Pulsed Laser Induced Deformation in an Fe-3 Wt Pct Si Alloy

Originally published by Metallurgical Transactions A, 8A, 119-125 (1977). See: Laser Peening
Authored by A. H. Clauer, B. P. Fairand, and B. A. Wilcox

The plastic deformation produced by laser induced stress waves was investigated on an Fe-3 wt pct Si alloy. The intensity and duration of the stress waves were varied by changing the intensity and pulse length of the high energy pulsed laser beam, and also by using different overlays on the surfaces of the specimens. The resulting differences in the distribution and intensity of the deformation caused by the stress waves within the samples were determined by sectioning the specimens and etching (etch pitting) the transverse sections. The magnitude of the laser shock induced deformation depended on the laser beam power density and the type of surface overlay. A combination transparent plus opaque overlay of fused quartz and lead generated the most plastic deformation. For both the quartz and the quartz plus lead overlays, intermediate laser power densities of about 5 x 108 w/cm2 caused the most deformation. The shock induced deformation became more uniform as the thickness of the material decreased, and uniform shock hardening, corresponding to about 1 pct tensile strain, was observed in the thinnest specimens (0.02 cm thick). 200 ns laser pulses caused some surface melting, which was not observed for 30 ns pulses, the pulse length used in most of the experiments. Deformation of the Fe-3 wt pct Si alloy occurred by both slip and twinning.

HIGH power pulsed lasers generate stress or shock waves in a target sample by subjecting it to very short pulses of intense laser light. The resultant disturbance in the sample is primarily mechanical since the thermal effects are small and confined to a region within a few micrometers from the laser irradiated surface.

The shock wave is created by the momentum impulse imparted to the specimen’s surface when a thin surface layer is rapidly vaporized by the intense laser light. Since this process depends on effectively coupling the laser energy into the specimen’s surface, the magnitude of the pressure is affected by such factors as the surface condition, its reflectivity, and the target material’s sublimation and ionization energy. Also, phenomena occurring within the blowoff material which absorb, reflect, or reradiate energy, such as the formation of a plasma, limit the amount of energy reaching the surface of the specimen. For these reasons, it was anticipated that the modification of surface conditions through the use of various overlay materials, and blowoff confinement by transparent overlays would affect the peak pressures. In addition, modifications of the pulse length and shape could also influence the magnitude and length of the pressure pulse.

Various recent studies have examined methods of modifying the material surface conditions to enhance the magnitude of laser induced shock waves. 1-4 It has been shown that a 7075 aluminum alloy was shock hardened after irradiation with a high power, Q switched laser through a transparent quart overlay covering the target specimen.5 Another study showed that very short (~1 x 10-9 s), high fluence pulses could cause back surface spalling in thin aluminum discs irradiated in vacuum.6

This paper reports on a series of experiments which relate the magnitude and extent of shock induced plastic deformation produced in an Fe-3 wt pct Si alloy to the laser pulse length and power density for several different types of surface overlays. The overlays studied include a transparent overlay to confine the blowoff material to the sample surface and an overlay of lead, which has a low sublimation energy. The etch pit patterns on the Fe-3 pct Si specimens after shocking and sectioning are qualitatively correlated with the measured and predicted shock pressures. A previous paper7 presents the computer code calculations, laser physics, and pressure environments for the experiments discussed here.

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