Aluminium alloys with lithium addition have been known for nearly 100 years in the aerospace industry. The main advantage of lithium addition is the reduction of the alloy density and the improvement of the mechanical properties, especially the modulus of elasticity and strength. Each added percent of lithium reduces the density of the alloy by 3 %. Improved mechanical properties are caused by the precipitation of the Al3Li precipitate. The main disadvantage of these alloys is the high reactivity of lithium. Lithium reacts with oxygen and nitrogen and forms gases which cause gas porosity. There are composites that could in theory replace aluminium and lithium alloys, however these are less affordable.
The aim of the diploma work was to investigate the influence of cooling rate on the quality of AlSi7MgLi alloy and to describe the sequence of solidification of the alloy. The alloy was cast in three different moulds: steel mould, Croning mould and graphite mould. The method of simple thermal analysis allowed us to monitor the solidification process of the alloy. We found that a faster cooling rate delivers higher quality castings with less porosity. Using the Thermo-Calc program, we calculated the thermodynamic equilibrium of the alloys. We examined which phases are expected to occur in the microstructure of the samples. The samples were then analysed using a scanning electron microscope and a light microscope. This also proved the existence of certain phases and explained the course of solidification of the alloy. This begins with the solidification of the new AlLiSi phase. The solidification sequence continues with the solidification of the Al dendrites and eutectic (α-Al + AlLiSi). The iron phase Al9Fe2Si2 is then formed. Then follows the solidification of the ternary eutectic (α-Al + β-si + AlLiSi). After that comes the transformation of the iron phase Al9Fe2Si2 into the π phase (Al18Fe2Mg7Si10). At the end forms the Mg2Si phase. The mechanical properties of the alloy were measured using a Vickers hardness measuring device. It is seen that the hardness is higher in samples with a higher cooling rate.
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