Rôle d'addition de magnésium sur l'occurence de la fonte naissante dans les alliages expérimentaux et commerciaux Al-Si-Cu et son influence sur la microstructure et les propriétés de traction de l'alliage /

Abstract

Aluminum-silicon (Al-Si) casting alloys are widely used in automotive applications on account of their high strength-to-weight ratio and ability to be cast into complex shapes. Among these alloys, 319 alloy, belonging to the Al-Si-Cu system is popularly employed in such applications, where magnesium is often added to the alloy for strengthening purposes. These alloys are usually heat treated in order to obtain an optimum combination of strength and ductility. The excellent castability and mechanical properties of such Al-Si-Cu-Mg alloys has made them commercially popular for industrial applications.The mechanical properties of a casting are controlled by its microstructure which, in turn, is influenced by the alloy composition and the solidification conditions employed. In the case of 319 type Al-Si-Cu-Mg alloys, this would involve the fineness of the primary aAl dendrites (as determined by the dendrite arm spacing (DAS) value), the Al-Si eutectic structure (i.e., an acicular vs. a fibrous Si morphology), the CuAl2 and other copper intermetallics, as well as other second phase constituents (Fe-intermetallics, Mg 2 Si, etc.), depending upon the alloying and trace elements present in the alloy. The normally acicular form of the eutectic Si can be transformed or 'modified' to the fibrous form by the addition of certain 'modifiers' such as Na or Sr to the alloy melt, to improve the alloy ductility and strength. Magnesium is also observed to modify the Si eutectic. However, as it also causes segregation of the copper phases, this can lead to problems related to the melting of the latter (termed 'incipient melting').The properties of alloys containing elements such as Cu and Mg can also be enhanced through heat treatment, where the formation of fine precipitates of C11AI2 and Mg2Si during aging result in the precipitation hardening of the alloy. A typical heat treatment consists of solution treatment and quenching, followed by artificial aging. Solution heat treatment is carried out to achieve maximum dissolution of the Cu and Mg into the aluminum matrix. For this purpose, the solution treatment temperature must be kept as close as possible to the Al-CuAb eutectic temperature, but, at the same time, limited to a safe level below the maximum to avoid incipient melting of the copper phases which would result in the formation of cavities after quenching and lower the alloy soundness.The solution heat treatment process can be carried out in either a single step or in multiple steps. Unfortunately, the single step or conventional solution heat treatment used for 319 alloys (-498 °C) is not capable of maximizing the dissolution of the Cu-rich phases, nor is it able to sufficiently modify the Si particle morphology, both of which are required for enhancing the alloy properties. To overcome this, a two-step solution treatment (conventional solution treatment followed by a higher temperature solution treatment) was suggested, that significantly improves the dissolution of the Cu-rich phase, giving rise to better homoge...