Characterisation and Mechanical Testing of Open Cell Al Foams Manufactured by Molten Metal Infiltration of Porous Salt Bead Preforms: Effect of Bead Size

Abstract

Preforms made from porous salt beads with different diameters (0.5-1.0, 1.4-2.0 and 2.5-3.1 mm) have been infiltrated with molten Al to produce porous structures using pressure-assisted vacuum investment casting. Infiltration was incomplete for preforms with high densities. At higher infiltration pressures, penetration of molten Al occurred into beads of all sizes and was predicted using a simple model. The yield strength of the porous structures increased with increasing density and decreasing pore (bead) size. Despite the non-optimum distribution of metal in the porous structure, due to partial infiltration within the beads, the magnitude and density dependence of the yield stress were comparable with those for pure Al foams reported in similar studies. The structural efficiency was improved for structures produced at lower infiltration pressure, where the metal is predominantly distributed in the cell walls. The rate of salt dissolution from the preforms was high, in particular for high density preforms, large beads and preforms infiltrated at low pressures, owing to the ability of the porous beads to collapse as well as dissolve.

Discrete element modelling of the packing of spheres and its application to the structure of porous metals made by infiltration of packed beds of NaCl beads

“…Details of the materials and methods used to make the beads are given in [10,11,19]. Castings were made by pouring salt beads (with different volumetric fractions of large and small beads, mixed using a turbula mixer) into a 35mm diameter flanged stainless steel mould with a porous base.…”

“…This was necessary as two particles in very close proximity, but not actually touching, may end up becoming connected pores if the gap separating them cannot be filled with the infiltrating liquid. The minimum gap that can be filled with the infiltrating liquid was determined from the Laplace equation (shown in equation 1) which relates the applied pressure, P, to the capillary radius, rc, using established wetting data for the Al-NaCl system [10,11] to determine the liquid surface tension, LV and the contact angle, . A typical capillary radius for this system and process is of the order of 15 m and hence particle separations below 30 m were deemed to be "contacting".…”

“…The infiltration of liquid metal into a bed of sacrificial particles (often termed porogens or space fillers) offers a convenient and reproducible method for the manufacture of porous metals [8][9][10][11]. Using this method a "preform" is made from the porogen either from loose or tapped beads, or using subsequent…”

Assessing the potential for multi-functional “hybrid” porous Al-phase change material structures

“…Such structures made from pure aluminium, have compressive yield stresses between 1 and 5 MPa [8] and thus have the capacity (especially if sheathed in an outer Al skin) for both significant storage of PCMs and to support service loads.…”

The permeability of virtual macroporous structures generated by sphere packing models: Comparison with analytical models

“…This ability to fill the gap is controlled by the capillary radius, r c, determined from the Laplace equation. For the Al-NaCl system [9,10] an infiltration pressure of 1 bar leads to a capillary radius of approximately 20 m and hence particles separated by less than 40 m are deemed "contacting" for this infiltration condition. An extension to this previous study is the graphical representation of this connection through calculation of the radius of the zone that is excluded from infiltration by the liquid (using a simple geometrical model also presented in [4]) and adding the position of the centre of the contact and the radius to the data set.…”