Process And Forming Mechanism Of Non-supportive Overhang Part By Annualar Laser Cladding

Laser cladding forming of overhang structures is very promising in the industries of aerospace,military and civil use,as it doesn't only help the direct forming of complex parts but also the remanufacturing of high value-added parts.The overhang part was mainly formed based on the method of offsetting between the neighbor layers.However,the unsupported overhang could be made by applying a small offset which could only give out a limited inclination angle.What's worse,the offset end of each cladding layer deformed due to no constraint.For achieving larger inclination angle,auxiliary supports must be formed together with the part,who were removed afterward.In this way,the process complexity was greatly increased.Besides,the "step effect" and residual powder adhesion reduced the surface finish quality and shape accuracy of the formed overhang part.In this paper,a vertical tangential layering method(VTL)is firstly introduced based on the annular laser forming technology.The new laser cladding system exclusively for forming the overhand part is integrated.This method contributes a full cover between the neighborlayers by the continuously spatial moving of the inside-beam powder feeding nozzle,who gives the possibility of forming non-supportive overhang part with any inclination angle.An energy distribution model for annular laser mode is optimized.The temperature field under different defocus amounts and energy peak positions is numerically simulated.The results show that energy distribution exhibits a higher edge with a slightly lower center,namely "saddled shaped".Due to the heat loss compensation at the edge,the temperature field is evenly distributed in the vertical direction to the scanning direction,while lower in the front and higher at rear in the scanning direction.The experimental results align with the simulation.The powder convergence model is established by analyzing the force effect to the powder particle.With the increasing of the inclination angle,the powder convergence gets worse.The exist of the protection air helps the powder stream coupled with the laser beam,however,the powder density gets lower if the air pressure is over loaded,which also means a powder utilization drop.The liquid molten pool tends to drop by the drag force of its gravity,and the peak height offset is also studied.For cladding at different inclination angles,the effect of process parameters to the width and height shows a good accordance with the horizontal cladding.However,the height of the cladding layer is dramatically affected by the laser power,scanning speed and the inclination angle.The peak height offset of the molten pool increases firstly and then drops with the increasing of inclination angle,while continuously increases with the increasing of laser power and decreasing of the scanning speed.Based on Lambert-Beer and Mie's theory,a coupled heat transfer model among the annular laser,powder and the substrate is established.Meanwhile,a high-speed camera shooting system for monitoring the molten pool is constructed.The surface roughness of the annular laser formed part is analyzed.The results show that the interaction area between the annular laser and the substrate turns to a fully melted pool if the laser power is high enough,which absorbs most of the powder particles to the pool and brings a smooth surface finish.The attenuation rate of the powder to the annular laser increases due to the axial offset between the laser beam and powder stream,which makes an unbalanced energy distribution at the ring area.Its effect to the surface roughness is obviously directional.The increase of the powder carrier gas strengths the powder impact to the molten pool boundary,which reduces the probability of powders' being captured.Thereby residual powder adhesion at the edge of the molten pool is deducted.By the precisely mating process parameters such as the laser power,scanning speed,protection air pressure,and powder carrier gas amount,and controlling the movement of the working table where the substrate is seated,parts with classical overhang feature are successfully formed,whose maximum inclination angle reaches 150°.Besides,a stable forming process,smooth surface roughness at Ra<6.3?m,small dimensional error,fine and dense microstructures are achieved at the same time.The VTL method studied in this paper significantly expands the complexity and inclination angle limit of laser caldidng forming parts.What's more,the formig function,efficiency and accuracy are improved accordingly.This paper is of great significance to the richness the laser spatial forming theory,promotion and development of annular laser forming technology.