EP1696043A1 - Process for casting a Titanium-alloy - Google Patents

Abstract

To cast a beta -titanium alloy, containing 7.5-20.0% molybdenum, the alloy is melted at a temperature of >=1770[deg]C to be poured with fine casting into a mold with hot isostatic pressure. The molding is annealed and finally quenched.

Description

The invention relates to a method for casting objects from a β-titanium alloy, more particularly a titanium-molybdenum alloy.

Titanium alloys are becoming increasingly popular because of their many beneficial properties. In particular, because of their good chemical resistance, even at high temperature, and their low weight with excellent mechanical properties titanium alloys are used in all areas where high demands are placed on the material. Because of their excellent biocompatibility, titanium alloys are also preferably used in the medical field, in particular for implants and prostheses.

Various methods for shaping titanium alloys are known. In addition to machining these are mainly casting and forging processes. Basically, titanium alloys are forgings, so forging processes are mostly used. Because it has been shown that titanium alloys are difficult to pour. Mostly this approach is taken in complicated shapes, but this approach leads to limitations in the selection of suitable alloys. In particular, it was found that only unsatisfactory results are achieved in the casting of β-titanium alloys (US-A-2004/0136859).

The invention has for its object to provide an improved casting method for β-titanium alloys, the one Production of even complex shapes with good material properties allowed.

The solution according to the invention lies in a method having the features of the main claim. Advantageous developments are the subject of the dependent claims.

According to the invention, in a method of casting articles of a β-titanium alloy comprising titanium molybdenum having a molybdenum content of 7.5 to 25%, the alloy is melted at a temperature of over 1770 ° C, the molten alloy is poured into a is mold-molded appropriate hot mold, hot isostatically pressed, solution-annealed and then quenched.

Under article is understood in the present case a molded product for end use. In the field of aviation, for example, parts for engines, rotor bearings, wing boxes or other support structure parts or in the field of medicine can be endoprostheses, such as hip prostheses, or implants, such as plates and pins or dental implants. The term article for the purposes of the present application does not include bars which are intended for further processing by forming processes, ie in particular ingots not produced by chill casting for further processing by forging.

With the method according to the invention, a rational production of articles made of β-titanium alloys is achieved by precision casting. The invention thus makes it possible to combine the advantageous properties of β-titanium alloys, in particular its excellent mechanical properties, with the advantages of producing articles by precision casting. Also objects with complex shapes, which could not be made or not made meaningful by conventional forging processes, can thanks to the Invention are made of a β-titanium alloy. Thus, the invention also opens up the field of application of the complex shaped articles to the β-titanium alloys known for their excellent mechanical properties and biocompatibility.

The proportion of molybdenum in the alloy or its molybdenum equivalent is in the range of 7.5 to 25%. Thus, in particular with a molybdenum content of at least 10%, a sufficient stabilization of the β-phase up to the region of room temperature results. Preferably, the content is between 12 and 16%. This can be achieved by fast cooling after the investment casting a metastable β-phase. The addition of other alloying agents is usually unnecessary. In particular, it is not necessary that vanadium or aluminum be added. The absence of this has the already mentioned advantage that the toxicity emanating from these alloy formers can be avoided. The same applies to bismuth, which in terms of its biocompatibility likewise does not equal titanium.

It has been shown that with the β-titanium alloys which have hitherto scarcely been used for investment casting, even more complex shapes can be produced thanks to the invention than the α / β titanium alloys hitherto used for precision casting, such as TiA16V4. With the method according to the invention an improved mold filling capacity is achieved. Thus, thanks to the invention, in particular, sharp edges with higher quality can be produced during precision casting. The tendency to form voids during precision casting is also reduced thanks to the better mold filling capacity.

Conveniently, a cold wall crucible vacuum induction plant is used to melt the β-titanium alloy. With such a plant, the high temperatures required for a safe melting of titanium-molybdenum alloys for investment casting can be achieved. This is the melting point of TiMo15 at 1770 ° C. For this purpose, a surcharge of about 60 ° C is appropriate to achieve a safe investment casting. Overall, a temperature of 1830 ° C for TiMo15 must be achieved.

Hot isostatic pressing preferably takes place at a temperature which is at most as high as a beta transus temperature of the titanium-molybdenum alloy and at a minimum of 100 ° C. below the beta-transus temperature.

Hot isostatic pressing counteracts unfavorable effects due to an accumulation of molybdenum in dendrites while depleting the residual melt, by bringing interdendritic precipitates into solution. Favorable is a temperature below the β-transus temperature, up to 100 ° C below. For a titanium molybdenum alloy with 15% molybdenum content, temperatures in the range of 710 ° C. to 760 ° C., preferably of about 740 ° C., at an argon pressure of about 1100 to 1200 bar, have proved successful.

For the solution annealing, temperatures of at least 700 ° C up to 880 ° C have proven, preferably in the range of 800 ° C to 860 ° C. Argon is preferably used to generate a protective gas atmosphere. This achieves an improvement in the ductility of the alloy.

Appropriately, quenching of the article by water occurs after solution heat treatment. Preferably, cold water is used. By "cold" is meant the temperature of unheated tap water. Quenching has been shown to exert a strong influence on the ultimate mechanical properties of the article. Alternatively, it can also be quenched in inert gas, for example by argon cooling. However, the results achieved remain behind those achieved with cold water.

It may be appropriate to harden the article at the conclusion. This can be slightly increased if necessary, the modulus of elasticity. Preferably, the curing in a temperature range of about 600 ° C to about 700 ° C is done.

Fig. 1

eine Tabelle mit mechanischen Eigenschaften der erfindungsgemäßen feingegossenen Titanlegierung;

Fig. 2

eine Abbildung des Mikrogefüges in einem Gusszustand unmittelbar nach dem Giessen;

Fig. 3

eine Abbildung des Mikrogefüges nach dem Hippen;

Fig. 4

eine Abbildung des Mikrogefüges nach dem Lösungsglühen mit anschließender Abschreckung; und

Fig. 5

eine Darstellung von Liquidus- und Solidustemperaturen für eine Titan-Molybdänlegierung.

The invention will be explained below with reference to the drawing, in which an advantageous embodiment is shown. Show it:

Fig. 1

a table with mechanical properties of the finely cast titanium alloy according to the invention;

Fig. 2

an image of the microstructure in a cast state immediately after casting;

Fig. 3

an illustration of the microstructure after hipging;

Fig. 4

an illustration of the microstructure after solution annealing with subsequent quenching; and

Fig. 5

a representation of liquidus and solidus temperatures for a titanium-molybdenum alloy.

Hereinafter, a way to carry out the method according to the invention will be described.

Starting material is a β-titanium alloy with a molybdenum content of 15% (TiMo15). This alloy can be purchased commercially in the form of small ingots.

In a first step, an investment casting of the objects to be cast takes place. For melting and casting of the TiMo15 a casting plant is planned. Preferably, it is a cold wall crucible vacuum induction melting and casting equipment. With such a plant, the high temperatures required for a safe melting of TiMo15 for investment casting can be achieved. The melting point of TiMo15 is included 1770 ° C plus a surcharge of approx. 60 ° C for a safe investment casting. Overall, therefore, a temperature of 1830 ° C must be achieved. The investment casting of the melt is then carried out by means of known methods, for example with wax cores and ceramic molds as a lost form. Such investment casting techniques are known for investment casting of TiA16V4.

As can be seen in the figure (1000X magnification) in Fig. 2, dendrites are formed and in interdendritic zones significant precipitates appear. This is a consequence of the so-called negative segregation of titanium-molybdenum alloys. This effect is based on the special course of the liquidus and solidus temperature in titanium-molybdenum alloys, as shown in FIG. 5. Because of the illustrated course of the melting temperatures of the liquid phase (T _L ) and the solid phase (T _S ) solidify in the melt first, the areas with high molybdenum content, forming the recognizable in the figure dendrites. As a result, the residual melt is depleted, ie its molybdenum content decreases. The interdendritic zones have a molybdenum content of less than 15% in the cast structure, whereby the molybdenum content can drop to values of about 10%. As a result of molybdenum depletion, there is a lack of sufficient β-stabilizers in the interdendritic zones. This has the consequence that locally sets an increased a / β transformation temperature, whereby the precipitates to be recognized in Fig. 2 arise.

It is expedient to remove a possibly resulting during casting edge zone in the form of a hard, brittle layer (so-called α-case) by pickling. Usually, this layer has a thickness of about 0.03 mm.

In order to counteract the unfavorable effect of the negative segregation with the precipitates in the interdendritic zones, they are freed from the casting molds after investment casting Casting according to the invention subjected to a heat treatment. For this purpose, a hot isostatic pressing (HIP) is provided, namely at a temperature just below the β-transus temperature. It may range from 710 ° C to 760 ° C, preferably about 740 ° C. The undesired precipitates in the interdendritic zones go into solution again. An advance storage before or after the hipping is not required. However, during the cooling after the dipping, fine secondary phases are again precipitated, preferably in the original interdendritic zones (see FIG. 3, magnification 1000 times). This results in unwanted embrittlement of the material.

For this reason, the articles have a low ductility after being tipped.

In order to eliminate the interfering precipitations, the castings are annealed in a chamber furnace under a protective gas atmosphere (eg argon). For this purpose, a temperature range of about 700 ° C to 860 ° C is selected, with a duration of several, usually two hours. There is an opposite relationship between the temperature and the duration, at higher temperature is sufficient for a shorter time and vice versa. After solution heat treatment, the castings are quenched with cold water. In Fig. 4 (1000X magnification), the microstructure after solution annealing is shown. One recognizes primary β-grains and within the grains very fine interdendritically arranged precipitations (see cloud-like accumulation in the upper left corner of the figure). The articles finely cast with the method according to the invention have, in their crystal structure, β grains with an average size of more than 0.3 mm. This size is typical of the crystal structure achieved by the process of the invention.

The mechanical properties achieved after solution annealing are shown in the table in FIG.

It can be seen that the modulus of elasticity decreases with increasing temperature during solution annealing, to values up to 60,000 N / mm ² . The toughness values improve with decreasing strength and hardness. Thus, after two hours of solution annealing at 800 ° C., a modulus of elasticity of 60,000 N / mm ^{2 is achieved} with an elongation at break of approximately 40% and a breaking strength Rm of approximately 730 N / mm ² .

Claims (8)

A method of casting articles of a β-titanium alloy comprising titanium molybdenum having a molybdenum content of 7.5 to 25%,
marked by
Melting the alloy at a temperature above 1770 ° C,
Investment casting of the molten alloy into a mold corresponding to the article to be produced,
hot isostatic pressing,
Solution annealing and
subsequent quenching. Method according to claim 1,
marked by
Using a cold wall crucible vacuum induction machine to melt the β-titanium alloy. Method according to claim 1 or 2,
marked by
Performing hot isostatic pressing at a temperature as high as a beta transus temperature of the titanium molybdenum alloy and at least 100 ° C below the beta transus temperature. Method according to one of claims 1 to 2,
marked by
Performing solution annealing at a temperature of about 700 ° C to about 900 ° C. Method according to claim 4,
marked by
Performing the solution annealing at a temperature of 800 ° C to 860 ° C. Method according to one of the preceding claims,
marked by
quenching with preferably cold water after solution heat treatment. Method according to one of the preceding claims,
marked by
final hardening of the article. Method according to claim 7,
marked by
Performing curing at a temperature of 600 ° C to 700 ° C.

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