Koichiro NAMBU

In recent years, many problems have arisen, such as increased power consumption in commercial refrigerators due to water droplets on aluminum alloy surfaces, increased frost formation due to bacteria and mold, and increased transportation costs for bullet trains. To solve these problems, we turned our attention to surface texturing technology and shot blasting, a new surface component that can switch between hydrophobic and hydrophilic properties. Shot blasting can be applied to large areas at a lower cost than laser processing and can be used for surface texturing. This study reports the results of an evaluation of the effect of shot blasting on the surface wettability of aluminum alloys. Shot blasting was performed on aluminum alloy A5052 using various particles, and the wettability of the specimens was evaluated. It was found that the wettability after shot blasting varied with the particle material and particle size as in previous studies. Using the aspect ratio (dent depth/dent width) calculated from the surface roughness, it was found that the specimens became hydrophilic to hydrophobic up to an aspect ratio of 7 or less, and that the contact angle gradually increased to hydrophobic at an aspect ratio of 7 or greater.

Surface modification treatments using particle impact, such as shot peening and fine particle peening, are used in many parts such as automotive gears. However, the surface modification effect is affected by many factors such as the material of the projectile, particle size, and nozzle diameter. In the case of the air injection type, the airflow inside and outside the nozzle has a particularly large effect. However, few studies have examined the effects of airflow inside and outside the nozzle. In this study, the airflow inside and outside the nozzle was analyzed using the finite element method. As a result, it was found that the airflow velocity increases as the nozzle diameter increases. It was also clarified that the factors causing this increase were the lengthening of the potential core section and the deceleration caused by the wall resistance.

Fine Particle peening is a method to obtain surface modification effects, such as fatigue strength improvement, by bombarding the work material with particles at high velocity. However, there are many factors that affect the surface modification effect, making it difficult to select the optimum conditions. The particle velocity and particle flight behavior have not been clarified due to the large number of flying particles in addition to the extremely high particle velocity. Therefore, in this study, in addition to air flow analysis inside and outside the nozzle, particle velocity analysis using the particle method was conducted. ANSYS was used for the airflow analysis, and Particle Works was used for the particle method. The nozzle diameter and nozzle-to-work distance were varied. The nozzle diameter was varied from 3 to 10 mm. The nozzle-to-work distance was 50, 100, and 150 mm. The pressure at the nozzle entrance was set to 0.2 MPa, and air flow analysis was performed under incompressible fluid conditions. The particle method used iron-based particles with a particle diameter of 100 μm as a model for analysis. The results of the airflow analysis showed that the potential core area increases as the nozzle diameter increases. This was attributed to the shear layer caused by the wall resistance inside the nozzle. Next, particle velocity analysis showed that particle velocity tended to increase with increasing nozzle diameter. In addition, it was found that the particle velocity increased with increasing nozzle-to-work distance. Next, the particle flight behavior was analyzed, and it was found that the particles accelerated most at the parallel part of the nozzle and continued to accelerate after the nozzle exit. Finally, to verify the validity of the analysis, the particle velocities were compared with those measured by a high-speed camera. Although the geometry of the nozzle was slightly different, the measured and calculated velocities showed similar trends, suggesting that the present method is valid.

In this study, SCM440 steel specimens with different surface morphology and hardness were prepared by shot peening and fine particle peening, followed by induction hardening and tempering at different temperatures. Rotating bending fatigue tests were performed for these specimens, and the combined effects of the surface dent formed by peening and residual stress on the fatigue limit of the induction hardened steels were quantitatively investigated. It was found that the fatigue limit of the induction hardened steel tended to decrease with an increase in the size of the particles used in the peening. The parameter of surface morphology that showed a good correlation with the fatigue limit of the induction hardened steel was the waviness parameter, but not the roughness parameter. Furthermore, a fatigue limit estimation for induction hardened steels with different surface morphology was described. The improvement in the fatigue limit of steels with surface dents due to compressive residual stress was more significant as the hardness decreased, and the maximum fatigue limit improved by compressive residual stresses increased as the size of surface dents decreased.