Microstructural evolution during pressureless infiltration of aluminium alloy parts fabricated by selective laser sintering

Abstract

The infiltration pathway has been examined during the fabrication of aluminium components formed by a rapid prototyping technology. An AA 6061 alloy preform is first prepared by selective laser sintering. In a second operation, the aluminium precursor powders are transformed into a skeletal AlN structure, which is then pressureless infiltrated by AA 6061. The infiltration process was conducted under nitrogen, argon or a vacuum. The infiltration distance and infiltration pathway was determined by density measurements which were confirmed by metallographic examination and quantitative image analysis. Slug flow behaviour was not observed. Rather, the results suggest that the infiltrant first penetrates the entire sample length and then progressively fills the cross-section. Comparing the microstructures of the surface and the centre of the as-infiltrated samples indicates that the pathway into the porous perform is dependent on the infiltration atmosphere. Under vacuum, the infiltrant firstly fills the interior of the sample and then propagates to the surface. In contrast, under nitrogen or argon, the infiltrant firstly penetrates along the surface before filling the inside. In all cases, the infiltrant preferentially fills the pore regions with the highest curvature. This causes the highly convoluted initial pore structure to decompose into a large number of smaller, rounder pores, which eventually fill and disappear.