Microstructure and properties of vacuum electron beam welded WE43 magnesium alloy joint

This study presents the microstructure and properties of vacuum electron beam welded WE43 magnesium alloy joint. The process parameters of acceleration voltage 150 kV, electron beam current 120 mA and welding speed 35 mm/s are used for vacuum electron beam welding of WE43 rare earth magnesium alloy plate. In this study, the main compositions of the weld are α-Mg and a small amount of eutectic rare earth phase β-Mg24Y5. The mass fraction of the rare earth phase β-Mg24Y5 in the weld area is more than that of the base metal. Segregation of Zr-rich particles can occur in weld zone. The average hardness of the weld is about 27% higher than that of the base metal, and the hardness near the center line of the weld is the highest. The yield strength, tensile strength and elongation of the joint are higher than that of the base metal by approximately 17%, 14% and 41%. Tensile fracture morphology of the welded joint is characterized by ductile and brittle mixed fracture. So, electron beam welding can achieve the connection of WE43 magnesium alloy plate with excellent microstructure and performance. It may be of great significance for this study to expand the application of rare earth magnesium alloy. The microstructure and morphology of WE43 magnesium alloy electron beam welded joints are analyzed. No defects such as oxidation and sag are found in magnesium alloy welds. The test result show, WE43 magnesium alloy electron beam welded joints possess high mechanical properties. By analyzing the tensile fracture morphology of welded joint, it is found that there are both ductile and brittle fracture. The microstructure and morphology of WE43 magnesium alloy electron beam welded joints are analyzed. No defects such as oxidation and sag are found in magnesium alloy welds. The test result show, WE43 magnesium alloy electron beam welded joints possess high mechanical properties. By analyzing the tensile fracture morphology of welded joint, it is found that there are both ductile and brittle fracture.


Introduction
Growing environmental demand and higher fuel efficiency encourage the use of new lightweight load-bearing materials, so magnesium alloy has become an attractive light metal material and is increasingly widely used. Rare earth (RE) magnesium alloy not only has excellent properties of traditional magnesium alloys, but also has high-temperature mechanical properties and corrosion resistance [1]. WE43 magnesium alloy belongs to highstrength heat-resistant RE magnesium alloy with strong plasticity. Its main RE elements are Y, Nd, Gd. Because RE elements will affect the viscosity, thermal conductivity and other properties of magnesium alloy melts, the solidification process of RE magnesium alloys will be different from ordinary magnesium alloys. So RE elements will have a certain impact on the welding performance of magnesium alloys. There are few studies on weldability of RE magnesium alloy plates. Wu Guohua [2] studied the principle of the influence of RE elements on the weldability of magnesium alloys. The results showed that RE elements are surfactants of magnesium alloys, which can reduce the surface tension of magnesium alloys and reduce hydrogen reserves in liquid metals. Liu Jianing [3] studied the effects of various RE elements on the microstructure, mechanical properties and corrosive behavior of magnesium alloy. The results show that adding a certain amount of RE elements can reduce the temperature of the solid phase line and liquid phase line of RE magnesium alloy. Therefore, when the temperature of the molten pool remains unchanged, a small amount of RE elements in the magnesium alloy can make the fluidity of the molten pool better, thus improving the welding performance of RE magnesium alloys.
EBW (electron beam welding) is characterized by high energy density, strong penetration ability, fast welding speed,and can overcome the phenomenon that magnesium alloys are easily oxidized [4][5][6]. In order to fully understand the welding performance, microstructure and mechanical properties of the joints of WE43 magnesium alloy plates, this work adopts an optimized EBW process to weld WE43 magnesium alloy plates. The welding characteristics of WE43 magnesium alloy plates were studied, and the microstructure, joint morphology and mechanical properties of the electron beam welded joint were analyzed, which provided a reference for further understanding of the relationship between the welding properties and the microstructure evolution of the magnesium alloy [7,8].
In the next section, it contains sample preparation, experimental setup and procedure. Section 3 shows detailed discussions and results about the morphology of the welded joint, microstructure analysis of the weld and mechanical properties of the welded joint. Finally, in Sect. 4, we present the main conclusions of this study, illustrating future recommendations and directions for the research on welding technology of rare earth magnesium alloy.

Materials and experimental procedure
The WE43 magnesium alloy plates with a thickness of 4 mm were used as the substrat in this work, and the composition is listed in Table 1. Combined with actual production experience, 520℃ × 10 h solid solution treatment + 225℃ × 14 h manual aging treatment was adopted [9]. Before welding, polish the surface of the magnesium alloy workpiece with sandpaper to remove the oxide film, and then clean the surface of the test board with acetone to remove oil stains and impurities [10].
The welding mode of EBW was single-face butt welding. Before the EBW, no groove processing was conducted. During the EBW, no welding wire was applied [11]. The EBW machine model HN203 was used as the welding equipment used in this work. The power of the equipment is 60 kW, the maximum acceleration voltage of the electron gun is 150 kV, and the size of the welded vacuum chamber is 7550 mm × 2650 mm × 3550 mm, and its working vacuum degree is about 1.7 × 10 −2 Pa. The welding parameters are listed in Table 2.
After welding, the wire cutting method was used to sample the welding area, then grinding and polishing, and finally acetic picral (4.2 g picric acid,10 ml acetic acid,70 ml etha-nol and 10 ml distilled water) was used for etching, and the etching time is 10-20 s [12]. The microscopic morphology of the welded joint was observed by XSP-4XB metallographic microscope. ZEISS EVO 18 tungsten filament scanning electron microscope was used to observe the joint microstructure and tensile fracture morphology of welding parts. The phase structure of the samples was determined by X-ray Diffraction (XRD) using Rigaku Smartlab 9 kW Cuk-α radiation with scattering angles in the range of 20-90°, a scanning rate of 6°/min and the phase identification based on jade database. The CMT4204 microcomputercontrolled electronic tester was used for tensile test. The size of the sample is shown in Fig. 1. Wilson Hardness 401MVD Microhardness Meter was used to test the hardness distribution of WE43 magnesium alloy EBW joints.  Figure 2 shows the appearance of the front and back of WE43 magnesium alloy EBW joint. Figure 2a reveals that the width of the weld is very uniform, and the surface shape is small fish scales, and there are no defects such as biting edges. Figure 2b reveals that the welding parts have been welded through, and the width of the weld is also very uniform, narrower than the width of the front of the weld, the surface is not smooth on the front, and there are intermittent slight depressions. The main reason is that the surface tension of molten magnesium is small [13]. In the process of EBW, when metal vapor escapes from the weld, the impact force of liquid metal is greater than that of magnesium surface growth, resulting in the sputtering of some liquid metals, resulting in the smooth back of the weld not as smooth as the front of the weld, and some areas are slightly sunken. Figure 3 shows the morphology of WE43 magnesium alloy EBW joint. No defects such as oxidation, porosity and crack were found in magnesium alloy welds. The weld section is a typical wedge with a wider upper part and a narrower lower part, the width of upper surface of weld is 1.81 mm. The heat source model of EBW is a superposition of nonlinear point heat source and linear heat source [14]. Therefore, the temperature distribution in the direction of workpiece thickness and the formed weld shows the morphological characteristics is wide at the top and narrow at the bottom on the cross-section. And the workpiece has been welded through, which also reflects the characteristics of high energy density and strong penetration ability of EBW.  Figure 4c reveals that EBW can refine the weld structure and make the equiaxed grains smaller. When welding magnesium alloys with EBW, the cooling speed of the molten pool is relatively fast. When the molten   Table 3. From the results of EDS analysis, it can be seen that the main element in BM (region B) is Mg, and the content (mole fraction, the same below) is about 93.01%. The content of elements in BM (region B) and WZ (region C) is compared and analyzed. The decrease of the percentage content of Mg elements in WZ leads to a relative increase in the percentage content of Y, Zr, Nd and Gd elements. When welding magnesium alloy plates with EBW, as the temperature rises rapidly, the metal melts and then reaches the boiling point. Because the boiling point of Mg (1090 °C) is lower than that of Y (3345 °C), Zr (4409 °C), Nd (3100 °C) and Gd (3250 °C) [15], the evaporation of magnesium are more serious in high-temperature molten pool [16]. The enrichment of RE elements can hinder the growth of α-Mg grains, further play the role of refining grains in the welding area, and improve the mechanical properties of WE43 magnesium alloy welded joint [17]. The spherical phase (region D) is the Zr nucleus, in which Zr content is about 81.88% and Mg is only 18.12%. So   Figure 5 shows the XRD patterns of WE43 magnesium alloy BM and WZ. It can be presented from Fig. 5 that α-Mg is the main phase in both WZ and BM and the diffraction peak of β-Mg 24 Y 5 phase in BM is weaker than that of WZ. The above analysis results of the elements and phases in BM and WZ show that the use of EBW with extremely high energy density is beneficial to improve the element distribution inside the weld and promote the formation of low melting point compounds in β-Mg 24 Y 5 phase [18]. Figure 6 shows DSC curve of WE43 magnesium alloy. It can be seen that there are two heat absorption peak points in the sample during heating, with corresponding temperatures of 566.2 °C and 639 °C. A turning point A (556.13℃) appears above the first peak point, corresponding to 556.13℃, which is the initial temperature of phase transition. Based on this turning point and the first peak point, it can be concluded that the eutectic temperature range of the alloy is 556.13-566.2 °C. During the heating process, an exothermic peak appears near 639 °C. Because the point is within the melting temperature range of the matrix α-Mg, according to the two points of B (602.76 °C) and C (664.35 °C) near the second peak, it can be judged that the solid-liquid temperature range of the alloy is 602.76-664.35 °C, corresponding to the initial melting temperature of α-Mg and the temperature of all conversion to liquid phase [19]. Figure 7 shows stress-strain curve of the base metal and the joint. As shown in Fig. 7, non-apparent yield phenomenon appeared in the tensile processes of WE43 magnesium alloy EBW joint and base metal [20]. The mechanical properties of WE43 magnesium alloy EBW joint and base metal are listed in Table 4. @@  It can be seen from Table 4 that the tensile strength, yield strength and elongation of WE43 magnesium alloy at room temperature are 197 MPa, 81 MPa and 7.0% respectively. After vacuum electron beam welding treatment, the tensile strength of the joint is 237 MPa, 17% higher than that of the base metal, and the yield strength is 94 MPa, 14% higher than the base metal, the elongation rate is 11.8%, and 41% higher than the base metal. The results show that the mechanical properties of WE43 magnesium alloy electron beam welded joints are better than that of base metal. The improvement of the mechanical properties of the joint can be explained by Hall-Petch relationship: the yield strength of a polycrystal was inversely proportional to the grain size, that is, the smaller the grain, the higher the yield strength, and the larger the grain, the lower the yield strength [21,22]. After electron beam welding, the grain of WE43 magnesium alloy is obviously refined, so the yield strength of the welded joint is higher than that of the base metal. Figure 8 shows the tensile fracture morphologies of WE43 welded joint. Figure 8a, c and d show cleavage planes and small curved tearing edges. Figure 8b, c and d show that there are more dimples in the local area of the fracture. The existence of a large number of cleavage planes, tear ridges and dimples indicates that the fracture mode is ductile brittle mixed fracture [23][24][25]. Figure 9 shows the microhardness distribution in different zones. By comparing the hardness of the upper, middle and lower layer perpendicular to the weld, it can be presented that the hardness is symmetrically distributed on the sample. The hardness of the weld is higher than that of the BM, mainly because the grains in the weld zone are very small. Due to no obvious HAZ, division area is barely visible. The average hardness of the weld is about 27% higher than that of the BM, and the hardness near the center of the weld is the highest. The hardness values   [26]. When solidification occurs in the center of the weld, temperature gradient decreased toward the weld center and consequently the length of liquid-solid region increased, which is conducive to inhibiting the growth of dendrites and promoting the formation of fine equiaxed grains. so EBW can reduce the loss of alloy elements, refine the weld structure and improve the hardness of the weld [27,28]. Furthermore, another reason for the grain refinement in the weld center is the heterogeneous nucleus of rare earth in the weld center. Therefore, the microhardness of the weld obtained after electron beam welding of WE43 magnesium alloy can achieve the hardness performance requirements of the BM, even higher than the hardness of the BM.

Conclusions
In this work, the morphology, microstructure, rare earth phase distribution characteristics and mechanical properties of WE43 magnesium alloy EBW joints are studied. no obvious heat affected zone(HAZ) is founded. No defects such as oxidation and sag were found in magnesium alloy welds. The weld is narrow and the weld section is a typical wedge with a wider upper part and a narrower lower part, the width of upper surface of weld is 1.81 mm. The weld zone is mainly composed of α-Mg phase and a small amount of β-Mg 24 Y 5 phase. Element Zr mainly exists as a Zr nucleus and does not form compounds with other elements. The tensile strength, yield strength and elongation of the joint reaches 237 MPa, 94 MPa, 11.8%, respectively, higher than that of the base metal by approximately 17%, 14% and 41%. And the hardness of the welded joint is higher than that of the base metal. The tensile fracture morphology of the welded joint is characterized by ductile and brittle mixed fracture.
The study results will provide new ideas and theoretical basis for the deeper studies regarding the welding of magnesium dissimilar alloys, and will help expand their industrial application.
Author contributions All authors contributed to the study conception and design. Data collection and analysis were performed by Haili Xu, Shanshan Hu. Conceptualization, methodology were performed by Sheng Lei. Material preparation and formal analysis were performed by Zhengwei Xue and Zhonghao Huang. The first draft of the manuscript was written by Xiang Zhang and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.
Funding This work was funded by the Natural Science Foundation of the Higher Education Institutions of Anhui Province under Grant No. KJ2020ZD42.

Data availability
The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

Conflict of interest
The authors declare they have no financial interests.
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