Yn-peng Wng,Feng Li,b,?,Ye Wng,Xue-wen Li,Wen-win Fng

a School of Materials Science and Engineering,Harbin University of Science and Technology,No.4,Lin Yuan Street,Xiang Fang,Harbin,Heilongjiang 150040,China

b Key Laboratory of Advanced Manufacturing and Intelligent Technology,Ministry of Education,Harbin University of Science and Technology,Harbin 150080,China

Received 2 August 2019;received in revised form 27 April 2020;accepted 4 May 2020 Available online 15 August 2020

Abstract There are many problems with the conventional processes of magnesium alloy bending products,such as long processes and difficulty in controlling the product shape.This paper provides a staggered extrusion(SE)process to solve the above manufacturing bottlenecks.The effects of different extrusion ratios(λ)on the AZ31 magnesium alloy bending products prepared by the SE process was investigated in this paper.The results show that the bending radii of the AZ31 Mg bending products increase with the increase ofλat the same staggered distance(h=16mm).Whenλis increased from 11.11 to 44.44,the average bending radius of bending products is decreased from 14.7mm to 9mm,and the average grain size is decreased by 59.43%.After the SE process,the extruded fiber texture of the AZ31 Mg bending products is obvious,and the deformed texture is a mixed texture of{0001}10–10deformation texture and{10–11}11–20recrystallization texture.The results of XRD and EBSD showed that pyramidal slip is an important mode of crystal slip systems in AZ31 magnesium alloys during the SE process.It provided a scientific basis for forming AZ31 Mg alloy bending products with excellent microstructure by the SE process.

Keywords:Magnesium alloy;Staggered extrusion(SE);Bending behavior;Texture;Slip.

1.Introduction

Due to its inherent hexagonal close packed(HCP)crystal structure,magnesium alloys show different plastic deformation characteristics from cubic system metals.such as some dislocation slip systems are very sensitive to temperature[1],and the texture has a significant effect on the macro-mechanical properties,which leads to the anisotropy of macro-mechanical properties[2].The deformation of magnesium alloys at room temperature is usually based on the slip and the appearance of tensile twins,which follow the Mises criterion[3].In the early stages of deformation,twins,especially tensile twins,are prone to appear,which leads to the asymmetry in macro-mechanical properties such as tension and compression[4].

The most easily activated one is the{0001}basal plane in all slip systems of magnesium alloys,which is the densest atomic arrangement plane.The densest atomic arrangement orientation,11–20,is the easiest direction to slip,which mainly includes three basal planes{0001},three prismatic planes{10–10}and three pyramidal planes{10–11}.When the crystal deformation activates more than five independent slip systems,the plastic deformation of AZ31 magnesium alloy under any macroscopic conditions will become easier[5-8].The crystal plasticity theory of metallic materials is based on the micro-structure and micro-deformation,combined with the slip mechanism and plastic deformation characteristics,to explain the elastic-plastic deformation behavior in physical essence.It also provides a basic theory for researching the deformation behavior of metallic materials in micro-scale[9].Wang et al.[10]carried out tensile and compression experiments along the transverse direction(TD),compression experiments along the normal direction(ND)and compression experiments at the 45°with the ND.Cheng et al.[11]used the crystal plastic finite element method to simulate the nucleation of twins during the plastic deformation of AZ31 Mg alloys.They accurately obtained a model of intergranular stress and deformation,revealing the effect of grain orientations and grain boundaries on the formation of twins.

The extrusion process is very important for the mechanical properties of AZ31 magnesium alloy products[12,13].After the extrusion process,magnesium alloy products are easy to produce fibrous structure with asymmetric tensile and compression performance,which limits the application of the extruded products.Therefore,it is very important to study the effect of extrusion texture on the properties of magnesium alloy products[14,15].The initial texture of different magnesium alloys has a great influence on the deformation behavior.When the magnesium alloy is processed by hot extrusion,a fiber texture10–10or11–20parallel to the extrusion direction(ED)was formed.The accumulative back extrusion(ABE)[16,17]process leads to significant grain refinement with an average grain size of～2μm in the shear zone,which has a positive effect on improving the properties of extruded products.Cyclic extrusion and compression(CEC)[18]can significantly refine microstructure of ZK60 Mg alloy,and the initial fiber texture in products becomes random during the CEC and develops a new texture.Differential velocity sideways extrusion(DVSE)[19]has been developed on the basis of the upper bound method,which can predict the shape of rectangular bars with variable curvatures along their lengths.

In this paper,the texture of AZ31 magnesium alloy bending products obtained by the SE process is investigated,which is of great significance for the follow-up study of the SE process.

2.Experimental procedure

2.1.The principle of SE process

The SE process is a new process for extruding bending products.It is mainly used for solving the bottlenecks of traditional bending processes,such as difficult in manufacturing process,poor flexibility,difficult in controlling quality and high cost.The curvature of extruded products can be quantitatively controlled by adjusting the staggered distance at the end of the stem.A short integrated process consisting of extrusion and bending is achieved by the SE process,which has good process flexibility and forming quality.The micro-structure and properties of bending products are also affected by the SE process.The principle is shown in Fig.1.The staggered distance of the stem ish.

Fig.1.Schematic diagrams of the SE process:(a),(b)principle of SE process;(c),(d),(e)bending products with differentλ.

Due to the particularity of the stem structure,the deformation flow behavior of billets during the SE process is quite different from that of traditional extrusion processes.The flow velocity difference occurs at the die exit,which causes the extruded products to exhibit bending characteristics.By adjusting the h,bending products with different curvatures can be obtained[30].By adjusting D of the die exit,theλof the bending products in the SE process can be calculated,and the bending products with different deformation degree can be obtained.By combining the research of related fields[31,32]and the previous experimental results[30,33],it is determined that the value ofλis between 5 and 50.λis determined by the inner diameter(D)of the die,which is one of the important factors for influencing the formation of AZ31 magnesium alloys.λhas a direct influence on the sectional velocity and bending degree of the extruded products during the SE process.It also causes a change in the correlation between plastic deformation and microstructure.Therefore,this paper focuses on investigating the effects ofλon the bending characteristics and microstructure of AZ31 magnesium alloy bending products,which purpose is to prove that the SE process can break some bottlenecks in conventional processes.

2.2.Research program

Fig.2 shows the bending products obtained by the SE process and the stems used in the SE process.For experimental flexibility,the stems were designed as a split type[25].The upper stem is a flat stem and the lower stems were designed as step-like structure,as shown in Fig.2(a)and(b).It mainly realizes the stems with differenthand the dies with differentDcan be freely combined,which reduces the experimental cost.

Commercial AZ31 magnesium alloys are used as raw materials in this paper.The chemical composition is shown in Table 1.

The initialФ45 mm magnesium alloy bars were processed intoФ40 mm×40mm cylindrical billets.The billets were annealed at 673K for 12h prior to extrusion[29].Prior to the extrusion experiment,the billet was assembled with tooling structures,and then heated them to 623K in a box-type heat preservation furnace and kept for 30 min.The experimental temperature was controlled by a thermocouple.The box-type heat preservation furnace has an error range of±2K.The extrusion speed was 1mm/s and the staggered distance of the stem was 16mm.Due to the low extrusion speed,the temperature change of the billets during the SE process was neglected.The extrusion equipment stops immediately when stems went down to 30mm.The bending products were taken out for water quenching,and the curvature and microstructure of bending products with differentλwere investigated.In this paper,three conditions ofλ=44.44,λ=19.75 andλ=11.11 were used in the SE process.The bending products are shown in Fig.2(c).

Fig.2.The pictures of stems and bending products:(a)upper stem;(b)staggered stem;(c)bending products with differentλ.

Table 1The chemical composition of AZ31 magnesium alloy(wt%).

The samples with a length of 10mm,as shown in Fig.2(c),were polished in the direction perpendicular to and parallel to the ED,respectively.First,800# sandpapers were used for smoothing the surfaces of samples,then 1200#,2000#,5000# metallographic sandpapers were used for polishing the surfaces until the scratches were not visible.Samples were polished with 0.25μm diamond polishing agent until surfaces were bright and there were no scratches.The surfaces of the samples were corroded by metallographic corrosive solution.The metallographic corrosive solution was a mixture of 1ml acetic acid,1ml nitric acid,1g oxalic acid and 150ml water.

The samples with the length of 10mm were polished mechanically to cubes with size of 5mm×5mm×1mm.Then the surfaces of the cubes parallel to the ED-TD were electrolytically polished with electrolyte solution consisting of phosphoric acid and ethyl alcohol(volume ratio 3:5)at 0.5 A current for 2min then 0.2 A current for 2min.The microstructure was observed by electron backscatter diffraction(EBSD).The experimental equipment was Quanta 200 F field emission scanning electron microscopy.The scanning step length of EBSD was 1.2μm,the step areas were 200μm×200μm and 400μm×400μm respectively,the inclination of test bench was 70°,and the working distance was 14.24mm.

The samples were polished and X-Ray diffraction(XRD)were carried out on the surfaces of the ED-TD.The scanning range was 25°?80°.

3.Results and discussions

3.1.Bending products

Fig.3 shows the bending heights and bending radii of the three bending products.Three bending products with equal distance between two end points were measured,and a straight line was obtained by attaching point 1 to point 13.The three bending products were divided into 12 equal distances through 13 points,and theY-axis values of point 1 and point 13 of each bending product were set to 0mm.By measuring the bending height of each point on three bending products respectively,Fig.3(a)can be obtained.And by calculating the bending radius of each point to get Fig.3(b).

The loading modes of the staggered stem acting on the billet during the SE process is asymmetrical,which causes different flow velocities of the billets when the it flows out the die exit.This is why the extruded products are bend.In the SE process,with increasingλ,the plastic deformation degree of the billets increases.The bending height of bending products decreases with the decrease ofλwhenh=16mm,as shown in Fig.3(a).Whenλ=44.44,λ=19.75 andλ=11.11,the maximum bending heights of the three bending products are 2.48mm,1.90mm and 1.70mm,respectively.

Whenλ=44.44λ=19.75 andλ=11.11,the bending radii of three bending products fluctuate around 9.0mm,13.8mm and 14.7mm,respectively,as shown in Fig.3(b).The bending radius of the bending product decreases with the increase ofλath=16mm.It can be seen that the bending radii of three bending products can be controlled precisely by the SE process.This is of great significance to obtain bending products with no defects in a single pass.

Fig.3.Bending heights and bending radii of bending products:(a)bending heights;(b)bending radii.

3.2.Grain morphology

Fig.4 shows the metallographic images of AZ31 magnesium alloys under threeλ.The effect ofλon dynamic recrystallization in the SE process can be investigated by observing the grain morphology of AZ31 Mg bending products.The energy for dynamic recrystallization of metals comes from the storage strain energy that is not released after recovery.AZ31 magnesium alloy has low stacking fault energy,wide dislocation distribution,and is prone to dynamic recrystallization during hot deformation.The grains after dynamic recrystallization are generally equiaxed and uniformly distributed in AZ31 Mg bending products[20,21].

As-cast AZ31 magnesium alloys are consisted of a large number of irregular grains with different sizes and some fine Mg17Al12particles in Mg-matrix,as shown in Fig.4(a).Fig.4(b)shows that the grain morphology of AZ31 magnesium alloys annealed at 673K for 12h.After annealing,the grain size of AZ31 magnesium alloy increases,and the average grain size is 65.27μm.A large number of second phase particles dissolve in the Mg-matrix and the structure becomes more homogeneous,which provides good conditions for subsequent extrusion experiments.The grain size increases with the decrease ofλas shown in Fig.4(c)–(e).The average grain size of AZ31 bending products was measured by a linear intercept method.The average grain sizes perpendicular to the ED are 9.69(±2.5)μm,14.34(±2.5)μm and 23.89(±2.5)μm whenλ=44.44,λ=19.75 andλ=11.11,respectively.Metallographic images of the bending products parallel to the ED are shown as Fig.4(f)–(h).With the decrease ofλ,the grain size increases and fine strip grains appear in bending products along the ED as shown in Fig.4(f).It can be found that the grains are elongated along the ED by external force during the SE process.In the SE process,the billets were strongly sheared by the die.At the same time,the special step structure of the stem also causes the local shear deformation of the billets.With the high forming temperature,the grains are fully dynamic recrystallized and refined.

The unique staggered stem and loading modes of the SE process provide sufficient activation energy for plastic deformation of AZ31 magnesium alloys,which is conducive to dynamic recrystallization of the bending products.The result indicates that grain refinement efficiency after the SE process is conditional on deformation degree of AZ31 magnesium alloy ath=16mm.The storage strain energy of grains increases with theλincreasing,which drives the nucleation and growth of recrystallized grains.Therefore,dynamic recrystallization behavior is more adequate and recrystallized grains are more uniformly distributed in AZ31 bending products atλ=44.44.And the integration of extrusion process and bending process is realized by the SE process which plays an important role in grain refinement of bending products.

3.3.Grain orientation and dislocations

Grain orientation maps of AZ31 magnesium alloy bending products obtained by the SE process are shown as Fig.5.Fig.5(a)–(c)shows the grain orientations of three bending products with differentλ.The grains of each color in Fig.5(a)–(c)represent different grain orientations,and red represents the grains parallel to the basal plane(0001).The step-like structure of the stem contacts with the billet first under external force,which leads to the uneven flow velocity of the billet along the die exit.It is the core principle of the SE process.In addition,the shearing effects of the die and the staggered stem on the billets both are more obvious,which results in grain deformation.Different deflection angles of grains by external force produce different grain orientations.As can be seen in Fig.5(a)–(c),with the external force acting on grains increasing gradually during the SE process,the number of grains parallel to the{0001}basal plane increases.This is because the largerλ,the greater shear force acting on the grains by the die.Therefore,increasing the deformation degree of grains will increase the number of grains deflecting to the{0001}base plane.

Fig.4.Grain morphology observed by metallographic microscope:(a)commercial AZ31 Magnesium Alloy;(b)annealed AZ31 magnesium alloy at 673K for 12h;(c)perpendicular to the ED atλ=44.44;(d)perpendicular to the ED atλ=19.75;(e)perpendicular to the ED atλ=11.11;(f)parallel to the ED at λ=44.44;(g)parallel to the ED atλ=19.75;(h)parallel to the ED atλ=11.11.

Fig.5d-f are partial enlarged details of grain orientations in the black rectangular areas of Fig.5(a)–(c).The grain orientations in Fig.5(d)–(f)were characterized by 3D hexahedral crystal lattices with different angles.In this paper,the red grain boundaries represent low angle grain boundaries(LAGBs<15°),and the black grain boundaries represent high angle grain boundaries(HAGBs>15°).There are some LAGBs between adjacent grains as shown in Fig.5(d)and(e).The energy of transformation from LAGBs to HAGBs mainly comes from dislocation energy,which is determined by the misorientation between adjacent grains.The dislocations rearrange and form LAGBs in the region of grain boundary migration.Therefore,the grain boundary energy of LAGBs increases with the increase of misorientation[22].During the SE process,two adjacent grains rotate different angles around the same axis under continuous deformation conditions.Two adjacent 3D lattices with LAGBs have approximately the same orientation,as shown in Fig.5(d-)–(f).In Fig.5,some incomplete HAGBs are composed of LAGBs,which indicates that LAGBs gradually absorbed dislocations and grow into HAGBs during the grain rotation.The formation and migration of HAGBs promote the dynamic recrystallization.According to EBSD analysis software statistics,whenλ=11.11,λ=19.75,λ=44.44,the percentage of LAGBs between adjacent grains was 6.4%,7.7% and 15.5%,and the average orientation angles were 52.51°,50.65° and 44.90°,respectively.HAGBs is one of the criteria for judging dynamic recrystallization,it can be seen that after the SE process,with the decrease ofλ,the number of HAGBs and dynamic recrystallization structure of bending products decrease.And continuous dynamic recrystallization promotes grain rotation,and the small dynamic recrystallized grains gradually absorb energy for growth.Finally,the dynamic recrystallization structure becomes homogeneous.

Fig.5.The grain orientation maps:(a)λ=44.44;(b)λ=19.75;(c)λ=11.11;(d),(e),(f)partial enlarged details of grain orientation.

Fig.6.TEM images of dislocations in the[11–20]direction of three AZ31 magnesium alloy bending products:(a)λ=44.44;(b)λ=19.75;(c)λ=11.11.

Dislocations in the[11–20]direction of three AZ31 magnesium alloy bending products are shown as Fig.6.Slip deformation is the relative sliding of one part of a crystal relative to another along a certain crystal plane or direction[23].Slip deformation in AZ31 magnesium alloys is basically caused by dislocation slip.When the dislocation slips,the atoms on the slip planes move one by one,instead of the whole atomic layer moving at the same time.Therefore,dislocation slip requires much less shear stress than dislocation-free slip.Dislocation slip of AZ31 magnesium alloy bending products is started by external force,and the slip direction of dislocations along the slip planes.Continuous dynamic recrystallization nucleation also occurs during hot deformation of magnesium alloys.The driving force of dynamic recrystallization is the distortion energy difference between grains,which is determined by dislocation density and its distribution.When dislocation density is high,dynamic recrystallization grain is easy to nucleate.The SE process is carried out at 623K that is higher than the recrystallization temperature of AZ31 magnesium alloy.The external force and temperature provide high deformation activation energy for magnesium alloy,which is beneficial for dynamic recrystallization.The special staggered structure adopted by the SE process makes the billet produce shear deformation before being extruded into the die exit,which can break the original grains and promote the dynamic recrystallization in the subsequent deformation processes.Therefore,the research on the staggered structure of the stem is helpful to analyze the microstructure evolution of the extrusion products.

Fig.7.Schmid factor of AZ31 magnesium alloy bending products:(a)λ=44.44;(b)λ=19.75;(c)λ=11.11.

Whenh=16mm,the more slip systems are started in metals,the more spatial orientations that can be used during slip deformation.It is actually the result of a steady flow of dislocations moving along the slip surface under the shear stress.The grains rotate when relative displacement occurs along the slip planes.The direction of the grain rotation is consistent with the external force,which results in the change of the orientation of the grains.Therefore,with the progress of dislocation slip,the final grain orientation tends to be parallel to the ED after the SE process,as shown in Fig.5.The results show that slip deformation plays an important role in preferred orientations of recrystallized grains.It is beneficial to the generation of deformation texture in bending products.

The billets with larger deformation degree have higher dislocation density,as shown in Fig.6.With increasingλ,the shear force acting on the AZ31 Mg bending products increases,which is beneficial to grain refinement and dynamic recrystallization.The high-speed movement of dislocations forms LAGBs.During plastic deformation of AZ31 magnesium alloy,dynamic recrystallization behavior occurs continuously and dislocation density decreases sharply.By absorbing dislocations,LAGBs between grains can be transformed into HAGBs,which is consistent with the conclusions in Fig.5.

3.4.Deformation texture

Fig.7 shows the Schmid factor(SF)distribution along the ED of three AZ31 magnesium alloy bending products obtained by the SE process.The c/a value of magnesium alloys is 1.623,which is close to the ideal value of 1.633.Therefore,magnesium alloys have relatively stable properties and poor plastic deformation ability at room temperature.AZ31 magnesium alloys can startc+apyramidal slip systems under high experimental temperature and sufficient external force during the SE process.When the plastic deformation of the crystals is dominated by slip,the dislocations only observe Schmid’s law when they slip along the basal planes.The c-axis of the grains rotates during the SE process and gradually parallels the ED,so SF value of(0001)basal grain is smaller.In order to coordinate the deformation between adjacent grains,it is necessary to startc+apyramidal slip systems[28].The Schmid’s law can be used as a qualitative criterion for judging slip deformation in metal plastic deformation mechanism.The larger the SF value is,the more grains with soft orientation are,so the metal is more prone to slip deformation.When the plastic deformation of magnesium alloy is dominated by slip deformation,the basal slip systems with larger SF value are activated first,and then the pyramidal slip systems are activated.

Slip deformation is usually used for achieving plastic deformation of AZ31 magnesium alloy.In this paper,the range of the SF of slip systems is 0–0.5[24].The staggered stem adopted in the SE process results in local shear deformation of AZ31 magnesium alloys in processing bending products.It is found that[27]local shear deformation can change grain orientations and activate the basal slip systems more easily.This plays an important role in coordinating the deformation of magnesium alloys.It can be seen from Fig.7,with the decrease ofλ,the number of grains with SF>0.2 increases,while that with SF<0.2 decreases after the SE process.Whenλ=11.11,the average SF value of the bending product is the maximum and the number of grains with soft orientation is the largest.Whenλ=44.44,the bending product has the minimum average SF value and the number of grains with hard orientation is the largest.With the increase ofλ,the bending products are more difficult to deform.This is because with the increase of deformation degree of AZ31 magnesium alloy,the c-axis of grains rotates and tends to be parallel to the ED during the SE process.Therefore,the grain orientation along the basal planes is gradually changed from the initial soft orientation to the hard orientation under the external force,resulting in a smaller SF value of grains.The above phenomena show that the further slip deformation ability of AZ31 magnesium alloy bending products decreases after the SE process.And the slip deformation is conducive to the extrusion texture of bending products.

Fig.8 shows the pole figures and inverse pole figures of three bending products with differentλ.Texture evolution can quantitatively reflect the development of microstructure after deformation process,which is a useful means to research the plastic deformation mechanism of magnesium alloys[15,17].During plastic deformation,the grain orientations of magnesium alloys rotate under the external force,which results in the change of grain orientations.The grains have preferred orientation in the process of rotation.The structure with preferred orientation is called texture.

Fig.8.Pole figures and inverse pole figures of AZ31 magnesium alloy bending products:(a)λ=44.44;(b)λ=19.75;(c)λ=11.11.

Most grains of original billets deflect to the ED after the SE process,and fiber texture appears in bending products,as shown in Fig.8.In the pole figures of{0001}basal plane,the basal plane inclines to a certain angle along the ED during the SE process,and finally forms a stronger extrusion fiber texture finally.During extrusion process,thec-axis of most grains deflects under the external force and tends to be parallel to the direction of external force.After the SE process,the texture evolution of the{0001}basal plane of the bending products shows a trend that the texture direction is parallel to the ED.The slip direction of the basal slip systems is parallel to{0001}and perpendicular to<11–20>.Therefore,the SF value of basal slip systems along the ED tends to decrease,and the grain deformation along the c-axis cannot be coordinated.It is necessary to activate potential slip systems for further plastic deformation of magnesium alloys.In the SE process,AZ31 magnesium alloy activates the non-basal slip systems under the dual effects of experimental temperature and external force.The{hkil}11–23pyramidal slip systems can provide five independent slip systems,especially for grains with11–23slip direction,which can well coordinate thec-axis deformation of grains.The activation of potential slip systems leads to complex contribution of each slip system to plastic deformation,which has an important effect on texture.In Fig.8,the pyramidal slip systems of{10–13}and{10–11}were gradually parallel to the TD-ED,and the texture intensity of{10–13}is stronger than{10–11}.{0001}10–10deformation texture evolves into{0001}10–10deformation texture and{10–11}11–20recrystallization texture.As shown in Fig.8,with the increase ofλfrom 11.11 to 44.44,the texture intensity of{0001}basal texture increases,and the maximum texture intensities of the three bending products are 11.91,6.99 and 6.57,respectively.

Fig.9 shows the macro-texture of AZ31 magnesium alloy bending products with three differentλby XRD patterns.Diffraction intensity in XRD reflects the relative number of crystal planes that are parallel to a certain plane,and the strongest peak is preferred distribution of the crystal plane[26].Therefore,by analyzing the XRD diffraction intensity of bending products obtained under differentλ,we can understand the changes of crystal orientations in the SE process.It can be seen from Fig.9 that after the SE process,the strongest diffraction peak of AZ31 magnesium alloy bending products is(0002)which belongs to{0001}crystal planes,and the sub-strong diffraction peak is(10–11)which belongs to{10–11}crystal planes.And the macro-texture intensity of{0001}basal plane of three bending products increases with the increase ofλ,which is consistent with the conclusion of Fig.8.

Because of grain rotation and twinning,the preferred orientation of(10–12)and(10–13)is obvious after the SE process.The{10–10}belongs to the pyramidal slip systems,which indicates that the pyramidal slip is an important mode of crystal slip of magnesium alloy during the SE process.Various textures coexist in AZ31 magnesium alloy bending products after the SE process,which can weaken the macro-anisotropy.

Fig.9.XRD patterns of AZ31 magnesium alloy bending products:(a)λ=44.44;(b)λ=19.75;(c)λ=11.11.

4.Conclusion

1.The SE process is a new approach to realize the short integration,consisting of a bending process and an extrusion process.The results show that theλand the bending height of bending products changed in direct proportion at the same staggered distance(h=16mm).Whenλis increased from 11.11 to 44.44,the average bending radius of AZ31 magnesium alloy bending products is decreased from 14.7mm to 9.0mm.It is feasible to control the bending behavior of AZ31 magnesium alloy bending products by the SE process.

2.The SE process can improve the microstructure of AZ31 bending products.After SE process,there are uniform and equiaxed grains present in AZ31 magnesium alloy bending products.With increasingλfrom 11.11 to 44.44,the average grain size of AZ31 magnesium alloy bending products decreases from 23.89μm to 9.69μm,and grain refinement rate is 59.43%.The grain refinement rate is remarkable by the SE process.

3.During the SE process,the main plastic deformation of AZ31 magnesium alloy bending products is slip deformation.The average SF value of the basal plane along the ED decreases with increasingλfrom 11.11 to 44.44.After SE process,the grain orientation of AZ31 bending products is changed from initial soft orientation to hard orientation,and the number of hard orientation grains atλ=44.44 is the largest.

4.After the SE process,the extruded fiber texture in AZ31 magnesium alloy bending products is obvious,and the deformation texture is transformed into a mixed texture consisting of{0001}10–10and{10–11}11–20.With increasingλfrom 11.11 to 44.44,the texture intensity of the{0001}basal plane increases from 11.91 to 6.57.And the{10–10}crystal planes have obvious preferential orientation after the SE process,which is caused by the rotation of grains.Pyramidal slip systems are an important mode of crystal slip systems of AZ31 magnesium alloy bending products during the SE process.Starting pyramidal slip systems can coordinate the plastic deformation of AZ31 magnesium alloy bending products.

Declaration of Competing Interest

The authors declare no conflict of interest.

Acknowledgment

This work was supported by the National Natural Science Foundation of China(51675143)and the Fundamental Research Foundation for Universities of Heilongjiang Province(LGYC2018JQ011)and the Natural Science Foundation of Heilongjiang Province(LH2019E056).