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Journal of Materials Processing Technology 211 (2011) 1203–1209

Contents lists available at ScienceDirect

Journal of Materials Processing Technology

journa l homepage: www.e lsev ier .com/ locate / jmatprotec

nfluence of extrusion parameters on grain size and texture distributions of AZ31lloy

eiqin Tang, Shiyao Huang, Shaorui Zhang, Dayong Li ∗, Yinghong Pengtate Key Laboratory of Mechanical System and Vibration, School of Mechanical and Power Energy Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China

r t i c l e i n f o

rticle history:eceived 17 March 2010eceived in revised form 2 October 2010

a b s t r a c t

Grain size and texture distributions have great influences on the mechanical properties of extruded rods.In order to study grain size and texture evolution during the hot extrusion process, direct extrusiontests were carried out with a variety of extrusion parameters (extrusion ratio, temperature and veloc-

ccepted 15 January 2011vailable online 21 January 2011

eywords:Z31 magnesium alloyxtrusion

ity) for commercial as-cast AZ31 magnesium alloys. Extruded specimens were investigated by opticalmicroscopy (OM) and electron backscattered diffraction (EBSD). Experimental results show that extru-sion ratio is the most important parameter for grain size refinement. Basal fiber textures with various(0 0 0 2) pole intensities are observed in extruded rods. Maximum intensities increase with the decreas-ing extrusion ratio and the increasing velocity, while the influence of temperature depends on the value

ocity

rain sizeexture

of extrusion ratio and vel

. Introduction

Magnesium alloys are promising structural light metals inerospace and automobile applications because of their hightrength to weight ratio and recyclability. Presently, most partsade from Mg alloys are manufactured by the die casting process

ecause of its high productivity. But the casting products have poorechanical strength. Somekawa and Mukai (2005) suggested that

he poor strength is resulted from the amount of eutectic in theicrostructure and the presence of cast defects such as inclusions,

nterdendritic shrinkage voids, and porosity caused by gas entrap-ent during solidification. Compared with casting magnesium

lloys, wrought magnesium alloys have attracted increasing inter-sts due to their higher mechanical strength and better ductility.here are different technologies of processing wrought Mg alloys:olling, stamping, forging, extrusion, etc. Mueller and Mueller2007) pointed out that out of currently employed large scale man-facturing processes, extrusion provided a possibility to produce aide range of magnesium alloy profiles.

However, magnesium alloys exhibit limited formability at roomemperature due to the HCP structure. Thus, most of the form-ng processes are conducted at elevated temperature. Previous

esearches showed that magnesium alloys might undergo dynamicecrystallization (DRX) during hot forming processes. Two mainifferent DRX mechanisms can take place during deformation ofagnesium alloy, discontinuous dynamic recrystallization (DDRX)

∗ Corresponding author. Tel.: +86 21 34206313; fax: +86 21 34206313.E-mail address: dyli@sjtu.edu.cn (D. Li).

924-0136/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2011.01.014

.© 2011 Elsevier B.V. All rights reserved.

and continuous dynamic recrystallization (CDRX). For DDRX, grainboundaries act as obstacles to the movement of dislocations dur-ing extrusion. Dislocations accumulate and subsequently generatelocal stress concentrations on the grain boundaries, resulting in thegeneration of serrated boundaries. The development of serratedgrain boundaries in coarse grains leads to the nucleation of DDRXby bulging. CDRX is considered as a process where a progressiveincrease in misorientation from the center to the boundary of grainand the conversion of low angle boundaries into the high angle onesis realized.

Regardless of the recrystallization mechanism, both dynami-cally recrystallized grain size (dDRX) and the percentage of DRX(XDRX) are sensitive to deformation conditions. Barnett et al. (2004)showed that recrystallized grain size was mainly affected by tem-perature and strain rate, lower strain rate and higher temperaturewill generate larger recrystallized grains. For the given temperatureand strain rate, Lee et al. (2007) found that the recrystallized grainsremained constant with varied strains. Al-Samman and Gottstein(2008) showed that even at high temperature (400 ◦C) and lowstrain rates (10−4 s−1), AZ31 showed virtually no grain growth withthe increasing strain. On the other hand, XDRX is influenced by tem-perature, strain and strain rate collectively. Maksoud et al. (2009)showed that both lower strain rate, higher temperature would yieldlarger value of XDRX. Lower strain rate provides more time, highertemperature provides higher boundary mobilities, both of which

are favored for DRX. Beer and Barnett (2007) observed that whenthe strain rate and temperature are fixed, XDRX increases with theincreasing strain. XDRX is also influenced by the initial grain size.Spigarelli et al. (2007) concluded that the “necklace” DRX does notresult in a fully recrystallized structure unless the grain size is fine.

1204 W. Tang et al. / Journal of Materials Processi

Table 1Extrusion conditions.

Extrusion ratio Velocity (m/min) Temp. (K)

643 673 703

6.25 0.8 + +1.4 + + +2.2 + + +

25 2.5 + + +

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srnrtwwbtd

2

wTusrHiratMtta

4.5 + + +7 + + +

Recently, some studies have been done to show that grainize refinement and texture evolution during extrusion of mag-esium alloy has great impact on its mechanical properties. Inrder to improve material properties of extruded products, it isecessary to gain a better understanding about how extrusionarameters (extrusion ratio, temperature and velocity) affect theicrostructure distribution. Of the various commercial magnesium

lloys, those developed from the Mg–Al–Zn group (designated asZ series, e.g. AZ31) are widely studied. Murai et al. (2003) inves-

igated the effects of extrusion conditions on grain refinement,nd concluded that average grain sizes decrease with increasingxtrusion ratio, decreasing temperature and decreasing extrusionpeed. Uematsu et al. (2006) examined the effects of extrusionemperature on grain refinement of AZ31B magnesium alloy andhowed that grain size decreased with decreasing temperature.hen et al. (2007) evaluated the effects of extrusion ratio on grainize. The results indicated that higher extrusion ratio (≥39) is moreffective than lower extrusion ratio (≤24) to refine grain size.n addition to grain size refinement, the texture evolution dur-ng extrusion has also been studied. Laser et al. (2008) pointedut that the extruded rods exhibit basal fiber texture (c-axis inhe radial direction). Park et al. (2009) found that higher rampeed and lower exit temperature will result in a weaker fiberexture.

Although there have been a lot of progresses in this filed, wehould note that the influence of extrusion parameters (extrusionatio, temperature and velocity) on the texture intensity is stillot clear. Besides, grain size distribution after extrusion is seldomeported because most of the previous research is concentrated onhe average grain size. In the present study, direct extrusion testsere carried out for commercial casting AZ31 magnesium alloyith various extrusion parameters. Grain size and texture distri-

utions of extruded rods were investigated by experiments. Then,he influences of extrusion parameters on grain size and textureistributions were discussed, respectively.

. Experimental procedures

Commercial direct chill (DC) casting feedstock of AZ31 alloysas machined into billets (˚100 mm) for direct extrusion tests.

wo dies with different exit diameter (˚40 mm, ˚20 mm) weresed in this study in a bid to find out the influence of extru-ion ratio. The extrusions were performed at three kinds ofam speeds (the speed with which ram moves toward die).owever, this resulted in different extrusion velocities (veloc-

ty at which extruded rods leave the die) for different extrusionatios. As shown in Table 1, the casting billet was extrudedt 9 different conditions with each die. The billet was heated

o the temperature listed in Table 1 and lubricated with

oS2 prior to extrusion. The extrusion die was also heatedo the same extrusion temperature. After leaving the chamber,he extruded rods were cooled with water, and then in their.

ng Technology 211 (2011) 1203–1209

After the extrusion tests, optical microscopy (OM) and electronbackscattered diffraction (EBSD) were conducted for microstruc-ture analyses. Specimens for OM and EBSD were cut from centerof the extruded rods, the observation planes were parallel tothe extrusion direction. Specimens for OM were mounted inepoxy resin, grounded with 1200 grit SiC paper, then mechan-ically polished using 6, 3 and 1 �m diamond paste and 0.3,0.05 �m alumina solution. Specimens were etched for 5 s inacetic picral (10 ml acetic, 10 ml distilled water, 70 ml picral).Specimens for EBSD were ground with 2000 grit SiC paper,followed by mechanically polishing with 3 �m diamond paste.Specimens were prepared by electro-polishing in AC2 elec-trolyte for the final stage. The microstructure was imaged byLEO 1450VP SEM. Grain orientations were examined and ana-lyzed by Zeiss Suppa 55VP SEM equipped with HKL Channel5 system.

3. Results and discussion

3.1. Initial microstructure and texture distribution

Fig. 1 shows the microstructure of casting AZ31 billet. It haslarge grains and the average grain size is approximately 200 �m.As shown in Fig. 2, almost random texture distribution exists forthe casting AZ31 billet.

3.2. Microstructure distribution of extruded rods

Fig. 3 shows the partially recrystallized microstructures ofextruded AZ31 magnesium alloys. Large grains are stretched inthe extrusion direction (ED), and many fine recrystallized grainssurrounding coarse grains. This suggests that dynamic recrystal-lization occurs during the hot extrusion process. The new fine grainslook like a “necklace” along large-size grain boundaries. Al-Sammanand Gottstein (2008) also observed the formation of “necklace”type structure during compression of AZ31 alloy, where the nucle-ation of new recrystallized grains occurs at the original grainboundaries and subsequent nucleation occurs at the recrystallizedgrains.

There are two important DRX mechanisms for AZ31 alloy, DDRXand CDRX. Fatemi-Varzaneh et al. (2007) stated that DDRX isonly operative when the initial grain size of magnesium alloy islarge enough for crystallographic slip to be heterogeneous, and themicrostructural development during DDRX involves the formationof a “necklace” structure of grains. In this work, the initial grain sizeis large (about 200 �m), which can possibly lead to predominant ofDDRX during extrusion.

Since DRX is operating during extrusion process, dDRX can beconsidered as a characteristic of deformation. The relationshipbetween DRX grain size, temperature and strain rate is describedin the following equation:

dDRX = k(Z)−n (1)

where dDRX is grain size, k, n are material constants and Z repre-sents Zener–Hollomon parameter. Zener–Hollomon parameter isdefined as Z = ε̇ exp(Q/RT), where ε̇ is strain rate, Q is activationenergy, R is gas constant and T is temperature. However, strainrate cannot be measured directly during the extrusion process. Theequation published by Park et al. (2009) is adopted to estimate theaverage strain rate in this work:

˙̄ε = 6D2BV ln(ER)

D3B − D3

E

(2)

where DB and DE are the diameters of the casting billet and theextruded rod, V is the ram speed and ER is the extrusion ratio.

W. Tang et al. / Journal of Materials Processing Technology 211 (2011) 1203–1209 1205

re of

Blr

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w

ZirtbdeigTiwHisaadear

3

rrte

Fig. 1. Microstructu

esides, the temperature rise cannot be ignored. The equation pub-ished by Liu et al. (2007) is employed to determine the temperatureise:

T = 34.396 ln(V) − 1.07 (3)

here V is ram speed in mm/s.Fig. 4 reveals the relationship between dDRX and

ener–Hollomon parameter in the current study, where dDRX

s the average of recrystallized grain size, and the measuredegion is in the center of the extruded rods. As shown in Fig. 5,he average of recrystallized grain sizes are around 5–15 �m foroth two extrusion ratios. This suggests that the extrusion ratiooes not have a significant influence on dDRX. Fatemi-Varzaneht al. (2007) and Lee et al. (2007) found that dDRX of AZ31 alloys mainly affected by temperature and strain rate. Once the DRXrains are formed, dDRX is not changed with the increasing strain.he relationship between dDRX and Zener–Hollomon parameters generally described by Eq. (1), where the variation of dDRX

ith Zener–Hollomon parameter in logarithmic scale is linear.owever, drawing a linear relationship for data in Fig. 4 will result

n a distinct deviation because the distribution of dDRX is rathercattered in both Figs. 6(a) and 7(b). The possible reason is thatverage strain rate and temperature variation during extrusionre calculated by Eqs. (2) and (3), respectively, which can induceeviation between the real value and calculated value. Thus, anquation is required to give a better description between dDRX

nd extrusion parameters (temperature, velocity and extrusionatio).

.3. Grain size distribution

Although dDRX is mainly affected by temperature and strainate, XDRX is influenced by temperature, strain and strainate collectively. Material is subject to different tempera-ure, strain and strain rate with varied extrusion param-ters, which can lead to different grain size distribution

as-cast AZ31 billet.

after extrusion. The influence of extrusion ratio, temperatureand velocity is discussed, respectively, in the following para-graphs.

The influence of extrusion ratio on grain size distributions ispresented in Fig. 5. In the current study, extrusion velocities arehigher at the larger extrusion ratio than those at smaller ratio. Inorder to preclude the influences of velocities, two conditions (seeFig. 5) where velocities are of close values are chosen for com-parison. When extrusion ratio is 6.25, large grains (about 45 �m)are still visible at both temperatures. When extrusion ratio is 25,the maximum grain sizes observed are about 20 �m and the per-centages of small grains (<10 �m) are larger than those at thesmaller extrusion ratio. Chen et al. (2007) established an empiricalequation to express the relationship between strain and extrusionratio:

ε = ln(ER) (4)

where ER is the extrusion ratio. Hence, the strain increases withthe increasing extrusion ratio. On the other hand, the value of XDRX

increases with the increasing strain, and thus grain size decreasessignificantly at large extrusion ratio.

Fig. 6 shows how extrusion temperature affects grain size dis-tributions. Temperature does not have significant influence on thegrain size distribution. Although grain growth is favored at hightemperature, grain of an abnormally large size is not found inFig. 6 with the increase of temperature. This phenomenon canbe attributed to two aspects: (1) the value of Zener–Hollomonparameter reduces when temperature increases, resulting in alarger percentage of DRX and a finer grain size. (2) Al-Sammanand Gottstein (2008) showed that AZ31 showed virtually no grain

growth in comparison with pure magnesium during deformation.Besides, extrusion tests were finished within a short period due tohigh velocity. After leaving the chamber, the extruded rods werecooled with water, and then in the air. Grain growth is arrested bywater cooling.

1206 W. Tang et al. / Journal of Materials Processing Technology 211 (2011) 1203–1209

2) Pol

drewhpZontv

3

miacHs

Fig. 2. Texture distribution of as-cast AZ31 billet. (a) (0 0 0

Fig. 7 depicts how extrusion velocity impacts on grain sizeistributions. Higher extrusion velocity suggests higher strainate, which would reduce grain size according to Eq. (1). How-ver, as shown in Fig. 7, grain size does not reduce significantlyith the increasing velocity. Govind et al. (2008) showed thatigher velocity would generate more heat and raise the tem-erature to a higher degree which had an opposite effect onener–Hollomon parameter. Thus when extrusion velocity is thenly flexible parameter in hot extrusion test, higher velocity doesot ensure a rise in the value of Zener–Hollomon parameter. Hencehe grain size does not decrease significantly with the increase ofelocity.

.4. Texture distribution

Texture development is closely related to the activated defor-ation mechanism. At low temperatures, the dominate slip system

s basal slip. Tensile twinning also plays an important part inccommodating strain during the deformation, especially in theoarse-grained magnesium alloys. Jin et al. (2006) and Wang anduang (2007) showed that tensile twining is favored at the early

tage of deformation when the initial grain size is large, inten-

e figure. (b) (1 1 −2 0) Pole figure. (c) (1 0 −1 1) Pole figure.

sive twinning will reorient grain to a basal orientation. Meanwhile,Mackenzie and Pekguleryuz (2008) proved that some other mech-anisms, including double twins and deformation heterogeneitiessuch as shear banding and kink banding also operate to accom-modate c-axis compression when non-basal slip is limited at lowtemperate.

At an elevated temperature, DRX plays a vital role in the defor-mation of magnesium alloy. Perez-Prado and Ruano (2002), Yanget al. (2005) and Bohlen et al. (2009) showed that recrystalliza-tion in magnesium alloys does not lead to a significant change intexture and new grains formed at grain boundaries have similar ori-entation with their parent grains. Hence dynamic recrystallizationserves to strengthen the basal texture. On the other hand, Jain andAgnew (2007) stated that critical resolved shear stresses (CRSS) ofnon-basal slip systems decline dramatically at an elevated temper-ature, non-basal slip systems are more easily activated and tendto split basal texture. Both DRX and non-basal slip have significant

impacts on the texture evolution of magnesium alloy at elevatedtemperatures.

As can be seen in Fig. 8, (0 0 0 2) basal planes of extruded rods arealmost parallel to the extrusion direction, which suggests that basaltexture is formed during the direct extrusion test of AZ31 alloy.

W. Tang et al. / Journal of Materials Processing Technology 211 (2011) 1203–1209 1207

Fig. 3. Microstructure of extruded rods. (a) Temperature = 643 K, veloc-ity = 2.2 m/min. (b) Temperature = 673 K, velocity = 2.2 m/min.

Fig. 4. Relationship between Zener–Hollomon parameter and recrystallized grainsize. (a) Extrusion ratio = 6.25. (b) Extrusion ratio = 25.

Fig. 5. Influence of extrusion ratio on grain size distribution, temperature = 673 K.

Fig. 6. Influence of extrusion temperature on grain size distribution, ratio = 25,velocity = 2.5 m/min.

However, texture distributions and maximum texture intensitiesof extruded rods are different under varied extrusion conditions.Here, the influences of extrusion ratio, temperature and velocityare discussed, respectively.

When extrusion ratio increases from 6.25 to 25, (0 0 0 2) poleintensities reduce significantly as presented in Fig. 9. CRSS of non-basal slip systems is higher than that of basal slip system. Grainsare subject to larger strain at higher extrusion ratio, which suggeststhat non-basal slip is more easily activated when extrusion ratio is25. Besides, earlier discussion has also shown that higher extrusionratio will lead to larger percentage of recrystallized grains. Grainrefinement caused by DRX promotes stress concentration at grainboundary, which is also effective for the activation of non-basal slipsystems. High activities of non-basal slip systems serve to rotatethe grain to radial direction, thus reduce the intensity of (0 0 0 2)

component.

As shown in Fig. 10(a), the maximum intensities of (0 0 0 2)planes become weaker at higher temperatures. CRSS of

Fig. 7. Influence of extrusion velocity on grain size distribution, ratio = 25, temper-ature = 673 K.

1208 W. Tang et al. / Journal of Materials Processing Technology 211 (2011) 1203–1209

F , ver ) Ext1

nNpFsp

ig. 8. (0 0 0 2) Pole figures of extruded rods. (a) Extrusion ratio = 25atio = 25, velocity = 2.5 m/min, temperature = 673 K, max intensity = 9.39. (c0.85.

on-basal slip systems reduces with the increasing temperature.on-basal slip systems are more easily activated at higher tem-

erature and thus the intensity of (0 0 0 2) pole figure is reduced.ig. 10(b) gives the pole intensities of (0 0 0 2) planes when extru-ion ratio is 25, which indicates that the pole intensities of (0 0 0 2)lanes do not reduce with the increasing temperature. The main

Fig. 9. Maximum (0 0 0 2) pole intensities.

locity = 2.5 m/min, temperature = 643 K, max intensity = 13.8. (b) Extrusionrusion ratio = 25, velocity = 2.5 m/min, temperature = 703 K, max intensity =

reason is that extrusion velocities are higher when extrusion ratiois 25. Higher velocity will raise the temperature to a higher degree,which will lead to a larger percentage of recrystallized grains.Although non-basal slip is easily activated at higher temperature,larger percentage of recrystallized grains tends to retain the basaltexture. Hence at higher extrusion ratio and velocity, the maxi-mum texture intensity does not decrease against the increasingtemperature.

As presented in both Fig. 10(a) and (b), when temperatureand extrusion ratio are fixed, (0 0 0 2) pole intensity becomesstronger as velocity increases. This phenomenon can be explainedfrom two aspects. (1) Higher velocity will generate more heatand raise the temperature to a higher degree, resulting in largerpercentage of recrystallized grains, which serves to strengthenthe basal texture. (2) Non-basal slip is the main deforma-

tion mechanism which splits the basal texture. The highervelocity suggests that extrusion test is finished more rapidly,there is no adequate time for non-basal slip to completelyrotate grains to radial direction and thus the basal texture iskept.

W. Tang et al. / Journal of Materials Processi

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ig. 10. Maximum (0 0 0 2) pole intensities. (a) Extrusion ratio = 6.25. (b) Extrusionatio = 25.

. Conclusions

Direct extrusion tests of as-cast AZ31 Mg alloy were carriedut in 18 different deformation conditions. Grain size and textureistributions of extruded rods were investigated by EBSD. The fol-

owing conclusions were drawn:

1) Partially recrystallized microstructures are observed inextruded rods. The new fine grains look like a “necklace” alonglarge-size grain boundaries. The average of recrystallized grainsizes is around 5–15 �m for both two extrusion ratios. Thissuggests that the extrusion ratio does not have a significantinfluence on dDRX.

2) Extrusion ratio is the most important parameter for grainsize refinement. Higher extrusion ratios lead to larger per-centages of recrystallized grains and reduce the averagegrain size. In contrast, extrusion temperature and velocitydo not have significant influences on the grain size distribu-tions.

3) Basal texture with various (0 0 0 2) pole intensities are observedin extruded rods with different extrusion parameters. Maxi-mum intensities increase with the decreasing extrusion ratioand the increasing velocity. The influence of temperaturedepends on the value of extrusion ratio and velocity. At lowextrusion ratio and low velocity, the intensity of (0 0 0 2) pole

figure reduces against the increasing temperature. Nonethe-less, the maximum texture intensity does not decrease withthe increasing temperature at high extrusion ratio and highvelocity.

ng Technology 211 (2011) 1203–1209 1209

Acknowledgements

The authors would like to acknowledge the support of theNational Natural Science Foundation of China (Nos. 50821003,50405014), Shanghai Science & Technology Projects (Nos.10QH1401400, 10520705000, 10JC1407300), Program for NewCentury Excellent Talents in University (NCET-07-0545) and FordUniversity Research Program.

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