EP0119939B1 - Process for pressure-sintering aluminium alloy powder - Google Patents

Description

The present invention relates to a process for the sintering under pressure of aluminum alloy powders.

In addition to traditional methods of molding or wrought casting products, it is known that the shaping of aluminum alloys can be carried out by compression of powders followed by sintering.

• D'une part, de manière uniaxiale, c'est-à-dire que la charge de poudre placée dans la cavité d'une matrice est soumise suivant une direction déterminée à l'action d'un poinçon mobile se rapprochant de la table de presse. Dans ces conditions, on peut atteindre en moyenne des pressions relativement élevées, de l'ordre de 800 MPa. Cependant, ce type de compression présente un inconvénient dû au fait que les couches de poudres situées au voisinage du poinçon mobile étant les plus comprimées viennent s'appuyer sur les parois latérales de la matrice, et les frottements qui en résultent diminuent d'autant l'effort de compression transmis aux couches inférieures de sorte que la masse volumique de la pièce ainsi obtenue n'est pas uniforme et décroît suivant sa hauteur. Cette hétéro- généⁱté se traduit alors par des différences de propriétés mécaniques d'un point à un autre de la pièce. Certes, l'utilisation de presses à deux poinçons mobiles, ou encore l'addition de lubrifiant tel que des stéarates permettent d'atténuer ce défaut mais dans des proportions le plus souvent insuffisantes pour atteindre l'homogénéité souhaitée.

According to this last type of process, the compression of the powders can be done in two ways:

• On the one hand, in a uniaxial manner, that is to say that the powder charge placed in the cavity of a matrix is subjected in a determined direction to the action of a movable punch approaching the table hurry. Under these conditions, relatively high pressures can be reached on average, of the order of 800 MPa. However, this type of compression has a drawback due to the fact that the layers of powders located in the vicinity of the movable punch being the most compressed come to bear on the side walls of the matrix, and the friction which results from it decreases all the more compression force transmitted to the lower layers so that the density of the part thus obtained is not uniform and decreases along its height. This heterogeneous ⁱ ty is then reflected by differences in mechanical properties from one point to another of the room. Admittedly, the use of presses with two movable punches, or the addition of lubricant such as stearates makes it possible to attenuate this defect but in proportions most often insufficient to achieve the desired homogeneity.

On the other hand, the compression can also be carried out isostatically, that is to say that the powder is placed in a mold made of flexible and impermeable material such as rubber or a plastic material which is immersed in a fluid on which pressure is exerted.

The advantage of this isostatic compression results in obtaining a uniform density at all points of the part and, consequently, a homogeneity of its mechanical properties. However, the allowable pressures in this type of compression are at most 300 MPa and, therefore, lower than those that can be achieved in one-way presses.

Whatever the way of compressing it, the part resulting from a simple cold compression of powders is certainly relatively resistant but it totally lacks plasticity and has significant porosity. This is why, after compression, consolidation is carried out by a heat treatment called _" sintering during which the boundaries between powder grains disappear and a piece of lower porosity and better plasticity is produced, the properties of which are mechanical are still quite different from those of parts obtained by traditional metallurgy.

However, it has been observed that the properties of the sintered parts can be significantly improved by combining compression and sintering in a single operation, which is known as pressure sintering or hot compression.

This pressure sintering can be carried out according to the two types of compression mentioned above. Thus, in uniaxial, the pressure of the punch is applied to the powder while simultaneously heating the matrix, while in isostatic, the powder is first placed in a metal capsule, then the assembly is brought to the appropriate temperature. at the same time exerting pressure by means of a hot and inert gas such as argon or nitrogen.

However, there are hot defects inherent in each of the types of compression, namely, for the uniaxial, different compression states from one point to another of the part and, consequently, a heterogeneity of the mechanical properties; for the isostatic, a pressure limited to 300 MPa, a disadvantage to which will be added problems of rise and fall cycle times in pressure and in temperature relatively long compared to those required by uniaxial operations.

Admittedly, a compromise between the two types of compression allowing both the use of a unidirectional press and the obtaining of pressures of up to 1000 MPa was achieved in EP-A-039 014. In fact, this document describes a sintering process under pressure of metal powder, according to which the powder is placed in a capsule, degassing is carried out and the capsule is sealed, a medium is placed around the capsule, inside of an external capsule and the whole is pressed, at a temperature allowing the sintering of the powder, in a unidirectional press via another medium and where the first medium consists in particular of a vitreous material liquid at the sintering temperature or at least sufficiently viscous to transmit the pressures as a fluid would.

However, this process requires the presence of two compression media of relatively large thickness which require oversizing of the press.

This is why the applicant has sought to develop a pressure sintering process the aim of which is to achieve the cycle times and the pressures usually carried out in uniaxial fashion while conferring on the products produced the homogeneity of properties generally obtained in isostatic and avoiding oversizing of the press.

This process of pressure sintering of aluminum alloy powders in which the powder is placed in a capsule, seals and brings the capsule to the sintering temperature and then compresses the assembly in a unidirectional press. .tional is characterized in that the capsule filled with powder and sealed with a film of a fusible vitreous material stable and viscosity between 10 and 10 ⁴ Pascal-second is coated at the sintering temperature.

Thus, the invention consists first of all in "encapsulating the metal powder which has to be treated. One finds here one of the phases of hot isostatic compression during which the mass of powder is loaded into a capsule of shape close to the final part, loading which can moreover be carried out by vibrating the capsule so as to compact the powder and improve its compactness. After filling, the capsule is connected to a vacuum pump so as to extract the air it contains and degas the powder, then it is sealed.

The purpose of this operation is to avoid trapping gases inside the capsule capable of causing porosities within the sintered part.

The material constituting the capsule must be sufficiently ductile and thin to be able to deform without tearing under the action of the stresses to which it is subjected during heating and compression. It must also have no polluting action on the powder. This capsule can be produced for example from a sheet of A-G3, an aluminum alloy containing about 3% of magnesium.

In this encapsulation operation, a variant consists in introducing into the capsule powder having undergone cold compression beforehand, which does not dispense with the need to evacuate the gases before closing.

This capsule is then subjected to the method according to the invention, namely first of all that it is coated over its entire surface with a vitreous material. This material can have different compositions such as those known in particular in aluminum glass spinning techniques. However, it must have the properties of being fusible and of forming, at a temperature close to the sintering temperature of the treated powder, a liquid whose viscosity is between 10 and 10 ⁴ Pascal-seconds at the treatment temperature. The viscosity of this material must not be too low so as to adhere to the surface of the capsule, nor too high so as to remain fluid. In addition, this material must be stable (that is to say in particular not recrystallize) for the duration of the operation. A material of this type is described in particular in patent FR-A-2,339,446.

The coating of the capsule can be done by spraying with a gun, coating with a brush or by dipping in the molten material or by any other suitable process. It should preferably be uniform.

The capsule thus coated is then placed in the matrix of a unidirectional press, this matrix having a shape substantially similar to that of the capsule, then brought to a sintering temperature which can be between 450 and 550 ° G and subjected to the action of the punch which can exert a pressure of up to 800 MPa.

After having stayed for a predetermined time under suitable conditions of pressure and temperature specific to the nature of the powder used, the part obtained is then freed by machining or by chemical treatment of its coating constituted by the wall of the capsule and glassy material.

Examination of this part then shows that, surprisingly, the mechanical properties such as hardness, mechanical strength and elongation, measured in different parts of the part, each exhibit remarkable uniformity, as if the latter had been performed by isostatic compression. This phenomenon can be explained by the role played by the vitreous material which. when heated, behaves like the fluid used in isostatic compression, that is to say that it does not transmit the pressure exerted by the punch in a preferred direction, but distributes it regularly on all sides of the product . The result of the process according to the invention is therefore to make it possible to combine in the same process the advantages specific to each of the types of uniaxial and isostatic compression and to eliminate the drawbacks. In addition, this process gives the possibility of working in a uniaxial press simultaneously at very high temperatures and pressures.

• De la poudre d'aluminium alliée à du magnésium, du zinc et du cuivre de granulométrie 15 J.lm Fischer a été chargée dans une capsule cylindrique de diamètre 12,5 cm et de hauteur 30 cm, constituée par une tôle d'alliage d'aluminium de type A-G3, d'épaisseur 0,2 cm et présentant un couvercle muni d'une tubulure destinée à être reliée à un groupe de pompage sous vide. Après avoir mis sous vide la capsule, la tubulure a été fermée par soudure. La capsule a alors été chauffée et revêtue au moyen d'un pistolet d'un film d'une matière vitreuse qui fondait au contact de la tôle. La capsule ainsi revêtue a été placée dans la matrice d'une presse unidirectionnelle de diamètre voisin de celui de la capsule, puis portée à une température de 500 °C. Le poinçon de la presse a été alors abaissé de manière à exercer une pression de 700 MPa, pression que l'on a maintenue pendant 5 minutes. Le poinçon ayant été relevé, on a extrait la capsule et procédé à un usinage de manière à mettre à nu totalement la pièce frittée. Celle-ci a été ensuite soumise à un traitement thermique du type T6 et on a mesuré la dureté Brinell aux différents points de sa surface : les valeurs obtenues oscillaient entre 160 et 170.

The present invention can be illustrated with the aid of the following exemplary embodiment:

• Aluminum powder alloyed with magnesium, zinc and copper with a particle size of 15 J.lm Fischer was loaded into a cylindrical capsule with a diameter of 12.5 cm and a height of 30 cm, consisting of a sheet of alloy aluminum type A-G3, 0.2 cm thick and having a cover provided with a tube intended to be connected to a vacuum pumping group. After evacuating the capsule, the tubing was closed by welding. The capsule was then heated and coated by means of a gun with a film of a vitreous material which melted in contact with the sheet. The capsule thus coated was placed in the die of a unidirectional press with a diameter close to that of the capsule, then brought to a temperature of 500 ° C. The punch of the press was then lowered so as to exert a pressure of 700 MPa, pressure which was maintained for 5 minutes. The punch having been raised, the capsule was extracted and machined so as to completely expose the sintered part. This was then subjected to a T6 type heat treatment and the Brinell hardness was measured at the various points on its surface: the values obtained varied between 160 and 170.

This shows that it is possible, according to the invention, to achieve the homogeneity of the parts, the performance of an isostatic press by simply using a unidirectional press in a temperature range from 450 to 550 ° C and while benefiting from its advantages.

The invention applies in the metallurgy of powders obtaining parts having homogeneous properties and equivalent or superior to those obtained by traditional metallurgy, these parts can be blanks or finished parts.

Claims (6)

1. A process for pressure sintering of powders of aluminium alloys wherein the powder is placed in a capsule, the capsule is sealed and raised to the sintering temperature and then the assembly is compressed in a unidirectional press characterised in that the powder-filled and sealed capsule is covered with a film of a stable fusible vitreous material with a viscosity of between 10 and 10⁴ Pascal-second at the sintering temperature.

2. A process according to claim 1 characterised in that the powder which is placed in the capsule is degassed under vacuum before sintering under pressure.

3. A process according to claim 1 characterised in that the powder was subjected to a cold compression operation before being placed in the capsule.

4. A process according to claim 1 characterised in that the capsule comprises an aluminium alloy with 3 % of magnesium.

5. A process according to claim 1 characterised in that the assembly is compressed under a pressure which can attain 800 MPa.

6. A process according to claim 1 characterised in that the sintering temperature is between 450 and 550 °C.