Effects of content and particle size of Si crystal on damping property of powder forged Al-Si alloy

The effects of the Si content and Si crystalline size on the damping properties of powder forged (PF) Al-Si alloys has been investigated using a fast Fourier transformation (FFT) analyser. A quantitative image analysis method was used successfully to determine the mean diameter and distribution of Si crystals in the aluminium alloy. The damping capacity greatly depends on the mean diameter of Si crystals. The value of delta for heat treated specimens with Si crystals of 2-5 (mu)m is 4 to 25 times less than that for the as forged specimens with Si crystals of 1-5 (mu)m. This indicates that the vibration is absorbed at the interface between the Si particles and the aluminium matrix by a mechanism of internal friction. Therefore, a fine distribution of Si crystals and/or elimination of crystal coarsening should contribute to improved damping.