Ultrafast cooling and lubrication during hot die forging (T02#)

用于金属锻造的动力锤或模具必须在每个工作循环后冷却，以防止其热机械疲劳。此外，为了减少模具和金属工件之间的摩擦，模具必须涂覆一层薄而均匀的润滑剂，通常在冷却液中提供。同时冷却和润滑通常使用喷雾或射流冲击过程来实现。锻造行业要求选择最合适的喷淋系统和操作参数，既满足模具所需的冷却要求，又能减少润滑剂颗粒对环境的污染。实现这一目标的先决条件是在纯水和润滑剂的喷雾冷却期间模具温度演变的可靠预测模型，以便确定工作循环的允许最小持续时间，并根据特定的操作条件优化润滑剂的量以及润滑颗粒/液滴的正确尺寸和形式。在该项目中，提出了一种预测工具，该工具基于在合作研究中心SFB-TRR 75的项目C4框架内开发的经验证的纯水理论模型。然而，添加尺寸为1至100微米的固体颗粒形式的润滑剂可能会导致液滴冲击到加热表面上的状态图的显著变化。例如，这些颗粒充当成核位点，从而增强蒸汽泡的形成和蒸发，或者较大的惯性颗粒可以潜在地穿透在莱顿弗罗斯特状态中形成的蒸汽层。然后，吸积的颗粒可以导致初始接触线的钉扎。目前还不清楚带有润滑剂颗粒的胶体液滴如何影响冷却过程。该项目的主要目标是在对主要物理过程有基本了解的基础上，为行业提供一个可靠的喷雾冷却和模具润滑模型。该项目将由学术和互补的工业合作伙伴组成的联盟执行，每个合作伙伴都提供了重要而独特的贡献。达姆施塔特工业大学研究小组的具体任务包括：（i）在实验室研究不同润滑剂浓度下，单滴撞击非常热的光滑或涂层基底的热力学和流体动力学现象，包括描述热传递和润滑剂层的残余形状;（二）喷射冲击热基体时热通量的试验研究和模拟以及壁后残余颗粒层的特性薄膜蒸发应用合作伙伴的其他重要任务：（iii）设计和制备通用和最佳的工业润滑剂混合物;（iv）开发能够描述润滑剂喷雾冷却壁面过程的工业3D代码，以及（v）验证和转移到工业环境。

A power hammer or die used for metal forging must to be cooled after each working cycle to prevent its thermo-mechanical fatigue. Moreover, to reduce the friction between the die and the metal workpiece, the die has to be coated by a thin, uniform layer of lubricant, normally supplied in the cooling liquid. The simultaneous cooling and lubrication are often realized using a spray or a jet impingement process. The forging industry demands are to choose the most suitable spray system and operational parameters, which satisfy the required cooling requirements of the die and at the same time reduce environmental waste water with lubricant particles. A prerequisite to achieving this aim is a reliable predictive model for the evolution of the die temperature during spray cooling for pure water and for lubricants, in order to determine the allowed minimum duration of the working cycle and to optimize the amount of lubricant and the correct size and form of the lubricating particles/droplets, depending on the specific operating conditions. In this project a predictive tool is proposed, based on the validated theoretical model for pure water developed in the framework of the project C4 of the collaborative research centre SFB-TRR 75. Nevertheless, the addition of lubricants, in the form of solid particles of the size 1 to 100 micrometers, can potentially lead to significant changes in the regime maps of drop impact onto heated surfaces. For instance, these particles serve as nucleation sites, thus enhancing vapour bubble formation and evaporation, or larger inertial particles can potentially penetrate a vapour layer formed in the Leidenfrost regime. The accreted particles can then lead to pinning of the initial contact line. It is simply not yet known how colloidal drops with lubricant particles effect the cooling process. The main goal of this project is to provide the industry with a reliable model for spray cooling and for die lubrication based on a basic understanding of the main physical processes. The project will be performed by a consortium of academic and complementary industrial partners, each providing a significant and unique contribution. The specific tasks of the TU Darmstadt team include (i) laboratory investigation of the thermodynamic and hydrodynamic phenomena of single drop impact onto a very hot smooth or coated substrate at different lubricant concentrations, including description of heat transfer and residual shape of the lubricant layer; (ii) experimental study and modelling of heat flux during spray impact onto a hot substrate and characterization of the residual particulate layer after wall film evaporation. Further important tasks of the application partners: (iii) design and preparation of generic and optimal industrial lubricant mixtures; (iv) development of an industrial 3D code able to describe the wall cooling process by a lubricant spray, and its (v) validation and transfer an industrial environment.