Functionally graded (FG) porous aluminum (Al), consisting of a high porosity layer and low porosity layer in a single porous Al, was fabricated by a friction stir welding (FSW). From X-ray computed tomography (CT) observations of the pore structures of fabricated FG porous Al, it was found that the fabricated FG porous Al had two layers with high porosity of large pore diameter layer and low porosity of small pore diameter layer. From compression tests of the fabricated FG porous Al, the deformation started from the high porosity layer then spread to the low porosity layer. The stress–strain curves obtained in the dynamic compression tests revealed that the stress of the plateau region was lower during the deformation of the high porosity layer than that during the deformation of the low porosity layer. The stresses and absorbed energies in those two plateau regions during such deformations were almost the same as those of the uniform porous Al.
The plateau stress of aluminum (Al) foam is an important mechanical parameter that is closely related to its energy absorptivity. An estimation method for plateau stress was proposed by simply assuming that when the mean true compressive stress on a maximum-porosity cross section perpendicular to the direction of compressive loading reaches the critical value, the nominal compressive stress becomes equal to the plateau stress. In this study, using commercial-purity A1050 Al, A6061 Al alloy and ADC12 Al alloy die castings containing a large amount of gases, uniform Al foam and two types of functionally graded (FG) Al foam were fabricated by the friction stir welding (FSW) route precursor process. The local porosity distributions for the fabricated Al foams were obtained from the observation of X-ray computed tomography (CT) images, then drop weight impact tests were carried out to evaluate the plateau stresses. Through the comparison of the test results with the results obtained by the estimation method, the applicability of the estimation method for plateau stress to impact compression tests on uniform aluminum Al foam and each foam layer of FG Al foam was examined. It was shown that the plateau stresses can be evaluated approximately within ±20% error when the proof stress is employed as the critical value.
A spacecraft requires lightweight power generation system. We are developing a lightweight flexible power generation system that uses thin film photovoltaic cells for a planetary exploration spacecraft. Because thin film solar cell on polyimide film has asymmetric and multi-layer structure, it will bend with changes in temperature in the space environment. To make ultra-lightweight flat thin film solar array which can be used in a space environment, we study about shape control by surface coating utilizing oxide materials on the solar array and durability against space environment. We describe experimental results in order to calculate the internal stress of the coating layer, predict the shape of the coated films and confirm the durability against space environment of the coated thin film solar cells.