技術論文

High Strain Rate Blow Formabilityof Newly Developed Al-Mg-High-Mn Alloy
Tomoyuki Kudo

Akira Goto

Kazuya Saito

Abstract : Blow forming accompanied with superplasticity makes possible the forming of complex
parts, which cannot be formed by cold press forming. The conventional superplastic AA5083 alloy
ALNOVI®-1 developed by the Furukawa-Sky Aluminum Corp. shows high superplasticity becauseof its fine grain and is widely used for blow forming. However, for mass production of components,
an Al-Mg alloy with finer-sized grains is needed. In this research, the newly developed high Mn
version of the Al-Mg alloy ALNOVI-U is used, and this material possesses grains finer than thoseof the conventional AA5083 alloy. The effects of finer grain size on the blow formability at high strain
rates over 10-2/s and the properties of the resulting moldings were studied.

１. Introduction

potential of ALNOVI-U was evaluated by investigating in

The 2000 and 7000 series aluminum alloys, which have fineproperties, blow formability, and properties of moldings formed

grains and show high superplasticity, are used for special

detail the effects of finer grain size on high-temperature tensile
at high strain rates (10-2-10-1/s).

components such as those used in aircraft, and are used asmaterials for blow forming. It is relatively difficult to ensure
fine grains in the 5000 series aluminum alloys and the

２. Experimental

superplasticity is slightly inferior. However, the 5000 series

2.1 Materials

aluminum alloys have high corrosion resistance and

The chemical composition of the tested alloy ALNOVI-U

weldability; therefore, these alloys are in great demand as

used in the experiments and AA5083 alloy are shown inmaterials for blow forming common components. The

Table 1. The sample alloy was subjected to DC-casting,

Furukawa-Sky Aluminum Corp. has developed the superplastic

homogenization, and hot and cold rolling. The thickness ofAA5083 alloy ‘ALNOVI-1’ with a fine grain size of 10 μm orcold rolled sheet was 0.5 mm. The precipitates in the tested

less by a proprietary process and ALNOVI-1 is widely usedalloys and AA5083 are shown in Fig. 1. The precipitates in

globally. ALNOVI-1 shows not only high superplasticity at low

ALNOVI-U are more densely distributed because the amountstrain rates but also good elongation at high strain rates, as isof additive Mn is higher than that in AA5083. It is thought

required for high productivity. However, there was a need for athat these precipitates contribute to the decrease in grain size

material suitable for higher strain rate blow forming, because

by stabilizing grain boundaries and acting as nucleation sites

the 5000 series aluminum alloys came to be used for mass

for recrystallization. The cold rolled sheet of ALNOVI-U was

production items. Therefore, the Furukawa-Sky Aluminum
Corp. developed the Al-Mg-Mn alloy ‘ALNOVI-U’ which

Table 1 Chemical composition.

(mass%)

allows for grains to be made 2 μm smaller by adjusting theMaterial

Si

Fe

Mn

Mg

amount of additive Mn1) and attempts were made to enhance

ALNOVI-U

0.03

0.05

1.42

4.75

elongation at high strain rates (10-2-10-1/s). In this study, theAA5083

0.03

0.04

0.73

4.70

Furukawa-Sky Review No.9 201311

High Strain Rate Blow Formability of Newly Developed Al-Mg-High-Mn Alloyannealed and the grain size of the sheet’s alloy were

2.3 Blow Forming tests

manipulated to be 6 µm; this is finer than that of AA5083, 9

After the sheets were inserted between the mold plates,

μm which is equivalent to that of AA5083 and 13 μm which isheated in the blow forming machine, and kept hot for 10 min,coarser than that of AA5083. These samples are called ‘6 µm’

blow forming by N2 gas was conducted. The mold shape was

sample, ‘9 μm’ sample, ‘13 μm’ sample respectively in thissimilar to that of an automobile quarter panel. The blow

paper. The grain structures are shown in Fig. 2.

forming machine used in the experiments can exert high2.2 High temperature tensile tests

pressure in a few seconds and is therefore suitable for highTensile tests were conducted at 663 K, 703 K and 743 K. The

strain rate blow forming. In this research, high strain rate blow

deformation resistance and elongation at high temperatures

forming tests that are quicker than conventional tests were

-3

were measured by tensile tests at constant strain rates of 10 ,conducted; the forming times were 5, 10, 15, and 20 s. The

10

and 10 /s. The deformation mechanisms at highgas pressure in these tests was constant at 3 MPa, and the-2-1

temperatures were studied by analyzing the results of thestrain rate change tensile tests in the range 10-4 to 10-1/s.

temperatures were 663 K, 703 K and 743 K.
2.4 Molding Properties(a) Appearance
The moldings were inspected for breaks. In addition, themolding accuracy was evaluated by measuring the radius ofcurvature at the point that is formed at the end of the forming(a)

0.5 μm(b)

20μμm

as shown in Fig. 3.
(b) Cavitation
For evaluating cavitation volume, sections with different
thickness reductions were selected from the moldings andpolished. The cavitation area ratio was determined using an
area analyzer. In addition, the strain distribution in grains(c)

0.5 μm(d)

20 μm

around the cavitations was observed by electron backscatter
diffraction (EBSD).
(c) Grain structure and strength after forming
The grain structures in the selected sections were observedby Barker’s method and the initial and final grain sizes were
measured. In addition, the Vickers hardness of these sections

Fig. 1 Precipitates in ALNOVI-U and AA5083.
(a) Fine precipitates observed by TEM in ALNOVI-U.
(b) Coarse precipitates observed by SEM in
ALNOVI-U.
(c) Fine precipitates observed by TEM in AA5083.
(d) Coarse precipitates observed by SEM in AA5083.

was measured.

40 μm
Measured point(a) 6 µm(b) 9 µm(c) 13 µm

Fig. 2 Grain structures of ALNOVI-U manipulated byannealing. They were subjected to the experiments.

12

Furukawa-Sky Review No.9 2013

Molding

Fig. 3 Molding and measured point of curvature radius.

High Strain Rate Blow Formability of Newly Developed Al-Mg-High-Mn Alloyof 10-1/s. However, the 6 µm sample showed a larger elongation

３. Results and Discussion

than the other samples even at strain rates in the range 10-2–
10-1/s, which are higher than those used in conventional blow

3.1 Deformation resistance and elongation at highforming.

temperature
The stress-strain curves for constant strain rates are shown

3.2 Discussion of deformation mechanism at highin Fig. 4. At any strain rate, the deformation resistance of thetemperature

6 µm sample was the smallest. Comparing the characteristics

The relation between stress and strain rates, as deducedof the S-S curves, in particular, for the 9 µm and 13 µm

from measurements made in the course of strain rate change

samples, a large peak was observed in early deformation andtensile tests, are shown in Fig. 6. The slope m indicates thethe deformation resistance increased with the strain rates. In

strain rate sensitivity exponent. In each sample, m wascontrast, for the 6 µm sample the peak was small, even at

approximately 0.5 at low strain rates and 0.3 at high strainstrain rates of 10 /s. In addition, in the 6 µm sample, therates. The relevant equation at high temperature is represented

deformation resistance was small and the typical S-S curves inby Eq. (1)2), where, ε̇ is the strain rate, A is a constant, k is thegrain boundary sliding situations, which have a large n value,

Boltzmann constant, T is the temperature, G is the shear

were remarkably visible, even at strain rates of 10 /s.

modulus, b is the Burgers vector, d is the grain size, σ is the-2-2

The elongations of each sample are shown in Fig. 5. The

flow stress, p is the grain size exponent, D0 is the diffusion

sample with finer grains showed greater elongation, especiallyconstant, Q is the activation energy, and R is the gas constant.

at strain rates lower than 10 /s. The elongation of each sample

The m and p values of each deformation mechanism areconverged at lower temperatures or higher strain rates. The

summarized in Table 2 2)-5). From the values listed in Table 2,

difference between the elongations of the 9 µm and 13 µm

we deduced that grain boundary sliding occurs at low strain

samples decreased at strain rates greater than 10 /s.

rates in the area of m = 0.5, and solute drag creep occurs atFurthermore, the effect of grain size diminished at strain rates

high strain rates in the area of m = 0.3. It is assumed that in

-2

-2

80

(a)

(b)

13 μm

True stress,σt /MPa

70

(c)

9 μm

60
50

13 μm
9 μm

40

13 μm
9 μm

30

6 μm

6 μm

20

6 μm

10
0

0

0.5

1

1.5

0

0.5

True strain,εt

1

1.5

0

0.5

True strain,εt1

1.5

True strain,εtFig. 4 S-S Curves measured by constant strain rate tensile tests (703 K).
(a) 10-3/s, (b) 10-2/s, (c) 10-1/s.
400

(a)

Elongation (%)

350

(b)

300

9 μm

250

6 μm

(c)

6 μm

6 μm

9 μm

9 μm

200
150

50
0

10 -3

13 μm

13 μm

13 μm

100

10 -2

Strain rate,ε/s

-1

10 -1

10 -3

10 -2

Strain rate,ε/s

10 -1

10 -3

10 -2

Strain rate,ε/s

-1

10 -1

-1

Fig. 5 Elongation as measured in constant strain rate tensile tests.
(a) 663 K, (b) 703 K, (c) 743 K.

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High Strain Rate Blow Formability of Newly Developed Al-Mg-High-Mn Alloy
2
1.8

(a)

(b)

1.6

(c)

m 0.3

1.4

m 0.3

m 0.3

logσt

1.2
1
0.8

6 μm

m 0.5

0.6

m 0.5

m 0.5

9 μm
13 μm

0.4
0.2
0
-5

-4

-3

-2

-1

logε

-5

0

-4

-3

logε

-2

-1

0

-5

-4

-3

-2

logε

-1

0

Fig. 6 Strain rate sensitivity exponents measured by strain rate change tensile tests.
(a) 663 K, (b) 703 K, (c) 743 K.

Table 2 m and p value of deformation mechanism at hightemperature.2)-5)mbecause p is 0. It is thought that the reason why elongation ofpeach sample converged at a strain rate 10 -1 /s is that the

Nabarro-Herring creep

1

2

deformation mechanism of whole materials become one of

Coble creep

1

3

Grain-boundary

Lattice diffusion

0.5

2

solute drag creep.

sliding

Grain-boundary diffusion

0.5

3

Dislocation creep

Harper-Dorn creep

10contribution of grain boundary sliding is made larger andSolute drag creep

0.33

0

therefore large elongation and low deformation resistance isClimb-controlled creep

0.2

0shown, even at low temperature and high strain rates, by using

Diffusional creep

From the results of the test pieces described above, the

ALNOVI-U. It is expected that AlNOVI-U will enables thethis alloy, deformation proceeds with interaction between theblow forming temperature and time to be reduced. The

dislocation and the solute Mg. in the area of m = 0.3. It isformabilities and properties of moldings obtained in highfound that grain boundary sliding occurs even at higher strainstrain rate blow forming tests are shown below.

rates at higher temperature or finer grain size (see Fig. 5). The

3.3 Appearance

shift in the value of strain rate at which grain boundary sliding

The radius of curvature at the point formed in the end of theoccurs when the grain size becomes finer from 9 µm to 6 µm

forming process as a function of the forming time is shown in

is simply estimated by Eq. (1). It is found that it is shifted toFig. 7. The symbol × signifies that breaking occurred in thean approximately 2.3-3.4 times higher strain rate because p ismoldings. A small radius of curvature means that sharp edges

2-3 (see Table. 2).

can form faster and that there is good molding accuracy as

Gb
.
ɛ=A
kT

b
d

P

σ

1

shown in Fig. 8. Breaking was shown by the 13 µm sample asmG

D0 exp −

Q
RT

(1)

forming proceeded, but not in the 9 µm and 6 µm samples.
From this result, it is confirmed that the samples of finer grain
size have larger ductility, even at the high strain rates as those

The 6 µm sample shows the smallest deformation resistanceused in the forming tests. That corresponds to relationand the largest elongation even at high strain rates in the range

between grain size and elongation shown in Fig. 5. And it is

10 -10 /s because grain boundary sliding contributes atfound that the finer grains enable to form sharp edges faster orhigher strain rates in this sample. However, the contribution of

at lower temperature as shown in Fig. 7. This can be

grain boundary sliding is small and that of solute drag creep is

accounted for by the deformation resistance being smaller inlarge at strain rates over 10-2/s in coarse grained samples. It isthe case of finer grains, even at high strain rates as shown inthought that the large peak shown in the S-S curves are

Fig. 4.

-2-1

generated by interactions between dislocations and solute Mg,

3.4 Cavitation

causing an increase in deformation resistance at highCavitation area ratio as a function of thickness reduction oftemperature. From Eq. (1) and Table. 2, grain boundary

each part in the molding is shown in Fig. 9. Cavitation

sliding is affected by grain size d and solute drag creep is not,increased with thickness reduction, but the 6 µm sample

14

Furukawa-Sky Review No.9 2013

High Strain Rate Blow Formability of Newly Developed Al-Mg-High-Mn Alloy

(a)

7

(b)

D

1.8

6

Break

5
3

6 μm

2

9 μm

1

13 μm
0

5

10

Break

15

20 0

5

10

15

A

1.6

4

0

2.0

(c)

20 0

5

10

15

20

Forming time, t/s-1Fig. 7 Radius of curvature against forming time.
(a) 663 K, (b) 703 K, (c) 743 K.

Cavitation area ratio(%)

Radius of curvature, R/mm

8

B

1.2

C
D

1.0
0.8

Measured
point

0.6

9 μm
C

0.4
0.2

Large

13 μm

1.4

0.0

B

A

6 μm

0

20

40

60

80

Thickness reduction(%)

Fig. 9 Cavitation area ratio against thickness reduction.
(Forming time is 10 s and forming temperature is 703 K.)

40 μm
Radius ofcurvature

6 μm

(a)
Measured
point

9 μm

13 μm

Small

Fig. 8 Radius of curvature and appearance of molding.

showed the lowest values. Fig. 10 (a) shows the section at point
D in Fig. 9 observed by a optical microscope. It is found that

15 μm

(b)

fewer cavitations were generated at finer grain sizes. Fig. 10 (b)shows an Image Quality map of the 6 µm and 13 µm samples
observed by EBSD and it represents the average strain in thegrains by means of a gray scale. It is confirmed that initialcavitations were generated on the triple junctions of grainboundaries by grain boundary sliding. In the 13 µm sample,
the largely stretched grains accepting intragranular

6 μm

13 μm

Fig. 10 Cavitation of each sample.
(a) Image observed by optical microscope.
(b) Image quality map observed by EBSD.

deformation are shown and it is assumed that the initialcavitations were combined and became larger along the grain

addition, initial cavitations tend to be larger because ofboundaries. It is said that initial cavitations are generated byintragranular deformation. It is thought that initial cavitations dostress concentration on the triple junctions of grain boundaries

not tend to be larger because the contribution of intragranularor the interfaces of coarse precipitates .

deformation is relatively small, and therefore the cavitation

6)

However, it is thought that the distribution of precipitatesarea ratio in the 6 µm sample is the lowest.

are not largely difference because the chemical compositions

3.5 Grain structure and strength after formingof these samples are the same. Therefore, considering only the

The relation between thickness reduction and grain size

effect of grain size, it is assumed that initial cavitations tend toafter forming is shown in Fig. 11. Point A represents the grain

be generated because stress concentration occurs on the triple

size not deformed, points B, C and D represent grain sizes atjunctions of grain boundaries in the 13 µm sample and, inabout the same points of B, C and D in Fig. 9 respectively.
Furukawa-Sky Review No.9 201315

High Strain Rate Blow Formability of Newly Developed Al-Mg-High-Mn Alloy
86

14.0

A

84

B

Grain size, d/μm

12.0

82

13 μm
Vickers hardness /HV

13.0

11.0
10.0

C

9 μm

D

9.0
8.0
7.0

6 μm

6.0

78
76
74
72
70

5.0
4.0

40 μm

80

68
0

10

20

30

40

50

60

70

Thickness reduction(%)

Fig. 11 Grain size after forming.
(Forming time is 10 sec and formin temperature is703 K.)

66

6 μm

9 μm

13 μm

Fig. 12 Vickers hardness and microstructures of moldings
against initial grain size.

The grain sizes after forming were remarkably made finer asof an automobile quarter panel model, breaking was not

the initial grain size became coarser. The plots as the grain

generated and the mold was able to form faster and at

size becomes slightly larger are shown and the tendency forlower temperatures for the 6 µm sample.

refinement of the grain size is less in the 6 µm and 9 µm
samples. It is considered that this effect is due to the grain size

• The molding made by the 6 µm sample showed the least
cavitation and the largest hardness.

being made finer by dynamic recrystallization because theFrom the above, it is thought that ALNOVI-U facilitates

contribution of intragranular deformation is larger as the grain

blow forming at lower temperatures and shorter forming

size increases. The reason why the grain size becomes slightly

times, thereby rationalizing production and is, in addition,

larger in fine grain samples is thought to arise from theable to achieve the fabrication of components that are lightertransfer of grain boundary, occurring by an accommodation

but which have high strength. Effectiveness of ALNOVI-U toprocess. However, the grain size after forming was finer as thethe automobile panel production was evaluated by Honda

initial grain size was finer (see Fig. 11). Fig. 12 shows the

R&D Co., Ltd. and Honda Engineering Co., Ltd.

Vickers hardness of point B. The 6 µm sample has the largest
hardness because it has the finest grain (Hall-Petch relationship).

４. Summary
ALNOVI-U containing high Mn, which has a grain size of
6 µm was used, and its high-temperature tensile properties,
high strain rate blow formability, and properties ofcomponents in the samples with grain sizes of 6 µm, 9 µm,
and 13 µm were studied. The temperatures in the tests
conducted for this study were 663 K, 703 K and 743 K; thestrain rates were greater than 10-2/s, and the following results
were obtained.
• In the high temperature tensile tests, the sample which
involves the finest grain size of 6 µm showed the highest
elongation and the smallest deformation resistancebecause grain boundary sliding worked effectively evenat strain rates in the range 10-2-10-1/s.
• Even in the high strain rate blow forming tests, involving
a forming time of 10 s using the mold having the shape
16

Furukawa-Sky Review No.9 2013This article was published in Materials Science Forum, 735(2013), 271. (www.scientifc.net)
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1) K. Ichitani and T. Tagata: Materials Science Forum, 539-543(2007), 357.
2) H. Somekawa, H. Watanabe, M. Kohzu and K. Higashi: Materials
Science Forum, 419-422 (2003), 393.
3) K. Kitazono: Journal of Japan Institute of Light Metals, 59 (2009),
458.
4) T. Ito, M. Ishikawa, M. Otsuka, M. Saga and M. Kikuchi: Journal ofJapan Institute of Light Metals, 53 (2003), 114.
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Higashi: Journal of Japan Institute of Light Metals, 49 (1999), 57.

High Strain Rate Blow Formability of Newly Developed Al-Mg-High-Mn Alloy

Tomoyuki Kudo
Technical Research Division
Furukawa-Sky Aluminum Corp.

Akira Goto
Automobile R&D Center
Honda R&D Co., Ltd.

Kazuya Saito
Body Die & Molding Engineering Division
Honda Engineering Co., Ltd.

Furukawa-Sky Review No.9 201317