STUDY OF THE INFLUENCE OF ALLOYING ELEMENTS ON THE HIGH TEMPERATURE PROPERTIES OF WROUGHT ALUMINIUM ALLOYS

摘要

Aluminium is nowadays one of the most important materials in the aeronautical sector. Its lightness and good specific mechanical resistance have favoured its increased use in an industry very concerned with weight reduction. New potential applications have been identified that require new alloys with improved performance in terms of mechanical strength and creep resistance. The present work deals with a design of experiment approach to identify wrought aluminium alloys with good mechanical properties at high temperatures (200-250°C). Age hardened wrought alloys are much used for aeronautic applications. Age hardening gives the alloy a higher strength due to the precipitation of second phases that act as an obstacle for the dislocation movement and pin the grain boundaries avoiding the grain growth. However, the coarsening of these precipitates at temperatures above 180°C leads to an important reduction in the mechanical properties of the alloy when they are exposed to high temperatures during long periods. Several heat resistant aluminium based materials that have been developed in the last years do exist that might already be used at temperatures over 200°C. However, most of them are either based on the incorporation of expensive alloying elements such as rare earths, silver or scandium or on the addition of reinforcements (SiC, Al_2O_3, B_4C, etc.). Other alloys produced by mechanical alloying, rapid solidification or spray forming may also work at those working conditions. Notwithstanding, these solutions cannot be widely used as they are either too expensive or present technical drawbacks such as insufficient ductility, low machinability or recycling problems. The present work aims at developing such alloys through a methodology based on the identification of the effect of 12 different alloying elements and their combinations in the properties of aluminium alloys and the selection of the optimum combination of these alloying elements through the Taguchi methodology. The steps followed for the selection of the alloying elements and the maximum and minimum ranges are explained and the process of the production and selection of the alloys is explained. The different alloys were cast and extruded in order to obtain tensile specimens that were tested at 250°C. Eventually the analysis of the microstructure of the most promising alloys is presented.