Process for producing a metal or metal alloy and corresponding melting furnace

Abstract

Process for producing a metal or metal alloy and corresponding melting furnace The process includes the following steps: - melting of solid metal elements (7); - refining of the molten metal; - transfer of the molten metal into a molding ring (11) and drawing of an ingot (15); a height of a molten metal bath (23) in the molding ring (11) being automatically maintained within a predetermined range by automatically measuring the height of the molten metal bath (23), automatically determining a drawing speed of the ingot (15) and controlling the drawing of the ingot (15) using said drawing speed, the height of the molten metal bath (23) being measured by implementing the following operations: - (S10) periodic acquisition of images of the molding ring (11) showing an inner wall (20) of the molding ring (11) and the molten metal bath (23);- (S20) determination on the images of a position of a free surface (31) of said molten metal bath (23) along the inner wall (20); - (S30) evaluation of said position on a predetermined height scale. Figure for the abstract: 3;

Description

Title of the invention: Process for producing a metal or a metal alloy and corresponding melting furnace

[0001] The invention relates generally to the production of a metal or a metal alloy, preferably a titanium alloy or a nickel-based alloy, in a cold crucible furnace.

[0002] Such a melting furnace may include:

[0003] - a fusion zone;

[0004] - a device for introducing solid elements into the melting zone, the melting of solid elements in the melting zone producing molten metal;

[0005] - a refining zone intended for refining molten metal;

[0006] - a molding ring;

[0007] - a device for drawing an ingot of said alloy through a bottom of the molding ring.

[0008] To improve the skin quality of the ingot, it is desirable to maintain a level constant flow of molten metal in the molding ring.

[0009] It is also desirable that the speed of drawing the ingot be as constant as possible.

[0010] In addition, the flow rate of the molten metal from the refining zone into the molding ring must be quantified as precisely as possible, so as to verify that it conforms to the customer's specifications.

[0011] It is possible to visually assess the level of molten metal in the molding ring using video images. The assessment is carried out by an operator.

[0012] This approach does not allow for the quantitative determination of the level of molten metal in the molding ring. It provides non-reproducible results, which do not allow for precise control of the ingot drawing speed, nor for an accurate measurement of the pouring speed.

[0013] In this context, the invention aims to propose a method for developing a metal or a metal alloy, preferably a titanium alloy or a nickel-based alloy, in a cold crucible furnace allowing the height of the molten metal bath in the molding ring to be automatically maintained within a predetermined range.

[0014] A second objective is to reduce the variability of the ingot extraction conditions in the molding ring, and in particular the drawing speed.

[0015] Yet another objective is to improve the determination of the molten metal flow rate.

[0016] To this end, the invention relates to a process for producing a metal or a metal alloy in a cold crucible furnace, the process comprising the following steps:

[0017] - introduction of solid elements into a melting zone of the cold crucible furnace, and fusion of solid elements in said fusion zone, the fusion of solid elements producing a molten metal;

[0018] - refining of the molten metal in a refining zone of the cold crucible furnace;

[0019] - transfer of the molten metal from the refining zone into a molding ring, and drawing of an ingot by a bottom of the molding ring;

[0020] a height of a molten metal bath in the molding ring being automatically maintained within a predetermined range by automatically measuring the height of the molten metal bath in the molding ring, automatically determining an ingot drawing speed using the measured height of the molten metal bath and controlling the ingot drawing using said drawing speed,

[0021] the height of the molten metal bath in the molding ring being measured by carrying out the following operations:

[0022] - periodic acquisition of images of the molding ring showing an inner wall of the molding ring and the molten metal bath;

[0023] - determination on images of a position of a free surface of said metal bath melted along the inner wall;

[0024] - evaluation of said position on a predetermined height scale.

[0025] The determination on the images of the position of the free surface of the molten metal bath along the inner wall can be carried out automatically, and gives sufficiently accurate results.

[0026] This allows the height of the molten metal bath in the molding ring to be determined precisely.

[0027] The height of the molten metal bath can then be automatically maintained within the predetermined range, by automatically adjusting the ingot drawing speed.

[0028] This contributes to achieving stable operation of the cold crucible furnace.

[0029] The conditions for extracting the ingot are not very variable, which makes it possible to improve the skin quality of the ingot.

[0030] Knowledge of the height of the molten metal bath in the molding ring makes it possible to determine precisely the flow rate of the molten metal in the molding ring.

[0031] The manufacturing process may also have one or more of the following characteristics, considered individually or in all technically possible combinations:

[0032] - each image comprises a plurality of pixels, each characterized at least by a level in a first spectral band and a level in a second band spectral, the position of the free surface of said molten metal bath along the inner wall being determined on the images using only the levels in the first spectral band;

[0033] - the operation of determining the position of the free surface of said image The molten metal bath along the inner wall comprises the following sub-operations:

[0034] * calculation, along a plurality of lines distributed at different positions on an axis predetermined images, of a level in said first spectral band;

[0035] * formation of a curve indicating the level in said first spectral band in function of the position of the line along the axis;

[0036] * locating a step along the curve, the position of the free surface of the bath of molten metal along the axis being the position of the step along the axis;

[0037] - the operation of determining the position of the free surface of said image molten metal bath along the inner wall includes an image preprocessing sub-operation, during which pixels having a level above a predetermined threshold in the second spectral band are eliminated, the preprocessed images being used for the computation sub-operation;

[0038] - the refining zone of the cold crucible furnace is equipped with at least one component of heating, a heating power of said at least one heating element being determined automatically using the measured height of the molten metal bath;

[0039] - a flow rate of the molten metal entering the molding ring is determined using the ingot drawing speed and the height of the molten metal bath in the molding ring;

[0040] - the cold crucible melting furnace is a plasma arc melting furnace or a furnace electron beam fusion;

[0041] - the metal alloy is chosen from the following list: Ti-6Al-4V, Ti-2-3, Ti-5553, Ti6242, Til7, nickel-based superalloys 718, AD730®, René65®, Udimet® 720.

[0042] According to a second aspect, the invention relates to a cold crucible melting furnace for the production of a titanium alloy or a nickel-based alloy, the melting furnace comprising:

[0043] - a fusion zone;

[0044] - a device for introducing solid elements into the melting zone, the melting of solid elements in the melting zone producing molten metal;

[0045] - a refining zone intended for refining molten metal;

[0046] - a molding ring;

[0047] - a device for drawing an ingot of said alloy through a bottom of the molding ring;

[0048] - a control device configured to automatically maintain a height from a bath of molten metal in the molding ring within a predetermined range, the regulation device comprising a measuring device configured to automatically measure the height of the molten metal bath in the molding ring, a device configured to automatically determine an ingot drawing speed using the measured height of the molten metal bath, and a control device automatically driving the drawing device using said ingot drawing speed;

[0049] the measuring device being configured to measure the height of the molten metal bath in the molding ring by implementing the following operations:

[0050] - periodic acquisition of images of the molding ring showing an inner wall of the molding ring and the molten metal bath;

[0051] - determination on images of a position of a free surface of said metal bath melted along the inner wall;

[0052] - evaluation of said position on a predetermined height scale.

[0053] The melting furnace may further have one or more of the following characteristics, considered individually or in all technically possible combinations:

[0054] - each image comprises a plurality of pixels, each characterized at least by one level in a first spectral band and one level in a second spectral band, the position of the free surface of said molten metal bath along the inner wall being determined on the images using only the levels in the first spectral band;

[0055] - the operation of determining the position of the free surface of said image The molten metal bath along the inner wall comprises the following sub-operations:

[0056] * calculation, along a plurality of lines distributed at different positions on an axis predetermined images, of a level in said first spectral band;

[0057] * formation of a curve indicating the level in said first spectral band in function of the position of the line along the axis;

[0058] * locating a step along the curve, the position of the free surface of the bath along the axis being the position of the step along the axis;

[0059] - the operation of determining the position of the free surface of said image molten metal bath along the inner wall includes an image preprocessing sub-operation, during which pixels having a level above a predetermined threshold in the second spectral band are eliminated, the preprocessed images being used for the computation sub-operation;

[0060] - the refining zone of the cold crucible furnace is equipped with at least one element of heating, the control device being configured to determine a power of heating said at least one heating element automatically using the measured height of the molten metal bath;

[0061] - the cold crucible melting furnace is a plasma arc melting furnace or a furnace electron beam fusion.

[0062] Other features and advantages of the invention will become apparent from the detailed description given below, with reference to the accompanying figures, among which: - [Fig.1] The [Fig.1] is a simplified schematic representation of the melting furnace of the invention; - [Fig.2] [Fig.2] is a schematic representation of the molding ring of the melting furnace of the [Fig.l]; - [Fig. 3] [Fig. 3] is a flowchart for measuring the height of metal melted into the molding ring of the [Fig.2]; - [Fig.4] The [Fig.4] is an image of the molding ring acquired for the determination of the height of the molten metal bath in the molding ring; - [Fig. 5] [Fig. 5] is a photo of the molding ring, showing how the height scale of the free surface is determined; - [Fig. 6] The figure is an image similar to that of [Fig. 4], illustrating the method for determining the position of the free surface of the molten metal bath along the inner wall; and - [Fig.7] Fig.7 is a schematic representation of the curve giving the level of grey along the lines of [Fig.6], as a function of the position of the line on the vertical axis A.

[0063] The furnace illustrated in Figures 1 and 2 is a cold crucible furnace. It is intended for the production of a metal or a metal alloy, preferably a titanium alloy or a nickel-based alloy.

[0064] For example, the titanium alloy is chosen from the following list: Ti-6AL4V, TilO-2-3, Ti5553, Ti6242, Til7.

[0065] The nickel-based alloy is chosen from the following list: Nickel-based superalloys 718, AD730®, René65®, Udimet® 720.

[0066] In the description that follows, the term "metal" is used to refer to the metal or metallic alloy produced in the cold crucible furnace.

[0067] The melting furnace 1 comprises:

[0068] - a fusion zone 3;

[0069] - a device 5 for introducing solid elements 7 into the melting zone 3, the melting of solid elements 7 in the melting zone 3 producing a molten metal;

[0070] - a refining zone 9 provided for refining the molten metal;

[0071] - a molding ring 11;

[0072] - a device for drawing 13 of an ingot 15 of said metal, through a bottom of the ring molder 11.

[0073] The solid elements 7 are, for example, recycled elements.

[0074] Typically, these are offcuts from the cutting or machining of solid parts made of said titanium alloy or said nickel-based alloy. These solid elements are in the form of solid blocks or chips. The chips are, for example, compacted.

[0075] In this case, in addition to the recycled solid elements 7, other solid elements in said alloy and / or in at least one other metal are added in the melting zone 3, so as to obtain the desired composition for the molten metal.

[0076] Alternatively, the solid elements 7 are not recycled elements.

[0077] The introduction device 5 is of any suitable type: screw conveyor, etc.

[0078] The melting zone 3 is configured to melt the solid elements 7.

[0079] The refining zone 9 is configured to dissolve or remove impurities that may exist in the molten metal from the melting zone 3.

[0080] Part of the impurities, also called inclusions, are dissolved, that is to say, are gradually put into solution under the effect of heating the molten metal.

[0081] Another part of the inclusions is removed by gravity by decantation.

[0082] The molten metal passes from the melting zone 3 to the refining zone 9 by overflow.

[0083] The molten metal also passes from the refining zone 9 to the molding ring 11 by overflow.

[0084] The refining zone 9 can be divided into several crucibles, the molten metal passing from one crucible to another successively by overflow.

[0085] The melting zone 3, the refining zone 9 and the molding ring 11 are each equipped with one or more heating elements 17.

[0086] In the melting zone 3, the heating element(s) 17 provide the heat necessary to melt the solid elements 7.

[0087] In the refining zone 9, the heating element(s) 17 are provided to maintain the molten metal at temperature, and to allow the dissolution or decantation of inclusions.

[0088] In the molding ring 11, the heating element(s) 17 are provided to keep the molten metal in liquid form, until the molten metal is drawn out and solidified in the ingot 15.

[0089] The heating elements 17 are mobile relative to the furnace, and move so as to sweep the surface of the molten metal.

[0090] The melting zone 3 comprises a crucible containing the molten metal. The crucible is cooled by circulating a heat transfer fluid.

[0091] Similarly, the refining zone 9 includes a crucible into which the molten metal is received, this crucible also being cooled by the circulation of a heat transfer fluid.

[0092] Because the crucibles of the melting zone 3 and the refining zone 11 are cooled, the metal contained in the crucible forms a skin upon contact with the cooled surface of the crucible. This skin is in solid, pasty, or even thick liquid form. This skin is frequently referred to by the English term "skull." Inclusions that settle in the molten metal are trapped in the skin.

[0093] According to a first embodiment, the furnace is of the PAM-CHR type (“Plasma Arc Melting - Cold Hearth Refining”, i.e. cold crucible furnace and plasma arc melting).

[0094] In PAM-CHR type furnaces, the heating elements 17 are plasma torches.

[0095] The plasma torch generates a flow of ionized and excited gas, this gas flow being directed towards the free surface of the molten metal and heating the molten metal.

[0096] PAM-CHR type ovens are maintained under an inert atmosphere.

[0097] As shown in [Fig.2], the molding ring 11 has a cylindrical crucible 19, with a vertical central axis.

[0098] The cylindrical crucible 19 is open at the bottom.

[0099] The cylindrical crucible 19 is cooled by a heat transfer fluid, like crucibles of melting zone 3 and refining zone 9.

[0100] More specifically, the molding ring 11 has an inner wall 20, delimiting an internal volume 21 receiving the molten metal bath 23.

[0101] The inner wall 20 is defined by the cylindrical crucible 19. It is cylindrical.

[0102] The ingot drawing device 13 comprises a plate 25 constituting the base of the molding ring, and an actuator 27 arranged to move the plate 25 vertically relative to the cylindrical crucible 19.

[0103] The actuator 27 is configured to move the plate 25 from a position up, downwards, at a specific speed called the draw speed.

[0104] In the upper position, the plate 25 closes the internal volume 21 of the cylindrical crucible downwards, the molten metal thus being confined inside the internal volume 21. The molten metal solidifies on contact with the plate 25.

[0105] When the actuator 27 moves the plate 25 downwards, the solidified metal exits the molding ring 11 from the bottom, and a new layer of liquid metal solidifies in the lower part of the molding ring 11. The height of the ingot 15 thus increases progressively, according to the drawing speed. Concurrently, a flow 29 of molten metal from the refining zone pours into the molding ring, compensating for the amount of solidified metal to increase the height of the ingot 15.

[0106] As previously stated, the molding ring 11 is equipped with a heating element 17.

[0107] In the first embodiment, this heating element is a plasma torch, heating the free surface 31 of the molten metal bath 23. The heating element 17 moves relative to the cylindrical crucible 19, so as to sweep the free surface 31 of the molten metal bath 23.

[0108] According to the invention, the furnace 1 includes a control device 33 configured to automatically maintain a height of the molten metal bath 23 in the molding ring 11 within a predetermined range.

[0109] The height of the molten metal bath 23 typically corresponds to the height between the free surface 31 of the molten metal bath 23 and the upper edge 35 of the inner wall 20.

[0110] The predetermined range is centered on a reference height and is bounded by a maximum height and a minimum height. It is entered by an operator and remains constant throughout the production cycle. The operator enters a reference height, a maximum height, and a minimum height.

[0111] The control device 33 includes an automatic measuring device 36 for the height of the molten metal bath in the molding ring 11, a device 37 configured to automatically determine a drawing speed of the ingot 15 using the measured height of the molten metal bath, and a control device 39 automatically controlling the drawing device 13 using said ingot drawing speed.

[0112] The speed of drawing the ingot 15 corresponds to the vertical descent speed of the plate 25. It is expressed for example in millimeters per minute.

[0113] The device 37 is typically a calculating unit. The ingot 15 drawing speed is determined automatically, for example by a PID (Proportional Integral Derivative) type algorithm, using at least the measured height of the molten metal bath and a predetermined reference height.

[0114] The device 39 is a control-command element, piloting the actuator 27 so that the actuator 27 lowers the plate 25 at the drawing speed determined by the device 37.

[0115] The automatic height measurement device 36 for the molten metal bath will now be described.

[0116] The measuring device 36 is configured to measure the height of the molten metal bath in the molding ring 11 by implementing the following operations ([Fig.3]):

[0117] - S10: periodic acquisition of images of the molding ring 11 showing the wall inner 20 of the molding ring and the molten metal bath 23;

[0118] - S20: determination on the images of a position of the free surface 31 of the bath of molten metal 23 along the inner wall 20;

[0119] - S30: evaluation of said position on a predetermined height scale.

[0120] The measuring device 36 therefore includes a camera 43 configured for image acquisition. It also includes a storage unit 44 for the images acquired by the camera 43.

[0121] The level measurement device 36 also includes an image analysis device 45, processing the images acquired by the camera 43.

[0122] This image analysis device 45 is a software, operating on a computing unit.

[0123] Before the oven and therefore the measuring device 36 is put into service, the measuring device 36 is calibrated during a calibration operation S05.

[0124] The camera 43 is oriented so as to provide images showing both the upper part of the cylindrical crucible 19, the inner wall 20 and the free surface 31 of the molten metal bath 23. An example of an image is shown in [Fig.4].

[0125] The optics of the camera 43 are also adjusted to provide sharp images.

[0126] Throughout the entire production phase, i.e. throughout the period in which the control device 33 is active, the position of the camera 43 and the settings of the optics of this camera 43 remain unchanged.

[0127] The image analysis device 45 is also subsequently calibrated during operation S05.

[0128] To do this, a bracket 47 is placed on the molding ring 11, as illustrated in [Fig.5].

[0129] The bracket 47 has a horizontal arm 49, placed horizontally on the upper edge 35 of the inner wall 20. It also has a vertical arm 51, placed against the inner wall 20 of the molding ring.

[0130] The bracket 47 is placed in the field of vision of the camera 43. Its position is rigorously controlled, so that the arm 51 is vertical and the arm 49 is horizontal.

[0131] The bracket 47 is removed after the calibration operation S05, before production starts.

[0132] During the calibration operation, a vertical axis A is aligned with the images using template 47.

[0133] The vertical axis A is a fictitious axis, superimposed by the image analysis device 45 on the images.

[0134] More specifically, axis A is materialized on the image, as seen in [Fig. 5]. The position of axis A on the image is adjusted so that it is strictly parallel to the vertical branch 51 of the template on the image. The position and orientation of axis A relative to the image is memorized.

[0135] Similarly, an ellipse E is superimposed on the image.

[0136] The ellipse E is a fictitious shape, superimposed by the image analysis device 45 in the images.

[0137] The position of the ellipse E is adjusted so that this ellipse exactly overlaps the upper edge 35 of the inner wall 20, as illustrated in [Fig. 5]. The position of the ellipse E relative to the image is then stored.

[0138] Finally, several level features NS, Nmax, Nref and Nmin are materialized on the image.

[0139] The NS, Nmax, Nref and Nmin level features are fictitious features, superimposed by the image analysis device 45 on the images.

[0140] The orientation of these level lines is adjusted so that they are parallel to the horizontal branch 49 of the template on the images.

[0141] The NS level is positioned so as to coincide with the edge of the branch 49 which is supported on the upper edge 35 of the internal wall 20.

[0142] The vertical arm 51 of the template 47 bears several markings corresponding to the reference height, the maximum height, and the minimum height of the molten metal bath in the molding ring. The lines Nref, Nmax, and Nmin are aligned to coincide with these levels on the image, respectively. The positions of the lines Nref, Nmax, and Nmin relative to the image are then recorded.

[0143] The vertical axis A and the lines NS, Nref, Nmax and Nmin define the height scale used for measuring the height of the molten metal bath in the molding ring.

[0144] Once the S05 calibration operation has been carried out, the furnace 1 can be put into operational use.

[0145] The S10 operation, as illustrated in [Fig.3], comprises a sub-operation SI 1 for image acquisition, and a sub-operation S12 for prior image control.

[0146] During suboperation SI 1, images are acquired periodically, for example ten images per second.

[0147] The SI 1 acquisition sub-operation is performed by camera 43.

[0148] Each image comprises a plurality of pixels, each characterized by at least one level in a first spectral band and one level in a second spectral band.

[0149] Typically, each pixel is also characterized by a level in a third spectral band.

[0150] These levels are recorded by camera 43, for each pixel.

[0151] These levels are light intensity levels or grey levels.

[0152] The first spectral band is, for example, the green band.

[0153] The second spectral band is, for example, the blue band.

[0154] The third spectral band is, for example, the red band.

[0155] The green band typically corresponds to wavelengths between 497 and 570 nanometers, the red band to wavelengths between 660 and 780 nanometers, and the blue band to wavelengths between 427 and 476 nanometers.

[0156] For example, the camera is an RGB (Red, Green, Blue) camera, recording images in the red, green and blue spectral bands.

[0157] Alternatively, each pixel is characterized using more or less than three spectral bands, or spectral bands corresponding to windows of wavelengths different from those indicated above.

[0158] In sub-operation S12, the level of each image in the first spectral band is evaluated.

[0159] Images with too low a level are not used for level measurements.

[0160] For example, a level that is too low is understood to mean an average level for all pixels of the image that is less than 20% of the maximum level.

[0161] The pre-sorting sub-operation S12 is performed by the image analysis device 45.

[0162] Images that are not eliminated at sub-operation S12 are recorded in storage 44.

[0163] Operations S20 and S30 are performed by the image analysis device 45.

[0164] The S20 operation for determining the position of the free surface on the images 31 of said molten metal bath along the inner wall 20 comprises the following sub-operations: - S21: calculation, along a plurality of lines distributed at different positions on a predetermined axis of the images, of a level in said first spectral band;

[0165] - S22: formation of a curve indicating the level in said first band spectral as a function of the position of the line along the axis;

[0166] - S23: locating a step along the curve, the position of the free surface of the bath along the axis being the position of the step along the axis.

[0167] Advantageously, when the images include pixels having a level above a predetermined threshold in the second spectral band, said pixels are not used to determine on the images the position of the free surface 31 along the inner wall 20.

[0168] Indeed, as illustrated by [Fig.4], the images may contain areas F of high light intensity which disrupt the analysis and determination of the level of the free surface.

[0169] In particular, for PAM-CHR type furnaces using a plasma arc heating element 17, the plasma flow appears on the images as an area of ​​very high light intensity.

[0170] The S20 operation for determining the position on the free surface on the images thus includes a sub-operation S24 for pre-processing the images, during which pixels having a level above a predetermined threshold in the second spectral band are eliminated, the pre-processed images being used for the calculation sub-operation S21.

[0171] Sub-operation S24 is performed before sub-operation S21.

[0172] The S24 sub-operation is implemented only for images not eliminated at sub-operation S12.

[0173] The preprocessed image, also called the mask image, is formed by analyzing the level of each pixel in the second spectral band and comparing it to the predetermined threshold. If the level is higher than the predetermined threshold, that pixel is marked as excluded for the free surface level determination operation.

[0174] The predetermined threshold is for example equal to 50, the value 100 being the maximum level value.

[0175] The mask image is saved in storage 44.

[0176] The S20 operation further includes a sub-operation S25 for eliminating mask images covering too small a part of the upper edge 35 of the inner wall 20.

[0177] Sub-operation S25 is performed after sub-operation S24, and before sub-operation S21.

[0178] This sub-operation is performed by superimposing the ellipse E onto the mask image. The number of pixels excluded from the analysis along said ellipse E is then determined. If this number of pixels exceeds a predetermined threshold, the mask image is discarded and is not used for the analysis to determine the free surface level.

[0179] For example, if more than 70% of the pixels along the ellipse are eliminated, then the image will not be considered for analysis.

[0180] The S20 operation further includes a sub-operation S26 of summing and normalizing the images over a given time interval.

[0181] Sub-operation S26 is performed after sub-operation S25, and before sub-operation S21.

[0182] All mask images recorded during a time range are used to constitute a normalized image, with the exception of mask images eliminated in sub-operation S25.

[0183] For example, the time range is five seconds. If ten images are recorded per second, and all the images are retained, the normalized image will be reconstructed from the fifty mask images corresponding to the fifty recorded images.

[0184] Because the camera is fixed, the images cover essentially the same field of view. They contain the same number of pixels, each pixel corresponding to the same point in the field of view.

[0185] In other words, a given pixel in one of the images has a corresponding pixel in each of the other images, located in the same row and column. These different corresponding pixels correspond to the same point in the camera's field of view.

[0186] The levels in the first spectral band of the corresponding pixels in the different images are added together. They are then divided by the number of images considered for the time window.

[0187] In other words, the average level in the first spectral band is determined by averaging the levels of all the mask images retained for the time interval considered, and this for each pixel of the images.

[0188] This normalized image is recorded in storage 44.

[0189] In sub-operation S21, a level in the first spectral band is calculated along a plurality of lines L1, L2, L3, L4 distributed at different positions on a predetermined axis of the images.

[0190] The S21 sub-operation is performed for each normalized image recorded in storage 44.

[0191] The predetermined axis is shown in [Fig.6]. It corresponds to axis A superimposed on [Fig.6]. This axis is vertical.

[0192] The lines are all identical, that is to say, of the same size and of the same shape.

[0193] These lines correspond to a fraction of the ellipse E. In the example shown, each line corresponds to the upper half of the ellipse E. Alternatively, the lines correspond to a fraction of the upper half of the ellipse E.

[0194] The lines, as shown in [Fig.6], are regularly spaced along axis A.

[0195] The different lines are deduced from each other by translation along the axis A, for example by translation of 10 pixels.

[0196] The first line is exactly superimposed on the ellipse E (line Ll in the example of [Fig.6]). It corresponds to the upper half of the ellipse E, or to a fraction of this upper half.

[0197] The other lines are regularly shifted downwards, starting from the first line.

[0198] In the example shown in [Fig.6], line L2 is obtained by translating line L1 downwards along axis A, line L3 by translating line L2 downwards along axis A, and line L4 by translating line L3 downwards along axis A.

[0199] Only four lines are shown in [Fig.6]. However, the calculation is typically done on a large number of lines.

[0200] The level value considered for each line corresponds to the average level in the first spectral band for all pixels in the line. Of course, pixels excluded in sub-operation S24 are not considered when calculating the average.

[0201] The sub-operation S22 is carried out after the sub-operation S21. The levels determined for the different lines are used to form a curve indicating the level in the first spectral band as a function of the position of the line.

[0202] This curve is shown in [Fig.7]. The abscissa corresponds to the position of the line along axis A. The ordinate corresponds to the level calculated for the line in substep S21.

[0203] The curve is made up of discrete points. It is not continuous.

[0204] The sub-operation S23 for locating a step along the curve is carried out after the sub-operation S22.

[0205] The step corresponds to a significant level variation, taking place over a reduced position range.

[0206] The level variation is for example at least 20, and occurs over for example 30 pixels or less.

[0207] The position of the step corresponds to the position of the free surface of the bath along axis A.

[0208] The existence of this step simply reflects the fact that, in the first spectral band, the free surface 31 of the molten metal has a grey level that is distinctly different from that of the inner wall 20. On the other hand, the grey level of the varies little along the free surface 31 of the molten metal bath, and the grey level varies little along the inner wall 20.

[0209] In [Fig. 7], the first part PI of the curve corresponds to the lines closest to the upper edge 35. They are located above the boundary L separating, in the images, the free surface 31 of the molten metal and the inner wall 20 (the boundary L is indicated in [Fig. 4]). The gray level corresponds approximately to the gray level of the inner wall 20. It is approximately constant.

[0210] The third part P3 of the curve corresponds to the lowest lines, located below the boundary L separating the free surface 31 of the molten metal and the inner wall 20 in the images. The corresponding gray level is close to the gray level of the free surface 31 of the liquid metal. It is substantially constant and much higher than the grey level of the first part PI.

[0211] The second part P2 of the curve forms the transition between the first part P1 and the third part P3. It corresponds to the position of the free surface 31 of the liquid metal along the inner wall 20. The gray level varies markedly and rapidly along the second part P2 because the lines have exactly the shape of the boundary L between the free surface 31 and the inner wall 20 in the image. Thus, a shift of a few pixels along the axis A moves a line located below the boundary L to a line located above the boundary L.

[0212] The position of the level step is located by calculation, for example by calculating the slope at each point of the curve and by identifying the breaks in slope.

[0213] During operation S30, the height scale determined in operation S05 is superimposed on the image.

[0214] In other words, the level lines NS, Nref, Nmin, and Nmax are placed along the vertical axis A. This allows the position of the free surface 31 to be evaluated on this height scale. The origin of the height scale corresponds to the level line NS. The height at which the free surface 31 is located is determined by calculation, for example, by comparing the distance between NS and the free surface 31 along the axis A with the distance between NS and the level line Nref along the axis A.

[0215] Advantageously, the control device 33 is configured to determine the heating power of said at least one heating element 17 equipping the refining zone 9 automatically, using the measured molten metal bath height.

[0216] The heating power is calculated for example using a PID type control using the measured molten metal bath height and the reference height.

[0217] When the display area 9 has several heating elements, only the heating element located closest to the molding ring is controlled according to the height of the measured molten metal bath.

[0218] Alternatively, the power of all heating elements in the refining zone is controlled according to the measured height of the molten metal bath.

[0219] Advantageously, the control device 33 is configured to determine the flow rate of the molten metal entering the molding ring 11 using the ingot draw rate and the height of the molten metal bath in the molding ring.

[0220] The pouring speed V corresponds to the flow rate of molten metal flowing from the refining zone 9 into the molding ring 11.

[0221] For example, the flow rate V is determined using the following equation:

[0222] V=p^S+pVtS

[0223] Where V is the pouring speed in kg / min

[0224] p is the density of the molten or solid metal in kg / m3

[0225] Ah is the change in height of the molten metal bath in the molding ring during the time interval At, expressed in m / min

[0226] S is the cross-section of the molding ring, taken perpendicular to the vertical direction, in m2

[0227] Vt is the ingot drawing speed, in m / min.

[0228] According to a second aspect, the invention relates to a method for producing a titanium alloy or a nickel-based alloy in a cold crucible furnace.

[0229] The process comprises the following steps:

[0230] - introduction of solid elements 7 into a melting zone 3 of the cold crucible furnace, and melting of the solid elements 7 in the melting zone 3, the melting of the solid elements producing a molten metal;

[0231] - refining of the molten metal in a refining zone 9 of the cold crucible furnace 1;

[0232] - transfer of the molten metal from the refining zone 9 into a molding ring 11, and drawing of an ingot 15 by a bottom of the molding ring 11.

[0233] The cold crucible furnace 1 is as described above. The melting zone 3, the refining zone 9 and the molding ring 11 are as described above.

[0234] Advantageously, the height of the molten metal bath 23 in the molding ring is automatically maintained within a predetermined range by automatically measuring the height of the molten metal bath 23 in the molding ring and automatically determining a drawing speed of the ingot 15 using the measured height of the molten metal bath.

[0235] The drawing speed is determined as described above.

[0236] The height of the molten metal bath 23 in the molding ring 11 is measured in implementing the following operations:

[0237] - S10: periodic acquisition of images of the molding ring 11 showing a wall inner 20 of the molding ring 11 and the molten metal bath 23;

[0238] - S20: determination on the images of a position of a free surface 31 of the bath of molten metal 23 along the inner wall 20;

[0239] - S30: evaluation of said position on a predetermined height scale.

[0240] Operations S10, S20 and S30 are as described above.

[0241] They are preceded by an S05 calibration operation, carried out as described above.

[0242] Advantageously, the heating power of at least one heating element 17 of the refining zone 9 is determined automatically using the measured height of the molten metal bath.

[0243] The heating power is determined as described above.

[0244] Advantageously, a flow rate of the molten metal entering the molding ring 11 is determined using the ingot drawing rate and the height of the molten metal bath in the molding ring 11.

[0245] The flow rate is determined as described above.

[0246] The cold crucible melting process and furnace have been described for a PAM-CHR type cold crucible melting furnace.

[0247] Alternatively, the cold crucible melting furnace is of the EB-CHR type (Electron Beam - Cold Hearth Refining).

[0248] This oven is similar to the PAM-CHR type oven, but differs from it in the following characteristics.

[0249] The heating elements are electron beam type. In other words, the heating elements generate electron beams directed towards the molten metal or towards the solid elements being melted in the melting zone. These electron beams strike the material to be heated with a very high power density. The electron beam is displaced and sweeps across the surface of the molten metal.

[0250] EB-CHR type ovens are kept under vacuum.

[0251] The measurement of the height of the molten metal bath in the molding ring is simplified.

[0252] Indeed, there are no areas of high light intensity in the images of the molding ring. These areas of high light intensity in PAM-CHR type furnaces correspond to the gas flow in the plasma state. The electron beams in EB-CHM type furnaces are not visible, or do not generate any glare areas in the images. The S24 sub-operation for eliminating pixels with a level exceeding a predetermined threshold in the second spectral band is not necessary.

[0253] It can, however, be implemented.

Claims

Demands

1. A method for producing a metal or a metal alloy in a cold crucible furnace (1), the method comprising the following steps: - introducing solid elements (7) into a melting zone (3) of the cold crucible furnace (1), and melting the solid elements (7) in said melting zone (3), the melting of the solid elements (7) producing a molten metal; - refining the molten metal in a refining zone (9) of the cold crucible furnace (1); - transferring the molten metal from the refining zone (9) into a molding ring (11), and drawing an ingot (15) through a bottom of the molding ring (11);a height of a molten metal bath (23) in the molding ring (11) being automatically maintained within a predetermined range by automatically measuring the height of the molten metal bath (23) in the molding ring (11), by automatically determining a drawing speed of the ingot (15) using the measured height of the molten metal bath and controlling the drawing of the ingot (15) using said drawing speed, the height of the molten metal bath (23) in the molding ring (11) being measured by implementing the following operations: - (S10) periodic acquisition of images of the molding ring (11) showing an inner wall (20) of the molding ring (11) and the molten metal bath (23); - (S20) determination on the images of a position of a free surface (31) of said molten metal bath (23) along the inner wall (20); - (S30) evaluation of said position on a predetermined height scale.;

2. A manufacturing method according to claim 1, wherein each image comprises a plurality of pixels each characterized by at least one level in a first spectral band and one level in a second spectral band, the position of the free surface (31) of said molten metal bath along the inner wall (20) being determined on the images using only the levels in the first spectral band.

3. A method of preparation according to claim 2, wherein the operation of determining on the images the position of the free surface of said molten metal bath along the inner wall (S20) comprises the following sub-operations: - (S21) calculation, along a plurality of lines distributed at different positions on a predetermined axis of the images, of a level in said first spectral band; - (S22) formation of a curve indicating the level in said first spectral band as a function of the position of the line along the axis; - (S23) marking a step along the curve, the position of the free surface (31) of the molten metal bath (23) along the axis being the position of the step along the axis.

4. A method of preparation according to claim 3, wherein the operation of determining on the images the position of the free surface of said molten metal bath along the inner wall (S20) comprises a sub-operation (S24) of image preprocessing, during which pixels having a level above a predetermined threshold in the second spectral band are eliminated, the preprocessed images being used for the calculation sub-operation (S21).

5. A manufacturing process according to any one of the preceding claims, wherein the refining zone (9) of the cold crucible furnace (1) is equipped with at least one heating element (17), a heating power of said at least one heating element (17) being determined automatically using the height of the measured molten metal bath.

6. A manufacturing method according to any one of the preceding claims, wherein a flow rate of the molten metal entering the molding ring (11) is determined using the ingot drawing rate and the height of the molten metal bath (23) in the molding ring (11).

7. A method of preparation according to any one of the preceding claims, wherein the cold crucible melting furnace (1) is a plasma arc melting furnace or an electron beam melting furnace.

8. A manufacturing process according to any one of the preceding claims, wherein the metal alloy is selected from the list following: Ti-6AI-4V, TilO-2-3, Ti5553, Ti6242, Til7, nickel-based superalloys 718, AD730®, René65®, Udimet® 720.

9. Cold crucible melting furnace (1) for the production of a metal or a metal alloy, the melting furnace comprising: - a melting zone (3); - a device (5) for introducing solid elements (7) into the melting zone (3), the melting of the solid elements (7) in the melting zone (3) producing a molten metal; - a refining zone (9) provided for refining the molten metal; - a molding ring (11); - a device (13) for drawing an ingot (15) of said alloy through a bottom of the molding ring (11);- a control device (33) configured to automatically maintain a height of a molten metal bath (23) in the molding ring (11) within a predetermined range, the control device (33) comprising a measuring device (36) configured to automatically measure the height of the molten metal bath (23) in the molding ring (11), a device (37) configured to automatically determine an ingot drawing speed using the measured height of the molten metal bath, and a control device (39) automatically driving the drawing device (13) using said ingot drawing speed; the measuring device (36) being configured to measure the height of the molten metal bath (23) in the molding ring (11) by implementing the following operations: - (S10) periodic acquisition of images of the molding ring (11) showing an inner wall (20) of the molding ring (11) and the molten metal bath (23);- (S20) determination on the images of a position of a free surface (31) of said molten metal bath along the inner wall (20); - (S30) evaluation of said position on a predetermined height scale.;

10. A melting furnace according to claim 9, wherein each image comprises a plurality of pixels, each characterized by at least one level in a first spectral band and one level in a second spectral band, the position of the free surface (31) of said molten metal bath along the inner wall (20) being determined on the images using only the levels in the first spectral band.

11. Melting furnace according to claim 10, wherein the operation of determining on the images the position of the free surface of said molten metal bath along the inner wall (S20) comprises the following sub-operations: - (S21) calculation, along a plurality of lines distributed at different positions on a predetermined axis of the images, of a level in said first spectral band; - (S22) formation of a curve indicating the level in said first spectral band as a function of the position of the line along the axis; - (S23) locating a step along the curve, the position of the free surface of the bath along the axis being the position of the step along the axis.

12. Melting furnace according to claim 11, wherein the operation of determining on the images the position of the free surface of said molten metal bath along the inner wall (S20) comprises an image preprocessing sub-operation (S24), during which pixels having a level above a predetermined threshold in the second spectral band are eliminated, the preprocessed images being used for the calculation sub-operation (S21).

13. Melting furnace according to any one of claims 9 to 12, wherein the refining zone (9) of the cold crucible furnace (1) is equipped with at least one heating element (17), the control device (33) being configured to determine a heating power of said at least one heating element (17) automatically using the measured height of the molten metal bath.

14. Melting furnace according to any one of claims 9 to 13, wherein the cold crucible melting furnace (1) is a plasma arc melting furnace or an electron beam melting furnace.

Images