Fatigue Performance of Probeless Friction Stir Spot Welded Joints in 2198 Aluminum Alloy

LU Tian

doi:10.3969/j.issn.1674-6457.2026.01.015

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Journal of Netshape Forming Engineering ›› 2026, Vol. 18 ›› Issue (1) : 160-168. DOI: 10.3969/j.issn.1674-6457.2026.01.015

Advanced Joining Technology

Fatigue Performance of Probeless Friction Stir Spot Welded Joints in 2198 Aluminum Alloy

LU Tian^*

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Abstract

The work aims to investigate the high-cycle fatigue performance of 1.8 mm thick 2198-T8 Al-Li alloy joints fabricated by probeless friction stir spot welding (PFSSW), analyze the effects of different rotational speed and welding time combinations on the fatigue life of the welded joints, and reveal the mechanisms of fatigue crack initiation and propagation, thereby providing a theoretical basis for optimizing welding process parameters. The PFSSW technique was employed to weld 2198-T8 Al-Li alloy under three rotational speed/welding time combinations of 950 r/min-6 s, 950 r/min-9 s, and 1 180 r/min-6 s. High-cycle fatigue tests were conducted to establish the S-N curves of the joints and evaluate their fatigue life. The fatigue fracture surfaces were observed through scanning electron microscopy (SEM) to analyze the crack initiation sites and transient fracture zones, and to determine the fracture modes of the joints. The S-N (stress amplitude vs. fatigue life) curve expressions for the three welding parameter combinations were as follows: 950 r/min-6 s (lg S=9.113-1.208lg N), 950 r/min-9 s (lg S=4.172- 0.208lg N), and 1 180 r/min-6 s (lg S=4.386-0.253lg N). When the welding time increased from 6 s to 9 s or the rotational speed increased from 950 r/min to 1 180 r/min, the slope of the S-N curves significantly increased, indicating enhanced sensitivity of fatigue performance to loading. Under the same stress amplitude, the 950 r/min-6 s joint exhibited the best fatigue performance but was also the most sensitive to fatigue loading. Fatigue cracks primarily initiated at Hook defects, and the transient fracture zones exhibited dimples, quasi-cleavage, and cleavage features, indicating a mixed fracture mode. In conclusion, the PFSSW process parameters significantly affect the fatigue performance of 2198-T8 Al-Li alloy welded joints. The 950 r/min-6 s combination shows the best fatigue performance under the same stress amplitude but is highly sensitive to fatigue loading. Increasing welding time or rotational speed enhances the sensitivity of joint fatigue performance to loading. Fatigue cracks mainly initiates at Hook defects, and fracture surface analysis reveals a mixed fracture mode. The findings provide important references for optimizing the PFSSW process parameters for 2198-T8 Al-Li alloy.

Key words

Al-Li alloy / probeless friction stir spot welding / fatigue performance / fracture mode

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LU Tian. Fatigue Performance of Probeless Friction Stir Spot Welded Joints in 2198 Aluminum Alloy[J]. Journal of Netshape Forming Engineering. 2026, 18(1): 160-168 https://doi.org/10.3969/j.issn.1674-6457.2026.01.015

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