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近年来，轨道列车高速化及轻量化已成为现代交通运输业的主要发展方向[1]. 铝合金因比强度高、耐蚀性好，具有优良的可成形性和可加工性，适用于制造大型中空型材而广泛应用于高速列车和地铁车体的轻量化制造中[2-3].
铝合金中空型材和熔化极气体保护焊(MIG焊)结合的制造方法，虽然减轻了列车重量，但接头强度低、耐冲击韧性差，对高速列车运行安全性不利. 搅拌摩擦焊(FSW)是英国焊接研究所(TWI)发明的一种新型固相焊接方法，与常规熔化焊相比，FSW焊接过程因母材未发生熔化可有效避免产生气孔、夹渣及裂纹等冶金缺陷[4-6]. 针对铝合金焊接，FSW具有焊缝成形平整美观、焊接残余应力低、焊接变形小、接头强度高以及绿色环保等显著优势[7-8]. 现阶段，在国内外高速列车和地铁铝合金车体的制造领域，FSW已成为主要焊接工艺之一[9-10]. 铝合金搅拌摩擦焊技术研究吸引了国内外大量研究学者，并取得了丰硕的研究成果[11-13].
随着FSW在高速列车铝合金车体制造中的规模化应用，产品结构设计也日益多样化和复杂化，出现了MIG焊缝与FSW焊缝的重叠甚至是交叉等特殊结构形式. MIG焊缝叠加FSW焊缝时，因FSW焊缝组织再次经历焊接热循环，其组织及性能将会发生复杂变化，从而影响重叠接头的力学性能. 但目前，国内外鲜有关于MIG焊叠加对FSW焊接头组织及性能影响的相关报道，尚需开展深入研究以拓展FSW工艺在铝合金车体制造中的应用范围.
文中主要研究MIG焊叠加对4 mm厚6A01-T5铝合金FSW焊接头组织和性能的影响，MIG焊叠加位置分别位于FSW焊缝中心、FSW焊缝前进侧热力影响区、FSW焊缝后退侧热力影响区，目的是为焊接结构设计提供理论依据，为实际生产提供技术支持.
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试验母材为6A01-T5铝合金中空大截面型材，焊接区厚度为4 mm，试板尺寸为600 mm × 1000 mm，MIG焊填充材料为ER5356铝合金焊丝，直径为1.2 mm. 试验母材与填充焊丝化学成分如表1所示.
6A01-T5铝合金型材FSW接头形式为对搭接接头，采用ESAB动龙门式双机头搅拌摩擦焊机和轴肩直径15 mm、针长4.2 mm的搅拌头；叠加MIG焊采用半自动焊方式，焊接设备为KEMPPI 3200型MIG电焊机. FSW和MIG焊均采用经过评定合格的焊接工艺规程.
表 1 试验母材与填充焊丝化学成分(质量分数，%)
Table 1. Chemical compositions of base metal and filler wire (mass fraction, %)
材料 Si Fe Cu Mn Mg Cr Zn Ti Al 6A01-T5 0.60 0.25 0.20 0.40 0.68 0.20 0.10 0.08 余量 ER5356 0.10 0.40 0.10 0.15 4.80 0.10 0.10 0.13 余量 为保证叠加MIG焊熔深，在MIG焊叠加位置，即在FSW焊缝中心、FSW焊缝前进侧热力影响区、FSW焊缝后退侧热力影响区位置分别开2 mm深的V形坡口，MIG焊叠加位置及坡口形式示意图如图1所示. MIG焊叠加前采用铝合金清洗剂和不锈钢丝刷对焊件表面进行清理.
图 1 FSW焊缝开坡口位置及叠加MIG焊位置示意图(mm)
Figure 1. Schematic diagrams of the groove and MIG welding superposition of the FSW Friction stir weld. (a) at FSW center; (b) at advancing side of FSW thermo-mechanically affected zone; (c) at retreating side of FSW thermo-mechanically affected zone
焊后分别截取FSW焊接头、FSW焊缝中心叠加MIG焊接头、FSW焊缝前进侧热力影响区叠加MIG焊接头和FSW焊缝后退侧热力影响区接头的横截面，制备金相试样并用Keller试剂腐蚀，然后采用OLYMPUS-BX51 M金相显微镜观察焊缝成形及微观组织特征. 利用FM-700型显微硬度仪测试接头的显微硬度，载荷为100 g，保持时间为15 s，测试位置距离工件上表面1 mm. 拉伸试验采用WDW-300E电子万能试验机，拉伸速度为1 mm/min，取5次试验结果平均值为最终试验结果，拉伸采用去除余高的光滑试样，试样尺寸如图2a所示. 图2中拉伸试样尺寸符合国家标准GB/T 2651—2008(ISO 4136:2001)，疲劳试样尺寸参考Q/SF 71-064—2013《铝合金材料试验规范》，试样厚度为3.8 mm，确保去除了焊缝表面下凹、搭接界面曲钩的影响. 疲劳试验采用QBG-50型全数字高频疲劳试验机，疲劳方式为恒负荷轴向拉伸疲劳，应力比R = 0，加载频率范围99 ~ 102 Hz，指定寿命1 × 107周次，疲劳试样均采用保留余高试样，试样尺寸图2b所示. 文中疲劳试验依据Q/SF71-64铝合金材料试验规范的要求，分别采用成组法和升降法绘制接头S-N曲线及获得指定寿命107周次下接头的疲劳强度，其中成组法采用4 ~ 5级应力水平，每个应力水平下取3 ~ 5个有效试样；升降法每组接头取13 ~ 15个有效试样，要求至少获得6组有效的对子数. 拉伸试样断口特征利用ZEISS SUPRA 55扫描电子显微镜分析.
图 2 拉伸及疲劳试样尺寸示意图(mm)(板厚: 3.8 mm)
Figure 2. Dimension diagrams of the tensile and fatigue samples (mm) (thickness: 3.8 mm). (a) tensile specimen size; (b) fatigue specimen size
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FSW焊接头、FSW焊缝中心叠加MIG焊接头、FSW焊缝前进侧热力影响区叠加MIG焊接头和FSW焊缝后退侧热力影响区叠加MIG焊接头的宏观形貌和微观组织分别如图3～图6所示. 可以发现，四种接头的焊缝熔合良好，MIG焊/FSW叠加位置未出现气孔等缺陷，分别如图3a～图3d所示. 图3a中搅拌摩擦焊接头前进侧的搭接的原始界面处有0.2 mm向上弯曲的界面曲钩.
图 3 FSW焊接头宏观形貌及微观组织特征
Figure 3. Macro-morphology and micro-structure characteristics of the FSW joint. (a) macroscopic morphology of the joint; (b) weld nugget zone (A zone); (c) advancing side of FSW thermo-mechanically affected zone (B zone); (d) retreating side of FSW thermo-mechanically affected zone (C zone); (e) heat affected zone (D zone)
图 4 FSW焊缝中心叠加MIG焊接头宏观形貌及微观组织特征
Figure 4. Macro-morphology and micro-structure characteristics of the MIG/FSW joint with the superposition at the FSW weld center. (a) macroscopic morphology of the joint; (b) weld nugget zone (A zone); (c) advancing side of FSW thermo-mechanically affected zone (B zone); (d) retreating side of FSW thermo-mechanically affected zone (C zone); (e) heat affected zone (D zone)
图 5 FSW焊缝前进侧热力影响区叠加MIG焊接头宏观形貌及微观组织特征
Figure 5. Macro-morphology and micro-structure characteristics of the MIG/FSW joint with the superposition at the FSW TMAZ (AS) . (a) macroscopic morphology of the joint; (b) weld nugget zone (A zone); (c) MIG fusion line zone near the FSW weld nugget (B zone); (d) advancing side of FSW thermo-mechanically affected zone (C zone); (e) heat affected zone (D zone); (f) retreating side of FSW thermo-mechanically affected zone (E zone); (g) MIG fusion line zone near base material (F zone)
图 6 FSW焊缝后退侧热力影响区叠加MIG焊接头宏观形貌及微观组织特征
Figure 6. Macro-morphology and micro-structure characteristics of the MIG/FSW joint with the superposition at the FSW TMAZ (RS) . (a) macroscopic morphology of the joint; (b) weld nugget zone (A zone); (c) MIG fusion line zone near the FSW weld seam (B zone); (d) advancing side of FSW thermo-mechanically affected zone (C zone); (e) heat affected zone (D zone); (f) MIG fusion line zone near the FSW weld seam (E zone); (g) MIG fusion line zone near base material (F zone)
FSW焊接头的微观组织由焊核区、热力影响区、热影响区和母材组成. 焊核区因晶粒在搅拌作用下来不及长大而破碎，呈细小的等轴晶且均匀分布，如图3b所示. 热力影响区受到搅拌头的搅拌作用明显小于焊核区，导致其晶粒发生一定程度的扭曲变形，从图3c和3d所示，前进侧热力影响区与母材分界线明显，而后退侧热力影响区与母材分界线较模糊，这主要因为焊接过程中前进侧和后退侧的塑性金属的流动状态有所差异造成的. 热影响区出现了较为明显的晶粒长大、粗化，如图3e所示.
MIG焊缝叠加FSW焊缝时，因MIG焊二次热输入的影响，FSW焊核区均存在晶粒长大的现象，且距离MIG焊缝越近，等轴晶粒的尺寸越大，如图4b，图5b，图5c，图6b和图6f所示；热影响区晶粒也发生进一步长大而变得更加粗大，如图4e，图5e和图6e所示. 当MIG焊缝叠加位置位于FSW焊缝中心时，FSW焊缝前进侧和后退侧热力影响区与MIG焊叠加位置附近出现了典型的MIG组织特征，并且前进侧叠加时更加明显，如图4c和图4d所示.
当MIG焊缝叠加位置位于FSW焊缝前进侧热力影响区时，靠近MIG焊缝位置的FSW前进侧热力影响区几乎完全消失，如图5d所示；FSW焊缝后退侧因远离MIG焊缝，其组织特征未发生明显变化，如图5f所示；而母材侧MIG焊熔合线附近呈现较为明显的MIG焊组织特征，如图5g所示. 当MIG焊缝叠加位置位于FSW焊缝后退侧热力影响区时，其组织特征变化与前进侧叠加相类似，但母材侧MIG熔合线附近出现的MIG焊组织更为明显，如图6d，图6f和图6g所示.
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FSW焊接头、FSW焊缝中心叠加MIG焊接头、FSW焊缝前进侧热力影响区叠加MIG焊接头和FSW焊缝后退侧热力影响区叠加MIG焊接头的硬度测试结果如图7所示. 可以发现，FSW焊接头焊核区硬度值最高，热力影响区和热影响区硬度值较低，这是因为该区域晶粒粗化、部分析出相长大或重溶，缺少焊核区的细晶强化及位错强化效果，同时析出强化效果减弱，因此表现为硬度降低，相比前进侧区域的硬度值略低，如图7a所示. MIG焊叠加FSW焊接头时，叠加区域位置的硬度值均出现了明显的降低，尤其是叠加后FSW焊接头的热力影响区和热影响区，成为整个接头的薄弱环节，如图7b ~ 图7d所示，这主要是因为MIG焊二次热输入进一步加剧了该区域组织晶粒的粗化、长大及合金元素的损失. 这表明，MIG焊叠加FSW焊，加剧了FSW接头的软化现象，将导致叠加接头的力学性能会有所降低.
图 7 FSW及MIG/FSW叠加焊接头的硬度分布
Figure 7. Microhardness profiles of the FSW and MIG/FSW joints. (a) FSW joint; (b) MIG weld superposed at the FSW center; (c) MIG weld superposed at advancing side of FSW thermo-mechanically affected zone; (d) MIG weld superposed at retreating side of FSW thermo-mechanically affected zone
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FSW焊接头、FSW焊缝中心叠加MIG焊接头、FSW焊缝前进侧热力影响区叠加MIG焊接头和FSW焊缝后退侧叠加MIG焊接头的拉伸试验结果如图8所示，四种接头的平均抗拉强度分别为219.8，188.0，195.4和191.4 MPa，分别可达到母材的87.8%，75.1%，78.1%和76.5%；四种接头的平均断后伸长率分别为9.0%，8.3%，10.1%和8.9%. 拉伸试验结果表明：MIG焊在FSW焊缝叠加，均降低了FSW焊接头的抗拉强度，且在FSW焊缝中心位置叠加时下降更多，这主要是因为中心叠加时MIG焊接过程的热输入对前进侧和后退侧热力影响区及热影响区带来的影响更为显著所致.
图 8 FSW及MIG/FSW叠加焊接头的拉伸试验结果
Figure 8. Tensile test results of the FSW and MIG/FSW joints
四种接头拉伸断裂位置及断口如图9所示. FSW焊接头断裂于前进侧热力影响区，主要是因为该区域因搅拌高温加热区较宽使其软化所导致的，断口分布大量韧窝，整体呈现韧性断裂的特征，如图9a所示.
图 9 FSW及MIG/FSW叠加焊接头的断裂位置及断口特征
Figure 9. Fracture locations and fracture characteristics of the FSW and MIG/FSW joints. (a) FSW joint; (b) MIG weld superposed at the FSW center; (c) MIG weld superposed at advancing side of FSW thermo-mechanically affected zone; (d) MIG weld superposed at retreating side of FSW thermo-mechanically affected zone
FSW焊缝中心叠加MIG焊接头均断裂于MIG焊缝热影响区和FSW焊缝前进侧热影响区，其原因是该区域分别是MIG焊接头和FSW焊接头的薄弱环节，断口存在大量较浅的韧窝，并伴随有解理面，整体仍呈现韧性断裂特征但塑性有所下降，如图9b所示. FSW焊缝前进侧和后退侧热力影响区叠加MIG焊时，叠加接头拉伸试样均起裂于靠近母材的MIG焊热影响区，沿着热影响区扩展直至断裂，其断口均整体呈现韧性断裂特征，如图9c和9d所示.
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在给定循环应力比r = σmin/σmax的条件下，焊接接头所加疲劳载荷的名义应力范围Δσ与疲劳寿命N之间服从幂函数关系[14]，即
$${\left( {\Delta \sigma } \right)^m}N = C$$ (1) 式中：m，C为材料常数.
将式(1)两端取对数并整理得
$$\lg N = A + B\lg \Delta \sigma $$ (2) 式中：A，B为拟合常数. 式(1)与式(2)中各参数的对应关系为
$$m = - B$$ (3) $$C = {10^A}$$ (4) 显然，在双对数坐标中, Δσ与N成对数的线性关系. 根据疲劳试验数据参照式(2)并利用最小二乘法原理拟合各组试样的S-N曲线(存活率50%)，如图10所示.
图 10 FSW及MIG/FSW叠加焊接头疲劳S-N曲线
Figure 10. Fatigue S-N curves of the FSW and MIG/FSW joints
由式(3)，式(4)计算出S-N曲线斜率m和材料常数C，并依据式(2)拟合出107循环周次下各组接头的疲劳强度，以及通过升降法疲劳试验获得的疲劳强度值，见表2.
表 2 S-N曲线参数及疲劳强度
Table 2. Parameters of S-N curves and the fatigue strength
接头组别 m C 疲劳强度$\Delta {\sigma _0}/{\rm{MPa}}$ 拟合值 试验值 FSW 17.66 1.86 × 1040 76.5 76.7 FSW中心 + MIG 10.21 2.45 × 1025 63.4 65.0 FSW前进侧 + MIG 10.86 4.66 × 1026 64.9 67.5 FSW后退侧 + MIG 9.99 1.64 × 1025 66.5 65.0 由表2可知，FSW焊接头、FSW焊缝中心叠加MIG焊接头、FSW焊缝前进侧热力影响区叠加MIG焊接头和FSW焊缝后退侧热力影响区叠加MIG焊接头在107周次下疲劳强度的试验值分别为76.7，65.0，67.5和65.0 MPa，而计算值与试验值拟合良好. 显然，FSW接头经过MIG焊叠加后使FSW接头的疲劳强度降低，但仍符合不低于63.6 MPa的试验规范要求.
各组接头疲劳断裂的位置如图11所示. 其中，FSW接头疲劳断裂主要位于FSW前进侧热力影响区，这是由于热力影响区在FSW热循环和部分塑性变形的作用下形成的组织不均匀，晶粒形状和取向的差异造成局部区域变形程度不均匀而导致应力集中，且前进侧的硬度相比于后退侧还要低些，更利于滑移在此软化部位启动进而形成疲劳裂纹，因此热力影响区的前进侧更容易发生疲劳破坏[14]. 而分别在FSW焊缝中心、FSW焊缝前进侧热力影响区和FSW焊缝后退侧热力影响区进行MIG叠加焊后的接头，其疲劳裂纹均启裂于MIG叠加焊缝的焊趾部位. 显然，与FSW焊接头相比，在FSW接头上MIG叠加焊缝的焊趾处应力集中是导致接头疲劳强度下降的主要原因，这与通常铝合金MIG焊接头疲劳断裂的位置相同[15].
图 11 FSW及MIG/FSW叠加焊接头疲劳断裂位置
Figure 11. Fatigue rupture locations of the FSW and MIG/FSW joints. (a) FSW joint; (b) MIG weld superposed at the FSW center; (c) MIG weld superposed at advancing side of FSW thermo-mechanically affected zone; (d) MIG weld superposed at retreating side of FSW thermo-mechanically affected zone
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(1) MIG焊叠加FSW，叠加焊缝熔合良好，叠加位置未出现气孔等缺陷；FSW焊核区及热影响区组织发生长大. MIG焊叠加位置在FSW前进侧/后退侧热力影响区时，前进侧/后退侧热力影响区几乎完全消失，后退侧/前进侧组织未发生明显变化.
(2) FSW焊接头焊核区硬度值最高，热力影响区和热影响区硬度值较低. MIG焊叠加FSW焊接头时，叠加区域位置的硬度值均出现了明显的降低，尤其是叠加后FSW焊接头的热力影响区和热影响区，成为整个接头的薄弱环节.
(3) FSW焊接头、FSW焊缝中心叠加MIG焊接头、FSW焊缝前进侧热力影响区叠加MIG焊接头和FSW焊缝后退侧叠加MIG焊接头的平均抗拉强度分别为219.8，188.0，195.4 MPa和191.4 MPa，MIG焊叠加降低了FSW接头的拉伸性能. 断口呈现大量的韧窝特征，整体边纤维韧性断裂特征.
(4) FSW焊接头、FSW焊缝中心叠加MIG焊接头、FSW焊缝前进侧热力影响区叠加MIG焊接头和FSW焊缝后退侧热力影响区叠加MIG焊接头在107周次下疲劳强度分别为76.7，65.0，67.5和65.0 MPa. FSW接头叠加MIG焊使接头的疲劳强度有所降低.
Effects of MIG welding superposition on microstructure and property of 6A01-T5 FSW joint
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摘要: 研究了MIG焊叠加对6A01-T5铝合金FSW焊接头组织及性能的影响. 结果表明, MIG/FSW叠加焊缝熔合良好，叠加位置未出现气孔等缺陷，FSW焊核区及热影响区组织发生粗化，叠加位置附近微观组织出现明显改变；叠加区域硬度明显降低，尤其是FSW焊缝热力影响区和热影响区. FSW、中心叠加、前进侧热力影响区叠加和后退侧热力影响区叠加MIG焊接头的抗拉强度分别为219.8, 188.0, 195.4和191.4 MPa，MIG焊叠加降低了接头的抗拉强度，断口均表现韧性断裂特征；FSW焊接头及带有MIG叠加焊缝余高的三种接头中值疲劳强度分别为76.7, 65.0, 67.5和65.0 MPa，MIG焊叠加也使FSW接头的疲劳性能有所下降.Abstract: Effects of MIG welding superposition on microstructure and property of 6A01-T5 FSW joint was researched in this work. The results indicated that the MIG/FSW joints formed well without porosity defects near the superposition. The microstructure of the FSW weld nugget and heat affected zone became coarse and which near the superposition was changed obviously. The hardness value of the superposition was reduced significantly, especially for FSW thermo-mechanically affected zone and heat affected zone. The tensile strengths of the FSW joint, MIG superposition on the FSW weld center, MIG superposition on the FSW advancing side thermo-mechanically affected zone, and MIG superposition on the FSW retreating side thermo-mechanically affected zone were 219.8 MPa, 188.0 MPa, 195.4 MPa and 191.4 MPa, respectively. MIG superposition reduced the FSW joint tensile strength, and the fracture appearance of all joints belonged to ductile fracture. The median fatigue strengths of above the FSW joint and three joints with MIG weld reinforcement were 76.7 MPa, 65.0 MPa, 67.5 MPa and 65.0 MPa respectively. The MIG superposition was also reduced the fatigue properties of the FSW joints.
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Key words:
- Aluminum alloy /
- Frication stir welding /
- Superposition welding /
- Mechanical property /
- MIG welding
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图 1 FSW焊缝开坡口位置及叠加MIG焊位置示意图(mm)
Figure 1. Schematic diagrams of the groove and MIG welding superposition of the FSW Friction stir weld. (a) at FSW center; (b) at advancing side of FSW thermo-mechanically affected zone; (c) at retreating side of FSW thermo-mechanically affected zone
图 2 拉伸及疲劳试样尺寸示意图(mm)(板厚: 3.8 mm)
Figure 2. Dimension diagrams of the tensile and fatigue samples (mm) (thickness: 3.8 mm). (a) tensile specimen size; (b) fatigue specimen size
图 3 FSW焊接头宏观形貌及微观组织特征
Figure 3. Macro-morphology and micro-structure characteristics of the FSW joint. (a) macroscopic morphology of the joint; (b) weld nugget zone (A zone); (c) advancing side of FSW thermo-mechanically affected zone (B zone); (d) retreating side of FSW thermo-mechanically affected zone (C zone); (e) heat affected zone (D zone)
图 4 FSW焊缝中心叠加MIG焊接头宏观形貌及微观组织特征
Figure 4. Macro-morphology and micro-structure characteristics of the MIG/FSW joint with the superposition at the FSW weld center. (a) macroscopic morphology of the joint; (b) weld nugget zone (A zone); (c) advancing side of FSW thermo-mechanically affected zone (B zone); (d) retreating side of FSW thermo-mechanically affected zone (C zone); (e) heat affected zone (D zone)
图 5 FSW焊缝前进侧热力影响区叠加MIG焊接头宏观形貌及微观组织特征
Figure 5. Macro-morphology and micro-structure characteristics of the MIG/FSW joint with the superposition at the FSW TMAZ (AS) . (a) macroscopic morphology of the joint; (b) weld nugget zone (A zone); (c) MIG fusion line zone near the FSW weld nugget (B zone); (d) advancing side of FSW thermo-mechanically affected zone (C zone); (e) heat affected zone (D zone); (f) retreating side of FSW thermo-mechanically affected zone (E zone); (g) MIG fusion line zone near base material (F zone)
图 6 FSW焊缝后退侧热力影响区叠加MIG焊接头宏观形貌及微观组织特征
Figure 6. Macro-morphology and micro-structure characteristics of the MIG/FSW joint with the superposition at the FSW TMAZ (RS) . (a) macroscopic morphology of the joint; (b) weld nugget zone (A zone); (c) MIG fusion line zone near the FSW weld seam (B zone); (d) advancing side of FSW thermo-mechanically affected zone (C zone); (e) heat affected zone (D zone); (f) MIG fusion line zone near the FSW weld seam (E zone); (g) MIG fusion line zone near base material (F zone)
图 7 FSW及MIG/FSW叠加焊接头的硬度分布
Figure 7. Microhardness profiles of the FSW and MIG/FSW joints. (a) FSW joint; (b) MIG weld superposed at the FSW center; (c) MIG weld superposed at advancing side of FSW thermo-mechanically affected zone; (d) MIG weld superposed at retreating side of FSW thermo-mechanically affected zone
图 8 FSW及MIG/FSW叠加焊接头的拉伸试验结果
Figure 8. Tensile test results of the FSW and MIG/FSW joints
图 9 FSW及MIG/FSW叠加焊接头的断裂位置及断口特征
Figure 9. Fracture locations and fracture characteristics of the FSW and MIG/FSW joints. (a) FSW joint; (b) MIG weld superposed at the FSW center; (c) MIG weld superposed at advancing side of FSW thermo-mechanically affected zone; (d) MIG weld superposed at retreating side of FSW thermo-mechanically affected zone
图 10 FSW及MIG/FSW叠加焊接头疲劳S-N曲线
Figure 10. Fatigue S-N curves of the FSW and MIG/FSW joints
图 11 FSW及MIG/FSW叠加焊接头疲劳断裂位置
Figure 11. Fatigue rupture locations of the FSW and MIG/FSW joints. (a) FSW joint; (b) MIG weld superposed at the FSW center; (c) MIG weld superposed at advancing side of FSW thermo-mechanically affected zone; (d) MIG weld superposed at retreating side of FSW thermo-mechanically affected zone
表 1 试验母材与填充焊丝化学成分(质量分数，%)
Table 1. Chemical compositions of base metal and filler wire (mass fraction, %)
材料 Si Fe Cu Mn Mg Cr Zn Ti Al 6A01-T5 0.60 0.25 0.20 0.40 0.68 0.20 0.10 0.08 余量 ER5356 0.10 0.40 0.10 0.15 4.80 0.10 0.10 0.13 余量 
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表 2 S-N曲线参数及疲劳强度
Table 2. Parameters of S-N curves and the fatigue strength
接头组别 m C 疲劳强度 $\Delta {\sigma _0}/{\rm{MPa}}$ 拟合值 试验值 FSW 17.66 1.86 × 1040 76.5 76.7 FSW中心 + MIG 10.21 2.45 × 1025 63.4 65.0 FSW前进侧 + MIG 10.86 4.66 × 1026 64.9 67.5 FSW后退侧 + MIG 9.99 1.64 × 1025 66.5 65.0
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