The Process And Characterization Of Coating Structure And Properties Of High-speed Laser Cladding

Laser cladding can use metallurgical bonding and low dilution rate to prepare high-quality,non-porous and crack-free coatings in a variety of materials,effectively reducing the loss of materials in the industry due to wear and corrosion.However,the thickness of the traditional laser cladding coating is too thick,and the low surface preparation efficiency limits its use range.The Fraunhofer Laser Technology Research Institute in Aachen,Germany has developed an ultra-high-speed laser cladding technology,which can effectively increase the thickness of the cladding coating and greatly improve the coating preparation efficiency,effectively broadening the application range of laser cladding.This paper uses high-power YLS-6000 fiber laser,ABB robot,double barrel powder feeder,new type high-speed laser cladding powder feeding device and micro-machine tool to build a high-speed laser cladding experiment platform.Using high-speed laser cladding technology to cladding Ni45 alloy coating on the surface of 45#steel shaft with a diameter of 40mm,the structure and distribution of high-speed laser cladding were explained through the classic solidification theory.The influence of process parameters and cooling rate on the coating morphology,size,microstructure and phase structure,and the difference in microhardness,wear resistance and corrosion resistance of the coatings prepared by traditional and high-speed laser cladding under the same material.The main research results are as follows:High-speed laser cladding can achieve a cladding speed of 76.86 m/min and a cladding efficiency of 156.79 cm²/min.Compared with traditional laser cladding,the speed and efficiency are increased by 128 times and 8.7 times,respectively.Compared with traditional laser cladding,the surface of high-speed laser cladding coating is smoother and has a lower roughness.The relative positions of the laser focus and powder focus and the workpiece have a great influence on the high-speed laser cladding forming.When the two focal points are merged 4 mm above the workpiece,a well-formed cladding layer with a low dilution rate can be obtained.The increase in cladding speed during high-speed laser cladding will effectively reduce the thickness of the coating.When the cladding speed is 76.86 m/min,a uniform coating with a thickness of approximately 53.53μm can be obtained.Similar to the traditional laser cladding,the high-speed laser cladding coating is a columnar crystal grown perpendicular to the interface at the interface area,the central area is transformed into dendrites with no obvious directionality,and the top area is very fine dendrites and equiaxed crystals.The difference is that at a high laser cladding rate,the R value（growth rate of the dendrite tip）increases,resulting in a decrease in the G/R ratio（supercooled tissue）,and the microstructure tends to be an equiaxed dendritic structure.Therefore,as the scan rate increased from 37.68 m/min to 76.86 m/min,the microstructure of the coating was gradually refined to a much greater degree than conventional laser cladding,and the size was observed at the top area when the cladding speed was 76.86m/min Nanocrystals around 20-100 nm.In high-speed laser cladding,the cooling rate difference of each area of the coating is far more than that of traditional laser cladding.The distance between the secondary dendrite arms from the interface area to the top area gradually decreases.The cooling rate is exponential as the secondary dendrite arms decrease.When the cladding speed is 76.86 m/min,the top cooling rate can reach 1.68×10⁶℃/s,which exceeds 100 times the bottom cooling rate.The average microhardness of the surface of traditional laser cladding Ni45 is only337HV0.2,while the microhardness of the surface of high-speed laser cladding is much larger than that of traditional laser cladding,up to 543 HV_0.2.However,the difference in the cooling rates of the high-speed laser cladding regions results in a large difference in the size of the microstructure.In addition,the top region generates nanocrystals at a very high cooling rate,which greatly enhances the microhardness.As a result,the microhardness of each section of the high-speed laser cladding coating section is different,and its top hardness>middle hardness>bottom hardness.With the increase of the cladding speed in high-speed laser cladding,the microstructure is continuously refined,and the coarse columnar crystals are transformed into fine dendrites,and the formation of nanocrystals in the top area greatly increases the hardness of the cladding coating,The denser microstructure also improves the wear resistance of the cladding coating.The corrosion resistance of the high-speed laser cladding coating is better than that of traditional laser cladding,and the presence of nanocrystals in the surface layer of the cladding layer with a laser cladding speed of 76.86 m/min increases the element spreading rate,making the surface thicker The Si O₂surface film can effectively inhibit the corrosion process of metals and enhance the corrosion resistance of the coating.