Residual stress improvement in metal surface by underwater laser irradiation

doi:10.1016/S0168-583X(96)00551-4

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms

Volume 121, Issues 1–4, 2 January 1997, Pages 432-436

https://doi.org/10.1016/S0168-583X(96)00551-4 Get rights and content

Abstract

Laser shock processing of water-immersed material was developed for improving the residual surface stress of metal components. The process changes the stress field from tensile to compressive by means of impulsive pressure of laser-induced plasma generated through the ablative interaction of the intense laser pulse with the material. The plasma, generated by the irradiation of second harmonic of a Q-switched Nd:YAG laser (SH-YAG, λ = 532 nm) on an SUS304 test piece, was directly observed by imaging the plasma radiation with a gated image intensifier and a charged coupled device (CCD) camera. Comparing the observed image to the plasma expansion velocity calculated with an analytical model, we deduced that about 20% of the plasma internal energy would represent the thermal energy. The calculation of the plasma pressure with this result showed that it exceeded 2 GPa in water and the yield stress of SUS304, when a typical laser pulse of the SH-YAG impinged on a water-immersed SUS304 test piece. A residual compressive stress exceeding 200 MPa was built over 200 μm in depth, by scanning the SH-YAG focused on a spot of 0.75 mm diameter with a power density of 60 TW/m² and a pulse duration of 5 ns.

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Citation Excerpt :
Although initially developed to improve the resistance to fatigue cracking of materials used in aeronautical applications (specifically in aluminum alloys), the LSP technique has shown favorable results in materials such as stainless steels [8] and titanium alloys [9]. This improvement in the lifetime of laser-treated parts has led to numerous industrial applications enabled by the commercial availability of new powerful laser sources capable of delivering pulse intensities above the GW/cm2 level [2,10]. A significant example of these applications is the LSP treatment of aircraft gas turbine engine components to mitigate foreign object damage (FOD).
Laser Shock Processing (LSP) is increasingly applied as an effective technology for the improvement mechanical and surface properties of metallic materials for different types of components, mostly as a means of enhancement of their fatigue life behavior. As reported in previous contributions, a main effect resulting from the application of the LSP technique consists in the generation of relatively deep compression residual stresses fields allowing an improved mechanical behavior. In this paper, the special case of Ti-6Al-4 V alloy is considered with specific consideration of the microstructural changes and residual stresses fields justifying those macroscopic effects. In particular, the effect of the application of different typical LSP intensities on the microstructure and residual stresses fields introduced in this material and their possible correlation to the associated surface effects are analyzed. Emphasis is placed on the study of the thermal stability of these fields after an aging heat treatment at typical high temperature working conditions. The results show that, according to expectations, a certain level of residual stresses and dislocation density remains after in-work thermal aging. The combination of different material characterization techniques has made possible to obtain a correlated set of results highlighting the complementarity of different scale approaches from surface (X-ray) to bulk (neutrons) methods. A model based on the evolution of immobile dislocation density is proposed to rationalize the distribution of dislocation densities, pointing out that plastic deformation is retained. The results clearly show such material improvement stability under typical heavy-duty conditions, thus endorsing the use of the LSP for Ti-6Al-4 V as a suitable technology for industrial applications in relatively high temperature conditions.
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We applied single laser pulse irradiation and laser peening without coating (LPwC) to materials that had different coefficients of thermal expansion (CTEs) to confirm the thermal effects on the surface residual stress (SRS) of the materials. First, 1000 MPa-grade high-strength steel (HT1000) and silicon nitride ceramics (Si₃N₄) were irradiated underwater with one pulse of a frequency-doubled Q-switched Nd:YAG laser. Si₃N₄ was also subjected to multiple-pulse laser irradiation of 2, 4, and 10 pulses on the same spot. The SRS within the laser-irradiated spots was measured by X-ray diffraction (XRD); the results showed that the SRS on HT1000 ranged from 600 to 700 MPa in tension, whereas the SRS on Si₃N₄ with a lower CTE was approximately 300 MPa in compression. Next, Fe-Ni low-expansion alloys with different CTEs were fabricated and subjected to LPwC followed by XRD; their characteristics were compared with SUS304, Alloy600, and Si₃N₄. The results showed that the SRS was closely correlated with the CTE of the materials. The SRS of Fe-Ni low-expansion alloys tended to be compressive, even under LPwC conditions where the SRS would not be sufficiently compressive for materials that had a high CTE, such as SUS304 and Alloy600, suggesting that CTE is an important index that is closely related to SRS status after LPwC. The SRS of Si₃N₄ with a low CTE was always approximately 300 MPa in compression, regardless of the pulse density, including single- and multiple-pulse laser irradiation. These results indicated that the SRS of Si₃N₄ became saturated in a compressive state after single pulse irradiation.

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Residual stress improvement in metal surface by underwater laser irradiation

Abstract

J. Appl. Phys.

J. Appl. Phys.