Laser Prototyping Technology Of Nickel-base Single Crystal Superalloys And Simulation

Using single crystal superalloy turbine blade with complex cavity has become a key approach to improve the overall performance of aero engines and gas turbines,while the traditional investment precision casting technology is cumbersome and complicated to prepare turbine blades.The procedure of casting is long,and internal cooling cavity structures cannot be arbitrarily designed.The laser 3D printing technology of metal materials provides a new approach for the preparation of complex cavity turbine blades with its unique process features.However,breakthrough has not yet been made in this technology.In order to promote the substantial progress of this technology in engineering,and to address the key problems faced in laser forming of single crystal superalloys,this work combines experimental research with computational simulation to develop corresponding experimental research methods and establish relevant theoretical models.The main contents include:(1)A scan path generation software was developed by using C language.According to the input laser prototyping parameters,it can automatically generate the numerical control code used for laser prototyping.Based on this software,this article can achieve the full control of process parameters and meet specific process design requirements.(2)A vectorization algorithm was proposed.Based on this algorithm,the influence of crystallographic orientation of the substrate on the temperature gradient,dendrite growth rate,and stray grain tendency of the single crystal superalloy during surface remelting was theoretically investigated and then we could carry out a comparative study of theoretical predictions and experimental results further.The results show that the algorithm can accurately predict the dendrite growth direction,and can preliminarily predict the stray grain tendency in the remelted microstructure.(3)Combining the above vectorization algorithm with finite element software,the effect of laser scanning speed on remelting microstructure of single crystal superalloy was studied.The results show that there was a layer of planar interface solidification outside the remelted microstucture.The thickness of the planar interface and the primary dendrite spacing decrease with the increase of the scanning speed.Based on the solidification parameters calculated by the simulation,the constant term in the calculation formula of the primary dendrite spacing was obtained.In addition,a calculation model for predicting the interface thickness was also proposed.(4)A method for screening laser cladding parameter of single crystal superalloy based on laser remelting results was proposed.In this method,the laser remelting is independent of the deposition process.The remelted microstructure is the basis for the laser deposition process.If there is a lot of stray grain in laser remelting,it is not recommended to use this process to prepare single crystals.Based on the process parameters obtained by the remelting process,laser deposition is used to prepare single crystal superalloys.The set process parameters in deposition allow each layer of the stray grain to be remelted in the cladding process of the next layer,thus getting dendrites with the same orientation as the matrix.(5)The phenomenon of dendrite deflection in laser-deposited samples was found in the present work.The EBSD results showed that the crystallographic orientation changed by more than 20°.This change in crystallographic orientation destroys the uniformity of single crystal orientation.The present studies show that the phenomenon of dendrite bending is attributed to the growth of dendrites in shear flow caused by the surface tension.According to the characteristics of dendrites growth in shear flow,a solution for increasing the deposition layer thickness and improving the proportion of overlapping areas was given.In addition,the dendrite deflection and its orientation change were successfully suppressed.(6)In the finite element software,a real-time simulation model of laser deposition was established,and an in-situ preheating method for the laser forming process of metal materials was proposed.This method realizes in-situ preheating of the machining process by controlling parameters such as boundary conditions of the laser forming process,laser power,laser scanning speed,laser spot size,and laser on-time interval.This in-situ preheating method can achieve a preheating temperature of more than 1000 ?,without additional design of resistance heating or induction heating device.Based on the in-situ preheating method,single-crystal superalloy bulks with crack-free and no-recrystallization were successfully prepared.(7)The microstructure and properties of laser-deposited single crystals and directionally solidified single crystals were compared and analyzed.The results showed that the solution treatment makes the' phase becoming irregular in the depostion.After solid solution and aging,the '?-phase size is larger and the volume fraction is slightly lower in the depostion.As a result,the yield strength and tensile strength of laser-deposited single crystals is slightly less than that of directional solidification single crystals,but the plasticity of laser-deposited single crystals is better.