Foamability of MgAl 2 O 4 (Spinel)-Reinforced Aluminum Alloy Composites
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INTRODUCTION
MAKING closed-cell aluminum alloy foams directly from molten alloys promises to be economical as fewer processing steps are required compared to manufacturing routes based on metal powders.[1] However, ceramic or intermetallic particles are required for foam stabilization,[2–5] which can be a disadvantage especially when SiC particles are used. Although SiC particles stabilize aluminum foams efficiently, they make the resulting foam brittle and difficult to machine, and the manufacture of composites is also expensive.[1] Particle-containing alloys can be foamed either by direct gas injection into melts (sometimes called the ‘‘Alcan route’’)[6,7] or by ‘‘foaming of reinforced metals by gas release in precursors,’’ known under the acronym ‘‘Formgrip.’’[8] Recent investigations of foam stabilization suggest that smaller particles (sub-micron-sized) with lower volume fractions than used in Alcan[6,7] or Formgrip processes[8] could provide sufficient stability too.[9] Dispersing SiC particles in aluminum melts is difficult and expensive,[10] especially when the particles are very small. Instead of adding particles from outside, TiC and TiB2 particles were recently synthesised in situ in an Al-Si melt and were shown to provide good foam stability.[11] However, the higher processing costs and the toxic gas produced during that synthesis made the processing route G.S. VINOD KUMAR, Postdoctoral Researcher, is with the Structural Properties of Materials Department, Berlin Institute of Technology, 10623 Berlin, Germany, is also with the Institute of Applied Materials, Helmholtz Centre Berlin for Materials and Energy, 14109 Berlin, Germany. Contact e-mail: [email protected] M. CHAKRABORTY, Professor and Director, is with the Indian Institute of Technology, Bhubaneswar 751013, India. F. GARCIAMORENO, Head of Metallic Foam Group, and J. BANHART, Professor and Head, are with the Structural Properties of Materials Department, Berlin Institute of Technology, are also with the Institute of Applied Materials, Helmholtz Centre Berlin for Materials and Energy. Manuscript submitted September 1, 2010. Article published online May 3, 2011 2898—VOLUME 42A, SEPTEMBER 2011
questionable. In addition, the thermal instability of TiC and the agglomeration tendency of TiB2 particles in the melt did not yield consistent foaming properties. Recently developed aluminum composites containing fine MgAl2O4 particles[12–14] appear to be a promising alternative system. MgAl2O4 (spinel) is a reaction product that forms during the reaction of Mg with oxides such as SiO2 or Al2O3 in aluminum matrix composites.[15] Owing to its good wetting with aluminum alloy melt and its good thermal stability, MgAl2O4 has found a place as a reinforcement component in Al composites. An additional advantage is that the particles can be formed in situ by a reaction of oxides with a molten Al-Mg alloy. Unlike the process used for producing TiC and TiB2 particles, the process of generating solid MgAl2O4 in Al by in-situ synthesis is simple, inexpensive, and nontox
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