A main effect resulting from the application of the Laser Shock Processing (LSP) technique consists in the introduction of relatively deep compressive residual stresses (RS) fields into metallic components, improving its mechanical behaviour.

Although several work has been contributed describing the effective introduction of such compressive RSs fields in this material by LSP, from a more fundamental point of view, the full elucidation of the microstructural changes justifying these macroscopic effects and their observed relative stability face to thermal cycles is needed in order to gain a better predictive insight for the technique.

The group of the author has experimentally succeeded to introduce compressive residual stresses fields in different materials (Al2024, AISI316L, Ti6Al4V) with peak values in the range of 500 MPa and in material ranges beyond 1 mm depth that have conferred the treated specimens with clearly observable mechanical properties enhancement [1]. The results relative to residual stresses measurements were normally determined on the basis of the application of the ASTM E837-13 Standard (strain gauge hole drilling method), able to provide RSs components perpendicular to the drilling direction and for which the proposers have developed a suitable uncertainty analysis procedure [2].

Additionally, the same group conducted a previous study [3] with the double purpose of, first, to confirm the order of magnitude of the RSs fields induced in the material by LSP processes of a given intensity in rapport to the values measured by the ASTM E837-13 technique and, second, to confirm the relative stability of such RSs fields upon aggressive thermal cycles. For such study, previously annealed samples (RSs total relaxation) directly treated by LSP at a 12 kJ/cm2 and the corresponding specimens subject to respective stabilization thermal cycles at 595ºC (close to the working limit of the alloy) and 710ºC (beyond its estimated maximum service temperature) were produced and the corresponding RSs measured on selected specimens by energy-dispersive diffraction using synchrotron X-ray radiation at the EDDI beam line of BESSY II (Berlin, Germany).

Apart from initial surface microstructural analyses, the near-surface RSs fields for either condition were obtained (see figure 1). The effective equivalence of the level of intense compressive RSs fields induced in the first 120 μm of the samples by the initial LSP treatment in comparison to the range measured by the hole drilling technique was proved. Also, the effective ability of the LSP treatments to induce high stability transformations in the metal structure essentially different from those induced by thermal/mechanical transformations at relatively low-to-moderate deformation rates. The applied high deformation rates were then shown to induce microstructural transformations responsible for the found remanence of compressive residual stresses fields even after the application of thermal aging cycles normally implying a total relaxation of residual stresses of thermal origin.

On the basis of these results, an additional degree of insight into the process is required by increasing the depth of exploration until at least few mm depth in order to observe the RSs recovery and the knowledge of the triaxial behaviour or the RSs fields. Neutron diffraction is hence the appropriate experimental approach to continue with, but in contrast with previous work [4], now the effect of heat treatments will be emphasized. The correlation of experimental results with the available numerical simulation tools is critical in order to predict safely the mechanical behaviour of the treated samples along different preferential working directions.

In this Seminar, experimental results on the residual stress profiles created in a typical material under different irradiation conditions are presented along with a model-based discussion on the limitations imposed to the treatment by the associate thermal effects deriving both from laser interaction and shock heating itself.

References :

[1] J.L. Ocaña et al. : “Improvement of surface and mechanical properties of high strength metallic alloys by laser shock processing”. Advances in Materials and Processing Technologies, 3, 12–22 (2017).

[2] D. Peral et al. : “Uncertainty analysis for non-uniform residual stresses determined by the hole drilling strain gauge method”. Measurement, 97, 51–63 (2017).

[3] J.L. Ocaña et al. : “Compressive Residual Stresses and Associated Surface Modifications Induced in Ti6Al4V by Laser Shock Processing”. Materials Science Forum, 879, 1408-1413 (2017).

[4] A.D. Evans et al.. “Near Surface Residual Stress Determination of Laser Shock Peening by Neutron Diffraction” Journal of Neutron Research, 11, 229-233, (2003).

Contact : dubouloz@ill.fr