CA2371805C

CA2371805C - Method and device for moulding titanium parts

Info

Publication number: CA2371805C
Application number: CA002371805A
Authority: CA
Inventors: Marcel Garnier; Christian Garnier; Roland Ernst
Original assignee: Centre National de la Recherche Scientifique CNRS
Current assignee: Centre National de la Recherche Scientifique CNRS
Priority date: 1999-02-19
Filing date: 2000-02-18
Publication date: 2009-02-17
Anticipated expiration: 2020-02-18
Also published as: DE60000505T2; CA2371805A1; FR2789917A1; FR2789917B1; WO2000048771A1; DE60000505D1; ES2183789T3; EP1152855B1; EP1152855A1; AU3169400A

Abstract

The invention concerns an appliance for moulding parts made of very reactive metal or mixture of metals, comprising, inside a vacuum chamber (15) a cold induction-heated crucible (1) whereof the base comprises a removable sectored plug (6), said crucible being configured to enable the levitation of said metal or mixture of metal in smelted state (5); a mould (20) provided under the crucible (1); means for driving the mould in rotation; and outside the chamber, a coil (13) arranged at the lower part of the crucible is designed to heat by induction said metal or said mixture.

Description

METHOD AND DEVICE FOR MOULDING TITANIUM PARTS

The present invention relates to a method and a device for casting pieces made of a strongly reactive metal or metal mixture/alloy, such as titanium.
The use of titanium has often been suggested in fields where one and/or the other of its specific properties is desired to be exploited. For e.xample, in the field of aeronautics, titanium is considered with great interest due to the high value of the breaking load-to-density ratio. In the field of chemistry, titanium is advantageous .for its properties of low reactivity with sane materials. Also, titanium has a good biological campatibility which makes it adapted to forming prostheses such as dental protheses. In these various fields, and especially in the last one, it is often desired to form pieces of small dimensions and of complex shapes but titanium is a material which is very difficult to work and tests of direct adaptation of the various known casting methods result in pieces that are heavily oxidized in their bulk and/or at their surface.
Thus, the'present invention aims at providing a tita-nium casting method which ensures a good imnunity against the axidation of titanium during the melting phase and during the casting phase.
To achieve this object, the present-invention provides a method for casting pieces of a very reactive metal or metal mixture, including the steps of placing in a vacuum chamber a cold induction- heated crucible containing said metal or said mixture and the bottom of which includes a removable sectorized plug, this crucible being designed to ensure the levitating of said metal or mixture in the molten state; arranging in the chamber, under the cold crucible, a mould likely to be rotated around a vertical axis; arranging outside of the chamber and at the level of the crucible and especially of the crucible bottom an induction heating winding; exciting the winding to melt said metal or said mixture, having it levitate and overheating it by at least 500C above its melting point; and opening the plug to end the levitation and have said overheated molten metal or mixture flow into the rotating mould and fill the cavities thereof.
According to an embodiment of the present invention, the very reactive metal is titanium.
According to an embodiment of the present invention, the mould is of the lost wax mould type.
According to an embodiment of the present invention, the cast pieces are intended for orthopedic applications.
According to an embodiment of the present invention, the titanium pieces are intended for dental applications.
The present invention also aims at a device for imple-menting this method and at the pieces obtained by this method.
An advantage of the titanium pieces obtained by the method according to the present invention is that they are not oxidized in their bulk and exhibit an extremely thin surface oxide layer, only due to possible reactions between the mould and the molten titanium having penetrated into this mould. Oxide thicknesses smaller than one micrometer have been obtained while thicknesses greater than several tens of micrometers are obtained with conventional methods.
This is an important advantage of the present invention since it is generally desired to deliver pieces which are not oxidized at their surface, which requires an operation of sand-blasting or of polishing by another method, which can be long and expensive if the oxide layer and especially the titanium dioxide layer (Ti02) is thick (of a thickness over a few m) .
The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.
Figs. 1A and 1B show a conventional induction furnace operating in levitation; and Fig. 2 schematically and partially shows a titanium melting and casting equipment according to the present invention.
In the following, the present invention will be explained in relation with the melting and casting of titanium.
More generally, the present invention applies to the melting and casting of any strongly reactive metal or metal mixture.
To melt a strongly reactive metal or metal mixture, device of induction-heated cold crucible type, and more specifi-cally crucibles likely to operate in levitation, are currently used. Such a crucible is shown in cross-section view in Fig. lA
and in simplified top view in Fig. 1B. Crucible 1 has a rounded bottom and, as seen from above, is divided into sectors, each sector being made of a hollow conductive material cooled down by the circulation of a liquid and electrically isolated from the adjacent sectors. A winding 3 in which a high-frequency electric current flows ensures the heating by induction and, due to the sectorizing and the rounded shape of the crucible bottom, the levitating of molten substance 5.
The use of such a crucible is described in the art, for exanple, in US patent 5,058,127 of Marcel Garnier, Isabelle Gleizes, Patrick Paillere, and Pierre Vernay.
This prior patent, which has some inventors in ccxtIInon with the present application, more specifically describes the forming of a crucible including a sectorized bottom forming a plug, enabling simple opening and fast flow of the levitating mass, the levitation disappearing when the bottom is opened.
This patent especially suggests in its second exarrple the melting of a titanium and aluminum mixture. However, this patent says nothing about the use of a possible mould in which the obtained material would be poured. Indeed, this patent considers levitation essentially in that it enables ensuring a stirring in conditions of great purity to abtain a homogeneous mixture/alloy of several metals.
On the other hand, this patent does not describe how the inductive windings are arranged around the crucible. It can thus be thought that these windings are arranged conventionally, around the crucible and as close as possible to the crucible.
The present invention aims at using a bottom-opening crucible of the type described in the above-mentioned patent, but which is adapted to a casting operation after the melting.
For this purpose, the present invention provides a device, the general diagram of which is symbolically shown in Fig. 2. In this drawing, the means for supporting the various elements are not shown but can be easily implemented by those skilled in the art.
Fig. 2 shows a crucible 1 of the same type as that illustrated in Fig. 1A and 1B containing a liquid mass 5 likely to levitate. Bottom 6 of the crucible, in accordance with the teachings of US patent 5,058,127, includes a cooled down and removable sectorized plug 6. Plug 6 for example moves along with a rotating arm 8 articulated around an axis 9. Arm 8 is driven by mechanical, electromechanical, or magneto-mechanical means to be able to very quickly tip over. It should be noted that axis 9 is also linked to water supply means (not shown) for cooling down sectorized bottom 6. Similarly, water supply means for the sectors of crucible 1 have not been shown.
According to the present invention, inductive windings 13 are arranged outside of the vertical walls of a chamber 15.
This is different from conventional assemblies in which, as illustrated in Fig. 1A, windings 3 are arranged inmediately around the crucible. Chamber 15 includes a bottam 17 and a cover (not shown) which can be connected thereto by seals to create vacuum inside of it. The chamber is connected to outlet means intended for forming a primary vacuum therein. These means are not shown either.

The present inventors have found that, conversely to an established prejudice, the fact of arranging windings 13 outside of the walls of chamber 15 does not modify the possibility of obtaining a heating by induction and a levitating of mass 5. The 5 power applied to the winding just has to be slightly increased.
It should be noted that induction systems in which an inductive winding is placed outside of a chamber for heating are known (see, for exanple, US-A-4,609,402) but not in levitation systems.
The fact of arranging the windings outside of the chamber has several advantages.
According to a first advantage, it is useless to provide crossings of the chamber, which is particularly eco-nomical, in particular in the case where the chamber is made of quartz.
According to a second advantage, this avoids risks of creating a plasma or causing electric breakdowns. Indeed, when the winding is arranged inside of the vacuum chamber, especially in the case where the metal to be molten is titanium, a very fast vaporization of this titanium appears, whereby a reactive plasma may be created and there are strong risks of short-circuits between the conductors of the inductive winding, within the chamber. To solve this problem, various ways of protecting the windings have been tried in prior art, for example, by embedding them in an isolator. This however has proven difficult to imple-ment and often inefficient.
Further, the present invention provides arranging under the opening provided in the bottam of crucible 1 a mould 20. This mould includes an inner cylindrical neck 21 having a relatively large diameter, to ensure that when the titanium flows through opening 6, it entirely penetrates into the neck. The mould preferably is a mould obtained by the so-called lost wax technique and includes various hollowings of calibrated shapes, two of which, 23 and 24, are visible in the cross-section view.
Mould 20 is assembled on a rotating device not shown and rotates relatively fast around axis 30 of the device. Thus, the molten titanium penetrates into openings 23, 24 and fills them up. There is no air discharge and oxidation problem since the entire system is under vacuum. Thus, at the end of the operation, there only is a thin titanium layer on the inner walls of neck 21 of the chamber and in each of casting openings 24.
According to an inportant aspect of the present inven-tion, before opening plug 6 to let the titanium flow into lost wax mould 20, this titanium is overheated much higher than its melting temperature. For example, if for pure titanium the melt-ing temperature is on the order of 16600C, the titanium is preferably heated up to a temperature on the order of from 1700 to 1800 C. All the disadvantages which would be due to a prema-ture cooling down in a contact of the molten titanium with the mould before this titanium completely fills the casting cavities are thus avoided.

Claims

1. A method for casting pieces of a very reactive metal or metal mixture, including the steps of:
placing in a vacuum chamber (15) a cold induction-heated crucible (1) containing said metal or said mixture and the bottom of which includes a removable sectorized plug (6), this crucible being designed to ensure the levitating of said metal or mixture in the molten state;
arranging in the chamber, under the cold crucible, a mould (20) likely to be rotated around a vertical axis (30);
arranging outside of the chamber at the level of the crucible and especially of the crucible bottom an induction heating winding (13);
exciting the winding (13) to melt said metal or said mixture, having it levitate and overheating it by at least 50°C
above its melting point; and opening the plug (6) to end the levitation and have said overheated molten metal or mixture flow into the rotating mould and fill the cavities thereof.

2. The method of claim 1, wherein the very reactive metal is titanium.

3. The method of claim 1 or 2, wherein the mould is of the lost wax mould type.

4. The method of any of claims 1 to 3, wherein the cast pieces are intended for orthopedic applications.

5. The method of claims 2, 3, and 4, wherein the titanium pieces are intended for dental applications.

6. A device for casting pieces of a very reactive metal or metal mixture, including:
- inside of a vacuum chamber (15):
a cold induction-heated crucible (1), the bottom of which includes a removable sectorized plug (6), this crucible being designed to ensure the levitating of said metal or mixture in the molten state;
a mould arranged under the crucible;

means for rotating the mould;
- outside of the chamber:
a winding (13) arranged at the level of the low portion of the cold crucible and intended for heating said metal or said mixture by induction.

7. The device of claim 6, wherein the very reactive metal is titanium.

8. The device of claim 6 or 7, wherein the mould is of the lost wax mould type.

9. The device of any of claims 6 to 8, wherein the cast pieces are intended for orthopedic applications.

10. The device of claims 7, 8, and 9, wherein the titanium pieces are intended for dental applications.