Patent Application
20170056971
The present invention relates to the manufacturing of cast aluminum components and, more particularly, to methodologies and technologies to produce porosity-free aluminum castings with fine microstructure without using degassing equipment or grain refiners.
Porosity and grain structure have long been recognized as factors affecting mechanical properties, especially fatigue performance, of cast components. Porosity forms due to volume shrinkage from liquid to solid during solidification, while undesirable grain structures (including large and columnar grain formations) can form if solidification temperatures are not carefully controlled. These problems are particularly acute in the casting of lightweight metal alloys (such as aluminum-based alloys in general and the Al—Si alloys (319, 356, 390 or the like in particular) that are used to make—among other things—automotive cylinder blocks and heads.
Regarding porosity, the evolution of dissolved gases as a result of the significant decrease in solubility of the gases in the solid as compared to the liquid metal is often the primary cause. This is especially true for aluminum-based castings, where hydrogen-induced porosity is the dominant form due to hydrogen being the only gas that is appreciably soluble in molten aluminum. As such, there are several methods that are currently employed to reduce inclusion and hydrogen content in liquid aluminum. These methods include various degassing techniques, including rotary impeller degassing, tablet (such as hexachloroethane (C2Cl6)) degassing, vacuum degassing and spray degassing. Although such degassing methods have demonstrated effectiveness to varying degrees in refining aluminum-based melts, they can cause environmental problems (for example, due to Cl2 gas release) or involve significant capital investment.
Regarding grain structure, it is desirable to pursue fine and equiaxed grain structure in aluminum-based castings as a way to minimize shrinkage, hot tearing and fatigue susceptibility, as well as giving a more uniform distribution of fine scale second phases and microporosity. These in turn improve yield strength, fracture toughness and other useful mechanical properties. Generally, any factor which increases the number of nucleation sites or reduces growth rate has a tendency to yield fine grains in an as-cast aluminum alloy. Commonly-used techniques include using a chill or related insert in the mold to increase local solidification rate (which in turn tends to promote grain size reduction and related mechanical properties). For instance, in a sand-cast engine block, the bulkheads near the crankshaft journal areas are formed with heavy metal chills to assure the required mechanical properties. Unfortunately, when chills are used, undesirable local columnar grain structure may form; such structure can significantly reduce the fatigue performance of the material. Therefore, in practice grain refiners in the form of chemical or elemental additives (such as Ti, B, C or combinations thereof) are often placed in the liquid metal or mold prior to mold fill when a chill is employed. Because the addition of such a grain refiner to a liquid metal melt in the furnace tends to lead to sludge settling over time, such an approach can significantly contribute to furnace and recirculation pump maintenance costs. Likewise, in-mold grain refinement tends to produce more oxides (which can contribute to undesirable bi-film formation) and microstructure segregation in the casting. As such, the present inventors believe that both of these approaches to grain refining should be avoided.
According to an aspect of the invention, a molten metal degassing and grain refinement system is disclosed. The system includes a conveyance assembly configured to fluidly couple a source of said molten metal to a mold, as well as an apparatus configured to impart ultrasonic vibrations to portions of the conveyance assembly (or to the molten metal that flows through a travel path defined by the conveyance assembly) such that during at least a portion of its travel between the source and the mold, the molten metal interacts with the vibrations to experience a reduction in the hydrogen gas presence as well as (upon subsequent solidification) its grain size. Such a configuration takes advantage of the fact that the solubility of hydrogen, oxides or other impurities (which are a significant contributor to as-cast porosity) is dramatically higher in the liquid or molten state relative to the solid state. In this way, the induced vibrations along the molten metal travel path tend to force the less dense gaseous impurities out prior to the metal being solidified (hardened) in the mold. As such, by the time that a substantial entirety of the metal contained within the ladle has solidified, most (or all) of the hydrogen (or other gaseous impurities) that was previously held within the molten metal has been liberated. Moreover, placement of the vibrations adjacent portions of the mold further help to ensure small, equiaxed grains upon component solidification. In this way, reductions in both hydrogen gas presence and grain size are imparted to the melt. Thus, depending on the location of the vibration-inducing equipment, the degassing and grain refinement can take place continuously or locally. For example, in situations where degassing or grain refinement of the entire cast component is desired, the ultrasonic vibration actuators may be located in numerous places along the casting process travel path, including the launder tube, pouring ladle or pouring basin. Likewise, local degassing or grain refinement at select component locations (such as the bulkheads in an engine block or combustion chambers in a cylinder head) may be achieved by placing the ultrasonic vibration actuation equipment adjacent particular portions of the mold that correspond to various component regions. In one exemplary form, they may be located on the chill or related heat sink insert that is placed adjacent an engine block bulkhead, cylinder head combustion chamber or the like. Significantly, the operation of the vibration-inducing equipment allows such degassing and grain refinement to take place without complex additional machinery or the use of grain refiner additives to the molten metal.
According to another aspect of the invention, a method of producing a cast aluminum component is disclosed where both degassing and structural refinement is performed using ultrasonic vibrations. The method includes conveying an aluminum-based melt (also referred to as a molten metal or liquid metal precursor to the finished component) from its source to a mold, applying ultrasonic vibrations to the melt during at least a portion of its travel path between the source and mold as a way to achieve hydrogen degassing in the melt relative to no such vibrations being present, and applying ultrasonic vibrations to the melt during its solidification in the mold such that at least a portion of the component exhibits reductions in grain size relative to no such vibrations being present.
The following detailed description of specific embodiments can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which:
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The ultrasonic vibration imparting apparatus 500 includes numerous discretely-placed vibration exciters or actuators such as probes, transducers, or the like. In an alternate embodiment, the vibrations from the ultrasonic vibration imparting apparatus 500 may be imparted by a magneto-hydrodynamic stifling mechanism such as a magnet-based electromagnetic coil or the like. Furthermore, in situations where the ultrasonic probes, transducers or coils are placed in locations where they would be exposed to very high temperature environments such as the casting mold, they may further include cooling mechanisms (such as by forced water cooling). In one form as shown with particularity in
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In the present context, each of the parts that make up the conveyance assembly 200 are shown as separate, discrete components. Nevertheless, it will be appreciated by those skilled in the art that the various functional attributes of these components (including, the launder tube 230, ladle 240 and pour basin 250) may be subsumed into larger, more structurally-integrated components as a way to mimic the various liquid metal-conveying functions described herein, and that any combination of such integrated components that perform such function or functions is deemed to be within the scope of the present invention, irrespective of whether the components performing such function are physically separable from one another. For example, the distal end of the launder tube 230 may be outfitted with an integrally-formed nozzle or related dispenser (not shown) that acts as the ladle 240 such that the molten metal 400 that traverses the launder tube 230 is dispensed from the ladle 240 and into the pour basin 250. The fact that the ladle 240 is either (a) integrated in a structural sense with the launder tube 230 or (b) not even present does not detract from the fact that its functional attributes are still present if the molten metal 400 is delivered from the furnace 210 and through the launder tube 230 to the suitable pour basin 250. As such, the presence of these functional attributes, rather than their respective structural component in discrete form, is what determines whether they form a part in the overall conveyance assembly 200, and that as long as such function is in evidence, the corresponding structure is deemed to likewise be present.
The placement of the vibration-inducing apparatus 500 in general (with the aforementioned probe or transducer portion of the apparatus in particular) is recited and described as being “at” one or more discrete locations along the molten metal 400 travel path, as well as “in” or “at” one more locations within the mold 340 that is used to give the component its shape. Within the present context, this does not mean that such apparatus must physically be embedded into the corresponding conveying assembly 200 or mold 340 (although they can be), but merely means that such apparatus is placed close enough to ensure the efficacy of the generated ultrasonic vibrations relative to the molten or solidifying metal; as such, placement of the vibration probe, transducer or coil of the vibration-inducing apparatus 500 may be on or adjacent an inner or outer surface of the assembly 200 or mold 340.
By using a suitable controller (not shown), the casting operation may be automated. In such case; the controller may be equipped with a central processing unit (CPU), and content-addressable memory (for example, in the form of read-only memory (ROM) for storing a program which controls the operation of the overall apparatus, and a random-access memory (RAM) having a data storage area). The CPU is connected to an input/output interface (which may perform one or both of discrete and analog input and output), while additional signal-processing apparatus, such as an analog-to-digital (A/D) converter and one or more filter circuits. Such a controller may function as a digital signal processor, an application specific integrated circuit, a field programmable gate array, any suitable programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof. In one preferred form, the controller is configured to instruct the ultrasonic vibration imparting apparatus 500 how to stage its operation, including frequencies and vibration amplitude. In one exemplary form, such vibration is performed with a minimum acoustic intensity of 10 W/cm2, 60% amplitude (i.e., a measure of the energy range of the vibrations) at a vibration frequency of at least about 15 kHz, and more preferably at least about 20 kHz. The present inventors have determined that to be able to perform adequate degassing and grain refinement of cast aluminum alloys, the frequency needs to be at least 15 kHz, and that while higher frequency is helpful, practical limits on ultrasonic vibration equipment capital costs need to be taken into consideration. In the present context, an upper limit frequency of about 100 kHz should suffice for a balance of high frequency capability coupled with reasonable equipment costs. Likewise, the desired amplitude range is between about 50% and 100%.
It is noted that terms like “preferably,” “commonly,” and “typically” are not utilized herein to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to highlight alternative or additional features that may or may not be utilized in a particular embodiment of the present invention.
For the purposes of describing and defining the present invention it is noted that the term “device” is utilized herein to represent a combination of components and individual components, regardless of whether the components are combined with other components. For example, a “device” according to the present invention may comprise an electrochemical conversion assembly or fuel cell, a vehicle incorporating an electrochemical conversion assembly according to the present invention, etc.
For the purposes of describing and defining the present invention it is noted that the term “substantially” is utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. The term “substantially” is also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue.
Having described the invention in detail and by reference to specific embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. More specifically, although some aspects of the present invention are identified herein as preferred or particularly advantageous, it is contemplated that the present invention is not necessarily limited to these preferred aspects of the invention.
