Fatigue performances of FSW and GMAW aluminum alloys welded joints: competition between microstructural and structural-contact-fretting crack initiation

The fatigue properties of gas metal arc welded and friction stir welded assemblies made of aluminum alloy 6061-T6 structural extrusions were examined. The mechanical performances of welded joints were obtained using uniaxial tensile and force-controlled constant amplitude axial fatigue tests. Microstructural and fractographic analyses were conducted to document the influence of the process on microstructure evolution, fatigue crack initiation sites, and propagation mechanisms leading to the final rupture of the assemblies. Microhardness measurements and digital image correlation techniques paired with interrupted tensile tests were also used to investigate the complex heterogeneous local mechanical behavior and to highlight the fact that the crack initiation mechanism was driven by the microstructural state of the joint as well as by the structural-contact-fretting occurring at the notch root. The corresponding fatigue strengths at 2 million and 10 million cycles were evaluated respectively at 10% and 20% higher for friction stir welded assemblies versus gas metal arc welded assemblies. Fractographic analyses revealed that the fatigue cracks were initiated from microstructural features (pores for the GMAW configuration and banded structure on the crown side for the FSW configuration), or from large sub-surface grains in a shallow region below the structural-contact-fretting occurring at the notch root.本研究针对6061-T6铝合金结构挤压件制备的熔化极气体保护焊（Gas Metal Arc Welding, GMAW）与搅拌摩擦焊（Friction Stir Welding, FSW）焊件的疲劳性能展开了测试分析。通过单轴拉伸试验与力控恒幅轴向疲劳试验，获取了焊接接头的力学性能数据。借助显微组织与断口形貌分析，阐明了焊接工艺对焊件显微组织演化、疲劳裂纹萌生位置以及最终断裂前的裂纹扩展机制的影响。此外，通过显微硬度测试、结合中断拉伸试验的数字图像相关（Digital Image Correlation, DIC）技术，研究了焊件复杂的非均匀局部力学行为，并证实：裂纹萌生机制同时受接头显微组织状态与缺口根部发生的结构接触微动的共同驱动。经测试，搅拌摩擦焊焊件在200万次与1000万次循环下的疲劳强度，分别较熔化极气体保护焊焊件高出10%与20%。断口形貌分析结果显示，两类焊件的疲劳裂纹萌生源分为两类：一类为焊件自身的显微组织缺陷，其中熔化极气体保护焊焊件的裂纹萌生于气孔，搅拌摩擦焊焊件的裂纹萌生于焊冠侧的带状组织；另一类萌生于缺口根部结构接触微动区域下方浅层处的粗大亚表层晶粒。