In this paper, laboratory and semi-industrial processes for the preparation of aluminium foam samples and core-foamed panels with closed porosity were investigated. The samples were prepared starting from the accumulative hot-roll bonded precursors, with titanium hydride (TiH2) or dolomite (Ca0.5Mg0.5CO3) powder added as the foaming agent. The formation of the precursors was performed in three steps. In the initial stage, titanium dihydride or dolomite particles were deposited on a single side of a selected number of aluminium strip samples made from the alloy AA 1050. In the second step, by putting together in pairs, single-sided coated strips, precursors with a two-layered structure were prepared. The samples were hot-rolled to a final thickness of 1.9-3.8 mm, introducing a total deformation of about 45-49 % by a process well-known as accumulative hot-roll bonding. In the third stage of the precursor's formation, the desired multilayered precursor's structure was achieved by hot-roll multi-passing, i.e., by repeating (with 2-16 passes) the accumulative hot-roll bonding procedure. The obtained precursors were foamed in an electrical furnace, under different foaming conditions, based on the initial temperature of the thermal decomposition of the foaming agent. The microstructure of the obtained foam samples was investigated with optical and scanning electron microscopy. According to the accumulated experimental results, one can conclude that the usage of dolomite powder as a foaming agent with a higher temperature of thermal decomposition ()750 °C) compared to TiH2, which thermally decomposed even at the temperatureof hot-rolling ()350 °C), enabling the formation of multilayered precursors at higher temperatures of hot-rolling without any intermediate annealing. This consequently increases the productivity of the foamed core panel production without influencing their final quality.
F.01 Acquisition of new practical knowledge, information and skills
COBISS.SI-ID: 15665942In this work, the viability of CaCO3 powder as a cost-effective alternative to TiH2 foaming agent was investigated. Closed cell aluminium foam samples were prepared starting from solid, foamable precursors synthesized by (i) powder metallurgy and (ii) the melt route. Precursors obtained by the melt route were machined and additionally cold isostatically pressed in order to improve their density. In all cases, the resulting precursors consisted of an aluminium matrix containing various fractions (3, 5, 7 and 12 wt.%) of uniformly dispersed CaCO3 powders of various average particle size (d50= 38, 72 and 120 ìm). Precursors were foamed by inserting them into a cylindrical stainless steel mould and placing them in a pre-heated batch furnace at 750 °C for 10 min. The quality of the foamable precursors was evaluated by determining their initial density and the foaming efficiency (the relative density of the foam obtained, ñ, calculated by dividing the apparent density of the foam, ñF, by the density of aluminium, ñAl). In addition, the quality of the foams obtained was characterised by their density, microstructure (the average pore size) and mechanical properties (uniaxial room temperature compression stress-strain curve, compressive strength and energy absorption after a 30% strain). The experimental findings confirmed that aluminium foams synthesized with CaCO3 powder as a blowing agent can be prepared by both powder metallurgy and the melt route, as well as showing that the density, microstructure, compression strength and energy absorption capacity are quite comparable with the corresponding counterparts foamed by TiH2.
F.02 Acquisition of new scientific knowledge
COBISS.SI-ID: 14642454Aluminium foams are prospective class of cell materials that offer a variety of applications in fields such as lightweight structures. Due to interesting combination of physical and mechanical properties, such as high stiffness at low density, high impact energy absorption, flame and heat resistance, sound absorption, aluminum foams are usually used in aircraft and aerospace industries, for armored vehicles, in car and shipbuilding industries, in civil engineering, as well as in biomedical industry. Production of aluminum foams by the powder-metallurgy process depends on preparation of pre-cursors. In general, pre-cursors consist of compacted metallic powder that is sintered at pre-determined temperature. Due to high temperature of sintering, foaming agent decomposes into a solid component that is built into the matrix material, and a gas component that causes foaming of the matrix material. Powder metallurgy is method for making close-cell foams and it is the best selection, especially for production of net-shape parts, providing excellent quality of end products. In doctoral thesis, powders, precursors and foams, made from different metallic powders (Al 99.7 % purity, AlSi12 alloy) with different foaming agents (TiH2, CaCO3, CaMg(CaCO3)2), were examined with light and electron microscopy. Density of porous material, size and distribution of pores was determined. Further, EDS analyses of distribution of single elements in cellular walls were also made. Results of characterization of single pre-cursors and obtained foams were mutually compared. Special attention was deveted to precursor preparation using different compacting techniques and I compared green densities of precursors on foaming effect. Foaming effect, size and distribution of pores, were checked with precursors made with various fractions of different blowing agents. I have observed differences in volumes of precursors during heating in heating microscope. Also AES analyses of oxides on the surface of powder particles and on the surface of pore walls were made for better understanding of foam stability. Results of analyses and of materials characterization during all the three steps of the powder-metallurgy foaming process (powders, precursors and foams) have revealed the most important parameters in aluminium foams preparation and the influence of foaming agents on foaming process and stability of foams.
F.02 Acquisition of new scientific knowledge
COBISS.SI-ID: 256890880