Effects Of Graphite Types And Carbon Content On Thermal Fatigue And Wear Properties Of Bionic Laser-processed Gray Cast Iron

The working environment of the brake drum is very harsh,and it will also undergo repeated heating and cooling during the frequent braking friction process,which would cause the material to expand and shrink in the temperature gradient.The cyclic stress and strain are formed by the material itself,and the thermal fatigue crack is formed on the brake surface.When the wear of the brake drum reaches a certain extent,the effectiveness of the brake will be affected,and the propagation of the thermal fatigue crack will eventually lead to the rupture of the brake drum.Therefore,improving the thermal fatigue resistance and wear resistance of brake drum material is the most important part of improving the life and safety performance of brake drum.Gray cast iron is the main material for the production of brake drums due to its low price,excellent casting performance,easy cutting,good shock absorption and wear resistance.Therefore,improving the thermal fatigue resistance of gray iron and the wear resistance after thermal fatigue are the core of improving the service life of the brake drum.Investigating the gray cast iron,it is found that it has a unique charm due to the very characteristic of the graphite and the existence of the representative phase.the graphite is a double-edged sword,on one hand,the gray cast iron has the advantages of good damping property,thermal conductivity,wear resistance;On the other hand,graphite exists as a soft phase in the matrix,similar to hole defects.During the working process,the internal stress is concentrated at the tip of the graphite,which induces microcracks and causes damage to the material.In the thermal fatigue test of gray cast iron,with the increase of the number and size of the graphite,the thermal conductivity of the gray cast iron is better,but at the same time,the splitting effect on the matrix of gray cast iron is getting stronger and stronger.Graphite is the initiation source of the thermal fatigue crack,and the initiation and expansion of the crack are closely related to the size,quantity and distribution of the graphite.When the gray cast iron is cracked during the thermal fatigue process,the material structure is changed,which directly affects the change of the wear resistance of the material.Therefore,in the study of the effect of graphite on the thermal fatigue resistance of gray cast iron,it is also necessary to study the wear resistance of the graphite cast iron at the same time.Due to the difference of solidification conditions and carbon content,the graphite type and graphite size in gray cast iron are greatly different.In order to study the thermal fatigue properties of laser bionic gray cast iron and the wear performance after thermal fatigue,after determining the laser bionic model as the net + crack arresting ring,the effects of A-E graphite on the thermal fatigue properties of gray cast iron were studied and the modes of crack initiation and propagation were analyzed.In addition,in order to improve the thermal fatigue properties of gray cast iron,the anti-fatigue mechanism of laser bionic structure in gray cast iron materials was studied in depth.The thermal fatigue properties are measured by the number,length and width of the thermal fatigue cracks.Through the study of this part,we found that A-type graphite has good thermal fatigue resistance.And on the non-bionic surface,the coarse C-graphite and dense D-type graphite are more likely to increase the number of cracks.The cracks are most likely to expansion between dendrites,so the D-type graphite gray cast iron has the longest thermal fatigue crack.The material containing E-type graphite has the deepest crack in the late stage of thermal fatigue.The bionic surface has strong anti-crack initiation ability and hindrance to crack propagation.The most obvious improvement in thermal fatigue resistance is the gray cast iron with D-type graphite,while the bionic surface has the worst effect on the gray cast iron with coarse C-type graphite.The thermal fatigue resistance of laser bionic gray cast iron with graphite type was studied,and the wear resistance of the cast iron was studied at the same time.It was found that the D-type graphite gray cast iron on the non-bionic surface had the worst wear resistance,but the C-type graphite had the best wear resistance,while the bionic surface had the most obvious improvement on the wear resistance of the D-type graphite gray cast iron.Through the study of the effect of graphite type on thermal fatigue and wear resistance,it is found that the comprehensive properties of A-type graphite gray cast iron are the best,but the composition of cast iron is often different due to burning loss in casting process.Therefore,we further investigated the microstructure and hardness,oxidation resistance,thermal fatigue resistance and wear resistance of laser bionic gray cast iron with A-type graphite during thermal fatigue under different carbon content.It is found that with the increase of carbon content,the amount of thermal fatigue cracks produced by gray cast iron in the thermal cycling process,but the crack propagation rate of the sample with lower carbon content is faster,which may be related to its lower thermal conductivity.With the increase of carbon content,the area of graphite increases,the hardness of matrix increases,and the wear resistance becomes better and better.However,the cracks and the graphite pits produced during the thermal cycle have a negative effect on the wear resistance,and there are serious ploughing wear around them.On the bionic surface,the thermal fatigue resistance and wear resistance of gray cast iron materials are greatly improved by bionic unit.With the increase of carbon content,the hardness and wear resistance of the unit increase,but it is easier to crack in the thermal cycling process.It can be concluded that the matrix of gray cast iron brake drum is pearlite and all A graphite.In the range of A graphite,the wear resistance increases with the increase of carbon content,and the number of thermal fatigue cracks increases,but the crack propagation rate is slower.The laser bionic model can improve the wear resistance and thermal fatigue resistance of gray cast iron with different carbon content.