Rare Earth And Tungsten Carbide Reinforced Fe-Mn-Si Memory Alloy Composite Coating Developed By Laser Cladding

In view of the problem that the residual stress of laser cladding is too large and the coating is easy to crack,the research group has developed the Fe-Mn-Si shape memory alloy coating,which can release residual tissue and heat stress through"stress adaptive characteristics".The coating has fine wear resistance and contact fatigue strength.However,the microhardness of the coating is only about 240 HV_0.2,which limits its industrial application.In this paper,the Fe-Mn-Si shape memory alloy coating was doped with micron-WC,nano-WC and Y₂O₃ to study the influence of composition powder on the microstructure and mechanical properties of the composite coating,revealing the structure and mechanical properties.The interaction law between the two layers,the evolution of the microstructure and mechanical properties of the composite coating after solid solution treatment;the phase transition of austenite to martensite induced by residual stress during the composite coating process,revealing the phase transformation mechanism of microstructure during the process of change.Through the above research,a method for controlling the preparation of low residual stress and high mechanical properties coating is constructed.First,micron-WC and Y₂O₃ reinforced Fe-Mn-Si memory alloy composite coatings were prepared on the surface of 304 stainless steel.By adjusting the energy input of the laser,the optimal process parameters are laser power P=4 kW,scanning speed v=15 mm/s,spot size 10 mm×2 mm,and overlap ratio 30%.When the composite coating was prepared by doping micron-WC in Fe-Mn-Si shape memory alloy,the fluidity of the composite powder was deteriorated.The convection property was limited after the formation of the molten pool,and the macroscopic morphology of the coating was uneven.The coating structure is very uneven.Y₂O₃ was added to the Fe-Mn-Si/micron-WC memory alloy composite coating to obtain a macroscopically well-formed coating.The coating formed a good metallurgical bond with the substrate.Austenite and martensite were present in the coating.And Fe6W6C phase formation,no WC phase exists;coating microstructure from the substrate to the top of the coating are plane crystal,cell crystal,cell dendrites,equiaxed crystals and dendrites at the top of the coating,no micron-in the coating WC particles indicate complete decomposition of micron-WC.Fe-Mn-Si+10 wt.%WC+0.8 wt.%Y₂O₃composite coating on Fe-Mn-Si/micron-WC/Y₂O₃ composite coating hardness pure Fe-Mn-Si memory alloy coating The wear resistance and corrosion resistance are the best.The W element solid solution strengthening,the second phase strengthening,the dispersion strengthening of the rare earth oxide and the"stress adaptive property"of the Fe-Mn-Si memory alloy work together to improve the wear resistance of the coating;adding Y₂O₃ makes the coating surface The points are consistent,the grain boundary is purified at the same time,the number of defects is reduced,and the corrosion resistance of the coating is improved.Secondly,in order to solve the problem of tissue inhomogeneity caused by micron-WC,Fe-Mn-Si/nano-WC/Y₂O₃ composite coating was prepared by reducing the particle size of WC from micron to nanometer.It was found that the particle size of WC was reduced to nanometer level,the macroscopic shape of the coating was good,the surface was flat without root nodules,the Y₂O₃ macro molding was further optimized,and the microstructure was more uniform.Comparing the XRD results of WC of two particle sizes,it is known that no new phase is formed,and the coating consists ofγaustenite,εmartensite and Fe₆W₆C.Due to the tendency of the added nano-WC to change to pure W,a large amount of tungsten is dissolved in the composite coating,and the thermal conductivity of the coating is lowered,so that the solid-liquid interface influence factor G/R becomes large,and there are a large number of composite coatings.Refined dendrites.The solid solution strengthening caused by tungsten and carbon and the strength of the second phase make the hardness of the composite coating more than three times higher than that of the pure Fe-Mn-Si memory alloy coating.The solid solution of tungsten is a stabilizing element of austenite,which limits the stress adaptive properties of the Fe-Mn-Si shape memory alloy.Although the microstructure and strengthening of the composite coating with nano-WC were improved,the wear resistance was improved compared with the Fe-Mn-Si/micron-WC/Y₂O₃ composite coating.The composite coating has a significantly improved corrosion resistance of the composite coating compared to the Fe-Mn-Si/nano-WC/Y₂O₃ composite coating due to the nano-WC-induced microstructure refinement and densification and the combination of Y₂O₃.Thirdly,the Fe-Mn-Si/-micron-WC/Y₂O₃ composite coating was solution treated at 1000°C,and the microstructure changes before and after treatment were compared.The results show that there areγ-austenite,ε-martensitic and Fe₆W₆C phases in the coating.The SSMA composite coating precipitates a large amount of Fe₆W₆C phase in the grain boundary and crystal.The precipitation of Fe₆W₆C phase destroys the continuity of the structure and produces Defects such as pores.The precipitation of a large number of second phases increases the microhardness of the SSMA composite coating by 200 HV_0.2,but the precipitation of a large amount of Fe₆W₆C causes cracks and holes in the SSMA composite coating,and limits the austenite of the shape memory alloy during solution treatment.The stress-induced martensitic transformation of Fe-Mn-Si shape memory alloy during friction and wear is also inhibited,resulting in a decrease in wear resistance of the SSMA composite coating after solid solution.In summary,for the ceramic powder and rare earth reinforced Fe-Mn-Si composite coating,it is not suitable for the heat treatment process,the heat treatment process is easy to precipitate the solid solution element and inhibit the microstructure of the Fe-Mn-Si shape memory alloy.change.Finally,studying the microstructure changes of Fe-Mn-Si shape memory alloy and the phase transformation during solidification,the results show that the martensitic transformation of Fe-Mn-Si shape memory alloy occurs during laser cladding.The mechanism is to generate extended dislocations under the action of residual stress.The dislocations are wound bundles at the austenite grain boundaries,and stacking faults are generated to form martensite.γaustenite andεmartensite have parallel crystals.The surface（111）_γ//（0001）_εprovides a channel for the slip between the two phases,and the martensite transformation is completed by dislocation slip.