| Large crawler engineering vehicles are widely used in construction,mining,petroleum and other engineering fields.Due to the bad working environment,the tooth surface of the driving ring gear driven by the crawler has serious wear in a short time under the action of high impact load and friction.The existing induction hardening process of the tooth surface can not meet the work needs.In order to solve the problem of short-term failure of gear surface and improve the service life of gear teeth,ZG42 CrMoA,which is commonly used in large gear ring,was taken as the research object in this paper.In order to improve the surface wear resistance,impact resistance and corrosion resistance of the material,laser cladding technology was adopted to carry out the coating material selection and basic process optimization,wear-resistant particle selection,rare earth element control,pulse laser cladding.The main results are as follows:(1)In order to improve the wear resistance and impact resistance of the tooth surface of the active gear ring,a sandwich composite structure coating was designed,which includes interface connecting layer,toughening layer and wear-resistant layer.The interface connecting layer connects the cladding coating and the substrate.In order to eliminate the metallurgical defects such as porosity and inclusions of substrate,the high dilution ratio is used to eliminate the defects,and a good metallurgical combination is formed at the interface.Toughening layer is used to buffer external force and enhance the impact resistance of the coating.Wear resistant layer is used to improve the wear resistance of the coating.The toughening layer and wear-resistant layer are prepared by alternating lamination.Ni201 powder with good toughness and wettability was selected as the material of the connecting layer and toughening layer.Ni45 alloy has relatively good wear resistance,impact resistance and corrosion resistance,so it was used as the main material of wear-resistant layer.The wear resistance is enhanced by adding WC particles.The impact resistance is improve by adding rare earth.(2)The best flow rate of powder carrier gas in laser cladding process is 600 L/h by combining numerical simulation and process test.Based on the orthogonal test,the laser power,scanning speed and powder feeding amount of laser cladding are optimized,and the best laser cladding parameters of Ni45 coating and Ni201 coating are obtained.The Ni45 coating selects 2100 W of laser power,300 mm/min of scanning speed,8.87 g/min of powder feeding rate;Ni201 coating selects 2700 W of laser power,300 mm/min of scanning speed and 4.72 g/min of powder feeding rate.The optimal lap rate of 40% was obtained by using the above parameters to optimize the multi lap test.The cracking of 40%lap rate coating was solved by preheating and slow cooling at 300°C.(3)In order to improve the wear resistance of laser cladding Ni45 coating,the effects of micro and nano WC particles on the microstructure and properties of the coating were studied.Because of the high hardness and low bonding probability of micro WC particles,the micro WC particles have good adhesion resistance,which can effectively improve the wear resistance of Ni45 coating.But WC particles with high brittleness will form high stress concentration point in the coating,which makes the impact resistance of Ni45 coating decrease obviously.However,the nano WC particles have small size and large specific surface area,which will adhere to the front of the solid-liquid interface to hinder the growth of the grains,which makes the coating structure be refined significantly.The impact resistance and corrosion resistance of the coating are improved significantly while the wear resistance of the coating is improved.Compared with Ni45 coating,the wear rate of the coating decreased by 53.17%,impact toughness increased by 13.4% and corrosion current density decreased by 34.12%.(4)In order to improve the impact resistance of laser cladding Ni45 coating,the effects of yttrium and its oxides on the microstructure and properties of the coating were studied.Yttrium can inhibit the grain growth and refine the coating structure,but it can also produce many hard precipitates.Because the hard phase will become the stress concentration point in the coating,the cracking tendency of the coating will be increased under the impact force,which limits the improvement of the impact resistance of the coating.At the same time,the hard phase will increase the precipitation of Cr element,aggravate the phenomenon of poor Cr in the coating,and then can not significantly improve the corrosion resistance of the coating.Because yttrium oxide is refractory and does not react with other metals,it will hinder the grain growth and become heterogeneous nuclear particle.Therefore,yttrium oxide can effectively refine the coating structure and inhibit the precipitation of hard phase,alleviate the stress concentration and Cr poor phenomenon of the coating,and improve the impact resistance and corrosion resistance of the coating.Compared with Ni45 coating,the wear rate of the coating with 0.4 wt.% yttrium oxide is only reduced by 2.86%,the impact toughness is improved by 53.8%,and the corrosion current density is reduced by 56.24%.(5)In order to further control the comprehensive properties of Ni45 coating,the influence mechanism of pulse frequency on the microstructure and properties of the coating was analyzed.The pulsed laser makes the molten pool have greater temperature gradient and cooling rate,so it can effectively refine the structure and reduce the precipitation of hard phase.Compared with the continuous laser cladding coating,the wear rate of the coating with pulse frequency of 80 Hz decreases by 26.63%,the impact toughness increases by 29.94%,and the corrosion current density decreases by 40.08%.(6)The composite coatings with homogeneous structure and sandwich structure were prepared by the optimized process.The results show that the microstructure of homogeneous composite coating is fine and uniform,and the size and quantity of W rich and Cr rich phases are small.Mo element is enriched in the grain boundary of the toughened layer in the sandwich composite coating,which can effectively prevent the diffusion of Cr element.The wear rate and corrosion current density of the homogeneous composite coating are 76.94% and 87.98% lower than those of the substrate,and 72.80%and 92.71% lower than those of the high frequency quenched substrate.The sandwich composite coating has the best impact resistance,which is 8.21% and 14.67% higher than that of homogeneous composite coating and high frequency quenching sample,respectively.(7)The fixture for laser cladding of large ring gear surface was designed.The tooling has the advantages of simple structure,fast installation and positioning,stable movement,and linkage of ring gear and powder feeding head.According to the motion trajectory control method of the ring gear and the powder feeding head,the uniform and equal thickness sandwich composite coating was prepared on the tooth surface.The stress distribution of three kinds of gear surface and track pin in the process of impact and friction were compared and analyzed by finite element simulation.Compared with high frequency quenching and laser cladding homogeneous coating,the toughened layer of sandwich composite coating will have micro plastic deformation in the process of impact and friction,so as to disperse the internal stress of the coating and effectively relieve the stress concentration on the tooth surface.Therefore,the coating can obtain good impact resistance while ensuring excellent wear resistance. |
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