# Patent

## Title
Aluminum-silicon alloy having reduced microporosity and method for casting the same

## Abstract

An aluminum-silicon lost foam casting alloy having reduced microporosity and a method for casting the same is herein disclosed. A preferred lost foam cast alloy consists essentially of 6 to 12% by weight silicon and preferably 9.0 to 9.5% by weight silicon, 0.035-0.30% strontium, 0.40% maximum iron, 0.45% maximum copper, 0.49% maximum manganese, 0.60% maximum magnesium, 3.0% maximum zinc, and the balance aluminum. Most preferably, the lost foam alloy is free from iron, titanium and boron. However, such elements may exist at trace levels. Most preferably, the alloy is lost foam cast with the process that applies at least 10 atmospheres of pressure during solidification. However, the range may be 5 to 60 atmospheres. The strontium addition is greater than 0.005% by weight and most preferably greater than 0.05% by weight. In accordance with the present disclosure, alloys having substantially decreased tensile liquid failure defects and substantially decreased surface puncture defects in comparison to conventional lost foam cast aluminum silicon alloys are obtained. Further, hydrogen porosity formation is substantially completely suppressed and surface porosity defects are substantially decreased in comparison to conventional lost foam silicon alloys when casting lost foam cast alloys in accordance with the claims of the instant disclosure. The instant disclosure further discloses aluminum silicon alloys that may be utilized in both the lost foam with pressure and the die casting processes.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application is a continuation-in-part of U.S. application Ser. No. 10/429,098 filed May 2, 2003 now U.S. Pat. No. 6,923,935.


STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC
Not Applicable
BACKGROUND AND SUMMARY
Aluminum silicon (AlSi) alloys are well known in the casting industry. Metallurgists are constantly searching for AlSi alloys having high strength and high ductility and that can be used to cast various parts at a relatively low cost. Herein is described an AlSi alloy for use in a lost foam casting process and a method for utilizing the same.
Most AlSi die casting alloys contain magnesium (Mg) to increase the strength of the alloy. However, the addition of Mg also decreases the ductility of the alloy. Further, during the die casting solidification process, Mg-containing AlSi alloys experience a surface film that forms on the outer surface of the molten cast object.
Since most aluminum alloys contain some Mg (generally less than 1% by weight), it is expected that the surface film that forms is MgO—Al2O3, known as “spinel”. During the beginning of the solidification process, the spinel initially protects the molten cast object from soldering with the die casting die. However, as the molten cast object continues to solidify, the moving molten metal stretches and breaks the spinel, exposing fresh aluminum that solders with the metal die. Basically, the iron (Fe) in the dies thermodynamically desires to dissolve into the iron-free aluminum. To decrease this thermodynamic driving force, the iron content of the aluminum alloy traditionally is increased. Thus, if the aluminum alloy already contains the iron it desires (with traditionally, a 1% by weight Fe addition), the aluminum alloy does not have the same desire to dissolve the iron atoms in the dies. Therefore, to prevent die soldering, AlSi alloys, and even Mg-containing AlSi alloys, traditionally contain iron to prevent soldering of the alloy to the die casting molds. Significantly, in the microstructure of such alloys, the iron occurs as elongated needle-like phase, the presence of which has been found to decrease the strength and ductility of AlSi alloys and increase microporosity.
The solidification range, which is a temperature range over which an alloy will solidify, is the range between the liquidus temperature and the invariant eutectic temperature. The wider or greater the solidification range, the longer it will take an alloy to solidify at a given rate of cooling. During a hypoeutectic (i.e. containing <11.6% by weight Si) AlSi alloy's descent through the solidification range, aluminum dendrites are the first to form. As time elapses and the cooling process proceeds, the aluminum dendrites grow larger, eventually touch, and form a dendritic network. During this time frame, and sometimes even before the precipitation of the primary aluminum phase, the elongated iron needle-like phase also forms and tends to clog the narrow passageways of the aluminum dendritic network, restricting the flow of eutectic liquid. Such phenomena tends to increase the instance of microporosity in the final cast structure.
In general, tensile properties of any alloy decrease as the cooling rate decreases. This occurs because it is difficult to obtain a fine cast microstructure at very slow feeding rates, and because of the increased tendency for castings to be less sound if they freeze thoroughly, particularly if hydrogen porosity is not suppressed.
A high degree of microporosity is undesirable, particularly when the alloy is used for engine blocks, because high microporosity causes leakage under O-ring seals on machined head deck surfaces, and lowers the torque carrying capacity of machined threads. Further, hypoeutectic AlSi alloy engine blocks are designed to have electro-deposited material, such as chromium, on the cylinder bore surfaces for wear resistance. Microporosity prevents the adhesion of the electro-deposited chrome plating.
Similarly, AlSi alloys cast using a high pressure die casting method also result in a porous surface structure due to microporosity in the parent bore material that, if used in engine parts, is particularly detrimental because it contributes to high oil consumption. Conventionally, hypereutectic (i.e. containing >11.6% by weight Si) AlSi alloys have been used to produce engine blocks for outboard and stern drive motors in the recreation boating industry. Such alloys are advantageous for use in engine blocks as they provide a high tensile strength, high modulus, low coefficient of thermal expansion, and are resistant to wear.
Furthermore, microporosity in mechanical parts is detrimental because the microporosity decreases the overall ductility of the alloy. Microporosity has been found to decrease the ductility of a AlSi cast object, regardless of whether the object is cast from a hypoeutectic, hypereutectic, eutectic or modified eutectic AlSi alloy.
Nearly 70% of all cast aluminum products made in the United States are cast using the die casting process. As aforementioned, conventional AlSi die casting alloys contain approximately 1% by weight iron to avoid die soldering. However, the iron addition degrades mechanical properties, particularly the ductility of the alloy, and to a greater extent than any of the commercial alloying elements used with aluminum. As a result, die cast alloys are generally not recommended in an application where an alloy having high mechanical properties is required. Such applications that cannot traditionally be satisfied by the die casting process may be satisfied with much more expensive processes including the permanent mold casting process and the sand casting process. Accordingly, all AlSi die casting alloys registered with the Aluminum Association contain 1.2 to 2.0% iron by weight, including the Aluminum Association designations of: 343, 360, A360, 364, 369, 380, A380, B380, 383, 384, A384, 385, 413, A413, and C443.
Furthermore, experimentation has demonstrated that the tensile strength, percent elongation, and quality index of AlSi alloys decreases as the amount of iron increases. For example, an AlSi alloy having 10.8% by weight silicon and 0.29% by weight iron has a tensile strength of approximately 31,100 psi, a percent elongation of 14.0, and a quality index (i.e. static toughness) of 386 MPa. In contrast, an AlSi alloy having 10.1% by weight silicon and 1.13% by weight iron has a tensile strength of 24,500 psi, a percent elongation of 2.5, and a quality index of 229 MPa. In further contrast, an AlSi alloy having 10.2% by weight silicon and 2.08% by weight iron has a tensile strength of 11,200 psi, a percent elongation of 1.0, and a quality index of 77 MPa.
Therefore, it would be advantageous to reduce the iron content of die casting AlSi alloys so that the iron needle-like phases are reduced to facilitate interdendritic feeding and correspondingly reduce microporosity. However, it is also important to prevent die cast AlSi articles from soldering to die cast molds, a problem that is traditionally solved by adding iron to the alloy.
Additionally, AlSi alloys, and particularly hypoeutectic AlSi alloys, generally have poor ductility because of the large irregular shape of the acicular eutectic silicon phase, and because of the presence of the beta-(Fe, Al, Si) type needle-like phase. The aforementioned iron needles and acicular eutectic silicon clog the interdendritic passageway between the primary aluminum dendrites and hinder feeding late in the solidification event resulting in microporosity (as aforementioned) and also decrease mechanical properties such as ductility. It has been recognized that the growth of the eutectic silicon phase can be modified by the addition of small amounts of sodium (Na) or strontium (Sr), thereby increasing the ductility of the hypoeutectic AlSi alloy. Such modification further reduces microporosity as the smaller eutectic silicon phase structure facilitates interdendritic feeding.
U.S. Pat. No. 5,234,514 relates to a hypereutectic AlSi alloy having refined primary silicon and a modified eutectic. The '514 patent is directed to modifying the primary silicon phase and the silicon phase of the eutectic through the addition of phosphorus (P) and a grain refining substance. When this alloy is cooled from solid solution to a temperature beneath the liquidus temperature, the phosphorus acts in a conventional manner to precipitate aluminum phosphide particles, which serve as an active nucleant for primary silicon, thus producing smaller refined primary silicon particles having a size generally less than 30 microns. However, the '514 patent indicates that the same process could not be used with a hypereutectic AlSi alloy modified with P and Na or Sr, because the Na and Sr neutralize the phosphorous effect, and the iron content of the alloy still causes precipitation of the iron phase that hinders interdendritic feeding.
U.S. Pat. No. 6,267,829 is directed to a method of reducing the formation of primary platelet-shaped beta-phase in iron containing AlSi alloys, in particular Al—Si—Mn—Fe alloys. The '829 patent does not contemplate rapid cooling of the alloy and, thus, does not contemplate die casting of the alloy presented therein. The '829 patent requires the inclusion of either titanium (Ti) or zirconium (Zr) or barium (Ba) for grain refinement and either Sr, Na, or Barium (Ba) for eutectic silicon modification. The gist of the '829 patent is that the primary platelet-shaped beta-phase is suppressed by the formation of an Al8 Fe2 Si-type phase. Formation of the Al8 Fe2 Si-type phase requires the addition of Boron (B) to the melt because the Al8 Fe2 Si-type phase favors nucleation on mixed borides. Thus Ti or Zr and Sr, Na or Ba and B are essential elements to the '829 patent teachings, while Fe is an element continually present in all formulations in at least 0.4% by weight.
U.S. Pat. No. 6,364,970 is directed to a hypoeutectic aluminum-silicon alloy. The alloy according to the '970 patent contains an iron content of up to 0.15% by weight and a strontium refinement of 30 to 300 ppm (0.003 to 0.03% by weight). One of skill in the art understands that for this minimum amount of strontium to modify the eutectic silicon, it is absolutely imperative that phosphorus (P), which reacts with Sr and neutralizes it, must be present by less than 0.01% by weight. The hypoeutectic alloy of the '970 patent has a high fracture strength resulting from the refined eutectic silicon phase and resulting from the addition of Sr to the alloy. The alloy further contains 0.5 to 0.8% by weight manganese (Mn). Those of skill in the art will understand Mn is added to modify the iron phase to a “Chinese script” microstructure, and to prevent die soldering. The alloy disclosed in the '970 patent is known in the industry as Silafont 36. The Aluminum Handbook, Volume 1: Fundamentals and Materials. published by Aluminium-Verlag Marketing, & Kommunikation GmbH, 1999 at pp. 131 and 132 discusses the advantages and limitations of Silafont 36 and similar alloys: “ . . . ductility cannot be achieved with conventional casting alloys because of high residual Fe content. Thus new alloys such as AlMg5Si2Mn (Magsimal-59) and AlSigMgMnSr (Silafont 36) have been developed in which the Fe content is reduced to about 0.15%. In order to ensure there is no sticking [i.e. soldering], the Mn content has been increased to 0.5 to 0.8%, and this has the added, highly desirable effect of improving hot strength.”
During use, outboard marine propellers sometimes collide with underwater objects that damage the propellers. If the alloy that forms the propeller has low ductility, a propeller blade may fracture off if it collides with an underwater object of substantial size. High pressure die cast hypoeutectic AlSi alloys have seen limited use for marine propellers because they are brittle and lack ductility. Due to greater ductility, aluminum magnesium alloys are in general used for marine propellers. Aluminum magnesium alloys, such as AA 514, are advantageous as they provide high ductility and toughness. However, the repairability of such aluminum magnesium propellers is limited. The addition of magnesium to AlSi alloys has been found to increase the strength of propellers while decreasing the ductility. Thus, AlSi alloys containing magnesium are less desirable than the traditional aluminum magnesium alloys for propellers. Still, it has been found that aluminum magnesium alloys are significantly more expensive to die cast into propellers because the casting temperature is significantly higher and because the scrap rate is much greater.
For cost and geometrical tolerance reasons, propellers for outboard and stern drive motors are traditionally cast using high pressure die cast processes. Propellers may also be cast using a more expensive semi-solid metal (SSM) casting process. In the SSM process, an alloy is injected into a die at a suitable temperature in the semi-solid state, much the same way as in high pressure die casting. However, the viscosity is higher and the injection speed is much lower than in conventional pressure die casting, resulting in little or no turbulence during die filling. The reduction in turbulence creates a corresponding reduction in microporosity. Thus, it would be advantageous to be able to die cast, and particularly high-pressure die cast marine propellers.
Regardless of how marine propellers are cast, the propellers regularly fracture large segments of the propeller blades when they collide with underwater objects during operation. This is due to the brittleness of traditional propeller alloys, as discussed, above. As a result, the damaged propeller blades cannot be easily repaired as the missing segments are lost at the bottom of the body of water where the propeller was operated. Furthermore, the brittleness inherent in traditional die cast AlSi alloys prevents efficient restructuring of the propellers through hammering. Thus, it is desirable to provide a propeller that only bends, but does not break upon impact with an underwater object.
An outboard assembly consists of (from top to bottom, vertically) an engine, a drive shaft housing, a lower unit also called the gear case housing, and a horizontal propeller shaft, on which a propeller is mounted. This outboard assembly is attached to a boat transom of a boat by means of a swivel bracket. When the boat is traveling at high speeds, a safety concern is present if the lower unit collides with an underwater object. In this case, the swivel bracket and/or drive shaft housing may fail and allow the outboard assembly with its spinning propeller to enter the boat and cause serious injury to the boat's operator. Thus, it is a common safety requirement in the industry that an outboard assembly must pass two consecutive collisions with an underwater object at 40 mph and still be operational. Further, as the outboard assembly becomes more massive, this requirement becomes more difficult to meet. As a result, it is generally accepted that outboards having more than 225 HP have problems meeting industry requirements particularly if the drive shaft housings are die cast because of the low ductility and impact strengths associated with conventional die cast AlSi alloys. Accordingly, it would be highly advantageous to be able to die cast drive shaft housings with sufficient impact strength so that the drive shaft housings could be produced at a lower cost. Similarly, it would be advantageous to manufacture gear case housings and stern drive Gimbel rings for these same reasons.
In addition to the above, AlSi alloys are used in lost foam casting and lost foam casting with pressure processes to produce complex parts. However, parts manufactured using lost foam casting processes are traditionally brittle parts (such as blocks and heads) and has not been used to make damage tolerant, high impact resistant parts.
Currently, the industry standard for lost foam casting is aluminum alloy 356 (AA 356). Conventionally, during solidification, the hydrostatic tension in the liquid alloy builds up and fractures the surface skin. This fracture is due to the fact that the AA 356 alloy, with a silicon content of 6.5 to 7.5% by weight, has a relatively wide solidification range and therefore forms a relatively thin skin on the outer surface of the cast product during cooling. The skin tends to fracture because of internal liquid tension build-up, causing air to be pulled into the internal, liquid portion of the just cast product, leaving highly undesirable surface-connected porosity.
In lost foam casting methods, a foam pattern and gating system is ablated during filling and the heat from the casting must effect such ablation. U.S. Pat. No. 6,833,580 describes an apparatus and improved method for lost foam casting of metal articles using external pressure. That patent is hereby incorporated by reference. U.S. Pat. No. 6,883,580 allows for the application of super-atmospheric isostatic pressure onto a lost foam casting to reduce the hydrostatic tension in the molten alloy that produces undesirable surface porosity that forms if the hydrostatic tension in the molten alloy is not lowered.
However, even when AA 356 is cast using the advantageous method of U.S. Pat. No. 6,883,580, a substantial amount of misruns occur because of the relatively low heat of fusion of AA 356 to combat premature freezing from heat extraction during ablation of the foam pattern and gating system. Further, products cast with AA 356 have a significant amount of surface porosity and require restoration by spray welding. Accordingly, a better lost foam casting AlSi alloy is sought.
SUMMARY OF THE INVENTION
A lost foam cast AlSi alloy preferably containing by weight 6 to 12% silicon and preferably 9.0-9.5% silicon, 0.035-0.30% strontium, 0.40% maximum iron, 4.5% maximum copper, 0.49% maximum manganese, 0.6% maximum magnesium, 3.0% maximum zinc, and the balance aluminum. Most preferably, the lost foam cast alloy is free from iron, titanium and boron, however, such elements may exist at trace levels. Preferably, the alloy is lost foam cast with a process that applies at least 5 ATM of pressure during solidification and most preferably about 10 ATM of pressure during solidification.
A lost foam cast aluminum silicon alloy having the following composition and weight percent is also contemplated: 9.0 to 9.5% silicon, 0.005 to 0.30% strontium, and preferably 0.05 to 0.07% strontium, 0.30% by weight maximum iron, and preferably 0.20% by weight maximum iron, 0.05 to 4.5% by weight copper, and preferably 0.05 to 0.20% by weight copper, 0.05 to 0.50% by weight manganese, and preferably 0.25 to 0.35% by weight manganese, 0.05 to 0.60% magnesium, and preferably 0.10 to 0.20% magnesium, 3.0% by weight maximum zinc, and the balance aluminum. Preferably, this alloy is cast using a lost foam casting process that applies at least 5 ATM of pressure during solidification and most preferably about 10 ATM of pressure during solidification.
Further, a method of lost foam casting is contemplated. A polymeric foam pattern corresponding in configuration to an article to be cast is positioned in a vessel. A polymeric foam gating system is connected to the pattern in the vessel. A finely divided inert material, such as sand, is introduced into the vessel to surround the pattern and gating system and to fill the internal cavities in the pattern. A pouring cup is positioned in the vessel and the pouring cup is connected with the gating system. A molten aluminum silicon alloy consisting essentially of 6 to 12% by weight silicon, at least 0.005% by weight strontium, 0.40% by weight maximum iron, 4.5% by weight maximum copper, 0.49% by weight maximum manganese, 0.60% by weight maximum magnesium, 3.0% by weight maximum zinc, and the balance aluminum is poured into the pouring cup. The molten alloy operates to decompose the gating system and pattern with the molten alloy filling the void created by decomposition of the polymeric foam material. The products of decomposition pass into interstices of the finely divided material. The pressure vessel is then sealed with the pouring cup retained within the vessel and, simultaneously, a pressure equalization member is positioned over the pouring cup and in contact with the inert material. An external super-atmospheric pressure is then applied to the inert material in the vessel and accordingly to the molten alloy in the pouring cup. The pressure is maintained in the inert material and molten alloy until the molten alloy solidifies to produce a cast article corresponding in configuration to the pattern.
The method described above may alternatively comprise the step of pouring the molten aluminum silicon alloy consisting essentially of 9.0 to 9.5% by weight silicon and 0.05 to 0.07% by weight strontium, 0.30% by weight maximum iron, 0.20% by weight maximum copper, 0.25 to 0.35% by weight manganese, 0.10 to 0.20% by weight magnesium, and the balance aluminum. The method may further alternatively comprise the step of pouring the molten aluminum silicon alloy consisting essentially of 9.0-9.5% by weight silicon, 0.05-0.07% by weight strontium, 0.25% by weight maximum iron, 0.20% by weight maximum copper, 0.50% by weight maximum manganese, 0.35-0.45% by weight magnesium and the balance aluminum. In the method described herein, the step of sealing the pressure vessel and applying external pressure may comprise sealing the pressure vessel and applying external pressure immediately after the pouring cup is filled. The step of connecting the pouring cup preferably comprises connecting a pouring cup having a volume in the range of 25% to 75% of the combined volume of the pattern and gating system. The step of applying external pressure preferably comprises applying external pressure in the range of 5 to 60 ATM, and most preferably, at approximately 10 ATM.
The present disclosure is further directed to an aluminum silicon lost foam cast alloy having 70-93.965% by weight aluminum, 6-12% by weight silicon, 0.40% by weight maximum iron, 4.5% by weight maximum copper, 0.49% by weight maximum manganese, 0.60% by weight maximum magnesium, 3.0% by weight maximum zinc and the balance strontium of at least 0.035% by weight. This alloy is preferably cast with a lost foam casting process that applies at least 5 ATM of pressure during solidification of the alloy and also preferably applies approximately 10 ATM of pressure during solidification of the alloy. The constituency of the above noted alloy may be modified to comprise 9.0-9.5% by weight silicon and 0.20% by weight maximum iron.
The instant disclosure further provides for an aluminum silicon die cast alloy consisting essentially of 65-93.995% by weight aluminum, 6-22% by weight silicon, 0.40% by weight maximum iron, 4.5% by weight maximum copper, 0.49% by weight maximum manganese, 0.60% by weight maximum magnesium, 3.0% by weight maximum zinc and the balance strontium of at least 0.005% by weight. Such alloy substantially reduces soldering to die cast dies during the die casting process compared to conventional aluminum silicon alloys.
The instant disclosure further provides for an aluminum silicon alloy comprising 6-22% by weight silicon, 0.40% by weight maximum iron, and 0.035-1.0% by weight of an element from the group consisting of: lithium, beryllium, sodium, potassium, rubidium, strontium, cesium, barium, francium, radium, lead and bismuth; and the balance aluminum. The alloy may further comprise 4.5% by weight maximum copper, 0.50% by weight maximum manganese, 0.6% by weight maximum magnesium, and 3.0% by weight maximum zinc. The alloy may be die cast, and if die cast, the alloy substantially reduces soldering to die casting dies during the die casting process, compared to conventional aluminum silicon alloys. The above noted alloy may further be cast using the lost foam casting process that applies at least 5 ATM of pressure and such application substantially reduces surface porosity defects in comparison to conventional lost foam cast aluminum silicon alloys. If lost foam cast, the above disclosed alloy is preferably cast with an external pressure applied at approximately 10 ATM.
The alloys described above have substantially decreased tensile liquid failure defects in comparison to conventional lost foam aluminum silicon alloys, such as AA 356. The alloys also have substantially decreased surface puncture defects in comparison to conventional lost foam cast aluminum silicon alloys. Further, surface porosity defects are substantially decreased in comparison to conventional lost foam cast aluminum silicon alloys. It is believed that the substantial decrease in the above noted defects results from the substantially decreased solidification range of the alloys described herein in comparison to conventional lost foam cast aluminum silicon alloys.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention is described in relation to some examples and with reference to the accompanying figures in which:
FIG. 1 is a graph demonstrating the comparative impact strength of propellers die cast from AA 514 and die cast from an alloy as further described herein.
FIG. 2 is a graph demonstrating the comparative impact strength of a die cast alloy as further described herein relative to AA 514 and Silafont 36.
FIG. 3 is a graph from the American Society for Metals demonstrating the effect of added elements on the surface tension of aluminum.
FIG. 4 is a perspective view of a driveshaft housing manufactured from the XK360 alloy that was subjected to a static load until the driveshaft housing failed.
FIG. 5 is a perspective view of a driveshaft housing die cast from an alloy as further described herein that was subjected to the same and higher static load as the driveshaft housing of FIG. 4.
FIG. 6 is a graph demonstrating solidification characteristics of an aluminum silicon alloy relative to temperature and silicon content.



Various other features, objects, and advantages of the invention will be made apparent from the following detailed description.
DETAILED DESCRIPTION
A preferred AlSi lost foam cast alloy in accordance with the instant disclosure has the following formulation in weight percent:














Element
Range of Percentages









Silicon
6 to 12%



Strontium
0.035 to 0.30%



Iron
0.40% maximum



Manganese
0.49% maximum



Magnesium
0.60% maximum



Copper
4.5% maximum



Zinc
3.0% maximum



Aluminum
Balance










Another preferred AlSi lost foam cast alloy of the instant disclosure has the following formulation in weight percent:











Element
Range of Percentages







Silicon
9.0 to 9.5%


Strontium
0.005 to 0.30%


Iron
0.30% maximum; more preferably 0.2% maximum


Copper
0.05 to 4.5%


Manganese
0.05 to 0.50%


Magnesium
0.05 to 0.6%


Zinc
3.0% maximum


Aluminum
Balance









Yet another preferred AlSi lost foam cast alloy has the following formulation in weight percent:














Element
Range of Percentages









Silicon
9.0 to 9.5%



Strontium
at least 0.005%



Iron
0.40% maximum, preferably 0.20%



Copper
0.05% 0.20%



Manganese
0.25 to 0.35%



Magnesium
0.10 to 0.20%



Aluminum
Balance










Strontium percentages may be narrowed to 0.05 to 0.07% by weight strontium to optimally modify any trace of iron that may be present in the alloy. At this most preferred range, the strontium dissolves in an iron needle-like phase and changes the angle of the monoclinic cell and may even change the crystal structure of this iron phase. The copper constituency may be in the range of 2.0 to 4.5% by weight or may be as small as a 0.25% by weight, max., depending on the corrosion protection qualities that the metallurgist intends to impart on the cast product. Finally, the magnesium may be as low as 0.10% by weight maximum as magnesium is not necessary to prevent die soldering, and the low levels of magnesium increases the ductility of the alloy.
An AlSi alloy may also be formulated according to the instant specification for die casting hypereutectic aluminum-silicon alloy engine blocks, the AlSi alloy having the following formulation and weight percent.














Element
Range of Percentages









Silicon
16.0 to 22%



Strontium
0.05 to 0.10%



Iron
0.35% maximum



Copper
0.25% maximum



Manganese
0.30% maximum



Magnesium
0.60% maximum



Aluminum
Balance










Preferably the die cast alloy contains 18 to 20% by weight silicon and further comprises a hypereutectic microstructure, with polygon shaped primary silicon particles embedded in a eutectic with a modified eutectic silicon phase. In contrast, die cast hypereutectic AlSi alloys that are phosphorus refined contain polygon-shaped primary silicon particles embedded in a eutectic, wherein the eutectic silicon phase is not modified. Thus, the die casting hypereutectic engine blocks produce a unique microstructure for the above disclosed hypereutectic alloys.

An aluminum silicon alloy may also be formulated according to the instant specification for either a lost foam cast alloy or a die cast alloy, the aluminum silicon alloy having the following formulation and weight percent:














Element
Range of Percentages









Aluminum
65-93.995%



Silicon
6-22%



Iron
0.40% maximum



Copper
4.5% maximum



Manganese
0.49% maximum



Magnesium
0.60% maximum



Zinc
3.0% maximum



Strontium
Balance of at least 0.005%










If the aluminum silicon alloy is to be cast using a lost foam casting process, the alloy preferably contains 70-93.965% by weight aluminum, 6-12% by weight silicon, and at least 0.035% by weight balance strontium. The aluminum range may further be narrowed to 9.0 to 9.5% by weight and the iron constituency may be lowered to 0.20% by weight maximum iron.
An aluminum silicon alloy for either die casting or lost foam casting may be produced in accordance with the instant disclosure with the alloy comprising 6-22% by weight silicon, 0.40% by weight maximum iron, 0.035-1.0% by weight of an element in the group consisting of: lithium, beryllium, sodium, potassium, rubidium, strontium, cesium, barium, francium, radium, lead and bismuth; and the balance aluminum. Such an alloy may further comprise 4.5% by weight maximum copper, 0.50% by weight maximum manganese, 0.60% by weight maximum magnesium and 3.0% by weight maximum zinc. Such an alloy will substantially reduce soldering to die casting dies during the die casting process, compared to conventional aluminum silicon alloys, as further described herein. If the alloy is lost foam cast, it is cast with a lost foam casting process that applies at least 5 ATM of pressure, and preferably, approximately 10 ATM of pressure during solidification. With such a lost foam cast alloy, surface porosity defects are substantially decreased in comparison to conventional lost foam cast aluminum silicon alloys, as further described herein.
As one of skill in the art will notice from the formulations set forth above, a wide range of silicon percentages may exist for the above described aluminum alloys. It is contemplated that the eutectic composition of a die cast AlSi alloy can shift from 11.6 to 14% by weight silicon because of the rapid die casting cooling rates and because of the high strontium content. Thus, the microstructure of an alloy may be a modified eutectic silicon phase in either a eutectic aluminum-silicon microstructure, a hypoeutectic aluminum-silicon microstructure or a hypereutectic aluminum-silicon microstructure.
Further, all AlSi alloys specified above as die cast alloys are not grained refined and are therefore substantially free from any grain refinement elements such as titanium, boron or phosphorus.
As an aluminum alloy according to the present invention is cooled from solution to a temperature below the liquidus temperature, aluminum dendrites begin to appear. As the temperature decreases and solidification proceeds, the dendrites increase in size and begin to form an interdendritic network matrix. Additionally, if iron is present, iron phases form concurrently during solidification or prior to the primary aluminum precipitation.
The high levels of strontium significantly modify the microstructure of the alloy and promote a non-wetting condition to avoid soldering in the die casting applications because the strontium increases the surface tension of the aluminum alloy solution. The strontium addition of greater than 0.005% by weight, preferably greater than 0.035% by weight, more preferably 0.05% to 0.10% by weight and most preferably 0.05 to 0.07% by weight effectively modifies the eutectic silicon and provides monolayer coverage of the molten surface with strontium atoms which effectively produces the non-wetting condition to avoid soldering to die cast dies. In a conventional, unmodified, hypoeutectic, die cast AlSi alloy, the eutectic silicon particles are large and irregular in shape. Such large eutectic silicon particles precipitate into large acicular shaped silicon crystals in the solidified structure, rendering the alloy brittle. The strontium addition increases the surface tension of aluminum. As a result, eutectic silicon does not nucleate at the eutectic temperature, but the primary aluminum continues to grow. Eventually, the silicon precipitates after a significant amount of undercooling has developed. This produces a refined eutectic silicon morphology called the modified eutectic silicon microstructure, that is entirely different from the unmodified eutectic silicon microstructure that nucleates at the eutectic temperature with no undercooling.
Furthermore, and quite unexpectedly, the strontium addition of at least 0.005% by weight, preferably greater than 0.035% by weight and most preferably 0.05-0.07% by weight modifies the iron phase shape morphology, if iron is present, by dissolving the iron phase. Conventionally, the iron phase morphology is needle-like in shape. The strontium addition modifies the iron phase morphology by reducing the iron needles of the microstructure into smaller, blocky particles.
The presence of modified eutectic silicon and the iron phase morphology change has significant effects on interdendritic feeding. The reduction in size of the eutectic silicon particles, along with the reduction in size of the iron phase structures, greatly facilitates liquid metal movement through the interdendritic aluminum network during cooling. As a result, the increased interdendritic feeding has been found to significantly reduce the microporosity in cast engine blocks.
The lowering of the microporosity in the microstructure of the cooled AlSi alloy product greatly reduces the number of engine blocks that fail to meet porosity specifications. Microporosity is undesirable as it results in leakage of O-ring seals, reduction in the strength of threads, surfaces incapable of metal plating during production, and for parent bore applications, high oil consumption. Thus, engine blocks with substantial microporosity defects are scrapped. With the alloy described herein, it is anticipated that a scrap reduction of up to 70% may be obtained solely through the use of the die cast alloy described herein. The reduction of blocks that fail to meet the porosity specification corresponds to the reduction in amount of blocks scrapped, which in turn, results in a more highly economic production of cast engine blocks.
Additionally, the other elements present in the alloy formulations described above, contribute to the unique physical qualities of the final cast products. Specifically, elimination of grain refining elements prevents detrimental interaction between such elements and the highly reactive strontium.
Turning now specifically to die cast applications, the AlSi die cast alloys described herein also have the unexpected benefit of not soldering to dies during the die casting process, even though the iron content is substantially low. Traditionally, approximately 1% iron by weight was added to AlSi die cast alloys to prevent the thermodynamic tendency of the iron from the die casting dies to dissolve into the molten aluminum. The die castings made with the substantially iron-free alloys described herein have dendritic arm spacings smaller than either permanent mold or sand castings and possess mechanical properties superior to products produced in the permanent mold casting or sand casting processes.
During the die casting process, a surface layer oxide film forms on the outer surface of the molten cast object as the alloy is cast and exposed to the ambient environment. When AlSi alloys are die cast, a film of alumina Al2O3 forms. If the alloy contains Mg, the film is spinel, MgO—Al2O3. If the alloy contains more than 2% Mg, the film is magnesia MgO. Since most aluminum die cast alloys contain some magnesium, but less than 1%, it is expected that the film on most aluminum alloys is spinel. Such alloys solder to die cast dies because the moving molten metal in a just-cast alloy breaks the film and exposes fresh aluminum to the iron containing die which results in soldering.
Ellingham diagrams, which illustrate that the free energy formation of oxides as a function of temperature, confirm that alkaline earth elements of group IIA (i.e. beryllium, magnesium, calcium, strontium, barium and radium) form oxides so stable that alumina can be reduced back to aluminum and the new oxide takes its place on the surface of the aluminum alloy. Thus, in the die cast alloys described herein where very low levels of magnesium and iron are present, an aluminum-strontium oxide replaces protective alumina or even spinel film, preventing die soldering.
Additions of alkaline earth elements other than strontium were tested to see if such elements provided the same protection that strontium affords in the die cast process. For example, additions of beryllium, though highly hazardous to health, at levels of 50 ppm by weight caused the protective properties of the film on an aluminum-magnesium alloy melt to improve significantly, with the result being that oxidation losses are reduced. However, even with these improvements of the oxide coating against oxidation losses, beryllium containing die casting alloys (up to 0.005% Be) experience the soldering problem in the die casting process. However, it is expected that high levels of beryllium (i.e., greater than 0.005% by weight) will provide the same anti-soldering resistance feature that strontium has demonstrated. Likewise, lithium, potassium, rubidium, cesium, francium, radium, and particularly sodium are elements that dynamically raise the surface tension of an aluminum silicon alloy melt when added in the amount of 0.005-1.0% by weight. Furthermore, lead and bismuth may be advantageously added in a similar manner to beryllium. However, as one of skill in the art will realize, the toxicity of such additions must be evaluated in the context of when and where the alloy to be produced is to be used.
It is contemplated that when AlSi die cast alloys having high strontium concentrations (i.e., greater than 0.035% by weight) and a low iron content, alloy melts will be produced with thicker oxide films on them. Further, the melt side of the oxide films is “wetted” which means that the film will be in perfect atomic contact with the liquid melt. As such, this oxide film will adhere extremely well to the melt, and, therefore, this interface will be an unfavorable nucleation site for volume defects such as shrinkage porosity or gas porosity. In contrast, the outer surface of the oxide film originally in contact with air during the die casting process will continue to have an associated layer of adhering gas. This “dry” side of the oxide film is not likely to know when it is submerged, and therefore, will actively remove traces of any oxygen of any air in contact with it, consequentially causing the strontium oxide to continue to grow. Thus, the gas film will eventually disappear, resulting in contact of the die and strontium oxide coated molten aluminum. Effectively, the driving thermodynamic forces changed for soldering at the die interface and a dynamic oxide barrier coating or monolayer at the interfaces is formed.
Thermodynamically, at infinite dilution, the free energy of formation of any solution from its pure components decreases at an infinite rate with increase in the mole fraction of solute. This is tantamount to stating that there is always a thermodynamic driving force toward some mutual dissolution of pure substances to form a solution. Accordingly, unalloyed aluminum has a strong thermodynamic tendency to take into solution the iron in the steel dies commonly used in the die casting process. This also explains why metallurgists add approximately 1% iron to die cast AlSi alloys, as this addition drastically decreases the aluminum's tendency to want to take into solution more iron from the die. The problem with this solution is that the iron used to avoid die soldering decreases mechanical properties, particularly ductility and impact properties, of the die cast aluminum alloy. This is because the iron, which has a very low solubility in aluminum (approximately 38 ppm) appears in the microstructure with a “needle-like” phase morphology. The needle-like morphology may be modified to “Chinese script” morphology with the addition of manganese. A manganese addition, by modifying the needle-like morphology of the iron phase, helps increase ductility and impact properties, but does not provide the same advantages as if low manganese, high strontium, and slightly higher iron was used in the AlSi die cast alloy, because the modified manganese-iron phases are still “stress risers” in the microstructure. In fact, U.S. Pat. No. 6,267,829 to Backerud et. al points out that the total amount of iron containing inter-metallic particles increases with increasing amounts of manganese added, and further quotes from “The Effects of Iron in Aluminum-Silicon Casting Alloys—A Critical Review” by Paul N. Creapeau (no date) that Creapeau has estimated that 3.3 volume % inter-metallic form for each weight percent total (% Fe+% Mn+Cr) with a corresponding decrease in ductility.
To illustrate this point, an alloy according to U.S. Pat. No. 6,364,970 (i.e. Silafont 36) was die cast having the following composition: 9.51% by weight silicon, 0.13% by weight magnesium, 0.65% by weight manganese, 0.12% by weight iron, 0.02% by weight copper, 0.04% by weight titanium, 0.023% by weight strontium, balance aluminum. This high manganese AlSi alloy was compared in a drop impact test with a die cast alloy as described herein with the following constituencies: 9.50% by weight silicon, 0.14% by weight magnesium, 0.28% by weight manganese, 0.20% by weight iron, 0.12% by weight copper, 0.0682% by weight strontium, trace amounts of titanium, and balance aluminum. Both such alloys were further compared with AA 514, as demonstrated in FIG. 2. In spite of the fact that the iron was lower for the alloy composition having high manganese, and in spite of the fact that such alloy had a high manganese content to modify the iron phase morphology, the drop impact properties were not as substantial as the die cast alloy described herein. It was found that the die cast alloy described herein, with a 67% higher iron content and a 57% lower manganese content, had much higher impact properties. See, FIG. 2. The conclusion is that the higher impact properties are due to the 200% higher strontium content.
It is well known that the surfaces of phases (i.e. liquid phase or solid phase) generally differ in behavior from the bulk of that same phase because rapid structural changes occur at and near phase boundaries. Accordingly, surfaces have a higher amount of energy associated therewith. The excess energy associated with surfaces is minimized by reducing surface area and by reducing surface energy. Since only a small fraction of the overall materials is associated with the surface, only very small amounts of impurities are required to saturate the surface. It has been reported by Sumanth Shankar and Makhlouf M. Makhlouf in WPI Advanced Casting Research Center May 25, 2004 Report No. Pr.04-1 entitled Evolution of the Eutectic Microstructure During Solidification of Hypoeutectic Aluminum Silicon Alloys that 230 ppm strontium increases the solid/liquid surface energy (γ) from 0.55 N/m to 1.62 N/m at 598 degrees Celsius; from 1.03 N/m to 2.08 N/m at 593 degree Celsius; from 1.39 N/m to 2.59 N/m at 588 degree Celsius; and from 2.24 N/m to 3.06 N/M at 583 degree Celsius. For a constant strontium content, the natural log of these surface energy measurements varies linearly with the natural log of the temperature in degrees Kelvin, as follows:
Modified Al—Si Alloy: In γ=−36.728 ln(T)+249.14; R2 fit parameter=0.9911
Unmodified AlSi Alloy: In γ=−80.042 ln(T)+541.48; R2 fit parameter=0.9928.
Based on these surface energy measurements, it is clear that approximately 200 ppm of strontium can double or triple the solid/liquid surface energy. Thus, the Shankar/Makhlouf findings suggest that 0.05 to 0.10% by weight strontium may increase the surface energy of an alloy by an order of magnitude. Therefore, the surface energy increase associated with a strontium addition favors non-wetting of the molten aluminum and the steel dies. This behavior can be likened or compared to the behavior of droplets of mercury (Hg) versus the behavior of water, the latter which tends to spread out and “wet” a surface.
Since soldering is most likely to occur in the die casting process under conditions that favor wetting, part of the benefit of using high strontium containing AlSi die cast alloys is the non-wetting conditions that are produced by the strontium effect on the solid/liquid surface energy. It is further postulated that the high reactivity of strontium in liquid aluminum solution for oxygen is a factor influencing the low iron or iron free AlSi alloys so that the thermodynamic forces tending to dissolve the iron and soldering with the steel does not develop.
Based on a thermodynamic treatment of interfaces, the Gibbs adsorption equation (i.e. the Gibbs adsorption isotherm) expresses the fact that adsorption or desorption behavior of a solute and liquid metals can be assessed by measuring the surface tension of a metal as a function of solute concentration. According to the Gibbs adsorption equation, the excess surface concentration of a solute in a two-component system at constant temperature and pressure is given by:




  
    Γ
    s
  
  =
  
    
      -
      
        d
        
          
        
        ⁢
        γ
      
    
    
      RT
      ⁢
      
        d
        
          (
          
            ln
            ⁢
            
              
            
            ⁢
            
              a
              s
            
          
          )
        
      
    
  




where Γs is the excess surface concentration of solute per unit area of surface, γ is the surface tension, as is the activity of solute “s” in the system, R is the gas constant, and T is the absolute temperature in degrees Kelvin. In dilute solutions, the solute activity, as can be replaced by the solute's concentration in terms of weight percent. Therefore, at low concentrations of solute, i.e. for strontium in the alloys of the present invention, Γs to be taken to equal surface concentration of solute per unit interfacial area. As the Gibbs adsorption equation indicates, the excess surface concentration Γs can be assessed from the slope of the experimentally determined:





  
    d
    γ
  
  
    d
    
      (
      
        ln
        ⁢
        
          
        
        ⁢
        
          a
          s
        
      
      )
    
  




curve for





  
    d
    γ
  
  
    d
    
      (
      
        ln
        ⁢
        
          
        
        ⁢
        x
      
      )
    
  




values, where x is the weight percent.

Carefully obtained surface tension measurements made for an unmodified and modified AlSi alloy for four different temperatures by Shankar and Makhlouf determined that strontium additions of 230 ppm raised the isothermal surface tension of aluminum significantly higher for the modified alloy than the unmodified alloy. Further, Shankar's and Makhlouf's R2 goodness of fit parameter for the temperature dependence for the surface tensions was 0.9928 for the unmodified AlSi alloy and was 0.9911 for the modified AlSi alloy, which indicates an excellent fit.
Applying the teachings of Shankar and Makhlouf to the alloys described herein indicates that strontium increases the surface tension of aluminum. A closer inspection of Shankar's and Makhlouf's data demonstrates the following:













Temperature (K)












871
866
861
856















Change in Surface Tension (N/m)
1.07
1.05
1.20
0.82


(modified minus unmodified)









Thus, the average change in surface tension is 1.035 N/m with a coefficient of variation of only 15%. Since the unmodified alloy in Shankar's and Makhlouf's investigation had a strontium content two orders of magnitude lower than that of the modified alloy, of approximately 0.00023% by weight, the following is true:




  
    
      d
      γ
    
    
      d
      
        ln
        ⁡
        
          (
          x
          )
        
      
    
  
  =
  
    
      1.035
      
        (
        
          
            ln
            ⁢
            
              
            
            ⁢
            0.0230
          
          -
          
            ln
            ⁢
            
              
            
            ⁢
            0.00023
          
        
        )
      
    
    =
    
      
        1.035
        4.605
      
      =
      
        0.225
        ⁢
        
          
        
        ⁢
        N
        ⁢
        
          /
        
        ⁢
        m
      
    
  




Applying this information to the Gibbs adsorption equation where R equals 8.31451 J/K/mole, and where the average temperature equals 863.5 K, the excess concentration of strontium atoms,




  
    Γ
    s
  
  =
  
    
      
        -
        
          d
          γ
        
      
      
        RT
        ⁢
        
          
        
        ⁢
        
          d
          
            
          
          ⁢
          
            ln
            ⁡
            
              (
              x
              )
            
          
        
      
    
    =
    
      0.225
      
        
          (
          8.31451
          )
        
        ⁢
        
          (
          863.5
          )
        
      
    
  




=31.3×10−6 moles/m2. Therefore, the area per strontium atoms at the surface is the reciprocal of (31.3×10−6 moles/m2) (6.02×1023 atoms/mole), which is 5.31×10−20 m2/atom or 5.31 square Angstroms per atom.

The limiting concentration in a close packed monolayer of strontium atoms (Pauling atoms radius r=1.13×10−10 m for Sr+2 ions) is estimated to be 2√3r2=4.42×10−20 m2/atom. This corresponds to 37.54×10−6 moles per m2. A comparison with the surface strontium concentration in the monolayer of 31.3×10−6 moles per meter squared (as calculated with the Gibbs adsorption isotherm) indicates either an 83.4% coverage, an imperfect monolayer is formed, or the assumption of close packing in the monolayer is incorrect.
Those who are skilled in the art will recognize that the above postulates are suggestions for a strontium concentration of 230 ppm at a pressure of 1 atmosphere. The present disclosure suggests a strontium concentration of preferably 500-1,000 ppm ensuring full coverage by the surface monolayer. Further, knowing the aluminum-strontium phase diagram, and understating strontium's very limited solubility in aluminum, Al4Sr tetragonal phase is expected to occur in the microstructure of the alloy. This Al4Sr tetragonal phase has an a-lattice parameter of 4.31 Angstroms and a c-lattice parameter of 7.05 Angstroms. Thus, the Al4Sr tetragonal phase is not expected to exhibit a close packed plane in the solid state for any interface. However, the discussion of the surface monolayer and the AlSi alloy of the present invention pertains to the alloy in a liquid state, not a solid state. Also, the application of high pressures are present in die casting on the liquid, incorporating LeChatelier's principle. This principle states that if a system is displaced from equilibrium through the application of a force, that system will move in the direction that will reduce that force. Thus, because rapid structural changes occur in the surface layer compared to the bulk, it is postulated that the die casting pressures and/or lost foam casting under at least 10 ATM pressure is sufficient to cause a liquid monolayer of strontium atoms at the surface of the molten alloy to be close packed.
It is appreciated by those of skill in the art that when an element appears to concentrate in a surface layer on aluminum, there is an accompanying reduction in surface tension. This is illustrated in FIG. 3. FIG. 3 is taken from the text entitled Aluminum, Properties and Physical Metallurgy, page 209, published by the American Society for Metals, 1984. FIG. 3 demonstrates that apparently all elements except strontium appear to lower the surface tension of aluminum as they are dissolved in aluminum. Surprisingly, in dilute solutions, even a high-surface tension solute, such as a high-melting point metal, is expected to have little effect on the surface tension of aluminum solutions.
In contrast to this general phenomena, D. A. Olsen and D.C. Johnson, (J. Phys. Chem. 67, 2529, 1963; reported in The Physical properties of Liquid Metals by T. Iida and Roderick I. L. Guthrie, Clarendon Press Oxford, 1988) have studied the surface tension of mercury-thallium amalgams as a function of thallium content and found an increase in surface tension for amalgams with thallium content greater than that of the eutectic composition. The authors explained that if there are components in the melt that form compounds that are less stable in the surface layer than in the bulk, the surface tension of the mixture may be higher than that of the pure components. Thus, the authors conclude that it would appear that a mercury-thallium compound is formed that might be concentrated in the bulk of the amalgam. The formation of such a compound would remove thallium atoms on the surface layers and thereby raise surface tension values.
Using similar reasoning, it is suggested that in the AlSI alloys described herein, the aluminum-strontium compound, Al4Sr, like the mercury-thallium compound, is unstable in the surface monolayer for thermodynamic reasons, specifically, because the strontium atoms want to diffuse away from the surface monolayer. It is further suggested that to avoid die soldering, a close-packed monolayer of strontium atoms exhibiting nearly 100% coverage because of the preferred 500 to 1,000 ppm strontium content, is in place in a dynamic fashion. It is further postulated that the dynamic characteristic of the surface monolayer occurs partially because of the high pressures involved with die casting and/or lost foam casting under pressure. The close-packed surface monolayer creates non-wetting conditions and make it considerably more difficult for soldering to occur in the die cast applications, eliminating the need for iron in the die cast alloys described herein and preventing die soldering.
The qualities of the alloys described above lend the alloys described herein to be particularly useful in the lost foam casting process. It has been found that substantial strength and impact properties can be obtained at slower cooling rates typical of permanent mold in sand cast base processes, particularly lost foam casting processes. Most preferably, the lost foam casting process and apparatus for effectively accomplishing this process is described in U.S. Pat. No. 6,883,580, which is incorporated herein by reference.
The coupling of a high impact resistant, damage tolerant alloy, such as the alloys described herein, with the lost foam casting with pressure processes described in U.S. Pat. No. 6,833,580 achieve several benefits. The lost foam casting technology is extended from previously making only brittle parts to making damage tolerant, high impact resistant parts, such as brackets, mid-sections, and possibly automotive structural frame elements. The combination also reduces the defects on the surface of parts cast using a lost foam casting process with pressure. Because most parts cast using the lost foam casting process with pressure are stressed in bending, the reduction of surface defects in the highest stressed areas are decreased, thereby improving performance and reliability. Also, the surfaces and near surface regions of many lost foam with pressure cast parts are machined and sealed against gaskets. The reduction of surface defects prevents leakage and failure of such seals. Cast defects in sealing surfaces are a significant cause of casting scrap and re-work, and reduction of such defects is extremely cost effective.
Additionally, the alloys described in the present disclosure can be advantageously used in both the die casting process and the lost foam casting process with pressure. This can potentially simplify production as the number of alloys at a plant may be reduced.
In open cavity casting processes (i.e., not lost foam), as one goes from sand casting to permanent mold casting to die casting, the heat transfer co-efficient increases. As a result, as one moves from sand casting to permanent mold casting to die casting, the silicon content of the alloy is increased. As demonstrated in FIG. 6, as the silicon content of AlSi alloys increases, the solidification range of such alloys decreases. Narrow solidification range AlSi alloys are said to be skin forming alloys, whereas AlSi alloys having a wide solidification range are not considered skin forming alloys.
In lost foam casting processes, a foam pattern and gating system must be ablated during filling and the molten must provide sufficient heat to ablate the foam pattern and gating system. The lost foam cast alloy and method described herein provides increased silicon content relative to traditional lost foam cast alloys. The increased silicon content provides fluid life to the molten aluminum silicon alloy allowing the efficient ablation of the foam pattern and gating system and avoids misruns, that is, premature freezing of the alloy due to use of all the heat energy of the alloy in ablating the foam pattern. This increased silicon content also decreases the solidification range and, therefore, increases skin forming tendencies.
When an aluminum silicon alloy has a narrow solidification range and is, therefore, a skin forming alloy, the skin forms around the molten liquid and the solid/liquid interface is very strong. While the skin forms, molten liquid is contained within the skin, and this molten liquid desires to contract but is constrained by the skin and solid/liquid interface. Accordingly, the liquid goes into a state of hydrostatic tension, and eventually a tensile liquid failure occurs which cuts off any further feeding.
When a region of a casting is not well fed, the internal hydrostatic tension will increase, reaching a level at which either an internal pore will form because of the tensile failure of the liquid, or a surface puncture or skin fracture will occur, pulling air into the site of high hydrostatic tension and creating surface porosity. This is disadvantageous because, in particular, skin fractures and the subsequent surface connected porosity is the primary reason castings leak and are rejected as scrap. However, if the melt quality is good and efficient feeding occurs, the isostatic pressure lowers the hydrostatic pressure in the molten metal and no internal pore can easily form. The rise in the isostatic pressure will then cause a more general collapse of the casting skin. This general collapse of the casting skin is known by those of skill in the art as solid feeding. Solid feeding is generally desired, however, the skin must remain intact for solid feeding and the hydrostatic tension of the molten alloy must remain less than a specific critical value, which is facilitated by application of isostatic pressure, to allow for this solid feeding. Therefore, avoiding a premature failure of the skin is of paramount importance.
Accordingly, two major types of failure are identified as skin forming AlSi alloys. Tensile liquid failure occurs when the skin is intact and very strong. This creates internal porosity when the hydrostatic tension in the molten liquid within the solid skin becomes so high that the molten liquid fails in tension, effectively cutting off feeding throughout the casting. Surface puncture defects occur when the skin is fractured due to the high hydrostatic tension in the liquid. Air is drawn into the liquid metal and serious surface connected porosity defects occur. Finally, the solid skin may deform under the hydrostatic tension of the molten liquid, causing solid feeding.
The aluminum silicon alloy and method casting described in the instant specification alleviates the above discussed mechanisms of failure. The application of pressure during the lost foam casting process helps reduce tensile liquid failure by decreasing the hydrostatic tension of the molten liquid within the formed skin. This has the added benefit in that the formation of hydrogen porosity is completely suppressed, a feature absent from all other known sand cast processes. The higher silicon content, by decreasing the solidification range, helps create a stronger skin and stronger solid/liquid interface preventing surface puncture failures and increasing the odds of solid feeding. The application of super-atmospheric pressure also allows the solidification event to be prolonged because the hydrostatic tension is decreased and, therefore, the feeding period is extended. Further, skin failure is prevented with a planar solidification front of the type most commonly associated with a modified eutectic AlSi alloy. The alloy according to the instant specification provides a planar solidification front through the high levels of strontium.
As discussed in the Background, the tensile properties of an aluminum silicon alloy decrease as the cooling rate decreases. This occurs because (1) it is difficult to obtain a fine cast microstructure at very low freezing rates, and (2) the increased tendency of castings to be less sound if they freeze slowly, particularly if hydrogen porosity is not suppressed. Thus, the addition of strontium, from a thermodynamic standpoint, should enhance the deleterious effect of hydrogen pick-up. Further, it is generally accepted that modification improves the properties of heavily chilled castings, but, for sand castings, there is a deterioration of properties in unchilled castings due to lack of soundness caused by the increased tendency for porosity in modified alloys. In sharp contrast to the above expectation, the application of at least 5 ATM of pressure, and preferably 10 ATM of pressure in the slowly cooled, lost foam casting process of the instant specification substantially completely suppressed the formation of hydrogen porosity. Thus, the benefits of the modified eutectic silicon phase are obtained without the deleterious effects of increased hydrogen porosity. The isostatic pressure also allows for much higher levels of strontium to be used. These higher levels of strontium dissolve in the needle-like iron phase and change the iron phase's morphology in such a way that feeding is enhanced.
Accordingly, the alloy and method of the instant specification, which applies an isostatic pressure that decreases the hydrostatic tension in the liquid phase of a just-cast product delays tensile liquid failure. The alloy and method, therefore, enhances feeding and the production of porosity free castings. Furthermore, the alloy according to the present specification provides puncture resistance during solidification to aid feeding and, therefore, allow constant feeding to the point of deformation of the solid skin resulting in solid feeding. In this manner, the solidification event is pushed closer to completion while porosity formation is avoided.
When casting engine blocks using the AlSi alloy as described in the instant specification, the alloy demonstrates significant advantages in its physical properties. In the as cast condition, at 0.15% magnesium by weight, yield strength is 17 KSI, ultimate tensile strength is 35 KSI and elongation in 2 inches is 11%. At 0.30% by weight magnesium, yield strength is 18 KSI, ultimate tensile strength is 39 KSI and elongation in 2 inches is at least 9%. At 0.45% magnesium by weight, yield strength is 21 KSI, ultimate tensile strength is 42 KSI and elongation in 2 inches is 6%.
Aging the as cast alloy containing 0.30% magnesium by weight four to eight hours at 340° F. provides a yield strength of at least 28 KSI, an ultimate tensile strength of 45 KSI and an elongation in 2 inches of at least 9%. With this T5 heat treatment condition, no loss of ductility occurs over the as cast condition, and the ultimate tensile strength is increased by 15%, while the yield strength is increased by 50%. With T5 treatment, no solution heat treatment is affected.
The T6 heat treatment condition, aged at 340° F. for four to eight hours, increases the yield strength to 35 KSI, an increase of nearly 100% over the as cast condition, with no loss in ductility over the as cast condition. However, in the T6 heat treatment condition, solution heat treatment is affected.
The T7 heat treatment condition, aged at 400° F. for four to eight hours with solution heat treatment, and the T4 heat treatment condition, aged at room temperature for four to eight hours without solution heat treatment, both increase the elongation in 2 inches over 100% compared to the as cast condition while maintaining the equivalent yield strength of the as cast condition.
Hypoeutectic AlSi alloys manufactured in accordance with the instant specification can be employed to cast engine blocks for outboard and stern drive marine motors. When such engines are to be cast, the magnesium level of the alloy is 0.0-0.6% by weight and is preferably kept in the range of 0.20-0.50% by weight.
EXAMPLE 1
An alloy was prepared having the following composition in weight percent: 11.1% silicon, 0.61% magnesium, 0.85% iron, 0.09% copper, 0.22% manganese, 0.16% titanium, 0.055% strontium and the balance aluminum. Thirty-six four-cylinder die cast engine blocks were then produced from this alloy.
A control lot was prepared using an alloy having the following composition in weight percentage: 11.1% silicon, 0.61% magnesium, 0.85% iron, 0.09% copper, 0.22% manganese, 0.16% titanium and the balance aluminum. Significantly, no strontium was added to this alloy. Thirty-eight four-cylinder blocks were die cast under identical conditions as the blocks of the first alloy using a 1200 ton die casting machine. The only difference between the two sets of blocks is that the first set contained 0.055% by weight strontium and the control lot contained no strontium.
The control lot and the strontium-containing lot were machined and all machined surfaces, threaded holes and dowel pin holes were inspected according to a stringent porosity specification that allowed only two instances of porosity of a size that could extend across two thread spacings for certain M6, M8 and M9 threads.
The thirty-eight control lot blocks produced eight blocks with microporosity defects, a percentage of 21.1%. Of those eight blocks with defects, seven of those blocks failed the porosity specification. Those seven blocks were scrapped, indicating an 18.4% scrap rate for the control lot.
In comparison, the strontium containing lot produced four of thirty-six blocks with defects, a percentage of 11.1%. Of those four blocks, only two were required under the porosity specification to be scrapped. Thus, the scrap rate for the strontium containing lot was 5.6%.
The magnitude of scrap reduction, a reduction of 70% from 18.4% to 5.6% is an unexpected, yet extremely useful result indicating the high strontium level influence in reducing microporosity. This reduction in scrap is essential to a highly economic production of cast engine blocks.
EXAMPLE 2
An alloy was preparing having the following composition in weight percent: 10.9% silicon, 0.63% magnesium, 0.87% iron, 0.08% copper, 0.24% manganese, 0.14% titanium, 0.060% strontium, and the balance aluminum. Forty 2.5 L V-6, two stroke engine blocks were prepared from this alloy.
A control lot was prepared using an alloy having the following composition in weight percentage: 10.9% silicon, 0.63% magnesium, 0.87% iron, 0.08% copper, 0.24% manganese. 0.14% titanium and the balance aluminum. Significantly, no strontium was added to this alloy. Thirty-three 2.5 L V-6, two stroke engine blocks were prepared from this alloy.
Both lots were die cast under identical conditions using a 2500 ton die casting machine, at the same time, and were sequentially numbered. The only difference between the two lots is that the first lot contained 0.060% by weight strontium while the control lot contained no strontium. Both lots were machined together.
The head decks of the engine blocks were examined for microporosity defects. Engine blocks with microporosity defects having a range of 0.010 inches to 0.060 inches in diameter were repaired. Blocks with microporosity defects larger than 0.060 inches in diameter were scrapped. This stringent porosity standard is necessary as an O-ring seal must be placed on the head decks of the engine blocks. Any significant microporosity defects provide opportunity for leakage beneath the O-ring seal.
Thirty-three control lot engine blocks produced sixteen blocks that were scrapped as a result of microporosity defects, a percentage of 48%. In comparison, the lot of forty strontium containing engine locks produced fourteen blocks which were scrapped as a result of microporosity defects, a percentage of 35%.
The magnitude of scrap reduction for this example is 27%, from 48% to 35%. This reduction in scrap due to microporosity defects indicates that the addition of strontium has an extremely useful, while unexpected result. This fundamental effect of lowering microporosity defects is unmistakable and results in a reduction of scrap that is essential to a highly economic production of cast engine blocks.
EXAMPLE 3
An alloy was prepared having the following composition in weight %:11.3% silicon, 0.63% magnesium, 0.81% iron, 0.10% copper, 0.25% manganese, 0.11% titanium, 0.064% strontium, and the balance aluminum. Thirty-seven 2 L, 4 stroke engine blocks were prepared from this alloy.
A control lot was prepared using an alloy having the following composition in weight percentage: 11.3% silicon, 0.63% magnesium, 0.81% iron, 0.10% copper, 0.25% manganese, 0.11% titanium, and the balance aluminum. Significantly, no strontium was added to this alloy. Twenty-five 2 L, 4 stroke engine blocks were prepared from this alloy.
Both lots were die cast under identical conditions using a different die casting machine than the first two examples. The lots were cast at the same time, and were sequentially numbered. The only difference between the two lots is that the first lot contained 0.064% by weight strontium, while the control lot contained no strontium.
The head decks of the engine blocks were examined for microporosity defects. All machined surfaces, threaded holes and dowel pin holes were inspected. Engine blocks with microporosity defects having a range of 0.010 inches to 0.060 inches in diameter were repaired. Blocks with microporosity defects larger than 0.060 inches in diameter were scrapped.
Twenty-five control lot engine blocks produced twenty blocks with defects, a percentage of 80.0%. Six of the defective blocks were scrapped, resulting in a scrap percentage of 24.0%. In comparison, the lot of thirty-seven strontium containing engine blocks produced twenty-eight blocks with microporosity defects, a percentage of 75.7%. Only five of the thirty-seven blocks had to be scrapped, a scrap percentage of 13.5%.
The magnitude of scrap reduction for this example is 44%, from 24% to 13.5% on a very tough porosity specification. Although 0.010% by weight strontium is more than sufficient to produce the eutectic silicon phase modification noted earlier, this amount of strontium is insufficient to lower the porosity level or the scrap identified above. Therefore, the results identified in the above experiments are unexpected, particularly the magnitude of reduction of the scrapped blocks.
EXAMPLE 4
An AlSi alloy as described herein may also be used to cast propellers for marine outboard and stern drive motors used in the recreational boating industry. Traditionally aluminum-magnesium alloys are used for die casting propellers, particularly AA 514. When an AlSI alloy as described herein is intended for die casting marine propellers, the alloy preferably contains by weight 8.75-9.25% silicon, 0.05-0.07% strontium, 0.3% maximum iron, 0.20% maximum copper, 0.25-0.35% by weight manganese, 0.10-0-20% by weight magnesium and the balance aluminum, providing an alloy that is ductile yet durable for use in the propeller and that does not solder to die casting dies. High ductility is desirable in propellers so that the propeller will bend, but not break, upon impact with an underwater object. As a result, the damaged propeller blades may be more easily repaired. The propellers will not fracture into segments in collisions with underwater objects and may be hammered back into shape.
FIG. 1 exhibits the impact properties of propellers cast with the AlSi alloy as described herein, cast at 1,260 degrees Fahrenheit as compared with impact properties of AA 514 cast at the same temperature. The propellers were cast with an AA 514 alloy having the following specific composition in weight %: 0.6% maximum silicon, 3.5-4.5% magnesium, 0.9% maximum iron, 0.15% maximum copper, 0.4-0.6 manganese, 0.1% maximum zinc, balance aluminum. The AlSi alloy used to cast propellers had the following composition by weight %: 8.75 to 9.75% silicon, 0.20% maximum iron, 0.05-0.07% strontium, 0.15% maximum copper, 0.25 to 0.35% manganese, 0.10 to 0.20% magnesium, 0.10% maximum zinc, with trace amounts of tin and balance aluminum.
Two lots of V6/Alpha propellers were produced for each alloy, respectfully. The propellers were die cast in 900 ton die casting machines. The AA 514 alloy was cast at 1,320 degrees Fahrenheit, while the alloy manufactured in accordance with the present invention was cast both at 1,320 degrees Fahrenheit and at 1,260 degrees Fahrenheit. The V-6/Alpha propellers that were produced have a shot weight of approximately 11 pounds. The propellers from each lot were subsequently subjected to a drop impact test to measure the impact properties. As demonstrated in FIG. 1, the propellers die cast from the AlSi alloy described herein out-performed the traditional AA 514 alloy, 400 foot pounds to 200 foot pounds.
Subsequently, more than 250,000 propellers have been die cast ranging from small propellers having a shot weight of approximately 3 pounds, medium 50-60 HP propellers having a shot weight of 7 pounds and large V-6 alpha propellers having a shot weight of 11 pounds. None of the 250,000 die cast propellers die cast from the alloy according to the present invention had any soldering problems. This is truly remarkable because the new propeller alloy is very low in iron content and one of ordinary skill in the art would have expected soldering to be a problem.
EXAMPLE 5
Drive shaft housings for a 275 HP, four stroke outboard engine were die cast from an XK 360 alloy having a composition in percent weight of 10.5 to 11.5% silicon, 1.3% maximum iron, 0.15% maximum copper, 0.20-0.30% manganese, 0.55-0.70% magnesium, trace amounts of zinc, nickel, tin, lead and the balance aluminum.
A second lot of a drive shaft housings for a 275 HP, four stroke outboard engine were produced from an AlSi alloy as described herein and having the following composition of percent weight: 8.75-9.75% silicon, 0.20% maximum iron, 0.05-0.07% strontium, 0.15% maximum copper, 0.25-0.35% manganese, 0.35-0.45% magnesium, 0.10% zinc, trace amounts of iron, and balance aluminum. The drive shaft housings were cast on two different 1,600 ton die casting machines at 1,260 degrees Fahrenheit, and had a shot weight of approximately 50 pounds.
The two lots of drive shaft housings were subjected to a “log impact” test where the drive shaft housing is subjected to consecutive hits with an underwater object, simulating an outboard assembly colliding with a log located under water. The drive shaft housings prepared from the AlSi alloy manufactured in accordance with the instant specification passed the log impact test at 50 mph, whereas drive shaft housings cast from the XK 360 alloy failed at 35 mph. Squaring the ratio of these two velocities indicates that the AlSi alloy as described herein exhibits more than double the impact energy than the XK360 alloy.
The drive shaft housings manufactured from the two lots noted above were further subject to a test where the bottom portion of the drive shaft housing is bolted to a movable base and the top/front section of the drive shaft housing is statically loaded until failure occurs. The results obtained from this experiment demonstrated in FIGS. 4 and 5. The XK360 driveshaft housing (FIG. 4) failed suddenly in a fast propagation mode. As expected, crack initiation started at the front of the driveshaft housing where the stress is highest and progressed (upwardly in the picture) to the back of the driveshaft housing in milliseconds. In contrast, the driveshaft housing manufactured with the AlSi alloy as described herein (FIG. 5) failed in a slower, more stable manner. A crack first started at the perimeter of the circular hole feature and the crack stopped after growing approximately two inches. Subsequently, a second crack initiated on the front side of the driveshaft housing (similar to the crack initiation of the XK360) and this second crack grew several inches before it stopped. The driveshaft housing manufactured with the AlSi alloy in accordance with the instant specification (FIG. 5) was able to tolerate twice the static toughness (i.e. area under the load displacement curve) than the XK360 alloy (FIG. 4). Furthermore, after tolerating twice the static toughness, at a load higher than the load that failed the XK360 driveshaft housing, the driveshaft housing manufactured with the AlSi alloy according to the instant specification (FIG. 5) is, quite unexpectedly, still in one piece. This test has been repeated over twenty times and the results, as described above, are continuously duplicated.
In reviewing the results of the test described, above, it is recognized that the AlSi alloy manufactured in accordance with the present invention tolerates approximately twice static toughness and twice the impact properties as the die cast XK 360 alloy. Accordingly, one of skill in the art will realize that this AlSi alloy has demonstrated twice the static toughness and twice the impact properties of XK 360, the alloy that has been traditionally used for 20 years for drive shafts.
Approximately 10,000 drive shaft housings were cast with the alloy of the present invention on a 1,600 ton die casting machine at 1,260 degrees Fahrenheit. The approximate surface area where soldering could have occurred was over 1,600 square inches. In spite of the large surface area, and in spite of the alloy's very low iron content, no soldering was experienced in the castings. The dies were run at both hot and cold conditions, and it was found that the alloy of the present invention prefers the hot running condition. However, in both the hot and cold condition, no die soldering was observed.
EXAMPLE 6
Approximately 50-150 propellers were die cast with the following specific alloy formulations, and soldering to the die cast dies was not observed, despite the low iron content: a) 5.96% by weight silicon, 0.19% by weight iron, 0.081% by weight strontium, 0.17% by weight copper, 0.31% by weight manganese, 0.39% by weight magnesium, balance aluminum; b) 6.45% by weight silicon, 0.23% by weight iron, 0.070% by weight strontium, 4.50% by weight copper, 0.46% by weight manganese, 0.27% by weight magnesium, 2.89% by weight zinc, balance aluminum; c) 6.68% by weight silicon, 0.24% by weight iron, 0.054% by weight strontium, 3.10% by weight copper, 0.41% by weight manganese, 0.29% by weight magnesium, balance aluminum; d) 7.23% by weight silicon, 0.20% by weight iron, 0.072% by weight strontium, 0.21% by weight copper, 0.45% by weight manganese, 0.31% by weight magnesium, balance aluminum; e) 7.01% by weight silicon, 0.12% by weight iron, 0.069% by weight strontium, 0.10% by weight copper, 0.33% by weight manganese, 0.61% by weight magnesium, balance aluminum; f) 11.31% by weight silicon, 0.25% by weight iron, silicon, 0.25% by weight iron, 0.096% by weight strontium, 0.20% by weight copper, 0.28% by weight manganese, 0.31% by weight magnesium, balance aluminum; g) 12.21% by weight silicon, 0.24% by weight iron, 0.051% by weight strontium, 3.52% by weight copper, 0.53% by weight manganese, 0.30% by weight magnesium, and the balance aluminum.
EXAMPLE 7
Approximately 100 propellers were die cast with the following hypereutectic AlSi alloy composition: 19.60% by weight silicon, 0.21% by weight iron, 0.062% by weight strontium, 0.19% by weight copper, 0.29% by weight manganese, 0.55% by weight magnesium, balance aluminum. In all of the propellers die cast, soldering to the die casting dies was not observed, despite the low iron content. Unlike the equiaxed primary silicon particles embedded in an unmodified eutectic structure, typical of strontium free, phosphorus refined microstructure, the above noted alloy, when die cast, has a primary silicon in spherical form and the eutectic structure is modified. The strontium affected structure would be expected to have greater impact properties than the strontium free microstructure.
It should be apparent to those skilled in the art that the present invention as described herein contains several features, and that variations to the preferred embodiment disclosed herein may be made which embody only some of the features disclosed herein. Various other combinations, and modifications or alternatives may be also apparent to those skilled in the art. Such various alternatives and other embodiments are contemplated as being within the scope of the following claims which particularly point out and distinctly claim the subject matter regarded as the invention.

## Claims

1. An aluminum silicon lost foam cast alloy consisting essentially of: 9.0-9.5% by weight silicon, 0.05-0.300% by weight strontium, 0.20% by weight maximum iron, 0.05-0.25% by weight copper, 0.05-0.50% by weight manganese, 0.05-0.60% by weight magnesium, 3.0% by weight maximum zinc, and the balance aluminum, wherein the alloy is cast with a lost foam casting process that applies approximately 10 ATM of pressure during solidification of the alloy to substantially completely suppress the formation of hydrogen porosity and substantially decrease the incidence of porosity defects and narrow the solidification range in comparison to conventional lost foam cast aluminum silicon alloys.

2. An aluminum silicon lost foam cast alloy according to claim 1, wherein tensile liquid failure defects are substantially decreased in comparison to conventional lost foam cast aluminum silicon alloys.

3. An aluminum silicon lost foam cast alloy according to claim 1, wherein surface puncture defects are substantially decreased in comparison to conventional lost foam cast aluminum silicon alloys.

4. An aluminum silicon lost foam cast alloy according to claim 1, wherein surface porosity defects are substantially decreased in comparison to conventional lost foam cast aluminum silicon alloys.

5. An aluminum silicon lost foam cast alloy according to claim 1, wherein the solidification range of the alloy is substantially decreased and hydrogen porosity formation is completely suppressed in comparison to conventional lost foam cast aluminum silicon alloys.

6. An aluminum silicon lost foam cast alloy according to claim 1, wherein the alloy consists essentially of: 0.05-0.07% by weight strontium, 0.25-0.35% by weight manganese, 0.10-0.20% by weight magnesium, and the balance aluminum.

7. An aluminum silicon lost foam cast alloy consisting essentially of 9.0-9.5% by weight silicon, 0.20% by weight maximum iron, 0.25% by weight maximum copper, 0.25-0.35% by weight manganese, 0.05-0.60% by weight magnesium, 0.1% by weight maximum zinc, 0.05-0.07% by weight strontium, trace amounts of titanium, nickel and phosphorus, and the balance aluminum, wherein the alloy is cast with a lost foam casting process that applies approximately 10 ATM of pressure during solidification of the alloy to completely suppress the formation of hydrogen porosity and substantially decrease the incidence of porosity defects and narrow the solidification range in comparison to conventional lost foam cast aluminum silicon alloys.