| Laser cladding prototyping technology is a complex physical, chemical, metallurgical process. Cladding parts are affected by the impact of various process parameters including mainly laser beam model, laser power, scanning speed and powder feeding rate. The effects on the cladding parts by process parameters can be reflected by laser molten pool. So the study on the molten pool has been an important part in the field of Laser cladding prototyping.Now, more circular beam is used in laser cladding prototyping, and lateral or coaxial powder feeding outside the laser beam are adopted which result in poor coupling of laser and powder reciprocity and coarseness of parts surface . To resolve it, a new process which is"hollow focusing laser, powder tubes being medial and inside-laser"are adopted. Based on this forming style, energy distribution of ring beam and circular beam were established, the relationship between the shape of the molten pool and the process parameters were researched, and the properties of cladding parts were analyzed.A mathematic model about laser energy was established, the intensity distributing of ring beam and circular beam was simulated and the result showed that energy distribution of ring laser was a saddle-like curve, while the one of circular laser was Gaussian shape. It proved that the ring beam energy distribution was more uniform and better than circular beam. The physical process of molten pool was researched, which showed that the surface tension and buoyancy are exist, and convection is caused mainly by tension and temperature gradient. Through the study of convection characteristics of the molten pool found that there was a link between process parameters and the shape of the molten pool. Molten pool convention mechanism under Circular and ring laser beam were simulated and contrasted, and the results show that molten pool under ring laser has four convection loops.Using rapid CCD to get molten pool images. In the process of laser cladding, there were noise caused by powder splash, arc light and dust, so it was necessary to pre-process images to get useful information. The better result was got by morphological filtering and edge detection, and programming in the Matlab. Using rapid CCD to get molten pool images of single channel laser cladding under different process parameters, the images showed that: when only increasing laser power or increase powder feeding rate, the width and depth of the molten pool both became lager; while increasing scanning speed, they both became smaller. Using this regulation, the process of forming the hollow conical shaped part were real-time observed through rapid CCD. The results of experiments showed that: higher accuracy forming parts could be got through real-time observation and laser power adjustment, the maximum height reached to 55mm.Under different ring laser duty cycle, cladding layers were detected and analyzed, the results showed that: the micro had typical characteristics of the rapid solidification. When increasing duty cycle, the microstructure changed from dendrite to cellular crystal and got coarse grain, but horizontal distributions of grains tended to uniform, and combination zone of layers became thinner. Considering the grain size, distribution uniformity and cladding layer dilution rate, found when duty cycle is 0.35, the microstructure of cladding layer was better. The combination of cladding layer and substrate was observed and found the both sides of longitudinal section of ring laser cladding layer was a little down asperity shape, while use traditional circular laser, the shape of the combination zone was like half moon. The hardness of the ring laser cladding layer was tested, and the results showed that: on vertical direction, top to down, the hardness was gradually reduce; on horizontal direction, the distribution of hardness was uniform. |
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