Determination of the heat transfer coefficient for the numerical design of compound forging processes

• Applicant: Professor Dr.-Ing. Bernd-Arno Behrens
• Subject Area: Primary Shaping and Reshaping Technology, Additive Manufacturing
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 496068488

In the scope of numerical process design, the heat transfer coefficient (HTC) is used to model the heat transfer. Currently, FE calculations of bulk forming processes are mostly based on the assumption of a constant HTC, as there is no full understanding of the correlations between the HTC and its influencing quantities, e.g. contact pressure, temperature and lubrication condition. This assumption leads to inaccuracies in the simulation result. Particularly in the case of compound forging for example of steel and aluminium, temperature profiles must be specifically adjusted in order to equalise the different forming properties. Setting incorrect boundary conditions in the compound forging process can lead to temperature profiles that result in melting of the semi-finished aluminium products when the solidus temperature is reached, which can in turn result in gap formation or insufficient compound strength. The exact knowledge of the HTC as a function of the process boundary conditions is thus of importance for the numerical process design, especially of compound forging processes. However, from an experimental point of view, the investigation of the HTC in forging processes is very challenging due to very short contact times and high heating rates in the process. Current approaches require a lot of effort in testing and have a low reproducibility. According to the state of the art, there is currently no methodology for determining the prevailing HTC in relation to a compound forging process as a function of relevant parameters with a reasonable amount of effort.The overall objective of the project is the fundamental and methodical investigation of effective parameters on the HTC in a compound forging process. In this context, the contact pressure, the workpiece temperature, the tool temperature and the lubricant condition are considered as relevant parameters. For this purpose, an experimental-numerical method is being developed. In contrast to current approaches, this method is intended to have a high level of accuracy, a very fast responding behaviour as well as a simple and well reproducible test procedure. With the help of this methodology, the heat transfer in the contact zone between the two forming materials themselves, on the one hand, and between the forming materials and the tool, on the other hand, are to be specifically investigated under variation of the effective parameters. On the basis of these investigations, a fundamental HTC model for improved numerical process design of compound forging processes with non-preformed semi-finished products is to be developed for the first time. The findings can also be used for conventional forging processes in order to predict tool surface temperatures. With the help of the obtained results, a contribution can be made with regard to the standardisation of HTC measurement.

• DFG Programme: Research Grants