Download pdf - Improvement of Ductility of Aluminum Wire for Automotive Wiring Harness by Alternate Drawing

Procedia Engineering 81 ( 2014 ) 706 – 711

1877-7058 © 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).Selection and peer-review under responsibility of the Department of Materials Science and Engineering, Nagoya University doi: 10.1016/j.proeng.2014.10.064

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11th International Conference on Technology of Plasticity, ICTP 2014, 19-24 October 2014, Nagoya Congress Center, Nagoya, Japan

Improvement of ductility of aluminum wire for automotive wiring harness by alternate drawing

Kazunari Yoshidaa,*, Kota Doib aPrecision Engineering, School of Engineering, Tokai University, 4-1-1, Kitakaname, Hitatsuka, Kanagawa, 259-1257, Japan

bGraduate Student, Tokai University

Abstract

As a production method of aluminum wires for an automotive wiring harness, “alternate drawing” which can give high ductility to a drawn wire was proposed. Alternate drawing promises to decrease the additional shearing strain by alternating the drawing direction of every 1 pass. To confirm its usefulness, a comparison of the ductility between alternately drawn wires and unidirectionally drawn wires were made through the results of mechanical tests and texture analysis. © 2014 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of Nagoya University and Toyohashi University of Technology.

Keywords: Alternate wire drawing; Aluminum wire; Wiring harness; Ductility

1. Introduction

In the automobile industry, research on the replacement of copper wires with aluminum wires for the conductive part of automotive wiring harnesses has recently been popular (Yamano et al., 2011; Horikoshi, 2006). This can reduce the weight of the car body and contribute to the strengthened environmental policies based on the improvement of fuel economy and the reduction in CO2 emission. In this study, we propose alternate drawing, a method of fabricating aluminum wires with high ductility and good fatigue characteristics that can be used for automotive wiring harnesses. In this method, the drawing direction is alternately reversed through multiple passes

* Corresponding author. Tel.:+81-463-58-1211(ex. 4390) ; fax: +81-463-59-8150 E-mail address: [email protected]

© 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).Selection and peer-review under responsibility of the Department of Materials Science and Engineering, Nagoya University

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707 Kazunari Yoshida and Kota Doi / Procedia Engineering 81 ( 2014 ) 706 – 711

to greatly improve both the strength and toughness of drawn wires. Kajino et al. (2006) have carried out intensive study on additional shearing strain generated in the surface layer of drawn wires from the crystallographic viewpoint. In addition, processes of introducing additional shearing strain to grow crystals in the desired orientation, such as asymmetric rolling (Ueno et al., 2010) and Equal Channel Angular Pressing (Horita, 2009), have been attempted. Alternate drawing is expected to suppress the generation of additional shearing strain and improve not only the strength but also the ductility and fatigue characteristics of drawn wires. This is because textures that cannot be obtained by conventional drawing methods are formed by alternate drawing.

In this study, we prepared two aluminum wires: one was unidirectionally drawn by a conventional method and the other was drawn with the alternately reversed drawing direction to examine the effects of alternate drawing on the mechanical properties of the drawn aluminum wire. The characteristics of the two wires were compared through various experiments and simulations, including ductility tests, cyclic bending tests, analysis of drawing by the finite element method (FEM), and texture analysis of the drawn wires by Electron Back-Scatter Diffraction .

2. Specimens and drawing conditions

Table 1 shows the chemical components of the aluminum wires used in this study. A carbide alloy die (die half-angle, 6o) and a petroleum-based hydrocarbon lubricant were used for drawing. The reduction in outer diameter per pass was approximately 20%. Mother wires with a diameter of 9.6 mm were thinned to 0.46 mm through 27 passes of drawing (total reduction Rt in outer diameter, Rt = 99.77%). In alternate drawing, the drawing direction was alternately reversed after every 1 pass of drawing (Fig. 1).

Table 1. Chemical composition of tested aluminum wire. Al Mn Fe Cr Si Ti+V Cu 99.65% 0.005% 0.25% 0.10% 0.10% 0.005% 0.005%

Fig. 1. Alternate wire drawing.

3. Results and Discussions

3.1. Mechanical properties of alternately drawn wires

A tensile test was performed for a thinned wire obtained after 27 passes of alternate drawing (Rt = 99.77%) and a wire obtained by conventional unidirectional drawing. Fig. 2 shows stress-strain curves for these wires and scanning electron microscopy (SEM) images of their fracture surface.

The tensile strength of the alternately drawn wire was approximately 20 MPa lower than that of the unidirectionally drawn wire. However, the tensile strain of the alternately drawn wire was approximately 0.01 higher than that of the unidirectionally drawn wire.

The mechanical properties required for wiring harnesses are a tensile strength larger than 160 MPa and a tensile strain larger than 0.015. The unidirectionally drawn wire satisfied the required tensile strength but did not satisfy the ductility requirement. In contrast, the alternately drawn wire satisfied both requirements. Therefore, the

Die

Wire 1pass 2pass 3pass


aluminum wire fabricated by alternate drawing had mechanical properties sufficient for use as the conductive part of wiring harnesses.

Fig. 2. Stress-strain curves and SEM images of fracture surface of each drawn wire (Rt = 99.77%).

3.2. Calculation of equivalent strain of drawn wires by FEM and its correlation with Vickers hardness

The deformation of the wire elements and the equivalent strain were analyzed by FEM to confirm that the generation of additional shearing strain in drawn wires is suppressed for alternate drawing.

In the FEM, the friction coefficient of a drawing die and a wire in contact was set to 0.1. The drawing die was assumed to be a rigid body, and the drawing conditions were as follows: a die half-angle of 6o, a 20% reduction in outer diameter per pass, and wire Young’s modulus of 70 GPa.

Fig. 3(a) shows the deformation of the elements when the wire was drawn by the two methods with Rt = 92.94% (12 passes) and SEM images of metal textures. The elements of the unidirectionally drawn wire greatly deformed forward along the drawing direction. In contrast, the forward deformation of the alternately drawn wire was suppressed. Fig. 3. (a) Comparison of element deformation by FEM and metal structure by experiment in each drawn wire (Rt = 92.94%); (b) Equivalent strain distribution by FEM and Vickers hardness distribution by experiment in each drawn wire (Rt = 92.94%).

0

200

160

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0 0.01 0.02

0

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Alternately drawn wire

Tensile strain

16

12

8

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Tens

ile st

ress

/ M

Pa

50 m

Observation by SEM

0.03 0 03Drawing rate 91.2%

Drawing rate 89.4%

i) Alternately drawn wire

ii) Unidirectionally drawn wire

Element (FEM) Metal structure (Experiment)

Viewing position Die

Wire C L Drawing direction

Vickers hardness HV Equivalent strain (FEM)

55 59 63 67

Surface

63 67

Center

4.49

1.59

2.12

CCL55 59 63 67


C 55 59 63 67

1.63

2.15

CL

Surface

63 67

Center

4.62


(a) (b)


Fig. 3(b) shows the distributions of equivalent strain of the drawn wires calculated by FEM and the distributions of Vickers hardness of the drawn wires measured experimentally.

In plastic processing, equivalent strain correlates with Vickers hardness (Yoshida et. al., 2011). This correlation was also observed in the experimental results of this study. As shown in Fig. 3(b), the equivalent strain and Vickers hardness were high on the surface of the unidirectionally drawn wire, which had a high forward slip ratio (Fig. 3(a)).

The Vickers hardness of the surface of the unidirectionally drawn wire was 10% higher than that of the center, whereas the difference was only 3% for the alternately drawn wire. This was due to the suppressed generation of additional shearing strain in the surface of the wire.

The above results indicate that the generation of additional shearing strain is suppressed in the alternately drawn wire.

3.3. Torsional and fatigue characteristics of alternately drawn wire

Materials of wiring harnesses are required to have good torsional and fatigue characteristics in addition to high strength and ductility.

(1) Torsional characteristics The wires were thinned by the two drawing methods with Rt = 99.77% (27 passes) and a torsion test was

performed for these wires. Figure 4 shows the number of rotations and SEM images of the fracture surfaces. With respect to the diameter of the wire (d), the gauge length of the wire under test was set to 100d (Ochiai, 2010). Because the variations in the results of torsion tests are generally large, the test was performed 20 times and the averages of the 20 measurements were compared.

As shown in Fig. 4, flat fracture surfaces were observed for the two wires, as normally obtained for high-ductility wires. The number of rotations for the alternately drawn wire was nearly double that of the unidirectionally drawn wire. Thus, the torsional characteristics were improved for the alternately drawn wire.

Fig. 4. Results of torsion test in each drawn wire (Rt = 99.77%).

(2) Fatigue characteristics A cyclic bending test with reversed loading was performed to examine the fatigue characteristics of the wires

thinned by the two drawing methods with Rt = 99.77% (27 passes). We prepared a simple fatigue tester that can bend a wire by 45o both upward and downward, and counted the

number of cycles until the wire fractured. The test was performed 20 times and the averages of the 20 measurements were compared. Figure 6 shows the test results and SEM images of the fracture surfaces of the drawn wires.

0 5 10 15 2000

10

20

30

40

Number of test / times

00

10

20

30

40

Num

ber o

f rot

atio

ns /

times

Unidirectionally drawn wireUnidirectionally drawn wirAlternately drawn wire

b)Unidirectionally drawn wire

a)Alternately drawn wire


Flat fracture surfaces were obtained for the two wires, as normally obtained in bending tests with reversed loading. The average number of cycles to fracture was approximately 49.3% higher for the alternately drawn wire than for the unidirectionally drawn wire. Thus, the fatigue characteristics were improved for the alternately drawn wire.

Fig. 5. Results of cyclic bending test for each drawn wire (Rt = 99.77%).

3.4. Texture analysis of drawn wires by EBSD

The alternately drawn wire showed mechanical properties and fatigue characteristics greatly different from those of the unidirectionally drawn wire. The textures of the two wires were analyzed by EBSD to discuss the factors for improving the ductility of the alternately drawn wire.

Figure 6 shows crystal orientation maps in the normal direction (ND) and inverse pole figures of the wires thinned by the two drawing methods with Rt = 99.77% (27 passes). The measurement area is a quarter sector of the cross section of the drawn wires and the step was 0.01.

Fig. 6. Crystal orientation map and inverse pole figure of each drawn wire (Rt = 99.77%).

Max = 7.702 5.480 3.900 2.775 1.975 1.405 1.000 0.712


Cross-section of drawn wire

Measurement area


(a) Crystal orientation maps

<111> <100> Random

<100> <111>

& R

andom

<111>

111

100 110


111

100

ii110


(b) Inverse pole figures

13.1 7.7

>

1.4

0.712

7.7

4

Max = 13.078 8.520 5.551 3.616 2.356 1.535 1.000 0.651

Wire (d=0.46mm)160

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0 5 10 15 20Number of test / times

160

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Num

ber o

f cyc

les t

o fr

actu

re /

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000





a)Alternately drawn wire

b)Unidirectionally drawn wire


In Fig. 6(a), the <111> crystal orientation is dominant over the entire measurement area of the unidirectionally drawn wire. The inverse pole figure also reveals a strong <111> with the maximum intensity of 13.1. A report showed that <111> fiber textures were uniformly formed in drawn aluminum wires with a final stable orientation (Inakazu 1987). In contrast, the crystals of the alternately drawn wire are <111> but also randomly oriented on the surface of the wire [Fig. 6(a)]. In addition, the crystals at the center of the drawn wire are <100>. In the inverse pole figure of the alternately drawn wire [Fig. 6(b)], the maximum intensity for the <111> is 7.7, which is lower than that for the unidirectionally drawn wire. The maximum intensity for the <100>, which is not observed in the unidirectionally drawn wire, is 1.4.

In alternate drawing, the crystals were not only <111> but also randomly (which results in the increased ductility) and <100>, resulting in the increased strength and ductility of the drawn wire.

4. Conclusions

Aluminum wires were thinned by alternate drawing with Rt = 99.77% (27 passes). Whether the drawn wires can be used for automotive wiring harnesses was examined. The findings obtained are summarized below. (1) The tensile strength of the wire alternately drawn with the drawing direction reversed after every pass was

approximately 20 MPa lower than that of the unidirectionally drawn wire. However, the tensile strain of the alternately drawn wire was approximately 0.01 higher than that of the unidirectionally drawn wire. The generation of additional shearing strain was suppressed and the ductility was improved for the alternately drawn wire.

(2) The aluminum wire obtained by alternate drawing satisfied the requirements of wiring harnesses, i.e., a tensile strength larger than 160 MPa and an tensile strain of larger than 0.015. In addition, the torsional and fatigue characteristics were also improved for the alternately drawn wire.

(3) The alternately drawn wire was more ductile than the unidirectionally drawn wire because of the difference in the orientation of the crystals formed by drawing. The crystals of the unidirectionally drawn wire were only <111>, whereas the crystals of the alternately drawn wire were not only <111> but also randomly oriented on the surface of the wire, and in addition, <100> at the center of the wire.

Acknowledgements

We are deeply grateful to the staff of Furukawa Electric, Co., Ltd., for providing valuable materials for our experiments in this study.

References Yamano Y., et al., 2011. Development of Aluminum Wiring Harness. SEI Technical Review, 81-88. Horikoshi T., et al., 2006. Development of Aluminum Alloy Conductor with High Electric Conductivity and Controlled Tensile Strength and

Elongation. Hitachi cable 25, 31-34. Kajino S., Asakawa M., 2006. Effect of Additional Shear Strain Layer on Strength and Ductility of Fine Drawn Wire. Current Advances in

Materials and Processes 19 (2), 389. Kajino S., Asakawa M., 2006. Effect of Additional Shear Strain Layer on Tensile Strength and Ductility of Finely Drawn Wire - Additional

Shear Strain Layer at Surface of Drawn Wire -. Journal of the Japan Society for Technology of Plasticity 47 (549), 953-957. Ueno T., et al., 2010. Rolling Characteristics and Curling in Asymmetric Rolling of Copper Sheet. Journal of Japan Copper and Brass

Association 49(1), 237-243. Horita Z., 2009. Kyushu University Introduction of High Strain Working Process. Journal of Japan Institute of Light Metals 59 (7), 385-388. Yoshida K., Yamashita S., 2011. Manufacturing Technology of Shaped Wire of Copper and Copper Alloy by Flat Rolling of Wire. Journal of

Japan Copper and Brass Association 50 (1), 130-135. Ochiai I., 2010. Manufacture and Testing of High Strength Steel Wires. Journal of the Japan Society for Technology of Plasticity 51 (593),

493-497. The Society of Materials Science Japan Committee on Fractography, 2008. Fractography - Analysis of fracture and breakage -, Maruzen, 22-24. Inakazu N., 1987. Drawing and fiber texture formation for aluminum. Journal of Japan Institute of Light Metals 37 (5), 381-393.