Physical Basis of Molybdenum and TZM Alloy Plastic Working

Before molybdenum and TZM alloy plastic should a certain understanding of its brittleness, toughness, fracture behavior and other physical properties, to understand the physical basis of the material, in order to better carry out plastic working. The plasticity of material refers to the deformation degree before breaking. The strength refers to a kind of ability to resist deformation and fracture. And toughness is the ability to absorb energy from plastic deformation to fracture of the whole process. Molybdenum and TZM alloy has high strength, but the plastic deformation is poor, namely poor toughness, having obvious brittleness.

The brittleness and toughness of material will changes as the temperature changes, namely, plastic-brittle transition temperature (DBTT). In the above DBTT temperature range, under high stress the plastic deformation is more smoothly, and the resulting products showed good toughness. Conversely, at temperatures below the DBTT to process deformation is prone to produce brittle fracture with different forms. Different metal materials, the plastic-brittle transition temperature is different, tungsten plastic-brittle transition temperature is generally about 400 ℃ and molybdenum DBTT is near the room temperature. High plastic-brittle transition temperature, the brittle of material is high which is not conducive to material processing. In order to reduce the plastic-brittle transition temperature of the material the measure is to overcome the brittleness and increase the toughness. The effect factors of material’s plastic-brittle transition temperature include purity, grain size, deformation degree, stress and alloying element material.

TZM alloy image

Molybdenum and its alloys low (or room) recrystallization brittleness is unlike copper, aluminum. After recrystallization annealed, copper and aluminum will form equiaxial recrystallization grain structure has excellent room temperature plastic processing, which can easily be processed at room temperature. However, molybdenum and TZM alloy after recrystallization has high brittleness at room temperature, so during the processing and using is prone to brittle fracture.

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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.

TZM image

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.

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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:

TZM image

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%.

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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.

TZM ALLOY PICTURE

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 ℃.

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Using Brazing Titanium Graphite and TZM Alloy to Produce Joint

Graphite is a low atomic weight materials, having low loudness density, high sublimation point (3850 ℃), high thermal conductivity, heat capacity and excellent impact resistance and other advantages. To produce a new type alloy material and this alloy is maked up by graphite W and Mo. As we know, graphite has good thermal performance and low-density advantages and other advantages. So doped with graphite into Mo or W to produce new type alloy is good for heat dissipation, but at the same time will reduce products’ quality, especially in some of the high-speed rotating components. Doped Ti, Zr and other trace elements in Mo, after powder metallurgy process and alloying process can produce Mo alloy, which is TZM alloying. And TZM alloy has good mechanical property, thermal physical property, thermal and electrical conductivity which is higher than other insoluble metal. Using TZM alloy and graphite to produce composite joints obtained by braze welding has widely applications in the aerospace field. Vacuum brazing is the preferred process for preparing high temperature composite joints. China domestic current has little research in this area. Smid, who studied the Mo-graphite composite materials for NET / ITER nuclear reactor to obtain joint which can use in 800 ~ 1200 ℃. Chan, who use 72Ag-28Cu and 95Ag-5Al brazing filer metal for Ti-6Al-4V and TZM alloy brazing, and the brazing temperature is 850 ℃ and 950 ℃. Besides, the weld of joint is good.

TZM ALLOY PICTURE

TZM alloy composition is (mass fraction, %) 99.4Mo-0.47Zr-0.1Ti, the rest is other trace elements. And the diameter is 100mm. After powder metallurgy sintering and 1400 ℃ forging, upsetting annealing can obtain TZM alloy. Graphite has high strength, high density, high purity, and its outer diameter is greater than 100mm. TZM alloy and Ti foil washed by ultrasonic with acetone, is placed in vacuum hot pressing brazing furnace sequentially. Chose titanium foil as brazing filler metal, and titanium foil thickness is 0.05mm and diameter is 100mm.

After brazing, under optical microscope and stereo microscope specimens found the joint is densification and has uniform width. The joint observed by SEM and microanalysis component analysis (EDX) showed that the brazing layer width is about 120um and complete penetration rate is above 95%, which has clearly two-layered structure. Graphite matrix is ​​loose, the brazing layer closing to graphite substrate side has deeper color. This is interface reaction layer produced by graphite substrate and brazing filler metal chemical reaction. This layer connects with graphite, so it is relatively smooth. Besides, there are a small number of cracks and voids. And the thickness of reaction layer occupies 1/3 of the entire brazing layer. Analyzed by EDS, the average component of mixed layer is (mole fraction, %) 46.22C-53.78Ti. And the mix layer is mixed by high melting point TiC and rest of brazing filler metal which is produced by interfacial reaction. The brazing layer closing to TZM matrix the color is lighter. This brazing layer is Ti-Mo solid solution, which accounted for 2/3 of the entire solder layer. Combined with TZM substrate, the structure is smooth, and there are knife-like carbide to grow up, improved joint performance. The EDS average component (mole fraction, %) is 51.48Ti-48.16Mo.

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Brazing Time and Temperature Affected Joints of Brazing Titanium Graphite and TZM Alloy

When the brazing filer metal is selected, process parameters of brazing process is the main reason to impact joint’s organization and performance. Use titanium alloy as brazing filer metal. The temperature of heat preservation, time of heat preservation and quantity of brazing filer metal will affect structure and properties of joint, and this joint is made up by graphite and TZM alloy braze welding. To analysis the mechanism and optimize the process parameters, making joint using temperature increases to 1400 ℃, and joint re-melting temperature is higher than 1600 ℃. Besides, the joint strength is about 141 ~ 150MPa.

Brazing process significantly affects the property and performance of joint. Brazing temperature and time determine the thickness and composition of the brazing layer. Combined with Fick law of diffusion and the research of ceramic and metal activity brazing found in certain reaction system the reaction layer thickness depends on the connection time and temperature. With the increase of the brazing time, temperature increasing, the thickness of the reaction layer is increase, till saturation. If the reaction temperature is low, or the time is short, and the reaction layer is not continuous. Besides, the wetting effect of brazing filler metal on graphite base material is insufficiency. What’s more, the interface bonding force is too small. Analyzed interface reaction layer’s thickness and morphology by SEM found within a certain range, the higher the temperature, the longer the time, the TiC reaction layer is more wider. However, the interface reaction does not increase with time and temperature without limit, when the reaction layer thickness reaches a certain extent will produce dense carbides to separate Ti and graphite. Ti difficult to penetrate or diffuse through the carbide layer to react with graphite, so long temperature preservation can not be significantly improved reaction layer thickness of the joint.

TZM ALLOY PICTURE

In addition, the brazing temperature and time also determines the thickness of the Ti-Mo solid solution layer. The weld of brazing and TZM substrate through diffusion reaction can change the composition of brazing filler metal to form a solid solution, and after isothermal solidification can obtain solid solution. Analyze the connection of TZM by TLP (transient liquid phase) diffusion mechanism. After brazing filler metal was melted, Mo of TZM element through the interface of brazing filler metal dissolves in liquid the brazing filler metal, and uniformly, resulting in solid solution melting point increase, solidification occurs in the brazing temperature. When the brazing filler metal thickness is 0.1mm, at different temperatures between 1700 ~ 1750 ℃, the thickness of the Ti-Mo layer has some differences. Showing from non-steady-state diffusion equation, the brazing temperature and time determines the diffusion amount of Mo to brazing filler metal. After solidification can form Ti-Mo solid solution layer with different thickness and composition. Brazing temperature and time optimized can produce interface reaction layer with appropriate thickness and the solid solution layer. Interface reaction layer of 30 ~ 40um is of TiC, and the joint is made up by 70 ~ 80um Ti-Mo solid solution layer has better performance. Further improve the brazing temperature and time to produce thicker interfacial reaction layer will weaken performance of joint and result in the TZM matrix grains to grow, reducing the mechanical properties.


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