Comparative study of the rotating bending fatigue performance of wire arc additively manufactured hollow shafts with and without TIG pre-joining

This study investigates the effect of Tungsten Inert Gas (TIG) pre-joining on the rotating bending fatigue performance of hollow shafts fabricated by Wire Arc Additive Manufacturing (WAAM) from ER70S-6 wire deposited onto stacked CCT34 low-carbon steel substrate rings. A four-factor, five-level Response Surface Methodology with Central Composite Design was employed across 30 runs, with welding current, bead overlap, deposition speed, and ring thickness as independent variables and fatigue life as the response. Analysis of variance confirmed that all four parameters and their quadratic terms significantly influenced fatigue life (p < 0.001), with deposition speed and welding current dominant. The optimal parameter set predicted a fatigue life of 505,427 cycles. Validation specimens (n = 5) achieved a mean fatigue life of 490,973 cycles under a bending stress amplitude of 208.5 MPa (R = -1), approximately 50% higher than WAAM-only specimens (n = 5, mean = 327,344 cycles). Weibull analysis indicated lower scatter for the TIG pre-joined configuration and a 62.9% higher B10 characteristic life, although the limited sample size means these estimates are preliminary. Metallographic sectioning revealed that WAAM deposition alone left an unfused region extending 0.175 mm from the bore at each ring-to-ring interface, whereas TIG pre-joining achieved complete penetration through the full ring wall. Microhardness profiling showed a hardened zone confined to the interface in the TIG pre-joined condition, 10.6 HV0.3 above the corresponding location in WAAM-only specimens (p = 0.004), while the two configurations were indistinguishable away from the interface (p = 0.85). Microscopy further showed a finer, more uniform polygonal ferrite structure with single-site crack initiation in the TIG pre-joined condition, against coarser and more widely distributed grain sizes with multi-site initiation in WAAM-only specimens. TIG pre-joining is therefore an effective strategy for enhancing both the fatigue life and the reproducibility of WAAM hollow shafts.