Modeling And Numerical Simulation On Laser Remelting And Cladding

Laser cladding is an advanced materials surface modification technology, in which the pre-selected cladding materials can be placed on the substrate by different way, by laser irradiating, the cladding materials are fused onto the substrate's surface. Owing to the peculiar characteristics of laser, laser cladding provides good metallurgical bonds, minimal dilution and low distortion of the materials, and the accompanying rapid solidification effects produce refine and novel microstructures with improved properties. In recent years, laser cladding develops very rapidly and shows a brilliant application future in wide fields of industry . Due to the difference of the way that the cladding material is put on the substrate, there are various kinds of laser cladding. This thesis is directed to laser cladding with powder by side injection and a CO2 laser as heat source.Actually, laser cladding with powder by side injection is a process including the interaction among laser beam, cladding materials and substrate, among which the interaction between laser beam and substrate, and the interaction between laser beam and the cladding material powder are the most important. With this in mind, both theoretical model and numerical simulation are established to investigate these two key processes, the relative comparative experiments are also carried out to verify the validity of the model.The main work are as follows1. A transient, moving laser beam, three dimensional mathematical model for convection-diffusion and phase change problem in laser melting pools and substrate was developed, which is suitable for the prediction of fluid flows,temperature field, liquid fraction field and phase change in solid,liquid phase and mash zone.2. A new fixed grid numerical method was established to solve the set of equations in above mathematical model3. A method was developed to calculate threshold condition for the laser remelting and cladding.4. The models were delveloped to analyze power attenuation by powder clouds and the temperature rise of cladding material powder particles on passing through the laser beam.5. With the above model and numerical method, the effect of various process parameters on the temperature field, fluid flow and the shape of melting pool were investingated.6. The relative comparative experimental research on laser remelting and cladding were carried out.The main conclusions are as follows:· The interaction between laser beam and substrate1. On liquid metals flow driven by the buoyancy, the buoyancy drives the liquid metal flow from center to the edge of the melting pool on the free surface ,whichModel and numerical simulation on laser remelting and cladding Zhenxia Liuresults in forming two eddies with the contrary flowing directions, but its function on melting pool shape ,fluid flow and temperature field is very weak. The velocity scale of the liquid metals near the free surface of melting pool only driven by the buoyancy is on the order of micron.2. The thermocapillary flow, which results from the variation of surface tension with temperature along the free surface due to the temperature gradient along the free surface,have very important influence on fluid flow, temperature field and the shape of melting pool. The effect is relevant to the scale and plus or minus of surface tension temperature coefficient:(1) When the surface tension temperature coefficient is minus, the fluid at the top of the melting pool flows from center to edge, and form two eddies with the contrary direction in the transverse section , i.e. the first order recirculating. The bigger the absolute value of the surface tension temperature coefficient is, the wider and shallower the melting pool is, the lower the free surface peak temperature is. There also exist second order recirculating that induce by the first order recirculating, which dig a pit in the bottom of the pool. The experimental results show a good ag...