(2) 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. 477. 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..
(3) 478. L. Cisar, Y. Yoshida, S. Kamado, Y. Kojima and F. Watanabe. 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.
(4) 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.. 479.
(5) 480. L. Cisar, Y. Yoshida, S. Kamado, Y. Kojima and F. Watanabe. 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..
(6) Microstructures and Tensile Properties of ECAE-Processed and Forged AZ31 Magnesium Alloy. 481. 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..
(7) 482. L. Cisar, Y. Yoshida, S. Kamado, Y. Kojima and F. Watanabe. 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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