Development of Automated Laser Induced Particle Impact Test (LIPIT) for Surface Treatment of Thin Plates

抄録

Since thin plate and foil have been widely used as mechanical component, improvements of mechanical property and long reliability are demanded. However, general surface treatments (surface peening and coating) may make a plate/foil deflect due to unbalance in internal residual stress. The thickness of surface hardening layer is critical for such a thin plate and foil. This study developed a surface treatment based on Laser Induced Particle Impact Test (LIPIT) to improve fatigue damage resistance. Using pulsed laser ablation, LIPIT enables us to conduct hypervelocity microparticle projectile which impacts on material surface. With LIPIT, microparticles (15 μm in diameter or smaller) impact on the material surface with the velocity of about 900 m/s. As a result, large plastic strain is introduced, and nano crystalline structure are developed in a target surface. This method can utilize small particles with micrometer and sub-micrometer. This appeared to locally introduce hardening layer at the surface, since the particle size is small and flying velocity is fast. On the specimen surface, particle impact induces a crater with large strain and large strain rate. This hardening layer is expected to improve mechanical properties and fatigue damage resistance of material. At first, LIPIT was conducted repeatedly as surface treatment for wide area of thin metallic plate. It is confirmed that the target plate does not deflect, even if surface hardening layer develops. To evaluate the mechanical properties, uniaxial tensile test was carried out to obtain stress-strain curves. It is found that LIPIT treatment improved the yield strength and flow stress. To investigate fatigue strength, we conducted cyclic fatigue testing. It is found that LIPIT treatment increased the fatigue strength. The developed surface work hardening layer is investigated from observations of SEM and SIM as well as computational method with FEM to discuss the mechanism.