CN1256209C - Densification method for gradient materials with continuously changing composition - Google Patents

Abstract

The present invention provides a densification method for functional gradient metal system materials with continuously varying components by different phase granule coprecipitation, which is characterized in that aiming for the main components forming the functional gradient metal system materials, sintering aid agents capable of realizing densification for granule homogeneous bodies formed by one or many main components are obtained by a mode of roughing selection. Then, sintering aid agents capable of realizing simultaneous densification for many components under the same conditions are further screened on the basis, and the adding content with which a single substance and composite materials achieve the best density under the conditions is used as a selection standard of a sintering mechanism and the adding content. Finally, the granularity distribution and the adding amount of sintering aid agent powder is calculated according to granularity sizes of each main component forming gradient materials and laminar flow precipitation conditions of granules in suspending liquid. The sintering aid agents are added to the granule precipitation bodies in a precipitation mode, and the granule precipitation bodies with continuous gradient structures are densified by vacuum hot pressing or discharge plasma sintering.

Description

Densification method for gradient materials with continuously changing composition

Technical field

The present invention relates to a method for densification of gradient materials with continuously varying composition.

Background technique

Obtaining a continuous gradient composite structure is an inevitable trend in the development of gradient materials, gradient composite theory and technology, and is a more stringent and finer requirement for gradient materials in several important application fields. From the perspective of thermal stress relaxation, the continuous distribution of components is the key to obtain excellent thermal insulation and thermal shock resistance, and it is also the goal that has been pursued in the field of gradient material preparation. The use of co-sedimentation of heterogeneous particles to achieve continuous distribution of gradient material components is a new preparation method developed in recent years. This method has the advantages of a wide range of applicable material systems, relatively simple process control, low cost, and flexible experimental methods. features.

Co-sedimentation method to prepare gradient materials is based on the Stokes free sedimentation formula under laminar flow:

$\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; D.</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; g</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&rho;</span>}}_{\mathrm{<span\; class="notranslate">\; Particles</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&rho;</span>}}_{\mathrm{<span\; class="notranslate">\; Liquid</span>}}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; 18</span>}\mathrm{<span\; class="notranslate">\; \&eta;</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

Among them, U represents the sedimentation velocity of the particles; D represents the particle size of the particles; η represents the viscosity of the suspension; g represents the acceleration of gravity. It can be seen from the above formula that solid particles with different particle sizes and densities have different settling velocities, so they start to accumulate at different times at the bottom of the settling vessel. By controlling the particle size distribution of the raw material powder and the process parameters in the sedimentation process, a particle deposit body with continuously changing components can be obtained.

For the sedimentary body formed by sedimentation, it needs to be densified by some technical means to meet the requirements of use. The deposit formed by metal powder is usually densified by sintering. However, due to differences in the sintering performance of different substances, scientists from various countries have adopted different methods in order to densify sediments formed by different components under the same conditions. German T.Jungling and others have used the method of increasing the sintering temperature to increase the density of the deposited body, but the effect is not ideal; the French first settle the substance with a higher melting point and graphite powder, and then sinter it to form continuous open pores. Change the porous material, and then heat the substance with a lower melting point to a molten state, and press the molten material into the gradient porous material under a certain pressure. Since the porosity of the gradient pores cannot reach 100%, the gradient material prepared by this method is dense in a certain range of components, but the continuity of the overall composition of the gradient material is destroyed, and this method is only applicable to in certain systems.

Contents of Invention

The invention provides a new method for preparing the overall densification of metal-based functionally graded materials by co-sedimentation of heterogeneous particles, so as to realize the overall densification of metal-based functionally graded materials with any specified structure. The process is:

The sintering aid capable of densifying the particle homogeneity formed by one or more main component metals is sequentially obtained through rough selection. Then, on this basis, the sintering aids that can realize the simultaneous densification of various main components under the same conditions are further screened out. Finally, according to the particle size of each main component of the metal-based functionally graded material, the particle size distribution and addition amount of the sintering aid powder are calculated by using the laminar flow sedimentation condition of the particles in the suspension. The sintering aid and the main components of the functionally graded material are deposited together in the way of sedimentation, and the densification of the continuous gradient structure particle deposition body is realized by vacuum hot pressing or spark plasma sintering.

Concrete invention content is described as follows:

1. Screening of sintering aids and determination of sintering mechanism

The screening of sintering aids is divided into two steps: rough selection and fine selection; rough selection is to screen out the types of sintering aids that have a densification effect on each main component of the metal-based functionally graded material within a wide range. Selection is within the scope of rough selection of sintering aids, to screen out sintering aids that can simultaneously densify multiple components under the same conditions (sintering temperature, pressure), and the selection of their sintering mechanism and content can be used in The added content of the simple substance and the composite material at this temperature is used as the standard, that is, after mixing the sintering aids with different contents and the simple substance (or composite material) uniformly, sinter at this temperature and test the density of each sintered sample. According to the situation of sintering, the addition amount of sintering aid corresponding to the compactness of simple substance (or composite material) is taken as the appropriate addition content.

2. Calculation of particle size distribution and content of sintering aids

Since the sintering aid is added to the particle sedimentation body by particle co-sedimentation, the deposition position of the sintering aid should correspond to the deposition position of its densified component, which requires the adjustment of the sintering aid according to the particle size distribution of the component. Particle size distribution is designed.

Based on the settling conditions of particles in laminar flow (Stokes formula, 1) and the continuity equation for settling of particles in suspension

$\frac{<span\; class="notranslate">\; \&PartialD;</span>\mathrm{<span\; class="notranslate">\; C</span>}}{<span\; class="notranslate">\; \&PartialD;</span>\mathrm{<span\; class="notranslate">\; t</span>}}<span\; class="notranslate">\; +</span>\frac{<span\; class="notranslate">\; \&PartialD;</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; CU</span>}<span\; class="notranslate">\; )</span>}{<span\; class="notranslate">\; \&PartialD;</span>\mathrm{<span\; class="notranslate">\; h</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

(wherein, C represents the concentration of particles in the suspension, which is a function of spatial position and time; h represents the position coordinates along the direction of gravity; t represents the settling time; U represents the settling velocity of particles), firstly, according to the main gradient material The particle size distribution of the components is calculated by calculating the mass distribution of each component in the sedimentary body after settlement, that is, the mass of each component at different positions in the sedimentary body along the thickness direction; then, according to the type of sintering aid determined in the first step and the added content, the mass distribution of the corresponding sintering aids is calculated from the mass distribution of each component (that is, the mass of the sintering aids at different positions of the deposit body) and the sum is obtained to obtain the total content of the sintering aids; finally, based on ( 1) and (2), and then deduce the corresponding particle size distribution from the mass distribution of the sintering aid. Based on the calculation results, the sedimentation classification of the sintering aid raw materials is carried out. By controlling the settling time of the particles, the particles with the required particle size distribution are selected. In terms of the added content of sintering aids, since the content of each component changes in gradients in the material, it is required that the added content of sintering aids also change in gradients with the corresponding components, so as to obtain a better sintering effect.

3. Formation and sintering of continuous gradient structure

According to the characteristics of the raw materials of the gradient material to be prepared, select an appropriate dispersant. The dispersion liquid used in the co-sedimentation method must be able to fully wet the particle surface of the functional gradient main component and sintering aid; it will not dissolve the particles, and there is no other Reaction; the powder particles have an appropriate settling velocity in the dispersion, that is, the viscosity of the dispersion is moderate.

Weigh the raw material powder with the specified particle size distribution according to the calculated mass, and disperse it into a suspension in the selected dispersant. After the suspension is fully dispersed by ultrasonic waves, it is added to the settling equipment for settling experiments. After the settling is completed, the liquid medium above the particle deposit is discharged from the settling equipment. After drying and pressing, the deposited body is moved to a graphite mold for sintering. The sintering mechanism is the same as that determined in step 1. A dense gradient material with a specified structure is finally obtained by hot-press sintering or spark plasma sintering.

By adopting the above method, the overall densification of the metal-based functionally graded material whose composition changes continuously can be realized.

Description of drawings

Figure 1: Particle size distribution of W powder

Figure 2: Particle size distribution of Mo powder

Figure 3: Particle size distribution of Ni-Cu alloy powder

Figure 4: EPMA diagram of the longitudinal section of the W-Mo gradient material without sintering aids: a is the Mo-rich end, b is the W-rich end

Figure 5: After adding the sintering aid Ni-Cu, the microstructure of the longitudinal section of the W-Mo gradient material a is the Mo-rich end, b is the W-rich end

Figure 6: EPMA diagram of the longitudinal section of the Mo-Ti gradient material without sintering aids: a is the Mo-rich end, b is the intermediate transition zone, and c is the Ti-rich end (1200 ° C, 30 MPa, vacuum sintering for 1 hour)

Figure 7: After adding the sintering aid Ni-Cu, the EPMA diagram of the longitudinal section of the Mo-Ti gradient material: a is the Mo-rich end, b is the intermediate transition zone, and c is the Ti-rich end (1200 ° C, 30 MPa, heat preservation for 1 hour vacuum sintering)

Specific implementation plan

Specific embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings.

Example 1:

The preparation of the W-Mo gradient material with continuously changing components, the specific process is described as follows:

(1) Determination of the types of sintering aids: in this system gradient material, its main components are W and Mo. At present, the sintering agent systems for sintering W alloys mainly include Ni-Fe, Ni-Cu, Ni-Co, Ni-Cr, etc.; in the research of preparing molybdenum alloys by powder metallurgy, Ti and Zr are usually used as sintering aids, while Ni -Cu also has a good sintering effect on Mo. Therefore, Ni-Cu is used as a common sintering aid for W and Mo components. Through the sintering research of W and Mo simple substances and their composite materials, it is determined that the added content is 3Wt.%Ni-2Wt%Cu, and the sintering temperature is 1200°C.

(2) Calculate the particle size distribution and content of the sintering aid Ni-Cu alloy (the ratio of Ni and Cu is 3:2): based on the continuity equation of the particles settling in the suspension, and the laminar flow condition (ie, the Stokes formula), for The particle size distribution of the Ni-Cu alloy powder calculated from W and Mo powders with a given particle size distribution (as shown in Figures 1 and 2) is shown in Figure 3. Then calculate the addition amount of Ni-Cu alloy powder according to the mass of W and Mo powder. Based on the calculation results, the Ni-Cu alloy raw materials are classified to obtain powders with required particle sizes.

(3) Formation and densification of W-Mo continuous gradient structure: because absolute ethanol has a good dispersion effect on W, Mo, Ni-Cu alloys, and no reaction occurs, the present invention selects absolute ethanol as the dispersion medium. Weigh the mass of the raw material powder and disperse it into a suspension in absolute ethanol. After the suspension is fully dispersed by ultrasonic waves, it is added to the settling equipment for settling experiments. After the settling is complete, the ethanol liquid above the particle deposit is drained. After the sedimentation body is dried and pressed, it is moved to a graphite mold, and the sedimentation body is vacuum hot-pressed and sintered under the condition of 1200°C-30MPa-1h.

(4) Structural test: For comparison, W and Mo particles without sintering aid Ni-Cu were sedimented and sintered under the same conditions. All samples obtained were analyzed by electron probe with energy spectrometer. Figure 4 is the microstructure of the W-Mo gradient material without sintering aids. It can be seen that there are more pores in both the W-rich end and the Mo-rich end. Figure 5 is the microstructure of the W-Mo gradient material obtained when Ni-Cu alloy is added as a sintering aid. It can be seen that the W-Mo material has been completely densified, and the black spots in the figure are the accumulations of sintering aids and Mo powder.

Example 2:

Preparation of Mo-Ti gradient materials with continuously changing composition:

(1) Determination of the type of sintering aid: the process is the same as that of Example 1, and finally Ni-Cu is determined as the common sintering aid of Mo and Ti components.

(2) Calculating the particle size distribution and content of the sintering aid Ni-Cu: the calculation process is exactly the same as that of Example 1.

(3) Formation and densification of Mo-Ti continuous gradient structure: Absolute ethanol is still selected as the dispersion medium. The process is the same as in Example 1.

(4) Structural test: Also for comparison, the Mo and Ti particles without sintering aid Ni-Cu were sedimented and sintered under the same conditions. All samples obtained were analyzed by electron probe with energy spectrometer. Figure 6 is the microstructure of the Mo-Ti gradient material without sintering aids. It can be seen that there are more pores at the Mo-rich end. Figure 7 is the microstructure of the Mo-Ti gradient material obtained when Ni-Cu alloy is added as a sintering aid. It can be seen that the Mo-Ti material has been completely densified.

The preparation of gradient materials of different systems above shows that the present invention proposes a new idea for the densification of gradient materials with continuously changing components, and the present invention can be used for preparing gradient materials with continuous components by co-sedimentation of heterogeneous particles. A bulk dense gradient material is obtained.

Claims (3)

1, a kind of densifying method that utilizes out-phase particle cosedimentation to prepare the continuous metal functional gradient material of component, it is characterized in that at each major components of constructing metal functional gradient material, obtain successively and can realize densified sintering aid by the mode of roughly selecting the particle isotropic body that one or more major components form, then on this basis, further filter out the kind that can under identical conditions, realize to the simultaneously densified sintering aid of multiple constituent element, addition and sintering mechanism, at last, according to the granule size of constructing each major components of functionally gradient material (FGM), utilize particle laminar flow setting condition in suspension to calculate the size distribution of sintering aid powder, mode by sedimentation is with the major constituent sedimentation together of sintering aid and FGM, realize densified to continuous gradient structure particles lithosomic body by vacuum hotpressing or discharge plasma sintering, wherein, based on particle under laminar condition setting condition and suspension in the continuity equation of particles settling be:

$\frac{\&PartialD;C}{\&PartialD;t}+\frac{\&PartialD;\left(\mathrm{CU}\right)}{\&PartialD;h}=0$

C represents the concentration of particle in suspension in the formula, and it is the function of locus and time; H represents the position coordinates along gravity direction; T represents the sedimentation time; U represents the sinking speed of particle.

2, densifying method according to claim 1, it is characterized in that in the selection of sintering aid, selection can make functionally gradient material (FGM) all components simultaneously fine and close sintering aid under temperature of the same race, the sintering mechanism of sintering aid and the selection of adding content with simple substance and composite under this condition the interpolation content when fine and close as standard.

3, densifying method according to claim 1 is characterized in that the size distribution of agglutinant is corresponding with the size distribution of FGM major constituent, the interpolation content of sintering aid and corresponding constituent content graded.

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