The Effects of Laser Ablation Coating Removal on the Fatigue Performance of a High Strength Structural Steel

Laser removal of protective coatings from naval vessels and bridge structural members is a promising alternative to traditional paint stripping methods such as abrasive blasting due to significant advantages in paint-layer selection, spatial control, and flexibility (reduced need for containment and hazardous waste clean-up). However, in order to substitute laser ablation coating removal (LACR) for legacy paint stripping methods, the laser-interaction effects on the morphological and mechanical properties of the underlying steel substrate have to be assessed. The main objective of this study is to determine the effect of LACR on the surface condition and mechanical properties (esp. fatigue) of high strength structural steel, obtained from two different lots having distinctive microstructures. Both materials were abrasive blasted (typical control condition prior to coating), using two different blasting protocols, i.e., laboratory controlled conditions of low-pressure (690 kPa, small particle size basting) versus typical industrial practice involving high-pressure (827 kPa, large particle size blasting). The results obtained from this study advance the knowledge and understanding of the process in a quantitative manner, enabling interested parties to assess the efficacy of LACR on low carbon structural steels.

The study showed that the LACR process was effective in the removal of a typical epoxy-based coating from the steel substrate. The efficiency of the coating removal depends on the laser processing parameters. However, multiple passes were required for complete removal of coatings on the order of 250 μm in thickness. At this thickness, a coating removal rate of 1.8 m2/hr was measured using a 7.6 cm laser scan width. The appearance of the underlying substrate surface is clearly changed at the microscopic level. However, conventional surface roughness (profile) measurements using stylus profilometry (or 3D optical microscope) indicate no statistically significant change in the roughness. Pneumatic Adhesion Tensile Testing Instrument (PATTI) studies performed on samples repainted after LACR confirm equivalent, or superior, performance to the abrasive blasted and painted samples.

Pull-pull fatigue testing of the samples revealed that the condition of the starting surface has a major impact on the fatigue performance after LACR. When low-pressure laboratory scale abrasive blasting is employed, the fatigue strength is unchanged by LACR. When industrial, high-pressure abrasive blasting is performed prior to LACR, the fatigue strength showed a knockdown of 12%. SEM observations of the surface and cross-section of abrasive blasted samples showed a high density of particles embedded in the near surface regions. Multiple, hard particle impacts during abrasive blasting produce beneficial compressive residual stresses (measured using depth-resolved X-ray diffraction methods), as well as surface defects such roughness, embedded particles, micro-cracks, etc. The laser treatment process results in surface melting and resolidification, which leads to changes in the surface residual stress state, i.e., the beneficial compressive residual stress is replaced by potentially harmful tensile residual stress state at the surface.

SEM observations of the fatigue fracture surface illustrate that, in cases where fatigue life remained unchanged by LACR, fatigue cracks initiated from the valleys of the surface profile in both abrasive blasted and LACR treated conditions. However, for the cases where the fatigue life decreased after LACR treatment, fatigue cracks tended to initiate at embedded abrasive media (~73% of the time). Even though this embedded media was present, it only sometimes (~25% of the time) served as the primary initiation site, prior to LACR treatment. Local residual stresses and geometrical stress concentrations of the impact-induced surface defects are shown to be the two root causes that result in such discrepancy in fatigue performances after LACR treatment.

Elastic stress concentration factor estimation using both an analytical function based on roughness parameters and finite element analysis (FEA) using real surface profiles obtained from SEM cross-sectional images revealed that kt,max (associated with the surface) decreases after LACR processing, due to surface smoothing. The counterbalancing effects of detrimental tensile residual stress and beneficial smoothing of the surface result in similar fatigue lives, in the cases where no embedded particles were observed. On the other hand, kt associated with embedded particles remains the same after LACR.

Finally, FEA was performed on the samples containing embedded particles to evaluate their local residual distributions. FEA results show that the residual stress in the proximity of the particles is typically highly compressive for the abrasive blasted surface conditions. Upon subsequent laser treatment, the local residual stress field relaxes. Depending on the size and depth of the particle, it can even undergo complete relaxation. Monotonic elastoplastic simulations were carried out to evaluate how the combined effects of the high value of kt and changes in residual stress state alter the local stress fields under tensile load. The results suggest that the preferred site for crack initiation will switch from the surface to the embedded particles following LACR, in agreement with observation. Thus, the high stress concentration associated with sharp, embedded particles combined with alterations in the residual stress state provides an explanation for the transition in fatigue behavior.