Research On The Microstructure And Properties Of Laser Cladding High Entropy Alloy Coating Resistant To Strong Acid Erosio

Phosphoric acid is usually prepared by a substitution reaction between sulfuric acid and phosphate rock.In the production process of phosphoric acid,a strong collision exists between the impeller blade and phosphate rock,resulting in severe erosion and corrosion.At present,904L super austenitic stainless steel is widely used in impeller blade material.Although it has sufficient corrosion resistance,its hardness is low(～190HV_0.2)and wear resistance is poor,resulting in short service life and an annual repair cost of more than 100 million yuan.The phosphorus chemical industry urgently needs alternative materials with wear resistance and corrosion resistance.High-entropy alloy can achieve both wear resistance and corrosion resistance through composition control,which is seen as the alternative material.However,the composition of high-entropy alloys is complex and the microstructure is changeable,so it is necessary to implement accurate composition design under the guidance of theory.In this paper,CoCrFeNi-M high-entropy alloy is taken as the research object,and the cluster-plus-glue-atom model is used to design the composition formula of high-entropy alloy by analyzing the chemical composition of 904L stainless steel.High-entropy alloy coating was prepared by wideband laser cladding technology.The microstructure and properties of high-entropy alloy coatings were studied by XRD,SEM,TEM,microhardness tester,friction and wear tester and erosion corrosion equipment.The main research contents and results are as follows:（1）By analyzing the chemical composition of 904L super austenitic stainless steel,the composition optimization design of CoCrFeNi-M system is carried out by using cluster plus connected atom model,and the composition formula of[Cr-Fe₄Co₄Ni₄]Cr_2.6-xAl_xMo_0.4 high entropy alloy with different Al elements is designed.Based on[Cr-Fe₄Co₄Ni₄]Cr_1.6Al_1.0Mo_0.4 alloy powder as experimental raw material,the optimized laser cladding technique is that dot size and scanning speed are fixed,laser output power is changed,and then dot size and laser output power are fixed,scanning speed is changed.The results show that,with the increase of laser output power,there exists no macroscopic cracks on the surface of the coating,and there are few microcracks inside the coating.The metallurgical bonding is formed between coating and substrate.The microhardness of the coating increases first,then decreases slightly.When laser output power is 2.5 k W,the highest hardness(～331.2 HV_0.2)is obtained.With the increase of laser scanning speed,there are no macroscopic cracks on the surface of the coating,and a good metallurgical bonding is formed between coating and substrate.When laser scanning speed is 5mm/s,the microhardness of the coating is the highest.So the optimized technological parameters are as follows:Dot size（20 mm×2mm）,laser output power（2.5 k W）and scanning velocity（5 mm/s）.（2）The phase analysis shows that[Cr-Fe₄Co₄Ni₄]Cr_2.6-xAl_xMo_0.4 high-entropy alloy coating is composed of FCC solid solution,and the typical dendrite and equiaxed cellular crystal exist in microstructure.Co,Cr,Feand Ni elements are evenly distributed,Mo element is mainly enriched at the grain boundary,and oxide particles derived from Al element are mainly near the grain boundary.TEM analysis shows that in addition to FCC solid solution phase,nano-precipitated phase,twins and dislocations appear in[Cr-Fe₄Co₄Ni₄]Cr_1.6Al_1.0Mo_0.4 high-entropy alloy coating(abbreviated as Al_1.0 coating).With the increase of Al element,the microhardness of the coating increases gradually.The hardness of Al_1.0 coating is the highest(331.2 HV_0.2),which is nearly 1.75 times than that of the substrate.The wear volume of Al_1.0 coating is the smallest（1.48 mm³）,which is approximately half of that of substrate.The erosion mass loss rate of Al_1.0 coating is 0.0265 mm³/h,which is lower than that of substrate（0.0372mm³/h）.（3）With the increase of TiC addition,the phase of[Cr-Fe₄Co₄Ni₄]Cr₃-x TiC high-entropy alloy coating is composed of FCC solid solution and TiC.The typical dendrites and TiC particles that is distributed on dendrite exist in the microstructure.From TEM results,TiC particles with long strip,polygonal and cluster shape are precipitated from FCC solid solution.The average hardness of[Cr-Fe₄Co₄Ni₄]Cr₃-15Vol.%TiC coating is the highest(357.9 HV_0.2).The coating has the lowest wear volume（0.397 mm³）and wear rate(3.974mm³·N^-1·m^-1).In 0.5mol/L H₂SO₄ solution,the corrosion resistance of the coating increases gradually.Under the condition of strong acid erosion,with the increase of TiC content,the erosion resistance of the coating is gradually improved.（4）The erosion corrosion test results under different conditions show that with the increase of H₂SO₄ solution concentration,Si O₂ particle size and Si O₂ particle addition amount,the erosion quality loss of[Cr-Fe₄Co₄Ni₄]Cr_1.6Al_1.0Mo_0.4 coating increases gradually at first and then gradually becomes stable,and the change trend of the erosion quality of the coating is lower than that of substrate.By counting the erosion quality of the coating under different erosion conditions,a linear relationship model among the erosion quality Y,the concentration of H₂SO₄ solution X₁,the size of Si O₂ particles X₂,and the amount of Si O₂ particles addition X₃ is constructed:Y=-0.65+4.39X₁+0.83X₂+10.05X₃.The computational fluid dynamics（CFD）simulation results show that the higher the solid particle concentration,the greater the erosion loss of the impeller blade,which is consistent with the experimental results of erosion corrosion test.Supposing that[Cr-Fe₄Co₄Ni₄]Cr_1.6Al_1.0Mo_0.4 high entropy alloy coating with a thickness of 10mm is fabricated by laser cladding technology on 904L stainless steel,according to the coating erosion mass loss model,the service life of coating is predicted to 5.78 years.