Efeito do teor de ferro na microestrutura e nas propriedades mecânicas de ligas Al-Si-Cu-Mg variando a taxa de resfriamento

Abstract

Aluminum is the metal of modern life. Heat-treatable aluminum alloys currently occupy a prominent place in the industrial sector and with even greater prospects for use in the future. The A356 alloy is extended to automotive, aerospace and other industrial areas due to its excellent properties, including high strength, low density and excellent castability. Aiming at production cost benefits, less energy consumption and elimination of environmental damage associated with the mining and refining sectors, the recycling of aluminum alloys has been widely explored. In recycled foundry alloys, iron is the main impurity found and, due to its low solubility in aluminum, its presence leads to the formation of second phases, such as β - Al5FeSi intermetallic compounds which are presented in the form of thin and long platelets (needles ) which have a fragile character, degrading the strength and ductility of the alloy, as they act as stress concentrators. The size of the β - Al5FeSi phase needles increases the higher the iron content in the alloy and the lower the cooling rate. Thus, optimal control of iron concentration in foundry alloys is essential. In order to evaluate the effect of iron content on the microstructure and mechanical properties of the A356 alloy, the alloy was cast, containing 1% and 3% iron in its composition and then heat treated. The microstructure after remelting and solidification presents intermetallic phases with acicular morphology of the AlSiMgFe and AlFeSi composition. On the other hand, the microstructure after heat treatment presents intermetallic phases in needle shapes with β -Al5FeSi composition. The additions of iron resulted in a decrease in the cooling rate. It was found that as the iron content increases, there is a reduction in the distribution of the volume fraction of precipitates and an increase in grain size. It is also verified that with different cooling rates, the volume fraction changes, concluding that the higher the cooling rate, the greater the volume fraction. After performing the microhardness test for post-remelting, it was possible to notice an increase in the values of alloys with iron contents. The microhardness values in relation to the cooling rates used in quenching were unexpected. The recycling simulation of this work and consequent iron contamination in the A356 alloy generated changes in the solidification sequence, enabling the appearance of primary phases formed of iron before the crystallization of aluminum grains.