The Ukrainian experience of alloying agents applied in aluminum smelting

IF WE DON’T MAKE IT OURSELVES, WE WILL HAVE TO IMPORT

The Ukrainian experience of alloying agents applied in aluminum smelting

Constantine MIKHALENKOV, candidate of technical sciences, the National Technical University of Ukraine

The Ukrainian aluminum industry clearly reveals a tendency towards a greater number of medium and small-size companies working aluminum scrap into ingots or making permanent-mold castings from remelted and reduced scrap alloys. These new business undertakings peacefully coexist with the old system of enterprises that make primary aluminum, semi-finished aluminum alloys and castings.

Aluminum alloy business is very alluring for small and medium firms owing to comparative simplicity of smelting and casting technologies, which need neither super-expensive equipment nor much power for operations. The main objective of small aluminum-making companies is to collect as much aluminum scrap as possible and to make chemistry of remelted aluminum be as close as possible to the composition required by Ukrainian technical standards. Ordinarily, this approach is applied when making technical-pure aluminum.

Yet, when it comes to manufacturing permanent-mold castings or ingots from specific alloy grades containing 3 to 10 alloying agents, a manufacturer inevitably faces a problem of preparing or acquiring the appropriate alloying agents, which inject higher-melting-point elements in aluminum metal. There are three ways of getting the needed alloys. Firstly, each individual company can single out the range of materials required to make alloying agents and arrange production of the required agents on its own. Secondly, the needed alloys can be purchased on domestic or foreign markets. Thirdly, the whole aluminum industry can put itself together and set up a specialized plant to manufacture and supply aluminum-based alloying agents. Though all the possible ways have their pros and cons, this article will consider none of them. The author is mainly trying to shed some light on the range and the purposes of materials applied in making of such alloys.

The European market for alloys is mainly ridden by commodities of the two leading manufacturers, namely London & Scandinavian Metallurgical Co. Ltd. (LSM), Rotherham, England, and Kawecki-Billiton Master Alloys B.V. (KBM), Delfzijl, the Netherlands. Besides, the European market is a place for a number of smaller companies that supply alloys to selected regions. One should keep in mind that virtually all the large aluminum-smelting corporations such as Germany’s VAW, France’s Pechiney, or Norway’s Norsk Hydro have their own facilities and technologies to manufacture alloys. Obviously, these corporations manufacture alloying agents for their own needs and not for sale. Nevertheless, these large corporations are an important factor on the alloy market because they make a wide range of alloys and enjoy highly sophisticated R&D divisions, which means that they can potentially launch the market with their alloys whenever they want to.

For the sake of analysis, alloying agents can be broken down into three groups of general-purpose, grain-refining and modifying alloys and fluxes.

General-purpose alloys

General-purpose alloys should have melting points below that of aluminum, have the greatest possible contents of secondary elements and have uniform chemistry and homogeneous volumetric distribution of middle phases. As a rule, general-purpose alloying agents comprise double-element alloys as mentioned in table 1.

Table 1. Conventional composition of general-purpose aluminum-based alloys

The range of products manufactured by LSM also embraces such alloying agents as Al-B (up to 5% B contents), Al-Cr (up to 20% Cr), Al-V (up to 10% V), Al-Fe (up to 30% Fe), Al-Zr (up to 15% Zr), Al-Ti (up to 10% Ti) and Al-Cu (up to 50% Cu). LSM exports these commodities i

KBM sells 18 types of aluminum-based alloying agents and six substances labeled as special alloys. Along with the ordinary Al-Si, Al-Cu and other alloys, the former group embraces such exotic and unusual materials as Al-Sb (up to 16% Sb contents), Al-Cd (up to 10% Cd) and Al-Li (up to 10% Li). The second group comprises such alloys as Al-Be (5% Be contents), Al-Y (8% Y), Al-Ag (10% Ag) and Al-Mo (10% Mo). All these alloying ents are shaped in 7-kg honeycomb ingots, with some of them being finished as rods, 10 mm in diameter, cut bars or continuous-cast ingots, 500 grams to 2.5 kilograms in weight.

It is also worth mentioning a rather young class of alloying tablets and briquettes. These materials show good performance in aluminum smelting owing to quick dissolution at aluminum-smelting temperatures; high efficiency of alloying agents (usually some 95-97% get involved in the proper reactions); negligible losses of the temperature of molten metal; accuracy, ease and neatness of utilization avoiding the weighing stage; precise weight, absence of any dust, relatively low costs and low transportation, delivery and storage expenditures.

LSM offers ALTABtm alloying tablets supplied as ALTABtm 75 and ALTABtm 80 (depending on the contents of the main component). For instance, ALTAB75 Cr contains about 74.0-76.0% chromium with aluminum accounting for the rest. These tablets have a density of 4.0-4.5 g/cm3. Trying to keep abreast of the competitors, KBM also sells alloying tablets and briquettes with the same chemical elements. Tablets weighing 50 grams, 75 g, 500 g and 1,000 g are packed in aluminum foil with 3-6 tablets in a single pack.

The Ukrainian market for general-purpose alloying agents has serious problems. As a matter of fact, nothing else besides Al-Si alloys made at Zaporozhye Aluminum Smelter is actually in supply on the market. Perhaps, the only exception is Al-Nb alloy manufactured by Donetsk Chemical-Metallurgic Works, though unfortunately this alloy has not found wide application in aluminum smelting.

Grain-refining alloys

Grain-refining alloys are applied to reduce the size of grains in aluminum or aluminum alloys. Despite the seeming straightforwardness of their application and manufacturing, these alloys can undoubtedly be referred to as hi-tech products.

In general, grain-refining alloys are used in foundry of wrought aluminum alloys because:

injection of shaped nucleating elements speeds up solidification of aluminum metal, which in turn boosts the pouring rate and accelerates the manufacturing process;

metal acquires fine grains due to a large number of injected nucleators that develop higher strength of hot alloys. This improves hot fracture toughness during extrusion of ingots, thus lowering the number of defective articles manufactured;

fine-grain solidification wards off development of liquation processes and ensures homogeneous distribution of alloying agents in cast ingot. As a result, mechanical properties of metal have a more uniform distribution along ingot’s cross section;

grain-refining alloys improve surface quality of ingots, which somewhat lowers the amount of required finishing efforts.

All the mentioned points do not consider how injection of grain-refining alloys affects mechanical properties of semi-finished commodities. Considering the conglomeration processes, infusion of small quantities of insoluble particles into permanent-mold metal will not call forth any appreciable strength enhancement. When being severely stressed, metal without any grain-refining alloys has practically the same strength as metal with such alloys.

There is no need to consider all the theoretical aspects related to selection of grain-refining agents and to requirements posed. Yet, it seems worthy to play up the two points, which directly influence production costs in smelting and casting. Firstly, it comes to fading effect of grain-refining alloys. The theoretical curve of grain reduction in aluminum alloys has two distinctively clear sections, that is to say the rise of the reducing effect until reaching the finest possible grain size and the ascending branch of the fading effect.

Up to the moment, it is still mainly considered that titanium is the best grain-refining alloy for aluminum smelting. Particles of Al3Ti intermetallic compounds in metal serve as nucleators for dendrite growth of aluminum crystals. As a matter of fact, all the double intermetallic compounds such as Al3Ti, Al3Zr, Al3Ta, etc. are inclined to enlarge and grow, which completely annuls their nucleating ability. Moreover, no external forces can restore nucleating potential of these compounds.

In this case, titanium diboride (TiB2) and titanium carbide (TiC) have a clear advantage over the other alloys. These alloys have an unlimited and everlasting ability to nucleate aluminum grains. These particles are still subject to gravitational settling, though simple mechanical or electromagnetic agitation can restore 92-95% of the nucleating effect.

The second point is the efficiency of grain-refining alloys, i.e. the number of potential agents that act as nucleators in solidification of aluminum metal. A good example to assessing alloy efficiency is the evaluation of Al-Ti performance as a grain-refining agent. Figures prove that, if Al3Ti particles have the average size of some 5 micrometers (which is quite feasible, by the way), there will be some 106 such particles per cm3 of the overall volume. The size of 200-215 micrometers is a good degree of aluminum grain reduction (in this case, there are some 105 particles per cm3). Therefore, out of the total quantity injected only each tenth Al3Ti particle served as a nucleator for growth of aluminum dendrites. To put it in other words, the efficiency of this alloy is roughly 10%. All the other particles were either engulfed by the growing dendrite crystals or were simply forced beyond the grain edges, i.e. turned out to be superfluous. Alloys with finer particles have the efficiency of only some 2-3%. Such poor efficiency figures reveal wastage of valuable agents and increase in production costs.

At the moment the developed European countries have mainly managed to solve the technological issues of aluminum grain reduction process. These countries have clearly defined the range of grain-refining elements and compounds, calculated the optimum consumption of alloying agents, disclosed the best possible composition of such agents and methods of their introduction into molten metal. Unfortunately, Ukrainian manufacturers of aluminum have yet to do all this. The reasons for such a lag behind the Europeans are quite simple. The domestic market has no demands for grain-refining alloys. Besides, there are no actual technological projects, which would enable commencement of at least small-scale production of these alloys at low initial outlay.

At this point of time Al-Ti-B triple alloy (with a usual Ti/B ratio of 5/1) is the most popular grain-refining agent for aluminum alloys. Table 2 shows the chemical composition of such alloying agents made by KBM. The TiBOR® alloy made by KBM (see sections a and b of figure 1) has a typical microstructure of large tetragonal crystals of titanium aluminide (Al3Ti) and conglomerates of titanium diboride (TiB2) in form of fine particles. The average size of the latter ranges from 1 to 2 micrometers. When the alloy is fed into a metal jet, the KBM company recommends using 0.25-3.0 kg of TiBOR® in rods per tonne of liquid metal. This alloy costs some DEM 6-12 per kg depending on terms of delivery. If an average of 1.5 kilos of alloy is consumed per tonne of metal, production cost of a tonne of aluminum gains approximately DEM 15.

LSM, the second largest European manufacturer of grain-refining agents, also offers triple Al-Ti-B agents under different trademarks, e.g. TiBAl™ (TiBAl 5/1, 5/0.2 and 3/1). The TiBAl™ alloys are supplied as 9.7-mm rods coiled into 180-kg bundles (see figure 2). The same alloys are also manufactured as 6.4-kg honeycomb ingots (see figure 3); cut bars, 0.3 meters long (67 grams in weight), 0.5 m (100 g) and 1.0 m (200 g); as well as Castcut™ nuggets, 40×30 mm in size and 0.2-3 kg in weight, which are in fact nothing but chopped continuous-cast ingots of alloy (see figure 4).

Table 2. Customary chemical composition of TiBOR® grain-refining alloys made by Dutch company Kawecki-Billiton Metaalindustrie

Figure 1. Structure of TiBOR® 5/1 alloy (in rod) made by KBM

It is not too hard to manufacture triple alloying agents, e.g. TiBAl 5/1. The only equipment needed is an induction furnace with a graphite or silicon carbide crucible. To make an alloy with 5% Ti and 1% B contents, one should add potassium fluorine-titanate or fluorine-borate (using the ratio of 6.87 to 1) to molten aluminum (usually, to technical-pure aluminum with no more than 0.3% of impurities) at the temperatures exceeding 730iN. After adding, the continuously agitated molten bath should be kept in furnace for 4-5 hours at the temperature of 800iN.

The nucleating ability of titanium carbide (TiC) has been known long ago, since the Khibula’s breakthrough studies in 1949. Yet, the Al-Ti-C industrial alloy was patented only a short while ago (A.Banerji, W.Reif and London & Scandinavian Metallurgical Co.: International patent appl. N PCT/GB 86/00108 dt.28.2.1986). Indeed, it took a long a time till the inventions found their application in industrial processes. The main reasons for such a lengthy period were the opportunity to get the required alloy quality faster if titanium diboride is added; technological difficulties of introducing carbon to liquid aluminum; the need to single out the optimal composition and shape of TiC particles to secure the best nucleating abilities. The structure of the patented triple alloy contains large crystal of titanium aluminide and TiC particles, approximately 1 micrometer in size.

Having about the same efficiency as Al-Ti-B, the TiC-based alloy benefits from two wonderful advantages, which are ease and environment safety of its manufacturing and absence of coarse-grain effect when this agent is introduced into chromium or zirconium-based alloys (unlike in case of alloying agents with TiB2 contents).

The LSM standard list of commodities mentions alloys with titanium carbide contents under the trademark of TiCAl™ 315. This alloying agent is finished and shaped just like the mentioned TiBAI. Recently, KBM has also patented and now engages in mass production of TiCar® grain-refining alloy with titanium carbide contents. At the same time, certain large intermediary companies such as Goodfellow also trade in grain-refining alloys. However, the intermediaries do not directly manufacture these alloying agents, thus their sale prices are unlikely to be lower than those of the two mentioned manufacturers.

Compared to the developed European industry of grain-refining alloys, the Ukrainian market for these commodities is just taking shape. As a matter of fact, Ukraine has no facilities to manufacture Al-Ti-B or Al-Ti-C alloying agents so far. A few Ukrainian producers have an experience of using Al-Ti-B alloys, purchased during barter settlements or acquired in insignificant volumes. Al-Ti and Al-Zr alloys shaped in honeycombs or ingots are the only available grain-refining alloys on the Ukrainian market. However, even these agents are manufactured at pilot plants, thus the possible purchase orders can hardly exceed 40-50 kg, while the price charged will be way above the prices of similar European alloys.

One should give special heed to Al-Sc alloying agent. A number of Russian studies have revealed that one can definitely improve serviceability of the end metal by adding scandium to aluminum alloys (especially to Al-Mg, Al-Li-Mg or Al-Li-Cu alloys). The author has not conducted any studies to check the scandium effects himself; thus the statement of favorable influence is based on reference publications solely. Unfortunately, they do not give enough data to assess how much the scandium’s grain-refining ability exceeds those of titanium or zirconium. Yet, it has been positively proven that scandium can considerably elevate the point of recrystallization. This chemical element and Al-Sc alloy (with Sc contents of 2%, as a rule) are of special interest because Ukraine has both deposits of scandium ores and the necessary facilities to manufacture these commodities at dispose. However, the actual consumers of these products, such as machine building and aerospace companies, have a shaky financial health, while scandium-based commodities are outrageously expensive. In consequence, the manufacturing plant has completely discontinued output of these commodities, while metal traders have stopped making any offers of scandium-based alloys.

Modifying alloys and fluxes

Almost 95% of permanent-mold castings are made using Al-Si (silumin) alloys, which sometimes contain magnesium or copper. This clearly proves that consumer properties of alloys depend not on the size of grain in solidified metal, but on the morphology of eutectic silicon. Therefore, application of the mentioned grain-refining alloys is unjustified in a number of cases. It is worth mentioning that the structure of metal (as well as metal properties) takes shape at the instance of pouring and solidification in mold, which disables any external influences on the process. Hence, it is much harder to alter the structure of foundry alloys.

At this point of time strontium is the most widespread modifier of Al-Si alloys (strontium is introduced in liquid aluminum as an Al-Sr or Al-Si-Sr alloy). Throughout the past 10 years strontium has gradually elbowed sodium from the market owing to two main reasons. Firstly, the effect of modification with strontium is much more long lasting than that with sodium. Secondly, it is much more advisable to use environment-friendly alloys than various flux mixes utilized to inject sodium.

LSM supplies Al-Sr modifying alloys with 3-10% strontium contents. Alloys with 3-5% Sr contents can be dispatched to clients as rods, cut bars and honeycomb ingots. The company advises in favor of feeding strontium-based alloys so that the total Sr contents in metal do not exceed 0.01-0.04%. We believe that the TiBAl alloy with up to 10% Sr contents (supplied in any finishing and shapes required) deserves a special attention. This alloy has a thorough effect on aluminum because, simultaneously with grain reduction in solid solutions, it is capable of modifying eutectic silicon. Although this alloy is not very efficient, most European aluminum foundries use it in their casting processes. KBM also offers its clients a wide range of alloys containing 3 to 10% Sr, as we

It is also economically justified to consider utilization of multipurpose fluxes in aluminum foundry owing to a huge number of small-size and medium Ukrainian companies that make finished casting or work aluminum alloy scrap.

The easiest way of flux treatment is when the flux materials are prepared directly by aluminum smelter. In this case, investment is only required to purchase raw materials and arrange technologies to prepare and apply fluxes. Yet, nothing guarantees that the flux make-up borrowed from reference sources will be the most suitable for spific smelting and casting processes or for quality of utilized furnace charge. Hence, it is worthy to consider business offers for supplies of fluxes by Schafer company, Germany. This firm offers its commodities with a relatively detailed manual on the possible fields and technologies of fluxes’ application. These fluxes cost some DEM 4-6 per kg. The author of the article has already subjected one of the Schafer fluxes to chemical analysis and has found out that, together with sodium and potassium, the flux contains carbon and chlorine, which is an obvious trace of hexachloroethane contents. It is wide known that the latter shows good performance in degassing and refining of aluminum alloys. Yet, it is also unbeatable that decomposition of this compound harms the environment with various pollutants. On average, a manufacturing company consumes 0.5-2.5 kg of fluxes per tonne of end metal. Calculations indicate that, depending on chemical composition, one kilo of used fluxes emits up to 652 milligrams of hydrogen chloride, up to 0.01 mg of hydrogen fluoride (fluorine value), 0.28 mg of soluble and insoluble hydrofluoric acids (fluorine value), up to 117 mg of aluminum compounds and up to 820 mg of various aerosols per hour of smelting operations.

Naturally, all the technical aspects of aluminum smelting and casting embrace much more than simple application of various alloys. Yet, this example shows that potential of the Ukrainian aluminum industry is frequently inferior to that European, which worsens the quality of Ukrainian-made aluminum. It has already been mentioned in this article that it is not a significant technical problem to arrange production of Al-Ti-B or Al-Ti-C grain-refining alloys. The author believes that arrangement of small-scale manufacturing of these alloying agents will not take more than a year (if the involved parties are truly interested in the project, it can take even less time). By commencing output of these alloys, aluminum smelters will abandon imports from abroad and will enhance product quality and productivity.

Ukraine already has several companies that can potentially manufacture aluminum-based alloying agents. Above all, this relates to Zaporozhye Aluminum Smelter and Donetsk Chemical-Metallurgic Works, as well to the young yet well-equipped companies Intersplav and Aphalina.

To sum up, one should not anticipate any strong interest in aluminum alloying agents as long as the quality of aluminum supplied to the domestic market or exported beyond Ukraine suits the clients. However, by launching production and industrial application of these alloys, Ukrainian companies will show themselves not as mediocre suppliers of semi-finished products, but as enterprises capable of meeting the European quality standards of castings and semis.

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