마찰교반용접재의 일정진폭하중하의 피로균열전파거동에 미치는 균열위치의 영향

Abstract

The Friction Stir Welding (FSW) is a method which is used by new principle that to become as intergration through to mix by using with nonexpendable rotation tool. It is a solid state joining process which is developed by The Welding Institute (TWI) in 1991. FSW's principle is that the rotation tool of the round bar which is inserted in weld zone, is moving as retation according to welding line between two materials. Hence, it is welded by the solid phase to heat, soften and plastic flow in the weld zone. This weld type is used in various transfer machine industry fields such as railroad vehicle, vessel, vehicle, aerospace and etc. due to many advantage as low crack/bubble arrising, low welding strain and etc. by especially the melting or clotting. Regarding the transfer machine's components for railroad vehicle, vessel, vehicle, aerospace and etc., the fatigue effect by the cyclic load, should be considered to apply in practical application. Numerous studies how been conducled on mechanical behaviors of friction stir welded aluminum allogs. Most of the researchs are the influence of residual stress, welding filler, grain refinement, stress ratio, environment and welding method which are effecting in fatigue crack growth behavior. Despite the practical significance relatively little research how been conducted on fatigue crack growth behavior of FSWed aluminum alloys. However, the researchs of fatigue crack propagation rate about this initial crack location, are very slight currently. This consideration about the fatigue crack growth behavior according to initial crack location, is one of the key factors in the application for future damage tolerance design of friction stir weld materials and etc. In this sense, the reached effect in the fatigue crack propagation from the initial cracks location are considered by constant amplitude loading of the fatigue crack propagation performing experiment of to collect the CT (Compact Tension) specimen based on ASTM standard in 3 different type about the welding material of LT direction in the Al7075-T651 aluminum alloy. The fatigue crack growth life of the friction stir weld material accoridng to crack location, is shown in the order of BM-CL, WM-CL and HAZ-CL specimens. In addition, the fatigue crack growth behavior is different according to initial crack location. The influence according to the crack propagation path of BM-CL, HAZ-CL and WM-CL's specimen, are checked by the macroscopic and microscopic's observation in the fatigue crack growth behavior according to the crack location.