Manufacture of near-net shape titanium alloy articles from metal powders by sintering at variable pressure

FIELD OF INVENTION

[0001] The present invention relates to powder metallurgy of titanium alloys, and can be used in aircraft, automotive, Naval applications, oil equipment, chemical apparatus, and other industries. More particularly, the invention is directed at the manufacture of near-net shape titanium articles from sintered elemental and alloyed powders.

BACKGROUND OF THE INVENTION

[0002] Titanium alloys are well known to exhibit lightweight, high resisdence to oxidation or corrosion, as well as the highest specific strength (the strength-to-weight ratio) amid all metals except beryllium. Previously, articles of titanium alloys have been produced by melting, forming and machining processes, or by powder metallurgy techniques. The first method is not cost effective but provides high levels of all properties of titanium alloys. The second method is cost effective but cannot completely realize all advantages of titanium alloys.

[0003] Various processes have been developed during the last three decades for the fabrication of near-net shape titanium articles with desirable density and mechanical properties. The use of elemental powder mixtures, controlling the particle size distribution, vacuum sintering, hot isostatic pressing, and special surface finishing are among those new developments. But all of these processes, as well as conventional powder metallurgy techniques, impose certain limitations with respect to the characteristics of the produced titanium alloys.

[0004] For example, a method for producing sintered articles from a titanium powder alloy disclosed in JP 06092605, 1998 includes molding a mixture of elemental powders, vacuum sintering, hot isostatic pressing of the alloy in α+β region, and shot pinning to heal surface porosity. The irregular porosity in the interior portion of the sintered articles is the drawback of this method, which decreases mechanical properties, especially the strength.

[0005] The method for producing titanium alloys from elemental powders disclosed in JP 129864, 1990, includes pressing of the powder mixture, vacuum sintering, quenching of the alloy in β-region, and hot pressing at a temperature over 800° C. The oxidation of resulting articles during the hot pressing results in the loss of mechanical properties.

[0006] The method described in the U.S. Pat. No. 4,432,795 includes grinding particles of light metals to the particle size less than 20 μm, mixing them faith particles of titanium based alloys having a particle size larger than 40 μm, and compacting the mixture by molding and sintering at temperatures less than that of a formation of any liquid phase. This method allows the manufacture of the alloy having a density close to the theoretical value but the resulting alloy, contaminated by oxygen, iron, and other impurities, also exhibits low mechanical properties.

[0007] The U.S. Pat. No. 4,838,935 describes the use of titanium hydride together with titanium powder in the primary mixture before molding and sintering. The molded article is heated in a hot-press vacuum chamber to a temperature sufficient for the dehydration of TiH2 to remove to gases. Then, the article is heated to a temperature of 1350-1500° C. while maintaining the pressure and vacuum. This method cannot completely prevent the oxidation of highly-reactive titanium powders during the second heating, because hydrogen is permanently outgassing from the working chamber. Besides, this method is not suitable for powdered mixtures containing low-melting metal and phases.

[0008] A preliminary partial sintering of titanium and titanium hydride powders with elemental powders of alloying metals is disclosed in U.S. Pat. No. 3,950,166. The “mother” alloy obtained in such a way is pulverized and remixed with powder metals such as Mo, V, Zr, and Al—V master alloy to achieve the final composition of titanium alloy. This mixture is molded in a predetermined shape and sintered at 1000-1500° C. in a vacuum. The preliminary sintering partially resolves one technical problem: how to improve uniform distribution of alloying components, but generates another: oxidation of the “mother” powder during pulverization. Several attempts have been made to improve the density and purity of sintered titanium alloys by using titanium hydride as the raw component, together with other alloying powders as in JP 07278609, 1995, or JP 06088153, 1994, or U.S. Pat. No. 3,472,705, 1969, or WO 9701409, 1997. All of these methods include vacuum heating and sintering accompanied with permanent outgassing. So, the “cleaning effect” of hydrogen is not used properly, and partial oxidation reoccurs after the removal of hydrogen from the vacuum chamber. Thus, these methods do not provide an effective improvement of mechanical properties of sintered alloys, in spite of the sintering promoted by thermal dissociation of titanium hydride.

[0009] Some specialized technologies were offered to manufacture titanium alloys in hydrogen atmosphere in JP 58034102, 1983 and CH 684978, 1995. These methods cannot prevent the contamination of sintered metals as well as the methods mentioned above: after the replacement of a hydrogen-containing atmosphere by an inert gas, the oxidation of reactive powders reoccurs.

[0010] All other known processes for making near-net shape titanium alloys from metal powders have the same drawbacks: (a) insufficient purity and low mechanical properties of sintered titanium alloys, (b) irregular porosity and insufficient density of sintered titanium alloys, and (c) low reproduction of mechanical properties that depend on the purity of raw materials.

OBJECTIVES OF THE INVENTION

[0011] The object of the invention is to increase the mechanical properties, particularly strength and plasticity, of near-net shape articles manufactured by sintering titanium alloys from elemental and/or alloyed metal powders.

[0012] In order to obtain a high level of mechanical properties, any oxidation or contamination of powdered components must be prevented during heating and sintering.

[0013] Another objective of the present invention is to provide low porosity and high-density structures of sintered titanium alloys to achieve the densities close to the theoretical value.

[0014] It is also an objective to provide the cost-effective manufacture of near-net shape articles using one run heating and sintering of powdered titanium alloys.

[0015] The nature, utility, and further features of this invention will be more apparent from the following detailed description, with respect to preferred embodiments of the invented technology.

SUMMARY OF THE INVENTION

[0016] The invention relates to the manufacture of near-net shape titanium articles from sintered powders containing titanium and all required alloying elements. While the manufacture of titanium alloys by sintering elemental and alloyed metal powders including titanium hydride has previously been contemplated as mentioned above, problems related to insufficient strength, irregular porosity, insufficient density, and cost reductions have not been solved.

[0017] The invention overcomes these problems by:

[0018] (a) mixing a titanium hydride powder having a particle size of ≦150 μm with alloying metal powders (master alloys or elemental powders) having a particle size in the range of {fraction (1/15)}-⅖ of the maximal particle size of said titanium hydride powder,

[0019] (b) compacting the resulting powder mixture by molding at the pressure of 400-1000 MPa,

[0020] (c) heating to the sintering temperature of the predetermined alloy composition at variable pressure in the furnace chamber: initially heating to 400° C. in vacuum of less than 10−2 Pa, then, increasing the temperature to a range of 400-900° C. with the pressure up to 104 Pa, which is controlled by hydrogen being emitted by the decomposition of titanium hydride contained in the compacted powdered alloy, and finally, heating to over 900° C. to the sintering temperature with the pressure continually decreasing to the starting vacuum level, and,

[0021] (d) sintering.

[0022] Heating to the sintering temperature is performed at the rate of 10-15 grad/min.

[0023] In another aspect of the invention, technology is provided to manufacture near-net shape sintered titanium articles in a cost-effective way.

[0024] In essence, the core of the invention is to control the purity and mechanical properties of sintered titanium alloys using (a) TiH2 powder having a predetermined particle size as the base component, (b) optimal ratio of particle size between TiH2 powder and alloyed metal powders, and (c) variable pressure of hydrogen in the furnace chamber during the heating and sintering.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS OF THE INVENTION

[0025] As discussed, the present invention relates generally to the manufacture of sintered titanium alloys using elemental metal powders and titanium hydride as raw materials. Optimal size distribution of raw metal powders and the prevention of their oxidation during heating and sintering play a very important role in such processes.

[0026] No previously known methods, also mentioned in References, tried to find out the optimal ratio between the particle size of titanium hydride powder and the particle size of metal powders alloying the titanium base. The known methods always used permanent outgassing of the vacuum chamber during heating and sintering. Therefore, a complete reaction is not achieved between metal powders and green titanium compacts with hydrogen, and the final structure of the sintered alloy contains oxides and irregular porosity.

[0027] On the one hand, the particle size of the titanium base and alloying metal powders should be as small as possible to enhance the chemical homogenization and to reach high final density of the sintered alloy. On the other hand, the smaller the particle size of raw metal powders, the more chemical contaminants in the powder mixture to be molded and sintered. We found that a combination of titanium hydride powder having a particle size of <150 μm with alloying metal powders having a particle size in the range of {fraction (1/15)}-⅖ of the size of said titanium hydride powder is the optimum to obtain a fully dense, strong structure of resulting titanium alloy. These sizes of raw metal powders achieve a high rate of homogenization and higher density of the sintered alloy accompanied with the limitation of impurities at the desired lower level.

[0028] Experimental testing of titanium hydride powder having a particle size >150 μm showed that the final density of the sintered alloy was decreased to 98% of the theoretical value and less. The use of other powdered components with a particle size less than {fraction (1/15)} of the particle size of the TiH2 powder resulted in unacceptable contamination of the sintered alloy.

[0029] The use of titanium hydride powder as the base component instead of titanium powder, promotes rapid phase formation and activation of sintering of powdered preforms. The titanium hydride is decomposed during the vacuum heating with the emission of hydrogen in the range of 400-900° C. that results in the formation of titanium having high density of crystalline defects, and hastens the acceleration of the diffusion process.

[0030] The emitted atomic hydrogen beneficially effects on sintering kinetics, reduces any oxides that are usually located on the surface of powder particles, and by doing so, is cleaning interparticle interfaces and enhances the diffusion between all components of the powder mixture.

[0031] In order to use this positive effect, it is necessary to maintain a high concentration of hydrogen in molded preforms and provide its permanent emission during the heating process to the sintering temperature. We increase the partial pressure of hydrogen in the furnace chamber up to 104 Pa in the temperature range of 400-900° C. to keep hydrogen in the crystalline lattice of titanium up to 900° C. High pressure of ambient hydrogen prevents a decrease in the concentration of hydrogen dissolved in titanium that usually happens with an increase in temperature. Further heating and sintering is carried out by outgassing hydrogen from the working chamber to the remaining pressure of 10−2 Pa to remove the hydrogen from the metal and to transform the multiphase powder mixture into a chemically homogeneous and fully dense alloy. Such change in the hydrogen pressure during the processing of titanium powders increases mechanical properties of the resulting alloy, especially the strength and plasticity. Thus, the positive effect of hydrogen is used in the heating stage. The hydrogen cannot be present in the vacuum chamber during the final stages of sintering in order to prevent its negative effect on properties of the solid sintered alloy.

[0032] On the other side, the particle size of alloying powders in the alloy mixture should not be larger than ⅖ of the particle size of base titanium powder to provide a complete solid-phase reaction of low-melting powders (for example, elemental aluminum) with titanium, before they reach their melting points to avoid a significant Kirkendal-type porosity. The use of larger low-melting powders resulted in the partial or even complete liquid-phase reaction with the titanium base, because coarse low-melting powders or their eutectics are melted during the heating earlier than when the solid-phase reaction would occur. This premature liquid-phase reaction on the heating stage resulted in an incomplete homogenization of the alloy composition that cannot be improved by subsequent sintering and annealing.

[0033] Thus, the above-mentioned ratio of particle sizes between titanium hydride powder and other metal powders in the raw mixture was experimentally proven, and can be considered as the optimal ratio.

[0034] Molding of powdered preforms to desired near-net shape is carried out at the pressure of 400-1000 MPa. The pressure less than 400 MPa is insufficient for molding. On the other hand cracks in the molded preforms occur at the pressure of >1000 MPa. The heating of molded near-net shape preforms is carried out at a rate of 10-15° C./min to provide the technological quality of the process. The preforms crack at the rate of >15° C. min because the hydrogen emission from the decomposed titanium hydride is too intense. The rate of <10° C./min is too low and has no affect on any properties of the processed alloy.

[0035] The hydrogen emission at a temperature less than 400° C. is insignificant, and the temperature higher than 900° C. is nearly complete. Therefore, the pressure of hydrogen is controlled in the working chamber in the temperature range of 400-900° C.

[0036] The return of the pressure in the working chamber to the level of less than 10−2 Pa accompanied with the heating from 900° C. to the sintering temperature results in the elimination of hydrogen from the sintered alloy. The absence of hydrogen prevents the deterioration of mechanical properties of titanium alloy, especially preventing a hydrogen-ignited brittleness.

[0037] The innovated technology allows the manufacture of chemically homogeneous titanium alloys with high densities and mechanical properties compared to properties of casting alloys.

EXAMPLE 1

[0038] Titanium hydride powder having a particle size of <150 μm was mixed with master alloys Ti—Al and Al—V powders having a particle size of −10 . . . −60 μm in the ratio providing the stoicheometric composition of the alloy Ti-6Al-4V. Powders are mixed for 6 hours, and compacted (molded) at 700 MPa in the near-net shape preform having a relative density of 74%. The preform was heated in a vacuum of 10−2 Pa at the rate of 10° C./min up to 1350° C. No liquid phases were at this temperature, yet. During the heating process, the pressure in the furnace chamber was increased to 104 Pa in the temperature range of 400-900° C. resulting in hydrogen being emitted from the titanium hydride. The pressure in the chamber was decreased gradually to 10−2 Pa during heating to over 900° C. Then, the preform was sintered for 4 hours at 1350° C. The obtained article was studied using microstructural analysis, X-ray, and microspectral analysis, which confirmed that the produced metal is a chemically and structurally homogeneous alloy Ti-6Al-4V having a density of 98.9% of the theoretical value. The tensile strength of the obtained alloy was 960 MPa and the elongation was 7%.

EXAMPLE 2

[0039] Titanium hydride powder having a particle size of <100 μm was mixed with aluminum and vanadium powders having a particle size of +10 . . . −30 μm in the ratio providing the stoicheometric composition of the alloy Ti-6Al-4V. Powders are mixed for 5 hours, and compacted (molded) at 800 MPa in the near-net shape preform having a relative density of 76%. The preform was heated in a vacuum of 10−2 Pa at the rate of 10° C./min up to 1250° C. During the heating process, the pressure in the furnace chamber was increased to 104 Pa in the temperature range of 400-900° C. resulting in hydrogen being emitted from the titanium hydride. Aluminum 45 powder reacts with titanium base at 600-620° C., which is lower than the melting temperature of aluminum. The pressure in the chamber was decreased gradually to 10−2 Pa during heating to over 900° C. Then, the preform was sintered for 4 hours at 1250° C. The obtained article was studied using microstructural analysis, X-ray, and microspectral analysis, which confirmed that the produced material is a chemically and structurally homogeneous alloy Ti-6Al-4V having a density of 98.7% of the theoretical value. The tensile strength of the obtained alloy was 990 MPa and the elongation was 3%.

EXAMPLE 3

[0040] Titanium hydride powder having a particle size of <150 μm was mixed with master alloys Mo—Al and Al—V powders having a particle size of +10 . . . −60 μm in the ratio providing the stoicheometric composition of the alloy Ti-3Al-5Mo-5V. Powders are mixed for 6 hours, and compacted (molded) at 700 MPa in the near-net shape preform having a relative density of 75%. The preform was heated in a vacuum of 10−2 Pa with the rate of 15° C./min up to 1300° C. No so liquid phases were at this temperature, yet. During the heating process, the pressure in the furnace chamber was increased to 104 Pa in the temperature range of 400-900° C. resulting in hydrogen being emitted from the titanium hydride. The pressure in the chamber was decreased gradually to 10−2 Pa during heating to over 900° C. Then, the preform was sintered for 7 hours at 1300° C. The obtained article was studied using microstructural analysis, X-ray, and microspectral analysis, which confirmed that the produced metal is a chemically and structurally homogeneous alloy Ti-3Al-5Mo-5V having a density of 98.4% of the theoretical value. The tensile strength of the obtained alloy was 920 MPa and the elongation was 5%.

EXAMPLE 4

[0041] Titanium hydride powder having a particle size of <150 μm was mixed with aluminum and molybdenum powders having a particle size of +10 . . . −30 μm in the ratio providing the stoicheometric composition of the alloy Ti-6Al-3Mo. Powders are mixed for 7 hours, and compacted (molded) at 600 MPa in the near-net shape preform having a relative density of 73%. The preform was heated in a vacuum of 10−2 Pa at the rate of 10° C./min up to 1250° C. During the heating process, the pressure in the furnace chamber was increased to 1 Pa in the temperature range of 400-900° C. resulting in hydrogen being emitted from titanium hydride. Aluminum powder reacts with titanium base at 620-640° C., which is lower than the melting temperature of aluminum. The pressure in the chamber was decreased gradually to 10−2 Pa during heating to over 900° C. Then the preform was sintered for 7 hours at 1250° C. The obtained article was studied using microstructural analysis, X-ray, and microspectral analysis, which confirmed that the produced metal is a chemically and structurally homogeneous alloy Ti-6Al-3Mo having a density of 98.2% of the theoretical value.

[0042] The innovated technology is suitable for applications both in a lab testing and a serial manufacture of sintered articles from titanium alloys.

Manufacture of near-net shape titanium alloy articles from metal powders by sintering at variable pressure

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