EP0633083B1 - Metallic powder for making parts by compression and sintering and process for obtaining the powder - Google Patents

Metallic powder for making parts by compression and sintering and process for obtaining the powder Download PDF

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Publication number
EP0633083B1
EP0633083B1 EP94420188A EP94420188A EP0633083B1 EP 0633083 B1 EP0633083 B1 EP 0633083B1 EP 94420188 A EP94420188 A EP 94420188A EP 94420188 A EP94420188 A EP 94420188A EP 0633083 B1 EP0633083 B1 EP 0633083B1
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EP
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Prior art keywords
granules
metallic powder
gelatin
powder
particles
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EP94420188A
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German (de)
French (fr)
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EP0633083A1 (en
Inventor
Christer Aslund
Claude Quichaud
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Metals Process Systems
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Metals Process Systems
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
    • B22F1/10—Metallic powder containing lubricating or binding agents; Metallic powder containing organic material
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
    • B22F1/14—Treatment of metallic powder
    • B22F1/148—Agglomerating
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F2301/00—Metallic composition of the powder or its coating
    • B22F2301/35—Iron

Definitions

  • the metal powder which is the subject of the invention relates to the production of parts by compression and sintering from stainless steels, other metals or alloys stainless or refractory and alloy steel, intended for the production of parts quality.
  • metal powders by spraying a metal or alloy liquid by means of a gas jet which can be, for example, a neutral gas such as argon or nitrogen or any suitable gas.
  • a gas jet which can be, for example, a neutral gas such as argon or nitrogen or any suitable gas. This gives powders having a much lower oxide content than powders obtained by spraying water and which thus have substantially the same purity as the starting metal.
  • the elementary particles of these powders have a substantially shaped spherical but in the state is not easily shaped.
  • these powders allow, after compression and sintering, to obtain by compression of parts having an apparent density at least equal to that allow to obtain the usual angular powders.
  • the raw part obtained from spherical particles has a mechanical resistance often insufficient to allow its manipulation and in particular its ejection from the mold, then transfer to the sintering furnace, without chipping or cracking.
  • Patent application GB 2 228 744 describes a similar method for joining together elements of alloys such as graphite, phosphorus or others with a base metal. According to this method, a mixture of an acrylic acid ester is used as binder with a methacrylic acid ester and an unsaturated polymerizable acid.
  • the aim is to avoid the segregation of finer or lighter particles such as graphite or others compared to a base metal, such as iron, by bonding these particles between them.
  • a paste is made by mixing metallic powder, lubricant of the stearate type, of cellulose gum and water, this paste is injected into a cold compaction mold, optionally drying and carrying out an operation of debinding - sintering of the raw parts obtained.
  • Such operations require wet pulp injection equipment which are similar to those of plastic injection molding and are different from cold powder compaction equipment.
  • the metal powder according to the invention consists of a set of granules each comprising a group of elementary metallic particles of shape spherical agglomerated with gelatin at a content of at least 0.5% of the weight of metallic powder.
  • the spherical particles are advantageously obtained by a spraying process, by means of a gas which can be air or a neutral or reducing gas such as N 2 , H 2 , NH 3 , Ar, or other, of a metal or liquid alloy.
  • a gas which can be air or a neutral or reducing gas such as N 2 , H 2 , NH 3 , Ar, or other, of a metal or liquid alloy.
  • the main metals or alloys that can be used are, for example, stainless steels, stainless or refractory metals or alloys or alloy steels with high mechanical characteristics.
  • the dimensions of these particles and those of the granules are chosen mainly according to the dimensions and characteristics in particular of density of the molded parts that it is to realise.
  • each granule contains a sufficient number of spherical particles agglomerated by the gelatin. he these granules must also be able to correctly fill the mold in its every corner. However, a certain number of granules can be formed isolated elementary particles coated with gelatin without affecting the quality of the final product obtained.
  • a maximum diameter "d 1 " of the elementary spherical particles and a maximum width "d 2 " of the granules obtained are determined. It is found that the powder according to the invention must preferably have a ratio of d 2 / d 1 ⁇ 3 so that the cold-pressure molded parts reach sufficient mechanical strength.
  • this ratio d 2 / d 1 can reach at least 4 or more.
  • a maximum width "d 2 " of the granules of 300 microns constitutes a lower limit.
  • the gelatin content should be determined based on the average size of the elementary spherical particles which are agglomerated by gelatin. This content also depends on the jelly strength of the gelatin. This jelly force is expressed in Bloom (standardized unit) and can vary between 50 and 250 blooms depending on the gelatin used.
  • the granules of which the elementary spherical particles are agglomerated by means of gelatin, do not stick to the walls of the molds, even when the latter heat up to temperatures of 100 ° C or higher, lubrication by a small amount of Zn stearate or other suitable lubricant can be used.
  • the invention also relates to the process for preparing a metal powder based on of elementary spherical particles suitable for cold forming by compression then sintering.
  • an agglomeration is carried out in granules of the particles elementary spherical.
  • the amount of water used being in the range of two to five times the amount of gelatin and the temperature of the water being 40 to 80 ° C.
  • the amount of gelatin to be used work depends on the size of the elementary spherical particles and also on the strength in gelatin jelly.
  • the mixture of elementary particles and gelatin solution the time necessary to obtain the wetting of the particles metal and during cooling the gradual formation of a gel.
  • Granules are thus formed, the drying of which is continued until elimination, preferably as complete as possible, water.
  • We end the operation preferably, by a final calibration which allows to individualize the granules by separating from each other and also giving them relatively dimensions regular.
  • the amount of gelatin contained in the granules is at least 0.5% of the weight of the metallic powder obtained.
  • the fines can be separated by a suitable opening sieve. It is noted that if the maximum diameter "d 1 " of the elementary spherical particles and the width "d 2 " of the granules obtained after drying are in a ratio d 2 / d 1 at least equal to 3 and, preferably, equal to or greater than 4, parts can be obtained by cold compression in a mold whose raw mechanical strength is much higher than in the case of the same powder with spherical particles not agglomerated into granules.
  • a lubricant such as zinc stearate.
  • this lubricant and the gelatin are removed by preheating the part. raw compressed at a temperature generally between 300 and 500 ° C.
  • Preheating can be carried out in air or in the presence of a neutral or reducing gas such as Ar, H 2 , NH 3 or the like. After complete elimination of the gelatin and of the lubricant, if any, sintering is carried out at a temperature appropriate to the material to obtain the desired densification.
  • a neutral or reducing gas such as Ar, H 2 , NH 3 or the like.
  • the examples below describe, without limitation, the characteristics of the metallic powder agglomerated into granules based on spherical particles elementary suitable for cold compression forming and subsequent sintering the invention.
  • the examples also describe, without limitation also, a mode for producing such an agglomerated powder and an embodiment of this powder for making parts by compression in a mold then sintering.
  • Example 1 - This example relates to the process according to the invention for preparation of a metallic powder agglomerated into granules from spherical particles elementary which has the properties according to the invention of aptitude for forming by cold compression and sintering.
  • Spherical particles obtained in known manner are used atomization by a neutral gas of a stainless steel bath whose composition corresponds to grade 316 defined by the ASTM standard.
  • a batch is prepared by sieving of these particles, the particle diameter of which is not more than 106 microns.
  • An aqueous solution based on deionized water containing by weight 30% is prepared a gelatin whose jelly strength is 50 blooms. We heat the solution between 50 and 70 ° C to completely dissolve the gelatin.
  • a mixture is produced containing 95% of 316 steel particles of not more than 106 microns in diameter and 5% aqueous solution, or 1.5% by weight of gelatin. We must make an intimate mixture in order to wet the entire surface of the elementary particles.
  • Granules are thus obtained.
  • the drying of these by cold or hot air is continued then a second sieving is carried out in order to separate the granules from each other of the others and to calibrate them by passing through a 500 micron mesh screen.
  • Granules are thus obtained whose dimensional ratio is at least 4.7 compared to the maximum diameter of metallic particles.
  • These dried granules are made up of agglomerated spherical metallic particles, firmly linked between them by gelatin films, certain granules can however be constituted by isolated elementary particles coated with gelatin.
  • the powder thus agglomerated into granules is capable of forming by cold compression in a mold of parts having raw mechanical strength much higher than that obtained with the starting metal particles.
  • Adding a small amount of a lubricant to the agglomerated powder in granules such as zinc stearate further facilitates forming.
  • the release is also facilitated by the fact that the solidified gelatin does not stick to the walls of the molds when these are heated.
  • the agglomerated powder in granules according to the invention is produced, added with about 0.75% by weight of zinc stearate a series of test tubes cold compression tensile in a mold according to standard ASTM B312 under two different compression ratios of 314 and 422 MPa.
  • a comparative test is made on the same grade of stainless steel 316 using spherical particles having a maximum diameter of not more than 150 microns.
  • Agglomeration with gelatin is carried out with the same concentrations of gelatin and the same sieving conditions as above, the diameter of the granules obtained not exceeding 500 microns.
  • the dimensional ratio between granules and spherical particles is therefore reduced to 3.3.
  • Example 2 A metallic powder is produced by the process described in Example 1 agglomerated into granules from stainless steel type 904 L which contains in% by weight: Cr 20; Ni 25; Mo 4.5; Cu 2: remains Fe.
  • the starting material contains spherical particles with a diameter not greater than 106 microns. Agglomeration is carried out in the same way as in the case of steel 316 described in Example 1, the final calibration of the granules being carried out by through a 500 micron mesh screen, the dimensional ratio between granules and spherical particles therefore being 4.7.
  • a corrosion test, carried out at room temperature, in 1% sulfuric solution does not show any attack after 48 hours.

Abstract

The invention relates to the production of parts by means of powder metallurgy and, more particularly, to the use of powders with spherical particles such as those obtained by spraying a liquid alloy or metal in a gas. The metallic powder according to the invention consists of granules, each of these including a group of spherical particles agglomerated by at least 0.5% of gelatin. This powder is prepared by mixing an aqueous gelatin solution with the spherical particles; in this way, a paste is obtained which is divided up so as to obtain granules by means of calibrating and drying. Application to the production of parts made from stainless steels or alloys of refractory or other types.

Description

La poudre métallique qui fait l'objet de l'invention concerne la réalisation de pièces par compression et frittage à partir d'aciers inoxydables, d'autres métaux ou alliages inoxydables ou réfractaires et d'aciers alliés, destinés à la réalisation de pièces de qualité.The metal powder which is the subject of the invention relates to the production of parts by compression and sintering from stainless steels, other metals or alloys stainless or refractory and alloy steel, intended for the production of parts quality.

Pour la réalisation de ces pièces par compression et frittage, on utilise de façon la plus courante des poudres à particules anguleuses obtenues par pulvérisation de métal liquide par jet d'eau sous pression selon diverses méthodes connues de l'homme de métier.For the production of these parts by compression and sintering, the most common powder with angular particles obtained by metal spraying liquid by pressurized water jet according to various methods known to the man of job.

Bien que les méthodes de pulvérisation de métal liquide par jet d'eau sous pression soient de très loin les plus utilisées pour la fabrication de poudres métalliques elles ont le grave inconvénient de conduire à des poudres métalliques ayant une forte teneur en oxygène.Although methods of spraying liquid metal by pressurized water jets are by far the most used for the manufacture of metallic powders they have the serious disadvantage of leading to metallic powders having a strong oxygen content.

On sait aussi préparer des poudres métalliques par pulvérisation d'un métal ou alliage liquide au moyen d'un jet gazeux qui peut être, par exemple, un gaz neutre tel que l'argon ou de l'azote ou tout gaz approprié. On obtient ainsi des poudres ayant une teneur en oxydes beaucoup plus faible que les poudres obtenues par pulvérisation à l'eau et qui présentent ainsi sensiblement la même pureté que le métal de départ.We also know how to prepare metal powders by spraying a metal or alloy liquid by means of a gas jet which can be, for example, a neutral gas such as argon or nitrogen or any suitable gas. This gives powders having a much lower oxide content than powders obtained by spraying water and which thus have substantially the same purity as the starting metal.

Cependant les particules élémentaires de ces poudres ont une forme sensiblement sphérique mais en l'état ne se mettent pas facilement en forme.However, the elementary particles of these powders have a substantially shaped spherical but in the state is not easily shaped.

En effet ces poudres permettent, après compression et frittage, d'obtenir par compression des pièces ayant une densité apparente au moins égale à celle que permettent d'obtenir les poudres anguleuses habituelles. Mais, au stade initial de la mise en forme par compression à froid de la poudre dans un moule, la pièce crue obtenue à partir de particules sphériques a une résistance mécanique souvent insuffisante pour permettre sa manipulation et notamment son éjection du moule, puis le transfert en direction du four de frittage, sans écaillage ou fissuration.Indeed, these powders allow, after compression and sintering, to obtain by compression of parts having an apparent density at least equal to that allow to obtain the usual angular powders. But, at the initial stage of the shaping by cold compression of the powder in a mold, the raw part obtained from spherical particles has a mechanical resistance often insufficient to allow its manipulation and in particular its ejection from the mold, then transfer to the sintering furnace, without chipping or cracking.

Pour certaines applications qui ne concernent pas les poudres à particules sphériques, on a proposé d'ajouter aux poudres utilisées pour l'obtention de pièces par compression et frittage, différents liants à base de composés organiques. Ainsi, le brevet US 4 456 484 décrit l'addition d'un liant organique à un mélange d'une poudre d'un carbure métallique réfractaire avec une autre poudre métallique, jouant elle-même le rôle de liant métallique. On emploie comme liant une amide qui est une cire. On effectue un broyage du mélange de ces 3 composants en présence d'un liquide tel que de l'eau dans lequel la cire est insoluble et, après séchage, la cire assurant une liaison entre les particules de carbure et celles du liant métallique, on effectue la compression puis le frittage.For certain applications which do not concern powders with spherical particles, it has been proposed to add to the powders used for obtaining parts by compression and sintering, different binders based on organic compounds. So the patent US 4,456,484 describes the addition of an organic binder to a mixture of a powder of a refractory metallic carbide with another metallic powder, playing the role itself of metallic binder. An amide which is a wax is used as a binder. We perform a grinding the mixture of these 3 components in the presence of a liquid such as water in which the wax is insoluble and, after drying, the wax ensuring a connection between the carbide particles and those of the metal binder, compression is carried out and then sintering.

La demande de brevet GB 2 228 744 décrit une méthode analogue pour solidariser des éléments d'alliages tels que le graphite, le phosphore ou autres avec un métal de base. Selon cette méthode, on utilise comme liant un mélange d'un ester d'acide acrylique avec un ester d'acide méthacrylique et un acide polymérisable non saturé.Patent application GB 2 228 744 describes a similar method for joining together elements of alloys such as graphite, phosphorus or others with a base metal. According to this method, a mixture of an acrylic acid ester is used as binder with a methacrylic acid ester and an unsaturated polymerizable acid.

Ces composants sont mis en solution dans du toluène.These components are dissolved in toluene.

Le but visé est d'éviter la ségrégation de particules plus fines ou plus légères telles que le graphite ou d'autres par rapport à un métal de base, tel que le fer, en liant ces particules entre elles.The aim is to avoid the segregation of finer or lighter particles such as graphite or others compared to a base metal, such as iron, by bonding these particles between them.

Il n'est, nulle part, envisagé dans ces documents d'utiliser de tels liants organiques pour améliorer la résistance mécanique crue de pièces moulées.Nowhere in these documents is it envisaged to use such organic binders for improve the raw mechanical strength of molded parts.

Par ailleurs, ces liants après mise en oeuvre, doivent être éliminées avant ou au cours du frittage et on constate souvent une contamination des poudres métalliques par certains composants des liants.Furthermore, these binders after implementation, must be removed before or during the sintering and contamination of metallic powders by some is often observed components of binders.

Enfin, on se rend compte qu'il ne suffit pas d'améliorer la liaison entre une petite quantité d'une poudre fine telle que le graphite et les particules de poudre par exemple de fer plus grossières pour améliorer la résistance mécanique d'une pièce moulée.Finally, we realize that it is not enough to improve the bond between a small quantity of a fine powder such as graphite and powder particles for example of iron more coarse to improve the mechanical resistance of a molded part.

On connaít par ailleurs par la demande de brevet EP 0029389 un procédé de fabrication par moulage par injection de pièces de forme non fragiles mettant en oeuvre des poudres métalliques à grains sphériques.We also know from patent application EP 0029389 a manufacturing process by injection molding of non-fragile shaped parts using metallic powders with spherical grains.

Selon ce procédé, on fabrique une pâte par mélange de poudre métallique, de lubrifiant du genre stéarate, de gomme cellulosique et d'eau, on injecte cette pâte dans un moule de compaction à froid, on étuve éventuellement et on effectue une opération de déliantage - frittage des pièces crues obtenues.According to this process, a paste is made by mixing metallic powder, lubricant of the stearate type, of cellulose gum and water, this paste is injected into a cold compaction mold, optionally drying and carrying out an operation of debinding - sintering of the raw parts obtained.

De telles opérations nécessitent des équipements d'injection de pâte humide qui s'apparentent à ceux de moulage par injection de matière plastique et sont différents des équipements de compaction à froid de poudres. De plus, il est nécessaire d'utiliser la pâte immédiatement après confection en respectant des conditions strictes de temps et de température pour obtenir des pièces de forme correcte à l'éjection des moules.Such operations require wet pulp injection equipment which are similar to those of plastic injection molding and are different from cold powder compaction equipment. In addition, it is necessary to use the dough immediately after making, respecting strict time conditions and temperature to obtain parts of correct shape when ejecting the molds.

On connaít en outre par la demande de brevet DE 40.27.887 des granulés constitués de poudres du genre herbicide, colorant ou sucre alimentaire agglomérées à l'aide de gélatine dans le but de réduire les poussières inhérentes à l'utilisation de telles poudres brutes ainsi que de rendre les granulés obtenus biodégradables.We also know from patent application DE 40.27.887 granules consisting of powders of the herbicide, coloring or food sugar type agglomerated using gelatin in order to reduce the dust inherent in the use of such powders as well as making the granules obtained biodegradable.

Mais ce document ne divulgue ni ne suggère aucune application ou aucun intérêt particulier à appliquer de tels granulés à des poudres métalliques pour réaliser par compaction à sec et à froid des pièces de forme qui auraient des propriétés particulières. On a recherché la possibilité de modifier les caractéristiques d'aptitude au formage d'une poudre métallique constituée de particules sphériques, en vue d'obtenir, après formage, des pièces présentant une résistance mécanique à cru bien supérieure à celle résultant de la compaction de ladite poudre utilisée telle quelle. But this document does not disclose or suggest any application or interest particular to apply such granules to metal powders to achieve by dry and cold compaction of shaped parts which would have particular properties. We looked for the possibility of modifying the aptitude for forming characteristics of a metallic powder made up of spherical particles, in order to obtain, after forming, parts with a raw mechanical strength much higher than that resulting compaction of said powder used as it is.

On a recherché aussi la possibilité de conférer à cette poudre métallique de telles caractéristiques de façon durable, cette poudre formable et frittable étant apte au transport sur de longues distances et au stockage sans perte de ses caractéristiques d'aptitude au formage puis au frittage.We also looked for the possibility of giving this metallic powder such characteristics in a sustainable manner, this formable and sinterable powder being suitable for long distance transport and storage without loss of its characteristics aptitude for forming then sintering.

On a recherché enfin la possibilité d'une mise en oeuvre facile d'une telle poudre, les additifs éventuels ne nécessitant pas de traitements particuliers avant frittage pour leur élimination en dehors éventuellement d'un traitement de courte durée, vers 300 à 500°C, d'élimination des lubrifiants tels que le stéarate de zinc, traitement habituel dans le procédé de compression-frittage de poudres métalliques.Finally, we looked for the possibility of easy implementation of such a powder, the any additives which do not require special treatment before sintering for their elimination possibly outside of a short-term treatment, around 300 to 500 ° C, elimination of lubricants such as zinc stearate, usual treatment in the compression-sintering process of metallic powders.

La poudre métallique à base de particules sphériques, apte au formage à froid par compression suivi d'un frittage, qui fait l'objet de l'invention ainsi que le procédé de préparation de cette poudre qui fait aussi l'objet de l'invention permettent de résoudre l'ensemble des problèmes ainsi posés.Metallic powder based on spherical particles, suitable for cold forming by compression followed by sintering, which is the subject of the invention as well as the method of preparation of this powder which is also the subject of the invention make it possible to resolve all the problems thus posed.

La poudre métallique suivant l'invention est constituée par un ensemble de granules comprenant chacun un groupe de particules métalliques élémentaires de forme sphérique agglomérées par de la gélatine à une teneur d'au moins 0,5 % du poids de la poudre métallique.The metal powder according to the invention consists of a set of granules each comprising a group of elementary metallic particles of shape spherical agglomerated with gelatin at a content of at least 0.5% of the weight of metallic powder.

Les particules sphériques sont avantageusement obtenues par un procédé de pulvérisation, au moyen d'un gaz qui peut être l'air ou un gaz neutre ou réducteur tel que N2, H2, NH3, Ar, ou autre, d'un métal ou alliage liquide.The spherical particles are advantageously obtained by a spraying process, by means of a gas which can be air or a neutral or reducing gas such as N 2 , H 2 , NH 3 , Ar, or other, of a metal or liquid alloy.

Les principaux métaux ou alliages qui peuvent être utilisés sont, par exemple, des aciers inoxydables, des métaux ou alliages inoxydables ou réfractaires ou encore des aciers alliés à hautes caractéristiques mécaniques. Les dimensions de ces particules élémentaires et celles des granules sont choisies principalement en fonction des dimensions et caractéristiques notamment de densité des pièces moulées qu'il s'agit de réaliser.The main metals or alloys that can be used are, for example, stainless steels, stainless or refractory metals or alloys or alloy steels with high mechanical characteristics. The dimensions of these particles and those of the granules are chosen mainly according to the dimensions and characteristics in particular of density of the molded parts that it is to realise.

On constate que pour obtenir, par formage sous pression à froid, une pièce présentant une résistance mécanique suffisante à cru, il est préférable que chaque granule comporte un nombre suffisant de particules sphériques agglomérées par la gélatine. Il faut aussi que ces granules soient aptes à remplir correctement le moule dans ses moindres recoins. Cependant un certain nombre de granules peuvent être constitués de particules élémentaires isolées enrobées de gélatine sans que cela nuise à la qualité du produit final obtenu.It can be seen that to obtain, by cold pressure forming, a part having sufficient raw mechanical strength, it is preferable that each granule contains a sufficient number of spherical particles agglomerated by the gelatin. he these granules must also be able to correctly fill the mold in its every corner. However, a certain number of granules can be formed isolated elementary particles coated with gelatin without affecting the quality of the final product obtained.

Dans la pratique, on détermine un diamètre maximal "d1" des particules sphériques élémentaires et une largeur maximale "d2" des granules obtenus. On constate que la poudre suivant l'invention doit avoir, de préférence, un rapport de d2/d1 ≥3 pour que les pièces moulées sous pression à froid atteignent une résistance mécanique suffisante.In practice, a maximum diameter "d 1 " of the elementary spherical particles and a maximum width "d 2 " of the granules obtained are determined. It is found that the powder according to the invention must preferably have a ratio of d 2 / d 1 ≥ 3 so that the cold-pressure molded parts reach sufficient mechanical strength.

De façon particulièrement avantageuse ce rapport d2/d1 peut atteindre au moins 4 ou davantage. Dans le cas, par exemple, d'une poudre dont les particules sphériques ont un diamètre maximal "d1" de 100 microns, une largeur maximale "d2" des granules de 300 microns constitue une limite inférieure.In a particularly advantageous manner, this ratio d 2 / d 1 can reach at least 4 or more. In the case, for example, of a powder whose spherical particles have a maximum diameter "d 1 " of 100 microns, a maximum width "d 2 " of the granules of 300 microns constitutes a lower limit.

Pour obtenir des résultats encore meilleurs, il faut donner aux granules une largeur maximale "d2" de, par exemple, 500 microns correspondant donc à un rapport d2/d1 =5.To obtain even better results, the granules must be given a maximum width "d 2 " of, for example, 500 microns therefore corresponding to a ratio d 2 / d 1 = 5.

La teneur en gélatine doit être déterminée en fonction de la grosseur moyenne des particules sphériques élémentaires qui sont agglomérées par la gélatine. Cette teneur dépend aussi de la force en gelée de la gélatine. Cette force en gelée s'exprime en Bloom (unité standardisée) et peut varier entre 50 et 250 blooms en fonction de la gélatine utilisée.The gelatin content should be determined based on the average size of the elementary spherical particles which are agglomerated by gelatin. This content also depends on the jelly strength of the gelatin. This jelly force is expressed in Bloom (standardized unit) and can vary between 50 and 250 blooms depending on the gelatin used.

L'utilisation de gélatines à plus fort Bloom peut permettre de réduire le pourcentage en gélatine des granules et donc porter à une valeur minimum la durée d'élimination de la gélatine avant d'atteindre la phase de frittage proprement dite à haute température.Using Stronger Bloom Gelatines May Reduce Percentage in gelatin granules and therefore bring to a minimum value the elimination time gelatin before reaching the actual high sintering phase temperature.

Bien que les granules, dont les particules sphériques élémentaires sont agglomérées au moyen de gélatine, ne collent pas aux parois des moules, même lorsque celles-ci s'échauffent à des températures atteignant 100°C ou davantage, une lubrification par une faible quantité de stéarate de Zn ou d'un autre lubrifiant adapté peut être utilisée.Although the granules, of which the elementary spherical particles are agglomerated by means of gelatin, do not stick to the walls of the molds, even when the latter heat up to temperatures of 100 ° C or higher, lubrication by a small amount of Zn stearate or other suitable lubricant can be used.

L'invention concerne aussi le procédé de préparation d'une poudre métallique à base de particules sphériques élémentaires aptes au formage à froid par compression puis au frittage. The invention also relates to the process for preparing a metal powder based on of elementary spherical particles suitable for cold forming by compression then sintering.

Selon ce procédé, on effectue une agglomération en granules des particules sphériques élémentaires. Pour cela on ajoute aux particules sphériques élémentaires de départ de la gélatine sous forme d'une solution aqueuse, la quantité d'eau utilisée étant de l'ordre de deux à cinq fois la quantité de gélatine et la température de l'eau étant de 40 à 80°C. Comme indiqué plus haut, la quantité de gélatine à mettre en oeuvre dépend de la grosseur des particules sphériques élémentaires et aussi de la force en gelée de la gélatine. On triture le mélange de particules élémentaires et de solution de gélatine le temps nécessaire pour obtenir le mouillage des particules métalliques et au cours du refroidissement la formation progressive d'un gel. On effectue, de préférence, un séchage partiel, par exemple, par soufflage d'un courant gazeux qui permet de donner au mélange une consistance pâteuse, puis on effectue une fragmentation de cette pâte, par exemple en la pressant sur un tamis dont la largeur de maille est déterminée en tenant compte du diamètre des particules sphériques élémentaires.According to this method, an agglomeration is carried out in granules of the particles elementary spherical. For this we add to the elementary spherical particles starting gelatin in the form of an aqueous solution, the amount of water used being in the range of two to five times the amount of gelatin and the temperature of the water being 40 to 80 ° C. As indicated above, the amount of gelatin to be used work depends on the size of the elementary spherical particles and also on the strength in gelatin jelly. The mixture of elementary particles and gelatin solution the time necessary to obtain the wetting of the particles metal and during cooling the gradual formation of a gel. We preferably performs partial drying, for example by blowing a stream gas which gives the mixture a pasty consistency, then fragmentation of this dough, for example by pressing it on a sieve whose mesh width is determined taking into account the particle diameter elementary spherical.

On forme ainsi des granules dont on poursuit le séchage jusqu'à élimination, de préférence aussi complète que possible, de l'eau. On termine l'opération, de préférence, par un calibrage final qui permet de bien individualiser les granules en les séparant les uns des autres et aussi de leur donner des dimensions relativement régulières. La quantité de gélatine contenue dans les granules est d'au moins 0,5 % du poids de la poudre métallique obtenue.Granules are thus formed, the drying of which is continued until elimination, preferably as complete as possible, water. We end the operation, preferably, by a final calibration which allows to individualize the granules by separating from each other and also giving them relatively dimensions regular. The amount of gelatin contained in the granules is at least 0.5% of the weight of the metallic powder obtained.

On peut séparer les fines par un tamis d'ouverture convenable. On constate que si le diamètre maximal "d1" des particules sphériques élémentaires et la largeur "d2" des granules obtenus après séchage sont dans un rapport d2/d1 au moins égal à 3 et, de préférence, égal ou supérieur à 4, on peut obtenir par compression à froid dans un moule des pièces dont la résistance mécanique à cru est beaucoup plus élevée que dans le cas de la même poudre à particules sphériques non agglomérées en granules.The fines can be separated by a suitable opening sieve. It is noted that if the maximum diameter "d 1 " of the elementary spherical particles and the width "d 2 " of the granules obtained after drying are in a ratio d 2 / d 1 at least equal to 3 and, preferably, equal to or greater than 4, parts can be obtained by cold compression in a mold whose raw mechanical strength is much higher than in the case of the same powder with spherical particles not agglomerated into granules.

On peut, avantageusement, avant formage, incorporer à la poudre ainsi constituée de granules, un lubrifiant tel que le stéarate de zinc.It is advantageously possible, before forming, to incorporate into the powder thus constituted of granules, a lubricant such as zinc stearate.

Après compression, on élimine ce lubrifiant et la gélatine par préchauffage de la pièce crue comprimée à une température généralement comprise entre 300 et 500°C. After compression, this lubricant and the gelatin are removed by preheating the part. raw compressed at a temperature generally between 300 and 500 ° C.

Le préchauffage peut être effectué à l'air ou en présence d'un gaz neutre ou réducteur tel que Ar, H2, NH3 ou autre. Après élimination complète de la gélatine et du lubrifiant, s'il y en a, on effectue le frittage à température appropriée au matériau pour obtenir la densification désirée.Preheating can be carried out in air or in the presence of a neutral or reducing gas such as Ar, H 2 , NH 3 or the like. After complete elimination of the gelatin and of the lubricant, if any, sintering is carried out at a temperature appropriate to the material to obtain the desired densification.

Après refroidissement, on constate, tous les paramètres étant inchangés par ailleurs, que les pièces ainsi obtenues, telles que par exemple des pièces en aciers inoxydables, ont une densité apparente en général supérieure à celle de pièces préparées à partir de poudres anguleuses de même composition et aussi des caractéristiques mécaniques supérieures en ce qui concerne la ductilité.After cooling, it can be seen, all the parameters being otherwise unchanged, as the parts thus obtained, such as for example steel parts stainless, have an apparent density generally higher than that of parts prepared from angular powders of the same composition and also superior mechanical characteristics with regard to ductility.

Il apparaít que pratiquement tous les métaux ou alliages inoxydables ou réfractaires susceptibles d'être pulvérisés sous forme de particules sphériques peuvent subir une transformation par métallurgie des poudres grâce au procédé d'agglomération en granules des particules sphériques suivant l'invention.It appears that virtually all stainless or refractory metals or alloys likely to be sprayed as spherical particles may undergo transformation by powder metallurgy using the agglomeration process into granules of spherical particles according to the invention.

Les exemples ci-après décrivent, de façon non limitative, les caractéristiques de la poudre métallique agglomérée en granules à base de particules sphériques élémentaires apte au formage par compression à froid puis au frittage suivant l'invention. Les exemples décrivent aussi, de façon non limitative également, un mode de réalisation d'une telle poudre agglomérée et un mode de mise en oeuvre de cette poudre pour la réalisation de pièces par compression dans un moule puis frittage.The examples below describe, without limitation, the characteristics of the metallic powder agglomerated into granules based on spherical particles elementary suitable for cold compression forming and subsequent sintering the invention. The examples also describe, without limitation also, a mode for producing such an agglomerated powder and an embodiment of this powder for making parts by compression in a mold then sintering.

Exemple 1 - Cet exemple concerne le procédé suivant l'invention de préparation d'une poudre métallique agglomérée en granules à partir de particules sphériques élémentaires qui présente les propriétés suivant l'invention d'aptitude au formage par compression à froid et au frittage.Example 1 - This example relates to the process according to the invention for preparation of a metallic powder agglomerated into granules from spherical particles elementary which has the properties according to the invention of aptitude for forming by cold compression and sintering.

On met en oeuvre des particules sphériques obtenues de façon connue par pulvérisation par un gaz neutre d'un bain d'acier inoxydable dont la composition correspond au grade 316 défini par la norme ASTM. On prépare par tamisage un lot de ces particules dont le diamètre des particules n'est pas supérieur à 106 microns. On prépare une solution aqueuse à base d'eau désionisée contenant en poids 30 % d'une gélatine dont la force en gelée est de 50 blooms. On chauffe la solution entre 50 et 70°C pour mettre en solution la gélatine de façon complète. Spherical particles obtained in known manner are used atomization by a neutral gas of a stainless steel bath whose composition corresponds to grade 316 defined by the ASTM standard. A batch is prepared by sieving of these particles, the particle diameter of which is not more than 106 microns. An aqueous solution based on deionized water containing by weight 30% is prepared a gelatin whose jelly strength is 50 blooms. We heat the solution between 50 and 70 ° C to completely dissolve the gelatin.

On réalise un mélange contenant 95 % de particules d'acier 316 de pas plus de 106 microns de diamètre et 5 % de solution aqueuse, soit 1,5 % en poids de gélatine. On doit réaliser un mélange intime afin de mouiller par la solution toute la surface des particules élémentaires.A mixture is produced containing 95% of 316 steel particles of not more than 106 microns in diameter and 5% aqueous solution, or 1.5% by weight of gelatin. We must make an intimate mixture in order to wet the entire surface of the elementary particles.

Le refroidissement progressif de la solution entraíne la formation du gel. On fait évaporer une partie de l'eau par soufflage d'air et on fait passer le mélange qui a une consistance pâteuse, à travers un tamis ayant des mailles d'environ 630 microns.The gradual cooling of the solution leads to the formation of the gel. We do evaporate part of the water by blowing air and pass the mixture which has a pasty consistency, through a sieve with meshes of about 630 microns.

On obtient ainsi des granules. Le séchage de ceux-ci par air froid ou chaud est poursuivi puis un deuxième tamisage est effectué afin de séparer les granules les uns des autres et de les calibrer par passage à travers un tamis à mailles de 500 microns.Granules are thus obtained. The drying of these by cold or hot air is continued then a second sieving is carried out in order to separate the granules from each other of the others and to calibrate them by passing through a 500 micron mesh screen.

On obtient ainsi des granules dont le rapport dimensionnel est d'au moins 4,7 comparé au diamètre maximal des particules métalliques. Ces granules séchés sont constitués de particules métalliques sphériques agglomérées, solidement liées entre elles par des films de gélatine, certains granules pouvant cependant être constitués par des particules élémentaires isolées enrobées de gelatine.Granules are thus obtained whose dimensional ratio is at least 4.7 compared to the maximum diameter of metallic particles. These dried granules are made up of agglomerated spherical metallic particles, firmly linked between them by gelatin films, certain granules can however be constituted by isolated elementary particles coated with gelatin.

On constate que la poudre ainsi agglomérée en granules est apte à former par compression à froid dans un moule des pièces ayant une résistance mécanique à cru très supérieure à celle qu'on obtient avec les particules métalliques de départ. L'addition, à la poudre agglomérée en granules, d'une faible quantité d'un lubrifiant tel que le stéarate de zinc facilite encore le formage. Le démoulage est également facilité par le fait que la gélatine solidifiée ne colle pas aux parois des moules lorsque ceux-ci sont échauffés.It is found that the powder thus agglomerated into granules is capable of forming by cold compression in a mold of parts having raw mechanical strength much higher than that obtained with the starting metal particles. Adding a small amount of a lubricant to the agglomerated powder in granules such as zinc stearate further facilitates forming. The release is also facilitated by the fact that the solidified gelatin does not stick to the walls of the molds when these are heated.

On réalise ainsi au moyen de la poudre agglomérée en granules suivant l'invention, additionnée d'environ 0,75 % en poids de stéarate de zinc une série d'éprouvettes de traction par compression à froid dans un moule selon la norme ASTM B312 sous deux taux de compression différents de 314 et 422 MPa.In this way, the agglomerated powder in granules according to the invention is produced, added with about 0.75% by weight of zinc stearate a series of test tubes cold compression tensile in a mold according to standard ASTM B312 under two different compression ratios of 314 and 422 MPa.

Les mesures de résistance mécanique à la traction effectuées à cru sur une partie de ces éprouvettes donnent des résultats de charge de rupture de respectivement 6,55 et 9,65 MPa (950 et 1400 psi). Ces valeurs sont tout à fait satisfaisantes car bien supérieures au minimum considéré comme acceptable de 3,44 MPa (500 psi). The mechanical tensile strength measurements taken bare on a part of these test pieces give breaking load results of 6.55 and 9.65 MPa (950 and 1400 psi). These values are completely satisfactory because greater than the minimum considered acceptable of 3.44 MPa (500 psi).

Les éprouvettes restantes, comprimées sous une charge de 422 MPa, sont préchauffées à l'air jusque vers 500°C pour éliminer la gélatine et le stéarate de zinc puis frittées par chauffage jusqu'à environ 1280°C. Des essais de traction effectués sur les éprouvettes ainsi frittées donnent les résultats moyens suivants :

  • Limite élastique 137,9 MPa (20 ksi)
  • Charge de rupture 344,7 MPa (50 ksi)
  • Allongement de rupture 25 %.
    • The remaining test pieces, compressed under a load of 422 MPa, are preheated in air to around 500 ° C to remove the gelatin and zinc stearate and then sintered by heating to about 1280 ° C. Tensile tests carried out on the specimens thus sintered give the following average results:
  • Elastic limit 137.9 MPa (20 ksi)
  • Breaking load 344.7 MPa (50 ksi)
  • Elongation at break 25%.

    • Selon la norme ASTM B525, les valeurs typiques de caractéristiques mécaniques de cet acier 316 sont :

  • Charge de rupture 413,7 MPa (60 ksi)
  • Allongement de rupture 7 %.
    • According to ASTM B525, the typical values of mechanical characteristics of this 316 steel are:
  • Breaking load 413.7 MPa (60 ksi)
  • Elongation at break 7%.

    • Un essai comparatif est fait sur la même nuance d'acier inoxydable 316 en utilisant des particules sphériques ayant un diamètre maximal pas supérieur à 150 microns.A comparative test is made on the same grade of stainless steel 316 using spherical particles having a maximum diameter of not more than 150 microns.

    L'agglomération par de la gélatine est effectuée avec les mêmes concentrations en gélatine et les mêmes conditions de tamisage que ci-dessus, le diamètre des granules obtenus n'étant pas supérieur à 500 microns. Le rapport dimensionnel entre granules et particules sphériques est donc réduit à 3,3.Agglomeration with gelatin is carried out with the same concentrations of gelatin and the same sieving conditions as above, the diameter of the granules obtained not exceeding 500 microns. The dimensional ratio between granules and spherical particles is therefore reduced to 3.3.

    On constate qu'on obtient dans ces conditions une résistance mécanique à cru des éprouvettes moulées sous pression beaucoup plus faible que dans l'exemple précédent et on constate de plus une tendance à la fissuration des éprouvettes de traction au cours du frittage.It is found that under these conditions a raw mechanical strength of the much lower pressure die-cast than in the example previous and there is moreover a tendency to cracking of the test specimens traction during sintering.

    Dans un autre essai comparatif, on utilise à nouveau de l'acier 316 composé de particules de diamètre pas supérieur à 106 microns mais on réduit la dimension des granules par calibrage final à travers des trous de 300 microns de côté. Le rapport dimensionnel est donc réduit à 2,8. On constate qu'en essayant de mouler par compression à froid des éprouvettes de traction on observe des ruptures de celles-ci dès le démoulage.In another comparative test, 316 steel composed of particles no larger than 106 microns in diameter but the size of the granules by final calibration through holes of 300 microns per side. The report dimensional is therefore reduced to 2.8. We find that by trying to mold by cold compression of the tensile test pieces, rupture is observed from the release.

    On voit donc que le rapport dimensionnel de 3 entre granules et particules élémentaires sphériques est au voisinage immédiat de la limite acceptable et que dans la pratique il est avantageux de choisir un rapport dimensionnel au moins égal à 4. So we see that the dimensional ratio of 3 between granules and particles elementary spherical is in the immediate vicinity of the acceptable limit and that in in practice it is advantageous to choose a dimensional ratio at least equal to 4.

    Exemple 2 - On réalise, par le procédé décrit dans l'exemple 1, une poudre métallique agglomérée en granules à partir d'un acier inoxydable type 904 L qui contient en % en poids : Cr 20 ; Ni 25 ; Mo 4,5 ; Cu 2 : reste Fe.Example 2 - A metallic powder is produced by the process described in Example 1 agglomerated into granules from stainless steel type 904 L which contains in% by weight: Cr 20; Ni 25; Mo 4.5; Cu 2: remains Fe.

    Le produit de départ comporte des particules sphériques de diamètre pas supérieur à 106 microns. On effectue l'agglomération de la même façon que dans le cas de l'acier 316 décrit dans l'exemple 1, le calibrage final des granules étant fait par passage à travers un tamis à mailles de 500 microns de côté, le rapport dimensionnel entre granules et particules sphériques étant donc de 4,7.The starting material contains spherical particles with a diameter not greater than 106 microns. Agglomeration is carried out in the same way as in the case of steel 316 described in Example 1, the final calibration of the granules being carried out by through a 500 micron mesh screen, the dimensional ratio between granules and spherical particles therefore being 4.7.

    Après formage d'éprouvettes de traction par compression à froid avec une pression de 422 MPa puis frittage, ces opérations étant effectuées comme dans le cas de l'exemple 1, la mesure des caractéristiques mécaniques en traction donne les résultats suivants :

  • Charge de rupture en traction 448,2 MPa (65 ksi)
  • Allongement de rupture : 15 %.
    • After forming of tensile specimens by cold compression with a pressure of 422 MPa then sintering, these operations being carried out as in the case of Example 1, the measurement of the mechanical characteristics in traction gives the following results:
  • Tensile breaking load 448.2 MPa (65 ksi)
  • Elongation at break: 15%.

    • Un essai de corrosion, effectué à température ambiante, en solution sulfurique à 1 % ne fait apparaítre aucune attaque après 48 heures.A corrosion test, carried out at room temperature, in 1% sulfuric solution does not show any attack after 48 hours.

    Ces résultats, en particulier l'ensemble 2, montrent que la poudre métallique suivant l'invention comportant des granules constitués de particules sphériques élémentaires liées par de la gélatine permet la réalisation par compression et frittage de pièces en acier inoxydable qu'on ne sait pas réaliser par la méthode habituelle qui s'applique aux poudres anguleuses, l'élaboration par pulvérisation dans l'eau de tels aciers fortement alliés étant difficile à envisager du fait de la teneur en oxygène qui en résulterait.These results, in particular assembly 2, show that the following metal powder the invention comprising granules made up of elementary spherical particles bonded by gelatin allows the production by compression and sintering of parts in stainless steel that we do not know how to make by the usual method that applies with angular powders, the development by spraying in water of such steels strongly allied being difficult to envisage because of the oxygen content which would result.

    De nombreuses modifications ou variantes peuvent être apportées aux caractéristiques de la poudre agglomérée en granules qui fait l'objet de l'invention et aussi au mode de préparation suivant l'invention de cette poudre agglomérée en granules à partir des particules métalliques sphériques qui la constituent.Many modifications or variations can be made to characteristics of the powder agglomerated into granules which is the subject of the invention and also to the mode of preparation according to the invention of this powder agglomerated in granules from the spherical metallic particles which constitute it.

    Ces variantes et ces modifications font également partie de l'invention.These variants and modifications also form part of the invention.

    Claims (11)

    1. A metallic powder, capable of sintering after cold compression forming, characterised in that it is constituted by an assembly of granules, each one comprising a group of spherically shaped elementary particles agglomerated by gelatin, the quantity of which is at least 0.5% of the weight of the metallic powder.
    2. A metallic powder according to Claim 1, characterised in that the maximum diameter d1 of the spherical elementary particles and the maximum width d2 of the granules are in a d2/d1 ratio of ≥ 3.
    3. A metallic powder according to Claim 2, characterised in that the d2/d1 ratio is ≥ 4.
    4. A metallic powder according to one of Claims 1 to 3, characterised in that the content of gelatin in the granules is 1 to 5% of the weight of the metallic powder.
    5. A metallic powder according to one of Claims 1 to 4, characterised in that it is a stainless steel or an alloyed steel.
    6. A metallic powder according to one of Claims 1 to 4, characterised in that it is a stainless or refractory metal or alloy.
    7. A process for the preparation of a metallic powder capable of sintering after cold compression forming, characterised by starting out with spherically shaped elementary metallic particles with which an aqueous solution of gelatin is mixed in order to moisten the entire surface of the spherical particles, and then after a gel starts to form which gives the mixture a pasty consistency, the paste is broken in order to obtain granules, each comprising a group of elementary particles which are then dried, the gelatin content of the granules being at least 0.5% by weight of the metallic powder.
    8. A process according to Claim 7, characterised in that the gelatin content is 1 to 5% by weight of the metallic powder.
    9. A process according to Claim 7 or Claim 8, characterised in that the paste is broken by passing therethrough at least one sieve in order that after drying granules are obtained of width d2 such that the ratio between that width d2 and the maximum diameter d1 of the spherical particles is at least equal to 3.
    10. A process according to Claim 9, characterised in that the ratio d2/d1 is at least equal to 4.
    11. A process according to one of Claims 7 to 10, characterised in that the metallic powder is an alloyed steel or a stainless steel or a stainless or refractory metal or alloy.
    EP94420188A 1993-07-06 1994-07-04 Metallic powder for making parts by compression and sintering and process for obtaining the powder Expired - Lifetime EP0633083B1 (en)

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