Manufacturing Aluminum Foams by Melt Gas Injection Process

Transcription

1 7th International Conference on Porous Metals and Metallic Foams 195 Manufacturing Aluminum Foams by Melt Gas Injection Process M. Malekjafarian 1, S.K. Sadrnezhaad 2*, M.S. Abravi 1, M. Golestanipour 1, H. Amini Mashhadi 1 1 Materials Research Group, Iranian Academic Center for Education, Culture and Research (ACECR), Mashhad Branch, P.O. Box , Azadi Square, Mashhad, Iran 2 Department of Materials Science and Engineering, Sharif University of Technology Azadi Ave., PO Box , Tehran, Iran 1 Introduction Aluminium foams are a new class of materials with low densities, large specific surface and novel physical and mechanical properties. Their applications are extremely varied: for light weight structural components, for filters and electrodes and for shock or sound absorbing products. Recently, interesting foaming technology developments have proposed metallic foams as a valid commercial chance; foam manufacturing techniques include solid, liquid or vapour state methods. The foams presented in this study are produced by Melt Gas Injection (MGI) process starting from melt aluminium[1,2]. The injected air causes' bubbles to rise to the surface of the melt, forming a liquid foam which is stabilized by the presence of solid ceramic particles on the gas liquid interfaces of the cell walls. The stabilized liquid foam is then mechanically conveyed off the surface of the melt and allowed to cool to form a solid slab of aluminum foam. The aluminum foam structure (cell size and cell wall thickness) is controlled by the process variables such as the volume fraction of the solid particles; foaming temperature, airflow rate, and impeller design the foam making process. Unfortunately, no publication has been found in the work on the influence of the process in variables on the cell structure of aluminum foam. The present study is aimed at investigating the effect of the concentration of SiC particles on the cell structure and mechanical properties. 2 Experimental 2.1 Materials Commercial A356 cast aluminum alloy was used as a base material. The reinforcement phase consisted of SiC particles with purity of 98.0 wt.% and mean mass particle size of 10 µm. Heating SiC particles for 1 h at 950 oc and then for 2 h at 650 oc was carried out to improve the wetting properties by removing the adsorbed gases from the surface of particles. SEM micrograph of heat-treated SiC particles powders is respectively shown in Fig. 1.

2 196 MetFoam Processing methodology Figure 1. SEM micrograph of SiC particles SiC p reinforced aluminum matrix composite slurry was prepared by conventional stir-casting techniques at oc, and then it was poured into a steel mould to obtain a composite ingot. This ingot was melted again at oc and then it was stirred at 700oC to achieve enough viscosity. The rotate speed of the stir-equipment was 1400 rpm. A content of 1 wt% of magnesium was subsequently introduced into the melt to improve the wet ability of liquid metal followed by stirring. When the molten composite had reached foaming temperature, it was then poured into foaming apparatus of Fig. 2 which was kept at foaming temperature in a resistance furnace. The crucible was made of low carbon steel and protected with zirconia coating guard. The compressed air was introduced into the composite melt through air injection shaft when the molten composite level was about 15 cm below the foaming spout. The foaming process was stopped when the aluminum foam build up on the melt surface in the foaming chamber was about to overflow over the foam spout. Figure 2. Schematic of the gas injection system The aluminum foam was then removed from the chamber after cooling and subjected to sectioning for the evaluations of cell structure and mechanical properties. The air flow rate was controlled at 2 and 4 l/min by a rotameter at the pressure of 0.2 MPa. The foaming

3 7th International Conference on Porous Metals and Metallic Foams 197 temperature was kept at 730 C. Different amounts of SiC particles (5, 10, 15 and 20 vol.%) was selected to produce composite foams with different relative densities and mechanical properties. 2.3 Density and percentage porosity The porosities of composite foams were calculated using the following equation: P 1 ( ) 100% s (1) Where P is the porosity of composite foams, ρ and ρs are the densities of composite foams and the cell wall material, respectively, and ρ/ρs, which is called the relative density of composite foams, indicates the ratio of the density of composite foams to the density of cell wall material. 2.4 Mechanical Properties DIN50134 standard single-axis pressure test on the foam cube samples with dimensions 50mm 50mm 40mm using a Zwick Z250 universal testing machine with computercontrolled with a strain rate of 1 mm / min. Was performed. At least seven times the size of a standard sample size of foam cells to the cell size effect on compressive properties of the foam. 3 Results and discussion 3.1 Microstructural features The microstructure of produced composite foam with density of 0.20 g/cm 3 is shown in Fig. 3. It can be found that the structure of cells is uniform (Fig. 3(a)) and SiC particles uniformly distribute in the cell wall of composite foams (Fig. 3(b)). 2cm (b) (a) Figure 3. a) Optical microscope and b) SEM microstructure of produced composite foam with density of 0.20 g/cm3.

4 198 MetFoam Cell size In the addition of different volume of silicon carbide particles, the cell size of the sample foamed at 730 C is increased with the increase in particle concentration, as shown in Fig. 4. Figure 4. Effect of volume fraction SiC particle on the cell size of the aluminum foam. The addition of the ceramic particles was originally designed at every concentration of the particles so as to study systematically the effect of volume fraction of the ceramic particles on the cell structure. The volume fraction of the SiC particle for stable foaming in the composite melt is over 15% for 10 µm particles at the foaming temperatures. In the stabilization of the aluminum foam, the wetability of the ceramic particles by the alloy melt is not the only dominant property of the behavior of the ceramic particles; foam stability is also greatly dependent on the drainage and rupture of the cell wall separating the air bubbles. Foam stability may increase or decrease, depending on the role of the particles on the drainage and rupture process. The presence of solid particles in the composite melt increases the bulk liquid viscosity, and the higher viscosity slows down liquid flow and thus retards the cell wall drainage before it solidifies. Apart from their influence on melt viscosity, the ceramic particles have an important impact on foam stability through their attachment to the gas/liquid interface of aluminum foam, which changes the interfacial curvatures and reduces the capillary pressure difference between the plateau border and the cell wall of the aluminum foam [3,4]. Therefore, for a given ceramic particle, a concentration range of the particles is critical for stable foaming of the composite slurry. However, the actual concentration required to form a stable foam depends largely on the immersion depth of the air exit, because the number of particles encountered and picked up by the gas bubbles is dependent on the distance traveled by the bubbles [5]. Furthermore, the rising bubbles become stable only when the critical surface coverage by particles is achieved. Therefore, the longer the path of bubble travels, the lower the critical concentration is required to produce stable foam. The affinity of the ceramic particles for the aluminum melt also plays an important part in the amount of particle addition for appropriate foaming of the melt.

5 7th International Conference on Porous Metals and Metallic Foams density and porosity Production of foam density in the range of 0.1 to 0.3 g/cm 3 was measured. Figure 5 show that the silicon carbide particles with increasing volume, density increases. Given the amount of porosity measurements for a range of samples is 88% to 96%. As can be seen Figure 6, with increasing volume percent silicon carbide particles decreases the amount of foam porosity. Figure 5. Effect of volume fraction SiC particle on the foam density of the aluminum foam. Figure 6. Effect of volume fraction SiC particle on the porosity of the aluminum foam. 3.4 Compressive properties The compressive stress strain curves of composite foams are shown in Fig. 8. During the compression loading of closed-cell foams, the strain tends to localize into a thin band, which causes buckling of cell walls [6]. A consequence of the topology of the cell structures is that the strain distribution tends, for all of foams, to be locally non-uniform. The onset site of local plastic deformation depends on the cell structure.

6 200 MetFoam 2011 ( a) (b) Figure 7. Elevation changes of aluminum foam samples tested (a) and after the pressure test (b) Figure 8. Compressive stress-strain curves of aluminum foams with density of 0.20 g/cm 3. Deformation initiates in a single band, which is in contact with the loading surface, and proceeds to other layer one after the other. The plateau region of the compressive stress-strain curve of composite foams is not very smooth and exhibits some serrations. The main reason is the addition of SiCp to Al alloy. According to the mechanical properties of matrix materials, metallic foams includes three types, i.e., elastic, plastic and brittle [7]. The brittleness of Al/SiCp composites is generally more than that of Al alloy [8], therefore, Al/SiCp composite foams belongs to brittle foams. When the stress of Al/SiCp composite foams reached the maximum, it came into the collapse plateau region. With the increase of the compressive strain, parts of cell walls produced cracks and suddenly brittle rupture, the space inside these cells decreased, and the stress was also reduced suddenly. Then the stress rose again with the increase of strain, and the next decrease of stress, which results from the sudden rupture of other cell walls, occurred at a larger strain. In addition, during straining, some cell wall fragments were ejected from the boundary cells, which also indicated the local brittle fracture of the Al/SiCp composite foams.

7 7th International Conference on Porous Metals and Metallic Foams 201 Consequently, the typical serration plateau was formed in the compressive stress-strain curve [9]. This is expected in view of the higher ductility associated with the cell wall microstructure of Al/SiC p composite foams. In the densification regions of the compressive stress-strain curves of foams, the internal surfaces of more and more cells touched each other, foams became more and more compact, and the stress rose when the complete compaction of forms commenced. The rates of densification are higher for the denser samples of composite foams. The effect of SiCp volume fraction on the plateau stress of Al/SiC p composite foams are respectively shown in Fig. 10. This indicates that SiC p has strengthening effect on the compressive properties of composite foams. Figure 10. Effect of SiC p volume fraction on the plateau stress of aluminum foams. 4 Conclusions In the light of this analysis the following conclusions can be drawn: (1) Stable foam does not occur until a critical particle concentration is reached for the SiC particles, while the excessive addition of the particles will lead to unstable foaming. Therefore, a range of the particle concentration is critical for stable foaming of the composite slurry at the foaming temperatures. (2) Cell size of the sample foamed increases with increasing SiC particle concentration at constant foaming temperature. (3) Cell wall thickness increases with increasing SiC particle concentration constant foaming temperature. (4) The compressive stress-strain curves of composite foams have obvious elastics region, plateau region and densification region. The plateau stress of composite foams increases with increasing relative density. The plateau stress of Al/SiCp composite foams increase with increasing SiCp volume fraction.

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