Study on refractory metal injection molding

Tungsten and molybdenum have high density, high strength, low thermal expansion coefficient, excellent corrosion resistance and thermal electron emission capability, and have been widely used in many industrial and defense fields such as aerospace, electronics, and chemical industry. However, due to the poor plasticity of tungsten at room temperature, the difficulty of processing and the poor oxidation resistance of high temperature, its application is limited.

Metal injection molding technology (Metal Injection Molding, referred to as MIM) is the traditional powder metallurgy and plastic injection molding technology combined and developed a new near net shape technology. MIM has the advantages of being able to form complex shapes, low cost, good batch production consistency, and isotropy. It is called “the hottest part forming technology today”. The metal powder injection molding process is applied to the preparation of pure tungsten and molybdenum parts, and the tungsten molybdenum parts with high density, uniform structure and complex shape are prepared at a low cost, which is of great significance for the research and development of the metal powder injection molding field.

In this paper, the preparation of pure tungsten and molybdenum and doped rare earth tungsten-molybdenum parts by injection molding method were studied, and their microstructure, mechanical properties and density were studied.

    First, the experiment

(1) Selection of raw materials

The powder selected for the experiment was a high-purity tungsten powder having a particle diameter of 2 to 3 μm, a high-purity molybdenum powder having a particle diameter of 3 to 5 μm, and a high-purity rare earth oxide powder having a particle diameter of 3 to 5 μm. The specific ratios are shown in Table 1. The binder selected in the experiment is a paraffin wax (PW) with low melting point and good fluidity as the main component, and a high-density polyethylene (HDPE) and polypropylene (PP) with a high melting point are added to provide sufficient strength of the green body. A small amount of surfactant stearic acid (SA) was also added. The binder has the characteristics of small swelling, good fluidity, good shape retention and easy removal.

Table 1 Experimental raw material composition table

(2) Experimental methods

The mixed powder was prepared according to the proportion of ingredients in Table 1, and ball-milled in a roller ball mill for 8 h at a rotation speed of 45 r·min ^-1 to thoroughly mix the rare earth oxide and the tungsten powder. The binder was added at a powder loading of 52%, and the powder and the binder were mixed on an X(S)K-160 type mixer to form an injection feed. The mixing temperature was 140-150 ° C, and the mixing time was 1.5. h. The mechanical properties of the green body and the part green body were obtained by injection on a CJ-80E injection molding machine. The shape and dimensions are shown in Fig. 1. The injection temperature was 165 ° C and the injection pressure was 65 MPa. The green body was degreased at room temperature and thermally degreased at 1200 ° C. Finally, in a hydrogen atmosphere, the tungsten product was sintered at 2300 ° C, and the molybdenum product was sintered at 1900 ° C.

Figure 1 Green size chart of sample (a) and complex shape part (b) (unit: mm)

(3) Sample testing

The samples were analyzed on the S-360 scanning electron microscope for morphology, fracture and energy spectrum. The mechanical properties were tested on the REGER3010 tensile bending machine. The C content was analyzed by infrared carbon sulfur analyzer, and the metallographic structure was observed with XJP-6A microscope. Density was measured by the Archimedes method on a TG-328A analytical balance.

    Second, the results and discussion

(1) Study on the injection molding process of tungsten and molybdenum

Figures 2 and 3 are SEM photographs of the powder and the sample solvent before and after degreasing, respectively. It can be seen from Figure 2 that the raw material powder is nearly spherical, the particles are fine and the particle size is not much different, and it is suitable for powder injection molding. As can be seen from Fig. 3(a), the binder uniformly wraps the powder particles and fills the gaps between the particles. Figure 3(b) shows that after solvent degreasing, a network of connected pores has been formed between the powders, which is formed after the solvent removes part of the binder. The connected pore network provides a guarantee for complete binder removal. .

Figure 2 SEM photo of the powder

(a)-tungsten powder; (b)-molybdenum powder

SEM photograph before and after the solvent degreased samples ^# 36 in FIG.

(a) - before solvent degreasing; (b) - after solvent degreasing

The powder loading amount (powder volume content) can be expressed by the following formula:

Where W _P and W _b are the weights of the metal powder and the binder, respectively, and ρ _P and ρ _b are the densities of the metal powder and the binder, respectively. The density of the feed can be calculated by the following formula.

ρ=ρ _b +ф(ρ _P -ρ _b ) (2)

In formula (2), ρ, ρ _P and ρ _b are the theoretical densities of feed, powder and binder, respectively, and the theoretical density of feed is linear with the powder loading. The critical loading is the volume fraction of powder particles when the binder is just filled with voids between the particles and the feed is in the closest packed state. Since the powder cannot flow freely to fill the entire space, when the powder content is too high, there is not enough binder to fill the gap between the particles. When the powder content is higher than the critical powder content, the feed density will be lower than the theoretical density due to the presence of voids. The powder loading at this time is the critical load of the powder. The critical loading of tungsten feedstock measured according to the tungsten feed density test is about 57% (Fig. 4). In practice, the optimum powder loading for powder injection molding is less than about 2% to 5% of the critical load of the powder. (Quality score). The powder loading selected herein was 52% by experiment.

Figure 4 Tungsten feed density versus load

Under the condition that the quality of the injection blank is satisfied, the lower injection temperature is favorable for the dimensional precision control of the product, and the injection temperature is selected to be 165 ° C by experiment. Figure 5 shows the effect of injection pressure on the weight of the green body, and the weight of the green body increases as the injection pressure increases. During the injection process, as the feed fills into the cavity, the pressure in the cavity gradually increases to prevent the feed from continuing to flow, and the injection pressure is increased, which can overcome the resistance formed by the filling in the cavity, that is, the filling time is increased. , more feed is pressed into the mold cavity, so that the weight increases. Experience has shown that a 0.2% change in the weight of the injection-molded green body can cause a dimensional change of 0.3%. Too high injection pressure may also increase the residual stress of the green body, causing the sample to be easily deformed when sintered. The injection pressure selected herein is 65 MPa. Ten samples were randomly selected from the samples and parts for dimensional inspection, and the dimensional accuracy of the green and sintered products were within ±0.3%.

Figure 5 Effect of injection pressure on green body weight

(II) Effect of rare earth elements on the properties of tungsten and molybdenum products

Figure 6 is a metallographic photograph of a tungsten article after sintering at 2300 °C. It can be clearly seen from the metallographic photograph that the crystal grains of the pure tungsten injection blank after sintering are coarse, and the crystal grains are obviously decreased after the addition of the rare earth element. For molybdenum sintered products, rare earth elements have the same effect. By performing a fracture scan (Fig. 7) and a backscattered electron scan (QBSD) on the 4 ^# sample (Fig. 8) and performing energy spectrum analysis on the grain boundary particles (Fig. 9 and Fig. 10), it can be confirmed that the uniform dispersion is distributed. The fine particles of the grain boundaries are rare earth oxides. The rare earth element acts as a dispersed second phase particle to hinder grain growth, refine grains, and stabilize substructures.

Fig. 6 Photograph of metallographic structure of tungsten sintered sample after sintering at 2300 °C

FIG 74 ^# sample of SEM FIG.

Figure 84 sample QBSD Photo ^#

Figure 9 Energy spectrum at P ₁

Figure 10 P ₂ energy spectrum

Fig. 10 is a fracture diagram of the 4 ^# sample. It can be seen from Fig. 10 that the fracture of the sample is a typical brittle fracture along the crystal fracture, and the equiaxed polycrystals are separated from each other along the crystal interface, and have a grain-like facet. appearance. In addition to the inherent brittleness of tungsten, the small amount of pores present in the sample is also responsible for the brittle fracture.

The experimental data in Table 2 shows that the mechanical properties of the tungsten injection sample are significantly improved after the addition of the rare earth oxide.

Table 2 Mechanical experimental data of tungsten sintered samples

In addition to significantly improving the mechanical properties, the density of tungsten and molybdenum is greatly improved after the addition of rare earth elements. It can be seen from Table 3 and Table 4 that the density of tungsten injection samples increased from 85.49% to 94.61%; the density of molybdenum injection samples increased from 90.18% to 95.09%; rare earth oxides can increase the density during tungsten sintering. Due to the presence of water vapor, the rare earth element chemically reacts with tungsten to form a tungstate oxide having a relatively low melting point. The presence of tungsten in the results of the energy spectrum analysis in Figures 8 and 9 confirmed that the above reaction occurred. At the sintering temperature of 2300 ° C in this experiment, these newly formed tungstate and tungstate oxides are in a liquid state, the liquid phase increases the atomic transport rate, effectively fills the internal pores, improves the sintering efficiency, and contributes to Increase in density. The addition of rare earth oxides has a similar effect on the sintering of molybdenum than tungsten.

Table 3 Density of tungsten samples after sintering at 2300 ° C

Table 4 Density of molybdenum samples after sintering at 1900 ° C

Table 5 shows that the tungsten content obtained by the metal powder injection molding process has a relatively low carbon content, the presence of carbon will affect the performance of the tungsten injection product, and the lower C content is beneficial to improve the overall performance of the material.

Table 5 Carbon content of sintered samples

    Third, the conclusion

(1) After the addition of rare earth oxides, the density of sintered samples after tungsten injection increased from 85.49% to 94.61%; the density of sintered samples after molybdenum injection increased from 90.18% to 95.09%.

(2) Using metal powder injection molding technology, the dimensional accuracy of tungsten and molybdenum products after sintering can be controlled within ±0.3%.

(3) After the addition of the rare earth oxide, the grain size of the sample after sintering is significantly reduced.

(4) After adding rare earth element oxide, the tensile strength and compressive strength of the tungsten sintered product can be significantly improved.

(5) Using a suitable powder injection molding process, tungsten and molybdenum products with lower carbon content can be obtained.