On Surface Integrity And Corrosion Resistance Of Laser Cladded Cr/Ni Alloy Finished By Hybrid Turning And Burnishing

With the development of human society and the deepening of industrialization process,the requirement of sustainable development has become the primary goal of continuing to develop the manufacturing industry.By using laser cladding method,it is possible to fabricate claddings with special properties covering worn-out parts or low-cost metal substrate,in order to realize the re-manufacturing of worn-out parts,but also to improve the corrosion and wear resistance,which embodies the superiority and advancement of laser cladding in the field of sustainable manufacturing.The use of machining methods can improve the surface quality of the laser cladded parts,to meet the size and accuracy requirements.Moreover,the combination of laser cladding and machining processes enables high-efficiency,high-precision and more flexible.The subsequent machining of laser cladded parts determines the final geometrical shape,surface roughness and stress state,etc.,and service performance.The purpose of this thesis is to prepare single-layer cladding on 45#steel,to achieve the repair/re-manufacturing of worn-out hydraulic column and obtain high corrosion resistance against aggressive environment.This method realize the recycling of resources,reflecting the advantages of energy saving and environmental protection,which has a broad significance for the construction of a conservation-oriented society.Firstly,a cladding layer with thickness of no less than 1.5 mm is fused on the 45#steel by using laser cladding process.The cladding has dense microstructure,no crack,low porosity,high bonding strength,high hardness,high mechanical strength and superior corrosion resistance.Through the characterization of the properties of the cladding material,the material foundation to analysis the machining processes is obtained.Secondly,the critical condition for two-layered machining of the cladding is proposed based on an orthogonal cutting model.The variation of the ratio between cutting depth and the minimum cladding thickness under different cutting depth is further analyzed,to reveal the difference between machining of laser cladded parts and whole homogeneous material.The results indicate that the ratio between cutting depth and the minimum cladding thickness increases with the increase of cutting depth and reach the maximum value of 2/3.With the increasing of cutting depth,the chip morphology transits from continuous chip to saw-tooth chip.As a result,the fluctuation of cutting force is intensified.The optimum depth of cut in turning of laser cladded parts is 0.2 mm based on the analysis of the cutting force spectrum by Fast Fourier Transform(FFT).Thirdly,the evaluation model of machined surface integrity is established to analyze the surface integrity of laser cladded parts by turning with conventional or wiper inserts,and low plasticity burnishing(LPB).The results indicate that both the surface roughness and porosity are decreased,while all the micro-hardness,compressive residual stress and interfacial bonding strength are improved after burnishing.Furthermore,the strengthening effect of LPB is related to the surface morphology by turning process.Thus,a new processing chain of turning with wiper insert at high feed rate and burnishing is put forward for finish machining of laser cladded parts.Fourthly,the surface residual area and the three-dimensional surface topography in turning with wiper insert under variable feed rates are modeled,to reveal the surface formation process in turning with conventional vs.wiper inserts.Considering the surface roughness,material removal rate and surface residual stress,the optimum feed rate in turning with wiper insert is 0.4 mm/r.Lastly,the relationship between the cladding layer after turning and burnishing processes and the corrosion resistance is established.The relationship between the machining-induced surface roughness parameters and corrosion potential is qualitatively analyzed based on the theory of electrical work function(EWF).An empirical model between the surface roughness parameters and corrosion resistance is also modeled based on the analysis of variation(ANOVA)and correlation coefficient.Moreover,the equivalent circuit model with three-capacitor is proposed based on the results of electrochemical impedance spectroscopy(EIS).The influence of machined surface integrity on corrosion resistance is analyzed based on this new model.From the viewpoint of corrosion resistance of the cladding layer,the optimized feed rate in turning with wiper insert is 0.35 mm/r.