EFFECT OF T5 AND T6 HEAT TREATMENTS ON COLD DEFORMATION OF A THIXOCAST ALUMINIUM ALLOY

摘要

Among cast aluminium alloys, 319 ranks as one of the commercially important alloys used in automotive applications, on account of its excellent casting characteristics and good mechanical properties (1,2,3). For these reasons, the 319 is one of the candidate alloys to perform the shaping of aluminium alloys in the semisolid state or thixocasting. In fact a thixotropic aluminium alloy presents some advantages with respect to a 100 percent melted alloy: a much higher viscosity; permitting the injection of the material at relatively high speed, a lower thermal content and less contraction during solidification (1).The use of this alloy is mainly justified by the search for maximum hardening obtained with T6 (at 160 deg C) or for stabilisation for the purposes of operation at high temperatures (220 deg C). In principle, under-ageing is not recommended as it results in a high sensitivity to stress corrosion. The T5 treatment is of particular interest in the case of thixocasting: the very high solidification rate leads to both a fine structure and a high Mg (and possibly Cu) content in solution in the as-cast state. This enable a simple ageing process to combine both high resistance and high ductility, avoiding the inconvenience and costs associated with solutionizing (1).The aim of this work was to test the mechanical behaviour of 319-thixocast alloy after different heat treatments and compare the results with cast alloy properties.A study of the microstructure evolution during solution treatment is illustrated iii figs. 1-3. A speroidisation of Si particles is the main structural change during exposure at high temperature, together with a redistribution of the solute elements from the eutectic to the globule (4-6). Hardness and electrical conductivity measurements have been performed on aged samples to follow the precipitation process (fig. 4-5). Hardness measurements revealed a delay in precipitation starting in T5 with respect to T6 due to the lack of supersaturation. After ageing, samples were prepared for tensile test at room temperature and the effect of thermal treatment on mechanical properties has been studied (fig. 6 and 7). T5 ageing treatment gives satisfactory results for elongation because the ductility values are comparable with the results from T6, while the UTS values are slightly lower From this point of view T5 is preferable to T6 because of time and cost saving. The thixocast 319 alloy has definitively good mechanical properties compared to the 319 alloy produced by other casting techniques (fig. 7). Fig. 8 is a fractography of the solution sample showing the ductile-like behaviour of this specimen.