A quantitative evaluation of the microstructure of aluminium foams and, particularly, any quantitative comparison is a very demanding and complex issue. In this work, the cell-size distribution (CSD) was proposed as the most efficient approach for their assessment. The foams were made by the powder metallurgy (P/M) route, by applying titanium hydride and dolomite powders of five different average particle sizes as the foaming agents. The average size of the pores and the pore-size distribution were estimated by assessing optical and scanning electron micrographs of as-polished foam bars by applying the point-counting method and image-analysis software. The uniformity of the CSD in the foamed samples with closed cells was studied as a function of the particle size distribution of the foaming agents, the average particle size of the applied AlSi12 powders, the concentration of the foaming agents, the foaming temperature and the foaming time. Generally, the samples foamed with the dolomite foaming agent had a more uniform cell-size distribution and a lower average bubble size. The most uniform cell-size distribution was achieved in the foam samples foamed with the minimum amount of the mass fraction (w = 0.5 %) of dolomite powder grades, having the lowest average particle size and a narrow particle-size distribution. In contrast, in samples made from coarser and less-uniform grades of foaming agents, the cell-size distribution was broader, with a significantly higher fraction of large bubbles. Longer foaming times and higher foaming temperatures also led to foam samples with a less-uniform microstructure. Based on the experimental findings and theoretical considerations regarding aluminium-foam microstructural development, the preconditions for stable bubble growth into a homogeneous and uniform foam structure were modelled and compared with the experimentally determined values.
COBISS.SI-ID: 915626
Aluminium foams, produced by the powder-metallurgy route, have a good potential for use in weight-sensitive structural parts. The goal of this study was to evaluate the properties and to optimize the preparation of precursors by a powder-compacting process. Various compacting pressures, from 200 MPa to 900 MPa, were used in the double-axial powder-compacting process for two different aluminium powders: pure aluminium and an AlSi12 alloy with the addition of 1 % of TiH2 as a foaming agent. The green density of the precursors and the distribution of the foaming agent were examined. The powder particles was also characterised. The results of the effective preparation of precursors are shown as the effectiveness of the foaming of the precursors. The relation between the powder characteristics, the aluminium-alloy properties and the preparation of precursors was studied by SEM/EDS analysis, powder-metallurgy standard testing of metallic powders, granulometry, etc. Different parameters were used for the precursor preparation and foaming. The foaming temperature varied between 680 °C and 770°C, and the foaming time was from 6 min to 13 min. The relation between the properties and the applied production parameters was studied in detail and is described in this paper.
COBISS.SI-ID: 14820630
In this work, the influence of the composition, density and porosity of foaming precursors on the foaming efficiency, microstructure development and mechanical properties of aluminium foams are presented and discussed. The foams were prepared, starting from precursors made either by powder metallurgy (PM) or by the melt route. Following the PM route, precursors were made by mixing Al powder and 3-10 % of volume fractions of dolomite or calcium carbonate particles of particle size from 20 [mu]m to 120 [mu]m and cold isostatically pressing the mixture at 700 MPa. In the case of the melting route, precursors were made by introducing dolomite or calcium carbonate particles directly into the molten aluminium at 700 [compositum]C. After melt stirring, the precursors were prepared by casting the semi-solid slurry into a cylindrical, water-cooled mould. Finally, aluminium foams were made in all cases by inserting precursors into a cylindrical stainless-steel mould and heating the arrangement at 750 [compositum]C for 10 min. After that, the mould was removed from the furnace and the foaming process was stopped by cooling in air to room temperature. The microstructure of the obtained foams was investigated by optical and scanning electron microscopy (SEM-EDS), while XRD was applied for a detailed identification of phases. The quality of the precursors was evaluated by determining their mechanical properties (uniaxial room-temperature compression stress-strain curve, compressive strength and energy absorption after a 30 % strain) and the foaming efficiency (the relative density of the foam obtained). The concentration of the foaming agent and the density of precursors were found to have a detrimental influence on the foaming efficiency as well as on the foam's microstructure and mechanical properties. The foaming of precursors with open porosity were inefficient.
COBISS.SI-ID: 856490
In this work, the viability of dolomite powder as cost-effective alternative to TiH2 foaming agent was investigated. Closed cells aluminium foam samples were prepared starts from solid, foamable precursors synthesized by powder metallurgy and melt route. Precursors obtained by melt route were machined and additional cold isostatic pressed in order to improve their density. In all cases, the resulted precursors consisted of an aluminium matrix containing various mass fractions of uniformly dispersed dolomite powders of various average particle size and 5 % of SiC particulates. Precursors were foamed by inserting into a cylindrical stainless steel mould and placing inside a pre-heated batch furnace at 700 °C for 10 min. The quality of foamable precursors was evaluated by determining their initial density and the foaming efficiency. On the other side, the quality of the obtained foams were characterised by their density, microstructure and mechanical properties. Experimental findings confirmed that aluminium foams synthesized with dolomite powder as blowing agent can be prepared by both powder metallurgy and melt route, as well as that the density, microstructure, compression strength, and energy absorption capacity are quite comparable with corresponding counterparts foamed by TiH2.
COBISS.SI-ID: 15446550
In recycling plants, especially those specializing in the recycling of low-grade aluminum scrap for wrought aluminum alloys, timely and accurate information about the amount of organics and other impurities in the incoming scrap is an important parameter in achieving both economic benefits and standard metallurgical quality of the recycled metal. To use aluminum scrap combined with organics as a source of aluminum for producing wrought aluminum alloys of standard quality, its metallurgical composition and the content of organic and other impurities should be quickly and cost-effectively analyzed on representative samples. In this work, an industrial thermogravimetric/differential thermal analysis of representative scrap samples was developed as an efficient analytical methodology for analyzing the humidity and organic impurities in incoming scrap. When performed in continuous mode, under a flowing atmosphere of argon with 1 wt.% of oxygen, this methodology enables a routine measurement of the humidity, the quantity of organics, and the carbon content in representative samples of incoming scrap in less than 15 min within an accuracy of 0.5%.
COBISS.SI-ID: 16152854