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The Impact of Machining on the Corrosion Performance of AA7150-T651 Aluminium Alloy
[Thesis]. Manchester, UK: The University of Manchester; 2015.
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Abstract
The impact of machining on the formation and evolution of the near-surface deformed layer in AA7150-T651 aluminium alloy has been investigated, and the corrosion behaviour of the machined AA7150-T651 aluminium alloy has also been examined.Characterization of the alloy revealed microstructure modification within the near-surface deformed layer. Such a deformed layer has a thickness of approximately 300-500 nm. Ultrafine equiaxed grains with the dimensions of 50-100 nm were formed in the deformed layer. Dynamic recrystallization occurred within the near-surface region of the work piece due to the severe plastic deformation and elevated temperature introduced by the machining operation, resulting in the grain refinement within the near-surface deformed layer. Segregation of magnesium and zinc at the grain boundaries was observed within the deformed layer. Dissolution of the strengthening MgZn2 particles in the alloy in the T651 temper occurred within the deformed layer during machining. The equilibrium ƞ phase can be observed within the region immediately beneath the deformed layer, possibly caused by the growth of the metastable ƞ’ phase due to thermal exposure at elevated temperature. Long-time natural ageing has promoted the precipitation of the alloying elements within the deformed layer.Corrosion evaluation showed decreased corrosion resistance of the deformed layer, caused by the increased fraction of grain boundaries due to the formation of ultrafine grains, elements segregation at the grain boundaries and increased density of dislocations within the deformed layer. Decrease of corrosion potential was recorded on the machined AA7150-T651 aluminium alloy by open circuit potential measurement in 3.5 wt. % NaCl solution. Preferential anodic dissolution of the near-surface deformed layer in 3.5 wt. % NaCl solution occurred during potentiodynamic polarization and immersion testing. Initiation and propagation of filiform corrosion were significantly promoted on the as-machined surface of the AA7150-T651 aluminium alloy, associated with the deformed layer with increased electrochemical activity, combined with higher roughness and higher population density of intermetallics on the as-machined surface. Orthogonal machining simulation of the AA7150-T651 aluminium alloy has been carried out to assess quantitatively the influence of the machining parameters, including feeding rate, cutting depth, cutting speed, clearance angle, on the microstructure evolution within the near-surface region and, consequently, the formation of the near-surface deformed layer. Near-surface deformed layers with the thicknesses varying from approximately 200 nm to 400 nm were formed during the simulation, depending on the parameters. The deformed layers are characterized by ultrafine grains or sub-grains with the dimensions less than 100 nm. Increases in feed rate and cutting speed resulted in the formation of a thicker deformed layer due to higher level of friction and energy input. A deformed layer with increased thickness was observed within the alloy subjected to orthogonal machining with the clearance angle of 0 degree compared with 45 degrees clearance angle, since more severe frictional interaction was involved in the process.