Patent Publication Number: US-2015083280-A1

Title: Solidification microstructure of aggregate molded shaped castings

Description:
This application is a continuation of application Ser. No. 11/505,019 filed on Aug. 16, 2006, which is still pending. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to metal castings. More particularly, the present invention relates to aggregate shaped metal castings having a fine solidification microstructure. 
     BACKGROUND 
     In the traditional casting process, molten metal is poured into a mold and solidifies, or freezes, through a loss of heat to the mold. When enough heat has been lost from the metal so that it has frozen, the resulting product, i.e., a casting, can support its own weight. The casting is then removed from the mold. 
     Different types of molds of the prior art offer certain advantages. For example, green sand molds are composed of an aggregate, sand, that is held together with a binder such as a mixture of clay and water. These molds may be manufactured rapidly, e.g., in ten (10) seconds for simple molds in an automated mold making plant. In addition, the sand can be recycled for further use relatively easily. 
     Other sand molds often use resin based chemical binders that possess high dimensional accuracy and high hardness. Such resin-bonded sand molds take somewhat longer to manufacture than green sand molds because a curing reaction must take place for the binder to become effective and allow formation of the mold. As in clay-bonded molds, the sand can often be recycled, although with some treatment to remove the resin. 
     In addition to relatively quick and economical manufacture, sand molds also have high productivity. A sand mold can be set aside after the molten metal has been poured to allow it to cool and solidify, allowing other molds to be poured. 
     The sand that is used as an aggregate in sand molding is most commonly silica. However, other minerals have been used to avoid the undesirable transition from alpha quartz to beta quartz at about 570 degrees Celsius (° C.), or 1,058 degrees Fahrenheit (° F.), that include olivine, chromite and zircon. These minerals possess certain disadvantages, as olivine is often variable in its chemistry, leading to problems of uniform control with chemical binders. Chromite is typically crushed, creating angular grains that lead to a poor surface finish on the casting and rapid wear of tooling. Zircon is heavy, increasing the demands on equipment that is used to form and handle a mold and causing rapid tool wear. 
     In addition, the disadvantages created by the unique aspects of silica and alternative minerals, sand molds with clay and chemical binders typically do not allow rapid cooling of the molten metal due to their relatively low thermal conductivity. Rapid cooling of the molten metal is often desirable, as it is known in the art that such cooling improves the mechanical properties of the casting. In addition, rapid cooling allows the retention of more of the alloying elements in solution, thereby introducing the possibility of eliminating subsequent solution treatment, which saves time and expense. The elimination of solution treatment prevents the quench that typically follows, removing the problems of distortion and residual stress in the casting that are caused by the quench. Related to the mechanical properties, the fineness of the cast microstructure is related to the rate of cooling and solidification. Generally, as the rate of cooling and solidification increases, the solidification microstructure of the casting becomes finer. 
     As an alternative to sand molds, molds made of metal or semi-permanent molds or molds with chills are sometimes used. These metal molds are particularly advantageous because their relatively high thermal conductivity allows the cast molten metal to cool and solidify quickly, leading to advantageous mechanical properties in the casting. For example, a particular casting process known as pressure die casting utilizes metal molds and is known to have a rapid solidification rate. Such a rapid rate of solidification is indicated by the presence of fine dendrite arm spacing (DAS) in the casting. As known in the art, the faster the solidification rate, the smaller the DAS. However, pressure die casting often allows the formation of defects in a cast part because extreme surface turbulence occurs in the molten metal during the filling of the mold. The presence of fine dendrite arm spacing may also be achieved by cooling the casting by a local chill or fin. Such techniques include the localized application of solid chill materials, such as metal lump chills or moldable chilling aggregates, and the like, that are integrated into the mold adjacent to the portion of the casting that is to be chilled. These methods, however, only provide a localized effect in the region where the chill is applied. This localized effect contrasts with the benefits of the invention discussed in this application, in which the benefits of fine microstructure can apply, if the invention is implemented correctly, extensively throughout the casting. This is an important aspect of the current application, because the ultimate benefit is that the casting displays properties that are not only generally higher, but are also essentially uniform throughout the product, and thus of great benefit to the designer of the product. The product now essentially enjoys the uniformity normally associated with forgings. 
     One variety of known permanent mold process in which the residual liquid phase in the structure may be subject to rapid cooling includes some types of semi-solid casting. In this process the metallic slurry is formed exterior to the mold, and consists of dendritic fragments in suspension in the residual liquid. The transfer of this mixture into a metal die causes the remaining liquid to freeze quickly, giving a fine structure, but surrounded by relatively coarse and separated dendrites, often in the form of degenerate dendrites, rosettes, or nodules. 
     However, all molds made from metal possess a significant economic disadvantage. Because the casting must freeze before it can be removed from the mold, multiple metal molds must be used to achieve high productivity. The need for multiple molds in permanent mold casting increases the cost of tooling and typically results in costs for tooling that are at least five (5) times more than those associated with sand molds. 
     Another common feature of the internal structure of conventional shaped castings, well known and well understood throughout the casting industry, is that those regions of larger geometric modulus (i.e., regions having a larger ratio of volume to cooling area) generally have a coarser structure. Such regions of the casting typically have significantly lower mechanical properties. Further, such regions commonly exhibit shrinkage cavities or pores because they are more easily isolated from feed metal at a late stage of freezing. Such regions are often seen, for instance, at hot spots formed by an isolated boss on a relatively thin plate, or in the hot spot that is found at the T-junction between two similar sections. Complicated castings are often full of such features, resisting the attainment of any degree of uniformity of properties. This problem greatly complicates the work of the designer of the product. For instance, the thickening of a section intended to increase its strength will lower properties and in the worst instance may even lead to defects, and so is, often to some indeterminate degree, counter productive. 
     In the locations of the casting where solidification is slow, not only is (i) the structure coarse, typified by a coarse DAS, but (ii) porosity is also present, and (iii) for those Al alloys that commonly suffer iron as an impurity, large plate-like crystals of iron-rich phases can form. All these factors are greatly damaging to the ductility of the alloy. 
     Extremely fine cast microstructures have been produced as laboratory curiosities in various scientific studies (for instance, the research paper by G. S. Reddy and J. A. Sekhar in Acta Metallurgica, 1989, vol. 37, Number 5, pp. 1509-1519 and the research paper by L. Snugovsky, J. F. Major, D. D. Perovic and J. W. Rutter; “Silicon segregation in aluminium casting alloy” Materials Science and Technology 2000 16 125-128.). 
     However, in contrast to such laboratory studies, the invention described in this application provides the unique conditions in which the solidification microstructures described herein are produced routinely by a production process that can be operated to produce one-off or volume-produced castings that are shaped in three-dimensions. 
     It is less generally known that the interior of castings can experience accelerated freezing as a result of a geometrical effect in shaped castings. The early solidification near the skin of the casting occurs substantially unidirectionally, and typically varies at a rate that decreases parabolically with time; i.e. the solidification rate reduces in speed as the thickness of the solidified layer increases. In contrast, the volume of remaining liquid in the center of a casting dwindles with time, and experiences increasing heat extraction from additional directions, so that the speed of freezing can be greatly increased. This effect is well described by one of the inventors. See  Castings,  John Campbell, pp. 125-126 (2 nd  Edition, 2003), published by Butterworth Heinemann, Oxford, UK, the entire disclosure of which is incorporated herein by reference. The behavior explains the so-called reverse chill effect in cast iron castings, in which the center of the casting, seemingly inexplicably, sometimes exhibits a white, apparently chilled, structure in contrast to the outer regions of the casting that remain grey, signifying a slow cooling rate. 
     As a result, it is desirable to develop a casting process and related apparatus that have the advantage of rapid solidification of metal molds, while also having the lower costs, high productively and reclaim-ability associated with sand molds. 
     It is also desirable to provide an aggregate molded shaped casting exhibiting a region of fine solidification microstructure over extensive regions of the casting, so as to promote substantially uniform properties akin to those of forgings. (Because of the relative insensitivity of properties to the variations in cooling rate at the high cooling rates used in this application, the variations that are discussed later, for instance in  FIG. 4 , do not significantly affect properties, conferring substantial uniformity of properties in the cast product). In particular, it is desirable to provide an aggregate molded shaped casting having a fine solidification microstructure that is finer than a structure produced by a conventional aggregate casting method, and possibly even finer than that produced by a permanent mold. 
     It is further desirable to provide an aggregate molded shaped casting having a fine solidification microstructure region that is substantially continuous from a distal point of the casting to the feeder or riser. 
     SUMMARY 
     The disclosure provides, in various embodiments, a shaped metal casting formed in an aggregate mold by an ablation casting process, the casting comprising a fine solidification microstructure that is finer than the microstructure of a casting of a similar metal having a similar weight or section thickness that is produced by a conventional aggregate casting process, wherein the fine microstructure comprises one or more of grains, dendrites, eutectic phases, or combinations thereof. 
     The disclosure also provides, in various embodiments, a metal casting exhibiting a cast microstructure, the microstructure comprising a first region located adjacent a surface of the metal casting, the first region comprising a coarse solidification microstructure; and a second region located internal to the first region, the second region comprising a fine solidification microstructure. 
     In a further aspect, the disclosure provides, in various embodiments, a metal casting formed from a eutectic-containing alloy, the casting comprising a dual solidification microstructure region, wherein the dual solidification microstructure region comprises (i) one or more regions containing coarse dendrites; and (ii) one or more regions containing fine eutectic. 
     Additionally, the disclosure provides a shaped metal casting made in a mold that is at least a partially aggregate mold, the casting comprising a dual solidification microstructure region, wherein the dual solidification microstructure region comprises at least one coarse solidification microstructure portion having a grain size and/or dendrite arm and/or eutectic spacing in the range commonly to be expected in a conventional aggregate or metal mold; and at least one fine solidification microstructure having a grain size and/or dendrite arm and/or eutectic spacing of less than one third of the conventional spacing for that portion of the casting. 
     In still another aspect, the disclosure provides a shaped metal casting made in an aggregate mold, the casting comprising a dual solidification microstructure region, wherein the dual solidification microstructure region comprises: at least one coarse solidification microstructure portion having a dendrite arm spacing in the range of about 50 to about 200 micrometers; and at least one fine solidification microstructure portion having a dendrite arm spacing of less than about 15 micrometers. 
     In another aspect, the disclosure provides a shaped metal casting formed in an aggregate mold by an ablation casting process, the casting comprising a fine solidification microstructure having a dendrite arm spacing that is finer than the dendrite arm spacing of a casting having a similar metal of a similar weight or section thickness that is produced by a conventional aggregate molded or permanent molded casting process. 
     Further, the disclosure provides a shaped metal casting with substantial soundness and with high and substantially uniform properties, to some extent resembling those features normally associated with forgings. 
     Other features and advantages of castings in accordance with the disclosure are further understood in view of the drawings, detailed description, examples, and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cooling curve for a solid solution alloy that undergoes dendritic solidification; 
         FIG. 1A  is a graph showing the relationship between dendrite arm spacing and freezing or solidification rate; 
         FIG. 2  is a micrograph (at ×200) showing the microstructure of a solid solution alloy cast by conventional casting methods; 
         FIG. 3  is a micrograph of a solid solution alloy comprising a fine solidification microstructure region (Dual DAS structure) produced by ablation; 
         FIGS. 4A-4E  are schematic representations of metal castings comprising fine solidification microstructure regions; 
         FIG. 5  is a cooling curve of a mixed dendrite and eutectic alloy; 
         FIG. 6  is a micrograph (at ×200) depicting the microstructure of a 356 alloy showing coarse eutectic silicon particles cast by conventional methods; 
         FIG. 7  is a micrograph (at ×200) depicting an ablation-frozen A356 alloy having regions of coarse plus fine dendritic material (dual DAS structure) and fine eutectic material; 
         FIG. 8  is a micrograph (at ×200) of a portion of an ablation-frozen A356 alloy showing uniform coarse DAS and uniform fine eutectic phase; 
         FIG. 9  is a micrograph (at ×200) of an ablation-frozen A356 alloy having fine eutectic regions, but which are somewhat coarsened after a solution heat treatment; 
         FIG. 10  is a micrograph (at ×200) of an ablation-frozen A356 alloy exhibiting coarse dendritic microstructure and mainly fine eutectic microstructure, but containing a trace of coarse eutectic phase; 
         FIG. 11  is a detail of the micrograph (at ×1000) of  FIG. 10  at higher magnification; 
         FIG. 12  depicts the solidification rate of various portions of a dendritic alloy formed in accordance with an exemplary embodiment; 
         FIG. 13  is a graph depicting the cooling rate of various sections of an exemplary embodiment casting; 
         FIG. 14  is a table showing the mechanical properties of various exemplary embodiment castings; 
         FIG. 15  is a bar graph and table comparing the mechanical properties of metal castings formed by various casting methods; 
         FIG. 16  is a chart showing the relationship between dendrite cell size and solidification rate for aluminum alloys; 
         FIG. 17   a  is a micrograph (at ×100) of a conventional cast permanent mold microstructure of a 2.75″ diameter bar of an aluminum alloy; 
         FIG. 17   b  is a micrograph (at ×100) of such a microstructure made with the ablation process for the same alloy at the start of the ablated part (first part in); and, 
         FIG. 17   c  is a micrograph (at ×100) of the ablated last part. 
     
    
    
     The figures are merely for the purpose of illustrating various embodiments of a development in accordance with the present disclosure and are not intended to be limiting embodiments of the development. 
     DETAILED DESCRIPTION 
     The disclosure relates to an aggregate molded, shaped casting comprising at least a fine solidification microstructure region. An aggregate molded shaped metal casting in accordance with the disclosure includes a solidification microstructure region that is finer than the solidification microstructure obtained by conventional aggregate molding methods. In some embodiments, an aggregate molded shaped metal casting in accordance with the disclosure has a solidification microstructure that is substantially free of shrinkage porosity. 
     The type or nature of the solidification microstructure will vary and depend on the metal and/or metal alloys undergoing solidification. Various microstructures include dendrites, eutectic phases, grains, and the like. In one embodiment, for example, a shaped casting may comprise only a single type of microstructure. Further, an alloy may exhibit a solidification microstructure comprising one or more different microstructures. For example, in one embodiment, a shaped casting may exhibit a microstructure comprising a combination of dendrites and grains. In another embodiment, a shaped casting may exhibit a combination of dendrites and eutectic phases. In still another embodiment, a shaped casting may exhibit a combination of dendrites, eutectic phases, and grains. These embodiments are not limiting embodiments, as other combinations and/or other microstructures may be possible. 
     As used herein, eutectic alloys includes any alloy that forms eutectic phases, including hypo-eutectic, near-eutectic or hyper-eutectic alloys. 
     An aggregate molded, shaped metal casting in accordance with the present disclosure may be formed by a method such as that described in U.S. application Ser. No. 10/614,601 that was filed on Jul. 7, 2003, and the entire disclosure of which is incorporated herein by reference. Generally, application Ser. No. 10/614,601 discloses a process for the rapid cooling and solidification of aggregate molded shaped castings. The method also provides for the removal of the mold. The process described in application Ser. No. 10/614,601 is referred to herein in as “ablation.” 
     Upon solidifying during an ablation process, a metal casting exhibits a fine solidification microstructure that is finer than the microstructure of a similar metal having a similar weight or section thickness that is produced by a conventional aggregate casting process. The fineness of a microstructure may be defined in terms of the size or spacing exhibited by a particular type of microstructure. For example, grains exhibit a grain size, dendrites exhibit a dendrite arm spacing, and eutectic phases exhibit a eutectic spacing. 
     With reference to  FIG. 1 , a cooling or solidification curve for a solid solution alloy is shown. Solid solution type alloys form only grains and/or dendrites during solidification. The cooling curve shows the cooling of a solid solution alloy with time, from the pouring temperature (T P ) through the liquidus temperature (T L ) to the solidus temperature (T S ), which is the point at which solidification is complete. 
     Cooling of a solid solution type alloy in a conventional aggregate mold is represented by the time/temperature profile “abcdef”. The total time for cooling using conventional methods is in the range of from minutes to hours and is, of course, particularly dependent on the thickness of the casting, and the rate at which heat can transfer into the mold. The rate of cooling slows at the liquidus temperature (T L ) as a result of the latent heat evolution during the formation of dendrites. Solidification is complete at the solidus temperature (T S ) and the rate of fall of temperature increases once the evolution of latent heat has subsided. 
     At point “e” on  FIG. 1 , the application of any rapid cooling is too late to have any effect on the solidification microstructure. Thus, for example, the cooling profile “el” would not have any effect on the solidification microstructure and is not a part of this patent application. Cooling profiles such as “el,” however, are commonly utilized in the casting industry where castings are taken from a metal die and quenched directly into water. 
     The solidified structure of solid solution alloys typically consists almost entirely of dendrites that are outlined by a negligible thickness of residual inter-dendritic material. The secondary dendrite arm spacing (often referred to more simply in this application as dendrite arm spacing, or DAS) is dependent on the freezing time, i.e. the solidification time, which is the time that a fixed location in the casting exists at a temperature between the liquidus T L  and the solidus temperature T S  of the alloy. In terms of the period of time in  FIG. 1   a,  it is seen to be t s =t e −t c . 
       FIG. 1   a  shows the approximate logarithmic relation between the local DAS and the local t s  in many common Al alloys. This figure illustrates that to reduce DAS by a factor of 10, t s  is required to be reduced by a factor of approximately 1000. (Unfortunately such a relationship has not been investigated for grains and eutectic spacing, so that a clear, quantitative description of the refinement of these other features of the solidification structure of some alloys cannot easily be made. Thus the quantitative predictions of refinement of structure by ablation described in this application concentrate on DAS. However, it is to be understood that similar but unquantified refinements are paralleled in grain size and eutectic spacing) Thus very large increases in cooling rate are required to substantially affect the fineness of the solidification microstructure. 
     While the quantitative relationship exemplified in  FIG. 1  is described with respect to dendrites and dendrites arm spacing, similar parallel refinements are expected in grain size and/or eutectic spacing in alloys comprising grains and/or eutectic phases. 
     For conventional aggregate casting methods, for a small aluminum alloy casting weighing a few pounds or kilograms, the local solidification time is typically of the order of about 1,000 seconds. These conventional aggregate casting methods produce castings having a DAS of around 100 micrometers, and sometimes in the range of from about 50 micrometers to about 200 micrometers. As used herein, a solidification microstructure having a DAS of greater than 50 micrometers is referred to as “coarse” microstructure.  FIG. 2 , is a micrograph depicting the coarse microstructure of a solid solution cast alloy A206 (a nominal Al-4.5 wt % Cu alloy) that was cast by conventional methods. 
     A casting in accordance with the present disclosure comprises the presence of fine solidification microstructure in at least a portion of the casting. That is, a casting may comprise from greater than 0% to up to 100% of fine solidification microstructure. In one embodiment, the casting is substantially free of any coarse solidification microstructure and comprises up to 100% fine solidification microstructure that is continuous throughout the casting. In another embodiment, a casting comprises a first region adjacent the surface of the casting that comprises up to 100% coarse solidification microstructure, and a second region internal to the first region wherein the second region comprises up to 100% fine solidification microstructure. In still another embodiment, a casting in accordance with the present disclosure comprises a continuous or at least a substantially continuous, region of fine solidification microstructure extending from a distal end of the casting to a proximal end thereof, i.e., the end adjacent the feeder or riser. 
     In other embodiments, a casting comprises a dual solidification microstructure region intermediate a coarse solidification microstructure region and a fine solidification microstructure region. As used herein, a dual microstructure region is a region that includes areas of coarse microstructure having one or more areas of fine microstructure interspersed therein. In still another embodiment, the dual solidification microstructure in a casting is substantially continuous throughout from a distal end of the casting to the feeder. 
     The dendrite arm spacing is generally dependent on the time over which solidification occurs. As shown in  FIG. 1A , the log/log relationship of dendrite arm spacing to freezing time is linear and, for aluminum alloys, for example, has a slope of approximately ⅓. The graph shows there is approximately a factor of 5 spacing reduction for each factor of 100 in freezing time. Thus, a particular casting section in a conventional mold may experience solidification in 1000 seconds giving a corresponding DAS of 100 micrometers. In the ablation casting techniques described in U.S. application Ser. No. 10/614,601, the same casting section would result in a local solidification time of only approximately 10 seconds, giving a dendrite arm spacing of about 20 micrometers. The relationship between spacing and freezing time remains constant over all experimental times.  FIG. 3  is a micrograph showing regions of both fine microstructure and regions of coarse microstructure. 
     With reference to  FIGS. 4A-4E , various exemplary embodiments of aggregate molded, shaped metal castings comprising a fine solidification microstructure region are shown. 
     With reference to  FIG. 4A , an embodiment is shown in which a casting comprises a small percentage of fine solidification microstructure. The casting in  FIG. 4A  represents a situation in which the rapid cooling process, such as ablation, is applied late in the casting process. In the casting in  FIG. 4A , some solidification has occurred prior to rapid cooling (e.g., ablation), and portions of the casting, such as those having a small geometric modulus (volume to cooling area ratio) conventionally freeze. Dual solidification microstructure regions occur where some portions have solidified prior to ablation but other portions remain liquid at the time ablation begins Even in an example such as  FIG. 4A  where a portion of the casting has solidified prior to a rapid cooling process, and thus constituting a far from optimum application of this invention, the presence of even a small amount of fine solidification microstructure is advantageous. Specifically, in conventional processes, the small percentage of residual liquid in metal alloys that remains is particularly difficult to freeze without creating defects such as shrinkage porosity. Applying a rapid cooling technique such as ablation, however, converts these regions from defective zones to fine structure zones. The presence of even small zones of fine structure provide good mechanical properties compared to those defective zones of a conventionally solidified casting. Although the trapped region of residual liquid illustrated in  FIG. 4A  will be expected to demonstrate some shrinkage porosity, ablation freezing of this region will reduce the extent of the shrinkage, and will replace it with a corresponding region of strong, sound material. The solidification microstructure profile of  FIG. 4A  would be expected where the rapid cooling step is applied rather late, closer to, but before, point “e” on the graph of  FIG. 1 . 
     In another embodiment, an aggregate molded shaped casting comprises a fine solidification microstructure region and a dual solidification microstructure region wherein the dual solidification microstructure region is substantially continuous from a distal end of the casting to a proximal end of the casting. The embodiment of  FIG. 4B  is an example of an embodiment of a casting having a substantially continuous dual solidification microstructure region. The solidification microstructure profile of  FIG. 4B  is a profile that would be expected from following the cooling profile “abcdjk” in  FIG. 1 . In  FIG. 4B , the freezing point arrives at and passes the point at which the natural freezing of the constricted section reaches the center of the section. The casting is frozen by a rapid cooling procedure, such as ablation, from the more distant parts of the casting up to the center point. If the fine structure zone produced by rapid freezing is terminated on reaching the modulus constriction (as it does in the embodiment in  FIG. 4B ) the casting will freeze soundly up to this point. Even though the dual solidification microstructure is absent in the local region of the constriction in the embodiment in  FIG. 4B , the rapid local solidification time throughout the remainder of the casting creates a substantially continuous zone of fine and sound alloy, free from shrinkage defects, throughout the remainder of the casting. Thus, by driving the solidification directionally from distal regions to those proximal to the feeder, the more distant portions of the casting exhibit fine solidification microstructure and mechanical soundness that would not have been achieved by conventional methods. 
     In the embodiment in  FIG. 4C , the casting includes a greater percentage of fine solidification microstructure, and the region of dual solidification microstructure is continuous through the constricted region of the casting. Such a desirable solidification microstructure may be achieved by applying a rapid cooling procedure, such as ablation, at a time earlier than that of  FIG. 4B . 
     In another embodiment, such as that of  FIG. 4D , a metal casting comprises a desirable fine solidification microstructure region that is substantially continuous from the distal ends of the casting to the feeder. Such a solidification microstructure may be achieved by applying a rapid cooling procedure early in the cooling profile. In this embodiment, the casting may also include regions of dual solidification microstructure and coarse solidification microstructure. The solidification microstructure profile of  FIGS. 4C and 4D  would be expected from applying a rapid cooling at an early time, such that the narrowest part of the casting would follow a path starting at a point between c and d on the cooling profile in  FIG. 1 . 
     In still another embodiment, such as the embodiment of  FIG. 4E , the entire solidification microstructure comprises a fine solidification microstructure. Such a desirable structure might be achieved by applying a rapid cooling method at Point “b” in the cooling curve of  FIG. 1  and following the profile “abghi.” This would occur if no freezing occurs due to loss of heat to the mold and the freezing occurs totally unidirectionally and at a high rate. Such a structure, however, is not easily achieved and has yet to be experimentally achieved by the inventors. Difficulties in achieving this solidification microstructure arise from the impingement of the liquid coolant directly on the surface of the still liquid casting. A casting comprising 100% fine solidification microstructure may be achievable under certain conditions such as using a highly insulating mold, and applying a highly directional solidification process. 
     An aggregate molded, shaped metal casting may comprise from about 1 to about 100% fine solidification microstructure. Even a small amount of solidification microstructure is desirable for enhancing the mechanical properties of a casting. This is especially the case where the creation of small amounts of fine solidification microstructure, denoting as it does in this invention the action of directional solidification, and thus optimal feeding, prevents defects such as shrinkage porosity from occurring in the casting. 
     Castings in accordance with the present disclosure that include a fine solidification microstructure region may be formed from any solid solution alloy that solidifies dendritically. These include both ferrous materials and non-ferrous materials. The dendrite arm spacing of both the coarse and fine solidification microstructure regions will vary depending on the metal that is used. With respect to aluminum alloys, coarse solidification microstructure regions typically have a dendrite arm spacing of greater than about 50 micrometers. In some embodiments the coarse solidification microstructure has a dendrite arm spacing of from about 50 to about 200 micrometers. Also in aluminum alloys, the fine solidification microstructure has a dendrite arm spacing of less than about 15 micrometers, and, in some embodiments, is from about 5 to about 15 micrometers. 
     Alloys in which solidification occurs partly by dendritic solidification and partly by eutectic solidification, as is typical of many Al—Si alloys, exemplified by Al-7Si-0.4Mg (A356) alloy, may also exhibit fine dendritic and/or fine eutectic microstructure. The conventional cooling curve for mixed dendrite/eutectic alloys is illustrated in  FIG. 5  as curve “a-h.” Starting at the pouring temperature (T P ) at point “a” the liquid alloy cools to the liquidus temperature (T L ), at point “c,” which is the point at which dendrites start to form. The dendrite growth is complete at point “e,” which is the eutectic temperature (T E ). The fall in temperature is arrested, forming a plateau until the completion of the eutectic solidification at point “g”. At this point the casting is completely frozen, and further cooling to room temperature follows “gh.” A second example alloy of an Al—Si alloy that benefits powerfully from the application of this invention is the widely used A319 alloy. This alloy also contains some copper. The alloy differs somewhat from A356 in having a eutectic formation region “eg” that is not isothermal, the horizontal plateau “eg” of  FIG. 5  being replaced by a steady downward slope. However, the same principles apply precisely. 
     This slow conventional cooling in, for instance, a silica sand mold, results in a structure of dendrites, having a DAS in the region of 200 down to 50 micrometers. The dendrites are surrounded by eutectic, which is characterized by a spacing in the region of 20 down to 2 micrometers. This is denoted the conventional or “coarse” eutectic microstructure for purposes of our description ( FIG. 6 ). 
     If it were possible to subject the alloy to total cooling by ablation, the cooling profile would follow the path “bijkl”′ so that the whole solidification microstructure would consist of fine dendrites and very fine eutectic. However, as discussed above, although this structure is not easily obtained, and has yet to be tested by the inventors, it may be achievable in special conditions. These conditions might include circumstances in which the mold is highly insulating, and the freezing is highly directional. Castings having excellent mechanical properties are, nevertheless, achievable without resort to these special conditions. 
     In general, a more practical cooling profile is illustrated by the cooling curve “abcdmno.” In this situation the prior cooling from “cd” creates coarse dendrites to strengthen the casting in its hot, partially solidified, and therefore weak state, prior to the application of the coolant. The subsequent dendrites and the eutectic are both then subjected to rapid cooling, so that the fine solidification microstructure includes both fine dendrites (DAS in the region of 30 down to 5 micrometers) and fine eutectic (not resolvable at 1000× magnification, since its spacing is around only 1 micrometer). 
     The two regions of the consequently dual microstructure form visually highly distinct areas when viewed under the microscope.  FIG. 7  shows a structure in which the ablation was applied in time to freeze some dendritic material, followed by the rapid freezing of all of the eutectic. The eutectic is so fine that it is not resolvable in this image, but appears as a uniform light grey phase (in this case the alloy had no refining action of the additions of chemical modifiers such as Na or Sr). Additionally, because all of the eutectic freezes along the path “mn,” the whole of the eutectic phase, between both the coarse and the fine dendrites, is seen to be uniformly fine in  FIG. 7 . 
     The uniform and extremely fine eutectic is a common feature of ablated solidification microstructures and is unique to ablation cooled alloys that have received no benefit from chemical modification by Na or Sr as an aid to refine the microstructure. It is seen in  FIG. 8 , in which some finely distributed dross and associated pores can also be seen in the structure. The mechanical properties of the castings appear to be remarkable insensitive to most defects of this variety and size.  FIG. 9  illustrates a similar fine eutectic after a solution heat treatment. In  FIG. 9 , the eutectic has coarsened somewhat to reduce its interfacial energy as is common for two-phase structures submitted to high temperature treatment. 
     In principle, although not normally desirable, it would be possible to allow all of the dendrites to freeze with the coarse structure, making only a late application of the ablation cooling, such as, for instance, at point “f” in  FIG. 5 . In this case some of the eutectic would have frozen with a coarse eutectic structure. Such a structure is shown in  FIG. 10 . The final regions of the eutectic that freezes with the benefit of ablation cooling adopt the extremely fine structure and are generally free from porosity and exhibit only fine iron-rich phases, which are generally too small to be seen in  FIG. 10 . At higher magnification, a few iron-rich phases can be seen, as shown in  FIG. 11 . 
     For mixed dendrite/eutectic alloys, most of the benefits of ablation are enjoyed by those structures seen in  FIGS. 10 and 11  because progressive solidification of the residual liquid will still be effective to feed the casting directionally. The alloy in  FIG. 10 , for example, has been heat treated, therefore to some extent coarsening all of the silicon particles in the eutectic phase. 
     In practice, however, it is desirable, and easily achieved, for some coarse dendritic structure to be formed prior to the application of the ablation (starting point “d” in  FIG. 5 ). The usual resulting dendritic microstructure is therefore dual in the sense described above and includes relatively uniformly fine eutectic. 
     As before, if coolant is applied extremely late, for instance following path “gq” in  FIG. 5 , the action of ablation cannot influence the solidification microstructure of the casting because, of course, the casting has fully solidified prior to any application of a coolant. Such cooling of a casting does not form part of this patent application, and falls into the casting production regime well known to those skilled in the art. 
     For conventionally cooled castings (those that adopt the cooling path ending in “h” or “q”) the last regions of the casting to solidify often contain porosity. In addition, such castings when made in a typical aluminum alloy such as A356 alloy, often contain thin platelets of beta-iron precipitates that further impair properties. 
     Ablation-cooled castings, including both dendritic castings and dendritic/eutectic castings, comprising a fine solidification microstructure are generally free of defects that are often found in castings formed by conventional casting methods. In one embodiment, a casting comprising a fine solidification microstructure portion is substantially free from porosity. The rapid freezing and directional feeding created by ablation reduces both gas and shrinkage porosity. In another embodiment, a casting comprising a fine solidification microstructure portion is substantially free of large damaging iron-rich platelets. In other embodiments, a casting is substantially free of both porosity and iron-rich platelets. Without being bound to any particular theory, the reduction in size of the iron-rich platelets may be the result of the more rapid quench of the liquid alloy. The reduction of porosity also benefits from this speed. In addition, it is significantly aided by the naturally progressive action of the ablation process, in which the cooling action of water (or other fluid) is moved steadily along the length of the casting to drive the solidification in a highly directional mode towards the source of feed metal. Furthermore, the maintenance of a relatively narrow pasty zone by the imposition of a high temperature gradient in this way is highly effective in assisting the feeding of the casting. 
     The substantial reduction or elimination of shrinkage porosity is significant, and may be restated as follows. Shrinkage porosity would normally be expected in regions of the casting such as an unfed hot spot. In principle, however, these regions can be fed if the freezing process is carried out directionally. The water or other cooling fluid is applied to ablate the mold and cool and cause solidification in the casting progress systematically, creating a uniquely strong directional temperature gradient. Thus, those regions that would have been isolated from feed liquid in a conventional casting are easily and automatically fed to soundness, or greatly improved soundness, when the benefits of the invention are correctly applied. 
     For this reason, alloys that cannot normally be cast as shaped castings because of hot-shortness problems, such as the wrought alloys 6061 and 7075, etc., or with long freezing range such as alloys 7075 and 852, can easily and beneficially be cast into a shaped form via ablation techniques. In addition, the ablated castings are characterized by a solidification microstructure that is immediately identifiable as being unique in a shaped casting. 
     An aggregate molded shaped metal casting comprising a fine solidification microstructure region is further described with reference to the following examples. The examples are merely for the purpose of illustrating potential embodiments of a shaped metal casting having a fine solidification microstructure region and are not intended to be limiting embodiments thereof. 
     EXAMPLES 
     In one example of the application of the technique described in this application, a single test bar was molded of diameter 20 mm and length 200 mm furnished with a small conical pouring basin at one end that was filled to act as a feeder. The mold material was silica sand bonded with a water-soluble inorganic binder as described in our co-pending application Ser. No. 10/614,601. 
     Thermocouples were inserted into the mold cavity at the base of the feeder, and at the base of the cavity. Two additional thermocouples were located at equal intervals along the axis. These four thermocouples were labeled TC1 (riser), TC2 (top midsection), TC3 (bottom midsection) and TC4 (bottom). 
     An aluminum alloy 6061 at a temperature of 730° C. (1350° F.) was poured into the cavity, arranged with its axis vertical. Within approximately 10 seconds, water at 20° C. (68° F.) was then applied from nozzles directed at the base of the mold so as to start the ablation of the mold from the base upward. The rate of upward progression of the ablation front was approximately 25 mm/s 
     Cooling traces of the four thermocouples were recorded as seen in  FIG. 12 . The thermocouple TC4 is seen to cool rapidly, signaling the freezing and cooling to below the boiling point of water in only about 2 seconds. At this time the thermocouple immediately above, TC3, still records that the metal is still molten, and that cooling has only just begun. This pattern is repeated successively up the mold. (The jump in temperature for TC2 records the unintentional momentary loss of cooling water in this experiment). The thermal traces confirm that the temperature gradient caused by the application of ablation was sufficient to freeze the melt and cool it to near ambient temperatures within a distance of less than the spacing between thermocouples (50 mm). Furthermore, the effect was easily and accurately sustainable for the length of an average automotive casting. 
     In a second example an automotive knuckle casting was made in alloy A356. Many knuckle castings have a reputation for being difficult to cast because of their complexity, having heavy sections distant from points where feeders can be added. This casting was no exception. The casting was filled with metal at 750° C. (1385° F.) on a tilt pouring station, taking 8 seconds to fill, resulting in an excellent surface finish. The feeder (riser) was located at the far end of the casting from the pouring cup and down-sprue. Thermocouples in the sprue and feeder are illustrated in  FIG. 13 . It is seen that freezing took place in the sprue, being the first to ablate, in approximately 20 seconds. Freezing was then caused to progress across the casting, arriving at the feeder approximately 90 seconds later, at which point the feeder is caused to freeze at a similar time of only about 20 seconds. 
     To achieve ablation for this casting, three banks of water spray nozzles were employed, with water pressure at approximately 0.7 bar (approximately 10 psi) and temperature approximately 40° C. (approximately 100° F.). 
     The casting was found to be completely sound, and with properties exceeding specification. 
     In a third example a control arm casting, a steering/suspension component of an automobile, was molded in the mold material as specified for Example 1. The mold was poured with A356 alloy of approximately composition Al-7Si-0.35Mg-0.2Fe at approximately 700° C. (approximately 1400° F.). Ablation cooling of this mold produced a casting that was subsequently cut up and machined to produce tensile test bars that were subject to a T6 heat treatment. The solution treatment was 538° C. (1000° F.) for 0.5 hours, water quench at 26° C. (78° F.) and age at 182° C. (360° F.) for 2.5 hours. Four test bars were cut from each of three castings numbered  45 ,  46  and  47 . The bars were subjected to tensile testing and the results are listed, together with averages, in  FIG. 14 . (The one low extension value of 9% was attributed to a large oxide inclusion since the control of the melt quality was known to be less than optimum on this occasion.) The results are compared with those from competitive casting processes in  FIG. 15 . The properties are clearly attractive, exceeding those of all current best competitive processes. 
     It should be appreciated that certain potential process conditions could result in a conventional microstructure from an ablated mold when using the ablation casting process. As an example, an 852 aluminum alloy (Al-6Sn-2Cu-1Ni-0.75Mg alloy) is known as a long range freezing alloy, wherein the eutectic freezes approximately at the melting point of tin (232 C, 610 F). This alloy was ablated using the ablation casting process. The mold was symmetrical, allowing the identical mold halves to be produced from a single sided pattern and then assembled. The metal was poured at or near 700 C (1275 F). The casting section thickness was approximately 75 mm (3 inch) in diameter. The pouring of the mold by gravity was achieved in 10 seconds. The mold was then left to sit for a period of nearly 180 seconds, to achieve a mostly solidified alpha phase. After this period of normal solidification rate being controlled by the molding aggregate (in this case silica sand), the mold was ablated. 
     The ablation conditions were as follows. Water pressure used for ablation was approximately 1 bar (15 psi). The spray volume was limited by the spray nozzles. However, the water volume is nearly insignificant as the pressure controls the water impingement against the as cast surface. This procedure captured a conventional but uniform cooling rate that yielded similar properties to that produced by a metal mold (i.e., a permanent mold, see  FIG. 16 ). In this connection,  FIG. 16  shows the spectrum of various casting processes and the relationship between dendrite cell size and solidification rate for aluminum alloys. A conventional permanent mold microstructure is illustrated in  FIG. 17   a .  FIGS. 17   b  and  c  show the same alloy but now created using the ablation process. In all three of  FIGS. 17   a - c,  the same magnification, 100×, was employed. The final eutectic structure was closely similar to that produced by a permanent mold. Although such a conventional microstructure can be achieved in the ablation process, in some conditions, those microstructures unique to ablation, including extremely fine phases possibly of dendrites, but more often of extremely fine eutectic, can also be observed. 
     To use the ablation process to capture a conventional microstructure (such as that resulting from a permanent mold), several variables have significant potential. An important parameter is the mold aggregate itself In addition, the volume, pressure and temperature of the cooling medium used in removing the mold while simultaneously causing solidification of the metal are, of course, also important. During their advance along the length of the casting, the dwell time of the cooling sprays upon the casting can be beneficially adjusted to allow for the local surface to volume ratio (the geometrical modulus). The rate of dissolution of the mold binder can be reduced to slow the rate of ablation of the mold, and so reduce the rate of thermal extraction. This could limit the rate so as to produce a conventional microstructure. Furthermore, the water pressure can be varied. At first, a higher pressure can be used to remove the aggregate of the mold. Then, the pressure can be reduced to create a cooling rate that would be akin to that of a conventional metal mold process. 
     Although a conventional microstructure may be attained in this way, there are significant additional benefits offered by the ablation process that make the ablation process uniquely desirable. First, there is improved control of the residual stress within the casting since the final solidification occurs with sufficient liquid ahead of the final solidification front to allow for a complete accommodation of thermal strains. Second, the porosity in the casting is significantly reduced (practically to zero in the majority of cases) because of the excellent directional solidification. This occurs as a result of the high applied temperature gradient that ablation uniquely achieves. 
     Aggregate molded or shaped castings having time solidification microstructure have been described with reference to the present disclosure and various exemplary embodiments. It will be appreciated that variations or modifications may be within the capabilities of a person skilled in the art and that the present application and claims are intended to encompass such modifications. It is intended that the development be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims and the equivalents thereof.