The Effect of Microstructure on Mechanical Propertiesof Forged 6061 Aluminum Alloy

Manabu Nakai1,2 and Goroh Itoh1

1Ibaraki University, Hitachi 316-8511, Japan2Kobe Steel, Ltd., Mooka 321-4367, Japan

The relationship between the microstructure and the yield strength after T6 tempering was investigated using 6061 aluminum alloymanufactured at various levels of temperature and strain rate during hot forging. Non-recrystallized structures (continuous recrystallizationstructure) were formed by hot forging at low Zener­Hollomon parameter (Z parameter) conditions, which consisted of fine grains surrounded byhigh angle grain boundaries and contained low angle grain boundaries inside. Increasing the Z parameter formed fine-grained structures,resulting in increased yield strength. Increasing further the Z parameter formed recrystallization, having coarse recrystallized structures(discontinuous recrystallization structure) of hundreds of micrometers in diameter with high angle grain boundaries, resulting in significantlyreduced yield strength. The yield strength of the material with recrystallized grain structures was less dependent on the grain size. On the otherhand, the yield strength of the material with non-recrystallized structures was severely dependent on the grain size, roughly in accordance withthe previous data. Subgrain strengthening appeared to be more effective than recrystallized grain strengthening. In consideration of the effect ofthe texture on the yield strength using Schmidt factor, ¸Acrss (the value equivalent to critical resolved shear stress in the slip direction.¸ACRSS ¹ ¸ACRSS0 = kAdA¹mA, ¸ACRSS = s··0.2, s: the averaged Schmidt factor in the tensile direction. dA: grain sizes in the slip direction) was lessdependent on dA. In consideration of the texture, the yield strength of 6061-T6 is essentially less dependent on the grain size as reportedpreviously. [doi:10.2320/matertrans.MA201324]

(Received September 2, 2013; Accepted November 1, 2013; Published December 25, 2013)

Keywords: 6061 aluminum alloy, Hall­Petch relation, subgrain, Zener­Hollomon parameter

1. Introduction

In 6XXX aluminum alloys, high strength is obtained byincreasing the amount of Mg2Si. It is reported that, however,there are problems of reduced fracture toughness andcorrosion resistance in 6X51 type aluminum alloy withexcess Si.1) The requirement is for materials which have notonly high strength but high fracture toughness and corrosionresistance for applications. It is known that those aluminumalloys which have high stacking fault energy easily formsubgrains due to polygonization during hot working.Forged material is manufactured mainly by hot forging andsubsequent T6 tempering. Generally, mechanical propertiesof hot forged material of heat-treatable aluminum alloyare greatly affected by the substructure formed during hotforging.

The 6061 moderate strength aluminum alloy have beenextensively used in vehicles, ships, land structures, etc. sinceit was AA registered in 1954. As a result of investigating therelationship between the microstructure and the properties of6061-T6,2­5) hot forging at high temperature and low strainrate formed mainly subgrains, resulting in significantlyincreased yield strength, compared to the recrystallized grainsobtained at low temperature and high strain rate conditions.Both the fracture toughness and the resistance to intergranularcorrosion were also increased. It was proved that a highstrength and reliable material could be obtained by control-ling microstructures, but not by increasing alloy constituents.

A modified Hall­Petch relation (eq. (1)) has beenproposed for improving the yield strength by subgrainformation.6,7)

·0:2 ¼ ·0 þKd�PS d�1=2 ¼ ·0 þKd�m

S ð1ÞWhere, ·0 is the yield stress of the tempered material thatdoes not contain any substructure, and k and m are

experiment constants. When m = 0.5 (p = 0), eq. (1) repre-sents the Hall­Petch relation. This means that the subgrainboundary is more effective as a resistance to dislocation thanthe recrystallized grain boundary. On the other hand, it issupposed that the dislocation tends to pass, but not be piled,at a low angle grain boundary that forms subgrains, and thelow angle grain boundary does not contribute to improve theyield strength so much as the high angle grain boundary.Furthermore, deformation texture may be developed in a non-recrystallized structure where such subgrains are formed.

The object of the present paper was to clarify theinterrelationship among alloy/process-subgrain microstruc-ture and mechanical properties combined with microme-chanics, and to improve strength and fracture toughness offorged 6061 aluminum alloy.

2. Material and Methods

2.1 Processing and experimental procedureThe present work has been carried out on an AA6061

alloy. The chemical composition is shown in Table 1. Thematerial which was prepared by melting and DC castingas ¤80mm billets were homogenized at 773K for 4 h, andwere then hot forged. The hot forging conditions were asfollows: All the specimens were hot forged at 583­773K, atinitial strain rates of 2.7 © 10¹4­2.7 © 10¹1 s¹1, with 71.1%reduction. Table 2 shows the detailed forging conditions. Inthis work, the hot forging conditions are expressed by Zener­Hollomon parameter, as shown by the following eq. (2):

Z � ðd¾=dtÞ � expðQ=RT Þ; ð2Þwhere Z is Zener­Hollomon parameter, d¾/dt is the initialstrain rate (s¹1), T is the temperature (K) in hot forging, Q isthe activation energy (144 kJ/mol),8) R is the gas constant(8.31 J/mol·K). The hot forged specimens were solution heat

Materials Transactions, Vol. 55, No. 1 (2014) pp. 114 to 119Special Issue on Strength of Fine Grained Materials ® 60 Years of Hall­Petch®©2013 The Japan Institute of Metals and Materials

treated at 813K for 3 h and subsequently quenched in waterat 298K. All the specimens were then aged at 453K for 8 h inT6 temper. Microstructure characterizations of the specimenswere made by optical microscopy (OM) and scanningelectron microscopy (SEM). OM observations were preparedusing caustic soda etching. Tensile tests were carried out at3.3 © 10¹3 s¹1 in the long transverse (LT) direction accordingto ASTM-E8. The fracture toughness Kc was estimated bythe following eq. (3):

Kc ¼ 36:6� NTR� 20:82; ð3Þwhere NTR is Notch tensile strength/Tensile yield strengthRatio. Figure 1 shows the observation part of microstruc-tures, geometry of tensile test piece, and its sampling part.

3. Results

3.1 MicrostructureFigure 2 shows the relationship between the fraction

recrystallized and the hot forging conditions, including other

test materials examined in the past. The judgment aboutwhether or not it was recrystallized was made using OMmicrographs after electrolytic etching. The fraction recrystal-lized increases sigmodially with the increasing Z parameter.The fraction recrystallized is almost zero when the Zparameter is 108 s¹1 or less, whereas it is almost 100% whenthe Z parameter is 1010 s¹1 or more.

3.2 SEM-EBSD micrographsIn Fig. 3, the grain boundary with an angle of 15° or more

is indicated with a thick line, whereas the grain boundarywith an angle of 2 to 15° is indicated with a thin line. High Z(2) shows recrystallized structures consisting mostly of highangle grain boundaries, where high angle (15° or more) grainboundaries account for about 98%. In addition, some coarsegrains with Y orientation and Z orientation were observed inHigh Z (2), which are usually observed in the surface layer ofrolled materials.9) Here, textures are displayed in the sameway as for rolling textures, where the compressive surfaceand the grain flow correspond to the rolled surface and therolling direction, respectively. Middle Z (2) and Low Z alsoconsist mostly of high angle grain boundaries, where theyaccount for about 75 and 85%, respectively. The grain size dL(ª = 15�) is 13 and 20 µm, respectively. The grain size dS(ª = 2�) is 10 and 14 µm, respectively. Either grain size is thesmallest in Middle Z (2), with intermediate Z parameteramong the 3 types of specimens. Middle Z (2) and Low Zform an ¡-orientation group from the Goss to the Brassorientation,9) which is usually observed in cold-rollingmaterials with high deformation. They are piled as high asabout 24% in Middle Z (2), compared to about 19% in LowZ. Table 3 shows the grain size measurement results alongwith various microstructure parameters described later.

Table 2 Testing conditions of hot forging, T: temperature, _¾0: initial strainrate, Z: Zener­Hollomon parameter.

Specimens T/K _¾0/s¹1 Z/s¹1

Low Z

773

2.7 © 10¹4 1.1 © 106

Middle Z(1) 2.7 © 10¹3 1.1 © 107

Middle Z(2) 2.7 © 10¹2 1.1 © 108

Middle Z(3) 2.7 © 10¹1 1.1 © 109

High Z(1) 698 2.7 © 10¹1 1.1 © 1010

High Z(2) 638 2.7 © 10¹1 1.1 © 1011

High Z(3) 583 2.7 © 10¹1 1.4 © 1012

Tensile direction (T)

Section for microstructural

charcterization (L-ST)

(L)

Flow direction (L)

(L)

Forged plate of

ϕ120mm×t26mm

Tnesile test piece

Fig. 1 Configuration of the tensile test piece and the section formicrostructural characterization with respect to the forged plate.

0

20

40

60

80

100

Z / s-1

Fra

ctio

n R

ecry

stal

lized

(%

)

Present study

Hosoda & Nakai2)

5 6 7 8 9 10 11 10 10 10 10 10 10 10 1012

Fig. 2 A fraction recrystallized as function of Z. Previous results2) are alsoshown.

Table 1 Chemical composition of the 6061alloy specimen in mass%. Composition range standardized by Aluminum Association is alsoindicated.

Alloys Si Fe Cu Mn Mg Cr Zn Ti

Specimen 0.74 0.22 0.23 ® 0.96 0.12 ® 0.02

Standard0.40

50:70.15

50:150.80 0.04

50:25 50:15¹0.80 ¹0.40 ¹1.20 ¹0.35

The Effect of Microstructure on Mechanical Properties of Forged 6061 Aluminum Alloy 115

3.3 Mechanical propertiesTable 4 shows the tensile characteristics of High Z, Middle

Z, and Low Z. The yield strength of Middle Z with finesubgrained structure is as high as 345 to 354MPa. Low Zwith slightly coarse subgrained structure shows 333MPa, andHigh Z with coarse recrystallized structure shows as low as308 to 310MPa.

3.4 Strengthening mechanismThe relationship between the yield strength · and grain

size d of the test materials, including High Z, Middle Z, andLow Z was analyzed following the Hall­Petch relation:

· ¼ ·0 þKyd�1=2 ð4Þ

where · is yield strength, ·0 is yield strength of the materialswith single crystal, Ky is the constant and d is the subgrainor grain sizes. The results are shown in Fig. 4. Solid lineshows the grain size dL (ª ² 15°) only, and broken line showsthe grain size dS (ª ² 2°). Figure 4 shows also the previousdata by Nakai and Eto10) using material with recrystallized

Low Z Middle Z

Brass (110)[1-12]

Goss (110)[001]

P (101)[-1-11]

PP (101)[-2-12]

RG (101)[-101]

RW (001)[-1-10]

SA (321)[-1-39]

SF (123)[-1-43]

Y (111)[-1-12]

Z (111)[0-11]

(2) High Z (2)L

ST

Fig. 3 Inverse pole figure maps of the three specimens. Grain boundary map is superimposed. : ª = 15�, : 15� > ª = 2�, where ª ismisorientation angle.

Table 3 Microstructural parameters of each specimen. dst: grain size in STdirection, ª: misorientation angle, s: Schmidt factor averaged based onvolume fraction of each grain.

Specimen

Opticalmicroscope

SEM-EBSD TEM

dST/µmdST/µm

s μ/cm¹2

ª = 2� ª = 15�

Low Z 14 14 20 0.432 7.2 © 108

Middle Z(1) ® 9 12 0.425 ®

Middle Z(2) 9 10 13 0.430 7.5 © 108

Middle Z(3) ® 9 12 0.426 ®

High Z(1) ® 71 75 0.452 ®

High Z(2) 121 123 141 0.447 6.1 © 108

High Z(3) ® 84 86 0.464 ®

Table 4 Mechanical properties of each specimen T6-tempered. ·u: ultimatetensile strength, ·0.2: 0.2% proof stress, ¤: elogation to failure.

·u/MPa ·0.2/MPa ¤ (%)

Low Z 364 333 15.6

Middle Z(1) 379 354 16.4

Middle Z(2) 373 346 15.9

Middle Z(3) 373 345 14.5

High Z(1) 327 310 16.0

High Z(2) 328 308 17.8

High Z(3) 339 308 17.5

280

300

320

340

360

380

0 5 10 15d -1/2 / mm-1/2

σ0.

2, σ

/MP

a

High Z (2)

Low Z

Present study

high angle grain (θlow and high angle grain (θ

Nakai & Eto10)

high angle grain

500 20 10d / μm

Middle Z (2)

15°)

2°)

100 50 5

Fig. 4 ·0.2 vs. d¹1/2 plots of each specimen. d represents dst measured bySEM-EBSD. Solid curve: ª = 15�, broken curve: ª = 2�. Previous data4)

of recrystallized 6061-T6 specimens are also shown.

M. Nakai and G. Itoh116

grains only. The yield strength of the material withrecrystallized grains only is less dependent on the grain size,and the increase in yield strength is as small as severalMPa even when the grain size is reduced to the minimum of15 µm (d¹1/2 = 8mm¹1/2). On the other hand, the yieldstrength of the material with subgrains is severely dependenton the grain size, even with high angle grain boundaries only.This means that high yield strength of subgrained 6061-T6alloy were due to subgrain strengthening. However, whenthe grain size is reduced to approximately 10 µm (d¹1/2 ²9mm¹1/2), the relationship between the yield strength andthe grain size (d¹1/2) is not linear, where the yield strengthincreases significantly with the slightly decreasing grain size,and it can no longer be represented by the Hall­Petchrelation.

The increase in yield strength by subgrains is oftenrepresented by the modified Hall­Petch relation (eq. (1)) asdescribed in the introduction. When m = 0.5 (p = 0), eq. (1)represents the Hall­Petch relation. ·0 is the yield stress of thematerial that does not contain any substructure. Accordingly,·0 was calculated using the relationship between the yieldstress and grain size of High Z mostly with recrystallizedgrain only. Firstly, the yield stress and grain size wereanalyzed using the Hall­Petch relation, and ·0 was calculatedwhen d¹1/2 = 0. Then, the slope m was determined froma double logarithmic plot of · ¹ ·0 = k · d¹m, using the ·0calculated above. Then, with this m, the yield stress and grainsize were analyzed using the relation of · ¹ ·0 = k · d¹m, and·0 was calculated when d¹m = 0. m and ·0 were calculatedin the same way until «m (n + 1) ¹ m(n)« < 0.01. n was 25times. The values of ·0 corresponding to the grain sizes dLand dS are 297 and 295MPa, respectively. For High Z (2),Middle Z (2) and Low Z, the yield stress and grain size wereanalyzed using the relation of · ¹ ·0 = k · d¹m, and plottedon a double logarithmic chart in Fig. 5. In the same way asshown in Fig. 4, solid line shows the grain size dL, andbroken line shows dS. For reference, Fig. 5 shows also therelated data in the previous studies.10­13)

In the previous studies, m is as large as 1.00 whendS ¯ 10 µm. Also, m is as large as 0.79 and 0.73, whend ¯ 20 µm in this study. It is believed that the strength of thesubgrain boundary increases with the decreasing subgrainsize. However, the orientation difference of the subgrainswith low angle grain boundaries is as small as severaldegrees. It is supposed that the dislocation tends to pass, butnot be piled at such boundaries. Therefore, it is difficult toattribute the increased yield strength in Low Z rather thanHigh Z, and further more in Middle Z, only to the growth ofsubgrains i.e., low angle grain boundaries. According to theboth studies by Nakai and Eto using the recrystallizedmaterial with high angle grain boundaries,10) and by Asadaand Yamamoto using the material with high angle grainboundaries,13) the results correspond to the Hall­Petchrelation as m = 0.5. Even when the data in this study isanalyzed in terms of the relationship (solid line) between theyield strength and the grain size dL with high angle grainboundaries only, m is as large as 0.79, when dL ¯ 20 µm, inthe same way as low angle grain is included. The recrystal-lized material used by Nakai and Eto was T6 tempered aftercold rolling. The solution heat treatment was by a rapid

heating process in a salt bath. Deformation textures are hardlydeveloped since recrystallization occurs in a high deforma-tion area around coarse second phase crystals. On the otherhand, deformation structures after hot forging remains in thedeveloped subgrain structure in Low Z and Middle Z, asshown in Fig. 2.

To take into consideration the effect of the texture on theyield strength, the relationship between the yield strengthand grain size was analyzed using the following relation(eq. (4)).

¸0CRSS ¼ ¸0CRSS0 þ k0 � dslip0�m0 ð5Þ¸0CRSS ¼ s � ·¸0CRSS0 ¼ s � ·0

Where, ¸ACRSS is the value equivalent to the critical resolvedshear stress, and s is the averaged Schmidt factor. The ¸ACRSSwas determined by multiplying the yield stress by theaveraged Schmidt factor. s is the averaged Schmidt factorin the tensile direction (LT direction) of each specimencalculated based on the volume fraction in each orientation,which is shown in Table 3. ¸ACRSS0 is the value equivalent tothe critical resolved shear stress of the material that does notcontain any substructure. kA and mA are the constants. dslipA isthe grain size in the slip direction. The grain sizes dl and dSare the values measured in the thickness direction (ST).Assuming that is the angle between the slip direction andthe tensile direction (LT), the grain size in the slip directionis represented as dLslipA = dL/cos ­ and dSslipA = dS/cos ­.Assume that ª is the angle between the normal line to the slipplane (111) of a grain and the tensile direction (LT). Thereare 4 equivalent slip planes: cos ª1, cos ª2, cos ª3, cos ª4, andthe maximum cos ª. cos ­ was determined based on therelation of s = cos ª · cos ­, and the grain sizes in the slipdirection dLslipA (= dL/cos ­) and dSslipA (= dS/cos ­) werecalculated. The crystal orientation {hkl} huvwi of a grain

1

10

100

1000

1 10 100 1000

m=1.00

m=0.79

σ 0.2 -σ 0 kd -m Present study

high angle boundary (θ low and high angle boundaries (θ

m=0.19

m=0.73

m=0.52m=0.21

2

1

m=0.48

low angle grain

McQueen et al.

Ball

Cotner & Tegart

high angle grain

Asada & Yamamoto

Nakai & Eto

1

1

1

5

σ0.

2−

0/ M

Pa

σ

15°)

2°)

Subgrain or Recrystallized Grain Diaeter, d / mμ

Fig. 5 Double logalithmic plots between ·0.2 ¹ ·0 vs. d, assuming that·0:2 � ·0 / d�m. ·0: stress derived from Hall­Petch relationship byextrapolating d¹1/2 ¼ 0. Previous data are also shown. Solid line: forrecrystallized grains. Broken line: for subgrains.

The Effect of Microstructure on Mechanical Properties of Forged 6061 Aluminum Alloy 117

representing each specimen was assumed to be the crystalorientation with the highest strength distribution by the ODFanalysis. The {hkl} plane is parallel to the forging surfacecorresponding to the rolling surface, whereas the huvwidirection is parallel to the tensile direction (LT). As shown inTable 3, the averaged Schmidt factor is low for Middle Zwith high yield strength, but high in High Z for low yieldstrength, as 0.464 for High Z(2), 0.426 for Middle Z (2),and 0.432 for Low Z.

Figure 6 shows the relationship between ¸ACRSS and dA(grain sizes in the slip direction) in a double logarithmicchart. Where, m is lower than the Hall­Petch relation as 0.39,but not as high as about 0.7. With high angle grainboundaries only, m corresponds almost to the Hall­Petchrelation as 0.47. Consequently, it is supposed that the lowestyield strength (LT direction) in High Z, and increasinglyhigher yield strength in Low Z and Middle Z in this orderare caused by the development of the texture correspondingto low Schmidt factor (LT direction), since deformationstructures after hot forging remain due to continuousrecrystallization. The development of such textures relatesto the formation of subgrains, and further fine subgrainedstructures. ¸ACRSS in consideration of the texture is lessdependent on the grain size.

3.5 Fracture toughnessAs shown in Fig. 7, estimated fracture toughness Kc of

subgrained Middle Z(2) and Low Z(2) were higher than forrecrystallized High Z. Kc was calculated by eq. (3). Thefracture toughness of three materials, recrystallized High Z,subgrained Middle Z(2) and Low Z(2) were 39.0, 40.5,40.0MPa·m0.5, respectively. In conventional alloys, if yieldstrength heightens, fracture toughness is lowered. But thesubgrained materials lead to both high strength and highfracture toughness.

4. Conclusions

The results of examining and analyzing the relationshipbetween the microstructure and yield strength after T6tempering, using the specimens of 6061 alloy manufacturedat various levels of temperature and strain rate during hotforging, are as follows.(1) When the Z parameter was 1.1 © 106, 1.1 © 108, and

1.1 © 1011 s¹1, the grain size (SEM-EBSD) was 14 µm,10 µm, 123 µm, and the yield strength was 333, 346,and 308MPa, respectively. The smallest grain size dand the highest yield strength were achieved whenusing the intermediate Z parameter during hot forging.

(2) The yield strength of the material with recrystallizedgrain structures was less dependent on the grain size.On the other hand, the yield strength of the materialwith fine grains containing subgrain boundaries wasseverely dependent on the grain size, roughly inaccordance with the previous data. Accordingly, finesubgrain strengthening appeared to be more effectivethan fine recrystallized grain strengthening.

(3) However, when the grain size is reduced to approx-imately 10 µm, the relationship between the yieldstrength and the grain size (d¹1/2) is not linear, wherethe yield strength increases significantly with theslightly decreasing grain size, and it can no longer berepresented by the Hall­Petch relation.

(4) In consideration of the effect of the texture on the yieldstrength using Schmidt factor, ¸Acrss was less dependenton dA. The yield strength of 6061-T6 is essentially lessdependent on the grain size as reported previously.

(5) Fracture toughness of subgrained materials were higherthan for recrystallized. The subgrained materials havehigh yield strength combined with high fracturetoughness.

1

10

100

1000

1 10 100 1000

τ'C

RS

S- τ

' CR

SS

0 /

MP

a

τ ' CRSS - τ 'CRSS0 ∝ k'd ' -m'

Present study

high angle boundary (θ low and high angle boundaries

(θ

m' =0.36m'=0.39

m'=0.34

m'=0.47

3

2

1

1

τ CRSS0 = sσ 0

τ '

'

CRSS = sσ 0.2 15°)

2°)

d ' / mμ

Fig. 6 Double logarithmic plots between ¸ACRSS ¹ ¸ACRSS0 and dA, assumingthat s � ð·0:2 � ·0Þ / d0�m0

. s: average Schmidt factor, ¸ACRSS ¹ ¸ACRSS0 isequal to s · (·0.2 ¹ ·0). dA: grain size in the slip direction. Solid line: forrecrystallized grains. Broken line: for subgrains.

38

39

40

41

42

280 300 320 340 360σ 0.2 / MPa

Kc

/ M

Pam

1/2

High Z (2)

Low Z

Sub-grained

recystallized

Middle Z (2)

Fig. 7 Kc vs. ·0.2 plots of each specimen. The fracture toughnessKc = 36.6 © NTR ¹ 20.82, NTR: Notch tensile strength/Tensile yieldstrength Ratio.

M. Nakai and G. Itoh118

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The Effect of Microstructure on Mechanical Properties of Forged 6061 Aluminum Alloy 119