NEAR-NET-SHAPE PROCESSES:AN APPROACH TO NEAR-NET-SHAPE PHILOSOPHY: PRINCIPLE AND POSSIBILITIES

AN APPROACH TO NEAR-NET-SHAPE PHILOSOPHY: PRINCIPLE AND POSSIBILITIES

The basic idea underlying the near-net-shape philosophy is to approximate, as far as possible, an initial material to the target shape of the final product by means of innovative techniques of shaping and structure formation. As a rule, all measures are estimated by their contribution to achieving economic effects such as minimized costs and higher productivity. With respect to the production of components, the trend is to displace chipping operation stages with much more efficient casting, powder metallurgy, and bulk metal forming processes. Inspired by technological progress, the term near-net-shape has also been widened to include other ranges such as the fabrication of special and semifinished materials. The criteria for deciding upon the techniques to be applied for creating a special shape and structure do not have to be those known from part manufacturing. Thus, for instance, in the process chain of near-net-shape casting of semifinished materials, the percentage of casting vs. bulk metal forming is greater than in conventional fabrication of semifinished material.

Component Shaping Techniques (Selection)

Casting and Powder Metallurgical Techniques (Primary Shaping) Primary shaping is the manufacturing of a solid body from an amorphous material by creating cohesion. Thus, primary shaping serves originally to give a component an initial form starting out from a material in an amorphous condition. Amorphous materials include gases, liquids, powder, fibers, chips, granulates, solutions, and melts. With respect to a product shape and follow-up proc- essing, primary shaping can be divided into three groups:

1. Products made by primary shaping that will be further processed by forming, cutting, and joining. With respect to shape and dimensions, the final product is no longer similar to the product originally made by primary shaping. In other words, shape and dimensions are also essentially altered by means of techniques from other main groups of manufacturing processes. The manufacturing of flat products from steel, which is widely realized by casting, is one near- net-shape application. Thereby, later shaping by rolling can be kept to a minimum.

2. Products made by primary shaping whose forms and dimensions are essentially similar to those of the finished components (e.g., machine parts) or final products. The shape of these products corresponds to the product’s purpose to the maximum extent. Obtaining the desired final form as well as final dimensions mostly requires only a few, as a rule chipping, operations to be carried out at functional surfaces.

3. Metal powders produced by primary shaping, whereby the powders are atomized out of the melt. From powder, sintering parts are produced as a result of powder metallurgical manufac- turing.

The fabrication of moldings from metals in foundry technology (castings) and powder metallurgy (sintered parts) as well as of high-polymer materials in the plastics processing industry yields sig- nificant economic advantages, such as:

• The production of moldings is the shortest way from raw material to finished product. Forming, including all related activities, is thereby bypassed. A form almost equal to the component’s final shape is achieved in only one direct operation, and masses ranging from less than 1 g to hundreds of tons can thus be handled.

• Maximum freedom for realizing shapes that cannot be achieved by any other manufacturing technique is enabled by manufacturing of moldings that are primarily shaped from the liquid state.

• Primary shaping can also be applied to materials that cannot be processed with other manufac- turing techniques. An advantageous material and energy balance is ensured by the direct route from raw material to the molding or the final product.

• Components and final products of better functionality, such as moldings of reduced wall thick- ness, diminished allowances, fewer geometric deviations, and enhanced surface quality (near- net-shape manufacturing), can be produced to an increasing extent due to the constantly advancing primary shaping methods.

• Great savings in materials and energy can be realized with castings and sintered parts. Positive consequences for environmental protection and careful use of natural resources can be traced back to these effects.

2.1.1.1. Primary Shaping: Manufacturing Principle In general, the technological process of primary shaping methods can be divided into the following steps:

• Supply or production of the raw material as an amorphous substance

• Preparation of a material state ready for primary shaping

• Filling of a primary shaping tool with the material in a state ready for primary shaping

• Solidification of the material in the primary shaping tool, e.g., usually the casting mold

• Removal of the product of primary shaping from the primary shaping tool For a survey of primary shaping methods, see Figure 3.

Bulk Metal Forming Techniques

Aiming at the production of parts described by defined geometry and dimensions, all bulk metal forming techniques are based on the plastic alteration of the form of a solid body (whose raw material is massive in contrast to sheet metal forming) whereby material cohesion is maintained. The form is altered (forming procedure) due to forces and moments applied from the outside and generating a stress condition able to deform the material—that is, to transform it into a plastic state, permanently deformable inside the region to be formed (forming zone). The form to be produced (target form) is

mapped by the form of the tool’s active surfaces and the kinematic behavior of each forming method. The requirements to be fulfilled by the material to be formed—summarized as the material formability property—result from the precondition that internal material cohesion not be destroyed during man- ufacturing. Formability of a material is influenced by the conditions under which the metal forming procedure is performed. Forming temperature, strain rate, and the stress condition within the forming zone are essential factors influencing formability.

A selection of essential bulk metal forming techniques to be used for realizing near-net-shape technologies in part manufacturing is given in Table 3.

All bulk metal forming techniques can be run at different temperature ranges dictated by the characteristics of each material (which in turn acts on the forming conditions), design of the process chain, and the forming result. According to their temperature ranges, cold, semihot, and hot forming processes can be distinguished.

Cold Forming In cold forming, the forming temperature is below the recrystallization temperatures of the materials to be formed. The workpiece is not preheated. In this temperature range, workpieces with close dimensional tolerances and high surface qualities can be manufactured. The cold forming process results in some work-hardening of the metal and thereby in enhanced mechan- ical components’ properties. As a minus, the forces and energy required for cold forming as well as the tool stresses are much higher than for hot forming, and the formability of the material in cold forming is less.

Semihot Forming The warm forming temperature range starts above room temperature and ends below the recrystallization temperature from which the grain of metal materials such as steels begin to be restructured and the material solidification due to deformation degraded. For most steel materials, the semihot forming temperature ranges from 600–900°C. In comparison to hot form- ing, higher surface qualities and closer dimensional tolerances are obtained. The forces and energy required for semihot forming as well as the tool stresses are less than for cold forming. However, the additional thermal stress acting on the tool is a disadvantage. Consequently, in manufacturing, the requirements for exact temperature control are high.

Hot Forming In hot forming processes, the workpiece is formed at temperatures above the temperature of recrystallization of the corresponding metal. For the majority of materials, form- ability is higher due to higher forming temperatures. The forces and energy required for hot forming, as well as the tool stresses, are essentially lower than those for cold and semihot forming. Surfaces of poor quality due to scaling and decarburized skin (for steel materials: reduced carbon content in marginal layers near the surface) and wider tolerances at the component’s functional surfaces, which in turn result in increased allowances for the necessary follow-up chipping operation, are disadvan- tageous. Further limitations are caused by the forming tools’ grown thermal stress and the high expenditure involved in heating and reworking the part.

2.2. Special Applications

The near-net-shape technologies of part manufacturing also involve applications whose type of shap- ing can be assigned to none of the conventional primary shaping or forming technologies. This concerns, for instance, the thixoforming method, which involves processing metals at temperatures between the molten and partially solidified state so that the working conditions are between hot forming and casting. Depending on whether the component is shaped on a die-casting or forging press, the procedure is termed thixocasting or thixoforging. A specific globular structure of the initial material is required to transform metals into a thixotropic state. During heating to produce the thix- otropic condition, the matrix phase, which becomes liquid at a lower temperature, is molten first and the metal becomes pulpy and easily formed.

Thixoforming is able to produce complex components of sophisticated form with minimum use of material and energy in only one operation.

Generic part manufacturing, represented by the rapid prototyping techniques, occupies a special position. The rapid prototyping principle is based on successive sedimentation of material (shaping layer by layer). These techniques are used not only for manufacturing of patterns but also for pro- ducing tools in a near-net-shape manner, such as for casting.