SE508223C2 - Controlling grain refinement of aluminium@ alloys - Google Patents

Abstract

Controlling the grain refinement of aluminium (Al) alloys comprises: (i) calibrating the grain sizes for different values of the grain growth index (GGI), for the casting process used, represented by formula (I); determining the GGI value for the particular aluminium base material by using the above formula; (iii) using the calibration data obtained in (i) to calculate the grain size of the aluminium base material and how the concentration of grain size affecting agents in the aluminium melt should be changed in order to obtain an aluminium casting having a desired crystal grain size; and (iv) adding the amount of grain size affecting agents calculated in (iii) to the melt. GGI = ~E miCi(ki-1) = m1C1(k1-1) + m2C2(k2-1) + .... (I) In (I): mi is the slope of the liquidus in the binary (Al-i) system; Ci is the concentration of its dissolved solute in the alloy; ki is the distribution coefficient of solute i between solid and liquid; and m1, C1 and k1, etc. represent the corresponding values for each alloy constituent.

Description

sus 223 2 aluminum. These two phases are titanium diboride (Al, TiBz) and titanium aluminide (TiAl3). The entire range of boride particles from AlB2 - TiB2 can be formed during the preparation of the alloy. 1 alloys with an excess of Ti compared to what is needed to form TiB; Most boride particles have a composition close to TiB2. For simplicity, this phase is considered in the following text. Essentially, the entire amount of titanium and boron in the alloy complexes is included in these crystals, because the solubility of boron and titanium in solid aluminum at room temperature is very small. This means that changing the alloy composition only changes the relative ratio of these two crystals which are added to affect the comfort.

Despite this simple fact, there has been a great deal of discussion and disagreement about what the Ti-B ratio in a pre-alloy should be in order to obtain the best grain refinement. This issue was considered extensively in U.S. Patent 4,612,073 and also in an article by M.M. Guzowski, G.K. Sigworth and D. A. Sentner, called "The Role of Boron in the Grain Re- nement of Aluminum" (published in Melallurgical Transactions, Volume 18A, pages 603-610). Guzowski and co-workers were of the opinion that boron acts by altering the shape of the TiAlg crystal and that TiB2 can also be an effective nucleating agent when there is a significant amount of dissolved Ti in the melt. This question (regarding optimal Ti / B ratio) has also been asked empirically by performing extensive grain refinement tests and then using the measured results to map the desired grain refinement technique. A typical example of this technique is an article entitled "Grain Refining Response Surfaces in Aluminum Alloys", published by W.C. Setzer and co-workers on pages 745-748 in Light Mezals (1989).

Despite the importance of this question, there is no idea what the optimal ratio of titanium to boron should be for a given alloy and for a given casting process. Over the years, our chemistry has led to the realization that the best grain refiner for one alloy may not be the best for another alloy. Commercial alloys range from relatively pure aluminum (such as foil and electrically conductive wire) to cast alloys, which can contain almost 20% dissolved elements. It has been found that alloys which refine well in pure aluminum do not work in heavily alloyed melts and vice versa. (see US-A-5,055,256, where a pre-alloy composition is shown only for an aluminum-based alloy containing a high Si content. This situation means that the grain refiner technique used in a foundry is in many cases far from optimal. At best you may use too much grain refiner and spend too much pre-alloy, in the worst case you may encounter casting problems, such as cracking or other defects on the final casting product.

OBJECTS OF THE INVENTION An object of the present invention is to present a detailed understanding of how the composition of aluminum alloys affects the coating. A further object of the present invention is to show a method by which the optimal grain refinement can be obtained. This purpose is shown in an algorithm or formula, which can be used to calculate the desired refining. Another object of the present invention is to provide a method for obtaining an optimal correlation by introducing the above-mentioned algorithm into a rod feeding mechanism.

Other objects can be understood by those skilled in the art from the following description of the invention, figures and examples.

SUMMARY OF THE INVENTION The present invention stems from the discovery that the combining of various aluminum-based alloys follows certain controlled patterns when the combing is considered in certain ways. A method for controlling the grain composition of certain aluminum alloys comprises the steps of: a) for the casting method used, determining the grain size for different values of grain growth index GGI, which are represented by the formula: GGI = Z miC ~, (ki-1) = m1C | (k1 -l) + m2C2 (kz-l) + where mi is the slope of liquidus in the binary (Al-i) system, Ci is the concentration of dissolved material in the alloy and kr is the coefficient of distribution of dissolved material between solid and ande surface and m1, ( 21, k, etc. correspond to values for each alloying constituent; b) determine the GGI value of the particular aluminum-based material using formula ia); c) use the information obtained in a) to calculate the grain size of the aluminum base material and how to change the concentration of the grain size agent in the aluminum melt to obtain an aluminum gum with the desired crystal grain size; and d) adding the amount of grain size influencing agent calculated in c) to the melt at the same time as a sufficient amount of nucleating agent is added if the original aluminum melt does not contain sufficient amount of such agents.

Grain size influencers are preferably Ti and / or B. The amount of Ti to be added to the aluminum melt should result in a GGI value in the grained alloy corresponding to a grain size less than equal to the desired grain size (GGld). This can be calculated by the formula: GGld - GGlb Quantity; '_- (krr Ûmn 10 15 20 25 30 5 508 223 where Quantity fi is the weight percent Ti to be added to the melt, GGld is grain size index obtained with aluminum ingot with a minimum grain size, GGlb is grain size index for the original aluminum base material, my, is the slope of liquidus in the binary (Al-Ti) system, kn is the coefficient of distribution of Ti between solid and fl surface material.

The calculations can also be done with thematic or multinear systems, which gives slightly different constants. The results are equivalent to the above-mentioned binary calculations.

This discovery is best illustrated by an example.

We first consider the addition of an Al-5% Ti-1% B alloy. From the stoichiometry of the TiBQ phase and the molecular weight of the two elements, we can find that 2.2% by weight of Ti in this precursor is bound by the boride compound. This is important because the boride is mainly insoluble in aluminum, which means that the titanium is not "free" to dissolve in the alloy. The remaining titanium (5.0% -2.2% = 2.8%) is present in the form of soluble titanium and titanium aluminide (TiAlg crystals, which dissolves easily in molten aluminum.

Experiments have been made in which a fixed amount of free titanium (0.01% dissolved Ti added in the form of an Al-5% Ti 1% B alloy) was added to a number of molten aluminum alloys. A series of experiments were performed in aluminum melts containing different amounts of dissolved silicon. A small ingot was prepared for each of these melts. A thermocouple placed in the melt showed that the cooling rate just before solidification was 1 ° C per second. The solidified casting is cut into two parts, polished in the cutting surface and etched to determine the grain size. The average grain size was determined by the line cut-off method. The grain size in micrometers (1000 micrometers = 1 millimeter) in the silicon-containing alloys is shown against the composition in curve (a) in Fig. 1. It appears that the minimum is obtained at about 3% by weight of silicon.

A second series of similar experiments took place at a higher addition level (0.05% Ti), the results shown in curve (b) in fi g. I was received. The grain size was much 10 IQ UI 508 223 smaller at lower silicon content, which showed a minimum at about 0.5% Si; and larger in alloys containing more than about 5% Si.

A third series of experiments took place at an intermediate addition level (0.03% Ti) and gave results lying between the first two shown in curve (c) in fi g. 1. It is interesting to note that there is an intersection point at the minimum of the curves.

Another series of experiments was performed in a series of Al-Si-Fe-Ti melts and also in some commercially available alloys containing different amounts of dissolved compounds. The inventors have discovered that the results shown in Fig. 1 and the results of all complex alloys (with one exception noted below) can be combined into a single curve by using a combined grain size index. The basis for this index is described below.

The composition (in sublimated examples,% Si and% Ti) of the base alloy affects the growth rate of the grains. First, an element is added to relatively pure aluminum. whereby grain growth becomes slower. This is because in alloyed melts, the diffusion of a dissolved element must occur before the growth of the solid phase. This diffusion process limits and delays the growth of new crystals and appears to allow borides to become active nuclei. I. Maxwell and A. Hellawell (in the article 'A Simple Model for Grain Refinement During Solidi fi catioph, published on pages 229-237 in Acta Melallurgica, volume 23, 1975) have proposed on theoretical grounds that crystal grain growth is inversely proportional to dependence on the product mC ( k- l), where m is the slope of the liquidus curve in the binary system (Al-Ti), C is the concentration of dissolved titanium in the alloy and k is the coefficient of distribution of solute between solid and fl surface titanium.

This grain growth index has been proposed on theoretical grounds, but its significance in the understanding and regulation of aluminum cladding has not been fully understood before. In the previous study by Maxwell and Hellawell, the samples were made only on relatively pure aluminum. It had not been realized that the rate of 10 v 508 223 grain growth in the process continued to decrease until a minimum grain size was reached. Nor had it been realized that higher additive levels lead to an increased growth rate. (This is probably due to a new mechanism in grain growth that is starting to become significant).

Finally and probably most important is that the inventors have discovered the effect that each dissolved element in aluminum is additive. In other words, the grain growth index of a fl first component alloy is the algebraic sum of each of the individual elements. These combined indices are represented mathematically by: Zmici (ki-1) = III | C | (cooling) + IIIQCQ (kg-1) "1" m3C3 (cooling) + The values of m and C for a number of alloying elements usually present in aluminum are shown in Table I. From these values and from simple calculations, it can be found that a addition of 0.10% Ti is equivalent to about 4% Si, as long as one considers a grain growth index. Thus, even small additions of Ti can have a great effect on barley growth. 10 508 223 metall- 'm (k-1) m max. component concentration, wt% ri z 30.7 245 0.15 Ta 2.5 70 105 0.10 V 4.0 10 30 z0.1 Hf 2.4 8.0 11.2: Oj M0 2.5 5 7.5 zone Zr 2.5 4.5 6.8 0.11 Nb 1.5 13.3 6.6 æ0.15 Si 0.11 -6.6 5.9 z12.6 Cr 2 3.5 3 , 5 = 0.4 Ni 0.007 -3.3 3.3 26 Mg 0.51 -6.2 3 = 34 Fe 0.02 -3 2.9 21.8 Cu 0.17 -3.4 2.8 33.2 Mn 0.94 -1.6 0.1 1.9 Zn 0.4 -1.6 0.96 250 (The above figures are compiled from TB Massalski: “Binary phase diagrams” volume 1 ASM lntemational (1990) ; M Johnsson, L. Bäckerud and GK. Sigworth: Metall. Trans. 24A (1993) pages 481-491; and LF Mondolfo: ”Aluminum Alloys: Structure and Properties”, Butterworth & Co (1976)). From theoretical and experimental studies on relatively pure aluminum, we find that the growth rate of the solid grains during solidification is proportional to I / ZmC (k-1). The grain size is proportional to the growth rate. When the grain size of aluminum-based alloys was plotted against the combined grain growth index, ZmC (k-1), it is found that all alloys which solidify at the same rate fall on the same curve. 10 15 20 9 sus 223 The results showed earlier in fi g. 1 can be re-inserted in fi g. 2 together with the results of other tests performed on multicomponent alloys. Data for two other previous cooling rates are also given in fi g. 2.

Tests performed so far show that the curves showed in fi g. 2 can be used to predict the conformability of Al-Ti-B additives in all aluminum-based alloys, with one notable exception. It has been found that in alloys having significant amounts of Zr, a precipitation of titanium-zirconium aluminide is obtained. This precipitate removes dissolved Ti and Zr from the melt so that the grain size is much larger than expected (as calculated from the base alloy composition and grain growth index). Thus, Zr can "poison" the effect of Ti. In this case, additional Ti must be added to compensate for the presence of Zr.

Examples of applications of the invention In order to fix the concepts as described above properly in memory and to clearly first how they are applied in practice, it may be practical to consider certain concrete examples of how this technology can be used in foundry technology.

EXAMPLE 1 We first consider the solidification of an 1100 alloy. This alloy is relatively pure and is often used to make ingots, which are rolled into foil. A sample of a molten alloy is taken from the furnace before casting and an analysis showed that it contained: Si 0.6% F e - 0.3% Cu - 0.05% Mn - 0.0l% Zn - 0 .06% Ti - 0.005% From the above compositions and values given in Table 1, we can calculate the grain growth index for the alloy. To illustrate how this is done, the calculations are given in the table below: 10 in 010 Element Q m at mQL-l) Si 0.60 5.9 3.54 Fe 0.30 2.9 0.87 Cu 0.05 2.8 0.14 Mn 0.01 0.1 0.001 Zn 0.06 0.96 0.058 Ti 0.005 245 1.225 total - - 25 8 This alloy is cast in a large piece, whose cooling rate is 1 ° C / sec and from previous experiments it turned out that the grain size must be less than or equal to 300 μm to obtain good results. From Fig. 2 we can find that the desired grain growth index must be greater than about 10. This means that we: next increase the "free" titanium content by adding a grain refiner, with an amount equal to: (io -5, s) / 24s = o, o17% Ti This addition can take place in a number of ways, but it is usually desirable to carry out the grain refining with as little boron as possible. High additions can cause needle sticks in the foil, because the boride particles are insoluble and destroy the end product. One possibility would be to add Al-IOTi flakes to the oven. Another possibility is to add Al-óTi rods to the tapping end of the oven.

The Ti content calculated above (0.017 ° / 0) represents the minimum desired content of "free" Ti. The maximum permissible value is obtained by looking at the right part of the curve shown in fi g. 2. We find that the grain growth index must be lower than about 36. Thus, the maximum Ti content that can be allowed is 10 l5 20 30 11 508 223 (36-5.8) / 245 = 0, 123% Ti.

This is important because on the surface of undissolved aluminides (TiAl3 particles) the titanium content will be about 0.15% Ti, greater than the maximum permissible value. This high titanium content of the surface in "duplex" particles was previously noted by Guzowski and co-workers (US-A-4,612,073) and the above article), but this mechanism does not appear to be suitable for this case.

Thus, the best granulation method for this alloy is to add an amount of about 0.02% Ti, in a form which dissolves readily in the metal. A fast-dissolving rod is suitable for addition in gutters. An addition of certain amounts of boron is also needed, since the borides act as nucleating agents in this alloy.

Commercial experience suggests that an addition level of around 20 ppm boron (or 65 ppm boron) would be appropriate. This can be added, for example, as Al-3 ° / 0Ti-1 ° / 6B or Al-5 ° / oTi-1% B-rods.

It appears that the optimal combining method for this alloy is obtained by making two separate additives. This can easily be done by feeding rods with two different alloys into the gutter. The two rods can be fed using two rod feeders or using a single rod feeder that can handle two rods (supply at different speeds). In either case, the addition rates (and rod feed rate) are controlled by a calculated algorithm, which contains the calculations logically described in the above example. The grain size agent and nucleating agent are added as a pre-alloy, a tube containing granules and / or particles or as a wire.

EXAMPLE 2 We can now consider the solidification of a 3005 alloy which has the following chemical composition: Si - 0.6 ° / 0 Fe - 0.7% Cu - 0.25% Mn-- 1.25% Mg - 0.45% Zn - 0.15% Ti - 0.05 The grain growth index for this alloy was calculated as follows: ßl fl lsm Q m kl i si 0.60 5.9 3.54 Fe 0, 70 2.9 2.03 Cu 0.25 2.8 0.70 Mn 1.25 0.1 0.125 Mg 0.45 0.1 0.045 Zn 0.15 0.96 0.144 Ti 0.05 245 12.28 total - - æl 8.8 An examination of Figure 2 shows that the grain growth index is very close to the optimal value and will give a grain size of about 150 μm at a cooling rate of 1 ° C / sec. In this case only a small addition of boride-containing particles without excess titanium is needed.

EXAMPLE 3 This example is the same as Example 2, except that the titanium content is very close to the maximum allowed for this alloy: 0.09% Ti. The grain growth index is therefore increased to 28.64. The grain size would thus increase to about 200 μm. This is also a rather low value and thus it may also be possible to accept only a small addition of boride-containing alloy. However, it would be possible to improve the performance by adding an amount of Al-B pre-alloy, which would react with dissolved Ti to form borides and thereby remove some of the "free" Ti. A similar result can also be obtained with a pre-alloy '13 sus 223 containing borides, which is a mixture of TiB; and AlBg. (Such a material is disclosed in US-A-5,055,256).

In this case, an AlStrB type stoichiometric alloy could be used with the advantage of a) supplying ZrBZ nucleating particles and b) simultaneously reducing the effects obtained with Ti, described above. It is also possible to use niobium.

Claims (10)

l0 508 223 M Patent claim A method of controlling grain refinement in certain aluminum alloys, comprising the steps of: a) determining for the casting method used grain size for different values of grain growth index GGI represented by the formula: 1Z mlCl (k1-1) = m1C1 (k1-1) + m2C2 (k2-1) + where m, is the slope of lividus in the binary (ALU system, C1 is the concentration of its dissolved amount in the alloy and k, the coefficient of distribution of the dissolved material between solid and liquid and ml, C b, determine the corresponding values for each alloying constituent, b) determine the GG1 value of the particular aluminum-based material using formula ia), c) use the information obtained ia) to calculate the grain size of the aluminum base material and how the grain size-influencing agent in the aluminum melt must be changed to obtain an aluminum ingot, with the desired crystal grain size; and that d) adding the amount of grain size influencing agent calculated in ic) to the melt while adding a sufficient amount of nucleating agents if the original aluminum melt does not contain sufficient amounts of such agents, and that, in the presence of Zr, Ti is added in an amount sufficient to correct the presence of Zr. l0 IJ, 15 sos 223 Method according to claim 1, characterized in that the grain size influencing agent is Ti and / or B. Method according to claim 2, characterized in that the grain size influencing agent is Ti. Process according to Claim 3, characterized in that the amount of free Ti to be added to the aluminum melt having a GG1 value lower than the GGI value obtained in aluminum ingots with a minimum grain size, in order to obtain an aluminum ingot of the desired grain size, can is calculated using the formula: GGId - GGlb Amount Ii (kTa-llmrt where Amount-y, is the weight percent Ti added to the melt, GGld is the cone growth index resulting in the aluminum ingot having a desired coin size, GGlb is the grain size base material of the original aluminum my, is the slope of liquidus in the binary (Al-TO system and SEK; is the coefficient of distribution of Ti between solid and liquid. A process for combining according to claim 1, wherein boron is added to the melt to remove excess titanium if the base grain growth index is greater than the calculated growth index so that the desired titanium additive obtains a negative value. The method of claim 5, comprising using zirconium and boron with stoichiometric excess of zirconium relative to ZrB2. Method according to one of the preceding claims, characterized in that the grain size-influencing agent and nucleating agent are added as a pre-alloy. 10 508 223 16 Method according to claim 7, characterized in that the pre-alloy is added in the form of a tube containing granules and / or particles, or as a wire containing the desired elements. The combining method of claim 1, wherein a calculated algorithm is used to control the type and amount of combining additives added to the aluminum melt prior to casting. Method for conveying according to claim 1, characterized in that a rod feeder is used which can supply separate rods at different speeds, the rods having different composition with respect to grain size influencing means.