Texture Randomization of AZ31 Magnesium Alloy Sheets

for Improving the Cold Formability by a Combination

of Rolling and High-Temperature Annealing

Masahide Kohzu, Kenji Kii*1, Yuki Nagata*2, Hiroyuki Nishio*3, Kenji Higashi and Hirofumi Inoue

Department of Materials Science, Graduate School of Engineering, Osaka Prefecture University, Sakai 599-8531, Japan

To improve the cold formability of AZ31 magnesium alloy sheets, we investigate texture control by rolling and annealing. The textureideal for forming close to random orientation was obtained by annealing at 773K before and after isothermal rolling at 298–573K. For therandomizing process, such a high temperature in pre-annealing was essential, whereas a slightly lower temperature was acceptable for finalannealing, assuming a sufficiently long annealing time. The randomized sheet could be obtained in a wide range of rolling temperatures andreductions. It could also be produced easily with a standard mill without roll heating. The microstructure of the randomized sheet consisted ofrelatively homogeneous grains 25–30mm large on average. In a 90 degree V-bending test, a well randomized sheet could be bent withoutcracking with a minimum bending radius per thickness R=t ¼ 1:4, which was about half of that in commercial AZ31D-O sheets, in spite ofmanganese content of over 0.6%. [doi:10.2320/matertrans.L-M2010802]

(Received July 3, 2009; Accepted December 14, 2009; Published March 3, 2010)

Keywords: AZ31 magnesium alloy, texture randomization, isothermal rolling, high-temperature annealing, cold formability, V-bending

1. Introduction

Magnesium alloys with high specific strength are alreadybeing used extensively as structural materials for portableelectronics. Potential application to transportation devices,especially to automotive parts, is now attracting attentionwith the goal of reducing CO2 production.

1) Most magnesiumalloy structures are produced by die-casting or thixomoldingeven for shell structures at present, although sheet formingvia rolling had been expected to improve productivity andtoughness early on. To replace magnesium casting oraluminum pressing with magnesium pressing, it is indispen-sable to lower the forming temperature preferably to roomtemperature.

Cold formability depends above all on texture.2,3) In arolled sheet, a basal texture, in which {0001} planes areoriented parallel to the rolled surface, often forms andconsequently seriously impairs cold formability. Rolling withadditional shearing strain to tilt the basal planes has beentried in order to control the texture. However, it did not leadto a significant improvement of cold formability.4–6)

Some authors have reported that commercial AZ31 rolledsheets with a double-peak texture had excellent low-temper-ature formability,2) although the cold forming was hard.However, such a sheet was confined to a subset of narrowcoiled strips. Thus, the process of texture evolution duringrolling deformation was systematically investigated byisothermal eccentric roll drawing as simulation rolling.7)

Double peak textures were obtained at 423–523K withrolling reductions of 22% and above. About the coldformability, there has been a report on AZ31 sheets annealedat high temperature (773K) being drawn with a drawing ratioof 1.75.8,9)

In this study, texture control for improving the coldformability of AZ31D magnesium alloy sheets has beeninvestigated, via a combination of isothermal rolling andhigh-temperature annealing. We have also studied the changein the microstructure in each procedure. In addition, coldformability has been evaluated by a V-bending test. Thisstudy is based on the idea that annealing is indispensable forsheet forming and final textures are not determined by plasticprocessing only.

2. Experimental Procedure

Figure 1 shows the scheme of the eccentric-roll drawing7)

(hereinafter called isothermal rolling), in which rollingreduction changes continuously, and the rolls can be heateduniformly in a furnace together with the specimen.

A commercial AZ31D-O rolled sheet 1.6mm thick wasused as a specimen after being cut into 200� 40mm2

rectangles along the rolling direction. Table 1 shows thechemical composition of the sheet. The specimens wereprocessed first by pre-annealing, by isothermal rolling andthen by final annealing under the conditions shown inTable 2. During the annealing, specimens were covered withaluminum foil to prevent oxidation. Annealing above 773K

Test pieceafter drawing

Fracture bytension

Fig. 1 The scheme of eccentric-rolls drawing.

*1Graduate Student, Osaka Prefecture University, Present Address: SRI

Sports Limited, Kobe 651-0072, Japan*2Graduate Student, Osaka Prefecture University*3Undergraduate Student, Osaka Prefecture University, Present Address:

Graduate Student, Osaka University, Suita 565-0871, Japan

Materials Transactions, Vol. 51, No. 4 (2010) pp. 749 to 755#2010 The Japan Institute of Light Metals

was not tried to avoid ignition and self-weight deformation.Isothermal rolling was performed at 100mm s�1 using theeccentric-roll drawing apparatus. Additionally, we carriedout rolling with a standard mill without heating the rolls at thesame rolling rate for specimens preheated at 523, 573, 623and 673K. The microstructures and {0001} pole figures ofthe rolled surfaces were examined at each stage of theprocess. The textures were analyzed by Schulz reflectionmethod using a copper X-ray tube operating at 40 kV and100mA. The normalized scan tilt angle range was 0�75

degree around the normal of the rolled surface. Thediffraction intensity was normalized to unity for magnesiumpowder. Specimens for microstructures observation wereetched with acetic picral after buffing, and the average grainsize was determined by the linear intercept method. Thesamples after rolling were taken from locations on thetapered specimen mainly corresponding to 4, 11, 22 and 32%reduction. The details of the experimental methods omittedhere are identical to those in our previous paper.7)

Test pieces for 90 degree cold V-bending were made upinto 20 (RD) � 12 � 0.9mm3 plates by cutting and grindingfrom the thickness tapered samples following texturemeasurements. Punches with tip radii R shown in Table 3were prepared, and a bending limit R=t was evaluated by astrict criterion as shown in Fig. 2. The test was conducted at0.17mm s�1 and was stopped when the bending load reachedabout five times its plateau level.

3. Results and Discussion

3.1 Discovery of texture randomizing phenomenonThe ‘‘texture randomization’’, in AZ31 was first discovered

by annealing at 773K for 1 h before and after isothermalrolling at 473K. Later, it was found that the final annealingfor 20min was sufficient and preferable for texture random-ization. Requirements for texture randomization were inves-tigated around this processing condition hereinafter referredto as the ‘‘standard randomizing process’’.

A texture of as-received sheet is shown as a {0001} polefigure in Fig. 3 together with its RD (rolling direction)section and optical microstructure. The pole figure exhibits atypical basal texture, consisting of concentric rings stronglyoriented toward its center (0 degree). The microstructureconsists of equiaxial crystal grains about 12 mm large onaverage.

3.2 Conditions for texture randomizationFigure 4 shows changes in {0001} pole figures, their RD

sections and optical microstructure by pre-annealing, iso-thermal rolling and final annealing. The left and the rightcolumns correspond to pre-annealing at 573K for 30min andat 773K for 1 h, respectively. In either column, rolling wasperformed at 473K and final annealing—at 773K for 20min.The right column corresponds to the above-mentioned

Table 1 The chemical composition of the specimen.

(mass%)

Al Zn Mn Si Cu Fe Ni Mg

3.5 0.9 0.64 0.01 0.01 0.001 0.002 Bal.

Table 2 The positive combinations of conditions in the texture controlling

process.

(a) Rolling temperatures for focused pre-annealing conditions.

Pre-annealing 573K 773K

Final-annealing 30min 1 h

473K 5min 473K 298, 393, 423, 473K

573K 30min 298, 473K 298, 473K

693K 24 h 473K

723K 30min, 1 h 473K

10 sec, 20 sec 473K

30 sec, 1min 298, 473K

773K 2min, 4min 473K

20min 298, 393, 473K 298, 393, 423, 473K

1 h 473K 473K

(b) Rolling temperatures for other pre-annealing conditions.

Pre-ann. 693K 723K 773K

Final ann. 24 h 1 h 2 h 30min

473K 5min 473 k 473 k 473 k

773K 20min 298, 393, 473K 298, 393, 473K 473 k 473 k

Standard rolling reductions for each rolling temperature: 298K-4, 7%;

393K-4, 11, 22%; >423K-4, 11, 22, 32%. Especially, 298K-4, 7, 9,

11%; 423K-4, 7, 11, 16, 22, 27, 32%.

Table 3 The tip-radii of punches and their ratios to sheet thickness, t=R, inV-bending (t ¼ 0:9mm).

4.8 3.8 3.0 2.4 R /mm

2.2 1.9 1.7 1.4 1.1 t/R

5.3 4.2 3.3 2.7 t/R

2.0 1.7 1.5 1.3 1.0 R /mm90°

R

14

t12

R/t = 4.2 3.3 2.7 2.2

Fig. 2 An example of the success/failure criterion in V-bending ( : rough

surface stopping short of cracking. In this case, the bending limit is

evaluated as R=t ¼ 4:2).

30 µmIntensity 8.4Peak

Fig. 3 The {0001} pole figure, its RD-section and microstructure of the

received AZ31D rolled sheet.

750 M. Kohzu et al.

30 µm

30 µm

30 µm

30 µm

30 µm

Pre-annealing at 573 K for 30 min

Aft

er p

re-a

nnea

ling

Aft

er r

ollin

g at

473

K

Intensity 8.9Peak

Intensity 7.5Peak

Intensity 6.4Peak

Intensity 8.2Peak

Intensity 8.9Peak

(c) 11 %

(a) beforerolling(0 %)

(b) 4 %

(e) 32 %

(d) 22 %

30 µm

30 µm

30 µm

30 µm

Aft

er f

inal

ann

ealin

g at

773

K f

or 2

0 m

in

Intensity 12.1Peak

Intensity 11.0Peak

Intensity 10.6Peak

Intensity 7.9Peak

(g) 11 %

(f) 4 %

(i) 32 %

(h) 22 %

30 µm

30 µm

30 µm

30 µm

30 µm

Pre-annealing at 773 K for 1 h

Intensity 7.3Peak

Intensity 12.9Peak

Intensity 7.3Peak

Intensity 6.6Peak

Intensity 9.5Peak

(C) 11 %

(A) beforerolling

(B) 4 %

(E) 32 %

(D) 22 %

30 µm

30 µm

30 µm

30 µmIntensity 6.4Peak

Intensity 4.7Peak

Intensity 3.6Peak

Intensity 2.7Peak

(G) 11 %

(F) 4 %

(I) 32 %

(H) 22 %

Fig. 4 The {0001} pole figure, its RD-section and microstructure after pre-annealing, rolling and final annealing (effect of high-

temperature annealing).

Texture Randomization of AZ31 Magnesium Alloy Sheets for Improving the Cold Formability 751

‘‘standard randomizing process’’. Low-temperature pre-an-nealing at 573K causes little change of texture and micro-structure, as shown in Fig. 4(a). However, high-temperaturepre-annealing at 773K produces a stronger basal texture anda coarse grain microstructure containing abnormally growngrains, as shown in Fig. 4(A). By rolling under highreduction, the texture formed by the high-temperature pre-annealing changed to a more pronounced double-peaktexture, as shown in Fig. 4(D)–(E), than that formed by thelow-temperature pre-annealing, as shown in Fig. 4(d)–(e).

For the low-temperature pre-annealed sheet, during rollingdynamic recrystallization progresses from the sites ofdeformation twins formed under low reduction. The twinsdisappear under high reduction, as shown in Fig. 4(b)–(e).For the high-temperature pre-annealed sheet, dynamicrecrystallization does not occur during rolling, and deforma-tion twins are ever-increasing with an increase in reduction,as shown in Fig. 4(B)–(E). By the final high-temperatureannealing, only samples rolled with medium reductions of11–22% after high-temperature pre-annealing are random-ized with a significant reduction of peak intensity. For eitherpre-annealing condition, the final annealing produces ahomogeneous grain structure of 25–30 mm, as shown inFig. 4(f)–(i) and (F)–(I) without mixing of abnormally growngrains as in Fig. 4(A).

Based on the standard randomizing process shown in theright column of Fig. 4, the pre-annealing, isothermal rollingand final annealing conditions are changed one by one. Adegree of randomization after final annealing is shown inFig. 5 as peak intensities of the pole figures, where thenumber in parentheses is the average grain size after pre-annealing (except abnormally grown grains). As shown inFig. 5(a), pre-annealing up to 723K does not lead to finaltexture randomization even if it is prolonged. Pre-annealingfor 30min is insufficient for final randomization even at773K. In addition, pre-annealing at 693K for 24 h isadequate to dissolve tiny amounts of �-phase (Mg17Al12)precipitated at grain boundaries. Grain size after this pre-annealing was comparable to that at 773K for 1 h, and waslarger than that at 773K for 30min. However, the final peakintensity was higher than that in the case of pre-annealing at773K for either pre-annealing time. From this it follows thatformation of a coarse grain structure does not always lead to afinal texture randomization. As shown in Fig. 5(b), rollingtemperature has little effect on the final peak intensity, whichsuggests that precise temperature control is not requiredduring rolling. As shown in Fig. 5(c), final annealing as lowas 693 or 723K can reduce peak intensity to almost the levelreached by annealing at 773K for 20min, but a lot more timeis required at lower temperature. At 773K, annealing timesgreater than 20min do not improve the randomization.Figure 5(a)–(c) shows that final peak intensities are low atmedium rolling reductions of 11–22%. It does not matter forrandomization whether the rolling texture becomes doublepeak or not, since the threshold is between 11 and 22%.

The wide range of rolling conditions for final random-ization suggests the potential applicability of standard rollingto this process. Table 4 shows the effect of pre-heatingtemperature on final peak intensity in standard rolling,without roll heating. The rolling rate is 100mm s�1 which is

the same as in the isothermal rolling. Rolling at pre-heatingtemperature between 523 and 573K leads to low final peakintensities comparable to those of the standard randomizingprocess using isothermal rolling. However, excess pre-heating at 673K creates a strong basal texture after finalannealing, although dynamic recrystallization does not occurduring rolling. A high-temperature annealed sheet has betterlow-temperature rollability than a normally annealed sheet.

3.3 Discussion of texture and microstructureFigure 6 shows changes of texture and microstructure in

the process for which pre-annealing in the standard random-izing process is replaced with 24 h at 693K. This pre-annealing serves as a solution treatment as described above.In the pre-annealed state of Fig. 6(a), the texture, existence of

0

5

10

15

Peak

Int

ensi

ty, I

693K-24h(26)723K-1h(21)723K-2h(25)

773K-1h(27)773K-30min(21)

Effect ofpre-annealing

( ) : grain size, d/µm

0

5

10

15

Peak

Int

ensi

ty, I

: 298K: 393K: 423K: 473K

rolling

0 10 20 300

5

10

15

Rolling reduction, r (%)

Peak

Int

ensi

ty, I

693K-24h723K-30min

723K-1h

773K-20min

Effect offinal annealing

Effect of

(a)

(b)

(c)

Fig. 5 The effect of experimental conditions in each procedure on the final

peak intensity (based on pre-annealing at 773K for 1 h, rolling at 473K

and final annealing at 773K for 20min).

Table 4 The effect of pre-heating temperature and rolling reduction on

final peak intensity in standard rolling without roll heating (pre-annealing

at 773K for 1 h and final annealing at 773K for 20min).

Temperature523K 573K 623K 673K

Reduction

17.5% 3.5 3.8 4.4 —

20.0% 2.9 3.4 4.0 11.9

752 M. Kohzu et al.

abnormally grown grains and average grain size of normallygrown grains differ little from those in the state of Fig. 4(A)pre-annealed at 773K for 1 h (which is sufficient forrandomization). However, in Fig. 6(a), subsequent rollingcauses dynamic recrystallization, shown in Fig. 6(b)–(c), andthe final texture is a concentric ring (basal texture), as shownin Fig. 6(d)–(e). This transition is similar to pre-annealing at573K for 30min shown in the left column of Fig. 4. but withlower peak intensities. Generally, in AZ31 magnesium alloyswith coarse grains, twins play a major role in a plasticdeformation. For rolling a sheet pre-annealed at or below723K, deformation twins disappear by being replaced withfine dynamically recrystallized grains.

In general, a larger strain is required for dynamicrecrystallization at lower temperatures,10) so only rollingtemperature was lowered to 398K in the process shown inFig. 6. The result is demonstrated in Fig. 7. In the micro-structure after rolling with 22%, many deformation twins areproduced without dynamic recrystallization (see Fig. 7(a)).Although this microstructure is similar to that in the standardrandomizing process (right column of Fig. 4), the finaltexture becomes a concentric ring (Fig. 7(b)) like those of

Fig. 6(d)–(e). The microstructures of Figs. 4(D) and 7(a)which contain many deformation twins are magnified andcompared in Fig. 8. In both cases, thin and partially-crossedtwins were mainly observed. The yet-unidentified type of thetwins, may be mainly the twin of f10�112g h10�111i, since theseplay a major role in plastic deformation with a limitednumber of slip systems.11,12)

Figure 9 shows the variation of the RD section of {0001}pole figure as a function of final annealing condition, wherepre-annealing and rolling conditions are as in the standardrandomizing process. Even for final annealing at 723 or693K, the texture may be randomized. Nevertheless, 773K ishighly favorable since there randomization progresses muchfaster than up to 723K. We note that annealing longer thanthe 20min in the standard randomizing process has no effect.

The above-mentioned requirements for final texturerandomization can be summarized as follows. (1) High-temperature pre-annealing at 773K (1 h) is absolutelynecessary. (2) High-temperature pre-annealing makes thegrain-structure coarse and leads to a microstructure contain-ing many deformation twins without dynamic recrystalliza-tion in the subsequent rolling process. (3) Coarse grainstructure and twinning which do not evolve into dynamicrecrystallization are not sufficient. (4) The range of success-ful rolling conditions (temperature and rolling reduction) iswide, hence standard mill without heating the rolls shouldalso be successful. (5) Although high-temperature finalannealing at 773K is not always necessary, it leads to arandomization much faster than up to 723K.

3.4 Correlation between peak intensity and bendabilityAlthough the excellent formability of a randomized

sample was exhibited by 90 degree V-bending test, astatistical correlation between the V-bending limit and thepeak intensity of a pole figure had not still been confirmeddown to such a low intensity level, since such AZ31 rollingsheets do not exist. By using many samples obtained insearch of satisfactory randomizing conditions, we can plotthe correlation diagram as in Fig. 10. The test pieces wereground down to a thickness of 0.9mm. The V-bending limit,R=t, shows a good correlation as a whole with peak intensity,

30 µm

30 µm

30 µm

30 µm

30 µm

Pre-

anne

alin

gfo

r 24

h a

t 693

KR

ollin

g at

473

KFi

nal a

nnea

ling

at 7

73 K

for

20

min

(b) 11 %

(c) 22 %

(d) 11 %

(e) 22 %

Intensity 5.5Peak

Intensity 6.3Peak

Intensity 7.6Peak

Intensity 11.0Peak

Intensity 12.6Peak

(a) beforerolling

Fig. 6 The {0001} pole figure, its RD-section and microstructure after pre-

annealing at 693K for 24 h, rolling at 473K and final annealing at 773K

for 20min.

30 µm

30 µmIntensity 5.5Peak

Intensity 7.6Peak

(a) Rollingwith 22 %

(b) After Finalannealing

Fig. 7 The {0001} pole figure, its RD-section and microstructure after pre-

annealing at 693K for 24 h, rolling at 398K and final annealing at 773K

for 20min.

Texture Randomization of AZ31 Magnesium Alloy Sheets for Improving the Cold Formability 753

I. The excellent bendability of R=t < 2:0 is limited to therandomized samples of I < 4:5. But in samples with higherintensity, the R=t distribution is wide because it is affectednot only by peak intensity of the pole-figure but also by itspattern.

4. Conclusion

Texture randomization of magnesium alloy sheets wasachieved with a standard commercial wrought alloy, AZ31,within the standard rolling process without the use of specialapparatus or technique. In this process, high-temperatureannealing at 773K before and after the final rolling passsignificantly weakens the basal orientation of rolling sheet.

This results in considerably improved cold-formability. Theallowed range of rolling conditions is wide, thereforeindustrial mass production is possible given demand. How-ever, the mechanism of this transition is still poorly under-stood and many questions remain to be answered. In thefuture, multiple verification of formability in view of thefinal practical application is required, in parallel with theexhaustive analysis of the mechanism.

Acknowledgments

We express our deepest appreciation to ProfessorMototsugu Katsuta of Nihon University and MitsubishiAluminum Co., Ltd. for their research cooperation.

11 %

22 %

As rolled573 K30 min

693 K24 h

723 K30 min

723 K1 h

773 K30 s

773 K1 min

Fig. 9 The variation in the RD-section of {0001} pole figure due to the final annealing (rolling at 473K after pre-annealing at 773K

for 1 h).

10 µm 10 µm

(a) Magnified microstructureof Fig. 4 (D)

(b) Magnified microstructureof Fig. 7 (a)

Fig. 8 The comparison between magnified microstructures of Fig. 4(D) and Fig. 7(a). (a): Rolling at 473K with 22% after pre-annealing

at 773K for 1 h. (b): Rolling at 393K with 22% after pre-annealing at 693K for 24 h.

754 M. Kohzu et al.

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2 3 4 5 6 7 8 9101.0

1.5

2.0

2.5

3.0

3.54.04.5

Peak intensity, I

V-b

endi

ng li

mit,

R/t

3057320030573

60, 2077320030573

20773200120, 60723

305732560773

3057320060773

60, 3072320060773

207732560773

20773150, 12060773

60, 2077320060773

tFA

/minTFA

/KTR /KtPA /min

TPA

/K

Finalannealing

RollingPre-annealing

Fig. 10 The correlation between the V-bending limit, R=t, and peak intensity of the {0001} pole figure, I (R: tip radius, t: thickness of

sheet).

Texture Randomization of AZ31 Magnesium Alloy Sheets for Improving the Cold Formability 755