Metal material sintering densification and grain size control method

Abstract

A method to achieve full densification and grain size control for sintering metal materials, wherein raw material powder is deagglomerated to obtain deagglomerated powder with dispersion. The deagglomerated powder is granulated by spray granulation. The granulated particles are processed by high-pressure die pressing and cold isostatic pressing. The powder compact is sintered by two-step pressureless sintering. The first step is to heat up the powder compact to a higher temperature and hold for a short time to obtain 75-85% theoretical density; the second step is to cool down powder compact to a lower temperature and hold for a long time. The two-step sintering can decrease the sintering temperature, so that the powder compact can be densified at a lower temperature. Thus, the obtained refractory metal product is densified, with ultrafine grains, uniform grain size distribution, and outstanding mechanical properties.

Claims

1. A metal material sintering densification and grain size control method, comprising the following steps: first, using metal powder as a raw material powder, processing the raw material powder by deagglomeration to obtain deagglomerated powder; granulating the deagglomerated powder by spray granulation and obtain granulated particles in spherical shape; processing the obtained granulated particles by die pressing and cold isostatic pressing to obtain a powder compact; sintering the powder compact by two-step pressureless sintering, wherein a first step of sintering comprises heating up the powder compact to a first temperature, holding the powder compact at the temperature for a first time and controlling a density at 75-85%, and a second step of sintering comprises cooling down the powder compact to a second temperature and holding the powder compact at the second temperature for a second time to further eliminate residual pores, so as to obtain ultrafine grain metal.

2. The method according to claim 1, comprising the following steps: S1: processing the raw material powder by deagglomeration with a high-speed helical blade mixer at 2,000-3,000 rpm blade rotation speed for 0.5-2 h, to obtain deagglomerated powder; S2: first, mixing a binder and deionized water homogeneously to prepare a solution A, in which the content of the binder is 5-15 wt. %; then, adding the deagglomerated raw material powder obtained in the step S1 into the solution A and stirring the solution mechanically to a homogeneous state, so as to obtain a slurry; granulating the obtained slurry by spray granulation with a centrifugal atomizing drier at 8,000-15,000 r/min. rotation speed, 100-300 kPa atomizing pressure, and 90-150° C. drying temperature; loading the granulated powder into a tube heating oven and adding hydrogen into the tube heating oven for degreasing and reduction at 550-700° C. processing temperature, 5-10° C./min. heating rate, and holding for 60-120 min., to obtain granulated particles in a spherical shape; S3: pressing the granulated powder by die pressing at 700-1,000 MPa pressing pressure, and holding for 0.5-1.5 min. to obtain a preformed powder compact, loading the preformed powder compact into a jacketed mold and performing cold isostatic pressing at 200-280 MPa, and holding for 5-10 min., to obtain a powder compact; and S4: performing two-step sintering: first, in the first step of sintering, heating up the powder compact obtained in the step S3 at a first heating rate to a temperature T1 and holding at the temperature T1, to obtain a one-step sintered compact; then, in the second step of sintering, cooling down the one-step sintered compact from the temperature T1 to a temperature T2 at a first cooling rate, and holding at the temperature T2, so as to obtain a metal material with ultrafine grains finally, wherein, the temperature T2 is lower than the temperature T1 by 50-250° C., and the holding time for T1 in the first step is shorter than the holding time for T2 in the second step.

3. The method according to claim 2, wherein in the step S1, the metal powder comprises a refractory metal; the particle size of the deagglomerated powder is smaller than 0.5 μm.

4. The method according to claim 2, wherein in the step S2, the binder is polyvinyl alcohol, polyethylene glycol, stearic acid or paraffin; the solid content in the slurry is 60-85 wt. %.

5. The method according to claim 2, wherein in the step S3, a density of the powder compact is higher than 50%.

6. The method according to claim 2, wherein in the first step of sintering in the step S4, the powder compact is sintered in a hydrogen atmosphere by heating up the powder compact to the temperature T1 at 5° C./min heating rate, the temperature T1 is 1,200-1,500° C. and the holding time for T1 is 1-2 h.

7. The method according to claim 2, wherein in the second step of sintering in the step S4, a shielding gas atmosphere is hydrogen or argon gas atmosphere, the temperature T1 is decreased to the temperature T2 at 15-25° C./min cooling rate, and the holding time for T2 is 10-60 h.

8. The method according to claim 2, wherein the density of the one-step sintered compact is 75-85%, and the grain size is 0.5-1 μm.

9. The method according to claim 2, wherein the ratio of the obtained ultrafine grain metal grain size to the one-step sintered compact grain size is less than or equal to 1.5.

10. The method according to claim 2, wherein the density of the ultrafine grain metal is higher than 98%.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The accompanying drawings illustrate one or more embodiments of the present invention and, together with the written description, serve to explain the principles of the invention. Wherever possible, the same reference numbers are used throughout the drawings to refer to the same or like elements of an embodiment.

(2) FIG. 1 is a process flow chart of the metal material sintering densification and grain size control method in the present invention;

(3) FIG. 2 is a schematic diagram of the two-step sintering process in the present invention. The first step of sintering is to rapidly heat up the powder compact to a higher temperature T.sub.1 and hold at the temperature for a short time to control the density at 75-85%, then immediately cool down the powder compact to a lower temperature T.sub.2 and carry out the second step of sintering, and hold at the temperature for a long time to further eliminate residual pores without grain growth. In the Figures, NS represents normal sintering, TSS-I represents the first step of sintering in the two-step sintering process, and TSS-II represents the second step of sintering in the two-step sintering process. Wherein, the sintering temperature T.sub.1 in the two-step sintering process is lower than the sintering temperature T.sub.0 of an normal sintering process by 300-500° C., and there is no obvious grain growth in the late stage of sintering in the two-step sintering process.

(4) FIG. 3 is a schematic diagram of the influence of the powder accumulation state on the sintering. Kingery and Francois first recognized that there was a critical coordination number Nc of the pores in the powder sintering process in 1967. Suppose a pore is surrounded by N grains, where, N is related with the powder accumulation state in the early stage. Then, if N<Nc for a pore (the powder accumulation state is a tight state), the interface between the pore and the grains is recessed toward the pore, and the pore shrinks; if N>Nc for a pore (the powder accumulation state is a loose state), the interface protrudes toward the pore, and the pore grows. Especially, in a case that the densification driving force is great so that pores with N>Nc shrink, surely the neighboring grains grow abnormally. (a) is a tight powder accumulation state, in which the pores are easy to shrink during sintering; (b) is a loose powder accumulation state, in which the pores are difficult to shrink during sintering, resulting in higher densification driving force and aggravated grain growth in the densification process. Therefore, reducing large pores with N>Nc is one of the key factors to control the grain growth in the sintering process. It is feasible to improve the density of the raw compact and the state of powder accumulation.

(5) FIG. 4 is a schematic diagram of pore structure change of the sintered compact in the two-step sintering process. The pores are usually difficult to shrink in the later stage of sintering, and the further densification process in the later stage of sintering inevitably results in grain growth. With the traditional sintering method, the grain growth rate is often related to grain boundary mobility. According to the Brook velocity criterion, the relative velocity of movement between the pore and the grain boundary has an important influence on the grain growth. In a first case, i.e., the movement velocity of the grain boundary is quicker than that of the pore, the grain boundary will be disengaged from the pore and move freely, and consequently the pores remain inside the grains and difficult to shrink, or the grains grow abnormally; in a second case, the movement velocity of the pores at the grain boundary is lower than the movement velocity of the grain boundary but without disengagement, and the pinning effect of the pores inhibit the growth of the grains. In a third case, the movement velocity of the grain boundary is lower that the movement velocity of the pore spaces, and the movement of the grain boundary controls grain growth. Different from the free movement of the grain boundary in the first case, the second case and the third case involve a slow grain growth process. The mechanism of the two-step sintering is: in the first step, a certain density is achieved at a higher temperature T.sub.1, i.e., closed pores are just formed. In the second step, at a lower temperature T.sub.2, grain boundary mobility (which enables grain growth) has a larger activation energy than grain boundary diffusion (which enables porosity reduction and sintering densification), possibly by 3-/4-grain junction pinning, so that a suppression of grain growth while an activation of densification can be achieve.

(6) FIG. 5 shows the SEM microstructure of pure tungsten treated through different sintering processes. The raw powder is pure tungsten powder with 50 nm average grain size, (a) shows the state of normal sintering (NS), the holding time at 1,600° C. hydrogen atmosphere is 2 h, and the grain size is about 3 μm; (b) shows the state of the first step of sintering (TSS-I) in the two-step sintering method, the holding time at 1,400° C. temperature in hydrogen gas atmosphere is 1 h, and the grain size is about 0.5 μm; (c) shows the state of the second step of sintering (TSS-II) in the two-step sintering method. The material is held at 1,400° C. temperature in hydrogen atmosphere for 1 h, and then immediately cooled down to 1,250° C. in argon gas atmosphere and held at the temperature for 10 h, and the grain size is about 0.7 μm.

(7) FIG. 6 shows the SEM microstructure of pure molybdenum treated through different sintering processes. The raw powder is pure tungsten powder with 30 nm average grain size, (a) shows the state of normal sintering (NS), the holding time at 1,500° C. hydrogen atmosphere is 2 h, and the grain size is about 5 μm; (b) shows the state of the first step of sintering (TSS-I) in the two-step sintering method, the holding time at 1,250° C. temperature in hydrogen gas atmosphere is 2 h, and the grain size is about 1.5 μm; (c) shows the state of the second step of sintering (TSS-II) in the two-step sintering method. The material is held at 1,250° C. temperature in hydrogen atmosphere for 1 h, and then immediately cooled down to 1,150° C. in argon gas atmosphere and held at the temperature for 40 h, and the grain size is about 2 μm.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(8) The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the present invention are shown. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure is thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference numerals refer to like elements throughout.

(9) Embodiments of the invention are illustrated in detail hereinafter with reference to accompanying drawings. It should be understood that specific embodiments described herein are merely intended to explain the invention, but not intended to limit the invention.

(10) Hereunder the technical scheme of the present invention will be further detailed in some embodiments, with reference to the accompanying drawings.

(11) The metal material sintering densification and grain size control method provided in the present invention comprises the following steps:

(12) first, processing raw material powder by deagglomeration to obtain deagglomerated powder with excellent dispersion; granulating the deagglomerated powder by spray granulation to improve powder flowability and density uniformity of powder compact and obtain granulated particles in an approximately spherical shape; processing the obtained granulated particles in an approximately spherical shape by high-pressure die pressing and cold isostatic pressing to obtain a powder compact; sintering the powder compact by two-step pressureless sintering, i.e., the first step of sintering is to heat up the powder compact quickly to a specified temperature, hold the powder compact at the temperature for a short time and control the density at 75-85%, and the second step of sintering is to cool down the powder compact to a specified temperature and hold the powder compact at the temperature for a long time to further eliminate residual pores, so as to a obtain high-density ultrafine grain metal.

(13) The specific steps of the method are:

(14) S1: using metal powder as a raw material, and processing the raw material powder by deagglomeration with a high-speed helical blade mixer at 2,000-3,000 rpm blade rotation speed for 0.5-2 h, to obtain deagglomerated powder;

(15) S2: first, mixing a binder and deionized water homogeneously to prepare a solution A, in which the content of the binder is 5-15 wt. %;

(16) then, adding the deagglomerated raw material powder obtained in the step S1 into the solution A and stirring the solution mechanically to a homogeneous state, so as to obtain a slurry;

(17) granulating the obtained slurry by spray granulation with a centrifugal atomizing drier at 8,000-15,000 r/min rotation speed, 100-300 kPa atomizing pressure, and 90-150° C. drying temperature;

(18) loading the granulated powder into a tube heating oven and charging high-purity hydrogen into the tube heating oven for degreasing and reduction at 550-700° C. processing temperature and 5-10° C./min. heating rate, and holding for 60-120 min., to obtain granulated particles in an approximately spherical shape;

(19) S3: pressing the granulated particles by high-pressure die pressing at 700-1,000 MPa pressing pressure, and holding for 0.5-1.5 min. to obtain a preformed powder compact, loading the preformed powder compact into a jacketed mold and performing cold isostatic pressing at 200-280 MPa, and holding for 5-10 min., to obtain a powder compact;

(20) S4: performing two-step sintering: first, in the first step of sintering, heating up the powder compact obtained in the step S3 at a specified heating rate to a temperature T.sub.1 and holding at the temperature, to obtain a one-step sintered compact; then, in the second step of sintering, cooling down the one-step sintered compact from the temperature T.sub.1 to a temperature T.sub.2 at a specified cooling rate, and holding at the temperature, so as to obtain a metal material with ultrafine grains finally, wherein, the temperature T.sub.2 is lower than the temperature T.sub.1 by 50-250° C., and the holding time in the first step is shorter than the holding time in the second step (as shown in FIG. 1).

(21) In the step S1, the metal powder comprises a refractory metal; the grain size of the deagglomerated powder is smaller than 0.5 μm.

(22) In the step S2, the binder is polyvinyl alcohol, polyethylene glycol, stearic acid or paraffin; the solid content in the slurry is 60-85 wt. %.

(23) In the step S3, the relative density of the powder compact is higher than 50%.

(24) In the first step of sintering in the step S4, the powder compact is sintered in a hydrogen atmosphere by heating up the powder compact to a temperature T.sub.1 equal to 1,200-1,500° C. and holding at the temperature for 1-2 h.

(25) In the second step of sintering in the step S4, the shielding gas atmosphere is hydrogen or argon gas atmosphere, the temperature is decreased from the temperature T.sub.1 to a temperature T.sub.2 at 15-25° C./min. cooling rate, and the holding time is 10-60 h.

(26) The density of the one-step sintered compact is 75-85%, the grain size is 0.5-1 μm, and the pore size and distribution are uniform.

(27) The grain size of the obtained ultrafine grain metal/one-step sintered compact is ≤1.5.

(28) The density of the ultrafine grain metal is higher than 98%.

Embodiment 1

(29) 50 nm pure tungsten powder is used as a raw material, the raw material powder is processed by deagglomeration with a high-speed helical blade mixer at 3,000 rpm blade rotation speed for 1 h, to obtain deagglomerated raw material powder with narrow grain size distribution. Polyethylene glycol and deionized water are uniformly mixed to make a solution A with 15 wt. % binder content, and then 50 nm raw tungsten powder is added into the solution A and the solution A is stirred mechanically to a homogenously mixed state to make a slurry with 85 wt. % solid content; the obtained slurry is granulated by spray granulation with a centrifugal atomizing drier at 15,000 r/min. rotation speed, 300 kPa atomizing pressure, and 90° C. drying temperature; the granulated powder is loaded into a tube heating oven, and high-purity hydrogen is charged into the tube heating oven for degreasing and reduction at 700° C. processing temperature and 5° C./min. heating rate, and the temperature is held for 120 min., to obtain granulated particles in an approximately spherical shape; the granulated particles is molded by two-way compression molding at 1,000 MPa pressing pressure, and the pressure is held for 1 min., to obtain a preformed powder compact. The preformed powder compact is loaded into a jacketed mold for vacuum encapsulation, and then processed by cold isostatic pressing at 280 MPa, and the pressure is held for 5 min., to obtain a powder compact; the powder compact is sintered by first-step sintering in hydrogen gas atmosphere at 1400° C. sintering temperature, 15° C./min heating rate, and the temperature is held for 1 h. Then, the compact is cooled rapidly at 20° C./min. cooling rate to the second-step sintering temperature 1,250° C., here, the sintering atmosphere is replaced with argon gas, and the holding time is 10 h. Finally, high-density ultrafine grain tungsten without grain growth is obtained. The microstructure is shown in FIG. 5, the density is 98%, and the average grain size is 0.7 μm.

Embodiment 2

(30) 50 nm pure tungsten powder is used as a raw material, the raw material powder is processed by deagglomeration with a high-speed helical blade mixer at 3,000 rpm blade rotation speed for 1 h, to obtain deagglomerated raw material powder with narrow grain size distribution. Polyethylene glycol and deionized water are uniformly mixed to make a solution A with 15 wt. % binder content, and then 50 nm raw tungsten powder is added into the solution A and the solution A is stirred mechanically to a homogenously mixed state to make a slurry with 85 wt. % solid content; the obtained slurry is granulated by spray granulation with a centrifugal atomizing drier at 15,000 r/min. rotation speed, 300 kPa atomizing pressure, and 90° C. drying temperature; the granulated powder is loaded into a tube heating oven, and high-purity hydrogen is charged into the tube heating oven for degreasing and reduction at 700° C. processing temperature and 5° C./min. heating rate, and the temperature is held for 120 min., to obtain granulated particles in an approximately spherical shape; the granulated particles is molded by two-way compression molding at 1,000 MPa pressing pressure, and the pressure is held for 1 min., to obtain a preformed powder compact. The preformed powder compact is loaded into a jacketed mold for vacuum encapsulation, and then processed by cold isostatic pressing at 280 MPa, and the pressure is held for 5 min., to obtain a powder compact; the powder compact is sintered by first-step sintering in hydrogen gas atmosphere at 1300° C. sintering temperature, 15° C./min heating rate, and the temperature is held for 1 h. Then, the compact is cooled rapidly at 20° C./min. cooling rate to the second-step sintering temperature 1,200° C., here, the sintering atmosphere is replaced with argon gas, and the holding time is 20 h. Finally, high-density ultrafine grain tungsten without grain growth is obtained. The density is 97%, and the average grain size is 0.6 μm.

Embodiment 3

(31) 200 nm pure tungsten powder is used as a raw material, the raw material powder is processed by deagglomeration with a high-speed helical blade mixer at 2500 rpm blade rotation speed for 1 h, to obtain deagglomerated raw material powder with narrow grain size distribution. Polyethylene glycol and deionized water are uniformly mixed to make a solution A with 10 wt. % binder content, and then 200 nm raw tungsten powder is added into the solution A and the solution A is stirred mechanically to a homogenously mixed state to make a slurry with 70 wt. % solid content; the obtained slurry is granulated by spray granulation with a centrifugal atomizing drier at 12,000 r/min rotation speed, 200 kPa atomizing pressure, and 120° C. drying temperature; the granulated powder is loaded into a tube heating oven, and high-purity hydrogen is charged into the tube heating oven for degreasing and reduction at 600° C. processing temperature and 5° C./min. heating rate, and the temperature is held for 120 min., to obtain granulated particles in an approximately spherical shape; the granulated particles is molded by two-way compression molding at 900 MPa pressing pressure, and the pressure is held for 1 min., to obtain a preformed powder compact. The preformed powder compact is loaded into a jacketed mold for vacuum encapsulation, and then processed by cold isostatic pressing at 250 MPa, and the pressure is held for 5 min., to obtain a powder compact; the powder compact is sintered by first-step sintering in hydrogen gas atmosphere at 1400° C. sintering temperature, 15° C./min heating rate, and the temperature is held for 1 h. Then, the compact is cooled rapidly at 20° C./min. cooling rate to the second-step sintering temperature 1,250° C., here, the sintering atmosphere is replaced with argon gas, and the holding time is 20 h. Finally, high-density ultrafine grain tungsten without grain growth is obtained. The density is 97%, and the average grain size is 1.5 μm.

Embodiment 4

(32) 400 nm pure tungsten powder is used as a raw material, the raw material powder is processed by deagglomeration with a high-speed helical blade mixer at 2000 rpm blade rotation speed for 1 h, to obtain deagglomerated raw material powder with narrow grain size distribution. Polyethylene glycol and deionized water are uniformly mixed to make a solution A with 5 wt. % binder content, and then 400 nm raw tungsten powder is added into the solution A and the solution A is stirred mechanically to a homogenously mixed state to make a slurry with 65 wt. % solid content; the obtained slurry is granulated by spray granulation with a centrifugal atomizing drier at 10,000 r/min. rotation speed, 150 kPa atomizing pressure, and 140° C. drying temperature; the granulated powder is loaded into a tube heating oven, high-purity hydrogen is charged into the tube heating oven for degreasing and reduction at 550° C. processing temperature and 5° C./min. heating rate, and the powder is held at the temperature for 120 min., to obtain granulated particles in an approximately spherical shape; the granulated particles is molded by two-way compression molding at 700 MPa pressing pressure, and the pressure is held for 1 min., to obtain a preformed powder compact. The preformed powder compact is loaded into a jacketed mold for vacuum encapsulation, and then processed by cold isostatic pressing at 200 MPa, and the pressure is held for 5 min., to obtain powder compact; the powder compact is sintered by first-step sintering in hydrogen gas atmosphere at 1400° C. sintering temperature, 15° C./min. heating rate, and the temperature is held for 1 h. Then, the compact is cooled rapidly at 20° C./min. cooling rate to the second-step sintering temperature 1,300° C., here, the sintering atmosphere is replaced with argon gas, and the holding time is 30 h. Finally, high-density ultrafine grain tungsten without grain growth is obtained. The density is 97%, and the average grain size is 1.2 μm.

Embodiment 5

(33) 30 nm pure molybdenum powder is used as a raw material, the raw material powder is processed by deagglomeration with a high-speed helical blade mixer at 2000 rpm blade rotation speed for 1 h, to obtain deagglomerated raw material powder with narrow grain size distribution. Polyethylene glycol and deionized water are uniformly mixed to make a solution A with 5 wt. % binder content, and then 30 nm raw tungsten powder is added into the solution A and the solution A is stirred mechanically to a homogenously mixed state to make a slurry with 60 wt. % solid content; the obtained slurry is granulated by spray granulation with a centrifugal atomizing drier at 12,000 r/min rotation speed, 150 kPa atomizing pressure, and 140° C. drying temperature; the granulated powder is loaded into a tube heating oven, high-purity hydrogen is charged into the tube heating oven for degreasing and reduction at 550° C. processing temperature and 5° C./min. heating rate, and the powder is held at the temperature for 120 min., to obtain granulated particles in an approximately spherical shape; the granulated particles is molded by two-way compression molding at 700 MPa pressing pressure, and the pressure is held for 2 min., to obtain a preformed powder compact. The preformed powder compact is loaded into a jacketed mold for vacuum encapsulation, and then processed by cold isostatic pressing at 200 MPa, and the pressure is held for 5 min., to obtain powder compact; the powder compact is sintered by first-step sintering in hydrogen gas atmosphere at 1250° C. sintering temperature and 15° C./min. heating rate, and the temperature is held for 1 h. Then, the compact is cooled rapidly at 20° C./min. cooling rate to the second-step sintering temperature 1,150° C., here, the sintering atmosphere is replaced with argon gas, and the holding time is 40 h. Finally, high-density ultrafine grain molybdenum without grain growth is obtained. The microstructure is shown in FIG. 6, the density is 97%, and the average grain size is 2 μm.

Embodiment 6

(34) 30 nm pure molybdenum powder is used as a raw material, the raw material powder is processed by deagglomeration with a high-speed helical blade mixer at 2000 rpm blade rotation speed for 1 h, to obtain deagglomerated raw material powder with narrow grain size distribution. Polyethylene glycol and deionized water are uniformly mixed to make a solution A with 5 wt. % binder content, and then 30 nm raw tungsten powder is added into the solution A and the solution A is stirred mechanically to a homogenously mixed state to make a slurry with 60 wt. % solid content; the obtained slurry is granulated by spray granulation with a centrifugal atomizing drier at 12,000 r/min rotation speed, 150 kPa atomizing pressure, and 140° C. drying temperature; the granulated powder is loaded into a tube heating oven, and high-purity hydrogen is charged into the tube heating oven for degreasing and reduction at 550° C. processing temperature and 5° C./min. heating rate for 120 min. holding time, to obtain granulated particles in an approximately spherical shape; the granulated particles is molded by two-way compression molding at 700 MPa pressing pressure, and the pressure is held for 2 min., to obtain a preformed powder compact. The preformed powder compact is loaded into a jacketed mold for vacuum encapsulation, and then processed by cold isostatic pressing at 200 MPa, and the pressure is held for 5 min., to obtain powder compact; the powder compact is sintered by first-step sintering in hydrogen gas atmosphere at 1350° C. sintering temperature, 15° C./min. heating rate and without holding time. Then, the compact is cooled rapidly at 20° C./min cooling rate to the second-step sintering temperature 1,200° C., here, the sintering atmosphere is replaced with argon gas, and the holding time is 40 h. Finally, high-density ultrafine grain molybdenum without grain growth is obtained. The density is 98%, and the average grain size is 1.2 μm.

Embodiment 7

(35) 50 nm pure molybdenum powder is used as a raw material, the raw material powder is processed by deagglomeration with a high-speed helical blade mixer at 2000 rpm blade rotation speed for 1 h, to obtain deagglomerated raw material powder with narrow grain size distribution. Polyethylene glycol and deionized water are uniformly mixed to make a solution A with 5 wt. % binder content, and then 50 nm raw tungsten powder is added into the solution A and the solution A is stirred mechanically to a homogenously mixed state to make a slurry with 60 wt. % solid content; the obtained slurry is granulated by spray granulation with a centrifugal atomizing drier at 10,000 r/min rotation speed, 150 kPa atomizing pressure, and 140° C. drying temperature; the granulated powder is loaded into a tube heating oven, high-purity hydrogen is charged into the tube heating oven for degreasing and reduction at 550° C. processing temperature and 5° C./min. heating rate, and the powder is held at the temperature for 120 min., to obtain granulated particles in an approximately spherical shape; the granulated particles is molded by two-way compression molding at 700 MPa pressing pressure, and the pressure is held for 2 min., to obtain a preformed powder compact. The preformed powder compact is loaded into a jacketed mold for vacuum encapsulation, and then processed by cold isostatic pressing at 200 MPa, and the pressure is held for 5 min., to obtain powder compact; the powder compact is sintered by first-step sintering in hydrogen gas atmosphere at 1350° C. sintering temperature, 15° C./min. heating rate and without holding time. Then, the compact is cooled rapidly at 15° C./min. cooling rate to the second-step sintering temperature 1,150° C., here, the sintering atmosphere is replaced with argon gas, and the holding time is 40 h. Finally, high-density ultrafine grain molybdenum without grain growth is obtained. The density is 98%, and the average grain size is 1.3 μm.

Embodiment 8

(36) 70 nm pure molybdenum powder is used as a raw material, the raw material powder is processed by deagglomeration with a high-speed helical blade mixer at 2000 rpm blade rotation speed for 1 h, to obtain deagglomerated raw material powder with narrow grain size distribution. Polyethylene glycol and deionized water are uniformly mixed to make a solution A with 5 wt. % binder content, and then 70 nm raw tungsten powder is added into the solution A and the solution A is stirred mechanically to a homogenously mixed state to make a slurry with 60 wt. % solid content; the obtained slurry is granulated by spray granulation with a centrifugal atomizing drier at 8,000 r/min. rotation speed, 150 kPa atomizing pressure, and 140° C. drying temperature; the granulated powder is loaded into a tube heating oven, high-purity hydrogen is charged into the tube heating oven for degreasing and reduction at 550° C. processing temperature and 5° C./min. heating rate, and the powder is held at the temperature for 120 min., to obtain granulated particles in an approximately spherical shape; the granulated particles is molded by two-way compression molding at 700 MPa pressing pressure, and the pressure is held for 2 min., to obtain a preformed powder compact. The preformed powder compact is loaded into a jacketed mold for vacuum encapsulation, and then processed by cold isostatic pressing at 200 MPa, and the pressure is held for 5 min., to obtain powder compact; the powder compact is sintered by first-step sintering in hydrogen gas atmosphere at 1250° C. sintering temperature and 15 r/min. heating rate, and the temperature is held for 2 h. Then, the compact is cooled rapidly at 25° C./min. cooling rate to the second-step sintering temperature 1,200° C., here, the sintering atmosphere is replaced with argon gas, and the holding time is 40 h. Finally, high-density ultrafine grain molybdenum without grain growth is obtained. The density is 97%, and the average grain size is 1.8 μm.

(37) While the present invention has been illustrated and described with reference to some embodiments, the present invention is not limited to these. Those skilled in the art should recognize that various variations and modifications can be made without departing from the spirit and scope of the present invention as defined by the accompanying claims. Therefore, the protected domain of the present invention shall be only confined by the claims.

(38) The foregoing description of the exemplary embodiments of the present invention has been presented only for the purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in light of the above teaching.

(39) The embodiments were chosen and described in order to explain the principles of the invention and their practical application so as to activate others skilled in the art to utilize the invention and various embodiments and with various modifications as are suited to the particular use contemplated. Alternative embodiments will become apparent to those skilled in the art to which the present invention pertains without departing from its spirit and scope. Accordingly, the scope of the present invention is defined by the appended claims rather than the foregoing description and the exemplary embodiments described therein.