Microstructure And Properties Of Laser Cladding Al_xCoCrFeNiCu High-entropy Alloy Coatings

Structural steel is widely used in industry,but due to increasingly complex working conditions,it is often subject to wear and corrosion in service conditions,resulting in damage or failure of workpieces,resulting in huge economic losses.In order to solve this practical problem,the Al_xCoCrFeNiCu（x=0,0.5,1,1.5,2）high-entropy alloy coating and in-situ ceramic particles（TiB₂,TiC）high-entropy coating were prepared on the surface of Q235 steel by laser cladding technology.Alloy composite coating.The macroscopic morphology and microstructure of the coating were observed by stereo microscope,metallographic microscope and field emission scanning electron microscope.The phase,hardness,friction and wear properties and corrosion resistance of the layers were characterized.Concluded as follow:（1）By designing a four-factor and three-level orthogonal experiment,and performing range and variance analysis on the results,the optimal laser process combination was obtained:powder thickness 1.25mm,scanning speed 180mm·min^-1,current size 220A,defocus amount-7mm.（2）The Al_xCoCrFeNiCu high-entropy alloy coating is compact and has no pores.With the increase of Alcontent,the strength of the coating increases,the toughness decreases,and cracks appear inside the high-alumina coating.The phase of the Al_xCoCrFeNiCu high-entropy alloy coating consists of a simple solid solution phase,and with the increase of Alcontent,the phase of the coating changes from a single FCC phase to FCC+BCC dual phase,and then to a single BCC phase.The coating structure is composed of equiaxed dendrites,columnar dendrites and planar crystals.With the increase of Alcontent,the grain size of columnar dendrites in the middle of the coating gradually decreases.When Al₂CoCrFeNiCu,the columnar dendrites disappear.Alelements are enriched on dendrites,Cu elements are segregated between dendrites,and other elements are uniformly distributed in the coating.（3）With the increase of Alcontent,the microhardness of Al_xCoCrFeNiCu high-entropy alloy coating increases gradually.Among them,the Al₂CoCrFeNiCu coating has the highest hardness,reaching 673HV_0.2,which is 3.3 times that of the CoCrFeNiCu coating.With the increase of Alcontent,the wear resistance of the coating is improved,the friction coefficient of the coating decreases gradually,and the wear rate decreases gradually.The Al₂CoCrFeNiCu coating has the lowest friction coefficient and the smallest wear rate,and its wear rate is only 2.3%of that of the CoCrFeNiCu coating.The wear mechanism of CoCrFeNiCu,AlCoCrFeNiCu coating is adhesive wear,the wear mechanism of AlCoCrFeNiCu coating is adhesive wear accompanied by abrasive wear,and the wear mechanism of high Al_1.5CoCrFeNiCu,Al₂CoCrFeNiCu coating Abrasive wear is dominant.（4）The addition of Alelement reduces the overall corrosion resistance of the high-entropy alloy coating.With the increase of Alcontent,the corrosion resistance of the coating first increases and then decreases.Among them,the AlCoCrFeNiCu coating has the most positive corrosion potential,the smallest self-corrosion current density,the largest impedance modulus value and the highest phase angle,showing the best corrosion resistance.The corrosion behavior of Al_xCoCrFeNiCu high-entropy alloy coating in3.5%Na Cl solution is mainly pitting corrosion.The Al₂CoCrFeNiCu coating has the largest number of corrosion pits per unit area,the deepest average depth and the worst corrosion resistance.（5）Tiand B₄C powders were added to the AlCoCrFeNiCu high-entropy alloy coating to prepare in-situ ceramic particle high-entropy alloy composite coatings with different mass fractions（10%,20%,30%）.The coating is well formed,and no defects such as pores,cracks and undercuts occur.The phase of the in-situ ceramic particle HEA composite coating is composed of FCC+BCC,TiB₂and TiC,and the content of BCC phase increases with the increase of Tiand B₄C mass fractions.The coating structure is composed of reticular tissue,and TiB₂particles with rectangular and hexagonal characteristics and TiC particles with symmetrical structures such as cross-shaped or fishbone-shaped grow uniformly in the inner of the reticular tissue.（6）Compared with the AlCoCrFeNiCu high-entropy alloy coating,the hardness of the in-situ ceramic particle（TiB₂,TiC）high-entropy alloy composite coating is significantly improved,and increases with the increase of the added Tiand B₄C mass fractions.Among them,AlCoCrFeNiCu+（Ti+B₄C）-30%（abbreviated as HEA-30%）coating has the highest hardness,reaching 1012HV_0.2.Compared with the AlCoCrFeNiCu high-entropy alloy coating,the in-situ ceramic particle high-entropy alloy composite coating shows a lower friction coefficient and a smaller wear rate,and the wear resistance is significantly improved,and the wear resistance increases with the increase of Tiand B₄C mass fractions Performance goes up.The wear mechanism of the in-situ ceramic particle HEA composite coating is mainly abrasive wear,accompanied by a small amount of adhesive wear.TiB₂grows with B₄C as the nucleus in the coating to form hexagonal and rectangular ceramic particles,which are formed preferentially in the system before TiC.The strengthening mechanism of in-situ ceramic particles in the coating is mainly dispersion strengthening and fine-grain strengthening.