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  • 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.13)

    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.612) 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).1719) 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 of80 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 mm150 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

    http://dx.doi.org/10.2320/matertrans.MC201004

  • 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 EDTD 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 200240mm. 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 EDTD 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 (10111) and (10110), respectively. After as-extrusion, thestrongest diffraction peak appears at (0002) after XRDscanning from the ED-TD planes. The intensity of crystalplanes 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 its 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 hu