Patent Application
20100288461
The invention relates to aluminum alloys that can be cast into high-quality aluminum cylinder blocks, utilizing a low-cost low pressure sand casting process, for automotive engines having good mechanical properties and wear and scuffing resistance; so that according to the present invention the engine blocks can be manufactured without the need for insertion of iron (or costly aluminum) liners in order to have effective cylinder walls.
Most of the automotive and aviation cylinder engine blocks made out of aluminum alloys are currently manufactured by casting the block body in silica sand molds using sand cores and inserting a set of cast iron liners to form the cylinder-piston contact surfaces. Other processes for casting blocks have included gravity semi-permanent molds, high pressure die casting, low pressure die casting, the lost foam process and the zircon sand package molds; and the liners can be either inserted as “cast-in” or “pressed-in”. More recently, in a few high-end aluminum engine blocks liners made of aluminum have been substituted for cast iron liners. However, the high cost of the currently-available Al alloy needed to meet the requirements for such aluminum cylinder liners prevents such alloy from also being used to cast the remainder of the aluminum engine block (as do also some negative physical attributes if it were to be used in the remainder of the block). The cost of such Al alloy, even when limited to use as a liner, has also prevented it form being universally adopted to replace iron liners in spite of the lower weight and greater cooling advantages.
This practice of utilizing liners however requires a number of process and material measures that, if able to be eliminated without the indicated drawbacks, would provide many advantages to block manufacturers. For example, the inventory of liners would be eliminated, the scrap rate of blocks due to poor bonding between the aluminum body and the liners would decrease, the energy consumption for preheating the liners would also be eliminated, and the casting process would be simplified. Currently, preheating the liners is done by electrical induction and consumes time as well as adding complexity to the overall casting process. All of the foregoing is especially true relative to iron liners. The need exists therefore for an aluminum alloy composition and casting process which eliminates the need for liners in an aluminum engine casting thereby overcoming such technical and economic disadvantages of the prior art.
It is known from the patent and technical literature; that silicon added to aluminum beyond the eutectic composition increases hardness of the alloy and consequently increases the wear resistance of its surfaces. However the sole increase of Si concentration in the alloy does not provide all the desired properties to the cast blocks (concerning wear-resistance, machinability, castability and other mechanical properties). Such desired properties are governed by the type of microstructure formed in the solidified casting. Another process problem posed by Si, when alloyed with aluminum, is that it adds a greatly increased capacity for heat that must be dissipated from the alloy during solidification. This results in uneven cooling, especially in large complex castings such as automotive engine blocks, causing problems in properly developing the often competing desired properties of the bulk casting relative to the cylinder surface.
Some relevant prior art patents found by applicants regarding the alloy composition and the casting process are described below:
U.S. Pat. No. 4,068,645 issued Jan. 17, 1978 to David Charles Jenkinson, teaches that the microstructure of a hypereutectic Al—Si alloy can be modified with strontium and/or sodium for obtaining Brinell hardness in the range of 70-150 by including magnesium up to about 4 wt. %. This patent teaches that the desired microstructure must avoid the formation of primary aluminum or primary silicon phases and that there must be a high-volume fraction of finely dispersed eutectic silicon which provides the wear resistance to the cast article.
According to this patent, the desired microstructures are provided by careful selection and combination of four parameters: (a) silicon content, (b) modifier content, (c) growth rate during solidification and (d) temperature gradient at the solid/liquid interphase during solidification.
Several combinations of the above four parameters are disclosed which provide the desired microstructure. The teachings of this patent however are applicable to permanent and semi-permanent mold casting processes where a controlled temperature gradient may be achieved by programming the cooling rate of the mold at different zones, but it is not applicable to silica-sand molds casting processes (where conventionally the solidification rate is only able to be modified by the addition of thermal cores which absorb heat from the liquid aluminum in the mold). This patent clearly teaches away from chill-casting in order to obtain the desired absence of primary Si and primary Al phases.
U.S. Pat. No. 4,434,014 issued Feb. 28, 1984 to David M. Smith, et al. teaches that the properties of the cast articles regarding wear resistance and machinability are obtained by a composition comprising 12-15% Si; 0.001-0.1 Sr; 0.1-1.0 Fe; 1.0-3.0 Ni; 0.1-0.8% Mn; and other components.
This patent teaches also that Ni, Fe and Mn are interchangeable with each other, being the ranges as follows: Fe+Mn between 0.2 and 1.5%; Fe+Ni between 1.1 and 3.0%; and Fe+Ni+Mn between 1.2 and 4.0%.
Titanium is added to improve castability and the mechanical properties of this alloy. This alloy however has a high cost due to the high content of Ni, in contrast with the alloy of the present invention having less than about 0.4-0.8% Ni. The lower concentration Ni thus particularly makes the alloy of the present invention more competitive.
U.S. Pat. No. 4,648,918 issued Mar. 10, 1987 to Kasuhiko Asano, et al. teaches an abrasion-resistance aluminum alloy having a composition comprising: 7.5-15% Si; 3.0-6.0% Cu, 0.3-1.0% Mg, 0.25-1.0% Fe; 0.25-1.0% Mn; and a balance of Al and other components. The alloy of this patent is directed to improve the extrudability, forgeability and mechanical properties of ingots. The Cu content is higher than the alloy of the present invention and the heat treatment and final processing of this alloy are far different from the sand-casting process of the present invention.
U.S. Pat. No. 5,019,178 issued May 28, 1991 to John Barlow et al. discloses a production method of an aluminum-silicon liner produced from a melt consisting essentially of 14-16% Si; 1.9-2.2% Cu; 1.0-1.4 Ni; 0.4-0.55 Mg; 0.6-1.0% Fe; 0.02-0.1% Sr; and 0.3-0.6 Mn. The alloy of this patent is formed into cylinder liners under pressure during the solidification stage of the casting process. This patent does not teach or suggest that the whole engine block be made of the claimed alloy in a low-pressure sand-casting process.
U.S. Pat. No. 5,217,546 issued Jun. 8, 1993 to John A. Eady, et al. discloses a cast hypereutectic Al—Si alloy having 12-15% Si; more than 0.10% Sr; more than 0.005% Ti; 1.5-5.5% Cu; 1.00-3.00 Ni; 0.1-1.0 Mg; 0.1-1.0% Fe; and other components. According to this patent, the microstructure obtained is such that any primary Si formed is substantially uniformly dispersed and is substantially free of segregation, with the microstructure predominantly comprising a eutectic matrix. The alloy of this patent however relies on Ti and an excessive amount of Ni, which makes it too expensive an alloy for competitive mass production of engine blocks.
U.S. Pat. No. 5,316,070 issued May 31, 1994 to Kevin P. Rogers, et al. teaches a process for controlled casting of a hypereutectic Al—Si alloy in permanent molds. Permanent molds can be fully equipped with cooling systems and with precise temperature control so that a pre-established solidification program can be implemented and therefore the desired microstructure of the cast article may be achieved. The teachings of this patent can not be applied to sand-casting processes.
U.S. Pat. No. 5,484,492 issued Jan. 16, 1996 to Kevin P. Rogers et al. discloses a hypereutectic Al—Si alloy essentially having at least one element selected from a first group of elements consisting of 0.005% up to 0.25% of Cr, Mo, Nb, Ta, Ti, Zr, V and Al; at least one element selected from a second group of elements consisting of 0.1 to 3.0% Ca, Co, Cr, Cs, Fe, K, Li, Mn, Na, Rb, Sr, Y, Ce, elements of the Lanthanide series and elements of the Actinide series; and a third group of elements consisting of: 12-15% Si; 1.5-5.5 Cu; 1.0-3.0% Ni; 0.1-1.0% Mg; 0.1-1.0% Fe; 0.1-0.8% Mn; 0.01-0.1 Zr; 0-3.0% Zn; 0-0.2% Sn; 0-0.2% Pb; 0-0.1% Cr; 0.001-0.1% Sr or Na; a maximum of 0.05% B; a maximum of 0.03% Ca; a maximum of 0.05% P; and others with a maximum of 0.05%. The casting microstructure is such that any primary Si present is substantially uniformly dispersed and predominantly comprises a eutectic matrix. The present invention in contrast uses a different and lower range of Ni (0.8% maximum).
To the best of applicants' knowledge, none of the last three patents (assigned to Comalco) have ever been commercialized.
U.S. Pat. No. 6,399,020 issued Jun. 4, 2002 to Jonathan A. Lee et al. discloses an aluminum alloy suitable for high-temperature applications, such as pistons and other internal combustion engines applications, having the following composition: 11.0-14.0% Si; 5.6-8.0% Cu; 0-0.08 Fe; 0.5-1.5 Mg; 0.05-0.9 Ni; 0-1.0 Mn; 0.05-1.2 Ti; 0.12-1.2 Zr; 0.05-1.2 V; 0.05-0.9 Zn; 0.01-0.1 Sr; with the balance Al. In this alloy the ratio of Si/Mg is 10-25, and the ratio of Cu/Mg is 4-15. The alloy of the applicants' invention differs from the alloy composition disclosed in this patent, mainly in the Si/Mg ratio and in the amount of Sr. Since Sr is an expensive element, the alloy of the present invention is more cost-competitive. In addition, the present invention does not include Zr or V and has a maximum of 0.3% Mg.
U.S. Pat. No. 6,592,687 issued Jul. 15, 2003 and U.S. Pat. No. 6,918,970 issued Jul. 19, 2005, both to Jonathan A. Lee et al. disclose an aluminum-silicon alloy having the following composition in weight percent: 14-25.0 Si; 5.5-8.0 Cu; 0.05-1.2 Fe; 0.5-1.5 Ni; 0.05-0.9 Mn; 0.05-1.2 Ti; 0.05 1.2 Zr; 0.05-1.2 V; 0.05-0.9 Zn; 0.001-0.1 P; and with the balance being Aluminum. The '970 patent's alloy has an extended range of Si (6.0-25.0%) plus Sr (with a range of 0.001-0.1). The Si/Mg ratio is 10-25 and the Cu/Mg ratio is 4-15. This alloy has as key elements Ti, V and Zr that modify the lattice parameters of the aluminum matrix by forming compounds of the type Al3X having L12 crystal structures, wherein X stands for Ti, V or Zr.
U.S. Pat. No. 6,921,512 issued Jul. 26, 2005 and US Patent Publication No. 2005/0199318 published Sep. 15, 2005, both appearing in the name of Herbert William Doty, disclose an aluminum alloy suitable for casting and machining cylinder blocks for automotive engines. The alloy comprises by weight, 9.5-12.5% Si; 0.1-1.5% Fe; 1.5-4.5% Cu; 0.2-3% Mn; 0.1-0.6% Mg; 2.0% maximum Zn; 0-1.5% Ni; 0.25% maximum Ti; up to 0.05% Sr; with the balance being aluminum. An important feature of this Patentee's invention is the proportion of Mn to Fe. The weight ratio Mn/Fe is between 1.2 to 1.75 or higher when the Fe content is equal to or greater than 0.4% and the weight ratio Mn/Fe is at least 0.6 to 1.2 when the Fe content is less than 0.4% of the alloy. In contrast, the Si range of the present invention is 13-14%.
The desired microstructures in the Al—Si alloys are produced by a right combination of growth rate during solidification and temperature gradient.
Documents cited in this text (including the foregoing patents), and all documents cited or referenced in the documents cited in this text, are incorporated herein by reference. Documents incorporated by reference into this text or any teachings therein may be used in the practice of this invention.
It is an object of the present invention to provide a new hypereutectic Al—Si alloy suitable for low pressure casting processes utilizing silica-sand molds and cores to cast an engine block having the required combination of machining, casting and wear resistance properties so as also not to require wear liners.
It is another object of the present invention to provide such a new Al—Si alloy for manufacture of aluminum engine blocks with unlined cylinders that are competitive with current mass produced aluminum engine blocks with iron liners.
It is a further object of the present invention to provide a new Al—Si alloy which produces improved engine block castings with mechanical properties that avoid the necessity for cylinder liners made from a different alloy or metal, and that also are easier to machine than engine block castings made from existing hypereutectic Al alloys of the prior art.
Other objects of the invention will be pointed out or will be evident from the following description of the preferred embodiments and the accompanying drawings.
The proposed invention herein described and claimed is an aluminum-silicon alloy composition which, when cast, meets the manufacturing and performance conditions required for cylinder engine blocks and further can be cast using low-cost casting processes such as silica-sand molds.
The alloy of the present invention comprises (in weight percent):
the balance being aluminum (apart from a minor amount of any trace elements, impurities, residuals, and other ingredients which in the aggregate are known as the “remainders” and are present in amounts insufficient to substantially affect the efficacy of this alloy for its intended purpose, including its wear resistance).
Although the invention is herein described as applied to an aluminum alloy cylinder engine block casting through a low pressure sand casting process it will be understood that in its broader aspects it may also be applicable to other types of castings requiring similar properties and also to other casting processes.
It is known that increasing the concentration of silicon in an alloy of the type utilized for automotive engines casting generally increases the hardness and wear resistance of the resulting casting, and that the final properties thereof depend on the cooling rate of the casting.
The traditional sand-casting processes featuring low-pressure mold filling, for example the Cosworth process (and also the non-commercialized Comalco process), cannot produce good-quality blocks utilizing alloys having a high concentration of silicon, primarily due to the difficulties posed by the sand molds and cores for controlling the solidification rate, and therefore the microstructure of the castings. When utilizing the aluminum alloys of the prior art with high Si contents, the intricate geometry of the cylinder engine blocks combining thick and thinner sections cause the formation of primary silicon phases with undesirable grain and size distribution of the primary silicon phase, as well as a high porosity level of the casting.
Another problem related to the utilization of high Si concentration alloys is that their heat of fusion is high as compared with hypoeutectic alloys, therefore, the sand molds must be able to cope with and dissipate the high heat release during the solidification process.
The aluminum alloy blocks to be manufactured demand strictly controlled characteristics and mechanical properties in order to perform as expected in modern vehicles. Blocks without liner inserts must have high wear resistance in the running surfaces and withstand high pressures on the order of 100 to 200 bar in those engines having high peak firing pressures. The porosity level must be below 1% and the maximum pore size must be below 500 microns in the running surfaces.
It is necessary also that the aluminum alloy has a high thermal conductivity in order to sustain high heat transfer rates from the hot areas of the engine to the cooling liquid of the engine cooling system, as well as having good corrosion resistance to the cooling media. The high-efficiency modern engines also demand that the alloys from which the engine blocks are cast show high strength and high resistance to fatigue and creep at elevated temperatures, in the range of 180°-200° C.
The current challenge for the processes utilizing hypoeutectic alloys is that machining high-silicon alloys means greater wear of tools and high machining cost, as in the case of the A390 alloy. In the process of the invention, primary silicon formation is suppressed resulting in a fully eutectic microstructure despite its high silicon content. This characteristic of the microstructure of the castings of the invention assures good machinability. Tool life is comparable to machining an A356 alloy but with superior surface finish.
The alloy of the present invention is based on the Al—Si—Cu—Mg—Ni—Mn—Fe system to enhance maximum wear resistance. It provides the required characteristics demanded by modern engine blocks having unlined cylinders, while also maintaining a competitive low manufacturing cost.
The casting process of the invention utilizes a thermal core (or massive chill) in combination with silica-sand cores and molds. The chill provides the right direction of the solidification process as well as the necessary solidification rate which results in high fatigue properties of the castings.
The alloy of the present invention is particularly suited for the production of linerless aluminum alloy blocks at a lower cost than the currently used alloys. The following table 1 compares the typical concentration of the elements of the prior-art alloys with the composition of the present invention.
Alloy 390 (A) is the historical choice for wear-resistance cast motor elements, but as discussed above it is not applicable for sand casting processes.
Alloy 3HA (B) is also an alloy of choice for those applications, but its cost is high because of its high content of nickel (2%). The high concentration of Ni increases the alloy cost by 35% ($15,000 US/Ton of Ni), and the 2000 ppm of Sr further combines to make it even more expensive.
Near eutectic alloys (C) do not have sufficient silicon content to provide the required wear resistance.
Despite it being known that high Ni content would improve the wear resistance of the casting surfaces, the high cost of Ni discouraged its utilization, since about each 1% of Ni content increases by about 15% the cost of the cast block. Nickel also helps in avoiding Cu segregation during solidification and therefore some of the prior art alloys nevertheless tend to increase the nickel content. Therefore applicants have looked for a better new alternative. They found a new alloy composition containing no more than 0.8% Ni and 900 ppm's of Sr, which produces large complex castings with the desired microstructure and mechanical properties capable of manufacture by a sand casting process.
Referring to
Additionally, the challenge faced by applicants in developing a new alloy which overcomes the disadvantages of the alloys of the prior art when used in combination with a silica sand casting process was to find a composition such that, despite the high heat release and low cooling rate of the silica sand process, the intermetallic segregation and porosity in the casting are minimized.
With reference to
The alloy and casting method of the present invention present the following advantages: The wear resistance provided by the alloy avoids the necessity of inserting iron liners in the cylinder bores. Consequently, the manufactured blocks are smaller and lighter, (saving the weight and cost of iron liners) and can increase the engine capacity without increasing engine size (for example from 2.3 to 3.0 liters).
The alloy of the invention has better thermal characteristics regarding heat dissipation (particularly with the absence of iron cylinder liners). Applicants' blocks run about 10° C. cooler than currently used aluminum blocks having iron liners blocks, due to the fact that the interface between the iron liners and block is eliminated.
The alloy also allows for tighter clearances because the thermal expansion coefficients of both pistons and the blocks are similar (in contrast with the greater differentiation of thermal expansion coefficients between the piston aluminum alloy and the iron liners). This advantage provides a quieter engine operation and makes the engines environmentally cleaner.
There is no need for liner inventory and handling. Therefore there are important savings in the manufacturing process, not only due to avoiding the cost of iron liners but also because there is no need of preheating such liners by electric induction. The same is true of the more rarely used aluminum liners, which in addition are made from a more expensive alloy than the alloy of the reminder of the engine casting block.
The linerless engines made from the alloy of the present invention are also easier to recycle, since no separation of iron cylinder liners from aluminum is required.
The alloy of the invention further provides very good machining characteristics, and although the tool life is comparable and similar to machining of the currently-known A356 alloy, the surface finish in the cylinder bores is significantly better.
The manufacturing cost of unlined engine blocks is reduced by about 40% by using the alloy and method of the invention as compared with the manufacturing cost when using the known alloys of the prior art.
An Al—Si alloy was prepared according to the present invention and a block was cast in silica sand molds and cores. The alloy had the following composition (in weight percent):
Si=13.5% Sr=900 ppm; Fe=0.4%; Cu=2.5%; Ni=0.5%; Mn=0.4%; Mg=0.35%; with the balance being essentially only aluminum (plus minor amounts of any other essentially non-affecting elements, hereinbefore referenced as the “remainders”). The alloy was poured into the mold at a temperature of 750° C.
The results were as follows:
The microstructural segregation was reduced.
Modified eutectic cells were more evenly distributed, and the primary aluminum was reduced. Primary silicon particles were still observed, but they comprised less than 1% of the total silicon.
In order to test the wear resistance of the alloy of the invention, a series of single stage 20 hour duration tests were carried out using a Plint TE77 testing machine. The test set-up provides a reciprocating line contact between a dowel and a plate. The hardened dowel is used to simulate the piston ring while a flat ground plate is used to simulate the cylinder liner. The oil used was a commercially available automotive petrol engine mineral oil heated to 100° C.
Three different materials were evaluated: (1) cast iron liners for diesel applications, (2) a hypereutectic aluminum-silicon alloy (of the type currently being used as expensive liners in high performance engines; where the primary wearing resistance phase was a phase of primary silicon), and (3) the alloy of the present invention. Results indicate that qualitatively the wear scars obtained on all there materials have been similar and do not appear to be significantly different in magnitude between the materials tested.
It is of course to be understood that the invention has been specified in detail only with respect to certain preferred embodiments thereof, and that a number of modifications and variations can be made without departing from the spirit and scope of the invention which is defined by the following claims.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11499165 | Aug 2006 | US |
| Child | 12728975 | US |
