Patent Application
20180347011
The present disclosure relates to a system and method to stabilize transition metal precipitates in cast aluminum alloys during primary solidification.
This introduction generally presents the context of the disclosure. Work of the presently named inventors, to the extent it is described in this introduction, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against this disclosure.
Aluminum cast alloys have widespread applications for structural components in the automotive, aerospace, and general engineering industries because of good castability, corrosion resistance, machinability, and high strength-to-weight ratio. Regarding castability, alloy compositions having lower Silicon content have been thought to inherently produce poor castings due to a wider freezing range and the reduced latent heat. Lower Silicon alloys are difficult to cast, they possess less fluidity for mold filling and lower latent heat for feeding solidification shrinkage. Further, lower Silicon alloys are prone to a hot tearing defect during solidification in which a partially solidified casting pulls itself apart doe to the contraction of the casting as it cools, which leaves fissures that may result in leaking, reduced mechanical properties and lower fatigue life. The addition of transition metals to the metal composition exacerbates both castability and fatigue properties due to these manufacturing problems.
Alternatively, alloy compositions having higher Silicon content are increasingly difficult to machine and have lower ductility and fracture toughness due to coarser primary silicon particles. In general, aluminum alloy casting performance is based on several factors including alloy composition, casting and solidification conditions, and post-casting process or heat treating.
In attempting to expand or improve the use of aluminum alloys in additional applications that can reap the benefits that aluminum alloys offer, existing aluminum alloy casting composition and processes have fallen short of success in high temperature applications. Aluminum alloys having between about seven to ten percent by weight of Silicon are known to facilitate the casting process. The Silicon expands during solidification, which compensates for some of the overall shrinkage of the alloy during solidification, gives the alloy more energy, increases the fluidity of the melt to improve the filling of the mold, and the casting generally solidifies better than those castings which do not include Silicon. However, the presence of Silicon within the Aluminum alloy melt poisons or reduces the ability for the alloy to form high temperature phases. This reduces that capability for these alloys to be used in some high temperature applications. The reduction in high temperature phases reduces the durability of the casting in high temperature applications.
In an exemplary aspect, a system for casting an aluminum alloy includes a first chamber for containing a first melt at a first temperature, a second chamber for containing second melt at a second temperature that is lower than the first temperature, a mixing chamber in communication with the first chamber and the second chamber for simultaneously receiving and mixing the first melt from the first chamber with the second melt from the second chamber, and a mold chamber in communication with the mixing chamber and for receiving the mixed melt.
In another exemplary aspect, the first melt includes Aluminum and at least one peritectic transition metal element.
In another exemplary aspect, the first melt includes one of Zirconium, Scandium, Cobalt, Chromium, Niobium, Tantalum, Titanium, Vanadium, Tungsten, Molybdenum, Hafnium and Boron.
In another exemplary aspect, the second melt has a composition that includes a higher percentage of Silicon than the first melt.
In another exemplary aspect, the second melt has a composition that includes a higher percentage of Copper than the first melt.
In another exemplary aspect, the second melt has a composition that includes a higher percentage of Magnesium than the first melt.
In another exemplary aspect, the first temperature is higher than the liquidus temperature of an aluminum precipitate in the first melt and the second temperature is lower than the liquidus temperature of the aluminum precipitate in the first melt and above the liquidus temperature of the mixed melt.
In another exemplary aspect, aluminum precipitate includes at least one of Aluminum-Vanadium precipitate, an Aluminum-Zirconium precipitate, an Aluminum-Titanium precipitate, an Aluminum-Scandium precipitate, an Aluminum-Cobalt precipitate, an Aluminum-Chromium precipitate, an Aluminum-Niobium precipitate, and Aluminum-Tantalum precipitate, and Aluminum-Tungsten precipitate, an Aluminum-Molybdenum precipitate, an Aluminum-Hafnium precipitate, and an Aluminum-Boron precipitate.
In this manner, improved elevated temperature, wear durability and/or other properties of cast Aluminum alloys are provided. Further, the formation of primary precipitates improves the self-grain refining ability of the alloy which reduces or eliminates the need to add exogenous grain refining materials such as, for example, Titanium Di-Boride or Titanium-Aluminide particles through grain refining agents. Even if combined with such grain refining materials, the grain refinement that is further improved over what has previously been possible with exogenous additions alone. This provides improved castability and reduced casting defects.
Further areas of applicability of the present disclosure will become apparent from the detailed description provided below. It should be understood that the detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure.
The above features and advantages, and other features and advantages, of the present invention are readily apparent from the detailed description, including the claims, and exemplary embodiments when taken in connection with the accompanying drawings.
The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein:
In an exemplary method, the first melt and the second melt may be simultaneously introduced into the mixing chamber 106 whereby heat will be rapidly exchanged between the first and second melts during the mixing which initiates rapid nucleation of dispersoids of a stable aluminide precipitate from components in the first melt. In this manner, the mixing of the lower temperature second melt with the first melt performs a liquid quench in which high temperature stable precipitates are formed in the mixed melt. The mixed melt, which includes the high temperature precipitate, then flows from the mixing chamber into the mold where solidification of the mixed melt occurs. A quick transition from the mixing chamber into the mold minimizes the agglomeration of the high temperature precipitates (or “dispersoids”). Further, the resultant low diffusion of transition metal atoms minimizes transformation of dispersoids to more complex intermetallic phases during the mixed melt solidification in the mold and inhibits dissolution during any subsequent heat treatment or during temperatures experienced by the casting during use. This results in a casting that has stable high temperature properties.
In an exemplary embodiment, the first melt includes Aluminum and one of a peritectic transition metal element and is held within the first chamber 102 at a temperature which is above the liquidus temperature of the first melt and the second melt includes an Aluminum alloy and is held within the second chamber 104 at a temperature which is below the liquidus temperature of the first melt and above the liquidus temperature of the mixed melt.
The second melt may also include Silicon which is a known poison to the high temperature precipitate forming reaction, but may be provided within the second melt to improve castability of the mixed melt during the casting process. In this manner, the second melt, containing the higher Silicon composition is separately prepared. The second melt may also contain age-hardening elements such as, for example, Copper and Magnesium.
In this manner, the composition, temperature, and volume of the two precursor melts may be tailored to optimize aluminide volume fraction and dispersion to provide optimum properties. The lower melting point eutectic second melt being held at a lower temperature than the first melt causes a rapid reduction in the temperature of the first melt during the mixing phase which initiates nucleation of high temperature precipitate dispersoids in the mixed melt just prior to introduction into the mold and subsequent imminent solidification. Thus, the inventive process provides two mutually-exclusive solidification reactions that provide highly grain-refined primary transition metal aluminides in the resultant casting.
Table 1 illustrates an exemplary set of melt compositions and temperatures that when mixed together just before introduction into the mold cavity result in a target alloy having improved characteristics. The first melt is held at a temperature of 1000 degrees Celsius in the first chamber 102 and the second melt is held at a temperature of 600 degrees Celsius in the second chamber 104. The first melt contains transition metal elements, such as, for example Zirconium and Vanadium, that usually solidify through a peritectic reaction during the mixing with the second melt in the mixing chamber 106. Due to the nature of the peritectic reaction, only a very small amount of dispersoids may form during solidification and because of the high temperature nature of the phases that do form, the dispersoids phases cannot be subsequently formed using a solid state heat treatment process. The second melt, by contrast, contains a much higher concentration of Silicon, Copper, and Magnesium than the first melt. As explained above, those elements improve the castability of the mixed melt and provides precipitation strengthening benefits.
While
Referring now to
The subsequent immediate solidification of the casting ensues rapidly, which precludes any appreciable growth of the dispersoids or their transformation into more complex intermetallic phases. In this manner, the resultant dispersoid phases provide excellent nuclei for the primary aluminum solidification which aids in grain refinement of the casting.
The higher temperature melt is held at a temperature which is just above the liquidus of that melt. The composition of the melt may essentially include Aluminum along with any number of other peritectic transition metal elements, such as, for example, Zirconium, Scandium, Cobalt, Chromium, Niobium, Tantalum, Titanium, Vanadium, Tungsten, Molybdenum, and the like, without limitations. The total of all the elements in the alloy may be limited by the resultant liquidus temperature. In exemplary embodiments, the melt may be held below a temperature of about one thousand degrees Celsius, but may be heated to twelve hundred degrees Celsius or higher for short periods of time such as may be experience if the melt is produced using a melt-on-demand system which may only melt the amount of material needed for each cast. In such a system, the temperature is likely only to exceed a temperature of one thousand degrees Celsius for a short period of time.
The lower temperature melt may be held at a temperature that is above the liquidus temperature of that melt. The composition of that melt may be controlled to minimize the liquidus temperature to provide a near-eutectic Silicon composition of between about 10-12% Silicon with the mixed melt having a Silicon composition of between about 6-10% Silicon. Alloying elements in the lower temperature melt may further include elements which improve the hardenability such as, for example, Copper and Magnesium, but may also include other elements such as, for example, Silver, Zinc, Manganese and the like. In an exemplary embodiment, the lower temperature melt may include a mixed alloy composition of between about 0.5-5.5% Copper, between about 0.1-0.6% Magnesium, between about 0.1-3.0% Zinc, and/or between about 0.1-0.6% Manganese.
The mixed melt results in a temperature above the liquidus temperature of the mixed melt composition. The composition, temperature and volume may be determined by rules of mixtures considering the composition of the high temperature melt and the low temperature melt. Following mixing, casting may proceed immediately after to minimize agglomeration of the high temperature dispersoids. The low diffusion of transition metal atoms minimizes transformation of the high temperature dispersoids to more complex intermetallic phases during solidification and inhibits dissolution at heat treatment temperature and at service temperatures to produce stable elevated-temperature properties for the lifetime of the casting.
The composition, temperature and volume of the two precursor melts may be tailored to optimize aluminide volume faction and dispersion to obtain desired properties in the resultant casting.
Table 2 below illustrates exemplary ranges for a preferred set of melts:
Table 3 below illustrates one preferred embodiment of melts:
Table 4 illustrates another preferred embodiment for melts:
Table 5 illustrates one special exemplary set of melt conditions:
Additional exemplary melt compositions and resulting mixed melt compositions are illustrated by Tables 6 through 8:
In yet another exemplary embodiment, primary Silicon may be refined to improve, for example, wear resistant properties of the resultant casting. Referring back now to
In an exemplary embodiment, the present invention may produce components for an internal combustion engine, such as, for example, and Aluminum alloy cylinder head that has improved high temperature properties in comparison to that which was previously achievable. In this manner, the cylinder head is able to withstand higher temperatures, which improves the efficiency of the combustion process, which may provide improved fuel economy and/or improved performance of a vehicle incorporating an internal combustion engine with an aluminum cylinder head incorporating the features which are obtainable through the use of the present invention.
This description is merely illustrative in nature and is in no way intended to limit the disclosure, its application, or uses. The broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent upon a study of the drawings, the specification, and the following claims.
