Microstructure And Properties Of Ultrasonic-assisted Laser Cladding IN718-Hf Composite Coating

Due to controllable heat input,wide material selection and flexible processing,laser cladding technology has been widely used in the aerospace and automobile manufacturing fields.However,the oxide film on the coating surface is prone to excessive oxidation under high temperature condition due to coarse grain structure and severe element segregation,which hinders the application of laser cladding processing in the high-temperature surface engineering field.As a result,the reactive elements and ultrasonic field are added in the laser cladding process to prepare composite coating.The addition of Hf elements into coatings and application of ultrasonic vibration during laser cladding can effectively increase high temperature performances of IN718 coating.The main results of the paper are as follows:(1)Cavitation effect,sound flow enhancement effect and thermal effect induced by ultrasonic field and molten pool are systematically studied.According to ultrasonic theory,molten pool dynamics and rapid solidification theory,the influence of ultrasonic vibration on the undercooling and the grain refinement in the molten pool are all discussed.The prediction model of average grain size from cladding layer assisted-with ultrasonic vibration is established.(2)Based on the orthogonal experiment,the optimization of laser cladding process parameters was carried out.Laser power,scanning speed and powder feeding rate are acted as factors.Aspect ratio and dilution rate of the cladding layer are used to measure the cladding layer quality.Moreover,in order to obtain the optimal process parameters,the multi-target comprehensive balance method is applied to analyze experimental results.Based on the optimal single-layer cladding process parameters,the optimization of overlap rate from multi-layer cladding was carried out.Therefore,the optimal laser cladding process parameters of the multi-layer cladding layer are obtained.(3)The ultrasonic vibration-assisted laser cladding device was built and high temperature oxidation tests for IN718 coating,IN718-Hf coating and IN718-Hf coating with ultrasonic vibration were carried out.Microstructure and high temperature oxidation resistance of coatings were studied.Scanning electron microscope(SEM)equipped with energy spectrometer(EDS),electron probe micro-analysis(EPMA)and X-ray diffraction(XRD)were used to analyze oxidation kinetics,phase composition and morphology of the oxide film from coatings oxidized under 750?,800? and 850? for 100 h.The effect of adding Hf element and ultrasonic field on the high temperature oxidation resistance of coatings was explored.(4)The influence of Hf and ultrasonic field on the high temperature wear performance of cladding layers was deeply explored.Micro-hardness tester,dry sliding friction tester and ultra-depth of field three-dimensional microscope were used to analyze the high temperature wear behaviors of IN718 coating,IN718-Hf coating and IN718-Hf coating with ultrasonic vibration worn for 20 minutes at 800?.The influence of adding Hf and applying ultrasonic field during laser cladding process on micro-hardness distribution,average friction factor and wear volume loss of coatings were investigated.The mechanism of the effect of Hf and ultrasonic field on the high temperature wear resistance of the coating is clarified.