Abstract:
A foundry method of casting monocrystalline metal parts, the method including at least casting a molten alloy into a cavity of a mold through at least one casting channel in the mold, subjecting the alloy to heat treatment, and removing the mold, and wherein the heat treatment is performed before an end of mold removal.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the foundry field, and in particular to casting monocrystalline metal parts. 
     Traditional metal alloys are equiaxed and polycrystalline: in the solid state, they form a plurality of grains of substantially identical size, typically of the order of 1 millimeter (mm), but of orientation that is random to a greater or lesser extent. The joints between grains constitute weak points in a metal part made of such an alloy. The use of additives for reinforcing these inter-grain joints nevertheless presents the defect of reducing the melting temperature, which is particularly troublesome when the parts produced in this way are for use at high temperature. 
     In order to solve that drawback, columnar polycrystalline alloys were initially proposed in which the grains solidify with a determined orientation. By orienting the grains in the direction of the main load on the metal part, that makes it possible to increase the strength of such parts in a particular direction. Nevertheless, even in parts subjected to forces that are strongly oriented along a particular axis, such as for example turbine blades that are subjected to centrifugal forces, it can still be advantageous to provide greater strength along other axes. 
     That is why, since the end of the 1970s, new so-called “monocrystalline” metal alloys have been developed that enable parts to be cast that are formed as single grains. Typically, such monocrystalline alloys are alloys of nickel with a concentration of titanium and/or aluminum of less than 10 molar percent (mol %). Thus, after solidification, those alloys form two-phase solids, with an upsilon (γ) first phase and an upsilon prime (γ′) second phase. The γ phase has a face centered cubic crystal lattice in which the atoms of nickel, aluminum, and/or titanium can occupy any position. In contrast, in the γ′ phase, the atoms of aluminum and/or titanium form a cubic configuration, occupying the eight corners of the cube, while the atoms of nickel occupy the faces of the cube. 
     One of these new alloys is the “AM1” nickel alloy developed jointly by Snecma, les laboratoires de l&#39;ONERA, l&#39;Ecole des Mines de Paris, and Imphy SA. The parts made out of such an alloy can not only achieve particularly high levels of mechanical strength along all force axes, but they also present improved ability to withstand high temperatures, since they do not need any additives for binding their crystal grains together more strongly. Thus, metal parts produced on the basis of such monocrystalline alloys can advantageously be used in the hot portions of turbines, for example. 
     Nevertheless, even when using such special alloys, it can be difficult to avoid a recrystallization phenomenon during the production of such parts, giving rise once more to crystal grains and to new weak points in the part. In a conventional foundry method, the molten alloy is cast into a cavity in a mold through at least one casting channel in the mold, the mold is removed after the alloy has solidified so as to release the part, and the part is then subjected to heat treatment, such as quenching for example, in which the metal is initially heated in order to be subsequently cooled rapidly so as to homogenize the γ and γ′ phases in the monocrystal without causing it to melt. 
     Nevertheless, the mechanical impacts to which the parts are subjected after casting can locally destabilize the crystal lattice of the monocrystal. Thereafter, the heat treatment can trigger unwanted recrystallization in the locations that have been destabilized in that way, thereby losing the monocrystalline nature of the part and giving rise to points of weakness therein. Even while making considerable efforts, it is very difficult to avoid mechanical impacts in the handling of molds that may weigh several tens of kilograms, particularly since removal of the mold of itself involves the use of mechanical blows. Furthermore, on its own, a limited reduction in the temperature of the heat treatment does not make it possible to prevent those recrystallization phenomena significantly. 
     OBJECT AND SUMMARY OF THE INVENTION 
     The present invention seeks to remedy those drawbacks. For this purpose, the invention seeks to propose a casting method that makes it possible to limit to a great extent the phenomena of recrystallization following the heat treatment of the parts after the alloy cast into the mold has solidified. 
     This object is achieved by the fact that, in a foundry method in at least one implementation of the invention, the heat treatment is performed after the alloy has solidified in the mold but before the end of mold removal. 
     By means of these provisions, the heat treatment is performed before operations that might weaken the crystal structure of the monocrystal forming the part. Although the person skilled in the art might have thought that the presence of at least some remaining portions of the mold during the heat treatment would make the heat treatment less effective, it has been found that it is possible to perform the heat treatment earlier in this way without harmful effects on the metal part, and that on the contrary performing this heat treatment earlier makes it possible to avoid unwanted recrystallization occurring during the heat treatment. 
     In particular, if said removal of the mold comprises a first step of removal by hammering and a subsequent step of removal by water jet, said heat treatment may advantageously be performed at least before the water jet removal, which is found often to be the source of the recrystallization phenomena that occur during heat treatment performed subsequently. 
     In alternative implementations, it is nevertheless possible to envisage performing the heat treatment even before initial removal of the mold. Under such circumstances, such recrystallization phenomena should be combated by other means, in particular geometrical means. 
     In a second aspect of the present invention, said casting channel may include at least one transition zone adjacent to said cavity, the transition zone having a rounded portion of radius not less than 0.3 mm between said casting channel and said cavity in order to avoid a sharp bend in the flow of the molten alloy, which bend could give rise to a zone of recrystallization in the alloy. In particular, in this zone, the casting channel may present a section that is enlarged relative to an upstream section in the direction of a main axis of a section of the cavity that is perpendicular to the casting channel. More particularly, after casting, this transition zone may form at least one metal web that is thinner than the casting channel upstream, and more particularly at least one such metal web on each of two opposite sides of the casting channel. When the mold contains at least one core penetrating into said cavity and occupying a space adjacent to said casting channel for the purpose of forming a cavity in the metal part, said transition zone, after casting, may form at least one metal web adjacent to said core and thinner than the casting channel upstream. Each metal web adjacent to the core may present an outer edge following a substantially concave line adjacent on a surface of the core. The transition zone may form at least one metal web on each side of said core. Under such circumstances, said adjacent metal webs of the core may present outer edges that join together at their ends, so as to go around the core. 
     In this way, during casting, this transition zone makes it possible to fill the cavity in substantially simultaneous manner over its entire width, thereby avoiding irregularities being created in the crystal structure of the monocrystal during solidification of the alloy. During the heat treatment step, such irregularities could give rise to local recrystallization, thereby forming a weak point in the metal part. 
     In order to increase the production of metal parts, the mold may contain a plurality of cavities arranged like a bunch of grapes, so as to mold a plurality of metal parts simultaneously. 
     The method of the invention is particularly suitable for producing certain metal parts, such as turbine engine blades. The present invention also provides metal parts obtained by the method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be well understood and its advantages appear better on reading the following detailed description of an implementation given by way of non-limiting example. The description refers to the accompanying drawings, in which: 
         FIG. 1  shows a prior art foundry method; 
         FIG. 2  shows a foundry method in an implementation of the present invention; 
         FIG. 3  shows the connection between a casting channel and a molding cavity in a prior art mold; 
         FIG. 4  is a perspective view of a metal part produced using the method of  FIG. 2 ; and 
         FIG. 5  is a cross-section on plane V-V of the metal part shown in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A conventional foundry method, e.g. as used in the production of turbine engine blades and more particularly high pressure turbine blades is shown in  FIG. 1 . In a first step, a ceramic mold  150  is produced, typically by the lost wax method, although other conventional methods could alternatively be used. The ceramic mold  150  has a plurality of cavities  151  connected by means of casting channels  152  to an external orifice  153  of the mold  150 . Each cavity  151  is shaped to mold a metal part that is to be produced. Under such circumstances, since the parts to be produced are hollow, the mold  150  also includes cores  155  penetrating into each of the cavities  151 . After this first step, in a casting step, a molten alloy  154  is poured into the orifice  153  in order to fill the cavities  151  via the casting channels  152 . 
     After the alloy has solidified, in a third step, initial removal of the mold  150  is performed by hammering in order to release the metal parts  156  united as a bunch  157  from the mold  150 . In order to eliminate the last remains of the mold  150 , an additional step is then performed of removal by water jet. In the following step S 105 , the individual parts  156  are cut away from the bunch  157 . The cores  155  are then removed from each of the parts  156  in the following step, and the parts  156  are finally subjected to heat treatment. By way of example, this heat treatment may be quenching, in which the parts  156  are briefly heated and then cooled rapidly in order to harden the alloy of the part. 
     The alloys that can be used in this method include in particular so-called “monocrystalline” alloys that enable a part to be formed as a single crystal grain, or “monocrystal”. Nevertheless, in that prior art method, the heat treatment for the purpose of homogenizing the γ and γ′ phases of the monocrystal can trigger recrystallization phenomena that weaken the parts locally. In order to avoid that drawback, in a foundry method in an implementation of the invention as shown in  FIG. 2 , the order of the operations is modified by performing the heat treatment step earlier. 
     Thus, in this method shown in  FIG. 2 , the first step is likewise producing a ceramic mold  250 . As in the prior art, the ceramic mold  250  may also be produced by the lost wax method, or by some alternative method selected from those known to the person skilled in the art. In addition, and as in the prior art, the ceramic mold  250  has a plurality of cavities  251  connected by casting channels  252  to an external orifice  253  of the mold  250 . Each cavity  251  is also shaped for molding a metal part that is to be produced. In addition, since the parts to be produced are also hollow, the mold  250  also includes cores  255  penetrating into each of the cavities  251 . 
     After the first step, and still as in the prior art, a molten alloy  254  is cast into the orifice  253  during a casting step in order to fill the cavities  251  via the casting channel  252 . After the alloy has solidified, in a third step, initial removal of the mold  250  by hammering is likewise performed in order to release the metal parts  256  united as a bunch  257  from the mold  250 . Nevertheless, in this method, after this initial removal, the heat treatment step is performed directly. During the heat treatment, the metal parts  256 , still constituting a bunch  257  and still together with remaining pieces of the mold  250  are subjected directly to quenching, for example, in which the parts  256  are briefly heated and then rapidly cooled. 
     In order to eliminate the last remains of the mold  250 , it is possible in the following step to then proceed with removal by water jet. Finally, the individual parts  256  are cut away from the bunch  257  and the cores  255  are then removed from each of the parts  256 , which parts have already been subjected to heat treatment before removal by water jet. 
     Because the heat treatment step is performed earlier, it is possible to reduce recrystallization phenomena during this step. Nevertheless, in order to reduce this recrystallization even more completely and above all in order to do so reliably, it is also appropriate to give the casting channels  252  an appropriate shape. In  FIG. 3 , there can be seen the connection between a casting channel  152  and a mold cavity  151  in the prior art mold  150 . This connection forms very sharp bends between the channel  152  and the cavity  151 , which bends can lead to recrystallization zones  160  forming during the heat treatment. 
     In the mold  250  of the method shown in  FIG. 2 , in order to avoid forming such recrystallization zones in each part  256  around the casting channels  252 , the channels  252  may include transition zones adjacent to the cavities  251 . In a transition zone, the casting channel  252  becomes progressively enlarged towards a main axis X of a section S of the cavity  251  in a plane A that is perpendicular to the casting channel in such a manner that the radius of the rounded portion between the casting channel  252  and the cavity  251  is not less than 0.3 mm. In particular, in the implementation shown, in which the mold  250  also includes a core  255  adjacent to the casting channel  252 , this transition zone enlarges on either side of the core  255 , and also away from the core  255 . When the cavity  251  and the channel  252  are filled with metal, the metal thus forms a web  261  away from the core  255  and two webs  262  and  263  that are adjacent to the core  255 , one on either side of the core  255 , as shown in  FIGS. 4 and 5 . Perpendicularly to the axis X, these webs  261 ,  262 , and  263  are substantially thinner than is the casting channel  252  upstream from the transition zone. 
     During the casting step, the presence of the transition zone thus makes it possible to distribute the flow of molten alloy substantially throughout the width of the cavity  251 , thus avoiding subsequent formation of recrystallization zones. 
     The monocrystalline part  256  shown in  FIG. 4  is a turbine blade. It is shown in its rough state after unmolding, i.e. with the metal that has solidified outside the part in the casting channel  252 . This metal thus forms a central rod  275 , webs  261 ,  262 , and  263 , and an enlarged section  276  adjacent to the blade tip  265 . During casting, the molten alloy flows from the blade tip  265 , through the blade root  266  and on to a casting channel  252  connected to another cavity  251  further downstream. The flow of molten alloy thus follows substantially the direction of the main axis Z of the blade. The web  261  that extends towards the trailing edge  267  of the blade presents an outer edge  268  with a concave upstream segment and a convex downstream segment. In cross-section, this outer edge  268  has a radius of curvature R that varies only very gradually from the central rod  275  to the enlarged section  276 . The webs  262  and  263  that extend towards the leading edge  269  of the blade on either side of the core  255  present respective outer edges  270  and  271  that are substantially concave and that run along the core  255 . These outer edges  270  and  271  join together via their ends above the core  255  and in front of it, thereby forming two connections  272 ,  273  so as to surround the core  255 . In cross-section, the webs  262 ,  263  present radii of curvature R′ and R″ on the surfaces adjacent to the outer edges  270 ,  271  so as to avoid seeding undesirable metallurgical defects in the proximity of the core  255 . The transition surfaces  277  of the webs  261 ,  262 , and  263  and of the rod  275  at the enlarged section  276  are likewise rounded to avoid seeding such defects. 
     Among the alloys that can be used in this method, there are in particular monocrystalline alloys of nickel, such as in particular AM1 and AM3 from Snecma, and also others such as CMSX-2®, CMSX-4®, CMSX-6®, and CMSX-10® from C-M Group, René® N5 and N6 from General Electric, RR2000 and SRR99 from Rolls-Royce, and PWA 1480, 1484, and 1487 from Pratt &amp; Whitney, among others. Table 1 gives the compositions of these alloys. 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Compositions of monocrystalline nickel alloys in weight % 
               
             
          
           
               
                 Alloy 
                 Cr 
                 Co 
                 Mo 
                 W 
                 Al 
                 Ti 
                 Ta 
                 Nb 
                 Re 
                 Hf 
                 C 
                 B 
                 Ni 
               
               
                   
               
             
          
           
               
                 CMSX-2 
                 8.0 
                 5.0 
                 0.6 
                 8.0 
                 5.6 
                 1.0 
                 6.0 
                 — 
                 — 
                 — 
                 — 
                 — 
                 Bal 
               
               
                 CMSX-4 
                 6.5 
                 9.6 
                 0.6 
                 6.4 
                 5.6 
                 1.0 
                 6.5 
                 — 
                 3.0 
                 0.1 
                 — 
                 — 
                 Bal 
               
               
                 CMSX-6 
                 10.0 
                 5.0 
                 3.0 
                 — 
                 4.8 
                 4.7 
                 6.0 
                 — 
                 — 
                 0.1 
                 — 
                 — 
                 Bal 
               
               
                 CMSX-10 
                 2.0 
                 3.0 
                 0.4 
                 5.0 
                 5.7 
                 0.2 
                 8.0 
                 — 
                 6.0 
                 0.03 
                 — 
                 — 
                 Bal 
               
               
                 René N5 
                 7.0 
                 8.0 
                 2.0 
                 5.0 
                 6.2 
                 — 
                 7.0 
                 — 
                 3.0 
                 0.2 
                 — 
                 — 
                 Bal 
               
               
                 René N6 
                 4.2 
                 12.5 
                 1.4 
                 6.0 
                 5.75 
                 — 
                 7.2 
                 — 
                 5.4 
                 0.15 
                 0.05 
                 0.004 
                 Bal 
               
               
                 RR2000 
                 10.0 
                 15.0 
                 3.0 
                 — 
                 5.5 
                 4.0 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 Bal 
               
               
                 SRR99 
                 8.0 
                 5.0 
                 — 
                 10.0  
                 5.5 
                 2.2 
                 12.0  
                 — 
                 — 
                 — 
                 — 
                 — 
                 Bal 
               
               
                 PWA1480 
                 10.0 
                 5.0 
                 — 
                 4.0 
                 5.0 
                 1.5 
                 12.0  
                 — 
                 — 
                 — 
                 0.07 
                 — 
                 Bal 
               
               
                 PWA1484 
                 5.0 
                 10.0 
                 2.0 
                 6.0 
                 5.6 
                 — 
                 9.0 
                 — 
                 3.0 
                 0.1 
                 — 
                 — 
                 Bal 
               
               
                 PWA1487 
                 5.0 
                 10.0 
                 1.9 
                 5.9 
                 5.6 
                 — 
                 8.4 
                 — 
                 3.0 
                 0.25 
                 — 
                 — 
                 Bal 
               
               
                 AM1 
                 7.0 
                 8.0 
                 2.0 
                 5.0 
                 5.0 
                 1.8 
                 8.0 
                 1.0 
                 — 
                 — 
                 — 
                 — 
                 Bal 
               
               
                 AM3 
                 8.0 
                 5.5 
                  2.25 
                 5.0 
                 6.0 
                 2.0 
                 3.5 
                 — 
                 — 
                 — 
                 — 
                 — 
                 Bal 
               
               
                   
               
             
          
         
       
     
     Although the present invention is described with reference to a specific implementation, it is clear that various modifications and changes may be made to that implementation without going beyond the general scope of the invention as defined by the claims. For example, in an alternative implementation, the heat treatment could be performed even before initial removal of the mold. In addition, the individual characteristics of the various implementations of the method may be combined in additional implementations. Consequently, the description and the drawings should be considered in an illustrative sense rather than in a restrictive sense.