| Laser cladding technology can prepare a functional coating with high wear/corrosion resistance on the surface of cheap carbon steel,thus replacing the expensive integral alloy,effectively reducing the production cost,and having great application potential in green manufacturing and remanufacturing.However,laser cladding is a process of rapid melting and solidification of cladding materials,and the properties of cladding materials are often different from those of matrix materials.Therefore,in "rapid heating and shock cooling",it is inevitable to produce structural stress and thermal stress,resulting in defects such as micro-cracks and micro-pores in cladding layers.The surface of cladding layer formed directly is rough,so it needs to be machined after cladding.Cladding materials are mostly high-strength alloys with high hardness and poor plasticity.Focusing on the machining process of cladding layer after cladding,based on the traditional ultrasonic burnishing(UB),this thesis studies and puts forward the ultrasonic burnishing process assisted by temperature field,and explores two new processes assisted temperature field,namely ultrasonic warm burnishing(UWB)and ultrasonic warm burnishing coupled with sequential heat treatment(UWB/HT).The Fe-based alloy powder was selected as the cladding material,and the surface roughness,porosity,hardness,residual stress and grain size were taken as the characteristic parameters of the forming surface integrity of the cladding layer.The influence mechanism of two kinds of temperature field auxiliary methods on the forming surface integrity and corrosion resistance of the cladding layer was experimentally studied,and the optimal process was determined.Furthermore,the influence of temperature field parameters on the forming surface integrity and corrosion resistance of the cladding layer in the optimal process was explored.The main research contents and conclusions include:(1)The influence mechanism of temperature field on the forming surface integrity of Fe-based cladding layer during ultrasonic burnishing is studied.The forming surface integrity of UB,UWB and UWB/HT cladding layers was analyzed and quantitatively evaluated by using fine turning as the contrast process.The results show that the three ultrasonic rolling processes all improve the forming surface integrity of the cladding layer in different degrees.Compared with UB process,UWB and UWB/HT process have better surface strengthening effect,where UWB/HT process has the best strengthening effect.In UB process,the static load coupled with ultrasonic vibration induces the acoustic-plastic effect,which forcing the cladding material to plastic deformation,and realizing surface burnishing and strengthening.In the UWB process,the acoustic plasticity produced by ultrasonic vibration and the thermal plasticity produced by temperature field acting on the surface of the material together,which reduces the deformation resistance of the material,and thus material is easier to severe plastic deformation(SPD),so the surface strengthening process has better effect.Compared with the UWB process,the heat treatment after rolling in the UWB/HT process can promote precipitation strengthening and even stress distribution,thus further improving the surface strengthening effect.According to the evaluation results of forming surface integrity,compared to three post-treatment processes of cladding layer,UWB/HT process has the best process effect on improving the surface properties of cladding layer.(2)Based on Tafel extrapolation method,the polarization curves(CV)of cladding layers treated by different processes are fitted,and the corrosion potential,corrosion current and corrosion rate are measured.Meanwhile,the electrochemical impedance spectroscopy(EIS)and bode curve are compared and analyzed,and the equivalent circuit is fitted.The results show that the corrosion rate of the turning sample is 0.0674mm/year,while the UB,UWB and UWB/HT cladding layers are reduced to 0.0392,0.0136,0.0096 mm/year,respectively.Passivation occurs on the surfaces of the samples of the four cladding layers,and the passivation phenomenon of the samples treated by ultrasonic rolling is more obvious.The improvement of corrosion resistance of cladding layer is attributed to the improvement of surface roughness,porosity,grain size,residual stress and other forming surface integrity parameters.(3)A full-factor experiment with two-factor and three-level was designed and carried out,and the influence mechanism of temperature field parameters on the forming surface integrity of Fe-based cladding layer in UWB/HT process was studied.The experimental results show that the surface roughness,porosity and grain size of the cladding layer decreased with the increased in heating temperature,while the hardness and residual compressive stress increased with the increased in heating temperature.Roughness increases slightly with the increased in holding time,and residual compressive stress decreases with the increased in holding time.The effect of holding time on porosity and grain size is not significant.When the heating temperature is100℃ and 200℃,the hardness decreased with the increased in holding time.When the heating temperature is 400℃,the hardness first increased and then decreased with the increased in holding time.With the increase of heating temperature,the thermo-plastic effect of the material increases,the deformation resistance of the material decreased,and the plastic deformation increased.Meanwhile,at a higher temperature,the static strain aging strengthening can be obtained in the process of heat preservation.However,long holding time caused partial recovery of plastic deformation,thus reducing the forming surface integrity of cladding layer.(4)The influence mechanism of temperature field parameters on corrosion resistance of Fe-based cladding layer in UWB/HT process was studied.Passivation films were formed on the surfaces of nine groups of cladding samples.The corrosion rate of the sample decreased with the increased in heating temperature,and the improvement of the forming surface integrity of the sample is responsible for the improvement of the corrosion resistance.Meanwhile,Cr,Mo and other elements are oxidized during the corrosion process to form a passivation film,which is beneficial to the improvement of the corrosion resistance of the cladding layer.The corrosion resistance of the sample decreased with the extension of holding time.Using heating temperature and holding time as input parameters and corrosion rate as output response,based on the first-order polynomial function,a prediction model of temperature field parameters affecting corrosion rate of cladding layer was established. |
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