The industrial use of recycled aluminium is greatly limited by the degraded mechanical properties due to the increased impurities. Fe, one of the common impurity content in Al alloys, is difficult to eliminate once introduced into aluminium during primary production or recycling processes. Due to the low solid solubility of Fe in Al, the formation of Fe-rich intermetallic compounds (Fe-IMCs) is inevitable, which is one of the main causes for the deterioration of mechanical properties in various cast Al alloys. In order to obtain desirable mechanical properties of recycled Al alloys, modification and refinement of the Fe-IMCs are urgently required as the compact and refined morphologies of such intermetallics are generally non detrimental to Al alloy's performance. However, manipulating the solidification behaviour of the Fe-IMCs phases, including nucleation and growth, is very challenging because of the inherently more difficult heterogeneous nucleation of the Fe-IMCs compared with that of a pure metal or a solid solution; and the strong growth anisotropy. Limited understanding on mechanisms of nucleation and growth of the multicomponent Fe-IMCs is available in the literature. The aim of this study is to gain a deeper understanding on the heterogeneous nucleation and growth behaviour of Fe-IMCs in various Al alloys. The nucleation and growth of both primary and eutectic Fe-IMCs have been investigated during various solidification conditions including a number of different cooling rates and casting temperatures. Based on the experimental results of the solidification of several ternary and quaternary alloys, effect of Mg on the solidification behaviour of Fe-IMCs was investigated. Further the surface modified TiB2 particles were used to enhance the heterogeneous nucleation of Fe-IMCs in order to refine the Fe-IMCs particles. The dominant Fe-IMC in Al-5Mg-2Si-1.2Fe-0.7Mn alloy is identified, using transmission electron microscopy (TEM), as α-AlFeMnSi with a body centred cubic (BCC) lattice structure and lattice parameter of 1.256nm. In the current alloy system, the nucleation of primary α-AlFeMnSi occur at lower cooling rate (≤0.8K/s) when required nucleation undercooling is reached, as the slower cooling rate allows longer diffusion time for the solute to form a stable nucleation embryo. When casting with 20K superheat, the size of primary α-AlFeMnSi increases gradually from 24.5±3.1μm (870K/s) to 251.3±75.3μm (0.02K/s) and the size of α-AlFeMnSi eutectic increased gradually from 102.0μm (870K/s) to 623.3μm (0.02K/s). The Fe and Mn concentration in α-AlFeMnSi appears to reduce with the increased cooling rate due to the relatively insufficient solute supply when solute concentration is low (1.2wt.% Fe and 0.7wt.% Mn). Microstructure observation reveals that the {011} plane, especially on <111> orientation, is the preferred growth orientation of BCC primary α-AlFeMnSi, resulting in rhombic dodecahedral in 3D. The eutectic α-AlFeMnSi, prefers to initiate on the primary α-AlFeMnSi. In addition to the substantial nucleation undercooling, the research revealed that the nucleation of primary α-AlFeMnSi also rely on the local solute concentration and the solute diffusion. Compared with α-Al, the growth of α-AlFeMnSi is less sensitive to the cooling rate changes due to the complexities in multi-components interaction and different diffusion efficiency of different elements. The addition of Mg to Al-1.2Fe-0.7Mn and Al-2Si-1.2Fe-0.7Mn alloys was found to lead to a morphology change of Fe-IMCs. Al6(Fe,Mn), the predominant Fe-IMC in the Al-1.2Fe-0.7Mn-xMg alloy, changed from needle morphology to interconnected lamellar morphology when Mg composition increased from 0.004wt.% to 6.04wt%. A Mg-rich layer at about 5-20nm in thickness was commonly observed on the Fe-IMC/α-Al interface in the alloys with Mg content. The eutectic lamellar spacing for Al6(Fe,Mn) increases from 1.8±0.3μm to 4.5±0.8μm when Mg content increased from 0.004wt.% to 6.04wt.%. In the case of α-Al12(Fe,Mn)3Si, the predominant Fe-IMC in Al-2Si-1.2Fe-0.7Mn-yMg alloys, its lamellar spacing of the eutectic increased from 1.4±0.3μm to 3.25±0.8μm when Mg increased from 0.04wt.% to 5.41wt.%. Owing to the strong anisotropy of the Fe-IMC crystals, the segregation of solute Mg on preferred growth orientation is higher, causing greater growth restriction on this orientation. Consequently, the growth velocity on other orientations becomes relatively more significant. To optimise the morphology of Fe-IMCs in Al alloys, a novel Αl-Ti-B(Fe) grain refiner for Fe-IMCs has been developed to enhance the heterogeneous nucleation of Fe-IMCs. The addition of the novel grain refiner to an Al-5Mg-2Si-1.2Fe-0.7Mn alloy under controlled solidification condition results in a considerable refinement of the primary Fe-IMCs from 251.3±75.3μm to 110.9±45.5μm and from 127.3±36.2μm to 76.5±18.2μm at cooling rates of 0.02K/s and 0.15K/s, respectively. TEM investigations on the refiner reveal a Fe-rich adsorption monolayer in a zigzag fashion on the prismatic planes on the boride particles. This surface modification is beneficial for the heterogeneous nucleation of the Fe-IMCs. Further investigation of the Al alloy with this grain refiner addition revealed that there existed specific orientation relationships (ORs) between TiB2 and Fe-IMCs: (001)[020]Al13Fe4 // (11-20)[10-10]TiB2, and (001)[120]Al13Fe4 ∠6.05˚ (11-20)[10-11]TiB2; (0-11)[100]α-AlFeMnSi // (0001)[-2110]TiB2, and (0-11)[111]α-AlFeMnSi ∠4.5˚ (0001)[10-10]TiB2. The Fe adsorption on substrate particle, the observed ORs between TiB2 and Fe-IMCs, and the refinement of primary α-AlFeMnSi with the addition of modified TiB2 provide evidence of structure templating and composition templating required by heterogeneous nucleation of Fe-IMCs. This research has delivered contribution to the understanding and new approach for optimizing the morphology of Fe-IMCs in the Fe-containing Al alloys. Using the slow cooling rates (≤0.15K/s), the formation compact primary α-AlFeMnSi can be considerably encouraged. With a lower casting temperature, the size and volume fraction of large Chinese-script α-AlFeMnSi can be significantly reduced. With addition of reasonable Mg content the morphology of Fe-IMC can be modified. Particularly, with the addition of the Al-Ti-B(Fe) grain refiner in well-controlled condition, the primary α-AlFeMnSi can be significantly refined. Thus, by implementing these approaches, the optimized Fe-IMC morphology in the microstructure of Fe-containing Al alloy is able to offer promising mechanical performance.

The quality of aluminium cast billets is highly dependent on the cleanliness of the material. Hydro Extruded Solutions are casting billets in Sjunnen that are delivered for extrusion within the same company. In order to produce extruded profiles of high quality and also maintain the pressing tools, it is a good method to keep the billets as free from inclusions as possible. A fluxing agent in form of salt compounds is used to purify the material during melting, but also to protect it from further oxidation. Another task of fluxing agents is to minimize the aluminium amount in the dross phase. The purpose of this project was to investigate if the fluxing agent that is used by the cast house is having any impact on the quality of the billets. Therefore, for that purpose, samples with a variating amount of fluxing agent were processed by the cast house in order to be investigated. These samples were investigated with the Optical Microscopy-method in order to determine the area fraction of non-metallic inclusions inside them. The results did not reveal any significant difference in the quality of the different samples since most samples obtained a low OM-index after the investigation. That can be explained by the type of scrap that was used since new scrap is usually very clean and contains smaller amounts of inclusions. / Kvaliteten hos aluminiumgöt är starkt beroende av renheten i materialet. Hydro Extruded Solutions gjuter rundgöt i Sjunnen, som sedan levereras för strängpressning inom samma företag. För att producera strängpressade profiler av hög kvalitet och även upprätthålla pressverktygen, är en bra metod att hålla göten så fria från inneslutningar som möjligt. Ett flussmedel, i form av saltföreningar, används för att rena materialet under smältningen, men också för att skydda den från vidare oxidation. En annan uppgift som flussmedel har är att minimera mängden aluminium i slaggfasen. Syftet med detta projekt var att undersöka om flussmedelet som används av omsmältverket har någon inverkan på götkvaliteten. Därför, för detta ändamål har prover med en varierande mängd av flussmedel tillverkats i omsmältverket för att bli undersökta. Dessa prover undersöktes med den Optiskt Mikroskopiska-metoden för att bestämma areafraktionen av icke-metalliska inneslutningar inuti dem. Resultaten avslöjade inte någon signifikant skillnad i kvaliteten hos de olika proverna eftersom de flesta prover erhöll ett lågt OM-index efter undersökningen. Detta kan förklaras av vilken typ av skrot som användes, då process-skrot oftast är väldigt rent och består av mindre mängder innseslutningar.

Orientador: Maria Helena Robert / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-11-08T18:35:02Z (GMT). No. of bitstreams: 1 Ribeiro_DanielTadeudeCarvalho_M.pdf: 21302819 bytes, checksum: 8a7a0a9d878be3b8ccb6833173c35aef (MD5) Previous issue date: 2011 / Resumo: Este trabalho tem como objetivo desenvolver um método alternativo para a fabricação de espumas metálicas pela reciclagem de cavacos de Al e a caracterização mecânico-metalúrgica do produto obtido. O processo investigado envolve a espumagem de compactados de misturas de cavacos e pó de agente espumante, com a espumagem ocorrendo na liga em estado semi-sólido tixotrópico. Foram fabricados precursores espumáveis pela mistura de cavacos da liga de Al AA356, com duas distintas granulometrias, a 1,5wt% do agente espumante TiH2 e posterior compressão também em duas distintas condições: compressão simples a frio e compressão dupla, a frio e a quente. Os elementos precursores foram aquecidos, sofrendo espumagem em estado semi-sólido, pela dissociação do agente espumante formando gás H2 no interior do metal. As espumas metálicas produzidas foram analisadas quanto à sua qualidade interna, arquitetura celular (dimensões, geometria de poros, espessura de paredes metálicas), propriedades físicas (densidade, teor de porosidade, condutividade e difusividade térmicas) e mecânicas em compressão semi-estática. O comportamento mecânico em compressão e o comportamento térmico das espumas produzidas foram também estimados teoricamente utilizando modelos da literatura. Os resultados obtidos mostraram que o processo proposto é eficaz para a produção de espumas de boa qualidade. Os melhores resultados são obtidos com o emprego de compactação dupla e espumagem direcional do precursor. Não foi possível observar influência das dimensões dos cavacos utilizados, na faixa de tamanhos empregada, nas características e propriedades das espumas produzidas. O comportamento das espumas metálicas sob compressão semi-estática demonstrou a formação de um platô de deformação plástica ascendente, o que indica uma grande capacidade de absorção de energia mecânica; todas as amostras apresentaram condutividade térmica bastante inferior e difusividade superior à do metal maciço, representando boas propriedades isolantes térmicas. Os valores de propriedades mecânicas estimadas teoricamente diferem dos obtidos experimentalmente, indicando necessidade de cuidado no emprego destes modelos / Abstract: This work aims to develop an alternative method for manufacturing metal foams by Al alloy machined chips recycling and mechanical and metallurgical characterization of the product. The process investigated involves foaming a compressed mixture of Al chips and foaming agent powder, with foaming occurring with the alloy in semi-solid thixotropic state. Foamable precursors were manufactured by mixing Al alloy A356 machined chips in two different particle sizes, to 1.5 wt% of the foaming agent TiH2 and its posterior compression also in two distinct conditions: simple cold compression and double compression, cold and hot. The precursors were heated, being foamed in semisolid state, by dissociation of the foaming agent generationg H2 into the bath. The internal quality, cellular architecture (pore size and geometry, cell wall thickness), physical properties (density, porosity level, thermal conductivity and thermal diffusivity) and mechanical behaviour under semi-static compression were analyzed for the metal foams produced. The mechanical behavior under compression and thermal behavior of the foams produced were also calculated using theorical models from literature. The results showed that the procedure proposed is effective for the production of foams of good quality. The best results are obtained foaming directionaly precursors manufactured by double compression. It was not possible to observe the influence of the dimensions of the machined chips, considering the size range adopted, on the characteristics and properties of the foams produced. The mechanical behavior of metallic foams under semi-static compression showed the formation of a ascendent plateau of plastic deformation, which indicates a great capacity of absorbing mechanical energy; all samples showed significantly lower thermal conductivity and diffusivity compared to the solid metal, what represents good thermal insulation properties. The theorical estimative of the sample's mechanical properties differ from those obtained experimentally, indicating that the matematical models proposed has to be used carefully / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica