Response to Thermal Exposure of Ball-Milled Aluminum-Borax Powder Blends
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NTRODUCTION
AL-B alloys are essentially Al composites reinforced with AlB2 particles owing to the very limited solubility of boron in solid and liquid aluminum. Al-AlB2 composite has a low density, but a very high resistance to abrasion and is an attractive lightweight candidate for structural applications.[1–3] These composites are particularly attractive for neutron shielding since B is an excellent neutron absorber and is a potent replacement for cadmium which is becoming increasingly difficult to use because of its high cost and increasingly more strict health and safety regulations.[4] However, the popular commercial application of Al-B alloys is in aluminum foundries as master alloys. They are used in the manufacture of aluminum conductors to precipitate transition elements that have a detrimental effect on electrical conductivity when in solid solution.[5–7] Finally, Al-B alloys were shown to be very powerful grain refiners for aluminum foundry alloys.[8–14] Al-B master alloys are produced with the popular halide salt process. KBF4 salt is reduced by aluminum so as to form aluminum borides when added into molten aluminum.[15] The ‘‘Halide salt’’ route, however, suffers serious drawbacks and is not attractive.[16] A large volume of salt must be added to molten aluminum since KBF4 is lean in B. While KBF4 is an expensive compound which adds greatly to the raw material costs, a substantial portion of the charge is of no use, giving YUCEL BIROL, Senior Scientist, is with the Materials Institute, Marmara Research Center, TUBITAK, Gebze, Kocaeli, Turkey. Contact e-mail: [email protected] Manuscript submitted August 5, 2012. Article published online December 1, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS B
rise to intense fluoride emissions and troublesome dross. Besides, the cooling of the melt by a large volume of particulate material is hardly compensated for by the exothermic reaction it kicks off. Hence, there have been several attempts to replace the KBF4 salt with other B-rich compounds.[16–22] Among several alternative B sources offered to replace KBF4 salt,[16–22] the potential of B2O3 has been investigated both in the liquid and the powder metallurgy routes.[16] The response to thermal exposure of ball-milled Al/borax pentahydrate powder blend was investigated to explore the potential of the powder metallurgy processing route to produce Al-AlB2 alloys.
II.
EXPERIMENTAL
Borax pentahydrate (BPH, ETI Mine Co. Kırka Borax Works, 97 pct Na2B4O7.5H2O, 160 lm average particle size) was used as the boron source, while aluminum (99.7 pct purity, 120 lm average particle size) was used as the reducing agent. Powder batches of 5 g (1.33 g Na2B4O7Æ5H2O and 3.67 g Al) were weighed on a sensitive balance. Ball milling of powder mixture was carried out in a Spex 8000 D Mill at room temperature. The ball milling media were a hardened chromium steel vial (50 mL) with three steel balls (diameter: 9.5 mm). The ball-to-powder weight ratio and the rotational speed of vial were 2:1 and 500 rpm, respectively. The response
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