Patent Application
20080031768
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.
indicates data missing or illegible when filed
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 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”).
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.
