Made available in DSpace on 2016-12-08T17:19:17Z (GMT). No. of bitstreams: 1 introducao 1.pdf: 60637 bytes, checksum: 290595bc9c94456b344557782acd7d97 (MD5) Previous issue date: 2012-02-29 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The properties of aluminum-silicon alloys are strongly dependent on the casting process used, the chemical additions made to control eutectic structure, primary silicon and grain structure, and molten metal treatment to reduce hydrogen gas content and to remove inclusions. Modification treatments transform the flake eutectic silicon into a fibrous form producing a structure with increased ultimate tensile strength, ductility, hardness, and machinability. Elements in groups I and II and the rare earths europium, lanthanum, cerium, praseodymium, and neodymium modify, but only sodium and strontium produce a strong modifying action at the low concentration required for commercial application. In this investigation, the effect of strontium concentration on the modification of the eutectic silicon in the 356 alloy was studied. For this porpoise the 356 alloy in as-cast and modified conditions using different Sr concentrations were examined. The results were accessed by tensile tests and metallographic examinations. As evaluated the effect of strontium in the formation temperature and the supercoiling of eutectic Al-Si through DTA. The results indicate that the Sr content of from 0 012% by weight resulted in total of Si modification with substantial increases in the toughness of the alloy 356, and supercoiling of 7.1 ° C temperature of the eutectic Al-Si. The indication of from 0.031% by weight mechanical properties, and temperature of the eutectic microstructure way similar to the alloys of the semi - modified but still higher than the same properties of the alloy without modification. / A resistência mecânica e a qualidade de ligas fundidas alumínio-silício são determinadas pela microestrutura e pela sanidade interna das peças fundidas. Tratamentos de refino de grão e de modificação do silício causam alterações microestruturais que resultam em melhorias nas propriedades mecânicas das ligas Al-Si de fundição. O tratamento de modificação altera a forma do silício do eutético da forma acicular para forma fibrosa interferindo sobre o comportamento mecânico da liga. Diversos tipos de modificadores são descritos na literatura, os mais utilizados são estrôncio e o sódio, pois apresentam efeitos de modificação adequados para as baixas concentrações requeridas. O estrôncio é o mais empregado em fundições, em função da maior facilidade no manuseio e uma maior resistência ao desvanecimento. Entretanto, adições de Sr estão associadas com a formação da porosidade em ligas modificadas. O principal objetivo deste estudo é avaliar a influência de diferentes percentuais de estrôncio sobre a morfologia e a distribuição das partículas de Si na matriz e os efeitos destas alterações sobre as propriedades mecânicas. Também foi avaliado o efeito do estrôncio na temperatura de formação e no superesfriamento do eutético Al-Si através de DTA. Os resultados obtidos indicam que teores de Sr a partir de 0, 012% em peso resultaram na modificação total do Si, com incrementos consideráveis na ductilidade da liga 356 e superesfriamento de 7,1 °C na temperatura do eutético Al-Si. Com valores a partir de 0,031% em peso as propriedades mecânicas, microestrutura e temperatura do eutético apresentavam semelhança com as das ligas semi-modificadas, porém superiores à condição sem modificação.

The work is particularly aimed at metallurgical problems connected with manufacture of castings of weight about 1 up to 40 tons from high-alloy austenitic nickel cast irons also known under the trade name Ni-resist. The main aim is to check and complement the knowledge about manufacture of heavy castings from austenitic nickel cast irons in a field of control of tapping and casting temperature, carbon content control with regard to the prediction of eutectic composition and in a field of setting a suitable chemical composition of cast iron with regard to the formation of chunky graphite and stability of the austenitic matrix and further on also possibilities of cast iron filtration during pouring the castings. Based on done experiments the formulae were determined that enable to calculate eutectic and liquidus temperatures on the basis of chemical composition of the cast iron. The use of several formulae given in the literature for calculating the carbon equivalent of austenitic nickel cast irons was compared and evaluated. For utilization of determined results in practice a calculating programme has been compiled making possible to be used in operational practice. This programme is named Calculation of casting temperature of Ni-resist cast irons. After the entry of the melt chemical composition the programme calculates the all important parameters necessary to know and to be controlled for manufacture of high quality castings from austenitic nickel cast irons. Eutectic temperature, liquidus temperature and then based on it the tapping and casting temperatures are included among those parameters. Further on there are eutectic concentration of carbon, carbon equivalent, nickel equivalent, and a criterion evaluating a danger of chunky graphite formation.