Papers by Author: Kunio Funami

Abstract: The development of laminated composite Mg alloy sheets, prepared by solid diffusion and roll bonding, is an effective way of improving the stiffness and surface properties of these materials while retaining their lightness. Laminated composites consisting of a core of Mg alloy between sheets of A5083 alloy as the coating material with Ti foil interlayers were prepared by solid diffusion and roll bonding. The laminated material had a strength and was resistant to cracking during deformation. Compounds that were formed and dispersed at the bonding interface between the Al and Mg alloys subjected to grain refinement improved the fracture toughness and strength of the composites, and it was important that these compounds were formed discontinuously. The fracture toughness of the laminated composite was twice that of the base Mg alloy, and its Young's modulus was 57 GPa.

Abstract: In the present study, the grain refinement, grain growth behavior, and tensile properties of rolled and annealed AZX311 Mg alloys were investigated. The yield strength and ultimate tensile strength of the rolled material were 360 MPa and 370 MPa, respectively, and the total elongation was 5%. When annealing was performed at 423 K for 1hr, the yield strength and ultimate tensile strength were unchanged, but the elongation increased to 10%. Furthermore, the strength and elongation did not change for annealing temperatures of 473–673 K owing to Al₂Ca precipitations.

Abstract: The good formability and corrosion resistance of 6N01 Al alloy allow it to be utilized in high-speed train systems, and weight reduction of railway vehicles is possible by improving the strength of this alloy. This study examined the effect of the fine-grained structure on the mechanical properties of the alloy formed by a combination of heat treatment and severe plastic deformation such as forging and rolling. The role of the fine-grained structure in determining the plastic formability was also investigated. The 0.2% proof stress and tensile strength of the heat-treated and multi-axial alternative forging (MAF) processed materials were both greater than 300 MPa. Subsequent cold rolling of these alloys increased both the 0.2% proof stress and tensile strength to over 450 MPa with a grain size of less than 1 μm. The fine-grained structure was confirmed to be effective in improving the strength of the 6N01 Al alloy.

Abstract: Superplasticity in an AZ80 magnesium alloy subjected to friction stir processing (FSP) has been investigated. FSP was carried out at two traveling speeds of 150mm/min and 300mm/min for grain refinement. Optical microscopy on cross section to processing direction revealed obvious differences in size and feature between the stir zones at the two traveling speeds. The hardness of FSPed sample at the room temperature was about 30HV higher than that of as-received one. The maximum stress of the FSPed sample was reduced remarkably at lower strain rates compared with those of the as-received one at 573K and 673K. On the other hand, the elongation to failure of the FSPed sample showed ten to thirteen times larger than that of the as-received one at 573K and low strain rates. Further surface morphology near the fracture tip was observed by scanning electron microscopy to discuss deformation mechanism at high temperatures.

Abstract: High-strain conditions as a means of microstructure control have recently been investigated to improve the ductility and enhance the strength of magnesium alloys. The level of superplastic deformation and the fatigue properties of the wrought materials have also been studied. In comparison, only a small number of such reports are available on cast materials. As a part of the search for applications of magnesium alloys, comparisons of structural changes and mechanical properties should be made between wrought and cast materials. In the present study, the grain refinement of cast and extruded materials made from commercially available AZ31 magnesium alloy was conducted using a multi-axial alternative forging method. The relationships between the structural changes and working processes and the relationships between changes in the mechanical properties as well as grain sizes and fatigue properties are discussed. Both the cast and the extruded materials tended to exhibit uniform crystalline structures with an increasing number of working cycles. Dynamic recrystallization was observed during both working and static recrystallization during both reheating and holding. When an equivalent strain of 0.6 was applied, the localized formation of ultra-fine grains of 0.5 μm was observed. The tensile strength and yield stress had maximum values in the initial stage of the multi-axial alternative forging. Although ductility improved with higher numbers of working cycles, the strength decreased. This can be explained by the dynamic and static recrystallization processes and work softening.

Abstract: Magnesium alloys show promise in meeting the demand for materials of lighter weight and higher rigidity. Mg alloys are hard to process and normally require grain refining for improved formability and mechanical properties. To process these fine-grained Mg alloys effectively, it is important to relate their load stress and mechanical properties to changes in their microstructures. Using a biaxial tensile machine and cruciform specimens, to evaluate the mechanical properties, microstructure, and plasticity, in a high temperature biaxial stress state, used of AZ31 Mg alloy sheet. With biaxial deformation, grain boundary slide occurred more frequently than with uniaxial deformation, causing grain boundary separation and formation of micro-voids between the grains. In the vicinity of the cracks and at the locations of grain boundary separation, although deformation temperature at higher than the recrystallization temperature, fine grains (about 2 )m) showing in duplex grain structures were formed locally. The formation of duplex grain structures as a result of local formation of fine grains during the deformation process is a major issue to be solved from the viewpoint of plasticity processing.

Abstract: Grain refinement and high temperature deformation in two kinds of magnesium alloys subjected to friction stir processing (FSP) have been investigated. One was a rolled sheet of LA141Mg and another was a cast plate of AZ91Mg. FSP was developed by adapting the concepts of friction stir welding to obtain a fine grain size in a stirred zone. Grain refinement was achieved by FSP to give fine grain sizes of 11.4μm and 8.4μm for LA141 and AZ91 alloys, respectively. For LA141 alloy, the maximum stress of the FSPed sample was higher than that of the as-received one in the range of 300K to 523K while the elongation to failure of the former was considerably smaller than that of the latter. On the other hand, the elongation for the FSPed sample of AZ91Mg showed three times larger elongation with a lower maximum stress than the as-received cast one at 523K and 2.8×10-3s-1. Further difference in high temperature deformation for both magnesium alloys was discussed based on microstructural change and stress-strain curves.

Abstract: The grain boundary sliding and the formation of slipped bands and cavitations during biaxial tensile deformation were examined in fine grained Al-Mg alloy. Biaxial tensile testing was conducted with cruciform specimens at initial strain rates of 10-4 to 101s-1. It was found that at the same equivalent strain conditions, the number of cavities under biaxial tension is significantly greater than that under uniaxial tension. A greater prevalence of slipped bands and grain separations were clearly observed under biaxial stress than under uniaxial stress. It was suggested that development of slipped bands resulted from the formation of elongated cavities and multiple deformed bands under biaxial stress. Additionally, the m-value under biaxial stress remained at about 0.3 over a wide range of strain rates. The effects of grain separation and formation of cavities were related to the motion of grain boundary sliding, grain size and loading conditions.

Abstract: For the formation of ultrafine grain in Al alloys, various means have been investigated based on a process of continuous recrystallization using a high-strain technique that employs rigorous plastic working. However, utilization for practical application is difficult for small specimens that require constraining. In this study, the effects were studied of the use of constraining die walls in the multi-axial alternative forging process (MAF) on the formation of ultrafine grains and microstructural homogeneity. This technique has possible for scaling up to a practical scale. Our results showed that tensile strength and yield stress in these fabricated materials were tripled over those of the initial materials when strain was applied. The average grain size after strain application was 0.5 µm. We conclude that a loading technique that uses different applied directions is the key determinant in creating ultrafine grains.