Compressive Properties and Energy Absorption Behaviour of AlSi10Mg Open-Cell Foam

Transcription

1 Journal of Materials Science and Technology 2014, Vol. 22, No. 1, pp Compressive Properties and Energy Absorption Behaviour of AlSi10Mg Open-Cell Foam Lenko Stanev 1, Boris Drenchev 2, Anton Yotov 1 and Rumiana Lazarova 1 1 Institute of Metal Science, Equipment and Technologies with Hydro- and Aerodynamics Centre Acad. A. Balevski, Bulgarian Academy of Sciences, 67, Shipchenski Prohod Blvd, 1574 Sofia, Bulgaria, s: stanev@ims.bas.bg; lazarovaruniana@abv.bg 2 Institute of Electrochemistry and Energy Systems, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bl. 10, 1113 Sofia, Bulgaria, bdrenchev@abv.bg Abstract. In the present work, microstructure, compressive behaviour and energy absorption properties of the aluminum alloy (AlSi10Mg) open-cell foams are studied. The cellular material is obtained by replication method. Additional thermal treatment is applied to the foams before compressive testing. The microstructure is observed, and the compressive stress-strain curves determined for two materials with different porosity are compared to the one for the aluminum alloy. Energy absorption capacity and energy absorption efficiency are calculated and compared for both porous structures. The aim is to characterize this Al-alloy open-cell material with respect to its structure and compressive properties. It is concluded that thermally treated AlSi10Mg open-cell foam possesses satisfactory mechanical energy absorption capacity, and is suitable for various multi-functional applications. Keywords: open porosity, replication method, compressive properties. Corresponding author.

2 Compressive Properties and Energy Absorption Behaviour Introduction There is no doubt that porous materials and especially metallic porous materials will play a significant role in more and more engineering constructions as structural and functional elements. The increasing interest to these materials is based on their specific combination of mechanical and physical properties such as high strength-to-weight ratio, large specific surface area, controllable permeability, well expressed energy and sound absorption capacity, considerable vibration reduction capacity, high flame resistance and others [1 3]. The basic researches in this field are directed to design of composition as well as macro- and microstructures of the cellular materials. Large variety of production methods are developed [4, 5] and lots of metallic and nonmetallic materials are elaborated. Replication method [6, 7] is one of the production techniques very often used for obtaining of different metallic opencells materials [8 10]. Aluminum and nickel based metallic foams continue to be the most commercially available porous materials. Density (which means porosity) of given cellular material plays a key role for its mechanical characteristics. However, density cannot be varied in a large interval because of different technological limits or strength requirements. For this reason the efforts of researchers are directed to development of additional processing techniques such as mechanical and thermal treatment and/or tuning alloy composition. In the present study, the compressive behaviour of two type aluminum alloy open-cell foams is explored. The materials are obtained by replication method and are subjected to additional thermal treatment. The compressive stress/strain curves are determined for nominally non-porous samples and two foams with different porosity. Energy absorption capacity and energy absorption effectiveness are calculated and compared. The aim is to characterize this open-cell material with respect to its structure and compressive properties. The results contribute to the multi-functional applications of the aluminum alloy foams in the industry.

3 46 L. Stanev, B. Drenchev, A. Yotov, R. Lazarova 2. Experimental 2.1. Materials and liquid phase processing The replicated open-cell foams are made from AlSi10Mg alloys with composition as given in Table 1. This alloy is widely used in practice but is not so popular as material for manufacturing of porous structures. The leachable preforms (spaceholders) are made from common salt, NaCl. Table 1. AlSi10Mg alloys composition, wt% Alloy Si Cu Ni Mg Fe Mn AlSi10Mg Alloy Ti Zn Sn Pb Al AlSi10Mg the rest Two sizes of salt particles are used for the preform preparation. The porosity P = 0.61 (samples type S1) is realized by particles size in the range between 500 µm and 1000 µm, and the porosity P = 0.71 (samples type S2) is realized by particles size in the range between 200 µm and 500 µm, see Table 2. In order to remove moisture, the NaCl particles are preliminarily dried at 250 ± 3 C for 2 hours. Afterwards, the salt particles are poured into thinwalled steel cups with diameter 65 mm, dried again at 300 ± 1 C for 1 hour and sintered at 780 ± 1 C for 5 hours. The obtained preforms diameter is 65 mm and height is 40 mm in both cases S1 and S2, see Fig. 1. Samples type Table 2. Structure characteristics of the samples and the preforms Porosity P Preform particles size, µm Average preform particles size, µm Relative density ρ /ρ s S S Rem.: Here ρ is the apparent density of the cellular material and ρ s is the density of the nominally non-porous alloy. Squeeze casting machine is employed for the preforms infiltration by the molten alloy. The steel mold is preliminarily heated up to 200 ± 10 C and the temperature of the salt preform before fixing into the die is 680 ± 2 C. The

4 Compressive Properties and Energy Absorption Behaviour melt temperature before pouring is 760 ± 10 C and the squeeze pressure of 80 MPa is held for 60 sec. Fig. 1. Optical images of sintered salt preforms S1 (right) and S2 (left) 2.2. Specimens, microstructure characterization and compressive tests The cast specimens are 80 mm in diameter and 60 mm in height and consist of two regions: outer non-porous region and inner porous one, Fig. 2(a). The ageing is carried out for 12 hours at temperature 165 ± 3 C. Porous samples are cut and machined from the central section of the specimens for structure characterization and compressive tests. Then the salt preform is removed by dissolution in boiling water during 2 hours, and so obtained open-cell foam samples are dried for 30 minutes at 100 C after that they are used for structure observation and compressive tests. The microstructure of the materials is studied by the light microscopy PolyvarMet. The average pore size is determined automatically by the microscopy software Olympus MicroImage. The relative density defined by ratio ρ /ρ s is obtained for both open-cell foams, see Table 2. The cylindrical samples for compressive tests, see Fig. 2(b, d), are 12 mm in diameter and 12 mm in height, and the tests are carried out using MTS testing machine at room temperature. Nominally non-porous samples and samples of different porosity are tested. The contacting sample surfaces are preliminarily coated with petrolatum in order to reduce as much as possible friction between the sample and the compressive plates. A constant compressive rate is applied in all tests. This compressive rate provides a nominal strain rate of s 1.

5 48 L. Stanev, B. Drenchev, A. Yotov, R. Lazarova Fig. 2. Optical images of foams obtained: (a) cast specimen with high porous core with 0.61 porosity and dense outer region; (b) test sample of open-cell AlSi10Mg foam with 0.61 porosity; (c) cast specimen with high porous core with 0.71 porosity and dense outer region; (d) test sample of open-cell AlSi10Mg foam with 0.71 porosity 3. Results and discussion 3.1. Microstructure characterization Some micrographs of the cellular materials under consideration in this work can be seen in Fig. 3. The open pores with average diameter 570 µm in S1 samples and average diameter 338 µm in S2 samples, see Table 2, are dispersed homogeneously. The inner cell surfaces are smooth and free of cracks. The cell walls are solid which indicates effective liquid phase processing technology, i.e. the squeeze pressure of 80 MPa guarantees successful preform infiltration and sound solidification. The foams have almost the same cell size as the NaCl particles.

6 Compressive Properties and Energy Absorption Behaviour Fig. 3. Optical micrographs images of open-cell aluminum AlSi10Mg alloy materials with: (a) porosity P = 0.61; (b) porosity P = Compressive behaviour of open-cell foams obtained The compressive stress-strain curves for nominally non-porous (ρ /ρ s = 1) alloy AlSi10Mg and open-cell foams with ρ /ρ s = (porosity 0.61) and ρ /ρ s = (porosity 0.71) from the same alloy can be seen in Fig. 4. In this case, the conventional three regions are formed well, Fig. 4(b). The first region I is the elastic region in which the compressive stress increases steeply and almost linearly with the strain up to ε = The second region II represents the plastic plateau that occurs after yielding, from ε = up to ε = The densification region III where the stress increases sharply with increasing strain can be seen at the very end of the curve above ε = These three sections are typical for each porous material [1]. It should be mentioned that the elasticity appears at very low strain range between 0.00 and 0.02 for both porous structures. In this initial deformation stage the compressive stress increases sharply with increasing strain. The deformation mechanism of open-cell foams is closely related to the pore edges bending [1]. The deformation region II is characterized with a large plateau, where the stress steady increases with the strain because of the strain-harden effect. At this stage the cell edges buckle, and the foam network collapses which leads to reduction of the porosity. The deformation mechanism here is based on the bending and buckling of the cell walls and edges. Therefore, the energy absorption capability of the materials under consideration depends directly on

7 50 L. Stanev, B. Drenchev, A. Yotov, R. Lazarova Fig. 4. Compressive stress-strain curves for AlSi10Mg alloy: (a) nominally nonporous material; (b) open-cell foam with 0.61 and 0.71 porosity the slope, length and height of the plateau. After the final collapse of the cellular network the cell walls contact each other and the stress increases sharply with the stain which is the last section in the stress-strain curves Energy absorption capacity and energy absorption efficiency There are two main quantities used for evaluation of compressive behaviour of all porous materials, namely energy absorption capacity and energy absorption efficiency. The absorption capacity W of a certain porous structure is a function of the strain, and is defined by the integral [1]: W (ε f ) = ε f 0 σ(ε)dε. (1) The energy absorption efficiency E is defined as follows: E (σ f ) = 1 σ f ε f ε f 0 σ(ε)dε. (2) Here ε f is a fixed strain and σ f is the corresponding stress according to the certain stress-strain curve. For all calculations in the present work the final value of the strain ε f is 0.6. This strain is determined on the basis of the stress-strain curve shown in Fig. 4(b) and gives the limit of the plateau region which can be considered equal for both porosity. The state with strain ε > ε f is not interesting from energy

8 Compressive Properties and Energy Absorption Behaviour absorption point of view because beyond ε f is region of intensive compacting and cells destroying. The energy absorption capacity and the energy absorption efficiency of the samples S1 and S2 are shown in Fig. 5. It can be seen that the porosity plays an essential role in energy absorption capacity especially for higher strains during compression. The energy absorption capacity for the material with porosity 0.61 at strain σ = 0.5 is about 44% higher than the energy absorption capacity for the material with porosity Almost the same relation is valid at strain σ = 0.6. Fig. 5. Compressive characteristics of open-cell AlSi10Mg foams: (a) energy absorption capacity W; (b) energy absorption efficiency E The energy absorption efficiency also depends on porosity which can be clearly seen for σ > 0.1, Fig. 5(b). Here again the material with lower porosity possesses better absorption efficiency. In case of S1 sample (P = 0.61) a plateau region appears between σ = 0.06 and σ = 0.2. The absorption efficiency at σ > 0.2 is about 30% greater for the material type S1 (lower porosity) compared to the material type S2. The above discussed relations are typical not only for open-cell foams but also for other porous materials. For example, similar results for materials obtained by sintering of stainless steel fibers and open-cells Zn-alloy based foam can be found in [11] and [12], respectively.

9 52 L. Stanev, B. Drenchev, A. Yotov, R. Lazarova 4. Conclusions The microstructure, the compressive behaviour and the energy absorption properties of the aluminum alloy (AlSi10Mg) open-cell foam are studied. The cellular materials with different porosity are obtained by replication method. Additional thermal treatment is applied to the foams before compressive testing. The microstructure of the foam is described and the compressive stress/strain curves are determined for two materials with different porosity and are compared with the same curve for the nominally nonporous material. The inner cell surfaces are smooth and free of cracks which indicate that the squeeze pressure of 80 MPa guarantees successful preform infiltration and formation of sound solid phase. The compressive stress-strain curves are composed of three different regions: elastic region, stress plateau region and densification region. The energy absorption capacity and the energy absorption efficiency strongly depend on the porosity. The energy absorption capacity for the material with porosity 0.61 at strain σ = 0.5 is about 44% higher than the energy absorption capacity for the material with porosity The same relation is valid at strain σ = 0.6. In case of 0.61 porosity a plateau region appears between σ = 0.06 and σ = 0.2. The absorption efficiency at σ > 0.2 is about 30% greater for the material with 0.61 porosity compared to the material with 0.71 porosity. References [1] L. J. Gibson and M. F. Ashby, Cellular Solids: Structure and Properties, 2 nd edn, Cambridge University Press, Cambrodge (1997). [2] E. Amsterdam, J. Th. M. De Hosson and P. R. Onck, Scripta Mater. (2008) [3] H. Degischer and B. Kriszt, Handbook of Cellular Metals, Wiley VCH, Weinheim (2002). [4] M. F. Ashby, A. G. Evans, N. A. Fleck, L. J. Gibson, J. W. Hutchinson and H. N. G. Wadley, Metal Foams: A Design Guide, Worburn, Butterworth Heinemann (2000). [5] J. Banhart, Progress in Material Science (2001) [6] O. Diologent, E. Combaz, V. Laporte, R. Goodall, L. Weber, F. Duc and A. Mortensen, Scripta Mater. (2009) [7] H. Brothers, R. Scheunemann, J. D. DeFouw and D. C. Dunand, Scripta Mater. (2005) [8] F. Diologent, R. Goodall and A. Mortensen, Acta Mater. (2009) [9] A. Hassani, A. Habibolahzadeh and H. Bafti, Materials and Design (2012) [10] S. Soubielle, F. Diologent, L. Salvo and A. Mortensen, Acta Mater. (2011)

10 Compressive Properties and Energy Absorption Behaviour [11] J. C. Qiao, Z. P. Xi, H. P. Tang, J. Y. Wang and J. L. Zhu, Materials Transactions (2008) 49 (12) [12] S. Yu, J. Liu, M. Wei, Y. Luo, X. Zhu and Y. Liu, Materials and Design (2009) 30 (1) 87 90, doi: /j.matdes Received October 15, 2013