Studies On Mechanical Properties And Microstructure Of Laser Cladded Graphene Nanoplatelets Reinforced Inconel 625 Superalloy Composite Coatings

Laser cladding is an advanced and high-efficient surface modification technology to deposit a hard layer with controlled thickness on the selected area of a metal substrate.Mainly because of its superior characteristics,such as high power density,rapid cooling velocity as well as small heat-affect zone(HAZ)in the base material,this technique has wide applications in aviation,marine and turbine industries and so on.In recent years,laser cladded Inconel 625(IN625)have a wide application in turbine engines,chemical,marine and nuclear industries because of its superior wear,corrosion and oxidation resistance.Associated with the new concept of metal matrix composite(MMC)which combines its metallic properties with ceramic properties to display greater resistance to aggressive environment,higher hardness,advanced mechanical strength and improved wear properties were continuously founded beyond the pure material.However,the carbon nanofibre-based reinforcement in laser cladding has been seldom studied because the fibre structure is vulnerable in highly intensive laser beam.In this study,a protective coating was designed surrounded the carbon materials to prevent from damage during the laser cladding process.Specifically,an innovative and simplified method of electroless pure-nickel coating was stably and evenly plated on graphene nanoplatelets(GNPs).The nickel-coated graphene nanoplatelets(NiGNPs)were uniformly dispersed in Inconel 625(IN625)powder with the ratio of 1:9 by ball milling.Then laser cladding was used to fabricate the NiGNPs reinforced IN625 composite coating,GNPs added IN625 and pure IN625.The survivability of GNPs,morphology of precipitated phase,microhardness through transverse section,wear resistance at room and elevated temperatures and mechanical properties were studied.The results indicated that the NiGNPs were successfully incorporated into IN625 coatings by laser cladding.Part of them were deduced to be damaged or dissolved into molten pool,which benefited the formation of NbC,but most of them could be observed as exposed GNPs and well-distributed welded GNPs.Whereas without protective coating,less GNPs and more chromium carbide can be founded in the GNPs reinforced laser cladded samples because naked GNPs can be easily dissolved in the molten pool and enhanced the formation of carbides.Compared to laser cladded IN625 coating,the composite coating showed higher hardness at bottom region of cladded coating and smaller size of HAZ due to the improvement of thermal conductivity by additional NiGNPs.Comparative wear test demonstrated that friction coefficient(COF)decreased from 0.6,0.35,0.34 and 0.38 to 0.55,0.33,0.3 and 0.29 at room temperature,550°C,600°C and 650°C respectively.The wear rate were also decreased at both room and elevated temperatures.Combined with wrinkled,tiled and rolled GNPs observed on the worn surface,it was revealed that the wear properties were efficiently benefited from superior lubrication between interlayers of GNPs.Furthermore,the GNPs reinforced composite coating exhibited highest yield strength up to 593 MPa,but the elongation was seriously decreased from 33.1% to 25.5%.Whereas,the NiGNPs samples,with lower yield strength,demonstrated around two times of elongation which benefited from the protective coating on GNPs.Thus,GNPs could survive and act as reinforced in laser cladding process under protective coating,which effectively enhance the hardness,wear resistance and yield strength of IN625.