Study On Microstructure And Property Of Tunnel Boring Machine Hob Cutter By Laser Cladding

With rapid development of China's rail transit,full-face tunnel boring machines dominated by shield machines have been widely used in national major underground tunnel projects.As main part of tunneling work,the hob cutter is more prone to failure under the harsh working environment,which will greatly increases production costs.Therefore,surface enhancement of hob cutters by means of additive manufacturing methods has become the only way for sustainable development of the shield industry.With high processing precision,rich choice of cladding materials and good bonding strength,laser cladding technology has become one of the most important technologies to achieve precise manufacturing and rapid prototyping of parts.However,additive manufacturing using laser cladding technology often lacks efficiency.The alloy cladding is mostly a single layer,and its formability needs to be improved.Moreover,it is difficult to achieve high-performance multi-layer additive reinforcement.In addition,with low hardness,it is tough for products after additive manufacturing to adapt to the actual construction environment,which limits its application.In view of series of problems mentioned above,this thesis mainly focuses on addictive manufacturing of tunnel boring machines hob cutters.Experiments were carried out with a fiber laser device and the hob material M is used as the research object.The microstructure and properties of the cladding layer were systematically studied in three aspects,including processing parameters,alloy content of powder and ceramics.The main research work of this thesis includes:(1)A single-pass laser cladding experiment was performed using Fe-Cr-B powder.The influence of laser processing parameters on the formability of single-pass cladding,effective powder utilization rate and the microstructure of both heat affected zone and cladding zone were studied.Moreover,the strengthening mechanism of the organization within the cladding has been revealed.With large laser scanning speed,the lateral ductility of single-pass cladding is distinct and gradually becomes flat.Under higher laser energy densities,the cladding forms a better metallurgical bond with substrate and the diffusion of alloying elements on both sides of the interface is more complete.(2)Iron-based and nickel-based multi-layer reinforced cladding layers were prepared on the Fe-Cr-B transition layer.Effects of alloy powders with different compositions on macromorphology,microstructure,hardness and corrosion resistance of the cladding were studied.Results show that microstructure of the strengthened cladding has a significant refinement and contains a large amount of solid solution.Also,more dispersed eutectic structures were found between grains.Microhardness of the iron-based cladding has increased significantly,ranging from 700 HV to 850 HV.The nickel-based cladding has a better gradient transition trend.Strengthened cladding effectively improves the corrosion resistance of samples.Additionally,while iron-based cladding is with more durable passivation film,nickel-based cladding shows better performance in pitting resistance.(3)Multi-layer composite cladding layers were prepared by adding different content of WC particles to the nickel-based alloy powder as a strengthening phase.Effects of WC content on the microstructure,hardness and corrosion resistance of the cladding were studied.When WC content is 20wt%,the best coating formation effect was obtained.Microstructure of cladding mainly consists of ?-Ni phase,WC particles and many other compounds.Due to different dissolved amounts,white WC particles range from each other in shapes and distribute in the middle and lower parts of layer.A continuous gray nickel-based solid solution was found around WC.With the increase of WC content,microhardness of cladding increases significantly,up to 750 HV and above.Additionally,cladding's self-corrosion potential has changed little and its selfcorrosion current density gradually decreases as the amount of WC particles increases.In summary,composite cladding with 20wt% WC possesses the best corrosion resistance.This thesis includes 40 figures,11 tables and 78 references.