Casting properties of the high-strength AlZnMgCuNiSi alloys
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ˇestmı´r Barta and Cˇestmı´r Barta, Jr. Barta, Barta, Triska (BBT) Materials Processing, Doubicka´ 11, 184 00 Prague 8, Czech Republic
Jan Sˇera´k, Petr Janda, and Ondrˇej Eckert Institute of Chemical Technology, Department of Metals and Corrosion Engineering, Technicka´ 5, 166 28 Prague 6, Czech Republic (Received 25 April 2002; accepted 11 December 2002)
In this paper, the improvement of casting properties of the high-strength AlZnMgCu alloys is described. This improvement was achieved by additions of Ni, Mg, and Si, which formed eutectic phases, thus reducing the crystallization interval of the alloy. The comparison of the castability and hot-tearing tendency was made for three alloys: nearly single-phase AlZn6Mg2 alloy, quasi-ternary eutectic AlZn7Mg7Cu1Ni3Si3 alloy, and common casting AlSi10 alloy. It was shown that the castability and hot-tearing tendency of the quasi-ternary eutectic alloy are similar to those of the AlSi alloy.
I. INTRODUCTION
AlZnMg alloys are typical representatives of highstrength aluminum alloys. The microstructure of the cast material is formed by ␣(Al) primary dendrites and of a Zn, Mg-rich degenerate nonequilibrium eutectic along the boundary of dendrites (Fig. 1). The composition of the eutectic is controlled by the chemical composition of the alloy. Depending on Zn and Mg contents, MgZn2 and/or Mg3Zn3Al2 eutectic phases can solidify.1 In Al–MgZn2 and Al–Mg3Zn3Al2 quasi-binary phase diagrams, the eutectic temperatures of 470 °C and 489 °C have been identified, respectively.2 Therefore, AlZnMgCu alloys are characterized by wide crystallization intervals (approximately180 K). This leads to a poor castability and enhanced tendency to form hot tears. The difficulty in production of castings of these alloys is that they are mostly treated by rolling, extrusion, forging, etc. Therefore, the production of small series of special parts often appears to be very expensive. The demand for relatively simple shaped parts produced by a casting technology evoked the research and development of high-strength AlZnMgCu-based alloys with improved casting properties. To obtain such properties, the temperature interval of crystallization should be narrowed down as much as possible. One way to obtain this is to reduce the concentrations of Zn and Mg. As a consequence, however, this change of composition will also deteriorate the mechanical properties of that material after age hardening. Another way is to add silicon, but the formation of Mg2Si phase would decrease J. Mater. Res., Vol. 18, No. 3, Mar 2003
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Mg content in solid solution with a negative effect on the mechanical properties. Therefore, the practical realization consists in appropriate additions of elements, which lead to the formation of eutectic intermetallic phases (IP). The formation of such eutectics [␣(Al) + IP] results in a reduction of the liquidus temperature and because the temperature of nonequilibrium solidus is nearly constant, the width of the crystallization interval is d
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