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Thesis (M.S. in Aeronautical Engineering)--Naval Postgraduate School, September 2002. / Thesis advisor(s): Ramesh Kolar, E. Roberts Wood. Includes bibliographical references (p. 113). Also available online.

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Initial investigations were conducted to determine the feasibility of applying a post solution heat treatment stress relief stretching operation to near-net-shaped taper rolled aerospace aluminium plate, in terms of the possible process window and the impact on key mechanical properties. Finite element modelling (FEM) and simulations, validated using digital image correlation to measure the strain distribution in flat tapered tensile specimens, have demonstrated the existence of a suitable process window for the production of near-net-shape wing skin blanks. It was revealed that the limits of the available taper geometry are dictated by the work hardening behaviour of the specific aluminium alloy. In addition, in the case of two typical upper wing skin alloys AA2195 and AA7055, tensile and toughness testing revealed that the resultant strain gradient offers the potential for graded mechanical properties suited to this application, with a higher level of toughness at the wing root and an increase in the yield strength of the material at the wing tip. Further, investigations explore the effect of increased tensile pre-stretching, to the maximum achievable before tensile instability, on the strength, microstructure, and precipitation kinetics seen during ageing alloys AA2195 and AA7055. Subsequently, in both alloys, microstructure data have been used to deconvolute and model the effects of increasing pre-strain on the main strengthening components that contribute to the T8 yield strength of the material. In the case of the AA2195 alloy, the T8 yield strength was observed to increase continually as a function of pre-strain to a maximum value of ~670MPa at 15%. It was shown that refinement of the T1 phase continued to the maximum obtainable pre-strain, which was predicted to result in a reduction in the precipitate strengthening contribution. The increase in the T8 yield strength resulting from high levels of pre-strain was attributed to a large increase in residual strain hardening following artificial ageing, owing to a low level of recovery. In the AA7055 alloy, increased pre-strain led to a substantial increase in the T3 yield strength, which was revealed to be due to the dynamic precipitation of GP-Zones. In the T8 temper the yield strength of the alloy increased continually as a function of pre-strain to a maximum of ~607MPa at 24%. Increasing pre-strain was shown to result in a continual and significant increase in the residual strain hardening contribution to the alloys T8 yield strength, due to the occurrence of limited recovery. However, it was also shown that increasing pre-strain led to a progressive reduction in the precipitate strengthening contribution from the eta prime phase, due to an increase in the volume fraction of precipitate free zones (PFZs) with increased dislocation density. Therefore, the increase in the T8 yield strength, as a function of pre-strain, is attributed to an increased residual strain hardening contribution which outweighs a smaller progressive reduction in the precipitate strengthening contribution.

Large strain tensile tests were carried out on OFHC Cu and 99.99% Al with the aim of determining the first and second order work hardening and rate sensitivity coefficients. The tests were performed at room temperature and 473 K and at constant true strain rates in the range 5 x 10('-4) to 10('-1) s('-1). With the aid of a diameter transducer, which was set up to measure and control the rate of reduction of the diameter of the tensile specimen, the strain rate at the minimum cross-section was held constant well beyond the point of maximum load. A second diametral sensor was constructed for use at elevated temperatures. In order to extend the range of conditions covered, constant strain rate compression tests were also performed on Cu at 698 K. In a further series of experiments, tensile tests were carried out on Cu and Al samples at 293 and on Al specimens at 473 K in which the flow localization process was followed by photographic means. / It was observed that the values of the rate sensitivity of the work hardening rate B(,(sigma)) beyond the maximum load are not negligible, but that they are less than 1, in opposition to the theoretical predictions of Kocks et al('(47)). Furthermore, it is shown that, contrary to the suggestion of these workers, the rate sensitivity at constant work hardening rate N is not the material coefficient that controls the growth of strain rate gradients at large strains. / The material coefficients determined using the diametral transducer were employed for the numerical integration of the second order differential equation describing flow localization proposed by Kocks et al('(47)). This equation was integrated at the minimum cross-section of the sample, and the solution is compared with the one calculated by integrating the first order differential equation proposed earlier by Jonas et al('(10)). As expected, the strain measurements obtained from the flow localization experiments are reproduced more closely by the second order solution than by the first order one largely because of the non-negligible values of B(,(sigma)). However, at large deformations, there is a discrepancy between the experimental observations and the predictions of the second order theory. This is attributed to the development of triaxial stresses at these strains. A possible modification of the second order treatment is suggested, based on the gradient in the Bridgman correction term.

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<p> The effect of strain hardening rate and material strength on the development of surface roughness in AA6111 was investigated. No measurable change in the rate of roughening, or in the surface morphology was observed due to altering the strain hardening rate by using different test temperatures. Changing the material strength and strain hardening rate by altering the precipitation state also gave no significant change in either roughening rate or morphology with respect to strain. The development of surface roughness is also independent of strain history. Samples subjected to an intermediate polish after 20% true strain resumed roughening at the same rate regardless of amount of previous tensile strain. The development of surface roughness is dependent on only the strain level to which the sheet is deformed. The surface morphology seems to be controlled by the combination and distribution of texture components on the surface. The rate of roughening is grain size dependent and the surface grain size may provide a key to controlling roughening. </p> / Thesis / Master of Applied Science (MASc)

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The aim of the thesis is the understanding of the thermal, mechanical and metallurgical phenomena occurring during friction stir welding of a 6005A aluminum alloy and the determination of the mechanical properties of the welded joints. The forces and the torque needed for welding as well as the thermal cycles were measured in the various zones of the joint. A first model predicts the influence of the welding parameters on the thermal cycles, and especially on the asymmetry of the temperature evolution with respect to the weld centerline. A second model links the local microstructural evolutions, and in particular the precipitation kinetics, to the thermomechanical cycles. A third model relates the local microstructures to the local mechanical properties, based on a novel physics-based strain hardening model. Finally, the link between the mechanical properties of the joint in service, the local mechanical properties and the microstructures is modeled using a multi-scale approach including a micro-mechanics damage constitutive model. The models are calibrated and validated through in-depth microstructure characterization and mechanical tests on the base material, on heat treated samples and on the friction stir welds.