Casting ring for obtaining a product made of titanium alloy or a titanium-aluminium intermetallic compound and method using same

Abstract

The present invention relates to a method for obtaining a product made of titanium alloy or a titanium-aluminium intermetallic compound by plasma torch melting, the alloy having an oriented structure, the method comprising heating the molten alloy (1) in a casting ring (2) by means of a plasma torch (3); cooling a cold zone (21) of the casting ring over a length L1, the cooling forming a semi-solid crown (12) of alloy; heating a hot zone (22) of the casting ring over a length L2, thereby forming a solidification front (13), the flatness of which relative to a plane perpendicular to a drawing direction is less than 10°; and drawing the solidified alloy (14) at a speed of more than 10-4 m / s in the drawing direction. The present invention also relates to a facility for implementing said method.

Description

technical field

[0001] The present invention relates to the field of alloy production, in particular aeronautical alloys such as titanium-based alloys or TiAl intermetallics, in particular molding rings used to obtain ingots and processes using such molding rings. Previous technique

[0002] The production of alloys, particularly by ingot casting, mainly consists of heating a raw material in a crucible to melt it and pouring it into a molding ring which will give the ingot its shape.

[0003] Molding rings are generally made partly of copper and can be water-cooled. Copper is used because of its high thermal conductivity, which allows for good heat exchange, and also because of its good ductility, which facilitates its use by limiting the risk of breakage of this critical component. Therefore, copper is particularly suitable for manufacturing the areas of the molding ring that need to be cooled, known as cold zones.

[0004] With regard to the areas which need to be heated, called hot zones, foundry ceramics, such as alumina, yttria, zirconia or their derivatives and composites, are generally the most suitable for the manufacture of alloys.

[0005] Unfortunately, these materials present drawbacks for the manufacture of titanium-based alloys or TiAl intermetallic alloys. In fact, these alloys, in their molten state, react strongly with casting ceramics, leading to erosion of the mold ring and the incorporation into the alloy of solid ceramic inclusions torn from the mold ring wall. Even worse, since casting ceramics are oxides, the oxygen they contain contaminates the alloys and weakens them.

[0006] Furthermore, since foundry ceramics are not thermally conductive, the use of an external electrical resistance is necessary. If induction heating is desired, an additional susceptor surrounding the molding ring is required to prevent direct coupling with the alloy being solidified within the molding ring. Indeed, such coupling generates circulating vortices of the molten alloy, thus destabilizing the solidification front.

[0007] Refractory metals are occasionally used to create hot zones. However, the risk of chemical interactions between these metals and titanium-based or TiAl intermetallic alloys is high. In particular, low-melting-point eutectics can form and lead to the development of critical defects in these alloys.

[0008] Recently, aluminum nitride has been used in the manufacture of foundry crucibles, which has shown promise. However, this material is expensive.

[0009] Document US2006070716 discloses a molding ring for continuous casting of a copper-based alloy, the ring being made of an alloy in MAX phase. Summary

[0010] This disclosure improves the situation.

[0011] To this end, the present invention proposes a molding ring for molding a titanium-based or TiAl intermetallic alloy ingot, formed of a tube with a first and a second end and comprising: a first section in copper, extending from the first end, in particular over a length L1 between 0.065 and 0.09 m; a second section in a material in an alloy in MAX phase and extending from the first section, in particular over a length L2 between 0.17 and 0.3 m; in which the MAX phase is chosen from: Number 4 Al 1 C 3 , Nb 2 AlC, Ti 2 AlC and Ti 2 AlN.

[0012] Thanks to the use of such a molding ring, there is no risk of contamination of the manufactured alloy because the elements composing the molding ring material are elements generally present in titanium-based and TiAl intermetallic alloys. Therefore, there is no risk of the alloy being weakened by the inclusion of foreign elements, such as oxygen from foundry ceramics. Furthermore, such a molding ring exhibits good resistance to thermal shock and low thermal expansion.

[0013] Other optional and non-limiting features are described below.

[0014] The internal surface of the tube at the second section can be covered with one or more layers, each of the layers being in a material chosen from: Nb 4 Al 1 C 3 , Nb 2 AlC, Ti 2 AlC, Ti 2 AlN and AlN.

[0015] When the material is Nb 4 Al 1 C 3, the internal surface of the tube at the second section can be coated, from the outside in: of a single layer in Nb 2 AlC; of a first layer in Nb 2 AlC and a second layer in Ti 2 AlC; of a first layer in Nb 2 AlC, a second layer in Ti 2 AlC and a third layer in AlN; or of a first layer in Nb 2 AlC, a second layer in Ti 2 AlC, a third layer in Ti 2 AlN and a fourth layer in AlN.

[0016] When the material is Nb2AlC, the internal surface of the tube at the second section can be coated, from the outside in: of a single layer in Ti 2 AlC; of a first layer in Ti 2 AlC and a second layer in AlN; or of a first layer in Ti 2 AlC, a second layer in Ti 2 AlN and a third layer in AlN.

[0017] When the material is Ti 2 AlC, the internal surface of the tube at the second section can be coated, from the outside in: of a single layer in AlN; or of a first layer in Ti 2 AlN and a second layer in AlN.

[0018] The first section and the second section can be connected to each other by a junction made by mechanical assembly or welding.

[0019] The molding ring may further include a third section extending from the second section to the second end, in particular over a length of at least 0.03 m, and made of a thermally conductive material.

[0020] The molding ring may further include a collar extending from the first end perpendicularly to the extension of the first section and outwards.

[0021] In another aspect, the present invention relates to a method of obtaining a titanium alloy or TiAl intermetallic product by plasma torch melting, the alloy having a directed structure.

[0022] The process includes: the selection of a molding ring as described above and whose length L1 is between 0.065 and 0.09 m and the length L2 between 0.17 and 0.3 m, and whose thickness e1 and e2 of the first and second sections is chosen according to the inequalities Math.1 and Math.2 below, where R is the inner radius of the molding ring, ΔT1 is the maximum desired thermal gradient in the first section, ΔT2 is the maximum desired thermal gradient in the second section, A1 is equal to 9 °Cm and A2 to 60 °C.m, L1 min is equal to 0.065 m, L1 max to 0.09 m, L2 min to 0.17 m, and L2 max to 0.3 m; heating of the surface of the molten alloy at the molding ring; cooling of the first section of the molding ring forming a cold zone, the cooling forming a semi-solid crown of alloy; heating of the second section of the molding ring forming a hot zone thus generating a solidification front of the alloy in this hot zone and whose flatness with respect to a plane perpendicular to a drawing direction is less than 10°; and drawing of the solidified alloy at a speed greater than 10 -4 < m / s along a drawing direction. . RexpL1minΔT1A1−1≤e1≤RexpL1maxΔT1A1−1 RexpL2minΔT2A2−1≤e2≤RexpL2maxΔT2A2−1 Brief description of the drawings

[0023] Other features, details, and advantages will become apparent upon reading the detailed description below and analyzing the attached drawings, on which: Fig. 1 [ Fig. 1 ] shows a diagram illustrating the cold crucible plasma torch melting process using the molding ring according to the invention. Fig. 2 [ Fig. 2 ] illustrates a molding ring according to the invention with a cold zone and a hot zone. Fig. 3 [ Fig. 3 ] illustrates a molding ring according to the invention with a cold zone, a hot zone and a second cold zone. Fig. 4 [ Fig. 4 ] shows the angle α formed by the solidification front with respect to a plane perpendicular to the direction of pull as a function of the length of the cold zone L1 and the length of the hot zone L2 at a pull speed of 0.00015 m / s. Fig. 5 [ Fig. 5] shows the angle α formed by the solidification front with respect to a plane perpendicular to the direction of pull as a function of the length of the cold zone L1 and the length of the hot zone L2 at a pull speed of 0.0003 m / s. Fig. 6 [ Fig. 6 ] shows the angle α formed by the solidification front with respect to a plane perpendicular to the direction of pull as a function of the length of the cold zone L1 and the length of the hot zone L2 at a pull speed of 0.00045 m / s. Fig. 7 [ Fig. 7 ] shows the angle α formed by the solidification front with respect to a plane perpendicular to the direction of pull as a function of the length of the hot zone L2 and the length of the cold zone L3 at a pull speed of 0.0003 m / s, for a cold zone length L1 of approximately 0.077 m.

[0024] In the figures 4 to 7Above, the lines are isopleth lines joining points of the same angular value. The solid line indicates the boundary between the region where the angle α is greater than 10° and the region where it is less than 10°. The darker the pattern, the larger the angle. Exposed

[0025] A molding ring according to the present invention is described below with reference to the figure 2 and to the figure 3 . Such a molding ring 1 is particularly suitable for molding a titanium-based or TiAl intermetallic alloy ingot, formed from a tube with a first end 11 and a second end 12.

[0026] The molding ring 1 comprises a first section 13 and a second section 14 of tubing. The first section 13 is made of copper and extends from the first end 11, in particular over a length L1 between 0.065 and 0.09 m. The second section 14 is made of a MAX phase alloy and extends from the first section 13, in particular over a length L2 between 0.17 and 0.3 m; the MAX phase being selected from: Nb₄Al₁C₃, Nb₂AlC, Ti₂AlC, and Ti₂AlN. These MAX phases are the most compatible with the compositions of titanium-based alloys and TiAl intermetallic alloys. Indeed, in addition to titanium and aluminum, such alloys include other elements, the most commonly used being zirconium, molybdenum, niobium, chromium, tungsten, vanadium, carbon, and boron. Thus, the only MAX phases considered all have aluminum at the A site.Moreover, these selected MAX phases are compatible with a clean temperature of the melting temperatures of titanium-based alloys and TiAl intermetallic alloys which are close to 1500°C.

[0027] The molding ring 1 may further include a third section 15 extending from the second section 14 to the second end 12, in particular over a length L3 of at least 0.03 m, and made of a thermally conductive material.

[0028] The lengths L1, L2, and L3 were determined by simulation, notably to obtain a solidification front perpendicular to the pulling direction, i.e., to the longitudinal axis of the molding ring 1. The results of these simulations are shown in the... figures 3 to 6These figures show the impact of the choice of lengths L1 and L2 on the flatness of the solidification front at different drawing speeds, respectively 0.00015 m / s, 0.0003 m / s, and 0.00045 m / s. The flatter the solidification front, the clearer the corresponding region. It can be seen that the higher the drawing speed, the smaller the region corresponding to a solidification front forming an angle of less than 10° with a plane perpendicular to the drawing direction. The angle is measured at the inner surface of the mold ring in a plane that includes the longitudinal axis of the drawn ingot, collinear with the drawing direction; this angle is that between a straight line resulting from the intersection of the plane in question and the plane perpendicular to the drawing axis, and a straight line tangent to the curve resulting from the intersection of the plane in question and the solidification front measured at the inner surface of the mold ring.The length ranges were defined to achieve a good compromise between the flatness of the solidification front and the range of drawing speeds over which the process is applicable. When lengths L1 and L2 are located within the aforementioned ranges, the angle is less than 10° for a wide range of drawing speeds.

[0029] The first section 13 This is a cold zone and serves, in particular, as a heat exchange surface between the alloy poured into the molding ring and a heat transfer fluid circuit, allowing the alloy temperature to be maintained at approximately 25°C at this point. Copper exhibits high thermal conductivity while also being ductile.

[0030] The second section 14 is a hot zone, that is to say a zone which is heated to remelt the alloy at this level, thus allowing to obtain a solidification front that is as flat as possible, in particular with an angle of less than 10°.

[0031] For alloys containing niobium and aluminum, the Nb₄AlC₃ and Nb₂AlC phases can be used alone. In other cases, it is preferable for the inner surface of the tube at the second section to be coated with one or more layers, each layer being made of a material chosen from: Nb₄AlC₃, Nb₂AlC, Ti₂AlC, Ti₂AlN, and AlN.

[0032] For example, when the material is Nb 4 Al 1 C 3, the internal surface of the tube at the second section 14 is coated, from the outside in: of a single layer in Nb 2 AlC; of a first layer in Nb 2 AlC and a second layer in Ti 2 AlC; of a first layer in Nb 2 AlC, a second layer in Ti 2 AlC and a third layer in AlN; or of a first layer in Nb 2 AlC, a second layer in Ti 2 AlC, a third layer in Ti 2 AlN and a fourth layer in AlN.

[0033] Another example, when the material is Nb 2 AlC, the internal surface of the tube at the second section 14 is coated, from the outside in: of a single layer in Ti 2 AlC; of a first layer in Ti 2 AlC and a second layer in AlN; or of a first layer in Ti 2 AlC, a second layer in Ti 2 AlN and a third layer in AlN.

[0034] As yet another example, when the material is Ti 2 AlC, the internal surface of the tube at the level of the second section 14 is coated, from the outside in: of a single layer in AlN; or of a first layer in Ti 2 AlN and a second layer in AlN.

[0035] The layer orders presented above are important. Indeed, they prevent the formation of secondary phases at the interfaces between the different layers; the existence of a continuous solid solution is essential between these phases.

[0036] Configurations featuring AlN in the innermost layer are particularly suited to drawing aluminum-free alloys with melting temperatures above 1600°C.

[0037] The layers preferably have a thickness between 50 µm and 1000 µm. For example: 60 µm, 70 µm, 80 µm, 90 µm, 100 µm, 250 µm, 500 µm, or 750 µm.

[0038] Furthermore, the choice of materials mentioned above also has the advantage of facilitating the manufacture of the molding ring. Indeed, all these materials are now available in powder form. Thus, to produce the molding ring, the various powders chosen can be densified or deposited as layers. The temperatures required to densify these different materials are relatively similar, between 1400 and 1700°C, which allows, in particular, for their co-sintering. In all cases, the following method can be implemented: the different materials are positioned concentrically in a mold that allows for high-temperature sintering of the powders. If thin layers are required (i.e., less than 250 µm), the cold spray process can be used. (cold spray(in English) can be used to create the necessary layers on the inner surface of the molding ring. For cases involving AlN, and if a thin thickness (i.e., less than 250 µm) is required, the high-power pulsed magnetron sputtering (HiPIMS) process can be implemented on the inner face of the molding ring.

[0039] For co-sintering, the flash sintering process (sparkplasma sintering (in English) can be used, for example, by applying the following densification cycle: Maximum sintering temperature: 1500-1600°C; holding time: 10-30 min; applied pressure: 30-100 MPa; atmosphere: vacuum.

[0040] An additional layer, not in contact with the molten alloy, can be added to the mold ring, for example, on the outer surface, but generally at any level, with the sole limitation that it must not be in contact with the molten alloy. This additional layer is made of a ferromagnetic material, particularly a ferromagnetic alloy. This additional layer promotes magnetic coupling with the mold ring. Examples of materials for such a layer are: pure iron, FeCo or FeSi alloys, etc. The additional layer preferably has a thickness of at least 250 µm, for example, 300 µm, 350 µm, 400 µm, 450 µm, or 500 µm. This additional layer can be obtained by thermal spraying or cold spraying. (cold spray in English).

[0041] The first section 13 and the second section 14 can be connected to each other by a junction 17achieved by mechanical assembly or welding. The joint 17 is preferably located within the cold zone of the molding ring. Indeed, this avoids limiting assembly techniques and also allows the ductility of copper to be taken advantage of, thus limiting bending stresses in the layer stacks during the MAX phase.

[0042] The third section 15, when it is planned is a cold zone for the cooling of the alloy.

[0043] The second end 12 The molding ring may have a chamfer to facilitate its insertion into the installation for producing alloy ingots by drawing. When the third section 15 is planned, the chamfer can be made in the third section 15, in particular so as to fully occupy the third section 15.

[0044] The molding ring 1 may also include a collar16 extending from the first end 11 perpendicular to the extension of the first section 13 and outwards. The collar 16 It is preferably circular, but not necessarily. It can be square, rectangular or triangular, optionally with rounded corners.

[0045] The light inside the forming ring gives the alloy ingot its shape. Since the ingot must be able to be pulled from one end to the other, the inner wall of the forming ring is a mathematical cylinder, that is, a surface generated by parallel generatrices around a closed curve and extending between the first and second ends. 11, 12.Although preferably the closed curve is a circle (the drawn ingot is therefore a right cylinder with a circular base), the present invention is not limited to such a shape. In particular, the closed curve may be a square, a rectangle, or a triangle. The corners may also be rounded.

[0046] The wall thickness at the first section 13, of the second section 14 and the third section 15 is preferably chosen based on the maximum temperature gradient that the molding ring 1 must support between its internal surface in contact with the alloy and its external surface. In particular, the thicknesses are chosen according to Math. 1 and Math. 2 above.

[0047] In general, the thickness e1 of the first section L1is less than the thickness e2 of section L2. Thus, a shoulder is formed between the first and second sections. This shoulder is preferably greater than 90° and preferably corresponds to the junction of the materials of the two sections.

[0048] The molding ring 1 described above can be advantageously used in a process for obtaining a titanium alloy product or TiAl intermetallic by plasma torch melting to obtain an alloy with a directed structure.

[0049] The process is shown schematically on the figure 1 includes: the selection of a molding ring 1 as described above, with a length L1 between 0.065 and 0.09 m and a length L2 between 0.17 and 0.3 m, and with a thickness e1 and e2 of the first and second sections chosen: R exp L 1 min Δ T 1 A 1 − 1 ≤ e 1 ≤ R exp L 1 max Δ T 1 A 1 − 1 R exp L 2 min Δ T 2 A 2 − 1 ≤ e 2 ≤ R exp L 2 max Δ T 2 A 2 − 1 where R is the inner radius of the molding ring, ΔT1 is the maximum desired thermal gradient in the first section, ΔT2 is the maximum desired thermal gradient in the second section, A1 is equal to 9 °C and A2 to 60 °C, L1 min is equal to 0.065 m, L1 max to 0.09 m, L2 min to 0.17 m, and L2 max to 0.3 m: the heating of the surface of the molten alloy at the molding ring, notably by a plasma torch 3 ; the cooling of the first section 13 of the molding ring 1 forming a cold zone, particularly through cooling methods 4, Cooling forms a semi-solid alloy crown; heating, particularly by heating 5, of the second section 14 of the molding ring 1forming a hot zone thus generating a solidification front of the alloy in this hot zone and whose flatness with respect to a plane perpendicular to a pulling direction is less than 10°; and the pulling of the solidified alloy at a speed greater than 10 -4< m / s along a pulling direction.

[0050] The process may also include cooling the third section 15 of the molding ring forming a second cold zone, notably by means of a second cooling method 6.

[0051] Prior to the steps described above, the process may include the supply of raw materials MP (particularly in the form of offcuts, briquettes, bars, a sponge / mother alloy mixture, etc.), heating the raw material MP (for example, by plasma torch) 8, by electric arcs, by induction, by electron bombardment, etc.) melting the raw material MP into a crude molten alloy, the refining of the crude molten alloy (including, for example, stabilizing the alloy's temperature and removing impurities), and the casting 2 of the refined molten alloy in the ring mold 1. These steps are known from the prior art and do not constitute the core of the present invention.

Claims

1. A casting ring for molding a ingot made of a titanium-based alloy or a TiAl intermetallic alloy, made of a tube with first and second ends and comprising: - a first section made of copper, and extending from the first end; - a second section made of a MAX phase alloy material and extending from the first section; wherein the MAX phase is selected from among: Nb4Al1C3, Nb2AlC, Ti2AlC and Ti2AlN.

2. The casting ring according to claim 1, wherein the inner surface of the tube at the second section is covered with one or more layer(s), each of the layers being made of a material selected from among: Nb4Al1C3, Nb2AlC, Ti2AlC, Ti2AlN and AlN.

3. The casting ring according to claim 2, wherein: when the material is Nb4Al1C3, the inner surface of the tube at the second section is covered, from the outside inwards, with: - one single layer of Nb2AlC; - a first layer of Nb2AlC and a second layer of Ti2AlC; - a first layer of Nb2AlC, a second layer of Ti2AlC and a third layer of AlN; or - a first layer of Nb2AlC, a second layer of Ti2AlC, a third layer of Ti2AlN and a fourth layer of AlN; when the material is Nb2AlC, the inner surface of the tube at the second section is covered, from the outside inwards, with: - one single layer of Ti2AlC; - a first layer of Ti2AlC and a second layer of AlN; or - a first layer of Ti2AlC, a second layer of Ti2AlN and a third layer of AlN; when the material is Ti2AlC, the inner surface of the tube at the second section is covered, from the outside inwards, with: - one single layer of AlN; or - a first layer of Ti2AlN and a second layer of AlN.

4. The casting ring according to one of claims 1 to 3, wherein the tube further comprises an additional layer of a ferromagnetic material.

5. The casting ring according to one of claims 1 to 4, wherein the first section and the second section are connected to each other by a junction made by mechanical assembly or welding.

6. The casting ring according to one of claims 1 to 5, further comprising a third section extending from the second section up to the second end, in particular over a length of at least 0.03 m, and a heat-conductive material.

7. The casting ring according to one of claims 1 to 6, further comprising an annular flange extending from the first end perpendicular to the extension of the first section and outwards.

8. A method for obtaining a product made of a titanium alloy or a TiAl intermetallic alloy by plasma torch melting, the alloy having an oriented structure, the method comprising: - selecting a casting ring according to one of the claims hereinabove, wherein length L1 is comprised between 0.065 and 0.09 m and length L2 is comprised between 0.17 and 0.3 m, and the thickness el and e2 of the first and second sections is selected according to: R exp L 1 min Δ T 1 A 1 − 1 ≤ e 1 ≤ R exp L 1 max Δ T 1 A 1 − 1 R exp L 2 min Δ T 2 A 2 − 1 ≤ e 2 ≤ R exp L 2 max Δ T 2 A 2 − 1 where R is the inner radius of the casting ring, ΔT1 is the desired maximum thermal gradient in the first section, ΔT2 is the desired maximum thermal gradient in the second section, Al is equal to 9°C.m and A2 to 60°C.m, L1min is equal to 0.065 m, L1max to 0,09 m, L2min to 0.17 m, and L2max to 0.3 m: - heating the surface of the molten alloy at the casting ring; - cooling the first section of the casting ring thereby forming a cold area, cooling forming a semi-solid crown of alloy; - heating the second section of the casting ring thereby forming a hot area and thus generating an alloy solidification front in this hot area and the flatness of which, with respect to a plane perpendicular to a drawing direction, is less than 10°; and - drawing the solidified alloy at a speed higher than 10-4 m / s along a drawing direction.

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