Irreversible Thermal Expansion of Replicated Aluminium Foam

Abstract

Irreversible thermal expansion of large items made of the replicated aluminium foam was detected during the extraction of soluble filler from Al-NaCl composite. Sources of the phenomenon were investigated The expansion discovered was caused by incomplete contraction of the porous metal due to the oxidation of its internal porous surface in air and water mediums during thermal cycling. Significant influence of oxide film defects on the expansion process was shown. The information about irreversible thermal expansion dependence on temperature of the extraction process and metal foam pore size was collected. Measurements of expansion dynamics showed its finite character. It was also noted that the expansion is limited by the thermal expansion coefficient of an alloy used. Finally, correction coefficients were obtained that, being applied to nominal sizes of a porous item, compensates the expansion.Keywords: aluminium, replicated foam, alumina film, micro-cracks, contraction, thermal expansion IntroductionThe effect of irreversible three-dimensional expansion of items was detected at the stage of soluble filler extraction during the manufacturing process of commercial replicated aluminum foam at Composite Materials LLC (Kirovgrad, Russia) [1]. Replicated aluminum foam technology involves mechanical treatment to final shape before the filler dissolving from the casting body. This treatment was applied during the process of mold fabrication for expandable polystyrene (EPS) molding with an overall dimension of 850 mm. Control measurement, performed by the customer, showed the overall dimension of 854 mm, not 850 mm, and the product was rejected. It was checked that the dimensional non-compliance was not a mistake at the machining process but the consequence of the filler extraction in accordance with the adopted manufacture technology. The case of the expansion was not identified by the other researchers of the replicated aluminum foam technology [2,3,4,5] due to insignificant sizes of laboratory specimens (not over 200 mm). In this case extraction of the filler is not a difficult task and may be realized within the mode of water external convection at room temperature. Such extraction process is ineffective if sizes of a casting are large, as mentioned above. Non-soluble aluminum hydroxide is formed in the pores while casting is immersed into water for a long time. The hydroxide blocks the process of the filler extraction. A casting is to be heated up to 200-300°С before immersion into the water in order to form a gap between the metal surface and the filler particles [5].Also formation of the gap is used for acceleration of wax model removing from the ceramic molds in the investment casting process [6].