High-energy High-speed Consolidation for TZM Alloy Manufacturing

Using hot isostatic for TZM mixed powder pressing due to time temperature deviation large, the mixed powder can be not fully densifying. And it will affect alloy’s mechanical properties. Using high-energy high-speed consolidation technology to replace the hot isostatic for mixed powder pressing, it is possible to produce high-density material and improve alloy’s strength. In this production process, the powder is pressed between the electrodes by means of high voltage which produced by unipolar generator to press the powder. When the voltage is about 25 to 5 volts, the unipolar generator can provide 100-150A/mm2 current density. Using high-energy high-speed consolidation method for TZM alloy powder pressing and the production process is as following:

1. TZM alloy powder was mixed evenly. And in TZM mixed powder the Ti content is 0.47%, Zr 0.109%, carbon 0.018%, oxygen 0.0016%, and average particle size is 74um.

2. The powder was placed in between the electrodes, and the current is about 10 megajoules to make the mixed solidified. During discharge, the copper electrode should to cover with a 250 microns molybdenum foil layer, or directly using tungsten electrodes. Besides, the weight of the powder is between 59 ~ 150g and molding pressure is between 270 to 690 MPa in the regulation.

Using high-energy high-speed consolidation for powder pressing, the powder has high density, moderate strength, low toughness and appropriate structural changes. After sintered, TZM alloy structure is unevenly, mainly because of copper electrode conduction cooling, so the copper electrode will affect the temperature of the powder.

With different pressures and unit input power, the midplane density of alloys is different. When the consolidation energy is higher than 3000 J/g, the densification is much larger than 98%. When consolidation energy is about 270-890 MPa can little affect the results.

When the local density is greater than 96%, the alloy shows good mechanical properties. In the sintering process, the alloy exhibits low plasticity, mainly because of the presence of excess oxygen. However, the oxygen content is too small Mo also suffers severe embrittlement.

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.

Using Powder Metallurgy Method for TZM Alloy Plate Manufacturing

TZM alloy is the most used widely and the best performance molybdenum alloy. TZM alloy has good high temperature strength, creep resistance and higher recrystallization temperature, so it is often made of nozzle, piercing point, mold, heat shields and high-power ceramic tube gates, widely used in industry, military and aerospace fields. TZM alloy production methods are melting and powder metallurgy. Currently most of manufacturers choose to use powder metallurgy method, mainly because it doesn’t need have consumable arc furnace, large presses and high-temperature furnace and other large equipment, and the process is simpler. Besides, using powder metallurgy for TZM alloy plate manufacturing the production cycle is short and energy consumption is low, having high productivity. In addition, the alloy which produced by powder metallurgy has similar properties with smelting method produced alloy. To obtain good system performance alloy plate should improve alloy’s recrystallization temperature and ductility, and reduce plastic-brittle transition temperature, so in the process of rolling should roll in changed directions and process intermediate heat treatment, making the alloy performance has improved. TZM alloy plate produce by powder metallurgy method and the production processes are as following:

1. Mixing 0.45% Ti, 0.08% Zr, 0.01% C and graphite powder with Mo for 6 hours is good for full mixing.
2. Using cold isostatic to press the mixed powder at 150MPa pressure obtain pressed blank.
3. Under the protection of hydrogen, the pressed blank is placed in sintering furnace at 2100 ℃ for 4 hours heat preservation and then to sinter to get TZM alloy blank.
4. Roll the 30mm blank at 1350℃, making the slab thickness to become 4.5mm, and the deformation is 83%. Then at 700 ~ 750 ℃, the slab is rolled to 1.2mm, and the total deformation is 95%.
5. Anneal at 900 ℃to eliminate stress, then at 600 ~ 700 ℃ longitudinal rolling to obtain 0.7mm.
6. After annealing at 850 ℃ to eliminate stress, then longitudinal cold rolling at 200 ~ 300 ℃ obtain 0.5mm alloy plate, and the total deformation is 98%.

Powder Metallurgy Produced TZM Alloy Plate Properties Analysis

Modern TZM alloy industrial production usually uses powder metallurgy method. In order to improve the properties of the alloy, should process rolling in changed direction and intermediate heat treatment, which not only can eliminate the stress of the alloy, but also can improve its mechanical properties.

Analyze the mechanical properties and process performance tests on TZM alloy, which produces by powder metallurgy. TZM alloy plate blank cold rolling at 45°orientation has a certain degree tensile property. After annealing at 850 ℃, the plate in all directions at room temperature has tensile strength.

With incomplete pole figure method estimates the orientation distribution function (ODP) of 0.5mm powder metallurgy produced TZM alloy plate in the processing state and eliminate stress state (850 ℃/h) showing that the states of the alloy plate blank will affect alloy plate’ texture And the states and orientation of plate blank’s affected the mechanical properties at room temperature of alloy plate is connected with texture type of plate blank.

Alloy state and orientation will great impact curved plastic-brittle transition temperature. When alloy at cold state, the bending performance has better direction. After 850 ℃ annealed, the plastic - brittle transition temperature is decreased, and the lateral amplitude transformation temperature is lowered significantly. After eliminated stress, in all directions of alloy plate transition temperature is similar and below 0 ℃, so the alloy plate is preferably used after stress relief.

After complete recrystallization, there is not elongation on alloy plate. And the curved plastic-brittle transition temperature is increased to above room temperature. So we should avoid use alloy plate above the recrystallization temperature. Alloy plate starting recrystallization temperature is 1200 ℃ and the ending recrystallization temperature is 1600 ℃.