Research On Friction Brake Behavior And Failure Mechanism Of Aluminum Matrix Composite

This paper studied SiC particle reinforced aluminum matrix composite(abbreviation as the aluminum matrix composite)brake disc used in China's urban rail train,and prepared the high quality aluminum matrix composite.The finite element method was used to simulate the service characteristics of aluminum matrix composite brake disc.The initiation and propagation mechanism of thermal fatigue crack of aluminum matrix composite under the service temperature of brake disc of urban rail train,and the effects of the third body films on the friction and wear properties of aluminum matrix composite were studied respectively.A new technology for the preparation of aluminum matrix composite by visualization vacuum semi-solid stirring was developed,and the high quality aluminum matrix composite were preparated by dilution method.The aluminum matrix composite prepared by the dilution method has uniform particle distribution.The tensile strength and elongation of the aluminum matrix composite prepared by the dilution method are 9%and 17%higher than composite prepared by direct stirring method.The working temperature range of aluminum matrix composite brake disc in a subway line for a full time trip was 30??180? while the temperature range was 30??260? for the two consecutive emergency braking condition,which all were simulated by the finite element method and ANS YS code.The thermal fatigue crack initiation and propagation of aluminum matrix composite at different temperatures were analyzed under the temperature range 30??300?.It was found that the crack length of 200?m is the boundary point of thermal fatigue crack initiation and propagation.Thermal fatigue crack initiation is a joined process of micro holes,micro cracks and other damages.The propagation of the crack showed the jump growth feature of Step-Like,which is a cyclic process of slow growth and rapid jump growth.In the slow growth stage,the thermal fatigue cracks mainly extend along the particle interface or through the a-Al,while the rapid jump growth is the connecting process of the main crack with the void and micro crack at the tip of main crack.The higher upper limit temperature of the cooling and heating cycle is,the faster of the thermal fatigue crack grows.The upper limit temperature of 200? is the critical temperature.The different friction tests under the temperature range of 30??30? were carried out,and the third body films with thickness of 3?m-5?m were found on the surface of aluminum matrix composites at the temperature below 100?.With the increase of the friction temperature,the third body films were gradually destroyed,and the friction and wear of the grinding pair were changed from the third body films lubrication friction to the serious third body abrasive wear of aluminum matrix composite.The 200? 200N and 1500rpm are corresponding to the critical transition temperature,load and speed respectively.Above the critical value,the wear rate increased rapidly.The appearance of rapid increase of wear rate can be delayed by adding air cooling around the friction pair.Based on the study of the microstructure and evolution of the third body films,it was found that third body films on the surface of aluminum matrix composite were an dynamic equilibrium mixed films formed by the process of transfer-accumulate-compaction from friction pair materials.The third body films have the positive effect on heat resistance,lubrication,stabling friction coefficient and reducing wear rate.The third body films contain a dense layer,a transition layer and a loose layer.The thickness of the third body films is up to 5?m?10?m,and 200N,1500rpm are the critical transition points for the effective action of the third body films.The study of this paper shows that the aluminum matrix composite with high quality was prepared by visualization vacuum semi-solid stirring and dilution method.The Step-Like Propagation Model of the thermal fatigue crack of aluminum matrix composite was established.The study revealed the positive effect mechanism of the third body films on the wear rate and proposed the critical transition value of the effective action of the third body films.The sliding friction and wear mechanism of the change from the third body films lubrication friction to the third body abrasive wear of aluminum matrix composite were also discussed.The all research findings provided a theoretical basis for analyzing of thermal fatigue failure behavior and stabling and regulating the friction and wear behavior of the aluminum matrix composite,and it also has a good guidance for the engineering application of the aluminum matrix composite in brake disc.