• No results found

Tensile Properties and Blow Forming of 5083 Aluminum Alloy Recycled by Solid State Recycling

N/A
N/A
Protected

Academic year: 2020

Share "Tensile Properties and Blow Forming of 5083 Aluminum Alloy Recycled by Solid State Recycling"

Copied!
7
0
0

Loading.... (view fulltext now)

Full text

(1)

Tensile Properties and Blow Forming of 5083 Aluminum Alloy

Recycled by Solid-State Recycling

Yasumasa Chino

1

, Mamoru Mabuchi

2

, Hajime Iwasaki

1

,

Atsushi Yamamoto

3

and Harushige Tsubakino

3

1Institute for Structural and Engineering Materials, National Institute of Advanced Industrial Science and Technology,

Nagoya 463-8560, Japan

2Department of Energy Science and Technology, Kyoto University, Kyoto 606-8501, Japan

3Division of Materials Science and Engineering, Graduate School of Himeji Institute of Technology, Himeji 671-2201, Japan

The tensile properties and blow forming characteristics of 5083 Al alloy recycled by solid-state recycling were investigated from the viewpoint of oxide contamination. Three types of machined chip with different volumes were recycled by hot extrusion and hot rolling in air. Oxide layers, which were contaminants from the machined chip surface, were distributed in the extrusion direction for the recycled specimens. Oxygen concentration in the recycled specimens increased with the total surface area of the machined chips per unit volume. From the result of the tensile tests performed at 773 K, the elongation to failure of the specimen made of smaller machined chips was lower, than that of the specimens made of larger machined chips, in spite of their same strain rate sensitivity of 0.5. Similarly, in the blow-forming tests at 773 K, the specimen made of smaller machined chips exhibited a lower formability. The low elongation to failure and formability of the recycled specimens made of smaller machined chips are likely to be attributed to a greater contamination of oxide. Thus, oxide contamination has a detrimental effect on the superplastic properties of recycled Al alloy.

(Received February 5, 2004; Accepted May 26, 2004)

Keywords: aluminum alloy, solid-state recycling, tensile test, superplasticity, blow forming

1. Introduction

For the reduction in CO2 emission produced by vehicles,

the application of Al alloy to the structural components of vehicles is increasingly becoming important due to the high specific strength of this alloy.1)Currently, Al alloy ranks the

second metal in term of consumption around the world,2)and

hence its recycling is an important technology for materials circulation.

The energy for recycling by remelting aluminum scraps is about104J/t, which is only 5% of the smelting energy from

bauxite.3) Hence, aluminum scraps should be recycled to

reduce environmental loads. Actually, the recycling of aluminum scraps is being carried out by some recycling processes based on remelting.3–6)

Effective recycling entails the production of high-perform-ance materials from scraps with low energy consumption. Solid-state recycling without remelting7–14) is a low-energy process. In this recycling, metal scraps are directly recycled by hot extrusion and other thermo-mechanical treatments. In our previous study,13) it was revealed that the solid-state

recycled 5083 Al alloy from machined chips showed a high strength due to grain refinement. Thus, solid-state recycling is a promising process for metal scraps.

Recently, superplastic blow forming has become one of the major near-net-shape forming processes for 5083 Al sheets.15,16)Because the solid-state recycled 5083 Al alloy

possesses a fine-grained microstructure, superplastic blow forming may be realized for solid recycled Al alloy. However, our previous study13)showed that the elongation to failure of a solid-state recycled Al alloy is lower than that of an extruded specimen processed from an as-cast ingot, particularly, at elevated temperatures. Oxide contamination is responsible for the poor formability of the solid recycled Al

alloy. In the present study, three types of 5083 Al alloy machined chip with different volumes are recycled by hot extrusion and hot rolling, and the tensile properties and blow forming characteristics of the solid recycled Al alloy are investigated from the viewpoint of oxide contamination.

2. Experimental Procedure

Chips were prepared as scraps by machining an as-received 5083 aluminum alloy block in a lathe without lubricants. Three types of machined chip with different volumes were prepared as shown in Fig. 1. The average volume, thickness and width of the machined chips are summarized in Table 1. The scraps cleaned by acetone were filled into a rectangular container with cross-sectional dimensions of 50mm30mm and then extruded at 723 K at an extrusion ratio of 6:1 in air. The cross-sectional dimensions of the extruded bar was 50mm5mm. For comparison, an as-received 5083 Al alloy block was also extruded under the same conditions as those of the extrusions of the scraps. After the extruded bars were annealed at 803 K for3:6104s, they were rolled at room temperature up to a

rolling ratio of 80%. Finally, the rolled specimens were annealed at 593 K for7:2103s. The rolling direction was

perpendicular to the extrusion direction (see Fig. 2). In the present study, the specimens recycled from the three types of machined chip and specimens from a virgin ingot are called the ‘‘specimen from small machined chips’’, ‘‘specimen from medium-sized machined chips’’, ‘‘specimen from large machined chips’’ and ‘‘virgin specimen’’, respectively.

The microstructure of the specimens was observed by OM (optical microscopy). The image of oxygen in the specimens was detected by EPMA (electron probe microanalyzer). Oxygen concentration in the specimens was measured by the

(2)

infrared absorption method. Tensile specimens of 10 mm gage length, 5 mm gage breadth and 1 mm gage thickness were machined. Tensile tests were carried out from room temperature to 773 K at initial strain rates of104102s1,

where the angle between the tensile direction and the rolling direction was 0 degrees. In addition, step strain rate tests were carried out at 773 K at strain rates of 105–101s1 to analyze strain rate sensitivity.

Discs of 70 mm diameter were machined in the blow forming tests. The specimens were clamped between two hollow dies and subjected to N2 gas pressure at 773 K to

induce bulging. Cups of 20 mm height were formed in a die cavity, which was 40 mm in diameter.

3. Results and Discussion

3.1 Microstructure and oxide contamination

The microstructures of the specimen from small machined chips and the virgin specimen are shown in Fig. 3, where the RD-TD and TD-ND planes are observed, respectively. The grain sizes were 11.5mm for the specimen from small machined chips, 13.6mm for the specimen from medium-sized machined chips, 14.7mmfor the specimen from large

machined chips and 13.4mmfor the virgin specimen, where grain size was measured on the RD-TD plane. It is known that dynamic recrystallization occurs during hot extrusion for Al-Mg aluminum alloys.17–19) In the as-received specimen before machining, the grain size was 390mm. It is therefore suggested that the fine-grained microstructures of recycled specimens are attributed to dynamic recrystallization during hot working. There was no difference in grain size between the specimens from machined chips and the virgin specimen. This indicates that an oxide contamination had no effect on recrystallization.

It is notable that black (dotted) lines in the specimen from small machined chips were observed parallel to the TD-ND plane, that is, the extrusion direction. The black (dotted) lines denote the oxide films of machined chips, as will be shown later. The interval of the black (dotted) lines was approx-imately 5 to 7mm for the specimen from small machined chips.

Figure 4 shows the SE (secondary electron) and oxygen

(a)

(b)

(c)

[image:2.595.91.519.65.493.2]

10mm

Table 1 Configurations of 5083 Al machined chips used for solid-state recycling.

Raw materials Average width (mm)

Average thickness (mm)

Average weight (g) Small

machined chips 1.4 0.2 4:910

1

Medium-sized

machined chips 3.0 1.2 1:310

1

Large

machined chips 4.5 1.8 3:410

2

Rolling

Direction

RD

ND

TD

TD-ND

plane

Extrusion Direction

RD-TD

plane

ND-RD

plane

Fig. 2 Schematic view of relationship between extrusion and rolling directions on the solid-state recycling.

[image:2.595.107.291.75.352.2] [image:2.595.47.291.396.490.2]
(3)

RD-TD plane

Virgin

specimen

Specimen from

small machined

chips

TD-ND plane

40µm

40µm 40µm

40µm

Fig. 3 Microstructures of 5083 Al alloy specimens.

Oxygen image

Virgin specimen

(TD-ND plane)

Specimen from

small machined

chips

(TD-ND plane)

Extrusion direction

Rolling direction

40

µ

m

40

µ

m

40

µ

m

40

µ

m

SE image

[image:3.595.78.519.75.350.2] [image:3.595.82.514.409.737.2]
(4)
[image:4.595.320.533.75.190.2]

images obtained by EPMA for the specimen from small machined chips and the virgin specimen, where the TD-ND plane is observed. In the specimen from small machined chips, the distinct peaks of oxygen were aligned to the extrusion direction. The interval of the distinct oxygen peaks was approximately 5mm to 10mm, which is in agreement with the interval of the black (dotted) lines shown in Fig. 3. Therefore, it is likely that the black (dotted) lines shown in Fig. 3 correspond to the oxide films contaminated during hot extrusion.

The oxide contaminants in the recycled specimens are considered to be introduced from the machined-chip surface. The total surface area of the machined chips introduced in the recycled specimen was estimated from the geometry of the machined chips listed in Table 1, assuming that the machined chips is rectangular. Figure 5 shows the relationship between the oxygen concentration of the recycled specimens and the total surface area of the machined chips in the recycled specimen per unit volume. The oxygen concentration increased with the total surface area of the machined chips in the recycled specimen, indicating that the size of the machined chips is one of the important factors in the control of the contamination level of oxides in solid-state recycling.

3.2 Tensile properties

Table 2 shows the tensile properties of the recycled specimens at room temperature. The recycled specimens showed a good combination of high ultimate tensile strength, high 0.2% proof stress and high elongation to failure. These

properties of the recycled specimens are almost the same as those of the virgin specimen. In general, the relationship between yield stress and grain size can be given by the Hall-Petch relation

y¼0þKd1=2; ð1Þ

where y is the yield stress for a polycrystalline, 0 is the

yield stress for a single crystal,d is the grain size andKis a constant depending on the type of material. The Hall-Petch relation between the 0.2% proof stress at room temperature and the grain size for 5083 alloy is shown in Fig. 6, where0

and K are 150 MPa20) and 63 MPamm1=2,21) respectively. For reference, the data in our previous research13) are included in Fig. 6. It can be seen that the data fit a line, suggesting that the dispersed oxides have little effect on the strength of the recycled specimens.

The variation in ultimate tensile strength and elongation to failure at1:7103s1are shown in Fig. 7 as a function of

[image:4.595.48.287.452.595.2]

testing temperature. The tensile strength of the recycled specimens from the machined chips was almost the same as that of the virgin specimen. However, the elongation to failure of the specimens from the machined chips was lower than that of the virgin specimen at elevated temperatures of more than 573 K. It should be noted that contamination of oxides had a detrimental effect on elongation at elevated temperatures, not at room temperature.

Figure 8 shows the variation in elongation to failure at 773 K as a function of strain rate. A large elongation more than 200% was attained at104s1 for the specimens from medium-sized machined chips and large machined chips. However, the specimen from small machined chips showed a lower elongation of 117% at104s1. As shown in Fig. 8,

elongation decreases with increasing oxygen concentration. Cavity nucleation is stimulated due to the oxides introduced from the machined-chip surface, resulting in premature fracture in the solid recycled specimen.11) Therefore, it is

suggested that a large amount of oxide contaminants in the specimen from small machined chips causes excessive cavity formation, resulting in a lower elongation.

The variation in flow stress at 773 K as a function of strain rate for all the specimens is shown in Fig. 9. A high strain rate sensitivity of 0.5 was obtained in the strain rate range from

104 to103s1 for all the specimens. Hence, superplastic behavior due to grain boundary sliding is considered to occur for all the specimens, regardless of the oxygen concentration

0 0.02 0.04 0.06 0.08

3 × 103

Oxygen Concentration (mass%)

T

otal surf

ace area of machined chips

in the rec

ycled specimen of 1m

3 (m 2)

y=1.7 × 105x + 83 R2= 0.99

Small machined chip Medium-sized machined chip Large machined chip

6 × 103 6 × 103 12 × 103 15 × 103

Virgin specimen

Fig. 5 Relationship between total surface area of machined chips per 1 m3

[image:4.595.45.290.676.787.2]

for and oxygen concentration for 5083 Al alloy specimens.

Table 2 Tensile properties at room temperature of 5083 Al alloy speci-mens.

Raw materials Ultimate tensile strength (MPa) 0.2% Proof stress (MPa) Elongation to failure (%) Small

machined chips 307 169 15

Medium-sized

machined chips 306 167 16

Large

machined chips 305 167 16

Virgin specimen 319 162 16

0 0.1 0.2 0.3 0.4 0.5

100 200 300

5083 Al alloy 22,23)

σy=150+63d -1/2 (MPa)

: This work : Chino et al. 14)

0.2% proof stress ,

σ0.2%

/ MP

a

d

/

1 / µm-0.5

(grain size)-1/2,

0.6 250

150

50

(5)

in the range investigated.

Another important result in Fig. 9 is that at a low strain rate below 104s1, stress depended on oxygen concentration

and strain rate sensitivity decreased with increasing oxygen concentration. This is likely to be associated with the presence of threshold stress. Threshold stress can be estimated by extrapolation to zero strain rate of a straight line which the data give asvs""_mon a double-linear scale,22)

whereis the stress,""_is the strain rate andmis the strain rate sensitivity. By using the data in Fig. 9, the threshold stresses of the specimens were estimated, as shown in Fig. 10. The threshold stress was estimated to be about 5 MPa for the specimen from small machined chips and about 2 MPa for the virgin specimen, respectively. Hence, the strengthening due to the dispersion of oxides is considered to be 3 MPa, which is lower than the flow stress at 104–105s1 (¼12

7MPa) for the specimen from small machined chips. Therefore, it is conclusively demonstrated that oxide con-tamination contributes little to the strengthening, but results in a low elongation to failure at elevated temperatures.

3.3 Blow forming

Blow forming tests were carried out at 773 K at a pressure of 0.5 MPa. The stress at the dome apex can be given by23)

c¼t¼PB

4S Hþ

1

H

; ð2Þ

wherecis the stress in the circumferential direction,tis the

stress in the thickness direction,Pis the forming pressure,B

is the die radius,Sis the thickness andHis the relative dome height. The strain rates at 0.5 MPa are of the order of104s1

100

UTS,

σ

/ MP

a

250

350

750

Temperature,

T

/ K

(a)

450

550

650

0

200

300

450

650

750

Temperature,

T

/ K

100

200

300

0

(b)

Elongation to f

ailure, (%)

250

350

550

: Virgin specimen

: Specimen from small machined chips

: Specimen from medium-sized machined chips

: Specimen from large machined chips

Fig. 7 The variation in ultimate tensile strength (a) and elongation to failure (b) at1:7103s1as a function of testing temperature for

5083 Al alloy specimens.

Strain rate, / sε. -1

10-5 10-4 10-3 10-2 10-1

0 100 200 300

Elongation to f

ailure, (%)

: Virgin specimen

: Specimen from small machined chips : Specimen from medium-sized machined chips : Specimen from large machined chips

Fig. 8 The variation in elongation to failure at 773 K as a function of strain rate for 5083 Al alloy specimens.

: Virgin specimen

: Specimen from small machined chips : Specimen from medium-sized machined chips : Specimen from large machined chips

0.5

1

log ( , / sε. -1)

-6 -5 -4 -3 -2 -1

0.4 0.8 1.2 1.6 2.0

log (

/

MP

a)

σ

[image:5.595.87.515.74.320.2] [image:5.595.60.284.372.543.2] [image:5.595.314.535.373.550.2]
(6)

from eq. (2) and the results in Fig. 9. The results of the blow forming tests are shown in Fig. 11. The cup-shaped virgin specimen was successfully formed. However, the specimen from small machined chips was fractured before it formed a cup shape. On the other hand, the specimens from medium-sized machined chips and large machined chips exhibited

better formability than that of the specimen from small machined chips. This trend of formability is the same as that of elongation at 773 K by as determined tensile tests. This indicates that poor formability for the specimen from small machined chips is attributed to a large amount of oxide contaminants. The results in the present investigation point

(Strain rate)1/2, ε1/2 / s-1/2

0 0.03

0.06 0.09

Flo

w stress ,

/ MP

a

σ

σ

20 40 60

(a) Specimen from small machined chips

σ

523 1/2 +

5.03

=

ε

(Strain rate)1/2, ε1/2/ s-1/2

0 0.03 0.06 0.09

Flo

w stress ,

/ MP

a

σ

σ

20 40 60

(b) Specimen from medium-sized machined chips

σ

1/2

+ 3.77

= 599

ε

(Strain rate)1/2,

ε1/2/ s-1/2

0 0.03

0.06 0.09

Flo

w stress ,

/ MP

a

20 40 60

(c) Specimen from large machined chips

σ

= 567

ε

1/2+3.21

(Strain rate)1/2,

ε1/2/ s-1/2

0 0.03 0.06 0.09

Flo

w stress ,

/ MP

a

20 40 60

(d) Virgin specimen

σ

1/2 +

2.29

= 544

ε

.

.

[image:6.595.85.505.69.392.2]

.

.

Fig. 10 The threshold stress analyses using the square root of strain rate for 5083 Al alloy specimens.

(a)

(c)

(b)

(d)

50mm

[image:6.595.115.481.437.660.2]
(7)

out the importance of the control of the contamination level of oxides for the formability of the solid recycled Al alloy. Further research is needed to reduce the detrimental effect of oxide contamination on formability in the recycled Al alloy.

4. Conclusions

The tensile properties and blow forming characteristics of 5083 Al alloy recycled by solid-state recycling were inves-tigated. The results are summarized as follows.

(1) Three types of machined chip with different volumes were recycled by hot extrusion and hot rolling, and oxide contamination was investigated. Oxide layers, which were contaminants from the machined chip surface, were distributed parallel to the extrusion direction in the recycled specimens. Oxygen concen-tration in the recycled specimens increased with the total surface area of the machined chips per unit volume. Therefore, the size of machined chips is an important factor for the control of the contamination level of oxides in solid-state recycling.

(2) The recycled specimens exhibited almost the same properties in term of strength and elongation to failure as the virgin specimen. However, at 773 K, the recycled specimens showed lower elongation than the virgin specimen, and the elongation decreased with increasing oxygen concentration. The same trend was found in the blow forming tests, that is, formability decreased with increasing oxygen concentration. Thus, oxide contam-ination has a detrimental effect on the formability of recycled Al alloy.

Acknowledgements

M. Mabuchi gratefully acknowledges the financial support from the project, ‘‘Barrier-Free Processing of Materials for Life-Cycle Design for Environment’’, by the Ministry of

Education, Culture, Sports, Science and Technology of Japan.

REFERENCES

1) K. Fukizawa, S. Koike and K. Shibata: Materia Japan39(2000) 17–24. 2) K. Osumi: KINZOKU66(1996) 117–128.

3) T. Ohnishi: J. JAPAN Institute of Light Metals46(1996) 525–532. 4) T. Nakamura: Materia Japan35(1996) 1290–1293.

5) F. Murata: J. JAPAN Institute of Light Metals46(1996) 551–556. 6) K. Nagai: KINZOKU63(No. 10) (1993) 65–70.

7) M. Mabuchi, K. Kubota and K. Higashi: Mater. Trans., JIM36(1995) 1249–1254.

8) Y. Chino, K. Kishihara, K. Shimojima, Y. Yamada, Cui’e Wen, H. Iwasaki and M. Mabuchi: J. Japan Inst. Metals65(2001) 621–626. 9) H. Watanabe, K. Moriwaki, T. Mukai, K. Ishikawa, M. Kohzu and K.

Higashi: J. Mater. Sic.36(2001) 5007–5011.

10) K. Kondoh, T. Luangvaranunt and T. Aizawa: J. JAPAN Institute of Light Metals51(2001) 516–520.

11) Y. Chino, K. Kishihara, K. Shimojima, Y. Yamada, Cui’e Wen, H. Iwasaki and M. Mabuchi: Mater. Trans.43(2002) 2437–2442. 12) T. Aizawa, T. Luangvaranunt and K. Kondoh: Mater. Trans.43(2002)

315–321.

13) Y. Chino, M. Mabuchi, S. Otsuka, K. Shimojima, H. Hosokawa, Y. Yamada, C. E. Wen and H. Iwasaki: Mater. Trans.44(2003) 1284– 1289.

14) Y. Chino, M Kobata, K. Shimojima, H. Hosokawa, Y. Yamada, H. Iwasaki and M. Mabuchi: Mater. Trans.45(2004) 361–364. 15) K. Osada: J. JAPAN Institute of Light Metals49(1999) 326–331. 16) Y. Ohnishi: J. JAPAN Institute of Light Metals49(1999) 346–348. 17) T. Sheppard, M. G. Tutcher and H. M. Flower: Met. Sci.13(1979)

481–490.

18) T. Sheppard and M. G. Tutcher: Met. Sci.14(1980) 579–589. 19) T. Sheppard, N. C. Parson and M. A. Zaidi: Met. Sci.17(1983) 481–

490.

20) Japan aluminum associationet al.ed.: Aluminum Handbook 4th ed. (Japan aluminum association, Tokyo, 1990) pp. 29–50.

21) T. Mukai, K. Ishikawa and K. Higashi: Mater. Sci. Eng.A204(1995) 12–18.

22) F. A. Mohamed: J. Mater. Sci.18(1983) 582–592.

Figure

Table 1Configurations of 5083 Al machined chips used for solid-staterecycling.
Fig. 3Microstructures of 5083 Al alloy specimens.
Figure 8 shows the variation in elongation to failure at
Fig. 8The variation in elongation to failure at 773 K as a function of strainrate for 5083 Al alloy specimens.
+2

References

Related documents

Results suggest that the probability of under-educated employment is higher among low skilled recent migrants and that the over-education risk is higher among high skilled

Field experiments were conducted at Ebonyi State University Research Farm during 2009 and 2010 farming seasons to evaluate the effect of intercropping maize with

Errors caused by transfer of grammatical rules were the only type of interlingual errors analyzed in this study, and the distribution of error frequencies in the two groups of

TRAP plate let ac tivatio n GpIIb /IIIa r ecep tor expo sure degra nulat ion ADP Arachidonic Acid Collagen ArA 1 COX TXA 2 TXA 2 COLtest ASPItest TRAPtest ADPtest ADPtest PGE1

One focuses on eliminating alternative hypotheses (or revising hypotheses). The other can be used to further specify relations between variables within the context of

19% serve a county. Fourteen per cent of the centers provide service for adjoining states in addition to the states in which they are located; usually these adjoining states have

Fig.3 shows the collaborative diagram for Intelligent Agent System for Smart Grid agents based on the information management domain and Table explains the