Wear-resistant eutectic aluminium-silicon alloy

Abstract: An improved eutectic aluminium-silicon alloy having a relatively high level of bismuth is provided which is particularly wear-resistant and sufficiently self-lubricating so as to be suitable for use in a wearing component even when poorly lubricated. The relatively high bismuth level within the alloy co-operates with the other elemental additions present so as to provide a sufficiently low-friction bearing surface (or self-lubricity), which significantly enhances the wear-resistant properties of the alloy. In addition, the preferred alloy also has relatively substantial additions of both nickel and copper, which results in the homogeneous distribution of hard wear-resistant nickel and copper phases throughout the alloy. The formation of components from the improved aluminium alloy should minimize wear and alleviate galling of those components during use.

Description: The present invention generally relates to eutectic aluminium-silicon alloys, particularly those alloys which are used for wear resistance in automotive environments. Air-conditioning systems are routinely employed within automobiles and other vehicles for creating comfortable conditions within the passenger compartment for the vehicle occupants. At outside temperatures above about 21.1°C (70°F), it is difficult to maintain a comfortable passenger compartment temperature without first cooling the air that is being blown into the passenger compartment. Typically, cooling of the air is accomplished by first compressing an appropriate refrigerant, such as the generally-used fluorocarbons (known commonly as freon) or another alternative refrigerant, using an engine-driven compressor which compresses the vaporized refrigerant. The materials and components within the air-conditioning system must be capable of withstanding extremely demanding conditions, particularly the materials used to form the components within the engine-driven compressor. The compressor contains many mating components which continuously wear against each other during operation of the air-conditioning system, whilst also being subject to significant pressures due to the compressed refrigerant. Appropriate lubricants are provided throughout the compressor at these bearing surfaces, so as to prevent excessive wear and galling between the mating materials. Typically in the past, a lubricant which is soluble in the refrigerant has been added directly in with the refrigerant when charging the compressor with the pressurized refrigerant prior to use. Since the conventional lubricants have been soluble within the refrigerant, the lubricant therefore moves freely through the compressor with the refrigerant, thereby providing lubrication where it is needed most between mating components. However, due to environmental concerns, the current fluorocarbon-based refrigerants are being eliminated from use. Alternative refrigerants which alleviate environmental damage have been tested, with a 1,1,1,2-Tetrafluoroethane refrigerant, known as R134A, being a likely substitute. Unfortunately, conventional lubricants which have been previously (and successfully) employed with the fluorocarbon-based refrigerants are not soluble within the R134A refrigerant. Therefore the lubricant does not freely move throughout the compressor components when the new refrigerant is used and does not lubricate mating surfaces, as was the situation when the fluorocarbon-based refrigerants were used. The result is that, during operation of the air-conditioning system with the new R134A refrigerant, the bearing surfaces of the mating components are not lubricated and correspondingly they experience significantly higher incidence of wear. Therefore, in the absence of an appropriate lubricant, it is necessary to provide a wear-resistant material which is essentially self-lubricating. The desired material must be capable of not only providing sufficient lubricity, but must also be sufficiently strong to resist wear and galling during operation of the compressor. In addition, there are certain applications wherein the material must also be sufficiently ductile to permit the formation of a component from the material such as by swaging or other forming techniques. Therefore, the requirements of this material are many. A wear-resistant eutectic aluminium-silicon alloy according to the present invention is characterised by the features specified in the characterising portion of claim 1. It is therefore an object of this invention to provide a wear-resistant eutectic aluminium-silicon alloy particularly suitable for use as a wearing component, such as in a compressor unit of an automobile air-conditioning system. It is a further object of this invention that such a eutectic aluminium-silicon alloy be sufficiently self-lubricating so as to prevent galling during use even when poorly lubricated. It is yet a further object of this invention that such a eutectic aluminium-silicon alloy be characterized by a uniform distribution of hard wear-resistant phases. In accordance with a preferred embodiment of this invention, these and other objects and advantages are accomplished as follows. According to the present invention, there is provided an improved eutectic aluminium-silicon alloy having a relatively substantial addition of bismuth. The aluminium-silicon alloy is particularly wear-resistant and sufficiently self-lubricating so as to be suitable for use as a wearing component, such as one which would receive a bearing member within a compressor unit of an automobile air-conditioning system. The improved eutectic aluminium-silicon alloy minimizes wear and alleviates galling during use, even when used in a poorly-lubricated environment. In addition, the improved alloy of this invention has relatively high levels of nickel and copper that produce hard, wear-resistant phases, NiAl₃ and CuNiAl₃, which are stable at high temperatures and which are dispersed uniformly throughout the alloy. The wear-resistant eutectic aluminium-silicon alloy is characterized by the following elemental composition, wherein the percentages are weight percents: from eleven to 13.5 percent silicon with about twelve to thirteen percent being preferred; from three to six percent bismuth with about four to about five percent being preferred; from two to five percent copper with two to about three percent being preferred; from one to three percent nickel with about 1.5 to about 2.5 percent being preferred; and from 0.005 to 0.020 percent phosphorus. In addition, the aluminium-silicon-copper alloy also consists of up to one percent iron; up to 0.5 percent manganese; and up to 0.25 percent titanium, with the balance of the alloy being aluminium. A particularly advantageous feature of the eutectic aluminium-silicon alloy of this invention is that the relatively high level of bismuth remains essentially as elemental bismuth within the alloy. The elemental bismuth provides a lubricating phase that results in a material having a low coefficient of friction at its surfaces. This property of self-lubricity for the alloy enhances the wear-resistant properties of the alloy. Another advantageous feature of the eutectic alloy is that the relatively high nickel and copper content within the alloy causes the formation of the extremely hard NiAl₃ and CuNiAl₃ phases, which are about half the relative hardness of the primary silicon particles. The hard wear-resistant nickel and copper phases are uniformly dispersed throughout the alloy, and therefore enhance the overall wear-resistance of the alloy. Further, the trace amounts of phosphorus within the alloy react with the aluminium to form aluminium phosphide which tends to uniformly precipitate the primary silicon particles throughout the alloy, thereby also enhancing the wear-resistance of the alloy. Other objects and advantages of this invention will be better appreciated from the following detailed description. According to the present invention, there is provided an improved eutectic aluminium-silicon alloy having a relatively substantial addition of bismuth, as well as substantial additions of copper and nickel also. The improved eutectic aluminium-silicon alloy exhibits good wear-resistance by being sufficiently self-lubricating and having a uniform dispersion of hard wear-resistant phases throughout, and therefore is particularly suited for use as a component subjected to wear during use thereof. More specifically, the self-lubricating, wear-resistant eutectic aluminium-silicon alloy of this invention is characterized by the elemental composition shown in Table I., wherein the percentages refer to weight percents. Si 11.0% - 13.5% Bi 3.0% - 6.0% Cu 2.0% - 5.0% Ni 1.0% - 3.0% P 0.005% - 0.020% Fe 1.0% (max.) Mn 0.5% (max.) Ti 0.25% (max.) Al Balance The silicon (Si) content of the eutectic aluminium-silicon alloy varies from eleven to 13.5 percent so as to ensure good wear-resistance of the material, with the range of about twelve to about thirteen percent being preferred. The eutectic point in a pure aluminium-silicon system is approximately 12.3 weight percent silicon within the pure alloy, however, due to the additional constituents within the alloy, it is believed that the actual eutectic point is somewhat lower in the alloy, possibly as low as about eleven weight percent of silicon. Therefore, the silicon content of the eutectic aluminum-silicon alloy should remain above or equal to eleven percent. Maintaining the silicon level above the eutectic point ensures that hard primary silicon particles will form in the alloy. These hard primary silicon particles contribute greatly to the wear resistance of the alloy. In addition, the silicon reacts with the aluminium to form hard aluminium-silicon particles which also enhance the wear-resistance of the alloy. The bismuth (Bi) content of the eutectic aluminium-silicon alloy may vary from three percent to six weight percent, with a range of about four to about five percent being preferred. It has been determined that the presence of bismuth within the alloy enhances the lubricity of the alloy by essentially remaining as elemental bismuth within the alloy. The elemental bismuth reduces the coefficient of friction on the bearing surfaces of the alloy. It is this high level of bismuth which enables the alloy to be essentially self-lubricating, thereby alleviating excessive wear and galling of the aluminium-silicon-copper base alloy during use. An advantageous feature of this invention is that many eutectic aluminium-silicon alloys of this type, which are designed for wear-resistance, also contain magnesium for strengthening purposes. However, with magnesium present, the bismuth content must be non-existent or at least limited, since it has been determined that the bismuth tends to react with magnesium so as to reduce the strengthening potential of the alloy by detrimentally reacting with magnesium. Therefore, it is generally necessary to eliminate the bismuth content within these types of alloys that require strength. Yet in the alloy of this invention, sufficient strength is achieved without the addition of magnesium, which then permits a relatively large amount of the lubricating bismuth to be used. Hence the alloy of this invention provides a strong yet self-lubricating material. The eutectic aluminium-silicon alloy of this invention contains relatively high levels of both copper (Cu) and nickel (Ni) which produce extremely hard, wear-resistant phases, NiAl₃ and CuNiAl₃, within the alloy. These phases are characterized by a hardness of about half the hardness of pure silicon. The copper content ranges from two to five weight percent, with two to about three percent being preferred; and the nickel content ranges from one to three weight percent, with about 1.5 to 2.5 percent being preferred. This amount of each alloy ensures that a sufficient amount of the desired hard phases, NiAl₃ and CuNiAl₃, will be present during formation of the alloy. It is to be noted that these hard phases tend to be stable at high temperatures and form in relatively equal amounts depending upon the ratio of nickel to copper within the molten alloy. The nucleation kinetics associated with the formation of these phases, NiAl₃ and CuNiAl₃, proceeds relatively independently of the cooling rate employed during the casting process for the alloy. Thus, the cast components which may be formed from the alloy are characterized by a uniform distribution of these wear-resistant particles throughout. This is particularly advantageous as the uniform distribution of these hard wear-resistant particles enhances the overall wear-resistance of the alloy. The eutectic aluminium-silicon alloy also contains a trace amount of phosphorus (P), from 0.005 to 0.020 percent with about 0.010 to 0.020 percent being preferred. The phosphorus reacts with the aluminium within the molten alloy to form aluminium phosphide. The aluminium phosphide nuclei precipitates the fine primary silicon particles, causing the primary silicon particles to be more homogeneously distributed throughout the alloy, which enhances the overall wear-resistance of the alloy. However, only a trace amount of the phosphorus is required to effect the fine distribution of the primary silicon particles, with the preferred phosphorus levels being sufficient for this purpose. Further, since it is difficult to add phosphorus directly to the molten alloy because of its fine powdery form, the phosphorus would most probably be added to the molten alloy using conventional phosphorus treatment methods, which include adding a phosphorus-containing compound such as a phosphorus-copper compound to the melt during casting. It is important that the phosphorus within the molten alloy be allowed to incubate within the melt for at least about five to ten minutes. This ensures an intimate reaction of the phosphorus within the molten metal so as to sufficiently activate the metal to allow formation of the aluminium phosphide particles. The iron (Fe) content within the aluminium alloy of this invention may vary up to 1.0 percent iron, with a maximum level of about 0.8 or less being preferred. The ductility of the alloy is typically impaired by the presence of iron within the alloy due to the formation of an aluminium-iron-silicon (Al-Fe-Si) compound. Therefore, it is desirable to minimize the iron content within the alloy, yet it is difficult to entirely eliminate the iron within the alloy since this level of iron is typically always present within the secondary aluminium used to form the alloy. The manganese (Mn) content within the eutectic aluminium-silicon alloy of this invention may vary up to 0.5 percent, preferably up to only about 0.4 percent, with as minimal a level practical being most preferred. It is noted that this small amount of manganese may be helpful in that the manganese tends to prevent formation of a brittle aluminium-iron-silicon intermetallic phase within the alloy. The titanium (Ti) content may vary up to 0.25 percent with a preferred maximum being about 0.2 weight percent. This small amount of titanium is desired since it provides a grain-refining effect within the preferred alloy. The balance of the alloy is aluminium. The preferred composition for the alloys, as discussed above, is summarized in Table II. Again, the percentages refer to weight percents. Si 12.0% - 13.0% Bi 4.0% - 5.0% Cu 2.0% - 3.0% Ni 1.5% - 2.5% P 0.01% - 0.02% Fe 0.8% (max.) Mn 0.4% (max.) Ti 0.2% (max.) Al Balance It is believed that the eutectic aluminium-silicon alloy could be heat-treated using a conventional Thigh aluminium alloy heat-treating schedule, so as to maximize the tensile and yield strengths of the alloy. It should be noted that the particular heat-treatment schedule employed on the alloy will vary depending on the intended application for the alloy. In particular, any of the T6 aluminium heat-treating schedules which basically solution heat-treat, quench and then artificially age the alloy would probably be suitable with the alloy of this invention. It is also presumed that, upon conventional metallographic examination, the microstructure of the alloy would exhibit well-dispersed primary silicon, aluminium-silicon and bismuth phases throughout the aluminium matrix of the alloy. The presence of these hard silicon particles within the alloys of this invention have been found to significantly improve the wear and galling-resistant properties thereof. In addition, it is believed that the cast alloy would exhibit uniform distribution of the hard wear-resistant particles, NiAl₃ and CuNiAl₃, throughout the alloy. An advantage of this alloy is that the formation of these hard copper/nickel/aluminium phases occurs relatively independent of temperature, so that, after cooling, these hard phases can be found in regions where other elements (such as silicon) may be depleted, i.e., specifically at the cast surfaces which cool most rapidly during casting - particularly when die-casting the alloy. The presence of these hard phases results in an increase in the matrix strength of the cast component and improved wear-resistance even under severe conditions where little lubrication is present, such as within automotive air-conditioning compressor components. Therefore, the alloy of this invention should exhibit enhanced wear and galling-resistance in an actual wearing environment, due to the uniform distribution of hard particles and high aluminium matrix strength of the alloy, particularly when coupled with its lubricity. In summary, there are many advantageous features associated with the eutectic aluminium-silicon alloy of this invention. The relatively high level of bismuth within the alloy co-operates with the other elemental additions by providing a sufficiently self-lubricating, low-friction surface which, in turn, enhances the wear and galling-resistant properties of the alloy, as well as the machinability thereof. Furthermore, in addition to the hard primary silicon and aluminium-silicon particles which provide wear-resistance, the relatively high nickel and copper content within the alloy causes the formation of uniformly-dispersed, extremely hard NiAl₃ and CuNiAl₃ phases. Because the formation of these hard wear-resistant particles is relatively independent of cooling rate, the preferred alloy is well suited for die-casting techniques. Die-cast components formed from the alloy of this invention would be essentially ready to use after casting without the requirement for further etching to expose the wear-resistant particles. Therefore, whilst the present invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art, such as by modifying the aluminium alloy within the ranges of element concentrations, or by modifying the processing steps, or by employing the alloy in an alternative environment. Accordingly, the scope of the present invention is to be limited only by the following claims, in which it is to be understood that the compositions given include any impurities such as usual impurities present in such alloys.

Claims: A wear-resistant eutectic aluminium-silicon alloy having sufficient lubricity so as to prevent substantial wear and galling thereof even when poorly lubricated, the wear-resistant eutectic aluminium-silicon alloy containing a substantially uniform dispersion of wear-resistant particles throughout and comprising the following by weight: from eleven to 13.5 percent silicon; from three to six percent bismuth; from two to five percent copper; from 1.0 to three percent nickel; from 0.005 to 0.020 percent phosphorus; at most 1.0 percent iron; at most 0.5 percent manganese; at most 0.25 percent titanium; and the balance aluminium and incidental impurities; and, in said wear-resistant eutectic aluminium-silicon alloy, the bismuth component being in its elemental form within the wear-resistant alloy so as to provide lubricity to the wear-resistant alloy, the silicon and aluminium components being in the form of a hard primary silicon phase, and the nickel and copper components being in the form of hard NiAl₃ and CuNiAl₃ phases and relatively independent of temperature, the hard nickel and copper phases being substantially homogeneously dispersed throughout said wear-resistant eutectic aluminium-silicon alloy. A wear-resistant eutectic aluminium-silicon alloy according to claim 1, in which said silicon ranges from substantially twelve to thirteen weight percent. A wear-resistant eutectic aluminium-silicon alloy according to claim 1, in which said bismuth ranges from substantially four to five weight percent. A wear-resistant eutectic aluminium-silicon alloy according to claim 1, in which said copper ranges from two to substantially three weight percent. A wear-resistant eutectic aluminium-silicon alloy according to claim 1, in which said nickel ranges from substantially 1.5 to 2.5 weight percent. A wear-resistant eutectic aluminium-silicon alloy according to claim 1, in which said wear-resistant eutectic aluminium-silicon alloy comprising the following by weight: from twelve to thirteen percent silicon; from four to five percent bismuth; from two to three percent copper; from 1.5 to 2.5 percent nickel; from 0.01 to 0.02 percent phosphorus; at most 0.8 percent iron; at most 0.4 percent manganese; at most 0.2 percent titanium; and the balance aluminium and incidental impurities.

Inventors: ALABI MUFTAU MASUD, ALABI, MUFTAU MASUD
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1992
Main classification: C22C21
IPC codes: C22C21