METHOD FOR THE POWDER-METALLURGICAL PRODUCTION OF COMPONENTS FROM TITANIUM OR TITANIUM ALLOYS

Abstract

A method for the powder-metallurgical production of a component from titanium or a titanium alloy is disclosed. In this method, following the customary procedure, first a green part is formed by using metal powder formed from titanium or the titanium alloy and is densified and compacted in a subsequent sintering step. Metal powder of titanium or the titanium alloy with an average grain size of <25 m is used for producing the green part and the sintering step is carried out at a sintering temperature of up to a maximum of 1100 C. for a sintering at a sintering duration of 5 hours in an atmosphere that is under a reduced pressure in comparison with normal pressure. These measures achieve the effect that the grain structure of the material obtained, and consequently also the material properties, can be selectively influenced.

Claims

1. A method for the powder-metallurgical production of a component from titanium or a titanium alloy, wherein first, using metal powder from titanium or the titanium alloy, a green part is formed and the green part is densified and compacted in a subsequent sintering step, wherein for producing the green part, metal powder from titanium or titanium alloy with a mean grain size of <25 m, measured using laser diffraction according to ASTM B822-10, is used and the sintering step is performed at a sintering temperature up to a maximum of 1100 C. in an atmosphere under a reduced pressure in comparison with normal pressure.

2. The method according to claim 1, wherein the maximum grain size of the metal powder from titanium or the titanium alloy is <30 m.

3. The method according to claim 1, wherein the sintering step is performed under a vacuum with a pressure of 10.sup.3 mbar.

4. The method according to claim 1, wherein the sintering step is performed in an inert gas atmosphere at a pressure of 300 mbar.

5. The method according to claim 1, wherein for producing the green part, metal powder from titanium or the titanium alloy with a mean grain size of <20 m is used.

6. The method according to claim 1, wherein the sintering duration is 3.5 h.

7. The method according to claim 1, wherein the sintering duration is at least 1 h.

8. The method according to claim 1, wherein the sintering temperature is up to a maximum of 1050 C.

9. The method according to claim 1, wherein the sintering temperature amounts to at least 860 C.

10. The method according to claim 1, wherein in the sintering step, the sintering temperature is adjusted in the range below a p-transition temperature of the titanium or titanium alloy material.

11. The method according to claim 1, wherein the component after the sintering step has a material density of >97%.

12. The method according to claim 1, wherein in the sintering step, a sintering temperature of below 950 C. is selected and that to achieve a material density in the component of >97%, after the sintering step the component is exposed to an additional step with pressure and optionally a temperature.

13. The method according to claim 1, wherein the component, following the sintering step, is subjected to a thermal aftertreatment.

14. The method according to claim 13, wherein the thermal aftertreatment is conducted in the form of one or more of the following treatment procedures: hot isostatic pressing (HIP), quench, and uniform rapid quench (URQ).

15. A component produced according to claim 1 from titanium or a titanium alloy having a globular -structure with a grain size of <30 m.

16. A component produced according to claim 1 from titanium or a titanium alloy having a grain structure with globular -structure with mean grain size of <30 m and lamellar (+) grain structure with a mean primary -phase grain size of <90 m.

17. A component produced according to claim 1 from titanium or a titanium alloy having a lamellar (+) grain structure with a mean primary -phase grain size of <120 m.

18. The method according to claim 3, wherein the sintering step is performed under a vacuum with a pressure of 10.sup.5 mbar.

19. The method according to claim 4, wherein the sintering step is performed in an argon atmosphere.

20. The method according to claim 5, wherein metal powder from titanium or the titanium alloy with a mean grain size of <10 m is used.

21. The method according to claim 20, wherein metal powder from titanium or the titanium alloy with a mean grain size of <5 m is used.

22. The method according to claim 6, wherein the sintering duration is 3 h.

23. The method according to claim 22, wherein the sintering duration is 2.5 h.

24. The method according to claim 7, wherein the sintering duration is at least 2 h.

25. The method according to claim 8, wherein the sintering temperature is up to a maximum of 1000 C.

26. The method according to claim 25, wherein the sintering temperature is up to a maximum of 950 C.

27. The method according to claim 11, wherein the component after the sintering step has a material density of >98%.

28. The method according to claim 28, wherein the component after the sintering step has a material density of 99%.

29. The method according to claim 12, wherein the additional step with pressure comprises one of cold isostatic pressing (CIP) and hot isostatic pressing (HIP).

30. The method according to claim 1, wherein the sintering step is performed for a sintering duration of 5 h.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0020] The figures show:

[0021] FIG. 1 a representation of a lamellar (+) grain structure of a Ti6Al4V sample with description of the gran structure fractions according to Sieniawski et al. [3];

[0022] FIG. 2 an enlarged photomicrograph of a standard sintered Ti6Al4V sample, produced by powder-metallurgically using powder particles <45 m and standard-sintered and confirms a lamellar (+) grain structure for this;

[0023] FIG. 3 a schematic representation of the effect of reducing the grain size by half (using the example of spherical particles) on the number of particles required to fill a defined volume;

[0024] FIG. 4 a schematic representation of the reduction in size of the hollow space between adjacent particles due to reducing the grain size by half (using the example of spherical particles);

[0025] FIG. 5 an enlarged polished micrograph section of a powder-metallurgically produced and sintered Ti6Al4V sample made from powder particles <20 m, confirming the formation of a distinct globular -structure; and

[0026] FIG. 6 an enlarged polished micrograph section of a powder-metallurgically produced and sintered Ti6Al4V sample made from powder particles <20 m, confirming the formation of a bimodal grain structure with a globular -structure and distinct lamellar (+) grain structure.

DETAILED DESCRIPTION

[0027] An essential prerequisite for implementing the process according to the invention and creating the possibility of influencing the material properties in the sintering process is the use of metal powder, produced from titanium or a titanium alloy, with a mean grain size of <25 m, so-called fine powder. In such fine powder used for the process according to the invention, the maximum grain size may in particular be <30 m. The maximum grain size is specified as a limit value by the manufacturers of such fine powders. At the same time, a small fraction of particles in such batches can always have grain sizes above this limit. Such a fraction, as a rule, is generally specified as a maximum of 1 to a maximum of 5 wt.-%.

[0028] The mean grain size may advantageously even be lower, especially <20 m, advantageously <10 m and particularly preferably even <5 m. The smaller the grain size of the metal powder is, the more readily high final densities can be achieved even at sintering temperatures markedly reduced compared to the relatively high sintering temperatures previously used.

[0029] The measurement of the grain sizes essential for the invention and the distribution thereof is performed by grain size testing using laser diffraction according to ASTM B822-10 (published 2010), valid at the time of this application. The grain size distribution is determined by wt.-% and according to D10/D50/D90, wherein D50 is the mean grain size. Specifically, the grain sizes given here in comparison tests were measured using the COULTER LS grain size analyzer made by Beckman Coulter and evaluated using the Fraunhofer theory according to ASTM B822-10.

[0030] For spherical particles, the grain size in the sense of the invention is specified as the particle diameter. For nonspherical particles, the grain size corresponds to the projected maximum particle dimension.

[0031] As a result of the reduced grain size, the surface area in the nonconsolidated component available for the sintering process increases, and thus so does the stored surface energy. Since the reduction of this energy is the driving force in the sintering process, the sintering process can then take place using little thermal energy.

[0032] An additional advantage of using fine powders of the sizes indicated above for forming the green part is that more powder particles can be introduced per unit volume. In addition to the enlarged surface, this leads to a higher number of contact points per unit volume, as shown in FIG. 3. There, in a schematic representation, the effect of reducing the grain size by half (using the example of spherical particles) on the particle count to fill a defined volume is shown.

[0033] The contact points of the particles in turn are the starting point and a necessary condition for the sintering process, which is driven by diffusion processes. The increased number of such contact points per unit volume therefore improves the starting conditions for the sintering process.

[0034] Through the use according to the invention of fine powders with mean grain sizes <25 m, when considering the ideal packing density in addition to the aforementioned advantages, the result also occurs that the volume enclosed by the powder particles, as shown in an idealized representation in FIG. 4, is decreased. In FIG. 4, in a schematic representation the decrease in size of the hollow space between adjacent particles is illustrated by reducing the grain size by half (using the example of spherical particles). Since this hollow space must be closed to achieve thehighmaterial density desired for the component following the sintering process must be closed by material transport during the sintering process, a smaller volume to be covered is an additional decisive reason for an improvement in the process result.

[0035] The sintering step typically takes place in a reduced-pressure atmosphere. This can be a vacuum with a pressure of 10.sup.3 mbar, especially 10.sup.5 mbar. However, it may also be a reduced-pressure inert gas atmosphere with a pressure of, e.g., 300 mbar. Argon gas in particular is considered as the inert gas here.

[0036] The sintering temperatures according to the invention are below 1100 C. They can in particular be a maximum of 1050 C., a maximum of 1000 C., and even a maximum of only 950 C. Preferably, however, to achieve a good sintering result, the sintering temperature selected advantageously should not be below 860 C. The sintering temperature may be kept uniform. In particular, however, it is also possible and falls within the meaning of the invention to vary the temperature during the sintering process. The sintering temperature is defined here as the temperature that the workpiece to be sintered has undergone. Depending on the sintering unit, in the unit control, an adapted process temperature is to be selected, which distinguishes the process temperature measured at a distance remote from the workpiece from the sintering temperature undergone by the workpiece.

[0037] The duration of sintering may especially be 3.5 h, often also 3 h or even 2.5 h. However, it was found that as a rule, for achieving good results, the sintering time should amount to at least 1 hour, preferably at least 2 hours.

[0038] After the sintering step, components from titanium or titanium alloys produced with the method of the invention generally have a final density of >97%. However, final densities above 98% may also be reached, even 99%.

[0039] To achieve a globular grain structure, the titanium components are sintered at less than the -transition temperature (e.g., at a temperature 30 C. below the -transition temperature.

[0040] For example, in initial experiments at a sintering temperature of 950 C., which is below the -transition temperature, and with a sintering duration of less than three hours, components with a final density of >97% were produced. These had a globular grain structure with an -grain size on average of 10.1 m and a max. size of 29 m. The grain structure of this material is shown in FIG. 5. These grain sizes fall in the order of magnitude of the powder particles used.

[0041] According to the literature, the -transition temperature of Ti6Al4V falls in the range of 985 C. to 1015 C. [3; 5]. This relatively wide range given in the literature is attributable, on one hand, to the distribution of the alloying elements in the titanium alloys. On the other hand, the ambient pressure is an additional influential factor. For example, Huang et al. describe that as a result of elevated process pressures (1500 bar), a reduction of the -transition temperature can be observed in the alloy Ti4Al8Nb [6].

[0042] The inventors now believe that depending on the process conditions, shifts in the -transition temperature of only a maximum of 20 C. will be observable due to pressure variations.

[0043] For creating a bimodal structure, the components were sintered close to the -transition temperature, but still below this.

[0044] For example, in order also to produce the lamellar grain structure with reduced primary -phase grain size of the Ti6Al4V alloy, which is also advantageous for many use cases, initially samples were produced in which the titanium components were sintered at a sintering temperature of 1000 C. (FIG. 6). As shown by studies of the samples obtained with respect to the grain structure formed, this sintering temperature was still below the -transition temperature, although only slightly. The bimodal grain structure formed is composed of globular -structure and small portions of lamellar (+) structures, wherein the mean -grain size is 81 m.

[0045] The density measurement was performed according to the specifications of ASTM B962 and ASTM B311. The grain size determination was performed according to the provisions of ASTM E112.

[0046] For creating a lamellar grain structure with the smallest possible grain size of the primary -phase grains, the components were largely sintered, i.e., for the greatest part of the time, below the -transition temperature, but with a minimal hold time that remained below 30 min, preferably below 20 min, especially below 10 min, and also above the -transition temperature in phases, so that the p-phase is entirely present, in order thus to create the lamellar grain structure, but also the primary -phase grain does not exceed the size range of a globular -structure with mean grain size of <30 m and lamellar (+) grain structure with a mean primary p-phase grain size of <90 m. The sintering above the p-transition temperature always took place at a temperature in excess of 1015 C. This temperature was always kept below 1080 C., but advantageously was below 1040 C. and especially 1020 C. was selected.

[0047] The possibilities mentioned above for influencing the phase composition in the sintered material by systematic adjustment of the sintering conditions at sintering temperatures below 1100 C., especially primarily below the -transition temperature, present a particular advantage of the process according to the invention. The prerequisite for this variability is that sufficiently compact titanium components can be produced below the -transition temperature, which is possible, as the inventors recognized, based on the use of the fine powder, essential to the invention, with grain sizes below <30 m.

[0048] Thus it has been shown that according to the method of the invention, powder-metallurgical moldings from titanium and titanium alloys can be sintered at sintering temperatures below the usual mark of beyond 1100 C., generally 1200 C. or more, advantageously below the p-transition temperature, and thereby components with good structural and other material properties can be obtained. It was possible to show that at distinctly lower set sintering temperatures compared with the sintering temperatures customary in the prior artunexpectedlycomponents with high final densities of >97% can be obtained. In particular it was shown that the method according to the invention makes it possible to vary the grain structure of the titanium component in the sintering process and drastically reduce the grain size, which makes it possible to optimize the mechanical properties of the components, e.g., the tensile strength, ductility and fatigue strength.

[0049] For example, within the scope of the invention, a particularly low temperature may also be selected for sintering, e.g., a temperature below 950 C., can be selected, and if the desired material density in the finished component (generally >97%) is not yet achieved in such a sintering step, further compaction of the material can be performed in the subsequently performed pressing step, in which the material is subjected to pressure and optionally a temperature, especially by cold isostatic pressing (CIP) or hot isostatic pressing (HIP). Here, for example, the material density after sintering may be at <97%, and it may be compacted to >97% by the pressing step after sintering.

[0050] In addition, following the sintering step, components produced according to the method of the invention may be subjected to additional thermal aftertreatments to further modify the properties of the materials. Such additional thermal aftertreatments can, for example, be one or more of the following methods: hot isostatic pressing (HIP), quench, uniform rapid quench (URQ).

[0051] The lower sintering temperature compared to the sintering temperatures from the prior art also result in additional environmental/financial and process technology advantages. On one hand, less thermal energy is required in the sintering process, leading to lower costs but also to shorter processing times. On the other hand, the method in accordance with the invention performed with reduced sintering temperature also allows the use of how-wall furnace designs which are once again more economical than furnaces designed for process temperatures >1100 C., where cold-wall furnaces are typically used.

[0052] The selective combination of fine powders with mean grain size <25 m, preferably also with maximum grain sizes <30 m, and reduced sintering temperatures compared with the prior art, to be classified as low, allows the unrivaled manipulation of the grain structure and thus of the material properties.

REFERENCES

[0053] [1] R. Gerling, T. Ebel, T. Hartwig: Method for producing components by metal powder injection molding. European patent EP1119429B1, 2003.

[0054] [2] H.-J. Blm: Method for combined debinding and sintering of glass-ceramic, ceramic and metal molded parts. European Patent EP1496325A2, 2004.

[0055] [3] J. Sieniawski, W. Ziaja, K. Kubiak, M. Motyka: Microstructure and Mechanical Properties of High Strength Two-Phase Titanium Alloys. Materials Science/Metals and Nonmetals Titanium AlloysAdvances in Properties Control, 2013, ISBN 978-953-51-1110-8.

[0056] [4] M. Marty, H. Octor, A. Walder: Process for forming a titanium base alloy with small grain size by powder metallurgy. U.S. Pat. No. 4,601,874, 1986.

[0057] [5] J. Lindemann: Titanium alloys, Laboratory Course on Lightweight Construction Materials, Department of Metals Science and Materials Technology Brandenburg Technical University Cottbus, 2012.

[0058] [6] A. Huang, D. Hu, M. H. Loretto, J. Mei, X, Wu: The influence of pressure on solid-state transformations in Ti-46Al-8Nb. Scripta Materialia, Vol. 56, 4.sup.th Ed., 2007, p. 253-324.