carbon fibres or hard material fibres, higher strength is obtained—improvement in tensile strength, increase in the yield point, increase in the elasticity module, improved high temperature strength and creep resistance—a reduction in the thermal coefficient of expansion. SiC particles, AlN, BN, TiB2, boron carbide, SiO2, tungsten carbide are typical as carriers of wear and tear or hard materials: fibres such as carbon fibres, metal fibres, ceramic or glass fibres.  
           [0032]    Suitable metal phases may be selected from aluminium, titanium, copper, beryllium, magnesium, calcium, nickel, lithium, chromium, molybdenum, tungsten, bronze, niobium, lead, zinc and cobalt.  
           [0033]    It is also possible to carry out continuous isostatic pressure sintering at temperatures of up to about 70% of the melting point of the main component of the initial metal powder mixture under an inert gas like a rare gas, nitrogen, carbon dioxide, if easily oxidising materials like Mg are to be sintered at temperatures of up to about 70% of the melting point of the main component.  
           [0034]    The method also allows a composite (preferably a powder press compact) consisting of several areas of differing composition—that is, a sintered part with layers, rings, strips etc to be sintered. This may be of interest, for example, if a hard layer is required as an external material—for example for cutting wheels or the like—but a cheaper, more ductile and more elastic material is required as internal material. Previously, such parts had to be produced separately and then joined—while the process according to the invention allows production in one step through joint continuous isostatic pressing, allowing several materials to be sintered together. Sintered parts with a hard cutting edge can also be produced from a different material composition than that used for other areas. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]    Further objects, features and advantages can be seen by viewing the following description and the claims together with the accompanying drawings. For a full understanding of the nature and objects of the invention, reference is made to the drawings, in which:  
         [0036]    [0036]FIG. 1 shows a schematic view of the process steps compared to the conventional sinter pressing of an aluminium-silicon alloy;  
         [0037]    [0037]FIG. 2 shows a microscopic picture of a solid microsection from an AlSi14% alloy—produced by melting metallurgy-type casting, after the method according to the invention and after conventional sintering.  
         [0038]    [0038]FIG. 3: hot pressure experiment with parts according to the invention  
         [0039]    [0039]FIG. 4: friction coefficient curves of AlSi14% against 100 Cr6—parts produced by the method according to the invention and parts produced by sintering  
         [0040]    [0040]FIG. 5: sintered products with non-homogeneous sections—a schematic view  
         [0041]    [0041]FIG. 6: diagrammatic process sequence in the production of sintered products with non-homogeneous sections. 
     
    
     DETAILED DESCRIPTION  
       [0042]    Subsequently a description of a preferred embodiment of the invention on basis of the production of AlSi14% formed parts is given, which up to now were usually hot sintered—but the invention is by no means restricted to this application, other sinterable metallic powders can also be processed using this method, for example Ti, Ta, Mg, Be, Cs, Cu.  
         [0043]    [0043]FIG. 1 shows a diagrammatic view of the process sequence according to the teaching of the invention. As shown, the method comprises the manufacture of a continuously sintered part which is produced by the continuous isostatic pressing of a sinterable material mixture without lubricant, from a sinter form closed by a die plate.  
       EMBODIMENTS  
     EXAMPLE 1  
     Production of AlSi14% Sintered Discs  
       [0044]    The initial material an inhomogeneously melting mixture of aluminium powder with 13 wt. % silicon powder (AlSi only melts homogeneously in the range 5-7%) is homogeneously mixed and then transferred to a powder press for the manufacture of powder billets not close to the final contour. There it is cold pressed under high pressure to form a billet-like green compact. The billet-like green compact is transferred to a system for continuous isostatic sintering—here an extruding press—and is sintered as it is pressed through the die plate. The AlSi14 sintered part leaves the die plate at temperatures of up to 70% of the melting point of the main component as a continuous sintered profile, the external contour of which is close to the final form. The continuously sintered profile is now mechanically separated according to the required disc height and the material discs are heat treated at 250° C. for 30 minutes. When the sintered discs come from the heat treatment they are then calibrated in a calibration press at a force of 150 KN—that is, the final form is achieved with very close dimensional tolerances. Unlike conventionally, hot isostatic pressed parts with the same composition, the sintered parts produced in this way do not have to be decapsulated and have suitable flow properties for calibration. They can then be used as finished parts without any further finishing.  
       EXAMPLE 2 (COMPARATIVE TEST)  
       [0045]    AlSi14% sintered parts were produced conventionally by sintering, by pressing a green compact, made of aluminium powder with 14wt. % of silicon with Hoechst Wachs C compression aiding material, into a disc, this disc was then treated for 20 minutes at 410° C. in a heat treatment stage, then sintered for 30 minutes at 590° C. in a sintering furnace and then heat-treated once more at 400° C. for 240 minutes, as a comparative product.  
         [0046]    [0046]FIG. 2 shows a a comparison of the microstructures of the AlSi14 sintered aluminium discs, produced with conventional hot sintering according to the comparative test, and produced by isostatic pressing according to the invention. It is clearly shown that the part produced according to the invention has a smaller grain size and fewer segregated areas—the sintered part produced according to the invention is therefore more homogeneous in its properties.  
         [0047]    [0047]FIG. 3 shows friction coefficient curves of hot isostatic pressed AlSi14% formed bodies and continuously isostatically pressure sintered AlSi14% formed bodies according to the invention against 100 Cr6.  
         [0048]    It can clearly be seen that the isostatically pressure sintered material initially has greater surface roughness, which however is quickly rolled out, so that as the friction test continues, the friction coefficient for the isostatically pressure sintered material is lower than for the hot isostatically pressed product. This speaks in favour of higher ductility of the isostatically pressure sintered material.  
                             TABLE 1                           Material parameter ranges for AlSi14%       (at room temperature, unless otherwise stated)            Composition   HIP AlSi14   AlSi14               acc. to the invention               Density g/c.c.    2.65    2.7       Hardness (Brinell)    95-150    80-105       Tensile strength N/mm 2     210-250   260-280       Yield point N/mm 2     130-210   160-180       Elongation %   0.5    3-12       E-module GPa   &lt;80   &lt;80       Si grain size in μm    10-20    5-10       Si distribution in structure   rel.   completely           homogeneous   homogeneous       Surface pressure N/mm 2  at 150° C.   780   550                  
 
         [0049]    It can clearly be seen from Table 1 that the sintered bodies produced according to the invention have a lower scatter—that is, they can be set more precisely and thus also supply fewer waste parts. The sintered parts are more homogeneous and can also be elongated more, thus providing improved elastic behaviour, as required in particular by mechanically stressed parts, such as chain wheels against steel chains, rotors and stators in a camshaft adjustment system or oil pump parts, bearing parts, pump wheels etc.  
         [0050]    Finally, a hot pressure test was carried out with the isostatically pressure sintered part—it was shown that after exposure of the produced AlSi14 products to air at 150° C. for 500 and 1000 h practically no change took place in heat pressure strength, pressure elongation or the pressure yield point (Table 1).  
         [0051]    Table 1 shows that the strength of the isostatically continuously sintered AlSi14 part produced according to the invention is considerably better than that of the hot isostatically pressed part.  
         [0052]    [0052]FIG. 5 shows the result of the manufacture of parts sintered according to the invention with different material areas—here in FIG. 5 a  a section of a round sintered part with a different external layer—in FIG. 5 b  a two-layered square sintered part; in FIG. 5 c  a tubular sintered part with different layers; FIG. 5 d  shows a striped distribution in sintered parts. A combination of different sintering materials is thus possible with the simultaneous production of the sintered part—for example the application of an external layer reinforced with hard materials, as a separate process stage, can thus be prevented by direct “application by sintering”—etc.  
       EXAMPLE 3  
       [0053]    Production of a sintered material disc with hard external material and easily workable internal material  
         [0054]    A powder mixture of alloyed AlMgl powder and 2 wt. % silicon powder for the mixture of the internal material, and a powder mixture of aluminium powder with 40% SiC for the external material are homogeneously mixed and pressed in a divided mould which produces the required powder billet with AlSi as core and AlSiC as outer layer. This powder billet is transferred to a continuous isostatic press with a round die plate and processed under high pressure at temperatures of up to 70% of the melting point of the main component to form a round sintered profile. The sintered profile produced in this way is cut into discs with a height of 15 mm by means of a water jet. These discs are suitable as pump gear wheels for oil and water pumps, said wheels having an easily workable internal zone for drilling holes, while the external zone with the SiC hard part phase is resistant to wear by friction.  
         [0055]    Obviously the invention is not limited to the exact design or the listed or described embodiments, but various modifications are obvious to the expert, without deviating from the essentials and the scope of protection.