Microstructures and Tensile Properties of ECAE-Processed and ...

Materials Transactions, Vol. 44, No. 4 (2003) pp. 476 to 483 Special Issue on Platform Science and Technology for Advanced Magnesium Alloys, II #2003 The Japan Institute of Metals

Microstructures and Tensile Properties of ECAE-Processed and Forged AZ31 Magnesium Alloy Lawrence Cisar1; * , Yu Yoshida1; *, Shigeharu Kamado1 , Yo Kojima1 and Fukashi Watanabe2 1 2

Department of Mechanical Engineering, Nagaoka University of Technology, Nagaoka 940-2188, Japan Hitachi Metals MPF, Ltd., Minamiuonuma-gun Muika-machi 949-6772, Japan

In order to achieve same level of high strength and high ductility as 6061 aluminum forging alloy that is currently used for automobile applications, AZ31 magnesium alloy rod with a large diameter of 40 mm was subjected to ECAE-processing, and the microstructures and mechanical properties of the ECAE-processed specimens were investigated. Furthermore, automobile knuckle arm was produced by forging using the ECAE-processed material, and the mechanical properties of the forged product and their strain rate dependencies were investigated under impact tensile load conditions. 4pass-ECAE-processed specimen has fine and uniform microstructure and a texture whose basal planes are mainly parallel to the extrusion direction with some inclined at angles up to 45 to the extrusion direction. Therefore, they show high ductility even if the tensile direction is parallel to the extrusion direction. The knuckle arm forged using the ECAE-processed material exhibits high elongation even in the high strain rate region. Furthermore, the tensile strength, fracture elongation and absorption energy of the forged product increase with increasing strain rate and their values are higher than those of T6-treated 6061 aluminum forging alloy specified by JIS. (Received October 23, 2002; Accepted January 6, 2003) Keywords: Mg–3 mass%Al–1 mass%Zn–0.4 mass%Mn alloy, equal channel angular extrusion, microstructure, texture, tensile properties, anisotropy

1.

Introduction

Magnesium is the lightest material among structural metallic materials and it has attractive potentials like high specific tensile strength, good machinability, high recyclability and so on. Therefore, it is drawing a huge attention as an eco-material.1) Recently, to deal with a environmental problems, mass reduction of automobiles has been intensified in order to improve fuel economy and minimize the emission of harmful gases. Consequently, a great number of researches on magnesium alloy is being carried out to develop suitable alloys such as heat resistant magnesium alloys,2) protium absorbing alloys,3) thixoforming alloys4,5) and so on, with the result that magnesium alloys could be used in several fields. However, magnesium has inferior cold workability because it has a hcp crystal structure and it has only one active slip plane at room temperature. At present most of the structural magnesium alloy products have been fabricated by casting and die-casting. Wrought products that are fabricated by plastic forming such as rolling, extrusion and forging are almost not available. Therefore applying magnesium alloys to suspension and steering parts that requires high strength and ductility has not been achieved. However, remarkable improvements of ductility of magnesium alloys have been reported in samples that addition of lithium to magnesium6,7) and have improved microstructure through grain refinement.8–10) Also, magnesium alloys with controlled texture have been reported to exhibit improved ductility.11–13) Previous researches14,15) show that in magnesium alloys processed by Equal Channel Angular Extrusion (ECAE)16) using specimens that have a small diameter of about 15 mm, the grains are refined and a texture in which the basal plane is inclined at 45 to the extrusion direction is obtained. As a result, if tensile test is carried out such that the tensile *Graduate

Student, Nagaoka University of Technology.

direction is parallel to the extrusion direction, basal slip occurs easily and elongation is remarkably improved.14,15) Due to the limited sample size, mechanical properties were evaluated using tensile specimens that were extracted such that the tensile direction was parallel to the extrusion direction and only the central part of ECAE-processed samples could be utilized. However, in order to effectively use ECAE-processed materials to make industrial products, larger materials are required. Therefore, the aim of the present research is to evaluate the mechanical properties of ECAE-processed AZ31 magnesium alloy samples with a large diameter. Extruded samples of AZ31 alloy having a diameter of 40 mm, which is about the size required for practical application, were used to investigate the effect of ECAE-process on microstructures and tensile properties. Furthermore, ECAE-processed materials were used to forge a steering part (knuckle arm) in order to evaluate the practicability of using ECAE-processed materials for industrial products. And using a high strain rate range of the order of 1  103 s 1 , the strain rate dependencies of the mechanical properties of the forged samples were also examined. 2.

Experimental Procedure

The magnesium alloy used in this study is commercial extruded AZ31 alloy bar. The nominal composition of the alloy is shown in Table 1. ECAE specimens were machined from the extruded bar into cylindrical specimens having a diameter of 40 mm and a length of 250 mm. ECAE processing condition was such that the extrusion speed was 20 mm/ Table 1

Nominal composition of AZ31 magnesium alloy (mass%).

Alloy

Al

Zn

Mn

Fe

Si

Mg

AZ31

3.0

1.0

0.4

0.003

0.001

bal

Microstructures and Tensile Properties of ECAE-Processed and Forged AZ31 Magnesium Alloy

min, while the temperature of both the specimen and the die was 523 K. ECAE was carried out 1, 2, and 4 times, respectively. The specimens were rotated 180 , 90 , and 180 successively after each pass. Microstructures of the obtained specimens were observed using optical microscope in order to evaluate the changes in grain size and the homogeneity of the microstructures and so on. In addition, X-ray diffraction was used to construct pole figures, which were then used to evaluate the texture. Then the relationship between the microstructures and the tensile properties were investigated. The tensile specimens were extracted from the top, middle and bottom parts of the extruded and ECAE processed samples, and they had a gage length of 10 mm and a gage diameter of 4 mm. The tensile tests were carried out at room temperature under low strain rate at an initial strain rate of 8:33  10 3 s 1 . Also, some of the specimens were extracted in such a way that the tensile direction is inclined to the extrusion direction at 0, 45, and 90 in order to evaluate the anisotropy of the tensile properties. Forging was carried out using 4-pass ECAE specimens, asreceived specimens and extruded 6061 aluminum alloy for comparison. The specimen temperature was 573 K for the magnesium alloy sample and 673 K for the 6061 aluminum alloy sample, while the die temperature was 423 K. Furthermore, the forged 6061 aluminum alloy was heated at 793 K for 1.8 ks, quenched in water, and then aged at 448 K for 28.8 ks in order to prepare T6 treated specimens. Microstructures of the forged samples were observed using optical microscope in order to evaluate the changes in the microstructures that occurred during forging. Furthermore, Extruded specimens, 4-pass ECAE specimens, Extruded + forged specimens, 4-pass ECAE + forged specimens and T6 treated 6061 aluminum alloy were subjected to tensile test under high strain rate range at an initial strain rate of

Fig. 1

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1  10 1 –103 s 1 in order to evaluate the strain rate dependencies of the mechanical properties of the specimens. The tensile specimens had a gage length of 6 mm and a gage diameter of 3 mm. 3.

Results and Discussion

3.1 Mechanical properties of ECAE samples 3.1.1 Microstructure Figure 1 shows the microstructures of the extruded sample before ECAE processing, and those of ECAE processed samples. In 1-pass ECAE sample, more fine grains are observed at the top than at the bottom part. This is because during ECAE processing, friction between the specimen and the die is higher at the outer channel than at the inner channel and as result, the upper part of the specimen that is in contact with the inner channel experiences more shear force such that a high amount of strain is induced in that part of the specimen.17,18) On the other hand, in 2-pass ECAE sample where the specimen was rotated 180 after the first pass, eventually, strain is evenly induced in the sample and as a result, a homogeneous microstructure is observed throughout the specimens. Although grain growth occurs at the bottom part of 4-pass ECAE specimens, a relatively homogeneous microstructure is also obtained. 3.1.2 Texture Figure 2 shows the (0002) basal plane pole figures obtained for the different parts of 1-pass ECAE sample. At the top part of the sample, the basal plane is inclined at an angle to the extrusion direction. But as the bottom part is approached, a higher density of the basal planes becomes parallel to the extrusion direction. Pole figures obtained for the central part of 1, 2, and 4-pass ECAE specimens are shown in Fig. 3. In all the specimens, although there is a high density of basal planes parallel to the

Microstructures of the extruded sample and those of ECAE-processed.

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Fig. 2 (0002) pole figures of specimens extracted from the top, center, and bottom parts of 1-pass ECAE sample.

Fig. 3 (0002) pole figure of 1-pass, 2-pass and 4-pass ECAE specimens extracted from the central part.

extrusion direction, as the number of passes increase there are also grains whose basal planes are inclined at angles up to 45 to the extrusion direction. However, the density of the basal planes that are inclined at 45 to the extrusion direction is not as high as that observed in specimens of small diameters.19) As described above, regions of the specimen close to the outer channel experience low shear force and in specimens with large diameters, the shear force is reduced further. Therefore, the difference in the textures of small diameter specimens and large diameter specimens is expected. 3.1.3 Tensile properties The stress-strain curves in Fig. 4 show the effect of extracted positions of specimens on the tensile properties of the extruded and ECAE processed samples. In all of the specimens, the tensile direction is parallel to the extrusion direction. In 1-pass ECAE sample, the specimen extracted at the top part exhibits a large elongation, however, as we go down to the bottom part, the elongation decreases but the proof stress increases. This is because of the fact that the basal plane is inclined to the extrusion direction in the upper part as shown in Fig. 2, such that during tensile test, the basal plane, which is the most active slip plane, experiences high shear force, resulting in more slip and high elongation. On the other hand, due to fine grains and the homogeneous nature of the microstructure of 2-pass ECAE sample, the elongation increases and differences in tensile properties associated with extracted positions of specimens are small. In 4-pass ECAE specimens, the grain growth observed at the bottom part results in smaller elongation, but the top and central parts exhibit similar tensile characteristics. Compared to 2-pass specimens, the 4-pass specimens exhibit larger elongation, because as the number of passes increases, the basal planes

are increasingly inclined at an angle to the extrusion direction. Figure 5 show the stress-strain curves of specimens extracted in different directions. In one-pass ECAE sample, the basal plane is parallel to the extrusion direction, therefore, if the specimen is extracted at 45 to the extrusion direction, its proof stress decreases, but elongation increases. In the case of 0 inclination, elongation decreases and but the proof stress increases. If the specimen is extracted perpendicular to the extrusion direction, the proof stress is almost the same as the specimen extracted at 45 inclination, but elongation is lower. In 2 and 4-pass samples, the proof stress of the specimens of 0 inclination decreases but elongation increases. This is because as the number of extrusion passes increases, the basal planes become more and more inclined to the extrusion direction as shown in Fig. 3. Furthermore, in all samples, the elongation of the specimens extracted perpendicular to the extrusion direction does not increase regardless of the low proof stress. This result is due to the effect of the heterogeneous microstructure of the specimens as we move from the inner to the outer channel. 3.2 Mechanical properties of forged samples 3.2.1 Microstructure Figure 6 shows the external appearance of the forged knuckle arm which is an automotive steering part. Also microstructures of a cross section of the forged specimens are shown in Fig. 7. In both forged samples, the central parts have more fine grains than the top and bottom parts because of forging effect. However, compared to the microstructures of the samples before forging grain growth occurs due to high forging temperature. In the extruded + forged sample

Microstructures and Tensile Properties of ECAE-Processed and Forged AZ31 Magnesium Alloy

Fig. 4 Relationship between the extracted positions of specimens and tensile properties.

Fig. 5 Relationship between the extracted directions of specimens and tensile properties.

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Fig. 6

External appearance of forged knucle arm.

Fig. 7

twinning occurs at the top part. In the extruded sample, in general, the basal plane is parallel to the extrusion direction and is also parallel to round surface of the sample.13,19,20) Therefore, when the forging direction is perpendicular to the extrusion direction, there are some grains with the basal plane parallel to the forging direction. In such case basal slip is difficult and tension twinning easily occurs. Generally, 4-pass ECAE + forged sample has a smaller grain size than the extruded + forged sample. 3.2.2 Tensile properties The stress-strain curves that are obtained from the high initial strain rate tensile test are shown in Fig. 8. The strain rate does not affect the tensile properties of the forged

Microstructures of forged samples of AZ31 alloy.

Microstructures and Tensile Properties of ECAE-Processed and Forged AZ31 Magnesium Alloy

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Fig. 8 Relationship between the strain rate and tensile properties, (a) forged A6061 aluminum alloy and (b) AZ31 magnesium alloy samples.

specimens of 6061 aluminum alloy. On the other hand, in specimens of AZ31 alloy, as the strain rate increases, the tensile strength and elongation increase. The relationship between strain rate and mechanical properties of the forged specimens investigated in the high strain rate region is clearly shown in Fig. 9. The 4-pass ECAE specimen and the specimen forged using ECAE processed sample exhibit lower tensile strength but higher

elongation, particularly higher uniform elongation and absorbed energy than the extruded sample and the specimen forged using the extruded sample. Although all of the investigated samples of AZ31 alloy do not exhibit higher tensile properties than the forged sample of 6061 alloy, their tensile properties are much higher than the standard values for the 6061 alloy specified by JIS.

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Fig. 9

4.

Relationship between strain rate and mechanical properties of investigated samples of AZ31 magnesium and 6061 aluminum alloy.

Conclusions

(1) In ECAE-processed specimens with large diameter, when the number of passes is small, a heterogeneous microstructure is observed, and the basal planes are parallel to the extrusion direction. However as the number of passes increases, the microstructure becomes homogeneous and there is a high density of basal planes that are inclined at an angle to the extrusion direction. Also, the anisotropy of the mechanical properties of the specimens decreases as the number of passes increases. (2) In the case of samples forged to knucle arm, the tensile properties of T6 treated 6061 aluminum alloy do not depend on strain rate. On the other hand as the strain rate increases, the elongation and absorbed energy of forged samples of 4-pass ECAE-processed AZ31 alloy increases. (3) Although the tensile properties of the forged samples of AZ31 magnesium alloy are lower than those of 6061 aluminum alloy, they are higher than the standard value specified by JIS and if forging temperature is lowered, forged AZ31 alloy will have higher tensile strength.

Thus, it is possible to apply forged AZ31 magnesium alloy to automobile steering parts. Acknowledgements This study is supported by New Energy and Industrial Technology Development Organization (NEDO) and Grantin-Aid for Scientific Research on Priority Area (B), ‘‘Platform Science and Technology for Advanced Magnesium Alloys’’ from Ministry of Education, Culture, Sports, Science and Technology of Japan. REFERENCES 1) Y. Kojima: Mater. Trans. 42 (2001) 1154–1159. 2) I. A. Anyanwu, S. Kamado and Y. Kojima: Mater. Trans. 42 (2001) 1212–1218. 3) H. Okumura, T. Tabata, A. Matsui, S. Kamado and Y. Kojima: Mater. Trans. 42 (2001) 1305–1311. 4) R. S. Rudi, S. Kamado, N. Ikeya, T. Araki and Y. Kojima: Mater. Sci. Forum. 350–351 (2000) 79–84. 5) S. Kamado, N. Ikeya, R. S. Rudi, T. Araki and Y. Kojima: Mater. Sci. Forum. 350–351 (2000) 205–214.

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