The Effect of Zr and Li on the Microstructure of AlMg5Si2Mn-Type Casting Alloys

In the article, the authors present results of microstructural studies of Al-Mg-Si-Mn casting alloys with Zr, Li, and TiB₂ additions on a broad scale. Zirconium content was set on two levels: 0.34 and 1.58 wt%, and Li was set 1.2 and 2.0 wt%. It was found that the addition of Zr shifts the eutectic melting temperature to a higher level, up to 611.3 °C at 1.6 wt% Zr. At the same time, Li addition leads to the depression of eutectic melting temperature: down to 587.2 °C at 2.0 wt% Li, what is a common effect of eutectic modification which was confirmed by means of structural examinations. The complex addition of Li and AlTi5B1 resulted in a eutectic melting temperature close to the equilibrium eutectic temperature for the Al-Mg-Si system (596.2 °C). The grain refinement effect of Zr is due to the nucleation of α-Al on the Zr(Al_1−x, Si_x)₃ phase. Crystals of this phase were detected in the grain centers of Zr-containing alloys. The Li addition does not affect α-Al grain size but changes the morphology of eutectic colonies from petal-like to fibrous. Observation of TiB₂ particles inside the primary Mg₂Si crystals gives direct experimental confirmation of nucleation of the primary phase on the surface of TiB₂ in the alloy after adding Li and AlTi5B1. Natural aging of the alloys resulted in the formation of fine precipitates detected close to dislocations. The most apparent supposition is that the mechanism responsible for their formation is heterogeneous nucleation in the stress field of dislocations. Hardness tests showed adding 2.0 wt% of Li is very effective, increasing hardness up to 113 HV0.2 in naturally aged condition, which is nearly double that of commercial Al-Mg-Si die-casting alloy. Several effects were proposed which may synergistically contribute to the rise of hardness in Li-containing alloys, such as solid solution strengthening, formation of primary LiAlSi phase and natural aging.