Study on microstructure and mechanical properties of new generation high-magnesium aluminum alloy cross-welded joints by friction stir welding-friction stir welding
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摘要: 为满足大型铝合金船舶壁板的制造需求，对新一代高镁铝合金进行了搅拌摩擦交叉焊接试验. 结果表明，交叉焊接头成形良好，搅拌区晶粒尺寸最小，热力影响区晶粒形态没有明显方向性，与单道搅拌摩擦焊相比，交叉焊接头搅拌区晶粒组织更细. 显微硬度测试结果表明，交叉焊接头显微硬度变化范围较小，前进侧接头软化明显；拉伸试验测试结果表明，交叉焊接头抗拉强度为340 MPa，为母材强度的87%，对比搅拌摩擦焊接头抗拉强度358 MPa略微降低，在热影响区断裂，断裂方式为45°韧性断裂；疲劳裂纹萌生于焊缝底部，在最大应力150 MPa下循环超2 × 106次未断裂，疲劳性能良好，瞬断区断裂方式为韧性断裂.Abstract: In order to meet the manufacturing requirements of large aluminum alloy ship panels, the cross-welded by friction stir welding-friction stir welding (FSW/FSW) tests of new generation high-magnesium aluminum alloy were carried out. The results show that a well FSW/FSW joint was obtained. Grain size in stir zone (SZ) is the smallest, and there is no obvious directionality of grains morphology in thermo mechanical affected zone (TMAZ). The grain size in the SZ of the FSW/FSW joint is smaller than that of the FSW. Hardness test results show that hardness in the FSW/FSW weld joint has a smaller variation range as compared to the FSW joint. A soft region appears in the advancing side (AS) of the FSW/FSW joint. Tensile test results show that the tensile strength of FSW/FSW joint is 340 MPa, which is around 87% of the base material. The tensile samples fracture in the heat-affected zone (HAZ) is a typical ductile fracture mode along the 45° to the tensile direction. The strength of the FSW/FSW joint is slightly lower than that of FSW joint (358 MPa). The fatigue cycle of the cross-welded joint is up to 2 × 106 times without fracture when the maximum stress is 150 MPa, indicating an excellent anti-fatigue performance. Fatigue cracks main initiate at the bottom of the joint, and the fracture mode of transient fault zone is ductile fracture.
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图 1 FSW/FSW接头拉伸及疲劳样品几何形状(mm)
Figure 1. The geometry of tensile and fatigue samples of FSW/FSW joints
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图 2 FSW与FSW/FSW接头横截面形貌
Figure 2. Cross-sections of FSW and FSW/FSW joints.(a) Cross-section of FSW joint; (b) Cross-section of FSW/FSW joint
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图 3 FSW各区微观组织
Figure 3. Microstructure of joints by FSW. (a) BM; (b) RS-TMAZ; (c) AS-TMAZ; (d) SZ
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图 4 FSW/FSW接头各区微观组织
Figure 4. Microstructure of joints by FSW/FSW. (a) 1st-SZ; (b) TMAZ; (c) HAZ; (d) SZ
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图 5 接头硬度分布
Figure 5. Microhardness distributions of joints. (a) crosswise microhardness distribution of FSW and FSW/FSW joint; (b) Lengthwise microhardness distribution of FSW/FSW joint
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图 6 接头抗拉强度
Figure 6. Tensile strengths of joints. (a) Tensile strength comparison; (b) Stress-strain curve
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图 7 FSW/FSW接头拉伸断口
Figure 7. Fracture morphologies of failure joints by FSW/FSW. (a) Fracture position of FSW/FSW; (b) Fracture morphologie of FSW/FSW; (c) Area a amplification; (d) Area b amplification
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图 8 新一代高镁铝合金FSW/FSW接头疲劳断口
Figure 8. The fatigue fracture of new generation high-magnesium aluminum alloy joints by FSW/FSW. (a) Macroscopic fatigue fracture; (b) Zoom in area a; (c) Zoom in area B; (d) Zoom in area C
表 1 新一代高镁铝合金化学成分(质量分数，%)
Table 1 Chemical composition of new generation high-magnesium aluminum alloy
Mg Mn Fe Si Zn Zr Cu Al 6.2 0.85 0.4 0.35 0.12 0.017 0.012 余量 
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表 2 FSW/FSW疲劳结果
Table 2 Fatigue test results by FSW/FSW
编号 应力σ/MPa 循环次数N/次 断裂情况 1 80 1 × 107 未断裂 2 100 5 × 106 未断裂 3 150 2 × 106 未断裂 1-1 135 1.1 × 105 断裂 2-1 100 5 × 104 断裂 3-1 150 4.2 × 104 断裂
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