Patent Application
20110129379
This application claims the priority of Italian Patent Application No. TO2009A 000908, filed on Nov. 24, 2009, the entirety of which is hereby incorporated by reference.
The present invention relates to a method for manufacturing massive components made of intermetallic materials, particularly for aerospace applications.
The materials used in aeronautic propulsion systems should have features of high mechanical strength and temperature creep resistance, fatigue resistance, oxidation and corrosion resistance, lightness and structural stability.
Traditionally, many materials are already used, and are differentiated according to composition and use requirements, but research and development activities continue for overcoming the limits posed by current technologies and for identifying new materials with increasingly better features.
A new class of materials having the features required for use in aeronautic engines is the intermetallic materials.
In recent years, the interest towards intermetallic materials has increased, especially due to the high weight saving potential which may be obtained by virtue of low density and high specific strength. The intrinsic fragility of this type of materials requires, in all cases, to set up particularly sophisticated manufacturing and inspection techniques and to implement important study and characterization campaigns to learn about the properties on statistically significant bases.
The most interesting intermetallic materials belong to the TiAl (titanium aluminides) family, useable instead of Nickel- or Cobalt-based superalloys up to temperatures close to 700° C., but other interesting intermetallic materials for the aeronautic field are those based on NiAl and Ni3Al, or FeAl systems.
Several uses of the intermetallic materials are known: intermetallic materials as precipitating phases, which are the reinforcing element of metallic alloys, and which allow to improve them by means of thermal treatments, or intermetallic materials as anti-wear, anti-oxidation or anti-corrosion coating systems.
Methods are known, e.g. from WO2006/109956, for the preparation of a metal matrix composite material wherein intermetallic compounds and ceramic powders are dispersed. In particular, there is disclosed the case where one metal element, which is intended to form the metal matrix of the composite—be it a single component or an element of a metal alloy—is made to react with other metallic particles via cold spray techniques. A much desirable dispersion reinforcement effect is thereby obtained through precipitation of the intermetallic compounds thus formed. However, while metal-matrix ceramic composites where intermetallic compounds are finely dispersed within the metal-matrix phase can be obtained by the method of WO2006/109956, no indication is provided as to how to prepare a massive component substantially wholly made of an intermetallic material, i.e. one having homogeneous chemical composition and mechanical properties throughout.
Known intermetallic mechanical components of massive type are typically obtained by solidification of mixtures of one or more metallic elements in molten state. These mixtures contain the metallic elements in a given proportion by weight, which characterizes a particular alloy, or corresponds to the characteristic atomic percentage composition of a given intermetallic compound, which may be formed from these elements. The intermetallic compounds are, in general, intermediate phases of systems with two or more metallic elements (see, for example, the state diagrams of bi-metallic systems in Figures from 1 to 4) and are characterized by a crystal lattice, in which points are tidily occupied by atoms of different metals. Thereby, the aforesaid metallic mixtures may be cast to form ingots intended to be then mechanically machined, or directly cast in moulds (ceramic shells) with investment casting techniques or the like, or even atomized to obtain powders to be consolidated, in a subsequent step, by massive sintering or rapid manufacturing techniques (such as Electron Beam Melting or Laser Sintering), or even by hot spraying.
The above technologies commonly used for manufacturing massive components made of intermetallic materials have sensitive critical states related to the high hardness, fragility and reactivity of these materials, which significantly limit machining possibilities by means of plastic deformation, such as forging, extrusion, rolling, etc., or lost-wax casting as well, which has a further drawback in the reaction of the intermetallic material with the ceramic shell.
Specifically, the techniques based on rapid manufacturing powder consolidation allow, to a certain extent, to obtain massive components characterized by high mechanical properties, and low faults and presence of machining allowances; these manufacturing procedures are very limited from the point of view of production capacities and require great system investments, accompanied by equally significant management and maintenance costs.
On the other hand, massive components obtained by the aforesaid direct hot spraying of intermetallic material powders generally have high internal stresses due to the contraction of the sprayed material and a high number of faults, both in the form of diffused porosity and of micro-cracks. These drawbacks decrease the reliability of this technique and its applicability from the industrial point of view.
In the art, there is a need to provide a method for manufacturing massive components made of intermetallic material which requires low system investments and low management and maintenance costs and which is able to ensure a high productivity.
Furthermore, in the aeronautic field, there is a need to provide a method for manufacturing massive components made of intermetallic material which have micro-structural and mechanical features such to satisfy the requirements imposed by particular conditions of use.
It is thus the object of the present invention to provide a method for manufacturing massive components made of intermetallic material, which allows to simply and cost-effectively satisfy at least one of the aforesaid needs.
The aforesaid object is achieved by the present invention as it relates to a method comprising the steps of:
a) preparing a mixture of powders of at least two metallic elements, the powders being present in the mixture in a proportion by weight corresponding to the atomic percentage in which the at least two metallic elements are present in a corresponding intermetallic compound which may be formed from these at least two metallic elements;
b) applying a plurality of layers of such a mixture of powders by cold spraying on a substrate (S) so as to obtain, on the substrate (S), a preform of metallic mixture of predetermined size;
c) thermally treating at least the preform of metallic mixture so as to cause the reaction between the metallic elements to form such an intermetallic compound; and
d) removing the substrate (S), thus obtaining the massive component made of intermetallic material.
Furthermore, the present invention relates to a massive component made of intermetallic material obtainable by the method.
For a better understanding of the present invention, a preferred embodiment will now be described below only by way of non-limitative example, and with reference to the accompanying drawings, in which:
Figures from 1 to 4 show, by way of non-limitative example, the phase diagrams of some two-component systems characterized by the possibility of forming an intermetallic compound.
It is worth noting that, in all cases, for each intermetallic compound the formation of which is thermo-dynamically possible, there is a specific stoichiometric ratio between the metallic elements which compose the same, which corresponds to a very precise percentage composition by weight, obtainable by simple stoichiometric calculations. For example, the Ti—Al intermetallic compound (see
Therefore, from the analysis of bi- and multi-metallic system state diagrams, which intermediate compounds is obtainable and which compositions, expressed in atomic percentages and/or by weight, of metallic elements correspond thereto may be easily assessed.
According to this analysis, a mixture of powders of at least two metallic elements is thus prepared, according to the method of the invention, the metallic powders being present in the mixture in a proportion by weight corresponding to the atomic percentage in which said at least two metallic elements are present in a corresponding intermetallic compound which may be formed from these metallic elements. Such a mixture of powders may be obtained by simply mixing pure constituents or by using mixtures in which the constituent elements are alloyed, or yet again by coated powders obtained by grinding, or by means of other known methods.
Given the above, it is worth noting that, in the context of the present invention, by “a mixture of powders of at least two metallic elements” reference is made to a mixture of powder metallic elements which is selectively prepared to be stoichiometrically and thermodynamically bound to react, under the operating conditions in accordance with step c) of the method as outlined above, to form substantially only the intermetallic compound at issue—if not for the possible formation of traces of other chemical species.
In other words, the “mixture of powders of at least two metallic elements” referred to herein is selectively prepared in order for the method of the invention to yield a massive component which is substantially wholly made (if not for the possible presence of traces of other species, impurities and the like) of the intermetallic compound at issue, i.e. a massive component which comprises at least 90% by weight of the intermetallic compound at issue.
According to the method of the invention, a plurality of layers L of the powder mixture is applied (see
The cold spraying technique generally comprises the steps of injecting the powder mixture into a nozzle and applying the mixture of powders onto the substrate by accelerating the powder mixture in non-molten state to a speed of the order of 300-1200 m/s by means of a flow of carrier gas crossing the nozzle.
For this purpose, apparatus 100 comprises a compressor 110, a system 120 for feeding the mixture of powders, and a nozzle 130 for spraying the powder itself.
Furthermore, apparatus 100 may comprise means 140 for heating the input gaseous flow.
Being compressed at a pressure higher than about 5 bars, the gas is put into contact with the constituent particles of the metallic powder mixture fed by system 120 and draws them by expelling them through the nozzle 130 at high speed. In order to generate a subsonic or supersonic flow, as shown in Figure Z, a converging-diverging nozzle 130 is advantageously used.
A portion of the gas flow is preferably heated before reaching nozzle 130.
The distance between the point where the gas and the particles come in contact with the base of the nozzle 130 may be typically varied to modulate the final temperature of the powders and is comprised in the range from 20 and 200 mm.
A monatomic inert gas is preferably used as the carrier gas, such as helium or argon, so as to exclude possible reactions with the components of the metallic powder mixture and have high gas speeds by virtue of its y ratio. However, if the contact times between components of the metallic powder mixture and the carrier gas are very short, more cost-effective carrier gases may also be used, such a nitrogen or air. Any proportion of the mixtures of the previously mentioned gases may be further used, and in particular nitrogen, air, argon, helium, neon, krypton.
The temperature at which the gas is heated by the carrier system 140 is directly related to the final temperature and speed of the sprayed particles. In cold spraying processes, the gas temperature is typically between 300 and 1200° C.
The powders remain in contact with the gas for a very short time, whereby the temperature of the powders, although not measurable, never reaches the gas temperature.
The deposition temperature is typically the lowest possible, compatibly with the need to obtain a minimum deformation level of the sprayed powder particles.
The temperature at which the powders are kept before coming in contact with the gas at the base of nozzle 130 is sufficiently low to minimize the possibility, of activating the reaction mechanisms which could lead to the early formation of the intermetallic compound; it is worth not exceeding a temperature equal to half the melting temperature of the lowest-melting metallic element.
With regards to the average size of the metallic particles forming the mixture of powders, this may be advantageously chosen in the range from 1 to 200 μm, so as to facilitate the dispersion and mixing in the step of preparing the powder mixture.
More preferably, as the powders are subsequently subjected to a thermal treatment to promote the reaction of forming the intermetallic compound, the average dimension of the metallic particles is in the range from 1 to 50 μm, so as to promote a more uniform, gradual reaction, since such a reaction involves scattering phenomena at the atomic level.
Indeed, a yet smaller size would be accompanied by an excessively light particle weight, and therefore the motion amount accumulated during the step of cold spraying and discharged when impacting against the substrate surface, i.e. against a previously applied layer of metallic powders, would be too small. On the other hand, if the average size of the particles is too large, the motion amount being high, the impact frequency and the area concerned by each impact would be too low.
The amount of metallic powder mixture deposited on the substrate will generally be such to form the envelope of the massive component to be manufactured, taking into account any possible deformations and volume variations which may be induced during the thermal treatment and any possible subsequent mechanical machining operation.
According to the method of the invention, a substrate made of a material which allows an easy release of the preform of metallic mixture at the end of the step of cold spraying may be advantageously used. For example, a support may be used which does not react with the metallic elements contained in the powder to be applyed by cold spraying, such a non-metallic material.
Alternatively, a (metallic or non-metallic) substrate may be coated with an insulating release layer. In this context, “insulating layer” means a layer of material which does not react with the metallic elements contained in the powder to be applied by cold spraying, such as for example a layer of an appropriate releasing agent paint, or even a substrate which is removable by chemical dissolution, etc.
Thereby, a step of mechanical releasing (e.g. by abrasion) is avoided, thus advantageously obtaining a simplification of the method and a reduction of machining times and costs.
The substrate typically consists of a material having sufficient rigidity to withstand the impact of the metallic powder mixture particles which are sprayed against its surface. Advantageously, the substrate consists of a mould, substantially having the complex shape of the massive component to be manufactured, with the exception of the changes to be obtained upon subsequent machining operations.
The method of the invention further comprises a step of thermally treating the preform of metallic mixture so as to cause the reaction between the metallic elements to form the intermetallic compound.
Such a thermal treatment is advantageously carried out once the preform of metallic mixture has been released from the substrate.
Alternatively, if the substrate is made of a material such as to withstand the conditions imposed by the thermal treatment and not react at all, during the thermal treatment itself, with the deposited metallic elements, the substrate (mould) may be removed at the end of the whole process of forming the intermetallic compound.
The release of the metallic mixture preform (if the release occurs before the thermal treatment) or of the massive preform made of intermetallic compound (if the release occurs after the thermal treatment) from the substrate may occur either manually or mechanically, by chemical dissolution, etc.
Alternatively, the release may also be carried out upon further mechanical machining operations performed before the thermal treatment, e.g. with the objective of obtaining a specific complex shape and/or for obtaining a surface finishing.
The temperature at which the thermal treatment is carried out depends on the composition of the single intermetallic compound, and generally depends on the corresponding eutectic or peritectic temperature.
In particular, since the metallic particles forming the powder mixture, upon repeated collisions imposed by the cold spraying deposition technique, have a high degree of deformation and, in general, a high concentration of voids and faults, the reaction of formation of the intermetallic compound may be thermo-dynamically promoted at temperatures even much lower than the eutectic or peritectic temperatures shown in the corresponding phase diagrams.
However, in order to increase the productivity and reduce the associated costs, the thermal treatment is carried out at a temperature substantially close to the eutectic or peritectic temperature related to the intermetallic compound which is intended to be formed.
As previously disclosed, the reason why the thermal treatment of the invention is preferably carried out close to the eutectic or peritectic temperature, which characterizes the specific intermetallic compound, is that, in participle, it is involved in no liquid phase thermo-dynamically balanced under such a temperature, and therefore it is adapted to promote obtaining the intermetallic compound in a highly dispersed form.
However, in real systems, as the involvement of the liquid phase at temperatures lightly higher than the eutectic or peritectic temperature is negligible, the formation of the intermetallic compound is not substantially influenced as well. Therefore, the expression “at a temperature substantially close to eutectic or peritectic temperature” used herein takes into account this phenomenon, so as to include the whole temperature range within which the method of the invention does not loose its efficiency.
The temperature stay times will generally depend on the selected temperature. More in particular, the temperature stay times will be such to allow the inter-scattering of the metallic elements forming the mixture, therefore they will depend on the degree of mixing the powders in the mixture.
Some hours of temperature stay are generally sufficient to obtain a complete conversion of the metallic mixture into intermetallic compound.
From the practical point of view, the treatment times may be optimized by experimentally evaluating the degree of progress of the formation of the intermetallic phases by means of diffractometry.
Furthermore, the thermal treatment aimed at forming the intermetallic compound may be advantageously combined with a thermal treatment aimed at conferring particular micro-structural features and mechanical properties to the intermetallic compound. For example, the thermal treatment for forming the intermetallic material Ti-48Al-2Cr-2Nb may be combined with the typical thermal treatment of this material, which is carried out at a temperature of 1205° C. for at least 2 hours, with subsequent cooling at controlled speed, in order to have an adequate, gamma-phase, lamellar microstructure.
The thermal treatment according to the invention may be carried out in an oven, i.e. using alternative thermal energy supply means, such as radiation, laser, etc.
After the step of cold spraying deposition and upstream of the above-described thermal treatment, the preform of metallic mixture may be subjected to a thermal, stress-relieving treatment in advance, having the objective of reducing the entity of internal stresses introduced by the cold spraying process itself and of improving the preform ductility, thus facilitating the mechanical stock removal and finishing and minimizing the risks of breakage. However, such a stress-relieving treatment should be carried out at a relatively low temperature in order to avoid the early intermetallic compound formation reaction.
The components produced by the method of the invention may then be subjected to a hot isostatic pressing treatment in order to reduce the porosity while increasing the material density.
As compared to the known methods for manufacturing massive compounds made of intermetallic material, the method of the invention results in a series of interesting advantages.
It is indeed known that the most critical aspects related to the prior art concern high hardness, fragility and reactivity of the intermetallic materials.
Despite their fragility, the intermetallic compounds offer an advantageous compromise between the properties of the ceramic materials and those of the metallic materials, in particular for conditions in which high hardness and high temperature resistance are particularly important. Moreover, some have particular magnetic and chemical properties which derive from their very tidy structure and from the nature of the involved bonds.
With respect to the known machining procedures, which involve plastic deformation operations, such as forging, extrusion, rolling, etc., the method of the invention advantageously suppresses the need to subject the intermetallic material to plastic deformation because the formation of the intermetallic compound occurs only by means of the reaction between its elementary components and, in essence, only once the component has been definitively processed with regards to forming and surface finishing.
Furthermore, as compared to the known solutions based on using lost-wax casting processes, the possibility of critical reaction interaction situations at high temperature between the molten intermetallic material and the ceramic shell is avoided by limiting the entity of machining allowances needed for removing the contaminated surface layer. The generation of faults is also advantageously minimized during the step of solidifying the intermetallic compound (e.g. micro-shrinkage).
Finally, as compared to the solutions based on rapid manufacturing techniques, such as Electron Beam Melting process, the method of the invention implies very low investment, management and maintenance costs and allows a significant increase of productivity.
It is finally apparent that changes and variations may be made in the system described and shown, without departing from the scope of protection of the independent claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| TO2009A000908 | Nov 2009 | IT | national |
