Laser Preparation And Tribological Properties Of CoCrFeMnNiTi_x Coating

In order to improve the surface wear resistance of 40Cr steel under harsh service conditions,Co Cr Fe Mn Ni Ti_x high entropy alloy coating was prepared on the surface by laser cladding technology.The effect of Ti content on the microstructure and properties of Co Cr Fe Mn Ni Ti_x high entropy alloy coating was researched,and the structural properties,thermodynamic properties,elastic properties and electronic structure of were calculated by first principles.The relevant studies are as follows:Firstly,the technological parameters of laser cladding are optimized.Co Cr Fe Mn Ni high entropy alloy coating was prepared by different power and scanning speed.The forming quality,phase structure and mechanical properties of the coating were taken as evaluation indexes to determine the best preparation parameters.The results show that the phase with laser power of 1500 W and laser scanning speed of 8 mm/s is mainly composed of a single FCC solid solution,and the microstructure is typical dendrites.The microhardness of the cladding layer is 343 HV_0.5,the friction coefficient and wear are 0.344 and 0.0104 g,respectively,and the mass loss is 0.40 times of the 40Cr material.The surface microhardness and wear performance are the best.Secondly,the formation of intermetallic compounds is predicted and the mechanism of Ti-enhanced coating properties is revealed from the microscopic point of view.The possible presence of intermetallic compounds are calculated in the alloy by using first-principles,the results show that the lowest values of H_form and H_cohcorrespond to Ni₃Ti and Fe₂Ti respectively.And they’re all toughness.The shear modulus G and young’s modulus E of Co_xTi（x=2,3）,Fe_xTi（x=1,2）and Ni₃Ti are larger,which means it has a high hardness,The Co-p and Ti-d of Co₂Ti were hybridized and showed obvious brittle behavior.The Co Cr Fe Mn Ni Ti_x high-entropy alloy model was established by virtual approximation method（VCA）to calculate the influence of Ti element on the properties.The results show that the lattice constant and density of Co Cr Fe Mn Ni Ti_x alloy increase with the increase of Ti content,and the hardness of Co Cr Fe Mn Ni Ti_x alloy increases gradually,and it always maintains the mechanical stability and toughness.Finally,the microstructure,microhardness and tribological properties of Co Cr Fe Mn Ni Ti_x cladding layer with Ti content were studied based on the optimized parameters and theoretical analysis at the micro level.The results show that the surface forming quality of the cladding layer is good,and the microstructure is equiaxed,columnar,cellular and planar from shallow to deep,with the increase of Ti content,the phase of the coating changed from a single FCC solid solution phase to the coexistence of FCC phase and intermetallics.EDS analysis showed that intermetallic compounds mainly included Co₂Ti,Ti Ni₃ and Fe₂Ti phases,which was basically consistent with the calculation results.When Ti content increases to x=0.75,the hardness of cladding layer reaches the maximum value(511 HV_0.5),which is 1.9 times of that of substrate(256HV_0.5).At this point,the friction coefficient and wear amount are the lowest（0.276 and 0.001g,respectively）.With the increase of Ti content,the wear mechanism of the coating changes from adhesive wear and abrasive wear to fatigue wear and abrasive wear.