Hypo-eutectic aluminium-silicon alloy having tin and bismuth additions

Abstract: An improved aluminium-silicon alloy having relatively substantial additions of tin and 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 tin and bismuth additions within the alloy co-operate 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 used to form 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 minimizes wear and alleviates galling of the socket plate (10) during use. In addition, the improved aluminium alloy is also characterized by good strength and sufficient 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 aluminium-silicon alloys, particularly those alloys which are used in automotive applications. 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 where rubbing occurs, 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 at their bearing surfaces. 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 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 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 so as 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 inserted within each of the five sockets. The socket plate material at each socket is then swaged around a balled end of each 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 balled ends of 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 in the absence of a lubricant, 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 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 alloy be sufficiently self-lubricating so as to prevent galling of the socket plate during use even in the presence of poor lubrication. Lastly, it is still a further object of this invention that such an aluminium 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 an 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 alloy as defined in claim 1 having relatively substantial additions of tin and bismuth, which 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 alloy minimizes wear and alleviates galling of the socket plate during use even in the absence of additional lubricants. In addition, the improved aluminium alloy is also characterized by good strength and sufficient ductility so as to permit swaging of the socket plate formed from the alloy. Preferred embodiments of the wear resistant Al-alloy are given in the dependent claims 2 to 6. The present invention relates also to a method of forming the claimed wear resistant Al-alloy as defined in claims 7 and 8. The wear-resistant aluminium alloy of this invention is characterized by the following elemental composition, wherein the percentages are weight percents: from seven to eleven percent silicon with seven to nine percent being most preferred; from one-half to three percent bismuth with one to two percent being most preferred; from one-half to three percent tin with one to two percent being most preferred; and from 0.3 to 0.6 percent magnesium with 0.45 to 0.6 percent being most preferred. In addition, the preferred aluminium alloy consists of up to 0.2 percent copper; up to 0.2 percent iron; up to 0.1 percent manganese; up to 0.1 percent zinc; and optionally between 0.005 and 0.015 percent phosphorus, with the balance of the alloy being aluminium. A particularly advantageous feature of the aluminium-silicon alloy of this invention is that the relatively high tin and bismuth additions co-operate with the other elemental additions so as to provide a sufficiently low friction surface (or self-lubricity) which enhances the wear-resistant properties of the alloy. Socket plates formed from the preferred alloy exhibit significantly diminished signs of wear during use when connecting rods 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 successfully swaged at room temperature. Furthermore, the strength of this preferred aluminium alloy is also 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", having five socket regions, formed from the preferred aluminium alloy of this invention, for use in an automotive air-conditioning compressor unit; 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 alloy having relatively substantial additions of tin and bismuth as defined in claims 1 to 6. The improved aluminium-silicon 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 regions in 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 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 swaging of each of the socketed regions 12 around the balled end 14 of the corresponding connecting rod 16. In particular, the self-lubricating, wear-resistant aluminium-silicon alloy of this invention is characterized by the elemental composition shown in Table I., wherein the percentages refer to weight percents. Si 7.0% - 11.0% Bi 0.5% - 3.0% Sn 0.5% - 3.0% Mg 0.3% - 0.6% Cu 0.25% (max.) Fe 0.2% (max.) Mn 0.15% (max.) Zn 0.15% (max.) Al Balance More particularly, the silicon (Si) content of this aluminium-silicon alloy may vary from seven to eleven percent to ensure good wear-resistance of the material, with the range of seven to nine 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 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 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 bismuth (Bi) content of the aluminium-silicon alloy may vary from one-half percent to three percent, with a range of one to two 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. However, although the lubricity of the alloy is enhanced by the bismuth and it would seem therefore that a greater amount is desired, the bismuth must also be limited since it tends to react with the magnesium to form Mg₃Bi₂ which reduces the strengthening potential of the alloy, since the magnesium generally provides the strengthening phase within the alloy. Therefore, it is desirable to limit the bismuth content to one to two weight percent within the preferred alloy. This range of bismuth provides the enhanced lubricity whilst minimizing the loss of strength of the alloy. The tin (Sn) content of the aluminium-silicon alloy may also vary from one-half to about three weight percent, with one to two percent being preferred. The effects of tin within the alloy are very similar to the self-lubricating effects associated with the presence of bismuth, as discussed above. The tin remains primarily as elemental tin within the alloy and tends to enhance the lubricity of the alloy by reducing the coefficient of friction of the bearing rubbing surfaces. However, some tin also tends to react with the magnesium thereby reducing the strengthening potential for the alloy. The range of one to two weight percent tin appears to optimize these competing concerns and therefore is again most preferred. The magnesium (Mg) content ranges from 0.3 to 0.6 weight percent, with a range of 0.45 to 0.6 percent being preferred. The magnesium reacts with the aluminium during heat treatment to form a strengthening compound, magnesium-silicide (Mg₂Si), which precipitates out and resides in the grain boundaries of the alloy. The preferred concentration of magnesium within the alloy appears to result in an adequate level of strength for the alloy used in the socket plate 10. However, depending on the intended application for the alloy, the strength requirements may vary which may accordingly necessitate a change in the magnesium content. The copper (Cu) content within the aluminium alloy of this invention may vary up to 0.25 percent with 0.2 percent being preferred. It is believed that the copper enhances the age-hardening effects of the alloy by precipitating fine particles of copper-aluminide (CuAl₂) homogeneously throughout the alloy, which is an additional strengthening phase for the alloy. Yet, an excess of copper may tend to degrade the overall characteristics of the alloy. Therefore optimal results appear to be achieved with a maximum copper concentration of 0.2 weight percent. The iron (Fe) content within the aluminium alloy of this invention may vary up to 0.2 percent iron. 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 alloy may each vary up to 0.15 weight percent, with a maximum of 0.1 percent being preferred for each element. These ranges for both the manganese and zinc again are normal levels found within the primary aluminium metal 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 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 the 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 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. Several samples of the self-lubricating, wear-resistant aluminium-silicon alloy of this invention were prepared using conventional casting techniques. The most preferred composition for the alloys, as discussed above, is summarized in Table II. Again, the percentages refer to weight percents. Si 7.0% - 9.0% Bi 1.0% - 2.0% Sn 1.0% - 2.0% Mg 0.45% - 0.6% Cu 0.2% (max.) Fe 0.2% (max.) Mn 0.1% (max.) Zn 0.1% (max.) P 0.005% - 0.015% Al Balance Test bar samples 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 these wear-resistant alloys. Several different heat-treatment schedules were tested for the preferred alloy in order to determine the optimum treatment. The following two T61 heat-treatment schedules having different solution treatment temperatures and durations, were employed on two sets of the preferred alloy and resulted in the optimal properties for the alloy in the socket plate 10 environment. Schedule A consisted of first solution-treating the alloy to about 527°C ± 2.5°C (980°F ± 5°F) for about 12 hours to form a solid solution of magnesium-silicide (Mg₂Si) therein, quenching in water, then naturally aging the alloy at room temperature for less than about 15 minutes, and finally artificially aging the alloy at about 154°C ± 2.5°C (310°F ± 5°F) for about eight hours to precipitate the magnesium-silicide throughout the alloy. Schedule B consisted of again first solution-treating the alloy to about 516°C ± 5°C (960°F ± 10°F) for about 8 hours so as to form the magnesium-silicide compound, quenching in water, then naturally aging the alloy at room temperature for less than about 15 minutes, and finally artificially aging the alloy at about 154°C ± 2.5°C (310°F ± 5°F) for about eight hours to precipitate the magnesium-silicide throughout the alloy. The natural aging step is not necessary, however it may occur due to the practicalities of moving these alloys from one processing station to another during the heat-treatment process. Both heat-treatment schedules resulted in the preferred alloy having the desired properties for the socket plate application. Therefore either heat-treatment schedule could be employed, as well as reasonable variations such as in the temperatures, durations, or quenching medium, with satisfactory results expected. After heat treatment, the preferred aluminium-silicon alloys of this invention were examined using conventional metallographic techniques. 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 preferred alloys of this invention contain these acicular-shaped particles throughout the entire alloy. The presence of these hard acicular silicon particles throughout these alloys has been found to significantly improve the wear and galling-resistant properties of the alloys, particularly as compared to conventional alloys. The mechanical properties for the preferred aluminium-silicon alloys heat-treated in accordance with Schedule B were determined and are as follows; an average yield strength of about 186,158.5 kPa (27,000 pounds per square inch (psi)), and an average tensile strength of about 227,527.1 kPa (33,000 psi), with an average elongation value of about six 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 69. The preferred alloy of this invention having the tin and bismuth additions, was tested against several other conventional aluminium alloys which would be considered for use in a bearing environment which requires good wear resistance. The preferred alloy of this invention exhibited significantly better wear and galling resistance. Lastly, it should be noted that the socket regions 12 of the socket plate 10 formed from the alloy of this invention were successfully swaged at room temperature around the balled ends 14 of the corresponding connecting rods 16, with the results of this forming operation being comparable to previous conventional materials which have been employed for this purpose. Therefore the alloy of this invention is sufficiently ductile as compared to the conventional materials. In summary, there are many advantageous features associated with the aluminium-silicon alloy of this invention. The relatively high tin and bismuth additions essentially remain in their elemental form within the alloy and co-operate with the other constituents to provide a sufficiently self-lubricating, low-friction surface which, in turn, enhances the wear-resistant properties of the alloy as well as its machinability. Socket plates 10 formed from the preferred alloy exhibit significantly enhanced wear and galling resistance during use when connecting rods 16 are engaged within the sockets 12 of the socket plate 10. In addition, the preferred aluminium alloy of this invention is sufficiently ductile so as to be successfully swaged at room temperature. Further, the strength of this preferred aluminium alloy is also sufficient to contain a high-pressure refrigerant. 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 preferred ranges of element concentrations, or by modifying the processing steps, or by employing the alloy in a different 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. 92201909.6 ≡ EP-A-0 528 453, based upon USSN 733,540, which discloses and claims a room temperature-formable, wear-resistant aluminium alloy containing silicon, copper 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): seven to eleven percent silicon; one-half to three percent bismuth; one-half to three percent tin; 0.3 percent to 0.6 percent magnesium; at most 0.25 percent copper; 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 tin and bismuth additions essentially remain in their elemental form within said wear-resistant aluminium alloy so as to provide lubricity to said wear-resistant alloy; said magnesium and silicon sufficiently co-operate with one another in the alloy to form a strengthening component of said wear-resistant alloy; and said wear-resistant alloy has adequate ductility, so as to permit said 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 seven to nine percent by weight of the alloy. A wear-resistant aluminium alloy according to claim 1, in which said bismuth and tin both range from one to two percent by weight of the alloy. A wear-resistant aluminium alloy according to claim 1, in which said magnesium ranges from 0.45 to 0.6 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-silicon 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 each respective bearing member. A method for forming a wear-resistant aluminium-silicon alloy according to claim 6, comprising the steps of: casting a homogeneous melt so as to form said wear-resistant aluminium-silicon alloy; solution-treating said aluminium-silicon alloy at a temperature and for a duration sufficient to form a solid solution of magnesium-silicide therein and then quenching said alloy in water; and age-hardening said aluminium-silicon alloy at a temperature and for a duration sufficient to precipitate particles of said magnesium-silicide 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 alloy according to claim 7, in which said solution-treating step occurs at a temperature from 510°C (950°F) to 529.5°C (985°F), and said age-hardening step occurs at a temperature ranging 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: B60H1, C22C21, F16C33