Effects of macroscopic pores on the damping behavior of foamed commercially pure aluminum

Abstract

Experiments have been carried out to study the effects of macroscopic pores on the damping behavior of foamed commercially pure aluminum (FA). The damping characterization was conducted on a multifunction internal friction apparatus (MFIFA). The FA specimens were prepared by an air pressure infiltration process. The size of the macroscopic pores was on the order of a millimeter and in large proportions, typically, up to 56 vol pct. The internal friction (IF) of the FA specimen was measured at low frequencies at room temperature. The measured IF shows that FA has a damping capacity, which is enhanced in comparison with bulk commercially pure aluminum, increases with increasing porosity, increases with decreasing macropore size, increases with increasing frequency, and increases with increasing strain amplitude. The microstructural analysis was performed using transmission electron microscopy (TEM). The TEM observations show that a moderate density of dislocation substructures exists near the grain boundaries in the FA. In order to explain these phenomena, the possible operative damping mechanisms in the FA are discussed in light of IF measurements and microstructural observations, and an approximate expression for IF is derived, which is based on the equation of plane waves in elastic material with voids.

Effects of Pore Characteristics Finely-Controlled by Spacer Method on Damping Capacity of Porous Aluminum

“…Amplitudedependent damping in porous metals and alloys is often related to dislocation-related damping. [3][4][5][6][7][8][9] If the strain amplitude of the specimen is sufficiently low, the dislocation line is held by weak pinners and undergoes damped motion. 23) This type of dislocation damping is amplitudeindependent.…”

“…This trend agreed with the results in previous works. [3][4][5][6][7][8][9] Also, specimen F with a residual porosity of 13.4% showed a higher damping capacity than dense Al when measured under similar testing conditions. 6) It has been pointed out that the amplitude of porous Al is inversely proportional to the pore size, [3][4][5][6] although a very weak sensitivity of pore size dependence has also been reported for porous steel.…”

“…1) Recently, it has been reported that the damping capacities of porous metals are enhanced, compared with those of dense materials. [2][3][4][5][6][7][8][9][10] Although pure Al or Al alloy is essentially a low damping material, [11][12][13] porous Al is of considerable practical interest as a damping material in severe conditions because of its lightweight and good corrosion resistance as well as its excellent damping capacity. In general, pore characteristics, such as porosity, pore size and pore shape, of the porous structure influence the damping capacity of cellular metallic materials.…”

“…However, there are some discrepancies in the dependence of the damping capacity on the pore characteristics. [3][4][5][6][7][8][9] Past relevant studies have mostly focused on the mechanical damping of foamed Al. [3][4][5]7) Foamed Al often suffers from heterogeneous pore characteristics because control of bubble expansion in liquid or semisolid metal is difficult.…”

“…In addition, the porous Al in the previous studies often had pores as large as millimeters. [3][4][5][6][7][8] The spacer method (or replication method), which has been employed since the 1960s, is one of the methods for fabricating porous Al. [14][15][16][17][18][19] Features of the spacer method are: (1) strict control of the porosity and pore size, and (2) distribution of homogeneous pores and small pores.…”