US3895941A

US3895941A - Aluminum silicon alloys

Info

Publication number: US3895941A
Application number: US402527A
Authority: US
Inventors: Gustaf Frederic Bolling; Jean Cisse
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1973-10-01
Filing date: 1973-10-01
Publication date: 1975-07-22
Anticipated expiration: 1992-07-22

Abstract

A method of preparing an improved hypereutectic aluminum-silicon alloy is disclosed, including the resulting composition. One mode comprises the addition of sintered aluminum powdered rods containing aluminum oxide particles, uniformly distributed throughout, to a hypereutectic melt of aluminum-silicon. It further comprises the addition of sodium in low concentrations added from a master Al-Si-Na alloy to the hypereutectic melt of aluminum-silicon. The melt is solidified producing a composition having both primary and eutectic silicon simultaneously finely divided and uniformly distributed throughout the casting.

Description

United States Patent 1 Bolling et al.

[451 July 22,1975

1 1 ALUMINUM SILICON ALLOYS [75} Inventors: Gustaf Frederic Bolling, Dearborn,

Mich; Jean Ciss, St. Germain en Laye, France [73] Assignee: Ford Motor Company, Dearborn,

Mich.

[22] Filed: Oct. 1, 1973 [21] Appl. No.: 402,527

[52] US. Cl. 75/148; 75/68 R; 75/141; 75/142; 148/32; 148/325 [51] Int. Cl. C22C 21/04 [58] Field of Search 75/148, 141, 142, 143, 75/68 R, 138, 146, 147; 148/32, 32.5

[56] References Cited UNITED STATES PATENTS 3,705,029 12/1972 Foerster.... 75/148 Primary ExaminerR. Dean Attorney, Agent, or FirmJoseph W. Malleck; Keith L. Zerschling [57] ABSTRACT A method of preparing an improved hypereutectic aluminum-silicon alloy is disclosed, including the resulting composition. One mode comprises the addition of sintered aluminum powdered rods containing aluminum oxide particles, uniformly distributed throughout, to a hypereutectic melt of aluminum-silicon. It further comprises the addition of sodium in low concentrations added from a master Al-Si-Na alloy to the hypereutectic melt of aluminum-silicon. The melt is solidified producing a composition having both primary and eutectic silicon simultaneously finely divided and uniformly distributed throughout the casting.

8 Claims, 4 Drawing Figures ALUMINUM SILICON ALLOYS BACKGROUND OF THE INVENTION Hypereutectic aluminum-silicon alloys have been deemed significant for use by the casting industry as well as the automotive industry. This principally results from the potential that such alloys hold for providing good wear resistance along with the conventional advantages derived from aluminum castings. Furthermore, it is well recognized that the casting characteristics of hypereutectic alloys are very good.

It is known that increasing the amount of silicon in aluminum-silicon alloys will increase wear resistance. The existence of the primary silicon phase has been considered the principal reason for the increase in wear resistance. Only limited or insignificant consideration has been given by the prior. art to the role played by eutectic silicon with respect to the wear characteristic.

The prior art has further appreciated that small and well dispersed particles of primary silicon in an aluminum-silicon eutectic matrix will improve wear resistance and other physical characteristics. To this end, commercial refiners or modifiers have been developed to effect either refinement of primary or eutectic silicon, but not refinement of both in a single casting. Phosphorous is one of the commercial refiners that has been used to achieve finely dispersed particles of primary silicon. A problem associated with the use of phosphorous is pollution resulting from phosphorous salts. Sodium is one of the commercial refiners that has been used to achieve refined eutectic silicon. But the refinement results from a lower growth temperature which changes the mode of growth, rather than the refinement resulting from an increase in the nucleation rate of the eutectic silicon. A problem associated with the simultaneous use of phosphorous and sodium is the formation of sodium phosphides which effectively inhibit the separate chemical or nucleating action of phosphorous or especially sodium insofar as it is a refiner of silicon in aluminum-silicon alloys. This may in part be explained by the fact that sodium will normally be added above the liquidus temperature and hypereutectic silicon requires an increased melting temperature; thus increased sodium losses occur due to its vapor pressure.

SUMMARY OF THE INVENTION One of the primary objects of this invention is to provide a hypereutectic aluminum-silicon alloy which has improved wear resistance and machineability and mechanical properties.

Another significant object of this invention is to provide an improved method for refining primary and eutectic silicon in an aluminum-silicon melt while overcoming the problems associated with prior-art refiners. A particular feature of the method is the introduction of aluminum oxide to the melt (in one preferred mode, the aluminum oxide is introduced in a finely divided and uniformly dispersed condition) for thereby providing nucleating sites for the primary silicon. A further and connected feature of the method is the provision for the refinement of the eutectic silicon made possible by the addition of sodium along with aluminum oxide; it has been discovered that sodium is no way rendered inert by reaction with the aluminum oxide inoculant. Simultaneous refinement of both the primary and the eutectic silicons is thereby provided.

DESCRIPTION OF THE DRAWINGS FIGS. 1, 2 and 3 show microstructures for an aluminumsilicon hypereutectic alloy under various magnifications, the microstructure depicting the inventive distinguishing characteristics of the novel composition herein and as a result of the novel method of preparation.

FIG. 4 illustrates the microstructure of a similar aluminum-silicon alloy microstructure which results from the addition of sodium influencing the eutectic silicon while the primary silicon is in a refined state and which is a significant result of the novel use of aluminum oxide as an addition to the hypereutectic melt.

ETAILED DESCRIPTION Evaluation of many commercial and experimental aluminum alloys indicates that only the hypereutectic aluminum silicon alloy system contains potential for promoting high wear resistance of this system is attributed to the extremely hard primary, the influence and presence of both silicon phases not being appreciated nor obtained by the prior art simultaneously. Generally, as the silicon content of these alloys is increased. wear resistance accordingly increases. Large amounts of silicon, however, can cause problems with machineability and castability. Therefore, it is important that a sufficient amount of primary silicon be used to impart the required improvement in wear resistance, but not so much as to cause casting and machining difficulties. However, the physical and metallurgical improvements obtained by this invention will be rendered in the broad range of silicon contents as long as the alloy is hypereutectic. To obtain optimum performance, a narrower range of silicon may be desirable, as will be described.

It has been discovered in accordance with this invention that if aluminum oxide is introduced to the hot melt during the formulation of the aluminum-silicon alloy, the aluminum oxide will function as a nucleant for primary silicon insuring that the majority of the silicon phases will be uniformly distributed and of a fine nature. For the first time, both phases (primary and eutectic) of the silicon can be simultaneously refined and modified to achieve unprecedented wear resistance and machineability by concomittent, precedent or subsequent melt additions.

Optimum machineability and good performance of the hypereutectic silicon alloy requires a reasonably small size and uniform distribution of the silicon phases. Two factors which govern the silicon particle size in the phases are (1) solidification rate and (2) artificial nucleation.

For example, in a 390 silicon aluminum alloy containing l618% silicon, 45% copper, 0.1% maximum manganese, O.6-l.l% iron, OAS-0.65% magnesium, 0.1% maximum Zinc, 0.2% maximum titanium, traces of phosphorous and the remainder aluminum, the primary silicon crystal formation will begin at approximately 1200F (liquidus temperature) and will be complete with the accompanying start of the first eutectic solidification at about 1050F. Without a prior refinement treatment, silicon particle size is controlled principally by the rate of cooling through this temperature range; more rapid rates will result in finer particles. Even in refined melts, where the size is also influenced by an increased number of nucleation sites, the ultimate primary silicon size will depend on the solidification rate. A die-cast process is preferred because it will provide an extremely rapid solidification rate and assist in producing a very fine particle size in conjunction with the use of aluminum oxide of this invention.

METHOD A preferred method in conformity with this invention comprises (a) preparation of a hypereutectic aluminum-silicon melt having a pouring temperature of at least 1200F.; (b) introducing A1 0, to said melt in an amount such that the sum of silicon and A1 0 is about from 19-21% by volume of the melt; (c) introducing a quantity of sodium in the range of (LS-1.5% by weight of the melt; and (d) pouring the melt mixture into a diecast machine.

Ideally, to avoid formation and segregation of a solid phase during flow and to solidify entirely at a rate influenced by conditions in a mold cavity, a hypereutectic aluminum silicon alloy should fill the cavity before cooling below its liquidus temperature. This requires that heat losses from the molten metal to the ladle, receiving chamber, runner system and during flow through the mold cavity itself, be considered in establishing the correct holding furnace temperature. If the pouring temperature of the melt is to be lowered, it is important that the total volume content or sum of the silicon and aluminum oxide in the melt be maintained constant, in the range of 19-21% of the melt, while the aluminum oxide content is increased.

In order for the aluminum oxide to be optimally effective, it must be present in the melt as surface active. well dispersed. very fine particles. However, nucleation of silicon on massive Al O particles is possible even though nucleation would seemingly be impossible on an apparently inert surface. These conditions are facilitated by the way in which the aluminum oxide is introduced into the melt. It has been discovered that by the use of sintered aluminum powder rods containing aluminum oxide (for example about 10% relative to the content of the master alloy) one effective mechanism results. The powdered rods are introduced to the melt containing aluminum and hypereutectic content of silicon (for example such that in the dissolved nature, the aluminum oxide will constitute about 3% of the melt). This has been found to be a very effective way for adding aluminum oxide particles less than 10 microns in size so that they will be immediately wetted or otherwise influenced to act as wetted by the melt liquid. The silicon is then capable of interacting immediately upon cooling below the alloy liquidus with the aluminum oxide in order to provide active surfaces where the aforementioned primary silicon nucleation takes place with little or no nucleation of the aluminum.

To achieve an object of reducing pollution problems, aluminum oxide can be used as a substitute for phosphorous which is conventionally used as a refiner in such types of hypereutectic aluminum alloys. Thus, the invention eliminates pollution problems arising when phosphorous salts are used.

COMPOSITION Referring now to the microstructural figures, and particularly FIG. 1, there is illustrated a metallographic illustration of a polished specimen, l50 magnification. Silicon represents 17% of the composition and aluminum oxide 3%. Those portions labeled a represent primary silicon nucleated from a cloud of aluminum oxide. Those portions labeled [2 represent coarse eutectic silicon. Those portions labeled 1' represent a cloud of aluminum oxide within the matrix.

In FIG. 2, the same composition was analyzed by scanning microscopy, the specimen being etched and photographed at 500 magnification. The primary silicon nucleated from a cloud of aluminum oxide as shown in detail, primary silicon being represented by a and a cloud of aluminum oxide being labeled 0 acting as a nucleant. In FIG. 3, again the same composition is analyzed at [00X magnification for purposes of comparing with FIG. 4 at a similar magnification, but incorporating sodium as an additive. In FIG. 3, the same primary silicon and coarse eutectic silicon is labeled a and b respectively; in FIG. 4 the interconnected primary silicon and aluminum oxide particles are labeled a and very fine eutectic silicon is labeled 1) (note the more globular shape of the primary silicon).

The preferred range of 16-18% silicon coincides generally with the range of greatest fluidity of the aluminum silicon system at normal casting temperatures, but an operational range is 16-19%. This is an important consideration for an alloy that is to be cast into complex shapes. The alloys represented in FIGS. l-3 can be used in the as-cast or artificially aged and stabilized temper.

We claim:

1. In a method of producing aluminum-silicon castings by preparation of a hypereutectic aluminumsilicon melt having a pouring temperature of at least I200F and pouring the melt into a casting machine, the improvement consisting of introducing aluminum oxide and sodium to said hypereutectic aluminumsilicon melt to nucleate both a primary and a eutectic silicon phase in said melt upon cooling.

2. In a method of producing aluminum-silicon castings by preparation of a hypereutectic aluminumsilicon melt having a pouring temperature of at least 1200F and pouring the melt into a casting machine, the improvement comprising:

a. introducing aluminum oxide to (a) said hypereutectic aluminum-silicon melt so that throughout the melt, the aluminum oxide is surface active or otherwise acts as so influenced, uniformly dispersed and has a particle size no greater than 10 microns.

3. A method of making an improved hypereutectic aluminum-silicon alloy, comprising:

a. introducing a quantity of sintered aluminum powder rods having about 10% aluminum oxide particles uniformly distributed throughout said rods, to a hypereutectic melt of aluminum and silicon so that the aluminum oxide constitutes about 3% of said melt. and

b. allowing said melt to solidify with clouds of aluminum oxide particles acting as nucleating sites for primary silicon, the aluminum alloy having improved wear resistance and increased machineability.

4. A method as in claim 2, in which the volume content of silicon plus aluminum oxide in said melt is maintained approximately constant, while increasing the aluminum oxide to permit lowering the pouring temperature of said melt.

5. A method as in claim 3, in which the aluminum oxide particles have a size no greater than 10 microns.

6. A method as in claim 3, in which sodium is added to said melt as a refiner for the eutectic silicon and inci- 6 and finely divided.

8. A hypereutectic aluminum-silicon alloy composi tion consisting essentially of l6l 9% silicon, 3% aluminum oxide. sodium in an effective amount less than 1.5% to refine the eutectic silicon, and the remainder aluminum, the microstructure of said composition being characterized by fine primary silicon being nucleated from a cloud of aluminum oxide.

Claims

1. In a method of producing aluminum-silicon castings by preparation of a hypereutectic aluminum-silicon melt having a pouring temperature of at least 1200*F and pouring the melt into a casting machine, the improvement consisting of introducing aluminum oxide and sodium to said hypereutectic aluminum-silicon melt to nucleate both a primary and a eutectic silicon phase in said melt upon cooling.

2. In a method of producing aluminum-silicon castings by preparation of a hypereutectic aluminum-silicon melt having a pouring temperature of at least 1200*F and pouring the melt into a casting machine, the improvement comprising: a. introducing aluminum oxide to (a) said hypereutectic aluminum-silicon melt so that throughout the melt, the aluminum oxide is surface active or otherwise acts as so influenced, uniformly dispersed and has a particle size no greater than 10 microns.

3. A method of making an improved hypereutectic aluminum-silicon alloy, comprising: a. introducing a quantity of sintered aluminum powder rods having about 10% aluminum oxide particles uniformly distributed throughout said rods, to a hypereutectic melt of aluminum and silicon so that the aluminum oxide constitutes about 3% of said melt, and b. allowing said melt to solidify with clouds of aluminum oxide particles acting as nucleating sites for primary silicon, the aluminum alloy having improved wear resistance and increased machineability.

6. A method as in claim 3, in which sodium is added to said melt as a refiner for the eutectic silicon and incidentally as a beneficial shape modifier for the primary silicon.

7. A HYPEREUTECTIC ALUMINUM-SILICON ALLOY COMPOSITION CONSISTING ESSENTIALLY OF 16-18% SILICON, AL2O3 IN AN AMOUNT SUCH THAT THE SUM OF SILICON PLUS AL2O3 IS GENERALLY ABOUT 20% BY VOLUME OF THE ALLOY, SODIUM BEING PRESENT IN AN EFFECTIVE AMOUNT LESS THAN 1.5% TO REFINE THE EUTECTIC SILICON, REMAINDER ALUMINUM, SAID COMPOSITION BEING CHARACTERIZED BY BOTH THE EUTECTIC SILICON AND PRIMARY SILICON BEING UNIFORMLY DISTRIBUTED AND FINELY DIVIDED.

8. A hypereutectic aluminum-silicon alloy composition consisting essentially of 16-19% silicon, 3% aluminum oxide, sodium in an effective amount less than 1.5% to refine the eutectic silicon, and the remainder aluminum, the microstructure of said composition being characterized by fine primary silicon being nucleated from a cloud of aluminum oxide.