Analysis of semi-solid diecasting runner-gate system as a function of solid fraction

The injection of alloys in the semisolid state, is highly effective in reducing casting porosity, particularly the one due to turbulences during die filling. On the other hand, the use of semisolid billet poses additional difficulties about either proper alloy re-heating and die design, especially runner and gate system. Obviously gates must be thicker than in traditional diecasting, because of the higher alloy viscosity, however no standards for a good die design are today available in the literature and each manufacturer refers to his own experience and know-how. The aim of this work is to evaluate the effect of the various runner and gate geometrical parameters on the filling pattern, taking also into account the influence of different solid fractions, not only those related to not properly controlled re-heating temperature, but also those typical of rheocasting technology. The study was carried out by means of a numerical simulation commercial software (Procast), considering a typical aluminun-silicon alloy (A356), whose thermo-fluid dynamics properties needed for simulation are mostly available. The results obtained will be subsequently used for the fabrication of an experimental die, for the production of dumb bell and toughness specimens, for future tests in the University diecasting laboratory

Design and production of new aluminum thixotropic alloys for the manufacture of structural components by semisolid die casting

Semi-solid processing is nowadays a powerful technology for the realization of structural components; in addition to the increase in their mechanical properties, due to the globular structure instead of the dendritic one, further developments are most likely to be expected from alloy chemical composition adjustments in order to achieve higher performances compared with the industrially used A356 and A357. Aim of this research is to try to set up new aluminium. alloys for semisolid foundry applications, starting from the standard Al-Si system, at the base of all known casting processes. Different chemical compositions, based on either foundry or wrought Al alloys, have been investigated by means of computational thermodynamics (Pandat (R)), producing quantitative data about solidus-liquidus interval, solid fraction as a function of temperature, phase diagrams i.e. potential for age hardening, etc.. Some selected alloys, fitting the needs of good castability, proper concentration of hardening elements in the alpha phase and, obviously, easy production of feedstock material have been mechanically stirred by means of an experimental apparatus designed and self-constructed in the foundry laboratory of the university; the effect of different stirring tool configurations on the semi-solid state obtainment has also been assessed. Subsequently, the manufactured alloys have been reheated and cast into a simple die, properly designed, for the production of small samples. Microstructural investigations have been done on the stirred alloy (before and after re-heating), on the as cast and the heat treated samples to evaluate the efficiency of the designed system and of the defined alloys. Experimental tests on the processed alloys have been carried out by means of an instrumented crucible in order to verify the predicted thermodynamic properties supplied by simulation study (i.e. fs-temperature curve)