Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/002—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working by rapid cooling or quenching; cooling agents used therefor
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D21/00—Casting non-ferrous metals or metallic compounds so far as their metallurgical properties are of importance for the casting procedure; Selection of compositions therefor
  • B22D21/002—Castings of light metals
  • B22D21/007—Castings of light metals with low melting point, e.g. Al 659 degrees C, Mg 650 degrees C
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D15/00—Casting using a mould or core of which a part significant to the process is of high thermal conductivity, e.g. chill casting; Moulds or accessories specially adapted therefor
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D15/00—Casting using a mould or core of which a part significant to the process is of high thermal conductivity, e.g. chill casting; Moulds or accessories specially adapted therefor
  • B22D15/04—Machines or apparatus for chill casting
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D25/00—Special casting characterised by the nature of the product
  • B22D25/06—Special casting characterised by the nature of the product by its physical properties
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D27/00—Treating the metal in the mould while it is molten or ductile ; Pressure or vacuum casting
  • B22D27/04—Influencing the temperature of the metal, e.g. by heating or cooling the mould
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D30/00—Cooling castings, not restricted to casting processes covered by a single main group
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D46/00—Controlling, supervising, not restricted to casting covered by a single main group, e.g. for safety reasons
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C21/00—Alloys based on aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/04—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon

Definitions

• the present invention relates to metal castings. More particularly, the present invention relates to aggregate shaped metal castings having a fine solidification microstructure.
• molten metal is poured into a mold and solidifies, or freezes, through a loss of heat to the mold.
• the resulting product i.e., a casting
• the casting is then removed from the mold.
• 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.
• 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. [0005] 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. [0006] The sand that is used as an aggregate in sand molding is most commonly silica.
• the elimination of solution treatment avoids the need for the quench that typically follows, removing the problems of distortion and residual stress in the casting that are caused by the quench.
• the fineness of the cast micros gagture 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.
• 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.
• 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.
• DAS fine dendrite arm spacing
• 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.
• 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.
• 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.
• 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.
• the solidification rate reduces in speed as the thickness of the solidified layer increases.
• die 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the disclosure provides a shaped metal casting formed in an aggregate mold by an ablation casting process, the casting comprising a fine solidification micros gagture 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.
• 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.
• FIGURE 1 is a cooling curve for a solid solution alloy that undergoes dendritic solidification
• FIGURE IA is a graph showing the relationship between dendrite arm spacing and freezing or solidification rate
• FIGURE 2 is a micrograph (at X 200) showing the microstructure of a solid solution alloy cast by conventional casting methods
• FIGURE 3 is a micrograph of a solid solution alloy comprising a fine solidification microstructure region (Dual DAS structure) produced by ablation;
• FIGURES 4A-4E are schematic representations of metal castings comprising fine solidification microstructure regions;
• FIGURE S is a cooling curve of a mixed dendrite and eutectic alloy
• FIGURE 6 is a micrograph (at X 200) depicting the microstructure of a 356 alloy showing coarse eutectic silicon particles cast by conventional methods
• FIGURE 7 is a micrograph (at X 200) depicting an ablation-frozen A356 alloy having regions of coarse plus fine dendritic material (dual DAS structure) and fine eutectic material;
• FIGURE 8 is a micrograph (at X 200) of a portion of an ablation-frozen A356 alloy showing uniform coarse DAS and uniform fine eutectic phase;
• FIGURE 9 is a micrograph (at X 200) of an ablation-frozen A356 alloy having fine eutectic regions, but which are somewhat coarsened after a solution heat treatment;
• FIGURE 10 is a micrograph (at X 200) of an ablation-frozen A356 alloy exhibiting coarse dendritic microstructure and mainly fine eutectic microstructure, but containing a trace of coarse eutectic phase;
• FIGURE 11 is a detail of the micrograph (at X 1000) of FIGURE 10 at higher magnification;
• FIGURE 12 depicts the solidification rate of various portions of a dendritic alloy formed in accordance with an exemplary embodiment
• FIGURE 13 is a graph depicting the cooling rate of various sections of an exemplary embodiment casting
• FIGURE 14 is a table showing the mechanical properties of various exemplary embodiment castings
• FIGURE 15 is a bar graph and table comparing the mechanical properties of metal castings formed by various casting methods
• FIGURE 16 is a chart showing the relationship between dendrite cell size and solidification rate for aluminum alloys
• FIGURE 17a is a micrograph (at XlOO) of a conventional cast permanent mold microstructure of a 2.75" diameter bar of an aluminum alloy;
• FIGURE 17b is a micrograph (at XlOO) of such a microstructure made with the ablation process for the same alloy at the start of the ablated part (first part in); and,
• FIGURE 17c is a micrograph (at X 100) of the ablated last part.
• FIGURE 18 is a photograph of an ablation frozen B206 alloy automotive control arm showing the location (boxed) from which tensile test bars were machined after heat treatment.
• FIGURE 19 is a table showing mechanical properties obtained from a very thick section in a specific exemplary embodiment B206 aluminum alloy casting.
• FIGURE 20 is a table showing data from the literature for A206 separately cast tensile bars produced via various casting methods.
• FIGURE 21 is a micrograph taken from an as-cast (F-temper) B206 casting obtained from the center of the thick section shown in FIGURE 18 and having both coarse and fine dendritic material (dual DAS structure).
• 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.
• 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.
• a shaped casting may comprise only a single type of microstructure.
• an alloy may exhibit a solidification microstructure comprising one or more different microstructures.
• a shaped casting may exhibit a microstructure comprising a combination of dendrites and grains.
• a shaped casting may exhibit a combination of dendrites and eutectic phases.
• a shaped casting may exhibit a combination of dendrites, eutectic phases, and grains.
• 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 Serial No. 10/614,601 that was filed on July 7, 2003, and the entire disclosure of which is incorporated herein by reference.
• Application Serial 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 Serial No. 10/614,601 is referred to herein in as "ablation.”
• a metal casting 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.
• FIGURE 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 (Ts), 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 die 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 (Ts) and the rate of fall of temperature increases once the evolution of latent heat has subsided.
• 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 Ts of the alloy.
• tj t e - t c .
• Figure 1 a shows the approximate logarithmic relation between the local DAS and the local tg in many common Al alloys. This figure illustrates that to reduce DAS by a factor of 10, ts is required to be reduced by a factor of approximately 1000. (Undoubtedly 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.
• FIGURE 2 is a micrograph depicting the coarse microstructure of a solid solution cast alloy A206 (a nominal Al - 4.5wt%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.
• 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.
• a casting comprises a dual solidification microstructure region intermediate a coarse solidification microstructure region and a fine solidification microstructure region.
• a dual microstructure region is a region that includes areas of coarse microstructure having one or more areas of fine microstructure interspersed therein.
• 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 FIGURE IA, the log/log relationship of dendrite arm spacing to freezing time is linear and, for aluminum alloys, for example, has a slope of approximately 1/3.
• FIGURE 3 is a micrograph showing regions of both fine microstructure and regions of coarse microstructure.
• FIGURES 4A-4E various exemplary embodiments of aggregate molded, shaped metal castings comprising a fine solidification microstructure region are shown.
• a casting comprises a small percentage of fine solidification microstructure.
• the casting in FIGURE 4A represents a situation in which the rapid cooling process, such as ablation, is applied late in the casting process.
• some solidification has occurred prior to rapid cooling (e.g., ablation)
• 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.
• 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 FIGURE 4B is an example of an embodiment of a casting having a substantially continuous dual solidification microstructure region.
• the solidification microstructure profile of FIGURE 4B is a profile that would be expected from following the cooling profile "abcdjk" in FIGURE 1. In FIGURE 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 FIGURE 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 FIGURE 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.
• a rapid cooling procedure such as ablation
• the casting includes a greater percentage of fine solidification microstructure, and die region of dual solidification microstructure is continuous through the constricted region of the casting.
• a desirable solidification microstructure may be achieved by applying a rapid cooling procedure, such as ablation, at a time earlier than that of FIGURE 4B.
• a metal casting comprises a desirable fine solidification microstructure region that is substantially continuous from the distal ends of the casting to the feeder.
• a solidification microstructure may be achieved by applying a rapid cooling procedure early in the cooling profile.
• the casting may also include regions of dual solidification microstructure and coarse solidification microstructure.
• the solidification microstructure profile of FIGURES 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 FIGURE 1.
• the entire solidification microstructure comprises a fine solidification microstructure.
• a desirable structure might be achieved by applying a rapid cooling method at Point "b" in the cooling curve of FIGURE 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 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.
• coarse solidification microstructure regions typically have a dendrite arm spacing of greater than about 50 micrometers.
• the coarse solidification microstructure has a dendrite arm spacing of from about 50 to about 200 micrometers.
• 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 FIGURE 5 as curve "a-h.”
• the liquid alloy cools to the liquidus temperature (T L ), at point "c,” which is the point at which dendrites start to form.
• T L liquidus temperature
• T E eutectic temperature
• the fall in temperature is arrested, forming a plateau until the completion of the eutectic solidification at point "g”.
• the casting is completely frozen, and furtiier 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 Figure 5 being replaced by a steady downward slope.
• 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 (FIGURE 6).
• FIGURE 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 FIGURE 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 FIGURE 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.
• FIGURE 9 illustrates a similar fine eutectic after a solution heat treatment. In FIGURE 9, the eutectic has coarsened somewhat to reduce its interfacial energy as is common for two-phase structures submitted to high temperature treatment.
• FIGURE 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 FIGURE 10. At higher magnification, a few iron-rich phases can be seen, as shown in FIGURE 11.
• FIGURE 10 for example, has been heat treated, therefore to some extent coarsening all of the silicon particles in the eutectic phase.
• FIGURE 5 The usual resulting dendritic microstructure is therefore dual in the sense described above and includes relatively uniformly fine eutectic.
• the last regions of the casting to solidify often contain porosity.
• 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.
• 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.
• a casting comprising a fine solidification microstructure portion is substantially free of large damaging iron-rich platelets.
• a casting is substantially free of both porosity and iron -rich platelets.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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 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 TCl (riser), TC2 (top midsection), TC3 (bottom midsection) and TC4 (bottom).
• 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. [0094] In a second example an automotive knuckle casting was made in alloy A356.
• 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-O.35Mg-O.2Fe at approximately 700 0 C (approximately 1400 0 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 0 F) for 0.5 hours, water quench at 26°C (78°F) and age at 182 0 C (360 0 F) for 2.5 hours.
• an 852 aluminum alloy (Al-6Sn-2Cu- lNi-0.75Mg alloy) is known as a long range freezing alloy, wherein the eutectic freezes approximately at the melting point of tin (232C, 610F).
• 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 700C (1275F).
• the casting section thickness was approximately 75mm (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. [0099]
• 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).
• FIGURE 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 FIGURE 17a.
• FIGURES 17b and c show the same alloy but now created using the ablation process. In all three of FIGURES 17a-c, the same magnification, 10OX, 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.
• 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.
• 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.
• 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.
• an automotive suspension control arm casting was molded in the material as specified for Example 1. Molds were poured with B206 alloy of approximate composition Al-4.8Cu-0.4Mn-0.28Mg-0.07Fe at approximately 1265F plus or minus 15F. Ablation cooling of these molds produced castings that were subsequently subjected to T4 and T7 heat treatments. A photograph of the part in question appears in Figure 18 with a box showing the position from which tensile samples were subsequently machined. Tensile properties obtained from the parts are shown in Figure 19 and are compared to standard tabulated values obtained from separately cast test bar data from the open literature for this type of alloy in Figure 20.
• FIG 21 A micrograph taken from the center of the thick section of one of the parts prior to heat treatment appears in Figure 21.
• the properties are again; clearly attractive, almost equaling those produced from separately cast permanent mold test bars and exceeding those in all other molding medias listed. These values are particularly attractive considering that they were obtained from an area with a section thickness of over 2 inches. Under conventional sand and permanent mold processes, those skilled in the art would expect to achieve far lower values of ductility from bars machined from a section this massive than are listed for the separately cast test bar data in Figure 20 in which 1 A inch thick gauge diameter bars with an as-cast surface are the norm.
• the micrograph in Figure 21 displays the aforementioned dual microstructure seen in ablatively solidified parts.
• a course DAS averaging 43 um with spacings as high as 85um is host to much finer patches with a DAS averaging on the order of 22um.
• the mechanical properties reported in Figure 17 are more typical of those that would be expected from the finer DAS.
• the inte ⁇ netallics visible in Figure 19 are, in the main, Cu Aluminides which dissolve during the solution heat treatment applied as part of both the subsequent T4 and T7 tempers.
• Those skilled in the art of producing 200 series aluminum castings will recognize the advantage of a fine structure with respect to economy of heat treatment.
• the three stage solutionizing operation needed to dissolve the aforementioned Cu Aluminides for thick sand castings may be foregone in favor of the two stage treatment most commonly applied in the case of thin and/or rapidly solidified casting.

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