| Due to its excellent thermal and electrical conductivity,commercially pure copper(cp Cu)has been widely applied to many fields,such as metallurgy industry,marine equipment,military industry and so on.However,the copper parts would be damaged locally to reduce their service life because of its poor hardness and wear resistance,which restricts its application a lot.Compared to ferrous materials and titanium,there exists many problems to make laser-alloyed layer in good quality on the surface of cp Cu because of its high thermal conductivity and low absorption to laser.Nowadays,equipment updating,alloying material choosing and parameters optimizing could be the hot issues for researchers.Ni60A and Ni60A-x wt.%Ti(x=10,20,30)laser-alloyed layer has been made on the surface of cp Cu by laser surface alloying.In the research,the influence of different parameters on its microstructure evolution,wear resistance and corrosion resistance have been analyzed.SEM,EDS and XRD are used to characterize the microstructure,phases and element constitution of the laser-alloyed layer respectively.And abrasive tester and electrochemical workstation are used to test the wear resistance and corrosion resistance demonstrated by friction coefficient,mass loss and corrosion current density.Results indicated that the Ni60A layers were mainly composed of γ-Ni,Cr23C6,Cr2Ni3,B2Fe3Ni3,Cu0.81Ni0.19 and so on.There are lots of irregular massive structure mixed with dendritic and granule structure,which could become dendric and cellular structure with the increment of laser power and scanning speed.When laser power P=5.0kW and scanning speed v=600mm/min were applied,Ni60A laser-alloyed layer had the maximu microhardness 1179.2HV0.2,that was about 18 times than that of cp Cu and it had lower mass loss and friction coefficient than cp Cu.Besides,it showed that pitting corrosion happened on the surface of the laser-alloyed layer and the lower corrosion current density also indicated that it had better corrosion resistance.While it also found that the interface between the cp Cu substrate and Ni60A laser-alloyed layer was flat,showing that metallurgical bonding was not so well between them.So pure Ti powder was added to as-received Ni60A powder.It was found that the interface between the Ni60A-x wt.%Ti(x=10,20,30)laser-alloyed layer and the cp Cu substrate was concave,and at the same time the cp Cu substrate was melted more participating in the reaction of the molten pool so that better metallurgical bonding was obtained between them.The Ni60A-Ti laser-alloyed layer contains more kinds of phases,such as NiTi,Ni2Ti,Ni3Ti,TiCr2,TiC and other compounds.The precipitation and dispersion of these compounds could further improve the hardness and wear resistance of the alloying layer,and what’s more the intermetallic compounds such as NiTi,TiCr2 and so on could improve the corrosion resistance.Based on a large number of experiments,it was found that the Ni60A-10Ti,Ni60A-20Ti and Ni60A-30Ti samples were of the best quality when the scanning speed was v=500mm/min.When the laser power is too low,because of the insufficient melting of alloying materials,a large area of some elements such as Ti and Cr were formed.When the laser power is too large,the convection in the molten pool was violent and the organization was chaotic,undirected and non-uniform.Under the optimal process parameters,the cellular microstructure was evenly distributed in the matrix of Ni60A-10Ti laser-alloyed layer,and there was some lamellar and granular microstructure in the upper part and the bottom part densely precipitated granular microstructure.And the morphology of Ni60A-20Ti and Ni60A-30Ti laser-allyed layer was close.The lamellar crystal was formed on the matrix of the alloying layer,and the small amount of needle-like microstructure was interlaced,during which the flower clusters were precipitated.It was observed that the clusters were aggregated by granular precipitates.With the increase of Ti content,the hardness and wear resistance of the laser-alloyed layer were increased,the average value of the microhardness of the Ni60A-30Ti is 1305.1HV0.2,which is about 21 times that of the cp Cu substrate.The mass loss was much lower than the pure copper and the friction coefficient was stable at-0.33.The scratch was shallow and a small amount of tiny debris was retained,and slight abrasive wear and adhesion happened on the surface.It was known that the hardening mechanism of Ni60A-x wt.%alloying layer includes solid solution strengthening,precipitation strengthening and dispersion strengthening,in which the effect of precipitation strengthening and dispersion strengthening is more significant.The experimental results showed that the corrosion current density of Ni60A-x wt.%Ti alloying layers were not all lower than that of the pure copper.Due to the improper selection of the process parameters,the microstructure of the alloying layer could be inhomogeneous and the defects such as the pores and microcracks existed,which formed microelectric pool in the alloying layer because of the potential difference,causing the increment of corrosion current density.In the electrochemical corrosion process,the surface of the alloying layer in good quality was pitting on the surface evenly,with needle-like or short rod-like microstructure containing Ni,Ti,Cr,showing that the compounds of NiTi,Ni2Ti,TiCr2 and so on were corrosion-resistant phases,and the needle-ike or short rod-like microstructure were corrosion-resistant microstructure.When P=5.0kW and v=500mm/min,the corrosion current density of the Ni60A-20Ti alloying layer was 0.0395A·cm-2,which was lower than the pure copper,indicating that the alloying layer had lower corrosion rate.In addition,the corrosion potential of the alloying layer is generally lower than that of the pure copper,which is determined by the properties of the alloying layer itself,and could be used as the anode protection to protect the pure copper substrate under the corrosion condition.Due to the poor flow of pure metal powder,the fluidity of Ni60A-x wt%Ti mixed Ti powder melts would be affected,and so that the process parameters were more stringent.Results showed that the reaction of the molten pool would be promoted when as-received Ni60A powder were mixed with a certain amount of pure Ti powder.The pure copper substrate was melted more involving in the reaction of the molten pool so that a variety of phases were generated,which is benefit for the further improvement of microhardness,wear resistance and corrosion resistance.And more important,there was a better metallurgical bonding with concave interface between the Ni60A-xTi laser-alloyed layer and pure copper substrate. |
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