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

In numerical process design, the heat transfer coefficient (HTC) is used to model the heat transfer between different workpiece materials and the dies. In most FE calculations for forming processes, a constant heat transfer coefficient is currently assumed, as there is no complete understanding of the relationships between the heat transfer coefficient and its influencing variables (e.g. contact pressure, temperature and lubrication condition), which leads to inaccuracies in the simulation results. However, specifically set temperature profiles are necessary, particularly in the composite forming of aluminum and steel, in order to match the forming properties of the dissimilar semi-finished products. Knowledge of the HTC as a function of the process relevant conditions is therefore an essential prerequisite for numerical process design. However, the investigation of the HTC in hot forging processes represents a major challenge from an experimental point of view due to the very short pressure contact times and the high heating rates that occur. Previous approaches have shown a high experimental effort and low reproducibility.

The aim of this research project is the fundamental and the methodical investigation of variables that affect the heat transfer coefficient in a composite forging process. The contact normal stress, the workpiece and die temperature and the lubricant condition are considered as relevant effective variables. For this purpose, an experimental-numerical methodology is being developed. In contrast to current approaches, this should have a high accuracy, a fast response behaviour in temperature detection and a reproducible test execution. With the help of this methodology, the heat transfer in the contact zone between the two semi-finished products themselves and between the forming materials and the die will be specifically investigated by varying the effective variables. Based on these investigations, a basic HTC model for the numerical design of composite forging processes with non-prefabricated semi-finished products is to be developed for the first time. The findings can also be used for conventional hot forging processes to enable, for example, a more accurate numerical determination of tool surface temperatures.