| Heat-resistant magnesium alloys can be divided into two categories due to their different application fields: one is the low-cost aluminum-containing heat-resistant magnesium alloys for the automotive field;the other one is the high-performance rare earth heat-resistant magnesium alloys for the aerospace and aviation field.The long-term use of temperature of the aluminum-containing heatresistant magnesium alloys is between the temperature of 120℃ and 200℃.Among them,the AlSi series is the represent.The development time of such series of alloys is early and the application range is relatively wide.The service temperature of rare earth heat-resistant magnesium alloy is often above 200℃,and the main kind of rare earth heat-resistant magnesium alloy is the heat-resistant magnesium alloy containing LPSO structure.The WZ73 magnesium alloy is an alloy that was found earlier to contain LPSO structure.Since the WZ73 alloy has very high strength under rapid solidification conditions,this kind of alloy has attracted widespread attention.At present,the research on friction and wear of magnesium alloys is relatively limited,and there are few related reports.However,heat-resistant magnesium alloys contain a large amount of high melting point phases and have good thermal stability,and strength and toughness.Thus,more research about the influence of these properties on friction and wear behavior is needed.In addition to the matrix,the friction layer also has an important influence on the friction and wear properties of the material.The friction layers include surface and subsurface layers.The main friction phenomena all occur on the surface.The friction surface is an open,discrete and complex system containing local extreme conditions.At present,most of the tribological research is carried out on the friction surface.The surface layer is a thin layer close to the surface,where friction-induced microstructure evolution usually occurs.The mild to severe wear transition of magnesium alloys is often caused by the softening of the surface layer.The subsurface layer is the transition layer between the matrix and the surface layer,which has received the least attention at present.In the subsurface layer,there is mainly the deformed structure,which belongs to the category of metal thermal deformation.The strain energy of the deformation layer comes from the plastic work of the surface.At the same time,the properties of the deformation layer will also affect the wear mechanism of the surface.Therefore,how to establish a link between the two is an important subject.The frictioninduced deformation layer can be regarded as the “panorama” of the deformed structure of the material,showing the entire process of the material from being slightly deformed to severely deformed in a very thin deformation area.Due to the high degree of inhomogeneity of deformation,the study of the deformation layer needs to establish a complete constitutive relationship σ=F(ε,(?),T) and calculate the overall stress,strain and energy distribution of the deformation layer.This dissertation has studied the deformation layer and the entire friction system through establishing a steady-state wear model,searching for an energy balance equation and using energy.The main research contents and results of this dissertation are shown as follows:(1)The friction and wear behavior of the AS31 alloy has been studied by a pin-on-disk wear machine.The mild wear mechanisms of the AS31 alloy include: oxidation + abrasive wear,delamination wear and delamination wear accompanied by severe surface oxidation;the severe wear mechanisms include: severe plastic deformation and surface melting.When sliding at low speed(0.1 m/s)with the load of lower than 60 N,the wear mechanism is a mixed mode of wear + oxidation;when the load is higher than 60 N,the wear mechanism is changed into delamination wear.When sliding at intermediate speed(0.5-1.5 m/s)with a low load,the wear mechanism is a mixed mode of wear and oxidation;when the applied load is intermediate and high,the wear mechanism is delamination wear with severe surface oxidation.When sliding at high speed(2.0-4.0 m/s),the wear mechanism is mainly the severe plastic deformation and surface melting.When changing from mild wear to severe wear,the surface structure of the material changes from a deformed structure to a recrystallized structure,and recrystallization makes the hardness of the worn surface drop sharply.Whether the surface temperature of the AS31 alloy exceeds the recrystallization transition temperature determines whether the alloy can undergo a mild to severe wear transition.Under the existing experimental conditions,the critical temperature range of the AS31 alloy wear transition is between 293℃ and 326℃.As the sliding rate increases,the transition load from mild to severe wear of the AS31 alloy decreases rapidly.When the sliding rate is 0.8 m/s,the transition load is 160 N;when the sliding rate increases to 4.0 m/s,the transition load decreases to 40 N.(2)The friction and wear behavior of the WZ73 alloy has been studied by a pin-on-disk wear machine.The mild wear mechanisms of the WZ73 alloy include: oxidation + abrasive wear,exfoliative wear and severe oxidation and exfoliative wear.Severe wear mechanisms include: severe plastic deformation,severe plastic deformation + spallation of oxidation layer,and surface melting.Dynamic recrystallization is also the cause of the softening of the friction surface of the WZ73 alloy,and then the cause of transition from mild to severe wear.Although the Mg12 Zn Y long strip phase in the WZ73 alloy undergoes high temperature and severe deformation,it still maintains its structural integrity and has a similar fiber-reinforced effect.Therefore,the WZ73 alloy has better wear resistance.The critical temperature range of the WZ73 alloy wear transition is between 311℃ and 340℃.The change law of the transition load caused by the WZ73 alloy’s mild to severe wear is similar to that of the AS31 alloy.(3)The incremental constitutive relationship of the WZ73 alloy that was established by the elastoplastic transformation model.Through the elastoplastic transformation model,the elastic deformation and plastic deformation in the metal deformation process were decoupled.The incrementtype constitutive equation was established by using the Arrhenius type hyperbolic sine equation.By defining the elastoplastic factor,the nonlinear stress curve was transformed into a linear problem.By defining the traces,the broken lines of the elastoplastic factors of the metal deformation under different processing conditions were ordered.Based on WZ73 alloy,a complete increment-type constitutive relationship was established,that is,a set of relationships containing four variables at the same time.According to the specific experimental conditions(temperature and strain rate),the constitutive relationship was used to solve the stress-strain curve of the alloy.According to the stress value of the stress-strain curve when the ε= 1,it can be judged whether the sample is broken,so that a fracture diagram for judging the boundary conditions of the thermal processing process can be established.(4)An energy balance equation based on the characteristics of stable wear was established in this dissertation.Since the friction layer of the WZ73 alloy has a clear deformation structure,the deformation layer of the WZ73 alloy has been chosen as the research object of this dissertation.The strain model of the deformation layer was established and the plastic work in friction process was calculated by combining the constitutive relationship of the WZ73 alloy and the balance equation of stable wear.The plastic work of the WZ73 alloy only accounted for 0.1-10‰ of the friction work,indicating that the material transferred most of the energy into friction heat through the surface wear mechanism,which protected the matrix.Through the comparison of calculated data and experimental data,the relationship between plastic work,friction work and wear rate were obtained.Based on this,two formulas were derived,and the theoretical value of friction surface strain was calculated. |
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