Investigation on Damping Capacities of Spherical Mg2Si Magnesium Matrix Composites Prepared by Super-Gravity Method

Due to inherent high damping capacities of Mg, Mg-based alloys or composites can absorb enormous elastic energy, thus, can be used as damping materials playing important role in reducing vibration and noise. Spherical Mg2Si magnesium matrix composites were prepared by super-gravity method with fly ash floating beads with particle sizes of 80, 125 and 500 μm. The microstructures and damping capacities of prepared samples were discussed. Spherical and fragment-shaped Mg2Si phases were observed in the matrix by SEM micrograph. Increasing particle size of fly ash floating beads can lead to an obviously decreasing tendency of internal friction tan .The critical strain cr  =1.9×10−2 of  – tan curve and critical temperature cr T =150 °C of T – tan curve for tan changing from slowly to greatly increasing. Damping peaks occur in the vicinity of strain  =5.2×10-2 in  – tan curve whereas no damping peaks occur in T – tan curve.


Introduction
Due to inherent high damping capacities of Mg [1] , Mg-based alloys or composites can absorb enormous elastic energy, thus, can be used as damping materials playing important role in reducing vibration and noise.Relatively speaking, Mg-based composite material have better damping capacities due to either an addition of the second phase resulting in much more crystal defects, or inherent high damping capacities of the second phase itself, or absorption of vibration energy by phase interface [2,3] .The common preparation methods of Mg-based composite material are by usage of particles such as SiC, Al2O3, TiCp particles and so on [4,5] .Liao et al. [6] successfully synthesized in-situ Mg2Si/Mg-Al composites with Si particles, and obtained increasing Mg2Si content can effectively increase the damping capacities of the composites.However, little hard research has been done on how the second phase with hollow spatial structure can influence the damping capacities of Mg-based composite material.
In the present work, in-situ spherical Mg2Si magnesium matrix composites were successfully prepared in centrifugal apparatus by super-gravity method with hollow fly ash floating beads in sizes of 80, 125 and 500 μm, whose damping capacities has been investigated under conditions of changing temperature from ambient temperature am T to 300 °C, strain  from 1.0×10 −3 to 1.0×10 −1 .

Preparation of Spherical Mg2Si Magnesium Matrix Composites
Spherical Mg2Si magnesium matrix composites were prepared in centrifugal apparatus in a type of DL-8M equipped with a heating furnace [7] using pure Mg ingot with 99.95 mass % purity and fly ash floating beads with particle sizes of 80, 125 and 500 μm shown in Figure 1..The preparation procedures of spherical Mg2Si magnesium matrix composites shown in Figure 2. can be summarized as follows: (1) Several pieces of magnesium about 30 g were placed into the upper graphite crucible and fly ash floating beads were placed into the lower one, where the upper and lower graphite crucible were tied together tightly as whole special crucible with substantial sieve pores between them [7] .Afterwards, the whole crucible were put in the heating furnace of the centrifugal apparatus; (2) The heating furnace was heated to 660 °C at a heating rate of 10 °C/min, and maintained for 10 min to ensure magnesium completely melted; (3) The whole crucible was rotated at a speed of 1338 r/min for MATEC Web of Conferences 207, 03019 (2018)  https://doi.org/10.1051/matecconf/201820703019ICMMPM 2018 promoting molten magnesium to percolate down through the sieve pores into the space among fly ash floating beads and their hollow space.During rotation process, the heating furnace was held at 660 °C for 1 min and then cooled to 600 °C; (4) The whole crucible including magnesium matrix composites sample was taken out from the heating furnace and then cooled to ambient temperature am T in air atmosphere.

Characterization of Microstructures and Damping Capacities
The microstructures of Mg2Si magnesium matrix composites were determined by scanning electron microscopy (SEM, JSM−5610LV) equipped with energy dispersive X−ray spectrometer (EDS) operating at 20 kV.The dynamic mechanical analyzer (DMA, Q800, TA Instruments, USA) was used in the characterization of damping properties of spherical Mg2Si magnesium matrix composites specimens under forced vibration by the test mode of single cantilever beam in this study.The damping capacities of spherical Mg2Si magnesium matrix composites was measured by the method [8] of tangent phase angle  expressed by tan  .
The measured specimens of prepared spherical Mg2Si magnesium matrix composites were cut into the size of 10 mm×10 mm×10 mm for characterization of microstructures using SEM and 30 mm×10 mm×2 mm for determination of damping capacities using DMA by wire cutting electrical discharge machining (WEDM), and then ground with SiC abrasive paper in size of 800, 1600, and 2000 grits for successively polishing the surface of specimens, polished by silica suspension, and cleansed by distilled water and ethanol.
Two relationships have been studied between strain  and internal friction tan  of spherical Mg2Si magnesium matrix composites specimens at ambient temperature, between temperature T and internal friction tan  at constant strain  .Then the damping properties of spherical Mg2Si magnesium matrix composites specimen have been discussed.
The effect of strain  from 1.0×10 −3 to 1.0×10 −1 on internal friction tan  is determined at vibration frequency f =1.0 Hz.The influence of temperature T from ambient temperature am T to 300 °C on internal friction tan  is under conditions of the heating rate as 5 °C /min, and strain  =7.0×10 −2 .

Microstructures of Mg2Si magnesium matrix composite
The microstructure of the spherical Mg2Si magnesium matrix composite prepared with 125 μm fly ash floating beads is shown in Figure 3..It can be clearly observed that there are spherical phase and fragment-shaped phase in the matrix shown in blue circles.Due to effect of centrifugal force, the molten magnesium can be percolated into hollow space of fly ash floating beads with open pore structure in this study and formed spherical phase.However, some fly ash floating beads with lower strength and hardness were fragmented because of influence of centrifugal force and formed fragment-shaped phase.
Further EDS analysis was performed on the matrix region labeled A and the spherical region labeled B. The point EDS analysis result illustrates chemical compositions of region A are almost pure Mg phase containing traces of Al and O, and that of region B contains considerable Mg and Si with an atomic ratio of Mg:Si of approximately 2:1, which is confirmed as Mg2Si phase.
Due to the fact that main composition of the fly ash floating beads is SiO2, the reaction between Mg and SiO2 at high temperatures can occur: Excess magnesium can continue to react with the generated Si and form Mg2Si: The formation of the Mg2Si phase can be well explained by Eq. ( 1) and Eq. ( 2).

Damping Capacities of Mg2Si magnesium matrix composites
The effect of strain  from 1.0×10 −3 to 1.0×10 −1 on internal friction tan  of Mg2Si magnesium matrix composites prepared by 80, 125, and 500 μm fly ash floating beads under the condition of vibration frequency f =1.0 Hz shown in Figure 4.It can be evidently observed that damping capacities of three specimens have similar tendency with an increase of strain  .This implies that the internal friction tan  of three samples increase slowly with strain  increasing in a range of 1.0×10 −3 to 1.9×10 −2 ; however, the internal frictions tan  exhibit a greatly increasing trend with an increase of strain  from 1.9×10 −2 to 1.0×10 −1 and reach the maximum value of tan  followed by a slight decline.Moreover, three specimens have higher damping capacities in the full strain range and reach the damping peaks in the vicinity of strain  =5.2×10 -2 , which can be attributed to the spherical structure of Mg2Si in the matrix.It also can be observed that increasing particle size of fly ash floating beads can lead to an obviously decreasing tendency of internal friction tan  .It can be ascribed to the fact that the larger the particle size of fly ash floating beads with hollow structures, the less Mg2Si phase formed resulting in lower dislocation density, less interface area and smaller damping value.The influence of temperature T from ambient temperature to 300 °C on internal friction tan  of Mg2Si magnesium matrix composites prepared by 80, 125, and 500 μm fly ash floating beads under conditions of vibration frequency f =1.0 Hz and strain  =7.0×10 −2 is shown in Figure 5.It can be obviously observed that three specimens of magnesium matrix composites have higher damping capacities in whole temperature range especially tan  =0.09 of the specimen with a particle size of 80 μm at am T .In addition, the internal friction tan  of three samples increase slowly with temperature T increasing from am T to 150 °C; however, the internal frictions tan  exhibit a greatly increasing trend with an increase of temperature T from 150 °C to 300 °C but no damping peaks occur.It also can be observed that increasing particle size of fly ash floating beads can lead to an obviously decreasing tendency of internal friction tan  due to aforementioned reasons.

Conclusions
According to the experimental results of microstructure and damping capacities expressed by internal friction tan  of spherical Mg2Si magnesium matrix composite and related discussion, the main summary remarks can be obtained as follows: (a) Spherical and fragment-shaped Mg2Si phases were observed in the matrix, where fragment-shaped phases were formed owing to the reason that some fly ash floating beads with lower strength and hardness were fragmented because of influence of centrifugal force.(b) Increasing particle size of fly ash floating beads can lead to an obviously decreasing tendency of internal friction tan  ascribed to the fact that the larger the particle size of fly ash floating beads with hollow structures, the less Mg2Si phase formed resulting in lower dislocation density,less interface area and smaller damping value.(c) The critical strain cr  =1.9×10 −2 of  -tan  curve and critical temperature cr T =150 °C of Ttan  curve for tan  changing from slowly to greatly increasing.Damping peaks occur in the vicinity of strain  =5.2×10 -2 in  -tan  curve whereas no damping peaks occur in Ttan  curve.Spherical Mg2Si magnesium matrix composite have high damping capacities in the full strain and temperature range possibly ascribed to spherical structure of Mg2Si in the matrix.

Figure 1 .Figure 2 .
Figure 1.SEM micrograph of fly ash floating beads with particle sizes of 125 μm

Figure 3 .
Figure 3. SEM micrograph of spherical Mg2Si magnesium matrix composites and the corresponding point EDS spectra (a) area A, (b) area B

Figure 4 .
Figure 4. Effect of strain  on damping capacities tan  of spherical Mg2Si magnesium matrix composites specimens at vibration frequency f =1.0 Hz

MATECFigure 5 .
Figure 5.Effect of temperature T on damping capacities tan  of spherical Mg2Si magnesium matrix composites specimens at vibration frequency f =1.0 Hz and strain  =7.0×10 −2