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Surface Heat Treatment of Magnesium Alloys by Plasma Electrolysis from Phosphate Electrolytic Solution * 1 Makoto Hino 1 , Koji Murakami 1 , Atsushi Saijo 2 , Shuji Hikino 3; * 2 , Teruto Kanadani 3 and Masato Tsujikawa 4 1 Industrial Technology Research Institute of Okayama Prefecture, Okayama 701-1296, Japan 2 Hori Metal Finishing Industry Co., Ltd., Takahashi 721-8540, Japan 3 Faculty of Engineering, Okayama University of Science, Okayama 700-0005, Japan 4 Graduate 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, Mg 17 Al 12 ) 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 Recently, there has been a rapid expansion of the use of magnesium alloys due to the low environmental impact of these materials, for example, their light weight, rich resources, their exceptional recycling characteristics and their nontoxicity to humans, which leads to a good energy efficiency. 1) Particularly, in the field of transportation machinery, magnesium alloys as lightweight auto body materials are expected to improve fuel economy. At present, the AZ91D magnesium alloy containing aluminum and zinc has been widely used in all kind of magnesium alloys as die casting materials in order to improve the mechanical property and corrosion resistance in the transportation equipment field. 2) Since the AZ91D magnesium alloy contains about 9 mass% aluminum, heat treatment, such as solution and aging treatment, makes it possible to improve the mechanical properties. 3,4) However, at the present time, heat treatment such as the solution treatment for the die casting parts with the complex shape as well as thickness of 1 mm or less is scarcely carried out because of the deformation of the products due to the heating strain, the long heating time, etc. In the meantime, magnesium has the lowest electrochem- ical potential among all the common commercial metals and is extremely prone to corrosion. Therefore, corrosion protection is needed when magnesium is considered for industrial applications. In particular, the surface treatment is indispensable for the long-time service of transportation equipment. 5) The anodizing of magnesium alloys is used as a surface treatment technique to produce material that has a high resistance to corrosion. Typical treatments currently used include Dow17 6) and HAE. 7) The authors have already examined another method of protection of magnesium alloys by environmental-friendly anodizing using an electrolyte consisting of phosphate and ammonium salts without heavy metals and harmful chemical agents such as fluorides, and showed an excellent corrosion protective performance. 8,9) Since a film grows by uniform sparks during dielectric breakdown in this anodizing, this uniform sparking seems to make it possible to uniformly heat the magnesium substrate surface in the electrolytic solution. In the current investigation, an attempt was made to improve the mechanical properties of a magnesium alloy by a surface heat treatment with anodizing from a phosphate solution. 8) Effects of the anodic plasma electrolysis on the mechanical properties and microstructure of various AZ series magnesium alloys were examined. 2. Experimental Procedure Experiments were conducted using an AZ91D magnesium alloy plate made by die casting and three kinds of AZ series magnesium alloy plates (wrought materials). The chemical composition of each specimen is shown in Table 1. Figure 1 shows the microstructure of each specimen. The specimens were first subjected to a pretreatment by alkali cleaning and Table 1 Chemical composition of various magnesium alloys. (mass%) Al Mn Zn Fe Si Cu Ni Mg 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. AZ61 6.26 0.28 0.61 0.005 0.012 0.0011 0.0007 bal. AZ91D 9.1 0.28 0.75 0.004 0.05 0.025 0.001 bal. * 1 This Paper was Originally Published in Japanese in J. Japan Society for Heat Treatment 50 (2010) 505–510. * 2 Graduate Student, Okayama University of Science Materials Transactions, Vol. 52, No. 12 (2011) pp. 2168 to 2173 #2011 The Japan Society for Heat Treatment

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Surface Heat Treatment of Magnesium Alloys by Plasma Electrolysis

from Phosphate Electrolytic Solution*1

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

1Industrial Technology Research Institute of Okayama Prefecture, Okayama 701-1296, Japan2Hori Metal Finishing Industry Co., Ltd., Takahashi 721-8540, Japan3Faculty of Engineering, Okayama University of Science, Okayama 700-0005, Japan4Graduate 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 onmagnesium alloys. Effects of the anodic plasma electrolysis onto various AZ series magnesium alloys on the mechanical properties andmicrostructure 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. Thetensile strength of AZ61 and AZ91D substrate after anodic electrolytic treatment increased or decreased, and this change of tensile strength isattributable to the precipitation of intermetallic compound (� phase, Mg17Al12) as sparks occurred due to dielectric breakdown during anodicelectrolysis. 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

Recently, there has been a rapid expansion of the use ofmagnesium alloys due to the low environmental impactof these materials, for example, their light weight, richresources, their exceptional recycling characteristics andtheir nontoxicity to humans, which leads to a good energyefficiency.1) Particularly, in the field of transportationmachinery, magnesium alloys as lightweight auto bodymaterials are expected to improve fuel economy. At present,the AZ91D magnesium alloy containing aluminum and zinchas been widely used in all kind of magnesium alloys as diecasting materials in order to improve the mechanical propertyand corrosion resistance in the transportation equipmentfield.2)

Since the AZ91D magnesium alloy contains about9 mass% aluminum, heat treatment, such as solution andaging treatment, makes it possible to improve the mechanicalproperties.3,4) However, at the present time, heat treatmentsuch as the solution treatment for the die casting parts withthe complex shape as well as thickness of 1 mm or less isscarcely carried out because of the deformation of theproducts due to the heating strain, the long heating time, etc.

In the meantime, magnesium has the lowest electrochem-ical potential among all the common commercial metals andis extremely prone to corrosion. Therefore, corrosionprotection is needed when magnesium is considered forindustrial applications. In particular, the surface treatment isindispensable for the long-time service of transportationequipment.5) The anodizing of magnesium alloys is used as asurface treatment technique to produce material that has a

high resistance to corrosion. Typical treatments currentlyused include Dow176) and HAE.7) The authors have alreadyexamined another method of protection of magnesium alloysby environmental-friendly anodizing using an electrolyteconsisting of phosphate and ammonium salts without heavymetals and harmful chemical agents such as fluorides, andshowed an excellent corrosion protective performance.8,9)

Since a film grows by uniform sparks during dielectricbreakdown in this anodizing, this uniform sparking seems tomake it possible to uniformly heat the magnesium substratesurface in the electrolytic solution.

In the current investigation, an attempt was made toimprove the mechanical properties of a magnesium alloy by asurface heat treatment with anodizing from a phosphatesolution.8) Effects of the anodic plasma electrolysis on themechanical properties and microstructure of various AZseries magnesium alloys were examined.

2. Experimental Procedure

Experiments were conducted using an AZ91D magnesiumalloy plate made by die casting and three kinds of AZ seriesmagnesium alloy plates (wrought materials). The chemicalcomposition of each specimen is shown in Table 1. Figure 1shows the microstructure of each specimen. The specimenswere first subjected to a pretreatment by alkali cleaning and

Table 1 Chemical composition of various magnesium alloys. (mass%)

Al Mn Zn Fe Si Cu Ni Mg

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.

AZ61 6.26 0.28 0.61 0.005 0.012 0.0011 0.0007 bal.

AZ91D 9.1 0.28 0.75 0.004 0.05 0.025 0.001 bal.

*1This Paper was Originally Published in Japanese in J. Japan Society for

Heat Treatment 50 (2010) 505–510.*2Graduate Student, Okayama University of Science

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

pickling. Anodizing was conducted by direct current elec-trolysis using a solution of phosphate salt. The heat input tothe specimen was made by changing the final bias voltagefrom 250 V to 400 V. A SUS316L stainless steel sheet wasused as the cathode. The solution temperature was controlledat 298 K � 5 K. For comparison, another anodizing byDow17 (250 g/L ammonium hydrogen fluoride, 100 g/Lsodium dichromate, and 90 mL/L phosphoric acid) was alsoused to prepare the specimen. The conditions for the Dow17anodizing were as follows: Solution temperature: 348 � 5 K,Current density: 0.25 kA/m2, Final bias voltage: 100 V,Electrolysis time: 900 s.

The anodizing was conducted for the tensile test piecesshown in Fig. 2. The surface and the cross section of theobtained films were observed by SEM, and the followingmechanical properties for each treatment were evaluated. Thetensile test (crosshead speed: 0.5 mm/min) was performed inorder to determine the tensile strength and the elongation.The microstructure observation was carried out in order toexamine the effects of the anodic electrolysis on the metalstructure.

3. Results and Discussion

3.1 Morphology of various treated coatingsIt is possible to observe the reaction at the electrode

surface because of the transparent electrolytic solutionconsisting of the phosphate salt without any heavy metalsion in this study. When the anodic electrolysis was started,a white oxidation film was immediately formed on eachmagnesium alloy substrate along with the generation ofoxygen. The current gradually decreased since this oxidationfilm is not electrically conductive. However, the voltageincreased in order to counteract the reduced current becauseof the anodic electrolysis at constant current. A dielectricbreakdown occurred when the voltage reached approxi-mately 200 V. Subsequently, the current recovered again, andthen gradually approached the set value.

Figure 3 shows the appearance before the electrolytictreatment at the AZ91D substrate and its spark dischargeunder dielectric breakdown. The spark discharge uniformlyoccurred at the substrate surface due to the dielectricbreakdown. As the electrolysis voltage increased, this sparkdischarge was intensified. On the other hand, the sparkdischarge by the Dow17 treatment, like the electrolysis fromthe phosphate electrolytic solution, occurred, however, theDow17 electrolytic solution of dark yellow green leads todifficulties in observing the spark discharge.

Figure 4 shows the secondary electron image of thesurface of the AZ91D anodized at 350 V. The oxide filmswith large number of microscopic pores were observed, andthen these pores, which were formed by means of the sparkdischarge, expanded with the increase in the final bias

(b)

(c) (d)

100µm

(a)

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

15 15 15

15

60

R102.7

4

Fig. 2 Schematic drawing of tensile test piece.

Surface Heat Treatment of Magnesium Alloys by Plasma Electrolysis from Phosphate Electrolytic Solution 2169

voltage. This environmental-friendly anodizing is currentlyapplied on a massive scale for magnesium products becauseof its excellent corrosion protection.8–10) The Dow 17anodized film also had numerous microscopic pores in asimilar manner as Fig. 4.

3.2 Effect of anodic electrolytic treatment on themechanical property

The relationship between the tensile strengths of thevarious magnesium alloys obtained from the tensile test andthe anodic electrolysis are shown in Fig. 5. The tensilestrengths are the mean values of three samples each. Eachtensile strength of the AZ10 and AZ31B alloy specimens wasalmost unchanged by varying the final bias voltage, but thetensile strength after being anodized at 400 V was slightlyreduced.

On the other hand, each tensile strength of the AZ61 andAZ91D alloy specimens was changed by varying the finalbias voltage. For the AZ61 anodized at 350 V, the tensilestrength increased, and then was reduced at 400 V. Inaddition, those of the AZ91D anodized at 250 V and 350 Vincreased, but were also reduced at 400 V.

In this way, the results of varying the mechanical proper-ties depending on the electrolytic treatment indicated thatthis treatment will influence the metal structure.

Based on the magnesium-aluminum binary phase dia-gram,11) magnesium is capable of dissolving about 2 at%aluminum. In the case of the supersaturated solid solution,such as the AZ91D alloy containing 9 mass% aluminum, theheat treatment, such as the solution and aging treatment,makes it possible to improve the mechanical property.Conversely, for the magnesium alloy with a low aluminumcontent, such as the AZ10 and AZ31B alloys, it is notpossible to improve the mechanical property by a similar heattreatment. The foregoing result of tensile testing with theanodic plasma electrolysis is in good agreement with the heattreatment condition based on the magnesium-aluminumbinary phase diagram. In the case of the AZ61 and AZ91Dalloys, the electrolytic treatment near 350 V seems to causethe heat treatment of the solution and aging.

On the other hand, as for the Dow17 anodizing, whoseelectrolytic solution and electrolytic conditions differ fromthe phosphate electrolytic solution, no increase in the tensilestrength occurred, and the tensile strength by this treatmentinversely was reduced. These results indicate that the varietyof electrolytic solutions and solution temperatures are closelyrelated to the mechanical property besides the final bias

(a) (b)

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

10µm

Fig. 4 Secondary electron images of specimen showing the surface after

anodic electrolysis (AZ91D, 350 V).

150

200

250

150

200

250

250

300

350

100

150

200

Ten

sile

Str

eng

th[M

Pa]

AZ10 AZ31B

AZ61 AZ91D

No treatment250V350V400V

Fig. 5 Tensile strengths of magnesium alloy specimens before and after

the anodic electrolysis.

2170 M. Hino et al.

voltage during the electrolytic treatment with the sparkdischarge.

3.3 Effect of anodic electrolytic treatment on the micro-structure

From the results described in section 3.2, it was obviousthat the anodic electrolytic treatment from the phosphateelectrolytic solution should influence the mechanical proper-ties of the magnesium substrate. The spark discharge duringthe anodic electrolytic treatment heated the substrate surface,and the metal structure near the substrate surface thenchanges. Therefore, it can be speculated that the mechanicalproperties are changed by the anodic electrolytic treatment.

Figure 6 shows the cross-sectional microstructures of themagnesium alloy substrates before and after the anodicelectrolysis. For the microstructure before the anodicelectrolysis, it was easy to observe twins originating fromthe plastic deformation in the specimens of the AZ10,AZ31B, and AZ61 alloy produced in the form of wroughtmaterials. In addition, there was a large number of twinsclose to the surface. As for the AZ91D alloy specimen, notwins were observed because of being manufactured by thedie casting process, and its grain size was approximately5 mm or finer, depending on the rapid solidification by themetal mold.

On the other hand, each microstructure of every specimenwas changed by the anodic electrolytic treatment. The twins

of the AZ10, AZ31B, and AZ61 alloy specimens decreasedby this electrolytic treatment, and then the twins completelydisappeared at the final bias voltage of 400 V.

For the purpose of clarifying further details about thesechanges, an electron backscattering diffraction (EBSD)analysis was conducted of the AZ31B alloy (Fig. 7). Therewas a large number of twins before the anodic electrolytictreatment, then the twins decreased after this treatment. Inaddition, the twins completely disappeared and the grain sizeincreased at the final bias voltage of 400 V. Furthermore, anunderstanding of the grain coarsening of the AZ91D alloywith the increased final bias voltage was acquired in aprevious study.12) These results indicated that the heating,which recovers the plastic strain, occurs near the substratesurface, and the heat input into the substrate increases withthe increasing final bias voltage.

The result of the reduction in each tensile strength of everyspecimen, as shown in Fig. 5, is explained in terms of therelaxation of the strain and the grain coarsening shown inhere.

Secondly, TEM observations were taken of the AZ91Dalloy, because the change in the tensile strength of theAZ91D alloy was more remarkable than that of the otherspecimens.

Figure 8 shows the results of the cross-sectional TEMobservation of the specimen near the surface before and afterthe anodic electrolytic treatment at the final bias voltage of

AZ10 AZ31B AZ61 AZ91DN

o t

reat

men

t20

0V35

0V40

0V

50µm

Fig. 6 Cross-sectional microstructure of various magnesium alloys substrates before and after anodic electrolysis.

Surface Heat Treatment of Magnesium Alloys by Plasma Electrolysis from Phosphate Electrolytic Solution 2171

400 V. On the specimen without the anodic electrolytictreatment, precipitation of the intermetallic compound (�phase, Mg17Al12) was observed along the grain boundary.This intermetallic compound was also observed in thespecimen with the electrolytic treatment at 400 V, and theseintermetallic compounds then coarsened. The heating de-pending on the sparks that occurred due to dielectricbreakdown seems to give rise to the precipitation of theMg17Al12 from the supersaturated solid solution in the matrixand the grain coarsening.

Figure 9 shows the bright-field and dark-field TEM imagesnear the substrate surface after the anodic electrolysis at250 V. Fine particles, whose size were 20–50 nm, disper-

sively precipitated in �-phase matrix. Mg17Al12 compounds,which segregated in the grain boundary, when cast, weredissolved in the �-phase by the heating due to electrolytictreatment, and then fine particles consisting of the Mg17Al12

intermetallic compound shown in Fig. 9, precipitated fromthis supersaturated solid solution produced by the electrolytictreatment. In this way, the disappearance of the Mg17Al12

compounds, which segregated in the grain boundary, andprecipitation of fine Mg17Al12 compounds leads to theimprovement of the tensile strength.

Finally, the tensile strengths of the AZ10 and AZ31Balloys were not improved by the electrolytic treatment,however, those of AZ61 and AZ91D were improved. Theseresults are explainable in terms of the precipitation of theMg17Al12 compounds. That is to say, according to themagnesium-aluminum binary phase diagram,11) since mag-nesium can dissolve about 2 at% aluminum at ordinarytemperature, 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 phosphatesolution, it was possible that the mechanical properties ofthe AZ61 and AZ91D alloys were improved in terms of thesolution and aging treatment depending on the treatmentconditions (final bias voltage, current).

4. Conclusions

This study examined the possibility of a surface heattreatment by plasma electrolysis from a phosphate electro-lytic solution on various AZ series magnesium alloys inwhich the aluminum content changed. The spark dischargeby this electrolytic treatment was found to change the metalstructure as well as the heat near the substrate surface. At thattime, the heating by the electrolytic treatment makes itpossible to improve the mechanical property of the AZ61 andAZ91D alloys which dissolved the aluminum in the super-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.

No

tre

atm

ent

Grain boundary

350V

400V

100µm

{1012} Twins

Fig. 7 Grain boundary maps obtained by EBSD analysis for the substrates

near the surface before and after the anodic electrolysis.

2172 M. Hino et al.

�-phase together with the precipitation of finer �-phase(Mg17Al12) particles of 50 nm or less. On the other hand, forthe AZ10 and AZ31B alloys in which the aluminum contentswere low, the mechanical property was almost unchanged bythe same electrolytic treatment. These results were in goodagreement with the magnesium-aluminum binary phasediagram.

Heat treatment was not applied to the die-cast partsconsisting of the AZ91D magnesium alloy, which couldbe easily molded into complicated shapes because of theproblem of the strain and the heating time. Corrosionresistance on the corrosive magnesium alloy is improvedby this plasma electrolytic treatment. In addition, it ispossible that the metal structure near the surface is made tochange by the electrolytic treatment under optimum con-ditions, and then improve the mechanical property. Theseresults demonstrated the utility of this electrolytic treatmentas a new surface heat treatment process.

REFERENCES

1) Y. Kojima: J. Jpn. Inst. Light Met. 58 (2008) 526–548 (in Japanese).

2) S. Ito: J. Jpn. Inst. Light Met. 59 (2009) 464–475 (in Japanese).

3) T. Sato: J. Jpn. Inst. Light Met. 60 (2010) 202–210 (in Japanese).

4) Netsushori-Gijutsu-Binran, (The Japan Soc. for Heat Treatment, 2000),

p. 527 (in Japanese).

5) M. Hino, M. Hiramatsu, K. Murakami, A. Saijo and T. Kanadani:

J. Jpn. Inst. Light Met. 56 (2006) 386–391 (in Japanese).

6) The Dow Chemical Company, G.B.Pat. 762,195 (1956).

7) H. A. Evangelides: U.S.Pat. 2,723,952 (1955).

8) M. Hino, K. Murakami, A. Saijo and T. Kanadani: MOLTEN SALTS

52 (2009) 103–108 (in Japanese).

9) K. Murakami, M. Hino, M. Hiramatsu, A. Saijo, S. Kobayashi, K.

Nakai and T. Kanadani: Mater. Trans. 48 (2007) 3101–3106.

10) M. Hino, K. Murakami, A. Saijo and T. Kanadani: Mater. Trans. 49

(2008) 924–927.

11) I. K. Geissler: GIESSEREIFORSCHUNG 32 (1980) 167–170.

12) M. Hino, K. Murakami, Y. Mitooka, M. Hiramatsu, S. Sumioka,

T. Kanadani and A. Saijo: J. Jpn. Inst. Metals 70 (2006) 912–917

(in Japanese).

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.

Surface Heat Treatment of Magnesium Alloys by Plasma Electrolysis from Phosphate Electrolytic Solution 2173