ANALYSIS OF PNEUMATIC FINE PARTICLE PEENING PROCESS BY USING A HIGH-SPEED-CAMERA

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1 International Journal of Moern Physics B Vol. 24, Nos. 15 & 16 (21) Worl Scientific Publishing Comany DOI: /S ANALYSIS OF PNEUMATIC FINE PARTICLE PEENING PROCESS BY USING A HIGH-SPEED-CAMERA TATSUYA ITO * , Hiyoshi, Kohoku-ku, Yokohama-shi, Kanagawa, , Jaan * tatsuya52@a3.keio.j SHOICHI KIKUCHI ** , Hiyoshi, Kohoku-ku, Yokohama-shi, Kanagawa, , Jaan ** shoichi-k@z8.keio.j YO HIROTA *** , Hiyoshi, Kohoku-ku, Yokohama-shi, Kanagawa, , Jaan *** you118s@hotmail.com ATSUSHI SASAGO **** , Hiyoshi, Kohoku-ku, Yokohama-shi, Kanagawa, , Jaan **** a-sasago@mmm-keio.net JUN KOMOTORI Deartment of Mechanical Engineering, Keio University, , Hiyoshi, Kohoku-ku, Yokohama-shi, Kanagawa, , Jaan komotori@mech.keio.ac.j Receive 1 October 28 In this stuy, the eening behavior of shot articles in a fine article eening (FPP) rocess such as velocity an imact angles were analyze by using a high-see-camera. Results showe that the velocity of shot articles eene on a eening ressure; the higher the eening ressure, the higher the article velocity. The article velocity measure in this stuy was aroximately 12 m/s; this was much higher than that of the conventional shot eening (SP) rocess. This was because the air resistance of shot articles in the FPP rocess was higher than that of shot articles in the SP rocess. In orer to iscuss the surface moification effect of the FPP rocess, commercial-grae ure iron treate by the FPP rocess was characterize by micro-vickers harness tester an scanning electron microscoe (SEM). Thickness of harene layer treate with higher eening ressure was much higher than that of the lower ressure treate one. The unique microstructure 347

2 348 T. Ito et al. with stratification atterns, which was harer than that of the other art, was observe near the secimen surface. The reason for the microstructural changes by the FPP treatment was iscusse base on the kinetic energy of shot articles. Keywors: Surface treatment; eening; article velocity; flui ynamics; microstructure; harness. 1. Introuction Shot eening (SP) rocess has been wiely use as a one of the surface moification techniques to imrove the mechanical roerties of metallic materials. This rocess forms a thick harness layer with a high comressive resiual stress at the surface. Fine article eening (FPP) treatment introuce in this stuy is a similar rocess to conventional SP rocess. 1 The ifference is that the shot articles using in the FPP rocess is much smaller than that in the SP rocess; less than 2 µm in iameter. The FPP rocess harens the surface an generates a higher comressible resiual stress near the surface. The inuce comressive resiual stress is much higher than that of the orinary SP rocess. Therefore, this rocess is commonly use to imrove the fatigue strength of machine an structural comonents. 2 3 The egree of harness an comressive resiual stress by the FPP treatment are change eening on the eening arameters such as article velocity, imact irection, coverage an iameter of shot articles. In orer to evaluate eening effects, Almen-scale, which measures the arc-height, is wiely use 4 5 ; however, this value is qualitative, then arc-height is not an aroriate inex for eening intensity. Therefore, a quantitative inex such as energy of shot article is neee to evaluate eening effects quantitatively. Maea et al. have measure velocity of shot articles use in the FPP treatment by using a high-see CCD analyzer an calculate the change of substrate temerature in the eening rocess. 6 They faile to reort the quantitative evaluation of the eening effects on the basis of the energy of shot article. The aims of resent stuy are to erform irect observation of the shot articles in neumatic fine article eening rocess an to iscuss the surface moification effect uner ifferent eening conitions base on the energy of shot article. 2. Exerimental Proceures Shot article use in the FPP treatment was a high-see tool steel; SKH59 (M42; ASTM stanar), with mean iameter of 63 µm an harness of 876 HV. In orer to investigate the effect of article iameter on its velocity, the articles use for the orinary SP rocess, with mean iameter of 8 µm an harness of 75 HV, was also reare. Figure 1 shows the schematic illustration of exerimental set u for analyzing the behavior of shot articles in eening rocess. The tye of eening machine was a suctionressure tye. The secimen was set 1 mm away from the eening nozzle. The behavior of shot articles near the secimen surface was observe every 1/27 secons with a high-see camera. The observe area was aroximately 1 mm 3 mm. The substrate material use in this stuy was commercial-grae ure iron with a carbon content of.8 %. Disc-shae secimens (substrate) with a iameter of 15 mm

3 Analysis of Pneumatic Fine Particle Peening Process by Using a High-See-Camera 349 an thickness of 4 mm were olishe with emery aer (#32 to #12) an then anneale at 1223 K for 2 h in a vacuum furnace. FPP was then erforme on the samles accoring to the conitions given in Table 1. In orer to investigate the surface moification effect, surface microstructures of treate secimens were observe using a scanning electron microscoe (SEM). The istribution of the micro-vickers harness of FPP treate secimen was also measure at the cross-section of the secimen. 3. Results an Discussion 3.1. Analysis of shot article behavior in eening rocess Figure 2 shows the combine image of a shot article behavior in the FPP rocess observe with the high-see camera. The shot article flying angle, rojecting from the eening nozzle, was change after reflection at the secimen surface. This was may be cause by the rotation of the shot article. Figure 3 shows the relationshi between article velocity an eening ressure. The velocity of shot article was increase with an increase in eening ressure; the graient of velocity in the FPP rocess was higher than that in the SP rocess. The article velocity in the FPP rocess was much higher than that measure in the orinary SP rocess. In orer to iscuss the reason for this, the behavior of shot article was evaluate on the basis of flui ynamics. The riving force of shot article is erive from the air resistance; D. Therefore, the equation of motion of shot articles is efine as in Eq. (1). Peening nozzle Shot article High-see camera Light 1 mm Secimen Work holer Fig. 1. Exerimental set u. Shot article Table 1. Peening conition. Shot article FPP SP Pressure [MPa].1,.2,.3,.4,.5,.6 Fee rate [g/s] 5 2 Peening time [s] 3 3 Shot article.3,.6 Observation area (1.3 mm 1.2 mm) Secimen surface 3 µm Secimen Work holer Fig. 2. Behavior of shot article observe by high-see camera.

4 35 T. Ito et al. ( ) Ma D ρa CD π N va v (1) 2 4 where ρ a, ρ, C D,, N, v a an v are air ensity, article ensity, resistance coefficient, article iameter, number of articles, air velocity an article velocity, resectively. M is article mass, an it is given by Substituting Eq. (2) to Eq. (1), the following equation can be obtaine Ma 3Mρa D 4ρ 2 ( v v ) When M, ρ a an ρ are constant, ΣD is affecte by C D /. In the case of.6 MPa in eening ressure, the value of air velocity (v a ) is assume subsonic see; 3 m/s. Then, the figures of the Reynols number in FPP an SP are calculate as follows: Re 1 M ρ π 6 ρ ρa va vsp SP ReSP 128 (5) µ a where µ a is viscosity coefficient of air. From the results of the Reynols number, the value of C D / in FPP an SP are calculate as follows: ( v v ) a a FPP FPP FPP µ a ( ) 3 N a C 735 D (2) (3) (4) C D FPP FPP 8159 (6) C D SP SP Consequently, the value of C D / of the FPP rocess is much higher than that of the orinary SP rocess. Therefore, the air resistance which accelerates FPP articles becomes larger than that of SP articles, resulting in higher velocity. Figure 4 shows the velocity of FPP article before an after reflection at the secimen surface. The article velocity ecrease significantly after reflection. The egree of article velocity ecrease at.6 MPa was larger than that at.3 MPa. This result inicates that the translational kinetic energy at.6 MPa was high comare to that at.3 MPa. The angular velocity before an after reflection was also measure, an then the rotational kinetic energy was calculate. As a result, the rotational kinetic energy of a shot article ( J) was negligibly-small in comarison to the translational kinetic energy of a shot article ( J) Evaluation of FPP conitions on surface moification effect In orer to investigate the effect of the FPP treatment conition on surface moification ability; surface roughness was measure using a scanning laser microscoe. Figure 5 shows the relationshi between eening ressure an surface roughness Ra. The surface 573 (7)

5 Particle velocity, m/s FPP SP Analysis of Pneumatic Fine Particle Peening Process by Using a High-See-Camera Peening ressure, MPa Fig. 3. Relationshi between article velocity an eening ressure (mean ± SD). Particle velocity, m/s roughness treate FPP increase comare to the olishe secimen; the higher the eening ressure, the larger the surface roughness. Figure 6 shows the relationshi between Vickers harness at cross-section of the secimen an eening ressure. The FPP treate secimen, the harness value at the 2 µm from the surface ( ), was higher than the untreate secimen; however, no noticeable ifferences were observe between.2 MPa an.6 MPa series. Deth of harene layer was thicker with an increase of eening ressure. In orer to clarify the egree of lastic eformation near the surface, microstructures of the FPP treate secimens were observe at cross-sections. Figure 7 shows the tyical features of the FPP treate surfaces. In the case of the FPP treate secimen with eening ressure of.2 MPa to.6 MPa, local stratification atterns forme by a concentration of lastic eformation were observe. Toaka et al. reorte that this unique microstructure is generate as a result of a foling u an thrusting rocess. 7 These tyes of microstructure became thicker with an increase in eening ressure. The harness of this microstructure was higher than that of outsie the structure. Therefore, the harness value of the.1 MPa series, microstructure without local stratification atterns, was lower than that of the secimen treate with eening ressure of.2 MPa to.6 MPa. Surface roughness, work harening an microstructural changes by FPP treatment Surface roughness Ra, µm Polishe (.44 µm) Peening ressure, MPa Fig. 5. Relationshi between surface roughness an eening ressuure (mean ± SD). Vickers harness, HV (.245 N) Before reflection After reflection.3 MPa.6 MPa Fig. 4. Particle velocity before an after imact (mean ± SD) Untreate (15 HV) 2 µm 5 µm 1 µm Peening ressure, MPa Fig. 6. Relationshi between Vickers harness an eening ressure..7

6 352 T. Ito et al. were closely relate to eening ressure. This suggests that the surface moification by FPP treatment was controlle by the kinetic energy of shot articles. 4. Conclusion In this stuy, the behavior of shot article in the eening rocess was irectly observe with a high-see camera. The result inicate that the higher the eening ressure, the higher the article velocity. The article velocity of the FPP rocess was much higher than that of the orinary SP rocess. This was because the air resistance accelerating FPP articles was high in comarison to that of the SP article. The effect of eening ressure on harness an microstructural changes of eene secimen were also iscusse. Harene layer an the local stratification atterns became thicker with the increase in eening ressure. These results inicate that the surface moification by an FPP rocess eens on the kinetic energy transmitte by the reflection of shot article. References Fig. 7. SEM observations of microstructures of the FPP treate secimen at cross-sections. 1. J. L. Liu, M. Umemoto, Y. Toaka, K. Tsuchiya, Journal of Materials Science, 42, 7716 (27). 2. D. Yonekura, J. Komotori, M. Shimizu an H. Shimizu, Transactions of the Jaan Society of Mechanical Engineers, A66, 118 (2), in Jaanese. 3. Y. Haraa, K. Fukaura an S. Haga, Journal of Materials Processing Technology, 191, 297 (27). 4. M. Guagliano, Journal of Materials Processing Technology, 11, 277 (21). 5. W. Cao, R. Fathallah, L. Castex, Materials Science an Technology, 11, 967 (1995). 6. H. Maea, N. Egami, C. Kagaya, N. Inoue, H. Takeshita an K. Ito, Transactions of the Jaan Society of Mechanical Engineers, C67, 27 (21), in Jaanese. 7. Y. Toaka, M. Umemoto, Y. Watanabe an K. Tsuchiya, Transactions of the Materials Research Society of Jaan, 29, 3523 (24).