Koichiro NAMBU

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.

Shot peening (SP) and fine particle peening (FPP) were performed as post-treatments on induction-hardened and tempered AISI 4140 steels. Furthermore, the combined effects of surface characteristics including changes in surface morphology, compressive residual stress, and hardness on the fatigue limit were quantitatively examined. The surface characteristics of the prepared specimens were investigated using a laser and stereomicroscope, a micro-Vickers hardness tester, and an X-ray device for residual stress measurements. The rotating bending fatigue properties were also examined. FPP increased the fatigue limit of induction-hardened and tempered steel samples by introducing compressive residual stress and increasing their surface hardness. Conversely, the fatigue limit of the steel treated with SP was lower than that of the electrochemically polished sample owing to the formation of large dents. The fatigue limits of post-peened induction-hardened and tempered steels can be estimated using a modified Goodman diagram considering changes in surface properties.

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.

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.

The mechanism of residual stress relaxation for carburized steels was elucidated and their fatigue limit was estimated. The compressive residual stress in the carburized steel after rotating bending fatigue tests is mainly influenced by the relaxation during the compressive loading phase, although it increases at the early stage. The fatigue limit of carburized steels can be estimated according to the modified Goodman diagram considering the residual stress relaxation. The proposed equation shows the maximum value of the fatigue limit (1013 MPa) of carburized steel and the maximum value of the compressive residual stress contributing to the increased fatigue limit.

Fine particle peening (FPP) using hydroxyapatite (HAp) shot particles was performed to improve the fatigue strength and form a HAp transfer layer on a beta titanium alloy (Ti–22V–4Al). The surface microstructures of the FPP-treated specimen were characterized using scanning electron microscopy, micro-Vickers hardness testing, energy dispersive X-ray spectrometry, X-ray diffraction, and electron backscattered diffraction. A HAp transfer layer with a thickness of 5.5 μm was formed on the surface of the Ti–22V–4Al specimen by FPP. In addition, the surface hardness of the Ti–22V–4Al was increased, and high compressive residual stress was generated on the specimen surface by FPP. Rotating bending fatigue tests were performed at room temperature in laboratory air over a wide cycle-life region (103–109 cycles). In the long cycle-life regime, the fatigue strength at 107 cycles of the FPP-treated specimen became higher than that of the untreated specimen. This result is attributed to the formation of a work-hardened layer with high compressive residual stress by FPP. However, the fatigue strength was not improved by FPP in the short cycle-life regime, because fatigue cracks were initiated at surface defects formed during the FPP process. The fatigue fracture mode of the FPP-treated specimens shifted from surface-initiated fracture to subsurface-initiated fracture at a stress amplitude level of 600 MPa.

Fine particle peening (FPP) using hydroxyapatite (HAp) shot particles can form a HAp layer on room-temperature substrates by the transfer and mi-crostructural modification of the shot particles. In this study, FPP with HAp shot particles was applied to form a HAp surface layer and improve the fatigue properties of Ti–6Al–4V extra-low interstitial (ELI) for use in bio-implants. The surface microstructures of the FPP-treated specimens were characterized by micro-Vickers hardness testing, scanning electron microscopy, energy-dispersive X-ray spectrometry, X-ray diffraction, and X-ray photoelectron spectroscopy. FPP with HAp shot particles successfully formed a HAp layer on the surface of Ti–6Al–4V ELI in a relatively short period by shot particle transfer at room temperature however, the thickness and elemental composition of the HAp layer were independent of the FPP treatment time. The original HAp crystal structure remained in the surface-modified layer formed on Ti–6Al–4V ELI after FPP. Furthermore, FPP increased the surface hardness and generated compressive residual stresses at the treated surface of Ti–6Al– 4V ELI. Four-point bending fatigue tests were performed at stress ratios of 0.1 and 0.5 to examine the effect of FPP with HAp shot particles on the fatigue properties of Ti–6Al–4V ELI. The fatigue life of the FPP-treated specimen was longer than that of the un-peened specimen because of the formation of a work-hardened layer with compressive residual stress. However, no clear improvement in the fatigue limit of Ti– 6Al–4V ELI occurred after FPP with HAp shot particles because of subsurface failures from characteristic facets.

Fine particle peening (FPP) and plasma spraying using hydroxyapatite particles were applied to a beta titanium alloy, Ti-22V-4Al, to form the hydroxyapatite (HAp) layer on the surface. As a result, HAp layer was formed on the specimen surface by the both treatments. The thicknesses were 5 μm and 100μm, respectively for FPP treated and plasma sprayed specimens. In the FPP treated specimen, Vickers hardness was increased by FPP compared with that of the untreated specimens, resulting in work-hardening. Rotary bending fatigue tests were carried out on both treated and untreated specimens. The FPP treated specimens exhibited higher fatigue strength than the untreated specimens. It is due to the increase in hardness and compressive residual stress by FPP. On the other hand, significant deteriorations of the fatigue strength for the plasma sprayed specimen was observed in comparison with the result for the untreated specimen. As a result of fracture surface observation in plasma sprayed specimen, the defect formed by blasting before the plasma spraying at interface between substrate and HAp layer was observed at crack initiation site. Thus, the defect plays a role as the crack starter in the case of plasma sprayed specimen.

<p>SUP10, which has surface defects, was subjected to a fatigue test and the effects of fine particle peening were analyzed. Moreover, a comparison with shot peening was performed.</p><p>The surface defect was made with a twist drill （0.2, 0.4, 0.6 mm）. The defect depth is half the diameter of the twist drill bit. High-speed steel balls were used for fine particle and shot peening.</p><p>When each processing was performed after introducing a surface defect in the sample, it was found that fine particle peening suppresses fatigue strength reduction more than shot peening. The reason for this is considered to be an improvement in the hardness of the drill bottom and the influence of residual stress.</p>

Fine particle peening (FPP) using hydroxyapatite (HAp) shot particles was introduced to form the HAp surface layer and improve the fatigue properties of commercially pure (CP) titanium. The surface microstructure of the FPP-treated specimens was characterized using a micro-Vickers hardness tester, optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX), X-ray diffraction (XRD), and non contact scanning white light interferometry. FPP could create a HAp layer on the surface of CP titanium within a relatively short time (1 s) by shot particle transfer. In addition, FPP increased the surface hardness and generated compressive residual stress at the treated surface. Four-point bending fatigue tests were performed at a stress ratio of 0.1 in air at room temperature to examine the effect of FPP using HAp shot particles on the fatigue properties of CP titanium. It was found that the fatigue limit for the FPP-treated specimen was higher than that for the unpeened specimen. The fatigue fracture mechanism for the CP titanium treated with FPP was discussed from the viewpoint of fractography. The HAp layer remained on the surface without delamination after the fatigue tests. (C) 2016 Elsevier B.V. All rights reserved.

Shot peening using particles 10 mu m in diameter (ultra-fine particle peening: Ultra-FPP) was introduced to improve the fatigue properties of 5056 aluminum alloy. The surface microstructures of the Ultra-FPP treated specimens were characterized using a micro-Vickers hardness tester, scanning electron microscopy (SEM), X-ray diffraction (XRD), non-contact scanning white light interferometry, and electron backscatter diffraction (EBSD). The Ultra-FPP treated specimen had higher hardness than the conventional FPP treated specimen with a short nozzle distance due to the high velocity of the ultra-fine particles. Furthermore, the surface hardness of the Ultra-FPP treated specimen tended to increase as the peening time decreased. Fatigue tests were performed in air at room temperature using a cantilever-type rotating bending fatigue testing machine. It was found that the fatigue life of the Ultra-FPP treated specimen tended to increase with decreasing peening time. Mainly, the Ultra-FPP improved the fatigue properties of 5056 aluminum alloy in the very high cycle regime of more than 10(7) cycles compared with the un-peened specimens. This is because the release of the compressive residual stress is small during fatigue tests at low stress amplitudes. (C) 2015 Elsevier B.V. All rights reserved.

The aim of this research is to clarify the influence of vacuum carburizing on the fatigue-crack progress characteristics of DSG2 steel. The test specimen tempering material (QT material) and vacuum carburizing material (VC material) has been used. The fatigue-crack progress was examined by subjecting the samples to four-point bending. The loading-capacity fixed experiment was done using a maximum load of P&lt inf&gt max&lt /inf&gt = 4000-7000 N. The ΔK fixed experiment was done using a load of ΔK = 18–36 MPa√m. The crack progress speed of VC material fell, after the high crack progress speed was shown, and after it showed the minimum, it showed the tendency to go up again. This is considered to be what is depended on the compressive residual stress given to the carburizing layer. From this, it is thinkable that there is a crack progress depression effect in a carburizing layer. In VC material, a carburizing layer has a crack progress depression effect from a plunger-helix bottom to about 2.6 mm, and it turned out that it is larger than an effective carburizing layer. Moreover, in each ΔK, it was shown that depression effect revelation differs and the crack progress process accompanying it was able to be shown typically.

In late years a study of advantageous high Si hard-drawn wire is pushed forward in comparison with oil tempered wires on a cost side. In this study, influence of the minute notch which gave it to rotary bending fatigue strength of the high Si hard-drawn wire and influence of the shot peening were investigated. The test bar which we gave shot peening treatment and notch to was prepared, and each hardness, residual stress, fatigue strength were investigated. A Nakamura-type rotary bending fatigue testing machine was used in fatigue test. SEM was used for fracture surface and organization evaluation. The fatigue limit of shot peening materials rose 45% in comparison with untreated material. When the linear notch of the 30 degrees ∼ 90 degrees notch angle was given in the case of notch angles less than 60 degrees, the fatigue limit of untreated material and the electrolytic grinding material became equal to smooth specimen. Influence of the wire drawing structure is considered as a reason.

The FPB (Fine Particle Bombarding) process attracts attention as a new surface-preparation technique. The influence of surface modification by the FPB process on various characteristics of SUP10 was investigated. The surface roughness of FPB treatment materials decreased more than shot peening treatment materials. As a result of comparing the shot time of FPB process, it proved that surface roughness increased as the shot time became long. On every projection conditions of FPB process, the hardness near the surface rose higher than the shot-peening. Hardness was the maximum at the time of shot time 20 seconds, and residual stress was the maximum at the time of 5 seconds. The maximum compressive residual stress was a value equivalent to a shot-peening process. The fatigue strength of the FPB material rose than shot-peening material by low surface roughness and high surface hardness. These results showed that a FPB process was effective in the improvement in fatigue strength of spring steel.

Surface modification by fine particle bombardment is mainly influenced by the velocity and diameter of a particle as well as the material properties. In this study, numerical calculation of particles velocity through a convergent nozzle is conducted by considering air as a compressible fluid. Because the axial flow of the convergent nozzle was much more dominant than that of a flat nozzle, the modeling of particle bombardment was accomplished based on a one dimensional isentropic fluid flow. The apparatus presupposed here is a direct pressure-type shot peening machine in which the air inside the nozzle expands and accelerates to the critical speed. Then, the particle accelerates by drag received from air, and the particle velocity increases up to the air velocity. The results of our numerical calculations show the particle velocity obtained from our numerical calculations values is close to that of the experimentally measured values. This demonstrates that our simple and easy model is reasonably accurate and can be used to give insight into the mechanism and selection of optimum drive parameters such as size and material properties of the particles used in the shot peening process.

In this study, the decreasing rate of fatigue strength by the directionality of a notch and the betterment effect of the fatigue strength by a shot-peening were examined. The straight notch with a notch angle of 30 -90 ° was given to the specimen, and the rotating bending fatigue test was carried out. As a result, in the case of 90° , it fractured at the notch, and the fatigue limit was reduced 14%. In the case of 60 ° or less, it fractured from the outside of notch with stress of near the fatigue limit, and the fatigue limit was equivalent to the plain wire. However, in the case of the oil tempered wire without remarkable drawn structure, the fatigue limit became decreased as the notch angle became large. This fact shows that the high Si hard drawn-wire with a notch angle of 60 ° or less did not fractured at a notch by the effect of drawn structure. When shot peening process was performed after giving the notch, the fatigue limit was higher than the untreated wire. This showed that a shot peening process was effective in the improvement of the decrease of the fatigue strength by a small defect.