The shock wave transforms to a composite shock wave when reflecting back and forth in the cavity (shown as Figure 3), which repeatedly acts on the surface of the workpiece. It is realized that multiple shock pulses are obtained with one laser pulse, and the energy efficiency is significantly improved. Furthermore, since the fluid is confined in the cavity with fixed shape, the issue in the present process such as poor adaptability, lack of rigidity and thickness uncontrollability can be resolved effectively.


laser spot used is circular with the radius of 2 mm. The peak pressure of the first shock wave was set to be 3 GPa. Every single shock lasts 160 ns, and the composite shock wave was supposed to be comprised of five conventional shock waves. A typical distribution of residual Von Mises stress on the workpiece treated with the traditional LSP method is shown in Figure 4. While, if the new method proposed here is adopted, the residual Von Mises stress distributed on the same workpiece increases, as shown in Figure 5.


Figure 3. A sketch for waveform comparison between the new method and the conventional method


The related properties of the composite shock wave, formed in the cavity, may have significant influences on the surface hardening effect. A preliminary simulation was carried out to study two important parameters of the composite shock wave, the time interval between adjacent waves τ (the time from the end of the former shock to the start of the latter shock) and the wave attenuation factor η (the ratio of the peak pressure of the latter shock to that of the former one). In the simulation, the


Figure 5. The distribution of Von Mises stress on the workpiece after the new LSP process (η=0.7, τ=320 ns)


When the wave attenuation factor η=0.5 was fixed, the computations with respect to different τ were executed. The calculated radial residual stress distributing on the surface of the workpiece is shown in Figure 6, in which “normal” represents the single shock in the traditional LSP processing and “long interval” represents that there is a very large interval between the two adjacent shock waves. As the value of τ increases, the


Figure 4. The distribution of Von Mises stress on the workpiece after the traditional LSP process


Figure 6. The calculated radial residual stresses distributed on the surface of the workpiece with respect to different τ (η=0.5)


(Continued on page 12) www.lia.org 1.800.34.LASER 11


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