Microstructural and thermodynamic investigations on friction stir welded Mg/Al-jointsWith friction stir welding it is possible to join light metals in the solid state. However joints between dissimilar metals are aspecial challenge. At the Institute of Materials Science and Engineering of the University of Kaiserslautern the friction stir weldability of magnesium-aluminium-joints was investigated. For the realizationofhigh strength joints between these materialst he knowledge of the developing microstructure in the weldingzone is of especially high interest.T herefore, light microscopic and scanning electron microscopic investigationswerecarried out and the results compared withp hase diagrams and phase fractions for the bondingz onew hich werec alculated at Institute of Metallurgy of the University of Clausthal.

Due to the increasing shortage of energy resources weight reduction is a central development target of new environment-friendly products. But for the integration of lightweight metals in complex structures appropriate joining technologies are necessary. The friction stir welding (FSW) technology is a suitable method to realize high-quality joints between lightweight metal components in solid state (T welding < T melting ). [1,2] The FSW process based on the friction of a rotating tool with a pin positioned in a butt seam. This leads to an increasing temperature which enables plastic deformation of the welding zone. The result is a high strength weld. The side of the joint where the rotation of the tool and the feed rate summate is called ''Advancing Side (AS)'' and the opposite side of the joint is the ''Retreating Side (RS)''. The main process parameters are the welding feed rate, the rotational speed, the tilt angle of the tool and the welding force, as shown in Figure 1. Their interaction influences the resulting microstructure in the welding zone and hence the mechanical properties of the joints.At the WKK the friction stir weldability of the die cast magnesium alloys AZ91D, MRI153M, and MRI230D and additionally of hybrid joints between the die casted magnesium alloy AZ91D and the rolled aluminum alloy AA5454 were investigated. To characterize the mechanical properties of the FSW-joints monotonic tests with different specimen alignments were carried out and the microstructure in the bonding zone was analyzed with light microscopy and scanning electron microscopy (SEM). Welding EquipmentA modified milling machine from the type Deckel Maho DMU80T (Fig. 2) is used at the WKK to realize friction stir welds. To ensure a constant welding force during the welding process the machine was adapted with z-axis force control system. COMMUNICATION [*] O. [**] The support of the DFG Priority Program 1168: ''Extending the Range of Applications of Magnesium Alloys'' is gratefully acknowledged.In a mobile world weight reduction is a predominant target of innovative products. In this context appropriate joining techniques are necessary for the integration of lightweight metals in complex mechanical components. Friction stir welding (FSW) is a newly established well suited process to realize high-quality lightweight metal joints in solid state. In a research project of WKK the friction stir weldability of similar joints using die casted AZ91-Mg-alloy and MRI-Mg-alloys was investigated.Additionally the joining of hybrid joints between AZ91 and AA5454 aluminum alloy was performed.To describe and optimize the FSW-process the welding temperatures and welding forces were recorded online during the process. The investigations of the monotonic properties of AZ91/AZ91-joints and MRI/MRI-joints yielded in tensile strength values at the level of the parent materials. For dissimilar joints an extreme increase of the nugget hardness was measured. By SEM investigations and EDX element mappings it could be proved that this is caused by inter...