Published papers

To create a coating of titanium nitride, atmospheric-controlled induction-heating fine particle peening using fine particles of pure titanium was conducted for low-alloy steel in a nitrogen atmosphere, and the surface characteristics of the treated specimens were investigated. The formation of hard titanium nitride coatings with a thickness of 100–200 μm was achieved by peening at 1373 K for 10 s and subsequent heating at 1373 K for 120 s. Titanium particles adhered to the treated surfaces during peening at the high temperature, which reacted with nitrogen in the atmosphere during the heating after peening. Ball-on-disk wear tests were conducted to examine the wear resistance of the specimens, and it was found that the created coating by AIH-FPP improved the wear resistance of low-alloy steel owing to its high hardness. The improvement was more significant at high testing temperatures because the high testing temperature and frictional heat caused oxygen in the air to react with titanium in the coating during the wear test. Immersion of the specimens into sodium chloride solution was also conducted to investigate the corrosion resistance, which revealed that the coating created improved corrosion resistance of low-alloy steel.

Hyperthermia can be induced to exploit the thermal intolerance of cancer cells, which is worse than that of normal cells, as a potential noninvasive cancer treatment. To develop an effective hyperthermia treatment, thermal cytotoxicity of cells should be comprehensively investigated. However, to conduct such investigations, the culture temperature must be accurately regulated. We previously reported a culture system in which the culture temperature could be accurately regulated by employing metallic culture vessels. However, appropriate temperature conditions for hyperthermia depend on the cell species. Consequently, several experiments need to be conducted, which is a bottleneck of inducing hyperthermia. Hence, we developed a cell culture system with temperature gradation on a metallic culture surface. Michigan Cancer Foundation-7 cells and normal human dermal fibroblasts were used as cancer and normal cell models, respectively. Normal cells showed stronger thermal tolerance; this was because the novel system immediately exhibited a temperature gradation. Thus, the developed culture system can be used to investigate the optimum thermal conditions for effective hyperthermia treatment. Furthermore, as the reactions of cultured cells can be effectively assessed with the present results, further research involving the thermal stimulation of cells is possible.

Various culture devices have been developed as fundamental technologies for facilitating bioengineering studies. Culture devices are designed to prepare specific culture environments. Thus, both macrostructures and surface micromorphology should be considered in the device design. Although fabricating devices with elaborate designs incurs high production costs, disposable materials are typically used for culture devices. However, some metallic materials are strong, stable, and biocompatible. Bioengineers have not applied these materials to culture devices because of the difficulty of processing. An emerging technology using three-dimensional (3D) printing has been developed, which can produce complex designs using metal. We demonstrate the applicability and potential of metal 3D printing for fabricating culture devices toward the development of the bioengineering discipline. As a specific example, we fabricated metallic culture devices where the environment of cultured tissues can be improved. One of the biggest factors determining the culture environment is active media supply. To attain active media supply to the tissue, devices having culture surfaces with mesh structures having holes far larger than cells were proposed. Cell sheets were cultured as tissue models, realizing tissue culture with such structures. The cultured tissue showed increased metabolism, indicating enhanced media supply owing to mesh surfaces. The biocompatibility of the 3D printed metal device was confirmed by viability assays on cultured cells, and reusability of the device was confirmed by mechanical and biochemical evaluations. We believe this study serves as a reference for using metallic 3D printing as an option for fabricating culture devices, which will promote bioengineering research.

In this study, we attempted to nullify the harmful influences of processing marks on the fatigue strength of Ti-6Al-4V alloy by particle collision treatments. As the surface treatments, fine particle bombarding (FPB) and shot peening (SP) were applied to form hardened layers and introduce compressive residual stress. The surface of the objective material was polished to a mirror surface to eliminate the influences of machining on specimens and was then grinded with emery papers (#80) to make uniform processing marks. After the particle collision treatments, the surface conditions, hardness distributions and residual stress were systematically examined, and their relationships with the fatigue strength were considered in detail. On observation of the surfaces, the processing marks were eliminated by the particle collision treatments. At the same time, the surface hardness was increased and high compressive residual stress was introduced. As a result, the fatigue strength was markedly improved by the treatments beyond the level of the material with the processing marks without deterioration of the mechanical properties. The improvement rates of the fatigue strength were high, at 75% by FPB treatment and 58% by SP treatment.

Atmospheric-controlled induction-heating fine particle peening (AIH-FPP) was performed for low-alloy steel, and the effects of the peening temperature, the microstructure of steel, and thermal history during AIH-FPP on the resultant microstructure of treated steel were investigated. Regardless of the prior microstructure and thermal history, the grains near the surfaces were refined by AIH-FPP at peening temperatures from 923 K to 1023 K owing to dynamic recrystallization. To examine the effect of AIH-FPP on the fatigue properties of steel, plane-bending fatigue tests were performed. Compared with the steel mirror-polished, treated with FPP at RT, or only induction-heated, the fatigue properties of steel treated with AIH-FPP were improved. This is because of the formation of a thick and uniform grain-refined layer, the generation of compressive residual stress, and hardening of the surface by AIH-FPP. Moreover, owing to the grain refinement progress, the fatigue properties of steel improved with increasing peening temperature.

Maraging steel components fabricated by laser powder-bed fusion, which is a technique of additive manufacturing, are expected to be used widely because of their high strength, hardness and toughness. To apply maraging steel manufactured by laser powder-bed fusion in wider industrial fields and prevent the components from fracture, a study was conducted to improve the wear resistance and fatigue properties of the steel by an atmospheric-controlled induction-heating fine particle peening surface modification technique. The technique developed can create hard intermetallic compound layers and introduce peening effects, which are expected to improve the wear resistance and fatigue properties of maraging steel fabricated by laser powder-bed fusion. The prepared specimens were examined using optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, laser microscopy, X-ray diffraction, micro-Vickers hardness testing, nano-indentation testing, reciprocating ball-on-disk wear tests and axial fatigue tests. It was revealed that atmospheric-controlled induction-heating fine particle peening using mechanically milled particles formed Fe-Al intermetallic compound layers with high hardness, introduced compressive residual stresses and increased the hardness of the maraging steel substrate owing to age hardening. In conclusion, atmospheric-controlled induction-heating fine particle peening can simultaneously improve the wear resistance and fatigue properties of maraging steel fabricated by laser powder-bed fusion within a few minutes. These achievements can expand applications of maraging steel fabricated by laser powder-bed fusion.

In this study, the surface of industrial pure iron was treated with atmospheric-controlled induction heating fine particle peening (AIH-FPP) using mechanical coating (MC) particles of carbon/steel obtained by mixing carbon powder and fine steel particles using mechanical milling. The surface modification effect and the mechanism of its effect were examined and considered by analyzing of the treated surface. Results showed that a modified layer in which carbon elements were diffused was formed near the treated surface by the AIH-FPP treatment using MC particles. In addition, by examining the influence of the treatment conditions on the formation of the modified layer by the design of experiments, the treatment temperature showed the most significant influence among the treatment temperature, peening time, and gas flow rate. The higher the treatment temperature, the deeper the carbon diffused layer. In addition, when treated at ≥1273 K, the microstructure near the surface became pearlite, and Vickers hardness increased. The time required for carbon diffusion in the AIH-FPP carburizing process using MC particles of carbon/steel was approximately the same as that of a general carburizing treatment using a reactive gas. In the AIH-FPP carburizing process, carbon is considered to be transferred from the particles to the surface, and the grain boundaries and dislocations increased by fine particle preening (FPP) are used as channels to diffuse inside the specimen. This mechanism is entirely different from the conventional carburizing methods.

Fine particle peening (FPP) at room temperature was introduced to create finer grains at the surfaces of Fe–Cr alloys that were then treated by atmospheric controlled induction heating FPP (AIH-FPP). The effect of the proposed treatment on the grain refinement behavior of the alloys was investigated using optical microscopy and micro-Vickers hardness testing. The FPP pretreatment at room temperature increased the effectiveness of the grain refinement via dynamic recrystallization during subsequent AIH-FPP. Grain refinement at the surface of Fe–Cr alloys progressed with an increase in the cycles of the treatment. The addition of Cr in Fe–Cr alloys promotes the formation of fine grains during the treatment. Specimens with fine grains created via static recrystallization were also prepared and compared to grains formed via dynamic recrystallization by the room temperature FPP pretreatment and subsequent AIH-FPP to evaluate the effectiveness of the proposed treatment. The grains formed through dynamic recrystallization by the proposed treatment were finer than those formed through static recrystallization. These results indicate that the proposed combination of room temperature FPP and AIH-FPP is effective for the formation of fine grains at the surfaces of Fe–Cr alloys.

This work characterized the surface-modified layers generated on a titanium alloy during gas blow induction heating (GBIH), so as to elucidate the rapid nitriding mechanism. The surface microstructures of GBIH nitrided specimens were analyzed by X-ray diffraction, micro-Vickers hardness testing, scanning electron microscopy and X-ray photoelectron spectroscopy. The data indicate that applying this process for only several minutes can forms nitrided layers on Ti-6Al-4V alloy surfaces, the characteristics of which are similar to those of layers produced by ordinary gas nitriding for several hours. The temperature inside the material was determined to be higher than that on the surface during GBIH nitriding. However, the thickness of the nitrided layer exceeded the estimated value based on the diffusion coefficient at this high internal temperature. This especially thick nitrided layer can possibly be attributed to eddy currents generated in the metal by the application of a high-frequency current. Passivation films on the surface of the Ti-6Al-4V alloy were found to be removed during GBIH nitriding, which is ascribed to the diffusion of oxygen atoms in the passivation film into the substrate. Thus, the data show that GBIH nitriding is able to modify a Ti-6Al-4V alloy within a short period of time based on the effect of an eddy current and the disappearance of the passivation film.

In this study, the particle collision treatments (fine particle bombarding (FPB) and conventional shot peening (SP)) were performed for Ti–6Al–4V alloy in order to comprehensively investigate the influences of their treatment conditions on the surface conditions, microstructures, hardness distributions and residual stress. Moreover, we examined the relationship between the surface characteristics of the treated materials and their mechanical properties including fatigue strength in detail. According to the obtained experimental results, the values of the maximum height waviness increased with increasing in the size of collision particles and injection pressure. Among all the treated materials, the differences in hardness distribution were relatively small because the work hardening exponent of Ti–6Al–4V alloy is low and the degree of work hardening is insensitive to the amount of plastic deformation. The absolute values of compressive residual stress generated at the surfaces became higher when finer particles were used as collision particles. The increase in injection pressure increased the surface temperature and released the introduced residual stress. As the particle collision treatments affected only the surface regions, the mechanical properties of the treated materials were almost the same as those of the annealed one. In the range of the conditions examined in this study, the most effective treatment to improve the fatigue strength of Ti–6Al–4V alloy was FPB under a relatively low injection pressure. This was because the value of the maximum height waviness was the smallest and the highest compressive residual stress was introduced at the same time.

© 2018 CIRP A modified surface layer using a Yb nanosecond fiber laser beam under atmospheric conditions was fabricated by applying a surface nitriding process on disc-shaped Ti–6Al–4V samples. On the generated surface, a melt layer comprising a TiN dendrite structure and a heat affected zone containing a needle-like α phase were detected. The modified layer exhibited optimal characteristics with the treatment at a laser output power of 12 W, resulting in a hardness of 12 GPa. The results of friction wear evaluation tests revealed that the layer exhibited superior abrasion resistance compared with an untreated Ti–6Al–4V substrate.

© 2018 The Japan Society for Heat Treatment. The effect of the intermetallic compound layer formed by atmospheric controlled induction heating fine particle peening (AIH-FPP) treatment on the wear resistance and high-temperature oxidation resistance of titanium alloy was investigated. To form an intermetallic compound layer on a titanium alloy by AIH-FPP treatment, pure titanium, aluminum and nickel particles were mechanically milled using a planetary ball mill, and the prepared particles were used as shot particles in the AIH-FPP treatment. The results showed that intermetallic compound layers, consisted of Ti3Al and Ti2NiAl3, were formed on titanium alloy by AIH-FPP treatment using the prepared particles. The wear resistance and high-temperature oxidation resistance of the AIH-FPP treated specimen were higher than those of the untreated ones.

The effect of atmospheric pressure nitrogen plasma in conjunction with femtosecond laser surface modification of Ti–6Al–4V samples was investigated. It was suggested that the superior friction and wear characteristics of the LP series (the sample subjected to both laser and plasma irradiation) were induced by the synergistic combination of the following three factors: the increased smoothness induced by homogenizing the laser-induced periodic surface groove shape structure, the high hardness caused by grain refinement and the high toughness and low friction properties conferred to the sample by the formation of oxynitride.

© 2018 Elsevier B.V. In order to develop a rapid nitriding process for titanium alloy at a low temperature, fine particle peening (FPP) was performed prior to gas blow induction heating (GBIH) nitriding. The surface microstructures of a Ti-6Al-4V alloy treated by FPP and GBIH nitriding were characterized using X-ray diffraction, optical microscopy, scanning electron microscopy, laser microscopy and micro-Vickers hardness testing. The results indicate that nitrogen diffusion into titanium during GBIH nitriding was accelerated by the pre-treatment with FPP, and a nitrided layer was formed on the surface of the Ti-6Al-4V alloy at a relatively low temperature (923 K), which grain coarsening and phase transformation did not occur and a nitrided layer was not formed only by GBIH nitriding. This is because a fine-grained surface layer was generated by the pre-treatment with FPP. Reciprocating ball-on-disk wear tests were carried out to examine the wear resistance of the prepared specimens. GBIH nitriding at a low temperature improved the wear resistance of the Ti-6Al-4V alloy following the pre-treatment with FPP, due to the formation of a hard nitrogen compound layer at the surface. The results show that the proposed surface treatment process was effective at modifying the surface properties of a titanium alloy at a low temperature and within a short span of time while avoiding grain coarsening and phase transformation.

© 2018 Elsevier B.V. Atmospheric-controlled induction-heating fine particle peening (AIH-FPP) using Cr and Mo shot particles was introduced to improve the electrochemical characteristics of AISI 1045 medium carbon steel in NaCl solution. AIH-FPP can form transferred and diffused layers of shot particles on the surface of carbon steel. The corrosion potential of the AIH-FPP-treated steel was higher than that of the untreated steel in a NaCl environment. Furthermore, a passivation region was observed in the carbon steel treated with AIH-FPP using Cr shot particles after peening with Mo shot particles through the formation of a surface-modified layer with high Cr and Mo concentrations in 20% NaCl solution.

© 2018 Atmospheric-controlled induction-heating fine particle peening (AIH-FPP) was conducted in a N2 atmosphere to form ultrafine grains on the surface of Fe-Cr alloys. The surface microstructure of Fe-2%Cr alloy and Fe-10%Cr alloy treated with AIH-FPP at 973 K and 1073 K was characterized using optical microscopy, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). AIH-FPP can be used as a thermomechanical process to form ultrafine grains on the surface of Fe-Cr alloys because strain is induced by AIH-FPP at a high rate and high temperature during dynamic recrystallization. In particular, the grain size of Fe-Cr alloys tends to decrease with increasing AIH-FPP treatment time. This is attributed to the increase in the strain induced by AIH-FPP with increasing AIH-FPP treatment time during dynamic recrystallization. Thus, the AIH-FPP-induced stratification structure composed of fine grains becomes more pronounced with increasing treatment time. Furthermore, the AIH-FPP treatment temperature at which fine grains are formed depends on the Cr concentration of the Fe-Cr alloys.