In many industries, surface modification methods as carburizing, nitriding and surface heat-treatment by laser or high frequency induction were used. By surface modification, surface of press mould, auto parts and other metallic materials were hardened with ductile matrix. However, the such methods need high processing temperature and long manufacturing time. Therefore, friction stir process (FSP) is one of methods to dissolve these problems. Because surface modified by FSP method improves mechanical properties such as strength, hardness and wear resistance by using frictional heat and plastic deformation.
In this study, the surface modification of commercial steels was conducted by friction stir process.
In the chapter 3.1, sound FSP conditions to modify the surface without defect were founded through observations of microstructure and mechanical properties of FSP zone. Optical microscope and scanning electron microscopy were used to observed microstructure. Mechanical properties such as tensile strength, hardness and impact energy were measured by tensile test, hardness tester and impact tester respectively.
In the chapter 3.2, surface modification of SUS 409 steel was conducted by FSP. The SUS 409 steel is not phase transformation during FSP. But microstructure was changed by FSP due to frictional heat and plastic deformation. The residual stress of FSP zone was similar to that of base metal (BM) due to release the stress by dynamic recrystallization. And compressive residual stress was formed at thermal-mechanical effected zone (TMAZ). Therefore, fatigue property of FSPed specimen was improved to the BM.
In the chapter 3.3, the effect of carbon content on phase transformation of mild steel (SS400 steel) and carbon steel (SM45C steel) by FSP was observed. The phase transformation of FSP zone was analysed by using XRD and EBSD. And the mechanical properties of FSP zone was measured by hardness test and impact test. The microstructure of FSP zone of SS400 steel was ferrite like BM. However, the FSP zone of SM45C steel consisted of ferrite, pearlite and martensite unlike BM. Also, the hardness, impact properties of FSP zone of SM45C steel were higher than that of FSP zone of SS400 steel. Those are because FSPed SM45C was the formation of martensite phase and smaller grain size than FSPed SS400.
In the chapter 3.3~4, the addition effect of WC powder in SM45C steel on microstructure of friction stir processed zone was observed. Friction stir process (FSP) was conducted, 400 RPM and 100~150 mm/min. The mechanical properties and microstructure of friction stir processed zone of SM45C steel were analyzed. Mechanical properties as hardness, tensile and abrasion properties of FSPed specimen with WC powder were measured by Rokwell hardness test, tensile test and abrasive wear test. And optical microscope and scanning electron microscopy was used to observed microstructure. By addition of WC powder in SM45C steel, hardness and wear property og FSPed specimen improved.