New Hybrid Molybdenum Powder Used For TZM Alloy Manufacturing

In industry, manufacturers usually add alloy elements and carbon for TZM alloy manufacturing. However, this method has its shortcomings, which is prone to melting and segregation during melting, to form grain boundary segregation, so alloy's fragility and recrystallization temperature will increase. Especially in the case of Ti presence in the alloy, due to Ti high vapor pressure, making it difficult to control the final content in the alloy, so the producer usually only by virtue of the adjustment smelting parameters and alloying elements doped quantity.

The new hybrid molybdenum powder is conductive to produce powder mixture in metallurgy technique or it can adjust alloy’s structure during production, so it is possible to reduce the brittleness of the alloy. Using impregnation and carbon nitride, with the corresponding method of sintering and smelting process, and can be produced TZM alloy having a low N2 content.

Using new hybrid molybdenum powder for TZM alloy manicuring:
1. A certain percentage of TiC, TiN and ZrN is added to TZM powder mixture, and using the mill to grind them into powder particles less than 5um.
2. In the mixer, 31% TiC, 48% TiN, 14% ZrN and 7% of fine carbon black is prepared a premix. Then 15% premix and 85% MoMP is slowly added to the mixer, mixed for one hour to prepare a mixture. The mixture is used as the parent group for the final 1:20 of TZM powder mixture production.
3. 5% mixture and 95% MoMP was placed in a mixer and mixed for 15 minutes, then after -150um sieved to obtain new hybrid molybdenum powder for TZM alloy manufaturing.
4. Using powder metallurgy method produce sintered blank, then after subsequent processing to produce TZM alloy.

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TZM Alloy Strengthening Ways

TZM alloy has three strengthening mechanism, which is solid solution strengthening and second phase strengthening caused by titanium, zirconium, carbon and deformation strengthening caused by subsequent processing of produce. However, there are other strengthening ways, such as adding rhenium, carbide, rare earth oxides, staple fibers and neutron radiation and so on. These methods can improve alloy's properties.

Rhenium Elements Strengthening

Doped alloying elements in TZM alloy will great improvement molybdenum brittleness and improve the performance of the alloy. Besides, doped rhenium in the alloy, not only can improve the high temperature properties of the alloy, and can significantly improve the properties of the alloy at room temperature. Rhenium occur solid solution reaction with the molybdenum substrate to form toughness and dense solid solution crystal structure. During bending fracture, molybdenum-rhenium solid solution is mainly transgranular fracture. And molybdenum-rhenium solid solution absorbs a lot of breaking energy can effectively improve the properties of the alloy at room temperature. However doped rhenium in alloys can inhibit titanium, zirconium solid solution in molybdenum, resulting in a large number of pores and a second phase particles gathering in rhenium-molybdenum alloy grain boundaries, which have an adverse effect on the strength of the alloy.

Rare Earth Oxide Strengthening

Doped with rare earth oxides in TZM alloys is in favor of grain refinement, and with doped quantity of rare earth oxides increase, the grain refining effect is more obvious. The common rare earth oxide is La2O3. Doped a certain amount of La2O3 particles in the alloy, these particles will also evenly distributed in the grain boundary and grain, so that the toughness of TZM alloy has been significantly strengthened.

Neutron radiation strengthening

In the range of 25 ~ 450 ℃ TZM alloy after neutron irradiation, the elongation sharp declines, but the breaking strength increases with test temperature increases. In addition, the alloy at 700 ℃ neutron radiation, a significant increase in yield strength, but the temperature exceeds 700 ℃, its strength did not change significantly.

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TZM Alloy Manufacturing - Raw Materials Preparation

In TZM alloy manufacturing, to prepare high quality raw materials has great significance for obtaining high-strength sintered alloy blank. Fine powder particles can reduce the sintering temperature, but also easy reunion, is not conducive to mix powder. At the same time, the powder impurities as low as possible, which can improve the density and strength of the alloy, to ensure that composition of the alloy is not out of control. Thus, molybdenum powder particle size is about 2.0 ~ 3.5um and purity is 99.95%, which is most conducive to producing high strength TZM alloy.

Heating TiH2 and ZrH2 powder, Ti and Zr will break down form TiH2 and ZrH2 and add to Mo powder. Some Ti, Zr will react with matrix to form solid solution strengthening. Another Ti, Zr, will react with O, C to form carbide which is second phase and to produce second phase strengthening. Ti, Zr added by chemical reaction, so the size of Ti and Zr has reached the atomic or molecular level. The minimum fee's size of TiH2, ZrH2 can be purchased on the market is smaller than 10um and purity is about 96%.

During mixing powder, the lanthanum nitrate solution and stearic acid solution was added into mixed powder, instead of La2O3 and graphite powder, because solution can make powder mixed evenly. On the other hand, lanthanum nitrate can produce La2O3 during the sintering process, and stearic (C18H36O2) acid can be decomposed into C elements. These elements are produced by reaction, so that more uniform mixed in powder and finer, which is conducive to form solid solution strengthening and second phase strengthening. Further, stearic acid can act as a lubricant and release agent during pressing.

During mixing, using ethanol as a solid - liquid mixed media, can prevent and reduce powder oxidation. What’s more, stearic acid is insoluble in water, but it is soluble in alcohol.

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TZM Alloy Boat

TZM alloy has many good properties, such as high temperature resistance, high strength, high melting point, high elastic modulus, low linear expansion coefficient, good corrosion resistance and good thermal conductivity, so it is widely used in various fields. TZM alloy is also often prepared to a boat for molybdenum powder production. As we knew, molybdenum powder should produce at high temperature environment, which is at least 1000 ℃. The boat used for containing molybdenum powder should have good high temperature resistance, not easily deformed, and at high temperature there is not oxidation reaction to produce impurities.

TZM alloy not only meet the above requirements, and using TZM alloy to make bout for molybdenum powder production can improvement the strength of powder, and also does not increase the impurity content of molybdenum powder. Titanium zirconium molybdenum alloy is made in a boat usually using a combination method, which is to mix stamping method with welding method. TZM alloy boat production processes are as following:

1. To obtain TZM alloy plates by vacuum furnace melting method or a powder metallurgy method.  The alloy boat molding temperature is about 600 ℃ and heating type is isothermal heating.
2. Select the GTAW welding process to weld TZM alloy. When the welding current at 210A, welding speed is 4mm/s and the argon gas flow rate is 8-12L / min, the weld of TZM is the best;

3. TZM alloy will oxidation at high temperatures, so during process, should do some oxidation resistance treatment. At 0 ~ 400 ℃, the oxidation rate of the alloy is slow and the surface will generate volatile MoO2. At 400 ~ 750 ℃, the alloy is rapidly oxidized and the surface will generate volatile MoO3. At 800-1000 ℃, alloy quality significantly fewer, because MnO3 is  evaporation.

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TZM Alloy Mechanical Property Influence Factors

There are many influence factors affecting TZM alloy's mechanical property, including production method, deformation processing, annealing temperature and neutron radiation treatment.

Using different production method to produce TZM alloy the mechanical property of alloy is vary. Using arc melting method and powder metallurgy method to produce TZM alloy the high temperature property and mechanical property has great improved, which is better than pure molybdenum’s. When the temperature is lower than 1000 ℃, the performance of the two methods obtained TZM sheet is considerably, but using arc melting method obtained TZM bars strength is better than powder metallurgy obtained. When the temperature is higher than 1200 ℃, powder metallurgy method produced TZM plates and bars performance is equivalent, but they poorer than using arc melting method produced.

TZM alloy after hot-forging and extrusion process, the elongation at room temperature up to 33%. Compared with the un-deformation TZM alloy the elongation has been greatly improved, indicating that proper treatment can improve the plastic deformation of the alloy. In addition, the annealing process can also affect the mechanical property and structure of the alloy. As the annealing temperature increase, TZM alloy’s hardness and tensile strength decreased, plastic increased. TZM alloy after neutron irradiation, strength and hardness significantly increased, while the elongation decreased sharply, but fracture toughness decreases to a lesser extent.

In addition, the strain rate has a greater influence on the TZM alloy’s mechanical property. At 400 ℃, the tensile strength of the alloy substantially does not vary with strain rate changes. At room temperature and 1000 ℃, the strength of the alloy with the change of strain rate were increased 30% and 20% separately. At 1200 ℃, the strain rate increases, the intensity of TZM alloy is increased 80%. At 1400 ℃, the strength of the alloy increased from 20MPa to 135MPa. It is shows that when the temperature is below 1000 ℃, strain rate has little effect on the alloy ‘s strength. When the temperature is higher than 1200 ℃, with temperature increasing, strain rate influence on the strength is more obvious.

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TZM Alloy Deoxidation Mechanism Analysis during Vacuum Sintering

The experiment found that TZM alloy during powder metallurgy sintering process mainly has two kinds of deoxidation mechanism: the first one, metal oxide of carbon reduction system to form metal carbides and CO; the second one, MoO2 happening disproportionation reaction at vacuum high temperature to form metal Mo and MoO3 gas, where the MoO3 gas will be discharged by vacuum system.

Firstly, analysis the influence of carbon content on TZM alloy deoxidation during vacuum sintering. Experiment arrangement as follows: adding 0.5% TiHx and 0.09% ZrHx into molybdenum powder, and then dividing the sample into three parts, in the sample were added 0.04%, 0.07%, 0.10% different proportions of carbon, and finally made TZM alloy rods. Tested oxygen content and the carbon content of TZM alloy bar found C element content and TZM alloy deoxidation effect is proportional to, but excess carbon can cause alloy component failure.

In addition, the oxygenium of molybdenum mixed powder mainly decisions by oxygen content of molybdenum powder. Molybdenum powder after the reduction reaction, the mainly existing way of oxygen is molybdenum oxide, which the most stable is MoO3 and MoO2, and MoO3 in a vacuum and high temperature will volatilize into a gas, so at high temperature molybdenum oxide mainly exists by MoO2 form. The experiment found that in the case of high temperatures and the absence of air, MoO2 happen disproportionation reaction to produce metal Mo and gaseous MoO3, and gaseous MoO3 will be discharged by vacuum system.

TZM alloy deoxygenation process mainly achieved by carbon reaction and disproportionation reaction of MoO2 which occurs in high-temperature vacuum sintering process. The reaction temperature and air pressure of product in vacuum furnace will have great impact on the quality of these two reactions. Reducing the partial pressure of product and improving vacuum degree can reduce the deoxygenation reaction temperature of the beginning which is good for reducing oxygen content of TZM alloy.

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TZM Alloy TIG Welding

TZM alloy because of its good high-temperature properties are widely used in high temperature mold industry, aerospace, industrial machinery and other industries. TZM alloy as a structural material used in these areas typically require the desired shape and configuration which can achieved by welding skills. The experiment found that, in order to prevent TZM oxidation or inhaled nitrogen and other impurities in the welding process, a better method is TIG welding. Using TIG welding, it can make TZM alloy during welding process protect by inert gas (argon) to cut off the air and to prevent oxidation or nitrogen elements intrusion. The experiment found that the optimum welding parameters are as follows: welding speed 4mm / s; argon gas flow: 10L / min; welding current: 210A.

TZM alloy after TIG welding to do seam X-ray inspection found that when the welding current is 210A, the weld is flat and there is no holes and cracks on TZM surface, smooth appearance on the alloy. Observing the microstructure of welded joint found, the weld zone has coarse grains and there are some small holes, but weld microstructure is dense and no cracks. On the other hand, TZM alloy coarse grained zone and the transition zone affected by the heat of the welding process to virtually eliminate the original structure and to formed a relatively coarse equiaxed crystalline, while the original base material still maintaining crystalline sheet organizational structure.

Observing its tensile strength found that the average tensile strength of welding is significantly less than the original base material. The increase of the elongation of the alloy is due to the thermal effect of the welding process to eliminate the portion of fiber texture which is formed by rolling, so that the edge of the weld is "softening." Observing the tensile fracture of TZM alloy after welding found the welding coarse grained area is not lamellar crystals, but still showing intergranular brittle fracture morphology. Coarse equiaxed grain replaced lamellar crystals, thus the weak strength of the material, but at some certain extent, improving the plastic of material can basically meet the requirements of ordinary structural.

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Ferro Molybdenum Price in China - Feb. 02, 2018

Trading in the international molybdenum market is thin owing to the weak demand and the price of ferro molybdenum and molybdenum oxide rises slightly. In China, the whole market is quiet and price maintains weak adjustment.

For molybdenum concentrate, with the coming of Chinese Spring Festival, miners not only reduce the production, but also are reluctant to sell, which support the high price of raw materials. Most miners are mainly producing according to previous orders.

The price and quantity of steel biddings fail to meet the market expectations, and thus ferro molybdenum price shows signs of decline. Steel mills’ plans for stocking up are almost completed, so manufacturers remain low willingness in purchasing, the market trading tends to be quiet.

Supported by high raw material price, molybdenum chemical and manufacturing enterprises raise product quotations and have not any plan to sell. Terminal customers are hard to buy low-price products, so the enquiry becomes less.

Prices of molybdenum products on Feb. 02, 2018

Picture of ferro molybdenum

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