Grain refining of Al-4.5Cu alloy by adding an Al-30TiC master alloy
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TRODUCTION
In a previous article, we reported on an aluminum matrix composite achieved by in situ synthesis of TiC particles in the preform. The TiC particles were fine and relatively free of agglomeration caused by sintering, which led us to expect the composite to work as an excellent ‘‘master alloy’’ for subsequent dilution in an aluminum melt. The present study was intended to confirm the performance of that composite as a master alloy and focused on its employment as a grain refiner of aluminum alloys. The use of grain refiners is the most common method of achieving grain refinement in aluminum alloys, and a number of AlTi-based master alloys are widely used to improve castability and mechanical properties and to increase tear resistance. Most grain refinement theories are based on the formation of aluminum crystals on nucleant particles. However, in spite of much work over the past half century, the detailed mechanism or mechanisms of grain refinement are still not wholly understood.[2–10] Generally, aluminum seems to nucleate on the Al3Ti particles at hyperperitectic compositions (.0.15 Ti wt pct), where Al3Ti exists in the melt stably. At hypoperitectic compositions, where Al3Ti cannot exist in the melt stably, TiC or other potential nucleant compounds seem to work as a nucleant instead of Al3Ti. According to the ‘‘carbide’’ theory of Cibula, the formation of TiC particles occurs by reaction of Ti and residual carbon in the melt, even where no deliberate carbon addition is made and the TiC particles work as a nucleant.[2] After Cibula, a number of studies have been made in this area and the following compounds have been reported as a potential nucleant: Al3Ti, Al3(Ti,B), AlB2, TiB2, and TiC.[3] In the industry, Al-Ti-B ternary master alloys are widely used and commonly accepted as the most effective aluminum grain refiner for most alloys. Although the role of B is not very clear, the addition of B improves the refining effect of the Al-Ti refiner in practice. However, TiB2 ag[1]
glomerates can lead to problems including damage to production of aluminum foil. In addition, when Zr, Cr, or Mn presents as an alloy element, TiB2 nucleants tend to be poisoned and to lose their refining effect.[4,5] Therefore, there is interest in developing improved nonboron containing alloys including alloys in the Al-Ti-C family.[6,7,10] As a master alloy for grain refining, the composite Al30TiC (Al-24Ti-6C) studied herein has a relatively high amount of TiC particles formed before introduction into the metal melt. Thus, it is anticipated that unlike other inoculants, which are presumed to nucleate on insitu formed TiC, this inoculant should require no holding time to achieve optimum grain refining efficiency and should be an effective refiner. II.
EXPERIMENTAL PROCEDURE
Preforms weighing 1.5 grams were formed from 0.8 g of titanium powder (99.7 pct), 0.2 g of crystalline graphite powder (99 pct), and 0.5 g of aluminum powder (99.9 pct). These were then infiltrated by being dipped into a pure aluminum (99.9 pct) melt at 1000
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