Heating and heating is a key link for powder forging, must ensure that the preform in the heating without oxidation, no decarburization or carburizing, heating should be uniform, the temperature is appropriate.
In order to ensure the quality of heating, heating usually take protective atmosphere (decomposition of ammonia, hydrogen, gas, nitrogen, etc.) or spray a layer of graphite or non-graphite-based protective layer. Heating speed should not be too fast or too slow,electric cookware press otherwise it will cause uneven heating or internal organization grain and other consequences. Usually in the furnace temperature rose to the specified temperature, the cross-sectional size should be 0.5 ~ 1mm / min heating rate to determine the holding time, heating temperature should ensure that the preform in the forging has a good, generally by forging temperature and then increase 50 ~ 100 ℃ To determine the heating temperature.
Powder forging There are two main heating methods: induction heating and furnace heating. Induction heating can be quickly and quickly loaded for simple parts with symmetrical shapes, such as the clutch of the automatic transmission and the clutch ring of the stator. The furnace heating can control the temperature better, use the controllable atmosphere to protect the parts, , Complex parts that are suitable for asymmetrical shapes, such as synchronizing rings for engine links and gears.
The choice of powder metallurgy forging equipment is essential in the whole process, related to the success or failure of forging. At present, there is no general way to choose the powder forging equipment, can only consider the following aspects of the characteristics of choice.
Equipment impact force and impact energy powder forging In order to obtain the desired shape and meet the requirements of the high density of the workpiece, must have enough to combat energy, or forgings the sharp corners of the parts will appear full of dissatisfaction or microcracks, resulting in forging failure. Therefore, the choice of forging equipment must first be a comprehensive analysis of the forging process and the equipment of the force and energy characteristics, so that the selected equipment force and energy characteristics in line with process requirements.
The blow energy produced by the equipment is used for the deformation of the blank and the other is used to overcome the frictional force between the preform and the mold. In order to obtain optimum forging performance, it is advantageous and sometimes necessary to change the shape of the preform. The combination of the high density of the forging and the particles requires a large hydrostatic stress component, which has a very important effect when forging a fully dense workpiece with excellent machinability.
In the final stage of densification (relative density> 0.9), the required pressure is approximately : P / σf = (2/3) ln [1 / (1-D) where P is hydrostatic stress; σf For the yield stress; D for the relative density. The size of the forging equipment is limited by the energy, and the forging hammer is graded according to the impact energy. The impact can be determined by the quality and speed of the forging hammer, which is usually converted to blank deformation energy Wd during the forging, The elastic deformation energy Ee and the kinetic energy of the forging hammer Ea. That is, Er = Wd + Ee + Ea, where Ea is approximately 0.05Er, and most of the blows can be seen as the elastic deformation energy of the machine.
Forging Strike Speed and Pressure Hold Time During powder forging, the plastic deformation of the powder particles can cause compression of the preform. The plasticity increases with the increase of the strike speed. Under the same degree of deformation, the density of the forging increases with the increase of the strike speed, and the greater the degree of deformation, the faster the density increases . If the surface of the preform is too cold due to the forging time, the yield stress of the powder will rise rapidly and the deformation of the particles will be suppressed, resulting in forging failure.
The porosity of the surface area of the powder forging is affected by the deformation rate and the contact time between the preform and the relatively cool mold. Meanwhile, the pressure holding time is also an important factor in determining the forging density. Different forging equipment in the forging hammer speed is very different, resulting in the same process conditions forging the same preform billet deformation rate of different. The table lists the four kinds of equipment in forging the same preform blank when the deformation rate of the blank and equipment pressure to maintain the time comparison. From the table can be seen, the equipment to reduce the holding time, blank deformation rate increased; forging near the surface area of the forging hammer more advantages; for thin and thin powder metallurgy forgings, the pressure to keep the shorter the equipment The more advantages.
The applicability of automated production To achieve automatic forging of powder preforms, forging equipment must have precise forging hammer guide mechanism, part stripping device, precise impact control and high yield and so on.
Improve the production can be considered from the following aspects: select the equipment to combat speed, as far as possible forging equipment and heating equipment between the transport distance and time, select each blow when the preform and machine tool contact time is short.
Environmental protection In the current environment of environmental protection in the world, the environmental protection of equipment becomes more and more important. The equipment selected for powder forging process should be able to minimize noise. Compared with the traditional forging, powder forging equipment in reducing noise, the solution is more feasible. Powder forging for the middle of the strike, can reduce the structural parts of the sound excitation force; in the automatic powder forging the operator and equipment separation, through the sound barrier to reduce noise.
Due to the economic advantages and the effective use of materials, the application of powder forgings has been expanding. At present, the production of powder forging parts are mainly used in automotive transmissions, engines and differentials. Some of the powder forgings produced and used in industry are: engine connecting rods and seats, stator lugs for transmissions, rings of tapered roller bearings, various gears, clutch hubs for torque converters, and so on.
After nearly 40 years of continuous development, powder forging has developed into an advanced metal processing technology, adding many new applications. Its development has tended to use the powder forging process more and more in the initial design of the parts. The main reasons for this trend are: (1) in the long-term high pressure fatigue applications, the use of records to prove that the performance of powder forgings is reliable. (2) net forming or near net forming capacity, unique metallurgical properties and combined with the heat treatment process, reducing the total manufacturing costs. (3) powder forging avoids a lot of capital investment compared to conventional processing methods. (4) powder forging material performance and process parameters have been widely recognized.
The application of the advanced CAD method in the process allows the technician to design the optimal preform in a very short time, and in the case of considering the production line, the expected performance of the part, and the final cost, The feasibility and the required equipment, the stress of the blank situation. Using the finite element model to identify and reduce the high stress area of the blank; pre-select the process parameters such as workpiece quenching oil flow. And the calculated values can be compared with the actual value, such as calculating the stress and the photoelectric or thermal analyzer measured stress comparison, the design of the optimal route.
The future development trend of powder forging is mainly the extension of process and application. The development of high impact / high speed forging will make the powder forging smaller parts possible. Compared with the current technology, high temperature sintering can further reduce oxidation, obtain higher hardness and impact strength, and can produce unique Performance and low cost parts.
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