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ALUMINUM/SILICON alloys are the most commonly diffused foundry alloys in the automotive industry since they respond to the need of decreasing vehicle’s weight without compromising strength resistance. A great contribution to the use of Al-Si alloys comes from high-pressure die-casting (HPDC), which allows to increase the production by lowering the cycle time and to manufacture complex-shaped castings with thin wall thickness. Unfortunately, the limit to large diffusion of HPDC remains the final integrity of castings. In HPDC, the metal is injected at high speed into a steel die, thus the associated turbulence represents the major source of inner and surface-casting defects. Further, to be competitive in the market by reducing the cycle time even more, foundries tend to increase the filling velocity, believing that the casting quality will not change. In this way the product requirements will never be satisfied and the amount of scraps increases with a progressive reduction of productive efficiency. There are several critical features in HPDC, such as forced convection during the filling phase, high speed of molten metal at ingates (up to ~50 ms1),[1–3] high cooling rate (up to ~103 K/s[4,5]), and high pressures (up to ~120 MPa) applied to the biscuit during the intensification stage to assist the feeding of solidification GIULIO TIMELLI, Assistant Professor, and ALBERTO FABRIZI, Postdoctoral Student, are with the Department of Management and Engineering, University of Padova, Stradella S. Nicola, 3, 36100, Vicenza, Italy. Contact e-mail: [email protected] Manuscript submitted January 24, 2014. Article published online August 15, 2014 5486—VOLUME 45A, NOVEMBER 2014

shrinkage in the die cavity.[6,7] All these features produce castings with both microstructures and defects that are markedly different from those created by other foundry processes. It is widely accepted that a fine-grained, defect-free layer for ms near the casting surface producing a microstructure that is commonly referred to as ‘‘the skin’’ or ‘‘the surface layer’’.[8,9] Furthermore, one of the interesting microstructural characteristics created during HPDC is the narrow bands of positive macrosegregation that commonly follow the contour of components.[10] The central region of die-castings, or core, contains a mixture of fine grains and externally solidified crystals (ESCs), which mainly form in the shot sleeve and are injected into the die cavity during die filling.[11,12] Generally, this region is also characterized by great amount of casting defects, such as porosity,[7,13] which have deleterious effects on the mechanical properties of die-castings.[14] In addition, the core also shows a large fraction of coarse intermetallic compounds containing Fe, Mn, and Cr, which are present as impurity elements in recycled Al foundry alloys, such as AlSi9Cu3(Fe) alloys.[15] The formation of these crystals, also named sludge, can occur throughout the HPDC process, such as in the holding furnace, in the ladle (or feed pipe) during the melt transfer to cold cham