Damping and Strain Response Mechanism of Mg-based Metal Materials

Author:Wang Yun Si

Supervisor:duan dong ping

Database:Doctor

Degree Year:2018

Download:24

Pages:124

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The society has brought more environmental pollution while developing and progressing,and the control of vibration,impact and noise is increasingly becoming a complex and urgent problem to be solved.So the development and research of damping materials will have great significance to social development and personal health.Magnesium has better damping capacity among metal materials.Based on the analysis of the internal structure of metal,the corresponding point defect and line defect model were established.Based on this mechanism,it was inferred that the macro defect and microstructure intervention would inevitably have an important impact on the magnesium-based metal material.Based on this,the preparation method of magnesium-based high damping metal was designed and studied in order to provide reference for the development and research of damping materials.Porous magnesium alloy was prepared by stirring casting method.The relationship between porosity and pore size and the mass fractions of Mg,Al and Sr was investigated.The results indicated that the porosity and the pore diameters increase with an increase of Mg content from 25.0 to 55.0 and then decrease sharply with an increase of Mg content from 55.0 to 70.0 under the condition of[%Sr]=3.0%.Moreover,the porosity and the pore diameters of porous Mg-based ternary alloys increase with an increase of Sr content from 1.0 to 7.0.The formed pores are not in size of round and become abnormal large in Mg-based ternary alloys with[%Sr]>5.0.The mechanism of pore formation in the porous Mg-based ternary alloys can be summarized as that the released heat from oxidization of Sr by O2 in air can promote the vaporization of Mg-based ternary alloys,and then the vaporized Mg-based ternary alloys produce the embodied pores in various sizes.The damping capacity expressed by loss factor is generally independent of temperature from ambient temperature to 70℃,nevertheless,above 70℃loss factor begins to increase rapidly until peak value and sharply decreases.The loss factor is dependent of vibration frequencies and generally decreases as vibration frequencies increase.The specimens with contents of[%Mg]=50%,[%Sr]=3%,and[%Mg]=50%,[%Sr]=5%not only have the highest porosity and but also exhibit both higher beginning and peak of loss factor curves.Open-cell porous magnesium has more superior performance and a wider range of applications than closed-cell porous magnesium.The open-cell porous magnesium was prepared by the Super-gravity method.NaCl particles were squeezed under the centrifugal force,and the density of NaCl particles became larger,so that the porosity of the actual porous magnesium sample was larger than the theoretically calculated porosity.Through testing the compression properties of porous magnesium with different pore sizes,it was observed that the stress decreased with the increase of the pore size under the same strain.By testing the compression properties of open-cell porous magnesium under different gravity coefficients,it was demonstrated that the compressive stress increased slightly as the gravity coefficient increases under the same compressive strain during the compression process.It should specially pointed out that regardless of the pore size and supergravity coefficient,all the open-cell porous magnesium prepared by this method had a longer plateau compression zone,and the strain lags heavily behind the stress.Meanwhile,the platform region exhibited a smooth and slowly rising,which suggested that this porous magnesium sample had good energy absorption and damping properties.At the same time,the porous magnesium prepared by this method did not undergo brittle collapse and fracture during the compression process,indicating that it had a strong tough metal skeleton.In order to investigate the effect of micro-interface on the damping properties of Mg-based metal materials,SiCp/Mg matrix composites with oxides were prepared by stir casting method.Microstructure observation and composition analysis were carried out.It indicated that a network structure with oxide attached was distributed at the grain boundary of the prepared SiCp/Mg composite,and the network structure increased with the increase of SiCp content.And the distribution was more dispersed with the more SiCp content.The damping characteristics results indicated that with the increase of frequency,the damping value decreased,and with the increase of SiCp content,the initial damping value increased.Meanwhile with the increase of the strain,the damping value gradually increased.A peak appeared in the strain range when the SiCp content was 3%and 4%,and when the SiCp content was 4%,the peak value was smaller than 3%when the strain is small.The damping value gradually decreased after crossing the peak,and the higher the frequency,the slower the speed.As the temperature increased,the damping value gradually increased,and the first damping peak appeared at higher temperature as the SiCp content increased.All samples exhibited high damping values over the full strain and full temperature range.Meanwhile,the G-L line of the specimen shew a small range of strain amplitude variation as a straight line,and the other part was a curve.This indicated that the mechanism of damping of the specimen was part of the dislocation damping mechanism,and the other part is the interface sliding damping mechanism.Mg-based composite materials mostly use solid particles,fibers,whiskers and as added phases.In order to prepare a Mg-based composite material with a hollow structure as an additive phase,hollow fly ash floating beads were used as an additive phase,and in-situ generated spherical Mg2Si-MgO magnesium matrix composites were prepared by Super-gravity method.The microstructure observation and analysis of the specimen showed that the Mg matrix contained both spherical Mg2Si-MgO phase and a small amount of finely divided Mg2Si-MgO phase.The damping characteristics illustrated that the damping value increased rapidly with the increase of the strain,and decreased slightly after reaching the maximum value.The damping value increased with the increase of temperature.By the influence of the spherical structure of Mg2Si-MgO in the matrix,the specimens exhibited high damping values in the full strain and full temperature range.Through drawing the G-L line of the specimen,it indicated that the damping mechanism of the Mg-based metal material was also a combination of the dislocation damping mechanism and the interface damping mechanism.