Spheroidal graphite cast iron is widely used for various purposes due to its excellent mechanical properties. To improve its mechanical properties, rare earth is added.

In this study, the effects of rare earth content on fatigue strength were investigated. Test specimens were added with 2.0%, 0.5%, 0.3% and 0% rare earth, and fatigue tests were performed using a plane bending fatigue test machine. The stop condition in the fatigue tests was 10⁷ times of repetitions or fracture of specimen. Fatigue strength was taken to be load stress of specimens that had not fractured.

  From the results of the plane bending fatigue test, the fatigue strength of test specimens containing rare earth was found to be about 307MPa. The fatigue strength of that without rare earth was 293MPa, which is a decrease of 8% compared to test specimens containing rare earth. The factors decreasing fatigue strength were studied by the observation of the fractured surface of the test specimens. As a result, casting defects were observed in the fractured surface. We therefore applied the √area parameter model to casting defects, evaluated using stress intensity factor ⊿K and threshold stress intensity factor ⊿K_th. As a result, it was found that the difference between ⊿K and ⊿K_th increased in test specimens not containing rare earth, and this is thought to lower fatigue strength.

  In addition, test specimens not containing rare earth showed less nucleation effects of graphite than those that did, suggesting that the volume expansion of the graphite during crystallization decreases expansion pressure, resulting in the large casting defects.

  Precision casting is one of the most important processes for the production of aero engine and turbo-charger parts. The IHI Group has been developing and improving the manufacturing process for the last three decades according to demands for higher mechanical properties, higher quality, and lower production costs of casting parts. However, in recent years, new and higher-level demands for casting parts are seen accompanying the need to reduce environmental load, save energy, and reduce weight. In this article, three process development topics in IHI Group are introduced. With respect to the turbo-charger, new processes for achieving defect-free and low cost production processes for aluminum compressor impellers produced with plaster mold castings are needed. We therefore developed the Low-pressure and Vacuum casting process for the mass production of compressor impellers. Furthermore, as for aero engine parts produced with ceramics mold castings, we developed new low-cost alloys for the single crystal casting blade, and a new mold system for titanium alloy castings. The new alloy for single crystal castings showed equivalent creep properties to commercial alloys with lower rare earth elements contents. Material cost is significantly decreased, and it is expected to be applicable to new blade materials. The new molding system for titanium alloys castings prevent alpha case formation during solidification. This reduces post-processing costs and keeps mechanical properties higher.

  In these developments, IHI Group has contributed to the recent technology progress of precision castings, and we will make further contributions to the advancement of the casting industry in Japan.