MXPA98010852A - Intermetallic alloy granules of estroncio-alumi - Google Patents

Abstract

Granules of enriched aluminum-strontium master alloy for use mainly in eutectic phase modification in aluminum-silicon die-cast alloys. The master alloy granules are predominantly intermetallic compounds of A14, Sr, Al2Sr or ALSr and mixtures thereof. The use of such dominant intermetallic alloys in a granulated state results in rapid dissolution in alumino-silicon alloy melts. The composition of the master alloy can be added directly to a molten content or injected thereto. The composition of the alloy can also be mixed with aluminum granules and extruded into a rod or drawn into a cast aluminum molding. The master alloy is also useful as an inoculant for cast iron gr

Description

INTERMETALLIC STRONTIUM-ALUMINUM ALLOY GRANULES TECHNICAL FIELD This invention relates to aluminum-strontium alloys for use primarily to modify the eutectic phase in aluminum-silicon smelting alloys or to modify the intermetallic bases in forged aluminum alloys. The aluminum-strontium alloys are also useful as inoculants for gray and ductile iron.

TECHNICAL BACKGROUND Due to its excellent melting properties and fluidity, aluminum-silicon eutectic and hypoeutectic alloys are widely used in the production of aluminum smelters. In an unmodified state, the eutectic silicon phase is present as thick plates with sharp sides and ends often referred to as acicular silicon. The presence of acicular silicon results in smelters that have low elongation percentage, low impact properties and poor machining properties. Strontium has been shown to be effective in refining or modifying thick acicular silicon in a fine, fibrous, interconnected structure. In general, small amounts of strontium of between 100 to 200 ppm are sufficient to produce a thin, fibrous eutectic silicon which in turn significantly improves the mechanical properties and machining characteristics of aluminum casting. U.S. Patent No. 3,466,170 issued to Dunkel et al. On September 9, 1969 recognizes the benefit of adding strontium either as a pure metal or as an AISr alloy with 7 percent net of Mr. Since the strontium metal is very reactive with oxygen, nitrogen and moisture, its use as a modifying agent is limited. In most cases, strontium is added in the form of a master alloy. The publication of Pekguleryuz and collaborators "Conditions For Strontium Master Alloy Addition to A356 Melts", Trans. Am. Poundrymen's Soc. (1989) considers the use of a 55 weight percent Al alloy of 45 weight percent Al as a master alloy for modifying aluminum-silicon alloys. This alloy largely comprises the intermetallic AI4Sr and AI2Sr. This document does not describe, however, the alloy being in the form of granules or powder. U.S. Patent No. 3,567,429 issued to Dunkel et al. On March 2, 1971 teaches the use of a silicon-aluminum strontium master alloy which has a strontium content greater than 7 percent. The silicon-aluminum strontium master alloys are no longer widely used to modify aluminum-silicon smelting alloys, since in most cases strontium is present as an intermetallic phase of high melting temperature such as AI2Sr2Si or SrSi2 which it dissolves very slowly at molten aluminum process temperatures, typically 760 ° C or lower. As reported by John E. Gruzleski and Bernard M. Closset ("The Treatment of Liquid Aluminum-Silicon Alloys," American Foundrymen's Society Incorporated, 1990, pages 31-39), a binary master alloy % strontium-aluminum dissolves twice as fast in an aluminum-silicon A356 cast alloy as a ternary master alloy of strontium-14 percent silicon-aluminum at all melting temperatures ranging from 670 to 775 ° C. Similar results were found in U.S. Patent No. 5,045,110, issued to Vader et al. On September 3, 1991, which reports dissolution times between 20 and 30 minutes for master alloys of strontium-14% silicon-aluminum in the form of an ingot. In contrast, U.S. Patent 4,576,791, discussed below, teaches that 5-10 percent strontium-aluminum binary alloys in the form of a rod and which contain refiners in titanium and boron grain dissolve in one minute. In addition, the customary processes used to produce master strontium-silicon alloys result in substantial amounts of detrimental impurities that include iron., barium and calcium being frequently present in the master alloy. U.S. Patent 4,108,646 teaches the use of a master composition consisting of strontium-silicon in the form of particles compressed in a briquette with aluminum or aluminum-silicon particles. The briquettes, which have a master composition of between 3 to 37 percent strontium by weight, are then added to an aluminum-silicon casting alloy to modify its structure. This master composition is less efficient than the aluminum-strontium binary master compositions since strontium is present as SrSi particles which, as discussed above, dissolve slowly and contain detrimental impurities that include up to 4 percent iron and 1 to 3 percent calcium. Aluminum-strontium binary alloys are now widely used to modify aluminum smelters; however, it has been difficult to increase the strontium content of these binary master alloys. This is best explained in the context of the aluminum-strontium binary equilibrium phase diagram of Figure 1. The phase diagram contains two eutectics of low melting point, one at about 3.5 percent strontium, the second at 90 percent strontium. On the rich aluminum side, the eutectic containing alloys range from about 0 percent to 40 percent strontium. On the rich strontium side, the eutectic-containing alloys range from about 77 percent to 100 percent strontium. In the final solidified state, these eutectic alloys contain in different proportions a eutectic phase which is finely divided and melted at low temperatures, 650 ° C in the case of the eutectic mixture rich in aluminum and 580 ° C for the eutectic mixture rich in strontium. These finely divided eutectic phases are more ductile and dissolve more rapidly than the intermetallic alloy phases of higher melting point which are present between about 44 percent to 77 percent strontium. Since these intermetallic alloys do not contain finely divided eutectic phase of low melting point, they are more brittle and dissolve much more slowly than alloys containing eutectic mixture. The presence of these high-melting intermetallic alloys has placed a significant limitation in the amount of strontium that can be effectively contained in commercial aluminum-strontium binary master alloys. In this specification, the term "intermetallic alloys" denotes alloys containing between about 40 percent to 81 percent strontium by weight. These alloys are dominated by the intermetallic AI Sr, AI2Sr and AISr and contain only minimal eutectic phase or do not contain. As discussed in "Phase Diagrams for Ceramists" compiled by the National Bureau of Standards, published by The American Ceramic Society Inc., volume 1, pages 9-14, Figure 1 as a binary equilibrium phase diagram shows the relationships between composition and temperature assuming that all the phases are in equilibrium with each other. These compositional relationships are valid only if the solidification regime is slow enough to allow the phases to reach compositional equilibrium at each instant. A faster rate of solidification will lead to very different compositional results. As shown in Figure 1, when a liquid alloy containing 10 percent strontium is cooled, the solidification begins at about 815 ° C. The first solid phase that precipitates is the AI Sr primary intermetallic containing about 44 percent strontium. As the melting temperature continues to decrease during solidification, more and more of this primary intermetallic phase AI4Sr precipitates. Primary intermetallic AI4Sr is present as massive interconnected plates or needles that are shown in two dimensions in the photomicrograph given in Figure 2. A three-dimensional view of the interconnected network of primary plates AI4Sr is shown in Figure 3 taken using a stereomicroscope. When the melting temperature cools to 654 ° C, the primary intermetallic phase AI4Sr stops precipitating and the remaining amount of liquid alloy solidifies as a ductile eutectic phase, very finely divided. The eutectic phase is shown in Figure 2 by the light regions surrounding the large primary needles of AI4Sr. The eutectic phase is divided much finer than the intermetallic phase AI Sr as evidenced by the lack of resolution of the eutectic phase at a magnification of 50 times. The amount of primary intermetallic phase AI Sr present in the final solidified alloy will depend on the rate at which the cooling took place between 815 ° C to 654 ° C. If the alloy is allowed to cool very slowly in such a way that the equilibrium is reached at each instant of cooling, then the amount of primary intermetallic phase AI4Sr in the final alloy will be given from the equilibrium phase diagram in Figure 1 using the lever rule, ie for a 10 percent strontium alloy. % of primary phase AI4Sr in final alloy = (10% minus 3.5%) = 16% (44% minus 3.5%) % of eutectic phase in final alloy (by difference) = 84% As discussed earlier in "Phase Diagrams for Ceramists," a faster solidification regime that does not allow phase equilibrium at all times will lead to very different composition results. A faster solidified alloy will contain less than 16 percent primary intermetal phase AI4Sr with the amount of primary AI Sr decreasing as the cooling rate increases. This reduction in the amount of the primary intermetallic phase AI Sr according to the solidification regime increases is due to the shorter period of time consumed by the alloy which is cooled in the temperature range of 815 ° C to 684 ° C where the Primary AI Therefore, rapid solidification leads to less primary intermetallic phase and correspondingly an increase in the amount of eutectic phase in the final solidified alloy. For a master alloy of 10% strontium-90% aluminum, the maximum amount of primary intermetallic phase AI4Sr is 16 percent, and correspondingly the minimum amount of eutectic phase is 84 percent, which occurs when the cooling rates are slow enough to allow phase balance. U.S. Patent 4,576,791 states that a 10 percent strontium-aluminum alloy rod, which contains a maximum of only 16 percent primary intermetallic phase AI4Sr and in the finely divided eutectic phase the same minimum of 84 percent , it dissolves normally very slowly to be unsuitable for use as a master alloy in the form of a rod. This is due to the presence of relatively large crystals of primary intermetallic phase of AI Sr that They vary from 5 to 300 microns seen through a two-dimensional microscope. The patents face this problem by providing 0.2 to 5 percent titanium and up to 1 percent boron in the master alloy to refine the typical two-dimensional size of primary intermetal Al4Sr glass from 20 to 100 microns. Reducing the size of the primary intermetallic AI4Sr increases the ductility of the rod, thus allowing it to be rolled and unrolled during feeding and also shortening the dissolution time to approximately 1 minute which is required for washing additions. The addition of titanium and boron allows strontium concentrations in the master alloy to be increased up to 20 percent strontium by weight, in the preferred embodiment up to 10 percent strontium. Retreating the size of the primary intermetallic phase AI4Sr is effective up to a maximum of 20 percent strontium beyond which the alloys are unsuitable for use in the form of a rod. In U.S. Patent 4,576,791 the primary phase crystals AI Sr are referred to as varying from 5 to 300 microns in size. It is important to note, however, that this size description can be misleading since it is based on a two-dimensional microscopic view of a polished sample (Figure 2). In fact, the primary intermetallic phase is first formed during solidification as a three-dimensional network of crystals. Even though a two-dimensional microscopic view, the Al4Sr intermetallic ones appear as discrete needles with sizes smaller than 300 microns, in fact these intermetallic crystals form an interconnected network of plates surrounded by eutectic phase divided very finely which is the last phase to solidify. Figure 3 shows the three-dimensionally interconnected plates of the primary intermetallic phase of AI4Sr present in a 10 percent strontium-90 percent aluminum alloy. The amount of three-dimensional interconnection increases as the concentration of strontium in the alloy increases. From here, in the prior art there has been a limit of strontium concentration. Beyond this higher strontium concentration limit, the three-dimensional network of finely divided eutectic mixture amount surrounding the very small plates makes the alloy unusable due to the slow dissolution and brittleness of these large intermetallic networks. A different approach to the problem caused by AI4Sr plates is found in U.S. Patent Nos. 5,045, 1 10 and 5,205,986, issued on September 3, 1991 and April 27, 1993, respectively, in the name of Shell Research Ltd These patents teach that the concentration of strontium in binary master alloys rich in strontium aluminum can be increased to 30 percent or 35 percent of Sr by weight by further refining the grain size and reducing the amount of the primary intermetallic phase of AI Sr as a result of atomizing the liquid alloy at very fast cooling rates of 102 to 104 ° C second. By this process both the amount and size of the primary intermetallic phase of AI4Sr that precipitates first is reduced and the amount of eutectic phase more ductile, finely divided is proportionally increased.

Figure 4 is a photomicrograph taken at a 500-fold magnification of a 10 percent strontium-90 percent aluminum alloy rod produced from a rapidly solidified atomized alloy as in U.S. Patent Nos. 5,045, 1 10 and 5,205,986. When compared to Figure 2 which is a photomicrograph taken at only a 50-fold amplification (an amplification 10 times smaller than Figure 4) of an aluminum alloy casting of 10 percent strontium-90 percent aluminum in a permanent mold at moderate solidification rates, it is evident that the rapid solidification rates resulting during atomization greatly reduce the size and amount of primary intermetallic phase of AI4Sr. Titanium and boron can also be added to the master alloy to further refine the structure. By reducing the amount and retreating the size of the primary intermetallic phase of AI Sr and also increasing the amount of ductile, finely divided eutectic phase, the patents teach that the concentration of strontium in aluminum-strontium master alloys can be increased up to 32 percent of strontium by weight. The atomized solid particles, each of which contains both an intermetallic phase of finely divided AI4Sr and a eutectic phase, are consolidated by an extrusion process on a rod for in-line addition to a scrubber, this rod has "sufficient ductility to allow rolled and unrolled. " Although as detailed by Gruzleski and Closset above, a 90 percent strontium-rich aluminum master alloy is also available, but it is of limited use as a master alloy. This strontium-rich master alloy consists of a 100 percent finely divided eutectic phase with no intermetallic phases present and has very limited application since it can be used only when the melting temperature of the aluminum-silicon die-cast alloy is below approximately 700 ° C. When added to a molten aluminum alloy, the 90 percent strontium alloy melts first and the 90 percent strontium enriched liquid is then dissolved to dilute levels of 150-200 ppm strontium. During this dissolution, the local liquid composition must be diluted to 90 percent strontium unless 0.02 percent strontium (150-200 ppm strontium). During this dissolution, the local melted composition must pass through the range of high melting intermetallic alloy compositions from 77 percent to 44 percent strontium and these intermetallic phases will precipitate during dissolution as solid intermetallic phases that stop or retard additionally the dissolution of strontium. At melting temperatures below 720 ° C, the 90 percent strontium alloy dissolves exothermically releasing enough heat to raise the melting temperature of the aluminum-silicon alloy locally to a sufficiently high level to prevent the formation of the intermetallic phases of high melting of AI4Sr and AI2Sr. Accordingly, at melting temperatures below 720 ° C, the alloy of 90 percent strontium-10 percent aluminum dissolves rapidly with high recovery. At melting temperatures above about 720 ° C this exothermic reaction decreases and insufficient heat is generated. This results in the formation of intermetallic phases of AI2Sr and AI4Sr during dissolution. The presence of high-melting intermetallic phases of AI Sr and AI2Sr effectively retards dissolution and results in poor strontium recovery. Thus, as taught by the prior art discussed above, the presence of primary intermetallic phases of high melting point of between 44% and 77% of strontium by weight placed significant limitations on the use of aluminum-strontium master alloys. Until now, the useful master alloys of aluminum-strontium have been alloys containing substantial amounts of low-melting, divided, ductile eutectic phase. In the case of aluminum strontium-rich master alloys ranging from 5 to 35% strontium, the alloy consists of a mixture of primary SR intermetallic phases surrounded by finely divided eutectic phase. The primary intermetallic phase AL4SR is present as a three-dimensional network of interconnected plates which under normal solidification regimes can be very thick in size. All prior art teaches that the only method of increasing the strontium concentration in these aluminum-rich master alloys while allowing acceptable dissolution rates of the alloys in molten aluminum is to minimize the amount and refine the size of the interconnected network of AI4Sr plates. and maximize the amount of the very fine, ductile eutectic phase. A separate use of the aluminum-strontium alloy of this invention is as an inoculant for cast iron.

Inoculation is a process in which the formation of metastable carbides in molten iron is suppressed. Instead, the graphite formation that is the equilibrium phase is allowed. Industrial solidification regimes that are usually between 0.1 to 10 ° C / sec normally do not allow the formation of graphite in very thin sections of foundries. Inoculation provides substrates or cores for graphite nucleation. These substrates or nucleation sites are believed to be sulfides. Strong sulphide formers such as calcium and strontium are added to the molten iron for inoculation. A good source of information on the mechanisms and practice of nucleation possible to inoculate cast iron can be found in "The Modern Inoculating Practices for Gray and Ductile Iron", Proceedings of AFS-CMI Conference, 6-7 February 1979. The Patent US Pat. No. 4,666,516, issued to Hornung et al. on May 19, 1987, also discusses the manner in which the shape taken by the carbon present in molten iron greatly affects its characteristics. If it is in the form of iron carbide (known as "tempered"), the cast iron is brittle ("white cast iron") but in the form of flake graphite the cast iron is soft and machinable ("gray cast iron"). ). The spherical shape of graphite produces greater strength and improved ductility ("ductile cast iron"). Ferrosilicon has been used as an inoculant to promote the formation of graphites particularly in spherical and nodular form and U.S. Patent No. 3,527,597, issued to Dawson et al. On September 8, 1970, teaches that an important Incoulant can be obtained by including strontium metal from 0.1 to 10% and maintaining a low calcium content. The commercial grade of ferrosilicon contains calcium as an impurity. As noted in the patent: "Pure ferrosilicon has very little inoculating effect when added to molten iron, the commercial grade foundry ferrosilicon depends on its content of small amounts of minor elements, notably aluminum and calcium, to stimulate the inoculating effect" . It has now been found that by using the strontium-aluminum alloy of this invention an additional improved inoculating effect can be obtained. It has been found that the most useful composition is ferrosilicon together with 80 percent strontium alloy. In the case of gray cast iron, tempering was eliminated and the amount of type D graphite minimized by combined addition. In the case of ductile iron, the combined addition reduced the amount of tempering and increased the number of nodules. In U.S. Patent No. 3,527,597, metallic strontium was added to gray iron together with FeSi. The required amount of strontium was much greater than if it had been allied with FeSi and Si. In the case of alloying with silicon only a maximum of 65 percent Sr could be obtained in the final alloy due to the nature of the dissolution reaction. In the present invention, the additive may contain up to 80 percent strontium and only a small amount is required for successful inoculation. U.S. Patent No. 3,527,597 teaches the only grade of FeSi that can be alloyed with Sr as a potent inoculating agent is a low calcium FeSi which normally contains less than 0.35 percent calcium. Above this calcium concentration the potency of the inoculant is decreased. The patent also describes the separate addition of the SiSr alloy alone or together with FeSi containing low and normal calcium concentration. SiSr with low calcium FeSi produced a significant inoculant effect in gray iron.

DESCRIPTION OF THE INVENTION The present invention is based on the discovery that dominant intermetallic alloys characterizing compositions of between about 40 percent to 81 percent strontium and consist mainly of the intermetallic phases AI4Sr, AI2Sr and AISr which were previously considered as detrimental to conventional Al-Sr master alloys, due to their slow dissolution characteristics, can be adapted to be used to add strontium to modify foundries of aluminum-silicon smelting alloys. Unlike the prior art which is based on alloys containing large amounts of eutectic phase, the alloys of the present invention contain only minimal amounts and in most cases no eutectic phase. The intermetallic phases are present as adjoining discrete phases and illustrated in a non-interconnected eutectic phase matrix in a network of platelets embedded in a eutectic phase matrix. Figure 5 shows a 125-fold photomicrograph of amplification of an intermetallic alloy of the present invention that It contains 55 percent strontium and 55 percent aluminum. This alloy contains two intermetallic phases AI Sr and AI2Sr with no eutectic phase present and has a microstructure that is significantly different from the known eutectic aluminum-strontium mixture alloys previously shown in Figure 2. Surprisingly, it has now been determined that all the intermetallic alloy compositions, ie AI Sr at 44 percent strontium by weight, AI2Sr at 62 percent strontium by weight and AISr at 77 percent strontium by weight, as well as mixtures of the intermetallic alloys with small Eutectic phase quantities and characterized by a global composition of 40 percent to 81 percent strontium by weight, dissolve rapidly when added as discrete granules to aluminum-silicon smelting alloy foundries. The rapid dissolution of these intermetallic alloys is surprising for several reasons: The discussion of the prior art of strontium-rich master alloys indicates that their operation is determined by the size and amount of the primary intermetallic phase AI4Sr which is present as a network of platelets. interconnected within the eutectic phase matrix. The present invention is capable of using strontium alloys from 40 to 81 percent strontium concentration because the intermetallic alloys AI4Sr, AI2Sr and AISr and their mixtures are present as discrete three-dimensional particles, not as an interconnected network of platelets. In the present invention, the intermetallic alloy particles are part of a global composition that has between 40 to 81 percent of strontium can be as large as 5000 microns or 50 to 500 times larger than the size of the AI intermetallic particles discussed in the prior art. It is therefore surprising, based on the prior art, that particles as large as 5000 microns containing strontium concentrations as high as about 81 percent dissolve so rapidly with high strontium recovery. The dissolution of the 10 percent strontium-rich aluminum alloy eutectic, which is richer in strontium than the intermetallic alloys of the present invention is effective only when added to smelters at temperatures below 720 ° C. This is because the alloy releases exothermic heat below 720 ° C which locally raises the melting temperature of aluminum above the melting points of the intermetallic alloys. At melting temperatures above 720 ° C, insufficient exothermic heat is released to raise the local melting temperature. As a result at melting temperature above 720 ° C, as the 90 percent strontium enriched liquid coming from the melting of the master alloy is diluted during dissolution, the high temperature intermetallic phases of AISr (77 percent of strontium), AI2Sr (62 percent strontium) and AI4Sr (44 percent strontium) precipitate as a solid and greatly retard the dissolution rate and decrease the recovery of strontium. Based on this information, it would be expected that intermetallic alloys containing up to about 81 percent strontium would also require the release of exothermic heat to dissolve quickly and therefore would be ineffective at melting temperatures above 720 ° C. The same exothermic effect is not evident, however, when using intermetallic alloy granules of the present invention since the strontium recovery and dissolution rate are excellent even at melting temperatures of 750 ° C. Also the dissolution rate of the intermetallic alloy granules with strontium concentration greater than 44 percent is not hindered by the formation of higher melting point phases during dissolution as would be expected based on the 10 percent aluminum eutectic alloy -strontium.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows the phase diagram of aluminum-strontium binary equilibrium. Figure 2 shows intermetallic needles of AI4Sr in a finely divided eutectic mixing matrix for a master alloy of 10 percent strontium-90 percent aluminum as seen in a 50-fold amplification. Figure 3 shows the three-dimensional network of interconnected primary intermetallic plates of AI4Sr present in alloys containing eutectic mixture seen through a stereomicroscope. Figure 4 shows a photomicrograph at a 500-fold amplification of a 10 percent strontium-90 percent aluminum alloy rod prepared by atomization and subsequent extrusion according to U.S. Patent Nos. 5,045, 110 and 5,205,986.

Figures 5, 6, 7 and 8 are photomicrographs of master alloys according to the invention. Figure 5 shows the microstructure at a 125-fold amplification of an alloy of 55 percent strontium-45 percent aluminum containing two intermetallic phases AI4Sr and AI2Sr and no eutectic phase. Figure 6 shows the microstructure at a 50-fold amplification of a 10 percent strontium-90 percent aluminum alloy rod prepared by mixing the appropriate amounts of aluminum granules and intermetallic alloy granules of AI4Sr and subsequently continuously extruding the mixture into a diameter rod of 0.95 cm. Figure 7 shows the microstructure at a 50-fold amplification of a 20-percent strontium-80-percent aluminum alloy prepared by dragging the appropriate amount of IA-nickel alloy solid granules into a 10 percent strontium molten liquid. 90 percent aluminum.

MODES FOR CARRYING OUT THE INVENTION The intermetallic alloy granules according to the present invention are first produced by melting and then alloying. The alloys can be prepared either by starting with an aluminum smelter and alloying it with the appropriate amount of strontium metal or by first melting the strontium metal and subsequently alloying it with the appropriate amount of aluminum metal. Care must be taken to ensure that strontium-rich smelters are protected from the atmosphere by an inert gas such as argon. In addition, you must have Care, especially of aluminum-rich foundries, to limit the amount of hydrogen collected from atmospheric moisture. The alloying process is usually conducted at melting temperatures with at least 50 ° C of superheat above the temperature at which solidification begins. Since these intermetallic alloys are brittle in the solid state, the granules can be produced by grinding using normal pressing and grinding techniques. The optimal mesh size distribution of the strontium-aluminum intermetallic alloy granules depends on the method of use. The applications involve direct addition on the surface of the foundry or in a vortex stirred in the foundry or very deep below the melt surface or a spill method where the melt is poured on top of the granules, granules with A mesh size distribution of approximately 150 microns or less are acceptable. In the preferred embodiment, however, the granules by these application methods are sized to approximately 2500 microns or less. For applications where strontium-aluminum intermetallic alloy granules are premixed with other granular matepales such as aluminum for briquette compression or the like or consolidated by extrusion into rods or other shapes, a mesh size distribution of approximately 2500 microns is acceptable. and less while 500 microns and less and 150 microns and less is preferred.

For applications where strontium-aluminum intermetallic alloy granules are introduced into the melt using pneumatic submerged injection through a suitably designed lancet or rotary degasser, a mesh size distribution of 2400 microns or less is acceptable, with 800 microns or less being preferred. For applications where the strontium-aluminum intermetallic alloy granules are physically entrained towards the aluminum melt or an aluminum-strontium alloy melt for subsequent use as an enriched aluminum-strontium master alloy, a mesh size distribution of about 3000 microns or less is acceptable with 500 microns or less preferred. In many of the above applications, it may also be desirable but not necessary to minimize the amount of ultrafine particles that may be present in strontium-aluminum intermetallic alloy granules such as below 74 microns or more preferably below 43 microns. Since strontium is known to be a highly reactive metal, the reactivity with atmospheric oxygen, nitrogen and moisture of the intermetallic aluminum alloy granules was also tested. As shown in Table I below, the strontium-aluminum intermetallic alloy granules with a size distribution of 147 microns and more were exposed to the atmosphere at room temperature for a period of up to 240 hours.

Table I Granules of Intermetallic Alloy Strontium - Alloy (- 147 microns); Gain in Weight Due to Atmospheric Exposure at room temperature.

The results indicate that, as expected, the reactivity of the master alloy granules increases with the increase in strontium concentration. Surprisingly, however, the degree of reactivity even for intermetallic alloy granules of 80 percent strontium-20 percent aluminum is not excessive and is well within the tolerance limits suitable for commercial production and use. In comparison, the eutectic alloy of 90 percent strontium-10 percent previously known aluminum is very reactive with air and can ignite spontaneously if exposed to excessive moisture or a spark. This 90 percent strontium alloy is classified as hazardous and must be protected from the atmosphere by impermeable protective packaging. The strontium-aluminum intermetallic alloy granules are used in a variety of methods depending on which method is best suited to the application. These methods include but are not restricted to the following: Directly as a master alloy enriched in strontium. Typical methods for direct addition include the addition to the surface of a melt at rest or agitated, addition to a swirl created by mechanically mixing or otherwise melting, pneumatic injection through a submerged device such as a lancet, nozzle or properly designed rotary degasser, a spill method where the liquid metal is poured over the top of the granules, and melting the granules below the melt surface using a properly designed device such as a basket or pot. Mix the intermetallic strontium-aluminum alloy granules with other aluminum granule particles. These metal mixtures can then be compressed into briquettes, tablets or the like or consolidated by hot or cold extrusion into rods or other suitable shapes. These compressed or consolidated mixtures are subsequently used as the addition of master alloy to add strontium to the melt. Figure 6 shows a photomicrograph of 10 percent strontium-90 percent aluminum alloy rod prepared by continuous extrusion of a mechanical mix of aluminum granules and intermetallic alloy granules of 45 percent strontium-55 percent aluminum (AI4Sr) . The discrete three-dimensional nature of the intermetallic phase in Figure 6 is distinctively different from the three-dimensional network of the interconnected primary intermetallic plates of AI Sr which exists in known eutectic strontium-aluminum alloys cited by the prior art. Physically drag strontium-aluminum intermetallic alloy granules to a melt whose temperature is maintained below the melting point of the intermetallic alloy granules. This melt may consist of but is not restricted to an aluminum strontium-aluminum alloy. By physically dragging the intermetallic alloy granules to a melt maintained below the melting point of the granules, the intermetallic alloy granules through proper care can be effectively maintained in physical suspension. as solid strontium-aluminum intermetallic alloy particles discrete three-dimensionally within a molten base alloy which may or may not contain strontium. This liquid-solid mixture can then be poured into ingots, moldings and the like and can be used as a strontium enriched master alloy, either directly or after extrusion of the rod moldings or the like. Figure 7 shows that this type of strontium-rich aluminum alloy is different from the strontium-aluminum master alloys known which, as shown in Figure 2, contain an interconnected network of primary plates of AI4Sr in a eutectic matrix. With this invention, strontium is present in intermetallic particles rich in discrete three-dimensional strontium which are not interconnected. The matrix can be either aluminum or aluminum-strontium alloy. Figure 8 illustrates how AI Sr intermetallic plates are broken when a 20 percent strontium alloy is prepared by dragging the appropriate amount of AI4Sr intermetallic granules into a 10 percent strontium-90 percent aluminum base alloy which forms matrix. The strontium-aluminum intermetallic alloy granules can be used with these methods to add strontium to a melt for applications such as but not restricted to modification of acicular silicon in an aluminum-silicon melt and to modify intermetallic phases in aluminum extrusion alloys. An important distinction between the present invention and the eutectic containing master alloys cited in the prior art is the size acceptable of the primary phase metallical alloy. In the prior art, repeated attempts were made to reduce the size and quantity of the primary phase intermetallics of AI Sr by the addition of titanium, boron and by atomization. As indicated in U.S. Patent No. 4,576,791, the two-dimensional size of the primary metallic phase of AI Sr viewed through a microscope had to be reduced to 100 microns or less to allow an increase in the strontium concentration of the master alloy at 20 percent strontium by weight. An additional size reduction to 10 microns and less was required to achieve 35 percent strontium concentrations. Unlike these earlier teachings, the present invention uses discrete intermetallic alloy granules of three dimensions with minimal or no eutectic phase which, depending on the method of use, allows rapid dissolution and high strontium recovery at sizes of approximately 5000 microns ( 5mm). As illustrated in Table II of Example I below, when added directly to an agitated whirlpool, intermetallic alloy granules as described in the present invention usually dissolve faster when they are between -1651 + 147 microns than when they have a size of minus 147 microns. This improvement with increasing mesh size is completely unexpected given the efforts cited in the prior art to reduce the size of the primary intermetallic phase of Al4Sr present in alloys containing conventional eutectic mixture.

In the following examples, the strontium-aluminum intermetallic alloy granules were prepared by melting and alloying to the correct composition, pouring into blocks and grinding and crushing the blocks into granules.

EXAMPLE I: Direct addition of intermetallic strontium-aluminum alloy granules to a vortex. Several experiments were conducted in which the strontium-aluminum intermetallic alloy granules were added directly to a 356 aluminum-silicon alloy melt in a vortex created by mechanical mixing operating at approximately 300 rpm. As detailed in Table II below, the experiments were conducted at two melt temperatures (700 and 750 ° C) using granules of alloy composition and different mesh size distribution.

Table II: Results of Direct Addition of Intermetallic Alloy Granules from Sr-Al to a Vortex in Al-Si Alloy Melts 356 The results of the above experiments indicate excellent strontium recovery for all intermetallic alloy compositions. Very rapid dissolution (2 minutes or less) was achieved at melt temperatures of 700 ° C and 750 ° C for larger granules having mesh sizes ranging from -1651 μ + 147 μ. The excellent strontium recovery is also achieved with granules of size -147 μ; however, the dissolution time is probably longer as a result of surface tension difficulties that give an increase in the poorer wettability of the fine granules by the melt. The results of this example are surprising. The improvement in dissolution regime with the size of the increase of the intermetallic alloy is unexpected given the teaching of the prior art based on alloys containing eutectic mixtures. Also the absolute size of -1651 μ for the intermetallic alloy granules is significantly greater than the intermetallic phase size allowable in the prior art of -100μ for alloys containing eutectic mixture with a maximum of 20 percent strontium and nominally -10μ for alloys containing eutectic mixture with a maximum of 35 percent strontium. Excellent dissolution and strontium recovery rates at melt temperatures greater than 720 ° C are also unexpected for intermetallic alloys containing more than 44 percent strontium.

EXAMPLE II: Direct addition of intermetallic strontium-aluminum alloy granules by pneumatic injection. Several experiments were conducted in which strontium-aluminum intermetallic alloy granules were directly added to aluminum-silicon alloy 356 melt by pneumatic injection. These injection tests were carried out by blowing granules of Intermetallic strontium alloy suspended by the central perforation in the arrow of a rotary degasser at which point the granules were released below the surface to the melt. The granules were injected for a period of approximately 30 seconds and subsequently melt samples were taken and analyzed for strontium. With 62 percent strontium-38 percent aluminum alloy granules with sizes of minus 1651 microns, the 72 percent recovery of strontium was achieved within approximately two minutes after the end of the alloy injection period. During this test, the degasser impeller was rotating at 300 rpm and the melt temperature was maintained at 760 ° C. A second test was conducted with an impeller speed of 150 rpm and a strontium recovery of 70 percent was achieved in two minutes.

EXAMPLE III: Direct addition of strontium-rich master alloy by stripping solid aluminum-strontium intermetallic granules in a base melt. Strontium-rich master alloys containing up to 23 percent strontium were produced by first mechanically entraining the appropriate amounts of strontium-aluminum intermetallic alloy granules to an aluminum base melt or strontium-aluminum alloy at temperatures below the stripping points. fusion of the granules. The resulting mixture of granules enriched in strontium liquids and solids were subsequently cast in ingots and moldings. The moldings were subsequently extruded to a 0.95 cm diameter rod. This resulted in a new type of strontium-rich master alloys containing discrete three-dimensional particles of intermetallic strontium alloy granules. These alloys differ from conventional strontium-aluminum alloys since strontium is present in three-dimensional form as discrete granules enriched in strontium while in conventional alloys strontium is present as a three-dimensional network of intermetallic needles or plates interconnected in a eutectic matrix. Table III summarizes the test results where master alloys enriched with strontium prepared in accordance with this invention whether in ingots or extruded form were added either to the surface or submerged in an aluminum-silicon alloy melt at 760 ° C. The results confirm that the master alloys enriched with strontium either in the form of extruded bar or ingot produced by entrainment of discrete solid intermetallic alloy granules in a base melt dissolve rapidly giving a high strontium recovery.

Table III EXAMPLE IV: Direct addition of tablets formed by compression of mechanical mixtures of strontium-aluminum intermetallic granules. A mechanical blend of 33 weight percent intermetallic alloy granules of 60 percent strontium-40 percent aluminum and 67 weight percent aluminum metal granules (both nominally minus 1651 μ) were prepared and compressed into tablets. The bulk composition of the tablets gave on average 20 percent strontium by weight. The tablets were then added to the surface of an agitated aluminum-silicon 356 alloy melt (300 rpm) maintained at 730 ° C.

Strontium recovery of 91 percent was achieved in four minutes after the tablets were added.

INOCULATION OF FIERRO COLADO The strontium-aluminum alloy of this invention has also been found to be useful for modifying the gray and ductile cast iron structure. In the following examples gray and ductile cast iron were inoculated using FeSi and Sr / AI alloys alone or in combination. The efficiency of each inoculation practice was evaluated by cold tests and metallographic examination. It was found that the most useful configuration is the addition of FeSi together with the alloy of 80 percent strontium-20 percent aluminum. In the case of gray cast iron, the cooling was eliminated and the amount of graphite type D reduced by the addition combined In the case of ductile iron, the combined addition reduced the amount of cooling and increased the graphite nodule count. The examples demonstrate that the alloys of this invention can be useful inoculants alone or in combination with additions of FeSi. In the following examples, the gray cast iron contained 3.7% C and 2.5% Si and the ductile cast iron contained 3.7% C and 2.5% Si as is typical of gray and ductile cast iron. In each example, cast iron was cast in ASTM 3.5 wedge molds in green sand. The wedges were broken from the middle and the extension of the cooling zone was determined in multiples of 0.08 cm. In each example, the pouring temperature was around 1375 ° C and one and one-half minutes for the inoculant solution was left in a pouring cauldron. The size range of inoculating agents was between 0.95 and 0.158 cm. Ferrosilicon contained 86% Si, 0.6% Ca and 0.7% Al.

EXAMPLE V. Gray Cast Iron Molten metal was cast into the mold without inoculation. Depth of cooling: 2.38 cm.

EXAMPLE VI. Gray Cast Iron Molten metal was inoculated into the transfer cavity with FeSi in an amount that is 0.75% of the total weight of the melt. Cooling depth: < 0.08 cm EXAMPLE VII. Gray Cast Iron Molten metal was inoculated with 65% Sr / 35% Al alloy. The amount of strontium added was 0.0225% of the total weight of the metal. Depth of cooling: 1 .032 cm.

EXAMPLE HIV. Cast Iron Gray Cast metal was inoculated with a combination of 65% Sr / 35% Al alloy and FeSi. The aggregate amounts of strontium and FeSi were 0.0225% and 0.75% of the total weight respectively. Cooling depth: < 0.08 cm EXAMPLE IX. Cast Iron Gray Cast Iron was inoculated with a combination of 80% Sr / 20% Al and FeSi in the same amounts as in Example 8. Cooling Depth: < 0.08 cm It will be seen from the above examples that Sr / AI alloys are useful inoculants alone or in combination with FeSi. In gray cast iron, it is important to have evenly distributed, randomly oriented graphite flakes defined as type A graphite. Among the less desirable forms of graphite is graphite type Del which is segregated into interdendritic regions in a random orientation. The following table shows the extent of type D graphite from the apex of the wedge to the point where type A graphite occurs.

Inoculant agent Graphite extension Type D (cm) FeSi 1 .508 65% Sr / 35% At 2.30 FeSi + 65% Sr / 35% At 0.714 FeSi + 80% Sr / 20% Al < 0.08 It is clear that the best inoculation practice for gray cast iron is the combined addition of FeSi with 80% Sr / 20% Al alloy.

EXAMPLE X. Ductile cast iron As in example 5. Cooling depth: 2.302 cm.

EXAMPLE XI. Ductile cast iron As in example 6. Cooling depth: 1 .905 cm.

EXAMPLE XII. Cast Iron Ductile Cast metal is inoculated with 80% Sr / 20% Al alloy. The amount of strontium addition was 0.0225% of the total weight of the metal. Cooling depth: 1 .27 cm.

EXAMPLE XIII. Ductile cast iron As in example 8.

Depth of cooling: 1 .667 cm.

EXAMPLE XIV. Ductile cast iron As in example 9. Cooling depth: < 0.08 cm In the case of ductile iron it is clear that the alloy 80% Sr / 20% Al alone is a more potent inoculation agent to reduce cooling than FeSi. Its power increases when combined with FeSi. The effect of inoculation on the graphite nodule count is shown in the following table: Inoculation agent Number of nodules / cm ^ None 22, 100 FeSi 25,600 80% Sr / 20% At 28,500 FeSi + 65% Sr / 35% At 38,700 FeSi + 80% Sr / 20% At 31,000 As can be seen, the Sr / AI alloy is a more effective inoculation agent for nuclear graphite nodules than FeSi and its potency is increased with the combined addition of FeSi. The process for adding the inoculant to a gray cast iron melt can use the inoculant in the form of granules, briquettes, tablets or core wire covered with steel. In the latter case, the Core wire may include other inoculation compositions, such as rare earth salicylicides, as well as the aluminum alloy strontium and ferrosilicon. The above examples are presented for the purpose of illustration, without limitation, of the product and methods of use of the present invention. It is understood that changes and variations may be made without departing from the scope of the invention as defined in the following claims.

Claims (10)

1. CLAIMS 1. A composition suitable for use as a master alloy addition for silicon aluminum alloys or as an inoculant for cast iron, comprising granules of intermetallic alloys selected from the group of AI Sr, AI2Sr and AISr, the composition containing from 40 to 81% in strontium weight, and having minimal amounts of eutectic phases of strontium aluminum. A composition according to claim 1, wherein the alloy extends no more than 4% in the range of composition of alloys containing eutectic mixtures. 3. The composition according to claim 1, which essentially has no eutectic phase of AISr. 4. A composition according to claim 1, wherein the granules are less than 5,000 μ in size. 5. A composition according to claim 1, wherein said granules are in free flowing form, not connected. 6. A composition according to claim 1, wherein said granules are incorporated into an extruded rod which also contains aluminum granules. 7. A composition according to claim 1, wherein said granules are drawn into a cast aluminum alloy molding. 8. A composition according to claim 1, wherein the granules are between 147 and 1650 μ in size. 9. A composition according to claim 1, wherein said discrete granules tri-dimensionally. A process for refining aluminum-silicon alloys, comprising adding to the free-flowing granules of inter-metallic alloys selected from the group of AI4Sr, AI2Sr and AISr, and containing from 40 to 81% strontium by weight, and only minimal amounts of eutectic aluminum-strontium phases. eleven . A process according to claim 10, wherein the granules are less than 5,000 μ in size. 12. A process according to claim 10, wherein the granules are between 147 and 1650 μ in size. 13. A process according to claim 10, wherein said granules are discrete three-dimensionally. 14. An inoculant for cast iron comprising the composition of claim 1 together with calcium. 15. An inoculant as set forth in claim 14 which further contains FeSi commercial grade. 16. An inoculant as set forth in claim 14 wherein the intermetallic alloy contains 65% Sr and 35% Al. 17. An inoculant as set forth in claim 14 wherein the intermetallic alloy contains 80% Sr. and 20% Al. 18. A process for inoculating gray cast iron comprising adding to the free-flowing granules of intermetallic alloys selected from the group of AI4Sr, AI2Sr and AISr, and containing from 40 to 81% by weight of strontium , and only minimal amounts of phases Eutectic aluminum-strontium. 19. A process according to claim 18, wherein the granules are less than 5,000 μ in size. 20. A process according to claim 18, wherein the granules are between 147 and 1650 μ in size. twenty-one . A process according to claim 18, wherein said granules are discrete three-dimensionally.