Characteristics of a near-net-shape formed Al–Al 3Fe eco-functionally graded material produced over its eutectic melting temperature

Abstract

With the concept of an eco-material in mind, this work looks into the possibility of manufacturing Al–Al 3Fe functionally graded material (FGM) products by semi-solid extrusion. For this purpose, an Al–Al 3Fe FGM thick-walled tube of 90 mm outer diameter × 90 mm long × 15 mm thickness was fabricated utilizing a vacuum centrifugal method. Al–Al 3Fe FGM billets of 40 mm diameter × 12 mm thickness, with graded distribution from 0 to 40 vol% of coarse brittle Al 3Fe particles in the thickness direction, were machined from the thick-walled FGM tube. The billets were pressed in the container at temperatures of 660, 670 and 680 °C, where Al alloy melt and solid Al 3Fe particles co-existed. A near-net-shape product of Al–Al 3Fe FGM, in the form of a cup, was successfully fabricated through the semi-solid backward extrusion process during which lower test temperatures produced better results. The volume fraction concentration of Al 3Fe appears to be independent of the test temperature and over 60 vol% around the bottom cup region, decreasing gradually towards the cup wall region. The shape of coarse Al 3Fe particles changes to a more fine fibrous shape at some locations whilst remaining unchanged in others. The shape change mechanism appears not to be due to melt flow erosion, dissolution and/or re-crystallization, but rather due to viscous melt flow splitting of coarse particles. The brittleness of the coarse Al 3Fe intermetallic compound that makes the application of conventional plastic deformation methods impossible appears not to be the case during near-net-shape forming over eutectic melting point. Shore hardness increases with increasing volume fraction of Al 3Fe particles and following semi-solid forming the product appears to be harder than before. This appears to be the result of refining of Al 3Fe particles through shear stress introduced by liquid Al flow during semi-solid forming. Fabrication of FGMs having fine fibrous Al 3Fe phase, appears to be possible if the forming process is carried out at an optimum condition.