In Situ Study of Microstructure Evolution in Solidification of Hypereutectic Al-Si Alloys with Application of Thermal An
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UTECTIC Al-Si alloys containing up to 16 to 20 pct Si exhibit attractive service properties under conditions of high temperature, tension and compression load cycles, and vibration. These alloys are extensively used for demanding applications in the automotive industry where superior tribological performance under high operating temperatures is required.[1–6] In addition to being light weight, such alloys have good thermal conductivity and are utilized for rapid heat removal in internal combustion engines. Examples of components include monolithic engine blocks cast using high-pressure die casting (HPDC), cylinder bore inserts for sand cast engine blocks, engine pistons made using low-pressure permanent mold (LPPM), and hot forgings.
DIMITRY G. SEDIAKO, Senior Research Officer, is with the Canadian Neutron Beam Centre, Chalk River, ON, Canada. Contact e-mail: [email protected] WOJCIECH KASPRZAK, Director of Operations, is with the CanmetMATERIALS, Natural Resources Canada, 183 Longwood Road South, Hamilton, ON, Canada. Manuscript submitted August 9, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
Typically, hypereutectic alloys contain, in addition to about 16 to 20 pct Si, up to 3 pct Cu and up to 0.5 pct Mg, which are added to increase strength via the precipitation strengthening mechanism. In addition, 0.01 pct P is used in order to achieve uniform size, shape, and distribution of the primary Si crystals that nucleate on AlP3 particles during the alloy solidification process. Under equilibrium or semi-equilibrium conditions, the microstructure of this alloy consists of a-Al dendrites, primary Si, Al-Si eutectic, Fe-based phases as well as Cu- or Mg-based intermetallics’, i.e., Al5FeSi, Al8Mg3FeSi6, Al5Mg8Cu2Si6.[7–9] Secondary phases precipitate during heat treatment processing, that is, AlCu2 and Mg2Si which are present with morphologies and in quantities depending on given heat treatment parameters. Commercial hypereutectic alloys contain typically up to 20 pct of Si but some research studies focused on higher concentrations. Early studies in the 1960s were conducted by Gosh and Telang for binary-type Al-Si alloy system containing up to 22 pct Si and up to 0.2 pct phosphorus. These studies focused predominantly on fundamentals of the primary Si refinement as well as the effect of melt processing parameters. It was found that melt heating above approximately 1173 K (900 C) prior to casting resulted in coarsening of the primary
Si for the test sample solidified inside the water cooled steel crucible. It was suggested that the AlP particles started to disintegrate or even melt above 1173 K (900 C) and caused a coarsening effect of primary Si crystals.[10,11] Intensive research in the 1980s led to commercial application of hypereutectic alloy in engine blocks in passenger vehicles.[12,13] These engine blocks were cast using the low-pressure die casting process. On the other hand, advancements in high-pressure die casting technologies resulted in the commercial application of DiASil in the production of liner-l
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