采用标准紧凑拉伸试样对比了超低碳贝氏体钢激光电弧复合焊接接头的电弧主导区、激光主导区、热影响区和母材区的疲劳裂纹扩展速率,总结了各微区疲劳裂纹的扩展特征,分析了裂纹扩展路径发生偏折的原因。结果表明:随着应力强度因子增大,各微区的裂纹扩展速率均提高,但热影响区和母材区裂纹扩展速率的增长率随着应力强度因子的升高而减慢;当应力强度因子较低时,各区疲劳裂纹扩展速率由快到慢的顺序为母材、热影响区、电弧主导区、激光主导区;随着应力强度因子升高到一定值,各区疲劳裂纹扩展速率由快到慢的顺序变为激光主导区、电弧主导区、母材、热影响区;裂纹在热影响区内扩展时的扩展方向会发生明显偏折,并随着应力强度因子增大而产生大量二次裂纹,在焊缝的裂纹中仅发生小幅偏折,这与材料的成分和组织不均匀有关;通过观察各微区的断口认为二次裂纹的产生与硬质相颗粒有关,二次裂纹的产生也是疲劳裂纹扩展速率变化的重要原因。

Standard compact tensile (CT) samples were tested to compare the fatigue crack growth rates (FCGRs) of the arc dominated zone (ADZ), laser dominated zone (LDZ), heat affected zone (HAZ), and base metal (BM), which were sampled on the laser arc hybrid welding joints of an ultra-low-carbon bainitic steel. The growth features of the fatigue crack in each microcell were summarized, and the reason for the deflected path of crack growth was explained. The result shows that the FCGRs increase but the accelerated velocities of the HAZ and BM decrease with the growth of the stress intensity factor (SIF). At the low SIF, the FCGRs decrease in the following sequence: BM, HAZ, ADZ, and LDZ. When the SIF is sufficiently large, the sequence changes to: LDZ, ADZ, BM, and HAZ. The crack-path deflection in the HAZ is observed clearly, and the secondary cracks increase when the SIF increases. However, the crack deflection in the weld zone is not observed clearly. This difference is attributed to the non-uniformity in the material composition and microstructure. The observation of the fracture explains that the generation of the secondary cracks is associated with the brittle phases. Further, the secondary cracks are primarily responsible for the change in the FCGRs.