Patent Publication Number: US-2002000273-A1

Title: Process for nodulizing silicon in casting aluminum silicon alloys

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
     [0001] This application is a continuation of allowed application Ser. No. 09/570,631, filed May 15, 2000. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] This invention relates to methods of casting Aluminum-Silicon alloys and more particularly, to a process for nodulizing silicon crystals in the casting of Al—Si alloys.  
       BACKGROUND OF THE INVENTION  
       [0003] The eutectic Al—Si alloy as conventionally cast, is an excellent wear-resistant light metal, which is widely used for moving and easily worn-out parts in automobiles, airplanes and spacecraft. However, there has long been a need in industry for Al—Si alloys having better resistance to wear coupled with higher tensile strength and other improved mechanical properties. This improvement is required, particularly, to meet present day demands of equipment such as automobile engines and aircraft mechanical controls, as well as for many industrial fabrication applications.  
       [0004] In pursuit of this need, over the past several decades, metallurgists have paid much attention to Al—Si alloys, and particularly to the effects achieved by varying the silicon content of an alloy. When an increase is made in its silicon content, the alloy&#39;s resistance to wear rises. However, the silicon crystals also tend to become coarse and angular, which leads to a weakening of the alloy&#39;s mechanical properties, particularly ductility, castability and machinability. To overcome the problems associated with increased silicon content, metallurgists in several countries have investigated methods of refining the silicon crystals in the alloy in order to improve the properties and microstructure of the Al—Si alloy, and further to obtain nodular silicon.  
       [0005] In the 1950&#39;s and 1960&#39;s, many different impurities such as Na, Sr, rare earth elements (RE) and others or their combination, were added to the molten Al—Si alloy prior to casting in an attempt to obtain nodular silicon crystals in eutectic or hypereutectic alloys. These efforts were not successful, resulting in a poor quality Al—Si alloy having a tendency toward porosity and being unsuitable for industrial use.  
       [0006] Other methods have been tried to nodulize silicon crystals in a eutectic Al—Si alloy (Si&lt;12%), including heating the alloy to high temperatures, near the melting point of the alloy. However, this approach produced only coarse clusters of nodular silicon. It was found that high temperatures will not nodulize or blunt the primary silicon crystals, and the effort was dropped as unsuccessful.  
       [0007] Of late, several patents directed to methods of improving the quality of high-silicon (hypereutectic) Al—Si alloys have been issued. Although these patented Al—Si alloys offer generally better properties than existing conventional high-silicon alloys, their silicon crystal morphology still exhibits an angular form and tends to be clustered, not favoring wear resistance, so that any improvement over the conventional high-silicon alloys is marginal at best in the critical characteristic of wear.  
       [0008] Therefore, there remains an important industrial need for a process for nodulizing silicon in casting aluminum-silicon alloys that will produce alloys having significantly improved mechanical properties such as resistance to wear, tensile strength, low porosity and machinability coupled with low cost.  
       SUMMARY OF THE INVENTION  
       [0009] A process for nodulizing the silicon crystals in casting aluminum-silicon (Al—Si) alloys is described, together with test results of the produced alloys. In the process of the invention, an Al—Si alloy melt is to be refined by the addition of small percentages of P, rare earth elements (RE), Ti, Zr and B in a master form or in salt bearing elements, followed by heating at a low temperature to nodulize the silicon crystals, and solution treatment and aging. The silicon crystal produced in the process is generally spheroidal in shape and forms primary silicon crystal nodules that are blunted and well distributed. Test results of the nodular silicon eutectic and hypereutectic Al—Si alloys produced show a significantly higher ultimate tensile strength at room temperature and at 300° C., and greatly improved resistance to wear compared with conventionally produced Al—Si alloys.  
       [0010] Accordingly, it is a principal object of this invention to provide a method of nodulizing silicon in aluminum-silicon alloys that will produce alloys having the greatly improved mechanical properties needed by industry.  
       [0011] A major advantage of this inventive process is that the silicon crystals in Al—Si alloys can be spheroidised without clustering.  
       [0012] Further objects and advantages of the invention will be apparent from studying the following portion of the specification, the claims and the attached drawings. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0013]FIG. 1 is a photomicrograph of a eutectic Al-12-Si alloy (12% by weight silicon) that was produced by the inventive process, showing the wide distribution of silicon crystal nodules;  
     [0014]FIG. 2 is a photomicrograph of a hypereutectic Al-18-Si alloy (18% by weight silicon) that was produced by the inventive process;  
     [0015]FIG. 3 is a photomicrograph of a hypereutectic Al-22-Si alloy (22% by weight silicon) that was produced by the inventive process;  
     [0016]FIGS. 4A, 4B and  4 C are graphs showing the length, breadth and average aspect ratio 1/b of silicon nodular crystals in the eutectic Al—Si alloy shown in the FIG. 1 photomicrograph;  
     [0017]FIG. 5 is a graph showing the effect of silicon crystal aspect ratio on the ultimate tensile strength of a Al—Si alloy at 20° C. and at 300° C., temperatures;  
     [0018]FIG. 6 is a graph showing the ultimate tensile strength of eutectic and hypereutectic Al—Si alloys versus alloy silicon content, for Al—Si alloys produced according to the present invention; and  
     [0019]FIG. 7 is a table showing the mechanical properties of eutectic and hypereutectic Al—Si alloys produced according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0020] As was described above, it is extremely advantageous to be able to produce hypoeutectic, eutectic and hypereutectic Al—Si casting alloys having well distributed nodular eutectic silicon and blunted primary silicon, and which have good castability, machinability and greatly improved mechanical characteristics, particularly high resistance to wear.  
     [0021] The present inventive process overcomes the disadvantages of the techniques presently in use wherein the nodulization of eutectic silicon as well as refinement and blunting of primary silicon are performed in a single step at a high temperature. In the present inventive process, two major steps are employed: first, a master alloy with combined elements is added to the Al—Si melt; second, the mixture is heated at a low temperature to nodulize the silicon, avoiding coarsening and clustering of the silicon crystals.  
     [0022] The inventive method for nodulizing the silicon in Al—Si alloy casting, for Al—Si alloys having a silicon content of 8.0 to 23% (the alloys also including amounts of copper, magnesium, and iron) comprises the following process steps:  
     [0023] a) forming a melt by heating a selected starting amount of Al—Si alloy in a graphite crucible in an electrical resistance furnace to the Al—Si alloy liquidus temperature;  
     [0024] b) adding a master alloy mixture into the melt at a master alloy temperature of 150° C. above the starting Al—Si alloy liquidus temperature, using a master alloy mixture composition comprising: Al—Ti 1-10% weight titanium and 90-99% weight aluminum; Al—B 0.2-3.0% weight boron and balance aluminum; Al-RE 4.0-10% weight rare earth elements and 90-96.0% weight aluminum; Al—Zr 1.0-5.0% weight zirconium and the balance aluminum; and Cu—P 5.0-8.0% weight phosphorous and 92.0-95.0% weight copper, stirring the added mixture in the melt and holding for a short time;  
     [0025] c) forming a treated molten by degassing the melt with nitrogen;  
     [0026] d) forming a casting of a new alloy by pouring and casting the treated molten into a mold;  
     [0027] e) treating the casting in solution by:  
     [0028] (1) heating the casting at a temperature of 500-530° C. and holding that temperature for 6-8 hours, causing dissolution of the Cu, Mg, Ni and other elements into the Al matrix and obtaining a solid solution of Al—Si alloy, and  
     [0029] (2) quenching casting by putting the new Al—Si alloy casting at high temperature into water; and  
     [0030] f) aging the new Al—Si alloy casting by heating the casting at a temperature of 130-230° C. for 6-9 hours, to obtain the desired alloy mechanical characteristics.  
     [0031] In selecting amounts of the added elements in the master alloy mixture, it is important to make the selection so that a chemical analysis of an alloy casting produced by the foregoing process includes a particular amount of these elements. This amount has been established as being 0.0050-1% phosphorous, 0.03-0.30% mixed rare earth elements, 0.020.30% titanium, 0.001.-0.10% boron and 0.02-0.10% zirconium by weight. These concentrations of modifier elements have been found to produce the optimum alloy mechanical characteristics, particularly with regard to wear resistance.  
     [0032] The following comments apply to the foregoing procedure steps:  
     [0033] In step a), the starting Al—Si alloy, which is the initial alloy to be modified, will have a liquidus temperature depending upon its chemical composition. For the range of Al—Si alloys to which the above process applies, the liquidus temperature has a range of 700-830° C. and would be known and selected by the engineer.  
     [0034] In step b), the overall weight of the added master alloy components is about 3% of the weight of the molten. The exact amount of modifier elements, such as, Ti, B etc. is selected by the engineer depending on the weight and composition of the starting Al—Si alloy molten, to achieve the required chemical composition of elements in the resulting new Al—Si alloy.  
     [0035] Instead of using metallic master alloys, master alloys may be added into the melt in the form of salt bearing elements as a matter of convenience. These would include titanium alkali-fluoride, alkali boron fluoride, zirconium alkalifluoride, rare earth chloride, and phosphide. The percentage of Ti, B, Zr, RE and P contained therein would be the same as listed above for the metallic master alloy.  
     [0036] An alternative master alloy that may be used comprises: Al—Ti 1-10% weight titanium and 90-99% weight aluminum; Al—B 0.2-3.0% weight boron and balance aluminum; Al-RE 4.0-10% weight rare earth elements and 90-96.0% weight aluminum; Al—Zr 1.0-5.0% weight zirconium and balance aluminum, and incidental impurities. The foregoing weights of Ti, B, RE, and Zr may instead be added in the form of salt bearing elements as described above.  
     [0037] In step f), the heat aging of the Al—Si alloy casting obtained by quenching is necessary to raise the strength of the alloy to meet given requirements. These requirements will vary for different Al—Si alloy compositions and their applications. Thus, the heating temperature and holding time will vary over a range, and are selected depending on the desired alloy characteristics.  
     [0038] The foregoing procedure for the nodulizing of silicon crystals in Al—Si alloys differs from the current conventional procedures primarily by two major features; the adding of a certain master alloy mix to the start melt before pouring, and the subsequent lower temperatures of heat treating the casting. These two features make it possible to achieve the nodulizing of the silicon crystals and their distribution without clustering required to obtain the desired improvements in Al—Si alloy mechanical strength and wear resistance.  
     [0039] To demonstrate the practicality and properties of the Al—Si alloys produced using the inventive nodulizing method, a number of eutectic and hypereutectic Al—Si alloys were produced using the forgoing procedure and their mechanical characteristics tested. A brief description of two examples of these alloy productions and a short summary of their test results is now presented:  
     EXAMPLE  1   
     [0040] For the initial melt, a number of eutectic Al—Si alloys were produced, having a composition with the following ranges: 11.0-13.0% Si, 0.8-1.5%Cu, 0.6-I.0% Mg, &lt;0.5% Mn, and &lt;0.7% Fe. Each alloy melt was prepared in a crucible and metallic master alloys including Al—Ti, Al—B, Al-RE, Al—Zr and Cu—P were added into the melt at 700-720° C. The melts were then degassed with nitrogen. After stirring and holding for  15  minutes, the treated moltens were poured and cast followed by heat treatment, solid solution and aging treatments to produce a new Al—Si alloy casting.  
     [0041] Refer now to FIG. 1, which is a photomicrograph of a eutectic Al-12-Si alloy (12% by weight silicon) that was produced by the inventive process. As shown, the alloy has nodular eutectic silicon crystals well distributed throughout the matrix. This produces excellent mechanical properties, particularly in wear resistance. The shapes of the silicon nodules shown in the photomicrograph are clearly blunted. This aspect is quantified in the graphs of FIGS. 4A, 4B and  4 C. These graphs show the lengths, breadths and the average aspect ratio 1/b of the silicon nodular crystals shown in FIG. 1 vs. their % frequency of occurrence. Approximately 70% of the nodules had an aspect ratio 1/b between 1.5 and 1.9 indicating a high degree of blunting.  
     [0042] Refer now to FIG. 5, which plots the results of tensile strength tests performed on the Al—Si alloys. As shown, the Al—Si alloys which had most nodulized Si crystals have an aspect ratio range of 1.5 to 2.8. An aspect ratio between 1.5 and 1.9 corresponds to the highest levels of tensile strength at alloy temperatures of 20° C. and at 100° C.  
     EXAMPLE 2  
     [0043] In Example 2, a number of hypereutectic Al—Si alloys were produced for the initial melt having a composition with the following ranges: 18.0-23.0% Si, 0.8-1.6% Cu, 0.6-1.0% Mg, &lt;0.5% Mn, and &lt;0.7% Fe. Each alloy melt was prepared in a crucible and metallic master alloys including Al—Ti, Al—B, Al-RE, Al—Zr and Cu-P were added into the melt at 850° C. to achieve the desired levels of titanium, boron, rare earth and phosphorous in the final Al—Si alloy. The melts were then degassed with nitrogen. After stirring and holding for 15 minutes, the treated moltens were poured at 820° C. and cast, followed by heat treatment, solid solution and aging treatments to produce new Al—Si alloys.  
     [0044] Refer now to FIGS. 2 and 3, which are photomicrographs of a hypereutectic Al—Si alloy with 18% silicon content and a hypereutectic Al—Si alloy with 22% silicon content, respectively. In both micrographs, the nodulized silicon crystals are well distributed and blunted, indicating a good wear resistance characteristic. The average nodule aspect ratio is somewhat higher than for the eutectic Al—Si alloys, but results in an alloy ultimate tensile strength only a little below the eutectic alloy strength at 20° C., as shown in FIG. 6, which plots the range of measured ultimate tensile strength vs percent silicon content for the tested alloys referred to above.  
     [0045] A table of the tested and measured mechanical properties of eutectic Al—Si (12% Si) and hypereutectic Al—Si (22% Si) alloys which were produced using the inventive process, is presented in FIG. 7. It is apparent that alloy resistance to wear is greatly improved, having about  60 % less wear compared to conventionally produced Al—Si alloys.  
     [0046] The ultimate tensile strength at 20° C. and at 300° C. is 30% higher than conventional Al—Si alloys with a similar composition, lending itself to possible new alloy applications requiring high tensile strength and endurance.  
     [0047] The machinability of the new alloys is greatly improved as evidenced by the ductility and hardness characteristics. In the hypereutectic test samples, which would normally have angular shaped silicon crystals that worsen machinability, nodular silicon crystals replace angular silicon and the alloy machinability is greatly improved and can be machined by a hard metal tool. At a silicon level less than 16%, the new alloy has very good machinability.  
     [0048] Regarding castability of the alloys, an examination of the cast tested alloys found few defects such as porosity, pinholes and shrinkage, which are regularly found in conventional hypereutectic Al—Si alloys. Thus, complex castings such as cylinder heads and blocks can be cast of the new alloys, which represents an advance in automobile engine manufacturing techniques of considerable significance for cost and weight reduction.  
     [0049] Finally, it should be understood that the elements used for the added master alloy are commonly used in foundries and are not expensive. It is easy to produce the metallic master alloy comprising the five elements. The melting point of the master alloy at about 650° C. is much lower than the melting temperature of the new alloy. Also, the amount of master alloy added is about 3% or less of the molten weight, making it easy to add the master to the molten alloy, with no pollution or smoking problems.  
     [0050] In view of its generally excellent qualities as described above, combined with a low cost and simple technology, it is anticipated that many automobile manufacturers will be interested in using the aluminum alloys that can be produced by this invention process.  
     [0051] From the foregoing description, changes and various modifications may be apparent to those skilled in the art. These alternatives and modifications are considered to be within the spirit and scope of the present invention.