Mechanical behavior and microstructure evolution for extruded AZ31 sheet under side direction strain

Qingshn Yng,Bin Jing,Bo Song,Dling Yu,Sensen Chi,Jinyue Zhng,Fusheng Pn

a School of Metallurgy and Material Engineering,Chongqing University of Science and Technology,Chongqing,401331,China

b National Engineering Research Center for Magnesium Alloys,Chongqing University,Chongqing,400044,China

c School of Materials and Energy,Southwest University,Chongqing,400715,China

d School of Engineering Technology,Purdue University,West Lafayette,IN,47907-2021,United States

ABSTRACT AZ31 magnesium alloy sheets were prepared by a conventional extrusion(CE) and a novel integrated extrusion with side direction strain (SE).The microstructure characterizations,crystallographic texture and mechanical property tests were carried out and compared between the extruded Mg alloy sheets processed by CE and SE.The results indicated that the SE sheets exhibited an excellent combination of strength and ductility.To reveal the side strain effect,the finite element model was employed to investigate the effective stress and strain behavior of the AZ31 magnesium alloy sheets during CE and SE processes.It was found that the SE process was effective in weakening the stress and strain concentration.This implied that it developed an additional side direction strain through the sheet thickness during the hot extrusion.Meanwhile,the side strain shear paths could promote the local accumulation of dynamically recrystallized grains and increase the random high-angle boundaries to achieve weak (0002) basal texture.Important factors including the side strain path and extrusion parameters need to be taken into account to understand the deformation mechanism and microstructure evolution.

Keywords:AZ31 Mg alloy Microstructure Texture Extrusion Side direction strain

1.Introduction

Developing lightweight,low-cost,and high-performance structural metals is considered as one of the most effective ways to improve the energy efficiency in transportation [1-3].As the lightest structural metal,Mg and its alloy have obvious advantages over other metals due to their low density (about one-fourth of steel or two-thirds of aluminum) and high specific strength properties which significantly promoted their utilization in aircraft and automobiles industries [4-6].However,the low ductility of wrought sheets resulting from the limited numbers of slip systems intrinsic to hexagonal close packed (hcp)structure of Mg alloys at room temperature restricted their extensive use in many fields [7,8].For most Mg alloy components,particularly the semi-manufactured components (i.e.,plate,strip and bar) are required to extend beyond the fabrication of structural products.In addition,Mg and its alloys easily give rise to basal texture when deforming with traditional manufacturing methods,such as extrusion and rolling [9,10],resulting in the anisotropic plasticity and poor formability.Thus,enhancing the strength and room temperature formability is crucial for Mg alloy sheet comprehensive applications.

In the past,various thermo-mechanical methods have been developed in order to improve the room temperature ductility of Mg alloy sheets by stimulating the activity of non-basal slips,such as high-speed rolling (HSR) [11],asymmetric rolling (ASR) [12]and equal-channel angular pressing (ECAP) [13],etc.Because of shear strain introduction during deformation,these methods have been proven to be effective in tailoring the texture and refining the grains.For example,Huang et al.employed differential speed rolling (DSR) process in Mg-4Y-3RE magnesium alloy sheets at 823 K to obtain random basal texture [14].Chen et al.reported that the ductility of pre-forged Mg alloys was enhanced by a slow speed extrusion [15].The principle behind these techniques is introducing sufficiently shear strain along the sheet direction by different deformation speed between the top and bottom surfaces of the sheets,which successively promotes the prismatic slips and/or twinning activities and subsequently,weakens the basal texture[16,17].However,the above-mentioned techniques are either not costeffective or requiring complex deformation facilities,and thus,not suitable for extensive promotions are a bit complex and required to multiple processing.

Recently,a newly integrated model is developed based on hot extrusion that combines a die bearing equipped with side direction strain path for extruded Mg alloy sheets.During side strain extrusion (SE)process,the side shear strain was introduced along transverse direction(TD) and Mg alloy sheets were deformed with a single step extrusion simultaneously.This massive confined strain deformation was expected to activate dynamically recrystallization and randomize the crystallographic texture.In this work,controlling strain deformation was utilized to investigate the microstructure evolution and mechanical behavior of the extruded Mg alloy sheets.

2.Experimental procedure

The material used in the present work was commercial cast ingots of AZ31(Mg-3 Al-1 Zn in wt%)alloy with a diameter of 82 mm.Mg alloy cast ingots were homogenized at 693 K for 1 h in an electronic furnace.Subsequently,the as-received AZ31 ingots were processed by common extrusion (CE) and integrated extrusion equipped with side direction strain(SE)path processes at 703 K to the sheet of 56 mm in width.The Mg alloy was extruded to 1 mm in thickness with extrusion speed of 20 mm/s and the extrusion ratio of 94:1.Fig.1 shows the cross sections of deformation processing dies of CE and SE.It can be seen that the structure of the CE die is symmetric.The schematic map for SE process is shown in Fig.1(b).It can be observed that the side direction strain was concentrated on both sides of the extrusion die bearing along extrusion direction during SE process.The side direction strain deformation zone is also displayed in the vertical view.The aim of independent control of the strain path is to increase the possibilities to acquire random grain orientations owing to the additional side strain at the outlet of SE die under hot extrusion process.

To study the mechanical properties (strength and ductility) of the processing,the tensile samples with a gauge length of 12 mm and a width of 6 mm were machined from extruded AZ31 magnesium alloy sheets.The tensile tests were measured by universal testing machine at room temperature with a strain rate of 10−3s−1.The uniaxial tension tests were carried out at different angles,i.e.,0°,45° and 90° to the extrusion direction.Here,ED,TD and ND represent extrusion direction,transverse direction and normal direction,respectively.For all mechanical results discussed in this work,at least three stress-strain curves were chosen for the evaluations.The n-value was calculated from the uniform plastic deformation region of the stress-strain curves.Standard Erichsen (IE) tests were performed by a hemispherical punch to examine the room temperature stretch formability of extruded Mg alloy sheets.The crystal orientation and texture were measured using the electron backscattered diffraction(EBSD)with a JEOL JSM 7800F-SEM equipped with HKL Chanel 5 system.

3.Results

3.1.Finite element analysis

The effective stress-strain behavior of the AZ31 magnesium alloy sheet was investigated using a finite element model (FEM) during CE and SE.DEFORM-3D software was employed to examine different extrusion processes which were encoded on the basis of the rigid-visco plastic finite element method [18].The flow stress data of AZ31 Mg alloy were imported into finite element simulation.The effective stress and strain behavior of AZ31 Mg alloy processed by FEM during CE and SE deformation is illustrated in Fig.2.It can be seen that there was a significant fluctuation of the effective strain along the extrusion direction during CE process,and the effective strain from CE was noticeably higher than that from SE.After extrusion,the extruded sheets were both 56 mm in width.During the SE process,as shown in plan view of Fig.1(b),Mg alloys were initially extruded to 58 mm in width and subsequently deformed to 56 mm in width from the side strain path.The side direction strain was 3.4% (position P1 to P2).When the Mg alloy reached the die bearing zone(side direction strain zone,P2 to P3),the effective strain gradient was drastically enhanced,which means that the effective strain was accumulated at the two sides of the die bearing.Therefore,it may be believed that SE process is effective to reduce the stress and strain concentration during hot extrusion process owing to the additional side direction strain gradient developed through the sheet thickness along TD.

3.2.Microstructure evolutions during CE and SE processes

Fig.3 shows the optical micrographs of CE and SE sheets.The CE sheet contains a mixed microstructure with plenty of dynamically recrystallized(DRXed)grains(about 8 μm)embedded in the coarse grains(～25 μm).In contrast,the SE sheet comprises a multi-recrystallized microstructure with a large number of finer and uniform DRXed grains(about 5 μm in diameter) embedded in the relatively large deformed grains with a diameter of approximately 20 μm.The finer grain size of the alloy may be due to the grain subdivision and the side strain gradient introduced by the SE path.The inverse pole figure(IPF)map and the orientation of c-axis correspond with the grains of extruded sheets are presented in Fig.4.It is interesting to note that compared with the grains which had strong basal texture along the ND from CE process,the grains from SE process had very diverse orientations,especially the DRXed grains(blue or green).For instance,the c-axis of grain A,B,C,D and E inclined about 49°,89°,86°,39° and 78° away from the normal direction of the(0002)basal plane(Fig.4(b)),respectively.It indicates that the SE process can effectively introduce new texture components,which may be attributed to the DRXed grains under side direction strain plastic deformation during hot extrusion.Here,the red grain F displays the normal direction of (0002) basal plane,which illustrates the CE sheets to have a strong (0002) basal texture,and the c-axis of most grains are roughly perpendicular to the sheet plane (TD-ED).This would dramatically restrict the basal slip in the subsequent sheet forming,such as rolling and stamping.

Fig.5 shows the texture of the extruded sheets processed by CE and SE from EBSD maps.The SE sheets had different texture features in term of the texture intensity,tilted direction and inclined angle of caxis.It can be found that the texture of the extruded sheet had strong(0002) basal texture after the CE process.There was no distinction to the texture feature of AZ31 Mg alloy sheets under common deformation process such as rolling at room temperature.Nevertheless,the basal texture of AZ31 sheets processed by SE showed double intensity peaks with relatively weak intensity towards TD.The maximum intensity of the basal poles decreased from 25.2 to 14.7.The decrease in maximum peak intensity can be attributed to the reduction of the fraction of the unDRXed grains that generally show a strong basal texture,while the DRXed grains display weak(0002)basal texture.During the SE process,the side strain deformation can be regarded as a compression load perpendicular to the c-axis of basal plane.It indicates that the variation in extrusion paths could significantly activate the normal direction of(0002)basal plane of Mg alloy sheets to trend to the imposed side strain direction in a single consecutive extrusion deformation pass under SE process [19,20].During the shear strain plastic deformation,most grains of Mg alloy sheets usually prefer the prismaticslips to basalslips [21,22].It is worthwhile to notice that the prismaticslip was the primary mode of the strain accommodation processed by SE process.Meanwhile,the prismaticslip of extruded Mg alloy sheets can stimulate c-axis of (0002) basal plane to deviate from the normal direction to side direction owing to the introduced SE strain.

To further study the deformation mechanisms,the distributions of the misorientation angle of grain boundaries processed by CE and SE was analyzed,and the results are exhibited in Fig.6.In general,grain boundaries were classified by different misorientation angles of grain boundaries:low-angle grain boundaries (LAGBs<15°) and high-angle grain boundaries (HAGBs>15°) [23].It can be seen that most grain boundaries are characterized by the misorientation angles<40°,and there were no grains with misorientation angles>60° for CE sheets.Moreover,it can be observed that the fraction of LAGBs concentrates on misorientation angles with the deviation from 25 to 40°.Nevertheless,the grain boundaries of SE sheets have a relatively random distribution.Most grains have the misorientation angles of about 30° and 90°.In addition,a high volume fraction of boundaries with>60° appeared compared with the boundaries in SE sheets,which indicates that DRX is easily activated due to their random large-angle boundaries.During the hot extrusion,DRX generally takes place in the Mg alloys associated with low stacking fault energy.Thus,the fine HAGB-grains can be acquired through the hot extrusion processes.

3.3.Mechanical properties of extruded Mg alloy sheets

The stress-strain curves of the CE and SE sheets tensioned along 0°(ED),45° and 90° (TD) at room temperature are displayed in Fig.7.It can be seen that the mechanical properties of these extruded sheets have the remarkable distinctions owing to differences of the initial textures.The 45° tensioned samples have the highest ductility on account of the low critical resolved shear stress (CRSS) for the basal slip.The basal slip is the most easily activated deformation mode during plastic deformation at room temperature in Mg alloy sheets.The reason is that when the angles between the stress direction and both slip direction and slip plane are both 45°,the value of schmid factor (SF) can reach the maximum [24,25].

The room temperature mechanical properties of the CE and SE sheets,such as ultimate tensile strength (UTS),the 0.2% proof stress(yield strength,YS)and the uniform elongation(Eu),the Erichsen value(IE)and the strain hardening exponent value(n-value)are summarized in Table 1.The average values of mechanical properties are signified as follows [26]:

where X is UTS,YS,Euand n-value.It can be noted that the SE sheets have a higher ductility and n-value in each direction compared with those of the CE samples.The YS value decreased and the Euvalue increased for the SE sheets along the loading direction compared with those of the CE sheets.The reason is that the strong(0002)basal texture leads to a poor plastic deformation ability and strong anisotropy for CE sheets.This indicates that the strong basal texture is unfavorable for the basal slip under plastic deformation at room temperature [27,28].However,the SE sheet has a relative weak basal texture with c-axis aligned to TD.It has been observed that the value of IE sharply increased from 2.6 to 4.6 with a simultaneous increase of the n-value from 0.32 to 0.38.Therefore,the ductility and formability were enhanced by the weak basal texture processed through the side direction strain paths.During the SE process,an extra side strain deformation was induced along TD,which could activate the non-basal slip systems and affect the dislocation storage and dynamic recovery.Subsequently,the strain hardening ability was influenced by an enhancement of prismatic slip increased with high dynamic recovery behavior.And also,the dynamic recovery can activate (0002) basal plane to deviate from the sheet plane (ED-TD) [29].

It can be concluded that the mechanical properties are mainly dependent on the texture of the extruded sheets.The double intensity peak and weak basal texture can be produced by side direction strain during the SE process.And the prismaticslip could accommodate the introduced side strain deformation along TD.The enhanced extrusion formability from the SE process is as result of the transformation of strain orientation relationship between the side direction and sheet thickness direction.

4.Discussion

Fig.8 shows the FEM analysis of the AZ31 Mg alloy during the SE process and the schematic diagram of the evolution of grain orientations under side direction strain.It can be noticed that the effective strain at different positions (center P1,near side strain zone P2,side strain zone P3) was successively increased during the SE process,as shown in Fig.8(a).Meanwhile,the effective strain was enhanced from the extrusion die entrance to the exit of sheet forming zone.It can be also observed that the velocity of the Mg alloy flow in the extrusion die is obviously along TD.This indicates that the side direction strain zone can generate the greatest deviation,while the center of the Mg alloy sheets generates the least average.The velocity evolutions near the side strain zone were obtained from the ED-TD plane during the SE process,as shown in Fig.8(a).The direction of the flow velocity was always parallel to ED during the hot extrusion process.The direction of flow velocity was tilted away from ED to side strain direction.Thus,the additional flow velocity along the side strain direction can be generated as result of the introduced side strain paths in the CE process,which is beneficial to the formation of the grains with the c-axis tilted away from ND to TD.Therefore,the formation of the new texture components of the Mg alloy sheets with tilted weak basal texture was promoted in SE path.

The sheet formation can be divided into A and B zones,as shown in Fig.8(b).Initially,the grain orientation was random in the Mg alloy ingots(AI).When the Mg alloys were extruded to sheets(AII),the c-axis of most DRXed grains changed to be parallel to ND.Subsequently,when the Mg alloy sheets were squeezed into the side direction strain zone(BI),the grain orientations due to the dynamic recrystallization and nucleation of new grains in SE can differ from those of the parent grains.A majority of dynamic recrystallization primarily is derived from the side strain deformation and the dislocation [6],which would create a mass of shear strain gradient deformation throughout the sheet plane along the side strain flowing path during SE process.On the other hand,the side strain shear paths can facilitate localized DRXed grains(BII) and migrating grain boundaries to be accumulated along TD[30].The side direction strain shear deformation is also principally concentrated in these zones as result of the local(0002)basal slip activities that resulted in the repetitive DRX [31].Generally,the non-basal slips(prismaticand pyramidaslip)of the Mg alloy sheets were thermally activated beyond the basal slip where Mg alloy deformed at above the temperature of 573 K.During CE process,the presence of strong basal texture can lead to a high fraction of LAGBs.However,under side strain path in SE process,in addition toslips,the prismaticslip can accommodate the shear strain deformation in the side strain zone[32,33].Therefore,the extrusion formability of the SE sheets is enhanced by the side strain accommodation along TD to increase the activity of slip systems during the hot extrusion.Further investigations of the strain analysis and microstructure evolution are to be performed to ascertain the plastic deformation mechanisms and grain orientation evolutions of Mg alloy sheets during SE and CE processes.

5.Conclusions

A novel integrated extrusion method that can introduce a weak texture for the extruded AZ31 Mg alloy sheets is developed.Themicrostructure evolution,crystallographic texture and mechanical properties have been investigated from conventional extrusion(CE)and side direction strain extrusion (SE).Through the introduction of side direction strain along TD in a single path under SE process,the room temperature ductility and formability are remarkably enhanced.The additional side strain deformation can effectively stimulate a compression load perpendicular to the c-axis,and this load can alter the(0002) basal plane to the introduced strain direction and consequently weaken the basal texture.

Table 1 Mechanical responses of the tensile tests carried out along different directions,0°,45°and 90° to ED.

Authors' statement

No conflict of interest exits in the submission of this manuscript,and manuscript is approved by all authors for publication.I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously,and not under consideration for publication elsewhere,in whole or in part.

Acknowledgements

The authors are grateful for the financial supports from National Natural Science Foundation of China(51701033,5170135),Chongqing Science and Technology Commission (cstc2018jcyjAX0022,cstc2017jcyjAX0216),Chongqing Municipal Education Commission(KJQN201901504,KJQN201801523).