Dissertations / Theses on the topic '6061-T6 aluminium alloy'

Dans le cadre de la démonstration de l’intégrité des composants nucléaires les plus sensibles, une analyse de la présence d’un défaut potentiel de type fissure peut être requise par la sureté nucléaire. Ceci est particulièrement le cas en présence de jonctions soudées. Pour assurer un conservatisme de cette analyse, la position du défaut postulé doit être la plus pénalisante possible. Les analyses réalisées pour des démonstrations similaires sur des structures en acier reposent sur une approche de type mono matériau utilisant le comportement du métal de base. Cette approche est la plus pénalisante dans le cas d’une soudure en overmatch mais doit être remise en cause dans le cas d’une soudure en undermatch. Dans ce cadre, cette thèse propose une méthodologie expérimentale et numérique permettant l’identification de la configuration la plus pénalisante vis-à-vis de la mécanique de la rupture d’une soudure en undermatch. L’application de cette méthode a été réalisée sur une soudure en faisceau d’électrons en Al6061-T6. Un gradient de propriétés mécaniques le long de la jonction soudée a été dans un premier temps identifié permettant la conduite d’une analyse fine basée sur une approche multimatériau. Dans un second temps, le comportement en ténacité de la jonction soudée a été étudié sur éprouvettes CT. La transférabilité du paramètre J à l’amorçage à une autre géométrie d’éprouvette a été démontrée ce qui constitue une base importante pour l’hypothèse de transférabilité vers des structures. Pour finir, une étude numérique sur un tube de grandes dimensions avec un défaut semi-elliptique a été développée en prenant en compte les contraintes résiduelles de soudage. Les résultats montrent que la zone affectée thermiquement à 13 mm du centre de la soudure considérée est la plus sensible en mécanique de la rupture, ceci remet par conséquent en question les méthodes traditionnelles menées dans des analyses à la rupture brutale qui consistent à considérer un défaut dans la zone fondue<br>For the demonstration of the integrity of the most sensitive nuclear components, conventional defects, as cracks for example, must be considered within the design step as required by the nuclear safety authority. This phase is particularly crucial for dimensioning of welded structures. To ensure a conservative prediction, the position of the initial crack within the welded joint must be the most detrimental in fracture behavior. Commonly used analyzes consider homogeneous structure with the behavior of the base metal of the welded joint, considered as the weakest metallurgical zone in the case of an overmatched weld. In contrast, similar analysis is not conservative in case of undermatched weld. The thesis contributes by the development of an experimental and numerical methodology allowing the identification of the detrimental metallurgical zone in fracture behavior of an undermatched welded joint. The methodology proposed is applied to an electron beam welded joint on Al 6061-T6. To reach this goal, the gradient of the mechanical behavior along the welded joint was first identified. This is particularly interesting to conduct an advanced analysis based on a multimaterial approach. In a second step, the fracture behavior of the welded joint was studied on CT specimen. The transferability of the J integral at initiation was approved on another geometry: this represents an important foundation for the transferability assumption to structure. Finally, a numerical analysis on full scale tube was developed. Residual welding stresses and structural effects were considered. The results demonstrate that the heat affected zone located at 13 mm from the middle of the welded joint is the most detrimental zone for fracture analysis. This contradicts the conventional methods conducted on fracture analysis which consider a conventional defect within the fusion zone

L'alliage d'aluminium 6061-T6 a été retenu pour la fabrication du caisson-coeur du futur réacteur expérimental Jules Horowitz (RJH). L'objectif de cette thèse est de comprendre et modéliser le comportement et l'endommagement de cet alliage en traction et en ténacité, ainsi que l'origine de l'anisotropie d'endommagement. Il s'agit de faire le lien entre la microstructure et l'endommagement du matériau à l'aide d'une approche micromécanique. Pour ce faire, la microstructure de l'alliage, la structure granulaire et es précipités grossiers ont été caractérisés en utilisant des analyses surfaciques (Microscopie Électronique à Balayage) et volumiques (tomographie/laminographie X). Le mécanisme d'endommagement a été identifié par des essais de traction sous MEB in-situ, des essais de tomographie X ex-situ et des essais de laminographie X in-situ pour différents taux de triaxialité. Ces observations ont notamment permis de montrer que la germination des cavités sur les précipités grossiers de type Mg2Si est plus précoce que sur les intermétalliques au fer. Le scénario identifié et les grandeurs mesurées ont ensuite permis de développer un modèle d'endommagement couplé, basé sur l'approche locale de la rupture, de type GTN intégrant la germination, la croissance et la coalescence des cavités. Le lien entre l'anisotropie d'endommagement et de forme/répartition des précipités a pu être montré. Cette anisotropie microstructurale modifie les mécanismes : Pour une sollicitation dans le sens long l'endommagement est majoritairement intergranulaire alors que dans le sens travers on observe un endommagement mixte intergranulaire et intragranulaire. La prise en compte des mesures de l'endommagement dans la simulation a permis d'expliquer l'anisotropie d'endommagement. Ce travail servira de référence pour les études futures qui seront menées sur le matériau irradié<br>The AA6061-T6 aluminum alloy was chosen as the material for the core vessel of the future Jules Horowitz testing reactor (JHR). The objective of this thesis is to understand and model the tensile and fracture behavior of the material, as well as the origin of damage anisotropy. A micromechanical approach was used to link the microstructure and mechanical behavior. The microstructure of the alloy was characterized on the surface via Scanning Electron Microscopy and in the 3D volume via synchrotron X-ray tomography and laminography. The damage mechanism was identified by in-situ SEM tensile testing, ex-situ X-ray tomography and in-situ laminography on different levels of triaxiality. The observations have shown that damage nucleated at lower strains on Mg2Si coarse precipitates than on iron rich intermetallics. The identified scenario and the in-situ measurements were then used to develop a coupled GTN damage model incorporating nucleation, growth and coalescence of cavities formed by coarse precipitates. The relationship between the damage and the microstructure anisotropies was explained and simulated

Aujourd'hui, les industries désirent réaliser la meilleure finition de surface et un taux élevé d'usinage. Ces facteurs sont utilisés pour évaluer la performance des processus d'usinage. Lorsque le taux d'enlèvement de matière augmente lors de l'usinage, la température de l'outil s’élève aussi et le refroidissement devient nécessaire. Pour réduire les effets de la chaleur sur les outils de coupe et la durée de vie des machines, une application efficace et rentable du fluide de coupe est nécessaire. En outre, pour faire face aux questions environnementales et de sécurité au travail, les nouvelles méthodes de lubrification et de refroidissement telles que la quantité minimale de lubrifiant (MQL) et la quantité minimale de refroidisseur (MQC) ont été proposées. L'usinage presque sec ou microlubrifiée avec la quantité minimale de refroidisseur ou de lubrifiant (MQCL), fournit une diminution considérable de la consommation de fluide de coupe par rapport à un refroidissement conventionnel ainsi que l'augmentation de l'usinabilité en comparaison avec d'usinage complètement à sec. La rugosité de surface, la réduction de la chaleur générée dans l'outil de coupe et l'émission de la poussière et d'aérosols dans l'atelier, comme un indicateur de la qualité du processus d'usinage, dépendent largement des conditions d'usinage. La présente recherche sorte sur l'étude des effets des différentes conditions de coupe, en utilisant de la quantité minimale de refroidisseur, sur la rugosité de surface, la température de l'outil de coupe et la concentration des aérosols et des poussières lors du tournage d'alliage d'aluminium 6061-T6. Les résultats sont comparés a ceux obtenus en tournage à sec et ceux obtenus en tournage sous lubrification abondante. Il est trouvé qu'il y a des conditions optimales pour lesquelles l’usinage MQC donne de meilleur fini de surface que l’usinage à sec et l’usinage sous lubrification abondante; l’usinage MQC et l’usinage à lubrification abondante produisent plus d’aérosols (liquide et solide) comparé à l’usinage à sec; et enfin, l’usinage MQC refroidit moins la coupe que l’usinage lubrifié traditionnel. Cependant l’usinage MQC demeure préférable en raison de la réduction des risques pour la sécurité du travail et pour l’environnement.

L’alliage d’aluminium 6061-T6 a été choisi comme matériau de structure du casier et du caisson du cœur de réacteur Jules Horowitz (RJH). Transparent aux neutrons, il doit ses bonnes propriétés mécaniques à la précipitation de fines aiguilles nanométriques appelées béta'' contenant Mg et Si et à la présence de dispersoïdes Al(Cr,Fe,Mn)Si jouant un rôle important dans la résistance à la recristallisation. Le caisson et le casier seront soumis à de forts flux neutroniques à une température avoisinant les 50°C. L’objectif de cette thèse est d’étudier les évolutions microstructurales de l’alliage sous irradiation et plus particulièrement la stabilité des précipités. Pour cela, des études analytiques par irradiations in-situ et ex-situ aux électrons et aux ions à température ambiante et forte dose ont été réalisées ainsi qu’une étude du comportement des précipités sous irradiations aux neutrons à faible dose. La caractérisation fine des précipités par Microscopie Electronique en Transmission a montré que les dispersoïdes sont stables sous irradiation, cependant ils présentent une structure cœur/coquille avec un cœur riche en (Fe, Mn) et une coquille riche en Cr qui s’accentue sous irradiation par accélération de la diffusion. En revanche, les nano-phases type béta’’ sont déstabilisées par l’irradiation. Elles sont dissoutes par irradiation aux ions au profit de l’apparition d’amas riches en Mg, Si, Al, Cu et Cr participant à l’augmentation du durcissement de l’alliage, tandis qu’elles tendent à se transformer en précipités cubiques sous irradiation aux neutrons<br>The 6061-T6 Aluminium alloy, whose microstructure contains Al(Fe,Mn,Cr)Si dispersoids and hardening needle-shaped beta” precipitates (Mg, Si), has been chosen as the structural material for the core vessel of the Material Testing Jules Horowitz Nuclear Reactor. Because it will be submitted to high neutron fluxes at a temperature around 50°C, it is necessary to study microstructural evolutions induced by irradiation and especially the stability of the second phase particles. In this work, analytical studies by in-situ and ex-situ electron and ion irradiations have been performed, as well as a study under neutron irradiation. The precipitates characterization by Transmission Electron Microscopy demonstrates that Al(Fe,Mn,Cr)Si dispersoids are driven under irradiation towards their equilibrium configuration, consisting of a core/shell structure, enhanced by irradiation, with a (Fe, Mn) enriched core surrounded by a Cr-enriched shell. In contrast, the (Mg,Si) beta” precipitates are destabilized by irradiation. They dissolve under ion irradiation in favor of a new precipitation of (Mg,Si,Cu,Cr,Al) rich clusters resulting in an increase of the alloy’s hardness. beta’’ precipitates tend towards a transformation to cubic precipitates under neutron irradiation

Master of Science<br>Department of Chemical Engineering<br>John Schlup<br>This study investigated the specular reflectance properties of 6061-T6 aluminum alloy anodized in accordance with military specification MIL-A-8625 as a function of both etch process time and anodization process potential. Both process parameters affect the specular reflectance characteristics when measured using a 660 nm, collimated diode laser source. The etch process time, when varied between 0.5 to 20 minutes, resulted in a decrease in specular reflectivity with increasing time. The anodization process potential was varied between 10 and 21 volts, with a 15 volt condition producing samples with the greatest specular reflectivity. Between the two parameters, the etch time had the greater effect. Additionally, the dependence of the incident beam angle on specular reflectivity was shown not to have a significant effect when compared to the etch process time and process potential.

Aluminum alloy 6061-T6 is a common engineering material used in aerospace, automotive, structural applications. Despite its wide use, little has been published about the effects of damage from surface corrosion on its fatigue life. An investigation was performed where 6061-T6 extrusions were exposed to a 3.5% NaCl solution at pH 2 for 2 days and 24 days. The length of time and pH were chosen in order to create distinct surface flaws. The effect of these flaws on the fatigue life was then investigated and analyzed using scanning electron microscopy (SEM) and Weibull statistics. It was determined that samples corroded for both 2-days and 24-days exhibit fatigue lives that can be described using a 3-parameter Weibull distribution. The result of which was the determination of a threshold value for fatigue as well a general understanding of flaw geometry.

Friction Stir Welding (FSW) is a new solid-state welding process that shows great promise for use in the aerospace and transportation industries. One of the primary benefits of this process is that mechanical properties of the base material are not as severely degraded as they are with conventional fusion welding. However, fatigue crack initiation and growth properties of the resulting weld nugget are not fully understood at this time. The primary goal of this project is to characterize the fatigue crack growth properties of friction stir welds in 6061-T6 aluminum as relates to the microstructural evolution of the weld. This was accomplished by producing friction stir welds and testing fatigue crack growth response in different crack orientations with respect to the weld. In addition, residual stress measurements were conducted for all cases, using both the crack compliance and contour methods. The results from the methods were compared in order to evaluate the accuracy of each method. Being an immature technology, the potential for discovery of new applications for the FSW process exist. With this in mind, novel applications of the FSW process, including the addition of particles during welding were explored. The first step was the investigation of property changes that occur when secondary cast phases are refined using the FSW process. The FSW process successfully refined all secondary phases in A380 and A356, producing an increase in hardness. Next, methods for the creation of particle metal matrix composites using FSW will be investigated. Nano-scale alumina particles were successfully added to the matrix and homogenously distributed. Using multiple weld passes through the composite was found to increase the uniformity of particle distribution. However, the alumina particle composite failed to provide any statistically significant hardness increase over the base material. The FSW process was also evaluated for weldability of traditionally difficult alloy systems. FSW was found to show very good weldability for dissimilar cast and wrought alloys, as well as for high-pressure die castings. Lastly, the feasibility of friction stir welding/processing in repairing crack defects in complex structural members in combination with cold-spray technology was determined. Friction Stir processing was used on a cold spray 6061-T6 block, resulting in significant increases in hardness over the base material, as well as a reduction in porosity. In addition, FSP was shown to eliminate crack-type defects in cold spray materials, a finding that has important applications in part repair. The deliverables of this work include an understanding of the fatigue crack growth response of FSW/FSP 6061-T6, as well as a feasibility study exploring novel uses for the FSW/FSP process. In addition, the deliverables include CNC code, fixtures, procedures, and analytical code for the creation and analysis of FSW/FSP joints. This will be important for the continuation of FSW/FSP work at WPI.

碩士<br>國立高雄應用科技大學<br>機械與精密工程研究所<br>98<br>In this paper, Our main research is focus on properties of anodic oxide film investigated about a single-step anodizing process of aluminium alloy(6061-T6) used by mixed acid (oxalic and sulphuric acid). We have discussed the influence of process parameters for anodic aluminium alloy oxide( AAAO) like film thickness, surface morphology, hardness and breakdown of voltage. And the process parameters to be discussed include: electrolyte type, concentration, temperature and operating voltage. In the post, anodizing technologies are used single acid for electrolyte such as sulfuric acid, oxalic acid and phosphoric acid. Up until recent years have switched to mixed acid or organic acid anodizing electrolyte was improved. Because mixed acid can reduce the oxide film dissolution rate, hardness increase and growth rate on film properties. So this paper will discuss and comparison the mixed acid and other single acid properties under different treatment conditions of oxalic acid, sulfuric acid and mixed acid. Anodized aluminum oxide was an amorphous pattern in our experiment, so the process parameters on the film was relatively more important. The results showed that current denisity and electrolyte temperature are important factors for properties of oxide film. Current denisity is main factor to control speed of ions reaction at barrier layer. When current density is rise to increase reaction rate and heat, Corrosion rates increases will lead to porous size and porosity increased and the film will loose. So To improve the quality of oxide film properties, how to control the reaction temperature stability and heat dissipation are key. We also found that the thickness increases will cause excessive stress to produces cracks. Using the mixed acid can change the stress cracks caused by excessive problems. The two different pore size characteristics of oxalic acid and sulfuric acid can reduce the stress accumulation rate of crack generation and heat. The results showed that under the same conditions using mixed acid were excellent than oxalic acid or sulfuric acid at same operating conditions.

博士<br>國立中央大學<br>機械工程學系<br>104<br>Tungsten inert gas welding and vacuum brazing butt joints and T-joints of Al–Mg–Si alloy 6061 in the artificially aged condition T6 were studied. Microhardness, tensile, constant amplitude and variable amplitude fatigue loading tests were performed. The experimental S-N curves were compared with the fatigue design curves recommended by the International Institute of Welding, British Standard, and Eurocode 9. Two mean stress correction methods, Goodman and Gerber, were evaluated. For the tungsten inert gas welding joints, the area with the lowest microhardness was the HAZ. For the vacuum brazing specimen, no clear HAZ was observed. Because the weldment was heated to a uniform temperature in a vacuum, so local overheating did not occur. In terms of the tensile properties of butt joints, the tensile strength of the vacuum brazing specimen increased with specimen thickness, whereas that of the tungsten inert gas welding welding specimen decreased. Tungsten inert gas welding butt joints and T-joints of AA 6061-T6 achieved higher fatigue strength as compared to the fatigue design curves of IIW, BS 8118, and Eurocode 9. In the lower life region (N = 104 ~ 105), it was found that the fatigue strength of vacuum brazing butt joints was lower than that of IIW FAT 45, but still higher than those of the BS 8118 class 42 and Eurocode 9 category 56-7. When tungsten inert gas welding or vacuum brazing joints of aluminum 6061-T6 was subjected to a variable amplitude loading with tensile mean stress, Goodman method was suitable to modify the mean stress effect. For T-joints, the fatigue life of the tungsten inert gas welding specimens given the bracket history can be predicted using the S-N method. The fatigue life of the vacuum brazing specimens under the bracket history can be predicted using the Goodman mean stress correction method. In terms of the size effect on the fatigue life, the thickness correction method recommended by the IIW was applicable to the tungsten inert gas welding joints of aluminum 6061-T6. This article proposed an innovational thickness correction method based on the ratio of the ultimate tensile strengths of specimens with different thickness. For butt joints of vacuum brazing, the tensile strength–based thickness correction method was better than the thickness correction methods recommended by the International Institute of Welding. For T-joints, vacuum brazing is required to carry out thickness correction in the higher life region.

There is an urgent need to develop new non-chromating coating methods for the corrosion protection of aluminum alloys, and this thesis reports studies to establish optimal working conditions for forming zinc phosphate coatings on 6061-T6 aluminum alloy. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and corrosion tests were used to characterize the treated 6061-A1 surface, and to assess the effects of different parameters in the coating bath, as well as different pre-treatments and post-treatments. The initial study investigated different working conditions for the zinc phosphating, and this built on other studies from this laboratory. An initially considered coating bath had 16.0 ml H3P04 (85%), 5.36 g ZnO and 0.5 g NaF per liter, and optimal conditions for coating involved dipping the 6061-A1 samples into this bath at 60 °C for 6 min. But the addition of more fluoride was shown to be effective for increasing the zinc phosphate coverage on the aluminum alloy; the optimal range of F" being from 400 ppm to 600 ppm for these conditions. Based on the working condition study, the effects of pre-treatments, posttreatments, and associated procedures involved with the zinc phosphating of 6061-A1 alloy on the quality of the final coating were investigated. Comparisons were made between acid etching and mechanical polishing in the pre-treatment stages, and the mechanical polishing involved was done by either machine polishing or hand polishing. New post-treatment procedures were investigated using methyltriethoxysilane. It results in a considerable increase in the zinc phosphate coverage and coating thickness on the aluminum surface. This gave a greatly improved corrosion resistance by the coating compared with samples that did not have the silane post-treatment. A related silane treatment can also be used at an intermediate stage in the zinc phosphating immersion. In order to improve the final coating, Cu²⁺ and Ni²⁺ ions were studied as accelerators. Cu(N0₃)₂ (0.002 wt%) and Ni(N0₃)₂ (0.0004 wt%) were separately introduced to the basic phosphating baths containing H₃PO₄; ZnO and NaF, and each was shown to not only increase the phosphating speed, but also to improve the corrosion resistance and adhesion, insofar as the coating contained smaller crystalline grains and increased coverage of zinc phosphate on the surface. Of these two accelerators, Ni²⁺ appeared especially effective at increasing the corrosion protection ability of the final coating. Although NO³⁻ has some capabilities as an accelerator it was shown that its effect was negligible in this context at the concentrations used.

碩士<br>中興大學<br>機械工程學系所<br>95<br>This study discusses the aluminum alloy 6061-T6 in HSM, and the surface roughness is adopted as the quality characteristics. In order to obtain the optimal machining parameters for good surface roughness, we selected the machining parameters (e.g. type of milling, cutting speed, feed per edge, depth of cut and width of cut etc.) via the Taguchi method, and utilized T500 3D instruments for roughness measuring. The results show that the effect upon the depth of cut, feed per edge, and width of cut have more influence to surface roughness. In contrast, the types of milling and cutting speed have less influence. We obtain the optimal machining parameters of surface roughness are: type of milling (down milling), Vc : 350 m/min, fz : 0.04 mm/tooth, ap : 0.1 mm, ae/d : 30%. Experiment to obtain the optimal roughness quality is Rmax=1.46~1.72 μm.

碩士<br>元智大學<br>機械工程研究所<br>87<br>Obtaining a mirror-like surface roughness by using a milling machine to cut a 6061-T6 aluminum alloy is the goal of this study. Traditionally, this is not doable by the milling process. However, by a series of systematic experiments for setting the cutting parameters, it is observed that a surface roughness of 0.05μm Ra instead of traditional 0.8μm Ra was obtained. Also, its repeatability was proved when using the cutting parameters found. Among the machining parameters, such as cutting tool, spindle speed, cutting depth, cutting fluids, and blade arrangement…etc., all could affect the surface roughness of a workpiece. We use a CNC machining center with inserted diamond face milling blade as the cutting tool. Then varying all the above parameter settings by using Taguchi method to perform the tests. Different types of cutting tools and various arrangements in the tool holder were tested. Three settings for each of the cutting speeds, feed rates, and cutting depths were also conducted in the experiments. Through the study, we have a better understanding on the influence of those parameters to the workpiece surface roughness. For example, the order of influence weighting to the surface roughness is found as: the tool arrangement, feed rate, depth of cut, and then the spindle speed accordingly. It is also known that to obtain a surface roughness within the range of grinding or polishing is possible for a milling process. This saves time and cost due to there''s no need to use grinding after the milling process.

碩士<br>國立屏東科技大學<br>車輛工程系所<br>98<br>Recently, the application of lightweighting materials to improve fuel efficiency and reduce pollutions has become the key issue in automotive industry. Aluminum alloy, magnesium alloy and composite, are then progressively used instead of steel for body structures. With the high strength, excellent corrosion resistance and easy recycling, aluminum alloy attracts the most attention in automotive industry. The deformation after welding significantly affects the quality of auto-body assembly. Friction stir welding (FSW) is a novel solid-state welding process. It takes the advantages of low heat input, high quality and small deformation and, is very suitable for aluminum alloy welding. But it can be problematic for thin aluminum alloy sheets. In this study, the shadow moiré technique is employed to investigate the FSW deformation characteristics for A6061-T6 aluminum alloy. Several process parameters such as tool’s rotation speed, welding speed, holding force and back plate material are investigated. A FEM model is first used to obtain the reasonable ranges of welding parameters. Then, the welding under different process parameters is performed and the warpage of the workpieces are measured. The results show that the temperature distribution is highly depends on the tool’s rotation speed and the situation of heat dissipation. In addition, the deformation is mainly affected by the longitudinal residual stress. Using a back plate with lower heat conductivity can speed the welding process and results in a smaller deformation.

碩士<br>國立成功大學<br>機械工程學系碩博士班<br>101<br>6061-T6 aluminum alloy has been used extensively as structural materials for internals of experimental nuclear reactors, aircraft fittings, amour systems and high-speed machinery for years due to its superior mechanical properties. In this study, the high temperature deformation and micro-structural evolution of 6061-T6 aluminum alloy under high strain rate loading condition are investigated by means of a split-Hopkinson bar. The specimens with longitudinal direction are heated using a clam-shell radiant-heating furnace. Impact tests are performed at strain rate ranging from 1×103 to 5×103 s-1 and temperatures between 100℃ and 350℃. The experimental results indicate that the flow response of 6061-T6 aluminum alloy is related to temperature and strain rate. At a constant temperature, plastic stress, work hardening rate and strain rate sensitivity all increase with the increasing strain rate, while the thermal activation volume decreases. However, at a constant strain rate, plastic stress, work hardening rate and strain rate sensitivity decrease with increasing temperature, while the thermal activation volume increases. The observed that high temperature and high strain rate deformation behavior of 6061-T6 aluminum alloy can be adequately described using the Zerilli-Armstrong constitutive equation. Transmission electron microscopy observations reveal that the dislocation density increases with an increasing strain rate, but decreases with an increasing temperature. The strengthening effect observed at higher strain rates and lower temperatures is attributed to a greater dislocation density. The stacking fault is also found in high temperature and high strain rate. A linear relationship between the square root of the dislocation density and the true stress is also found.

碩士<br>國立成功大學<br>機械工程學系碩博士班<br>101<br>In this study, a spit-Hopkinson bar is utilized to study the effect of temperature and strain rate on impact response and microstructural characteristics of 6061-T6 aluminum alloy at different cryogenic temperatures of 0℃, -100℃and -196℃, under strain rates of 1000s-1, 3000s-1 and 5000s-1, respectively. The experimental results indicate that the mechanical properties are related to temperature and strain rate. At a constant temperature, flow stress and strain rate sensitivity all increase with the increasing strain rate, while the work hardening and thermal activation volume decreases. However, at a constant strain rate, flow stress and strain rate sensitivity decrease with increasing temperature, while the thermal activation volume increases. In addition, the observed impact deformation behavior of this alloy under current testing conditions can be described by the Zerilli-Armstrong equation. Optical microstructural observations reveal that the formation of adiabatic shear band and morphology of deformed grain of 6061-T6 aluminum alloy are strongly dependent on temperature and strain rate. Transmission electron microscopy (TEM) observations show that the dislocation density increases with increasing strain rate, but decreases with increasing temperature. Based on the experimental mechanical properties and microstructural characteristics, it is found that the correlation between the dislocation density and flow stress obeyed by the Bailey-Hirsch type relation. In addition, the flow stress, strain rate sensitivity and thermal activation volume are also related to the observed dislocation substructure.

Friction Stir Welding (FSW) is an emerging solid state welding process. It has been a proven method for welding high strength aluminium alloys which were previously not recommended for conventional fusion welding. Since the invention of the process by The Welding Institute, United Kingdom, in 1991, a number of studies have been conducted on the material flow, microstructural evolution and mechanical properties of friction stir welds. However, there is not enough conceptual background available on FSW process for physical understanding of the mechanism of weld formation. In addition to that, FSW welds of high strength precipitation hardenable aluminium alloys suffer from reduced joint efficiency due to overaging in the heat affected zone. In the present investigation, experimental analysis has been carried out to understand the mechanism of weld formation and parameter optimization for aluminium alloys 7020-T6 and 6061-T6. For this purpose the investigations have been made on both the process aspects and the material aspects. The process aspects are analyzed with the objective of learning the mechanism to produce defect free welds. For this purpose experiments have been carried out to analyze the effect of FSW parameters, material flow and the frictional characteristics between the tool and base metal. Preliminary experiments are conducted on aluminium alloy 7020-T6 with different tool geometries to analyze the interaction of the tool with the base metal using a knee-type vertical milling machine. Then, the tool geometry which produced defect-free weld is used for further experimentation. The role of tool pin, shoulder and axial load on the formation of defect free weld is analyzed in an innovative experiment, where the tool and base metal interaction is continuously increased by continuously increasing the interference between the tool and base metal. In another experiment the initial abutting interface position with respect to the tool is continuously varied to study the interaction of the tool with the initial interface and to find the positional information where the initial interface is completely eliminated. Further, the tool metal interface condition is studied using a specially designed tribological experiment which simulates the FSW condition. From the base metal point of view, due to the strain, strain rate and temperature imposed on the base metal during the process, the microstructure is altered. In precipitation hardenable aluminium alloys the strengthening precipitates are dissolved or overaged in the weld region depending on the peak temperature in the region, which reduces the joint efficiency. However, the dissolution and overaging are kinetic process. In order to analyze this time dependant softening behavior of the base metal 7020-T6 and 6061-T6, isothermal annealing and differential scanning calorimetric studies are performed. In order to obtain FSW welds with maximum joint efficiency, the welding temperature should not exceed the “softening temperature” of the base metal. But, to produce defect free welds favorable material flow in the weld nugget is necessary. The material flow and consolidation depend on the process temperature. Hence, for a given tool to produce defect free weld there is a need for minimum temperature. If the weld formation temperature is less than the base metal softening temperature, the weld can be made with 100% joint efficiency. In order to optimize the FSW parameter which gives defect free weld with lowest possible temperature, an instrumented programmable FSW machine is designed and developed. The machine is designed in such a way that welding parameters – rotation speed, traverse speed and plunging depth – can be continuously varied from the start to end of the weld between given two values. This reduces the number of experimental trials, material and time. Based on the experimental results the following conclusions are derived. 1.The minimum diameter of the pin required for FSW depends on the base metal and tool material property for a given set of parameters. If the pin diameter is insufficient for a given set of welding parameters, it fails during plunging operation itself. 2.There is a minimum diameter of the shoulder for a given diameter of the pin which produces defect free weld. The ratio of pin to shoulder to produce a defect free weld is not a constant value. It changes with tool geometry and process parameters. 3.Increasing the area of contact between the tool and shoulder for a given set of parameters increases the heat input and results in increased weld nugget grain size. 4.Initial abutting interface of the base metal is eliminated at the leading edge of the tool. However, new surfaces are generated due to interaction with the tool and the newer surfaces are consolidated at the trailing edge of the tool. Importantly, the weld strength is controlled by the defects generated due the improper elimination of newly generated surfaces. 5.Optimal axial load is required to generate the required pressure to consolidate the transferred material at the trailing edge of the tool and should be equal to the flow stress of the material at the processing temperature. The optimal axial load is 8.1kN for a tool having 20mm diameter shoulder with 6mm diameter frustum shaped pin. 6.Only the material that approaches the tool at the leading edge on the advancing side is stirred and the remaining material is simply extruded around the tool. Further, the initial abutting interface is completely removed only when it is located in the stirring zone, otherwise the initial abutting interface is not eliminated. In the present study the interface is completely stirred when it is located on the advancing side of the tool between 0.5mm away from the centerline and edge of the tool. 7.The temperature and pressure at the tool–base metal interface is above the temperature and pressure required for seizure to occur for given tool material (H13) and base metal (7020-T6). Hence, it is clear that during FSW the base metal transfers on to the tool and interaction occurs between transferred layer on tool and base metal. The coefficient of friction between the given tool material and base metal in FSW condition is in the range of 1.2 – 1.4. 8.The minimum temperature requirement for FSW of 7020-T6 is 400oC and 6061-T6 is 430oC. However, 7020-T6 and 6061-T6 softens at lower temperatures than that of the minimum FSW temperature. 7020-T6 softens 30% in 7min at 250oC, 4min at 300oC, 2min at 350oC and 1min at 400oC. After softening 30%, there is 10% recovery in hardness and the hardness remains constant thereafter. Whereas 6061-T6 softens gradually up to 47% in 7min at 350oC and 400oC, below the temperature of 250oC for 7020-T6 and 350oC for 6061-T6 there is no softening observed in 7min. 9.The maximum joint efficiency of the 7020-T6 weld is 82% and 6061-T6 weld is 60%. 10. The reduction in joint efficiency is attributed to overaging of the material in the heat affected zone.

碩士<br>國立臺北科技大學<br>製造科技研究所<br>89<br>Friction stir welding process is one of the most recent welding technologies in joining aluminum alloys. It can produce low-distortion, high-quality, low-cost aluminum joints. During the welding process, welding beads are formed in solid-phase through the crushing, stirring and forging on the joint by the interaction forces of the base metal and the stirring bar. The process is conducted without fumes, magnetic arc blow, radiation, running off of alloying elements and porosity. The newly developed process contributes its credits on environmental protection. 　　This study aims to join 6061-T6 aluminum alloys plates in butt joint type by using friction stir welding technology. Special contours of stirring bars have been designed and produced in this project. 　　The optimal welding parameters of travel velocities, rotating speeds and welding depths have been tested and determined.

碩士<br>國立成功大學<br>工程科學系碩博士班<br>101<br>This thesis investigates the mechanical behavior and buckling failure of 6061-T6 aluminum alloy tubes with six different sharp-notched lengths subjected to symmetric cycling bending. The tube bending machine and curvature-ovalization measurement apparatus were used to control, measure and collect the experimental data. It can be observed from the experimental moment-curvature curve that the tube exhibits cyclical hardening during the symmetric cycling bending and becomes steady after a few cycles. Next, for the experimental ovalization-curvature curve, the ovalization of the tube cross-section increases in a ratcheting manner with the number of cycles. When the tube ovalization reaches to a critical amount, the tube buckles. In addition, it is shown in the experimental controlled curvature-number of cycles to buckling failure relationship that the larger controlled curvature leads to a fewer amount of the number of cycles to buckling failure. It can be observed from the relationship in a log-log scale that the controlled curvature-number of cycles to buckling failure relationships show parallel straight lines when the notch depth less than or equal to 1.0 mm; However, the controlled curvature-number of cycles to buckling failure relationships show nonparallel straight lines when the notch depth less than or equal to 1.2 mm. Finally, by referring the theoretical formulations proposed by Shaw and Kyriakides【5】, Lee et al.【6】and Fan【14】and including the factor of the sharp-notched lengths, a theoretical formulation was proposed to simulate the relationship between the controlled curvature and the number of cycles to buckling failure. By comparing the theoretical analysis with the experimental data, it is shown that the theoretical formulation can properly simulate the experimental results.

碩士<br>國立成功大學<br>工程科學系碩博士班<br>101<br>This paper presents the experimental investigation of the buckling of the 6061-T6 aluminum alloy tube with the unnotched and sharp notch subjected to symmetrical curvature-controlled cyclic bending. There are five different sharp-notched depths (0.4, 0.8, 1.2, 1.6 and 2.0 mm) and unnotched (0.0 mm) of 6061-T6 aluminum alloy tubes. The curvature-rate for the experiment is a constant (0.012 m-1/s) for obtaining the relationship between the ovalization and number of cycles. It can be observed from the experimental curve of the ovalization - number of cycles curve can divide into three stages, the initial, secondary and tertiary stages. It can be seen that the trend of the curve is similar to that tested by Lee et al. 【1】for 304 stainless steel. Finally, the theoretical model proposed by Lee et al. 【1】is used for simulating the ovalization - number of cycles relationship for 6061-T6 aluminum alloy tubes with different sharp-notched depths in initial and secondary stages. By comparing the theoretical simulation with the experimental data, good agreement between the experimental and theoretical results has been achieved.

碩士<br>國立臺灣師範大學<br>工業教育研究所<br>82<br>In order to explore the best brazing parameter, torch brazing is used to braze the wrough aluminum alloy plates of 1 100 and 6061(T6) by using filler metals of BAlSi-4 and TA1OO in the type of lap joint. The joint shear strength was compared through the tensile test. Further more, the change of microstructure was examined by optical microscope and micro hardness test. The result showed that there is no different between the joint shear strength using the filler metals BAlSi-4 and TA1OO for 1100 aluminum alloy. For the wrough 6061 (T6) aluminum alloy plates, the joint shear strength of brazing filler metal TA 100 is higher than filler metal BAlSi-4. There are reduction in the strength and hardness of base metal for bath 1100 or 6061 (T6) wrough aluminum alloy plates. Because the wrough 1100 aluminum alloy plate is strengthened by cold working, brazing heat caused recrystallization and increase in grain size. The decrease of mechanical properies of wrough 6061 (T6) aluminum alloy plate was obtained by heat treatment during brazing. In addition, there is diffusion of Silicon in the weidment of wrough 1100 aluminum alloy plates and promote the increase of hardness of base metal. But no diffusion was observed in 6061 (T6) aluminum alloy.

碩士<br>國立宜蘭大學<br>機械與機電工程學系碩士班<br>106<br>In metal cutting, the use of hard tools to remove excess material from the workpiece is the most common and convenient method of processing. In this study, the carbide blade of WINDIA TT25 was used to medium carbon steel shank by high-frequency welding, and the cutting experiment of aluminum alloy 6061-T6 was carried out by boring mode to explore the change of the cutting edge angle and the nose radius of the main cutting edge. When cutting aluminum alloy 6061-T6, record the cutting force and surface roughness results, observe the cutting characteristics of each tool, and find the best cutting tool. In the study, six main cutting edge chamfering tools and three nose nose radii were used. A total of 18 turning tools were experimental tools in the experiment, and the cutting force was measured by three-dimensional cutting dynamics. The results show that the angle increases to a positive value, the cutting force is low, and when the edge chamfer angle increases too much, the contact length between the tool and the workpiece also increases with it, causing the cutting force to increase, and the cutting angle of the main cutting edge is 10° (Cs= 10 °), it is ideal. When the feed rate is increased, the radius of the nose increases to obtain a more stable surface roughness. However, when the nose radius R=0.3mm, it is still affected by the built-up edge (BUE), and the effect is less obvious. The nose radius R = 0.5 mm is selected.

碩士<br>國立成功大學<br>機械工程學系碩博士班<br>101<br>In this study, the effect of temperature and strain rate on plastic deformation and microstructure of 6061-T6 aluminum with Transverse direction is evaluated. A split Hopkinson pressure bar tester is utilized to investigate the macro-mechanical properties and microstructural variation of the specimens under high strain-rate loadings over wide temperature range. The specimens are deformed at 100 ℃, 200 ℃ and 350 ℃ under the strain rates of 1000 s^(-1), 3000 s^(-1) and 5000 s^(-1). The experimental results indicate that the mechanical properties are related to temperature, strain rate and strain. At a constant temperature, plastic stress, work hardening rate and strain rate sensitivity all increase with the increasing strain rate, while the thermal activation volume decreases. However, at a constant strain rate, plastic stress, work hardening rate and strain rate sensitivity decrease with increasing temperature, while the thermal activation volume increases. A Zerilli-Armstrong constitutive equation is used to predict flow behavior under different temperatures and strain rates. OM observation results indicate that the morphology of deformed grain and recrystallization varied with strain rate and temperature. Moreover, grain refinement and recrystallization are observed of influencing largely intensity at high temperature and high strain rate. TEM microstructure observations reveal that the dislocation density increases with the increasing strain rate, but decreases with the increasing temperature. In addition, stacking defects and Widmanstatten structure generate at high temperature and low strain rate. The higher dislocation density prompts a reduction in the dislocation cell size. The relationship between flow shear, grain size and dislocation cell can be described by a modified Hall-Petch equation. Furthermore, the effect of grain size on the stacking fault energy is also evaluated.

碩士<br>國立成功大學<br>工程科學系碩博士班<br>101<br>In this study, the finite element software ANSYS Workbench 12.0 is used to analyze the mechanical behavior of sharp-notched 6061-T6 aluminum tubes under curvature-controlled symmetrical cyclic bending. Different notch depths have 0.4, 0.8, 1.2, 1.6 and 2.0 mm. The controlled curvature of the cyclic bending is between +0.1 to -0.1 m-1. According to the experimental data from Lin Bo-Yan【28】, the moment-curvature relationship exhibits increasing manner with the number of cycles and gradually becomes steady. The notch depth is inversely proportional to the moment. For the relationship of the ovalization and curvature, it exhibits the ratcheting, and increasing phenomenon with the number of cycles. The relationship shows symmetrical at shallow notch, however, the relationship shows unsymmetrical at deeper notch. In addition, as the notch depth increases, the growth speed of ovalization will be faster. Finally, the ANSYS analysis is compared with the experimental data. Although some differences between the experimental and simulated results, but the trends are very similar between the two.

碩士<br>國立臺灣師範大學<br>工業教育研究所<br>89<br>Because of the advantages of ‘high strength to weight’ ratio and good corrosion resistance, aluminum and alloys have been applied extensively in industries. The properties of high coefficient of thermal expansion (CTE) and thermal conductivity in aluminum alloys make them need high welding current to supply the heat loss. The kinetic-energy machine is the best choice, however, it costs a lot. Consequently, 90% of resistance spot machines in industries are alternating—current transformer machines, and the application of resistance spot welding in connecting aluminum is limited. The aims of this study is to explore the reliability of AC spot welding in 6061-T6 aluminum alloy under different welding parameter control, and to find the best welding parameters. For each welding parameter, 20 data points of tensile-shear strength were collected to analysis the data distribution. According to the Weibull analysis, we will discuss the reliability of aluminum alloy made by AC spot welding. Furthermore, the fracture modes of specimen have been observed by SEM. The fracture surface of the material identified through the SEM show that the broken is only in the nugget, beginning from the defects in the inner nugget. The thicker (t=1.6mm) the 6061 aluminum alloy plate is, the larger the range of welding parameter control is, and the defects occur more in the inner nugget. However, the thinner (t=1.0mm) the 6061 aluminum alloy plate is, the narrower the range of welding parameter control is. Cracks occur easily if the welding parameter control is not in fitting control. According to the reliability analysis, the Weibull modulus, m is over 1 for 6061 aluminum alloy which the plates’ thickness (t) are 1.2mm (m=3.52) and 1.0mm(m=3.43), indicating the wear-out mode. However, the plate’s thickness 1.6mm(m=0.89) needs more welding current machine, then the reliability will improve.

碩士<br>國立成功大學<br>機械工程學系碩博士班<br>100<br>The dynamic shear deformation behaviour and fracture characteristics of rolled 6061 aluminum alloy plate under different crystallographic orientations and temperature. Cylindrical torsion specimens will be prepared from rolled plate with three different crystallographic orientations, i.e. longitudinal(L), transverse(T) and through-thickness(TT). After machining, all specimens will be solution heat treated at 565℃ for 1.5 h and aged at 171℃for 16 h. The torsion tests will be carried out using a split-Hopkinson torsional bar system. Shear deformation is conducted at temperatures of -150℃, 25℃ and 300℃ under strain rates ranging from 2×103s-1 to 4×103s-1, respectively. The experimental results indicate that the shear flow response is found to be sensitive to the strain, strain rate and temperature. The flow stress, fracture strain, work hardening rate, strain rate sensitivity all increase with the increasing strain rate for a fixed temperature, but decrease with the increasing temperature under a constant strain rate. The fracture stress of T, L and TT are not significant but the fracture strain of transverse direction is higher than L and TT. From the SEM observations, it is found that the fracture surfaces are characterized by a dimple structure, which is indicative of a ductile failure mode. The morphology and the density of these dimples are influenced greatly by strain rate and temperature conditions. Optical microscopy observations reveal that grain of the fracture surfaces are twisted into band like features.

碩士<br>國立宜蘭大學<br>機械與機電工程學系碩士班<br>104<br>The most common way for metal working is to use the tools which are made from harder materials to remove the unnecessary workpiece. The study use carbide cutting tools to do cutting test on aluminum Alloy 6061-T6. The goal is to find out the cutting force and roughness average when using different side cutting edge angle and nose radius to cut aluminum Alloy 6061-T6, and to record influences of cutting characteristics when using various cutting tool angles. In this study, researcher used six different side cutting edge angles with three different nose radii as the test cutting tools, and use 3-dimensional dynamometer to measure cutting force in this test. According to the experimental result, the increasing of side cutting edge angle is helpful for the decreasing of cutting of cutting force, but over increase the angle will raise the friction and same time. Therefore, the ideal side cutting edge angle is 10°. The increasing of nose radius will also heighten the cutting force, but is helpful for decreasing the roughness average. However, it is easy to form BUE and bring about the shoddy roughness average, when side cutting edge angle approach to 0° and nose radius is 0.3mm.

碩士<br>國立臺灣科技大學<br>機械工程系<br>105<br>Grinding process results in high heat generation at the workpiece-grain interface and most of the heat goes into the workpiece, another fraction is absorbed by the grain and removed by MQL. These high temperatures to the workpiece causes rapid wear of the abrasive grains, surface and sub-surface alteration of the workpiece. A model of the working temperature in both dry and minimum quantity lubrication was developed. In this study, NaHCO_3 particles were spread out with high-pressure air when the threshold of 40°C was exceeded. The results show that the utilization of the surfactant can reduce the working temperature and heat-affected zone efficiently. The hardness of the workpiece is high when the surfactant is applied due to the reduction of the rubbing behavior or efficient dispassion of heat. The simulated results indicate that during dry grinding the temperature rise is rapid as compared to the MQL grinding. The high temperature in dry grinding might be because of immediate wear of the coated abrasive grain, which results in an increase of frictional heat due to large contact area between the abrasive grain and workpiece. The effects of grinding parameters such as mesh size are also analyzed in this study. It is observed that using an abrasive disc of 150 mesh size high temperatures were recorded as compared to 600. The simulation results were verified by the experiments.