Research On Microstructure And Corrosion Resistance Mechanism Of AISI431 Stainless Steel Coating Prepared By Extra High Speed Laser Cladding

At present,the preparation technology of corrosion resistant coating is mainly chromium electroplating.However,the pollution of chromium electroplating industry has been paid more and more attention.Both China and The European Union have restricted the chromium electroplating industry.There is an urgent need for surface modification technology which can replace or reduce the application of chromium electroplating technology.The traditional laser cladding technology in the industry has a certain application,through the adoption of large flare semiconductor lasers partly solved the problem of low efficiency of traditional laser cladding coating preparation.However,because of its high heat input,the surface roughness of coatings,the dilution rate and the coating microstructure inhomogeneity are relatively serious.Therefore,the processing time,the cost of materials and performance are still a problem to be solved.The Extra High Speed Laser Cladding（EHLA）is a new surface modification technology developed based on Conventional Laser Cladding（CLA）.Compared with traditional laser cladding,ultra-high cladding speed（15～200m/min）has the characteristics of high efficiency（0.82～1.5m²/min）,low coating dilution rate（2.2J/mm～10J/mm）and high powder utilization,and has broad industrial application prospects in corrosion resistance,wear-resistant coating preparation and heterogeneous material cladding.Ultra-high speed laser cladding technology can obtain very high cladding speed by increasing the rotating speed,so at present it is mainly used in the preparation of surface coatings for rotating bodies such as shaft and disk.AISI431 duplex stainless steel corrosion resistant coatings were prepared on 27SiMn steel by EHLA technique.The macroscopic characteristics of ultra-high speed laser cladding coating,including surface morphology,dilution rate and heat affected zone,were analyzed.The effect of the cladding velocity on the microstructure,composition uniformity and phase composition of the coating was studied systematically.The corrosion resistance of ultrafine laser cladding coating was characterized by electrochemical method and salt spray corrosion,and the correlation between the coating structure and corrosion resistance was established.The growth mechanism of passivating film on coating surface in air and the corrosion behavior of passivating film in solution were analyzed.The internal defects of ultrafine laser cladding coating were observed,the corrosion process of defects was analyzed in situ,and the mechanism of pitting induced by defects was revealed.The results show that the extra-high cladding speed has great influence on the macroscopic properties and internal structure of the coating.The surface of EHLA coating is relatively flat with only a certain amount of adhesive powder.When the cladding velocity exceeded 15m/min,the macroscopic characteristics of the coating surface did not change much,the coating heat affected area was between 50～70μm,and the coating dilution rate was only less than 1%when the cladding velocity was above 30m/min.With the increase of the cladding speed,the cladding efficiency increased from 0.05m²/h to0.84m²/h.The microstructure of AISI431 coating is mainly dendrite structure,and the increase of cladding velocity is conducive to the refinement of dendrite size.The dendrites in the overlapped zone are coarsened by repeated thermal action.The dendrite size decreased from 5.4μm to 0.9μm with the increase of the cladding speed.The compositions of dendrites and dendrites were different.The content of Cr and Mn was higher between dendrites,while the content of Fe,Ni and Siwas less.The refinement of dendrite size is helpful to reduce the difference between dendrite and dendrite.The coating phase is a martensite/ferrite biphasic structure.The internal dendrites of the coating are mainly composed of ferrite,and martensite is lamellar separated in the dendrite interior by the interdendrite structure.Therefore,with the increase of cladding velocity,the size and content of martensite in the coating gradually decrease,and the hardness of the coating decreases as well,from 664HV at 1.5m/min to 460HV at100m/min.EHLA AISI431 stainless steel coatings provided excellent corrosion resistance.The electrochemical and impedance spectroscopy tests show that the coating prepared under the condition of high-speed laser cladding has better corrosion resistance and the passivation film generated on the coating surface is more compact.The improvement of corrosion resistance of coating is mainly due to the refinement of coating dendrite structure and the improvement of composition uniformity.The finer dendrite structure is conducive to the nucleation of the passivation film,and the more uniform element distribution is conducive to the improvement of the density of the passivation film.After the salt spray corrosion test,the ultra-high speed laser cladding coating did not corrode even when the coating thickness decreased to 90μm.The dense passive film formed on EHLA coating is the key to the excellent corrosion resistance of coating.The density of passivation film is closely related to the internal structure of coating.EHLA coating surface passivation membrane overall average thickness of about 14 nm,coating internal dendritic segregation caused by uneven distribution of elements will result in uneven coating surface passivation membrane thickness,general interdendritic form passivation film is more thicker,the dendritic size is small,therefore,the thickness of oxide film on the surface of the coating is more uniform and higher corrosion resistance.The passivation film has obvious double layer characteristics,the outer layer is thicker and the inner layer is thinner.The key barrier of chromium-rich oxide layer in the passivation film can effectively prevent the erosion of Cl^-ion in the corrosive environment.The content of chromium between dendrites is relatively high and the thickness of chromium-rich oxide layer is larger than that of dendrite position.The defects in the coating are the main causes of pitting damage.The possible process defects in the EHLA coating are mainly pores,SiO₂ inclusions,cracks and compound defects,etc.,which are basically micro-defects with the size between 1μm and10μm.The main defects causing pitting corrosion of the coating are SiO₂ inclusions and cracks.Inclusion and crack defect locations usually make the passivation film discontinuous or weak,making it more vulnerable to damage by Cl^-ions,and thus causing pitting corrosion.