We have developed a new semi-solid injection process which can obtain high material yield of about 90% for magnesium alloys. The morphology of solid particles in the semi-solid slurry has significant effects on rheological behavior. In this study, semi-solid injection moldings of AZ91D magnesium alloy were conducted at injection speeds 220mm/s, 300mm/s, 400mm/s and fraction solids f_s = 0.3, 0.4 and 0.5 to investigate the effects of shear stress on microstructure. With high injection speeds, α-phase solid particles became spherical and average particle size decreased. The average roundness of α-phase solid particles was correlated to shear stress calculated on the assumption that slurry corresponds to the steady state in the nozzle. The decrease of the average particle size suggests that solid particles were broken up and spheroidized by shear stress. Under all experimental conditions, the distribution of α-phase particles on the plane perpendicular to flow direction became denser at the center of the specimen than at the surface. On the other hand, the distribution of α-phase particles on the plane parallel to flow direction depended on injection speed. A high fraction solid zone was generated on the end of the specimen at the high injection speed.

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  This study aims to elucidate the effects of vanadium carbide (VC) on the fatigue strength of spheroidal vanadium carbide cast iron (SCI-VCrNi) added with chromium (Cr) and nickel (Ni). The VCs in SCI-VCrNi were examined for the effects of fatigue strength compared with SUS304 using a plane bending fatigue test machine.
  As a result of plane bending testing, the fatigue limit of SCI-VCrNi was found to be330MPa.The fatigue limit of SUS304 was 200MPa. The fatigue strength of SCI-VCrNi was higher than that of SUS304. The results of fracture surface observation indicated the crack initiation point to be the cluster of VC. We therefore quantitatively calculated the size at which the cluster of VC becomes the crack initiation point by using the √area parameter. Moreover, we also introduced artificial small defects in a SCI-VCrNi specimen, to consider how the behavior of crack growth affects the VC. ΔK_th was considered from the ΔK and crack growth was calculated from the √area parameter of the artificial small defect. The results showed that the ΔK_th of SCI-VCrNi may be 5 to 6MPa √m. Although the fatigue strength of SCI-VCrNi is enhanced by the VC in this matrix, the cluster of VC became the crack initiation point, and the distribution morphology of VC affected the fatigue strength.

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  Giga-cycle fatigue properties were investigated for spheroidal graphite cast irons of FCD400 and FCD800. Fatigue tests were carried out by ultrasonic fatigue testing at 20kHz and conventional servo-hydraulic fatigue testing at 30Hz using specimens with 3 and 6mm diameters under the stress ratio of R ＝ －1. The results of the ultrasonic fatigue testing showed negligible difference from those of the conventional fatigue testing. Accordingly, ultrasonic fatigue testing could be applicable to spheroidal graphite cast iron. Decrease of fatigue strength in the giga-cycle region was in range from 10 to 20%, Namely, fatigue limit was not confirmed even up to 1 × 10¹⁰ cycles. Fracture origins were microshrinkage in most cases, while the 3mm specimens of FCD800 showed graphites at the origin as well as microshrinkage. The microshrinkage size was larger in FCD800 than in FCD400, and larger in the 6mm specimens than in the 3mm specimens. The graphite size was much smaller than the microshrinkage, The fatigue strength of FCD800 was lower than that of hot-rolled steels on the fatigue strength versus tensile strength diagram, while that of FCD400 was almost equal to it. Namely, in contrast to FCD800, the fatigue strength of FCD400 was as excellent as that of the hot-rolled steels. This was likely to be caused by difference of the microshrinkage size and the microstructure between FCD400 and FCD800. The effect of specimen size on fatigue strength was not remarkable. However, the fracture sites were graphite and microshrinkage in the 3mm specimens, while all of them were microshrinkage in the 6mm specimens. This difference of the fracture sites was a kind of size effect.

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  I In this study, the lap joint friction stir welding of high silicon ductile cast iron (FCD) and austenitic stainless steel (SUS304) was investigated by using a tool made of cemented carbide without the screw to examine microstructures.
  As a result, it was found that the cementite crystallized by eutectic solidification doeswas not formed in all welding conditions. The microstructure of the stir zone of FCD was a brittle one which consistinged of elongated graphite and martensite. TheAs a results of tensile test indicated that, the specimens broke at the interface between the stir zone and thermo-mechanically affected zone

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