Modeling Current Status and Future Needs

Current models of the spray process, which rely on a combination of empirical inputs and physically modeled processes, appear to describe the shape (Ref 3, 27) of all types of spray formed deposits and the thermal conditions in billets (Ref 15, 27) and plates (Ref 27). The modeled thermal conditions in the spray have been found to control the two major unknowns in the process: sticking efficiency, which determines the yield, and trapped gas porosity, which is the major structural defect (Ref 34). These successes were achieved by use of carefully and expensively measured intermediate process parameters--notably the gas velocity field, PSD, and RMF (Ref 3, 27). If each modeling attempt requires this level of prior investigation, then the models will probably have limited application. Renewed experimental and theoretical studies that can lead to reliable predictions of these parameters in different spray forming chambers and with alloys of interest in spray forming are needed. It may be possible and more desirable to make progress directly in such studies by direct investigative techniques in actual gas atomization sprays. Techniques using lasers (Ref 47, 48), high-speed photography, and thermal imaging (Ref 15) appear to offer potential for direct measurements of droplet sizes, velocities, and deposit temperatures from which a process model should be able to back-calculate the needed process parameters and their variation with process variables, such as atomization, gas pressure, and metal flow rate.

The three-dimensional shape models need to be developed further to give thermal models that, once tested and validated, will facilitate improved process design to avoid undesirable features, such as near substrate porosity, and to promote finer and perhaps more uniform grain sizes. The features that limit grain coarsening in the high solid fractions of spray formed deposits also need further study. This is desirable both as a scientific puzzle (Ref 29) and as a potential means of further refining spray formed grain sizes. Given the growing commercial successes of the process (Ref 1), further detailed scientific analysis of all aspects of spray forming is warranted.

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