Microstructural Development in Friction Welded AZ31 Magnesium Alloy

Materials Transactions, Vol. 47, No. 4 (2006) pp. 1071 to 1076 Special Issue on Platform Science and Technology for Advanced Magnesium Alloys, III #2006 The Japan Institute of Metals Microstructural Development in Friction Welded AZ31 Magnesium Alloy Shinji Fukumoto 1, Soshi Tanaka 1; *, Toshitsugu Ono 1; *, Harushige Tsubakino 1, Tomoki Tomita 2, Masatoshi Aritoshi 2 and Kozo Okita 3 1 Graduate school of Engineering, University of Hyogo, Himeji 671-2201, Japan 2 Hyogo Prefectural Institute of Industrial Research, 3-1-12 Yukihira, Suma, Kobe, Hyogo 654-0037, Japan 3 Institute of Industrial Research, Osaka Sangyo University, 3-1-1 Nakagaito, Daito, Osaka 574-8530, Japan Microstructural development of friction welded AZ31 alloy was studied. The microstructures near weld interface consist of mainly three regions that are recrystallized fine-grain, mixed-grain and twin regions. The most impressive microstructural feature is grain refinement. Fine grains whose size was approximately 2 mm were produced at the weld interface due to a hot heavy working, resulting in the increase of micro- Vickers hardness. New fine grains were born at the shear bands that were introduced during the friction and upset processes. The grain size depended on the welding condition, especially the upset pressure. The smaller grains were obtained with higher upset pressure and shorter friction time. Although Hall Petch relation was basically realized in friction welded AZ31, it is necessary to consider the effect of work hardening. (Received November 7, 2005; Accepted February 1, 2006; Published April 15, 2006) Keywords: friction welding, magnesium alloy, Hall Petch relation, grain refinement, microstructure 1. Introduction Table 1 Chemical composition of AZ31 (mass%). Magnesium and its alloys are attractive in automobile, aircraft and electronic housing due to their superior specific strength and recyclability. However, it is hard to weld magnesium alloys by commercial fusion welding processes such as arc welding since magnesium is extremely active metal. Recently friction stir welding (FSW) is often applied to the welding of magnesium alloys since it is solid state process and the traveling speed is fast. 1) However, basically FSW has been developed for welding of plate. On the other hand, the friction welding can be applied for welding of round bars and it is also one of the solid state bonding processes. It has been reported that the sound AZ31 friction welded joints were obtained. 2 4) Therefore, it is supposed to be more common process to weld magnesium alloys. Although the friction welded joints show superior mechanical properties, the most attractive microstructural feature is grain refinement. It is well known that grain refinement is found in hot-heavy worked metals. For example, the submicron-grained microstructures were obtained in aluminum alloys by the torsion straining, equal-channel angular pressing (ECAP) and the accumulative roll bonding processes. 5 8) Copper with ultra fine grains was obtained by ECAP, too. 9) The grain refinement phenomena were found in Armco iron and magnesium alloy by the torsion straining and ECAP processes, too. 10 12) The friction welding is one of the hot-heavy working processes. The authors reported the microstructural developments in friction welded aluminum and magnesium alloys, which were completely different from those by the other solid state bonding processes. 13 15) The microstructural changes that are mainly grain refinement would affect on the mechanical properties. The purpose of this study is to clarify the grain refinement mechanism of AZ31 magnesium alloy by the friction welding *Graduate Student, University of Hyogo Al Zn Mn Fe Si Ni Cu Mg 2.92 0.85 0.46 0.002 0.006 <0:001 0.001 Bal. and the relation between microstructural development and hardness near the weld interface. 2. Experimental Procedures The AZ31 magnesium alloy extrusion bar whose extrusion temperature was 653 K of 16 mm in diameter was supplied to the continuous drive friction welding. The chemical composition of AZ31 is shown in Table 1. The friction pressure, P 1, and upset time, t 2 were 50 MPa and 6.0 s, respectively. The friction time,, was varied from 0.5 to 2.5 s. The rotational speed, N, of 40 and 60 s 1 and the upset pressure, P 2,of0, 100 and 150 MPa were selected, respectively. In the case of t 2 ¼ 0 s P 2 ¼ 0 MPa, the friction welding was performed without the upset process. Cross sections of joints were etched with the acetic Picral aq. after polishing with alumina powder for the optical microscopy. Grain size was measured on the optical micrographs by a section method. In this work, only grains that were surrounded with high angle grain boundaries were counted. The micro-vickers hardness near the weld interface was measured under the load of 0.245 N. Some joints were annealed at 623 K for 7.2 ks in Ar atmosphere to eliminate strain. 3. Results and Discussion 3.1 Microstructural development The typical macroscopic view of a friction welded AZ31 is shown in Fig. 1. The parameter x is defined as distance from the weld interface as shown in Fig. 1. In other words, x ¼ 0 mm indicates the weld interface. The symmetrical flash was uniformly formed around the entire weld circumference. The

1072 S. Fukumoto et al. Fig. 1 Weld interface (x=0) 10 mm Cross sectional view of friction weld joint. x were formed by dynamic recrystallization due to a hot-heavywork process. Grain size is smaller with higher upset pressure [see Figs. 2(a) and 3(a)] and the details will be described later on. Mixed-grain region adjoins the fine-grain region and consists of small and large grains [Figs. 2(b) and 3(b)]. Large grains were surrounded by small grains, which showed mesh structure. It is believed that the mesh structure is originally from shear band. Recrystallized grains were born at the shear bands as the nucleation site. In general, shear bands are introduced at an angle of 45 degree to the rolling direction when a plate is extend by applying pressure. Since the deformation mode in the friction welding is more complicated than that of rolling process, lots of shear bands could be introduced in various directions. The last one located between base alloy and mixed-grain region. A number of twins are observed there [Fig. 3(d)]. In some cases, not only twins but also jagged grain boundary was observed [Figs. 2(c) and 3(c)]. This area expanded with higher upset pressure. Those twins and shear band like structure are characteristic features in magnesium alloys that were not observed in the friction welded aluminum joints, 14) which would help to clarify the grain refinement process. microstructural flow from center to periphery was observed. Microstructures gradually changed toward the weld interface. Typical regions are shown in Figs. 2 and 3. Cross sections showed three characteristic regions, that is, fine-grain, mixed-grain and twin regions. Fine grain region is located at the weld interface and consists of new fine grains. They 3.2 Effect of friction time and upset pressure on grain refinement The relation between mean grain size and the distance from weld interface is shown in Fig. 4. Grain size decreased with decreasing x. The smallest grains whose size is approximately 2.5 mm are formed at the weld interface with (a) (b) (c) (d) Fig. 2 Microstructures of friction weld AZ31 near friction weld interface, ¼ 1:5 s P 1 ¼ 50 MPa and P 2 ¼ 100 MPa and N ¼ 40 s 1. (a) x ¼ 0 mm, (b) x ¼ 2 mm, (c) x ¼ 5 mm, (d) x ¼ 11 mm.

Microstructural Development in Friction Welded AZ31 Magnesium Alloy 1073 (a) (b) (c) (d) Fig. 3 Microstructures of friction weld AZ31 near friction weld interface; ¼ 1:5 s P 1 ¼ 50 MPa and P 2 ¼ 150 MPa and N ¼ 40 s 1. (a) x ¼ 0 mm, (b) x ¼ 2 mm, (c) x ¼ 5 mm, (d) x ¼ 11 mm. Mean grain size, d /µm 20 15 10 5 0 0 5 10 Fig. 4 Distance from weld interface, x /mm Change in grain size near weld interface. =0.5s 1.5s 2.5s the friction time of 0.5 s. The range of 0 to 2 mm of x corresponds approximately to the recrystallized region, that is, fine and mixed-grain regions. Recrystallized fine grain microstructures at the weld interface (x ¼ 0) are shown in Fig. 5. A microstructure of AZ31 base alloy is shown in Fig. 5, too. Although the base alloy was extrusion bar, the grains were almost equiaxed and the mean grain size was 25 mm. The grains grew with increasing friction time; mean grain size of 2.5, 2.9 and 3.2 mm were formed with 0.5, 1.5 and 2.5 s of friction time, respectively. Such a drastic grain refinement is general phenomenon in friction weld joints, for example, grain size of 1 mm has been reported in the case of friction welded commercial purity aluminum due to the dynamic recrystallization. 13,14) Since the dynamic recrystallization occurs due to hot heavy work, the upset process that has charge of most amount of deformation could play a great role on it. The effect of upset process on the grain refinement is shown in Fig. 6. Although the grain refinement was observed at the weld interface of even without upset process, it was more remarkable with upset pressure than without. Moreover, the grain refinement region was extended when the upset pressure was loaded. As a result, the grain refinement process is described as follows. A lot of shear bands are introduced at the temperature below recrystallization temperature at the beginning of the friction process due to a large amount of complex deformation. When the temperature reaches the recrystallization temperature, the weld is still under deformation process. So new fine grains are born at the shear bands during mainly the upset process. Since weld process is completed within just ten seconds, new grains will not grow up significantly, resulting in leaving fine grains there. 3.3 Hall Petch relation Asahina et al. reported no significant change in micro- Vickers hardness was detected in AZ31 friction welded joints. 2) Ogawa et al. reported that AZ31 was hardened a little bit due to work hardening near the friction weld interface. 16) However, the friction welding caused microstructural changes drastically as shown in Figs. 2 and 3, which must

1074 S. Fukumoto et al. (a) (b) (c) Fig. 5 Microstructural refinement at weld interface against frictional time; P 2 ¼ 100 Mpa, (a) ¼ 1:5 s (b) ¼ 2:5 s (c) Base alloy. a b c d 50 µm Fig. 6 Microstructures of friction weld AZ31: ¼ 1:0 s, P 1 ¼ 50 MPa (a) P 2 ¼ 150 MPa, x ¼ 0 mm (b) P 2 ¼ 150 Mpa, x ¼ 2 mm (c) P 2 ¼ 0 MPa, x ¼ 0 mm (d) P 2 ¼ 0 MPa, x ¼ 2 mm.

Microstructural Development in Friction Welded AZ31 Magnesium Alloy 1075 HV (load: 0.245 N) 75 70 65 60 =0.5 s =1.5 s =2.5 s 55 0 5 10 Distance from weld interface, x/ mm Fig. 7 Micro-Vickers hardness near friction weld interface; P 1 ¼ 50 MPa, P 2 ¼ 100 MPa and N ¼ 40 s 1. affect on the hardness. The distribution of micro-vickers hardness near weld interface is shown in Fig. 7. The micro- Vickers hardness of base alloy was approximately HV55. The hardness at the region of x < 2 mm was considerably larger than the base alloy, which corresponded to where remarkable grain refinement occurred (Fig. 4). The tendency was more outstanding with the condition of the shorter friction time. In particular, the maximum hardness of HV75 was obtained with the shortest friction time of 0.5 s at x ¼ 0. The grains at the weld interface contain few dislocations as shown in Fig. 8(a), which means they have been almost recrystallized. On the other hand, the microstructure at x ¼ 5 mm shows lots of dislocations [Fig. 8(b)]. Thus the increasing of hardness is most likely due to the balance of the grain refinement and work hardening. So the decreasing in hardness near the weld interface with longer friction time might be due to the grain growth. The joint with the condition of ¼ 1:5 s was annealed at 623 K for 7.2 ks to eliminate the effect of the work hardening. The microstructures after the annealing are shown in Fig. 9. Although the grain growth occurred, the grain size at the weld interface was still smaller that that at the other regions. The twins disappeared owing to the elimination of strain. Anyways, the recrystallized grains in various grain size with few dislocations were obtained. The relation between grain a size and HV in the annealed and as welded specimens are shown in Fig. 10. The HV of the annealed joints was directly proportional to the grain size which are shown as solid circle in Fig. 10 and the relation that is calculated by least square method is illustrated in the following equation: HV ¼ 45 þ 30d 1=2 ð1þ The Hall Petch coefficient in yield strength of magnesium alloy, for example AZ91, ZK60 and so on, were varied from 200 to 300 MPamm 1=2. 17 20) Since it is hard to estimate the yield strength at each region of the joints, the coefficient could not be compared to this work directly. Park et al. found the Hall Petch relation in the friction stir welded AZ91D, which is almost similar to the eq. (1). 21) The slight difference in the coefficient might be due to the dislocation density as Park mentioned. Anyway, Hall Petch relation in the hardness seems to be found to the friction welded magnesium alloy. The hardness in as-weld joints are larger than eq. (1). The difference is small in the fine-grain and base alloy regions whose grain size are several and several tens mm, respectively. The plots that show larger hardness than eq. (1) correspond to the twin region where are work-hardened at relatively low temperature that contains a lot of dislocations as shown in Fig. 7(b). So the difference became smaller with longer friction time due to annealing effect since the temperature during the welding must be higher with longer friction time. On the other hand, since fine grain region has almost been recrystallized as mentioned above, the difference is quite small from eq. (1). 4. Summary (1) The grain refinement occurred at the friction weld interface by the dynamic recrystallization, resulting in the increasing of hardness. The mean grain size was approximately 2 mm at the weld interface. The recrystallized small grains were born at the shear bands that were introduced during the friction process. (2) Smaller grains were obtained with higher upset pressure. The upset process played a great role on the grain refinement. (3) The Hall Petch relation was found in the friction weld joints of AZ31 including the effect of work hardening. b 1 µm Fig. 8 TEM images of friction weld; (a) at weld interface (x ¼ 0 mm), (b) x ¼ 5 mm.

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