Geometry Characteristics And Mechnism Of Spatial Multi-direction Cladding Layer By Annular Laser Powder Feeding

In the fields of aerospace,military industry,metallurgy and shipbuilding,some large or super large parts will inevitably be damaged.There is a huge market demand for the rapid in-situ repair of parts due to their high added-value.For the repair of the parts which are inconvenient to disassemble or move and that of parts with complex spatial structures,the existing laser cladding technology is still unable to meet the demand.In this paper,a spatial multi-directional inside-beam powder feeding laser cladding technology is proposed,which can keep the repaired parts in place,while the laser cladding nozzle performs in a wide spacial range,to achieve in-situ repair of large parts.The detailed research contents,methods and results are as follows:(1)Based on inside-beam powder feeding cladding nozzle,the method of adding protective covers and a circumferential protective air curtain is introduced,to prevent splashing materials from falling into the cavity of the cladding nozzle.It can achieve longterm stable cladding work under a large inclined angle and expand the application range of inside-beam powder feeding laser cladding system.Therefore the most important equipment problem of "in-situ repair by laser" is solved.(2)The influence of the nozzle tilt angle on the powder concentration distribution and the laser-powder coupling are studied.Through the gas-solid two-phase flow simulation method,the powder concentration distribution in different directions in space is obtained.As the tilt angle of the nozzle increases,the powder concentration center gradually deviates from the center axis of the powder tube,and the maximum deviation of the powder concentration is at 90°.Studies have shown that the collimated air pressure has a greater impact on the deviation of the powder concentration in the process of spatial multidirectional cladding.The adjustment method of "laser-powder coupling eccentricity" is proposed,that is,when the collimation gas pressure is in the range of 0.0125 MPa～0.0625 MPa,by adjusting its value,the powder concentration distribution can be effectively changed and the laser-powder coupling eccentricity can be reduced.(3)Models are established between aspect ratio,the forward and backward contact angles,the vertex offset of the spatial multi-directional single cladding bead and the tilt angle of the substrate and the process parameters.Through single-factor experiments,the influence of process parameters such as substrate tilt angle,laser power,scanning speed,powder feeding rate,collimation air pressure and defocus amount on the laser bead aspect ratio,forward and backward contact angles and vertex offset are analyzed.Orthogonal experiments are used to analyze the weights of the above factors on the aspect ratio,the forward and backward contact angles and the vertex offset of the cladding layer.The results show that laser power,scanning speed,collimation air pressure,and the tilt angle of the substrate have great influences on the aspect ratio;the tilt angle of the substrate,the powder feeding rate,defocus amount and scanning speed have great influences on the forward and backward contact angles as well as the vertex offset.The response surface method is used to establish a function model of the aspect ratio,the forward and backward contact angles,the vertex offset of the cladding bead with the key parameters.(4)From the force condition on the spatial molten pool,the cause and mechanism of the cross-sectional morphology of the spatial multi-directional laser bead are derived theoretically for a relationship model.The cross-sectional profile model is established.The double-ellipse model and the double-eccentric-circle model are used to fit the geometry profile of the spatial laser bead.The mathematical relationship between the overlapping ratio and the aspect ratio,contact angle,vertex offset is obtained.The overlapping ratio model is verified by experimental methods.The results show that according to the data calculated by the overlapping ratio model,the overlapping cladding layer can be obtained with a flat surface.(5)The continuous laser cladding of the spatial curved surface is carried out by changing the process parameters in segments with the tilt angle of the substrate.In the range of different inclination angles of the substrate,different process parameters are selected to carry out the spacial cladding experiment.The results show that the cladding layer is bright and smooth.There are no unmelted region or pit defects in the cladding layer,and the thickness is uniform.Through the combination of theoretical research,simulation analysis and experimental verification,the geometry characteristics and mechanism of spatial multi-directional cladding layer are studied in this paper.The relationship between the cross-section geometry characteristics of spacial single cladding bead and process parameters are established.The mathematic model of multi-layer overlapping ratio is obtained with cross-section geometry characteristics.Finally,the fine planeness of spacial cladding layers is realized according to the process method of changing process parameters in different directions.The research results of this paper have certain practical significance for realizing laser in-situ repair of large structural parts.