Recently, optimization using computational fluid dynamics (CFD) simulators is gaining attention in various fields. However, solution space is unstable because of the loss of trailing digits or calculation error. Thus, even if the solutions are close to each other, costs do not decrease or increase constantly. Specifically, the costs are about the same, but sometimes the quality of the experimental results differs dramatically. We therefore proposed an algorithm which can realize high calculation quality in actual experiments taking into account the effects of the loss of trailing digits or calculation errors, and applied it successfully to the die casting process.

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  Fluid flow analysis using MPS (Moving Particle Semi-implicit) method was carried out, and its applicability to mold filling problems was reviewed. Dam break problem, which is a representative benchmark problem for free surface flow, was firstly calculated. As a result, calculation instability occurs when detached free surface particles move too close to each other, generating unnatural pressure and velocity between them. Therefore, potential force between particles (PF) and velocity correction method (VC) were introduced. As a result, calculation stability was improved and calculation could be continued without failing. In addition, unnatural velocity decay known to occur with the MPS method is also improved, by weighted potential force depending on the average velocity of surrounding particles. Finally, mold filling into a thin-tilted plate was calculated, and the results were compared with those obtained by FDM (Finite Difference Method). As a result, FDM can calculate stably but unreal flow patterns are generated due to stair-step boundary appearance at the tilted plane. Calculation using conventional MPS fails immediately after it is started, whereas calculation with the proposed methods is stable and proper results can be obtained.

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  Analysis was carried out on defect formation and its causes, as well as on microstructures, mechanical properties and the like, for butt welds formed by the laser welding of dissimilar metal using copper alloy castings (CAC 403) and austenitic stainless steel (SUS 304).
  As a result, weld cracks were partly identified in the heat-affected zone between the CAC 403 and SUS 304 materials. The cracks are thought to be due to heat stress caused by the variation of linear expansion coefficients between the two base metals.
  The fusion zone affects fine dendritic microstructures, and a number of small and large white spheric microstructures were identified in such dendritic microstructures. In addition, EPMA analysis of the fusion zone indicated that areas with smaller traces of Cu, Sn, and Zn tend to contain larger amounts of Fe, Ni, and Cr contrastively.
  On the other hand, welded joints presented a tensile strength of 178MPa and had an average joint efficiency of 89%, for CAC 403 base metal before welding. Moreover, the joints had an elongation of 5.8% on average, and breaking of the joints began from a position in the base metal. Furthermore, welded joints had impact values slightly lower than those of the CAC 403 base metal before welding. In addition, the welded joints decreased in plane bending fatigue limit to as low as approximately 10MPa, compared to that of the CAC 403 base metal before welding. Finally, the welded joints had a plane bending fatigue limit ratio of 0.42.

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