Behavior Of Materials
Characterization of material behavior during its residence in a unit process allows for an accurate understanding of workpiece response to process conditions. For instance, the constitutive relationship describing material behavior is an essential starting point for simulation of a unit process (see Chapter 10). In many instances, the material behavior is nonlinear and dependent on its past history in the unit process. Therefore, each increment along the process route requires a more complex, complete solution of the governing equations. This chapter discusses material characterization for unit process simulation, focusing on the concept of microstructure evolution.
Microstructure evolution is an extremely important factor in unit manufacturing process science. Product properties and resulting performance are ultimately governed by microstructural features, which can be characterized by their respective composition, size, shape, and distribution.
In a processing system, which consists of several unit processes chained together, the evolution of microstructure is a serial, progressive phenomenon. The microstructure can be considered the ''carrier'' of the process history from one unit process to another. The final product properties, as determined by the final microstructure, are the integration of the incremental microstructure evolution of each unit process in the system. Knowledge of this material microstructure evolution sequence is essential to the understanding of the intermediate properties, as well as to the final resulting properties of the product.
The workpiece surface and interior microstructure comprise many components, each with its representative chemistry, size, shape, and distribution. The structures and their evolution may be viewed at several different levels of detail. The levels range from the macroscale down to the atomic scale or nanoscale. Each level has its characteristic microstructural features and
quantitative descriptions. For example, shape changes resulting from a deformation process are readily understood on a continuum level, while the crystallographic texture of a highly mechanically worked product can be understood from quantitative information at the polycrystalline level.
Defects and damage are also very important microstructural features and are typically dependent on process conditions and history. Process maps that incorporate process criteria and a mapping of defects and damage in terms of process parameters are useful tools. These criteria can be combined with simulation output (i.e., distributions of material conditions of stress, strain, and temperature) to identify conditions corresponding to formation of defects or damage.
Microstructure changes caused by a unit process involve both the interior and the surface regions of the workpiece. The interior represents the bulk of the workpiece material and governs many of the properties and performance aspects of the product. The surface region is typically subjected to a wide variety of conditions imposed by workpiece interactions with tooling, molds, lubricants, heat sources, atmospheres, and other physical and chemical process agents. The effects of these agents influence the mechanisms of heat transfer and friction, as well as the chemical reaction products that are unique to the surface region of the workpiece. The resulting surface microstructure may differ from the interior microstructure, as would the performance of the surface material. Surface conditions also influence the energy flux from the unit process to the interior region and thus affect the microstructural evolution of the bulk material.
A description of the surface of a part involves three components: the topography or geometry, defined as surface finish plus waviness; the metallurgical state of the material; and the residual stresses produced by processing. For example, mass-change processes can alter all three factors by introducing concentrated, localized, high-energy gradients on the workpiece surface, which, in turn, create high surface temperatures with extended thermal gradients, plastic deformation accompanied by plastically deformed debris, and chemical reactions with subsequent diffusion into the workpiece surface.
The mathematical descriptions developed to model the material behavior of the workpiece in the unit process (i.e., the constitutive models) typically do not include the influence of microstructural evolution, although recent efforts have begun to consider the evolving microstructure of the workpiece. The next step would be to develop the understanding and mathematical description of the surface material behavior, including the chemical, tribological, and thermal transfer characteristics of the workpiece surface. Specific needs include the description of the "mushy" liquid-solid state that occurs during the casting process. Similar characterization of the visco-elastic flow of polymers is also required for injection molding.
Based on the preceding discussion of research status and needs of unit process material behavior, the following areas emerge as strong candidates for future research emphasis.
- Quantification of the material microstructure as it evolves. An important research area is developing the capability to model a microstructure as it evolves during the course of a unit process. Ideally the entry microstructure and process parameters for a unit process would be specified; the model could then predict the resulting microstructure at any stage of the process.
- Systematic representation of the relationship between the microstructural features developed during processing and the resulting constitutive behavior. This representation would incorporate into the constitutive models the microstructural evolution of the workpiece. These tools would incorporate fracture mechanics concepts and damage mechanisms into their phenomenological models of damage initiation and growth.
- Process maps that contain defect and damage criteria. Processing maps are needed to identify processing windows (i.e., appropriate combinations of process parameters) to ensure defect-free (at the surface as well as the interior) products. These maps could be developed in part through the application of the microstructure evolution models and enhanced constitutive relationships discussed above.
- Characterization of boundary conditions. Understanding the mechanisms occurring at workpiece interfaces (e.g., heat transfer, friction, and chemical reaction products) is essential to a thorough understanding of a unit process, since these boundary conditions often determine the process limits.
- Materials property databases. Databases are required that contain the many physical properties of materials under a variety of conditions (e.g., temperature, strain rate, defect density) that are needed for process modeling and control. These databases should be readily accessible by researchers and design engineers.