Microstructure And Mechanical Properties Of Composite Coatings On INVAR Alloy By Laser Cladding

In order to strengthen Invar alloy and expand its application scope without changing its low coefficient of thermal expansion,nanostructured carbide-strengthened cobalt-based cladding layers were fabricated on an Invar alloy by laser cladding.The cladding materials are mainly composed of nano WC,VC,Cr₃C₂ and Co based alloy?Co,Si,B,ZrC,and Y₂O₃?.After cutting,polishing,ultrasonic cleaning and corrosion,the microstructure of cladding layer and interface was observed and analyzed by optical microscopy and scanning electron microscopy,and the phase,composition and structure of cladding layer were characterized by X-ray diffraction,energy-dispersive spectroscopy and field-emission transmission electron microscopy.When the powder thickness is 1mm,the recommended parameters after the laser cladding process optimized are:a laser power of 1.4 kW;a laser scanning speed of 5 mm/s;and a protective gas:15 L/min of argon.The main defects of the cladding layer are cracks and a small number of pores.The cladding layer near the Invar alloy is prone to form cracks with a grain boundary direction.The results of microstructure analysis show that the cladding layer consists of?-?Fe,Ni?and Co-based solid solution,W₂C,Cr₃C₂,V₈C₇,CoC_x and NbC.Elemental diffusion,carbide strengthening,and interface strengthening through the dislocations and plastic deformability at the carbide/matrix interface were believed to be the surface strengthening mechanism of Invar alloys.Friction and wear test results indicated that compared with Invar alloy the wear resistance of the cladding layers is obviously improved and VC-doped cladding layers exhibited better wear resistance than that of cladding layers with Cr₃C₂ or VC and Cr₃C₂.The wear mechanism of Invar alloy is mainly adhesive wear and fatigue wear,while that of cladding layers is abrasive wear.The results of high temperature oxidation resistance experiments show that under 800?,the high temperature oxidation resistance of the cladding layers was significantly improved compared with that of Invar alloy.The thickness of the oxide layer in cladding layers was 18.4?m,while that of Invar alloy was 88.38?m.Cr₃C₂ displayed higher oxidation resistance than that of VC and a mixture of Cr₃C₂ and VC,it is the dense oxide layer Cr₂O₃ generated in interface that helped hindering the further diffusion of oxygen.