Published Article / The heat treatment cycles that are currently applied to semi-solid processed components are mostly those that are in use for traditional dendritic alloys. These heat treatments are not necessarily the optimum heat treatments for SSM processing. The T6 heat treatment of aluminium alloys is a relatively expensive process and a reduction in treatment times would be advantageous. In order to improve the T6 heat treatment cycle for rheocast alloy A356, the effect of various parameters on the quality index were investigated. These included solution treatment time, natural aging time, artificial aging temperature and artificial aging time.

The influence of recycling on aluminium alloys and subsequent influence on the microstructure and corrosion performances have been investigated. The investigation was commenced by taking two block cast, recycled aluminium alloys (AA5050 and AA6011) and rolling them into 1mm gauge plate. In the case of AA6011, the plate was subjected to subsequent solution heat treatment and artificial aging steps, in order to attain certain temper specifications. To replicate the automotive paint bake industrial practice, a sample was subjected to a 2% tensile stretch followed by heat treatment for 30 minutes at 180˚C. Microstructural observations revealed Al-Fe-Mn-Si intermetallics to be the dominant secondary phase in both alloys. The size, distribution and composition of these were unaffected by artificial aging. Mg2Si was found in a coarse, localised form in both alloys also, albeit in much less amounts in AA5050. The presence of this phase was likely due to poor homogenisation during thermomechanical processing. HR-TEM of AA6011 revealed needle/rod shaped precipitates, aligning in the [001]Al lattice direction. This is consistent with β''/β' hardening precipitates consisting of magnesium and silicon. Circumstantial evidence was found for the copper-containing Q phase precipitate also. An additional, unidentified precipitate was observed, nucleating on the {111} habit plane of the aluminium matrix. The high iron content of AA6011 retarded the precipitation hardening response by capturing elements associated with hardening precipitates in the Al-Fe-Mn-Si intermetallics. Electrochemical corrosion experiments revealed the materials had a high susceptibility to localised corrosions, with the open circuit potential and breakdown potential possessing similar values. Atmospheric corrosion experiments showed that artificial aging had a large influence on the preferred corrosion mechanism. Non-heat treated samples showed susceptibility for pitting corrosion. This was particularly true for the -T4P temper, which showed large scale pitting. Heat treated samples saw an introduced susceptibility to intergranular corrosion. This was attributed to precipitation at grain boundaries, which would then form a microgalvanic couple with adjacent depleted zones. In the case of the -T8P temper, tensile stretching introduced defects into the sub-grain microstructure. This resulted in intergranular corrosion fronts of increased width, where grains with higher stored energy undergo preferential dissolution alongside the grain boundary attack. Overall, the detrimental effects of high iron content need to be overcome before AA5050 and AA6011 can be seriously considered for use in the automotive industry. However, the corrosion performance of AA6011-T8P is encouraging.

Made available in DSpace on 2014-06-11T19:32:11Z (GMT). No. of bitstreams: 0 Previous issue date: 2010-09-23Bitstream added on 2014-06-13T20:42:52Z : No. of bitstreams: 1 teixeira_dg_dr_araiq.pdf: 2545330 bytes, checksum: a6a1d20f601e5055143107f1be1c8ecb (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Este trabalho está basicamente dividido em duas partes. A primeira está relacionada à limpeza da superfície da liga de alumínio AA6063T5 e sua influência na proteção contra a corrosão. A segunda parte se refere à preparação de revestimentos híbridos orgânicoinorgânico pelo processo sol-gel e deposição sobre a superfície da liga de alumínio por dipcoating. Três amostras de liga AA6063T5 foram investigadas: como recebida, debastada e polida mecanicamente e tratada quimicamente. O tratamento mecânico foi realizado utilizando lixas SiC de granulometria 320 a 4000 e o químico foi realizado utilizando uma solução aquosa contendo uma mistura de hidróxido, sulfato e cloreto de sódio. Todas as amostras foram caracterizadas fisicamente por difratometria de raios X (XRD), medidas de dureza, microscopia eletrônica de varredura (SEM) e microscopia óptica (MO), e por medidas de potencial de circuito aberto (OCP) e espectroscopia de impedância eletroquímica (EIS) em solução 3,5% NaCl. As imagens SEM mostraram que a presença de partícula intermetálicas na superfície da liga diminui após o tratamento químico comparado ao tratamento mecânico. Os resultados de EIS mostraram que o tratamento químico oferece maior resistência à corrosão do que as submetidas aos outros tratamentos, porque o ataque alcalino utilizado dissolveu a maioria dos precipitados contendo Fe ou Cu, elementos responsáveis por sítios catódicos. A lavagem com HNO3 pela qual passou a amostra tratada quimicamente provavelmente reforçou a camada de óxido, tornando a superfície da liga mais resistente ao ataque por cloreto. Os revestimentos híbridos foram somente depositados na liga que recebeu tratamento químico. Esses filmes híbridos foram preparados com tetraetoxi-silano (TEOS) e 3-metacriloxipropril trimetoxi silano (MPTS), pela hidrólise e policondensação em meio ácido seguido por... / This work is basically divided in two parts: the first one is related to the surface treatment of AA6063T5 aluminum alloy and its influence on the protection against corrosion. The second concerns to the preparation of organic-inorganic hybrid coatings by the sol-gel process that were deposited on the aluminum alloys by dip-coating. Three samples of AA6063T5 aluminum alloys were investigated, the as-received one, after mechanical and chemical treatment, respectively. The mechanical treatment was realized using SiC paper grades 320 to 4000 and the chemical one was done by an aqueous solution containing a mixture of sodium hydroxide, chloride and sulfate. All samples were physically characterized by X-ray diffractometry (XRD), hardness measurements, scanning electron microscopy (SEM) and optical microscopy (OM), and electrochemically by open circuit measurements (OCP) and electrochemical impedance spectroscopy (EIS) in 3.5% NaCl solution. The SEM images depicted that the presence of intermetallic particles on the alloy surface decreased after the chemical treatment compared to the mechanical one. The EIS results showed that the sample chemically treated possesses a higher corrosion resistance than those obtained using other procedures. EIS results showed that the chemically treated sample presented higher corrosion resistance than the ones submitted to other procedures. The alkaline treatment dissolves the great part of precipitates containing iron or copper, which are responsible by the cathodic sites formation. The washing of the sample with nitric acid solution after the alkaline treatment removes impurities from the surface and can give to the oxide layer higher resistance against the action of the chloride ion. The hybrid coatings were only deposited on the alloy that received the chemical treatment. Hybrid films containing tetraethoxysilane (TEOS) and 3-methacryloxypropyltrimetho... (Complete abstract click electronic access below)

The effect of microstructure on corrosion resistance and anodising behaviour of a 3rd generation lithium-containing aluminium alloy, AA 2099-T8, has been studied. Microstructural characterisation of the alloy has shown elongated grains with high angle grain boundaries and approximately equiaxed subgrains with low angle grain boundaries. Two types of constituent particles of high and reduced contents of copper were found, which are present individually or in the form of multiphase particles. The alloy also contains rod-like Al-Cu-Mn-Li and spherical (Al3Zr) dispersoids, and fine age hardening precipitates of T1 (Al2CuLi), (Al3Li) and (Al2Cu).Immersion testing in a chloride-containing solution revealed two types of localised corrosion sites: one was initiated from intermetallics and quickly became passivated; the other lasted for the course of testing, with extensive boundary attack. The latter has been correlated to grains and subgrains with relatively high stored energy.Alkaline etching removed intermetallics from alloy surface; generated a copper-enriched layer in the alloy matrix immediately beneath the residual alumina film; and gradually developed a surface nanotexture. The surface nanotexture has been correlated to the formation of copper-rich nanoparticles within the copper-enriched layer.Anodising the alloy in tartaric-sulphuric acid disclosed that copper in the alloy matrix could be occluded in the anodic film material as copper-rich nanoparticles or be oxidized and incorporated into the film material as copper ions, depending on the anodising voltage. In the latter case, the process was accompanied by oxygen gas generation within the film material, forming anodic films with lateral porosity. Intermetallics of different compositions behaved differently during anodising, leading to modification of the morphology and composition of anodic films. Concerning the effect of alkaline etching on anodising behaviour of the alloy, the removal of intermetallics increased anodising efficiency; the pre-developed copper-enriched layer advanced the establishment of the steady-state film/alloy interface. Variation of copper-enriched layer structure is revealed by the detection of copper-rich nanoparticles in the copper-enriched layer. The copper-rich nanoparticles have structures consistent with either θ", θ' or θ phases. Machining damage can be detected not only by chromic acid anodising (CAA) but also by tartaric-sulphuric acid anodising (TSAA). Neither CAA nor TSAA can be used for detecting shallow corrosion pits.

A heterogeneous microstructure is intentionally developed in AA2024-T3 aluminium alloy during solidification and thermomechanical processes to obtain good mechanical properties. As a consequence, the alloy is susceptible to localized corrosion, which is the major nucleus for onset of stress corrosion cracking and fatigue cracking.In this research, electron microscopy was employed to observe intermetallic particles and their periphery and monitor the initiation and development of intermetallic particle induced localized corrosion in AA2024-T3 aluminium alloy. In-situ optical microscopy, energy dispersive X-ray spectroscopy (EDX), X-ray microtomography and atomic force microscopy were also used to provide supportive evidence.Intermetallic particles with different electrochemical natures and geometries were found. The main coarse intermetallic particles are identified as S (Al2CuMg), θ (Al2Cu) and α (Al-Cu-Fe-Mn-(Si)) phases. θ (Al2Cu) and α (Al-Cu-Fe-Mn-(Si)) phases showed a relatively inert nature compared with S-phase particles. It was observed that continuous localized corrosion is associated with large clusters of S-phase and θ-phase intermetallic particles that are buried beneath surface but connected to the alloy surface. Propagating away from the intermetallic particles, corrosion developed preferentially along selected grain boundaries.Electron backscatter diffraction (EBSD) was employed to further investigate the relationship between the grain structure and the intergranular corrosion susceptibility. It was revealed that intergranular attack occurred at the grain boundaries that surround grains of relatively high stored energy. Corrosion was not confined within the region immediately adjacent to the grain boundaries, but had developed into the grains of relatively high stored energy, suggesting that grains with relatively high levels of defects are more susceptible to corrosion.

Innovative materials with novel properties are in great demand for use in the criticalcomponents of emerging technologies, which promise to be more cost-effective and energyefficient.A controversial issue with regard to manufacturing complex industrial products isto develop advanced materials with optimised manufacturability in addition to the requiredmechanical and physical properties. The objective of this research study was to develop andoffer new solutions in material-processing-related issues in the field of mechanical andelectrical engineering. This was achieved by investigating the new opportunities affordedby a recently developed rheocasting method, RheoMetalTM process, with the goal of comingto an understanding of the critical factors for effective manufacturing process. A study of the evolution of microstructure at different stages of the rheocasting process,demonstrated the influence of multistage solidification on the microstructural characteristicsof the rheocast components. The microstructural investigation onquench slurry showed itconsists of the solute-lean coarse globular α-Al particles with uniform distribution ofalloying elements, suspended in the solute-rich liquid matrix. Such inhomogeneous slurryin the sleeve seems to play a critical role in the inhomogeneity of final microstructure. Inthe rheocast component, the separation of the liquid and solid parts of slurry during fillinginfluenced on the microstructural inhomogeneity. The relationship between the microstructural characteristics and properties of the rheocastcomponents was investigated. The study on the fracture surfaces of the tensile-testedspecimens showed that the mechanical properties strongly affected by microstructuralinhomogeneity, in particular macrosegregation in the form of near surface liquid segregationbands and subsurface porosity. The thermal conductivity measurement showed variation ofthis property throughout the rheocast component due to variations in the ratio of solute-leanglobular α-Al particles and fine solute α-Al particles. The result showed silicon in solidsolution have a strong influence (negative) on thermal conductivity and precipitation ofsilicon by heat treatment process increase the thermal conductivity. / RheoCom

Research focused on the selective laser melting (SLM) of stainless steels and aluminium alloys. For steels, the possibility of creating a magnetically graded material was demonstrated as well as the ability to improve consolidation with austenitic and martensitic stainless steel powder mixtures. Stainless Steel/CoCr hybrid samples were also manufactured and tested to investigate the advantages of functionally graded materials (FGMs). Al alloy research began with examining the requirements for successful Al alloy consolidation in SLM and through experimentation it was found that Al alloys with good welding properties were the best choice: pure Al was found to be completely unsuitable. 6061 Al alloy was then used as a base material to manufacture Al-Cu alloy samples. Single layer SLM samples were produced first, which resulted in recognised Al-Cu microstructures forming. Multilayer Al alloy SLM research resulted in the discovery of the theorised ability to manufacture Al-Cu alloy parts with a nanocrystalline Al matrix with dispersed Al2Cu quasicrystals, resulting in a material comparable to a metal matrix composite that showed excellent corrosion resistance and compressive strength. Finally, a demonstration part was made to test the capability of the SLM process producing an aerospace type geometry using a customised Al alloy. Observations during manufacture and post process analysis showed that Al alloys were susceptible to changes in mechanical properties due to the geometry of the manufactured part.

Através de microscopia ótica utilizando luz polarizada e de metalografia quantitativa, determinou-se a cinética de recristalização a 400 ºC de uma liga de alumínio da série 3000 contendo dispersão de precipitados, para diferentes graus de deformação produzidos por laminação. Mediram-se parâmetros, para vários tempos de recozimento, que permitiram determinar: fração volumétrica recristalizada, quantidade de interfaces por unidade de volume entre regiões recristalizadas e não recristalizadas e, entre regiões recristalizadas, assim como velocidade média de migração de interfaces. Fizeram-se também medidas de microdureza em todas as amostras. Os resultados experimentais mostraram que à medida que se diminui o grau de deformação, a quantidade de interfaces por unidade de volume responsáveis pela recristalização diminui exigindo portanto um crescimento através de distâncias maiores para a recristalização se completar, tornando a cinética de recristalização mais dependente do crescimento. Constatou-se a ocorrência de recuperação, durante a recristalização, das regiões não recristalizadas e consequente diminuição da velocidade média de migração de interfaces com o tempo de recozimento. Com a diminuição da deformação, ocorreram frações não recristalizadas residuais, as quais eram maiores para graus de deformação menores, chegando a atingir mais de 80% para 34,4% de redução em espessura. Os resultados sugerem que, para baixas deformações, a velocidade de crescimento tem um papel importante na cinética de recristalização de ligas com dispersão de precipitados. A comparação entre os resultados de metalografia quantitativa e medidas de microdureza evidenciou algumas limitações desta última técnica para estudo de recristalização. / Using polarized light optical micrsocopy and quantitative metalography, kinetics of recrystallization at 400 ºC of an aluminum alloy of 300 series were determined, for different rolling deformation. Recrystallized volume fraction, amount of interfaces per unit volume, between recrystrallized and unrecrystallized regions and between recrystallized grains, and also mean velocity of interface migration were determined. Microhardness measurements were made in all specimens. The results have shown that with decreading deformation, the amount of interfaces responsible for recrystallization, per unit volume, decreases and so the interfaces must sweep larger distances for complete recrystallization, making kinetics more dependent on growth. Recovery of the unrecrystallized regions was observed, during recrystallization, causing decrease in the average velocity of interface migration with annealing time. With decreasing deformation residual unrecrystallized volumes have been observed, after long time anneals. The fraction of residual unrecrystallized region were large than 80-% for 34,4% thickness reduction. The results suggest that, in alloys with precipitates, the growth velocity has an important role in recrystallization kinetics. Some shortcomings of hardness measurements, as a tool to study recrystallization, were shown by comparison between the quantitative metallography and hardness measurements.

Extrusion is a continuous solid state deformation process which is widely used in the aluminium industry. The demand for aluminium extrudates are growing and extruders are pressurized to extrude products as fast as possible without lowering the quality of the product. Important extrusion parameters and conditions are exit temperature, extrusion speed and alloy composition. It is widely accepted in extrusion industry that extrusion surface defects increase when the extrusion speed and exit temperature are increased for a constant alloy. One of the major surface defects is the so-called die pick-up and it is presently uncertain if increase with extrusion speed (from a low 25m/min) would result in an increase of the number of die pick-up defect.Die pick-up appears like a scratch mark or comet on the surface of the extrudate which damages the appearance. Previous research suggests that second phase particles, eutectic reactions (555°C - 600°C), extrusion process conditions and die conditions may influence the cause of die pick-up. However the influencing factors for die pick-up are not well established.The research started by determining the lowest melting temperature for AA6060 alloy as this temperature limit the highest temperature above which incipient melting starts. This temperature corresponds to the eutectic melting temperature for AA6060 alloy. Eutectic melting was only detected above 610°C and therefore the exit temperature could be increased to a maximum of 610°C. For an AA6xxx alloy system the lowest melting temperature is 555°C if Mg2Si and excess silicon were present. However as Mg2Si may have fully dissolved into the solid solution, no reaction can take place.A preliminary investigation was conducted to study the characteristics of the newly installed extrusion control and monitoring system. Through this study the relationship between the set extrusion speed and the actual extrusion speed was established. It was found that the actual extrusion speed was lower than the set extrusion speed and was further complicated by the capacity limit of the extrusion pressure. Exit temperature measurements were accurate, however it was measured about 1m away from the die exit. Experiments were carried out to estimate the exit temperature drop and hence the exit temperature measurements were corrected accordingly.Thus, the aim of the present research was to establish the relationship between die pick-up and extrusion conditions (extrusion speed, exit temperature and die condition) and to propose the likely formation mechanism for die pick-up.In this research AA6060 alloy was used and was extruded at 25m/min, 30m/min, 35m/min, 40m/min and 45m/min. The exit temperature was found to increases from 542°C to 567°C. Three types of die pick-up was identified which were named as normal pick-up, die line pick-up and lump pick-up. Normal pick-up occurred regardless of the extrusion speed and exit temperature; however the amount of normal pick-up did not increase when the extrusion speed was increased. Die line pick-up occurred when the extrusion speed was 45m/min and appeared only on the die lines. Lump pick-up is not significant since it was very rare.AA6060 (0.4%Mg and 0.5%Si) alloy has about 0.27% excess silicon and therefore at 555°C, Mg2Si particles react with aluminium and excess silicon to form liquid. However normal pick-up and die line pick-up still occurred at temperatures lower and higher than 555°C and therefore it confirms that eutectic reactions do not influence formation of pick-up. Therefore die pick-up is most likely to be caused due to a mechanical process rather than a metallurgical process.

Extrusion is a continuous solid state deformation process which is widely used in the aluminium industry. The demand for aluminium extrudates are growing and extruders are pressurized to extrude products as fast as possible without lowering the quality of the product. Important extrusion parameters and conditions are exit temperature, extrusion speed and alloy composition. It is widely accepted in extrusion industry that extrusion surface defects increase when the extrusion speed and exit temperature are increased for a constant alloy. One of the major surface defects is the so-called die pick-up and it is presently uncertain if increase with extrusion speed (from a low 25m/min) would result in an increase of the number of die pick-up defect.Die pick-up appears like a scratch mark or comet on the surface of the extrudate which damages the appearance. Previous research suggests that second phase particles, eutectic reactions (555°C - 600°C), extrusion process conditions and die conditions may influence the cause of die pick-up. However the influencing factors for die pick-up are not well established.The research started by determining the lowest melting temperature for AA6060 alloy as this temperature limit the highest temperature above which incipient melting starts. This temperature corresponds to the eutectic melting temperature for AA6060 alloy. Eutectic melting was only detected above 610°C and therefore the exit temperature could be increased to a maximum of 610°C. For an AA6xxx alloy system the lowest melting temperature is 555°C if Mg2Si and excess silicon were present. However as Mg2Si may have fully dissolved into the solid solution, no reaction can take place.A preliminary investigation was conducted to study the characteristics of the newly installed extrusion control and monitoring system. Through this study the relationship between the set extrusion speed and the actual extrusion speed was established. It was found that the actual extrusion speed was lower than the set extrusion speed and was further complicated by the capacity limit of the extrusion pressure. Exit temperature measurements were accurate, however it was measured about 1m away from the die exit. Experiments were carried out to estimate the exit temperature drop and hence the exit temperature measurements were corrected accordingly.Thus, the aim of the present research was to establish the relationship between die pick-up and extrusion conditions (extrusion speed, exit temperature and die condition) and to propose the likely formation mechanism for die pick-up.In this research AA6060 alloy was used and was extruded at 25m/min, 30m/min, 35m/min, 40m/min and 45m/min. The exit temperature was found to increases from 542°C to 567°C. Three types of die pick-up was identified which were named as normal pick-up, die line pick-up and lump pick-up. Normal pick-up occurred regardless of the extrusion speed and exit temperature; however the amount of normal pick-up did not increase when the extrusion speed was increased. Die line pick-up occurred when the extrusion speed was 45m/min and appeared only on the die lines. Lump pick-up is not significant since it was very rare.AA6060 (0.4%Mg and 0.5%Si) alloy has about 0.27% excess silicon and therefore at 555°C, Mg2Si particles react with aluminium and excess silicon to form liquid. However normal pick-up and die line pick-up still occurred at temperatures lower and higher than 555°C and therefore it confirms that eutectic reactions do not influence formation of pick-up. Therefore die pick-up is most likely to be caused due to a mechanical process rather than a metallurgical process.