ES2328955T3 - PROCEDURE FOR THE MOLDING BY COLADA OF A TITANIUM ALLOY. - Google Patents

Abstract

Procedure for the casting by casting of objects from a ß-titanium alloy with a molybdenum content of 15%, characterized by a fusion of the alloy at a temperature above 1,770 ° C, a fine casting molding of the molten alloy inside a casting mold corresponding to the object to be produced, a hot isostatic pressing, a solution annealing at a temperature between 760 ° C and 800 ° C and subsequent quenching.

Description

Procedure for casting a titanium alloy

The invention relates to a process for casting of objects from an alloy of β-titanium, more accurately said of a titanium and molybdenum alloy.

Titanium alloys, because of their numerous and advantageous properties enjoy every popularity getting older In particular, because of its good chemical stability, also under a high temperature, and of its small weight, along with outstanding mechanical properties, alloys of titanium in all sectors, in which they are raised requirements for the material. Because of his outstanding Biocompatibility, titanium alloys, are also used for preferred way in the medical sector, particularly for implants and prosthesis.

Different methods are known for conformation of titanium alloys. Along with a treatment with chip removal, these are mostly molding procedures by casting and forging. Basically, titanium alloys are forging alloys, therefore they are used in the largest Part of the forging procedures cases. Since it has shown that titanium alloys are difficult to mold by wash. In most cases this route is covered in the case of complicated forms, but this route leads to restrictions on make the choice of appropriate alloys. In particular, it showed that by casting cast alloys of β-titanium can only be achieved Unsatisfactory results (patent application document of The USA. US-A-2004/0136859).

From the appointment of Donachie and collaborators, Titanium, a Technical Guide (Titanium, a technical guide), 2000, pages 39, 41, 42 different sections of the treatment are known thermal titanium alloys.

The invention is based on the mission of provide an improved casting process for β-titanium alloys, allowing a production even of complex shapes along with good properties of the materials.

The solution to the problem posed according to invention is in a process that has the characteristics of the main claim. Some advantageous improvements are subject to the claims high schools.

According to the invention, in the case of a procedure for casting objects from a β-titanium alloy with a content of 15% molybdenum, the alloy is expected to be melted at a temperature above 1,770 ° C, that the molten alloy is mold by fine casting in a casting mold corresponding to object to be produced, isostatically hot pressed, undergo a solution annealing at a temperature of 760-800 ° C and then cool sharply (se quenching).

An object means in the present case a product shaped for final use. It can be treated by example in the aeronautics sector of parts and parts for propeller groups, rotor supports, wing boxes or other parts of the support structure, or in the medical sector of stents, such as hip prostheses, or implants, such as plates and plugs, or dental implants. The concept of the object in The meaning of the present invention does not cover ingots that are designed for further treatment through procedures of change of form (conformation), ie ingots in particular not produced by ingot casting, for further treatment by forging.

With the process according to the invention, get a rational production of objects from alloys of β-titanium in the molding process fine casting (precision). The procedure provides by consequently to the possibility of combining the advantageous properties of β-titanium alloys, in particular its outstanding mechanical properties, with the advantages of a production of objects in the fine casting molding process. Also some objects with complex shapes, which could not be produce, or could not be produced conveniently, by usual forging procedures, can be produced thanks to invention from a β-titanium alloy. Accordingly, the invention opens to alloys of β-titanium, which are known for their excellent mechanical properties as well as its biocompatibility, also the field of applications of the shaped objects of a complex mode

The proportion of molybdenum in the alloy is located at 15%. In this way a sufficient one is established stabilization of the β phase until reaching the region of the room temperature. Therefore, by rapid cooling after fine casting molding can be achieved a metastable β phase. The addition of other elements Alloy trainers are expendable as a rule. In In particular, it is not necessary to add vanadium or aluminum. The waiving this has the advantage already outlined that it can be avoided the toxicity that comes from these forming elements of alloys The corresponding is valid for bismuth, which in its Biocompatibility does not reach titanium either.

It has been shown that with the alloys of β-titanium, which until now is hardly used For fine casting molding, they can be produced thanks to invention even more complex forms than with the alloys of α / β-titanium, such as for example TiAl6V4, so far used for fine casting molding. With the process according to the invention achieves an improved filling capacity of the molds. So, thanks to the invention, in the case of fine casting molding can occur in particular Sharp edges with a higher quality. It also decreases the tendency to the formation of blows or blows in the case of fine casting molding, thanks to the best filling capacity of baking tin.

Conveniently, for alloy fusion of β-titanium an installation of vacuum induction in cold-walled crucibles. With one such installation can reach high temperatures, which are necessary for a safe fusion of titanium alloys and Molybdenum intended for fine casting molding. So, the point of Melting of the TiMo15 is located at 1,770 ° C. For this it is necessary still an addition of about 60 ° C, in order to achieve A safe fine casting molding. In total, a 1.830 ° C temperature for TiMo15.

Preferably, hot isostatic pressing is performed at a temperature, which is at most as high as a beta transus temperature of the titanium and molybdenum alloy and at least 100 ° C below the beta transus temperature.

Unfavorable effects are counteracted by hot isostatic pressing due to an enrichment of molybdenum in dendrites mediating impoverishment of the remaining melt, interdendritic segregations being dissolved. A temperature below the temperature of β transus and certainly up to 100 ° C below it is favorable. For a titanium and molybdenum alloy with a molybdenum ratio of 15%, temperatures in the range of 710 ° C to 760 ° C, preferably at approximately 740 ° C, with an argon pressure of approximately 1,100 to 1,200 bar have been accredited.

For annealing by dissolution they have accredited temperatures of at least 700ºC to 880ºC (range not in accordance with the invention), preferably located in the range of 800 ° C to 860 ° C (range that is not according to the invention). For the production of a gas atmosphere Argon is preferably used protector. This way it achieves an improvement in the ductility of the alloy.

Conveniently, after annealing by dissolution an abrupt cooling of the object is effected by Water. Preferably, cold water is used. For the concept of "cold" means in this context the water temperature unheated current. Sudden cooling has been shown exerts a strong influence on the mechanical properties of object, finally achieved. Alternatively, it can be done also a sudden cooling in a protective gas, for example by cooling with argon. The results obtained from this way, however, they remain behind those achieved With the cold water.

It may be convenient to harden the object still as final. In this way, if necessary, you can raise the modulus of elasticity somewhat. Preferably, for this, hardening is performed in a temperature range from about 600 ° C to about 700 ° C.

The invention is explained below by reference to the drawing, in which an advantageous example is represented of realization. In this:

Fig. 1 shows a table with properties Titanium alloy mechanical cast by fine casting, according to the invention;

Fig. 2 shows a reproduction of the microstructure in a molded state, immediately after laundry;

Fig. 3 shows a reproduction of the microstructure after hot isostatic pressing;

Fig. 4 shows a reproduction of the microstructure after dissolution annealing with subsequent sudden cooling; Y

Fig. 5 shows a representation of liquidus and solidus temperatures for a titanium and molybdenum alloy.

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Next, a way to lead to Perform the process according to the invention.

The starting material is an alloy of β-titanium with a molybdenum ratio of 15% (TiMo15). This alloy can be acquired in a usual way in trade in the form of small ingots (in English ingots).

In a first stage a molding is carried out by fine casting on one of the objects to be cast by casting. For melting and casting molding of the TiMo15, a casting molding installation. Preferably it is about a vacuum induction casting and casting system in thin-walled crucibles. With one such facility, they can reach high temperatures, which are necessary for a safe fusion of TiMo15 for fine casting molding. Point of fusion of the TiMo15 that is located in 1,770ºC to which it has to add an addition of approximately 60 ° C for safe molding by fine wash In total, a temperature must therefore be reached of 1,830 ° C. The casting by fine casting of the melt is carried out then by a procedure known per se, by example with wax males and ceramic molds as molds lost Such fine casting molding techniques are known. for the thin casting of TiA16V4.

As can be recognized in reproduction (with a 1,000-fold increase) in Fig. 2, dendrites form and in the interdendritic areas considerable segregations are shown. This is a consequence of the so-called negative segregation of titanium and molybdenum alloys. This effect is based on the special evolution of liquidus and solidus temperatures in the case of titanium and molybdenum alloys, as depicted in the \ hbox {Fig. 5.} Due to the represented evolution of the melting temperatures of the liquid phase (T L) and the solid phase (T S) in the melt, the areas with a high proportion of molybdenum are first solidified , forming dendrites that can be recognized in reproduction. As a result, the remaining melt is impoverished, that is, its molybdenum content decreases. The interdendritic zones have a molybdenum content of below 15% in the molded structure, and the molybdenum content can be reduced to values of approximately 10%. As a consequence of the impoverishment in molybdenum, a sufficient amount of β stabilizing elements is lacking in the areas between dendrites (interdendritic). This has the consequence that a high transformation temperature of α / β is adjusted locally, resulting in the segregations that can be recognized in Fig. 2.

It is convenient to remove an area of pickling edge, possibly resulting when casting, in the form of a fragile and hard layer (the so-called \ alpha box, in English α-case). Usually this layer has a thickness of 0.03 mm.

In order to counteract the unfavorable effect of negative segregation with segregations in interdendritic areas, cast bodies, which after fine casting have been released from the casting molds, are subjected according to the invention to a heat treatment For this, a hot isotactic pressing (HIP, acronym for heiss isostatisches Pressen) is planned and certainly at a temperature very little below the temperature of β transus . It may be in the range of 710 ° C to 760 ° C and is preferably about 740 ° C. In this way, the unwanted segregations in the interdendritic zones again dissolve. No prior aging is necessary before or after hot isostatic pressing (in German Hippen). However, in the case of cooling after hot isostatic pressing, secondary secondary phases are segregated again, and certainly preferably in the original interdendritic zones (see Fig. 3, with a 1,000-fold increase). This results in an unwanted embrittlement of the material.

For this reason, the objects, after hot isostatic pressing, they have only a small ductility.

In order to eliminate the disturbers segregations, casting bodies are subjected to a annealed in a chamber oven under an atmosphere of a gas protector (eg argon). For this, an interval of temperatures of 760ºC to 800ºC, with a duration of several hours, in Most of the cases two hours. There is in this case a reciprocal connection between temperature and duration, at a higher temperature is sufficient a shorter period of time and the other way. After annealing by dissolution, the bodies cast molded are sharply cooled with cold water. In the Fig. 4 (with a 1,000 fold increase) the structure is represented after annealing by dissolution. Grains are recognized primary β and within the grains some very segregations thin arranged between dendrites (= interdendritically) (see a cloud-like accumulation on the left above in the figure). The objects molded by fine casting with the conforming procedure the invention has in its crystalline structure some β grains with an average size of more than 0.3 mm. This size is typical for the crystal structure achieved with the procedure according to invention.

The mechanical properties achieved after solution annealing are reproduced in the Table of
Fig. 1.

It is recognized that the modulus of elasticity decreases with an increasing temperature when annealing dissolution, and certainly at values up to 60,000 N / mm2. The toughness values improve with mechanical resistance and a decreasing hardness. Thus, after an annealing by dissolution for two hours at 800 ° C an elastic modulus of 60,000 N / mm2 with a break elongation of approximately 40% and with a breaking strength Rm of approximately 730 N / mm2

Claims (6)

1. Procedure for casting molding of objects from a β-titanium alloy with a molybdenum content of 15%, characterized by a fusion of the alloy at a temperature located above 1,770 ° C, a fine cast casting of molten alloy inside a casting mold corresponding to the object to be produce, hot isostatic pressing, an annealing by dissolution at a temperature between 760ºC and 800ºC and subsequent quenching

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2. Procedure in accordance with the claim 1, characterized by the use of an induction installation vacuum in cold-walled crucibles for alloy fusion of β-titanium.

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3. Procedure in accordance with the claim 1 or 2, characterized by performing hot isostatic pressing at a temperature that is at most as high as a beta transus temperature of the titanium and molybdenum alloy and at least 100 ° C below the temperature of beta transus .

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4. Procedure according to one of the preceding claims, characterized by an abrupt cooling with water preferably cold after annealing by dissolution.

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5. Procedure according to one of the preceding claims, characterized by a final hardening of the object.

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6. Procedure in accordance with the claim 5, characterized by carry out hardening to a temperature of 600ºC to 700ºC.