Research On Corrosion Fatigue Short Crack Initiation And Growth Behavior Of A7N01P-T4 Aluminum Alloy Welded Joints

A7N01P-T4 is a kind of Al-Zn-Mg alloy.Its yield strength and tensile strength can reach to 295MPa,407.5MPa respectively,and its elongation can reach to 11.8%.It has been used in load-carrying part of high-speed train,such as body understructure,because of its high strength,great processing properties and good welding properties.During the service,the body of high-speed train is acted by applied stresses as well as residual stresses caused during the welding,sand blasting or random loading.When aluminum alloys run with loading stress,combined with residual stress in corrosive environments,it is much more sensitive to fatigue failure than in the air.Generally speaking,load frequency during the operation of high-speed train is below 10Hz,so corrosion fatigue failure can only be found after the high-speed train is in service for more than millions of kilometers.As a result,the research on the corrosion fatigue failure on the body of high-speed train is still in the prototype stage.In this paper,the corrosion fatigue crack initiation and short crack propagation behavior of A7N01P-T4 aluminum alloy welded joints in 3.5wt.%NaCl solution has been researched.Corrosion fatigue short crack was directly monitored using a digital camera and 250 ?m as a critical value of fatigue crack initiation.An 810 Material Test System low-frequency fatigue test machine was used in fatigue test and the specimens were loaded with the stress ratio R = 0.4,0.5,0.6 and 0.7 at frequency of 5 Hz.On the basis of calculation,fatigue crack initiation threshold value(??eqv)thcf and fatigue crack initiation resistant factor Cicf were 49 MPa and 6.97×1012,respectively.So the fatigue crack initiation life curve of A7N01P-T4 welded joints can be described as Ni = 6.97 ×1012"[??eqv 1.739-49 1.739]-2.A JSM-6490LV QUANTA FEG 250 scanning electron microscopic(SEM),electron back-scattered diffraction(EBSD)and JEM-2100F transmission electron microscopy(TEM)were used to research the mechanism of corrosion fatigue crack initiation.SEM and TEM tests revealed that several corrosion fatigue cracks formed asynchronously and the first crack does not necessarily develop into the leading crack.A uneven reticular dislocations produced by fatigue-loading was prone to piling up and tangling near the grain boundaries or the second phases and form the "High Dislocation-Density Region(HDDR)".which acted as an anode in micro batteries and dissolved to form small crack.Thus the etching pits,HDDR near the grain boundaries and second phases were confirmed as the main causes inducing the initiation of fatigue crack.According to the corrosion fatigue short crack growth testing,it was found that the corrosion fatigue short crack growth rate of A7N01P-T4 aluminum alloy welded joints was turned out to be cyclical growth.SEM and EBSD tests were conducted to study the mechanism of corrosion fatigue short crack growth.A zig-zag growth path was found and almost all the dispersed second phase particles were arranged along the grain boundaries.During the corrosion fatigue short crack growth,second phase particles were easily dissolved,so intergranular cracking was more likely to happen.However,transgranular cracking is more likely happen if there were coarse grains in the matrix.