Effect of Continuous Rotation Evolutional Control on the Surface Color of Anodized Al Mg Alloys* 1

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1 Materials Transactions, Vol. 5, No. 7 (29) pp. 792 to 797 #29 The Japan Institute of Metals Effect of Continuous Rotation Evolutional Control on the Surface Color of Anodized Al Mg Alloys* Satoshi Oue, Hiroaki Nakano, Daisuke Kurai 2; * 2, Hisaaki Fukushima, Katsuaki Nakamura and Masataka Masuda Department of Materials Science & Engineering, Kyushu University, Fukuoka 89-95, Japan 2 Department of Materials Process Engineering, Kyushu University, Fukuoka 89-95, Japan User Science Institute, Kyushu University, Fukuoka , Japan Al Mg alloys treated by continuous rotation evolutional control (CREO) were anodized and colored by alternating current electrolysis at V in sulfate solutions containing Cu, Sn, Ni, or Co at K. Electrolysis turned the anodized Al Mg alloys to rufous (Cu), yellow to gold (Sn), or reddish brown (Ni and Co). Their lightness was enhanced significantly by CREO, regardless of the metal deposited. The gloss of the Al Mg alloys decreased probably due to increase in surface waviness resulting from CREO, and the enhancement in lightness with CREO is attributed to this decrease in the glossiness. SEM images showed that the micropore density in anodic oxide films on Al Mg alloys decreased as a result of CREO. The decrease in the micropore density is assumed to enhance the lightness of colored Al Mg alloys treated by CREO. [doi:.22/matertrans.mra28469] (Received December 5, 28; Accepted March, 29; Published June, 29) Keywords: aluminum-magnesium alloy, continuous rotation control, color, lightness, anodizing. Introduction Al alloys are generally anodized by electrolysis to improve properties such as corrosion resistance and abrasiveness. ) In porous anodic oxide films, Al alloys can be colored by depositing metals into micropores by applying alternating current. 5) In the development of new materials, appearance parameters, such as color and tone, are important to users in addition to corrosion resistance and abrasiveness. On the other hand, reducing the grain size of metallic materials to the submicrometer or even nanometer scale by continuous rotation evolutional control (CREO) is being increasingly studied as a method for improving mechanical properties such as strength and ductility. 6 ) In CREO, torsion strain can be concentrated in a zone of a columnar specimen heated locally using an induction coil. The CREO process is practical, because large strains can be applied continuously to the entire specimen by moving the columnar specimen in the longitudinal axis direction. Anodized Al alloys processed by CREO should exhibit a different appearance from conventional alloys after coloring, because their structure is significantly changed by CREO. Regarding the coloring of anodized Al alloys, many studies have been conducted on the coloring methods, 4, ) coloring mechanism, and the effect of structure 4 7) of anodic oxide films and surface roughness 8) of Al alloys. However, there are few reports on the effects of severe plastic deformation on the color of anodized Al alloys. In this study, anodized Al Mg alloys with CREO were colored by depositing Cu, Sn, Ni, or Co into micropores of Al oxide films by applying alternating current. The effect of CREO on the color of the anodized Al Mg alloy and the reasons for color change due to CREO were investigated using a colorimeter and by surface analysis. * This Paper was Originally Published in Japanese in J. Japan Inst. Metals 72 (28) * 2 Graduate Student, Kyushu University water spray for cooling shearing Fig. 2. Experimental induction coil for local heating continuous movement Schematic diagram of CREO. rotating AA556 (.% Cu,.6% Si,.4% Fe,.6% Mn, 4.94% Mg,.% Zn,.6% Cr, <:5% other elements, rest Al) was used as a specimen of Al Mg alloy. Figure illustrates the principle of the CREO technique. A portion of a rod ( mm 2 mm) was rotated at 25 rpm with respect to the other portion around its longitudinal axis. Torsion strain was introduced in the zone, which separated the two portions rotating in opposite directions. The torsion straining zone was localized by softening the zone more than other portions by local heating (inner 588 K, outer 58 K) and cooling, as shown in Fig.. While producing the torsion straining zone, the rod was moved at 5 mm/s along the longitudinal axis, which produced continuous severe plastic strain throughout the rod. To create torsion strain efficiently, the torsion straining zone must be narrow, and the rotation of the rod should be fast relative to the speed of the rod. The initial grain sizes of the Al Mg alloy prior to CREO were 5 mm. The average grain size of Al Mg alloy after CREO was confirmed by TEM observation to be. 2. mm. 9)

2 Effect of Continuous Rotation Evolutional Control on the Surface Color of Anodized Al Mg Alloys 79 Table Electrolysis conditions for coloring of anodized Al Mg alloy. Bath composition CuSO 4 5H 2 O H 2 SO 4.2 mol/l. mol/l Prior to anodization, the Al Mg alloy was carefully polished using No. 5 emery paper and immersed in a.75 mol/l NaOH solution at 298 K for s. Then, the alloy was neutralized in a.48 mol/l HNO solution for s and electropolished at 29 K in a solution containing methanol and perchloric acid (MeOH : HClO 4 ¼ 4:) at V for 5 min. Anodization was performed in a solution containing. mol/l of H 2 SO 4 and.85 mol/l of Al 2 (SO 4 ) 6H 2 O at 29 K under a constant cell voltage of 5 V for min; the solution was agitated at rpm using a magnetic stirrer. The anodized Al Mg alloy was colored by depositing Cu, Sn, Ni, or Co into micropores of Al oxide films by applying alternating current. Table shows the conditions for the alternating current electrolysis. The color of the anodized Al Mg alloy was evaluated based on JIS-Z using the colorimeter (CM-6d; Konica Minolta Co.). A pulsed xenon lamp was used as a light source, and the color was measured in an area with a diameter of 25.5 mm. Both the specular component exclude () and specular component include () methods were utilized for color evaluation. In the method, only diffuse reflectance light, with an exception of regular reflection, was measured to approach the visual estimation of a human; in the method, the total reflectance light, including regular reflection, was measured to evaluate the inherent color of the material. The color was presented with reference to the L a b system based on JIS-Z-8729:24. The gloss was measured at an angle of incidence of 45 using a digital variable gloss meter (Suga Test Instrument Co.). The surface roughness of electropolished Al Mg alloy was measured using a SURFCOM5DX-DF (Tokyo Seimitsu Co.). The surface roughness was evaluated at a cutoff value of.8 mm and measurement length of 4 mm. The surface morphologies of electropolished Al Mg alloy with and without anodization were observed by SEM. The Al Mg alloy was immersed in a solution containing nitric and phosphoric acid (HNO : H PO 4 ¼ 94 : 6) at 58 K for 2 min to observe the subgrain boundaries.. Results and Discussion Operating conditions Peak voltage of alternating current Frequency Temperature V 5 Hz C Electrolysis duration 2 s SnSO 4.47 mol/l (Standard duration Cu: 6 s, Sn: 8 s) H 2 SO 4. mol/l Counter electrode Pt NiSO 4 6H 2 O. mol/l Peak voltage of alternating current V H BO.49 mol/l Frequency 5 Hz CoSO 4 7H 2 O.8 mol/l Temperature C H BO.4 mol/l Electrolysis duration 5 s Glycerin.22 mol/l Counter electrode Pt. Coloring of anodized Al Mg alloy by alternating current electrolysis Anodized Al Mg alloy was colored by alternating current Table 2 Effect of CREO on color of anodized Al Mg alloy with Cu, Sn, Ni or Co depsoition. Cu Sn Ni Co a : : b a : b (+) a-value! ( ) (+) b-value! ( ) (Red) (Green) (Yellow) (Blue) Electrolysis Duration of Cu, t/s Fig. 2 Relationship between lightness of anodized Al-Mg alloy colored by Cu deposition and duration of electrolysis. electrolysis. Visual observation confirmed that electrolysis of Cu, Sn, Ni, or Co turned the alloy red (Cu), yellow to gold (Sn), or reddish brown (Ni and Co). Table 2 shows the effect of CREO on the color of anodized Al Mg alloys by alternating current electrolysis of Cu, Sn, Ni, or Co. The red coloration increases with the a-value, while green coloration increases when the value becomes more negative. On the other hand, yellow coloration increases with the b- value, and blue coloration increases as the b-value turns negative. In coloring by Cu electrolysis, the both the a- and b-values evaluated by the method were higher for the alloys treated with CREO than those without CREO, while evaluation by showed the a- and b-values to be hardly affected by CREO. In coloring by Sn, Ni or Co electrolysis, CREO had little effect on the a- and b-values, irrespective of the method for evaluation used. Figure 2 shows the relationship between the lightness of anodized Al Mg alloy colored by Cu electrolysis and the electrolysis duration. The method showed decrease in the lightness with an increase in the duration of electrolysis, whether or not CREO was applied, reaching almost zero for durations longer than s. For durations of 2 and 6 s, the lightness of anodized Al Mg alloy was higher with CREO than without. On the other hand, in method to evaluate the inherent color of materials, the lightness decreased with an increase in the duration of electrolysis. At the electrolysis durations longer than 6 s, the lightness of the anodized Al Mg alloy was higher with CREO than without. At 2 s, the lightness evaluated by was not affected by CREO; however, the lightness evaluated by was higher with

3 794 S. Oue et al Cu Sn Ni Co Gloss, G Electrolysis Duration of Sn, t/s Fig. Relationship between lightness of anodized Al-Mg alloy colored by Sn deposition and duration of electrolysis Ni Ni Co Co without with without with without with without with CREO CREO CREO CREO CREO CREO CREO CREO Fig. 4 Effect of CREO on lightness of anodized Al-Mg alloy colored by Ni or Co deposition. CREO than without. This indicates that CREO decreases the intensity of regular reflection, and gloss. Figure shows the relationship between the lightness of the anodized Al Mg alloy colored by Sn electrolysis and the electrolysis duration. The method indicated lightness to be higher with CREO than without for all electrolysis durations, while the lightness evaluated by showed little effect of CREO, except for s of electrolysis. Although the inherent lightness of the anodized Al Mg alloy colored by Sn electrolysis was hardly affected by CREO, the lightness, evaluated by, increased with CREO. Figure 4 shows the effect of CREO on the lightness of the anodized Al Mg alloy colored by Ni or Co electrolysis. The evaluation by the method showed that the lightness of the alloy colored by Ni or Co electrolysis was higher with CREO than without. On the other hand, the evaluation by the method showed that the lightness of the alloy colored by Ni electrolysis was higher with CREO than without, but the alloy colored by Co electrolysis was hardly affected by CREO. As mentioned above, in all electrolytic colorings of Cu, Sn, Ni, and Co, the lightness of anodized Al Mg alloys evaluated by was higher with CREO than without without with without with without with without with CREO CREO CREO CREO CREO CREO CREO CREO Fig. 5 Effect of CREO on gloss of anodized Al-Mg alloy colored by Cu, Sn, Ni or Co deposition. Reflectance (%) (c) (d) Wave Length, λ /nm Fig. 6 Effect of CREO on reflectance of anodized Al-Mg alloy colored by Cu, Sn, Ni (c) or Co (d) deposition (Reflectance evaluated by ). Figure 5 shows the effect of CREO on the gloss of the anodized Al Mg alloy colored by Cu, Sn, Ni, or Co electrolysis. In all electrolytic colorings of Cu, Sn, Ni, and Co, the gloss was lower with CREO than without, although there was a difference in the degree of the effect of the CREO treatment. In method, the regular reflection is excluded to approach the visual evaluation of a human. The lightness of colored Al Mg alloy evaluated by increased with CREO, as shown in Figs This is attributed to the decrease in gloss caused by CREO. Figure 6 shows the reflectance of anodized Al Mg alloy with electrolytic coloring, as evaluated by. The reflectance was higher with CREO than without for almost all wavelengths, regardless of the metal deposited, corresponding to the increase in lightness evaluated by with CREO.

4 Effect of Continuous Rotation Evolutional Control on the Surface Color of Anodized Al Mg Alloys 795 Reflectance (%) (c) (d) Wave Length, λ /nm Fig. 7 Effect of CREO on reflectance of anodized Al-Mg alloy colored by Cu, Sn, Ni (c) or Co (d) deposition (Reflectance evaluated by ) Al-Mg Al-Mg Anodized Al-Mg without with without with without with without with CREO CREO CREO CREO CREO CREO CREO CREO Fig. 8 Effect of CREO on lightness of electropolished Al-Mg alloy with anodization and without anodization. Figure 7 shows the reflectance of anodized Al Mg alloy with electrolytic coloring. The reflectance was evaluated by. In Cu and Ni electrolysis, reflectance of the total reflection, including regular reflection, was higher with CREO than without, showing that CREO increases the inherent lightness of colored Al Mg alloy..2 Effect of CREO on the appearance of Al Mg alloy without coloring The effect of CREO on the lightness, gloss, and surface roughness of the Al Mg alloy prior to coloring was investigated to clarify the reason why the lightness and gloss of anodized Al Mg alloy with electrolytic coloring were affected by CREO. Figure 8 shows the effect of CREO on the lightness of electropolished Al Mg alloys with and without anodization. Both Al Mg alloys with and without anodization showed increase in lightness by CREO, regardless of the Table Effect of CREO on gloss of electropolished Al Mg alloy with anodization and without anodization. Gloss Al Mg Anodized Al Mg Table 4 Effect of CREO on surface roughness Ra of electropolished Al Mg alloy with anodization and without anodization. Ra (mm) Al Mg Anodized Al Mg evaluation methods. The increase in lightness of anodized Al Mg alloy with electrolytic coloring due to CREO is attributed to an increase in lightness of the Al Mg alloy itself. Table shows the effect of CREO on the gloss of electropolished and anodized Al Mg alloys. The gloss of Al Mg alloys with and without anodization decreased by CREO. The increase in lightness () of Al Mg alloy by CREO is attributed to this decrease in gloss. The surface roughness was measured to determine the reason for the decrease in gloss due to CREO. Table 4 shows the effect of CREO on the surface roughness, Ra, of electropolished Al Mg alloys. CREO decreased the surface roughness of the alloys. The gloss generally increases with decreasing surface roughness; however, the gloss of Al Mg alloys in this study decreased, despite the decrease in surface roughness by CREO. Therefore, the profile of surface roughness was then investigated. Figure 9 shows the primary profiles of electropolished Al Mg alloys with and without CREO. Although CREO decreased the surface roughness of the Al Mg alloy, the surface waviness increased. Because the surface waviness of Al Mg alloy increased by CREO, the gloss decreased despite a decrease in surface roughness.. Effect of CREO on the morphology of Al Mg alloys It seems that the enhancement of lightness of colored Al Mg alloys with anodization from CREO is attributable to a decrease in gloss. However, the inherent lightness () of material colored with Cu and Ni electrolysis was also enhanced by CREO, as shown in Figs. 2 and 4. The morphology of electropolished Al Mg alloys with and without anodization was observed by SEM to determine the reason for enhancement of the lightness of colored Al Mg alloys due to CREO. Figure shows the SEM images of electropolished Al Mg alloys with and without CREO. A number of subgrains smaller than mm in size were observed, irrespective of whether CREO was applied or not. The size of subgrains was smaller with CREO than those without CREO. Figure shows SEM images of anodized Al Mg alloys, in which the SEM images of, and (c) is for the surface

5 796 S. Oue et al. Height, h/µm Distance, d/mm Fig. 9 Primary profiles of electropolished Al-Mg alloy with CREO and without CREO. treated with CREO and of (d), (e), and (f) for that without CREO. The SEM magnification varies in to (c) and (d) to (f). Anodic oxide films with pores about nm in diameter were formed, irrespective of CREO. A number of white lines were observed over all anodic oxide films. TEM observation confirmed that the area of white line was salient, and the area surrounded by white line was reentrant. The area of the white line indicated by arrows in Figs. (c) and (f) has a lower density of micropores in anodic oxide films. The white line increased by CREO, and the total density of micropores in anodic oxide films consequently decreased. The enhancement of lightness () of the Al Mg alloys colored with electrolysis of Cu and Ni due to CREO is ascribed to the decrease in density of the micropores in anodic oxide films. Since the shape of white line is similar to that of subgrain boundary of the Al Mg alloy shown in Fig., the white line seems to be formed on the subgrain boundary. As mentioned above, the lightness of anodized Al Mg alloy colored by alternating current electrolysis was enhanced due to CREO, which is attributed to both the decrease in gloss of the Al Mg alloy and the decrease in the micropore density in the anodic oxide films. 4. Conclusion Fig. SEM images of electropolished Al-Mg alloy with CREO and without CREO. Continuous rotation evolutional control (CREO) affects the appearance of anodized Al Mg alloy colored by alternating current electrolysis. Electrolysis colored the anodized Al Mg alloys rufous (Cu), yellow to gold (Sn), or reddish brown (Ni and Co). The lightness of the colored Al Mg alloys was significantly enhanced by CREO, regardless of the metal deposited. The gloss of the Al Mg alloys decreased probably due to increase in the surface waviness resulting from CREO. The enhancement of the lightness of colored Al Mg alloys treated by CREO is attributed to the decrease in the glossiness. SEM images showed that the density of the (c) (d) (e) (f),,(c): (d),(e),(f): Fig. SEM images of anodized Al-Mg alloy with CREO and without CREO.

6 Effect of Continuous Rotation Evolutional Control on the Surface Color of Anodized Al Mg Alloys 797 micropores in anodic oxide films on Al Mg alloys decreased as a result of CREO. This decrease in the density of the micropores may be assumed to enhance the lightness of the colored Al Mg alloys treated by CREO. Acknowledgments This work was supported by a Research Promotion Grant from User Science Institute of Kyushu University. REFERENCES ) T. Asada: Kinzoku Hyomen Gijutsu 2 (97) ) M. Nagayama, H. Takahashi and M. Koda: Kinzoku Hyomen Gijutsu (979) ) N. Baba: Anodic Oxide Films Produced by Electrolysis, (in Japanese, Maki Syoten, Tokyo, 996) pp ) K. Wada, Y. Matsui, M. Tsutsumi and R. Uchida: Kinzoku Hyomen Gijutsu (98) ) K. Wada, Y. Matsui, M. Tsutsumi and R. Uchida: Kinzoku Hyomen Gijutsu (98) ) K. Nakamura, K. Neishi, K. Kaneko, M. Nakagaki and Z. Horita: Mater. Trans. 45 (24) ) K. Nakamura, K. Neishi, K. Kaneko, M. Nakagaki and Z. Horita: Mater. Sci. Forum 5 54 (26) ) Y. Kawasaki, K. Neishi, Y. Miyahara, K. Nakamura, K. Kaneko, M. Nakagaki and Z. Horita: Mater. Sci. Forum 5 54 (26) ) Y. Miyahara, N. Emi, K. Neishi, K. Nakamura, K. Kaneko, M. Nakagaki and Z. Horita: Mater. Sci. Forum 5 54 (26) ) K. Neishi, A. Higashino, Y. Miyahara, K. Nakamura, K. Kaneko, M. Nakagaki and Z. Horita: Mater. Sci. Forum 5 54 (26) ) H. J. Gohausen and G. C. Schoener: Surf. Finish. (984) ) T. Kawaguchi, S. Ono, T. Satoh and N. Masuko: Kinzoku Hyomen Gijutsu 4 (99) ) S. Ishida and S. Itoh: Kinzoku Hyomen Gijutsu 4 (99) ) K. Kawai, M. Yamamuro and K. Wada: Hyomen Gijutsu 49 (998) ) M. Yamamuro and S. Morisaki: J. Jpn. Inst. Light Met. 5 (2) ) Y. Tsukamoto and K. Ebihara: Kinzoku Hyomen Gijutsu 5 (999) ) S. Van Gils, P. Mast, E. Stijns and H. Terry: Surf. Coat. Technol. 85 (24). 8) M. Yonehara, S. Kumai, H. Idono, T. Sugibayashi and N. Igata: J. Jpn. Inst. Light Met. 56 (26)