Papers by Author: Hiroshi Utsunomiya

Abstract: The lubricant thickness in cold forging was estimated by machine learning of the in situ captured images of the die–workpiece contact interface. The images were in situ captured by a high-speed camera from the backside of the transparent glass die during forging of commercially pure aluminum workpiece. On the other hand, the images of the lubricated workpiece were individually captured as training images for random forest with classification. The classification accuracy of the lubricant thickness was confirmed to be approximately 75% (classification ability: 5–10 μm in lubricant thickness) in the training images with 22,500 px (50 px/mm). The in situ captured images of the die–workpiece contact interface during forging were classified by random forest using the training images. The estimated lubricant thickness of the in situ captured image almost agreed with the lubricant thickness estimated from the mean brightness value of the in situ captured image.

Abstract: To fabricate sandwich-structure composite of porous metal with nonporous surface layer, the nonporous skin layer was formed on surface of open-cell type nickel foam from aluminum powder by friction stir powder incremental forming (FSPIF) process. In this process, the surface pores of the foam were filled with the powder, then the powder and the cellular matrix near the surface of the foam were incrementally hammered by a rod-shaped tool without rotation. After that, the hammered surface of the foam was incrementally stirred by the tool at a very high rotation rate. The formed skin layer was composed of two layers; the friction stirred layer (relative density: above 0.90) in the upper part and the compact layer (relative density: 0.60–0.90) in the lower part. The friction stirred layer with a maximum thickness of 0.8 mm was formed on the surface of the foam without deforming the cellular matrix of the inside of the foam under the forming conditions; a tool rotation rate of 8000 rpm, a tool feed rate of 60 mm/min, a tool pushing pitch of 0.1 mm, and a total forming depth of 5.0 mm.

Abstract: Recently, it was reported that Cu-Al alloys of low stacking fault energy (SFE) processed by severe plastic deformation show excellent tensile properties due to TWIP (Twinning induced Plasticity) phenomenon. In this study, Cu-15at% Al sheets were heavily processed by conventional multi-pass cold rolling up to 90% in reduction in thickness without annealing. In order to reveal the change in mechanical properties and the microstructure evolution, tensile test, hardness test, optical microscopy (OM) and electron backscattering diffraction (EBSD) analysis were performed. Deformation twinning due to low SFE is observed even in the case of low reduction in thickness. As the reduction increases, grains are refined by intersections of shear bands. It is found that the balance of strength and elongation of the processed sheets is comparable to those by severe plastic deformation followed by annealing in literature.

Abstract: To fabricate aluminum foam having nonporous surface layer (sandwich structure), the selective laser melting (SLM) was applied to fill surface pores of a commercial closed-cell type aluminum foam with aluminum. A commercially pure aluminum powder was continuously melted and solidified by irradiating with a pulsed Nd:YAG laser with a maximum average power of 50 W. As a result, the aluminum foam having nonporous surface layer (SLM surface layer) was successfully fabricated. The compressive deformation behavior of the fabricated aluminum foam having the SLM surface layers was investigated with uniaxial compression test. The plateau stress of the aluminum foam having the SLM surface layers was improved by approximately 20%, compared with that of the aluminum foam without the SLM surface layers.

Abstract: To improve mechanical properties of a closed-cell type aluminum foam, the surface pores are filled with aluminum powder by selective laser melting (SLM). The relationship between the laser irradiation conditions and solidification characteristics of aluminum powder is investigated with one-line laser irradiation on the aluminum powder with a thickness of 5 mm. The aluminum powder is continuously melted and solidified with laser spot diameter of 0.6 mm and scanning speed of 10–20 mm/s. According to the successful melting and solidification conditions, the SLM is applied to the surface pores of a closed-cell type aluminum foam supplied with aluminum powder. The supplied aluminum powder is confirmed to be successfully melted and welded to the aluminum foam. As the result, the aluminum foam with nonporous surface layer (sandwich structure) is fabricated by the SLM with aluminum powder. The compressive behavior of the fabricated aluminum foam with nonporous surface layer is investigated.

Abstract: Distribution of residual stress through the thickness of a cold-rolled aluminum sheet is analyzed by the elastic-plastic finite element method under plane strain condition. Single-pass rolling of 2mm-thick aluminum sheet is considered. Influences of roll diameter D, reduction in thickness r, and friction coefficient μ are investigated. When the friction is low (μ = 0.1 and 0.2), and the case with smaller rolls (D = 130 mm) and low reduction (r = 5%), the residual stress in the rolling direction is compressive at surface and tensile around the layer quarter deep from the surface. While in the case with larger rolls (D = 310 mm) and high reduction (r = 30%), the stress is tensile at surface and the stress decreases to compressive with increasing depth from surface. In other words, with low friction, the residual stress distribution strongly depends on the aspect ratio (contact length / mean thickness) of the roll bite. On the other hand, when the friction coefficient is high (μ = 0.4), the residual stress is compressive at surface regardless of roll diameter and reduction. It means that the friction makes the residual stress at surface more compressive. It is found that the relationship between the residual stress at surface and the aspect ratio is almost linear, and that the slope depends on the friction coefficient.

Abstract: For the fabrication of lightweight components such as hollow components, we proposed a cold extrusion method for forming of deep holes that utilizes a punch with an internal channel for the supply of liquid lubricant using a servo press. The relationship between the punch ram motion and the punch wear in the proposed forming method is investigated by the finite element analysis in this study. The punch wear is determined by using the Archard’s equation. Although low friction at specimen–punch interface realizes in the forming with pulse punch ram motion (proposed forming method), the punch wear in the backward extrusion with pulse punch ram motion is 1.1–2.4 times larger than that with no pulse punch ram motion (conventional forming method). The influence of punch ram motion and friction on the punch wear is discussed.

Abstract: In hot rolling processes, oxide scale shows complicated deformation behavior and may cause surface defects on sheet. It influences the friction and the heat transfer on the interface between the sheet and the rolls. To reveal the mechanism, it is necessary to investigate the scale deformation during the hot rolling. In this study, the microstructure of the scale before / after the hot rolling was preserved by glass coating and analysed by scanning electron microscopy (SEM)/ electron backscatter diffraction (EBSD) technique. Using this technique, the microscopic deformation of oxide scale during hot rolling is discussed. Electrolytic pure iron sheets were rolled at a thickness reduction of 30% at 1273 K after oxidation in air for 0 s to 40 s. The scale consists of columnar grains which cover mostly throughout the thickness. The scale preferentially grows to the thickness direction at the initial stage of oxidation (<10 s). After the initial stage (>10 s), scale grains grow parallel as well as perpendicular in the thickness direction (ND). The scale grains are deformed at a fixed volume during hot rolling. The lower ductility of the thick scale results in the fracture of the scale and extrusion of matrix sheet to the outermost surface.

Abstract: Microstructural evolution of a copper alloy processed by accumulative roll-bonding (ARB) was investigated by EBSD analysis. The grains became thinner and elongated to the rolling direction with increasing the number of ARB cycles. The subdivision of the grains to the rolling direction actively begins to occur after 5 cycles of the ARB, resulting in formation of ultrafine grains with small aspect ratio. After 8 cycles, the ultrafine grained structure with the average grain diameter of 250nm developed in almost whole regions of the sample. In addition, the fraction of high-angle grain boundaries increased with the number of ARB cycles and reached about 0.7 after 8 cycles. The texture development of the ARB processed samples was different depending on the number of ARB cycles and the positions in the thickness.

Abstract: Lotus-type porous aluminum with cylindrical pores oriented in one direction was deformed by Equal Channel Angular Extrusion (ECAE) through a 150° die with sequential 180° rotations, and the pore morphology and Vickers hardness after the extrusion were investigated. The Vickers hardness increases with increasing number of passes in the extrusions both parallel and perpendicular to the pore direction, accompanied by the decrease of porosity. The densification occurs more easily in the perpendicular extrusions than in the parallel extrusions, and the large deformation by the densification gives rise to the large increase in the Vickers hardness for the perpendicular extrusions.