Study On The Fatigue Property And Strength Evaluation Of Al-Si-Mg Aluminum Alloy Used In Positioning Device Of High Speed Railway Overhead Wire

Due to the superior characteristic of corrosion resistance,mechanical and fatigue properties,Al-Si-Mg alloys have been widely used in machinery.With the increase of service life and the deterioration of service conditions,fracture of supporting devices for overhead wire usually occur due to fatigue failure,which endangers the safe operation of high-speed railway.The main raw material for supporting device of high-speed railway overhead wire is Al-Si-Mg alloy,so it is necessary to carry out the fatigue performance and strength evaluation research of the alloy.In this study,rotating bending fatigue tests were conducted on two Al-Si-Mg casting alloys,with different Mg contents of 0.3 and 0.6 wt%,the fracture surfaces of the specimens were observed by scanning electron microscopy(SEM),the influence of Mg content on the fatigue properties and behaviors in Al-Si-Mg alloy was investigated.Two different fine particle shot peening(FPSP)treatments were performed on Al-Si-Mg alloy specimens,rotating bending fatigue tests were conducted,and the fatigue fracture surfaces were observed using scanning electron microscopy(SEM),the crack initiation and propagation behavior were investigated using the replica method,the strengthen effects of fine particle shot peening on fatigue properties in Al-Si-Mg alloy was investigated.Artificial defect was introduced in Al-Si-Mg alloy,and the rotating bending fatigue tests were conducted.The effects of defect size on fatigue strength and the threshold value of stress intensity factor range in Al-Si-Mg alloy were investigated.The critical defect size for reducing the high cycle fatigue strength of specimen is proposed,and the empirical expression for predicting the fatigue strength and stress intensity factor threshold range by Vickers hardness and defect size is optimized.The results are as follows:(1)The increase of Mg content can improve the yield strength and ultimate tensile strength,whereas it can decrease the elongation.The fatigue strength at 5×10~7 cycles is80MPa for Al-7Si-0.6Mg alloy,which is 20MPa higher than Al-7Si-0.3Mg alloy.For both Al-Si-Mg casting alloys,fatigue failure can initiate from crystal slip or casting defects.The increase of Mg content slows the fatigue crack growth speed and changes the crack propagation mode from transgranular to intergranular.(2)Compared with ceramic shot peened specimens,steel shot peened specimens have deeper residual stress and work-hardened layer,higher maximum residual stress value,as well as higher surface roughness.The fatigue strength at 5×10~7 cycles is 80MPa for Al-7Si-0.3Mg fine particle shot peened specimens,which is 20MPa higher than unpeened specimens.In steel shot peened specimens,both crystal slip and casting defects can contribute to fatigue failure,while only casting defects can cause failure in ceramic shot peened specimens.The crack initiation sites move to the subsurface after fine particle shot peening treatment.The threshold value of stress intensity factor range can be improved by fine particle shot peening treatment;as a result,crack initiation and propagation are restrained.(3)The surface artificial defects can significantly reduce the fatigue properties of Al-Si-Mg specimens.The fatigue strength reduction and the threshold value of stress intensity factor range of the high cycle of the specimens also increase gradually with the increase of the defect size.The fatigue strength prediction expression was revised based on the fatigue test results.The evaluation results were more consistent with the test results.