Date

2019

Author

İlhan, Çağrı

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7050 T7451 is a solution heat treated, stress relieved, and overaged aluminum alloy which has extensively used in aerospace industry. Chromic Acid Anodizing (CAA) is a treatment against corrosion by producing aluminum oxide layer (Al2O3) at the surface. Although corrosion resistance is increased by CAA, fatigue life of the material is affected inversely due to CAA process. CAA process consists of pre-treatments (degrease, etching etc.) and anodizing steps which have different effects on fatigue life of the material. The effect of Chromic Acid Anodizing (CAA) surface treatment on 7050 T7451 aluminum alloy was presented in this study in terms of fatigue behavior. In this study, eight different CAA processes were examined with regard to etching stage of pre-treatments by using an alkaline etchant and/or acid etchants during various immersion time. Optical microscopic examination was applied in order to measure pit depths, caused by pretreatments, and to determine pitting characteristics for selection of CAA process parameters before fatigue tests. After CAA process was selected among 8 processes in terms of pitting characteristics and conformity for ASM Volume 5 Anodizing, selected process was used as surface treatment of fatigue specimens. Four groups were determined for fatigue tests which were as-machined, pre-treated, anodized and anodized with extended immersion time. These groups were to research bare condition of the material, only pretreated condition of CAA, standard CAA applied condition and effect of immersion time respectively. Constant amplitude axial fatigue tests, ASTM-E466, were conducted to specimens at 91 Hz with resonant test machine at stress ratio ( R) of -1 until run-out criteria which was 106 cycles. Fatigue test results were evaluated according to AGARD-AG-292 for curve fitting in order to obtain S-N curve in which comparison of each fatigue groups was done. Fatigue limit reduction was detected due to pretreatments of CAA. While, fatigue limit reduction was not influenced by immersion time of CAA. Fracture surfaces of the specimens were examined by scanning electron microscope (SEM) to investigate morphology, crack initiation sites and striation counting. Crack propagation stage of fatigue failure was found to be independent from surface treatment by analyzing striation counting.

Subject Keywords

Aluminum alloys., Chromic acid., Fractography.

URI

http://etd.lib.metu.edu.tr/upload/12623003/index.pdf
https://hdl.handle.net/11511/27950

Collections

Graduate School of Natural and Applied Sciences, Thesis