EP0030060B1 - Process and apparatus for automating a vacuum degassing stage of aluminium alloys - Google Patents

Abstract

Apparatus and process for automating a vacuum degasification cycle for metal alloys, particularly aluminum alloys. The process comprises adjusting the degasification speed to sequentially correspond to a plurality of sets of degasification speed parameters. Each set of degasification speed parameters corresponds to a predetermined desired degasification speed. The degasification speed may be adjusted by adjusting the vacuum surrounding the alloy. The degasification speed is triggered to change from corresponding to one set of parameters to another set of parameters by the sensing of a series of predetermined partial pressures of gas in the alloy. The apparatus includes an inlet-outlet assembly for transforming given indications into a numerical form for the parameters of the degasification cycle and a calculator assembly adapted to transform the speed of degasification into variations of theoretical partial pressure and to regulate the pressure in the enclosure of the furnace to obtain an identical variation to that required based upon the indications of the standard curve placed in the assembly memories. Several furnaces may be regulated. The temperature of the metal may be regulated at a level equivalent to that required before casting. The apparatus can include a microprocessor.

Description

The present invention relates to vacuum degassing of aluminum alloys and its regulation.

It is known that aluminum alloys dissolve gases, in particular hydrogen. These gases being less soluble in solid metal than in liquid metal, can be released during solidification and give microporosities.

• - ou effectuer un dégazage chimique par l'introduction dans le métal de produits qui, en se décomposant, donnent un élément susceptible de se combiner avec l'hydrogène (en général C1₂ pour donner HCI, le chlore étant sous forme de chlore naissant);
• - ou effectuer un dégazage physique par un barbottage de gaz (azote, argon, chlore en général); la pression partielle de l'hydrogène dans la bulle de gaz barbotteur étant inférieure à celle du métal, l'hydrogène du métal diffuse dans la bulle;
• - ou effectuer un dégazage sous vide. Le métal est introduit dans un four étanche dans lequel on fait le vide ou on pose sur le creuset un couvercle et on fait le vide sous celui-ci. Les niveaux de vide résiduels sont de 1 à 30 millibars. Le vide est arrêté après un temps tel que la solidification sous vide, après opération, d'un lingotin prélevé dans le métal, soit satisfaisante (concavité de la surface, mesure de la densité, tranche radiographique).

To reduce the gas and essentially hydrogen content of aluminum alloys, you can:

• - or carry out a chemical degassing by the introduction into the metal of products which, by decomposing, give an element capable of combining with hydrogen (in general C1 ₂ to give HCI, the chlorine being in the form of nascent chlorine) ;
• - or carry out a physical degassing by bubbling gas (nitrogen, argon, chlorine in general); the partial pressure of the hydrogen in the bubbling gas bubble being lower than that of the metal, the hydrogen of the metal diffuses in the bubble;
• - or perform a vacuum degassing. The metal is introduced into a sealed oven in which a vacuum is created or a cover is placed on the crucible and a vacuum is created under it. Residual vacuum levels are from 1 to 30 millibars. The vacuum is stopped after a time such that the solidification under vacuum, after operation, of an ingot taken from the metal, is satisfactory (concavity of the surface, measurement of the density, radiographic section).

If the result is not satisfactory, the degassing is resumed.

Such vacuum degassing is illustrated, for example, by the document "FOUNDRY, vol. 30, No. 350, November 1975, K. ALKER: vacuum degassing of foundry aluminum alloys."

In this document, the first bubble method is used, which only applies at the start of degassing because then the bubbles are released under an oxide plane and can no longer be observed. This process therefore consists in observing a posteriori that the degassing operation has ended and does not allow this operation to be directed according to its measured evolution.

• - après avoir au préalable enregistré les courbes optimales de dégazage, c'est-à-dire les courbes de pression partielle en fonction du temps, de l'alliage considéré, au cours d'un stade de mise au point par enregistrement des cycles d'essais et établissement des corrélations existant entre ces enregistrements et les propriétés d'un lingotin solidifié sous vide et celles d'une éprouvette présentant des gradins d'épaisseurs différentes;
• - à décomposer les courbes de dégazage en quatre phases: une première phase pendant laquelle l'alliage passe de la pression partielle d'origine P₁ à une pression P₂ avec une vitesse V₁ suffisamment lente pour éviter tout bouillonnement de l'alliage, une seconde phase entre les pression P₂ et P₃, effectuée avec une vitesse V₂ supérieure à V₁, une troisième phase entre les pressions P₃ et P₄, effectuée avec une vitesse V₃ inférieure à V₂ et une quatrième phase de maintien à la pression partielle P₄ avec une vitesse de dégazage nulle et à mettre en mémoire les valeurs _Pt, _P2, _P3, _P4 ainsi que _Vt, _V2 ^V3^;
• - au stade opérationnel de dégazage d'un alliage donné dans un four, à mesurer à intervalles de temps donnés (At) la valeur de la pression partielle d'hydrogène au cours du dégazage, à calculer la variation de pression partielle pendant le temps At, à comparer ces valeurs avec les valeurs optimales mémorisées correspondantes et à agir en conséquence sur le vide à l'intérieur du four de manière que lesdites valeurs mesurées suivent constamment lesdites valeurs optimales correspondantes jusqu'à l'obtention, dans la phase finale de maintien, de la pression partielle d'hydrogène prédéterminée.

The object of the present invention is a process for automating a degassing cycle in an aluminum alloy vacuum furnace, characterized in that it consists:

• - after having previously recorded the optimal degassing curves, that is to say the partial pressure curves as a function of time, of the alloy considered, during a development stage by recording the cycles d 'tests and establishment of the correlations existing between these records and the properties of an ingot solidified under vacuum and those of a test piece having steps of different thicknesses;
• to decompose the degassing curves into four phases: a first phase during which the alloy changes from the original partial pressure P ₁ to a pressure P ₂ with a speed V ₁ sufficiently slow to avoid any bubbling of the alloy, a second phase between the pressures P ₂ and P ₃ , carried out with a speed V ₂ greater than V ₁ , a third phase between the pressures P ₃ and P ₄ , carried out with a speed V ₃ less than V ₂ and a fourth phase of maintaining partial pressure P ₄ with zero degassing speed and storing the values _Pt , _P2 , _P3 , _{P4 as} well as _Vt , _V2 ^V 3 ^;
• - at the operational degassing stage of a given alloy in a furnace, to measure at given time intervals (At) the value of the partial pressure of hydrogen during degassing, to calculate the variation in partial pressure during the time At , to compare these values with the corresponding optimal stored values and to act accordingly on the vacuum inside the furnace so that said measured values constantly follow said corresponding optimal values until obtaining, in the final maintenance phase , the predetermined partial pressure of hydrogen.

The device for implementing the above method comprises a sealed oven, a crucible containing the degassing alloy and a source of vacuum, and is characterized in that it further comprises a pilot regulating an interposed valve between the vacuum source and the sealed oven containing the crucible with the alloy to be degassed, a hydrogen metering electrode immersed in the alloy and connected to a partial hydrogen pressure recorder itself connected to the pilot, pilot comprising an input-output assembly allowing the introduction of the values of the parameters, a computer assembly, and a memory assembly where the values of the corresponding parameters of the optimal curves are stored.

To the present description are attached Figure 1 showing the degassing diagram according to the invention, Figure 2 illustrating an embodiment of the invention, Figure 3 illustrating the principle of regulation and Figure 4 showing a stepped plate.

• - obtenir dans trois premières phases des vitesses de dégazage déterminées;
• - obtenir dans une quatrième phase, phase finale de maintien, une pression partielle déterminée;
• - réguler par un automatisme ces phénomènes;
• - avoir une agitation de la masse du métal;
• - ne pas détruire la modification du silicium dans le cas des alliages siliciés.

In the present invention, vacuum degassing is carried out to achieve the following objectives:

• - obtain in the first three phases determined degassing rates;
• - obtain in a fourth phase, final maintenance phase, a determined partial pressure;
• - regulate these phenomena automatically;
• - have an agitation of the mass of the metal;
• - do not destroy the modification of silicon in the case of silicon alloys.

It appeared in fact that the degassing, expressed by the partial pressure of hydrogen in the metal as a function of time, must be carried out according to the diagram in FIG. 1.

PHASE 1

Degassing at slow speed V ₁ to avoid any bubbling leading to the creation of oxides in this phase where the pressure in the enclosure leaves too much partial pressure of oxygen in the atmosphere above the metal.

This phenomenon would also cause the disappearance of the sodium introduced to ensure the modification of the shape of the silicon of the alloy in the case of silicon alloys.

PHASE 2

When a partial pressure of hydrogen P ₂ in the metal is reached, application of a rapid degassing speed V ₂ until a partial pressure P _{3 is} obtained, boiling having been avoided by the passage from P ₁ to P ₂ .

PHASE3

When a partial pressure of hydrogen P ₃ is reached, application of a slow speed V ₃ to the pressure P ₄ so as to then allow the establishment of a zero speed.

PHASE4

When the partial pressure of hydrogen is P ₄ , establishment of a zero speed to maintain in the metal a residual hydrogen pressure.

It has indeed appeared that with a too low hydrogen content in the metal, defects appear during solidification, characterized by a localization instead of a dispersion in the form generally of indentations marked in connecting radii or of coves.

PRODUCTION

• 1. un four étanche 1 assurant de préférence le chauffage par induction pour maintenir une température (en général voisine de 750°C) et assurer des mouvements de la masse de métal liquide pour renouveler les couches en contact avec le vide.
  • Le four comprend ou contient:
    • - l'inducteur 2;
    • - le creuset 3;
    • - le métal 4;
    • - le couvercle 5;
    • - le joint 6 résistant à des températures de l'ordre de 300°C (polymère à base de silicone de préférence) protégé de l'atmosphère du four par des briques isolantes;
    • - le thermocouple 7 permettant d'assurer l'enregistrement de la température et sa régulation par l'enregistreur-régulateur 8 qui commande le circuit de l'inducteur 2.
• 2. un circuit de vide comprenant:
  • - la prise 9 dans le four;
  • - le réfrigérant 10 par exemple à eau;
  • - les filtres 11 pour neutraliser les produits qui peuvent subsister sous forme de flux dans le métal ou sur les parois du creuset;
  • - un réservoir 18;
  • - une pompe à vide 19 branchée sur ce réservoir;
  • - un ensemble de mesure et de régulation de vide dans le réservoir 18 par:
  • . l'indicateur de pression 20;
  • . le pressostat régulateur 21 qui commande la pompe, le vide dans le réservoir étant par exemple de l'ordre de 2 millibars;
  • . une vanne automatisée 22 mettant le réservoir en communication avec le four;
  • . un robinet 23 permettant d'isoler le four.
• 3. un système de mesure et de régulation comprenant:
  • - une électrode à dosage d'hydrogène immergée (14, 15)
  • - un enregistreur de pression partielle d'hydrogène 16;
  • - un pilote 17 régulant la vanne 22.
  
  le pilote comprend par exemple:
  • - un ensemble entrées-sorties;
  • - un ensemble calculateur;
  • - un ensemble mémoires;
  • - et peut être construit autour de microprocesseurs et d'horloges électroniques;

The realization includes according to figure 2:

• 1. a sealed oven 1 preferably providing induction heating to maintain a temperature (generally close to 750 ° C.) and ensuring movements of the mass of liquid metal to renew the layers in contact with the vacuum.
  • The oven includes or contains:
    • - inductor 2;
    • - crucible 3;
    • - metal 4;
    • - the cover 5;
    • - The seal 6 resistant to temperatures of the order of 300 ° C (preferably silicone-based polymer) protected from the atmosphere of the oven by insulating bricks;
    • - the thermocouple 7 enabling the temperature to be recorded and regulated by the recorder-regulator 8 which controls the circuit of the inductor 2.
• 2. a vacuum circuit comprising:
  • - socket 9 in the oven;
  • - The refrigerant 10, for example water;
  • - The filters 11 to neutralize the products which may remain in the form of flux in the metal or on the walls of the crucible;
  • - a reservoir 18;
  • - a vacuum pump 19 connected to this tank;
  • a set of measurement and regulation of vacuum in the reservoir 18 by:
  • . the pressure indicator 20;
  • . the regulator pressure switch 21 which controls the pump, the vacuum in the tank being for example of the order of 2 millibars;
  • . an automated valve 22 putting the reservoir in communication with the oven;
  • . a valve 23 for isolating the oven.
• 3. a measurement and regulation system comprising:
  • - an immersed hydrogen metering electrode (14, 15)
  • - a hydrogen partial pressure recorder 16;
  • - a pilot 17 regulating the valve 22.
  
  the pilot includes for example:
  • - an input-output set;
  • - a computer assembly;
  • - a set of memories;
  • - and can be built around microprocessors and electronic clocks;

INPUT-OUTPUT ASSEMBLY.

• a) Pressions P₂ fin de la phase I;
  • P₃ II;
  • P₄ III;
• b) Vitesses V₁ pour la phase I;
  • V₂ II;
  • V₃ III;
• c) L'unité de temps de mesure de pression partielle: Δt

The parameters of the basic curve in Figure 1 are introduced:

• a) P ₂ pressures at the end of phase I;
  • P ₃ II;
  • P ₄ III;
• b) V ₁ speeds for phase I;
  • V ₂ II;
  • V ₃ III;
• c) The partial pressure measurement time unit: Δt

CALCULATOR ASSEMBLY

• - receives the indication of actual variation in partial pressure over time ΔT, ie ΔPR;
• - calculates the theoretical partial pressure variation to be obtained at the same time ΔT or ΔP _T by the relation: ΔPT - V.ΔT
• - compare ΔP _R and ΔP _T ;
• - ordered:
  • . closing the automated valve 22 if ΔP _R > ΔP _T
  • . opening the valve if ΔPR ≤ ΔPT.

The principle of regulation is represented by the diagram in Figure 3.

The pilot can receive his information from several vacuum furnaces and regulate it like the first from the automated valves 24, 25, etc.

MEMORY SET

• - chaque type d'alliage;
• - chaque dimension de creuset.

It can receive all input indications without going through the display, the parameters being stored for:

• - each type of alloy;
• - each crucible dimension.

Obtaining the optimal vacuum is then controlled by these two parameters alone.

USE OF THE EQUIPMENT

• 1) Un lingotin de métal est prélevé dans le creuset 3 figure 2 par une capsule d'acier 26 selon figure 2. Il est solidifié sous 2 millibars en branchant l'enceinte 27 où il va se solidifier sur le réservoir 18.

The curves giving the partial pressure of hydrogen as a function of time are recorded and compared during the development with the results of two significant tests:

• 1) A metal ingot is taken from the crucible 3 in FIG. 2 by a steel capsule 26 according to FIG. 2. It is solidified under 2 millibars by connecting the enclosure 27 where it will solidify on the reservoir 18.

The enclosure is provided with a glass cover 28 through which the solidification can be followed.

• - le temps d'apparition des premières bulles;
• - la concavité de la surface;
• - la densité du lingotin.

• 2) Une plaque à gradins de 200 mm par 200 mm par exemple selon figure 4, constituée de gradins d'épaisseur variable de 20, 16, 12, 8, 4 mm par exemple est coulée dans un moule en sable de préférence en basse-pression.

We notice:

• - the time of appearance of the first bubbles;
• - the concavity of the surface;
• - the density of the ingot.

• 2) A 200 mm by 200 mm stepped plate for example according to FIG. 4, made up of steps of variable thickness of 20, 16, 12, 8, 4 mm for example is poured into a sand mold preferably in low- pressure.

une dégradation éventuelle de la modification du silicium ne devant apparaitre que dans le gradin de 20 mm. Les caractéristiques mécaniques des différents gradins sont par ailleurs comparées aux profilés des courbes. Les courbes optimales à mettre en mémoire pour les différents alliages peuvent être déterminées par exemple par ces tests.The plaque is examined by radiography. A satisfactory metal must lead with an AS7606 alloy for example to microporosities:

a possible degradation of the modification of the silicon which should only appear in the 20 mm step. The mechanical characteristics of the different steps are also compared to the profiles of the curves. The optimal curves to be stored for the different alloys can be determined for example by these tests.

OVEN SUPPLY

The furnace can be supplied with metal from the melting furnace by pump, electromagnetic for example. Likewise for the evacuation of the metal after degassing under vacuum, for example to the crucible of a low-pressure machine.

Claims (2)

1. Process for automating a degassing cycle in a vacuum furnace for aluminium alloys, characterised in that it consists of:

- after having first recorded the optimum degassing curves, that is the curves of partial pressure against time, of the alloy involved in the course of a stage of investigation by the recording of test cycles and the establishment of correlations between these recordings and the properties of an ingot solidified under vacuum and those of a test piece which has steps of different thickness;

- dividing the degassing curves into four phases: a first phase during which the alloy passes from the original partial pressure P₁ to a pressure P₂ at a rate V, sufficiently slowly to avoid any boiling of the alloy, a second phase between the pressures P₂ and P₃ at a rate V₂ higher than V₁, a third phase between P₃ and P₄ at a rate V₃ lower than V₂, and a fourth phase during which the partial pressure P₄ is maintained at a degassing rate of zero, and recording in a store the values P₁, P₂, P₃, P₄, and V_f, V₂ and V₃;

- during the actual degassing of a given alloy in a furnace, measuring at given time intervals (A t) the value of the hydrogen partial pressure in the course of the degassing, calculating the variation in partial pressure over the time A t, comparing these values with the corresponding optimum values stored, and adjusting appropriately the vacuum inside the furnace so that the measured values always follow the corresponding optimum values until, in the final holding phase, the predetermined hydrogen partial pressure is obtained.

2. Apparatus for putting into effect the process according to claim 1, comprising a sealed furnace, a crucible containing the alloy to be degassed and a vacuum source, which further comprises a control (17) regulating a valve (22) lying between the vacuum source (18, 19) and the furnace (1) containing the crucible (3) with the alloy to be degassed (4), a hydrogen electrode (14) immersed in the alloy and connected to a recorder of the hydrogen partial pressure (16), itself connected to the control (17), the latter having an array of input/output terminals for receiving parameter values, a calculator unit, and a number of stores which receive the values of the parameters corresponding to the optimum curves.

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