Microstructure and mechanical properties of friction stir welding of AZ31B magnesium alloy added with cerium

Microstructure and mechanical properties of friction stir welding of AZ31B magnesium alloy added with cerium

JOURNAL OF RARE EARTHS, Vol. 28, No. 2, Apr. 2010, p. 316 Microstructure and mechanical properties of friction stir welding of AZ31B magnesium alloy ...

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JOURNAL OF RARE EARTHS, Vol. 28, No. 2, Apr. 2010, p. 316

Microstructure and mechanical properties of friction stir welding of AZ31B magnesium alloy added with cerium YU Sirong (Ѣᗱ㤷)1, CHEN Xianjun (䰜ᰒ৯)1, HUANG Zhiqiu (咘ᖫ∖)1,2, LIU Yaohui (߬㗔䕝)1 (1. Key Laboratory of Automobile Materials (Jilin University), Ministry of Education, and College of Materials Science and Engineering, Jilin University, Changchun 130025, China; 2. College of Materials Science and Engineering, Jiamusi University, Jiamusi 154007, China) Received 19 June 2009; revised 13 December 2009

Abstract: The AZ31B magnesium alloy sheet added with 0.5 wt.% Ce was welded with friction stir welding (FSW). The microstructures and mechanical properties of the welded joint were investigated. The results showed that the microstructures in the weld nugget zone were uniform and with small equiaxed grains. The grains in the heat-affected zone and the thermo-mechanical affected zone were coarser than those in the base metal zone and the weld nugget zone. The ultimate tensile strength of AZ31B magnesium alloy added with 0.5 wt.% Ce was 270.41r4.48 MPa, and its elongation to fracture was (17.71r0.60)%. The ultimate tensile strength of FSW joint was 237.97r2.53 MPa, and its elongation to fracture was 6.63r0.60%. The fracture locations of FSW joint were in the heat-affected zone. The ductile fracture was the main fracture mode. The ultimate tensile strength of the sample along the direction of the weld was 270.02r1.45 MPa, and its elongation to fracture was (17.08r0.39)%. The microhardness in the weld nugget zone was slightly lower than that in the base metal. The microhardness in the thermo-mechanical affected zone and heat affected zone was lower than that in the weld nugget zone. The microhardness increased from the surface to the bottom of the weld. Keywords: AZ31B magnesium alloy; friction stir welding; microstructure; mechanical property; rare earths

Magnesium and magnesium alloys are the lightest structural materials in industrial applications[1]. They have low density, high specific strength, high specific stiffness and excellent machinability[2]. Therefore, magnesium alloys are widely used in the aerospace, automotive, and electronics industry fields[3,4]. In order to reduce the weight for the purpose of energy saving, one of the important measures is to replace steel and iron parts with magnesium alloy parts[5]. Most of magnesium alloy parts were mainly produced using the casting process now[6]. After hot deformation treatment, the microstructures were refined and the casting defects were eliminated to produce magnesium alloys with higher strength and better ductility[7]. Therefore, the wrought magnesium alloys are expected to be new types of high-performance structural materials in this century. As one of the wrought magnesium alloys used widely, AZ31B magnesium alloy has good ductility, but poor strengths at room temperature and high temperature[8]. Thus rare earth, such as Ce DQGLa, is used to refine the grains of AZ31B wrought magnesium alloy and enhance its mechanical properties[9,10]. The welded structure is inevitably used in the engineering application of wrought magnesium alloys. However, magnesium alloys are of poor weldability because of their exceptional thermal and electrical properties and oxidation characteristics. Especially, the traditional welding methods can induce the residual stress, pores, slag, splash, and other defects[11]. These limited greatly the application scope of mag-

nesium alloys. Therefore, it is very important to investigate the welding behavior of magnesium alloys. Friction stir welding (FSW) is a solid connection method invented by the Welding Institute, UK, in 1991[12]. Compared with the traditional melting welding, FSW exhibits many advantages, such as no splash, no smoke, and no oxidation, and it does not need the wire and the protection of gas. In this sense alone, FSW is suitable for the welding of magnesium alloys. In fact, it is difficult to obtain sound FSW joint of magnesium alloys because the close-packed hexagonal structure of magnesium alloy results in the lack of slip plane during plastic processing. In the past few years, a number of studies have been conducted to evaluate the feasibility of FSW of magnesium alloys[11,13], and the results showed that sound FSW joints with uniform microstructure and good mechanical properties can be achieved under optimized FSW parameters. To our knowledge, hardly no evaluation has been carried out on the weldability of AZ31B magnesium alloy added with Ce via FSW. In this work, we reported the results of the microstructure characteristics in different zones and the mechanical properties of FSW joints of magnesium alloy added with 0.5 wt.% Ce. 1 Experimental AZ31B magnesium alloy sheet added with 0.5 wt.% Ce was used as the base metal, and its chemical composition is

Foundation item: Project supported by the Support Plans for Science and Technology of Changchun City (2007KZ07), the Program for New Century Excellent Talents in University, and 985 project of Jilin University Corresponding author: YU Sirong (E-mail: [email protected]; Tel.: +86-431-85095862) DOI: 10.1016/S1002-0721(09)60104-6

YU Sirong et al., Microstructure and mechanical properties of friction stir welding of AZ31B magnesium alloy added with…

listed in Table 1. The dimensions of the samples for FSW were 200 mm (length)×60 mm (width)×5 mm (thickness). The tool of FSW with a screwed pin was made of hot work die steel. The shoulder diameter, pin diameter and pin height were 16, 7, and 4.7 mm, respectively. The processing parameters affecting the microstructures and properties of FSW joints mainly include the tool rotation rate, the travel speed and the applied pressure[14]. In this work, the applied pressure of the tool is hard to determine, so the moving down distance of the tool shoulder was used as one of the processing parameters. A horizontal XTJ-05 metallographic microscope was employed to observe the microstructure of the welded joints. The tensile properties of the welded joints were measured using MTS810 electro-hydraulic servo test system at room temperature. The shape and size of the tensile specimen are shown in Fig. 1. The position of the tensile samples taken from the welded joint is shown in Fig. 2. The speed of the crosshead was 0.5 mm/min. The tensile test of every material was repeatedly carried out three times, and the average of the tensile properties of three samples was served as the final reported result. The morphologies of the tensile fracture surfaces were analyzed using JSM-5310 scanning electron microscope (SEM). The microhardness of the welded joints was measured using FUTURE-TECH FM-700 digital microhardness meter under the conditions of 100 g of the applied load and 15 s of the loading time. Table 1 Chemical composition of AZ31B magnesium alloy added with 0.5 wt.% Ce Elements Al

Zn

Content 3.19

0.81 0.5

Ce

Mn

Si

0.334 0.02

Fe

Cu

0.005 0.05

Ca

Be

Mg

0.04

0.1

Bal.

Fig. 1 Dimension of the tensile specimen (mm)

Fig. 2 Position of the tensile samples taken from the welded joint

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2 Results and discussion 2.1 Microstructure of FSW joint The welded joint bears the mechanical agitation of the tool and the friction and pressure of the shoulder at the same time during FSW. Compared with the traditional welding methods, the microstructures of FSW joint exhibit unique characteristic. The schematic diagram of the typical zones in the cross section of a FSW joint of magnesium alloy is shown in Fig. 3. The microstructures of the FSW joint can be divided into four zones. The zone A is the base metal zone (BMZ), the zone B the heat-affected zone (HAZ), the zone C thermo-mechanical affected zone (TMAZ), and the zone D the weld nugget zone (WNZ). The microstructures of the typical zones in the cross-section of FSW joint of AZ31B magnesium alloy added with 0.5 wt.% Ce under a tool rotation speed of 1300 r/min, welding speed of 40 mm/min, and a shoulder moving down distance of 0.17 mm are shown in Fig. 4. It can be seen that the microstructures of AZ31B magnesium alloy added with 0.5 wt.% Ce are mainly fine dynamic recrystallization grains, and there are small amount of coarse grains and elongated grains (Fig. 4(a)). In the FSW process, the metal in the HAZ was neither stirred by the pin nor rubbed by the shoulder, and it experienced a heating process only due to the heat conduction. Therefore, the crystal grains in the HAZ grew up and were coarser than those in the BMZ (Fig. 4(b)). The metal in the TMAZ is located at the edge of the tool. The plastic flow and deformation of grains occurred because of the rotation and stir of the tool. Some grains were stretched along the direction of the moving tool. Most of the microstructures in the TMAZ experienced the dynamic recovery and dynamic recrystallization process, and some grains even grew up (Fig. 4(c)). In the FSW process, the metal in the WNZ bore not only the pressure and the rotating and moving friction force of the shoulder, but also the rotation friction and shear force of the pin of the tool, so the crystal grains were elongated and crushed. Broken grains produced the dynamic recovery and dynamic recrystallization under the effects of the stir and friction heat and the plastic deformation. A inconspicuous growth of the recrystallization grains can also be found. Therefore, the microstructures in the WNZ were uniform with small equiaxed grains (Fig. 4(d)), and the grain size is slightly bigger than that in the base metal. Because the mixing effect of the tool in the TMAZ was not strong enough and the deformation energy stored was not enough, the grain size in the TMAZ was big and nonuniform as compared with that in the WNZ.

Fig. 3 Schematic diagram of the typical zones in the cross section of a FSW joint of magnesium alloy

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JOURNAL OF RARE EARTHS, Vol. 28, No. 2, Apr. 2010

Fig. 4 Microstructure in the typical zones of FSW joint cross section of AZ31B magnesium alloy added with 0.5 wt.% Ce (a) BMZ; (b) HAZ; (c) TMAZ; (d) WNZ

2.2 Tensile properties of FSW joints The tensile results are shown in Table 2. It can be found that the ultimate tensile strength of the base metal is 270.41r4.48 MPa, and its elongation to fracture is (17.71r0.60)%. Under optimized process parameters for welding, the ultimate tensile strength of the welded joint (Specimen 1) is 237.97r2.53 MPa, the elongation to fracture is 6.63r0.60%. That is to say, both the ultimate tensile strength and the elongation to fracture of the welded joint are lower than those of the base metal. This result is in agreement with that of Ref. [15]. Through observing the tensile fracture surface, it can be found that the tensile fracture position is in the HAZ. The microstructure in the HAZ is the coarsest, so the HAZ in the welded joint is the weakest position and is easy to fracture when the applied load reaches the strength limit. It can also be found that the ultimate tensile strength of the weld (Specimen 2) is 270.02r1.45 MPa, and the elongation to fracture is (17.08r0.39)%. This indicates that the ultimate tensile strength in the WNZ is high because the microstructure in this zone is fine. The ultimate tensile strength in the WNZ is about 99.86% of that of the base metal.

The tensile curves of AZ31B magnesium alloy added with 0.5 wt.% Ce and its FSW joints are shown in Fig. 5. The morphologies of the tensile fracture surfaces of the base metal and FSW joint of AZ31B magnesium alloy added with 0.5 wt.% Ce are shown in Fig. 6. It can be seen that there are a large number of dimples on the fracture surface of the base metal, and its fracture mode belongs to the ductile fracture (Fig. 6(a)). Whereas for the tensile fracture surface of FSW joint, there are not only dimples but also a few cleavage patterns, and the fracture mode is mainly ductile fracture (Fig. 6(b)). Therefore, the elongation to fracture of FSW joints is less than that of the base metal. 2.3 Microhardness The microhardness distribution curve of the welded joints of AZ31B magnesium alloy added with

Table 2 Tensile properties of the base metal and welded joints of AZ31B magnesium alloy added with 0.5 wt.%Ce Samples

Ultimate tensile strength/MPa Elongation to fracture/%

Base metal

270.41r4.48

17.71r0.60

Specimen 1

237.97r2.53

6.63r0.60

Specimen 2

270.02r1.45

17.08r0.39

Fig. 5 Tensile curves of AZ31B magnesium alloy added with 0.5 wt.% Ce and its welded joints

YU Sirong et al., Microstructure and mechanical properties of friction stir welding of AZ31B magnesium alloy added with…

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Fig. 6 Fracture surfaces of AZ31B magnesium alloy added with 0.5 wt.% Ce (a) and FSW joint (b)

Fig. 7 Microhardness distribution in FSW joint of AZ31B magnesium alloy added with 0.5 wt.% Ce

0.5 wt.% Ce at a tool rotation speed of 1400 r/min, a welding speed of 40 mm/min, and a shoulder moving down distance of 0.19 mm is shown in Fig. 7. It can be seen that the microhardness in the WNZ is slightly lower than that in the BMZ. The reason is that the grain size in the WNZ is bigger than that in the BMZ[13]. The grains in the TMAZ and HAZ grew up because of the stir and friction heat effects, so the microhardness is lower than that in the WNZ. The microhardness distribution in the WNZ from the surface to the bottom of the weld is shown in Fig. 7(b). It can be seen that the microhardness showed an increasing trend from the surface to the bottom of the weld. The distribution of the stir and friction heat because of the strong mixing of the tool and the friction of the shoulder is nonuniform in the weld, and the temperature is lowered in gradient from the top to the bottom of the weld, so the grain size in the upper surface of the weld is bigger than that in the bottom, resulting in the microhardness increasing from the upper surface to the bottom. 3 Conclusions (1) The microstructure of AZ31B magnesium alloy added with 0.5 wt.% Ce was mainly small dynamic recrystallization, and there were small amount of coarse grains and elongated grains. The microstructures in the weld nugget zone were uniform with small equiaxed grains. The grains in the heat-affected zone and the thermo-mechanical affected zone were coarser than those in the base metal zone and the weld

nugget zone. (2) The ultimate tensile strength of AZ31B magnesium alloy added with 0.5 wt.% Ce was 270.41r4.48 MPa, and its elongation to fracture was (17.71r0.60)%. The ultimate tensile strength of FSW joint was 237.97r2.53 MPa, and its elongation to fracture was (6.63r0.60)%. The fracture locations of FSW joint were in the heat-affected zone. The ductile fracture was the main fracture mode. The ultimate tensile strength of the sample along the direction of the weld was 270.02r1.45 MPa, and its elongation to fracture was (17.08r0.39)%. (3) The microhardness in the weld nugget zone was slightly lower than that in the base metal. The microhardness in the thermo-mechanical affected zone and heat affected zone was lower than that in the weld nugget zone. The microhardness increased from the surface to the bottom of the weld.

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