MX2007010366A

MX2007010366A - Method for casting titanium alloy.

Info

Publication number: MX2007010366A
Application number: MX2007010366A
Authority: MX
Inventors: Sevki Baliktay
Original assignee: Link Waldemar Gmbh Co
Priority date: 2005-02-25
Filing date: 2006-02-27
Publication date: 2007-10-17
Also published as: JP2008531288A; AR052391A1; RU2402626C2; EP1851350B1; DK1851350T3; EP1696043A1; TW200643182A; EP1851350A1; BRPI0607832A2; KR101341298B1; PL1851350T3; KR20070105379A; WO2006089790A1; CA2597248C; ATE438746T1; CN100594248C; RU2007135062A; TWI395821B; DE502006004443D1; CN101128609A

Abstract

The invention relates to a method for casting objects from a ??-titanium alloy containing titanium molybdenum with a molybdenum content of 7.5 to 25 %. According to the invention: a melting of the alloy is carried out at a temperature of higher than 1770 degree C; the molten alloy is precision cast into a mold corresponding to the object to be produced, and this cast object is subjected to a hot-isostatic pressing, solution annealing and subsequent quenching. An efficient production of objects made from ??-titanium alloys in the precision casting process is achieved using the inventive method. The invention thus creates the possibility of combining the advantageous properties of ??-titanium alloys, particularly their excellent mechanical properties, with the advantages of a production of objects in the precision casting process. Even objects having complex shapes, which could not or could not be sensibly produced by conventional forging methods, can be produced from a ??-titanium alloy thanks to the invention.

Description

METHOD OF FOUNDRY FOR AN ALLOY OF TITANIUM DESCRIPTION OF THE INVENTION The invention relates to a method for casting objects of a β-titanium alloy, more precisely, of a titanium-molybdenum alloy. Titanium alloys enjoy increasing popularity thanks to their large number of advantageous features. In particular for its good chemical resistance, even at high temperatures, and its small weight despite excellent mechanical characteristics, titanium alloys are used in all those fields where there are great demands on the material. Thanks to their excellent biocompatibility, titanium alloys are also preferably used in the field of medicine, in particular for implants and prostheses. Various methods are known for forming titanium alloys. In addition to machining with chip removal, these are, above all, foundry and forging methods. The alloys of titanium are, fundamentally, alloys of forge, reason why methods of forging are generally used. Because it has been found that it is difficult to produce castings of titanium alloys. This route is usually attempted through complex molds, but this route restricts the selection of appropriate alloys.

It has been shown in particular that only unsatisfactory results are obtained in the casting of titanium β-alloys (US-A-2004/0136859). The invention is based on the objective of creating an improved casting method for titanium ß alloys that allows the production also of complex shapes preserving good material characteristics. The inventive solution follows from a method having the characteristics of the main claim. Advantageous improvements are the object of the dependent claims. It is envisaged inventively, in a casting method for objects of a ß-titanium alloy, comprising titanium-molybdenum having a molybdenum content of 7.5 to 25%, that the alloy is melted at a temperature above 1770 ° C, the molten alloy it is precision molded into a mold that corresponds to the object that must be produced, that is, it is subjected to isostatic compression of high temperatures, to annealing of solution and then it cools sharply. By object, it is understood, in the present context, a product formed for its final use. This can be, for example, in the field of aeronautics, components for turbines, rotor bearings, wing boxes or other structural load components, or in the field of endoprosthesis medicine as a hip prosthesis, or implants such as plates and nails, or dental implants. The concept of object in the sense of the present application does not include bars that are designed for further processing by deformation methods, that is, in particular not ingots produced by die casting for further processing by forging. With the inventive method a rational production of objects of titanium alloy ß is achieved by the precision casting method. The invention thus creates the possibility of combining the advantageous characteristics of the titanium ß alloys, in particular their excellent mechanical characteristics, with the advantages of object production by the precision casting method. Even objects, having complex shapes, which could not be produced, or not in a rational manner, by means of conventional forging methods can be produced, thanks to the invention, of a titanium ß alloy. The invention thus opens the field of applications of complex forming objects for the titanium-β alloys known for their excellent mechanical characteristics. The proportion of molybdenum in the alloy, respectively its equivalent of molybdenum, is in the area of 7.5 to 25%. This produces, in particular with a Molybdenum content of at least 10%, a sufficient stabilization of the ß phase up to the ambient temperature area. The content is preferably between 12 and 16%. This allows a metastable ß phase to be achieved by rapid cooling after precision casting. The addition of other alloying agents is usually dispensable. In particular, it is not necessary to add vanadium or aluminum. To dispense with this has the advantage, already mentioned, that the toxicity associated with these alloying agents can be avoided. The corresponding is also true for bismuth that also does not compete with titanium in terms of its biocompatibility. It has been found that even more complex forms can be produced with the titanium ß alloys which were hitherto practically impossible for use in precision casting than with the a / b titanium alloys used so far for the purposes of the invention. precision casting, such as, for example, TIA16V6. An improved ability to fill the mold is achieved by the inventive method. It is thus possible, thanks to the invention, to produce, by precision casting, particularly sharp edges of higher quality. Also the tendency to form cavities in precision casting is reduced, thanks to the better ability to fill the mold.

Conveniently, a cold wall crucible vacuum induction installation is used to melt the titanium alloy. Such an installation allows to reach the high temperatures required to reliably melt titanium-molybdenum alloys for precision casting. The melting point of TiMol5 is located at 1770 ° C. An increase of approximately 60 ° C is desirable to achieve a reliable precision casting. In total, it should be reached, in this way, at a temperature of 1830 ° C for TiMol5. Preferably, the application of isostatic compression at high temperatures is carried out at a temperature which is, at most, as high as the beta phase transition temperature of the titanium-molybdenum alloy, and at least 100 ° C below the Beta phase transition temperature. Isostatic compression at high temperatures counteracts the inconvenient effects due to the concentration of molybdenum in the dendrites that dilutes it in the rest of the melt, by causing the precipitations in the regions between the dendrites to dissolve. A temperature below the ß phase transit temperature is convenient, namely up to 100 ° C below it. For a titanium-molybdenum alloy having 15% molybdenum component, temperatures in the area of 710 ° to 760 ° C, preferably about 740 ° C, gave a good result at an argon pressure of about 1100 to 1200 bar. For the annealing of dissolution, temperatures of at least 700 ° C to 880 ° C, preferably in the area of 800 ° C to 860 ° C, gave good results. Argon is preferably used for the generation of a protective gas atmosphere. This improves the ductility of the alloy. Conveniently, after the solution annealing, the object is quenched with water. Preferably cold water is used. By "cold" is meant here the water temperature of the unheated network. It has been discovered that the sudden cooling has a very important effect on the mechanical characteristics, finally obtained, of the object. As an alternative it is also possible to carry out the cooling in protection gas, for example an argon cooling. But the results obtained with it are not as good as those obtained with cold water. It may be convenient to harden the object still to finish. In this way it is possible to increase the modulus of elasticity a bit, if required. Hardening is carried out for this, preferably in a temperature range from about 600 ° C to about 700 ° C. The invention is explained below with reference to the drawing, in which an advantageous application example is represented. In this show: Fig. 1 a table showing the mechanical characteristics of the inventive titanium alloy, precision casting; Fig. 2 an image of the microstructure in a state of casting immediately after casting; Fig. 3 an image of the microstructure after isostatic compression at high temperatures; Fig. 4 an image of the microstructure after annealing of solution, followed by abrupt cooling; and Fig. 5 a representation of liquid and solid temperatures for a titanium-molybdenum alloy. Next, a route to carry out the inventive method is described. The raw material is a titanium ß alloy having a molybdenum component of 15% (TiMol5). This alloy can be purchased commercially in the form of small bars (ingots). In a first stage, a precision casting of the objects to be molded is carried out. To melt and strain the TiMol5 provides a casting facility. Preferably it is a cold wall crucible vacuum induction melting and casting installation. With such an installation, the high temperatures required to reliably melt the TiMol5 for precision casting can be achieved. The melting point of TiMol5 is located at 1770 ° C, plus an increase of approximately 60 ° C for a reliable precision casting. In other words, a temperature of 1830 ° C must be reached in total The precision casting of the melt is then carried out by methods known per se, for example with wax cores and ceramic molds such as lost wax. Precision casting methods of this type are known for the precision casting of TIA16V4. As seen in the image (1000-fold magnification) in Fig. 2, dendrites are formed and considerable rainfall is observed in the zones between dendrites. This is the consequence of the so-called negative segregation of titanium-molybdenum alloys. This effect is due to the particular curve of the liquid and solid temperature in the titanium-molybdenum alloys, as shown in Fig. 5. Due to the curve represented by the melting temperatures of the liquid phase (T_) , solidify in the melt first the regions with a high molybdenum component, forming in this the appreciable dendrites in the image. Consequently, the rest of the melt has an impoverished content, that is, its proportion of molybdenum decreases. The zones between the dendrites have a molybdenum content of less than 15% in the casting structure, the molybdenum content being reduced to values of approximately 10%. As a result of the impoverishment in molybdenum a sufficient quantity of ß-stabilizers is lacking in the zones between dendrites. This has the consequence that a higher a / β conversion temperature is locally adjusted, which generates the appreciable precipitations in Fig. 2. It is convenient to eliminate a marginal zone, possibly generated during casting, in the form of a hard layer and rigid (the so-called layer a) by chemical attack. This layer usually has a thickness of approximately 0.03 mm). To counteract the unfavorable effect of negative segregation with precipitation in the zones between dendrites, the molded bodies, released after the precision casting of the molds, are subjected to an inventive heat treatment. For this, high-temperature isostatic compression (HIP) is predicted, and at a slightly temperature lower than the ß phase transition temperature. This can be located in the area of 710 ° C to 760 ° C, preferably it rises to approximately 740 ° C. In this, the undesirable precipitations in the zones between dendrites go back into solution. It is not necessary to rest before or after the high temperature isostatic compression. Certainly, upon cooling after this treatment, fine secondary phases are precipitated again, and preferably in the original zones between dendrites (see Fig. 3, 1000-fold increase). This results in an undesirable increase in stiffness of the material. For this reason, the objects have little ductility after high temperature isostatic compression. To eliminate troublesome precipitations, the molded bodies are annealed in a chamber furnace in a protective atmosphere (eg, argon). A temperature area of about 700 ° C to 860 ° C is selected for this, lasting several hours, generally two hours. There is an opposite relationship between temperature and duration: the higher the temperature, the shorter the time, and vice versa. After annealing the solution, the melting bodies are cooled with cold water. In Fig. A (1000-fold magnification) represents the structure after the solution annealing. The primary ß grains are appreciated and within the grains very fine precipitations between dendrites (note the accumulation in the form of clouds at the top left in the image). The objects produced by precision casting have grains ß having an average size no greater than 0.3 mm in their crystalline structure. This size is typical for the crystalline structure achieved with the inventive method. The mechanical characteristics obtained after the solution annealing are shown in Fig. 1. It can be seen that the modulus of elasticity decreases as the temperature increases during the annealing of solution, and to values up to 60,000 N / mm2. Tenacity values improve as they decrease strength and hardness. Thus, after annealing the two-hour solution at 800 ° C, a modulus of elasticity of 60,000 N / mm 2 with a breaking strength of approximately 40% and a breaking strength Rm of approximately 730 N / mm 2 is achieved.

Claims

CLAIMS 1. Casting method for objects of titanium-molybdenum titanium-alloy having a molybdenum content of 7.5 to 25%, characterized by melting the alloy at a temperature above 1770 ° C, casting precision of the molten alloy in a mold of cast iron that corresponds to the object to be produced, isostatic compression of high temperature, annealing of solution and finally to cool abruptly. Method according to claim 1, characterized by the use of a cold wall crucible vacuum induction installation for melting the titanium ß alloy. Method according to claim 1 or 2, characterized by the performance of high temperature isostatic compression at a temperature that is as high as maximum, as the beta phase transition temperature of the titanium-molybdenum alloy and at least 100 ° C below the beta phase transition temperature. TO . Method according to one of claims 1 to 2, characterized by carrying out the annealing of solution at a temperature of about 700 ° C to about 900 ° C. 5. Method according to claim A, characterized by carrying out the annealing of solution at a temperature of 800 ° C to 860 ° C. Method according to one of the preceding claims, characterized by a sudden cooling, preferably with cold water, after annealing of dissolution. Method according to one of the preceding claims, characterized by a final hardening of the object. 8. Method according to claim 7, characterized by carrying out the hardening at a temperature of 600 ° C to 700 ° C.