On the Formation of Sludge Intermetallic Particles in Secondary Aluminum Alloys
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FE is a common impurity in commercial aluminum– silicon alloys, forming brittle intermetallic compounds with Al and other elements that are known to be detrimental to mechanical properties.[1] Even with a low amount of Fe in the melt, the monoclinic particles of b-Al5FeSi crystallize into a platelet-like morphology during solidification. These phases act as stress raisers, causing a general reduction of ductility and ultimate tensile strength.[1–5] Formation and amount of porosity are also affected by the Fe content.[6–8] However, Fe in aluminum die-casting alloys is a desirable and necessary element which helps to prevent or alleviate die soldering.[9,10] Therefore, inexpensive secondary alloys with an already relatively high Fe content (in the range of 1 wt pct) are used. The Al-Si-Fe eutectic composition occurs at about 0.8 wt pct Fe. When Fe content is above this level, the molten metal has little or no tendency to dissolve and solder die steel when the two materials are in intimate contact. It has been shown that the size and quantity of Fecontaining phases is strongly influenced by the solidification rate[5,11,12] and that alloying elements such as Mn, Cr, Ni, Mo, Co, Sr, K, and Be can change the morphology of intermetallic phases or enhance the precipitation of phases which are less detrimental than STEFANO FERRARO, Research Fellow, is with the Department of Management and Engineering, University of Padova, Stradella S. Nicola, 3, 36100 Vicenza, Italy. Contact e-mail: [email protected] ANTON BJURENSTEDT, Ph.D. Student, is with the Department of Mechanical Engineering, Materials and Manufacturing, Casting, Jo¨nko¨ping University, P.O. Box 1026, 551 10 Jo¨nko¨ping, Sweden. SALEM SEIFEDDINE, Associate Professor, is with the Department of Mechanical Engineering, Materials and Manufacturing, Casting, Jo¨nko¨ping University, and also Researcher and Research Coodinator with the Swerea SWECAST, P.O. Box 2033, 55002 Jo¨nko¨ping, Sweden. Manuscript submitted January 26, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
b-Al5FeSi.[5,11–19] Moreover, a cooling rate higher than 20 K/s can suppress the platelet-like morphology and generates a less harmful a-phase form.[20] Fe, Mn, and Cr, the main source of which is recycled aluminum scrap, are normally present in secondary aluminum alloys as impurities. Fe, Mn, and Cr can substitute one another when present in a crystal structure that is body-centered cubic, a-Al15(Fe, Mn, Cr)3Si2; the morphology of this may be Chinese script, star-like, or polyhedral[1,21,22] However, these complex intermetallic compounds have a high specific gravity and tend to segregate to the bottom of aluminum melts and holding furnaces and the intermetallics, generally called sludge, thus reduce the effective capacity of the furnace.[23–26] Since sludge crystals are hard and brittle compact inclusions,[27] they can also compromise machining operations and have a considerable effect on cutting tool life, and degrading the mechanical and physical properties of the component.[2–4,26,28–30] The p
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