Study On Microstructure And Properties Of Laser Cladding Composite Coatings With TiC Reinforcement On Ductile Iron

The hot roller of ductile iron is in service under the harsh working conditions of high temperature,high load,thermal fatigue and strong wear.It is easy to fail due to crack,peeling and wear.The ceramic reinforced metal composite coating can improve the high temperature resistance,corrosion resistance and wear resistance of the material.It is an effective measure to improve the reliability and service life of metallurgical spare parts.In this paper,the selection of Fe based alloy with better economic performance and Co base alloy with good high temperature performance,the Ti C reinforced composite coating has been prepared on the surface of ductile iron by laser cladding technology.The effects of adding Ti C directly and in-situ generation of Ti C on the structure and properties of the metal-based coating have been studied.(1)The effect of different Ti C doping amount on Fe based coatings has been studied.Firstly,the laser power process was optimized.With the increase of laser power,the microstructure of cladding layer become more compact.However,excessive laser power led to a large number of longitudinal cracks.When the laser power was 1.35 k W,there is no cracks and other defects on the surface of cladding layer,and the hardness of cladding layer reached 681 HV_0.2.The cladding layer is composed of?-Fe,?-Fe,Cr₂₃C₆,and Cr₇C₃phases.Then,under the optimal laser power condition,the effects of different Ti C doping amount on the microstructure and high temperature wear resistance of Fe based coatings have been explored.The composite coatings with 3.0 wt.%and 5.0 wt.%Ti C had good surface forming quality.The structure is more refined than that of Fe based coating.A large number of Ti C rich particles were evenly distributed in the structure.However,when10.0 wt.%Ti C was added,a large number of macro cracks appeared on the surface of the coating,which could not guarantee the quality of the cladding.The results show that Ti C was uniformly distributed in the coating,which has promoted the formation of new phase of Fe_xTi.The phases were mainly composed of?-Fe,?-Fe,Cr₂₃C₆,Cr₇C₃,Ti C and Fe_xTi.Under the condition of high temperature friction and wear,the friction coefficient of the composite coating was reduced after adding Ti C.the friction coefficient of the Fe based composite coating with 5.0 wt.%Ti C was only 0.17,which shows the best wear resistance at high temperature.(2)The effects of different Ti doping amounts on Fe based coatings have been studied.The Fe based composite coatings with 3.0 wt.%and 5.0 wt.%Ti had good macro morphology,and the microstructure was refined.A large number of black particles aggregated by Ti C have been produced in the cross-sectional structure of the coating.However,a few macroscopic cracks appeared on the surface of Fe based composite coating with 10.0 wt.%Ti.The Fe based composite coating with Ti are composed of?-Fe,?-Fe,Cr₂₃C₆,Ti C,and Fe_xTi phases.The average hardness of the Fe based composite coating with 5.0 wt.%Ti was increased to 741 HV_0.2.The high temperature wear resistance of the Fe based composite coating with Ti is significantly better than that of the Fe based composite coating with the same mass fraction of Ti C.The high temperature wear resistance of the Fe based composite coatings with 5.0 wt.%Ti is the best,and the wear rate of the composite coating is about twice as high as that of the Fe based coating.The wear mechanism is abrasive wear and oxidation wear.(3)The effects of different Ti C doping amounts on the Co based coatings have been studied.Firstly,the laser scanning speed technology was optimized.There are a lot of fine martensite structure appeared in the heat affected zone of the Co based coating,which led to the increase of hardness in the heat affected zone.With the increase of scanning speed,the microstructure is refined first and then coarsened.The Co based coating consists of face-centered cubic?-Co and Cr₂₃C₆ and Cr₇C₃ phases.When the scanning speed is 120mm/min,the microstructure of the Co based coating is the smallest,the density is the best,and the wear resistance and anti-friction performance of the cladding layer is the best.Then,the Ti C reinforced composite coating was prepared by directly adding Ti C to the Co based alloy powder.No cracks,pores and other defects on the surface of the Co based composite coating doped with 3.0 wt.%Ti C.The composite coatings doped with 5.0 wt.%Ti C has a small amount of pore defects,and the composite coatings doped with 10.0 wt.%Ti C has a large number of cracks and pores.The Co based composite coating with Ti C is composed of?-Co,Cr₂₃C₆,Cr₇C₃,Ti C and Co₂Ti phases.The structure of the cladding layer has been refined by the addition of Ti C.A large number of Ti C hard particles have been dispersed in the structure,resulting in particle strengthening,which improves the hardness of the Co based coating.The Co based composite coating doped with 5.0 wt.%Ti C has the lowest friction coefficient and wear rate,and the best wear and anti-friction performance at high temperature.(4)The effects of different Ti doping amounts on Co based coatings have been studied.By adding different mass fractions of Ti powder,the structure can be refined,and Ti C hard particles can be generated in-situ in the structure,resulting in particle strengthening,which can improve the hardness of Co based coating.The surface of the Co based composite coatings doped with 3.0 wt.%and 5.0 wt.%Ti has a good cladding quality.However,there are a lot of cracks and holes on the surface of Co based composite coating doped with 10.0wt.%Ti.The Co based composite coating with different mass fractions of Ti are composed of?-Co,Cr₂₃C₆,Ti C and Co₂Ti phases.Under the condition of 500?friction and wear test,the wear rate of the Co based composite coating doped with 5.0 wt.%Ti is more than twice as that of the Co based coating,and is better than that of Co based composite coatings doped with 5.0 wt.%Ti C.The wear resistance at high temperature is the best,and the main wear mechanism of the cladding layer is abrasive wear.In conclusion,the ceramic reinforced metal composite coating with good forming quality prepared by direct adding and in-situ generating an appropriate amount of Ti C ceramic reinforcing phase to the metal base alloy powder.The microstructure is refined,the microhardness and high temperature wear resistance and antifriction performance are improved.Among them,the Co based composite coating doped with 5.0 wt.%Ti has the best high temperature wear resistance,slightly better than the Fe based composite coating dope with 5.0 wt.%Ti,and significantly better than the Co based composite coating doped with the same quality of Ti C.The properties of laser cladding Ti C ceramic reinforced metal composite coating are better than that of matrix,which can provide the optimal preparation process and certain theoretical support for the high quality remanufacturing of ductile iron.