Study On The Process Optimization Of Laser Cladding On The Tread Of Train Wheel And The Mechanism Of Thermal-Mechanical Coupling Damage

With the increase of train speed,axle load and traffic volume,tread is easy to cause local damage in the process of wheel operation.Laser cladding repair technology has obvious advantages in the repair of local damage of train wheels.The high temperature sliding condition caused by tread braking is a severe test for the coating of damaged wheels repaired by laser additive.Therefore,it is of important economic value and scientific significance to explore the influence mechanism of complex braking environment on the service performance of cladding layer to improve the ability of train wheel and rail system to resist extremely harsh environment.In this paper,the laser cladding repair technology was used to prepare the repair coating on the damaged part of train wheel tread.Taking Co-,Fe-and 316L stainless steel alloy powder as cladding materials,firstly,the optimization of cladding foundation process and the selection of coating materials were discussed.Then,the influence of different temperature rise,room temperature and high temperature braking conditions on the friction and wear properties of the repaired sample and the contact surface damage mechanism were studied,and the main research results were obtained as follows:（1）Co-,Fe-and 316L stainless steel alloy cladding layers were prepared on wheel materials by laser cladding repair technology.Through orthogonal test analysis,three laser cladding process parameters were optimized,and three optimal repair process schemes were obtained:The parameters of Co-based cladding repair coating are 1500W laser power,500mm/min scanning speed,powder delivery rate of 25g/min.The parameters of Fe-based cladding repair coating are 1500W laser power,500mm/min scanning speed and 20g/min powder delivery rate.The coating parameters of 316L stainless steel cladding repair are1500W laser power,500mm/min scanning speed,15g/min powder delivery rate.Then,the optimization work of multi-pass lap was completed,and finally the optimal lap rate of the three repair coatings was 40%.The process parameters of three different alloy powder materials were optimized.（2）Three different alloy repair coatings were prepared according to the optimum process parameters.The laser repaired coatings have dendrite and eutectic structures,and the structure is compact and uniform,the metallurgical bond is firm,and the forming quality is good.The microstructure of Co-based coating isγ-Co,Fe Ni₃and Cr₂₃C₆,while Fe-based coating is dominated byα-Fe,（Fe,Ni）,Cr₇C₃and other solid solutions,316L stainless steel coating is austenite and carbide(M₂₃C₆),after laser cladding repair,Fe-based up to 795.5HV_0.1,The hardness of Co-based coating and 316L alloy coating is also increased by 115.8%and 49.9%,indicating that the microhardness of the prepared coating is higher,and the improvement effect is more significant.（3）By analyzing the comparison of wear resistance and corrosion resistance of the three repair coatings,it is found that the properties of different cladding layers are significantly different,among which the Co-based alloy coating has the best wear resistance(friction coefficient of 0.52 and wear rate of 0.96×10^-5mm³/N·m)and the strongest corrosion resistance（corrosion rate is only 1/40 of the substrate）.By comparing the performance of the three coatings,it is concluded that the Fe-based alloy coating has good wear resistance,poor corrosion resistance and strong hardness.However,in the practical engineering application,the reliability and safety of high-speed train are mainly considered,and the hardness of wheel material matches the coating.The 316L stainless steel in the study also shows excellent wear and corrosion resistance.Therefore,316L stainless steel powder is still preferred.（4）The friction and wear properties of the repaired specimens were studied under room temperature and high temperature test conditions of temperature rise and braking conditions（axle load and braking frequency）.It was found that with temperature rise,the friction coefficient and wear rate showed a gradual decrease.When 600℃,the friction coefficient（value 0.36）and wear rate(1.44×10^-5mm³/N.m)were the lowest.Wear surface damage is minor.The wear rate increases with the increase of axle load,indicating that the greater the contact load,the more serious the wear.With the increase of the braking frequency,the wear of the sample increases first and then decreases.When the frequency is 1Hz,the maximum value at room temperature is 1.66×10^-5mm³/N·m.More peeling layers appear on the wear surface and a large number of wear fragments are randomly distributed,which shows that the wear mechanism is characterized by abrasive wear and adhesive wear.