The design of new engineering materials resistant to both wear damage and corrosion degradation becomes increasingly demanding in complex service conditions. Unfortunately, there is typically a tradeoff between wear and corrosion resistance, even for important passive metals such as Al alloys. This is because the presence of precipitates hardens the material but at the same time lead to unfavorable galvanic coupling between the precipitates and the matrix, resulting in accelerated corrosion. This work showed that Al (or Mg) supersaturated solid solution formed using non-equilibrium methods exhibited enhanced corrosion resistance without compromising strength. For Al, alloying with Mn up to ~ 20.at.% simultaneously increased the wear resistance of Al as well as the protectiveness of the passive layer, thus improving the overall tribocorrosion resistance. For Mg, alloying with Y (4.67 wt.%), Zr (0.45 wt%), and Nd (1.79 wt%) in solid solution led to ~ 8 fold increment in corrosion resistance in physiological environment. Magnetron-sputtered aluminum (Al) and aluminum–manganese (Al-Mn) films with structures ranging from nanocrystalline to amorphous were obtained by tuning the Mn% up to 20.5 at.%. Corrosion behavior of the films was investigated in 0.6 M and 0.01 M NaCl aqueous solutions by potentiodynamic polarization (PD) and electrochemical impedance spectroscopy (EIS). Pitting corrosion was found to be strongly affected by alloy composition. The amorphous Al–20.5 at.% Mn exhibited the best pitting resistance during short term exposure. However, over longer immersion in 0.01 M NaCl up to 108 hrs, nanocrystalline Al–5.2 at.% Mn showed the highest corrosion resistance. The dual-phase Al-11.5 at % Mn alloy was found to have higher nominal corrosion rate compared to its nanocrystalline or amorphous counterparts. The effects of Mn alloying on the tribocorrosion behavior of magnetron-sputtered Al-Mn thin films with 5.2 at.% and 20.5 at.% Mn were investigated in 0.6 M NaCl aqueous solution. Tribocorrosion resistance of Al-Mn was found to be strongly affected by the alloying composition and applied potential. Higher Mn content increased H/E ratio and promoted the formation of denser and more compact passive film, hence improving tribocorrosion resistance of Al. In particular, alloying with 20.5 at.% Mn led to an increase of the corrosion resistance by ~ 10 times and the hardness ~ 8 times compared to pure Al. The total material loss during tribocorrosion was found to increase with applied potential. When the applied potential was increased from cathodic to anodic, simultaneous contribution of the mechanical and the electrochemical wear leads to accelerated material loss. A galvanic cell model was used to investigate the depassivation-repassivation kinetics during tribocorrosion. It was found that alloying with 5.2 at.% Mn led to more than 10-fold reduction in the current density required to re-passivate similar worn areas compared to pure Al. The origin of wear-corrosion synergy was discussed based on these observations. Magnesium alloys such as WE43 are considered for biomedical applications including cardiovascular stents and bone implants due to their biocompatibility, good cell adhesion, and mechanical properties close to that of bones. Unfortunately, their high degradation rate and subsequent loss of structural integrity in physiological environments hinders such applications. To improve the corrosion resistance of WE43 magnesium alloy, its microstructure was optimized to prevent micro-galvanic coupling between Mg matrix and precipitates. Chemically homogeneous WE43 with nanoscale surface roughness was obtained by magnetron sputtering with high effective quench rate. The effect of chemical heterogeneity on the corrosion resistance of biodegradable WE43 magnesium alloy was studied by performing corrosion tests in blood bank buffered saline using samples from two metallurgical states, cast and deposited. The microstructure of all samples was investigated by grazing incidence X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The deposited samples, prepared by magnetron sputtering using targets with the same global composition as cast WE43, exhibited chemically homogeneous microstructure without the formation of secondary phases typically observed in the cast alloy. The corrosion behavior was studied by PD and EIS tests. It was found that the deposited alloy showed enhanced corrosion resistance, ~8-fold reduction in corrosion rate compared to the cast alloy, owing to the elimination of micro-galvanic coupling between the Mg matrix and the precipitates. In-situ monitoring of hydrogen bubble evolution during corrosion indicated significantly reduced cathodic reaction kinetics in the deposited alloy. Post-corrosion surface and cross-sectional SEM studies showed that the high corrosion rate in the cast alloy was associated with the formation of severely cracked corrosion products preferably around Zr- and Y-containing precipitates.

This master's thesis is focused on the modeling and tracks of thin-walled components. Used softwares are CATIA V5R20 from Dassault Systemes for Experimental Component Modeling and PowerMILL 2018 from Autodesk for Draw and NC Programming. The production efficiency between the precision casting and the workpiece of the cube blank based on the material utilization factor and machining time is compared in this work.

The thesis focuses on the corrosion research of AlCu4Mg1 aluminium alloys of square shape and sheet metal coated in 99.5% aluminium. The theoretical part deals with the characteristics of aluminium, the classification and naming of aluminium alloys and the impact of admixtures on aluminium alloys. In the following chapter a general introduction to metal corrosion, its differentiation according to the type of corrosion attack, mechanisms of corrosion, the impact of corrosive environment, according to the choice of anti-corrosion protection, etc. is given. The main part is devoted to the corrosion of aluminium with a special importance of the impact of admixtures on aluminium alloys, the types of corrosion attack, the impact of corrosive environment as well as of the anti-corrosion protection of aluminium alloys. The corrosion tests in controlled atmosphere are considered in the next chapter, while the last one deals with the evaluation methodology of the process of corrosion. At the beginning of the experimental part all the used samples and materials are introduced. The thesis continues then with an overview and a description of the methods used for experiments. The measurement procedure and the results of the experiments are presented in the following chapter that focuses on the metallographic analysis of the aluminium alloy samples before the experiment and after the corrosion attack. The main part of the chapter deals with the dependence of corrosion speed of the individual aluminium alloy samples on time after being exposed in the climate chamber and the salt fog chamber

<div>Aluminum alloy sheets are typically manufactured from cast slabs by multi-step rolling and annealing process. This process is very energy intensive, especially in the homogenization process after casting, which usually is conducted at 480 to 580 ℃ for up to 48 h. To reduce the </div><div>processing steps and energy, a shear-based single-step deformation process, large strain extrusion machining (LSEM), was used to create strips from AA6013-T6 with and without preheating of the workpiece. Continuous strips were obtained from this alloy with low workability. Flow patterns through the thickness of the strips exhibited primary shear with grains inclined steeply to the faces of the strips, modified to varying degrees by secondary shear from friction with the tools at the surfaces. Through control of the deformation parameters (strain, strain rate and temperature), a wide range of microstructure could be achieved. In high-temperature LSEM, dynamic recrystallization occurs at lower temperatures than in commercial hot rolling processes. </div><div><br></div><div>LSEM was performed directly on the as-cast AA6013 without homogenization. By appropriate combination of strain and strain rate, continuous strips were obtained in a single step without preheating directly from the as-cast workpiece. The highly deformed LSEM strip has enhanced workability. It can be cold rolled with at least 73% reduction in a single step without cracking. The strips were characterized by strong shear texture with partial {111} and <110> fibers. After annealing, a mixed texture containing simple shear texture and other texture develops. The annealed strips are expected to have better formability than commercial ones made by rolling.<br></div><div><br></div><div>In comparison, multi-step warm-rolling and cold rolling were performed on the as-cast AA6013. The as-cast material was preheated to 300 ℃ and rolled with 12% reduction per pass till the same effective strain as the LSEM. The warm-rolled strips were then cold-rolled with the same reductions as those on LSEM strips. The results showed that during warm rolling process, cracking occurs on the strips before reaching the same effective strain and the warm-rolled strips can only be further cold rolled with reduction less than 26% before cracking, compared with 73% reduction without cracking for LSEM strips.<br></div><div><br></div><div>Based on the simple shear LSEM process, a novel way to produce aluminum strip/sheet material is introduced. The alloys are cast into disk-shape workpiece and then transferred to the LSEM line. In this line, continuous strips/sheets are obtained in a single step at room temperature. The materials are then coiled if needed and cold rolled to the final gauge. Finally, the strips/sheets are solution treated for further deformation processing. In this method, the conventional homogenization and hot rolling, including reversing and multi-stand hot rolling, are replaced by a single-step LSEM process at room temperature.<br></div>

This licentiate thesis is concerned with residual stresses in aluminum alloy 6061-T6 resistance spot welded joint. Several topics related to mechanical strength of welded structures are treated such as; nugget size and microhardness and microstructures of weld zone and their influence on mechanical strength of welded structure, failure load measurement using tensile-shear test, resistance spot welding simulation, residual stress measurement by X-ray diffraction method and analysis effect of welding parameters on the mechanical strength and the residual stresses. To investigate the effect of resistance spot weld parameters on mechanical strength of welded structures, various welding parameters e.g. welding current, welding time and electrode force are selected to produce welded joints with different quality. According to the failure mode, the empirical equation was used to prediction of failure load base on nugget size and hardness of failure line. Microstructure study has been carried out to investigate microstructural changes in the welded joints. Microhardness tests are done to find hardness profiles due to microstructural changes and determine the minimum hardness. In addition, an electro-thermal-structural coupled finite element model and X-ray diffraction residual stress measurement have been utilized to analyze residual stresses distribution in weld zone. The electrical and thermal contact conductance, as mandatory factors are applied in contact area between electrode-workpiece and workpiece-workpiece to resolve the complexity of the finite element model. The physical and mechanical properties of the material are defined as thermal-dependent in order to improve the accuracy of the model. Furthermore, the electrodes are removed after holding cycle using the birth and death elements method. Moreover, the effect of welding parameters on maximum residual stress is investigated and a regression model is proposed to predict maximum tensile residual stresses in terms of welding parameters. The results obtained from the finite element analysis have been used to build up two back-propagation artificial neural network models for the residual stresses and the nugget size prediction. The results revealed that the neural network models created in this study can accurately predict the nugget size and the residual stresses produced in resistance spot weld. Using a combination of these two developed models, the nugget size and the residual stresses can be predicted in terms of spot weld parameters with high speed and accuracy. / <p>QC 20131014</p> / No

<p>The lack of a thorough understanding of the solidification behaviors of the proeutectic Al₃Sc and the Al-Al₃Sc eutectic in a hypereutectic Al-Sc alloy stimulates the present dissertation. The major findings for the single-phase growth of the proeutectic Al₃Sc is summarized as follows: At a low cooling rate (~1 ºC·s^-1), the proeutectic Al₃Sc phase’s formation was governed by the lateral growth, exposing six flat {100} facets. At an intermediate cooling rate (~400 ºC·s^-1), the proeutectic Al₃Sc grew in a dendritic manner, with well-defined backbones extending in eight <111> directions and paraboloidal dendrite tips, although the dendrite tips and side-branches turned into faceted steps at a late growth stage,when the lateral growth prevailed. At a high cooling rate (~1000 ºC·s^-1), the proeutectic Al₃Sc primarily crystallized into an entirely seaweed-structured particle, which was composed of interior compact seaweeds and exterior fractal seaweeds. In order to verify the proposed dendritic and seaweed growth mechanisms for the proeutectic Al₃Sc, various morphological stability criteria were used, and fair agreement between the observed and the estimated characteristic length scales was reached.</p><p>On the Al-Al₃Sc eutectic side, it was found that a rod-typed Al₃Sc eutectic phase prevalently existed in an as-cast hypereutectic Al-Sc alloy that solidified via both slow cooling in air (~1 ºC·s⁻¹) and rapid cooling in a wedge-shaped copper mold (up to ~3000 ºC·s⁻¹). Al-Al₃Sc eutectic dendrites were identified within a narrow region near the edge of the wedge. The eutectic dendrites had an equiaxed dendritic contour and a rod eutectic structure inside. Quantitative assessments revealed that an interface undercooling of 48.2 ºC was required to form the eutectic dendrites, or in other words, to enter the coupled zone of the Al-Al₃Sc phase diagram. Furthermore, a phenomenon of scientific interest was discussed: When crystallizing under a near-equilibrium condition, the eutectic Al₃Sc phase formed a non-faceted morphology, in contradiction to its faceted nature. Based on the competitive growth criterion, it was deduced that the non-faceting of the eutectic Al₃Sc phase essentially reduced the interface undercooling for the resultant regular eutectic, in comparison to an otherwise irregular eutectic that would contain a faceted eutectic Al₃Sc phase.</p>

Fatigue-crack-growth-rate tests were conducted on compact tension specimens made of 7050-T7451 aluminum alloy and 9310 steel. Compact tension specimens were tested over a wide range of load ratios (0.1 less than, or equal to R less than, or equal to 0.9) to generate crack-growth-rate data from threshold to near fracture. Three methods were used to generate near threshold data. A crack-closure analysis was performed on both materials using the FASTRAN crack-closure model. The crack-growth-rate data for each material correlated very well and each collapsed onto a nearly unique curve in the low- and mid-rate regimes using the strip-yield model in the FASTRAN life-prediction code. For the 7050 alloy, a constraint factor of α = 1.8 was required, while for the 9310 steel α = 2.5 worked very well in correlating the test data over a very wide range in R values and rates from threshold to near fracture.

Fatigue and crack-growth tests were conducted on 7050-T7451 aluminum alloy under a wide range of loading conditions. Crack-growth tests were conducted on compact, C(T), specimens under constant-amplitude loading, single-spike overloads, and a simulated aircraft spectrum loading. Fatigue tests were also conducted on single-edge-notch bend, SEN(B), specimens under constant-amplitude loading and three aircraft load spectra. The FASTRAN, life-prediction code, was used to make crack-growth predictions on the C(T) specimens; and to make fatigue-life calculations using a 12-micrometer initial flaw size at the center of the edge-notch on the SEN(B) specimens. The predictions agreed fairly well with most of the tests, except the model was unconservative on the single-spike overload tests and the severe spectrum Mini-TWIST+ Level 1 tests. The discrepancy was suspected to be caused by a low constraint factor and/or crack paths meandering around overload plastic zones. A roughness- and plasticity-induced crack-closure model would be needed to improve the model.

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