Hypo-eutectic aluminium-silicon-copper alloy having bismuth additions

Abstract: An improved aluminium-silicon-copper 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 so as to provide a sufficiently low-friction bearing surface (or self-lubricity), which significantly enhances the wear-resistant properties of the alloy. In particular, the preferred aluminium alloy is suited for use in a socket plate (10) which receives high-strength steel bearing members (16) within a compressor unit of an automobile air-conditioning system. The improved aluminium alloy should minimize wear and alleviate galling of the socket plate (10) during use. In addition, the improved aluminium alloy should have sufficient strength and ductility so as to permit swaging of the socket plate (10) formed from the alloy around respective balled ends (14) of the high-strength steel bearing members (16).

Description: The present invention generally relates to room temperature-formable, wear-resistant aluminium alloys such as an aluminium-silicon alloy which is suitable for use in a wearing environment. 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 commonly-used fluorocarbons (known as freon) or other alternative refrigerants. Within an automobile, the engine-driven compressor compresses the vaporized refrigerant, thereby significantly raising the temperature of the refrigerant. The refrigerant then flows into a condenser where it is cooled and returned to its liquid state; thus, the heat added to the refrigerant in the compressor is transferred out of the system. The cooled liquid refrigerant is then sprayed through an expansion valve into an evaporator where it is again vaporized. The heat of vaporization required for vaporizing the refrigerant is drawn from the incoming outside air, which is blown around the evaporator. Any excess humidity contained within the incoming air is removed as condensation on the evaporator, thereby also drying the incoming air. The cooled, dry air then enters the passenger compartment of the vehicle. 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. More particularly, in a five cylinder compressor which is in use within several automotive air-conditioning systems, known generally as a "Wobble Plate" compressor, there is a "wobble plate" which has five pocketed regions. As shown in Figure 1, the "wobble plate" has five sockets and is accordingly also referred to as a socket plate. As shown in Figures 2 and 3, five high-strength steel connecting rods are engaged within each of the five sockets, so as to reciprocate within each of the sockets. The socket plate material at each socket is then swaged around a ball end of the connecting rod. Typically the socket plate has been formed from an aluminium-silicon alloy such as A356 or A357, and has performed quite satisfactorily with the previous refrigerant/lubricant combinations. However, in the absence of a suitable lubricant for use with the new refrigerant, excessive wear and even galling has occurred between the connecting rods and the aluminium-silicon sockets, whereby the softer socket material repeatedly attaches and welds itself to the harder steel connecting rod during use. This is not surprising since the conventional aluminium-silicon alloys are known for their good wear resistance, but only when lubrication is present since their hard surfaces can be quite damaged when lubrication is not present. Therefore what is needed is an aluminium alloy for use in this socket plate which is particularly tolerant even when poorly lubricated, and which resists galling and wear by providing a certain degree of self-lubricity. In addition, the aluminium alloy must be sufficiently ductile to permit swaging of the material, yet sufficiently strong to contain the high-pressure refrigerant over the repeated thermal cycling experienced within a typical automotive environment. It is therefore an object of this invention to provide a wear-resistant aluminium-silicon-copper alloy particularly suitable for use as a wearing component, such as a socket plate, in a compressor unit of an automobile air-conditioning system. It is a further object of this invention that such an aluminium-silicon-copper alloy be sufficiently self-lubricating so as to prevent galling of the socket plate during use even when poorly lubricated. Lastly, it is still a further object of this invention that such an aluminium-silicon-copper alloy be characterized by sufficient ductility so as to permit swaging of the socket plate formed from the alloy, yet by sufficient strength to contain a pressurized refrigerant. 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 aluminium-silicon-copper alloy having a relatively substantial addition of bismuth. The aluminium-silicon-copper alloy is particularly wear-resistant and sufficiently self-lubricating so as to be suitable for use as a wearing component, such as a socket plate which receives bearing members within a compressor unit of an automobile air-conditioning system. The improved aluminium-silicon-copper alloy having the bismuth additions minimizes wear and alleviates galling of the socket plate during use even when poorly lubricated. In addition, the improved aluminium-silicon-copper alloy of this invention is also characterized by good strength and sufficient ductility so as to permit room temperature-swaging of the socket plate formed from the preferred alloy. As defined in claim 1 the wear-resistant aluminium-silicon-copper alloy of this invention is characterized by the following elemental composition, wherein the percentages are weight percents: from 6.5 to twelve percent silicon with 6.5 to 10.5 percent being most preferred; from two to five percent copper with three to four percent being most preferred; from one to five percent bismuth with three to four percent being most preferred; at most 0.2 percent iron; at most 0.15 percent manganese; at most 0.15 percent zinc; optionally 0.005 to 0.015 percent phosphorus; and the balance being aluminium with incidental impurities; said bismuth essentially remains in its elemental form within said wear-resistant alloy so as to minimise the coefficient of friction of said alloy. In addition, the preferred aluminium-silicon-copper alloy consists of up to 0.15 percent iron; up to 0.1 percent manganese; and up to 0.1 percent zinc, with the balance of the preferred alloy being aluminium. A particularly advantageous feature of the aluminium-silicon-copper alloy of this invention is that the relatively high level of bismuth remains essentially as elemental bismuth within the alloy, which 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 preferred alloy enhances the wear-resistant properties of the alloy. Preferred embodiments of the alloy defined in claim 1 are given in the dependent claims 2 to 6. The invention also relates to a method for forming the wear resistant Al-Si-Cu-Bi alloy defined in claims 7 and 8. It is presumed that socket plates formed from the preferred alloy would exhibit significantly diminished signs of wear during use with connecting rods which are engaged within the sockets of the socket plate. In addition, the preferred aluminium alloy of this invention is sufficiently ductile so as to be swageable at room temperature. Furthermore, the strength of this preferred aluminium alloy is sufficient to contain a high-pressure refrigerant even over extended thermal cycling which is characteristic of an automotive environment. Other objects and advantages of this invention will be better appreciated from the following detailed description. The above and other advantages of this invention will become more apparent from the following description taken in conjunction with the accompanying drawings, in which: Figure 1 shows a socket plate, known as a "wobble plate", formed from the preferred aluminium alloy of this invention, for use in an automotive air-conditioning compressor unit wherein the socket plate has five socket regions; Figure 2 shows a cross-sectional view of a socket region shown in Figure 1; and Figure 3 shows the socket plate of Figure 1 having five high-strength steel connecting rods inserted within each of the five socket regions. According to the present invention, there is provided an improved aluminium-silicon-copper alloy having a substantial addition of bismuth. The improved aluminium-silicon-copper alloy exhibits good wear-resistance by being sufficiently self-lubricating, and therefore is particularly suited for use as a wearing component. As shown in Figure 1, the preferred alloy of this invention is used to form a socket plate 10 for use in an automotive air-conditioning five cylinder compressor unit. The socket plate 10 has five pocketed regionsin the form of sockets 12. As shown in Figure 3, the socket regions 12 of the socket plate 10 receive five high-strength steel connecting rods 16. A balled end 14 of each of the five connecting rods 16 engages a corresponding socket 12 on the socket plate 10, as illustrated in Figure 3 and cross-sectionally in Figure 2. The dimensional tolerances between the socket 12 and the balled end 14 of the connecting rod 16 are at most only a few hundreths of a millimetre (a few thousandths of an inch). The connecting rods 16 reciprocate within the socket regions 12 of the socket plate 10 and are continuously subjected to high pressures from the refrigerant during operation of the automotive air-conditioning compressor unit. Therefore the balled end 14 is essentially a bearing member which bears against the socket 12. (It is to be noted that although the socket plate 10 is the intended application for the alloy of this invention, it is foreseeable that this alloy could be used in a multitude of various applications, such as those which require wear-resistance.) The improved aluminium-silicon-copper alloy of this invention minimizes wear and alleviates galling of the socket plate 10 during use even when poorly lubricated. In addition, the improved aluminium alloy is sufficiently ductile so as to permit room temperature-swaging of each of the socketed regions 12 around the balled end 14 of the corresponding connecting rod 16 which is engaged within each of the socket regions 12. In particular, the self-lubricating, wear-resistant aluminium-silicon-copper alloy of this invention is characterized by the preferred elemental composition shown in Table I., wherein the percentages refer to weight percents. Si 6.5% - 12.0% Bi 1.0% - 5.0% Cu 2.0% - 5.0% Fe 0.2% (max.) Mn 0.15% (max.) Zn 0.15% (max.) Al Balance More particularly, the silicon (Si) content of the preferred aluminium-silicon-copper alloy of this invention may vary from 6.5 to twelve percent so as to ensure good wear-resistance of the material, with the range of 6.5 to 10.5 percent being most preferred. The silicon within the alloy reacts with the aluminium to form hard aluminium-silicon particles which increase the wear-resistance of the material, as discussed more fully later. The silicon content of the preferred aluminium-silicon-copper alloy should remain below 12.3 percent, which is the eutectic point in the aluminium-silicon phase diagram, making the preferred alloy a hypo-eutectic aluminium-silicon-copper alloy. Maintaining the silicon level below the eutectic point ensures that hard primary silicon particles will not form within the preferred alloy. In addition, the ductility of the alloy is reduced as the silicon content is increased, accordingly it is desirable to maintain this hypo-eutectic level of silicon. Sufficient ductility is required so that the material of the socket plate 10 is capable of being swaged, for use in the compressor unit of the air-conditioning system. Therefore, in the preferred alloy, the presence of primary silicon particles is not desired and so to ensure that the formation thereof does not occur, as well as to ensure a sufficient level of ductility, the silicon content is kept well below the eutectic point. The preferred copper (Cu) content within the aluminium alloy of this invention may vary from two to five weight percent with three to four weight percent being most preferred. The presence of copper within the alloy provides a strengthening mechanism by reacting with the aluminium to form copper aluminide (CuAl₂). The copper appears to also enhance the age-hardening affects of the alloy by precipitating many fine particles within the alloy which results in the homogeneous dispersion of the copper aluminide throughout the alloy. The bismuth (Bi) content of the preferred aluminium-silicon-copper alloy may vary from one percent to five weight percent, with a range of three to four percent being most 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 preferred alloy to be essentially self-lubricating, thereby alleviating excessive wear and galling of the preferred aluminium-silicon-copper base alloy during use. An advantageous feature of this invention is that many 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 then be limited, since bismuth tends to react with magnesium so as to reduce the strengthening potential of the alloy by detrimentally tying up the magnesium. Therefore, it is generally desirable to eliminate the bismuth content within these types of alloys that require strength. Yet in the preferred 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 preferred iron (Fe) content within the aluminium alloy of this invention may vary up to 0.2 percent iron, with a maximum level of 0.15 or less being most preferred. The ductility of the alloy is typically impaired by the presence of iron within the alloy due to the formation of the 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 primary aluminium metal used to form the alloy. In addition, the manganese (Mn) and zinc (Zn) contents within the preferred alloy may each vary up to 0.15 weight percent, with a maximum of 0.1 percent being most preferred for each element. These ranges for both the manganese and zinc again are normal levels found within the primary aluminium after smelting from bauxite prior to casting of the alloy, and are therefore going to be normally present within the final alloy. Optionally, the alloy of this invention may include between 0.005 and 0.015 weight percent phosphorus (P), with a concentration of 0.010 percent being most preferred. This trace amount of phosphorus will tend to increase or induce the formation of some primary silicon so as to enhance the wear-resistance of the alloy without significantly reducing the ductility or coefficient of friction of the alloy. The phosphorus would be added to the alloy by conventional phosphorus treatment methods, which include adding a phosphorus-containing compound, such as a phosphorus-copper compound, to the melt during casting, since it is difficult to add phosphorus directly to the melt because of its fine powdery form. The addition of phosphorus is optional depending on the desired application for the material and considering the competing concerns of ductility versus wear-resistance. It is also to be noted that trace levels of strontium could be substituted for the phosphorus within the alloy with essentially the same results obtained. The strontium would provide a structure which is slightly higher in ductility, but a slight loss in the galling or wear-resistance of the alloy would also be observed. Depending on the application, this may be desirable. The balance of the preferred alloy is aluminium. The most preferred composition for the alloys, as discussed above, is summarized in Table II. Again, the percentages refer to weight percents. Si 6.5% - 10.5% Cu 3.0% - 4.0% Bi 3.0% - 4.0% Fe 0.15% (max. Mn 0.1% (max.) Zn 0.1% (max.) Al Balance Test bar samples, weighing about 50-60 grams each, of the preferred alloy of Table II. were heat-treated using a conventional T61 aluminium alloy heat-treating schedule as described by the following steps, so as to maximize the tensile and yield strengths of the alloy whilst maintaining adequate elongation. 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 for particular applications. Several different heat-treatment schedules were tried on the preferred alloy in order to determine the optimum heat-treatment schedule, described below, for the alloy in the socket plate 10 environment. The preferred alloys were heat-treated by first solution-treating the alloy to about 532°C ± 5°C (990°F ± 10°F) for about 12 hours to form a solid solution of copper-aluminide (CuAl₂), quenching in water, and then artificially aging the alloy at about 154°C ± 2.5°C (310°F ± 5°F) for about fifteen hours to precipitate the copper-aluminide throughout the alloy. Although this heat-treatment schedule resulted in the preferred alloy having the desired properties for the socket plate 10 application, it is foreseeable that other heat-treatment schedules could be employed having reasonable variations, such as in the temperatures, durations, or quenching medium, with satisfactory results expected. The mechanical properties for the preferred aluminium-silicon alloys heated treated in accordance with the above procedure were determined and are as follows; an average yield strength of about 268,895.6 kPa (39,000 pounds per square inch (psi)), and an average tensile strength of about 351,632.8 kPa (51,000 psi), with an average elongation value of about four percent. The strength of the alloy is sufficient to contain the high-pressure refrigerant during operation of the compressor even over repeated thermal cycling, as compared to previous conventional materials which have been employed for the socket plate 10. The average hardness, as determined by standard Brinell measurements (using a 500 kilogram load and 10 millimetre indentor for 30 seconds) was about 86. Upon conventional metallographic examination, the microstructure of the alloy exhibited well-dispersed aluminium-silicon and bismuth phases throughout the aluminium matrix of the alloy. It is believed that the aluminium-silicon particles within the alloys of this invention are characterized by an acicular or needle-like shape. These acicular particles are characterized by a high surface area, or high aspect-ratio, which improves the wear-resistance of the alloy. The presence of these hard acicular silicon particles within the alloys of this invention have been found to significantly improve the wear and galling-resistant properties of the alloys. It is believed that the alloy of this invention would exhibit enhanced wear and galling-resistance in an actual wearing environment, such as a socket plate 10 within an air-conditioning compressor, due to the hardness and high strength of the alloy when coupled with its lubricity. The self-lubricating qualities of the alloy are provided by the bismuth additions. In addition, it is believed that the alloy of this invention is sufficiently ductile and would permit successful room temperature-forming operations, such as swaging, as compared to the formability of previous conventional materials used in the socket plate 10. In summary, there are many advantageous features associated with the aluminium-silicon-copper 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 its machinability. Socket plates which would be formed from the preferred alloy should exhibit significantly enhanced wear and galling-resistance during use when connecting rods are engaged within the sockets of the socket plate. Therefore, while 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 preferred 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 scope of the following claims. Applicants have a co-pending European patent application on file of even date, European patent application No. 92 201 910.4, based upon USSN 733,539,(1991), which discloses and claims a room temperature-formable, wear-resistant aluminium alloy containing tin and bismuth.

Claims: A room temperature-formable, wear-resistant aluminium alloy particularly suitable for use as a wear-surface component in an automotive air-conditioning compressor unit, said wear-resistant aluminium alloy consisting of (by weight) : 6.5 to twelve percent silicon; two to five percent copper; one to five percent bismuth; at most 0.2 percent iron; at most 0.15 percent manganese; at most 0.15 percent zinc; optionally 0.005 to 0.015 percent phosphorus; and the balance being aluminium with incidental impurities; said bismuth essentially remains in its elemental form within said wear-resistant alloy so as to minimise the coefficient of friction of said wear-resistant alloy, thereby enhancing the self-lubricity and wear-resistant properties of said alloy, whilst all of said constituents sufficiently co-operate with one another within said alloy to provide adequate ductility, so as to permit the room temperature-forming operations of said alloy, and sufficient self-lubricity so as to prevent wear and galling of a wear-surface component formed from said alloy even when said component is poorly lubricated. A wear-resistant aluminium alloy according to claim 1, in which said silicon ranges from 6.5 to 10.5 percent by weight of the alloy. A wear-resistant aluminium alloy according to claim 1, in which said bismuth ranges from three to four percent by weight of the alloy. A wear-resistant aluminium alloy according to claim 1, in which said copper ranges from three to four percent by weight of the alloy. A wear-resistant aluminium alloy according to claim 1, in which said silicon sufficiently co-operates with said aluminium in said alloy so as to form an aluminium-silicon phase which is primarily acicular in shape. A wear-resistant aluminium alloy according to any one of claims 1 to 5, when used in an automotive air-conditioning unit having a socket plate formed from the wear-resistant alloy, wherein the socket plate has a plurality of sockets, each of which receives a corresponding bearing member, said wear-resistant aluminium alloy having sufficient lubricity so as to prevent wear and galling between the sockets and bearing members even when poorly lubricated, whilst also having sufficient ductility so as to permit swaging of each socket around the corresponding bearing member. A method for forming a wear-resistant aluminium-silicon-copper alloy according to claim 6, comprising the steps of: casting a homogeneous melt so as to form said wear-resistant aluminium-silicon-copper alloy; solution-treating said wear-resistant aluminium-silicon-copper alloy at a temperature and for a duration sufficient to form a solid solution of copper-aluminide therein, and then quenching said alloy in water; and age-hardening said wear-resistant aluminium-silicon-copper alloy at a temperature and for a duration sufficient to precipitate particles of said copper-aluminide within said alloy and such that said silicon and aluminium co-operate with one another to form a primarily acicular-shaped aluminium-silicon phase within said alloy. A method for forming a wear-resistant aluminium-silicon-copper alloy according to claim 7, in which said solution-treating step occurs at a temperature from 521°C (970°F) to 538°C (1000°F), and said age-hardening step occurs at a temperature from 151.5°C (305°F) to 157°C (315°F).

Inventors: ALABI MUFTAU MASUD, ALABI, MUFTAU MASUD
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1992
Main classification: C22C21
IPC codes: C22C21, B60H1, F16C33