Measuring the cell wall mechanical properties of Al-alloy foams using the nanoindentation method

Al-foam is a super light weight material, promising for reinforcing thin-walled structural members in diverse engineering fields including aerospace and automobile industries. Measuring the material and mechanical properties (e.g. hardness and Young’s modulus of elasticity) of the cell wall of an Al-foam is a challenging task, as there are no standard methods available for such measurements. Nanoindentation is a method used for measuring the mechanical properties of thin films and surface layers, which is believed to be applicable for measuring the cell wall mechanical properties of Al-foams as well. However, there has been no detail investigation on applicability of nanoindentation measurements to cellular materials. In this paper, the nanoindentation technique has been used for measuring the cell wall mechanical properties of Al–Si–Cu–Mg alloy foams. The foams were produced using the powder metallurgical method. Hardness and elastic modulus of the specimens were evaluated using both the continuous stiffness measurement (CSM) and the unloading stiffness measurement (USM) method. Uni-axial tensile tests were performed on the precursor materials, and Vickers hardness of the specimens was measured. The experimental results revealed that the hardness and the elastic modulus of both the Al–3% (wt.)Si–2%Cu–2%Mg and the Al–5% (wt.)Si–4%Cu–4%Mg alloy foam cell walls are higher than those of their respective precursor materials. The elastic modulus obtained using the nanoindentation test was lower than that of the uni-axial tensile test. The hardness obtained using the nanoindentation method, however, was higher than the corresponding micro Vickers hardness. The average hardness of the foam cell wall was found to increase with the increase of foaming time regardless of the foaming temperatures and the alloy compositions.