Influence of rolling speed on microstructure and mechanical properties
of AZ31Mg alloy rolled by large strain hot rolling
Fei Guo a,Dingfei Zhang a,b,n,Xusheng Yang a,Luyao Jiang a,Sensen Chai a,Fusheng Pan a,b
a College of Materials Science and Engineering,Chongqing University,Chongqing400045,China
b National Engineering Research Center for Magnesium Alloys,Chongqing University,Chongqing400044,China
a r t i c l e i n f o
Article history:
Received10March2014
Received in revised form
2April2014
脸长的女生适合什么发型Accepted4April2014
Available online13April2014
Keywords:
Magnesium alloys结婚要买些什么
Rolling speed
Microstructure
Twinning
Dynamic recrystallization
a b s t r a c t
The influence of the rolling speed on microstructure and mechanical properties of as-rolled AZ31Mg
alloys sheet was critically examined.Samples were hot rolled by nearly55%thickness reduction in a
single rolling pass at rolling speeds varying between3.5and12.1m/min.The results showed that
twinning dominated the deformation at lower rolling speed and dynamic recrystallization was
葡萄酒怎么做constricted in shear bands.While,at high rolling speed,dynamic recrystallization behavior was largely
extended and microstructure became homogeneous.The yield behavior of as-rolled sheets was affected
by the recrystallized grains formed during rolling.The tensile ductility was gradually improved as rolling
speed increased,due to the enhancement of dynamic recrystallization.The dynamic recrystallization is
an effective way to consume the deformation energy,which may suppress edge crack initiation.
&2014Elsevier B.V.All rights reserved.
1.Introduction
Mg alloys usually exhibit poor formability and ductility at room
temperature due to their limited slip system[1].Therefore,the
production offlat workpiece is generally performed by hot or
warm rolling.In the rolling process,the thickness is reduced by
multi-pass rolling with small reduction per pass,accompanied by
intermediate annealing,in order to maintain the pass-to-pass
workability and to suppress edge cracks or fracture of the material.
Because of the low efficiency and high manufacture cost,Mg alloy
flat production is not currently widespread use[2].
To eliminate the disadvantages in conventional rolling of Mg
alloy and improve the mechanical properties of rolled sheets,
蜜丽娅miliyamany researches were carried out in past a few years,which can
be classified into two categories.One was to use new rolling ways
or rolling routes to cover the shortages in conventional rolling.
Huang et al.[3,4]demonstrated that large shear strain was
induced in the rolling process by differential speed rolling(DSR),
resulting in a weak of basal texture and a distinctive grain
refinement.The strength of rolled sheets effectively increased
and microstructure became more homogeneous through multi-
pass of DSR[5].Zhang et al.[6,7]reported that the ductility of Mg
alloy sheets was enhanced by three-directional rolling routes,due演员王海燕
to the weakness of basal texture.The formability of Mg alloys
sheet was improved by cross-roll(CR)rolling,as it changed the
strain distribution in the rolling process[8,9].Although a number
of drawbacks of conventional rolling were overcome by new
rolling ways listed upon,it was difficult to establish these
techniques into industrial manufacture as it required specific
rolling device or may not be effective.Therefore,the other kind
of researches was to study the effect of rolling parameters on
conventional rolling,such as temperature and rolling reduction,
and tofind out the optimum craft.Researchers had confirmed that
rolling temperature largely affected the workability of Mg alloys
[10–14].Xu et al.[15]found that dynamic recrystallization(DRX)
behavior was dramatically enhanced by high temperature rolling
in Mg–Gd alloys,leading to a significant soft in rolled sheets.It was
suggested that remarkable grain refinement and weak basal
texture were obtained through warm or high temperature rolling,
due to the contribution of non-basal slip and DRX.The thickness
reduction was another factor in the rolling process[16–18].The
fraction of dynamic recrystallization was apparently increased as
rolling reduction increased in hot rolling[17].Thefinal rolling
reduction was largely affected the intensity of texture and
mechanical properties in high temperature rolling of Mg–RE alloys
[19].Del Valle et al.[18]reported that the great grain refinement
was obtained by large strain rolling,due to complete continuous
dynamic recrystallization(CDRX).The effects of rolling tempera-
ture and thickness reduction on microstructure evolution have
Contents lists available at ScienceDirect
journal homepage:www.elsevier/locate/msea
Materials Science&Engineering A
/10.1016/j.msea.2014.04.024
0921-5093/&2014Elsevier B.V.All rights
reserved.
n Corresponding author at:College of Materials Science and Engineering,Chongqing
University,Chongqing400045,China.Tel.:þ862365112491.
E-mail address:zhangdingfei@cqu.edu(D.Zhang).
Materials Science&Engineering A607(2014)383–389
been widely discussed in the past few years,but the rolling speed,which is another important rolling parameter,was paid less attention.Recently,it is found that rolling speed largely affected the texture evolution and kinetics of recrystallization in Mg –Zn –Ce alloys [20].However,it still requests more experiments to under-stand the different dynamic recrystallization behaviors at various rolling speeds and a reasonable explanation on the distinction of mechanical properties for as-rolled sheets should be put forward.
Therefore,the aim of the present study is to explore the in fluence of rolling speed on microstructure and mechanical properties of as-rolled AZ31Mg alloys sheets.In the present work,AZ31Mg alloys were hot rolled by nearly 55%thickness reduction in a single rolling pass at different rolling speeds.It seems that mechanical properties and rolling feasibility of the sheets largely affected by the rolling speed.Twinning and dynamic recrystalliza-tion behaviors during rolling process were discussed.The differ-ence in deformation energy consumption at various rolling speeds was critically evaluated.
2.Experimental procedure
Hot rolled AZ31Mg alloy (Mg –2.96wt%Al –0.94wt%Zn)plates with a thickness of 2.5mm were used as the initial material.Prior to hot rolling,the plates and the rollers were preheated to 798K and 573K,respectively.Then the samples were hot rolled to 1.1mm through a single rolling pass.Nearly 55%reduction was carried out in one pass with rolling speeds of 2.1,3.5,4.9,7.5,9.8and 12.1m/min,respectively.The plates were quenched into water immediately to obtain deformation microstructure after rolling process.
Mechanical properties were examined by tensile tests at room temperature with a constant speed equal to an initial strain rate of 1.8Â10À3s À1.Tensile specimens with a gauge length of 14mm were machined out of the sheets along the rolling direction.Electron Back-Scatter Diffraction (EBSD)observation was per-formed in a Zeiss s AURIGA FIB-SEM (scanning electron
microscopy)machine operating at 25kV.The inverse pole figures were measured at a step size of 0.2μm.
3.Results and discussion
3.1.Rolling feasibility
Fig.1presents the appearance of AZ31plates rolled at different rolling speeds.It can be seen that large edge cracks propagated at low (2.1m/min)rolling speed.As increasing the rolling speed to 3.5m/min,the crack depth was decreased,indicating that the sheets had a better workability.Only minor edge cracks were observed on the sheets rolled at 4.9and 7.5m/min.By increasing the rolling speed to Z 9.8m/min,the sheets were produced without any obvious edge cracks.The rolling feasibility of the sheets can be judged by the number of edge cracks and the total crack length.Both of two factors were measured along the edge for 100mm on each sheet and the results were shown in Fig.2.The number and total length of the cracks decreased
with
Fig.1.The appearance of as-rolled AZ31Mg alloy rolled at rolling speeds of 2.1,3.5,4.9,7.5,9.8and 12.1
m/min.
Fig.2.The statistical figure on rolling feasibility of the sheets rolled at different rolling speeds.
F.Guo et al./Materials Science &Engineering A 607(2014)383–389
384
increasing the rolling speed,indicating that the rolling feasibility was signi ficantly improved.3.2.Microstructure
Fig.3shows the optical microstructure of the AZ31sheets rolled at different rolling speeds.At low rolling speed (2.1and 3.5m/min),few grains re fined and several twins,including paral-lel twins and intersecting twins,appeared in large original grains.Two kinds of twins are commonly observed in deformation of Mg alloys.{1012}twins produced in grains whose c -axis were nearly perpendicular to the normal direction (ND)and made the whole
matrix close to ‘basal orientation ’.{1011}
twins were formed in grains close to ‘basal orientation ’and became favorable sites for recrystallizatio
n [21].Shear bands (presented as red arrows)were found in the region between large original grains (presented as yellow arrows),which formed as a ‘sandwich ’structure.These shear bands were consisted of ultra fine grains and twins,which had suffered intensive shear stress [21,22].Ion et al.[10]believed that the new grains within shear bands were formed because of
rotation dynamic recrystallization (RDX)during hot working.Large strains can be accommodated along the re fined grains,because the new orientation of RDXed grains was favorable for basal slip.By increasing the rolling speed to 7.5m/min,the shear bands were replaced by re fined grains.Recrystallized grains were clustered and obviously separated by unrecrystallized grains.At high rolling speed (12.1m/min),the re fined grains with an average grain size of $2.6μm were settled around large
original
Fig.3.Optical micrographs for the sheets rolled at different rolling speeds.(a)2.1m/min,(b)3.5m/min,(c)7.5m/min,(d)12.1m/min.All optical micrographs are shown the microstructure in the TD plane (red arrows represent shear bands;yellow arrows represent large original grains).(For interpretation of the references to color in this figure legend,the reader is referred to the web version of this article.)
Table 1
The effective grain size of the samples rolled at different rolling speeds.Rolling speed (m/min)f r D r (μm)f l D l (μm)D e (μm)3.50.360.70.6411.67.64.90.39 1.60.6111.77.77.50.46 1.80.5410.1 6.39.80.63 3.10.3811.7 6.312.1
0.71
2.6
0.29
9.0
4.5
F.Guo et al./Materials Science &Engineering A 607(2014)383–389385
grains,formed as a ‘necklace ’structure.No shear band can be observed and the quantity of twins was reduced.DRXed grains were commonly seen in the areas with high density of dislocation,such as grain boundaries (GBs)and twin boundaries (TBs).Twins were observed to be potent sites for recrystallized nuclei at high speed rolling,as presented in Fig.3(d).
As microstructure can be obviously divided into recrystallized grains and large original grains,an effective grain size was assigned to the as-rolled samples taking into account the mixed mode of the grain structure as well as the grain size.The following
equation was used to assign the effective grain size (D e ):D e ¼½f r ðD r Þþf l ðD l Þ
ð1Þ
where f r is the area fraction of recrystallized grains;D r the average grain size of recrystallized grains;f l the area fraction of large original grains;and D l is the average grain size of large original grains.
The results of the grain size of samples rolled at different rolling speeds were shown in Table 1.The recrystallization was much extended with rolling speed increased.At low rolling
奚梦瑶和窦骁谈过恋爱吗speed,
Fig.4.Inverse pole figure maps,distributions of grain boundary misorientation and (0001)pole figures of (a)all the grains,(b)the dynamic recrystallized grains,(c)unrecrystallized grains in the sheet rolled at 12.1m/min.Thin white lines correspond to the boundaries of misorientation Z 21,black lines Z 151.
F.Guo et al./Materials Science &Engineering A 607(2014)383–389
386
large original grains occupied a major area in the samples.When the rolling speed increased,the recrystallized grains grew larger and occupied more area.The fraction of recrystallized grains exceeded that of large grains as the rolling speed increased to 9.8m/min.The grain size of large original grains maintained nearly similar of about10μm at different rolling speeds.The effective grain size decreased from7.6μm to4.5μm.The drop of the effective grain size was mostly attributed to the increase of the fraction of recrystallization.It is obvious that DRX behavior was largely extended by increasing rolling speed.
In order to study the DRX behavior in large strain hot rolling, the sample rolled at12.1m/min was cho
sen for a further investi-gation due to its drastic recrystallization.The microstructure and texture of the sample rolled at12.1m/min were observed using EBSD.Fig.4(a)shows the inverse polefigure maps,the distribu-tions of grain boundary misorientation and the(0001)polefigures of the sheet.The EBSD images associated with the DRXed grains and the unrecrystallized grains are extracted from Fig.4(a),as presented in Fig.4(b)and(c).The thin white line in inverse pole figures represents the low angle grain boundary(LAGB),whose grain boundary misorientation less than151,while the black line represents the high angle grain boundary(HAGB).It can be seen LAGBs in unrecrystallized grains were much more than that in recrystallized ones and the average misorientation angle increased from17.41to30.71through DRX.This was because the LAGBs were turned to be the HAGBs by continuous accumulated the disloca-tions in the DRX process.As most of the LAGBs converted to be the HAGBs in the recrystallized grains,LAGBs can only exist in grains that barely recrystallized.Newly recrystallized grains were formed as the sub-grain boundary misorientation increased.It is indicated that the dominant DRX behavior in large strain hot rolling for AZ31 Mg alloys was CDRX,which is commonly observed in low stack fault energy materials during hot deformation[23].
The texture of the sample rolled at12.1m/min was double peak basal textures with the basal pole incl
ining from the normal direction(ND)to the rolling direction(RD),which was commonly observed during the hot rolling process of Mg alloys.The inclina-tion of the basal pole may relate to the activation of pyramidal 〈cþa〉slip and double twins[24].Compared with the unrecrys-
tallized region,the intensity of the texture significantly decreased in DRX region.It is suggested that the new grains formed in CDRX have more orientations than large original grains.The weakness of the texture is likely to be mainly resulted from activated non-basal slip system for the sheets rolled at798K.With the temperature increases,critical resolved shear stress(CRSS)for prismatic and pyramidal slip decreases[25].As twinning is not sensitive to temperature,non-basal slip should take an important role in this high temperature rolling process.Non-basal dislocations were propagated in the severe deformation and clustered along the GBs and the TBs,which provided the driving force for CDRX.New refined grains nucleated by consuming the non-basal dislocations, which may alter the basal pole to some new orientations[12].
3.3.Mechanical properties
The tensile properties of the as-rolled AZ31sheets were pre-sented in Fig.5.The samples rolled at 3.5m/min obtained the highest yield strength(YS)and ultimate tensile strength(UTS), while the elong
ation(EL)was quite low.It shows that the strain hardening behavior was significantly enhanced in low speed rolling process.With increasing rolling speed,the YS and UTS of the samples decreased and EL significantly increased.At the intermedi-ate rolling speed(7.5m/min),the YS and the UTS maintained at$250MPa and$295MPa,respectively.With increasing the rolling speed to12.1m/min,the sample obtained the highest EL of about$20%.
The mechanical properties of as-rolled AZ31Mg alloy sheets were largely affected by rolling speed.The sample rolled at3.5m/ min obtained the highest strength of281MPa and356MPa for YS and UTS,respectively.As the area fraction of large original grains was remarkably higher than that of CDRXed grains,the mechanical properties were dominantly affected by large original grains.Huge number of twins formed in the large original grains might be responsible for the high strength.The twinning drastically acti-vated at low speed rolling may be caused by the inhomogeneous strain distribution.It was reported that twinning played a major role in the initial stage of hot rolling[26]and the critical strain for twinning was lower than that for recrystallization[27]. At low rolling speed,large amounts of shear bands were formed, indicating that the distribution of strain was inhomogeneous.The orientation of the DRXed grains within shear bands was favorable for basal slip,leading toflow localization in this region[18].Less strain was distributed in large original grains an
d not attained to the critical strain for DRX.This strain cannot afford the critical strain for DRX but twinning.It was confirmed that the TBs are effective barriers for dislocations movement[28,29].Large original grains were separated by TBs into pieces,as shown in Fig.3(a).The movement of dislocations is severely suppressed within small regions surrounded by TBs and GBs,resulting in an increase in strength.
The EL of the as-rolled sheet increased from4.8%to16.2%as the rolling speed increased to7.5m/min.The increase of ductility may relate to the extension of CDRX behavior.The CDRX led to afl
ow Fig.5.Mechanical properties of AZ31Mg alloys rolled at different rolling
speeds.
Fig.6.Hall–Petch plots showing the influence of dynamic recrystallized grain size on yield stress for the samples rolled at different rolling speeds.
F.Guo et al./Materials Science&Engineering A607(2014)383–389387
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