Massively Parallel Simulation of the Melt Pool Area during Laser Beam Welding using the Lattice Boltzmann Method

激光焊接作为一种灵活、无接触的连接技术，越来越受到人们的重视。大熔点合金的凝固开裂倾向给其加工带来了挑战。凝固裂纹的形成是由于枝晶组织的临界应力和应变状态以及枝晶间熔体。尽管与工业高度相关，但只有从冶金或结构方面解决问题的单一方面的办法。“激光束焊接过程中的凝固开裂——高性能工艺的高性能计算”研究单位旨在对凝固开裂机理及其与工艺参数的关系进行定量的工艺理解。该子项目旨在以大约一微米的分辨率模拟熔池的动力学。这是使用由大约109个计算单元组成的数值模型来实现的，这些计算单元必须计算超过105个时间步。该模拟将模拟熔化和凝固时的相变、激光输入的能量、熔体池中的膨胀和收缩以及热量和质量的传递。如此复杂的模拟需要并行高性能系统的计算能力，并且只能通过优化的并行算法和现代软件技术来实现。在这个子项目中，晶格玻尔兹曼方法（LBM）将被使用和扩展，以正确地捕捉熔体池中的各种物理效应。与其他数值格式相比，LBM非常适合并行计算和包含硬件加速器的现代计算机体系结构。该实现将基于HPC框架waLBerla，该框架是专门为实现复杂的多物理场应用而设计的。使用抽象层和代码生成概念，用waLBerla开发的软件是可持续的，也就是说，可移植到未来的计算机架构中。该项目的一个重要方面在于验证新实现的模型和算法，以及与研究单元中合作伙伴的模型的互操作性。这将通过与邻近子项目的密切合作来实现。

As a flexible and contact-free joining technology, laser beam welding has increasingly gained importance. Processing of alloys with large melting range poses a challenge due to their solidification cracking tendency. Solidification cracks form due to critical stress and strain states of the dendritic microstructure with interdendritic melt. Despite the high industrial relevance, there are only approaches addressing single aspects of the problem, metallurgically or structurally oriented. The research unit "Solidification Cracking during Laser Beam Welding – High Performance Computing for High Performance Processes" aims at developing quantitative process understanding of the mechanisms of solidification cracking and their relation to process parameters.The sub-project aims at simulating the dynamics of the melt pool with a resolution of about one micron. This is achieved using a numerical model consisting of approximately 109 computational cells that have to be computed for more than 105 time steps. The simulation will model the phase change at melting and solidification, the energy input from the laser, the expansion and contraction, and the heat and mass transport in the melt pool. Simulations of such complexity require the computational power of parallel high-performance systems and can only be realized using optimized parallel algorithms and modern software technologies. In this sub-project, the lattice Boltzmann method (LBM) will be used and extended to correctly capture the various physical effects in the melt pool. Compared to other numerical schemes, the LBM is well-suited for parallel computing and modern computer architectures that include hardware accelerators. The implementation will be based on the HPC framework waLBerla specifically designed for implementing complex multi-physics applications. Using abstraction layers and code generation concepts, software developed with waLBerla is sustainable, i.e., portable to future computer architectures. One of the significant aspects of this project lies in the validation of newly implemented models and algorithms, and in the interoperability with models from the partners in the research unit. This will be achieved via the close cooperation with neighboring sub-projects.