No abstract

The 7xxx aluminum alloy series is applied in aircraft components that demand high specific mechanical strength and fatigue resistance. The corrosive environment and friction forces imposed on those components reduce the service life. Therefore, anodizing is frequently used to enhance resistance against corrosion and wear. However, anodizing decreases the fatigue life of aluminum alloys. Plasma immersion ion implantation (PIII) is an alternative surface treatment that has been used to improve surface hardness, corrosion resistance and reduce wear damage. The PIII treatment consists of accelerating positive ions towards a sample by applying negative high voltage pulses in plasma. The shot peening (SP) process is a surface treatment that can be used with other surface treatments to improve the fatigue strength. The present work aimed at investigating the fatigue behavior of AA 7050-T451 aluminum alloy treated with SP, PIII, and a combination of both SP and PIII surface treatments. The combination of SP and PIII treatments (SP+PIII) improved the fatigue performance with respect to the base material and was more efficient than PIII in improving the fatigue strength for high-cycle fatigue regime. The fatigue strength improvement is related to the induced compressive residual stresses at the surface layers, which was responsible for delaying the nucleation and early fatigue crack propagation periods. The residual stress at the surface changed from −62 MPa to −130 MPa and −210 MPa for the SP and SP+PIII conditions, respectively. Compared to chromic anodizing, the SP+PIII condition increased the fatigue life in about 4 times at a maximum stress level of 281 MPa. The combination of SP and PIII treatments tested in this work is more advantageous regarding fatigue behavior than chromic anodizing, which reduces the fatigue strength of the material.

The influence of the electrodeposition of cadmium and zinc-nickel and the stress concentration effect on the fatigue behavior of AISI 4140 steel threaded components were studied. Axial fatigue tests at room temperature with a stress ratio of R = 0.1 were performed using standard and threaded specimens with and without nut interface under base material, cadmium, and zinc-nickel-coated conditions. Finite element analysis (FEA) was used, considering both elastic and elastoplastic models, to quantify the stress distribution and strain for threaded specimens with and without a nut interface. The numeric results were correlated to the experimental fatigue data of threaded components with and without the nut interface, to allow the oil & gas companies to extrapolate the results for different thread dimensions, since the experimental tests are not feasible to be performed for all thread interfaces. Scanning electron microscopy (SEM) was used to analyze the fracture surfaces. The stress concentration factor had a greater influence on the fatigue performance of threaded components than the effect of the Cd and Zn-Ni coatings. The fatigue life of studs reduced by about 58% with the nut/stud interface, compared to threaded components without nuts. The elastoplastic FEA results showed that studs with a stud/nut interface had higher stress values than the threaded specimens without a nut interface. The FEA results showed that the cracks nucleated at the regions with higher strain, absorbed energy, and stress concentration. The substitution of Cd for a Zn-Ni coating was feasible regarding the fatigue strength for threaded and smooth components.