Study On The Solidification Microstructure And Residual Stress Characterization Of Laser Cladding Cobalt-based Alloy Coatings Of The Hermetic Surface Of Valves

Laser cladding is a promising method for surface modification of material. Cobalt-based alloy is a kind of important materials used in elevated temperature. Generally, the wear resistant properties of regular metal or alloy surfaces can be improved by cladding a Co - based alloy layer using laser modification. This technique has been widely used in industry.In the present work, the solidification microstructure, elements distributing, fracture characteristic and microhardness characteristic of the hermetic surface of valves was studied by Scanning Electron Microscope (SEM), Energy Disperse Spectroscopy (EDS) and microhardness testing technique. And the difference between laser cladding and plasma spray welding was investigated for giving support for the application of cobalt-based alloy laser cladding on repairing of the hermetic surface of valves. The experiment results show that the size of grain of laser cladding layer is finer than that of the plasma spray-welding layer, and no crack is found at the interface of those two layers from the profile of fracture. Therefore, laser cladding is suitable for repairing of the hermetic surface of valves if using proper processing technics. We also found the microstructure of the laser cladding inherit the growth orientation and pattern of the matrix, and is strongly influenced by that.Residual stress is a internal stress that commonly exists in material, how to measure that is very concerned by many scientists. Microindentation hardness testing is a well known experimental method used for determination of the constitutive properties of conventional materials, which can be used for macroscopic, microscopic and even nanoscopic by nanoindentation developed recent years. In the present research, a new method was developed for measuring the residual stress of the nonhomogeneous material by microindentation hardness testing. The relationship between residual stress and microhardness was studied, and a formula was given at last. That method was used for calculating the residual stress of laser cladding layer of the hermetic surface of valves. There is good agreement between our results and the others quoted from literature.