| Magnesium alloy is the lightest metal structural material,which possesses many outstanding advantages,such as low density,high specific strength and specific rigidity,strong electromagnetic shielding ability and good recyclability.At present,magnesium alloys are mainly used as weight-saving structural materials in automotive applications,in order to reduce vehicle weight and reduce energy consumption.In the research and application of magnesium alloys,it is found that it also has excellent damping capacity,so,it is a typical weight-saving green material which integrates structure and vibration damping functions.Magnesium alloys are used in automobile manufacturing,which not only helps to reduce energy consumption and vibration noise,but also improves the comfortable and safety of automobile,it has been regarded as one of the most promising metals in automobile and aerospace applications.However,because people's understanding of damping capacity and damping mechanism of magnesium alloys and their applications still remain on a qualitative level,there are some key problems,which still cannot be reasonably explained.So,it leads to difficult to control,utilize,and tap the damping capacity and vibration damping property of magnesium alloys and magnesium alloys parts,it causes inefficient use and a huge waste of magnesium alloys.Funded by National Natural Science Foundation of China “Research on Damping Mechanical Behavior and Vibration Damping Characteristics of Magnesium Alloys”(No.51175533).Based on the research status of dislocation damping mechanism of magnesium alloys,its application in internal combustion engine parts,and theirs existing problems,this dissertation focuses on the dislocation damping capacity and dislocation damping mechanism of magnesium alloys used in automotive internal combustion engine,the vibration damping property of magnesium alloys parts,and the advantage of vibration damping property of magnesium alloys parts,the main work of this dissertation is as follows:(1)The damping capacity of AZ61 magnesium alloy,AZ91 D magnesium alloy and ADC12 aluminum alloy are researched with Dynamic Mechanical Analysis(DMA)and impulse response decay method(IRDM),the damping capacity-strain amplitude spectra and damping capacity-frequency spectra of magnesium alloys and aluminum alloys are obtained,and the advantage of damping capacity of magnesium alloys are quantitatively determined,in the average state,the damping capacity of AZ91 D is 2.3 times as ADC12,it determines the damping parameter value in finite element simulation study,which is used to study the vibration response characteristics of the AZ91 D valve cover and ADC12 valve cover.At the same time,the damping capacity characteristics of magnesium alloys,the important of the damping capacity transition ranges and their corresponding stress and strain amplitudes stages are determined,it determines the important basis for the study of the dynamic behavior evolution of dislocations and the mechanical model of dislocation damping of magnesium alloys.(2)An observation of the micro-dynamic behavior evolution of dislocations caused by shear stress in magnesium alloys is studied,and it is employed to study dislocation damping capacity and establish dislocation damping mechanism theoretical model of magnesium alloys.The research process is as follows,firstly,based on the magnitudes of the cyclic shear stress in the damping test specimen of magnesium alloy during damping capacity test and in the rectangular beam during cyclic bending deformation,by comparing damping test data and dislocation observations under the same shear stress amplitude,the direct relationship between the important transition region of the damping capacity of magnesium alloys and the dynamic behavior evolution of dislocations is determined.Secondly,based on the mechanism of change of dislocation number and dislocation morphology in the stress state,the law of internal energy dissipation in magnesium alloys caused by the microscopic dynamic evolution of dislocations is studied.Finally,combined with the G-L dislocation damping model,the complete mechanical model of the dislocation damping mechanism of magnesium alloys is constructed.It can clearly explain that magnesium alloys have better damping capacity and the changing rules of the damping capacity of magnesium alloys,especially the damping capacity under large strainamplitude conditions.(3)Because the AZ91 D magnesium alloy is used to directly replace ADC12 aluminum alloy in manufacturing engine valve cover,the finite element model of the cover is established,and statics and vibration response simulation calculations are studied.Firstly,the finite element model of the valve cover is verified by the means of computational modal analysis and experimental modal analysis.Secondly,the simulating calculation of the change of cover stiffness after AZ91 D magnesium alloy direct replacement of ADC12 aluminum alloy is carried out,the availability of magnesium alloy cover is determined.Finally,the vibration responses of AZ91 D magnesium alloy cover and ADC12 aluminum alloy cover by frequency response analysis method are computed and compared,the results show there is no significant difference in the vibration response in the low and medium frequency bands.But the AZ91 D magnesium alloy cover obviously has better vibration damping property in the frequency band of above 1500Hz-1700 Hz,the damping capacity advantage of the AZ91 D magnesium alloy cover has been fully utilized in this frequency band is responsible for it.(4)The vibration response of AZ91 D magnesium alloy valve cover and ADC12 aluminum alloy valve cover are tested and compared by using engine bench test method.The results show the advantages of vibration damping property AZ91 D cover are mainly reflected in the engine operating conditions which is greater than 1800r/min,and as the engine speed increases,the advantage increases.When the engine speed is 4000r/min,the root mean square value(effective value)of acceleration of the measurement point 1 on the AZ91 D cover is 84% of the root mean square value of acceleration of the measurement point 1 on the ADC12 cover.The contrast and analysis of frequency spectrum of acceleration signal show the advantage of vibration damping property of AZ91 D cover mainly concentrates in the band from 1700Hz-2500 Hz when the engine speed is greater than 1800 rmp,and it is the direct reason of that the advantage of vibration damping property of AZ91 D cover gradually increases as the increase of engine speed.The finite element simulation results are verified with the measurement datas,it shows that the finite element model of the valve cover and related calculation parameter settings are reasonable,the simulation results can reflect the actual dynamic response characteristics of the two kinds of covers.Combining theoretical analysis conclusions,finite element calculation results,and measurement results,it shows that under large excitation loads and in mid-high response frequency bands,the damping capacity advantages of magnesium alloy can be more fully exerted,thereby significantly controlling the resonance of each resonance point in the middle and high frequency band,this is the fundamental reason of the AZ91 D magnesium alloy cover has better vibration damping property.The study shows that the AZ91 D magnesium alloy and AZ61 magnesium alloy have excellent damping capacity,and the dynamic behavior and evolution of the internal high-density mobile dislocations under cyclic shear stress causes the high damping capacity of magnesium alloys.Based on the simulation results and measurement datas,it is quantitatively determined that the vibration damping property of the AZ91 D cover is better than the ADC12 cover,it provides basic data supporting for it used in manufacturing engine parts,and it is helpful in promoting magnesium alloy used in automobile manufacturing in wider range and higher quality. |
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