Patent Publication Number: US-2004055724-A1

Title: Semi-solid metal casting process and product

Description:
PRIORITY  
     [0001] This application claims priority to the provisional U.S. patent application entitled, Semi-solid Metal Casting Process and Product Thereof, filed Sep. 20, 2002, having a Ser. No. 60/411,872, the disclosure of which is hereby incorporated by reference. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] The present invention relates generally to the process of casting metal alloys. More particularly, the present invention relates to a method of casting aluminum-silicon alloys for semi-solid metal rheocasting.  
       BACKGROUND OF THE INVENTION  
       [0003] Semi-solid metal (SSM) aluminum alloy castings outperform, in both cost and performance, other casting techniques, such as conventional die casting, gravity permanent mold casting, and squeeze casting. SSM casting methods, when utilized for the manufacturing of hypereutectic aluminum (Al) alloy products/castings have advantages over other casting techniques because SSM castings tend to exhibit higher mechanical properties in the areas of strength and wear resistance, ductility, and reduced porosity than castings produced by the above-listed other methods.  
       [0004] Of the several methods to achieve cast components with SSM alloys, thixocasting is the most common approach. Thixocasting involves the heating of a metal alloy to the liquid state and then the electromagnetic stirring of the melt during solidification/freezing. These billets are subsequently cut into slugs, and re-heated to a semi-solid state before being injected for casting. Alternatively, rheocasting, which is also known as “slurry” or “slurry-on-demand” casting, eliminates several steps required by thixocasting techniques. This process involves singularly heating a metal to a liquid state and then cooling the molten metal to the required SSM phase, before injecting the semi-solid metal into the mold/die cavity.  
       [0005] The mechanical and metallurgical properties of hypereutectic SSM castings are predicated, in part, by the microstructures of primary Si in the final part. The size and morphology of these particles can be controlled by the cooling rate of the hypereutectic alloy to the required temperature and the isothermal hold time at the SSM temperature. Because solid primary phase particles are a part of the semi-solid metal being injected into a mold/die cavity, the microstructure of the primary phase of an aluminum alloy prior to injection into a mold/die is indicative of the microstructure of the primary phase of the resulting aluminum alloy casting. Thus, the mechanical properties of a casting can be predicted before a casting is even produced. Accordingly, many attempts have been made to improve methods to achieve the requisite microstructure. Known strategies including electromagnetic stirring and addition of grain refiners.  
       [0006] One concern in the casting of hypereutectic aluminum-silicon (Al—Si) alloys is to achieve a homogeneous distribution of primary silicon (Si) both in the melt and in the final part. Uneven distribution of large globular primary Si aggregates can seriously compromise the mechanical integrity of a casting. The morphology of the primary Si depends on the imposed temperature gradient, presence of impurities, and ease of nucleation. Most of the research in this regard has been, however, related to conventional casting of Al—Si alloys and little has been learned regarding SSM casting of Al—Si alloys. Therefore, the conventional casting of Al—Si alloys has been successful in industry, while significant challenges remain in rheocasting of these alloys, particularly in controlling the microstructure of Al—Si alloys during rheocasting of SSM materials.  
       [0007] Accordingly, it is desirable to provide a method of utilizing the rheocasting method of SSM hypereutectic Al—Si alloys that can impart desirable mechanical properties. In particular, there is a need for a process to control for the nucleation of primary Si particles in hypereutectic Al—Si alloys with high Si content to limit the primary Si size. Further still, it is desirable to provide a method of producing products with Al—Si alloy castings by rheocasting techniques wherein the temperature and the final composition of the product can be control led.  
       SUMMARY OF THE INVENTION  
       [0008] It is therefore a feature and advantage of the present invention to provide an SSM casting process to generate a desirable hypereutectic Al—Si composition by mixing together a hypoeutectic liquid with one that is hypereutectic.  
       [0009] It is another feature and advantage of the present invention to provide an SSM casting process to control the temperature and cooling rate of a hypereutectic Al—Si alloy melt from mixing a cooler alloy with a hotter one.  
       [0010] It is another feature of the instant invention to manufacture Al—Si alloy products of by mixing a hypoeutectic Al—Si liquid with one that is hypereutectic  
       [0011] There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described below and which will form the subject matter of the claims appended hereto.  
       [0012] In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting.  
       [0013] As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0014]FIG. 1 is a phase diagram of the composition versus temperature of the alloys used in the mixing experiments.  
     [0015]FIG. 2 shows the time versus temperature plot for various experiments.  
     [0016]FIG. 3 shows representative microstructures from the castings produced from the experiments outlined in FIG. 2. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION  
     [0017] The present invention provides a method for controlling the composition, temperature and microstructure of Al—Si alloys prior to SSM casting to control the mechanical properties of the final cast product. Generally, this is accomplished by mixing a hypereutectic Al—Si alloy with a hypoeutectic Al—Si alloy. By definition, aluminum alloys with less than about 12.6 percent Si are considered hypoeutectic whereas those with greater than about 12.6 percent Si are considered hypereutectic (FIG. 1).  
     [0018] The metallic composition of alloys used in current methods for SSM casting are limited to the availability and composition of the starting materials. In contrast, according to the present invention, a broad range of metallic compositions are achievable from the same starting materials. This is because the combination of a hypereutectic solution into a hypoeutectic allows for the manipulation of the final concentration of Si in the Al—Si alloy by controlling the composition and mass of the two liquids or semi-solid slurries. The final concentration of Si present in the alloy determines many of its mechanical properties. For example, increasing amounts of Si provides greater wear-resistance and strength with lower expansion rates.  
     [0019] In one embodiment, the final, mixed alloy composition is about 17 percent to about 18 percent Si in aluminum, formed by combining a hypereutectic aluminum alloy comprising about 23 percent to about 25 percent Si and a hypoeutectic aluminum alloy comprising about 7 percent to about 8 percent Si. A hypereutectic alloy can contain about 12.6 percent to over 25 percent Si in aluminum. Conversely, a hypoeutectic alloy can contain about 12.6 percent or less Si in aluminum. One example of a hypoeutectic alloy with about 7% Si is developed by Elkem (under the trademark of SIBLOY®), and is preferable for SSM processing of hypoeutectic Al—Si alloys because the alpha aluminum formed in the melt is independent of the hold time.  
     [0020] In addition to imparting unique physical properties to the end product, the concentration of Si in aluminum has consequences in the phase profile of any given alloy at any given temperature. FIG. 1 is a phase diagram showing the composition of alloys as varied by temperature. According to FIG. 1, about 12.6 percent Si in aluminum defines the eutectic point, which is defined as the lowest melting point possible between two substances in an alloy or solution.  
     [0021] The phase diagram also indicates the temperature to which the alloys need to be raised in order to be entirely in the liquid state; this consists of the area designated above the liquidus line  1 . The shaded areas  2 ,  3  indicate the temperature and composition where the alloy is in a semi-solid phase, containing both liquid and solid matter. For any given Al—Si alloy that is in the SSM range, the semi-solid phase is where deposits of one of the metals in the alloy begin to form. For example, hypereutectic Al—Si alloys begin to develop large Si particles as they begin to cool below the liquidus and into the SSM range. The instant invention teaches a method of mixing two Al—Si alloys at different temperatures together so that the amount of time the mixture spends in the transitional semi-solid phase is minimized.  
     [0022] Temperature control of the alloys can also be achieved by mixing a hypereutectic alloy with a hypoeutectic alloy as in the present invention. Generally, one alloy is heated to a liquid state and then mixed with an alloy of cooler temperature to bring the combined melt within the SSM range. The hypoeutectic alloy is generally maintained at a lower temperature than the hypereutectic alloy. Preferably, the hypereutectic alloy is generally poured into the hypoeutectic alloy, however, it is also possible to pour the hypoeutectic alloy into the hypereutectic alloy.  
     [0023] In one embodiment, the hypereutectic alloys are heated to a range of about 800° C. to about 900° C. and combined with hypoeutectic alloys which are heated in the range of 350° C. to about 580° C. Preferably, the hypereutectic alloy is raised to about 800° C. and the hypoeutectic alloy to about 500° C. This large temperature gradient allows for a quicker extraction of heat from the parent hypereutectic alloy and decreases the time necessary for the liquid alloy to drop in temperature to a semi-solid/slurry processing temperature.  
     [0024] As mentioned above, the growth of Si particles in the semi-solid phase is directly correlated to the time in addition to the temperature of the alloy. Longer time periods in the semi-solid phase is conducive for undesirable growth of large Si particles. Alternatively, shortening that period minimizes the growth of large Si particles by maximizing the number of nucleating events, producing more Si particles of smaller size. During the casting process, Al—Si alloys can spend a defined length of time in the casting machinery/device in addition to the imposed cooling times. Therefore, in addition to temperature control, it is preferable to define the time parameters (i.e. cooling rates) within which the desirable properties of the alloy are realized.  
     [0025] Specific processing parameters including the composition, temperature, and holding times of the alloys were varied and the microstructure of the cast alloys were therefore analyzed (FIGS. 2 and 3). Each alloy was heated to the temperature indicated and the hypereutectic melt was then poured into the hypoeutectic alloy. The temperature was then recorded and plotted as a function of time for seven experiments shown (FIG. 2). The hypereutectic alloy used had a composition of about 25 percent Al—Si and the hypoeutectic alloy carried about 7 percent Si. Holding times ranged from about 200 seconds to about 400 seconds to simulate real-world holding times and to study primary Si particle formations after allowing significant time for their growth. Seven experiments are presented (FIG. 2).  
     [0026]FIG. 3 shows the microstructure of the alloys from the experiments described after they had been quenched. Microanalysis of the casting from experiment 7 (FIG. 3A) shows that the primary Si particles range in size from about 60 microns to about 100 microns in diameter. The primary Si are also relatively evenly distributed with minimal aggregate formation as compared with controls. These results are comparable to the final parts obtained in thixocasting which are prepared to contain Si particles of desirable size and distribution.  
     [0027]FIG. 3B shows the morphology of primary Si from experiment 6 to be radiating from a given point (star-shaped). This is generally observed when the cooling rates are slow and were controlled by elevating the temperature of the hypoeutectic solution to about 570° C. The star shaped primary Si structures were reduced by decreasing the temperature of the hypoeutectic alloy from 570° C. to 500° C. as shown in FIG. 3A from experiment LM #7. Results from experiment 5 show that the amount of dissolved aluminum can be controlled by regulating the temperature of the hypoeutectic solution. FIG. 3C shows the structures obtained when the hypoeutectic alloy is heated to 350° C. and then mixed into the hypereutectic alloy. In this instance, the greater heat required to melt the primary aluminum in the hypoeutectic alloy led to a final microstructure that displayed regions of undissolved primary aluminum appearing as white spots. FIG. 3D similarly shows results from experiment 4 where undissolved primary aluminum of the hypoeutectic alloy remain in the final casting. In this case, the final temperature was 615° C. Small primary Si can be seen on the primary aluminum, indicating that the heat extracted by the primary aluminum provided local undercooling and assisted in the nucleation of the primary Si. Lastly, FIG. 3E is a representative example of the microstructure from experiments LM #1-3 and shows the dissolution of primary aluminum as the melts were held at a higher temperature (ranging from about 625° C. to about 636° C.). In addition, these conditions led to an uneven distribution of primary Si size and shape. Therefore, preferable characteristics of SSM cast hypereutectic alloys can be attained by controlling the temperatures of the hypo- and hypereutectic solutions and the hold times at the SSM temperature during casting.  
     [0028] A more rapid drop in temperature results in greater nucleating events than if the temperature is dropped gradually. This has the desirable effect of generating multiple Si particles that are smaller in size (width and length), but also generally uniformly distributed through out the alloy. The even distribution of the Si particles allows for better prediction of mechanical properties with less likelihood of mechanical failure which in effect limit the average growth of the Si particles and diminished the likelihood of globular aggregates.  
     [0029] The many features and advantages of the invention are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirits and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.