Sessions focus on environment, new computer applications.
There was, however, another presentation by A. Ghosh, AKG Assoc., and S. Dutta, University of Windsor, that revealed a sobering side of the complex environmental issues drawing a restrictive regulatory net around the foundry industry. The authors called on foundry managers to consider "sustainable manufacturing," which they defined as a process of creating harmony between economic goals and social/ecological concerns.
Instead of current practices of singularly focusing on waste, materials and technologies, their concept seeks to unite these elements so that economics and environmental goals blend as co-equal determinants of a safe and prosperous society.
They faced head-on the flawed adage, "The solution to pollution is dilution," by recognizing the potential for environmental damage at the beginning of a product life cycle. The authors stressed working toward "clean" rather than "add-on or cleanup" technologies--stopping toxic substances from entering the environment by not producing them.
One of the critical elements of sustainable manufacturing is asset management, which implies product design and processes with minimal or no environmental impact. For example, the mass of a typical automobile has decreased by more than a half ton since 1975. About 20% of the decrease is due to substitution of aluminum and plastics for iron and steel.
Lighter cars use less fuel, but steel and iron are easy to recycle, whereas composites and plastics often resist reuse. The net result may be a drop in fuel consumption but an overall increase in the amount of permanent waste created and resources consumed.
Many German and Japanese companies have responded to the ecological crisis by inventing pollution control equipment that they market worldwide. Quoting from Costing the Earth by Frances Cairncross, Ghosh and Dutta counseled North American companies to do the same under the assumption that environmental standards will rise everywhere and an inevitable environmental market is waiting.
A presentation by H. Huang and J. Berry of the University of Alabama-Tuscaloosa examined minimizing casting microporosity formations caused by the failure of interdendritic feeding and/or precipitation on dissolved gases. Their investigation into the problem led them to the application of criteria functions (certain combinations of thermal parameters) and dimensionless numbers as they relate to solidification modeling to predict properties as well as faults in castings.
An especially troublesome porosity defect in long freezing range alloys (such as aluminum alloy, A356) can be correlated with criteria functions. One of the most effective ways to minimize such defects, the authors said, is to design a feeding system using solidification modeling based on previous experience. A casting of a long freezing range, high thermal conductivity alloy is likely to possess a shallow thermal gradient and a large mushy zone containing an extensive dendritic network that resists interdendritic feeding.
A computer model can determine possible porosity sites in a casting design (size and shape) and modify the design to eliminate the defect. Appropriate feeding systems can be designed before the first casting is made. Huang and Berry concluded that criteria functions are often associated with thermal parameters such as cooling rate, thermal gradient, solidus velocity and local solidification times. Further microporosity criteria evaluation is necessary to establish reliability and applicability to predict and avoid other defects like hot tearing.
V. Suri, Concurrent Technologies Corp., in his presentation on modeling and prediction of micro/macro scale casting defects, reported on the important applications of casting simulations to predict possible defects in a casting that then can be remedied before production.
Microporosity defects, Suri said, result from the combined effects of shrinkage and the rejection of dissolved gases during solidification. Bulk defects also can occur due primarily to the shrinkage of liquid metal during solidification, leaving voids in the casting that are inadequately fed.
The defect prediction is based on a comprehensive 3-D fluid flow, heat transfer and solidification kinetics model incorporating mold cavity filling and subsequent solidification. In micropore modeling, it is assumed for a stable pore to exist, the interdendritic liquid pressure has to be balanced by reflected gas pressure and gas-liquid surface tension.
P. Hansen, Technical University of Denmark, P. Sahm, Gieserei Institut RWTH, and E. Flender, MAGMA Giesserei Tech. GmbH, also discussed criterion functions and the power of using dimensionless numbers in fluid dynamics. Numerical modeling and the simulation of solidification processes in casting is gaining adherents in the foundry industry.
The authors reported that predicting and characterizing casting quality requires "criterion functions" to connect the output of numerical models (temperatures, gradients, solidification times, etc.) to empirical findings (microstructure, porosity, shrinkage, faults, etc.).
Using a series of mathematical equations, the authors stated that when working with the solidification process of castings, different phenomena relate differently to casting size or scale and casting shape.
A new computerized optimal gating and risering design system using 3-D geometric casting analysis was the subject of a paper by G. Upadhya and A. Paul, Concurrent Technologies Corp. The system directly links gating and risering to a simulation of a solid casting model using a knowledge base (rules and specific limits) as the input.
This results in a complete rigging design for a casting including risers, runners, ingates and sprue.
The comprehensive simulation system includes various phenomenological aspects (fluid flow, heat transfer and solidification kinetics) and allows verifications of any casting design for effectiveness by emulating its casting process in the computer.
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|Title Annotation:||CastExpo '93: 97th AFS Casting Congress, Chicago|
|Date:||Jun 1, 1993|
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