Effect Of Cr Content On Microstructure And Properties Of Laser-deposited Cu-Ni-Fe-Mo-xCr Alloy Coatings

Copper based alloy coating is widely used in industry because of its high conductivity,thermal conductivity and good machinability.However,poor wear resistance and oxidation resistance limit the service life of copper-based alloy under extreme conditions.Therefore,how to maintain the high conductivity and thermal conductivity of copper alloy as well as improve its wear resistance and oxidation resistance is the main direction and subject of the development of modern copper processing industry.In this paper,Cu-Ni-Fe-Mo-x Cr alloy coating with different Cr content was prepared on the surface of carbon steel by laser deposition using self-made Cu based alloy powder and Cr powder as deposition materials,and the powder composition was determined and the process parameters were optimized.The effect of Cr content on macro-quality,microstructure,phase structure,hardness,wear resistance and high temperature oxidation resistance of the coatings was revealed by macro photography,optical microscopy(OM),environmental scanning electron microscopy(ESEM)incorporating with EDX microanalysis,X-ray diffraction(XRD)analysis.The results show that when the laser power is 3400W,the surface morphologies of the coatings with Cr content ranging from 0 to and 5 wt.%are characterized by smooth and crack free while the surface morphologies of coatings with higher Cr content of 7.5 wt.%and 10 wt.%appear obvious cracks.Good metallurgical interface exists between coating and substrate.The microstructures of the coatings consist of dark grey matrix phase and white second phase,furthermore,both number and size of the second phase increase with an increase in Cr content.The second phase content in the coating is very low and the matrix phase mainly exhibits dendrite structure with Cr content of 0 wt.%.With the increase of Cr content,the microstructure of Cu(Ni)matrix changes from dendrite to cell.The phase composition of the coating without Cr is Cu(Ni),(Fe,Ni,Si)₂Mo and small amount of?-(Ni,Fe)while the coating containing Cr is Cu(Ni),(Fe,Ni,Si,Cr)₂Mo and small amount of Cr,?-(Ni,Fe,Cr),?-(Fe,Cr).The distribution of the added Cr is poor in the matrix area but rich in the second phase area.Cr promotes the liquid phase separation to produce more hard second phase,which improves the macro hardness of the coating.With the increase of Cr content,the high temperature oxidation resistance of the coating is enhanced.The oxide film on the surface of the Cu(Ni)is composed of Cu O.In addition,a thin Ni Cr₂O₄ oxide film is formed on the surface of(Fe,Ni,Si,Cr)₂Mo,which is reported to be a dense spinel structure so as to prevent further oxidation inward.The wear resistance of the coating increases with the increase of Cr content.The matrix and the second phase display typical wear characteristics of oxidation and abrasive,respectively.The(Fe,Ni,Si,Cr)₂Mo phase has high hardness and good oxidation resistance,which can improve the wear resistance of the coating.The best addition of Cr is 5%.In this case,15vol.%(Fe,Ni,Si,Cr)₂Mo is formed.The coating presents good oxidation resistance and wear resistance,together with few defects as cracks and holes in it.