Preparation,Microstructure,and Properties Of FeCoCrNiMnTi High Entropy Alloys By Laser Cladding

The high entropy alloys（HEAS）are the novel alloys that contain five or more primary elements with equimolar or nearly equimolar ratios.Because of the high entropy effect,the simple solid solution structure can be obtained,giving the HEAs greater overall properties than conventional alloys.Because of the relatively high cost of raw materials used in the preparation of HEAs,as well as the high cost of producing the entire high entropy alloy component,the application of HEAs in surface engineering can fully exploit its advantages,and the preparation of high entropy alloy films and coatings becomes a new development trend.Laser cladding is a revolutionary surface processing technology with high energy density,quick solidification,low dilution,and superior controllability.It can make a high entropy alloy cladding layer with a compact structure,consistent composition,good substrate adhesion,and good performance on a low-cost metal substrate.In this thesis,laser cladding technology was used to prepare a cladding layer of FeCoCrNiMnTix（B4C）y series high entropy alloys on Q355 steel.X-ray diffraction（XRD）,metallographic microscope（OM）,scanning electron microscope（SEM）,and X-ray energy dispersive spectrometer（EDS）were used to examine the phase structure and microstructure of the cladding layer.The cladding layer’s microhardness,wear resistance,and corrosion resistance were tested and analyzed using Vickers microhardness tester,wear testing machine,and electrochemical workstation respectively.It was shown that the Fe Co Cr Ni Mn high entropy alloy cladding layer consisted of facecentered cubic（FCC）solid solution.The addition of Ti promoted the production of bodycentered cubic（BCC）and hexagonal phases.BCC,FCC,and hexagonal solid solutions make up the cladding layer.The microstructure was mostly equiaxed fine grain structure.When the Ti content was higher（x=0.8）,the Laves phase formed.The addition of the B4 C ceramic phase to FeCoCrNiMnTix（x=0.4,0.6,0.8）refined the cladding layer’s microstructure,and promoted the formation of the carbides and the borides,but prevented the formation of Ti based hexagonal phase.The cladding layer consisted of FCC phase,BCC phase,carbide,and boride.The cladding layer’s microstructure changed from equiaxed crystal to needle-like,the needle-like phase was BCC solid solution consisting mainly of Cr and Fe elements.The microhardness of the FeCoCrNiMnTix（x=0.5,0.6,0.8,1.0）high entropy alloy cladding layers increased as the Ti content increased,which could be attributed to the effect of solution strengthening,microstructure strengthening,and grain refinement.The wear resistance of the cladding layer increased at first and then decreased as the Ti content increased,with Ti0.6alloy having the best wear resistance.When the Ti content was less than x=0.6,the alloy’s wear resistance was positively correlated with its hardness.However,as Ti content reached x=0.6,the brittle and hard Laves phase was formed,and the larger lattice distortion increased the internal stress,which resulted in flake peeling of the wear surface.As a result,for Ti0.8 and Ti1.0cladding layers,the wear resistance decreased.With the addition of B4 C,the microhardness of the FeCoCrNiMnTix（B4C）y（x = 0.6,0.8,1.0;y = 0.2,0.4,0.6）HEAs cladding layer decreased at first and then increased.When a small amount of B4C（y = 0.2,0.4）was added,the amount of carbide and borides less,and the strengthening effect of the second phase was not notable.The precipitation of carbide and boride,on the other hand,reduced the solid solution content of Ti,Cr,and other elements in the solid solution,and induced the reduction or even disappearance of the harder hexagonal phase in the alloy,which resulted in a tendency to decrease in alloy hardness.When the B4 C content reached y=0.6,the carbide,boride,and BCC phase content increased,as did the alloy’s hardness.Because the alloy contained a large number of fine hard phases（carbides and borides）,the wear resistance of the cladding layer was improved.When y=0.6,the content of the second phase increased,making the cladding layer too brittle,and the wear resistance of the alloy decreased.The corrosion resistance of the alloy increased initially and then decreased as the B4 C content increased.The（B4C）0.4 alloy had the best corrosion resistance.The corrosion resistance was closely related to the alloy’s microstructure.The finer the structure of the alloy,and the more uniform the distribution of the elements,the better the corrosion resistance.To summarize,the FeCoCrNiMnTi0.6（B4C）0.4 cladding layer could achieve excellent overall performance.