Surface Heat Treatment of Magnesium Alloys by Plasma Electrolysis from Phosphate Electrolytic Solution*1

Materials Transactions, Vol. 52, No. 12 (2011) pp. 2168 to 2173 #2011 The Japan Society for Heat Treatment

Makoto Hino1, Koji Murakami1, Atsushi Saijo2, Shuji Hikino3;*2, Teruto Kanadani3 and Masato Tsujikawa4

1Industrial Technology Research Institute of Okayama Prefecture, Okayama 701-1296, Japan 2Hori Metal Finishing Industry Co., Ltd., Takahashi 721-8540, Japan 3Faculty of Engineering, Okayama University of Science, Okayama 700-0005, Japan 4Graduate School of Engineering, Osaka Prefecture University, Sakai 599-8531, Japan

This study examined the possibility of surface heat treatment by the plasma electrolysis from phosphate electrolytic solution on magnesium alloys. Effects of the anodic plasma electrolysis onto various AZ series magnesium alloys on the mechanical properties and microstructure were examined. The tensile test revealed that the anodic electrolytic treatment at final bias voltage from 250 V to 400 V influenced the tensile strength. The tensile strength of AZ61 and AZ91D substrate after anodic electrolytic treatment increased or decreased, and this change of tensile strength is attributable to the precipitation of intermetallic compound ( phase, Mg17Al12) as sparks occurred due to dielectric breakdown during anodic electrolysis. These results demonstrate the utility of this electrolytic treatment on AZ61 and AZ91D magnesium alloys. [doi:10.2320/matertrans.H-M2011826]

(Received July 19, 2011; Accepted September 2, 2011; Published October 26, 2011) Keywords: magnesium alloy, plasma electrolysis, surface heat treatment

1. Introduction high resistance to corrosion. Typical treatments currently used include Dow176) and HAE.7) The authors have already Recently, there has been a rapid expansion of the use of examined another method of protection of magnesium alloys magnesium alloys due to the low environmental impact by environmental-friendly anodizing using an electrolyte of these materials, for example, their light weight, rich consisting of phosphate and ammonium salts without heavy resources, their exceptional recycling characteristics and metals and harmful chemical agents such as fluorides, and their nontoxicity to humans, which leads to a good energy showed an excellent corrosion protective performance.8,9) efficiency.1) Particularly, in the field of transportation Since a film grows by uniform sparks during dielectric machinery, magnesium alloys as lightweight auto body breakdown in this anodizing, this uniform sparking seems to materials are expected to improve fuel economy. At present, make it possible to uniformly heat the magnesium substrate the AZ91D magnesium alloy containing aluminum and zinc surface in the electrolytic solution. has been widely used in all kind of magnesium alloys as die In the current investigation, an attempt was made to casting materials in order to improve the mechanical property improve the mechanical properties of a magnesium alloy by a and corrosion resistance in the transportation equipment surface heat treatment with anodizing from a phosphate field.2) solution.8) Effects of the anodic plasma electrolysis on the Since the AZ91D magnesium alloy contains about mechanical properties and microstructure of various AZ 9 mass% aluminum, heat treatment, such as solution and series magnesium alloys were examined. aging treatment, makes it possible to improve the mechanical properties.3,4) However, at the present time, heat treatment 2. Experimental Procedure such as the solution treatment for the die casting parts with the complex shape as well as thickness of 1 mm or less is Experiments were conducted using an AZ91D magnesium scarcely carried out because of the deformation of the alloy plate made by die casting and three kinds of AZ series products due to the heating strain, the long heating time, etc. magnesium alloy plates (wrought materials). The chemical In the meantime, magnesium has the lowest electrochem- composition of each specimen is shown in Table 1. Figure 1 ical potential among all the common commercial metals and shows the microstructure of each specimen. The specimens is extremely prone to corrosion. Therefore, corrosion were first subjected to a pretreatment by alkali cleaning and protection is needed when magnesium is considered for industrial applications. In particular, the surface treatment is indispensable for the long-time service of transportation Table 1 Chemical composition of various magnesium alloys. (mass%) equipment.5) The anodizing of magnesium alloys is used as a Al Mn Zn Fe Si Cu Ni Mg surface treatment technique to produce material that has a AZ10 0.9 0.31 0.3 0.005 0.008 0.001 0.0008 bal. AZ31B 2.87 0.38 0.85 0.003 0.014 0.0004 0.0003 bal. *1This Paper was Originally Published in Japanese in J. Japan Society for AZ61 6.26 0.28 0.61 0.005 0.012 0.0011 0.0007 bal. Heat Treatment 50 (2010) 505–510. AZ91D 9.1 0.28 0.75 0.004 0.05 0.025 0.001 bal. *2Graduate Student, Okayama University of Science Surface Heat Treatment of Magnesium Alloys by Plasma Electrolysis from Phosphate Electrolytic Solution 2169

(a) (b)

100µm

Fig. 1 Microstructure of magnesium alloy substrates. (a) AZ10 (b) AZ31B (c) AZ61 (d) AZ91D.

2.7 3. Results and Discussion R10 3.1 Morphology of various treated coatings 4 15 It is possible to observe the reaction at the electrode surface because of the transparent electrolytic solution 15 15 15 consisting of the phosphate salt without any heavy metals 60 ion in this study. When the anodic electrolysis was started, a white oxidation film was immediately formed on each Fig. 2 Schematic drawing of tensile test piece. magnesium alloy substrate along with the generation of oxygen. The current gradually decreased since this oxidation pickling. Anodizing was conducted by direct current elec- film is not electrically conductive. However, the voltage trolysis using a solution of phosphate salt. The heat input to increased in order to counteract the reduced current because the specimen was made by changing the final bias voltage of the anodic electrolysis at constant current. A dielectric from 250 V to 400 V. A SUS316L stainless steel sheet was breakdown occurred when the voltage reached approxi- used as the cathode. The solution temperature was controlled mately 200 V. Subsequently, the current recovered again, and at 298 K Æ 5 K. For comparison, another anodizing by then gradually approached the set value. Dow17 (250 g/L ammonium hydrogen fluoride, 100 g/L Figure 3 shows the appearance before the electrolytic sodium dichromate, and 90 mL/L phosphoric acid) was also treatment at the AZ91D substrate and its spark discharge used to prepare the specimen. The conditions for the Dow17 under dielectric breakdown. The spark discharge uniformly anodizing were as follows: Solution temperature: 348 Æ 5K, occurred at the substrate surface due to the dielectric Current density: 0.25 kA/m2, Final bias voltage: 100 V, breakdown. As the electrolysis voltage increased, this spark Electrolysis time: 900 s. discharge was intensified. On the other hand, the spark The anodizing was conducted for the tensile test pieces discharge by the Dow17 treatment, like the electrolysis from shown in Fig. 2. The surface and the cross section of the the phosphate electrolytic solution, occurred, however, the obtained films were observed by SEM, and the following Dow17 electrolytic solution of dark yellow green leads to mechanical properties for each treatment were evaluated. The difficulties in observing the spark discharge. tensile test (crosshead speed: 0.5 mm/min) was performed in Figure 4 shows the secondary electron image of the order to determine the tensile strength and the elongation. surface of the AZ91D anodized at 350 V. The oxide films The microstructure observation was carried out in order to with large number of microscopic pores were observed, and examine the effects of the anodic electrolysis on the metal then these pores, which were formed by means of the spark structure. discharge, expanded with the increase in the final bias 2170 M. Hino et al.

(a) (b)

Fig. 3 Photographs showing (a) before anodic electrolysis, (b) spark discharge by anodic electrolysis.

250 250 AZ10 AZ31B No treatment 250V 350V [MPa] 400V 200 200

150 150 350 200 AZ61 AZ91D Tensile Strength Tensile

300 150

250 100

Fig. 5 Tensile strengths of magnesium alloy specimens before and after the anodic electrolysis. 10µm

Fig. 4 Secondary electron images of specimen showing the surface after In this way, the results of varying the mechanical proper- anodic electrolysis (AZ91D, 350 V). ties depending on the electrolytic treatment indicated that this treatment will influence the metal structure. voltage. This environmental-friendly anodizing is currently Based on the magnesium-aluminum binary phase dia- applied on a massive scale for magnesium products because gram,11) magnesium is capable of dissolving about 2 at% of its excellent corrosion protection.8–10) The Dow 17 aluminum. In the case of the supersaturated solid solution, anodized film also had numerous microscopic pores in a such as the AZ91D alloy containing 9 mass% aluminum, the similar manner as Fig. 4. heat treatment, such as the solution and aging treatment, makes it possible to improve the mechanical property. 3.2 Effect of anodic electrolytic treatment on the Conversely, for the magnesium alloy with a low aluminum mechanical property content, such as the AZ10 and AZ31B alloys, it is not The relationship between the tensile strengths of the possible to improve the mechanical property by a similar heat various magnesium alloys obtained from the tensile test and treatment. The foregoing result of tensile testing with the the anodic electrolysis are shown in Fig. 5. The tensile anodic plasma electrolysis is in good agreement with the heat strengths are the mean values of three samples each. Each treatment condition based on the magnesium-aluminum tensile strength of the AZ10 and AZ31B alloy specimens was binary phase diagram. In the case of the AZ61 and AZ91D almost unchanged by varying the final bias voltage, but the alloys, the electrolytic treatment near 350 V seems to cause tensile strength after being anodized at 400 V was slightly the heat treatment of the solution and aging. reduced. On the other hand, as for the Dow17 anodizing, whose On the other hand, each tensile strength of the AZ61 and electrolytic solution and electrolytic conditions differ from AZ91D alloy specimens was changed by varying the final the phosphate electrolytic solution, no increase in the tensile bias voltage. For the AZ61 anodized at 350 V, the tensile strength occurred, and the tensile strength by this treatment strength increased, and then was reduced at 400 V. In inversely was reduced. These results indicate that the variety addition, those of the AZ91D anodized at 250 V and 350 V of electrolytic solutions and solution temperatures are closely increased, but were also reduced at 400 V. related to the mechanical property besides the final bias Surface Heat Treatment of Magnesium Alloys by Plasma Electrolysis from Phosphate Electrolytic Solution 2171

AZ10 AZ31B AZ61 AZ91D No treatment 200V 350V 400V

50µm

Fig. 6 Cross-sectional microstructure of various magnesium alloys substrates before and after anodic electrolysis. voltage during the electrolytic treatment with the spark of the AZ10, AZ31B, and AZ61 alloy specimens decreased discharge. by this electrolytic treatment, and then the twins completely disappeared at the final bias voltage of 400 V. 3.3 Effect of anodic electrolytic treatment on the micro- For the purpose of clarifying further details about these structure changes, an electron backscattering diffraction (EBSD) From the results described in section 3.2, it was obvious analysis was conducted of the AZ31B alloy (Fig. 7). There that the anodic electrolytic treatment from the phosphate was a large number of twins before the anodic electrolytic electrolytic solution should influence the mechanical proper- treatment, then the twins decreased after this treatment. In ties of the magnesium substrate. The spark discharge during addition, the twins completely disappeared and the grain size the anodic electrolytic treatment heated the substrate surface, increased at the final bias voltage of 400 V. Furthermore, an and the metal structure near the substrate surface then understanding of the grain coarsening of the AZ91D alloy changes. Therefore, it can be speculated that the mechanical with the increased final bias voltage was acquired in a properties are changed by the anodic electrolytic treatment. previous study.12) These results indicated that the heating, Figure 6 shows the cross-sectional microstructures of the which recovers the plastic strain, occurs near the substrate magnesium alloy substrates before and after the anodic surface, and the heat input into the substrate increases with electrolysis. For the microstructure before the anodic the increasing final bias voltage. electrolysis, it was easy to observe twins originating from The result of the reduction in each tensile strength of every the plastic deformation in the specimens of the AZ10, specimen, as shown in Fig. 5, is explained in terms of the AZ31B, and AZ61 alloy produced in the form of wrought relaxation of the strain and the grain coarsening shown in materials. In addition, there was a large number of twins here. close to the surface. As for the AZ91D alloy specimen, no Secondly, TEM observations were taken of the AZ91D twins were observed because of being manufactured by the alloy, because the change in the tensile strength of the die casting process, and its grain size was approximately AZ91D alloy was more remarkable than that of the other 5 mm or finer, depending on the rapid solidification by the specimens. metal mold. Figure 8 shows the results of the cross-sectional TEM On the other hand, each microstructure of every specimen observation of the specimen near the surface before and after was changed by the anodic electrolytic treatment. The twins the anodic electrolytic treatment at the final bias voltage of 2172 M. Hino et al.

400 V. On the specimen without the anodic electrolytic sively precipitated in -phase matrix. Mg17Al12 compounds, treatment, precipitation of the intermetallic compound ( which segregated in the grain boundary, when cast, were phase, Mg17Al12) was observed along the grain boundary. dissolved in the -phase by the heating due to electrolytic This intermetallic compound was also observed in the treatment, and then fine particles consisting of the Mg17Al12 specimen with the electrolytic treatment at 400 V, and these intermetallic compound shown in Fig. 9, precipitated from intermetallic compounds then coarsened. The heating de- this supersaturated solid solution produced by the electrolytic pending on the sparks that occurred due to dielectric treatment. In this way, the disappearance of the Mg17Al12 breakdown seems to give rise to the precipitation of the compounds, which segregated in the grain boundary, and Mg17Al12 from the supersaturated solid solution in the matrix precipitation of fine Mg17Al12 compounds leads to the and the grain coarsening. improvement of the tensile strength. Figure 9 shows the bright-field and dark-field TEM images Finally, the tensile strengths of the AZ10 and AZ31B near the substrate surface after the anodic electrolysis at alloys were not improved by the electrolytic treatment, 250 V. Fine particles, whose size were 20–50 nm, disper- however, those of AZ61 and AZ91D were improved. These results are explainable in terms of the precipitation of the Mg17Al12 compounds. That is to say, according to the Grain boundary magnesium-aluminum binary phase diagram,11) since magnesium can dissolve about 2 at% aluminum at ordinary {1012} Twins temperature, it seems that precipitation of the Mg17Al12 compounds shown in Fig. 9 is not generated. As the summary shows, based on the attempt to conduct the surface heat treatment with anodizing from a phosphate

No treatment solution, it was possible that the mechanical properties of the AZ61 and AZ91D alloys were improved in terms of the solution and aging treatment depending on the treatment conditions (ﬁnal bias voltage, current).

350V 4. Conclusions

This study examined the possibility of a surface heat treatment by plasma electrolysis from a phosphate electrolytic solution on various AZ series magnesium alloys in which the aluminum content changed. The spark discharge 400V by this electrolytic treatment was found to change the metal structure as well as the heat near the substrate surface. At that 100µm time, the heating by the electrolytic treatment makes it possible to improve the mechanical property of the AZ61 and Fig. 7 Grain boundary maps obtained by EBSD analysis for the substrates AZ91D alloys which dissolved the aluminum in the super- near the surface before and after the anodic electrolysis. saturation solution due to dissolving the aluminum in the

β phase (Mg17Al12)

(a) (b)

Fig. 8 TEM images of the substrates near the surface (a) No treatment (b) anodic electrolysis at 400 V. Surface Heat Treatment of Magnesium Alloys by Plasma Electrolysis from Phosphate Electrolytic Solution 2173

TEM-BF TEM-DF

Fig. 9 Bright-ﬁeld and dark-ﬁeld TEM images of the substrate near the surface after anodic electrolysis at 250 V.

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