The main aspects of precision forging

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The article concerns the directions of development the forging and problems with precision forging like tool and preform temperature, slug geometry, press settings, process speed, lubrication and cooling, and tool shape and quality. It was introduced the present state of knowledge as well as crucial problems of the technology. Suggestions solving these problems as well as researches carried out in Metal Forming Processes Department of Wroclaw University of Technology were presented.

Introduction

The main advantage of precision forging over conventional die forging is its lower (by as much as 60%) material consumption. This is owing to the lack of flash and to the fact that the end product has minimum machining allowances. The forgings generally have very good service properties. Because of its advantages, precision forging is the most popular technology for producing car parts from relatively expensive materials. Connecting rods, skew and straight bevel gears, worm gears, tripods, turbines, alternators, constant-velocity joints and so on (Figure 1) are manufactured in this way [1], [2], [3], [4], [5], [6].

Besides its obvious advantages, precision forging has some drawbacks of which the most serious one is the too short life of the forming tools, mainly dies, punches (Figure 2). The tools used in precision forging wear out long before the end of their expect life.

The life-span of the tools depends on the forging conditions, workmanship of staff, tools material, the shape of the preform and that of the slug, etc. The low durability of the tools lowers the quality of the forgings, in spite of their still control. The most common forging defect due to low tool durability are, i.e. shorts, laps, burrs, bends, cracks, delamination, micro and macro-fractures and so on. This, in turn, affects the functionality of the final product made from the forging. Because of the large number and variety of factors (and their interactions) having an influence on precision forging the process is very difficult to analyze. Therefore a whole range of computer tools, such as CAD/CAM/CAE, mostly based on FEM and physical modelling, are used for the design, analysis and optimization of the forging process [2], [7], [8], [9], [10], [11].

Section snippets

Precision forging

In conventional precision forging the material is formed at ambient temperature or in semi-hot conditions. In the case of very complicated parts, a properly prepared charge is hot formed in isothermal conditions. Sometimes the super plasticity of the formed material is exploited. Initially, enclosed dies were used for forming [5], [12]. Thanks to the material savings and the lower costs of manufacturing products with enhanced properties at competitive prices, precision forging was increasingly

Choice of process parameters

As shown above, each of the stages in the forging process is critical. Any shortcomings even at one of them may result in bad quality of the forged products, press jamming and production stoppages. Therefore the process specifications must be adhered to during production, which requires proper shopfloor customs and technical culture. For example, if the hot forging die is improperly lubricated and preheated and there is improper cooling during production, the die heats up excessively, which

Heat treatment

Heat treatment has a decisive effect on tool life. Figure 9 shows a heat treatment diagram for a hot-work tool steel.

For instance, cracks which appear on the surface of a ground tool made of tool steel can be caused by improper tempering or by overheating during austenitizing. Such heat treatment faults can limit the possibilities of grinding the tool, even if proper precautions are taken.

Stress relief annealing. Tool steels are usually delivered annealed; further treatment is done by the user

Optimum die profile

One of the main parameters having an influence on the forging process is the shape of the adopted die. The magnitude of deformation obtainable in a single operation is limited not by material decohesion (cracks) but by material strength (too large hoop stresses). For this reason, instead of producing a forging in one operation, multioperation industrial precision forging is used. Still this does not sufficiently reduce the great forming forces and thereby the wear of the tools.

Die shape

Die design

Currently, in order to increase the crack resistance of precision forging tools, pre-stressed dies (i.e. reinforced with a single concentric ring or a larger number of such rings with thermo-compression or forced-in joints in-between) are used. It turns out that by changing the state of stress one can significantly reduce tool cracking [7]. The application of an appropriately high pre-stress (compressive hoop stress) during assembly should compensate for the very high tensile hoop stresses

Conclusion

The growing market demand, particularly from the automotive industry, has led to very intensive development of precision forging. Its advantage over other technologies is that it offers considerable material savings owing to the fact that there is no flash and that the end product is almost finished whereby finishing is not needed or reduced to minimum. Currently, precision forging is used mainly as multi-operation industrial forging to manufacture CV joint tulips, gear wheels, connecting rods

Acknowledge

The licences of programmes in article were used the MSC. The MARC as well as ProEngineer of Wrocław Centre for Networking and Supercomputing.

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