Study on the influence of weld toe radius on the evolution behavior of fatigue short cracks
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摘要:
焊趾半径对焊接结构的疲劳性能有重要影响.综合泰森多边形法和子模型技术，采用粘塑性率相关的晶体塑性本构，在宏观的十字形焊接接头焊趾局部构建了二维微观RVE(representative volume element)子模型.基于滑移带平均的疲劳指标参量计算滑移带的成核寿命和扩展寿命，通过降低具有最小寿命的滑移带的弹性模量模拟裂纹的形成，实现了宏观-微观跨尺度疲劳短裂纹演化行为仿真.分析表明：当焊趾半径小于0.5 mm时，成核寿命随着焊趾半径增大而增大；当焊趾半径大于0.5 mm时，成核寿命主要受到晶粒形态与取向的影响呈现波动性变化.焊趾RVE子模型可划分为焊趾圆弧影响区和非影响区.在焊趾圆弧影响区，裂纹演化行为主要受到焊趾半径影响，成核位置有所差异，扩展速率随裂纹循环次数呈线性上升；随着裂纹扩展，焊趾半径影响逐渐减小，晶粒形态与取向影响逐渐增大；当裂纹扩展到非影响区，裂纹演化行为主要受到晶粒形态和取向的影响，扩展速率呈现较大的波动性.
Abstract:The weld toe radius has a significant impact on the fatigue performance of welded structures. By integrating the Voronoi tessellation method and submodeling technique, a two-dimensional microscopic representative volume element (RVE) submodel was constructed at the macroscopic cruciform welded joint weld toe using a viscoplastic rate-related crystal plasticity constitutive model. The average fatigue indicator parameters of the slip band were calculated to determine the nucleation life and extension life of the slip band. The formation of cracks was simulated by reducing the elastic modulus of the slip band with the shortest life, realizing the simulation of macro-micro scale fatigue short crack evolution behavior. Analysis shows: when the weld toe radius is less than 0.5 mm, the nucleation life increases with the increase of the weld toe radius; when the weld toe radius is greater than 0.5 mm, the nucleation life mainly fluctuates due to the influence of grain shape and orientation. The weld toe RVE submodel can be divided into the weld toe arc influence zone and the non-influence zone. In the influence zone, the crack evolution behavior is mainly affected by the weld toe radius, with different nucleation positions and the extension rate increases linearly with the number of crack cycles; as the crack extends, the influence of the weld toe radius gradually decreases, and the influence of grain shape and orientation gradually increases; when the crack extends to the non-influence zone, the crack evolution behavior is mainly affected by the grain shape and orientation, and the extension rate shows significant fluctuations.
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Keywords:
- Fatigue indicator parameter /
- Crystal plasticity /
- Weld toe radius /
- Short crack
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0. 序言
搅拌摩擦焊(friction stir welding, FSW)作为一种固相焊接技术，具有焊缝质量高、变形小等优点[1-2]. 目前加工制造业对焊接智能化、高效化的要求日益上升，机器人搅拌摩擦焊得以更普遍的应用.在实际大型结构的FSW生产中，由于接头形式、板材加工精度以及工装夹具装配质量问题，焊接过程容易产生较大的间隙，对接头的成形和性能极为不利[3-4]，当工件之间的间隙超过工件厚度的10%时，很难获得无缺陷质量良好的接头[5]. 间隙的存在导致焊核区(weld nugget zone，WNZ)材料流动不充分，焊缝出现孔洞和隧道等缺陷[6]. 同时，工件被塑化的材料流入间隙，弥补材料缺失使得焊缝位置减薄严重，降低接头承载能力[7].
研究人员[8-9]采用粉末、焊丝或者补偿条作为填充材料对大间隙下的工件进行FSW，得到成形良好无缺陷的接头，接头与常规FSW接头力学性能吻合，然而，当焊接速度过快时，这些填充材料很容易飞出间隙，从而形成缺陷. 同时填充材料需要在焊前放置在间隙内，针对复杂结构间隙及焊接过程中产生的间隙，填充材料的尺寸以及填料的连续性受到限制.
基于传统搅拌摩擦焊方法，填充材料旁轴送料，将FSW与填料过程同时进行，实现大间隙机器人搅拌摩擦填丝焊，并对其接头进行盐雾腐蚀试验，分析搅拌摩擦填丝焊接头不同区域的腐蚀行为差异.搅拌摩擦填丝焊提高了FSW对工况条件的适应性，适用于高铁、船舶和飞机上大型及复杂结构焊缝，有望为工程实际应用提供理论依据和技术支撑.
1. 试验方法
试验材料为5A06铝合金轧制板材，尺寸为300 mm × 70 mm × 3 mm，填充材料为直径1.6 mm的5B06丝材. 机器人搅拌摩擦填丝焊焊接过程示意图及焊具尺寸如图1所示，对接板材焊接间隙为2 mm. 填充丝材经过高推力送丝系统从送丝孔连续输送到储料腔内部，高速旋转的螺杆将金属丝材剪切成粒状材料，粒状材料在自身重力及与螺杆侧壁的摩擦力的影响下，在储料腔内塑化从底部的缝隙流出. 轴向压力使储料腔与板材之间产生挤压效果，粒状材料发生变形堆积并被塑化. 在旋转的搅拌针的驱动作用下，塑化的填充材料发生流动并实现与基材的连接. 试验所采用的焊接工艺参数为转速3 000 r/min，焊接速度200 mm/min，送丝速度1.8 m/min，轴向压力5 000 N，倾角1.5°.
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图 1 焊接过程示意图及焊具结构Figure 1. Welding process and the welding tool structure. (a) schematic illustration of wire-feeding friction stir welding; (b) dimensions of the welding tool2. 试验结果与分析
2.1 焊缝成形
图2为机器人搅拌摩擦填丝焊接头焊缝表面形貌. 焊缝表面光滑成形良好，无沟槽缺陷，在搅拌针的驱动作用下，塑化的填充材料发生流动后沉积弥补了间隙位置材料缺失，同时焊缝有一定程度的增厚，提高了接头的承载能力.
2.2 焊缝微观组织
图3为焊缝整体微观形貌及不同区域的微观组织. 焊接接头填充材料与基体母材结合良好，焊缝无孔洞及隧道缺陷，由于搅拌针的存在，搅拌针促进塑化的丝材和基材发生流动，提高了填充材料与基材的结合效果. 丝材经过螺杆的剪切及静轴肩的挤压作用，与焊核区受到搅拌针的搅拌作用一样，填充材料也经历了大塑性变形，发生动态再结晶，形成细小的等轴晶.
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图 3 搅拌摩擦填丝焊接头微观组织Figure 3. Microstructures of wire-feeding friction stir welding. (a) microstructures of the cross-section; (b) top interface; (c) thermo-mechanically affected zone interface; (d) filler materials zone2.3 焊缝腐蚀性能
搅拌摩擦填丝焊接头经过7天盐雾腐蚀试验后接头各区域腐蚀形貌如图4所示. 接头表面均发生了点蚀坑的萌生, 表面出现腐蚀产物；焊核区及填充材料区域的点蚀坑尺寸较小，且分布较为均匀；母材点蚀坑分布不均匀，尺寸较大.热力影响区(thermo- mechanically affected zone，TMAZ)的点蚀坑随晶粒分布特征呈流线分布，热影响区(heat-affected zone，HAZ)的点蚀坑尺寸较大，且出现一定的聚集现象，点蚀坑发生扩展.焊核区和填充材料区表现出更好的耐腐蚀性能.
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图 4 不同区域盐雾腐蚀形貌Figure 4. Salt spray corrosion morphologies in different zones. (a) WNZ; (b) filler materials zone; (c)TMAZ; (d) HAZ; (e)BM第二相分布及尺寸对点蚀坑的形成有巨大影响，第二相和基体之间形成微电偶会导致腐蚀现象发生.焊核区经过塑性变形后第二相颗粒被打碎，尺寸较小分布也更均匀，进而发生腐蚀现象后点蚀坑分布均匀细小；填充材料区域拥有更小且弥散分布的第二相颗粒，填充材料的加入增强了焊核区的耐蚀性.经过轧制后的母材中第二相颗粒尺寸较大且分布不均匀，耐蚀性较差易形成较大的点蚀坑；热力影响区点蚀坑呈流线分布，而热影响区第二相颗粒发生聚集长大，发生点蚀后有利于点蚀坑的扩展,导致热影响区的耐蚀性较差.
图5为热影响区点蚀坑SEM图及附近元素分布.发现在第二相Al6(FeMn)附近产生了明显的腐蚀现象, 点蚀坑发生扩展. 在盐雾环境中，铝合金表面虽然存在一层氧化膜，但是随着溶液中Cl−的侵入，Cl−破坏了表面氧化膜，促进点蚀现象发生. 同时热影响区第二相颗粒Al6(FeMn)与铝基体之间存在腐蚀电位差形成原电池，由于Al6(FeMn)电位高于铝基体[10]，第二相颗粒在腐蚀过程中充当阴极，促使周围基体发生腐蚀，因此在第二相附近形成环形腐蚀区域产生腐蚀坑并向四周扩展. 当第二相尺寸较大时，周围基体溶解的范围增大，点蚀坑的尺寸也会更大. 基于元素分布图可以看出，在腐蚀坑附近Al元素含量减少，点蚀坑内金属发生溶解，点蚀孔内阳离子浓度升高，Cl−就会不断侵入以维持平衡.随着Cl−浓度的升高发生水解，导致点蚀坑内部氢离子浓度升高，溶液酸化，促使基体进一步溶解，点蚀坑发生扩展.
2.4 接头力学性能
图6为经过7天盐雾腐蚀接头、未腐蚀接头及母材的拉伸测试结果.未腐蚀接头抗拉强度为388.9 MPa ± 1.4 MPa，断后伸长率为20.5% ± 0.4%，分别达到母材的99%及94%. 经过7天盐雾腐蚀后接头抗拉强度降低到356.6 MPa ± 1.2 MPa，断后伸长率为18.1% ± 0.9%，盐雾腐蚀后接头强度降低了8.3%，断后伸长率下降了11.7%，盐雾腐蚀试验后接头仍保持较优的力学性能. 盐雾腐蚀环境造成焊缝表面出现点蚀坑，而富Cl−环境使基体金属进一步溶解，点蚀坑发生扩展，减少了接头有效承载面积，在承受载荷时其易成为薄弱位置，裂纹在点蚀坑位置产生，降低了接头承载能力.
3. 结论
(1) 实现了大尺寸间隙下机器人搅拌摩擦填丝焊，焊接过程与填料过程同时进行，提高了搅拌摩擦焊对接头间隙的容忍性，消除了焊缝减薄问题.
(2) 填充材料与基材实现了良好的冶金连接，经过剧烈塑性变形后，焊核区和填充材料发生动态再结晶，表现为细小的等轴晶粒.
(3) 未腐蚀接头抗拉强度达到388.9 MPa ± 1.4 MPa，断后伸长率为20.5% ± 0.4%，分别达到母材的99%及94%. 在腐蚀过程中焊核区和填充材料区耐腐蚀性能优于热影响区与母材，点蚀坑细小且均匀分布，7天盐雾腐蚀后接头保持优异的耐蚀性能.
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图 2 基于滑移带平均的结果参量
Figure 2. Result parameter of PSB-based average. (a) Von Mises; (b) FIP
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图 3 微观短裂纹投影长度与扩展投影速率
Figure 3. Crack length and propagation rate in projection direction in MSC
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图 4 不同焊趾半径的疲劳寿命分布云图
Figure 4. Fatigue life distribution diagram of different weld toe radius. (a) R = 0.2mm; (b) R = 0.4 mm; (c) R = 0.6 mm; (d) R = 0.8 mm
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图 5 成核阶段时焊趾圆弧边界处滑移带寿命统计
Figure 5. Life statistics of slip band at weld toe radius boundary in nucleation stage
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图 6 焊趾半径对裂纹扩展路径影响
Figure 6. Crack propagation path of different welding toe radius. (a) R = 0.1mm; (b) R = 0.2 mm; (c) R = 0.3 mm; (d) R = 0.4 mm
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图 7 焊趾半径对裂纹扩展阶段行为的影响
Figure 7. Effect of welding toe radius on crack propagation behavior. (a) crack length in projection direction; (b) propagation rate in projection direction
表 1 铁素体{110}[111]滑移系特性参数
Table 1 Characteristic parameters of ferrite {110}[111] slip system
弹性模量E/GPa 初始硬化模量h0/MPa 饱和应力$R_{\tau _s} $/MPa 屈服应力$ R_{\tau _0} $/MPa 硬化常数q 应变速率敏感指数m 剪切应变速率参考值${\dot \gamma _0} /{s^{ - 1} }$ $ \begin{gathered} {C_{11}} = 230.1 \\ {C_{12}} = 134.6 \\ {C_{44}} = 116.6 \\ \end{gathered} $ $ {h_0} = 180 $ $ {\tau _s} = 148 $ $ {\tau _0} = 65 $ $ \begin{gathered} q = 1 \\ {q_1} = 1.4 \\ \end{gathered} $ $ m = 0.1 $ $ {\dot \gamma _0} = 0.001{s^{ - 1}} $ 
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表 2 裂纹演化仿真结果(R = 0.6mm)
Table 2 Simulation results of crack evolution (R = 0.6mm)
晶粒
编号滑移带长度
d/μm裂纹实际长度
a0 /μm裂纹投影长度
a/μm扩展实际速率
(da0 /dN)/(μm·万次−1)扩展投影速率
(da/dN)/(μm·万次−1)晶粒循环次数
n/万次裂纹循环次数
N/万次占比
p(%)1 14.68 14.68 6.73 1.00 0.46 14.74 14.74 34.03 2 31.24 45.92 19.07 3.62 1.43 8.64 23.38 19.95 3 8.95 54.87 21.85 2.55 0.79 3.51 26.89 8.11 4 18.62 73.49 36.48 3.64 2.86 5.12 32.01 11.81 5 23.80 97.29 58.86 7.81 7.34 3.05 35.06 7.04 6 24.36 121.65 75.45 14.16 9.65 1.72 36.78 3.97 7 16.87 138.52 89.56 6.93 5.79 2.44 39.21 5.62 8 10.61 149.13 98.61 11.29 9.63 0.94 40.15 2.17 9 24.96 174.10 105.93 22.29 6.54 1.12 41.27 2.59 10 17.53 191.62 118.91 13.80 10.22 1.27 42.54 2.93 11 12.07 203.69 128.24 15.67 12.12 0.77 43.31 1.78
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