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零件之间的摩擦，模具必须涂上一层薄而均匀的润滑剂，通常在冷却液中供应。同时冷却和润滑通常是通过喷雾或射流冲击过程实现的。锻造行业的要求是选择最合适的喷涂系统和操作参数，既能满足模具的冷却要求，又能减少含有润滑剂颗粒的环境废水。实现这一目标的前提是建立可靠的纯水和润滑剂喷雾冷却过程中模具温度变化的预测模型，以确定允许的最小工作循环时间，并根据特定的操作条件优化润滑剂的量以及正确的润滑颗粒/液滴的大小和形状。在该项目中，根据在合作研究中心SFB-TRR 75的C4项目框架内开发的经验证的纯水理论模型，提出了一种预测工具。然而，添加尺寸为1至100微米的固体颗粒形式的润滑剂，可能会导致液滴撞击加热表面的区域图发生重大变化。例如，这些粒子充当成核点，从而促进蒸汽泡的形成和蒸发，或者更大的惯性粒子可能穿透在莱登弗罗斯特体系中形成的蒸汽层。然后，附着的颗粒会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，为行业提供可靠的喷雾冷却和模具润滑模型。该项目将由一个学术和互补性行业合作伙伴组成的联盟执行，每个合作伙伴都做出了重大而独特的贡献。TU Darmstadt团队的具体任务包括(I)在实验室调查不同润滑剂浓度下单滴撞击到非常热的光滑或涂层基材上的热力学和流体力学现象，包括描述润滑层的传热和残余形状；(Ii)喷雾冲击高温基材时热流密度的实验研究和模拟，以及壁膜蒸发后残余颗粒层的表征。应用合作伙伴的其他重要任务：(Iii)设计和制备通用和最佳工业润滑剂混合物；(Iv)开发能够描述通过润滑剂喷雾进行壁面冷却过程的工业3D代码，以及(V)验证和转移工业环境。

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.