표면 피닝된 Al 6061 합금의 잔류응력 변화 연구

Abstract

Residual stress is the stress that remains in materials after original causes of the stress have been removed, such as tensile stress and compressive stress. Tensile residual stresses are highly likely to promote fatigue failure by small scratches, but compressive residual stresses have the effect of preventing fatigue failure by increasing rigidity. Therefore, the mechanical surface hardening method is used to locally plastically deform the area near the surface to generate compressive residual stress to improve the properties of the surface layer and the life of the material. Types of failure such as fatigue, wear, corrosion and erosion originate at the material surface, and the structure and properties of the surface greatly affect the material. Therefore, by using the surface deformation machining process as the peening process, a beneficial compressive residual stress is applied to the material of the mechanical structures to improve the performance and life of the material. Various peening techniques have been developed to create microcrystalline structures with improved mechanical properties by converting the coarse grain structure of bulk materials into fine grains without changing the overall chemical composition of the material. However, since a long time ago, the uncertain effect of fatigue properties and the deterioration of the lifespan of the material frequently occur due to the indiscriminate peening and under-peening and over-peening process, and some side effects have been reported. As such, the peening process imparts beneficial compressive residual stress to the mechanical structures to improve the material performance and lifespan, or to improve the corrosion resistance by improving the metal flow, pore closure, and the density of the material. However, by reducing the compressive residual stress through the peening process that is not suitable for the material, it not only reduces the life of the material, but also causes surface depression, peeling, and cracking, which can adversely affect the corrosion properties. There are many studies that compare and evaluate various characteristics through variables such as peening process time and peening process intensity. But, most studies correlating the characteristics according to the effect of residual stress are lacking, and studies comparing and evaluating various peening processes are also lacking. Therefore, in the present study, shot peening, laser shock peening and ultrasonic peening were performed on the commercially available surface of 6061 aluminum alloy to compare and evaluate the effect and characteristics of residual stress. Shot peening, laser shock peening, and ultrasonic peening of Al 6061 alloy were subjected to microstructure analysis through X-ray diffraction analysis, SEM, and EBSD. Also, surface roughness measurement and residual stress measurement were performed. For mechanical properties, micro Vickers hardness machine and nano indentation were used. Electrochemical polarization corrosion test was achived to evaluate the corrosion properties. All specimens peened by plastic surface processing increased the FWHM of the (311) diffraction plane by X-ray diffraction, and the peak shifted to a lower angle. This is due to the grain refinement and increase in lattice defects caused by local plastic deformation and the effect of compressive residual stress, which is supported by the Debye ring, optical micrographs and EBSD analysis. Although a large compressive residual stress was generated in the surface layer due to the peening process, shot peening showed –23 MPa, unlike laser shock peening and ultrasonic peening, which showed compressive residual stresses of –117 MPa and –180 MPa near a depth of about 600 μm. Laser shock peening and ultrasonic peening showed higher hardness values than un-peened specimens even at a depth of 1 mm, unlike shot peening, which showed a sharp decrease in hardness, indicating a plastic region deeper than 1 mm due to peening. All the peened specimens showed excellent corrosion resistance than the un-peened specimens, which seems to be the effect of the compressive residual stress generated in the surface layer. Among the peened specimens, the shot peened specimen had the low corrosion resistance, which is due to the effects of rough surface roughness and surface defects.