The fatigue characteristics of Al-6Si-4Cu alloy casting solution-treated at 807K, which is approximately 17K higher than the ternary eutectic temperature, were investigated. The influence of pore configuration on fatigue fracture was studied by X-ray computer tomography in three dimensions. The fatigue limit of the specimen solution-treated at 807K decreased approximately by 18%, compared to alloys solution-treated with the standard T6 conditions. This is thought to be due to the eutectic dissolution resulting from solution-treatment above the eutectic point. Significant difference was observed in fatigue strength between alloys with and without high-temperature solution treatment in the high cycle region, whereas the difference was negligible in the low cycle region. When the volume fraction of pore exceeded 0.2% and/or macro-pores existed near the casting surface, fatigue life was found to shorten in both the high and low cycle regions. The negative effects of high-temperature solution treatment appeared to be dominant over the positive effects for fatigue.
The effects of joining conditions on joint strength in molten pure Al with AIN substrates were examined. In the bending test of laminated specimens, high maximum shearing stress was obtained under high pressure and/or high temperature. The maximum shearing stress also showed a similar tendency to thermal cycling performance. Both the maximum shearing stress and thermal cycling performance were considered to be reflected in the joint strength. These findings indicate that high joint strength was obtained under high pressure and/or high temperature. On the other hand, the maximum shearing stress showed greater variation under a long holding time, suggesting that the progress of oxidation decreases the reliability of joint strength. From the appearance and optical micrograph of the bending specimens, finely-fractured Al layers were found adhered on both delaminated surfaces under the conditions where higher maximum shearing stress was measured. This result suggests that the joint strength has the potential to be as high as the shearing strength of pure Al. Weibull plots of the maximum shearing stress implied that high-reliability joining can be achieved by both good wettability and prevention of oxidation.
In this study, a method of analyzing heat conduction in exothermic mold materials was proposed, and a analytical solution for moving steady state heat conduction in a plate of semi-infinite thickness was obtained. In addition a numerical solution was developed for non-steady heat conduction in exothermic materials of different geometries. Satisfactory agreement between the analytical and numerical solutions was confirmed in a range of conditions where an essentially steady state prevails. For verification, small spheres of exothermic materials were ignited in an electric furnace and the temperature at the sphere center was recorded. The basic thermal behavior was found to qualitatively agree with the theoretical predictions, including the overheating phenomenon, whereas some behaviors more complicated than those predicted by simple theory were also observed. The in-furnace combustion test was found to be useful for determining the ignition temperature of exothermic materials and also for the qualitative comparison of the thermal characteristics of different exothermic materials. On the other hand, the test was found not fit for the quantitative determination of combustion heat and other characteristics.
In recent years, molten metal flow and solidification analysis by computer simulation is becoming a very important tool for optimal casting and casting plan design. The analysis accuracy is, however, insufficient because of complicate phenomenon and the need for difficult experiments especially in the die-casting process. In this work, the effects of various analysis conditions in simple aluminum sand casting were investigated using existing experiment results. In addition, actual die-casting experiments to measure molten metal pressure, air-pressure, solidification, and casting defects especially inclusion and shrinkage defects were conducted. The results for air-pressure, solidification, and casting defects showed good agreement with computer simulation results.
Salt mixtures with various compositions in the NaBr-KBr-Na2CO3-K2CO3 system were made by the gravity casting process. Their fracture strength, fracture toughness, and dissolution rate in water were examined by bending test, bending test of precracked specimens, and dissolution of the salt mixtures in water, respectively. Salt mixtures with high fracture strength over 15MPa were found in the composition area in the primary carbonate crystal and K+ ion rich region. Such salt mixtures also showed high fracture toughness and only a few surface cracks. In the case of salt mixtures with low fracture strength, many cracks were observed on the surface and fracture toughness did not have a strong correlation with fracture strength. These findings suggest that the surface cracks formed during solidification lead to low fracture strength, and higher saturation solubility limits lead to higher dissolution rate of salt mixture in water. Salt mixtures of the NaBr-KBr-Na2CO3-K2CO3 system are expected to serve as core materials with good expendability as well as high strength, because the composition area showing higher fracture strength has a relatively high saturation solubility limit.
Frozen mold casting provides industrial advantages such as improvement of working conditions and reduction of factory waste. However, to put it to practical application, it is important to establish economical and productive freezing technique for casting sand molds. The differential pressure freezing method which enables sand molds to be frozen in the short time at lower costs is drawing attention as a useful way to produce frozen molds, but there still remain some problems with regard to mold surface conditions. In this study, the authors reviewed a new differential pressure freezing method which is suitable for the core molds, whose entire surface is usually covered by molten metal in the casting. A hollow core was prepared and pressure difference was given between the hollow portion and outer space to cause aeration in the radius direction. It was confirmed that frozen core molds produced by this process had uniform lateral surfaces with good conditions. Moreover, the hollow frozen core molds did not break during casting work, and successfully provided the desired cavity in castings. We also developed an air-through mold form material to facilitate the above freezing method. This air-through material has eased most of the regulations on core geometry in the freezing method, enabling production of frozen mold with irregular and/or curved surfaces easily.