Micromechanical assessment of fatigue in airframe fusion welds

Abstract

The present study concentrates on micro-mechanical aspects of fatigue performance in a fusion welded airframe alloy, 2024-T351.  Within the overall program the initiation and growth of short fatigue cracks was studied for two forms of welding:  Metal Inert Gas (MIG) and Variable Polarity Plasma Arc (VPPA).  Fatigue testing has been carried out on relatively small samples in 3-point bending at constant amplitude and at a R-ratio equal to 0.1.  During the tests, replication techniques were used to determine crack initiation points and crack length as a function of the number of fatigue cycles.  Secondary electron and back-scattered imaging observations, in association with electron back-scattered diffraction mapping, were carried out on failed samples to determine the crack initiation sites and assess the associated microstructural interactions.  Optical microscopy, differential scanning calorimetry and transmission electron microscopy studies have also been carried out to elucidate the local microstructural conditions of the different elements of the welds.  Thereby a balance between aging, overaging, re-solutionising and re-precipitation has been identified across the heat affected zone.

It is concluded that several fatigue crack initiation processes may occur within the welds, each with its own implications for performance/lifting.  Fatigue life of the MIG welds is controlled in the fusion zone by the combined effects of interdendritic defect sizes, crack coalescence, and, to a lesser degree, residual stresses.  In the VPPA case, the fusion zone presents a much finer, lower density of crack initiating defects, and although crack initiation is indeed clearly seen in the fusion zone, fatigue life then becomes limited by the high peak residual stress levels of the VPPA heat-affected zone (HAZ) and the ‘naturally occurring’ defecting population of the parent material.  In the context of multiple crack interactions, a engineering micromechanical modelling has been successfully developed to predict the fatigue life of the MIG fusion zone and autogeneous VPPA HAZ (1mm crack length) as a function of the probability of initiation and the density/distribution of pores (or intermetallic particles).  Semi-empirical crack growth rate approximation and a microstructural crack growth rate approach are used.  First order estimates of short crack closure behaviour are used to introduce a residual stress influence on crack growth rates model.

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