Numerical and Experimental Investigation of Unsteady-State Material-Laser Interactions with Application to Laser Cutting Process

9500181 Yao这个项目旨在推进非稳态条件下材料-激光相互作用的知识。选择激光切割工艺进行研究，是因为在激光切割过程中，非稳态条件对材料-激光相互作用的影响比激光焊接等其他类似工艺更为明显。具体目标是(1)提高对材料-激光相互作用的几何效应的认识，(2)了解使用可变工艺参数来抵消非稳态材料-激光相互作用有害影响的机制，(3)探索一般振荡现象的性质，特别是激光切割过程中条纹的形成，(4)扩大提高工艺可重复性所需的技术基础，消除反复试验的需要。并对非稳态条件下的工艺进行了优化。该项目将开发一个瞬态三维模型，包括熔融液体层、固体工件和气体射流的耦合连续性、动量和能量方程。液体/固体将被建模为具有相变焓-孔隙率公式的单一域。将采用基于控制体积的对流项幂律格式有限差分算法和压力校正算法求解方程组。熔融层的不稳定性将通过引入扰动进行数值研究。该模型能较准确地预测切口几何形状、能量输运和熔体流动。该模型将用于研究上述各种非稳态现象、计算机数控（CNC）插补的影响以及非稳态条件下的优化策略。氧气辅助二氧化碳激光切割低碳钢将是重点。将进行各种非定常条件下的实验研究，包括边界侵蚀、转弯、轮廓切割和可变工艺参数。实验技术包括高速拍摄以研究切割锋面的流动性和条纹的形成；采用近焦高温计和热电偶研究非稳态条件下的切削前沿温度和工件温度分布。激光的集中热传递能力使其非常适合在非常选定的局部区域修改材料。如此高的能量集中也给不稳定过程的连续控制带来了困难。由于该项目开发的预测模型，可控激光切割的成功演示将对激光在材料加工行业的使用产生重大影响。

9500181 Yao This project is aimed at advancing knowledge of material-laser interactions under unsteady state conditions. The laser cutting process has been chosen for investigation because the effects of unsteady state conditions on material-laser interactions are more pronounced in the laser cutting process than other similar processes like laser welding. Specific objectives are (1) advance knowledge of geometric effects on material-laser interactions, (2) understand the mechanism of using variable process parameters to offset the detrimental effects of unsteady-state material-laser interactions, (3) explore the nature of oscillatory phenomena in general and striation formation in particular in the laser cutting process, (4) expand the technology base necessary for improving process reproducibility, eliminating the needs for trial-and-error, and optimizing the process under unsteady state conditions. The project will develop a transient three-dimensional model embracing the coupled continuity, momentum and energy equations for the molten liquid layer, solid workpiece and gas jet. The liquid/solid will be modeled as a single domain with an enthalpy-porosity formulation for the phase change. A control-volume based finite-difference algorithm with power-law scheme for convective terms and pressure correction algorithm will be used to solve the equations. Instability of the molten layer will be numerically studied by introducing perturbations. The model can accurately predict the kerf geometry, energy transport and melt flow. The model will be used to investigate various unsteady state phenomena as mentioned above, the effects of Computer Numerical Control (CNC) interpolations, and optimization strategies under unsteady state conditions. Oxygen assist carbon dioxide laser cutting of mild steel will be the focus. Experimental investigation under various unsteady state conditions will be undertaken, including boundary encroachment, cornering, contour cuts and variable process parameters. Experimental techniques to be used include high speed filming to investigate cutting front mobility and striation formation; close focus pyrometer and thermocouples to investigate cutting front temperature and workpiece temperature distribution under unsteady state conditions. Focused heat delivery capability of lasers makes it ideal for modifying materials in a very selected local zones. Delivery of such high energy concentration also poses difficulties in continuously controlling the unsteady process. Successful demonstration of controlled laser cutting as a result of the predictive model developed in this project will significantly impact the use of lasers in materials processing industries.