Advanced Materials Processing and Manufacturing

An introduction to the area of materials processing and manufacturing that has been of considerable importance in the growth and advancement of technology is given. The critical significance of transport phenomena in understanding the processes involved and in the prediction, control, and design of manufacturing systems is discussed. Different materials and products that are of particular interest in engineering are indicated, including new and emerging materials and applications. Various manufacturing techniques, from traditional processes like casting and forging to more recent advanced ones like thin film deposition and fabrication of optical fibers, are outlined. Practical aspects that are important in manufacturing and that must be addressed for a realistic, acceptable, and optimal process are indicated. Since the area of manufacturing is an extensive one, the scope and thrust of this book are briefly discussed.

The importance of heat and mass transfer and of the associated fluid flow and material transformations, including chemical conversion, in a wide range of materials processing techniques is stressed in this chapter. In many circumstances, such as melting and solidification that are encountered in casting and crystal growing, the heat transfer to and from the material is at the very core of the process, since it determines the rate of phase change. In addition, the temperature distribution and the buoyancy-driven flows that arise in the molten material due to temperature and concentration differences affect the characteristics of the solid-liquid interface and the microstructure of the product. In processes like food extrusion, hot rolling, thermal spray coating, and soldering, the thermal transport again determines the rate of fabrication and the characteristics of the final product. Thus, it is necessary to develop mathematical models for these processes in order to understand the basic mechanisms and thereby lay the foundation for analysis, numerical simulation, and experimentation. Computational models are developed on the basis of the mathematical models. Numerical solution techniques are generally needed since complexities that arise in common materials processing systems make it very difficult to obtain analytical results, which are applicable largely for very idealized and simplified systems. Mathematical models also guide the design of relevant experiments and the selection of relevant data to be obtained. They help in generalizing the experimental and numerical results, ultimately leading to greater insight into the basic processes involved and the framework to use the results for improving existing processes and for developing new ones. These aspects are presented in detail in this chapter.

In Chap. 2, the transport mechanisms that form the basis for materials processing were discussed. The governing equations, based on the conservation principles, were presented, along with many additional effects that are of particular interest in various manufacturing processes. The solution to these equations is needed in order to provide information on the behavior of the system and its dependence on the important parameters in the problem. The results also provide guidelines for choosing the various boundary conditions to obtain the desired product and the inputs needed for the design and optimization of the process. Though experimental results can be obtained in a few selected cases, analytical and numerical approaches, particularly the latter, are extensively used to obtain the desired information and inputs. This chapter considers analytical and numerical, as well as experimental, methods for studying materials processing systems. It presents the various approaches that may be employed. The focus is on the computational approach that is necessary for the simulation of a wide variety of manufacturing processes. Different solution methods, challenges to be overcome, and typical results are presented.

Manufacturing processes that involve phase change of materials to obtain the desired product are considered in this chapter. Phase change arises in a fairly wide range of materials processing applications, such as casting, crystal growing, welding, soldering, thermal sprays, injection molding, and additive manufacturing. Extensive work has been done on melting and solidification processes to determine the basic characteristics of the process, time taken for solidification, quality of the solidified material, flow and temperature fields, thermal stresses, and the nature of impurities and defects in the product. Simple conduction models that have been traditionally used to determine solidification times and eliminate voids due to material shrinkage on solidification are first discussed. The buoyancy-driven flow in the melt is then considered since it is critical to the nature and characteristics of the solid-liquid interface and to the rate of solidification. The basic analytical and numerical approaches that may be employed are discussed, considering interface tracking and single-domain methods. The latter methods are based on enthalpy and are particularly suited to mixtures, polymers, and alloys that do not have a fixed melting point, and solidification occurs over a range of temperatures. For interface tracking, grid transformations are used to obtain simpler domains. Some simple examples are taken from mold casting to illustrate the basic ideas. Essentially stationary interfaces arise in processes such as continuous casting and crystal growing in which the material moves as phase change occurs across a stationary interface whose stability, shape, and other characteristics are important in determining the quality of the product, as well as the rate of production. These aspects are discussed in terms of typical problems and systems. The solidification of mixtures such as alloys is discussed due to their importance in various industrial applications and the stringent demands placed on the quality of the casting obtained. The basic approaches that have been employed are discussed here, along with some characteristic results. Some experimental results are presented in terms of benchmark problems for the validation of the numerical models.

Continuous materials processing, as contrasted with batch processing, is discussed. There is growing interest in continuous processes because of higher production rates and lower costs. A wide range of processes can be considered for continuous processing. These include hot and cold rolling, extrusion, wire and fiber drawing, heat treatment of moving materials, and deposition on continuously moving surfaces. The problem is time dependent at the start of the process, but generally approaches steady-state conditions as time elapses. Both situations are of practical interest. Fluctuations and instabilities can also lead to time dependence. Of particular interest is the thermal field in the moving material, since the resulting thermal stress and microstructure are determined by the temperature distribution. The temperature rise or decay with distance is also critical in designing the system to maintain temperatures above certain values, such as the recrystallization temperature for hot rolling. Though a convective heat transfer coefficient, obtained from empirical data and correlations, may be employed at the surface, the problem is actually a conjugate one and both the fluid flow and the thermal field in the solid have to be considered. Though boundary-layer assumptions are valid in some cases, solution of the full elliptic equations is generally needed for a realistic simulation. Buoyancy effects are important in many circumstances and an additional forced flow, in an extensive environment or in a channel, may be employed to enhance the heat transfer. Experiments are used to validate the analytical and numerical models, to provide physical insight into the basic mechanisms, and to provide realistic and practical operating conditions. The basic formulation, solution strategies, and typical results are presented on this materials processing technique. The relevance of such results in the operation, control, design, and optimization of practical continuous manufacturing processes is discussed.

The processing of polymers, which include a wide range of materials such as plastics, food, rubber, resins, silicones, and biopolymers, is discussed. Because of the extensive use of these materials, many important processing techniques are available and are widely used to manufacture products for many different applications. Substantial literature exists on the fundamental aspects involved in polymer processing and on the design and operation of the relevant systems. The focus here is on the three major processes of extrusion, injection molding, and thermoforming. Extrusion is considered in significant detail in order to discuss the characteristics of the materials involved, major concerns, requirements and constraints, process modeling, and typical simulation and experimental results that may be used for the control, design, and optimization of the processes. Extrusion also serves as the feeding mechanism in several injection molding systems. The discussions on phase change, transport phenomena in polymers, and channel flow can also be applied to injection molding processes. Thermoforming is a very different process, since it depends on forming and shaping after adequately heating the plastic beyond the glass transition temperature. A few examples are given to illustrate this process as well as other similar ones.

The basic considerations in micro-/nanoscale fabrication are considered. The discussion is focused on the chemical vapor deposition process for the fabrication of thin films for materials such as silicon, titanium nitride, and gallium nitride. The different types of reactors, modeling of the flow, heat and mass transfer, and chemical reactions are considered in detail. Numerical and experimental results are presented for a variety of conditions and geometries. Of particular interest are the deposition rate and the quality of the deposited film. The chemistry is a critical component in these processes and, though the chemical pathways, chemical kinetics, and properties are known to a fair degree of accuracy for some important materials, the information available for other materials is often quite limited. The modeling of the transport processes without the chemical reactions is relatively straightforward, but the inclusion of chemical reactions generally makes the simulation much more complicated and difficult. Experimental data are available for materials like silicon due to their extensive use, but the data for the wide variety of materials of current and future interest are somewhat limited. Though the focus is on CVD systems, similar considerations arise in other fabrication techniques, such as flame synthesis of various materials. The basic considerations of quality versus production rate are common to all these processes. The link between the product characteristics and the operating conditions is an important one and the needed future effort is discussed. The approaches presented here may be extended, with suitable modifications, to other processes and applications.

This chapter discusses the fabrication of optical fibers, focusing on the drawing, cooling, and coating of fibers. The basic transport mechanisms that arise are discussed, along with results from analytical, numerical, and experimental studies. Starting with the fabrication of the preform, this chapter discusses the thermal transport in the draw furnace and the flow in silica glass. Of particular interest are the neck-down profile, defects generated during the process, feasibility of the process, and possible optimization of the process. The consideration is also extended to hollow, microstructured, and doped fibers that are of interest in different applications. The cooling and the liquid polymer coating processes are similarly considered, to discuss the underlying phenomena and present characteristics results. In all these considerations, a particular aspect that is brought up is the quality of the fiber, as well as of the coating, in terms of defects, imperfections, entrapment of bubbles in the coating, dopant distribution, viscous rupture, and so on. Also considered is the production rate, as indicated by the drawing speed and the ranges over which operating conditions may be varied to obtain a high-quality coated fiber with low transmission loss. Even though the presentation is directed at the fabrication of optical fibers, many of the basic aspects, methods for analysis, concerns, and trends are similar to those applicable to other manufacturing processes, such as wire drawing. Some of these similarities are discussed in other chapters as well in order to link the basic aspects of different manufacturing processes and thus impact on new and existing processes and systems.

This chapter discusses several manufacturing processes, besides those discussed in detail in earlier chapters, which are employed in industry for the fabrication of a wide range of parts, using different materials. Most of these processes have strong similarities with the processes considered earlier, and, therefore, the methods for analysis, simulation, and experimentation outlined earlier may be employed for these as well. Also, several of the results presented earlier indicate the trends, concerns, solutions, and approaches that may be adopted for these. This chapter presents heat treatment, material bonding, and fabrication of microchannels in some detail, whereas many other additional processes are briefly outlined. Most of the processes mentioned in this chapter, as well other similar ones that have not been mentioned, are important materials processing techniques in their own right, and extensive work has been done on understanding the basic mechanisms involved, the effects of various parameters, the desired characteristics of the product, and methods to improve product quality and production rate. Many of the references given here may be consulted for further information on these processes, since a more detailed discussion is beyond the scope of this book. However, the basic considerations and detailed discussions of several manufacturing processes given in earlier chapters will provide the basis for further work on these additional processes.

This chapter focuses on the system considerations related to manufacturing processes. Material transformations generally occur at the micro-/nanometer length scales, whereas the boundary and initial conditions are imposed at the engineering or macroscale. The manufacturing systems are also at engineering scales, even though the devices like thin films and optical fibers are microns in thickness or diameter. Thus, a multiscale problem arises in modeling and simulation, linking the processes at different length and time scales. To complement the simulation, experimentation is needed for validation of the models, for physical insight, and for providing inputs to simplify the analysis if the model is unavailable, difficult, or inaccurate. An important aspect in manufacturing is the design and optimization of the system needed to achieve the desired process. The inverse problem that arises in order to choose the appropriate boundary conditions is briefly discussed. The resulting feasible design domain and optimization of the process are also discussed. In many cases, multiple objectives are of interest and multi-objective optimization with trade-offs is needed. Uncertainties in design parameters and operating conditions may arise and must be considered to obtain realistic and practical results. These aspects are discussed in detail. A consideration of appropriate objective functions for different manufacturing processes, along with multi-objective optimization using available and new methods, is critical. This discussion may be used to address the needs for new materials and devices, particularly for new and emerging applications. Since uncertainties inevitably arise in practical systems, it is important to consider this aspect, along with sensitivity to the governing parameters, to obtain realistic designs.