1 Introduction In the past few years, there has been a considerable increase in using metal foams for lightmass structural components and energy absorption parts for their wide plateau in the compressive stress-strain curve<1-3>. It has been shown that, e.g. the compression strength is connected to the density of a foam, thus allowing to adjust this property within a certain range. However, because density cannot always be varied freely and in order to gain more control over the properties of metallic foams, adjustment of other variables seems desirable, namely of alloy composition, foam morphology (size and shape of cells) and the metallurgical state of the matrix metal<4-10>. There have been some results on the heat treatment of metal foams published in the past, almost all of them focused on the properties of foams under quasi-static loading or cyclic loading<11-14>. However, there have been no prior efforts to examine the effects of heat treatment on the dynamic behavior of open cell aluminum foams. The purpose of this study is to evaluate the influence of heat treatment on the dynamic compressive properties and energy absorption characteristics of two kinds of open-cell aluminum alloy foams. This will be beneficial to the multi-functional applications of aluminum alloy foams in industry. 2 Experimental 2.1 Sample preparation Cellular solids can be produced by various methods. Two kinds of open-cell aluminum foams produced by infiltrating process were used in this study. The compositions (analyzed using ICP-AES) of these foams are listed in Table 1. Fig.1 shows the microstructures of these foams observed using LEO438VP SEM. All test samples were electro- discharge machined from blocks of aluminum alloy foams. Heat treatments of samples were carried out in two different ways. One was followed a conventional precipitation hardening cycle which consisted of three steps, namely solution heat treatment, quenching and ageing. The other consisted of only one step of ageing. In our study the foams were solution heat treated at 515
℃ for 30 min and quenched in water at room tem- perature. Warm ageing was chosen and done at 160 ℃ for 16 h.160CAO Xiao-qing, et al/Trans. Nonferrous Met. Soc. China 16(2006) 159-163 Table 1 Chemical analysis of foamed aluminum alloys (mass fraction, %) Alloy Mg Si Cu Fe Mn Ca Al-Mg-Si alloy 0.48 1.77 0.17 0.34 0.23 0.35 Al-Cu-Mg alloy 0.12 0.67 0.45 0.29 0.05 0.02 Fig.1 Microstructures of aluminum alloy foams used in this study in as-fabricated condition: (a) Al-Mg-Si alloy foam; (b) Al- Cu-Mg alloy foam 2.2 Compression test The split Hopkinson pressure bar(SHPB) was used for dynamic compression test of aluminum alloy foams for strain rates above several hundred per second. The samples were 35 mm in diameter and 6 mm in height for dynamic compressive tests. Using thin specimen was to obtain high strain rate. The apparatus was mainly composed of a gas gun, a striker bar, an incident pressure bar, a transmitter bar, a momentum trap and the data acquisition system (Fig.2). The diameters of the striker, incident and transmitter bars were 37 mm, and their lengths were 800 mm, 2000 mm and 2000 mm, respectively. The striker bar, accelerated by the gas gun, hit the incident bar and generated a compressive pulse. As the wave reached the end of the incident bar, a portion of the incident pulse was reflected back and a portion was transmitted through the specimen to the transmitter bar. The reflected wave traveled back as a tensile wave. The incident stress wave generated in the incident bar was recorded by the resistance strain gauge attached at the incident bar. A semiconductor strain gauge attached at the transmitter bar recorded the portion of wave that has transmitted the specimen, and at the same time, a strain gauge attached at the incident bar recorded the reflect wave. All the bars were made of aluminum alloy. 3 Results and discussion The compressive stress-strain curves of aluminum foams under high strai
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