METHOD

BACKGROUND OF THE INVENTION [0001] Field of Invention

[0002] The present invention relates to a method for making tungsten-refractory metal alloy powders and to tungsten-refractory metal alloy powders made by the method.

[0003] Brief Description of Related Art

[0004] Processing of high melting temperature alloys, such as steel and Inconel, requires tooling made of materials that can survive under the applied loads and elevated temperatures required for such processing, which can be above 1,000°C for example. Tungsten-rhenium alloys are candidates for such applications. However, these tungsten-rhenium alloy tools require certain densities and grain structures to process steel and Inconel, and the cost for producing such tooling can be generally high. This may be due to the traditional sintering process required to consolidate/densify tungsten and rhenium powders, which is lengthy because of the low diffusion rates of tungsten, and thus is costly. For example, tungsten-rhenium alloys are typically sintered at 2000°C for more than 24 hours before alloying/sintering is complete. It has been proposed to reduce the particle size of these powders, i.e. to nanocrystalline powders, so as to reduce the diffusion distance and thus shorten the sintering time. However, long ball milling times (typically in excess of 20 hours) are required to produce nanocrystalline powders, and this can result in powder contamination from the milling media. Densification can be further inhibited due to the increased surface area of the nanocrystalline powders, which increases the amount of native surface oxides. Thus, most tungsten-rhenium alloys have only about 90% relative density with grain growth that is not favorable for mechanical properties of the alloy. Additional costs can be incurred in machining the resulting solid specimen starting bar stock in order to produce the final tooling configuration.

[0005] Therefore, it would be desirable to have a method of making a fine grained, single-phase of tungsten-rhenium alloy in a reduced time period without compromising the mechanical properties of the solid phase alloy. BRIEF SUMMARY OF THE INVENTION

[0006] In an aspect, the present subject matter provides a method for forming a tungsten-refractory metal alloy powder and a tungsten-refractory metal alloy powder produced by the method. The method includes (a) mixing a majority portion by weight of a base tungsten powder with a minority portion by weight of a base refractory metal powder to form a mixture, said base refractory metal powder being formed of a refractory metal other than tungsten; (b) milling the mixture from step (a) for a period of time sufficient to at least partially mechanically alloy the base tungsten powder and base refractory metal powder together to form at-least-partially- mechanically-alloyed particles; (c) heat treating the at-least-partially-mechanically- alloyed particles from step (b) to a temperature sufficient to promote diffusion between tungsten and the refractory metal other than tungsten and obtain agglomerations of particles having only a tungsten phase; and (d) milling the agglomerations of particles having only a tungsten phase from step (c) to break up the agglomerations of particles and obtain the tungsten-refractory metal alloy powder.

[0007] In another aspect, the present subject matter provides a tungsten-refractory metal alloy powder formed according to the methods.

[0008] The alloys and methods of making them as described herein, provide [0009] The foregoing and other features of the invention are hereafter more fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the present invention may be employed.

BRIEF DESCRIPTION OF THE DRAWINGS [0010] FIG. 1 shows an X-ray diffraction (XRD) plot for 2Q = 35-45° for an example tungsten-rhenium metal alloy powder according to the present subject matter.

[0011] FIG. 2 shows (a) a SEM bright field image and (b) EDS mapping of the example tungsten-rhenium metal alloy powder of FIG. 1.

[0012] FIG. 3 shows XRD plots for example tungsten-rhenium alloy powders including (a) 2 weight percent hafnium carbide powder, (b) 4 weight percent hafnium carbide powder, and (c) 6 weight percent hafnium carbide powder according to the present subject matter.

[0013] FIGS. 4a-c shows optical microscopy images of the tungsten-rhenium alloy powders of FIGS. 3a-c.

[0014] FIG. 5 shows an optical microscopy image of a comparative example tungsten-rhenium alloy powder.

[0015] FIG. 6 shows XRD plots for example solid specimens prepared by DCS consolidation of the example powders of FIGS. 3a-c.

DETAILED DESCRIPTION OF THE INVENTION [0016] The present subject matter provides methods for forming a tungsten- refractory metal alloy powder, and tungsten-refractory metal alloy powders produced by the methods. The tungsten-refractory metal alloy powder can be used to form a consolidated solid body with small sized grains and improved mechanical property over traditional methods.

[0017] The methods of the present subject matter include in a step (a), mixing a base tungsten (W) powder with a base refractory metal powder to form a mixture.

The mixture may include a majority portion by weight of the base tungsten powder, and a minority portion by weight of the base refractory metal powder. The base tungsten powder and the base refractory metal powder may be blended using a Turbula Type T2F shaker-mixer. The powders may be mixed together to make the mixture homogeneous or substantially homogeneous.

[0018] The base refractory metal powder may include one or more refractory metals other than tungsten, i.e. niobium, molybdenum, tantalum, rhenium. The base refractory powder may include rhenium (Re) powder, molybdenum (Mo) powder, or combinations thereof. The mixture may include 20.00% to 27.00%, 20.00 to 25.75%, or 25±1 % of the rhenium powder by weight. The mixture may include 20.00% to 40.00%, 25.00-35.00%, or 30±1% of the molybdenum powder by weight.

[0019] The base tungsten powder and the base refractory metal powder may have various particle sizes and shapes, and may each have an initial average particle size (D50) of 30-50 microns (pm), 35-47 microns, or 38-45 microns. The base tungsten powder and the base refractory metal powder may be sieved to obtain these initial average particle size ranges. [0020] When the base refractory metal powder includes rhenium powder, the methods may also include adding hafnium carbide (HfC) powder to the mixture in step (a). The hafnium carbide may be obtained from Materion (MT-B-982), and be included at up to 2%, up to 4%, 0.01-2%, or 0.01-4% by weight of the mixture.

[0021] When the base refractory metal powder includes molybdenum powder, the methods may also include adding zirconium oxide (ZrC ) powder to the mixture in step (a). The zirconium oxide powder may be included at up to 2%, or 0.01-2%, by weight of the mixture.

[0022] The hafnium carbide powder or zirconium oxide powder may be mixed into the mixture using a V-blender. The hafnium carbide powder and the zirconium oxide powder may have the same average particle sizes as the base tungsten powder and the base refractory metal powder.

[0023] Once the mixture is obtained, the methods may include in a step (b), milling the mixture obtained from step (a) for a period of time sufficient to at least partially mechanically alloy together the base tungsten powder and the base refractory metal powder to form at-least-partially-mechanically-alloyed particles. Milling may also reduce the average particle size of the base tungsten powder and the base refractory metal powder in the mixture.

[0024] Milling in step (b) may be performed using a ball mill, e.g. a planetary ball mill. However, this is not required. The ball mill may employ stainless steel jars as the container, and milling media including stainless steel balls or tungsten carbide balls. The ball mill may be a planetary ball mill, for example one from Across International (Livingston, NJ 07039). Milling may be performed for up to 24 hours, less than 24 hours, less than 8 hours, or 1-4 hours. Milling may be performed to reduce the average particle size of the base tungsten powder and the base refractory metal powder so that at the end of step (b), the at-least-partially- mechanically-alloyed particles may have an average particle size (Dso) of less than 20 microns, less than 16 microns, less than 10 microns, less than 5 microns, 0.01-20 microns, 0.01-16 microns, 0.01-10 microns, or 0.01-5 microns. Average particle size (Dso) of the at-least-partially-mechanically-alloyed particles can be determined by using a Cilas 1064 particle size analyzer (Madison, Wl 53711).

[0025] Subsequently to step (b), the methods include in a step (c), heat treating the at-least-partially-mechanically-alloyed particles from step (b) to a temperature sufficient to promote diffusion between tungsten and the refractory metal other than tungsten, and to obtain agglomerations of particles having only a tungsten phase. [0026] The heat treatment may include heating the at-least-partially-mechanically- alloyed particles to a temperature of 900°C-1800°C, 1000°C to 1700°C, 1600°C to 1700°C, or 1000°C-1650°C. The heat treatment may last for 1-15 hours, 1-12 hours, 1-5 hours, 2-4 hours, or 5±0.5 hours. Such heat treatment may cause the at-least- partially-mechanically-alloyed particles to agglomerate and form agglomerations of particles. These agglomerations of particles may have a tungsten-only phase, where no base refractory metal has alloyed into the tungsten phase. Heating may be accomplished using a CM furnace (Bloomfield NJ 07003).

[0027] After the heat treatment in step (c), the methods include in a step (d), milling the agglomerations of particles having only a tungsten phase to breaking up the agglomerations of particles and reduce their particle size, and thereby attain the tungsten-refractory metal alloy powder. This step (d) may be accomplished using a ball miss, a planetary ball mill, or other types of mills. A ball mill may employ stainless steel jars as the container, and milling media including stainless steel balls or tungsten carbide balls. The ball mill may be a planetary ball mill, for example one from Across International (Livingston, NJ 07039). Milling may be performed for up to 24 hours, less than 24 hours, less than 8 hours, 1-4 hours, or 2±0.1 hours. Milling may be performed to reduce the average particle size of the agglomerations of particles so that at the end of step (d), the tungsten-refractory metal alloy powder may have an average particle size (Dso) of less than 20 microns, less than 16 microns, less than 10 microns, less than 7 microns, less than 5 microns, 0.01-20 microns, 0.01-16 microns, 0.01-10 microns, or 0.01-5 microns.

[0028] After milling in step (d), the methods may include forming spherical particles by spray freeze drying the tungsten-refractory metal alloy powder obtained in step (d). The spherical particles therefore include the tungsten-refractory metal alloy powder obtained in step (d).

[0029] The obtained tungsten-refractory metal alloy powder may be used to form a metal part, for example by using Direct Current Sintering (DCS) of the tungsten- refractory metal alloy powder. Further post processing of the metal part, such as subjecting the metal part to heat treatments, may be used to promote further solutionizing of the refractory metal alloy in the tungsten and to promote increased densification of the metal part. EXAMPLES

[0030] The following examples are intended only to illustrate the invention and should not be construed as imposing limitations upon the claims. The following experimental methods, conditions and instruments were employed in preparing the exemplary powdered alloy and a solid body as detailed below.

[0031] Several examples of tungsten-refractory metal alloy powder were prepared according to the methods described herein. These alloy powders were prepared from commercially available initial tungsten and rhenium powders, which were sieved to obtain a uniform size range of 38-45 pm. Each of these example powders were prepared by mixing 75 wt% tungsten powder with 25 wt% rhenium powder using a Turbula Type T2F shaker-mixer. Four example mixtures of these powders are listed below in Table I as Batch A, Batch B, Batch C, and Final. The four example mixtures were subject to ball milling using a planetary ball mill for a time between 1-4 hours as shown in Table I, and the Batch A example had a resultant powder size of 4.5 pm, and the Batch B example had a resultant powder size of 4.6 pm.

TABLE I

[0032] The Batch A example was initially analyzed using XRD analysis, but showed no alloying between the tungsten and rhenium powders after the ball milling. Therefore, in order to promote alloying in the example alloy powders, the example alloy powders were subjected to a heat treatment at different temperatures and durations as shown in Table I. The heat treatment of the example alloy powders produce agglomerations of the alloy powders, and thus after the heat treatment, the powders were subject to sieving and an additional milling step, thus helping refine the powders by removing and/or reducing the size of the agglomerations. As seen in Table I, the Final example powder, after the heat treatment, sieving, and additional ball milling, had an average particle size of 2pm.

[0033] An XRD analysis of this Final example after the milling, heat treatment, sieving, and the second milling step, is shown in FIG. 1, and indicates that the rhenium has alloyed into the tungsten phase. FIG. 2a shows a corresponding SEM bright field image of this Final example, and FIG. 2b shows a corresponding EDS mapping, which also indicates alloying of the starting powders.

[0034] Using the same parameters as that use for the Final example from Table I above, three additional examples were prepared with the further addition of varying amounts of hafnium carbide (HfC) obtained from Materion (MT-B-982). These three examples, indicated below in Table II as “Final+2%HfC”, “Final+4%HfC”, “Final+6%HfC”, each included 75 wt% tungsten powder with 25 wt% rhenium powder along with a V-blended addition of the indicated amount (i.e. 2 wt%, 4 wt%, and 6 wt%) of the hafnium carbide by weight of the total weight of the tungsten powder and rhenium powder. Each of these examples were processed like the Final example, by subjecting the mixture of powders to 4 hours of ball milling, followed by a heat treatment of 1650°C for 2 hours, followed by an additional 2 hours of ball milling. [0035] XRD analysis of each of these example powders are shown in FIGS. 3a-c, where FIG. 3a shows the XRD analysis of the Final+2%HfC example, FIG. 3b shows the XRD analysis of the Final+4%HfC example, and FIG. 3c shows the XRD analysis of the Final+6%HfC example. As seen in FIGS. 3a-c, peaks corresponding to the FlfC addition can be observed along with a minor trace of tungsten carbide (WC). [0036] These example powders were then consolidated into solid specimens using DCS at 1800 °C for 30 minutes, followed by hot isostatic pressing (FI IP) of the specimens at 2000°C for 6 hours. Density and phase analyses of the solid bodies were completed prior to FH IP processing. Table II below, summarizes the subsequent density measurements before and after HIP processing, along with the final hardness of the solid bodies. Hardness measurements were taken of the consolidated specimens using a Vickers tester at 1000 g. TABLE II

[0037] Optical microscopy images shown in FIGS. 4a-c confirm the presence of the HfC phase shown along the grain boundaries, which helps the solid phase alloy retain strength at elevated temperatures. The homogenous distribution of the HfC within the consolidated matrix is shown in FIGS. 4a-c.

[0038] A comparative example of a tungsten-rhenium alloy solid specimen was prepared by a conventional method, which included V-blending 75 wt% tungsten powder with 25 wt% rhenium powder, pressing the mixture into a greenware rod, sintering the rod, followed by HIP processing to produce the solid comparative example. An average density of 96.88 + 0.14% was measure for the comparative example. The grain size was measured for each of the examples including HfC, and for the comparative example, and these are shown below in Table III. The image processing software “ImageJ” from the National Institutes of Health in Bethesda MD, was used to analyze the grain size based on the feret diameter. The average grain size was based on measurements of 100 grains.

TABLE III

[0039] The finest, uniform equiaxed grains were obtained with the addition of 4 wt% HfC in the Final+4%HfC example. With an addition of 6% HfC in Final+6%HfC example, the grains have a more bi-modal distribution as can be observed in FIG. 4c and reflected by the larger average grain size in Table III above. This is well below the the resulting microstructure of the comparative example, which had an average grain size of 257pm, as shown in Table III and FIG 5.

[0040] Figures 6a-c shows the XRD phases analysis for the example solid specimens after DCS consolidation of the example powders at 1800 °C for 30 min followed by a subsequent HIP heat treatment at 2000 °C for 6 hrs at 30 psi, where FIG. 6a is the XRD phase analysis of the consolidated Final+2%HfC example powder, FIG. 6b is the XRD phase analysis of the consolidated Final+4%HfC example powder, and FIG. 6c is the XRD phase analysis of the consolidated Final+6%HfC example powder. Due to the small amounts of HfC, their presence cannot be determined in XRD. Although a slight indication is noted in Figure 6c for the Final+6%HfC example powder with the 6% HfC addition. The shift in the (110) W peak position, summarized in Table VIII, further indicates formation of the single solid phase of W-Re without the o phase.

Table IV

[0041] Many other benefits will no doubt become apparent from future application and development of this technology.

[0042] All patents, applications, standards, and articles noted herein are hereby incorporated by reference in their entirety.

[0043] The present subject matter includes all operable combinations of features and aspects described herein. Thus, for example if one feature is described in association with an embodiment and another feature is described in association with another embodiment, it will be understood that the present subject matter includes embodiments having a combination of these features.

[0044] As described hereinabove, the present subject matter solves many problems associated with previous strategies, systems and/or devices. Flowever, it will be appreciated that various changes in the details, materials and arrangements of components, which have been herein described and illustrated in order to explain the nature of the present subject matter, may be made by those skilled in the art without departing from the principle and scopes of the claimed subject matter, as expressed in the appended claims.