Papers by Author: Hisashi Serizawa

Abstract: In order to evaluate the corrosion fatigue property of dissimilar joint by the simultaneous fatigue-corrosion test, an original compact shear fatigue test machine has been developed and installed in the combined cycle corrosion equipment. The material used were an innovative ultra-high strength steel (i-UHSS) with the ultimate strength of 1.5 GPa & large elongation of 20 % and an innovative Al-Mg alloy with high ductility. The dissimilar lap joints were fabricated by resistance spot welding (RSW) or refill friction stir spot welding (RFSSW). An innovative adhesive was also employed for producing the weld-bonded (WB) joints. An accelerated corrosion condition conducted in this research was the climate of high temperature & high humidity. The natural corrosion fatigue tests were also performed outdoor on Miyakojima Island in Japan and indoor in an air-conditioned room. The accelerated corrosion fatigue tests suggest that the corrosion fatigue property of the dissimilar WB joint joined by RSW seems to be better than that of the WB joint joined by RFSSW because of the difference in the area of adhesive interface. In addition, it is considered that the crack would propagate in the adhesive interface and then lap joint might break suddenly after achieving the crack to the circumference of joint interface produced by RSW or RFSSW. Moreover, it can be concluded that the accelerated corrosion condition employed in the research seems to be an appropriate condition for examining the corrosion fatigue property of lap joint in the climate of high temperature and high humidity.

Abstract: As a result of R&D efforts about the advanced materials, an ultra-high strength steel (UHHS) and a carbon fiber reinforced thermos-plastic (CFRTP) have been developed. In this study, various advanced multi-material dissimilar lap joints using UHHS, CFRTP and an aluminum alloy of A5083P-O were fabricated by the resistance spot welding (RSW), refill friction stir spot welding (RFSSW), friction stir spot welding (FSSW) and laser irradiation assistance plastic welding (LIAPW). Where, the total heat input for each joining method was varied in order to examine the influence of welding condition on joint performances, which were the tensile shear strength, shear fatigue property and corrosion shear fatigue property. The tensile shear strength of UHHS and A5083P-O joined by RFSSW is almost two times of that produced by RSW, and this difference seems to be mainly caused by the difference of joint area. The difference of joint area also affects the tensile shear strength of UHHS & CFRTP or A5083P-O &CFRTP lap joints made by FSSW and LIAPW, and the joint strength of LIAPW is larger than that of FSSW. As for the shear fatigue tests, it is found that the fatigue strength seems to decrease almost monotonically with increasing the applied load regardless of the types of dissimilar materials. Moreover, through the corrosion shear fatigue tests for A5083P-O and CFRTP joined by FSSW or LIAPW, it is found that the corrosion fatigue property seems to be controlled by the crack propagation behavior.

Abstract: Friction stir welding (FSW) is one of the solid-state welding and it has been widely employed for joining aluminum alloys. In addition, as a result of R&D efforts about FSW tool, this method is expected to join the steels and/or various dissimilar materials. In order to examine the thermal and mechanical behavior in FSW, many numerical studies have been conducted and the heat generation behavior near FSW tool is precisely demonstrated by using the moving particle semi-implicit (MPS) method which is one of the particle method. In this research, in order to reduce the computational time, a new parcel method based on MPS is developed and its applicability is examined for simulating the friction stir welded dissimilar joint between V-ally and austenite stainless steel SUS316L. From the serial computational results, it is revealed that the influence of rotational speed on the heat generation during FSW seems to be larger than that of traveling speed. Moreover, the numerical result indicates that the sound dissimilar joint might be fabricated when V-alloy is set to be the retreating side (RS), the FSW tool is inserted in RS and the rotational speed increased appropriately although the two materials have not been joined in this welding condition of FSW experimentally.

Abstract: As for the fuel cladding in the light-water reactor, silicon carbide fiber reinforced silicon carbide matrix (SiC/SiC) composite is one of the promising candidates as a replacement of Zircaloy due to many superiorities, where it is necessary to develop the end-cap seal of SiC/SiC composite cladding. In this research, the caulking method was employed as the method for sealing the end cap of SiC/SiC composite tube by Zircaloy tube where the titanium micro-powder was inserted between two tubes. The fiber laser was circumferentially irradiated on the outer surface of zircaloy tube, and the insert method of titanium powder was varied by changing the cutting method of SiC/SiC composite tube. The examinations about the slit shape effect for the flat cutting suggested that the hook slit is considered to be the best cutting method for holding the titanium powder during the laser irradiation where the width of slit should be narrower than that of laser irradiation line.

Abstract: As examples of the most typical methods to determine the shear strength of SiC/SiC composite joints, the asymmetrical four point bending test of a butt-joined composite, the tensile test of a lap-joined composite, and the compression test of a double-notched composite joint were analyzed by using a finite element method with the interface element. From the results, it was found that the shear strength in the asymmetrical bending test was controlled by both the surface energy and the shear strength at the interface regardless of their combination while the strength in the tensile test or the compression test was governed by the surface energy when both the surface energy and the shear strength were large. In addition, the interface element was employed in order to examine the influence of the specimen geometry on the microstructural fracture morphology in nanoSiC/SiC composite during a miniaturized Double Notch Shear (DNS) test. From the serial computations, it is revealed that a relationship between the inter-laminar shear strength and the yield stress seems to be very important for selecting appropriate specimen geometry of the miniaturized DNS test.

Abstract: A fusion reactor is expected as one of the new electric power sources in next generation. Reduced activation ferritic/martensitic steel F82H is planned to be used as a structural material for the blanket modules set on the inner wall of the reactor. However, especially in the case of laser beam welding (LBW), the weldability of the steel was not completely clarified. On the other hand, although post weld heat treatment (PWHT) should be conducted for the welds of the steel in accordance with general standards for chrome steels, the heat treatment conditions were uncertain. Therefore, adaptability of LBW as a joining method for the steel and the applicable PWHT conditions for the welded joints were investigated in this study. The effect of LBW conditions on weld penetration behavior were ascertained by observation of cross sections in the welds. The adequate PWHT conditions were confirmed in consideration of both hardness distributions measured in welds and ductile-brittle transition temperatures (DBTT) evaluated using Charpy impact test. Full penetration without weld defects such as hot cracking, porosity etc. was obtained for plates with the thickness of 4mm of the steel by control welding conditions. That means laser beam is one of useful welding heat sources to realize sound weld joints of the steel. In addition, due to select appropriate PWHT conditions, the hardness in welds was suppressed to the level of base metal and the toughness in the welded joints was improved to a practical level without the damage to base metal.

Abstract: In order to simulate the large deformation surrounding the rotational tool of friction stir welding (FSW) precisely, the moving particle semi-implicit (MPS) method was employed and the temperature distributions near the tool were obtained. Also, the temperature distributions in the whole model except for the area computed by MPS were calculated by the finite element method (FEM) and then the elastic-plastic analysis was conducted using the temperature distributions obtained by MPS and FEM. The inhomogeneous temperature distributions through the thickness near the joint line could be simulated and the maximum temperature distributions computed had a good agreement with the experiments. In addition, the longitudinal plastic strain distributions indicates that this plastic strain near the tool is not governed by only the temperature distributions and the influence of plastic flow should be taken into account.

Abstract: As a numerical tool for examining the microstructural fracture behavior in the smart and intelligent materials, a finite element method with the interface element was developed and the applicability of this method was studied through the serial computations using virtual polycrystalline models. As the results assuming the influence of grain orientation on the grain boundary, it was found that the anisotropic mechanical property of grain boundary (interaction between opening and slipping deformations) would be a dominant factor of the fracture process. Also, by employing the theory of crystal plasticity for the mechanical property of grain, it was revealed that the stress concentrations caused by both the mismatch between neighbor grains and the slipping at grain boundary could be demonstrated by using this method.

Abstract: The magnetic pressure seam welding is one of the candidate methods to join thin sheet smart and multifunctional materials. In this research, to examine the mechanism of magnetic pressure welding from a dynamic viewpoint, numerical simulation of the impact was carried out by using a commercial Euler-Lagrange coupling software MSC.Dytran (MSC.Software) as a first step of the computational studies, where the joint between Fe and Al was employed according to the previous experimental researches. From the serial numerical results, it was found that the increase of temperature at the joint interface was not enough to melt Al or Fe in the range of collision velocity and angle studied in this report. Also, it was revealed that the very large mean stress occurred at the interface which could be considered as the pressure at joint interface and Al moved with high velocity along the interface. Moreover, it was found that there were two patterns of plastic strain distribution near the joint interface depending on the collision velocity and collision angle. Finally, it can be concluded that the plastic strain pattern might be related to the success of magnetic pressure seam welding.

Abstract: An in-house developed FE code, based on the idea of Iterative Substructure Method, was used to evaluate the effectiveness of fixture and pre-strain for reduction of welding distortion in a fillet welded structure. Comparison between the simulation results and experimental results has been performed to validate the basic FE model. Effects of two fixture conditions and various prestrain schemes were analyzed. The simulation results show that appropriate fixture can greatly reduce angular distortion of the work-piece and results in an appreciable reduction in residual deformation and the application of pre-strain can significantly reduce the residual distortion.