Patent Publication Number: US-10760153-B2

Title: Method for manufacturing turbomachine components, blank and final component

Description:
This application is a national phase of PCT/fr2016/050507, filed on Mar. 4, 2016 which claims priority to FR1552055 filed Mar. 12, 2015. The content of those applications are hereby incorporated by reference. 
     The present invention relates to a method for manufacturing metal turbomachine parts, and in particular movable turbine wheel blades of an aircraft turbojet engine or turboprop engine. 
     It relates to a titanium-aluminium intermetallic compound alloy. TiAl 48-2-2 is specifically concerned. 
     It also relates to an assembly comprising a blank for a turbomachine part made from such an alloy based on TiAl and a machined part resulting from the machining of this blank. 
     An alloy forms an intermetallic compound with certain chemical compositions and under certain pressure and temperature conditions. Unlike a conventional alloy, where atoms of different natures may be distributed randomly on the same crystallographic site, an intermetallic compound consists of a periodic alternation of atoms. Thus, when an elementary mesh is looked at, a crystalline structure can be noticed. 
     The formation by founding (casting) of a part made from titanium-aluminium intermetallic alloy is extremely difficult at the present time and does not make it possible to cast sufficiently fine thicknesses to produce, by founding (casting), parts with finished cast regions. 
     Managing to efficiently machine a part made by founding (casting) is furthermore difficult. 
     Two categories of problems result therefrom:
     those related to the founding (casting),   those related to the machining,   the whole to be considered in an economic context.   

     In the prior art, there exist in particular the following solutions: 
     1) a solution providing for the obtaining of an over-thick rough form by lost-wax founding (casting), and then machining this rough form in order to obtain the final part, such as a blade, 
     2) a solution consisting of the casting of a blank to the almost final form of the part (referred to as “near net shape”), next allowing machining that is certainly minimal (with little loss of material) of the final part, but which remains necessary, 
     3) and a solution by founding (casting) in a centrifuged permanent mould, where it is possible to make provision for manufacturing a plurality of turbomachine parts, following steps consisting of: 
     
         
         a) pouring the metal material into a centrifuged casting mould, 
         b) extracting therefrom a blank of elongate shape, preferably substantially cylindrical or polyhedral and/or with a circular or polygonal cross section, and 
         c) machining the blank until the final form of the part is obtained. 
       
    
     Solutions by forging also exist, but are tricky to implement because of the fragility of TiAl alloys. 
     One drawback of founding (casting) for parts based on TiAl is the very rapid solidification of the molten material. 
     The result of this is a high risk of porosity of the parts, a failure to achieve suitable filling of the moulds and therefore a tricky finalisation of the external form of the as-cast rough form (blank). 
     Furthermore, a hot isostatic compression (HIC) is next typically necessary in order to close up any porosities, implying significant cost. In addition, this treatment is not always sufficient, in particular if the porosities of the rough form are opening out. 
     As drawbacks of loss-wax founding (casting) (non-permanent mould), the following can be noted:
     the necessary use of rare materials for the mould shell (such as yttrium), with costs and supply problems,   the risk of weakening of the parts via the formation of inclusions: both those issuing from the mould/shell reactivity (specific to TiAl since it is very reactive) and those issuing from shell debris that fall into the moulds (specific to the lost-wax process),   the very specific development of the shell, with typically a compromise to be found between resistance to centrifugation force and the friability of the shell for facilitating removal from the mould,   the use of specific installations for casting by centrifugation.   

     Other points may also be mentioned:
     Drawbacks of solution 1): during the heat treatment consisting of hot isostatic compression (HIC) that this solution requires, residual stresses are stored in the part. Unpredictable deformations are too often discovered on machining.   Drawbacks of solution 2): sufficient excess thickness of material is not available on the as cast rough form (the blank) in order to avoid a lack of material on the finished part if the blank is slightly deformed and it is sought to machine this part in an automated manner. A risk of non-compliance with the dimensions of the finished part also exists.   Drawbacks of solution 3): a lengthy implementation before ending up (in particular if it is a case of a blade) with an optimised mould+part system leading neither to shrinkages of excessive size, nor to a chemical and macrostructural heterogeneity of the blank due to solidification.   

     One objective of the invention is to avoid or limit many of the problems mentioned above. 
     One solution for this is a method for manufacturing at least one metal turbomachine part, comprising steps consisting of: 
     a) keeping a TiAl (titanium-aluminium) intermetallic alloy melted by plasma torch in a retractable-bottom mould (or ring mould), 
     b) extracting therefrom an ingot, as cast, in a state cooled from molten, 
     c) cutting the ingot into at least one blank with an external shape that is simpler than the more complex one of the part to be manufactured, 
     d) and machining the blank in order to obtain the part with said more complex external shape. 
     The term “blank” must be understood here in a fairly broad sense. It designates a product that is not finished but the general form of which corresponds essentially to the appearance of the finished part. This means that a blank for a part as aforementioned is a metal product of the aforementioned type. This excludes neither the subsequent adaptation of the shape of this blank, for example by machining, nor the modification of this general appearance, for example by curving, bending or any other plastic deformation. It must rather be understood that a “blank” of a product of the aforementioned type is a part of this type that may undergo various shaping, machining or surface treatments in order to give rise to a finished product. 
     To supplement the aforementioned solution, it is advised that:
     at step c), the cut blank, from which the part of step d) is to be machined, should have a given external volume and/or mass A1,   at step d), the machined part should have a given external volume and/or mass A2, and   the ratio A2/A1 should be greater than 0.95.   

     One objective sought is a machining aimed at reduced losses of material. In this context, and in a more general context of saving on material, it is moreover recommended that:
     at step c), all the cut blanks should represent more than 95% of the external volume and/or of the mass of the ingot extracted, and/or   at step b), a substantially cylindrical or polyhedral ingot should be obtained.   

     Typically, the “ring moulds” mentioned above are referred to as PAM (plasma arc melting) furnaces. These PAM furnaces are normally, in the prior art, used for casting material for remelting, that is to say, after melting of the material in the PAM furnace, this material solidifies, and is then remelted in order be cast. The cast bars, or ingots, then have very large diameters (especially &gt;200 mm). 
     However, in order to comply with the requirements of a rough PAM bar or ingot, to be used with a view to direct machining, it has appeared useful to change the PAM method in order to make it more robust and better in a position to produce ingots without defects. 
     In this light, it is here proposed to cast PAM ingots of smaller diameters where the phenomena giving rise to defects are more easily controllable. 
     Thus it is in practice advised that, at step b), the extracted ingot should have a diameter of less than or equal to 200 mm or a cross section of less than approximately 32×10 3  mm 2  within 5%. 
     Applying the aforementioned PAM production in particular to such small diameters of ingots will make it possible to avoid the shrinkage and chemical segregations that are the two main technical difficulties in casting in a centrifuged permanent mould, with solidification that will take place sequentially in a small volume that will be referred to as solidification wells. 
     By using such a PAM method, it will therefore be possible to obtain semifinished products with very little porosity and very homogeneous. 
     Moreover, by proceeding with a heat treatment in one of more operations, as advised below, the obtaining of the desired microstructure and mechanical properties will be encouraged even further. 
     This treatment, applied a priori to the blank, will favourably comprise:
     a heat treatment to obtain a duplex microstructure consisting of gamma grains and lamellar grains (alpha2/gamma),   and/or a heat treatment for preparation for HIC (hot isostatic compacting) and then HIC (in order to close up the porosities).   

     As an alternative or in addition, it is however provided for the post-PAM treatment, on a blank consisting of a TiAl alloy with gamma grains having typically a composition containing between approximately (to within 5%) 47 and 49 percent aluminium (at %), to be as follows:
     heat treatment by heating to a temperature of approximately (within 5%) 1038° C. to 1149° C., for a period of approximately 5 to approximately 50 hours, the material next optionally undergoing hot isostatic compression (HIC) at a temperature of between 1185° C. and 1204° C.,   then another heat treatment at a temperature of between approximately 1018° C. and 1204° C. (still within 5%), without HIC.   

     If the melting step and the step of obtaining the ingot are properly carried out it could be unnecessary to apply pressure during the second aforementioned heat treatment step. 
     In the above global context, it is anticipated that the ranges comprising the manufacture of bars or ingots with a view to direct machining, after cutting into a blank or blanks of simple shape during step c), must be designed so as to comply with the requirements of the final parts since they are in this case directly transferred onto the blanks. The main requirements are:
     chemical homogeneity, which guarantees microstructural and mechanical homogeneity after heat treatment,   absence of inclusion or non-molten part (portion of the original material not melted in the PAM furnace),   few porosities on the as-cast bars/ingots and with sizes less than one millimetre,   practically no porosity on the blank, after HIC (if this compression takes place).   

     Concerning the assembly already mentioned including:
     a blank of a turbomachine part made from a TiAl intermetallic compound, obtained at the end of melting by plasma torch, and   a machined part issuing from the machining of such a blank, provision is made for the blank to have a determined external volume and/or mass A1, and the machined part having a determined external volume and/or mass A2, the ratio A2/A1 being greater than 0.95 and less than 1.   

     In correlation with the above, this assembly will favourably be such that the blank will have a diameter of less than or equal to 200 mm, preferably 120 mm, and a length of less than 300 mm, preferably between 220 mm and 240 mm. 
     This will assist a saving on material, particular in the context of the manufacture of a blade. 
     Before the aforementioned step a) of keeping the alloy molten, it will be possible to provide a series of plasma torches to melt the intermetallic compound and to keep it molten. 
    
    
     
       Other advantages and features of the invention will also emerge from a reading of the following description given by way of non-limitative example and with reference to the accompanying drawings, where  FIGS. 1 and 2  are dimensionally precise and correspond to industrial reality, like dimensioned drawings, and in which: 
         FIG. 1  shows schematically a PAM fusion furnace from which an ingot is extracted, 
         FIG. 2  is a schematic view in perspective of a block of material, or blank, issuing from a rough cut of the ingot extracted, 
       and  FIG. 3  is a table that presents and compares these cases of manufacture of a metal part in accordance with those mentioned above, intended for a turbomachine, in particular a movable turbine wheel blade of an aircraft turbojet engine or turboprop engine. 
     
    
    
     In the left-hand column in  FIG. 3 , the steps are listed involving remelting, with lost-wax moulding (temporary mould), of a rough ingot issuing from melting (other than PAM), at the initial step. 
     In the central column the steps also involving remelting are listed, with moulding in a centrifuged mould (permanent mould), of a rough ingot issuing from melting (other than PAM), at the initial step. 
     And in the right-hand column the steps of the present invention are listed without moulding or necessarily remelting, after a rough ingot issuing from PAM melting has been obtained at the initial step. 
     Thus:
     in the “lost-wax casting” prior art, the following steps are successively carried out: obtaining a rough ingot issuing from melting, and then production of wax models, then assembly of a wax cluster, then moulding of the shell, then firing the shell, then dewaxing of the shell, then remelting of the ingot—casting of the metal, then the mould is broken, then cutting the remelted ingot obtained into blanks, then heat/optionally HIC treatment, then dimensional check and machining;   in the “centrifugal permanent mould” prior art, the following steps are successively carried out: obtaining a rough ingot resulting from melting, then remelting an ingot—casting the metal in a permanent mould, then cutting the remelted ingot obtained into a blank, and then HIC heat treatment and machining;   the “invention” prior art, the following steps are successively carried out:   

     obtaining a rough ingot resulting from PAM melting, then cutting the remelted ingot obtained into a blank, and then heat treatment/optionally HIC and machining. 
     The solution in the favoured example in the right-hand column therefore consists of limiting the manufacture of this part to four steps making provision for: 
     a) initially casting a TiAl intermetallic compound in a ring mould (or PAM furnace), with melting by plasma torch, 
     b) extracting therefrom an as-cast ingot, in a state cooled from molten, 
     c) cutting the ingot into at least one blank with a simpler external shape than the more complex one of said part to be manufactured, 
     d) machining the blank in order to obtain the part with said more complex external shape. 
     As shown schematically in  FIG. 1 , the PAM melting  1  is here carried out with a material  3  that is TiAl, in this case 48-2-2 TiAl, therefore comprising 48% AI 2% Cr 2% Nb, at %). This raw material is introduced by means of a wide channel  5  where the material is poured, as shown in  FIG. 1 . A series of plasma torches  7  melt the metal provided and then keep it molten. There is at least one such torch above each vessel or receptacle  9  and refining hearth  11   a  and then  11   b , with its beam such as  8  directed towards the metal in the vessel or hearth. The circulation (see arrows) of the metal bath is done from vessel to vessel. The flow of the material and the stirring of the liquid make it possible to prevent problems of segregation and the presence of any inclusion of heavy metals (high density inclusion—HDI), these problems being well known in the conventional technology of a VAR (vacuum arc remelting) remelting arc furnace. It is thus possible to consider a single melting, whereas by the VAR method two or even three successive meltings (referred to as remeltings) are necessary. The PAM technique also makes it possible to limit the appearance of alpha-phase inclusions (hard phase inclusion—HPI). 
     A last plasma torch  70 , placed above a final mould or vessel, keeps the top of the bath arriving from the tanks  11   a  and then  11   b  molten therein. This final vessel is in the form a ring mould  13 . The ring mould  13  comprises a bottom  13   a  that is retractable or movable, for example axially, here with controlled vertical movement. The ring mould  13  is cold, typically cooled from outside, for example with water, via cooling means  15 . Under its bottom opening  13   b  and here by lowering of the movable bottom  13   a , the bottom of the bath flows, by gravity or other, then sufficiently cold to form an ingot  17 , as cast, in this state cooled from molten. The ring mould  13  may be made from copper. 
     By using the various vessels  9 , multiple refining hearths, such as here  11   a ,  11   b , and then the ring mould  13 , with plasma torches  7 ,  70  also multiple and placed above each of these receptacles, the travel of the material will be optimised, so as to completely melt it and to keep it therein at a substantially homogenous temperature. Reducing the number of inclusions or non-molten parts will also be possible by using, as illustrated, a plurality of overflow tanks. To guarantee an even greater quality, it will also be possible to make provision for carrying out successive meltings of the material. 
     Typically, the ingot  17  obtained will be substantially cylindrical or polyhedral. 
     In order to assist compliance with the requirements of a bar or ingot  17  intended for direct machining, and therefore with neither any intermediate moulding nor the conventional drawbacks of lost-wax founding (casting) (defects resulting from interactions with the mould, which is typically made from ceramic), or other defects characteristic of producing by casting in centrifugal permanent moulds (central shrinkage and chemical macrosegregation, in particular), it is here proposed to cast ingots of small sizes, in particular such that each ingot  17  extracted has a transverse dimension d (diameter or width for a square cross section) less than or equal to 200 mm, and preferably 120 mm, or, in cross section, less than approximately 32×10 3  mm 2  and 12×10 3  mm 2  within 5%, respectively. 
     It is next from such an as-cast ingot that one and preferably a plurality of blanks  21  will be directly cut (by basic tools), each with a simple shape, in particular once again substantially cylindrical or polyhedral and in any case with an external shape simpler than the more complex one of each of said parts to be manufactured, the result of the machining of each blank, such as the two blades  19   a ,  19   b  that can be seen by transparency in the blank  21  of  FIG. 2 , aiming at a maximum use of the material. 
     This objective and a search for optimisation of the manufacturing processes in particular of turbine blades, with shortening of the cycle times, has moreover led to preferring:
     that each blank  21  issuing from the ingot  17  should have a length L 2  of less than 300 m, preferably between 220 mm and 240 mm, and a cross section S (perpendicular to its length L 2 ) of less than 12×10 3  mm 2  within 5% (that is to say 1.2 dm 2 ),   that at step c) all the cut blanks  21  represent more than 95% of the external volume and/or of the mass of the ingot  17  extracted, and/or:   that at step c) the cut blank  21 ; that is to say therefore the block from which the part of step d) (blade such as  19   a  or  19   b ) is to be machined, should have a determined external volume and/or mass, referred to as A1,   that at this step d) the machined part  19   a  or  19   b  should have a given external volume and/or mass, referred to as A2, and   that the ratio A2/A1 is greater than 0.95 and less than 1.   

     From a reading of the above table it will moreover have been clear that, between the step of cutting the ingot into blanks and the machining of each blank, preferably heat treatment (in a single sequence or multiple sequences) of each of these blanks will occur. 
     As already indicated, one aim will be to thereby assist the achieving of the expected mechanical and microstructure criteria.) 
     In fact, it is recommended carrying out:
     a heat treatment so that the material of the blank has a duplex microstructure consisting of gamma grains and lamellar grains (alpha2/gamma),   and/or heat treatment for preparation for HIC (hot isostatic compacting) and the HIC (to close the porosities again).   

     One aim being therefore to obtain a duplex microstructure (intermetallic compound) consisting of gamma grains and lamellar grains (alpha2/gamma), and it is in practice advised to proceed as follows (with values supplied within 5%):
     a TiAl alloy with gamma grains, in particular the aforementioned one issuing from the PAM furnace  1 , typically having a composition containing between approximately 47 and 49 percent aluminium (at %), undergoes heat treatment at a temperature from approximately 1035° C. to approximately 1150° C., for a period of approximately 5 to approximately 50 hours,   then it undergoes another heat treatment at a temperature of between approximately 1000° C. and 1220° C.   

     Between the two steps of this heat treatment, the material will also have been able to undergo hot isostatic compression (HIC) at a temperature of approximately 1200° C., preferably between 1185° C. and 1204° C.