Developing a predictive weld-seam formation model: experimental support

In hollow aluminium extrusions produced with porthole dies, longitudinal weld-seams are present throughout the entire extruded length, due to rejoining of metal streams inside the extrusion tooling. These weld-seams, formed by a solid-state bonding process at elevated temperatures and under conditions of interfacial pressure and plastic deformation, are commonly the weakest areas in the extrusion cross-section. Therefore, predictive control of the weld-seam formation process is essential. For this purpose a conceptual model is developed, containing all relevant features pertaining to the solid-state bonding process as this occurs in aluminium extrusion. This paper describes an experimental programme aimed at exploring the effects of tooling geometry, process conditions and alloy composition on weld-seam performance. In lab-scale experiments, strips were extruded from AA6060 and AA6082 billets, using tooling geometries in which a bridge was incorporated to form a weld-seam. Keeping the profile geometry constant, the internal geometry of the tooling was varied, resulting in different bonding conditions. The obtained weld-seams were characterised by means of transverse tensile tests. Associated microstructural characterisation of the extrusions and inspection of fracture surfaces was performed in order to relate the mechanical performance to microstructural features. The results clearly show that the evolutionary microstructural response of the alloy, determined by the local thermomechanical conditions inside the die, has a significant effect on the weld-seam performance. It is therefore concluded that, besides a criterion for suitable flow conditions and interfacial pressure, a predictive tool for weld-seam quality must incorporate the evolutionary microstructural response of the alloy in question.