An Integrated Approach for Modeling Nano- and Femtosecond Laser Sintering of Metallic Micro- and Nanoparticles

美国国家科学基金会-化学运输系统司？颗粒多相工艺项目（1415）提案编号：0730143主要研究者：Zhang，Yuwen单位： 密苏里州哥伦比亚大学提案标题：一个集成的方法，用于模拟纳米和飞秒激光烧结金属微米和纳米粒子本项目的目标是开发一个集成的方法，用于模拟纳米和飞秒（ns和fs）激光烧结金属微米和纳米粒子。在理论研究中，首先模拟粉末颗粒和团聚体的堆积，然后对微粒（直径远大于激光波长）和纳米颗粒（直径与激光波长相当或小于激光波长）的激光-颗粒相互作用进行建模。将分子动力学和双温模型相结合的混合分子动力学模拟将被执行来表征过热、超快熔化、汽化或相爆炸、再凝固、过冷和结合强度的性质。利用分子动力学模拟的性质，建立了飞秒激光烧结的半经典双温热力学模型和纳秒激光烧结的简化经典热力学模型，并对激光光斑下的颗粒进行了数值模拟。粉末床/烧结区域中远离激光束的区域将被建模为连续体。将几个数量级的时间和长度尺度的模型进行整合，以开发一个连贯的框架，纳秒和飞秒激光烧结。在实验中，通过控制激光加工工艺和材料参数，可以制备出各种孔隙率可变和可控的金属零件和纳米多孔层。实验结果将被用来验证所开发的多尺度激光烧结模型。如果成功的话，结合建模和实验将提供一个通用的解决方案，在粒子科学和技术领域前所未有的多尺度模拟短脉冲激光烧结。在该领域，PI首次提出通过控制激光脉冲宽度、重复频率、激光强度和扫描速度来利用和控制激光烧结最终产品中的孔隙率。采用分子动力学/双温度混合模型研究飞秒激光脉冲与粒子相互作用过程中电子与声子之间的非平衡态。与将整个粉末床视为连续体的大多数现有模型相反，将使用耦合的半经典双温度热机械模型来执行离散区域中的每个颗粒的颗粒级建模。离散和连续区域之间的相互作用被认为是通过放置在连续区域中的颗粒层，靠近两个区域之间的界面，和颗粒层中的颗粒的温度被用来桥接离散和连续区域。所开发的技术的应用包括制造具有分级孔隙率的骨科植入物、燃料电池电极和纳米多孔表面。更广泛的影响所提出的综合方法建模ns和fs激光烧结将提供指导方针的预测和控制的孔隙率在能源，航空航天和生物工程的应用。PI将把拟议研究的材料整合到几门现有和新课程中，为机械工程教育奠定坚实的基础。PI计划开放他们的热制造实验室，在MU工程周项目期间向初中和高中学生展示激光烧结实验和可视化计算结果，并在年度女童子军工程日向女童子军展示，以激发她们攻读工程学位的兴趣。

National Science Foundation - Division of Chemical &Transport Systems ? Particulate & Multiphase Processes Program (1415)Proposal Number: 0730143Principal Investigators: Zhang, YuwenAffiliation: University of Missouri ColumbiaProposal Title: An Integrated Approach for Modeling Nano- and Femtosecond Laser Sintering of Metallic Micro- and NanoparticlesThe objective of this project is to develop an integrated approach for modeling nano- and femtosecond (ns and fs) laser sintering of metallic micro- and nanoparticles. In theoretical investigation, packing of powder particles and agglomerates will be simulated first, followed by modeling of laser-particle interaction for microparticles (diameters are much greater than the laser wavelength) and nanoparticles (diameters are comparable or smaller than the laser wavelength). A hybrid molecular dynamics simulation combining molecular dynamics and two-temperature model will be performed to characterize the properties of superheating, ultrafast melting, vaporization or phase explosion, resolidification, undercooling and bonding strength. With the properties from the molecular dynamic simulation, a coupled semi-classical two-temperature thermomechanical model for fs laser sintering and a reduced classical thermomechanical model for ns laser sintering will be developed for each particle in the region under the laser spot (discrete region). The region far from the laser beam in the powder bed/sintered region will be modeled as a continuum. The models ranging several orders of magnitude of temporal and length scales will be integrated to develop a coherent framework for ns and fs laser sintering. In experiments, various metallic parts with variable and controllable porosity and nanoporous layer will be fabricated by controlling the laser processing and material parameters. The experimental results will be used to validate the developed multiscale laser sintering model. If successful, the combined modeling and experiments will provide a general solution to model short-pulsed laser sintering in unprecedented multiscale in the field of particle science and technology. Intellectual Merit For the first time in the field, the PIs propose to utilize and control the porosity in the final product of laser sintering by controlling the laser pulse width, repetition rate, laser intensity, and scanning velocity. Nonequilibrium between electrons and phonons during fs laser pulse-particle interaction will be considered by a hybrid molecular dynamics/two-temperature model. In contrast to most existing models that treat the entire powder bed as a continuum, particle level modeling for each particle in the discrete region will be performed using a coupled semi-classical two-temperature thermomechanical model. The interaction between the discrete and continuum regions is considered by placing a layer of particles in the continuum region next to the interface between the two regions, and the temperatures of the particles in the particle layer are used to bridge the discrete and continuum regions. Applications of the developed technology include fabrication of orthopedic implants with graded porosity, electrodes for fuel cells, and nanoporous surfaces. Broader Impact The proposed integrated approach for modeling ns and fs laser sintering will provide guidelines on prediction and control of the porosity for applications in energy, aerospace, and bioengineering. The PIs will integrate materials from the proposed research into several existing and new courses to build a solid foundation for mechanical engineering education. The PIs plan to open their Thermal Manufacturing Lab to demonstrate laser sintering experiments and visualized computational results to middle- and high-school students during MU Engineering Week Program, as well as to Girl Scouts in the Annual Girl Scouts Engineering Day to stimulate their interests to pursue a degree in engineering.