The effect of microscopic defects such as pin-holes and shrinkage porosity on the fatigue strength of aluminum-silicon alloy (AC4A) castings was studied and anlyzed by fracture mechanics. The results are summarized as follows:  (1) Comparing with the sound castings, the fatigue life in terms of the number of cycles to failure of AC4A alloy castings with pin-holes is one tenth and that with shrinkage porosities is one hundredth.  (2) Experimentally obtained fatigue strength of the specimens containing pin-holes and shrinkage was higher than that predicted by Paris law.  (3) Therefore, the fracture mechanics can be a useful means of predicting the fatigue life of castings containing pin-holes and shrinkage porosities with an understanding that the prediction is on the safe side.

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  The influence of water vapor in air and air-carbon dioxide atmosphere on oxidation of molten Al-Mg alloys was studied by gravimetry and scanning electron and optical microscopies. Water vapor in air slightly suppresses oxidation of the alloys. Water vapor in air-carbon dioxide reduces the suppressive effect of carbon dioxide. The action of water vapor is weakened by increased carbon dioxide concentration. A thick oxide film is formed in air containing water vapor and enters into the molten metal locally and randomly. A thin oxide film is formed in air-carbon dioxide with and without water vapor. The MgO surface film formed on the molten metal absorbs water vapor and/or carbon dioxide and shields the metal from oxygen in air.

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  In order to elucidate the role of oxygen in promoting the growth of the flake graphite cast iron in austenite region, growth tests were carried out on as-cast and pre-grown irons in atmosphere of various concentration of oxygen introduced by an oxygen pump. A hypothesis of selfcarburization based on an irreversible graphite migration theory of growth was used for the explanation of the results.  (1) Both irons kept growing by repeated heating. The growth tendency was remarkable in dilute oxygen concentration and became saturated at oxygen higher than 10%.  (2) The growth promoting effect of oxygen was remarkable in the pre-grown cast iron where the growth in 2% oxygen was 3.6times that in 0% oxygen, while the figure was only 2.9 in the as-cast iron.  (3) The weight of both irons decreased by heating in an atmosphere with 2% oxygen or less, while appreciable growth occurred. On the other hand, the growth became saturated in more than 10% oxygen, in spite of the weight increase.  (4) Microstructures showed remarkable development of decarburized zone at the surface and pearlitic zone below it in the pre-grown cast iron in the high oxygen range. Oxide scale began to be observed on the surface around 4∼6% oxygen and internal oxidation occurred when oxygen was over 10%.  (5) Growth-promotion in dilute oxygen atmosphere is explained theoretically by a hypothesis of self-carburization due to an atmosphere formed internally by the reaction of graphite and oxygen. Internal carburization may increase nominal expansion and redistribution of graphite due to carbon migration during heating and cooling, and may promote growth consequently.  (6) Complicated effect of oxygen on growth in medium concentration must be analyzed by oxygen permeability of oxidized surface of cast iron.  (7) It became more certain than before that the growth, especially in dilute oxygen atmosphere, could be explained by a hypothesis of self-carburization based on the irreversible graphite migration theory of growth. The hypothesis does not interfere with the explanation by the oxidation theory.

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  Experimental work was done to determine the melting and casting conditions necessary for the formation of B-type rosette graphite groupings in gray cast iron. The results gained are summarized as follows.  (1) The optimum chemical composition of the five main components to obtain rosette graphites was 3.45% C, 2.25%Si, 0.40%Mn, 0.54%P and 0.12%S.  (2) More rosette graphite were found with decreased maximum melting temperature and decreased pouring temperature.  (3) The distance between each rosette graphite groupings was increased by vanadium addition.

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  Although starch has only been applied as a secondary binder of sand mold, attention has been recently paid to replacing convensional organic binders by starch, e.g. application to the hot box process. In this study, the basic properties of starches were examined to ascertain the possibility of their use as a binder of the dry sand mold. It was confirmed from the results that some kinds of starch can be applied as a binder of the dry sand mold and an addition of approximately 3% of starch resulted in a dry sand mold having enough compressive strength for use.

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  Low temperature welding was applied to overlay welding of cast iron using Cu-Mn base alloy which has a good brazing performance for cast iron. Satisfactory overlay welding was obtained under a low current of 60A and a low voltage of 10V by working with a welding rod of Cu-30%Mn-18%Ni-2%Co alloy. TIG welding machine was used with direct current of straight polarity. In welding gray cast iron which is easy to crack by thermal stress, short and straight beads, while peening every bead, are desirable to suppress the interpass temperature and to reduce welding distortion. Because the welding heat input was small, there were no hardened zones around the weld. Almost 100% joint efficiency was obtained by this low temperature welding method.

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