Study On Laser Depositied AlCrFeCoNiCu High-entropy Alloy Coatings System On Aluminum

Surface modification is the main method to improve wear resistance and corrosion resistance of aluminum-alloy mechanical components,and laser deposition is an important technology to prepare excellent coatings.However,traditional coating materials,such as Fe-,Ni-and Co-based alloys,have higher melting points than aluminum alloy,and exhibit negative enthalpy of mixing with aluminum?Al?.Al with low density in the substrate will float up to the molten pool during laser deposition,and it reacts with the above materials to easily form hard brittle intermetallic compound which will seriously affect the forming quality and service performance of the coating.Therefore,the preparation of excellent coatings on aluminum has always been a problem in the field of laser deposition.To solve this problem,a new material,high entropy-alloy?HEA?,is proposed as the coating to suppress the forming of hard brittle phase caused by the dilution of the substrate.Based on the optimization of process and coating composition,the technology of the preparation of coating with high forming quality and wear resistant and corrosion resistant by laser deposition on aluminum were studied.The main results are as follows:1.Based on the calculation and analysis of the alloy system of the phase structure determination parameters such as the entropy of mixing,enthalpy of mixing and the atomic radius difference,a coating composition,named AlCrFeCoNiCu HEA,suitable for the aluminum alloy for surface modification of aluminum alloys was designed.It was found that a small spot diameter of 1.2 mm can reduce the cracking tendency.Based on combining the signal-to-noise ratio analysis and the approximate ideal value method to optimize the dilution rate and contact angle of the coating,the optimal value closeness of the response can be increased from 0.5389 to 0.5647.And the coating formed a simple phase structure,which is FCC1+FCC2+BCC phase,avoiding the generation of intermetallic compounds with complex phase structure.The hardness and bonding strength of the coating are 509HV_0.2 and 100.4 MPa.2.To improve the bonding quality between the coating and the substrate,pulsed laser mode in laser deposition is proposed.Based on the adjustment of duty cycle and pulse frequency,the solidification behavior of molten pool and the structure of coating can be controlled.Under the laser power of 1850w,pulse frequency of 50Hz,duty cycle of 70%,scanning speed of 120mm/min and powder feeding rate of 5.6g/min,the coating shows FCC1 phase structure,the dendrite structure at the bottom and middle of coating is refined.The bonding strength of coating was 171.9 MPa,which was 31.3%higher than that of coating by continuous laser deposition.Under the above process conditions,based on the finite element simulation and high-speed camera technology,the mechanism of effect of the pulse frequency on forming quality of the coating was revealed.When the pulse frequency is less than 30Hz,due to the solid-liquid interface solidification between the droplets,the molten pool has a high temperature gradient,and the coating will generate great tensile stress during cooling,and cracks easily occur.when the frequency is greater than 50Hz,too low average temperature makes it difficult for the molten pool to be wetted and spread on the substrate,thus the coating is easy to form a stack height and its edge is stained with powder and spheroidized.When the frequency is between 30 Hz and 50 Hz,the molten pool has a small temperature gradient and good wetting spread,which can form a coating with excellent geometry and forming quality.3.To improve the wear resistance and corrosion resistance of coatings,based on the mechanism of phase control on HEAs,the effects of the main component Al in the AlCrFeCoNiCu HEA system on the microstructure and properties of the coatings were studied.The BCC phase can effectively prevent the micro cutting and micro shearing of the micro convex body to the coating.Therefore,the wear resistance of the coating is enhanced with the increase of Al,the wear mechanism changed from delamination fracture and adhesive wear to abrasive wear and adhesive wear.In 3.5%NaCl solution,the corrosion resistance of the coating increases first and then decreases with the increase of Al content.The intergranular corrosion of the coating is caused by copper rich FCC1 phase,and the corrosion resistance of Al_0.8CrFeCoNiCu coating is better.The results of Cauchy pressure and Paugh ratio of the HEA show that when the Al content exceeds 1,the HEA becomes brittle.;the results of material properties such as bulk modulus,shear modulus and Young's modulus show that the increase of Al content will increase the strength of the alloy.Based on the above analysis,the performance of Al_0.8CrFeCoNiCu HEA is relatively balanced,and the optimized content of Al is 0.8.4.To further improve the corrosion resistance of the coating,the Cu content was optimized.The increase of Cu content can promote the formation of FCC1 phase,improve the toughness of the coating,reduce the crack sensitivity,but reduce the corrosion resistance.The results show that proper reduction of Cu content can reduce the degree of interdendritic segregation,improve the corrosion resistance and promote the transformation of corrosion behavior from intergranular corrosion to pitting corrosion.The intergranular corrosion is caused by interdendritic segregation of Cu and the pitting corrosion is caused by active dissolution of Al element.When the content of Cu is 0.5,the toughness and corrosion resistance of the coating can be improved.5.To further improve the performance of coatings,the effects of Ti and Si and B content on properties were studied.The addition of a small amount of Ti has a limited effect on the properties of the HEA,but when the addition of Ti is more than 0.5 atomic ratio,the coating will form Laves phase,which can improve its hardness,but also reduce the corrosion resistance.Compared with the Cr₃Si phase in the as-cast Al_0.8CrFeCoNiCu_0.5Si_0.5 HEA,the as-deposited Al_0.8CrFeCoNiCu_0.5Si_0.5 HEA only has a simple phase structure.This is because under the rapid cooling conditions,the intermetallic compound phase incubation time in the supercooled melt is higher than the solid solution phase.Si and B can promote the formation of eutectic structure and boride in the coating,respectively,and appropriately increasing Si and B can improve the wear resistance of the coating.However,the addition of B will reduce the corrosion resistance of the coating,because the formation of boride will cause the micro-couple effect and destroy the continuity of the passivation film.The Al_0.8CrFeCoNiCu_0.5Si_0.3 HEA has the highest hardness,wear resistance and corrosion resistance than that of all materials studied under the premise of good bonding quality between the coating and the substrate,and its performance is far higher than that of the substrate,so it can be used as an optimal coating on aluminum by laser depositon.