Micro-mechanisms of fatigue and corrosion in a laser shock peened aerospace aluminium alloy.

Abstract

A laser shock peening treatment with ablative layer (LSP), and one without an ablative layer (LSPwC) were performed on AA7075-T651 (UNS A97075) to investigate their effects on the micro-mechanisms of fatigue, corrosion and fatigue of pre-corroded pits. Surface and microstructural characterisation techniques (micro-hardness, SEM-EBSD, contactprofilometry, incremental hole drilling, X-ray diffraction) showed the laser shock peening treatments generated high residual stress fields of up to –400 MPa, with LSPwC having 14% higher subsurface compressive residual stresses than LSP. Both showed limited hardness increase, with a limited surface roughness increase for LSP, and moderate for LSPwC. Fatigue testing showed a two-order magnitude increase in LSP AA7075 T651 overall life, due to the mechanism of crack initiation changing from surface second-phase particles to subsurface crack initiation dependent on the local stress field. Laser peening-induced defects in LSP (in some cases), and the surface roughness in LSPwC, were able to prevent AA7075- T651 from achieving maximum fatigue performance enhancement. This highlighted the importance of the interplay between surface features and residual stress on the micromechanism of crack initiation, and provided new insights into using this knowledge to maximise fatigue life extension. Modelling work by collaborators has used this experimental data to quantitatively evaluate the proposed mechanisms and is briefly reported in the discussion of the PhD results. LSP and LSPwC AA7075-T651 were also exposed to 3.5 wt.% sodium chloride solution under several electrochemical protocols (open-circuit potential, potentiodynamic sweep, galvanostatic control). The compressive residual stresses did not significantly affect the corrosion behaviour, corrosion pit morphology or pit depth. Additionally, laser shock peening-induced surface roughness was shown to have the most detrimental impact on corrosion performance. However, there was no clear indication of long-term changes in corrosion performance. The fatigue performance from pre-corroded pits in peened AA7075-T651 was also studied. It showed pits act as stress concentrations, causing cracks to initiate shortly after dynamic loading, reducing fatigue life by 50%. The laser shock peening treatments markedly increased fatigue life, by up to 400% compared to corroded untreated AA7075-T651, due to the residual stresses effectively counteracting the stress concentrations of the pits. Laser shock peened AA7075-T651, even after suffering subsequent pitting corrosion, can be expected to have fatigue performance as good as, or better than, standard untreated AA7075-T651.

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Identifiers

ORCID for Alvaro Sanchez Araujo: orcid.org/0000-0002-7246-7000

ORCID for Philippa Reed: orcid.org/0000-0002-2258-0347

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