Research On Damping Properties And Mechanism Of Austempered Ductile Iron After Two Step Austempering Heat Treatment

With the vigorous development of automobile industry,vibration and noise have become important technical indicators to evaluate automobile quality.The research shows that 30%of the vehicle noise comes from the gearbox,so how to reduce the gearbox noise has become one of the key points of vehicle vibration and noise reduction research.The noise of gearbox is mainly caused by the low meshing quality of gearbox gears and the low damping properties of gear materials.Therefore,finding alternative materials for transmission parts with high damping has become the key to the vibration reduction of gearbox.At present,transmission gears are mostly made of low alloy steel.Austempered ductile iron（ADI）is a kind of ductile iron with acicular ferrite and austenite structure as the matrix after austempering processing.Due to its special microstructure,there are a large number of interfaces between graphite and matrix and between different phases of matrix,and the matrix also contains a large number of dislocations.These crystal defects can consume a lot of energy in the process of vibration,resulting in good damping capacity.In addition,compared with low alloy steel,ADI also has the characteristics of low specific gravity（7.2 g/cm³）,high fatigue strength（480 MPa）and low cost.ADI can meet the requirements of safety,lightweight,low cost and comfort required by automobile components at the same time.It is an ideal material to replace alloy steel to manufacture transmission gears.However,at present,the damping capacity of ADI is rarely reported,and its damping mechanism has not been systematically studied and explained.Therefore,systematically studying the damping properties of ADI and revealing its mechanism is an important issue to be solved urgently,which has important academic and application values.In this study,ADI samples with different graphite morphology,quantity and matrix structure were obtained by adjusting the amount of spheroidizing agent,and casting wall thickness and heat treatment process parameters,and ADI samples with different microstructure and stress distribution were obtained by adjusting the shot peening process parameters.The effects of graphite morphology and quantity,matrix structure and shot peening on the mechanical and damping properties of ADI were systematically studied,and the damping mechanism of ADI was revealed.At the same time,based on the fractional derivative model,the mathematical model of ADI damping performance about temperature,frequency and amplitude was established,the empirical formula of ADI damping performance about the above variables was obtained,and the prediction of ADI damping performance is realized.By adjusting the amount of nodulizer and casting thickness,casting billets with different graphite morphology and quantity was obtained,and the influence of graphite morphology and quantity on the damping properties of ADI was studied.The results show that the shape of graphite has a great influence on the damping properties,while the number of graphite balls has a relatively small influence.The effect of graphite morphology on the damping properties is as follows:with the graphite morphology changing from vermicular to flocculent and finally to spherical,the damping properties of ADI decrease gradually.The maximum values of strain damping,frequency damping and temperature damping decreased from 0.042,0.157 and 0.014 to 0.025,0.143 and 0.013,respectively.The effect of the number of graphite balls on the damping properties is as follows:the damping properties of ADI increases with the increase of the number of graphite balls.The maximum values of strain damping,frequency damping and temperature damping increased from 0.018,0.302 and 0.015（34 graphite spheres/mm²）to 0.020,0.317 and 0.018（58 graphite spheres/mm²）,respectively.For ADI with different graphite morphology and quantity,the variation law of damping properties with strain amplitude,frequency and temperature is basically the same.The damping properties of ADI increases with the increase of strain amplitude and fluctuates with the increase of frequency,but the increase is small.At the same time,the resonance peaks appear at a specific frequency,and the frequencies are not affected by the morphology and quantity of graphite.The damping properties of ADI first increases and then decreases with the increase of temperature,and a damping peak appears near 210℃.The damping mechanism includes dislocation damping,intrinsic damping and interface damping.Dislocation damping conforms to G-L theory.The frequency response mechanism of ADI is intrinsic damping and interface damping,and its intrinsic damping can better meet the ROM law.The frequency response mechanism of ADI is intrinsic damping and interface damping,and its intrinsic damping can better meet the ROM law.The temperature response mechanism of ADI is dislocation damping and interface damping.When 50℃<T<170℃,dislocation damping is dominant.When 170℃<T<230℃,the proportion of dislocation damping decreases,the proportion of interface damping increases,and a damping peak appears at 210℃.When 230℃<T<300℃,the interfacial damping and dislocation damping are greatly reduced.ADI with different matrix structure was obtained by adjusting the austempering process parameters,and the influence of matrix structure on the damping properties of ADI was studied.The results show that with the change of austempering temperature and time,the content and morphology of ferrite and retained austenite in the matrix change,accompanied by the formation of martensite and Fe₃C.For ADI with different matrix structure,its damping performance is mainly related to the phase boundary（ferrite/austenite,martensite/austenite）and grain boundary area,also the movable dislocation density in the matrix.With the increase of interface area and movable dislocation density in the matrix,the damping properties of ADI increase.At the same time,the variation law of its damping performance with strain amplitude,frequency and temperature is basically the same,and the variation trend is similar to that of graphite.ADI with different microstructure and stress distribution were obtained by adjusting the shot peening time,and the effect of shot peening on its damping properties was studied.The results show that compared with that before shot peening,the damping properties of shot peened samples decreases in varying degrees,which is mainly related to the damage of graphite caused by shot peening.The contribution of graphite in ADI to damping properties is greater than that of matrix.With the extension of shot peening time,retained austenite transforms into martensite through trip effect.While the content of retained austenite decreases,the interface area and grain boundary area of martensite/austenite phase increase significantly,and the interface damping increases.However,due to the destruction of graphite in ADI,the graphite/matrix interface area is greatly reduced,resulting in the reduction of damping properties of ADI.Based on the fractional derivative model,the damping loss factor of temperature is divided into two parts,which are only related to frequency and frequency/temperature.At the same time,the damping loss factor related to amplitude is expressed by logarithmic function,and the mathematical model of damping loss factor about the above variables is established.On this basis,the structure factor is introduced to modify the above model and fitted with the test curves.Finally,the empirical formulas of ADI damping loss factor about frequency,temperature and strain amplitude are obtained.