Microstructure and Mechanical Properties of Magnesium

Alloy AZ31 Processed by Compound Channel Extrusion

Xiaofei Lei1;2, Tianmo Liu1;2;*, Jian Chen1;2, Bin Miao1;2 and Wen Zeng1;2

1College of Materials Science and Engineering, Chongqing University, Chongqing 400044, P. R. China2National Engineering Research Center for Magnesium Alloys, Chongqing University, Chongqing 400044, P. R. China

We report microstructure evolution and mechanical properties of Mg alloy AZ31 processed by a new severe plastic deformation techniquewhich combines forward extrusion, equal channel angular extrusion (ECAE) and change channel angular extrusion (CCAE). Under all of theprocessing temperatures ranging from 623 K to 723 K, the grain size of the as-extruded sample was remarkably refined, which is attributed to thegrain subdivision and dynamic recrystallization during the drastic deformation. We have also found the strength and tension-compressionasymmetry were improved. Simultaneously, micro-fracture of tensile exhibited the characteristics of ductility, indicating plasticity of Mg alloyAZ31 was improved by compound channel extrusion. [doi:10.2320/matertrans.MC201004]

(Received November 4, 2010; Accepted February 15, 2011; Published April 13, 2011)

Keywords: magnesium alloy AZ31, compound channel extrusion, microstructure and mechanical property, fracture behavior

1. Introduction

To fight against the energy consumption and environmentdegradation, improving efficiency and reducing cost havebecome an inevitable trend. As an extremely light metal,magnesium alloys are of excellent specific strength, excellentsound damping capabilities, good castability, hot formability,and excellent machinability which are very attractive in avariety of technical applications, especially in electronics,aerospace, transportation induries, sports industries, etc.1–3)

However, due to Hexagonal Closed-Packed crystal structure,Mg alloys exhibit poor plastic ability at room temperature.Therefore, most products are processed by casting, whichgreatly limits the wide use of Mg alloys. To expand theapplication field of Mg alloys and improve its mechanicalproperties, hot plastic deformation method is a trend ofdevelopment. Furthermore, mechanical properties ofwrought magnesium products was enhanced compared tothe cast products.4,5)

Recently, severe plastic deformation has become apromising processing to get ultrafine grains for improvingductility and strength of alloys.6–12) In addition, it providesa reliable method for improving the room-temperaturemechanical properties of hexagonal close-packed materials.A. Galiyev et al.13) reported that Mg alloy exhibits distinctplastic deformation mechanism at different temperaturerange. When the temperature over 498 K, the ductility ofMg alloy increases greatly with the temperature due to thebehaviors of recovery and recrystallization.14) Thus abovethis temperature, Mg alloys can be plastically deformedsmoothly, such as rolling15) and extruding.16) In the case ofextruded Mg alloys, many researches focus on equal channelangular extrusion (ECAE).17–19) The process involves push-ing the work-piece though two channels of equal crosssection that meet at an predetermined angle.20) However,ECAE technology cannot be applied to the actual productionowing to most ECAE billets needing initial extrusion.

Another reason is that that process cannot be extrudedcontinuously either. Considering some aspects like economicbenefit and efficiency, many passes of ECAE is not good forthe practical application.

In this work, we apply a new severe plastic deformationtechnology-compound channel extrusion, in order to reducethe passes of extrusion and improve the work efficiency.The microstructures, mechanical properties, the deformationbehavior and fracture behavior of Mg alloy AZ31 wereinvestigated.

2. Experimental Procedures

The material used for the current study was a commercialMg alloy AZ31 with the following chemical composition(in mass%): 3% Al, 1% Zn, 0.3% Mn and Mg (balance),supplied in form of cast ingots. In order to improve theuniformity of alloy composition, ingots were homogenizedat 673 K for 15 h. And then, these ingots were machinedinto cylindrical samples with a cross-section dimension of�80 mm� 150 mm as starting materials for the compoundchannel extrusion process. The compound channel extrusiondie is schematically shown in Fig. 1. We carried out thisexperiment using XJ-500 horizontal extruder with ratedextrusion pressure 20 MPa. The samples were �80 mm�150 mm cylinders, which were firstly deformed into �50 mmcylinders in diameter using routine forward extrusion. Andthen deformed samples went through into four changechannels which were the change channel angular extrusionto form four bars with �10 mm in diameter, and finally thefour bars were deformed by equal channel angular extrusion.The total extrusion ratio is 16 : 1, and the angle of eachcorner is 90�. Both the die and the extruder were preheated tothe extrusion temperature. Then the samples were put into thecontainer and kept for 30 min at the same temperature of thedie. As for lubrication, the graphite was used. Extrusionexperiments were carried out at 623 K, 673 K and 723 K re-spectively, with a velocity of 2 mm/min. Finally, as-extrudedsample extruded at 623 K was annealed at 623 K for 2 h.*Corresponding author, E-mail: tmliu@cqu.edu.cn

Materials Transactions, Vol. 52, No. 6 (2011) pp. 1082 to 1087Special Issue on Platform Science and Technology for Advanced Magnesium Alloys, V#2011 The Japan Institute of Metals

Microstructures of the samples were examined usingoptical microscopy (OM). The mean grain size from theoptical micrographs was determined by the followingprocedures presented in ASTM standard E112-95. TheVickers hardness (HV) was measured using a digital micro-hardness tester HXD-1000TM/LCD at a load of 0.98 N for20 s. Both the compressive and tensile directions werealigned parallel to the extrusion direction, which werecarried out by electronic universal testing machine CMT-5150 at room temperature. X-ray diffraction was used toanalyze the texture evolution parallel to the ED–TD plane. AD/Max-1200X diffractometry with copper target operatedat 40 kV and 30 mA was applied. Fracture morphology wasobserved by TECSAN-VEGAII scanning electron micro-scope (SEM).

3. Results and Discussion

3.1 Optical micro-structure and macro-textureFigure 2 presents the optical microstructures of Mg alloy

AZ31 of different processing states. Figure 2(a) shows themicrostructure of as-cast homogenization before compoundchannel extrusion. The average grain size of the AZ31 alloyprovided in the form of cast ingots is approximately 150 mm.After homogenization, the grain size grew up to 200�240

mm. The coarse microstructure after this combined channelsextrusion process has been greatly improved since theoriginal grains are replaced by newly generated equiaxedfine grains (Figs. 2(b), (c), (d)), where the ED–TD plane isobserved. The mean grain sizes of the AZ31 samplesdeformed at 623 K, 673 K and 723 K are determined to be4, 11 and 16 mm, respectively (Fig. 3). Therefore, grains ofMg alloy AZ31 have been refined obviously. After theannealing process for the extrusion parts, the grains re-grewinto 30 mm, as illustrated in Fig. 2(e).

The Mg alloys are prone to dynamic recrystallization byhot extrusion process.21) With the increase of the extrusiontemperature, the grain size became larger. The size of thegrains obtained at 723 K is two times larger than that of thegrains extruded at 623 K (Figs. 2(b), (d)), which indicates

that the extrusion temperature is an important factor incontrolling the microstructure of Mg alloys AZ31. We shouldnote that this new type of severe plastic extrusion processretains the advantages both ECAE and CCAE22) on grainrefinement at the same extrusion temperature, for example,the grain was refined to 9 mm under five passes by ECAE andwe can get grain size to 23 mm by one-pass CCAE, but we canmake grain fine to about 4 mm after one-pass this process at623 K.

As can be seen from Fig. 4, the strongest and the secondstrongest XRD diffraction peak of homogenized Mg alloysare (10�111) and (10�110), respectively. After as-extrusion, thestrongest diffraction peak appears at (0002) after XRDscanning from the ED-TD planes. The intensity of crystalplane’s diffraction reflects the relative number of thedistribution of crystal planes, which are parallel to thesurface. So the crystal planes corresponding to the strongestdiffraction peak are the preferred and the most powerfulcrystal planes. Figure 4 clearly shows that the as-extrudedsample has a preferred orientation along the extrusiondirection, and there is a fiber texture {0002} of basal plane.For the as-extruded Mg alloy after annealing, the strongestpeak remains the (0002), but the intensity decreasesobviously.

3.2 Micro-hardness and mechanical propertiesAs seen in Fig. 5, the hardness and compressive properties

of as-extruded Mg alloy AZ31 are better than those of theoriginal homogeneous state. Micro-hardness and compres-sive strength are greatly improved after extrusion, but thetotal strain-to-fracture declines a little compared to thehomogeneous state. In regard to the effect on the total strain-to-fracture, the theoretical strain-to-fracture decreases withthe extrusion temperature and the grain size increasesgradually. But at the same time, strain-to-fracture has acertain relationship with the uniformity of structural stressand heat stress. The structural stress is rather high owing tothe less homogeneous structure, thus it’s more easily togenerate pileup of dislocation and stress concentration, whichlead to the overall plasticity and toughness decrease. There-fore, we find that the grain size of as-extruded Mg alloy AZ31was fined hugely in Fig. 3, but the total strain-to-fractureshowed little change due to the structural and thermal stress.Although the grains of annealed Mg alloy AZ31 re-grew,total strain-to-fracture was excellent due to complete recrys-tallization and homogeneous structure. As above discussion,two factors affect the strain-to-fracture, total strain-to-fracture of Mg alloy AZ31 extruded at different temperaturechanged slightly during this process.

We further investigate the mechanical properties have astrong dependence on the texture. Fiber texture {0002}makes property of as-extruded weak. As to as-extruded Mgalloy after annealing, the strongest peak remains the (0002),but the intensity decreases obviously, which was not in favorof microstructure and mechanical properties. In short, the finegrains make strength, ductility and toughness of Mg alloyAZ31 improved greatly. Compared with available studies onhot extrusion, the yield strength and the UCS of Mg alloythrough this as-extrusion process at 623 K are 160 MPa and396 MPa higher than previously reported 130 MPa (yield

Sample

Die

Container

4x 10mm

ED

TDND

Fig. 1 A schematic drawing of the compound channel extrusion process.

The ED, ND and TD represent the extrusion direction, normal direction,

and transverse direction, respectively.

Microstructure and Mechanical Properties of Magnesium Alloy AZ31 Processed by Compound Channel Extrusion 1083

(a) (b)

(e)

(d) (c)

Fig. 2 Optical microstructures of Mg alloy AZ31 at different processing states: (a) homogenized at 673 K for 14 h, (b) (c) (d) extruded at

623 K, 673 K, 723 K parallel to the ED–TD plane, (d) annealed at 623 K for 2 h.

623K 673K 723K A H05

1015202530

216

218

220

222

224

Gra

in s

ize

(μm

)

Processing states

Grain sizes of AZ31

Fig. 3 Average grain sizes of Mg alloy AZ31 at different processing states.

(A-annealed at 623 K for 2 h, H-homogenized at 673 K for 14 h and as-

extruded at 623 K, 673 K, 723 K).

2θθ / degree30 40 50 60 70 80 90

Homogenized

As-extruded

Inte

nsit

y(a.

u.)

Annealed

0

0

0

45000

12500

7000(1011)

(1010)

(0002)

(1012)

(1013)

Fig. 4 The XRD pattern of Mg alloy AZ31 at different processing states:

homogenized at 673 K for 14 h, as-extruded at 623 K and annealed at

623 K for 2 h.

1084 X. Lei, T. Liu, J. Chen, B. Miao and W. Zeng

strength obtained by 6 repetitive ECAEs at 523 K),23) and375 MPa (UCS obtained by hot extrusion at 673 K),24) whilethe total strain-to-fracture 16.5% is also increased comparedto that 15%.24) As to annealing after extrusion, the compres-sive strength of the 623 K as-extruded sample after 623 K for2 h annealing increased from 396 MPa to 414 MPa, strain-to-fracture rose from 16.5% to 20.4%. Although the hardnessafter annealing cut down, the overall mechanical perform-ance was improved. In the practical application, as anecessary follow-up process after the extrusion, annealingis of great value.

3.3 Effect of compound channel extrusion on tension-compression asymmetry

Figure 6 shows the axial tension and compression stress-strain curves at room temperature of homogenized at 673 Kfor 15 h, as-extruded at 623 K and annealed 623 K for 2 h Mgalloy AZ31. According to Fig. 6(a), the tensile yield strengthat homogenized state is about 95 MPa, and this sampleruptures as strain reaches 14.9% with the tensile strengthbeing approximately 211 MPa. However, the compressiveyield strength is relatively low, about 42 MPa. The alloyyields under low stress and ruptures when strain reaches18.5% with the compressive strength being 270 MPa. Theexperiment suggests that homogenized Mg alloy has natureof tensile and compressive asymmetry with the value of�yc=�yt being 0.44 (�yc and �yt represent the compressiveyield strength and the tensile yield strength respectively). In asimilar way, we can observe that Mg alloy AZ31 exhibitedtensile and compressive asymmetry at as-extruded andannealed state, where the value of �yc=�yt is 0.76 and 0.65,respectively. But the asymmetry was improved when beingextruded through this process.

Figure 3 shows the grain size of the homogenized state is20 times larger than that at 623 K as-extruded state indicatingthe tensile and compressive asymmetry of Mg alloy AZ31 atas-extruded state improved.25) In addition, the existence oftexture has significant effect on the tensile and compressive

asymmetry. According to Fig. 6, during the extrusionprocess, fiber texture {0002} formed on the basal planealong the direction of extrusion greatly influences thetension-compression asymmetry of Mg alloy AZ31. For theMg alloy AZ31 with texture on the basal plane in as-extrudedcondition, the tensile yield strength is decided by the densityof basal texture. Higher texture density will cause moredifficulties in deformation and greater yield strength. Whenthe alloy is compressed, orientation factors causing slip isalso zero, while twinning strain at the time is the largest. So inthis case, deformation is very much likely to happen throughtwinning mode, and the tension force along the axis c is easyto form the tension twinning {10�112}(CRSS of which isextremely low). So the tensile yield strength is higher thanthe compressive yield strength. When being annealed, the Mgalloy AZ31 has relatively larger average gain size, but thetexture intensity decreases obviously. So the annealingprocess does not have tremendous influence on the tensileand compressive asymmetry. On the whole, the new com-pound channel extrusion process improves this nature of theMg alloy AZ31.

3.4 Fracture behaviorFigure 7(a)–(d) shows SEM fractography of Mg alloy

AZ31 under different conditions, including are homogenizedand as-extruded Mg alloy micro-fracture morphology ofthe tensile and compressive tests. The tensile fracture ofhomogenized sample exhibits complete dissociation charac-teristics (Fig. 7(a)). There are a large number of river patternsand cleavage terraces in SEM picture. The micro-fracture(Fig. 7(b)) of compressive fracture is similar to that of tensilefracture. We can observe cleavage planes and cleavageterraces (as pointed by the arrows in the Fig. 7(b)), but theriver pattern is not very clear. The tensile fracture of the as-extruded sample is shown in the Fig. 7(c), which has dimplesand displays the characteristics of the resembling microvoidcoalescence. So the plasticity is improved significantly.However, it still exhibits the characteristic of brittleness. So

623K 673K 723K A H0

10

20

30

40

50

60

70

80

90

Hardness

UCS

Total strain-to-fracture

Processing States

Tota

l str

ain-

to-f

ract

ure

/ % H

ardn

ess

/ Hv

0

50

100

150

200

250

300

350

400

450

Stress, σ / MP

a

Compressive yield strength

Fig. 5 The hardness, compressive yield strength, ultimate compressive

strength (UCS), and total strain-to-fracture of Mg alloy AZ31 processed at

different states. Hardness was performed in the ED–TD plane. Total strain-

to-fracture was represented by the compressive axis. (A-annealed at 623 K

for 2 h, H-homogenized at 673 K for 14 h and as-extruded at 623 K, 673 K,

723 K).

0

200

400

0

200

400

0 4 8 12 16 20

0 4 8 12 16 20

0

200

400

T

C

Homogenized

T

C

Stre

ss, σ

/ M

Pa

As-extruded

T

C

Annealed

Strain (%)

(a)

(b)

(c)

Fig. 6 Tensile (T) and compressive (C) stress-strain curves of homogen-

ized (a), as-extruded (b) and annealed (c) Mg alloy AZ31.

Microstructure and Mechanical Properties of Magnesium Alloy AZ31 Processed by Compound Channel Extrusion 1085

the tensile fracture of the as-extruded sample belongs toquasi-cleavage fracture. The compressive fracture of the as-extruded sample is of the typical transcrystalline fracture, inwhich a few cracks and elongated micro-cavities can beobserved. In short, as-extruded alloy exhibits more plasticbefore rupture compared to the homogenized state. Thuspeople also regard the compressive fracture of the extrudedalloy as quasi-cleavage fracture. From the above analysis, aconclusion can be made that the tensile and compressivefracture of the homogenized Mg alloy AZ31 belong tocleavage fractures, while that of the as-extruded alloy fallinto the quasi-cleavage fracture of the brittle-to-ductileassociative form.

4. Conclusions

We have applied a compound channel extrusion techniqueto Mg alloy AZ31 and investigated in details the micro-structure evolution and mechanical properties. Mg alloyAZ31 exihibits uniform fine microstructure and excellentmechanical properties using this new process. Simultane-ously, tension-compression asymmetry was greatly im-proved. Because of grain refinement, fracture of the as-extruded sample turned into quasi-cleavage fracture, whichcontains brittleness and ductility. In addition, this technologyis developed and designed basing on the previous ECAE andCCAE, we find that single-pass compound channel extrusioncan acquire better mechanical properties and continuousproduction compared to that obtained from the multi-passECAE and single-pass CCAE. Therefore, considering theshape of material, the production efficiency and the cost, thistechnique is worth exploring.

Acknowledgements

This work was supported in part by the National 973 MajorProject of China, ‘‘The Key Fundamental Problem ofProcessing and Preparation for High Performance Magnesi-um Alloy’’, under Grant No. 2007CB613700 and in part bythe Fundamental Research Funds for the Central Universities(CDJXS10131154) and a Distinguished PhD award fromMinistry of Education of China. One of the authors (ZengWen) thanks the Chinese Scholarship Council (CSC) project(LJC20093012) for scholarship support.

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