TZM Alloy Boat Bending-Stamping Forming

Sheet bending-forming has been widely used in the aerospace, automotive, electrical appliances, food packaging, household hardware, construction industry and other fields, and it is a very important machining technology. TZM alloy is an excellent molybdenum-based superalloy, and it is often used in aerospace, nuclear power industry, metallurgical industry, electronic industry and metal processing industry and other fields. TZM alloy boat has good high temperature performance, at 1000 ℃ used for containing molybdenum oxide for molybdenum powder production. TZM alloy boat production processes are as follows: mixing-ball-milling - isostatic pressing - sintering - rolling - welding or stamping molding. Considering the production process and product quality, employs bending-stamping on compacted TZM alloy sheet to obtained TZM alloy boat blank with quadrangle. After welding, we can obtain TZM alloy boat which has better performance.

TZM alloy has poor room temperature ductility processing performance and welding performance, so to obtain high performance materials boat, should be strictly controlled bending-stamping forming process.

1. During stamping forming, the seam on circular bead of boat too long or too short is not conducive to final product;

2. The friction coefficient increased from 0 to 0.3, sheet metal equivalent stress strain increases, but not obvious. But when it increased from 0.3 to 0.6, equivalent stress strain shows linearly increase and there are many areas wrinkled and perked.

3. The die gap in the appropriate range for forming effect is not obvious, but too small will cause scratches in sheet extrusion process, and too large it will lead to difficulties in welding.

4. TZM alloy plastic deformation at low-speed stamping is poor, and at high speed and medium speed, the plastic deformation ability is similar. If stamping speed is too low, the sheet metal will rupture, and high speed stamping will lead to overlap of corner angle, so the speed should be controlled within a reasonable range.

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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.

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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Molybdenum and TZM Alloy Heat Treatment after Plastic Deformation

Molybdenum and TZM alloy after processing to a certain extent, that is hardening to a certain extent, should to be treated by annealing treatment making material soften, so that it can continue to process machining deformation, to reduce reparation, breakage and other phenomena during using process. After plastic deformation molybdenum and TZM alloy should go through heat treatment at different temperatures and its structure and properties has different changes.

Heating hardening molybdenum and TZM alloy at a temperature below recrystallization temperature to basic eliminate internal stress and to reduce processing deformation resistance, namely stress annealing. After stress annealing, molybdenum and TZM alloy crystal defects can be certain to eliminate and distortion degree of the grains can be reduced. Besides, the resistance decreased and the internal stress reduced significantly. On the other hand, the grains shows substructure change, which to some extent reduced the distortion degree of the crystal grains. There is recrystallization brittle in molybdenum and its alloys, so during stretching in order to reduce the work hardening should relief portion stress, for plasticity improving and further processing, in which the heat treatment after the plastic deformation can only be stress annealing.

Recrystallization temperature of molybdenum and TZM alloy playing an important role on formulate annealing process and the heating temperature, so working out the recrystallization temperature has great significance. When the deformed material is heated to a certain extent, in the original deformation tissue will re-form a new grain without distortion, and its performance has also undergone dramatic changes, resulting in complete recovery, this process is called recrystallization. For pure molybdenum, the greater the deformation degree is, the lower the recrystallization temperature. Added trace metals into molybdenum can produce TZM alloy, which has higher recrystallization temperature than pure molybdenum. In general, the longer the heating time, the slower the heating rate, the recrystallization temperature is lower. The finer original grain before annealing, the recrystallization temperature is lower.

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TZM Alloy and Other Molybdenum Alloys

Pure molybdenum room temperature brittle and cold-brittle transition temperature is high, which greatly limits its processing and applications. In order to expand the application range of molybdenum material, manufacturers usually add a small amount of alloying elements into molybdenum to obtain molybdenum alloy. Molybdenum alloy not only improves the toughness of molybdenum and reduce its plastic-brittle transition temperature, but also can expand its application range. Molybdenum alloys can be classified by strengthening type and alloying elements. The following mainly describes the classification by alloying elements, which is binary molybdenum alloy, multi-element molybdenum alloy, doped molybdenum alloys, rare earth molybdenum alloy and so on.

Binary molybdenum alloy means adding single alloy elements into molybdenum to form molybdenum alloy. Adding tungsten and rhenium in molybdenum, this molybdenum alloy has large alloying elements added quantity and the hardness and heat resistant of alloys is excellent. The molybdenum alloy which added with titanium, zirconium, hafnium and other alloy elements has small alloying elements adding quantity.

Multi-element molybdenum alloy, there are two or more alloying elements added into molybdenum. Multi-element molybdenum alloy has a variety of categories, such as Mo-Ti-Zr-C series, which is an alloy added with a certain amount of zirconium and carbon on the basis of Mo-0.5Ti, including TZC and TZM. Mo-W-Hf-C series and the Mo-W-Hf-Zr-C series have good overall performance, but the preparation process is not mature, including M25WH1, M25WH, H2WM25, and M25WZH1 and so on. Mo-Hf-Zr-O series according to different hafnium doping quantity are divided into different type, including ZHM, ZHM4, ZHM7, and ZHM8.

Doped molybdenum alloy usually doped silicon, aluminum, potassium and other trace elements into molybdenum, which has better high temperature properties. Recrystallization temperature of this it can reach 1750 ℃, and after recrystallization it has some strength and ductility.

Rare earth molybdenum alloy has stable chemical structure and good strengthening effect, which has wide application range.

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Molybdenum and TZM Alloy Plastic Processing

When stress of the metal exceeds the elastic limit, there will have plastic deformation occurs. Adjacent atomic planes have relative tangential motion. Besides, when the shear stress is large enough, the atomic will from the equilibrium position go into another equilibrium position. But when the external force disappear the atomic planes which after tangential motion can not move back to the original position, thus producing irreversible permanent deformation, that is, plastic processing.

Deformation resistance refers to a kind of ability for resistance plastic deformation. The metal material in order to maintain the original state, there will produce deformation resistance to resist deformation at certain of temperature, speed and deformation degree. From the microscopic point, the external forces make the distance between atoms to change resulting in an imbalance of attraction and repulsion. Molybdenum and TZM alloy in the rolling process will produce deformation resistance, and the main factors which influence the deformation resistance is material chemical composition, metallurgical properties, geometry, friction of deformation outer zone, material work hardening and the deformation zone of thermodynamic parameters.

Molybdenum and TZM alloy plastic processing method mainly includes rolling, extrusion, drawing, stamping, forging, die forging and the equipments include used include rolling mill, extrusion machines, stretching machines, punching machine, forging machines and rolling machine and so on. Molybdenum and TZM alloy only through plastic processing can obtain the desired shape such as wire, strip, rods, tubes and other products with customized shapes and can apply to different areas as well. The main plastic processing method of TZM or molybdenum rod is rolling and model rolling. On the other hand, the wire and tubes usually through stretching machine to produce, and the products of sheet, strip and foil mainly through rolling method.

Using rolling method for material plastic processing mainly puts metal blank in the gap of a pair of rotating rollers (the desired shape), through the compression of rotating rollers to decrease cross-section and to increase the length of the material, then to obtain high quality products. Rolling method is mainly used for the production of bar, sheet and tube. Using rolling mill can obtain good quality product, having low metal burning rate, high productivity, high degree of mechanization, and therefore it is widely used. Rolling can be divided into cold rolling and hot rolling.

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Molybdenum and TZM Alloy Plastic Processing Technology

To rational design the plastic processing technology can effectively change the state structure of molybdenum and TZM alloy to improve its performance, especially improved brittleness. In molybdenum and TZM alloy plastic processing, deformation temperature, deformation degree and heat treatment temperature are important parameters affecting its structural state. During plastic processing there is must go through three temperature stages, which are hot working, warm working and cold working.

There are two ways of thermal working. One is processing above recrystallization temperature with large rolling reduction for cogging, and through hot forging, rolling and extrusion can obtain dense structure equiaxed coarse-grained organization, having low dislocation density and internal stress. The second is processing at proximity to or on the recrystallization temperature, through controlling dynamic recrystallization plastic deformation, so the deformation and recovery, recrystallization can reach dynamic equilibrium, so as to obtain significant fine grain structure or sub-grain organization and fine equiaxed grains under high temperatures recovered state. In the hot deformation process, strengthening and softening to achieve a balance, the temperature and deformation degree can control grain size, so in thermal processing is impossible to form the dislocation cellular tissue.

Warm working is processing at a temperature above the plastic - brittle transition temperature, but below the recrystallization temperature. Warm working, it is an important processing step, generally intermediate processing step, but also can be finished product procedure. Through warm processing can obtain sub-structure, so that molybdenum and TZM alloy having high strength and toughness.

Cold working is unlike general room temperature processing, which is processing above plastic - brittle transition temperature, but below t * characteristic temperature. Cold working at t * temperature the crystal lattice Peine strength and yield strength have been significantly improved and the dislocation density have been increased. With the increase of deformation, dislocation chaotically distributed on dislocation forest way, or formed dislocation tangles deformed cells. In the cold working process should follow with stress annealing, so that the process can smoothly.

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