Axial and Biaxial Fatigue Behavior of 6061‐T651 Cold Metal Transfer Welded Joints

ABSTRACT

Lightweight aluminum structures in transportation systems are increasingly exposed to complex service loading, yet the fatigue performance of welded joints is still predominantly assessed using uniaxial data. In this work, the axial and biaxial fatigue behavior of 6061‐T651 aluminum alloy joints produced by cold metal transfer (CMT) welding is investigated experimentally. Microhardness measurements reveal a localized softened region within the heat‐affected zone, resulting from welding‐induced precipitation degradation, which governs deformation and fatigue damage under uniaxial loading. Axial fatigue tests show a marked reduction in fatigue life and increased scatter for welded joints compared with the base material, with crack initiation consistently occurring in the softened heat‐affected zone. Biaxial fatigue experiments performed using an anticlastic bending configuration demonstrate a fundamentally different response. Although the base material exhibited longer fatigue lives under the investigated biaxial loading conditions when expressed in terms of equivalent uniaxial stress, this benefit does not extend to the welded joints. Instead, the biaxial fatigue response of the welds is controlled by the interaction between multiaxial constraint and welding‐induced mechanical heterogeneity, leading to competing crack‐initiation mechanisms in the heat‐affected and fusion zones and significantly increased fatigue scatter. The results highlight the limitations of uniaxial fatigue data and equivalent stress concepts for welded aluminum structures and underscore the need for explicit biaxial fatigue characterization in durability‐oriented design of lightweight welded components.