Temperature control for Friction Stir Welding of dissimilar metal joints and influence on the joint properties

Friction Stir Welding (FSW) is a suitable technology to join dissimilar metals such as aluminum and copper or aluminum and titanium. Since it is a solid state welding process, the solidus temperature is typically not exceeded and the formation of intermetallic phases can be minimized compared to fusion welding processes. However, an intermetallic layer is still formed at the joining interface. This layer determines the seam properties such as the joint strength or the electrical conductivity. The thickness of the layer is in the nanometer range and is mainly influenced by the welding temperature via an Arrhenius law. The process temperature mainly depends on the rotational speed and on the feed rate of the machine tool. In this study, a temperature control system for aluminum-copper lap joints was developed. A PI control system was used for this purpose to maintain the given welding temperature by adjusting the rotational speed. Consequently, a constant welding temperature was ensured along the entire seam and influences such as changes in workpiece geometry, environmental conditions, or material variations could be mitigated. Experiments with six different temperature levels (low - high) were conducted for one exemplary welding task in order to verify the proposed constant welding conditions. The joints were investigated by tensile shear tests as well as optical and electron microscopy. It was proven that temperature-controlled FSW ensures a constant thickness of the intermetallic compound layer.