Дисертації з теми "6xxx series aluminium alloys"

The present work has been carried out in order to investigate Al-Mg-Si alloys that are deformed at moderate temperatures. These temperatures are in the range between 200 C and 300 C. Also some experiments are performed at room temperatures. Two deformation models have been applied in the experiments: material deformation by compression testing and by forward extrusion.

The investigated alloys are AA6063, AA6082 and an alloy that is named “Alloy R” in this work. The latter alloy is the industrial alloy AA6082 without the Mn-addition (0.56wt%Mn in the AA6082). The “R” denotes the recrystallized microstructure in the material after hot forming operations.

The investigations show the effect of changing the temperature in the given temperature interval on the stress-strain relationship for each alloy. From the compression testing, it is found that none of the alloys AA6063 or Alloy “R” reaches a steady state condition as true strain approaches 0.8 for deformation temperatures between 200 C and 250 C. At compression testing performance at 300 C, the alloy “R” reaches a steady state condition at a true strain equal to 0.4.

As true stress-true strain relationship has been investigated for the “Alloy R” and the AA6063 at comparable deformation parameters, it is shown that the alloy “R”, with the highest Si-content, requires the highest true stress for a given true strain value (AA6063: 0.45wt%Si, Alloy “R”: 0.87wt%Si).

From the compression testing, the effect of Mn on the material properties in the AA6082-alloy has been determined. For the Alloy “R” and the AA6082, the true stress reached the same value after a certain amount of deformation. As deformation temperature increases, this common value of true stress corresponds to a decrease in true strain.

The AA6082 and Alloy “R” are also compared in experiments performed in forward extrusion. One observes that for the same deformation temperature and at identical die diameters, the ram force is identical. It is worth noticing that these alloys did not show the same relationship during the compression testing at low values of true strain (<0.8). On a microscopic scale, one concludes that Mn has no significant effect on the stress-strain relationship for the applied deformation parameters in the forward extrusion equipment.

Hardness measurements indicate that the age hardening potential in the extruded test specimen decreases as the deformation temperature increases. The hardness data is similar for both the AA6082 and the Alloy R, thus indicating that the Mn content has no significant effect on the strength of the material.

The deformed material has been annealed in order to investigate the recrystallization process in the AA6082 and the Alloy “R”. The recrystallization grain size in the Alloy “R” is significantly larger than in the AA6082 at comparable deformation parameters after annealing at 530 C for 15 minutes. This result is due to the effect of Mn-containing dispersoids in the AA6082. The recrystallization grain size in the Alloy “R” seems to be unaffected by the deformation temperature after annealing for 15 minutes. The observation of the AA6082 is quite different. A small increase in grain size is observed for both reduction ratios as the deformation temperature is elevated from 20C to 200 C and further to 250 C. At extrusion temperatures of 300 C the recrystallization grains are significantly larger.

Annealing experiments performed at 430 C on the AA6082 indicates that a change in the deformation temperature from 200 C to 250 C does not affect the amount of stored energy in the material significantly.

The Forge2 programme has been used to perform numeric simulations of the forward extrusion experiment. From this the temperature distribution, strain rate variation and true strain development in the test piece had been investigated. As the simulated true strain values are compared to the grain size in the annealed material, the recrystallization grain size is related to the amount of stored energy in the material in a very convincing way. It is also shown that the recrystallization grain diameter is related to the amount stored energy as the grain diameter is investigated in the radial and the extrusion direction separately.

Les alliages d’aluminium, présentant un rapport résistance-poids élevé, sont capables de répondre aux exigences de réduction de masse et d’augmentation de sécurité dans la construction de nouveaux véhicules. Cependant, lors des opérations d’emboutissage, ces alliages présentent une plus faible formabilité et un retour élastique plus élevé que les aciers traditionnellement utilisés. L’emboutissage à température moyenne (qualifiée de tiède) apparaît comme une solution très intéressante pour résoudre ces problèmes. Néanmoins, l’emboutissage pour les alliages d'aluminium de la série 6000, reste toujours un défi car la gamme de température utilisée est proche de celle utilisée lors des traitements thermiques de ces alliages. Ainsi l’augmentation de la température peut entraîner un durcissement par précipitation. En outre, ces alliages sont susceptibles de vieillir naturellement. Tous ces changements doivent être prédits pour éviter des variations dans le process de production. Dans ce contexte, l'objectif principal de ce travail a été d'analyser les conditions d’emboutissage à tiède de deux alliages d’Al-Mg-Si (l'EN AW 6016-T4 et l'EN AW 6061-T6), afin de d’améliorer la robustesse des opérations d’emboutissage. Des essais de traction et des essais d’emboutissage de godets cylindriques en température ainsi que des mesures de retour élastique (essai dit de Demeri) ont été effectués. Plusieurs paramètres comme le vieillissement naturel, la durée de chauffe et la vitesse d’emboutissage ont été étudiés afin de proposer des solutions permettant d’améliorer la robustesse des opérations d’emboutissage. L’emboutissage à tiède se révèle être une solution efficace pour améliorer la formabilité, réduire le retour élastique et la variabilité causée par le vieillissement naturel. Cependant, utiliser des vitesses d’emboutissage élevées et une chauffe rapide sont nécessaire pour éviter le durcissement par précipitation pendant l’emboutissage
Heat treatable aluminum alloys present a high strength-to-weight ratio, which replies to the requirements of mass reduction and safety increase in the construction of new vehicles. However, in sheet metal forming operations, these alloys have lower formability and higher springback than traditionally mild steels used. In this context, forming in warm temperature appears as an attractive solution to solve these problems. Nevertheless, there is still a challenge since the temperature range used in warm forming is similar to one used in the heat treatment of these alloys. Thus increasing the temperature can lead to precipitation hardening, which modifies the thermo- mechanical behavior of the material. In addition, these alloys are prone to natural aging that causes variability in forming operations and increases the amount of scrap. The present study addresses the warm forming of two heat-treated Al-Mg-Si alloys (EN AW 6016-T4 and EN AW 6061-T6), in order to propose solutions that can contribute to the increase of robustness of sheet metal forming operations. The influence of natural aging, temperature and exposure time has been studied by using uniaxial tensile tests, cylindrical cup tests and the split ring tests. The main goal is to propose solutions to improve the robustness of the sheet metal forming process. Warm forming proves to be an effective solution for improving formability, reducing the springback and variability caused by natural aging. However, high forming speeds and fast heating are necessary to prevent precipitation hardening during forming operations

Aluminium applications can be found in the vast majority of industries – particularly the automotive, aerospace and building sectors. Light weight, good corrosion resistance, high strength with good machining and weldability has led 6xxx series alloy to be the most widely used for extrusion products. Semi-continuous direct-chill (DC) casting is a well established process and the most widely used in the production of wrought aluminium extrusion billets. The techniques have continuously evolved since its invention in the 1930s. To ensure high productivity and a quality billet by DC casting, grain refiners are added during casting prior to solidification. It is efficient, cost effective and considered optimized in modern production techniques. However, some problems still persist, for example, macrosegregation, centerline cracking, porosity, hot tearing, etc. For surface finish critical products, particles in added grain refiners may cause surface defects during downstream processing. Additions of grain refiners are also not desirable for recycling of the end use products. As a novel DC casting technology, the melt conditioned DC casting (MC-DC) technology is developed to achieve uniform fine equiaxed grains without deliberate additions of grain refiners. The MC-DC process is implemented by submerging a rotor-stator high shear device into the mould assembly of a conventional hot-top vertical DC caster. In this work, the fundamentals of MC-DC process has been investigated by studying the flow patterns in the sump using computer modelling in combination with thermal field measurement and delineation of the sump profile. Followed is the microstructural evolution of the MC-DC castings. Then the formation of Fe-bearing intermetallics which are critical to the arrangement of homogenisation treatment are presented. The grain refining mechanism by MC-DC is due to enhanced heterogeneous nucleation on dispersed oxides and grain fragments by intensive melt shearing, in combination with dendrite fragmentation and transportation in a uniform temperature and solute field. By optimising MC-DC parameters, alleviation of macrosegregation can be achieved even compared with DC-GR castings. Another finding is the correlation between grain structure and the distribution of the Fe-intermetallic particles. It has been demonstrated that equiaxed dendritic grains with fine secondary dendritic arm spacings achieved in MC-DC are preferred rather than finer granular grains in grain refined material. MC-DC also promotes the formation of α- Fe-bearing intermetallics. All these offer the potential for the reduction of homogenisation practices currently required as part of the DC process.

The compressive creep behaviour of six sand cast zinc-rich alloys: No3 and No5, corresponding to BS 1004A and BS 1004B, respectively, alloy No2, ILZRO,.16 and two newer alloys ACuZinc5 and ACuZinc10 was investigated. The total creep contraction of the alloys was found to be well correlated using an empirical equation. On the basis of this equation, a parametrical relationship was derived which allowed the total creep contraction to be related to the applied stress, the temperature and the time of test, so that a quantitative assessment of compressive creep of the alloys could be made under different testing conditions. The primary creep and secondary creep rates were found for the alloys at different temperatures and stresses. Generally, the primary creep contraction was found to increase with copper content, whereas secondary creep rates decreased in the order No3, ACuZinc10, ACuZinc5 and No2. ILZRO.16 was tested only at the highest stress and two higher temperatures. The results showed that ILZRO.16 had higher creep resistance than all the other alloys. Thus, based on the above empirical equation, alloy No2 was found to have a substantially better total creep resistance than alloys No3 and No5, and slightly better than ACuZinc5 and ACuZinc10 for strains up to 1%. Both ACuZinc alloys had higher creep strength than commercial alloys No3 and No5. Alloy No5 had much higher creep resistance than alloy No3 under all conditions. The superior creep resistance of alloy No2 was considered to be due to the presence of small precipitates of -phase in the zinc matrix and a regular eutectic morphology. The stress exponents and activation energies for creep under different testing conditions were found to be consistent with some established creep-controlling mechanisms; i.e. dislocation climb for alloy No3, dislocation climb over second phase particles for alloys No5, No2, ACuZinc10, controlled by lattice diffusion in the zinc-rich phase. The lower creep resistance of alloy No3 was mainly due to the lower creep strength of copper-free primary particles having greater volume than eutectic in the microstructure. Alloys No5, ACuZinc5 and ACuZinc10 showed much better creep resistance than alloy No3, based on the precipitation-hardening due to the presence of small -phase precipitates. The primary dendrites in both ACuZinc alloys however were not of much benefit in improving the creep resistance of the alloys.

The desire to reduce weight in automotive products is driven by a need to improve efficiency. As such, to allow further weight reduction, higher performance aluminium alloys are in demand for sheet metal body structures. Due to their high strength to weight ratio 7xxx alloys are seen as an ideal candidate for this, however their use to date has been limited by poor formability. Previous work indicated that by moving to high temperatures (>350°C) or by using a soft temper (W), good formability could be achieved but the samples required further heat treatment post-forming. This work explored the warm forming temperature range to improve formability whilst developing the required properties during processing. The performance of a 7xxx candidate alloy, 7021, has been assessed in stretching and drawing operations, both at room temperature and over the elevated temperature range of 150-250°C. The microstructure and other properties of the alloy were investigated in W, T4 and T6 tempers, before and after testing, through a range of techniques, including DSC, DMTA, SEM, EBSD and TEM.In the T4 temper, UTS and proof stress increased with temperature up to 190°C, due to dynamic precipitation. Increasing temperature only provided a modest increase in strain to failure for both the T4 and T6 temper. Cup height was not significantly improved in the warm forming temperature range during Erichsen cup testing. By deep drawing at 250°C it was possible to fully draw a cup (with an LDR of 2.2) in both the T4 and T6 temper of 7021, with both tempers having comparable post-forming hardness. This indicates that at 250°C the starting condition has no impact on drawability. Although full drawability is achieved at 250°C the final product would require further heat treatment if it were to replace 6016. However, by deep drawing 7021-T4 at 190°C, a fully formed cup was produced with a hardness between that of the T4 and T6 temper. The microstructure of the formed cup showed no grain boundary precipitation and a fine distribution of the strengthening phase η', suggesting there is a dynamic effect on the precipitation during deep drawing at this temperature. In conclusion, the work has shown that warm forming does not significantly improve stretching behaviour of 7021, but by using warm forming temperatures deep drawing is improved.

A new family of commercial zinc alloys designated as ZA8, ZA12, and ZA27 and high damping capacity alloys including Cosmal and Supercosmal and aluminium alloy LM25 were investigated for compressive creep and load relaxation behaviour under a series of temperatures and stresses. A compressive creep machine was designed to test the sand cast hollow cylindrical test specimens of these alloys. For each compressive creep experiment the variation of creep strain was presented in the form of graphs plotted as percentage of creep strain () versus time in seconds (s). In all cases, the curves showed the same general form of the creep curve, i.e. a primary creep stage, followed by a linear steady-state region (secondary creep). In general, it was observed that alloy ZA8 had the least primary creep among the commercial zinc-based alloys and ZA27 the greatest. The extent of primary creep increased with aluminium content to that of ZA27 then declined to Supercosmal. The overall creep strength of ZA27 was generally less than ZA8 and ZA12 but it showed better creep strength than ZA8 and ZA12 at high temperature and high stress. In high damping capacity alloys, Supercosmal had less primary creep and longer secondary creep regions and also had the lowest minimum creep rate among all the tested alloys. LM25 exhibited almost no creep at maximum temperature and stress used in this research work. Total creep elongation was shown to be well correlated using an empirical equation. Stress exponent and activation energies were calculated and found to be consistent with the creep mechanism of dislocation climb. The primary and phases in the as-cast structures decomposed to lamellar phases on cooling, with some particulates at dendrite edges and grain boundaries. Further breakdown into particulate bodies occurred during creep testing, and zinc bands developed at the highest test temperature of 160°C. The results of load relaxation testing showed that initially load loss proceeded rapidly and then deminished gradually with time. Load loss increased with temperature and almost all the curves approximated to a logarithmic decay of preload with time.

Les séquences de précipitation d'alliages d'aluminium à durcissement structural appartenant aux séries 2XXX (Al-Cu), 6 XXX (Al-Mg-Si-(Cu)) et 7 XXX (Al-Zn-Mg-(Cu)) ont été étudiées par microscopie ionique et sonde atomique tomographique. Cet instrument permet de caractériser les matériaux métalliques avec une résolution subnanométrique et d'établir des cartographies tridimensionnelles des différents éléments dans le volume analysé. Nous avons déterminé les compositions des zones de Guinier-Preston et des phases stables et métastables formées lors des traitements de vieillissement à la température ambiante et lors de revenus de type industriel. Nous avons montré que les zones gp, en forme de disques monoplans riches en cuivre, de l'alliage Al-1,7 à T. % Cu admettent une certaine variabilité de compositions, entre 50 et 100 à T. % Cu. Les zones GP formées dans les alliages 6 XXX et 7 XXX étudies se présentent sous formes d'amas diffus de solutés (Mg et Si pour la série 6XXX Zn et Mg pour les 7XXX). Ces zones GP contiennent une proportion importante d'aluminium (de 75 à 80 at. % en moyenne). Les compositions des phases métastables (alliages 6xxx) et (alliages 7XXX) s'écartent notablement des compositions stœchiométriques prêtées aux phases stables Mg2Si et MgZn2 des diagrammes d'équilibre. La phase des alliages Al-Zn-Mg-(Cu) ne correspond pas non plus à la stœchiométrie attendue, tant par l'existence d'une teneur non négligeable en aluminium que par son rapport Zn/Mg, inférieur à 2. Nous pensons que la composition de la phase, telle qu'elle est caractérisée par les techniques de diffraction (RX et MET), est susceptible d'évolution au cours du revenu (température, nombre de paliers,), en particulier en fonction de son mode de formation. Ce peut être par transformation directe de la phase transitoire, et alors montre une grande similitude avec la composition de, ou par germination (sur ou autres enrichissements locaux de la solution solide) et alors la composition de apparaît plus proche de la stœchiométrie attendue. Dans l'alliage quaternaire, le cuivre participe à la composition des phases et et apparait, en comparaison avec l'alliage ternaire jumeau, favoriser la persistance des premiers stades de l'évolution de la cinétique. L'ajout de cuivre à l'alliage 6xxx ternaire que nous avons étudié augmente de façon spectaculaire la densité et la fraction volumique de la phase durcissante.

The objective of this research is to increase the understanding of the solidification behaviour of some industrially important wrought aluminium alloys. The investigation methods range from direct investigations of as-cast ingots to laboratory-scale techniques in which ingot casting is simulated. The methods span from directional solidification at different cooling rates to more fundamental and controlled techniques such as DTA and DSC. The microstructure characteristics of the castings have been investigated by optical and Scanning Electron microscopy. Hardness tests were used to evaluate the mechanical properties. The effects of adding alloying elements to 3XXX and 6XXX aluminium alloys have been studied with special focus on the effects of Zn, Cu, Si and Ti. These elements influence the strength and corrosion properties, which are important for the performance of final components of these alloys. Solidification studies of 0-5wt% Zn additions to 3003 alloys showed that the most important effect on the microstructure was noticed at 2.5 wt% Zn, where the structure was fine, and the hardness had a maximum. Si addition to a level of about 2% gave a finer structure, having a relatively large fraction of eutectic structure, however, it also gave a long solidification interval. The addition of small amounts of Cu, 0.35 and 1.0 wt%, showed a beneficial effect on the hardness. Differences have been observed in the ingot surface microstructures of 6xxx billets with different Mg and Si ratios. Excess Si compositions showed a coarser grain structure and more precipitations with possible negative implications for surface defect formation during DC casting. The comparison of alloys of different Ti content showed that the addition of titanium to a level of about 0.15 wt% gave a coarser grain structure than alloys with a normal Ti content for grain refinement, i.e. < 0.02 wt%, although a better corrosion resistance can be obtained at higher Ti contents. The larger grain size results in crack sensitivity during DC casting. A macroscopic etching technique was developed, based on a NaOH solution, and used in inclusion assessment along DC cast billets. Good quantitative data with respect to the size and spatial distribution of inclusions were obtained. The results from studied billets reveal a decreasing number of inclusions going from bottom to top, and the presence of a ring-shaped distribution of a large number of small defects in the beginning of the casting. The present study shows how composition modifications, i.e. additions of certain amounts of alloying elements to the 3xxx and 6xxx Al alloys, significantly change the microstructures of the materials, its castability, and consequently its mechanical properties
QC 20100901

碩士
國立臺灣大學
材料科學與工程學研究所
101
This research mainly discuss that, under different condition of heat treatment, using OM, SEM, TEM XRD, DSC,UTM and hardness tester… to observe and test the mechanical properties of 7075, 6066, 6061 aluminum alloy extruded bars’ microstructure and precipitations. The test result shows that after 120°C-72 hours of artificial aging, can get the best strength (over 680MPa). If after quenching, placed at room temperature for several days, and artificial aging, the precipitations are relatively small, the strength is decreased. Compare to T7 aging treatment, duplex aging treatment and RRA processing both make grain boundary precipitates bulky, too, but PFZ from it grow narrow which makes grain boundary strength increase. Within grain, still maintain and have more diffused organization and precipitation reinforcement. Containing higher Cu, 6066 can produce Q (Al-Mg-Si-Cu) phase after casting. Melting point of Q-phase is about 535°C. Because of such high solid solution temperature, Q-phase will melt with many micro-voids, thereby weakening its intensity. If given appropriate homogenizing treatment, the Q-phase can be eliminated, and the strength will increase to more than 500MPa, after rising its solid solution temperature to 560°C. Compare 6066 and 6061 after solid solution at 530°C and artificial aging. We can find that 6066 has more alloying elements, and dispersed particle. This can provide better solid solution and dispersion strengthening effect. According to DSC analysis, 6066 has higher Si, so under 90°C to 350°C, it will precipitate Si-cluster and silicon phase respectively. s’’ and s’ exothermic peak will lower than 6061. Therefore, 6066 has better precipitation effect. 6061 needs 24 hours to reach its peak aging, while 6066 needs only 8 hours. Using polarization measurement to test material stress corrosion resistance, we can find that 7075 will have current reversal phenomenon and intergranular corrosion phenomenon under T6 treatment. The current reversal phenomenon can be eliminated by T7 or RRA. However, After T6 treatment, 6066 and 6061 will not produce this phenomenon, too. Therefore, their ability to resist stress corrosion will be better than 7075. To explore the material properties of extruded bar after cold forging and heat treatment. In the experiment, bars are given different strains by rotary swaging, and then carry on the solution treatment, aging treatment. We can find that recrystallization occur, the original texture is destroyed and intensity is greatly reduced. When the strain getting higher, the recrystallized grains will become smaller , and the strength will increase. If changed to T8 treatment (solution treatment- quenching- cold forging- aging treatment ), the strength can be effectively improved, but ductile fracture will change to brittle-ductile coexistence mode, and the elongation will greatly decrease.

Tese de doutoramento em cotutela, na área de Engenharia Mecânica, no ramo de Tecnologias de Produção, apresentada ao Departamento de Engenharia Mecânica da Faculdade de Ciências e Tecnologia da Universidade de Coimbra e à Université de Bretagne-Sud, França
The transport industry faces sustainability challenges that demand vehicles’ weight reduction, while safety requirements impose the increase of their structural strength. One of the approaches adopted to address this goal is the continuous development of lightweight materials and their manufacturing technologies. Within lightweight materials, the aluminum alloys are characterized by presenting medium strength and good formability. However, when compared with the metallic alloys traditionally used in sheet metal forming operations, they present lower formability and higher springback. In this context, warm forming was proposed as a solution to solve these problems, with good acceptance, first, in the research community and, nowadays, in industry. However, the warm forming of heat treatable aluminum alloys is still a challenge, since the recommended range of temperature is similar to the one used for heat treatments. Thus, warm conditions can result in heat treatment changes during the forming operation, which need to be predicted during the process virtual try-out to avoid defects in the production line. Moreover, heat treatable alloys are prone to natural aging, which can lead to variability and defects in the production line. Thus, the main goal of this work is to analyze the warm forming conditions of heat treatment aluminum alloys, taking into account the natural aging, in order to propose solutions that can contribute to the increase of robustness of sheet metal forming operations. Two heat treatable Al-Mg-Si (6xxx series) alloys were selected for the study: the EN AW 6016-T4 (natural aging) and the EN AW 6061-T6 (artificial aging). Their thermo-mechanical behavior was characterized using uniaxial tensile tests between room temperature (RT) and 300 ºC at 0, 45 and 90 º to the rolling direction, in a strain rate range from 2x10-4 to 2x10-2s-1, with heating time of 20 seconds. This tests were performed in a Gleeble 3500 machine under monotonic load and with stress-relaxation stages. Additional uniaxial monotonic tensile tests were performed in an Instron 4505 machine coupled with a classical furnace, at RT and 200 ºC, enabling the analysis of heat-holding times of 10 and 30 minutes. The strain field was measured using optical 3D deformation analysis (gom-aramis 4M system). The cylindrical cup tests were performed in a Zwick/Roell Amsler BUP200 sheet metal testing equipment, adapted with specific tools for warm forming. The adoption of this geometry enabled the study springback using the split ring test. All warm forming tests were performed considering non-isothermal conditions, with the die and the blank-holder heated up to the desired temperature, while the punch is refrigerated to keep its temperature close to RT. The cylindrical cup tests were performed between RT and 250 ºC, for punch (ram) speeds between 0.1 and 10 mm/s and heat-holding times of 1, 10 and 30 minutes. During each test, the punch force and its displacement, the blank-holder force and the temperature were acquired, as a function of time. After the forming operation the cup thickness and the cup height were measured, as well as the springback. Finally, the influence of natural aging was evaluated for the period from 1 to 18 months, which required the duplication of some tests. Globally, it is possible to correlate the thermo-mechanical results with the ones obtained in the forming tests, validating the macroscopic approach adopted for the results analysis. The EN AW 6061-T6 mechanical behavior shows a small natural aging effect, while it has a strong effect for the EN AW 6016-T4 alloy, leading to an increase of the yield stress and tensile strength and, consequently, of the springback. In this context, warm forming tests considering a heat-holding time of 1 minute, at 200 and 250 ºC, prove to be an effective solution to reduce the variability caused by natural aging in: forming forces, formability and springback. The orthotropic behavior is not affected by the temperature increase or natural aging. Both alloys present a negligible strain rate sensitivity at RT, while it is positive for warm temperatures. The warm forming tests performed at 200 ºC (heat-holding time of 1 minute), show that high punch speeds are advantageous. Formability and springback remained stable or improved with the punch speed increased. In fact, low punch speeds lead to high exposure time and, consequently, to tools thermal equilibrium that counteract the effects of the non-isothermal conditions, and promotes dynamic precipitation hardening on the EN AW 6016-T4 alloy that leads to strength and springback increase. The tensile tests with stress-relaxation stages were extremely useful on understanding the dynamic precipitation phenomena at warm temperature. Moreover, whatever the punch speed adopted the variability caused by natural aging was negligible at warm forming, which validate the effectiveness of this solution. Concerning the analysis of heat-holding time at 200 ºC, globally the results show that for the EN AW 6061-T6, the increase of the heat-holding time up to 50 minutes has a negligible impact on the material behavior and, consequently, the formability and springback warm forming results are independent of this process variable. For the EN AW 6016-T4 the increase of the heat-holding time up to 30 minutes leads to an increase the yield stress and tensile strength, while the total elongation reduces, as a result of a heat treatment change from natural aged to artificial aging condition. Consequently, the warm forming test performed with similar conditions shows a reduced formability and a springback increase. Moreover, a heat-holding time of 30 minutes leads to the known negative-effect of natural aging in artificial aging, for a storage time of 18 months. Thus, the heating system selected for the warm forming must guarantee a high heating rate, which is also advantageous from the production point of view. Throughout this work the split-ring test was used to evaluate springback. In this context, a numerical study was performed, in order to improve knowledge concerning the impact of the ironing stage in the circumferential residual stresses in the ring. Globally, the results show that the introduction of an ironing stage changes the characteristic distribution of the residual stress component throughout the vertical wall, even for relatively small ironing strains. This change affects the trend observed for the opening value of the rings located at different heights. This study was important to the results analysis since it explained the lower sensitivity of the ring located in the top of the cup (ironed zone) to the changes in material and process conditions. The analysis of the other rings clearly shows that warm forming contributes to springback reduction, when performed under non-isothermal conditions, with high punch speeds and heating rates.
A indústria dos transportes enfrenta desafios de sustentabilidade que exigem a redução de peso dos veículos, ao mesmo tempo que os requisitos de segurança impõem o aumento da resistência estrutural. Uma das abordagens adotadas para atingir estes objetivos é o contínuo desenvolvimento de materiais de elevada resistência e das suas tecnologias de fabrico. As ligas de alumínio pertencem a este conjunto de materiais, onde se caracterizam por apresentarem uma resistência média e boa formabilidade. No entanto, quando comparadas com as ligas metálicas tradicionalmente utilizadas nas operações de conformação de chapas metálicas, apresentam menor formabilidade e maior retorno. A conformação a temperaturas moderadas foi proposta como alternativa para ultrapassar estes problemas, primeiro na comunidade científica e, atualmente, na indústria. No entanto, a utilização deste processo para ligas de alumínio tratáveis termicamente é ainda um desafio, uma vez que a gama de temperaturas recomendada é semelhante à utilizada nos seus tratamentos térmicos. Assim, a conformação nesta gama de temperaturas pode conduzir a alterações do tratamento térmico, que devem ser previstas na conceção virtual para evitar a ocorrência de defeitos na linha de produção. Além disso, as ligas tratáveis termicamente são propensas a envelhecimento natural, que pode introduzir variabilidade e defeitos na linha de produção. O principal objetivo deste trabalho é analisar as condições de conformação a temperaturas moderadas deste tipo de ligas de alumínio, incluindo o efeito de envelhecimento natural, a fim de propor soluções que possam contribuir para aumentar a robustez das operações de estampagem de chapas metálicas. Foram selecionadas duas ligas Al-Mg-Si (série 6xxx) tratáveis termicamente: a EN AW 6016-T4 (envelhecimento natural) e a EN AW 6061-T6 (envelhecimento artificial). O seu comportamento termo-mecânico foi caracterizado com recurso a ensaios de tração uniaxial, realizados entre a temperatura ambiente (TA) e 300 ºC, com provetes orientados a 0, 45 e 90 º, em relação à direção de laminagem, para uma gama de velocidades de deformação de 2x10-4 a 2x10-2s-1 e um tempo de aquecimento de 20 segundos. Estes ensaios fora realizados num equipamento Gleebe, em condições de carga monótona crescente e incluindo estágios de relaxação de tensão. Foram também realizados ensaios monótonos de tração uniaxial numa máquina Instron, equipada com um forno, a TA e a 200 ºC, o que permitiu estudar tempos de permanência a 200 ºC de 10 e 30 minutos. O campo de deformação foi medido com um sistema ótico de análise (gom-aramis 4M system). Foram ainda realizados ensaios de estampagem de taças cilíndricas, num equipamento Zwick/Roell Amsler BUP200, com recurso a ferramentas adaptadas para conformação a temperaturas moderadas. A adoção desta geometria permitiu o estudo do retorno elástico, com base no ensaio de corte de anel. Todos os ensaios foram realizados em condições não-isotérmicas, com a matriz e o cerra-chapas aquecidos, até à temperatura desejada, e o punção refrigerado para manter uma temperatura próxima da TA. Estes ensaios de estampagem foram realizados entre TA e 250 ºC, para velocidades de deslocamento do punção compreendidas entre 0.1 e 10 mm/s e tempos de manutenção à temperatura de aquecimento de 1, 10 e 30 minutos. Em cada ensaio, foram adquiridos os resultados correspondentes à evolução com o tempo, da força e do deslocamento do punção, da força do cerra-chapas e da temperatura. Após a operação de conformação, foi medida a evolução da espessura ao longo da parede da taça, bem como o perfil das orelhas e o retorno elástico. Finalmente, foi avaliada a influência do envelhecimento natural para o período de 1 a 18 meses, o que exigiu a duplicação de alguns ensaios. Globalmente, a abordagem macroscópica adotada para a análise de resultados foi validada pela correlação entre os resultados dos ensaios de caracterização termo-mecânica e os de conformação. O comportamento mecânico da liga 6061-T6 é pouco sensível ao envelhecimento natural, enquanto a liga 6061-T6 apresenta um aumento da tensão limite de elasticidade e máxima não desprezável, que resulta no aumento do retorno elástico. Neste contexto, a conformação a quente a 200 e 250 ºC, com um tempo de manutenção de 1 minuto, revela-se uma solução eficaz para reduzir a variabilidade induzida pelo envelhecimento natural na força de estampagem, na formabilidade e no retorno elástico. O comportamento ortotrópico não é afetado pelo aumento da temperatura nem pelo envelhecimento natural. As ligas apresentam uma sensibilidade à velocidade de deformação negligenciável à TA, que passa a ser positiva para 200 ºC. Os testes de conformação realizados a esta temperatura (tempo de manutenção de 1 minuto) mostram que a utilização de uma velocidade do punção elevada pode ser vantajosa, uma vez que a formabilidade e o retorno elástico permanecem estáveis ou melhoram. De facto, velocidades do punção reduzidas conduzem a tempos de exposição elevados que conduz ao equilíbrio térmico das ferramentas, o que neutraliza os efeitos pretendidos com as condições não-isotérmicas e promove o endurecimento por precipitação dinâmica da liga EN AW 6016-T4, i.e. a força e o retorno elástico aumentam. Os ensaios de tração com estágios de relaxação de tensão permitem uma melhor compreensão dos fenómenos de precipitação dinâmica em função da temperatura. Além disso, qualquer que seja a velocidade seccionada para o punção, a variabilidade causada pelo envelhecimento natural é negligenciável para 200 ºC, o que valida a eficácia desta solução. Em relação à influência do tempo de manutenção a 200 ºC, para a liga EN AW 6061-T6, o aumento deste tempo até 50 minutos tem um impacto negligenciável no comportamento mecânico do material e, consequentemente, a formabilidade e retorno elástico são independentes desta variável de processo. Para a liga EN AW 6016-T4, o aumento do tempo de manutenção até 30 minutos resulta num aumento da tensão limite de elasticidade e máxima, com a redução do alongamento total, como resultado da alteração do tratamento térmico de envelhecimento natural para artificial. Logo, o ensaio de conformação realizado em condições semelhantes apresenta uma formabilidade reduzida e um aumento de retorno elástico. Além disso, um tempo de manutenção de 30 minutos resulta no efeito negativo do envelhecimento natural no envelhecimento artificial, para a liga EN AW 6016-T4 com um tempo de armazenamento de 18 meses. Assim, o sistema de aquecimento selecionado para a conformação a temperaturas moderadas deve garantir uma velocidade de aquecimento elevada, o que também é vantajoso do ponto de vista industrial. Ao longo deste trabalho, o ensaio de corte de anel foi utilizado para avaliar o retorno elástico. Neste contexto, foi realizado um estudo numérico, para melhorar o conhecimento sobre o impacto da etapa de estiramento na distribuição das tensões residuais circunferenciais no anel. Os resultados mostram que a introdução da etapa de estiramento altera a distribuição característica das tensões residuais ao longo da parede vertical, mesmo para deformações reduzidas. Esta alteração afeta a tendência observada para o valor de abertura dos anéis cortados a diferentes alturas. Este estudo foi importante para a análise dos resultados, uma vez que justifica a menor sensibilidade do anel localizado no topo do copo (zona estirada) às mudanças nas condições de comportamento do material e de processo. A análise dos outros anéis mostra claramente que a conformação a temperaturas moderadas, em condições não isotérmicas, contribui para a redução do retorno elástico, se realizada com velocidades do punção e de aquecimento elevadas.
French Ministry of Higher Education

Dissertação de Mestrado Integrado em Engenharia Mecânica apresentada à Faculdade de Ciências e Tecnologia da Universidade de Coimbra
Nos dias de hoje os processos de estampagem são uma componente de grande importância para a indústria. As ligas de alumínio são dos materiais mais utilizados na indústria automóvel. No entanto, as da série 5xxx são apenas usadas para os painéis interiores, uma vez que apresentam defeitos de superfície, enquanto as da série 6xxx são utilizadas nos painéis exteriores das viaturas. O principal objectivo deste estudo é caracterizar mecanicamente ligas da série 6xxx e avaliar as potencialidades de recorrer à sua estampagem a quente, de modo a tentar dar resposta a um conceito de utilização de apenas uma liga de alumínio na indústria (6xxx), com consequentes impactos positivos do ponto de vista económico e de reciclagem. O estudo descrito neste trabalho resulta de uma parceria entre o CEMUC e o LIMATB, com o objectivo de estudar a influência da temperatura no processo de estampagem de taças cilíndricas da série 6016-T4 e 6061-T6, tendo sido estudadas duas temperaturas: ambiente e 200ºC. O estudo envolveu a realização de ensaios experimentais de estampagem de taças cilíndrica e a simulação do processo, admitindo condições isotérmicas. Os parâmetros analisados foram a evolução da força do punção com o seu deslocamento, a evolução da espessura ao longo da taça e o perfil das orelhas de estampagem. O retorno elástico foi estudado experimentalmente, com o auxílio do teste Demeri. Todas as simulações numéricas foram realizadas com o programa DD3IMP,e a caracterização dos materiais foi realizada com o programa DD3Mat. A análise experimental foi realizada no LIMATB. Os resultados permitem concluir que a realização da estampagem a 200ºC conduz a uma redução da força de estampagem e do retorno elástico, maioritariamente para o 6061-T6.
Nowadays the deep drawing processes are a component of huge importance to industry. The aluminium alloys are one of the most used materials in the automotive industry. However, the 5xxx alloys are only used in inner panels, since they present superficial defects, while the 6xxx alloys are used in the outer panels of cars. The main goal of this study is to characterize the mechanical behaviour of 6xxx alloys and evaluate the potential of using warm deep drawing, in order to respond to a single material use concept of aluminium alloys in industry (6xxx), with positive consequences regarding the economic and recyclability point of view. The study presented in this work results from a partnership between the CEMUC and the LIMATB, with the purpose of studying the influence of temperature in the deep drawing process of cylindrical cups for the 6016-T4 and 6061-T6 alloys, based on two different temperatures: room temperature and 200ºC. This work involved performing deep drawing of cylindrical cups experimental tests and the numerical simulation of the process, assuming isothermal conditions. The analysed parameters were the evolution of the punch force with the punch displacement, the evolution of thickness along the cup’s wall and the ears profile. The springback was study experimentally, with the help of the Demeri test. All the numerical simulations were performed using the in-house code DD3IMP, and the material characterization was performed using the DD3Mat. The experimental analysis was accomplished in LIMATB. The results allow us to conclude that performing the deep drawing at 200ºC leads to a punch force and springback reduction, mainly for the 6061-T6.