Optimizing the composition of master alloys for grain refining aluminium

Optimizing the composition of master alloys for grain refining aluminium

Scripta Materialia, Vol. 35, No. 9, pp. 1053-1056,1996 Elsevier Science. Ltd Copyright 0 1996 Acta Metallurgica Inc. Printed in the USA. All rights re...

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Scripta Materialia, Vol. 35, No. 9, pp. 1053-1056,1996 Elsevier Science. Ltd Copyright 0 1996 Acta Metallurgica Inc. Printed in the USA. All rights resewed 1359~6462/96 $12.00 + .OO

Pergamon PI1 S1359-6462(96)00270-9

OPTIMIZING

THE COMPOSITION OF MASTER ALLOYS FOR GRAIN REFINING ALUMINIUM T. Sritharan and H. Li

Division of Materials Engineering, School of Applied Science Nanyang Technological University, Nanyang Avenue, Singapore 639798 (Received February 26,1996) (Accepted May 13,1996) Introduction

Aluminium alloys are grain refined by inoculating the melt with specially prepared master alloys, called Tibor alloys, which contain high levels of Ti and B. Such Tibor grain refiners are commercially available in various chemical compositions and are widely used by aluminium smelters, alloy makers and even foundries. Systematic studies have been done (1-3) to evaluate the grain refining potency of several of these commercial Tibor alloys. The general fmding had been that the inoculation level, i.e. content of Ti and B in the matrix alloy after inoculation, has a marked effect on the resulting grain size. Ti and B in solution will readily combine to form TiB2 under these conditions (4,5) and hence, the presence of free Ti and B simultaneously in solution is an unlikely prospect. Depending on the prevailing B/Ti ratio, either free Ti, or free B, is likely to occur except when the ratio is the ideal stoichiometric atom ratio of 2/l for TiB2. Commercial grain refiners have B/Ti atom ratios lower than 2/l, i.e. they contain an excess of Ti. Evaluations of grain refmers have shown that an excess of Ti is essential for efficient gram refinement (1,6). On the other hand, eliminating B totally from the grain refiner also dirninishes its potency. Therefore, B plays an important role in improving the gram refinement potency of Tibor alloys but, too much of it may lessen the effect. An optimum B/Ti ratio may then exist. This study was undertaken to determine this optimum value. Experimental

Commercial purity ahnninium was melted in fifteen small clay-graphite crucibles. Each crucible contained approximately 200 g of Al and was inoculated to one of the three levels of Ti, O.O5wt%, 0.lOwt% or O.l5wt%. Then, different amounts of B were added to the crucibles to form the following five B/Ti atom ratios for each level of Ti inoculation: O/l, l/6, l/3, l/l and 2/l. Note that the B/Ti ratio of 2/l corresponds to the stoichiometry for TiB2. All other melts contained Ti in excess while the first melt had no B at all. Al-lOwt%Ti master alloy was used to inoculate the melt to the required Ti levels while B was added in the form of aOAl-5wt%Bmaster alloy. The melt was inoculated at 750 C and was stirred manually with a graphite rod to ensure complete dissolution of ibe master alloys. After holding at that temperature for 15 min., the crucibles were re1053

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moved from the furnace and were left out for the melt to solidify at room temperature. A convenient sample was cut and polished from the centre of the solid metal from each crucible for metallography. The samples were macro-etched in Poulton’s reagent and the grain size was determined by the mean linear intercept method using a stereo-microscope. Results and Discussion

The average grain size obtained from each melt depended on two parameters: the Ti content and the prevailing B/Ti ratio. Fig 1 shows the results where the grain size is plotted against the B/Ti atom ratio for each level of Ti inoculation. A minimum grain size is shown for each Ti level at a B/Ti atom ratio of l/2 approximately, while the stoichiometric atom ratio for TiB;! is 2/l (see Fig 1). Since, the grain size obtained for the stoichiometric B/Ti ratio of 2/l is larger than the grain sizes exhibited by the other casts of the same total Ti content but with lower B/Ti ratio, it is confirmed that a Ti content in excess of that required for TiB2 formation is necessary for the best grain refining action. However, small amounts of B are clearly beneficial since, as the B/Ti atom ratio increased from zero, the grain size decreased until the optimum value was reached. Hence, B is an essential component of the grain refiner but its content must be optimized in relation to its Ti content. The optimum B/Ti atomic ratio of l/2 implies that approximately one in four Ti atoms will combine with the B to form TiBz while the balance will be dissolved in the Al melt. As the B/Ti atom ratio is reduced from this optimum value, keeping the total Ti unchanged, the dissolved Ti content will increase while the amount of TiBz particles in the melt will decrease. Consequently, the grain refining efficiency decreases because the TiB2 particles are heterogeneous nucleation sites (73). On the other hand, increasing the B/Ti ratio from the optimum value of l/2 without changing the total Ti content means increasing the number if TiBz particles in the melt while decreasing the content of dissolved Ti. This action also decreases the efficiency of the grain refining master alloy resulting in coarser grains. Note that a grain refiner with stoichiometric B/Ti ratio will give the highest amount of TiB2 particles for any given total Ti content. However, Fig 1 shows that a stoichiometric composition will lead to a grain size even larger than a grain refiner without any B but having the same Ti content.

0

0.5

1

1.5

2

25

B/Ti Atom Ratio

Figure 1. Plot of grain size against the prevailing B/X atom ratio for the three total Ti contents O.OSwt%,0.lOwt?/aand 0.1 Swt%. Note that the Bali ratio is converted to single decimal numbers for ease of graph plotting. The stoichiometric atom ratio for TiB2 is indicated on the graph.

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Thus, although1 TiB2 particles may act as heterogeneous nucleants, they are not as efficient as the mechanism which operates when B is absent. This latter mechanism is thought to be one involving peritectic cells Icentered around ALTi embryos which may occur even when the Ti content is less than the peritectic composition of O.l5wt%. The nucleation mechanism proposed for grain refinement in the presence of Ti132particles and dissolved Ti is called the “peritectic hulk theory” (9). The results obtained in this study indicate that this is the most efficient grain refining mechanism compared to the one that operatIes in the absence of any B (peritectic theory) (10) or the one that could operate in the absence of any dissolved Ti (particle theory) (7,8). Detailed evaluations and analyses of grain refining mechanisms are not the aim of this paper. Increasing the total Ti content for any B/Ti atomic ratio decreases the grain size. In practice this means a larger inoculation level with any grain refining master alloy. This is very well known but the challenge is to increase the potency of the grain refiner so that a smaller addition will give a satisfactory grain refining performance. To achieve this aim the grain refming master alloy must be produced with an optimum B/Ti ratio. Two factors appear to control the grain refinement mechanism. One is the presence of sufficient number of nucleation sites in the form of TiBz particles. The other is the content of free Ti dissolved in the Al alloy melt. The individual roles of both these factors must be optimized through the appropriate contents of Ti
r

Figure 2. The grain size Catain Fig. 1 is replottedagainst the total Ti content for each B/Ti atom ratio. Note the steep increment in slope exhibited between O.lOwt%and 0.15 wt% for the three B/Ti ratios O/l, l/6, l/3 while for the ratio 2/l no change in slope is evident. ThleB/Ti ratio of l/l shows an intermediateincrementin slope.

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many mechanisms could give rise to grain refinement when Al is inoculated with Tibor alloys. The roles of each mechanism will depend on the available free Ti and the TiB2 particle content which are, in turn, determined by the total Ti content (which depends on the level of inoculation) and the prevailing B/Ti ratio (which depends on the composition of the Tibor alloy). Conclusions

For any given Ti level, an optimum B/Ti ratio exists for best grain refining performance in commercial purity Al. This optimum B/Ti ratio in atom content is approximately l/2 which corresponds to a Tibor alloy composition of approximately Al+wt%Ti- 1wt??B. Increasing the inoculation level increases the Ti content of the melt resulting in finer grain size but the actual grain size obtained for any total Ti level will depend on the B/Ti ratio of the Tibor gram refiner. Many different mechanisms appear to cause grain refinement. The predominance of one mechanism over the other is determined by the total Ti content and the B/Ti ratio of the grain refining Tibor alloy. References 1. 2. 3. 4. 5. 6. I. 8. 9. 10.

R. Mollard, W. G. Lidman and J. C. Bailey, Light Merub 1987 (Edited R. D. Zabreznik), p 749, TMS-AlME (1987). A. Cibula,J. Inst. Metals, 76,321 (1949-50). L. Amberg, L. Beckerud and H. Klang, Metals Tech, 9, 1 (1982). L. Christodoulu, Pruc. Muter. Res. Sm. Sjmp., 120,29 (1988). D. Lewis and M. Sin& In-Situ Composites: Science und Tecnologv (Edited M. Singh and D. Lewis), p 21, TMS-AlME (1994). D.G. McCartney, ht. Mater. Rev., 34,247 (1989). A. Cibula, J. Inst. Metals, 80,l (195 1). G.P. Jones and J. Pearson, Metoll. Trans. B, 7B, 223 (1976). L. Beckernd, Aluminium, 67,910 (1991). A. Grossley and L. F. Mondolfo, Trum. AIM?, 191, 1143 (1951).