Material Flow Interaction in Multi-Pin Extrusion: A Study on Pin Height Variability and Process Predictability

The increasing demand for lightweight multi-material structures, particularly in automotive and aerospace applications, requires robust and efficient joining technologies. Mechanical joining methods based on locally formed pin structures offer a promising solution, as they enable the joining of dissimilar materials such as steel and aluminium as well as steel with fibre reinforced plastics without auxiliary elements. As single-pin connections have already been well researched, the focus has now shifted to applying this principle to multi-pin connections that are more relevant to practical applications. However, the forming behaviour of such systems remains insufficiently characterised.In this work the formation of multi-pin structures by forward extrusion and their implications for subsequent joining applications is being investigated. It is shown that multi-pin forming cannot be interpreted as a simple superposition of single-pin processes. Instead, the results reveal that local pin geometry is governed by interaction effects between neighbouring pins, even under constant process conditions. These interactions arise from competition for the locally available material, leading to systematic variations in pin height depending on pin spacing and arrangement.A distance-based interaction model is proposed, enabling the prediction of local pin height and its spatial distribution with good accuracy. The findings further demonstrate that in addition to the local pin height, the homogeneity of the global pin height is a critical factor, as it directly affects the reliability and predictability of the resulting mechanical joint. The presented approach provides a foundation for the design and optimisation of multi-pin joining systems for lightweight applications.