선박 배기밸브의 리엔지니어링을 위한 레이저 클래딩에 관한 기초 연구

Abstract

Materials used in modern industry are commonly exposed to very aggressive environments such as high temperatures, extreme wear condition or high mechanical stresses, etc.. So, Cladding is applicable to many kinds of components. This process is widely used for improving the surface and near surface properties such as wear and corrosion resistance of a part or to re-surface a component that has become worn through use. Also, It involves the creation of a new surface layer on top of a base metal. Most traditional cladding can be divided into two-types technique, arc welding and thermal spraying. These are have its own advantages and drawbacks. But compared to with conventional surface technique including TIG welding, SAW, PTA and thermal spraying, laser cladding has some distinct advantages related to coating quality and process. Laser cladding is an advanced hard-facing process producing thick coating with metallurgical bond to the substrate and one of the industrial preferred surface engineering techniques for its ability to apply a chemically different materials as a layer onto a given substrate. Laser cladding combines the features and improves the drawbacks of existing techniques. It can produce denser coatings with little or no porosity, minimal dilution, minimal distortion of the workpiece, reduced heat affected zone, better surface quality, simplicity to automate and reduction in processing times. Furthermore, high power diode laser used for this process is the most proper laser source due to the higher efficiency, the larger absorption of the powder in the specific wavelength of the radiation emitted by the diode laser and the lower price.

This study is related to develop fundamental technologies on laser cladding for repairing of marine diesel engine exhaust valve. Stellite 6 powder injection laser cladding was conducted on heat resisting steel SNCrW substrate by using diode laser. The powder can be blown by carrier gas through coaxial nozzle.

The laser power, beam traveling speed, powder feed rate, overlapping ratio and layer number were regarded as dominant parameters, and the criterion of overlapping ratio was defined as beam size between width of clad layer and beam size by experiments. The effects of process parameters were studied by observing the geometry of clad and measuring the width, height and dilution ratio of clad layer. The heat input was proportion to width and dilution ratio of clad layer and inverse proportion to height and hardness. Hardness was measured in the longitudinal and transverse direction, EDS and EPMA on cross section of 1st, 2nd and 3rd layer & multi-pass clad was carried out to investigate characteristics of clad with various parameters.