Temperature Measurement And Research On Microstructure Controlling In Laser Rapid Forming Process

Laser rapid forming (LRF) is a new and advanced manufacturing technology that has been developed on the basis of high power laser cladding technology and rapid prototyping. The rapid fabrication of high density metal components with excellent performance and complex shape can be realized without dies. Therefore, LRF is a promising manufacturing technology in high tech fields, such as aeronautic and astronautic applications. In this paper, melt pool temperature in the LRF process was measured by temperature measurement system and the influence of processing conditions on the temperature was investigated systematically. With the combination of theoretical analysis, some discussion about how to control the forming precision and how to get unidirectional solidification microstructure was carried out. The main results are as follows:1. Building up a temperature measurement system of the LRF processThe temperature measurement system was built up by using a two-color infrared thermometer, and the real-time tracking measurement and fixed-point measurement were achieved in the LRF process.2. The influences of processing conditions on the melt pool thermal behavior The influences of powder feed rate , scanning velocity, laser power, spotdiameter, shielding gas flow rate and powder feeding position on the temperature of melt pool were revealed by temperature measurement.The heat cumulating during laser multi-layer cladding can cause layer size to increase and temperature gradient to decrease with layers, and primary dendritic space would augment with the fall of temperature gradient.3. Bringing out a feedback control proposalThe control of layer thickness by changing mass input. When the laser power and scanning velocity were constant, the temperature of melt pool was increasing with the decrease of layer thickness. So the feedback control of layer thickness can be achieved by controlling the powder feed rate or powder feeding position.The control of layer thickness by changing energy input. When the powder feed conditions were constant, the layer thickness was increasing with the increase oftemperature of melt pool. So the feedback control of layer thickness can be achieved by controlling the laser power or scan velocity.4. Unidirectional solidification research by LRFThe influences of crystallography orientation of the substrate, solid/liquid interface shape of the tail of melt pool and remelt depth on the solidification microstructure have been analyzed. And the results were consistent with those of experiments.With the combining of Lin Xin's columnar to equiaxed transition (CET) model during alloy solidification and the analysis of solidification velocity and temperature gradient in the tail of melt pool, it was found that the CET transition is easiest to occur at the interface of z = 0. By using the fixed-point temperature measurement, the optimized processing parameters with which the CET transition can't occur were obtained. With those parameters the unidirectional solidification microstructure was obtained and it was found that there was no new grain forming in the epitaxial growth region and the turning dendrite region, which is consistent with the results forecasted by the model.Based on the research about the influences of crystallography orientations of the substrate and the shape of solid liquid interface on the dendrite growth, further studies were performed about the influence of the substrate's interface and the crystalline preferred orientation on the solidification microstructure. The turning dendrite was eliminated successfully during the multilayer cladding experiment. The epitaxial unidirectional solidification was obtained with only a thin layer of the nucleation but without turning dendrite.5. The finite element simulation of the LRF temperature fieldThe temperature gradient of solid liquid interface of the melt pool was analyzed by the finite element simulation, and the solidification microstructure was predicted. Although the simulation results were basically consistent with the experimental results...