AU2006218029B2

AU2006218029B2 - Method for casting titanium alloy

Info

Publication number: AU2006218029B2
Application number: AU2006218029A
Authority: AU
Inventors: Sevki Baliktay
Original assignee: Waldemar Link GmbH and Co KG
Current assignee: Waldemar Link GmbH and Co KG
Priority date: 2005-02-25
Filing date: 2006-02-27
Publication date: 2011-07-21
Anticipated expiration: 2026-02-27
Also published as: AU2006218029A1; CN101128609A; RU2402626C2; AR052391A1; TWI395821B; KR101341298B1; BRPI0607832A2; CA2597248A1; JP2008531288A; KR20070105379A; MX2007010366A; EP1696043A1; DE502006004443D1; EP1851350A1; CN100594248C; ES2328955T3; CA2597248C; JP5155668B2; DK1851350T3; EP1851350B1

Abstract

The invention relates to a method for casting objects from a ß-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 °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 ß-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 ß-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 ß-titanium alloy thanks to the invention.

Description

- 1 Process for Casting a Titanium Alloy The invention relates to a process for casting objects from a S-titanium alloy, more specifically a titanium 5 molybdenum alloy. Titanium alloys are becoming more and more popular on account of their numerous advantageous properties. Titanium alloys are used in all fields in which high 10 demands are imposed on the material, in particular on account of their good chemical stability, even at high temperature, and their low weight combined with excellent mechanical properties. On account of their excellent biocompatibility, titanium alloys are also preferentially is used in the medical sector, in particular for implants and prostheses. Various methods for shaping titanium alloys are known. In addition to cutting processes, these primarily include 20 casting and forging processes. In principle, titanium alloys are forging alloys, for which reason forging processes are generally used, since it has been found that titanium alloys are difficult to cast. This approach is generally taken for complicated shapes but leads to 25 restrictions in terms of the choice of suitable alloys. In particular, it has been found that only unsatisfactory results are achieved when casting S-titanium alloys (US-A 2004/0136859). 30 It would be advantageous if the present invention would provide an improved casting process for S-titanium alloys which allows even complex shapes to be produced with good material properties. 35 According to a first aspect of the invention, in a process for casting objects from a S-titanium alloy comprising C:\NrPoribl\GHMatters\NARELLEV2442692_2.DOC 20/10110 titanium-molybdenum with a molybdenum content of from 7.5 to 25%, it is provided that the alloy is melted at a temperature of over 177 0 oC, the molten alloy is investment-cast into a casting mold corresponding to the 5 object to be produced, is hot-isostatically pressed, solution-annealed and then quenched. In the present context, an object is to be understood as meaning a product which has been shaped for final use. io The object may, for example in the aeronautical industry, be parts used for jet engines, rotor bearings, wing boxes or other supporting structure parts, or in the field of medicine may be endoprostheses, such as hip prostheses, or implants, such as plates or pins or dental implants. The is term object in the context of the present application does not encompass billets which are intended for further processing by shaping processes, i.e. in particular does not include ingots produced by permanent mold casting for further processing by forging. 20 The process according to embodiments of the invention achieves economical production of objects made from S titanium alloys using the investment-casting process. Embodiments of the invention therefore provide the 25 possibility of combining the advantageous properties of S titanium alloys, in particular their excellent mechanical properties, with the advantages of production of objects using the investment-casting process. Embodiments of the invention allow even objects of complex shapes, which have 30 been impossible to produce (economically) using conventional forging processes, to be produced from a S titanium alloy. Therefore, the embodiments of the invention also open up the application area of complex shaped objects to B-titanium alloys, which are known to 35 have favourable mechanical properties and biocompatibility. C:\NrPortbIGHMalters\NARELLEW\244292_2.DO 20/10/10 - 3 The molybdenum content in the alloy or its molybdenum equivalent is in the range from 7.5 to 25%. The result of this is that, in particular for a molybdenum content of at least 10%, the S-phase is sufficiently stabilized even as s far as the room temperature range. It is preferable for the content to be between 12 and 16%. This allows a meta stable S-phase to be achieved by rapid cooling following the investment casting. There is generally no need to add further alloy-forming elements. In particular, there is 10 not need to add vanadium or aluminum. Dispensing with these has the advantage mentioned above that the toxicity resulting from these alloy-forming elements can be avoided. The same correspondingly applies to bismuth, which also does not have the same biocompatibility as 1s titanium. It has been found that embodiments of the invention, using the S-titanium alloys which have hitherto been almost impossible to use for investment casting, allow the 20 production of even more complex shapes than the a/S titanium alloys which have hitherto been used for investment casting, such as for example TiA16V4. The process according to the embodiments of the invention achieves improved mold filling properties. This means 25 that as a result, in particular sharp edges can be produced with a higher quality during investment casting. The susceptibility to the formation of voids in investment casting is also reduced as a result of the improved mold filling properties. 30 It is expedient for a cold-wall crucible vacuum induction installation to be used to melt the S-titanium alloy. An installation of this type makes it possible to reach the high temperatures which are required for reliable melting 35 of titanium-molybdenum alloys for investment casting. For example, the melting point of TiMol5 is 1770 0 C. A supplement of approx. 60 0 C should expediently be added to C-\NrPortbIlGHMatters\NARELLEW\2442692_2 DOC 201 Oil 0 - 4 this to effect reliable investment casting. In particular, therefore, a temperature of 1830 0 C has to be reached for TiMol5. 5 It is preferable for the hot isostatic pressing to take place at a temperature which is at most equal to a beta transus temperature of the titanium-molybdenum alloy and is no more than 100 0 C below the beta transus temperature. 10 The hot isostatic pressing counteracts undesirable effects of concentrating the molybdenum in dendrites while depleting the remaining melts by dissolving inter dendritic precipitations. A temperature below the beta transus temperature, specifically at most 1000C below it, is is favourable. Temperatures in the range from 7100C to 760 0 C, preferably of approx. 740 0 C, at an argon pressure of approximately 1100 to 1200 bar have proven suitable for a titanium-molybdenum alloy with a molybdenum content of 15%. 20 Temperatures of at least 7000C to 8800C, preferably in the range from 800 0 C to 8600C, have proven suitable for solution annealing. Argon is preferably used to produce a shielding gas atmosphere. This improves the ductility of 25 the alloy. It is expedient for quenching of the object by water to be carried out after the solution annealing. It is preferable to use cold water. In this context, the term 30 "cold" is to be understood as meaning the temperature of unheated tap water. It has been found that the quenching has a considerable influence on the mechanical properties of the object which are ultimately achieved. Alternatively, quenching may also take place in shielding 35 gas, for example by argon cooling. The results achieved, however, are not as good as those achieved with cold water. C:\NrPortbl\GHMatters\NARELLEW2442692_2.DOC 20/10110 - 5 It may be expedient for the object finally also to be hardened. This may allow the modulus of elasticity to be increased slightly, if required, For this purpose, it is preferable for the hardening to be carried out in a 5 temperature range from approx. 600 0 C to approx. 700 0 C. The present invention provides in a second aspect a process for casting objects from a S-titanium alloy comprising titanium-molybdenum with a molybdenum content 10 of from 12% and 16%, characterised by melting the alloy at a temperature of over 17700C, investment-casting the molten alloy into a casting mold corresponding to the object to be produced, hot isostatic pressing, solution annealing at a temperature between 700 0 C and 9000C. 15 The invention is explained in more detail below with reference to the drawing, which illustrates an advantageous exemplary embodiment. In the drawing: 20 Fig. 1 shows a table which gives mechanical properties of the investment-cast titanium alloy according to an embodiment of the invention; Fig. 2 shows an image of the microstructure in a cast 25 state immediately after casting; Fig. 3 shows an image of the microstructure after hot isostatic pressing; 30 Fig. 4 shows an image of the microstructure after solution annealing with a subsequent quench; and Fig. 5 illustrates liquidus and solidus temperatures for a titanium-molybdenum alloy. 35 The text which follows describes a way of carrying out the method according to an embodiment of the invention. C:\NrPortbl\GHMatters\NARELLEW2442692 2 DOC 20/10/10 - 6 The starting material is a S-titanium alloy with a molybdenum content of 15% (TiMol5). This alloy can be obtained commercially in the form of small billets (ingots). 5 A first step involves investment casting of the objects that are to be cast. A casting installation is provided for melting and casting the TiMol5. This is preferably a cold-wall crucible vacuum induction melting and casting 10 installation. An installation of this type can reach the high temperatures which are required for reliable melting of TiMol5 for investment casting. The melting point of TiMol5 is 17700C, plus a supplement of approx. 600C for reliable investment casting. Overall, therefore, a is temperature of 1830 0 C has to be reached. The investment casting of the melt then takes place using processes which are known per se, for example, with wax cores and ceramic molds as lost molds. Investment casting techniques of this type are known for the investment casting of TiA16V4. 20 As can be seen from the figure (1000 times magnification) in Fig. 2, dendrites are formed, and considerable precipitations are evident in inter-dendritic zones. This is a consequence of what is known as the negative 25 segregation of titanium-molybdenum alloys. This effect is based on the specific profile of the liquidus and solidus temperatures of titanium-molybdenum alloys, as illustrated in Fig. 5. On account of the profile of the melting temperatures of the liquid phase (T1) and the solid phase 30 (T 5 ) illustrated, it is firstly the regions with a high molybdenum content which solidify in the melt, during which process the dendrites that can be seen in the figure are formed. This leads to depletion of the residual melt, i.e. its molybdenum content drops. The inter-dendritic 35 zones in the cast microstructure have a molybdenum content of less than 15%, and it is even possible for the molybdenum content to drop to approx. 10%. As a result of C:\NrPortb\GHMatters\NARELLE\2442692_2 DOC 20/10110 - 7 the molybdenum depletion, the inter-dendritic zones lack a sufficient quantity of S-stabilizers. The result of this is that an increased a/S transformation temperature is locally established, resulting in the formation of the 5 precipitations shown in Fig. 2. It is expedient for a surface zone which may form during casting as a hard, brittle layer, known as the a-cast, to be removed by pickling. The thickness of this layer is 10 usually approx. 0.03 mm. To counteract the unfavourable effect of the negative segregation with the precipitations in the inter-dendritic zones, according to an embodiment of the invention the castings, after the casting molds have been removed 15 following the investment casting, are subjected to a heat treatment. This involves hot isostatic pressing (HIP) specifically at a temperature just below the 9-transus temperature. It may be in the range from 710 0 C to 760 0 C and is preferably approximately 740 0 C. This causes the 20 undesirable precipitations in the inter-dendritic zones to be dissolved again. There is no need for any preliminary age-hardening before or after the hot isostatic pressing. However, fine secondary phases precipitate again during the cooling following hot isostatic pressing, 25 preferentially in the original inter-dendritic zones (cf. Fig. 3, 1000 times magnification). This leads to undesirable embrittlement of the material. The objects have only a low ductility following the hot 30 isostatic pressing. To eliminate the disruptive precipitations, the castings are annealed in a chamber furnace under a shielding gas atmosphere (e.g. argon). A temperature range from approx. 35 700 0 C to 860 0 C with a duration of several hours, generally two hours, is selected for this purpose. In this context, C:\NrPortbl\GHMatters\NARELLEV2442692_2.DOC 20/10/10 - 8 there is a reciprocal relationship between the temperature and duration; at higher temperature, a shorter time is sufficient, and vice versa. Following the solution annealing, the castings are quenched with cold water. 5 Fig. 4 (1000 times magnification) illustrates the microstructure following the solution annealing. Primary 9-grains and, within the grains, very fine inter-dendritic precipitations (cf. cloud-like accumulation in the top left of the figure) can be seen. The objects which have 10 been investment-cast using the process according to the invention have S-grains with a mean size of more than 0.3 mm in their crystal structure. This size is typical of the crystal structure achieved by the process according to the invention. 15 The mechanical properties achieved following the solution annealing are given in the table in Fig. 1. It can be seen that the modulus of elasticity drops with 20 an increasing temperature during the solution annealing, specifically as far as levels of 60,000 N/mm 2 . The ductility values improve with decreasing strength and hardness. For example, after solution annealing for two hours at 8000C, a modulus of elasticity of 60,000 N/mm 2 25 combined with an elongation at break of approx. 40% and a fracture strength Rm of approx. 730 N/mm 2 are achieved. It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute 30 an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country. C \NrPortbl\GHMatters\NARELLEW\2442692_ 2 DOC 20/10/10

Claims

1. A process for casting objects from a S-titanium alloy comprising titanium-molybdenum with a molybdenum 5 content of from 12% and 16%, characterised by melting the alloy at a temperature of over 17700C, investment-casting the molten alloy into a casting mold corresponding to the object to be produced, hot 10 isostatic pressing, solution annealing at a temperature between 700 0 C and 900 0 C and subsequent quenching.

2. The process as claimed in claim 1, is characterised by using a cold-wall crucible vacuum induction installation for melting the p-titanium alloy.

3. The process as claimed in claim 1 or 2, 20 characterised by carrying out the hot isostatic pressing at a temperature which is at most equal to a beta transus temperature of the titanium-molybdenum alloy and is not more than 100 0 C below the beta transus 25 temperature.

4. The process as claimed in claim 1, characterised by carrying out the solution annealing at a temperature 30 of from 800*C to 860 0 C.

5. The process as claimed in any one of the preceding C:\NrPortbl\GHMalters\NARELLEW\2442692_2.DOC 20/10/10 - 10 claims, characterised by quenching with preferably cold water following the solution annealing. 5

6. The process as claimed in any one of the preceding claims, characterized by final hardening of the object. 10

7. The process as claimed in claim 6, characterised by carrying out the hardening at a temperature of from 600 0 C to 7000C. 15

8. A process substantially as herein described with reference to one or more of the drawings. C:\NrPorbl\GHMatters NARELLEW2442692_2.000 20/11OMD