NEAR-NET-SHAPE PROCESSES:PROCESS CHAINS IN PART MANUFACTURING

PROCESS CHAINS IN PART MANUFACTURING

Part manufacturing is aimed at producing components for various destinations and functionalities. These components can be either autonomous products or parts of a more complex product. Com- ponents are characterized by their geometry and material properties. They have to cope with specific quality requirements depending on the corresponding part function and the individual application. The number of required components which determines the type of manufacturing—e.g., in small and medium series or large-lot production—depends on the part’s destination. In conclusion, the type of manufacturing mainly results from a wide range of possible preconditions.

The nature of manufacturing itself is to transform in steps the different variants of the initial material that is available for manufacturing technology into the final state predefined by the com- ponent’s target geometry. For this procedure, very different process stages, depending on the com- ponent itself and the manufacturing conditions, are required. These steps can be realized by various mechanical, thermal, and chemical manufacturing techniques. The process chain represents the tech- nological sequence of the individual process steps, which guarantees adequate fabrication of the component (see Figure 1).

As a rule, to manufacture a given component, different process chains are possible, posing the question of optimizing manufacturing processes and subsequently the process chains that should be applied. The first goal is to keep the process costs to a minimum at constant or even enhanced product quality. Another goal is to improve process productivity. Environmental protection, careful use of resources, and use of reproductive materials are increasingly important considerations.

Within this context, near-net-shape (nns), which involves optimizing the manufacturing process and particularly shortening the process chains, describes one trend in part manufacturing development.

Near-Net-Shape Processes: Definition and Limitations

For a better understanding of the near-net-shape principle, all explanations should be based on the different characteristics of the manufacturing techniques employed for shaping within the fabrication of components. These are methods that, on the one hand, create shapes by removing material from an initial state, including cutting methods such as turning, drilling, milling, and grinding as well as some special techniques. In contrast, shaping can be realized by distributing or locating material in an intentional way. The primary shaping methods (e.g., casting, powder metallurgy), metal-forming technology, as well as similar special techniques are based on this principle.

Starting with this distinction and bearing in mind the heterogeneous terminology in technical literature, we can define near-net-shape production as follows:

Near-net-shape production is concerned with the manufacture of components, whereby shaping is realized mostly by nonchipping manufacturing techniques and finishing by cutting is reduced to a minimum. Near- net-shape manufacturing involves such objectives as reducing the number of process stages, minimizing costs, and guaranteeing enhanced product quality.

Essentially nonchipping shaping is necessary if materials such as compounds and ceramics are characterized by poor machinability.

The near-net-shape principle carried to its limits finally results in net-shape production that makes finishing by cutting totally unnecessary. In net-shape manufacturing, shaping is performed only with nonchipping techniques. The decision on the extent of residual cutting work—that is, the extent to which finishing by cutting can be reasonably diminished or even completely eliminated—depends on the associated manufacturing costs.

Thus, in some cases it may be more economical to manufacture a shape element by cutting than by metal forming. Maintaining a certain minimum value for finishing by cutting may be necessary for technical reasons as well. For example, a disturbing surface layer (skin after casting or decar- burized skin caused by hot working) that results from a nonchipping shaping procedure will have to be removed by cutting.

Near-net-shape manufacturing can cover the whole component, but also only parts of it, such as functional surfaces of outstanding importance.

Goals and Benefits

To keep the manufacturing industry competitive in today’s global markets, innovative products must constantly be developed. But high product quality also has to be maintained at reasonable costs. At the same time, common concerns such as protecting the environment and making careful use of resources must be kept in mind. Near-net-shape manufacturing has to cope with these considerations.

The goals of near-net-shape production are to reduce the manufacturing costs and make the manufacturing procedure related to a given product more productive. These goals can be achieved by employing highly productive nonchipping shaping techniques and minimizing the percentage of cutting manufacturing, thus generating more efficient process chains consisting of fewer process steps. Value adding is thus shifted to the shaping methods as well. For that reason, enhancement of shaping procedures involves great development potential.

An increasing variety of advanced products with new functionality and design are being devel- oped. However, novel components characterized by higher manufacturing demands are emerging. Lightweight manufacturing, reduction of moving mass at energy-consuming assemblies or products, and the application of sophisticated multifunctional parts that save space and weight are typical manifestations. Frequently, when nonchipping shaping methods are used, geometrically complex com- ponents can be realized much better than would be possible with conventional cutting techniques perhaps used in conjunction with joining operations. With enhancing near-net-shape manufacturing as a must, the specific features of the different chipless shaping methods, such as low-waste and environmentally compatible production, increase in product quality, and better characteristics for use, can be taken advantage of.

Preconditions for Near-Net-Shape Manufacturing

As a precondition for developing near-net-shape manufacturing for and applying it to a given man- ufacturing task, efficient nonchipping shaping techniques that enable working accuracy commensurate with that of cutting techniques are needed. For guidelines for some achievable cutting accuracy values, see Table 1. Table 2 includes the corresponding values for nonchipping shaping techniques.

When the values in Tables 1 and 2 have been compared, a wide variety of feasible nns methods is available. At the same time, the differences that are still apparent pose a challenge to further development and improvement of nonchipping shaping methods. A trend toward still-higher accuracy and productivity of cutting techniques can also be observed. Thus, synergy effects that act mainly on the design of nns processes are emerging. The new quality of part-manufacturing development towards nns technologies is also related to the complex consequences of all those factors influencing quality and efficiency in part manufacturing. Figure 2 represents diagrammatically the principal de- pendencies and relationships.

From their origin, the influencing factors mentioned above can be assigned to workpiece, process, or equipment. In component design and when choosing the part material, apart from the component’s function, the requirements resulting from the foreseen near-net-shape production the workpiece must fulfill must be considered. In cases where shaping has to be realized by a metal forming procedure, the material should possess sufficient formability. Profound knowledge of the theoretical and practical

fundamentals of the manufacturing technique to be employed within the process chain is also very important. This expertise is a fundamental precondition for optimizing the entire process chain as well as each technique individually with respect to the manufacturing result and working efficiency. For such optimization tasks, FE simulation of manufacturing operations and production sequences is becoming more and more important. Finally, interaction between the development of manufacturing techniques and expanding the range of application of near-net-shape technologies exists.

With regard to equipment, the chosen parameters and manufacturing constraints must be main- tained in order to guarantee the required process reliability.