Fatigue of friction stir welded AA2024-T351 plate

Abstract

The fatigue crack initiation and propagation characteristics of Friction Stir Welds (FSW) and 13mm gauge 2024-T351 A1 alloy have been studied. Two failure locations have been identified: outside the weld nugget region and over the nugget region. The study shows that when failure occurs outside the nugget, fatigue crack growth is essentially conventional (mode I dominated), with initiation from S-phase particles. For failures over the nugget region initiations were linked to coarse intermetallics associated with macroscopic discontinuities in the weld flow pattern; with subsequent crack growth being seen to follow the curve of the banded structure within the weld nuggets region. A variety of microstructural and micromechanical contributions to fatigue failure have been identified, including the roles of local matrix ageing characteristics, intermetallic particle size and distributions, local flow stress characteristics and the residual stress distributions.

The effect of varying the welding parameters has also been studied in partial penetration 25mm gauge 2024-T351 Al alloy. Again, two failure locations have been identified: outside the weld nugget region and over the nugget region. Outside the nugget exhibited essentially conventional (mode I dominated) crack growth, with initiation occurring at S-phase particles. Failure over the nugget only occurred in samples from the slower of the two welding speeds, initiation was identified with linear defects at, or just below, the surface. Again a variety of microstructural and micromechanical facts were identified as having an influence on the subsequent growth, in particular the marked deflection from a conventional mode I path. The failure location however did not appear to significantly alter fatigue lives.

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