Study on preparation technology of molybdenum-tungsten alloy with different tungsten content

The molybdenum-tungsten alloy rods with different tungsten content are prepared by powder metallurgy. The effects of different sintering processes on the density and microstructure of alloy rods are studied. The effects of different spinning forging processes on the yield and mass loss rate of alloy bars are also discussed. The results show that the optimum sintering processes of Mo-20W and Mo-50W alloys are 2150°C, 8 h, and 2200°C, 8 h respectively. Under the optimum sintering process, the relative density of the alloy rod reaches more than 98%. The microstructure is uniform and fine, and the grain of molybdenum tungsten alloy decreases gradually with the increase of W content. The optimum spinning forging processes of Mo-20W and Mo-50W alloys are 1280~1360°C and 1440~1520°C respectively. Under the optimum spinning forging processes, the yield of alloy bars reaches more than 92%, and the mass loss rate is reduced to 1.05~1.10%.


Introduction
Molybdenum and tungsten are congeners with the same atomic radius (1.36 angstroms).They can form complete solid solutions at all temperatures and in all proportions [1] .Because molybdenum-tungsten alloys have the advantages of both molybdenum and tungsten alloys, their high-temperature strength and recrystallization temperature are higher than those of molybdenum alloys, and their weight and processability are better than those of tungsten alloys.The good substitution and high performance-price ratio make Mo-W alloy products have broad market application space [2] .Molybdenum-tungsten alloys are widely used in the melting industry [3] , in electrode or wiring materials such as Flat panel displays and semiconductor components [4] , and in aerospace applications [5] .Because of its excellent resistivity, good processability, and strong corrosion resistance, Mo-20W can greatly improve the sensitivity and reliability of liquid crystal display (LCD).Mo-50W has higher density, higher temperature strength, better ablation resistance, and flame erosion resistance than industrial tungsten alloys, and it can be used as a furnace liner, heat shield, gas rudder, and guard plate of solid rocket motor [6] .
The production of Mo-W alloy is increasing year by year because of its excellent properties and wide applications.However, there are still some problems in the production of Mo-W alloy blank.For example, fusing often occurs in alloy billet high temperature and heat preservation processes.The product yield is generally about 80%.During forging, the alloy billet often breaks or produces serious longitudinal cracks.Better billets also have fine longitudinal cracks, resulting in low-yield products.In order to improve the machinability of molybdenum-tungsten alloy billets with different tungsten content and improve the yield, in this paper, the method of sintering and rotary forging of Mo-W rod at different temperatures is used.The optimum sintering process and rotary forging process of Mo-W alloy with different contents have been determined.The goal of improving the rate of finished products is achieved.The effect of different sintering processes on the density and microstructure of Mo-W bars with Mo-20W, Mo-50W composition, and φ25.0 mm diameter is investigated, as well as the effect of different rotary forging temperatures on the yield and mass loss rate of φ8.0 mm molybdenum-tungsten rod.Finally, the reasonable sintering process and rotary forging process for Mo-W alloy with different tungsten content are determined.

Experiment
The raw materials of the experiment are the common molybdenum powder and the fine tungsten powder which are sold in the market.The particle sizes of the molybdenum powder and the fine tungsten powder are about 3.0~4.0μm and 2.5~3.5 μm respectively.Molybdenum and tungsten powders with 20% and 50% W content were stoichiometry respectively, and uniform molybdenum-tungsten alloy powders were prepared by a two-step powder mixing process.The alloy powder was pressed into the alloy bar billet by cold isostatic pressing, and the pressing process was 180 MPA and 240 s.The molybdenum-tungsten alloy bars with a diameter of φ25.0 mm were prepared by sintering the alloy bars in hydrogen atmosphere at 2100℃ and 8 h, 2150℃ and 8 h, 2200℃ and 8 h, 2250℃ and 8 h, respectively.Some physical and chemical indexes of molybdenum powder and tungsten powder as well as their micro-morphologies are shown by SEM in Table 1 and Figure 1 Table 1.Some Physical and Chemical Indexes of Molybdenum Powder and Tungsten Powder.Under the optimum sintering process, Mo-20W and Mo-50W of φ25.0 mm in diameter molybdenum-tungsten alloy bars were prepared by rotary forging at different temperatures, and the forging temperatures were 1200~1280℃, 1280~1360℃℃, 1360~1440℃℃, 1440~1520℃, 1520~1600℃ respectively.The deformation of alloy rods was 90% ~ 91%, and the dimension of molybdenum-tungsten rods was φ8.0 mm.The Mo-W bars were tested by drainage method, weighing method, Jeol JSM-7000F scanning electron microscope, and TP-BX2000 metallographic analyzer.

Effect of different sintering processes on the density of alloy bars
The Mo-W alloy bars with Mo-20W and Mo-50W were sintered at 2100℃ and 8 h, 2150℃ and 8 h, 2200℃ and 8 h, 2250℃ and 8 h, respectively.The density and relative density are shown in Tables 2  and 3 respectively.The formula for calculating the theoretical density of MO-W alloy is as follows: where M1 and M2 are the masses of the two doped metals, and ρ1 and ρ2 are the densities of the two doped metals.
The theoretical density of Mo-20W and MO-50W is 11.28 g/cm 3 and 13.33 g/cm 3 respectively.
From Table 2 and Table 3, it can be seen that with the increase of sintering temperature, the density of the alloy increases first and then decreases.The Mo-20W alloy has the highest density at 2150℃ and 8 h, and the Mo-50W alloy has the highest density at 2200℃ and 8 h.The sintering of Mo-W alloy belongs to binary solid-state sintering.Because Mo-W alloy does not change phase during sintering, we can regard Mo-W binary sintering as unit sintering.The main mechanism of unit system sintering is diffusion and flow, and their relationship with sintering temperature and time is very important.It is generally considered that the sintering process of the powder metallurgy can be divided into three stages, i. e. bonding stage, sintering neck growth and viscoplastic flow stage, closed pore spheroidization and shrinkage stage according to the temperature-time relationship.At the initial stage of sintering, i. e. the bonding stage, the diffusion of atoms on the surface of the alloy particles and the stress caused by surface tension make the particles flow to the contact point, so that the contact gradually expands to the surface, and the porosity correspondingly shrinks; With the increase of sintering temperature, during the period of sintering neck growth and visco-plastic flow, the alloy atoms migrate to the interface of particles, which makes the sintering neck grow and the particle spacing shrink.A continuous network of pores is formed in the alloy [7] .At the stage of closed pore spheroidization and shrinkage, the contact surface grows larger, and the pores continue to shrink and become spherical.The grain boundaries move across the pores as the grains continue to grow.Where the grain boundary is swept away, the pores disappear in great numbers.Finally, a compact sintered body is formed.If the sintering temperature continues to increase, it is easy to cause the densification of the alloy surface when the internal voids in the alloy have not completely disappeared.The results show that there are voids in the alloy and the density decreases.It can be seen from Figure 2 and Figure 3 that with the increase of sintering temperature, the grains of Mo-W alloy grow and the pores between grains shrink.The resulting gap is essentially gone (as shown in Figure 2a, Figure 2b, Figure 3a, Figure 3b, and Figure 3c).The high sintering temperature makes the surface of the alloy densify quickly, but the pores in the alloy can not disappear completely (as shown in Figure 2c, Figure 2d, and Figure 3d).This is consistent with the change of density with different sintering processes.The results show that the optimum sintering process of Mo-20W alloy is 2150℃ and 8 h, and that of Mo-50W alloy is 2200℃ and 8 h.

Effect of different sintering processes on the microstructure of alloy bars
Comparing the microstructure of Mo-20W and Mo-50W under the same sintering process, we can find that the grain size of Mo-W alloy decreases with the increase of W content.This is because the activation energy of W is significantly higher than that of Mo (385 kjꞏmol -1 and 263 kjꞏmol -1 , respectively).The existence of the W atom hinders the grain boundary diffusion and results in grain refinement [8] .

Effect of different rotary forging processes on yield of alloy bars
The Mo-20W and Mo-50W alloy bars prepared by the optimum sintering process are processed by rotary forging.The starting temperatures of rotary forging are 1200~1280℃, 1280~1360℃, 1360~1440℃, 1440~1520℃, and 1520~1600℃.Figure 4 shows the metallographic of the alloy rod at 1440~1520℃ in the rotary forging process, in which (a) is the Mo-20W alloy rod and (b) is the Mo-50W alloy rod.It can be seen that the grains of the alloy bars after rotary forging at 1440~1520℃ appear elongated, and the microstructure appears fibrous along the direction of rotary forging.The microstructure of the other alloy bars after rotary forging treatment is similar to that after rotary forging treatment at 1440~1520℃.  5 that the yield of molybdenum-tungsten bars with different tungsten contents increases with the increase of rotary forging temperature.The finished product rate reaches the highest value about 92%.If we continue to improve the rotary forging process, its yield basically remains unchanged.The yield of Mo-20W reaches the highest value at 1280~1360℃, and that of Mo-50W reaches the highest value at 1440~1520℃.The reason for the low yield is that alloy rod is easy to split during rotary forging.This is because when the alloy is processed by rotary forging, the grain becomes fibrous due to rotary forging tension [9] .This process includes the deformation of the alloy grains themselves and the deformation of the alloy grains due to relative movement.Due to the excessive concentration of deformation in the direction of rotary forging, the flow softening of the alloy in this direction increases.This results in shear failure of the alloy bars and cracking of the alloy along the rotary forging direction [10] .The results show that the spin forging temperature of 1200~1280℃ is too low for Mo-20W alloy.At this temperature, it is not enough to make the alloy deform without cracking, and it is not enough to eliminate the internal stress and improve the plasticity of the alloy.During the process of rotary forging, it will lead to work hardening and cracking.The plasticity and ductility of the alloy rod increase with the increase of the rotary forging temperature.When the rotary forging temperature rises to 1280~1360℃ or above, it is beneficial to reduce the deformation resistance of the alloy, which can make it not easy to crack.For Mo-50W alloy, the rate of the finished product can not be lower than 92% when the rotary forging temperature is 1440~1520℃ or above.4. It can be seen from Table 4 that the mass loss rate of Mo-W bars with different tungsten contents increases with the increase of rotary forging temperature.This is because the mass loss rate in the process of rotary forging is mainly manifested as the fire loss of the alloy.The greater the fire consumption is, the greater the mass loss rate of the alloy is.The rotational forging temperature of molybdenum alloy is generally 1150~1250℃, and the rotational forging temperature of tungsten alloy is generally 1500~1600℃.With the increase of rotary forging temperature, the molybdenum oxide waste slag in molybdenum tungsten alloy increases, and the fire consumption increases.This results in an increase in the alloy mass loss rate.In addition, the cost of rotary forging will increase as the temperature increases.
Combined with the yield, quality loss rate, cost, and other factors of molybdenum and tungsten products with different tungsten content, the best-spinning forging process of Mo-20W is 1280~1360℃, and the best-spinning forging process of Mo-50W is 1440~1520℃.Under the best process, the highest product yield is more than 92%, and the quality loss rate is between 1.05 and 1.10%.

Conclusion (1)
The sintering process has a very important effect on the density and microstructure of molybdenum tungsten alloy.With the increase of sintering temperature, the density of the alloy rod increases first and then decreases.The best sintering process of Mo-20W alloy is 2150℃ and 8 h.The best sintering process of Mo-50W alloy is 2200℃ and 8 h.Under the respective optimal sintering process, the relative density of the alloy rod reaches more than 98%.The microstructure is uniform and fine, and there is basically no cavity.
(2) The grain boundary diffusion activation energy of W is significantly higher than that of Mo.The presence of W atoms hinders grain boundary diffusion and leads to grain refinement.The grain size of the Mo-50W alloy rod is obviously smaller than that of the Mo-20W alloy rod.
(3) With the increase of rotary forging temperature, the yield of the alloy rod increases and reaches the highest value of about 92%.If the rotary forging process continues to improve, the yield of the product remains basically unchanged.The yield of Mo-20W reaches about 92% when the spinning forging process is 1280~1360℃, and the yield of Mo-50W reaches about 92% when the spinning forging process is 1440~1520℃.
(4) As the temperature increases, the mass loss rate increases.Combined with product yield, quality loss rate, cost, and other factors, the best-spinning forging process of Mo-20W and Mo-50W alloys is 1280~1360℃ and 1440~1520℃.

Figure 2 and
Figure 3 show the microstructures of Mo-20W and Mo-50W in different sintering processes.

Figure 4 .Figure 5
Figure 4. Metallographic of alloy bars with 1440~1520℃ rotary forging process.Mo-20W; (b) Mo-50WFigure5shows the change trend of the yield of Mo-20W and Mo-50W alloy bars with different rotary forging processes.It can be seen from Figure5that the yield of molybdenum-tungsten bars with different tungsten contents increases with the increase of rotary forging temperature.The finished product rate reaches the highest value about 92%.If we continue to improve the rotary forging process, its yield basically remains unchanged.The yield of Mo-20W reaches the highest value at 1280~1360℃, and that of Mo-50W reaches the highest value at 1440~1520℃.The reason for the low yield is that alloy rod is easy to split during rotary forging.This is because when the alloy is processed by rotary forging, the grain becomes fibrous due to rotary forging tension[9] .This process includes the deformation of the alloy grains themselves and the deformation of the alloy grains due to relative movement.Due to the excessive concentration of

Figure 5 .
Figure 5.The change trend chart of Mo-20W and Mo-50W alloy bar yield with different rotary forging processes.

3. 4
Effect of different rotary forging processes on the mass loss rate of alloy bars The Mo-20W and Mo-50W alloy bars which were rotated down to φ8.0 mm by different processes were directly weighed and compared with the sintered billets before.The effects of different rotary forging processes on the mass loss rate of alloy bars are studied.The results are shown in Table

Table 2 .
Density and Relative Density of Mo-20W Rod with Different Sintering Processes.

Table 3 .
Density and Relative Density of Mo-50W Rod with Different Sintering Processes.

Table 4 .
The Mass Loss Rates of Alloy Bars with Different Tungsten Contents Under Different