Effect of banded structure on mechanical properties of aluminum/magnesium dissimilar metal friction stir welding joint
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摘要: 采用搅拌摩擦焊(FSW)对厚度为4 mm的6061铝合金与AZ31B镁合金进行不同工艺的平板对接试验. 采用光学显微镜(OM)、扫描电镜(SEM)、X射线衍射仪(XRD)及能谱仪(EDS)对接头进行微观组织观察，采用电子万能试验机对接头力学性能进行测试. 结果表明，在接头焊核区(WNZ)中存在着明显的带状组织，带状组织是由插入镁基体中的铝合金条以及弥散分布在条带上的金属间化合物(IMCs)组成；IMCs主要为Al12Mg17和Al3Mg2；铝/镁异种金属FSW接头裂纹形核和扩展均发生在带状组织内；焊接工艺影响带状组织形态和IMCs尺寸及数量；随着转速(n)的增加或焊接速度(v)的降低，带状组织呈弯曲状，长度相对较短且呈不连续分布；当转速(n)过高或焊接速度(v)过低时，带状组织变细，但IMCs数量增多且尺寸变大；铝/镁异种金属FSW接头强度主要取决于带状组织形态和IMCs尺寸及数量.Abstract: The flat butt welding tests of different processes for 6061 aluminum alloy and AZ31B magnesium alloy with thickness of 4 mm were carried out by friction stir welding (FSW). Microstructure of the joints was observed and analyzed by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive spectrometer (EDS), mechanical properties of the joints were tested by electronic universal testing machine. The results show that, there are obvious banded structures in the weld nugget zone (WNZ), which are composed of aluminum slices inserted in the magnesium matrix and IMCs dispersed on the band. The types of IMCs are Al12Mg17 and Al3Mg2. Crack nucleation and propagation of aluminum/magnesium dissimilar metal FSW joints occur among the banded structure. Welding technology affects the morphology of banded structure and size and quantity of IMCs. With the increasing of rotational speed (n) or the decreasing of welding speed (v), the banded structure in the joint WNZ are curved, relatively short in length and discontinuously distributed. The banded structure becomes thinner, but the quantity of IMCs increase and the size becomes larger when the rotation speed (n) is too high or the welding speed (v) is too low. The ultimate strength of aluminum/magnesium dissimilar metal FSW joint mainly depends on the morphology of the banded structure and the size and quantity of IMCs.
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图 3 铝/镁FSW接头元素分布
Figure 3. Element distribution of Al/Mg FSW joint. (a) distribution of Mg element; (b) distribution of Al element
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图 6 不同转速下铝/镁FSW接头裂纹扩展路径
Figure 6. Crack propagation path of Al/Mg FSW joints under different rotation speed. (a) rotating speed 650 r/min-welding speed 20 mm/min; (b) rotating speed 750 r/min-welding speed 20 mm/min; (c) rotating speed 850 r/min-welding speed 20 mm/min
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图 7 不同转速下铝/镁FSW接头带状组织的形貌
Figure 7. Morphology of Al/Mg banded structure in FSW joints under different rotation speed. (a) rotating speed 650 r/min-welding speed 20 mm/min; (b) rotating speed 750 r/min-welding speed 20 mm/min; (c) rotating speed 850 r/min-welding speed 20 mm/min
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图 8 不同转速下工程应力应变曲线
Figure 8. Engineering stress strain curves under different rotating speed
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图 9 不同焊速下铝/镁FSW接头裂纹扩展路径
Figure 9. Crack propagation path of Al/Mg FSW joints under different welding speed. (a) rotating speed 750 r/min-welding speed 10 mm/min; (b) rotating speed 750 r/min-welding speed 20 mm/min; (c) rotating speed 750 r/min-welding speed 35 mm/min
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图 10 不同焊速下铝/镁FSW接头带状组织的形貌
Figure 10. Morphology of Al/Mg banded structure in FSW joints under different welding speed. (a) rotating speed 750 r/min-welding speed 10 mm/min; (b) rotating speed 750 r/min-welding speed 20 mm/min; (c) rotating speed 750 r/min-welding speed 35 mm/min
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图 11 不同焊速下工程应力应变曲线
Figure 11. Engineering stress strain curves under different welding speed
表 1 带状组织EDS分析
Table 1 EDS analysis of banded structure
点 Al元素原子分数a(％) Mg元素原子分数a(％) 相 1 59.31 34.056 Al3Mg2 2 29.31 42.43 Al12Mg17 3 31.406 45.124 Al12Mg17 4 34.35 52.8 Al12Mg17 + Al 5 10.53 70.32 Mg 
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