EP0635325B1 - Process for making a workpiece in titanium by sintering and a decorative article made by such a process - Google Patents

Description

The invention relates to a method for manufacturing a titanium part using powder technologies and in particular such a method making it possible to manufacture titanium parts by sintering titanium hydride powder (TiH ₂ ), parts whose porosity is less than about 2%. The invention also relates to a decorative article produced by such a method.

The process of the invention is more suitable particularly in the manufacture of semi-finished products made of titanium which are intended to produce articles decorative items such as watch cases, links bracelet, watch faces or the like, which once polishes have a surface of intense shine.

In recent years, the technologies of powder metallurgy, and in particular molding by injection of metal powder, made it possible to carry out titanium parts of complex shapes that could not be previously obtained only at the cost of long machining and expensive from a block of titanium.

Taking into account the pyrophoric characteristics of titanium powder and the delicate conditions of its handling and its implementation which result therefrom, the use of TiH ₂ powder, which does not present any risk of spontaneous ignition upon simple contact with l 'air, was developed for the production by sintering of titanium parts.

Such a process is described in the publication of Kei Ameyama et al., Entitled "INJECTION MOLDING OF TITANIUM POWDERS "and published by" Metal Powder Industries Federation "105 College Rd., East, Princeton, New Jersey 08540, USA, 1989, pages 121 to 126.

According to this process, the TiH ₂ powder is firstly mixed with a binder formed from a mixture of polymer, plasticizer and wax. The mixture thus obtained is then injected into a mold to obtain a part having the desired shape. The shaped part is then, firstly, freed of its binder by air heating and, secondly, it is introduced into an oven containing an atmosphere of argon, nitrogen or the vacuum in which it is gradually heated to about 1,100 ° C for sintering.

After analysis of the porosity of the parts obtained according to this process, it was found that the weakest parts porosities were obtained with sintering in the void or in the atmosphere of argon, and that the porosities reached were around 3%. This is due to the violent release of hydrogen from titanium hydride upon heating which creates a large number of bubbles or pores.

Despite this low porosity, it appears micropores on the surface of the parts after polishing these, these micropores producing a diffusion of the incident light which prevents reflection perfect specular of the light reaching the room. he this results in a mat or milky-looking surface unsightly.

The polishing of titanium parts obtained by this process therefore does not provide sufficiently smooth and shiny surfaces allowing their use as decorative pieces, so these parts are limited to technical uses in which the aesthetic aspect has no importance.

The main aim of the invention is therefore to remedy the disadvantages of the aforementioned prior art in providing a method of manufacturing by sintering a titanium part which has a very low porosity and which once polished has an aesthetic appearance fulfilling the requirements for the realization of decorative pieces.

• (a) mélanger une poudre d'hydrure de titane à un liant temporaire,
• (b) injecter le mélange obtenu dans un moule pour obtenir une pièce de forme souhaitée,
• (c) éliminer le liant ,
• (d) chauffer la pièce sous une atmosphère d'hydrogène avec un gradient de température compris entre 150°C et 250°C par heure jusqu'à une température de frittage comprise entre 1'000 at 1'400°C,
• (e) remplacer l'atmosphère d'hydrogène par du vide ou une atmosphère non réactive une fois que la température de frittage a été atteinte, et
• (f) refroidir la pièce dans l'atmosphère du gaz non réactif.

To this end the invention relates to a method of manufacturing by sintering a titanium part having a porosity of less than 2%. The process is characterized in that it consists in:

• (a) mixing a titanium hydride powder with a temporary binder,
• (b) injecting the mixture obtained into a mold to obtain a part of desired shape,
• (c) removing the binder,
• (d) heating the part under a hydrogen atmosphere with a temperature gradient of between 150 ° C and 250 ° C per hour to a sintering temperature of between 1,000 and 1,400 ° C,
• (e) replace the hydrogen atmosphere with vacuum or a non-reactive atmosphere once the sintering temperature has been reached, and
• (f) cooling the part in the atmosphere of the non-reactive gas.

According to another of its aspects, the invention relates a sintered titanium decorative article from a titanium hydride powder and having a polished surface obtained by the process described above.

Such an article thus presents, after polishing, a brighter than sintered titanium parts and polished obtained according to the processes of the prior art and is particularly well suited to the realization decorative items such as watch cases, links of bracelets or the like.

The invention will now be described in a manner detailed.

Titanium hydride powder (TiH ₂ ), having a high degree of purity (99.5%) and an average particle size of the order of a few microns, typically 10 microns, is conventionally mixed with a temporary binder in the form of granules until a homogeneous mixture is obtained.

Preferably, the binder is formed from a polymer or of a thermal copolymer but can also be formed of wax. This mixing is carried out at a temperature comprised between 120 ° and 180 ° C depending on the nature of the binder used. Typically, with a thermal copolymer, the temperature of the mixture is of the order of 170 ° C.

The paste mixture which is obtained is then conventionally injected into a mold having the shape of the part you want to obtain, for example a watch case, with dimensions that take into account shrinkage of the part during the stages subsequent to the process, narrowing which is typically around 15%. The injection is preferably carried out at a temperature of about 140 ° C.

We then proceed to the elimination of the binder contained in the shaped part. Elimination is done in agreement with the nature of the binder. Often this elimination of binder is carried out thermally. To do this, the room conformed is then introduced into an oven in which it is gradually brought to a temperature understood between 200 ° and 300 ° C. During this heating, the binder is gradually eliminated by evaporation and, to avoid not deteriorate the shape of the room, this heating takes place between 6 and 9 hours and from preferably within 8 hours. It is also important that removal of the binder is complete to avoid any pollution of the part by carbon and / or oxygen from binder, which can lead to deterioration of properties mechanical and corrosion resistance of the part to manufacture.

Preferably, the removal of the binder is carried out under vacuum or in a hydrogen atmosphere in order to part, to avoid any oxidation of the binder during its elimination, and secondly, to increase the speed of the process of removing the binder from the part without deteriorate the shape of the part.

According to a variant of the process and in particular in the If the binder is a thermal polymer, the latter can also be eliminated chemically, by decomposition using an appropriate acid vapor.

After complete removal of the binder from the part and according to a particularly important aspect of the invention, the atmosphere in the oven is replaced by an atmosphere hydrogen (if the removal of the binder has not already been performed in a hydrogen atmosphere) and preferably, this hydrogen atmosphere is produced under the shape of a flow circulating continuously in the oven. At the same time, the room temperature is gradually increased until reaching temperature desired sintering. The sintering temperature is between 1'000 and 1'400 ° C and preferably approximately equal to 1'200 ° C to avoid getting close too much of a temperature where the room would start to deform.

This heating lasts approximately 5 to 7 hours. During the heating, titanium hydride gradually releases its hydrogen. In this regard it is important according to the process of the invention that the heating is not too fast so as not to cause rapid release of hydrogen which could cause pore formation within the room and thereby alter the shine of the surface once polished. Preferably the speed of heating is between 150 ° C and 250 ° C per hour.

By heating the room in an atmosphere hydrogen, the hydrogen from titanium hydride is released gradually, which greatly decreases the tendency to form bubbles or pores within the room. In addition, given the high reactivity titanium at high temperature, the process according to the invention advantageously eliminates the risk of reaction of titanium with compounds other than hydrogen which could affect the purity of the part obtained.

The sintering temperature reached and the hydrogen of the part having been largely released, we replace at new furnace atmosphere, that is hydrogen, by a non-reactive atmosphere such as argon, or helium or vacuum. Argon will be preferred. The replacement of hydrogen by non-reactive atmosphere is done while keeping the room at its temperature of sintering. The duration of this plateau is between 5 and 80 minutes and is preferably about 20 minutes.

The part is then cooled down to the temperature ambient in said non-reactive atmosphere at a rate cooling of about 300 ° C per hour. During from this cooling, the part slowly releases the rest of its hydrogen which is eliminated progressively.

The sintered titanium part obtained by the process which just described has remarkably porosity low, less than 2%.

Thus, this part can be subjected to a polishing specular of its surface in order to obtain an article decorative such as a watch case, a link bracelet, dial or the like, having a surface of an intense polish and shine.

The following example is a preferred example of setting work of the manufacturing process by sintering a part titanium, object of the invention

Example

A binder formed from a copolymer comprising 32% by volume of polyethylene oxide (246 g) and 4% by volume of polypropylene (26 g) is prepared in a container. This binder is heated to a temperature of about 170 ° C to obtain a homogeneous mass. 64% by volume of TiH ₂ (1920 g) having a degree of purity of 99.5% is then added in addition, which is mixed with the binder until a homogeneous paste is obtained.

The mixture is then granulated cooled. The granules obtained are then introduced into an injection molding machine and injected into a mold, having by example the shape of a watch case, at a temperature about 140 ° C.

The shaped part is then introduced into an oven in which a vacuum of about 10 ^-2 millibar is made. The part is then brought to a temperature of approximately 300 ° C. by linear heating in 8 hours.

The part is then sintered. For this make, the vacuum of the oven is replaced by an atmosphere of hydrogen as a stream with a flow rate of 150 ml / min and the part is brought from 300 ° C to 1'200 ° C linearly in 4 hours. Once the temperature of 1'200 ° C reached, we replace the hydrogen atmosphere with an argon atmosphere in the form of a flow having a flow of 150 ml / min, and the temperature is maintained at 1,200 ° C. for about 20 minutes.

The part is then cooled linearly under the same argon atmosphere up to room temperature. The cooling rate is 300 ° C per hour and we thus obtains a sintered titanium part whose porosity is 1.5%.

The sintered element is finally subjected to a electropolishing to obtain a watch case having a intense shiny appearance.

In a variant of the above example, we use polyacetal as a binder and the latter is eliminated by decomposes in a nitric acid vapor at 120 ° C. The result obtained with this variant is identical to that obtained with the previous example.

Claims (10)

1. Process for the manufacture by sintering of a titanium part having a porosity less than 2 %, characterized in that it consists of:

   (a) mixing a titanium hydride powder with a temporary binding agent,

   (b) injecting the mixture obtained into a mould to obtain a part in the desired shape,

   (c) removing the binding agent,

   (d) heating the part in a hydrogen atmosphere with a temperature gradient comprised between 50°C and 250°C per hour up to a sintering temperature comprised between 1'000 and 1'400°C,

   (e) replacing the hydrogen atmosphere by vacuum or a non-reactive atmosphere once the sintering temperature has been reached, and

   (f) cooling the part in a non-reactive gas atmosphere, so that a sintered titanium part is obtained.
2. Process according to claim 1, characterized in that step (d) is carried out for between 4 and 8 hours.
3. Process according to any one of the preceding claims, characterized in that, during step (d), the hydrogen is supplied in the form of a continuous flow.
4. Process according to any one of the preceding claims, characterized in that, during step (e), the part is maintained at said sintering temperature for between 5 and 60 minutes.
5. Process according to any one of the preceding claims, characterized in that, during the steps (e) and (f), the non-reactive atmosphere is argon or helium.
6. Process according to any one of the preceding claims, characterized in that step (c) is carried out chemically or thermally.
7. Process according to claim 6, characterized in that step (c) is carried out in a vacuum at a temperature below 300°C.
8. Process according to claim 7, characterized in that step (c) is carried out for between 6 and 9 hours.
9. Process according to claim 6, characterized in that the binding agent is a thermoplastic polymer and in that step (c) comprises the chemical decomposition of the polymer by an acid vapour.
10. Process according to any one of the preceding claims 1 to 9, characterized in that it comprises a supplementary step (g) during the course of which the part is subjected to specular polishing.