Papers by Author: Masato Tsujikawa

Abstract: In ductile cast irons with copper, cementite stability was investigated against an annealing heat treatment used to obtain a fully ferritic matrix. Copper controls cast-iron mechanical properties, but its role in the matrix microstructure formation remains unclear. Some reports suggest the copper layer around graphite or cementite. They can be barrier to carbon diffusion at eutectoid reaction, however it is difficult to understand the mechanism of pearlite stability by copper. To confirm the existence of the barrier and effect of copper addtion, ten 9-mm-thick spheroidal graphite cast iron castings were prepared with different copper contents of 0.16 wt% – 0.69 wt%. The samples’ as-cast microstructures included spheroidal graphite, ledeburite, and pearlite. The pearlite fraction degreases to about 10% by heat treatment for ordinary ductile irons without intentional copper addition. The samples’ copper content and the pearlite fraction after heat treatment are not linearly related. The retained pearlite increased suddenly with increased copper content greater than 0.4 wt%. However, even the sample with the highest copper content showed no precipitation of a copper solid solution around graphite nodule or cementite.

Abstract: High-strength Mg-Y-Zn alloy plate was obtained by friction stir processing (FSP) after casting. In this study, the effect of processing order on the strength of processed specimens was discussed. It was revealed that the microstructures and strength of doubly stirred zone depended on the direction of overlapping pass. In the area of the doubly stirred zone that was affected by the thermal history of Retreating-side, the hardness was 121Hv. It was twice as hard as Base Metal, and harder than the doubly stirred zone that was affected by thermal history of Advancing-side.

Abstract: Strength of Al5083-O FSW joints was evaluated, in relation to FSW conditions. Static strength of the joints was found to be enough because base metal fracture occurred in the tensile tests of joints for some FSW conditions. However, the fatigue strength of those joints with fine static strength varied significantly. For example, in tensile strength of joints with the FSW condition with tool rotation: 800 rpm- welding speed: 100 mm/min (800-100), 800 rpm-200 mm/min (800-200) and 500 rpm- 100 mm/min (500-100), each condition of FSW joint efficiency were all hundred percent. In contrast, joint efficiency for fatigue strength varied from 75% for 500-100 to 31% for 800-200. Crack path in fatigue test was always initiated at the center of back surface of FSW weld zone, and propagated through stir zone. The variance of fatigue strength is believed to be attributed to the stirred condition at back surface.

Abstract: Friction stir processing (FSP) is the effective method of the grain refinement for light metals. The aim of this study is to acquire the fine grained bulk Mg-Y-Zn alloy by ingot metallurgy route much lower in cost. Such bulk alloy can be formed by the superplastic forging. The microstructure of as-cast Mg-Y-Zn alloy was dendrite. The dendrite arm spacing was 72.5 [(m], and there are the lamellar structures in it. FSP was conducted on allover the plate of Mg-Y-Zn alloy for both surfaces by the rotational tool with FSW machine. The stirring passes were shifted half of the probe diameter every execution. The dendrite structures disappeared after FSP, but the lamellar structure could be observed by TEM. The matrix became recrystallized fine grain, and interdendritic second phase particles were dispersed in the grain boundaries. By using FSP, cast Mg-Y-Zn alloy could have fine-grained. This result compared to this material produced by equal channel angular extrusion (ECAE) or rapid-solidified powder metallurgy (RS P/M). As the result, as-FSPed material has the higher hardness than materials produced by the other processes at the similar grain size.

Abstract: Bulk metallic glass (BMG) exhibits remarkable properties such as high strength, good stiffness and good corrosion resistance. However, the wear resistance of amorphous metals is not excellent as expected their high strength. It is thought that large local shear bands easily change into cracks for debris formation. The effective obstruction of shear band formation might be applied to improve the wear resistance of BMG. In this study, we tried to suppress shear band deformation by fine crystalline phase dispersion formed by semi-conductor laser irradiation. The microstructures of irradiated Zr-based BMG specimens were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The fine dispersions of crystalline phases are observed in the amorphous matrix. The optimum condition for laser irradiation was discussed.

Abstract: Fatigue strength of Al5083-O FSW joints was evaluated in the relation of the FSW conditions. Static strength of the joints in a range of FSW conditions was the same as that of base metal. Because the fracture of the joints occurred at the base metal in the tensile tests. Therefore the joint efficiency in tensile strength of joints with the FSW condition of tool rotation: 800 rpmwelding speed: 100 mm/min (800-100), 800-200 and 500-100 are all hundred percent. However, the fatigue strength of those joints with fine static strength varied very much. The joint efficiency for fatigue strength varies from 75% for 500-100 to 31% for 800-200. Crack path in fatigue test was always initiated at the center of back surface of FSW weld zone, and propagated through stir zone. Crack growth rate measurement and fracture surface analysis resulted the difference of the joint efficiency has a connection with the incomplete welding at back surface of joints.

Abstract: Grain refinement of magnesium alloy by Friction Stir Processing (FSP) was investigated. It is assumed that dynamic recrystallization (DRX) is occurred by frictional heat and plastic flow during FSP. This process is the effective method of the grain refinement for light metals. In this study, FSP was conducted to cast Mg alloys for and the difference of the grain refinement by DRX in these alloys was examined. As a result, in comparison with commercial Mg-Al-Zn alloy and Mg-Y-Zn alloy have finer microstructure. The grain size of FSP-ed Mg-Y-Zn alloy was ~1.7 [/m], however, that of AZ31 alloy was 20~30 [/m].

Abstract: Dynamic recrystallization behavior of Mg-3.1wt.%Al-0.1wt.%Y alloy was investigated. During extrusion, dynamic recrystallization occurred and very fine microstructure was obtained. We compared the relationship between Zener-Hollomon parameter, Z, and the dynamically recrystallized grain size, drec, and between the normalized recrystallized grain size, drec/d0, where d0 is the initial grain size, and Z for Mg-3.1wt. %Al-0.1wt.%Y with commercial Mg-3Al-0.5Zn alloy. As the result, the dynamically recrystallized grain size of Mg-3Al-0.1Y was finer than that of Mg-3Al-0.5Zn alloy under the equivalent Z, while the initial grain size of Mg-3Al-0.1Y was coarser than Mg-3Al-0.5Zn alloy. The normalized grain size for Mg-3Al-0.1Y alloy was much smaller than that of Mg-3Al-0.5Zn alloy under the equivalent Z. We conclude that small addition of Y to Mg-Al alloys is effective for grain refinement by dynamic recrystallization.

Abstract: The influence of tool control in non-linear friction stir welding (FSW) on mechanical properties of joints was investigated. FSW is widely applied to linear joints. It is impossible for five axis FSW machines, however, to keep all the FSW parameters in optimum conditions at non-linear welding. Non-linear FSW joints should be made by compromise with the order of priority for FSW parameters. The tensile test results of butt joints with rectangular change in welding direction on plate plane (L-shaped butt joints) with various welding parameter change. It was found that turn to the retreating side is encouraged when welding direction change. And the method of zero inclination tool angle is effective at non-linear and plane welding.

Abstract: Surface layer hardness and concentration profiles of austenitic stainless steels after plasma carburizing and /or nitriding at 673 K were investigated. Carbon and nitrogen concentration were measured by glow discharge optical emission spectrometry (GDOES) and carbides or nitrides were detected by x-ray diffraction analysis (XRD) and TEM. The state of carbon at the treated surface was investigated by Raman spectroscopy. Separation of carburized layer and nitrided layer was observed in a simultaneous carburizing and nitriding plasma treatment.