0　Introduction

Weight reduction has been one of the most important subjects in the aerospace industry. Al-Li alloy has been widely used in aerospace industry for its superior properties such as low density, high specific strength and excellent superplasticity[1]. But the bad formability of Al-Li alloy limits its application and increases the costs of forming. Technology of diffusion bonding combined with superplastic forming (SPF/DB) can fabricate the lightweight structure of multi-sheet structures[2-5]. However, with the development of the aviation industry, the application of SPF/DB process in Al-Li alloy has exposed a series of problems. For instance, the solid oxide film are easily formed on the surface of Al-Li alloy in high temperature, which will result in great difficulties on diffusion. A significant grain growth and subsequent dramatical mechanical properties reduction will happen due to the long time and high temperature during diffusion bonding process. What's more, the diffusion bonding limits the design complexity of parts.Therefore, studies on the combination of SPF with other welding methods are being conducted[6].

Vacuum electron beam welding can effectively prevent the oxidation of the weld, and the joints are excellent beacause of its narrow weld seam, fine microstructures and small deformation [7]. But the electron beam welded　joint of Al-Li alloy is inhomogeneity, which is significantly different from Al-Li alloy base metal. This study is aiming to investigate the microstructure evolution of electron beam welded joints during superplastic deformation.

1　Experimental

The material used in the present work was 2 mm thick sheet. And its chemical composition (mass fraction) is shown in Table 1. All of the joints were welded utilizing the ZD150-30C CV65M electron beam welding machine at the velocity of 25 mm/s, the accelerating voltage of 90 kV, the focusing current of 1 570 mA, and the electron beam current of 16 mA. The width of joint was about 3 mm. The dimension of hot tensile specimens is shown in Fig.1. Hot tensile tests were performed at temperatures ranging from 425 ℃ to 500 ℃ and strain rates ranging from 5×10-4 s-1 to 1×10-2 s-1. Microstructures of the specimens before and after tensile tests were observed by optical microscope.

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Table 1　Chemical composition of 5A90 Al-Li alloy (Mass fraction, %)

ElementMgLiZrSiFeAl5A90Al-Li4．5-6．01．9-2．30．08-0．15≤0．15≤0．2Balance

Fig.1　Dimension of samples for hot tensile tests(unit：mm)

2　Results and analysis

2.1　The high temperature mechanical properties of the joint

Fig.2 is the photo of welding specimens before and after superplastic deformation. Both sides of the specimen after deformation are tinily jagged, which indicates necking and anti-necking phenomenon during tensile process. Fig.3 shows the stress-strain curve of the welded specimens. It can be seen that extensive strain hardening takes place in the early stage of deformation, and the curves exhibit flow softening behavior in which the flow stress reaches a peak at a critical strain and then decreases with further straining. The specimens display low flow stresses, which displays the typical superplastic deformation.

Fig.2　Specimens before and after superplastic deformation

Fig.3　Stress-strain curves of samples

The microstructure of the welded joint before superplastic deformation is shown in Fig.5. the microstructure of weld bead is composed by equiaxed dendrites, α+ δ(AlLi) eutectic microstructure, and short clavite α+the δ(AlLi) + T (Al2MgLi) ternary eutectic microstructure, and the latter is located in the grain boundary. The coarse grain in HAZ is produced by the welding thermal cycle.

As shown in Fig.4b, the elongation of the weld joint increases first and then decreases with the increasingdeformation temperature, and the maximal elongation of 171.1% is obtained at the temperature of 450 ℃ and the strain rate of 5×10-3s-1. The increase of temperature contributes to promote grain boundary sliding and diffusion creep, and strengthen the dynamic recovery and recrystallization softening effect, and increase the deformation coordination of the internal microstructure of the joint. But when the temperature is too high, the grain grow severely, and the sample is easily oxidized, which leads to the elongation of joint decreased. The elongation increases first and then decreases with the strain rate decreasing. At low strain rate, the material has enough time to coordinate the deformation, which is beneficial to improve the elongation of the samples. However, when the strain rate is too low, the time of joint in high temperature environment is too long,which will result in the joint elongation decreased due to the grain grow severely.

纳米材料研究领域非常广泛,包括纳米金属材料、纳米有机和高分子材料、纳米陶瓷材料、纳米半导体材料和纳米复合材料等,以及这些材料在力学、光学、电学及生命科学等领域的应用[5]。在授课有限的时间内,教学内容不可能涵盖纳米材料研究的各个方面。课程学时少、内容多,不具备全面深入的讲解,一方面需要广泛覆盖,给学生一个纳米材料科学全面的概览,另一方面,作为一门研究生专业课程,为了避免浮于表面,导致课程变成一门纯介绍性的概论课程,又要能够突出重点,提高课程教学的深度。

Fig.6 shows the microstructures of weld bead with different strain at 450 ℃, 5×10-3s-1. It is found that the grain size of weld bead increases first and then decreases with the increase of strain. At the strain of 0.4, the microstructure of weld bead has almost transformed to coarse equiaxed grains (Fig.6a) and the eutectic structure that originally presented in the grain boundaries and dendrites are nearly disappeared. Moreover, the alloy elements in eutectic structure prone to cause grain boundary diffusion, and lead to grain boundary migration. As the deformation continues, the grain size further increases (Fig.6b,c). But as the strain reaches to 1.3, the coarse equiaxed grain become smaller and then the fine grain appears (Fig.6d). As the strain further improves to 1.7, the microstructure of weld has refined significantly (Fig.6e). These prove that the deformation mechanism of welded joint is grain boundary migration in the early stage, and then changes to thedynamic recrystallization when the strain beyonds 1.3.

Fig.4　Effects of tensile parameters on the peak flow stress and elongation-to-failure (a) Peak flow stress (b) Elongation

2.2　The microstructure evolution of the joint during superplastic deformation

The peak flow stress and elongation-to-failure of welded specimens are shown in Fig.4a and Fig.4b, respectively. From Fig.4a, it could be observed that the peak flow stress decreases with the deformation temperature increasing and the strain rate decreasing. This could be explained from that the kinetic energy of atoms increases and thermal activation enhances with the deformation temperature increasing, which would promote the grain boundary sliding and improve the ability of diffusion creep. Moreover, the softening degree caused by dynamic recovery and recrystallization also increases with deformation temperature increasing, and then result in the lower stress. The flow stress decreases from 15.9 MPa to 8.8 MPa with the strain rate increasing from 5×10-4s-1 to1×10-2s-1 at the temperature of 500 ℃, which is closely depended on the sensitivity of the strain rate. In the same deformation temperature, the dislocation density in grain increases rapidly at the higer strain rate, and then diffusion creep and dislocation slip could not coordinate the grain boundary sliding effectively due to dislocation pileup. Especially, there are large differences between the microstructure of weld bead and base metal, which will result in that the grain sliding and rotating process are easily blocked, and then cause stress concentration.

Fig.5　Microstructure of the joint before the superplastic deformation

知识可视化(Knowledge Visualization)作为在科学计算可视化、信息可视化和数据可视化基础之上发展起来的新兴研究领域，通过可视化的方式，知识可以被人们用来更好地存取、讨论、评估和日常管理，这就为应对信息时代人类所面临的挑战——快速获取和掌握知识提供了有力的支持[2]。知识可视化在缓解知识飞速增长所带给人们压力方面有着很重要的作用。

2.3　The microstructure evolution of HAZ during superplastic deformation

Fig.7 shows the microstructure of HAZ with different strain at 450 ℃, 5×10-3s-1. It is clearly observed from the Fig.7 that the microstructure of HAZ is inclined to be refined and equiaxed with the increase of strain. After deformation at the strain of 0.4, the HAZ consists of most coarse and irregular grains and a small number of fine grains (Fig.7a). When the strain increases to 1.0, most of the coarse grain has been further refined (Fig.7c). The refined coarse grain indicates that the coarse grain of HAZ recrystallizes during superplastic deformation. When the strain reaches to 1.70, the grain get further refinement and spheroidized (Fig.7e). This indicates that the superplastic deformation mechanism of the HAZ is dynamic recrystallization.

Fig.6　Microstructure of weld bead with different strain at 450 ℃, 5×10-3s-1 (a) 0.4; (b) 0.7; (c) 1.0; (d) 1.3; (e) 1.7

Fig.7　Microstructure of HAZ with different strain at 450 ℃, 5×10-3s-1(a) 0.4; (b) 0.7; (c) 1.0; (d) 1.3; (e) 1.7

3　Conclusion

(1) The welded joint of 5A90 Al-Li alloy has good superplasticity, and the maximal elongation of 171.1% is obtained at the optimal parameter 450 ℃, 5×10-3s-1.

文学作品在浩瀚的文献中占有极其重要的地位，在大多数公共图书馆的馆藏中也占有相当大的比重。编目机构在采用《中图法》来组织处理文学作品的归类时，历来存在下列四个方面的问题。

(2) The superplastic deformation mechanism of welded joint (weld bead and HAZ) is the grain boundary migration and dynamic recrystallization.

References

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[3]　Zhang L Y, Qi G G. Application of Al-Li alloys in aeronautical industry and advances on SPF/DB. Equipment Environmental Engineering,2001,19(3):1-3.

[4]　Wang C W,Zhang K F. Study of SPF/DB technology of Al-Li alloy. Journal of Plasticity Engineering,1998,5(4):1-6.

[5]　Ma Z Y,Mishra R S, Mahoney M W. Superplastic deformation behaviour of friction stir processed 7075Al alloy. Acta Materialia, 2002, 50(17): 4419-4430.

[6]　Cheng D H, Huang J H,Yang J, et al. Superplastic deformation mechanical behavior of laser welded joints of TC4 titanium alloys. Rare Metal Materials and Engineering,2010, 39(2): 277-280.

[7]　Cui L,Li X Y, He D Y, et al. Microstructure investigation of Nd: YAG laser welded 5A90 aluminium-lithium alloys[J].Transactions of the China Welding Institution,2010,31(9): 77-80.(in Chinese)