Koukichi Nakanishi

A knowledge-based mesh generation system for forging simulation

We developed a knowledge-based system GENMAI (Artificial Intelligence Mesh GENerator) to auto-generate two-dimensional structured meshes. GENMAI is easily applicable to various kinds of application domains. Mesh generation is one of the major tasks confronted in computational simulation. The quality of generated meshes affects computational accuracy and computing time. Since various kinds of domain knowledge are needed to generate high quality structured meshes, the knowledge-based approach has been found effective and successful. Before designing GENMAI, we analyzed mesh generation jobs in plastic deformation analysis and computational fluid dynamics. Then, we formulate GENMAI so that it searches feasible plural divided patterns combinatorially and selects the best pattern. The characteristics of GENMAI are as follows: the meta-inference mechanism and its knowledge representation are widely applicable to various kinds of application domains; and plural patterns can be efficiently obtained at the same time by a search technique based on global dependency and local dependency. We applied GENMAI to forging simulation and developed AI-FESTE, which integrated a rigid-plastic deformation analysis program and GENMAI. Forging designers can easily decide shapes of a forging product and dies and also plan the forming sequence using AI-FESTE. AI-FESTE automates a series of forging analysis operations and shortens the execution time from 1 or 2 day(s) to a few hours. As a result, not only can AI-FESTE shorten the turn-around time, but it can improve the quality of product and die design.

MRT letter: In situ observation method for microstructural changes of steel during hot deformation.

We report on the result of an in situ method for observing microstructural changes during hot deformation. The observation of microstructural changes of steel at 1,473 K under tensile strain is demonstrated using the reported method. The development of deformed structures and the formation of a new grain boundary, which subsequently moved with increased strain, were clearly observed. The effectiveness of this method was confirmed by the results of several examples.

Calculation for grain growth rate of carbon steels by solute drag model considering segregation effect of each substitutional element

Abstract Based on the solute drag model, a practical model incorporating the segregation effect is proposed to calculate grain growth rates in carbon steels. The segregation effect is modelled using two factors: the difference in atomic diameter between a solvent and a substitutional element, and the solubility of a substitutional element. By including the segregation energy, the proposed model enables the simulated retardation of grain growth by the addition of microalloying elements. The calculated grain growth rate by the proposed model shows reasonable correspondence between grain growth rates for experimental and calculated results. The temperature dependence of the grain growth rate is also well simulated.

In Situ Observation of Grain Growth and Recrystallization of Steel at High Temperature

An in-situ observation method for structures at high temperature is developed. The new observation device can reveal grain boundaries at high temperature and enables dynamic observation of these boundaries. Grain growth while maintaining microstructure at high temperature is observed by the new observation device with only one specimen for the entire observation, and grain sizes are quantified. The quantifying process reveals two advantages particular to the use of the new observation device: (1) the ability to quantify grain sizes of specified sizes and (2) the results of average grain size for many grains have significantly less errors because the initial structure is the same for the entire observation and the quantifying process. The new observation device has the function to deform a specimen while observing structures at high temperature, so that enables it to observe dynamic recrystallization of steel. The possibility to observe recrystallization is also shown.

Development of the Fast FE Method for Welding Deformation

We propose a fast finite element method for the prediction of welding deformation. To decrease the calculation time, this method allows the shell element to be used with the modeling for welding phenomena such as the heat source and joining. For the sake of further time savings, this method includes discrete thermal mechanically coupled analysis, which controls the frequency of mechanical calculations. Reasonable agreement between experimental results and calculation results has been obtained using simulated automotive parts with a short calculation time.