| Laser cladding is an advanced processing technology that uses the rapid fusion technology under the effect of high-energy laser,and achieve the high-performance materials or coatings preparation.Therefore,this technology has been widely utilized in many areas,such as additive manufacturing,crack repairing and remanufacturing,special materials or coatings preparation and so on.The technology plays an important role in the parts manufacturing,repairing and protective coating preparation in the fields of aviation,aerospace and navigation.After removing laser beam,the molten pool solidifies and obtain high-performance materials.However,it is difficult to solve some problems in materials by adjusting the laser cladding process parameters simply,such as the pores and residual stress in coatings,to exceed their inherent limits and obtain excellent materials.Based on the background,this thesis studied the controlling method of laser cladding on the microstructures,and proposed ‘graphene + electrostatic field’hybrid technology to achieve controlling the defects in cladding layer during cladding process.The related research and verification were based on 316 L stainless steel,a universal and commonly used material.At first,this thesis studied the effect of different graphene contents on the microstructure,phase and mechanical properties of 316 L.By comparing the changes in microstructures among the graphene-added cladding layers with different graphene proportions,the effect and mechanism of graphene on microstructure were analyzed,and the results showed that graphene can make the microstructures refined.The presence of graphene could become the core of nucleation so that promote new nucleation during the cladding process.And graphene could exist at the grain boundary,inhibiting the formation of columnar crystals and limiting the further growth of crystals.By comparing the microhardness,tensile strength and friction and wear coefficient,the effect of graphene on the mechanical properties of the cladding layers was analyzed,and the results showed that the cladding layer with 2 wt.% multilayer graphene has better mechanical properties.Compared with the non-graphene cladding layer,the microhardness,tensile strength and friction coefficient were increased by 44.91%,5.44%and 76.32% respectively.Obviously,the microhardness,tensile strength and wear resistance of the cladding layer have been improved dramatically.By analyzing fracture morphology and wear morphology,it was found that a certain proportion of graphene can enhance the 316L-based material obviously due to its excellent mechanical properties,assisting stress transmission and resisting deformation in materials.The two-dimension structure of graphene made it combined with more 316 L stainless steel substrates,and micro-nano scale graphene existed in the pores.Its excellent mechanical properties could assist the cladding layer to resist larger stress and prevent the propagation of micro-cracks.However,adding redundant graphene could result in the decline of the mechanical properties of cladding layer,which even lower than the cladding layer without graphene.Then,this thesis explored the effect of electrostatic fields with different directions and intensities on controlling the microstructure of laser cladding 316 L.The changes of pores,microstructure and grain orientation which controlled by electrostatic field in two different directions were analyzed.It was found that the porosity and the number of pores in the cladding layer decreased greatly under the control of electrostatic field.The porosity in the cladding layer without electrostatic field control was 6.347%,when the electrostatic field of 350 V/cm was applied in along and reverse molten pool moving directions,the porosity decreased to 2.753% and 2.102%,respectively.And it was found that the growth of microstructures in the cladding layer under the control of the electrostatic field had been changed,and the electrostatic field in different directions had different effects on microstructures.Under the control of the electrostatic field along the moving direction of the molten pool,the growth of microstructures in the cladding layers tended to be uniform,and tended to grow along the moving direction of molten pool.The angle between the microstructure and the substrate decreased from76° to 34° with the increasing of the electrostatic field strength.However,when the electrostatic field was applied in reverse the moving direction of molten pool,the direction of microstructure growth became disordered.Compared with other cladding layers,the isotropic region in the cladding layers which influenced by this electrostatic field tended to be expanded.When the intensity increased to 350 V/cm,the microstructures in some areas in the cladding layer appeared to grow against the moving direction of laser.These phenomena might be related to the effect of electrostatic field on molten droplets during the cladding process.The electrostatic field along molten pool direction promoted the flow of molten droplets along laser moving direction,which was the same as the influence of laser on the molten pool,so that the growth of microstructures in this direction was enhanced and the uniform growth morphology was formed.The electrostatic field in reverse molten pool direction promoted the flow of molten droplets against the laser moving direction,which was opposite to the influence of laser on the molten pool.Therefore,the convection in the molten pool was enhanced,and the isotropic areas in the cladding layers increased under the effect of electrostatic field.While the effect of electrostatic field on the molten pool was greater than laser,the cladding layer presented reverse flow morphology obviously.At last,this thesis explored the effect of “graphene + electrostatic field” hybrid technology on laser cladding 316 L,and found that this hybrid technology can release the residual tensile stress in the cladding layer obviously.While graphene and the electrostatic field along molten pool direction were used simultaneously,the residual tensile stress was released obviously.While the cladding layers were controlled by graphene and the electrostatic field reversing the laser moving direction,it could be found that there was residual compressive stress in the cladding layers.And the micrographs showed the squeeze deformation in the cladding layers.It can be inferred that the exist of graphene can further promote the effect of electrostatic field on molten pool.Overall,this thesis studied the influence and mechanism of graphene and electrostatic field on laser cladding 316 L,and initially explored the influence of‘graphene + electrostatic field’ on laser cladding.The results provide more options for exploring the controlling method of laser cladding metallic materials. |
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