Modeling of keyhole mode laser welding

高强度小孔模式激光焊接是指激光在熔池表面打出一个深的键形小孔，特别适用于厚板焊接接头的焊接。这种激光焊接的能力在该领域几乎是独一无二的。它使激光焊接变得非常有吸引力。然而，这种激光焊接模式通常与小孔不稳定有关，这往往会造成缺陷。本研究计划的主要目标是开发数值工具，用于模拟小孔模式下的激光焊接，特别是小孔的行为。它包括激光焊接过程中多物理现象的建模，先进数值方法的应用，以及C++内部面向对象代码的开发。该研究计划还包括大量的实验研究，以表征激光光源的行为、材料的行为以及过程中的热传递。为了达到主要目标，研究计划分为以下三个步骤。首先，将在COMSOL中开发和实现用于多物理建模的整个方程系统。它允许评估不同可用的小孔激光焊接过程模拟方法的能力和局限性。在这一阶段，将对铝进行一系列激光焊接试验，以确定激光热源的特征。它可以理解激光撞击并将能量传递给材料的方式。此外，还将利用高温计和高速相机进行一系列测试来表征铝液的表面张力。其次，将发展不同的方法来模拟激光热源与熔池的相互作用。这一步对于模拟锁孔的行为非常重要。脉冲激光的实验结果将被用来验证这些方法。最后，在前两步知识的基础上，将利用先进的数值方法在内部代码中开发和实施所提出的小孔激光焊接的多物理模型。脉冲激光焊接的实验结果将被用来验证激光焊接模拟在温度、小孔行为、焊缝截面和气孔方面的有效性。在研究计划结束时，通过实验和模型研究更好地了解激光焊接小孔的形成，将有助于改进工艺控制和开发，避免缺陷，提高焊接质量。这些知识将为该行业打开大门，使其能够从传统焊接过渡到激光焊接，这将大大提高生产率。此外，该研究框架是通用的，可以适用于其他材料制造过程，如切割、成形，特别是添加剂制造过程。关键词：激光焊接、小孔、不稳定性、缺陷、实验、建模、模拟

High intensity keyhole mode laser welding, in which the laser makes a deep key-shape hole from the surface of the melting pool, is especially used to make thick welded joints. This capability of the laser welding is almost unique in this field. It makes laser welding very attractive. However, this laser welding mode is typically associated with keyhole instability that often creates imperfections.  The main objective of this research program is to develop numerical tools for the simulation of the laser welding in keyhole mode, especially the behavior of the keyhole. It includes the modeling of multiphysics phenomena occurring in the laser welding process, the application of advanced numerical methods and the development of the in-house object oriented code in C++. The research program includes also a significant amount of experimental investigation to characterize the behavior of laser sources, the behavior of materials and the heat transfer during process. To reach the main objective, the research program is split into three following steps. Firstly, the whole system of equations for the multiphysics modeling will be developed and implemented in COMSOL. It allows evaluating the capability and the limits of different available approaches for keyhole laser welding process simulation. In this stage, a series of laser welding tests on aluminium will be performed to characterize the laser heat source. It allows understanding the manner the laser hits and transfers the energy to materials. Also, a series of tests will be carried out to characterize the surface tension of aluminium liquid using pyrometer and high speed camera. Secondly, different approaches for modeling the interaction of laser heat source and the molten pool will be developed. This step is very important for simulating the behavior of the keyhole. Experimental results of pulsed laser will be used to validate these approaches. Finally, based on the knowledge from the two first steps, the proposed multiphysics modeling of the keyhole laser welding will be developed and implemented in the in-house code using advanced numerical methods. Experimental results of pulsed laser welding will be used to validate laser welding simulations in terms of temperature, keyhole behavior, weld cross-section and porosity. At the end of the research program, a better understanding of the keyhole formation in laser welding through experimental and modeling investigations will allow improving process control and development, avoiding defects and increasing weld quality. This knowledge will open the door for the industry to be able to transfer from conventional welding to laser welding which makes an important gain in productivity. Moreover, this research framework is generic and could be adapted for other material manufacturing processes such as cutting, forming and particularly additive manufacturing processes. Keywords: laser welding, keyhole, instability, defects, experimentation, modeling, simulation