The development of laser peening without coating（LPwC） in the recent quarter century is summarized. Experiments showed that water on the laser-irradiated material confines ablation plasma and intensifies the pressure beyond the yield strengths of most metals. Time-dependent elasto-plastic FEM simulation reproduced the experimental results of the residual stress depth profile. LPwC significantly reduced the susceptibility of sensitized SUS304 to stress corrosion cracking（SCC） and improved the fatigue properties of SUS316L. LPwC was attempted using a microchip laser with a pulse energy of 1.7 mJ, which introduced compressive residual stresses and enhanced the fatigue properties of A7075 aluminum alloy.

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In this paper, social implementation of laser peening technology is described. Laser peening is the process to induce compressive residual stress to metal surface by using the effect of laser ablation. We developed several types of laser peening systems and applied to nuclear power plants as a countermeasure against stress corrosion cracking. We also found that fatigue strength was improved about 40 percent by introducing compressive residual stress by laser peening. We developed laser peening devices to treat inner surface of small pipe shape targets and applied to low pressure turbine blades for steam turbine components.

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Laser peening can introduce compressive residual stress on the surface of various materials and, therefore, is effective in enhancing the fatigue strength. This study clarifies whether the laser peening with lower pulse energy comparing to the preceding studies generates compressive residual stress, and whether such stress would account for prolonged fatigue life in the welded zone of high-strength steel HT780. Moreover, configuration and effect of a newly-developed portable laser peening device are introduced.

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In order to ensure the long-term safety of various industrial equipment, transportation machine and social infrastructure, safe life design is required to prevent fatigue fracture of them. The basic mechanism of fatigue fracture of metal is the generation and propagation of fatigue crack due to repeated loads during use. In other words, it is effective to prevent the generation and propagation of fatigue crack in order to prevent fatigue fracture. For that purpose, it is effective to generate compressive residual stress to the surface of component materials. The laser peening process developed in Japan that does not require a surface coating was developed to convert the tensile residual stress into a compressive residual stress near the weld line of a nuclear power generation facility. That is, it can be expected to improve fatigue property by laser peening. In this report, the improvement of high cycle fatigue property by laser peening was introduced especially the suppression effect on surface fatigue crack growth, using austenitic stainless steel and aluminum alloy as examples.

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We have successfully performed femtosecond laser peening on a 2024 aluminum alloy, which was not covered with any sacrificial overlays such as a protective coating, nor using water as a plasma confinement medium during the peening treatment, in air. The femtosecond laser peening process has a great potential to be applied in various fields where conventional peening methods cannot be used, as this process can be performed under ambient conditions without the use of a sacrificial overlay on the material.

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Significant growth of the Electric Vehicle （EV） market is making a valuable contribution to the efficient reduction of CO₂ emissions. Application of power semiconductors in EVs is attracting a lot of attention, since many silicon （Si） power semiconductor chips, packages and modules are implemented in the main traction inverter, electric power steering system （EPS）, battery management system, and other motor drivers that are key components for EVs. Recently, silicon Carbide （SiC） power devices are promising alternatives, and with a view to achieving the superior semiconductor performance, the transition from Si power devices to SiC devices is underway. The maximum operating temperature of Si power devices is around 175℃, that of SiC devices is expected to be as high as 250℃, taking advantage of their wide bandgap characteristics, which induces high thermal stress around a SiC chip owing to a Coefficient of Thermal Expansion（ CTE） mismatch and deteriorates the power module reliability. Al wire bond interconnections damage rapidly due to a thermal strain between the SiC chip and the wire material, which accelerates failures near the interface of the wire bonds in the power module, due to the T_j changes caused by higher current loads. In this study, Al-Clad Cu （AlCu） wire bonding structure on a thick ion-plated Cu over-pad metallization （OPM） layer is proposed. To demonstrate the power cycling durability, SiC-Schottky barrier diodes （SBD） with AlCu wire bonds on a 25 μm-thick Cu-OPM layer were assembled into the test vehicles. Applying finite element method （FEM） analysis, the reliability of AlCu wire bonds on Cu-OPM was demonstrated. The lifetime during power cycling tests of AlCu wire bonds on a 25 μm-thick Cu-OPM layer was 14-times longer than that of Al wire bonds on an Al pad at ΔT_j = 75℃. In light of inelastic stress-strain analysis, the lifetimes of the AlCu wire bonds on a 100 μmthick and a 300 μm-thick Cu-OPM layer during power cycling tests were predicted to be 42-times and 1,600 times longer than that of Al wire bonds on an Al pad, respectively.

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