Ying-Ping Guan，Zhen-Hua Wang，Bin Wu，Wei-Xin Wang，Wan-Tang Fu

(1.Key Laboratory of Advanced Forging＆Stamping Technology and Science(Yanshan University)，Ministry of Education of China，Qinhuangdao 066004，China;2.College of Mechanical Engineering and Applied Electronic Technology，Beijing University of Technology，Beijing 100124，China;3.CITIC Dicastal Wheel Manufacturing Co.，Ltd.，Qinhuangdao 066003，China;4.State Key Laboratory of Metastable Materials Science and Technology，Yanshan University，Qinhuangdao 066004，China)

1 Introduction

Aluminum alloy 6061 is a heat-treatable，wrought Al-Mg-Si alloy.Among the alloying elements，Mg and Si are the major solutes，which increase the alloy strength by precipitation hardening［1］.Other elements in the alloy，such as Mn and Cr，can inhibit the harmful effect of Fe，Cr and Zn provide improved strength，while Ti refines the microstructure［2］.Therefore，alloy 6061 is well-known for its superior properties such as high strength to mass ratio，good ductility，excellent weldability，good corrosion resistance，as well as resistance to stress corrosion cracking and deformability.This alloy is used widely in the automotive，aerospace，and food industries［14］.

In recent years，properties such as fatigue［1］，dynamic deformation［3］，ultrafine grain production［5－7］，thixoforming［8］，and heat treatment［9－11］of the Al-Mg-Si alloys have been investigated extensively，enriching our understanding of the alloy significantly.However，in the production of 6061 alloy forgings，grain coarsening usually occurs during SHT，as shown in Fig.1.Coarse grains of the alloy could result in an unappealing appearance and poor mechanical properties.Obvious differences in grain sizes can be found in different forging positions(Fig.1).This indicates that various microstructure evolutions occur for different deformation microstructures during SHT.Hence，the microstructures developed during hot deformation and subsequent SHT should be clarified to prevent grain from coarsening.

Fig.1 Macrostructure of 6061 alloy resulting from forging

A number of papers have been published on the effect of hot deformation and SHT.During hot deformation，at high strain rates(6－40 s－1)，the dominant mechanisms of the breaking up of grains for the 6061 alloy are continuous dynamic recrystallization and geometry dynamic recrystallization at a low and high Z parameter，respectively［12］.At low strain rates(0.005－5 s－1)，the dynamic flow softening mechanisms，for high-Cu 6061 alloys，are dynamic recovery and recrystallization［13］.Because higher levels of stored energy created during deformation easily induce static recrystallization，finer grains can be obtained in the evolution of the deformation microstructure during subsequent SHT［14］.This means that a finer grain size exists after deformation at higher strain rates or at lower deformation temperature and subsequent SHT.However，Lee［15］claimed that increasing the strain rate can lead to coarser grains after SHT for the 6061 alloy.Chun［16］indicated that fine static recrystallization grain of metals is independent of stored deformation energy.Therefore，the microstructure evolution of hot deformed structures during subsequent SHT is still under debate.Zhao［17］investigated the evolution of the deformation microstructure during subsequent SHT of the 6061 alloy without Ti.The effect of the Z parameter on final grain size is determined，and the method of inhibiting grain coarsening through the addition of Cr or Zr was provided.However，the applied strain is small(approximately 0.7)，and the influence of Cr or Zr addition on mechanical and chemical properties is still unclear.

In this study，a high-Ti 6061 alloy was rolled with strains up to 0.8－2.0 at 350－550℃.The deformed and solutionized microstructures were observed by optical(OM)and transmission electron microscope(TEM)，and the effect of deformation temperature，strain and SHT temperature on grain size was determined.The mechanism of grain coarsening was clarified and a method for the inhibition of grain coarsening through two-stage deformation was provided.

2 Experimental Procedures

The tested 6061 alloy was a continuous 200 mm diameter cast bar with chemical composition(wt%):Mg 1.1，Si 0.8，Cu 0.35，Cr 0.25，Zn 0.1，Ti 0.15，F(xiàn)e 0.2，balance Al.The bar was solutionized at 560℃for 240 min resulting in an average grain size of 200μm.Slabs with dimensions of 70 mm×40 mm were machined from the bar along the axial direction.The slabs were then rolled and heat treated according to the procedures described in Table 1.

Table 1 Rolling and heat treatment procedures

The deformation microstructures were examined by using a JEM-2010 TEM.The TEM specimens were machined parallel to both the normal(ND)and rolling(RD)directions.The specimens were ground mechanically to a thickness of～30μm and then twinjet electropolished in a solution consisting of 20 ml HNO3and 80 ml methanol at 20 V and－30℃.The grain sizes were observed by OM.The observation plane was parallel to both the ND and RD.Keller’s reagent was used as etchant.

3 Results

3.1 Influence of Deformation Temperature on Grain Size

Fig.2 shows typical macrostructures(in the ND and RD)of the 6061 alloy deformed at different temperatures to a strain of 0.8 and subsequently solutionized at 540℃for 120 min.

It is apparent that the initial rolling temperature has a significant effect on the macrostructure evolution of the 6061 alloy during SHT.The higher the rolling temperature is，the coarser the structure is.The specimen rolled at 400℃(Fig.2(a))has a fine structure，while an initial rolling temperature of 550℃yields“grain”sizes in the order of millimeters.

Fig.3 shows optical micrographs of the specimens which is in Fig.2.The 6061 alloy is fully recrystallized into a structure with elongated grains parallel to the rolling direction and a few small equiaxial grains.Hereafter，the length of the grain is defined as the grain size，and those grains larger than 200μm are termed as coarse grains and those smaller than 200μm as fine grains.It is important to note that the“grain”in the macrostructure(Fig.2)is composed of several grains of similar color(Fig.3).

Fig.2 Macrostructures of 6061 alloy deformed at different temperature to a strain of 0.8，followed by SHT at 540℃for 120 min(ND:normal direction;RD:rolling direction)

Fig.3 Microstructures of 6061 alloy deformed at different temperature to a strain of 0.8，followed by SHT at 540℃for 120 min

3.2 Influence of Strain on Grain Size

Fig.4 shows the microstructure of the 6061 alloy deformed to 1.2 and 1.6，followed by SHT at 540℃for 120 min.Compared to the results in Fig.3(a)，we can see that the grain size decreases with the increase of strain in the specimen rolled at 400℃.Increasing strain also leads to an increased fraction of equiaxial grains.However，at 500℃，the higher strain yields larger grains but grains become thinner in the ND.At 450 and 550℃，the relationships between grain size and strain are the same as those at 500℃，which are not shown here.

3.3 Influence of SHT Temperature on Grain Size

Fig.5 shows the microstructure of the 6061 alloy deformed under different conditions，followed by SHT at 560℃for 120 min.

A slight increase in SHT temperature to 560℃does not have a significant influence on grain coarsening.Recrystallized grains are therefore very stable when in contact with one another.

Fig.4 Microstructures of 6061 alloy deformed under different conditions，followed by SHT at 540℃for 120 min

Fig.5 Microstructures of 6061 alloy deformed under different conditions，followed by SHT at 560℃for 120 min

4 Discussion

4.1 Mechanism of Grain Coarsening

No recrystallization occurs in the 6061 alloy deformed in the temperature range 400－550℃and at 1 s－1to strains of 0.8－1.6(OM observation).Therefore，the deformation microstructures were examined by using TEM.The microstructures of the specimen rolled at different temperatures to a strain of 1.2 are shown in Fig.6.

Fig.6 Microstructures of specimens rolled at(a)400℃(b)450℃(c)500℃(d)550℃to a strain of 1.2

After rolling at 400℃，well-defined cells and subgrains with lower internal dislocation density were obtained(Fig.6(a)).Increasing the rolling temperature increases the extent of dynamic recovery.The microstructure of the specimen rolled at 450℃is composed of a high density of dislocations with a few subgrains(Fig.6(b)).A further increase in rolling temperature to 500 and 550℃yields a low dislocation density which is uniformly distributed in the deformed matrix.Subgrains and cells are rarely found.A higher deformation temperature results in a lower dislocation density.

In the investigation of the static recrystallization of warm-rolled pure Ti，Chun［16］found that a subgrain structure with higher deformation inhomogeneity accelerates the static recrystallization more than the uniformly distributed dislocation structure，even though the overall stored energy is lower in the former.This occurs because a higher deformation inhomogeneity can induce more sites for the nucleation of recrystallization.Combining the results in Fig.6 with Chun’s work［16］，it can be deduced that a specimen rolled at 400℃possesses more nucleation sites during SHT，thus leading to the formation of finer grains.In addition，the overall stored energy is higher in the specimen rolled at 400℃than that at 500℃.

To examine this inference，the specimens rolled to a strain of 1.2 were maintained at 540℃for 5 min，as shown in Fig.7.The specimen rolled at 400℃exhibits a higher density of recrystallized grains compared with that at 500℃.In Fig.7(a)(400℃)，some newly formed grains with equiaxial shape exist(indicated by arrows).In Fig.7(b)(500℃)，the newly formed grains are thin and long(indicated by arrows).From Figs.6 and 7，it can be deduced that a specimen with subgrain or cell structure possesses more static recrystallization nucleation sites.More nucleation sites not only induce fast recrystallization kinetics but also lead to finer recrystallized grains，because the grains stop growing when they contact one another(Fig.5).However，in the specimen rolled at 500℃，newly formed grains have more time and space to grow，as fewer nucleation sites exist for recrystallization，which results in the formation of coarse grains.

From the shape of the newly formed grains in Fig.7，it can also be inferred that the nucleation mechanisms for recrystallization are subgrain rotation in the specimen rolled at 400℃and grain boundary bulging in the specimen rolled at 500℃.Different nucleation mechanisms induce different dependences of grain size on strain(Fig.4).At 400℃(Figs.3(a)，4(a)and 4(b))，a specimen exposed to a strain of less than 1.2 may not reach steady state.The number of subgrains or cells acting as nucleation sites therefore increases with the increase of strain;larger strain leads to a finer grain size and a larger fraction of equiaxial grains.In the specimen rolled at 500℃(Fig.3(c)，4(c)and 4(d))，the larger the strain is，the smaller the distance between the parent grain boundaries is.Because the nucleation mechanism is grain boundary bulging，the density of nucleation sites of recrystallization increases with the increase of strain in the ND.Hence，the newly formed recrystallized grains have less space for growing in the ND，leading to a decrease in grain thickness in the ND with the increase of strain.However，in the RD，a larger strain induces a larger parent grain length and coarser grains can then be obtained after SHT.

Fig.7 Microstructures of specimens rolled at 400℃and 500℃to a strain of 1.2 and then held at 540℃for 5 min

Information on the grain sizes is given in Fig.8 for a range of testing conditions.In the specimen rolled at 400℃，the grains after SHT are fine and in the range of 60－110μm.Rolling at 550℃induces large grains with a size exceeding 500μm during subsequent SHT，with the largest grain being approximately 4 mm in size.One can infer that different nucleation mechanisms lead to an obvious inflection on the curves showing the relationship between grain size and deformation temperature.Although strain has a certain influence on final grain size，from the viewpoint of industrial engineering，deformation temperature is thought to be the most important factor which inhibits grain coarsening.

Fig.8 Relationships between grain size and processing condition

4.2 A Method for Inhibiting Grain Coarsening

Lowering the deformation temperature is effective in inhibiting grain coarsening(Fig.8).However，a lower deformation temperature may induce a higher deformation resistance and poor ductility，thereby leading to a failed forging process.It is noteworthy that most forgings are implemented in several stages.Therefore，in the early deformation stage，forging should proceed at high temperature to achieve a low deformation resistance and high ductility.In later stages，a lower deformation temperature can be used to inhibit the formation of coarse grains.

Additional experiments were conducted to examine this method.The specimens were firstly deformed at 500℃to strains of 1.2 and 1.6，which would form coarse grains if solutionized at 540℃，and then rolled at 400℃and 350℃to strains of 1.6 and 2.0，respectively.Finally，they were further solutionized at 540℃for 120 min.The resulting microstructures are shown in Fig.9.It is apparent that，after high temperature deformation，adding a further strain of approximately 0.4 at lower temperature can induce a fine structure.In addition，the grains of specimen rolled at 350℃are finer，which may be caused by more nucleation sites.

The specimens rolled in the temperature range of 450－550℃were further rolled at 400 and 350℃.The critical additional strains for obtaining fine grains are shown in Fig.10.It is apparent that，for the 6061 alloy forgings，the critical values of the additional strains imposed at 350 and 400℃for achieving fine grains are approximately 0.25 and 0.3，respectively.

Fig.9 Microstructures of specimens deformed at 500℃to strains of 1.2 and 1.6，further rolled at 400℃and 350℃to strains of 1.6 and 2.0，respectively，and solutionized at 540℃for 120 min

Fig.10 Critical additional strains for obtaining fine grains after high temperature deformation

5 Conclusions

1)Microstructure evolution during SHT depends mainly on the previous rolling temperature and it was found that the higher this temperature is，the coarser the grains are.The grain size decreases with the increase of strain at 400℃，while it increases with the increase of strain at 450－550℃.Increasing the SHT temperature from 540 to 560℃does not have a significant effect on the grain size.

2)After rolling at 400℃，well-defined cells and subgrains form.These cells and subgrains induce further sites for recrystallization nucleation.During subsequent SHT，the recrystallization mechanism is subgrain rotation.The newly formed grains are equiaxial and the final grain size is finer than 200μm.Increasing the rolling temperature to 500℃results in a low dislocation density，which is distributed uniformly in the deformed matrix.This lower dislocation density provides fewer nucleation sites.During subsequent SHT，the recrystallization mechanism is grain boundary bulging and the newly formed grains are thin and long.

3)To inhibit grain coarsening，the 6061 alloy forgings should be deformed at higher temperature，causing better ductility and lower deformation resistance，in the early stage，and then be deformed at lower temperature in the later stage.After higher temperature deformation，the critical values of the additional strains imposed at 350 and 400℃for achieving fine grains are approximately 0.25 and 0.3，respectively.

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