Method for preparing refractory metal pre-alloy powder through freeze drying

Abstract

The invention discloses a method for preparing refractory metal pre-alloyed powder by freeze drying, which specifically comprises the following steps: weighing soluble refractory metal salt according to the proportion of required alloy components, adding deionized water, heating and stirring to fully dissolve the soluble refractory metal salt, then dropwise adding the soluble refractory metal salt into a beaker filled with liquid nitrogen to instantly freeze and solidify the soluble refractory metal salt, and cooling to room temperature to obtain the refractory metal pre-alloyed powder. And then the frozen and solidified sample is rapidly put into a vacuum freeze-drying machine to be freeze-dried, the dried sample is ground and crushed, then hydrogen reduction is conducted, and the refractory metal pre-alloy powder is obtained. According to the method, the refractory metal pre-alloy powder is prepared through the liquid nitrogen quick freezing-vacuum freeze-drying-hydrogen reduction method, the mixed solution is instantly frozen and solidified under the action of low-temperature liquid nitrogen, the distribution uniformity of alloy elements is guaranteed, the defect that the elements of the pre-alloy powder prepared through a solid-solid method are difficult to distribute uniformly is overcome, and the preparation efficiency is improved. The use of an organic complexing agent in a traditional liquid-liquid method is avoided, and the method has the advantages that the technological process is simple, and the prepared pre-alloyed powder is uniform in component and higher in purity.

Description

Technical Field

[0001] The invention relates to the technical field of powder metallurgy, and in particular to a method for preparing refractory metal pre-alloyed powder by freeze drying. Background Art

[0002] Refractory metal alloys (such as tungsten-molybdenum, molybdenum-rhenium, tungsten-rhenium, tungsten-tantalum, etc.) have high melting points, high strength, excellent high-temperature mechanical properties and corrosion resistance, and are widely used in high-tech fields such as aerospace and nuclear industry. The methods for preparing refractory metal alloys mainly include powder metallurgy, arc melting, electron beam melting, suspension melting, etc. Among them, powder metallurgy is the most commonly used due to its many advantages such as simple process flow, low equipment requirements, low cost, and easy industrial scale-up. To prepare refractory metal alloys using powder metallurgy, it is first necessary to prepare refractory metal mixed powder or pre-alloyed powder, and pre-alloyed powder is more beneficial for subsequent sintering and uniform distribution of elements.

[0003] The methods for preparing pre-alloyed powders mainly include solid-solid high-energy ball milling, solid-liquid mixing and liquid-liquid mixing. The solid-solid high-energy ball milling method has high requirements for equipment and is easy to introduce other impurity elements, and the production capacity is not high; the solid-liquid mixing method is a method of mixing one or several metal powders with another or several metal salts in an aqueous solution to achieve the dispersion of different refractory metal elements. The method is simple, but the uniformity of element distribution is difficult to ensure. The traditional liquid-liquid mixing method usually uses a variety of metal salts to dissolve in water, adds a complexing agent to fully disperse the different metal ions in the solution, and then forms a uniform mixed precursor through high-temperature drying. Finally, the desired pre-alloyed powder can be prepared by hydrogen reduction. The disadvantage of this method is that the complexing agent used is often some organic matter. The carbon element in it is difficult to remove completely during the preparation process, and it will remain in the pre-alloyed powder, affecting the purity and physical and chemical properties of the powder, and then affecting the performance of the subsequent metal alloy. Therefore, how to prepare high-purity refractory metal pre-alloyed powder with more uniform element distribution is still a challenge. Summary of the invention

[0004] The purpose of the present invention is to provide a method for preparing refractory metal pre-alloyed powder by freeze drying. Due to its high melting point and strength, the solid phase method for preparing refractory metal pre-alloyed powder has high equipment requirements and low output. In the wet preparation, the traditional liquid-liquid method (such as sol-gel method, etc.) usually requires the addition of organic matter as a complexing agent to achieve a full and uniform mixing of multiple refractory metal elements. In this process, it is easy to cause carbon residue in the pre-alloyed powder, affecting the purity of the powder. The present invention applies vacuum freeze drying technology to the preparation of refractory metal pre-alloyed powder for the first time, and prepares refractory metal pre-alloyed powder by a method combining liquid nitrogen quick freezing-vacuum freeze drying-hydrogen reduction, which opens up a new way for the preparation of refractory metal pre-alloyed powder.

[0005] The present invention achieves the above-mentioned purpose through the following technical solutions.

[0006] The present invention provides a method for preparing refractory metal pre-alloyed powder by freeze drying. The preparation of the refractory metal pre-alloyed powder comprises the following steps:

[0007] S1) weighing a certain amount of soluble refractory metal salt according to the required alloy composition ratio, adding a certain amount of deionized water, heating and stirring to fully dissolve it, and obtaining a hot solution;

[0008] S2) adding the hot solution obtained in S1) dropwise into a vessel filled with liquid nitrogen to freeze and solidify it instantly, thereby preparing a pre-frozen sample;

[0009] S3) vacuum freeze-drying the pre-frozen sample obtained in S2);

[0010] S4) grinding, crushing and sieving the dried sample obtained in S3), and then performing hydrogen reduction to obtain refractory metal pre-alloyed powder of the designed composition.

[0011] Furthermore, the soluble refractory metal salt in S1) is two or more of ammonium metatungstate, ammonium paratungstate, ammonium tetramolybdate, ammonium heptamolybdate, ammonium perrhenate, and tantalum oxalate.

[0012] Furthermore, the weight ratio of the added deionized water to the soluble refractory metal salt in S1) is 5:1 to 2:1; and the heating and dissolving temperature is 60 to 90°C.

[0013] Furthermore, the hot solution in S2) is added dropwise into the liquid nitrogen at a rate of 5 to 10 ml / min.

[0014] Furthermore, the cold trap temperature of the vacuum freeze dryer in S3) is -45 to -80°C, and the drying time is 24 to 72 hours.

[0015] Furthermore, the powder after crushing in said S4) needs to be sieved with 40-200 mesh; the hydrogen flow rate of hydrogen reduction is 1-5 L / min, the reduction temperature is not less than 800-1000° C., and the reduction time is 2-5 h.

[0016] A refractory metal pre-alloyed powder is prepared by adopting the above-mentioned preparation method.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] The present invention utilizes a mixed solution of refractory metal salts to be instantly frozen and solidified under the action of low-temperature liquid nitrogen, and different refractory metal ions are mixed with each other in the solution in the form of liquid phase ions, achieving a dispersion degree at the atomic level. The solution droplets are instantly frozen by the low temperature of the liquid nitrogen, and the dispersion state of each element in the solution is preserved, thereby ensuring the uniformity of the distribution of the alloy elements to a great extent, overcoming the disadvantage that it is difficult to achieve microscopic uniform mixing of elements in the preparation of pre-alloyed powder by a solid-solid method, and also avoiding the use of an organic complexing agent in a traditional liquid-liquid method, avoiding the pollution of residual carbon, and having the advantages of simple process, uniform composition of the prepared pre-alloyed powder and higher purity. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 The present invention is a flowchart of a method for preparing refractory metal pre-alloyed powder by freeze drying.

[0020] Figure 2 The figure shows the morphology and microelement distribution of the molybdenum-rhenium pre-alloyed powder (Mo-14Re) prepared in the present invention.

[0021] Figure 3 The morphology and microelement distribution diagram of the tungsten-rhenium pre-alloyed powder (W-3Re) prepared in the present invention.

[0022] Figure 4 The morphology and microelement distribution diagram of the tungsten-tantalum pre-alloyed powder (W-5Ta) prepared in the present invention.

[0023] Figure 5 This is the morphology and microelement distribution diagram of the tungsten-rhenium pre-alloyed powder (W-3Re) prepared in Comparative Example 2.

[0024] Figure 6 This is the morphology and microelement distribution diagram of the tungsten-tantalum pre-alloyed powder (W-5Ta) prepared in Comparative Example 3. DETAILED DESCRIPTION

[0025] The present invention will be further described below in conjunction with specific embodiments, but the present invention is not limited thereto.

[0026] like Figure 1 As shown, the present invention provides a method for preparing refractory metal pre-alloyed powder by freeze drying, comprising the following steps:

[0027] S1) weighing a certain amount of two or more soluble refractory metal salts according to the required alloy composition ratio, adding a certain amount of deionized water, heating and stirring to fully dissolve them;

[0028] S2) adding the hot solution dropwise into a beaker filled with liquid nitrogen to freeze it instantly to prepare a pre-frozen sample;

[0029] S3) quickly placing the pre-frozen sample into a vacuum freeze dryer for vacuum freeze drying;

[0030] S4) Grinding, crushing and sieving the dried sample, and then performing hydrogen reduction to obtain refractory metal pre-alloyed powder of the designed composition.

[0031] The soluble refractory metal salt in S1) is two or more of ammonium metatungstate, ammonium paratungstate, ammonium tetramolybdate, ammonium heptamolybdate, ammonium perrhenate, and tantalum oxalate; the weight ratio of the added deionized water to the refractory metal salt is 5:1 to 2:1; and the heating and dissolving temperature is 60 to 90°C.

[0032] The rate of adding the hot solution into the liquid nitrogen in S2) is 5-10 ml / min.

[0033] The cold trap temperature of the vacuum freeze dryer in S3) is -45 to -80°C, and the drying time is 24 to 72 hours.

[0034] The powder after crushing in the step S4) needs to be sieved with 40-200 mesh; the hydrogen flow rate at the hydrogen reduction temperature is 1-5 L / min, the reduction temperature is not less than 800-1000° C., and the reduction time is 2-5 h.

[0035] The present invention will be described in detail below through specific examples.

[0036] Example 1

[0037] Preparation of 200 g of Mo-14Re pre-alloyed powder specifically includes the following steps:

[0038] S1) Weigh 316.49 g of ammonium heptamolybdate and 40.33 g of ammonium rhenate, add 1000 ml of deionized water, place in a water bath at 80°C, and stir thoroughly to dissolve.

[0039] S2) The completely dissolved and clarified hot solution is added dropwise into a beaker filled with liquid nitrogen at a rate of 5 ml / min to freeze and solidify it instantly to prepare a pre-frozen sample.

[0040] S3) The cold trap temperature of the vacuum freeze dryer is set to -65°C in advance. When the temperature is reached, the pre-frozen sample is quickly placed in the vacuum freeze dryer for vacuum freeze drying for 60 hours.

[0041] S4) The dried sample is ground and crushed to pass through a 40-mesh sieve, and then reduced at 400° C. for 1 h and at 950° C. for 3 h at a hydrogen flow rate of 2 L / min to obtain Mo-14Re pre-alloyed powder.

[0042] The SEM morphology of the Mo-14Re pre-alloyed powder obtained above and the corresponding EDS element distribution diagram are shown in the attached figure. Figure 2 As shown, it can be seen that the Mo and Re elements are evenly distributed without obvious segregation. The purity can reach more than 99.96% by glow discharge mass spectrometry (GDMS).

[0043] Example 2

[0044] Preparation of 200 g of W-3Re pre-alloyed powder specifically includes the following steps:

[0045] S1) Weigh 259.97 g of ammonium metatungstate and 8.64 g of ammonium rhenate, and add 600 ml of deionized water, place in a water bath at 85°C, and stir thoroughly to dissolve.

[0046] S2) The completely dissolved and clarified hot solution is added dropwise into a beaker filled with liquid nitrogen at a rate of 6 ml / min to freeze and solidify it instantly to prepare a pre-frozen sample.

[0047] S3) The cold trap temperature of the vacuum freeze dryer is set to -55°C in advance. When the temperature is reached, the pre-frozen sample is quickly placed in the vacuum freeze dryer for vacuum freeze drying for 36 hours.

[0048] S4) The dried sample is ground and crushed to pass through a 40-mesh sieve, and then reduced at 600°C for 1 h and 980°C for 3 h at a hydrogen flow rate of 1.5 L / min to obtain W-3Re pre-alloyed powder.

[0049] The SEM morphology of the W-3Re pre-alloyed powder obtained above and the corresponding EDS element distribution map are shown in the attached figure. Figure 3 As shown, it can be seen that W and Re elements are evenly distributed without obvious segregation, and the purity of the pre-alloyed powder can reach 99.98%.

[0050] Example 3

[0051] Preparation of 200 g of W-5Ta pre-alloyed powder specifically includes the following steps:

[0052] S1) Weigh 254.61 g of ammonium metatungstate and measure tantalum oxalate solution (Ta 5+ ion concentration 10g / L) 1000ml, place in 80℃ water bath and stir thoroughly to dissolve.

[0053] S2) The completely dissolved and clarified hot solution is added dropwise into a beaker filled with liquid nitrogen at a rate of 8 ml / min to freeze and solidify it instantly to prepare a pre-frozen sample.

[0054] S3) The cold trap temperature of the vacuum freeze dryer is set to -60°C in advance. When the temperature is reached, the pre-frozen sample is quickly placed in the vacuum freeze dryer for vacuum freeze drying for 50 hours.

[0055] S4) The dried sample is ground and crushed to pass through a 100-mesh sieve, and then reduced at 600° C. for 1 h and 1000° C. for 2.5 h at a hydrogen flow rate of 3 L / min to obtain W-5Ta pre-alloyed powder.

[0056] The SEM morphology of the W-5Ta pre-alloyed powder obtained above and the corresponding EDS element distribution diagram are shown in the attached figure. Figure 4 As shown, it can be seen that W and Ta elements are evenly distributed without obvious segregation, and the purity of the pre-alloyed powder can reach 99.95%.

[0057] Comparative Example 1

[0058] Different from the traditional sol-gel and other liquid-liquid methods for preparing pre-alloyed powder, one of the advantages of the present invention is that no organic matter needs to be added as a complexing agent during the preparation process, thereby avoiding the influence of carbon element contamination on the pre-alloyed powder. This comparative example prepares 200g of Mo-14Re pre-alloyed powder, which is different from Example 1 in that the Mo-14Re pre-alloyed powder is prepared using the existing sol-gel method, and anhydrous oxalic acid is added as a complexing agent, specifically comprising the following steps:

[0059] S1) Weigh 316.49 g of ammonium heptamolybdate and 40.33 g of ammonium rhenate, add 1000 ml of deionized water, place in a water bath at 80°C, and stir thoroughly to dissolve.

[0060] S2) adding 180 g of anhydrous oxalic acid and continuing stirring until the solution forms a gel.

[0061] S3) The colloidal sample is placed in a forced air drying oven and dried at 120° C. for 6 h. After drying, the sample is ground and crushed through a 40-mesh sieve to obtain a precursor powder.

[0062] S4) calcining the sieved precursor powder at 280° C. for 6 h (to remove carbon).

[0063] S5) The calcined sample is subjected to hydrogen reduction at a hydrogen flow rate of 2 L / min, at 400° C. for 1 h and at 950° C. for 3 h to obtain Mo-14Re pre-alloyed powder.

[0064] As mentioned above, the sol-gel method is prone to cause carbon residue problems in the pre-alloyed powder. Therefore, the carbon content in the samples of Example 1 and Comparative Example 1 was tested, and the results are shown in Table 1. It can be seen that the carbon content of the Mo-14Re pre-alloyed powder in Comparative Example 1 is one order of magnitude higher than that of the sample prepared in Example 1. It can be seen that the use of the method of the present invention to prepare pre-alloyed powder can effectively avoid the problem of excessive carbon content and improve the purity of the powder.

[0065] Table 1 Comparison of carbon content between Mo-14Re samples prepared by the present invention and samples prepared by sol-gel method

[0066] Example 1 Sample Comparative Example 1 Sample Carbon content (%) 0.0018 0.0116

[0067] Comparative Example 2

[0068] One of the innovative points of the present invention is to use liquid nitrogen to instantly freeze and solidify the precursor solution, thereby retaining the uniform distribution of each element in the liquid phase, and then remove the moisture in the solid phase precursor by vacuum freeze drying, so as to continue to retain the uniform distribution of each element in the alloy, and then the precursor powder with uniform distribution of elements can be converted into pre-alloyed powder with uniform distribution of elements by hydrogen reduction. This comparative example prepares 200g of W-3Re pre-alloyed powder, which is different from Example 2 in that the liquid phase precursor solution is prepared by conventional freeze drying method in the preparation process, and is pre-frozen in a vacuum freeze dryer or refrigerator, and then vacuum freeze dried, without the step of quick freezing the precursor with liquid nitrogen, and specifically includes the following steps:

[0069] S1) Weigh 259.97 g of ammonium metatungstate and 8.64 g of ammonium rhenate, and add 600 ml of deionized water, place in a water bath at 85°C, and stir thoroughly to dissolve.

[0070] S2) The cold trap temperature of the vacuum freeze dryer is set to -55°C in advance. When the temperature is reached, the liquid sample is quickly placed in the vacuum freeze dryer for pre-freezing and vacuum freeze drying. The drying time is 36 hours.

[0071] S3) The dried sample is ground and crushed to pass through a 40-mesh sieve, and then reduced at 600° C. for 1 h and 980° C. for 3 h at a hydrogen flow rate of 1.5 L / min to obtain W-3Re pre-alloyed powder.

[0072] The SEM morphology of the W-3Re pre-alloyed powder obtained above and the corresponding EDS element distribution map are shown in the attached figure. Figure 5 As shown, it can be seen that there is an obvious segregation phenomenon of Re element in the powder. This is because the pre-freezing in the vacuum freeze dryer takes a certain amount of time for the precursor to change from liquid phase to solid phase, which is slower than the liquid nitrogen quick freezing process. In the slow pre-freezing process, ammonium metatungstate and ammonium rhenate will crystallize out of the solution one after another due to the difference in solubility in water, which leads to the easy formation of element segregation in the pre-frozen solid phase. It is difficult to change the distribution of elements after subsequent grinding, crushing and hydrogen reduction, which eventually leads to the formation of element segregation in the pre-alloyed powder.

[0073] Comparative Example 3

[0074] The present invention innovatively uses liquid nitrogen to rapidly freeze and solidify the precursor solution. The concentration of the precursor solution has a great influence on the composition uniformity of the pre-alloyed powder prepared subsequently. The concentration of the precursor solution is controlled by adjusting the weight ratio of deionized water to the metal salt added during the dissolution of the refractory metal salt. The more suitable ratio is 5:1 to 2:1. In this comparative experiment, 200g of W-5Ta pre-alloyed powder was prepared. The difference from Example 3 is that the weight ratio of deionized water to the refractory metal salt added during the dissolution of the precursor is set to 1.5:1, which specifically includes the following steps:

[0075] S1) Weigh 254.61 g of ammonium metatungstate and measure tantalum oxalate solution (Ta 5+ ion concentration 23g / L) 433ml, placed in 80℃ water bath condition, stirred thoroughly to dissolve.

[0076] S2) The completely dissolved and clarified hot solution is added dropwise into a beaker filled with liquid nitrogen at a rate of 8 ml / min to freeze and solidify it instantly to prepare a pre-frozen sample.

[0077] S3) The cold trap temperature of the vacuum freeze dryer is set to -60°C in advance. When the temperature is reached, the pre-frozen sample is quickly placed in the vacuum freeze dryer for vacuum freeze drying for 50 hours.

[0078] S4) The dried sample is ground and crushed to pass through a 100-mesh sieve, and then reduced at 600° C. for 1 h and 1000° C. for 2.5 h at a hydrogen flow rate of 3 L / min to obtain W-5Ta pre-alloyed powder.

[0079] The SEM morphology of the W-5Ta pre-alloyed powder obtained above and the corresponding EDS element distribution diagram are shown in the attached figure. Figure 6 As shown, it can be seen that the distribution of W and Ta elements has obvious element segregation phenomenon, and tantalum element only appears on the matrix of some tungsten particles, which proves that the concentration of the precursor during liquid nitrogen quick freezing has a great influence on the uniformity of element distribution of pre-alloyed powder. The higher the precursor concentration, the more solutes are contained in the droplets dripping into the liquid nitrogen, which is equivalent to more solutes entering the liquid nitrogen for freezing and solidification per unit time. A large number of solutes with different compositions are more likely to precipitate successively during the liquid nitrogen freezing and crystallization process, resulting in greater microscopic composition differences, and ultimately leading to uneven element distribution in the reduced pre-alloyed powder.

[0080] The above is a detailed introduction to a freeze-drying method for preparing refractory metal pre-alloyed powder provided in the embodiment of the present application. The description of the above embodiment is only used to help understand the method and its core idea of ​​the present application; at the same time, for those skilled in the art, according to the idea of ​​the present application, there will be changes in the specific implementation method and application scope. In summary, the content of this specification should not be understood as a limitation on the present application.

[0081] For example, certain words are used in the specification and claims to refer to specific components. Those skilled in the art should understand that hardware manufacturers may use different nouns to refer to the same component. This specification and claims do not use differences in names as a way to distinguish components, but use differences in the functions of components as the criteria for distinction. As mentioned throughout the specification and claims, "including" and "comprising" are open-ended terms, so they should be interpreted as "including / including but not limited to". "Approximately" means that within an acceptable error range, those skilled in the art can solve the technical problem within a certain error range and basically achieve the technical effect. The subsequent description of the specification is a preferred embodiment of the present application, but the description is for the purpose of illustrating the general principles of the present application, and is not used to limit the scope of the present application. The scope of protection of the present application shall be subject to that defined in the attached claims.

[0082] It should also be noted that the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a product or system including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such a product or system. In the absence of more restrictions, the elements defined by the sentence "comprises a ..." do not exclude the existence of other identical elements in the product or system including the elements.

[0083] It should be understood that the term "and / or" used in this article is only a description of the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship.

[0084] The above description shows and describes several preferred embodiments of the present application, but as mentioned above, it should be understood that the present application is not limited to the form disclosed herein, and should not be regarded as excluding other embodiments, but can be used in various other combinations, modifications and environments, and can be modified within the scope of the application concept described herein through the above teachings or the technology or knowledge of the relevant field. The changes and modifications made by those skilled in the art do not deviate from the spirit and scope of the present application, and should be included in the claims attached to the present application.

Claims

1. A method for preparing refractory metal pre-alloyed powder by freeze drying, characterized in that: The preparation of the refractory metal pre-alloyed powder comprises the following steps: S1) weighing a certain amount of soluble refractory metal salt according to the required alloy composition ratio, adding a certain amount of deionized water, heating and stirring to fully dissolve it, and obtaining a hot solution; S2) adding the hot solution obtained in S1) dropwise into a vessel filled with liquid nitrogen to freeze and solidify it instantly, thereby preparing a pre-frozen sample; S3) vacuum freeze-drying the pre-frozen sample obtained in S2); S4) grinding, crushing and sieving the dried sample obtained in S3), and then performing hydrogen reduction to obtain refractory metal pre-alloyed powder of the designed composition.

2. The method according to claim 1, characterized in that The soluble refractory metal salt in S1) is two or more of ammonium metatungstate, ammonium paratungstate, ammonium tetramolybdate, ammonium heptamolybdate, ammonium perrhenate, and tantalum oxalate.

3. The method according to claim 1, characterized in that: The weight ratio of the added deionized water to the soluble refractory metal salt in S1) is 5:1 to 2:1; the heating and dissolving temperature is 60 to 90°C.

4. The method according to claim 1, characterized in that: The hot solution in S2) is added dropwise into the liquid nitrogen at a rate of 5-10 ml / min.

5. The method according to claim 1, characterized in that The cold trap temperature of the vacuum freeze dryer in S3) is -45 to -80°C, and the drying time is 24 to 72 hours.

6. The method according to claim 1, characterized in that The powder crushed in S4) needs to be sieved with 40-200 mesh; the hydrogen flow rate of hydrogen reduction is 1-5L / min, the reduction temperature is not less than 800-1000°C, and the reduction time is 2-5h.

7. A refractory metal pre-alloyed powder, characterized in that: The refractory metal pre-alloyed powder is a refractory metal pre-alloyed powder prepared by the preparation method according to any one of claims 1 to 6.

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