Friction Stir Processing (FSP) has been attracting attention as a useful surface modification process for cast alloys. It is well known that casting defects and abnormal structures are easily formed in the base metal. These defects significantly degrade the mechanical properties of the base metal. In this study, surface modification of the ADC12 die casting alloy by multi-pass friction stir processing (MP-FSP) was proposed and successful modification of the base material was achieved. The tensile and fatigue properties were investigated for MP-FSP samples in comparison with the base metal of the ADC12 alloy comprising 4mm thick plates. In all the MP-FSP samples, the tensile strength and elongation were much higher than those of the base metal, and the fatigue life was also superior to the base metal.

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  The influence of sulfur content on the matrix structure of cast iron was studied using spheroidal graphite and flake graphite cast iron samples whose chemical composition was 3.6mass%C and 2.0mass%Si. The sulfur content of the flake graphite cast iron samples was changed from 0.006mass% to 0.076mass% with FeS addition. The matrix structure was observed for the as cast and heat treated samples. The heat treatments were annealing (furnace cooled) and normalizing (air cooled), cooled from the different heating temperature, as 850°C, 950°C and 1050°C.
  The formation of pearlite structure was accelerated with sulfur in flake graphite cast iron samples. The formation of pearlite in spheroidal graphite cast iron was nearly identical with that of the low sulfur flake samples. When the heating temperature was 850°C, the pearlite morphology was granular and the pearlite formed from the higher temperature and the as cast sample, the morphology was lamellar. The difference should be affected by the solubility of carbon in austenite.

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  This study was conducted to clarify the effects of C content on heat treatment characteristics by using three kinds of spheroidal graphite cast irons containing 2.6mass%C to 3.6mass%C. The austenitizing condition was 1223K-900s. The ferrite, pearlite, bainite and martensite transformation regions existed in the CCT diagrams of all cast irons. Little influence of C content was seen for each transformation region in the CCT diagram, but in the case of the 3.5%C cast iron, the ferrite transformation region shifted slightly to higher temperature and shorter cooling time with the increase in C content. Pearlite started to precipitate at higher temperatures than ferrite in the cooling time range between 20 and 600s. The quenching hardness dropped slightly with an increase in C content, but the hardenability of spheroidal graphite cast iron did not influence C content in base materials.
  The effects of C content on quenching hardness and hardenability were slight. The transformation behavior and hardenability of spheroidal graphite cast irons containing 2.6mass%C to 3.6mass%C differ from those of steels, and this is considered due to graphite in the cast iron.

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  Decarburizing techniques of cast iron in old times were studied in view of reactive diffusion to clarify their mechanism. An ax excavated from remains of the Yayoi era shows decarburized microstructure of white cast iron. The time and temperature of decarburization calculated on the basis of reactive diffusion is considered reasonable for the era. Decarburization at solid-liquid coexisting temperature is regarded as a phenomenon of reactive diffusion which forms a solid layer. The rate of decarburization at this temperature, therefore remains low. Paddling of molten iron, which fractures the solid layer to replace it with fresh molten iron, seems to be an excellent technique for markedly increasing the decarburizing rate. Paddle Process of refining invented in England, as well as the refining process using reverberatory furnace to cast canons at the end of the Edo era in Japan, are considered to be advanced decarburizing processes by means of paddling. Sagé Process of refining in pre-modern times in Japan seems to be a decarburizing process which is accelerated by the included gas bursts at the front of the decarburized layer.

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