同轴送粉激光3D打印光粉耦合作用以及熔池气液界面追踪数值模拟的研究进展

Research Progress on the Numerial Simulation of a Melting Pool Coupling of Light-Powder and Gas-Liquid Interface Tracking by Coaxial Powder Metal Laser 3D Printing

金属激光3D打印作为一种"无需工具"的数字化制造技术,摆脱了传统加工方式的约束,将有可能改变产品的生产模式,给企业和消费者带来巨大的经济效益和社会效益。它利用层层堆积的精密加工模式,使得高精度复杂结构制造变为可能。这将极大简化产品设计环节,提高零部件的集成度,缩小产品的研发周期。相对于利用切削机床对毛坯进行加工的"减材制造",3D打印制造减少了原材料的使用量,降低了对自然环境的压力。3D打印技术实际上是一个"逐点扫描-逐线搭接-逐层堆积"的循环往复过程,在长时间的加工过程中,零件的不同部位材料均经受着一系列短时变温、非稳态、强约束、循环固态相变的微热处理过程。这种微热处理的加热及冷却速度极快、相变持续时间极短,且每一微热处理的相变温度、加热及冷却速度和相变持续时间均随热循环次数的变化而变化,使得激光3D打印的金属构件显微组织结构独特,并表现出对加工工艺条件强烈的依赖性,进而影响成型零部件的综合力学性能。因此,实现对激光3D打印金属零部件显微组织结构、冶金缺陷的主动控制是亟待解决的关键问题。其中掌握同轴送粉金属激光3D打印加工过程中粉末流与激光束的耦合作用,以及熔池气-液自由界面的传质和扩展特征,是选择最优加工参数,获得综合力学性能优良的金属零件的关键。然而,采用试验分析途径难以精确并定量地揭示上述问题,但数值仿真模拟却能有效揭示其微观规律。例如在光-粉耦合作用方面,目前大部分研究学者从单个粉末颗粒到粉末流对激光束产生衰减的角度出发,研究了激光束与粉末相互作用的机理;另一方面其他研究人员从粉末颗粒对激光束吸收和散射的角度建立数学模型,利用米氏散射理论和朗伯-比尔定律仿真分析计算了粉末流与激光束的相互作用。而包含粉末流对激光束产生衰减以及粉末流吸收并反射、散射激光等多角度多因素的数值模型却鲜有报道,因此,这些方面是学者今后研究的主要方向。本文主要综述了国内外研究学者对同轴送粉金属激光3D打印仿真模拟的研究进展,并详细阐述了3D打印过程中的光-粉耦合作用、熔池气-液界面和固-液界面追踪等。

As a“toolless”digital manufacturing technology,metal laser 3D printing that get rid of the limitation of traditional processing methods is very likely to change the production mode of products and highly benefit the social and consumers.The 3D printing technology makes the high precision complex structure manufacturing possible through the precision machining mode of layer stacking,which will consumedly simplify the product design process,improve the integration of components and accelerate the producting cycles.Furthermore,compared to the“subtract material manufacturing”of blanks with cutting machines,3D printing technology can reduce the amount of raw materials and greatly relieve the pressure of natural environment.Actually,3D printing manufacturing is a“point by point scanning-line by line lapping-layer by layer stacking”cyclic process.During the process of the long time manufacturing,different parts of the material have went through a series of micro-heat treatment processes,including short-term temperature changes,unstable state,strong constraint and cyclic solid phase transition.The heating and cooling rate of these micro-heat treatments are extremely fast and the time of phase transition is very short.And the phase transition temperature,heating and cooling rate and phase transition duration of each micro-heat treatment change with the number of thermal cycles.Thus,the laser 3D printed metal components obtain a unique microstructure and exhibit a strong dependence on the processing conditions,which in turn affects the mechanical properties of the molded parts.Therefore,achieving the active control of the microstructure and metallurgical defects of laser 3D printed metal parts is crucial issue to be solved.The mastering of the coupling effect of the powder flow and the laser beam in the 3D printing process of the coaxial powder feeding metal laser,and the mass transfer and expansion characteristics of the gas-liquid free interface of the molten pool are the pivotal to select the optimal processing parameters and hence lead to an excellent mechanical properties of the metal.However,it is hard to accurately and quantitatively reveal the above problems by the means of experimental analysis.Yet the numerical simulation is capable of effectively revealing the microscopic laws.For example,in the aspect of light-powder coupling,most researchers currently study the interaction mechanism between laser beam and powder from the perspective of laser beam attenuation from single powder particle to powder flow.On the other hand,other researchers establish the mathematical model from the perspective of powder particle absorption and scattering to the laser beam,and adopt the Mie scattering theory and the Lambert-Beer law to analyse and calculate the interaction between the powder flow and the laser beam.However,the reports revolve round numerical models including multi-angle and multi-factors such as powder flow attenuation on laser beam and absorption of powder flow and reflection and scattering of laser light is rare,therefore,the above aspects will be the main research direction of scholars in the future.This paper mainly reviews the research progress of 3D printing simulations of coaxial powder metal laser in domestic and foreign research scholars,and elaborates the light-powder coupling,the gas-liquid interface and the solid-liquid interface tracking algorithm etc in the 3D printing process.