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INTRODUCTION

IRON is one of the most common impurity elements in Al-Si foundry alloys. Actually, dilution seems to be the only practical, although uneconomic, method to reduce the Fe content in the alloy.[1] The maximum solubility of Fe in solid Al is between 0.03 and 0.05 wt pct at 928 K (655 C) and is less than 0.01 wt pct at 700 K (427 C).[2] Consequently, in commercial Al-Si foundry alloys, Fe forms intermetallic compounds with Al and other elements, which have long been known to be detrimental to mechanical properties.[3–8] A wide range of Al-Fe-Si particle types is reported in literature.[2,9,10] These can be generally grouped into three different morphologies: polyhedral crystals, Chinese script, or thin platelets. Even at low Fe content in the melt, the monoclinic b-Al5FeSi particles crystallize during the solidification. This platelet-like phase, which appears as needle in the microstructure, acts as stress raiser with a general reduction of the plastic properties of the cast alloy.[3–5] The formation and amount of porosity is also dependent on Fe-needles’ content.[11–13] Iron in Al die-casting alloys is a desirable and necessary element that helps to prevent or alleviate the die soldering.[14] Generally, an Fe content higher than STEFANO FERRARO, Ph.D Candidate, and GIULIO TIMELLI, Assistant Professor, are with the Department of Management and Engineering, Stradella S. Nicola, 3 I-36100 Vicenza, Italy. Contact e-mail:[email protected] Manuscript submitted March 2, 2014. Article published online December 10, 2014. 1022—VOLUME 46B, APRIL 2015

0.8 wt pct reduces the solution potential of the molten Al alloy for steel components of the casting machine and die.[14,15] Therefore, secondary alloys with higher Fe content (in the range of 1 wt pct) than primary ones are used. It has been shown that the size and amount of Fe-containing phases is strongly influenced by the solidification rate[4,5,16] and that alloying elements such as Mn, Cr, Ni, Mo, Co, Sr, K, and Be can change the morphology of the phases or enhance the precipitation of Fe-rich particles which are less harmful than b-Al5FeSi.[4–6,16–22] Manganese and chromium are normally present in secondary Al alloys as impurities due to the recycling process of aluminum scraps. Mn and Cr can also be intentionally added to the alloy because, singly or in combination, they neutralize the effect of Fe-needle particles by modifying the morphology and type of phase.[2,9] Fe, Mn, and Cr can substitute each other in the same bcc crystal structure, a-Al15(Fe,Mn,Cr)3Si2, which may appear as Chinese script, star-like or polyhedral morphology.[2,9,23] Modification of Fe-bearing compounds with Mn and Cr addition has also some disadvantages. The complex intermetallic compounds, like primary a-Al15 (Fe,Mn,Cr)3Si2, have a high specific gravity and tend to segregate to the bottom of molten alloy and holding furnaces; such particles are generally called sludge.[9,15,24–27] When sludge crystals are entrained into castings, they decrease the alloy’s fluidity and appear as hard incl