Study On Fatigue Performance Of Selective Laser Melting Of High-strength Aluminum Under Cyclic Loading

Electro Hydrostatic Actuator(EHA)is a high cycle fatigue actuator that operates under alternating cyclic loads and strong random disturbances during the active control of an aircraft.It requires 100000 hours of service in the aircraft.This thesis conducts fatigue performance experiments on high-strength aluminum materials used in aviation EHA,and explores the influence of process defects caused by selective laser melting(SLM)forming technology on fatigue life.Using experimental data,the distribution characteristics at different stress levels are obtained,and the fatigue life of hydraulic pipes is predicted considering reliability.Firstly,the S-N curve of Al Mg Sc Zr-2 high-strength aluminum alloy was obtained by fatigue test by group method,and the fatigue ultimate strength was obtained by the lifting chart as 134 MPa,which was about three times that of Al Si10 Mg.The fatigue fracture analysis by electron microscopy shows that the high stress spread is uneven,the fatigue life dispersion is not high,the low stress is relatively flat,and the dispersion is high.The types of fatigue source defects include pores,unfused and cracked,of which stomatal and unfused defects account for 93%.Then,the fatigue source defect is measured and characterized,and the ratio of defect-to-surface distance to 5% diameter is used to represent the position parameters,and the ratio of elliptical envelope area and true defect area is used to represent the shape parameters,and the G-N stress correction model is proposed,which improves the fitting accuracy from 0.44 to 0.90.Secondly,in order to consider reliability for life prediction,the commonly used probability distribution function is discussed and analyzed,and the nonlinear cumulative damage model is obtained by using the relationship between the Weibull distribution function and fatigue damage,and fatigue damage is replaced by reliability.Then,the equivalent load concept is used to obtain a multi-stage fatigue life prediction recursive formula.Through examples,the fatigue life data of Al Mg Sc Zr-2 alloy materials conform to the Weibull distribution in the low-stress region below 200 MPa,and the relative error of multi-stage fatigue life prediction based on the Weibull distribution is 0.25,which can meet the engineering needs.Finally,the nCode software and the prediction method based on Weibull are used to analyze the life of hydraulic pipes in aviation EHA,and the fatigue life of hydraulic pipes is 15507 h and 4108 h,respectively,compared with the reliability prediction based on Weibull distribution function.In addition,the wall thickness of the hydraulic pipe is used as the design parameter to optimize the size,and the fatigue life of 100,000 hours is the goal,and the wall thickness of the hydraulic pipe should be greater than or equal to 1.202 mm.