Fang Chai,Datong Zhang*,Yuanyuan Li

National Engineering Research Center of Near-Net-Shape Forming for Metallic Materials,School of Mechanical and Automotive Engineering,South China

University of Technology,Guangzhou 510640,China

Microstructures and tensile properties of submerged friction stir processed AZ91 magnesium alloy

Fang Chai,Datong Zhang*,Yuanyuan Li

National Engineering Research Center of Near-Net-Shape Forming for Metallic Materials,School of Mechanical and Automotive Engineering,South China

University of Technology,Guangzhou 510640,China

6 mm thick AZ91 casting alloy plates were subjected to normal friction stir processing(NFSP,in air)and submerged friction stir processing (SFSP,under water),and microstructures and tensile properties of the experimental materials were investigated.After FSP,the coarse microstructures in the as-cast condition are replaced by fne and equiaxed grains and the network-like eutectic β-Mg17Al12phases disappear and are changed into particles pinned at the grain boundaries.SFSP results in further grain refnement in comparison with NFSP,and the average grain sizes of the NFSP and SFSP alloys are 8.4±1.3 and 2.8±0.8μm,respectively.XRD results reveal that the intensity of β-Mg17Al12diffraction peaks in the SFSP specimen decreases compared with NFSP.Due to signifcant grain refnement,the tensile strength and elongation of the SFSP AZ91 alloy are increased from 262 MPa and 18.9%for the NFSP material to 282 MPa and 25.4%,and the tensile strength(282 MPa)is nearly three times that of the BM(105 MPa).SFSP is an effective approach to refne the grain size and enhance the tensile properties of AZ91 casting alloy.

Submerged friction stir processing;AZ91 magnesium alloy;Microstructure;Tensile properties

1.Introduction

Of all metallic materials,magnesium alloys have the lowest density and because of their excellent specifc properties,they have been widely employed in the automobile and aerospace industries to replace heavier materials[1–4].However,owing to the HCP crystal structure with limited number of independent slip system,both the formability and industrial application of magnesium alloys have been restricted to some extent.Fortunately,this can be solved by grain size refnement,which can improve the strength and ductility of magnesium alloys simultaneously.Some researches revealed that severe plastic deformation(SPD)such as equal-channel angular pressing(ECAP) [5,6]or accumulative roll bonding(ARB)[7],which can induce large-scale deformation during the process of magnesium alloys,has great grain refning capacity[8].Using this approach,materials with high strength and high ductility can be prepared,especially with excellent superplasticity.As a novel SPD technique,friction stir processing(FSP)gains high attention among researchers due to its energy effciency,environment friendless and versatility[9].Recently researches on FSP of magnesium alloys mainly focus on preparation of fnegrained materials,grain refnement mechanism,and properties of fne-grained materials[10–13];according to these different parameters of FSP,the average grain size of magnesium alloys varies in a range of 4–15μm.However,thermal accumulation applied on the samples may result in the growth of the grains and reduction of mechanical properties.Apparently,it is possible to prepare much fner-grained materials with improved strength through combining FSP technique with rapid cooling.

Aimed at preparing a much fner-grained material,submerged friction stir processing(SFSP)comes up as a new variation to FSP,which means that the entire processing plate is carried out underwater.For the convenience of statement,FSP performed in air is defned as normal FSP(NFSP).Tokisue et al.were the frst to use SFSP,and their result showed that it is possible to joinAl-6061 submerged[14].At present there are only a few reports about SFSP,and all those researches showed that SFSP has great potential in the preparation of fner-grained materials[15,16].However,most of the researches on SFSP are confned to aluminum alloys,and SFSP of magnesium alloys has been rarely reported until now.

Fig.1.Macroscopic features of the FSP AZ91 alloy:top surface of(a)NFSP and(b)SFSP;cross-section of(c)NFSP and(d)SFSP.

In this study,6 mm thick cast AZ91 magnesium alloy plates were subjected to NFSP and SFSP,and the microstructure and tensile properties of the experimental materials were examined and compared.

2.Experimental methods

AZ91 magnesium alloy plates with a thickness of 6 mm were machined from the cast billets,and the main chemical composition of the plate is 9.08Al-0.60Zn-0.27Mn-0.014Si-0.002Fe-0.012Ce(wt.%).The plates were subjected to NFSP and SFSP at a rotation speed of 600 rpm and a traverse speed of 60 mm/min, respectively.The FSP experiments were carried out on FSW-3LM-003 welding machine with a 5.8 mm diameter,5 mm length cone-thread pin and a concave shoulder 16 mm in diameter.A tilting angle of 2.5°was used and the plunge depth of the shoulder was controlled at～0.2 mm.The SFSP experiment was conducted in a tank with the plates underwater,and the fow rate of water was 29 ml/s.

The specimens used for microstructural examinations were cross sectioned perpendicular to the FSP direction.After being mechanically grinded and polished,the specimens were etched in a solution of 5 g picric acid,10 ml acetic acid,10 ml distilled water and 80 ml ethanol.The microstructures were observed by Keyence VHX-600 light microscopy,Leica DMI-500M horizontal metallurgical optical microscope(OM). Thin foils with a thickness of about 1 mm were prepared for transmission electron microscopy(TEM)specimens.After being mechanically ground to approximately 40μm,the foils were further ground to a thickness of 25μm by a Gatan-656 dimple grinder.Final thinning of the foils was performed by ion milling operated at 5 kV.TEM observation was carried out on a JEM-2200FS TEM operated at 200 kV.Tensile specimens with a gauge length of 5 mm,a width of 3.5 mm and a thickness of 1.5 mm were machined by electro-discharged machining parallel to the processing direction,with the gauge part consisting of the stir zone(SZ)only.Tensile tests were performed using a ShimadzuAG-X100kN computer-controlled universal testing machine at a strain rate of 1×10?3s?1.The tensile specimens at each processing condition were repeated fve times.The fracture surfaces of failed tensile specimens were examined by a Nova Nano 430 scanning electron microscopy (SEM).

3.Results and discussion

3.1.Macrostructure and microstructure of the experimental material

Fig.1 shows the top surface and cross-section appearance of NFSP and SFSP AZ91 magnesium alloys.From the fgure, defect such as voids and cracks are not observed on the top surface and cross-section both for the NFSP and SFSP AZ91 specimen.However,comparing Fig.1a with b,the NFSP specimen creates excessive fash(shown by arrows),especially on the retreating side(RS),while the surface of the SFSP AZ91 specimen exhibits very smooth quality and particular rings without prominences or depressions.Furthermore,due to the high tool rotation speed,both the NFSP and SFSP AZ91 specimens show elliptical stir shape and onion-ring patterns can be seen clearly in the SZ(Fig.1c and d).Based on the microstructural characterization,four zones,i.e.base material(BM),heat affected zone(HAZ),thermo-mechanically affected zone (TMAZ)and stir zone(SZ)are identifed.For the two FSP specimens,the boundaries around SZ are much more distinct in the advancing side(AS)than RS.

Fig.2 shows the optical images of as-cast,NFSP and SFSP AZ91 magnesium alloy.The alloy in the as-cast condition exhibits coarse and continuous eutectic network β-Mg17Al12phases in α-Mg dendrites(Fig.2a).The average grain size of magnesium grains is about 72±3μm.Due to dynamic recrystallization during FSP,the microstructures are greatly refned. The average grain size of the NFSP AZ91 alloy is about 8.4±1.3μm(Fig.2b).Compared with NFSP,SFSP results in further grain refnement,and the microstructures are more uniform.SFSP is similar to conducting FSP with immediate quench,in which the duration time at high temperature is very short for the newly-formed grains to grow.Through the lineintercept method,the average size of the SFSP AZ91 specimen is about 2.8±0.8μm(Fig.2c).Darras et al.also reported that more grain refnement was achieved under submerging condition,and they attributed it to two main factors:(1)submerging in water reduced the maximum temperature;(2)the time spent by the processed material above certain temperature was reduced[17].Moreover,the network β-Mg17Al12phases in the BM disappear after FSP.

Fig.2.Microstructures of AZ91 magnesium alloys:(a)BM;(b)NFSP;(c)SFSP.

Fig.3.Representative microstructures of the FSP magnesium alloys:SFSP of(a)SZ/TMAZ on theAS,(b)TMAZ and(c)HAZ;NFSP of(d)SZ/TMAZ on theAS, (e)TMAZ and(f)HAZ.

Fig.4.Backscattered electron images of the AZ91 magnesium alloy showing particles:as-cast condition;(b)NFSP;(c)SFSP.

Fig.3 shows the representative grained structures of the NFSP and SFSP specimens.From Fig.3a and d,it can be observed that the SZ and TMAZ have a sharp interface on the AS.The grains of theTMAZ are highly extruded and elongated for both NFSP and SFSP(Fig.3b and e).Compared with NFSP, SFSP results in much fner microstructures in the TMAZ.For HAZ,due to only experiencing processing thermal cycles without plastic deformation during FSP,both the NFSP and SFSP specimens exhibit similar grained microstructures with BM(Fig.3c and f).However,much fner grains are achieved under submerging in water due to the reducing heat accumulation of the cooling water.In addition,deformation twins can be observed in the HAZ(Fig.3c and f).

3.2.Evolution of the second phases of the experimental material during FSP

Fig.4 shows the backscattered electron images of as-cast, NFSP and SFSP AZ91 magnesium alloys.As for the as-cast condition,most of the β-Mg17Al12phases exist as network structures distributed at the grain boundary,while some β-Mg17Al12particles are distributed inside α-Mg grains (Fig.4a).Fig.4b and c shows the backscattered electron images of the NFSP and SFSP AZ91 magnesium alloys,respectively. The network β-Mg17Al12phases are changed into particles after FSP.It is considered that the morphology change of the β-Mg17Al12phases is mainly caused by dissolution,breaking-up and re-precipitation during FSP.Compared with the density of the β-Mg17Al12particles,NFSP results in much more volume fraction of β-Mg17Al12particles than SFSP.This is mainly due to the high cooling rate during SFSP which can hinder the re-precipitation.

Fig.5.TEM images of the SFSP AZ91 magnesium alloy showing particles on the grain boundaries.

Fig.6.XRD patterns of AZ91D magnesium alloy:(a)as-cast;(b)NFSP;(c) SFSP.

Fig.5 shows the TEM image of the SFSP AZ91 magnesium alloy.Fine equiaxed grains are observed in the fgure due to dynamic recrystallization.It is widely accepted that low dislocation density can be observed in the recrystallized grains [18,19].However,in the present investigation,the interior of the recrystallization grains contains few dislocations.From Fig.5,it also can be seen that the continuous networks have disappeared and changed into particles.The particles pinning on the grain boundaries can be seen clearly which are shown by arrows.The particles have an ellipsoidal shape,with size ranging from 180 to 550 nm.Zhang et al.reported that the different sizes of the second phase played a different role in dynamic recrystallization[20].At the beginning of FSP,due to the large second phase,the high strain energy in the matrix provides preferential nucleated sites of dynamic recrystallization.When the second phases change into small particles,the particles can retard grain growth.Therefore,relatively fner grains are prepared in FSP in comparison with BM.

Fig.6showstheXRDpatternsoftheAZ91magnesiumalloys, whichindicatethepresenceofα-Mgandβ-Mg17Al12phasesinthe BM,NFSP and SFSP specimens.After FSP,both the number and the intensity of β-Mg17Al12diffraction peaks decrease. Furthermore,the intensity of β-Mg17Al12diffraction peaks in SFSP AZ91 magnesium alloy is lower compared with the NFSP AZ91 alloy,suggesting the re-precipitation of β-Mg17Al12phases is retarded.This coincides with the results of Fig.4.

3.3.Tensile properties of the experimental materials

Fig.7a shows the strain–stress curves of the as-cast,NFSP and SFSP AZ91 magnesium alloys.Fig.7b summarizes the room-temperature tensile properties of cast and FSP AZ91 magnesium alloys.Due to the presence of the coarse eutectic β-Mg17Al12network at the grain boundaries,the as-cast AZ91 magnesium alloy exhibits lower yield and ultimate strengths(55 and 105 MPa)and elongation(15.2%).FSP results in a signifcant improvement in tensile strength.This is mainly attributed to the remarkable grain refnement and signifcant dissolution and breakup of theβ-Mg17Al12phases.First, the FSP results in much fner grain size than that of BM,and grain refnement plays an important role in material strengthening.Second,according to the Orowan hardening mechanism, the fne particles can reduce the possibility of crack under lower stress,thereby improving the strength of the FSP specimens. Moreover,compared with NFSP,the tensile strengths of the SFSP specimens are improved,with the yield strength and ultimate tensile strength from 132 and 262 MPa in the NFSP condition increasing to 151 and 282 MPa.It is worth noting that the ultimate tensile strength of the SFSP specimens is signifcantly improved,which is nearly three times that of BM.The tensile test results also indicate that FSP can enhance the ductility of AZ91 magnesium alloy.In NFSP the elongation is increased up to 18.9%,which is around 1.2 times the BM elongation.In addition,because much fner grains after SFSP can make the deformation more uniform,the elongation of the SFSP specimen(25.4%)is signifcantly improved.

Fig.8a shows the fracture surface of the as-cast AZ91 magnesium alloy.Some small cleavage planes(shown by arrows) and tearing edges can be observed,indicating a typical characteristic of quasi-cleavage fracture.Fig.8b and d presents the fracture surfaces of NFSP and SFSP specimens at the edge, respectively.It can be found the surface of the SFSP specimen is much more fat.This suggests that the SFSP specimen possesses better ductility than the NFSP specimen.Both the fracture surfaces of the NFSP and SFSP specimens at the center exhibit a ductile small-sized dimples with fne particles(shown by arrows),indicating plastic deformation occurs during tensile test(Fig.8c and e).

Fig.7.Tensile properties of the as-cast and FSP AZ91 magnesium alloys:(a)the strain-stress curves;(b)the histogram images

Fig.8.Fracture surfaces of the AZ91 magnesium alloy:(a)BM;NFSP specimen at the(b)edge and(c)center;SFSP specimen at the(d)edge and(e)center.

4.Conclusion

This work presents experimental investigation of NFSP and SFSP forAZ91 magnesium alloy,respectively.Microstructures and mechanical properties of the two FSP specimens are investigated in detail.The results are concluded as follows:

1 Compared with NFSP,SFSP has remarkable grain refnement effect.The average grain size of the NFSP and SFSP specimen is 8.4±1.3 and 2.8±0.8μm,respectively.Furthermore,the microstructures in theTMAZ and HAZ for the SFSP are much fner than those for the NFSP.

2 After FSP,the coarse network β-Mg17Al12phases in the as-cast condition are changed into particles pinned on the grain boundaries.The volume fraction of the β-Mg17Al12particles in the SFSP specimen is much less than the NFSP specimen,suggesting the re-precipitation of β-Mg17Al12phases is retarded.

3 The yield strength,tensile strength and elongation of SFSP AZ91 magnesium alloy with much fner-grained structure are much higher than those of as-cast and NFSP materials.

Acknowledgement

This work was sponsored by the Fundamental Research Funds for the Central Universities(No.2014ZG0028)and Research Fund for the Doctoral Program of Higher Education of China(No.20130172110044).

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Received 17 September 2014;revised 11 August 2015;accepted 25 August 2015 Available online 28 September 2015

*Corresponding author.Tel:+86 20 87112272;Fax:+86 20 87112111. National Engineering Research Center of Near-Net-Shape Forming for Metallic Materials,School of Mechanical and Automotive Engineering,South China University of Technology,Guangzhou 510640,China.

E-mail address:dtzhang@scut.edu.cn(Z.Datong).

http://dx.doi.org/10.1016/j.jma.2015.08.001

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?2015 Production and hosting by Elsevier B.V.on behalf of Chongqing University.