Friction stir welding of thixocast high performance magnesium alloys

Abstract

The aim of the present project is to investigate the friction stir weldability of novel light and recyclable high-performance materials based on the ZK60 magnesium alloy (Mg-6%Zn-1%Zr) modified with different mischmetal (RE) contents: 0.5, 1.5 e 2.5 wt%. For the production of such materials still unexplored in Brazil, it is proposed the use of a thixocasting procedure under mechanical stirring, followed by heat-treatments in the T4, T5 or T6 conditions. The ZK60 alloy represents an important matrix for the development of materials with applications in aeronautic light-weight structures. This is due to its highest specific strength among the traditional magnesium alloys, which nearly corresponds to that of the 7075 aluminum alloy. Its modification with RE additions enables the formation of intermetallics with high melting point, hot- and chemical stability, which are required to improve the mechanical strength at room and elevated temperatures, as well as the corrosion and oxidation resistance. The zirconium additions promote furthermore an effective grain refinement in magnesium castings, thus increasing the potential of the proposed alloys for a neat-net-shape processing route based on thixocasting. The effect of casting, heat-treatment and welding parameters will be quantified on the microstruture, mechanical properties, corrosion and oxidation behavior of the alloys and their weldments. It is proposed the acquisition of an X-ray diffractometer, which will enable the study of further weldment properties rarely investigated in Brazil, but particularly relevant to the performance of magnesium welds, such as: the formation of crystallographic texture in the stir zone and thermomechanical affected zones, and its influence on the mechanical properties; as well as the mechanical stress relief and its impact on the fatigue crack growth rates and corrosion behavior. This proposal is supported by Grupo Rima Industrial S/A and EMBRAER, which demonstrated great interest in this research and undertook to supply the raw materials for the alloy preparation and to take active part in the casting and welding procedures of the ZK60-RE alloys. The proponent has already experience in the welding and characterization of magnesium alloys. (AU)