FR2982280A1 - Copper and zinc alloy, useful for manufacturing molded metallic products e.g. shift fork of gearbox, comprises composition comprising copper, zinc, manganese, aluminum, iron, nickel, silicon, tin, lead, and impurities - Google Patents

Abstract

Copper and zinc alloy comprises a composition comprising copper (60-70 wt.%), zinc (15-24 wt.%), manganese (8-15 wt.%), aluminum (0.50-4 wt.%), iron (0.5-4 wt.%), nickel (1 wt.%), silicon (1 wt.%), tin (1 wt.%), lead (2 wt.%), and impurities (less than 1 wt.%). Independent claims are included for: (1) a molded metallic product made of copper and zinc comprising the composition and alpha phase percentage of 50-90%, preferably 53-87%; and (2) a process for manufacturing a molded metallic product made of copper and zinc alloy, comprising elaborating the alloy at a liquid state, and providing the alloy in a mold for forming the product under solidification and cooling conditions.

Description

The invention relates to metallurgy, and more particularly brass-like copper alloys containing mainly copper and zinc. For the production of molded parts that must resist corrosion and, above all, have good friction properties and high mechanical properties, such as forks for vehicle gearboxes, a copper alloy (aluminum bronze) is often used. ) of the CuA110Fe2-C type which, according to the NF EN 1982 standard, contains from 8.5 to 10.5% of Al, from 83 to 89.5% of Cu, from 1.5 to 3.5% of Fe up to 1% Mn up to 1.5% Ni. It must also not contain more than 0.05% Mg, 0.10% Pb (or 0.03% for molded parts to be welded), 0.20% Si, 0.50 % of Zn. All these percentages, and those which will be given in the rest of the text, are mass percentages.

These alloys are intended to be molded in shell and have the following mechanical properties: - tensile strength R,: at least 600 MPa; - conventional yield strength Rp0.2: at least 250 MPa; - elongation at break A: at least 20%; - Brinell hardness: at least 130 HB. This alloy is marketed under the name SUPER ACIOR®. Its advantages include having generally high mechanical characteristics, and not requiring complex processes for formatting. It can advantageously replace a steel that should be shaped by forging, which would be too difficult and too expensive for many applications. The SUPER ACIOR® only needs to be molded and rectified, which in its case are easy to perform. It is also not very fragile and therefore has a consistent and consistent hold during its usual jobs. However, one may wish to improve its hardness and elastic limit. One could think of doing so by increasing its content of alloying elements such as Al and Fe. But then its fragility would also be considerably increased and it would be possible to reveal phases y 2 which would degrade the mechanical strength. Also, the Al content is difficult to adjust very precisely during the foundry operations of Cu-Al alloys.

The abrasion resistance of the SUPER ACIOR® may also sometimes be considered insufficient, which can be remedied by surrounding the parts of the part that are the most stressed by a composite shell. But this is expensive, and requires to adapt the shape of the room in a way that weakens it. Above all, the material of this alloy is relatively expensive, because of the presence of Cu in very large quantities, this metal having a high price and subject to strong variations. The object of the invention is to find an alloy that can replace the SUPER ACIOR® for its main applications. It should have mechanical characteristics close to, or better, lower material cost, an implementation during the manufacture of parts requiring, if possible, no radical changes foundry processes and facilities usually used for the SUPER ACIOR® and do not entail a higher risk of defects, machinability and ease of cutting comparable to those of aluminum bronzes. For this purpose, the subject of the invention is an alloy of copper and zinc, characterized in that its composition is, in percentages by weight: - Cu = 60 to 70%; Zn = 15 at 24 ° / 0; Mn = 8 to 15%; - Al = 0.5 to 4%; - Fe = 0.5 to 4%; - Ni = 1% traces; - Si = 1% traces; Sn = 1% traces; - Pb = 2% traces; the remainder being copper and impurities resulting from the preparation, the total contents of said impurities not exceeding 1%. % Cu Its fictitious copper Tf - title is preferably between% Cu + IK.% I 55 and 64%, where i denote the various alloying elements, Ki denotes a typical coefficient of the element i and% i the content in the alloy of element i, K being equal to 1 for Zn, 10 for Si, 6 for Al, 2 for Sn, 2 for Mg, 1 for Pb and all the elements immiscible in the Cu matrix, 0 , 9 for Fe, 0.5 for Mn, -1.3 for Ni. The subject of the invention is also a metallurgical product molded from copper and zinc alloy, characterized in that its composition is in accordance with the abovementioned composition, and in that its percentage of phase a is between 50 and 90%, preferably between 53 and 87%. It can be a gearbox fork. The invention also relates to a method of manufacturing a metallurgical product molded from a copper and zinc alloy, characterized in that: - an alloy is produced in the liquid state, the composition of which is in accordance with the aforementioned ; and casting said alloy in a mold to form said product, under solidification and cooling conditions ensuring the presence on the solidified and cooled product of a phase proportion a of 50 to 90%, preferably of 53 to 15%; 87%. The different parts of said mold are preferably preheated to a temperature between 200 and 300 ° C, and the alloy is cast in the mold at a temperature of 750 to 880 ° C above the temperature of the coldest points of the walls of the mold. mold. The solidified and cooled product can then be machined. As will be understood, the solution chosen by the inventors was first of all to use a Cu-Zn-Mn-Al alloy of defined composition, with a Cu content that is significantly lower than that of SUPER ACIOR®, which can also be The inventors have sought to optimize the percentage of phase a which is likely to be found in the molded product after it has solidified under industrial conditions to give it properties which make it possible to obtain Fe and a small amount of Ni and Si. it is possible to replace the SUPER ACIOR® with an alloy according to the invention, for the manufacture of parts from which various mechanical characteristics are expected. It has been found that the fictitious Cu Tf title is a relevant element, at least in a first approximation, for determining the precise composition of the alloy to be used for a given application. The invention will be better understood on reading the description which follows, given with reference to the following appended figures: FIG. 1, which shows a micrograph of an example of an alloy according to the invention; - Figures 2 and 3 which show in isometric perspective a machined gearbox fork that can be made from a molded part by the method according to the invention. Regarding the elements of the alloy according to the invention, their choice was first dictated by the following criteria, determined by the inventors. It has been chosen to remain in the field of copper alloys, so as not to have to modify the processes and installations customarily used to make the SUPER ACIOR® parts, and to be better ensured to maintain a high mechanical strength. It was not necessary to use in very significant quantities dangerous alloy elements whose use is very strictly regulated by legislation or would be likely to become so. In particular, Pb, Cd, Hg, Be and Co should be avoided or avoidable. Likewise, alloying elements that are too reactive or hardly amenable to easy addition during melting, such as Ca, Mg, W, generally did not appear desirable. We can, however, make exceptions for borderline cases such as Mn, Zn or Cr where, in spite of certain technical difficulties rendering the fusion more complex, the addition of these elements can still be envisaged. Also, the desire to limit the material cost excluded the voluntary and significant introduction of elements such as Ag, Sn, Pd, Ni and Bi. That said, these elements had to be tolerable in the form of traces or small quantities resulting from the melting of the raw materials, so as not to have to make a drastic selection of these raw materials. Among the alloying elements, therefore remain "candidates" among others Al, Zn, Si, Fe, and Mn. Neither should be tolerable in small quantities. The machinability and the cutability of the alloy had to remain high. The bronzes of Al in shell have the particularity of having an acicular duplex structure, comprising phase needles a, of cubic face-centered structure, soft at a low temperature in a matrix derived from phase 3, of cubic structure centered, hard at low temperature, as well as ferrous precipitates of phase Ô. Indeed, phase 13 is thermodynamically stable only at high temperature and, in conventional shell foundry, partially transforms into a series of hard metastable derivatives, of which, mainly and for CuA110Fe2-C, the phase 3 '(disordered martensite to hexagonal structure). The percentage of phase after shell molding is typically in the order of 60%. Its softness at low temperature, associated with the reinforcing character of the phase 13 and its derivatives, favors machinability at room temperature, contrary to the case where these different phases would be solitary (too soft and plastic for a or too hard and fragile for 13 and its derivatives). The presence of phase R strengthens the room. The precipitates O increase the rigidity of the part, but we can not afford to have too much, because their main constituent element, namely Fe, deteriorates the viscosity of the alloy during its molding. It was therefore necessary to find for the alloy of the invention a structure that can be comparable to those encountered on the SUPER ACIOR® with the same grain characteristics. Careful balancing between the different phases present after the molding should therefore be sought by means of a suitable chemical composition. The resistance to wear and friction had to be optimized. This depends in particular on the size of the grains, their respective quantities and distributions, their type of crystalline structure, their intrinsic mechanical properties and the presence of precipitates and their more or less lubricating or embrittling natures.

More grains are small and evenly distributed, the better are usually the friction properties. Among the types of desirable precipitates, the inventors have thought that complex silicide manganese and iron precipitates would be potentially advantageous in their tribological properties.

Regarding the cost of alloying elements, it was necessary to minimize the presence of Cu, because of its price. A priori, an increase in the Fe and Al contents, low or moderate cost elements, was a path to explore. But the inventors have found that it does not provide good results. In particular, the significant increase in the Fe content provides, as has been said, a brittle and viscous alloy, thus not responding to the technical problems to be solved. This is due to the low solubility of Fe in the matrix of the alloy: increasing Fe will only increase the number, size and shape of the grains of precipitates thus showing certain aberrant forms of phase δ and concentrations in critical points, thus weakening the material.

The inventors then considered using Cu-Zn binary brass, the phase diagrams of which have similarities with those of the Cu-Al binary alloys, as their working base. In both cases, for certain concentration ranges, there is a duplex solidification a + 13 with a hard centered cubic phase and a soft centered cubic phase, the phase 13 being furthermore transformed, during cooling, into a 3 'phase. corresponding to a cubic centric ordered structure allowing it to reach a great hardness. This is an interesting finding in the perspective of the conservation of properties related to the presence of phase 13 and its hard derivative phases on the molded finished product. One notion has been the basis of the search for the optimization of the contents of the various elements: the notion of fictitious title copper Tf, and its corollary, the concept of zinc equivalent. The empirical notion of fictitious title Tf applies to brasses, and derives from the notion of copper T titer which is equal to the% of Cu in the alloy: for example, the copper T titer of SUPER ACIOR® is thus of the order of 88%. This notion of Tf makes it possible to relate to the composition of a given complex brass alloy, that of a simple binary brass of equivalent crystalline structure easily described by means of the equilibrium diagram Cu-Zn.

The fictitious title Tf is calculated according to the formula:% Cu where i denote the various alloying elements, Ki denotes a typical coefficient of the element i and% i the content in the alloy of the element i. For the coefficients Ki, the following values are taken for the various elements that can be envisaged: 1 for Zn, 10 for Si, 6 for Al, 2 for Sn, 2 for Mg, 1 for Pb and all the non-miscible elements with Cu which could be present as impurities, 0.9 for Fe, 0.5 for Mn, -1.3 for Ni. This means that, for example, the effects on Tf of the addition of 1% Al is similar to that of the 6% Zn addition, that the 1% effect of Mn is similar to that of the addition of 0.5% Zn, that the effect of 1% Ni has a diluting effect similar to that of 1.3% Zn, and that Pb has an effect similar to that of a neutral body Tf -% Cu +1% i dispersed in the alloy, thus lowering the overall copper content, but having no particular action on the existing phases in a Cu-Zn binary alloy. The inventors concluded that in an aluminum bronze, Al could be partially substituted by Zn and Mn to transform the aluminum bronze into a manganese brass having a phase distribution a + 13 similar to that which is observable in a binary aluminum cast in shell and used as a basis for reasoning. Knowing that a title of the order of 53 to 63% would, according to the shell mold experiment, to obtain a duplex brass of the type desired for the invention, one could, for example, replace 8% d Al by 11% Mn and 24% Zn, which would maintain a duplex structure a + 13 resembling the initial structure. That said, we can not rely entirely on this calculation to predict what the exact properties of the alloy will be. Indeed, the phase diagrams are established for structures in thermodynamic equilibrium which do not necessarily correspond to those obtained, during the molding, to the cooling conditions of an industrial part. It is therefore essential to compare these theoretical results with experimental results from industrial practice. Experimental results on Cu-Zn-Mn-Al alloys actually show that if one has a high Tf (of the order of 70% and more), one finds oneself with a pure structure as in low alloyed brass and if we have an average Tf of 50% or even more, we end up with a pure type 13 structure that will not be very deformable and will not meet the requirements on the mechanical properties that we asked.

It is therefore for Tf situated between these two values that we are, a priori, likely to find the desired acicular mixed structure, called "duplex structure" characteristic of brasses a + 13. In alloys of the type according to the invention, the prediction of the structures obtained for a given composition is, however, made even more difficult by the fact that these alloys comprise a multiplicity of components of which, for some, the influences combine in a complex manner. In particular, because we rely on the very simple model of Tf calculation, we can not certainly predict the possible formation of non-usual phases in simple brasses, ie non-a, 13, y, δ phases. , n or c containing Fe, Mn, Al, Ni, Si. The experimental results obtained under actual conditions of lodging are therefore decisive for the determination of the compositions likely to satisfy the needs expressed above. In any case, it is estimated, by default, that for the alloys according to the invention, aiming at a Tf of 55 to 64% will give the desired results in a preferential manner. The experiment actually shows that Fe, added to a Cu-Zn alloy for, inter alia, reducing its Tf at low cost and, therefore, decreasing the proportion of phase a, tends to form intermetallic compounds with Al, Mn, Ni and If. These precipitates increase the rigidity of the material, but they can make the alloy more difficult to machine. They can also be sources of fragility and breakthroughs. Another advantage, according to the inventors, of the transition from an aluminum bronze to a manganese brass is that the high levels of Zn and Mn make it possible to lower the liquidus temperature very easily. This thus makes it possible to reduce the temperature at which the liquid alloy must be maintained before casting, and thus to save energy. A finding was also that a too high content of Mn was likely to cause an increase in the formation of dirt during the implementation of the alloy, and also lead to a melting where the liquid alloy to obtain is difficult to homogenize, because of viscosity problems, or even after solidification to reveal a partial demixing of the matrix creating a new phase, or even, in the presence of iron, to facilitate the enlargement of the already existing precipitate. It was therefore necessary to carry out multiple experiments to find an alloy that could, without major disadvantages and with notable advantages, replace SUPER ACIOR®. As will be seen, alloys which, a priori, could have been suitable from the point of view of their chemical composition, proved to be insufficient on major points and could not be retained. For example, attempts have been made to cast metal gravity shells made of NiCr15Fe alloy (2.4816) in accordance with the NF EN 10095: 1999 standard with a conductive black pelletization and a pouring temperature oscillating between 1010 ° C. and 1050 ° C., alloy 1, composition Zn = 21.12%, Mn = 10.87%, Al = 3.95%, Fe = 1.28%, Ni = 0.262%, Si = 0.128%, Cu = 62.4%. The samples obtained are pieces having thicknesses ranging from a few mm up to 10 mm for a length approaching 140 mm but also cylindrical traction bars obtained in accordance with ISO 6892-1: 2009 and thickness 10 mm. This composition provides a Tf of 54.4%, which would therefore, a priori, likely to lead to a satisfactory product, or at least close to being (by excess of alloying elements). In fact, tensile specimens were obtained with Young's modulus E of 109,824 MPa, Rp0.2 of 545 MPa, Rm of 752 MPa and A of 3.7%. This alloy has, in fact, a purely p structure with precipitates, which makes it too fragile and rigid. In addition, the tracing of its stress-strain curve during a tensile test reveals that it is strongly subject to the Portevin-Le Chatellier (PLC) phenomenon, that is to say to obtaining a stress-deformation curve "serrated" during plastic deformation, likely to result in localized hardening during machining with lower plasticity and ductility. The consequences are a certain fragility when subjected to certain deformation forces at low temperature but also a tendency to have an irregular surface appearance after machining. A similar piece was then cast, under conditions similar to those mentioned above, an alloy B derived from alloy 1, but with a slightly smaller amount of alloying elements to obtain undoubtedly a mixed structure a + 13. composition of alloy B was Zn = 16.5%, Mn = 11.5%, Al = 2.8%, Fe = 1.2%, Ni = 0.3%, Cu = 67.6%, providing a Tf of 62.6%. It has been found that this modification of the composition actually makes it possible to ensure the obtaining of a duplex structure in the casting conditions which will be described. A preferred variant in the material loading order, known in the foundry of cuprous alloys, is to successively place part of the cuprous raw materials, the alloying elements Mn, Fe, Al and Ni, in an induction furnace. the rest of the cuprous material, so without initially adding the Zn. This way of doing so makes it possible to limit the fire losses on said alloying elements. Once the filler is melted, ingots or pieces of the target alloy are added, so as to bring the bath temperature back to about 1100 ° C, and that is when Zn is added. The liquid metal is then stirred under the effect of the magnetic field while maintaining the temperature of the bath, then the liquid metal is transferred to a pocket and then to the temperature maintenance oven, which is comprised of 2 9822 80 10 preferably between 1010 and 1050 ° C. If necessary, the contents of the alloying elements are corrected in the holding furnace, in particular the contents of Zn and Al which may have varied during the previous operations. Then the molding of the pieces is done. The mussels are prepared in the usual way; in particular they undergo a poteyage, that is to say the projection of a graphite preparation on their walls to protect and lubricate, and facilitate demolding. The alloy is poured into the preheated molds at a temperature substantially lower than that which is usual for the SUPER ACIOR® because of the difference between their liquidus temperatures (1075 ° C for the SUPER ACIOR®, against approximately 910 ° C for the new alloy according to the invention), which confirms the possibility of significant energy savings provided by the invention. The pouring temperature is generally about 100 ° C. higher than that of the alloy liquidus, but its exact value must also take into account the preheating temperature of the various parts of the mold, for its precise adjustment to the obtaining a good internal health for the room. The operation does not pose particular problems that would be unusual for the skilled person. Samples, identical to those previously mentioned, were thus made in a SUPER ACIOR® type alloy A for use as a reference, then in three alloys B, C and D likely to be of interest in the context of the invention. 'invention. Table 1 summarizes the compositions of the alloys A, B, C and D. Alloy Al Fe Ni Mn Cu Zn Si Sn Tf 0/0 0/0 0/0 0/0/0/0/0/0/0/0 0/0 A 10.0 2.8 <0.5 <0.5 Rest <0.10 <0.10 <0.01 Not applicable B 2.8 1.2 0.27 11.5 67.6 16.5 0.06 0.01 62.6 C 2, 6 1.6 0.25 12.8 64.1 18.6 0.07 0.01 60.2 D 3.3 1.6 0.23 12.6 63.0 19.1 0.08 0.01 57.3 Table 1: Compositions of the Alloys Tested Table 2 summarizes the mechanical properties measured on these alloys. E is the Young's modulus, Rp0.2 is the yield strength, T is the elongation at break, and T is a magnitude giving an insight into the intrinsic toughness of the material 2 9822 80 11 by calculating the integral located under a relative stress-displacement tensile curve. Alloy E Rp0.2 Rm T MPa MPa MPa% kJ / m 3 A 97 920 264 713 48.5 338 068 B 96 170 271 547 41.4 224 739 C 110 790 319 605 27.3 175 259 D 120 004 448 Table 2: Mechanical Properties of the Alloys Tested. Sample A is SUPER ACIOR®, and serves as a reference point for the other alloys obtained. Sample B exhibits very good ductility, although it is slightly lower than that obtained for reference A. Its micrographic analysis shows that it contains 76.5% of phase a: it is therefore clearly a duplex alloy. It also shows the presence of Fe-based fine precipitates. The metallurgical structure is consistent with that targeted. With respect to the sample A, the tensile strength is lower, but the yield strength is substantially the same as the Young's modulus. Toughness is less, while remaining correct. These mechanical properties could be improved if the percentage of phase a could be reduced, but this sample can very well substitute for SUPER ACIOR® A, while containing 20% less Cu. The main object of the invention, namely to reduce the Cu content of the alloy used without unacceptably degrading its important mechanical properties for SUPER ACIOR® applications is therefore reasonably achieved. Sample C has, relative to sample A, higher Zn, Mn, and Fe contents, which is normally such as to decrease the proportion of phase a. This is confirmed by the micrographic analysis which indicates the presence of 70.6% of phase a, as well as the presence of fine precipitates based on Fe. FIG. 1 shows a typical micrograph of this material after a chemical attack by a solution. nitric acid / ethanol with 3% acid. Compared to samples A and B, the Young's modulus is higher, the tensile strength is at an intermediate level between those of A and B, the elastic limit is higher. The elongation at break is substantially lower, while still remaining acceptable in the preferred applications of the invention. Tenacity is also diminished, while remaining acceptable in these same contexts. Overall, the material remains of sufficient ductility, and it is an important parameter to consider for the manufacture of gearbox forks for example. Indeed, these parts may, after demolding, have a deformation with respect to the desired shape, and require to be straightened by bending. This straightening must be done in the plastic field to be permanent, and it is therefore preferable that this area is relatively large to avoid certainly breakage of the part during straightening. A ductile material is more tolerant to small defects inherent in the foundry that could lead to such a break. The structure of the sample C is indeed a duplex structure, with a percentage of phase a of 70.6% and the presence of fine precipitates based on Fe. The structure is thus in conformity with that sought.

Sample D, for which the Al content has been increased relative to B and C, has a significantly lower Tf, but the structure is still duplex with a percentage of a phase of 49.9% and the presence of ends. However, its mechanical properties are unsatisfactory for all SUPER ACIOR® applications. In particular the elongation is too low and the elastic limit too high for the straightening of a molded part is easy, and the toughness is low. All this is related to an insufficient presence of phase a in the structure because of a weak Tf. This shows that the actual obtaining of a given structure is very dependent on the fine adjustment of the composition of the metal. This confirms what was found above on samples 1 and B.

However, this alloy can be used for applications that never require straightening of the parts obtained. Samples B and C are therefore to be regarded as part of the preferred variant of the present invention, the sample D belonging to a variant of the invention may be intended for applications for which an elastic limit and a tensile strength high prices are to be preferred. These are, for example, applications where the part is not supposed to deform plastically or very little and having a relatively high hardness. Examples of such applications include clamping type lug type and other tools requiring high values in elastic limit and rupture but not very stressed in fatigue. In general, it is considered that a Tf of at least 58%, corresponding to a phase percentage of at least about 57%, is necessary for a part to be straightened because it would have a lower yield point. or about 400 MPa. The correlation between Tf and the presence of phase a is confirmed when one wants to obtain a percentage of phase a of the order of 45 to 82%, but, as we have said, it is necessary nevertheless to confirm by experiments that a Tf a priori correct will provide the desired structure under the actual conditions of solidification. It has been seen, particularly in sample 1, that small compositional differences in solidification conditions far from those used to draw phase diagrams can result in significant deviations from predictions. In summary, the composition of the alloy according to the invention is as follows, in percentages by weight: - Cu = 60 to 70%; Zn = 15 to 24%; Mn = 8 to 15%; - Al = 0.50 to 4%; Fe - 0.5 to 4%; - Ni = 1% traces; - Si = 1% traces; Sn = 1% traces; - Pb = 2% traces; The remainder being impurities resulting from the preparation, the total contents of said impurities not exceeding 1%. One of the advantages of the invention is to provide a simplification of the book of shades proposed by the foundry that makes the parts. Indeed, it is possible, from the same basic shade, to obtain a multiplicity of sub-shades, each having substantially different mechanical properties and adapted to a wide range of applications, simply by playing on the adjustment of the Zn or Al contents. The solidification conditions making it possible to obtain the desired structure are, of course, variable according to various parameters, among which are the temperature difference between the liquid metal and the walls of the mold, the geometries of the part and its mold which govern the rates of solidification and cooling of the metal, therefore the type of structure obtained, in the various regions of the part, the residence time of the part in the mold up to the opening of the mold which triggers the cooling of the room by the ambient air ... A detailed example of the implementation of the process for producing a piece of dimensions that will be specified. As a first approximation, it can be estimated that a temperature difference of 750 to 880 ° C., for example 850 ° C., between the liquid metal during pouring and the coldest points of the walls of the mold is suitable for this purpose. for a preheating temperature of the mold ranging from 170 to 260 ° C according to the parts of the mold. Shell solidification under the conditions that have been said is fast enough that a small piece is already hard solidified at its ejection of the mold. The residence time of the piece in the mold must not be too high, so as to avoid sticking it against the walls of the mold. A machining of the solidified and cooled product can then be performed to give it its final shape, size and surface condition if necessary for its use.

By way of nonlimiting example, an example of the use of this new type of alloy and in particular the use of a shade similar to the preceding grade C in chemical composition, namely: Zn = 19, is described below. , 6%, Mn = 12.4%, Al = 2.1%, Fe = 1.3%, Ni = 0.3%, Cu = 64.3% for obtaining gravity by metal shells from ranges of automobile gearbox. This range is shown, in its final machined state, by the isometric views of FIGS. 2 and 3. This range of gearboxes, of known shape in itself, is constituted, as can be seen in FIGS. 2 and 3, a substantially cylindrical barrel 1 and in which is arranged a through hole 2 on either side of said barrel 1. This through hole 2 is formed directly during the molding of the fork, using a spindle shape forming part of the mold of the metal shell type used to manufacture said fork. Advantageously, the pin is of slightly conical shape to facilitate demolding of the piece after solidification. A butt 3 extends the barrel 1 and extends axially on one of the sides of the barrel 1. This butt 3 has a distal zone 7 roughed during molding, and which, after machining, of the demolded fork, makes it possible to obtain a notch made by milling and intended to transmit a translational movement to the fork. On the barrel 1 extend radially two arms 4, 5 also connected to a web 6 connected to the barrel 1 near its end opposite to which is connected the stick 3. The arms 4, 5 each end with a 8, 9 pads are intended to cooperate by sliding contact with the groove of a player of gearbox, not shown in the figure. The casting zone 10 of the casting not shown in FIG. 1, of rectangular shape, is situated at the base of one of the arms 4 on the width of the barrel 1. The barrel 1 of the piece is about 62 mm long. for about 24 mm wide, and the opening hole 2 slightly conical is of a width of the order of 19 mm. The butt 3, after a recess on a base of 20x20 mm, has a length of the order of 85 mm for a height of 12 mm and an average thickness of 7 mm. The roughing area of the notch 7 is located 11 mm from the end of the butt 3 and has an average width of 7 mm for 7 mm deep. Starting from the shaft 1, the arms 4, 5 are each 60 mm long and 11 mm wide, their thickness being of the order of 13 mm on their outer parts and 5 mm on their inner parts. The previously mentioned fabric 6 is also 5 mm thick. The pads 8, 9 have an average width of 12 mm for a length of 17 mm and an average thickness of 6 mm. The pads 8, 9 are spaced about 76 mm. The casting attack 10 is 10x6 mm in size and is located 16 mm from the base of the barrel 1. To do this, an impression of the piece is made in a mold of external dimensions 330x220 mm, separating into two clevises. 140 mm thick. These screeds are nickel-alloy NiCr15Fe (2.4816) in accordance with the NF EN 19095: 1999 standard and they are covered with a conductive black coating based on graphite, called poteyage, while being maintained at a temperature of the order of 200 ° C.

The elaboration of the alloy in the liquid state is carried out according to the procedure previously described. The Zn content of the liquid alloy under development is occasionally corrected when this content, regularly controlled, is found to be excessively lowered, so as to always remain close to the 19.6% content targeted. The liquid alloy is poured into the mold, preferably at a temperature between 1010 and 1050 ° C, the time of solidification and residence in the mold being 12s until opening of the latter. 18 seconds after the opening of the mold, the piece thus obtained is ejected from the screed that contains it.

The room is then allowed to cool to ambient air until its temperature becomes that of the environment. After observation of the pieces thus obtained, note the remarkable absence of significant porosities in the heart of the room after radioscopic analysis.

Similarly, few defects have been observed on the external appearance of the room. During the first tests, blowholes were sometimes observed inside the barrel 1 on the side of the birth of the arms 4, 5 and the casting attack 10. But this was very quickly determined as being linked to a training of air due to a too turbulent lodging. A more moderate pouring speed made it possible to eliminate this possible defect. Similarly, phenomena of "pumping" appeared, still inside the barrel 1, close to the massive areas such as the butt 3 or the arms 4, 5, but they were visibly linked to an alloy pour and a mold too hot (1075 ° C for the alloy, 280 ° C for the mold). These problems have been solved by lowering the pouring temperature and the mold temperature. Finally, cut-off lines on the barrel 1 on the side opposite to the butt 3 appeared punctually, and this as soon as the pouring temperature and / or the mold temperature were too low (less than 1010 ° C. for the temperature pouring and below 170C for mold temperature). The adaptation of these temperatures made it possible to eliminate this defect.

The remedies for the classic defects that have been noted on the first castings do not go beyond what is customary to achieve by the skilled person to optimize the casting conditions of a casting shell shell . The figures which have been given on the alloy and mold temperatures and the durations of the various operations are valid in the case of the detailed example described, but will, of course, have to be adapted by means of reflections and tests. routine molding parts other than those described and / or made of an alloy according to the invention of different precise composition.

Claims (1)

CLAIMS1.- Copper and zinc alloy, characterized in that its composition is, in percentages by weight: - Cu = 60 to 70%; Zn = 15 to 24%; Mn = 8 to 15%; - Al = 0.50 to 4%; - Fe = 0.5 to 4%; - Ni = 1% traces; - Si = 1% traces; Sn = 1% traces; - Pb = 2% traces; the remainder being copper and impurities resulting from the preparation, the total contents of said impurities not exceeding 1%. 2. An alloy according to claim 1, characterized in that its fictitious title in% C copper Tf - is between 55 and 64%, where i denote the different% Cu + EK.% I alloy elements, Ki designates a coefficient typical of the element i and% i the content in the alloy of the element i, K being equal to 1 for Zn, 10 for Si, 6 for Al, 2 for Sn, 2 for Mg, 1 for Pb and all the immiscible elements in the Cu matrix, 0.9 for Fe, 0.5 for Mn, -1.3 for Ni. 3. A metallurgical product molded from copper and zinc alloy, characterized in that its composition is in accordance with one of claims 1 to 2, and in that its percentage of phase a is between 50 and 90%, preferably between 53 and 87%. 4.- metallurgical product according to claim 3, characterized in that it is a gearbox fork. 5. A process for the production of a metallurgical product molded from a copper-zinc alloy, characterized in that: an alloy, the composition of which is in accordance with one of claims 1, is prepared in the liquid state, or 2 and casting said alloy in a mold to form said product, under conditions of solidification and cooling ensuring the presence on the solidified and cooled product of a phase proportion a of 50 to 90%, preferably at 87%. 6. A process according to claim 5, characterized in that the different parts of said mold are preheated to a temperature between 200 and 300 ° C, and in that the alloy is cast in the mold at a temperature greater than 750 to 880 ° C at the temperature of the coldest points of the walls of the mold. 7.- Method according to claim 5 or 6, characterized in that then proceeds to a machining said solidified and cooled product.