Study On The Improvement Of Corrosion Resistance Of Fe-based Ultrafine Grained Laser Cladding Layers Based On Composition Optimization

With the continuous improvement of industrial production levels,inexpensive and high performance steel components are widely used.Many components are directly exposed to high humidity and corrosive conditions,which can easily damage their surfaces and lead to overall failure.The modification and strengthening of component surfaces through laser cladding technology can significantly improve their surface properties,reduce the risk of failure and increase the life of the part and industrial productivity.In this thesis,the organisation and properties of in-situ autogenous VC-enhanced Fe-based ultrafine grain laser clad layers have been studied,and the ultrafine grain highly corrosion-resistant laser clad layers have been prepared by optimising the alloy powder.The effect of alloying elements on the structure and properties of the clad layer has been investigated,with emphasis on the corrosion resistance of the clad layer,and the improvement mechanism of the alloying elements on the corrosion resistance of the ultrafine crystal clad layer has been analysed.In addition,the effect of water-cooled substrates on the structure and properties of Fe-based ultrafine grain laser clad layers has been investigated by applying forced cooling during the cladding process.Three types of Fe-based ultrafine grain cladding layers have been prepared by varying the initial powder size.It has been found that the smallest initial powder particle size has a smaller matrix grain size,but the carbides in the clad layer appear to be agglomerated,making the clad layer performance degraded;the largest initial powder particle size has a significantly larger tissue size and the clad layer performance is poor;the gradient particle size clad layer shows the best overall performance.Based on the gradient particle size alloy powder,the addition of CeO₂ to the clad alloy powder has been shown to effectively improve the organisation and properties of the clad layer.As the addition of CeO₂ increases,the amount of slatted martensite in the clad layer increases,the VC changes from long to granular,and the hardness of the clad layer first increases and then decreases.At 1.5 wt.%CeO₂ addition,the best overall performance of the clad layer was achieved,with a significant refinement of the clad matrix grains and carbides to an average size of 0.83μm and 83 nm respectively,resulting in an increase in corrosion resistance of approximately 3.78 times and an increase in hardness of approximately 128 HV_0.2 compared to the clad layer without CeO₂ addition.A highly corrosion resistant laser clad layer has been prepared by adding Ni to the clad alloy powder.As the amount of Ni added increases,the amount of residual austenite in the clad layer changed and the hardness of the clad layer increased,then decreased and then increased again.Compared to the clad layer without Ni addition,the self-corrosion potential was significantly increased at 4.5 wt.%Ni addition and the corrosion resistance has increased by about 2.87 times.The addition of both CeO₂ and Ni in appropriate amounts significantly improved the corrosion resistance of the clad layer,and the corrosion resistance of the clad layer was significantly improved when 1.5 wt.%CeO₂ or 4.5 wt.%Ni was added to the clad alloy powder.The results showed that the best corrosion resistance was achieved when CeO₂and Ni were added to the molten alloy powder at 1.5 wt.%CeO₂ and 4.5 wt.%Ni.The corrosion resistance of the molten layer was improved by a factor of 5.53 compared to that of the molten layer without CeO₂ or Ni,and the molten layer had a flat corrosion surface with fewer corrosion pits.When water-cooling the substrate,the dilution rate of the clad layer was significantly reduced,the substrate grain size and carbide size were reduced,the grain refinement made it easier for the clad layer to form a dense passivation film to prevent further corrosion during the corrosion process,the reduction in carbide size reduced the corrosion cell area,galvanic corrosion was greatly reduced,the corrosion resistance of the clad layer was improved and the hardness of the clad layer reached 929 HV_0.2.