Age‐hardening Response of Replicated Microcellular Al‐4.5%Cu

Abstract

Replication processing of microcellular aluminium begins with the production of a porous preform of internally bonded NaCl powder; this is then infiltrated with aluminium or one of its alloys. [1,2] After metal solidification the NaCl preform is removed by dissolution in water to create a fine interconnected network of metal. One of the key parameters governing the mechanical properties of such a replicated foam is its microstructure, i.e. the inner microstructure of the metal it is made of. The replication process is very flexible in this regard, as it virtually allows the use of any aluminium alloy. Of particular interest in this regard are the classical age-hardening Al-Cu alloys, as these give strengths in the upper range of what is achieved in aluminium alloys.In this study we assess the response of replicated Al-4.5%Cu foams to age hardening, varying the heat-treatment parameters (time and temperature). We investigate in particular the influence of the cell size and morphology, together with that of the foam relative density, on the age hardening of these fine-scale microcellular metals. ExperimentalAl-4.5%Cu open-celled foams having cells 75 or 400 lm in diameter were produced by replication processing. [1,2] In brief, (i) cold-pressed porous NaCl powder preforms were infiltrated with molten Al-4.5%Cu, (ii) the resulting Al-4.5%Cu -NaCl composites were then machined into cylinders with 10 mm diameter and height and finally, (iii) the cylinders were immersed in water to dissolve the NaCl network. The NaCl particles used to produce the preforms are of two types, Fig. 1: (i) 400 lm particles (supplied by Fluka, St. Gallen, Switzerland) produced by NaCl precipitation and of generally equiaxed, cuboidal shape; (ii) 75 lm particles (supplied by Salines de Bex, Switzerland, trade name CP1) produced by grinding, the shape of which is less equiaxed and more irregular. This difference in particle shape is reflected in the mechanical properties of the foams, the more irregular 75 lm particles yielding, at given relative density, foams of somewhat lower Young's modulus than foams produced with equiaxed or spherical NaCl particles. [3] The cylindrical samples were solution heat-treated at 520°C for 4h under argon, and quenched before warm aging in air. Two aging temperatures (130 and 180°C), and seven aging times (2, 6, 10, 20, 54, 168 and 1000 h) were investigated. These samples were tested in compression on an MTS Alliance RT50 screw-driven testing machine, at a displacement rate of 5 lm/s (corresponding to a strain rate of 5 10 -4 ). The load was measured with a 5 kN load cell, and the strain was measured with three LVDTs fixed around the bottom compression plate and measuring the displacement of the upper plate. Table 1 gives the composition of the Al-4.5%Cu alloy used in this study, as measured by the supplier, Alusuisse, Neuhausen, Switzerland (now Rio Tinto Alcan). Figures 2a) and b) show the structure of two replicated Al-4.5%Cu foams: it is apparent that the foam mesostructure, meaning the geometrical metal di...

Influence of microstructural heterogeneity on the scaling between flow stress and relative density in microcellular Al–4.5 %Cu

“…Heat-treatment increases the flow stress by a factor near two, as was already reported in Ref. [15]; here again there is no visible systematic difference between the two solidification conditions.…”

“…These form the essentially insoluble Al 7 Cu 2 Fe phase, which remains after solutionization in the microcellular alloy, as was documented in earlier work [15].…”

“…Some samples were tested in the as-cast condition, while other samples were brought to the T6 condition by solution heat treatment at 525°C for 4 hours under argon, followed by quenching in water and age-hardening for 168h at 130°C under air. This heat treatment corresponds to peak of hardening of the alloy for this ageing temperature [15]. The four different microstructures tested are summarized in Table 1.…”

“…Microstructure variations will thus generally not change dramatically the elastic modulus of microcellular metals (since the stiffness of most engineering alloys is relatively insensitive to microstructure or composition) but will influence the strength of microcellular metals significantly, a priori in equal proportion to what is observed in dense metals and alloys. This is for example visible in the fact that significant increases in flow stress can be produced in microcellular metals by heattreatment if their constituent metal alloy is an age-hardening alloy [7][8][9][10][11][12][13][14][15][16][17][18]. Harnessing the microstructure of microcellular metals is thus an interesting way of optimizing their strength.…”

“…Often, this is not so: Thornton and Magee showed in their pioneering study of aluminium foams [19], as did Yamada et al for microcellular magnesium alloys [20,21], that heat treatment can affect the foam very differently than it does the bulk alloy; 6xxx series aluminium alloys have been shown to display atypical intergranular failure when in microcellular form [22,23] and local strut or cell wall properties have been found to differ from those of the same alloy in dense form [23,24]; fine-scale replicated microcellular Al-4.5wt%Cu was shown to solidify and respond to ageing differently to the bulk alloy when the pore size falls below roughly 100 µm [15,25]; and since brittle second phases are frequently located near strut surfaces in microcellular alloys [17,20,26,27], internal damage accumulates differently during deformation in a microcellular structure than it does in the same alloy in bulk form .…”