FIELD OF THE INVENTION
The present invention relates to the casting of metals, more specifically, to a novel method of producing sand castings in hot molds coupled with progressive cooling (HMPC).
BACKGROUND OF THE INVENTION
Large thin-walled shape castings free from shrinkage porosity are difficult to make, especially those with lengths much greater than their thickness. One example is thin-walled plate-shaped castings with lengths that are orders of magnitude greater than their wall thickness. The length of such castings is limited by the fluidity of the metal. The internal quality of such castings is affected by the feeding of the solidification shrinkage by a feeder.
Forging is capable of producing porosity-free thin-walled plate-shaped parts. Pores that form during solidification can be closed by plastic deformation during forging. However, forging is incapable of making parts of complex geometry. The costs associated with forgings are much higher than those of castings. In addition, forging is good only for manufacturing small parts.
Continuous casting and direct-chill casting processes are capable of making billets free from shrinkage porosity but are incapable of making thin-walled shape castings of complex geometry [1].
High-pressure diecasting (HPDC) is a process that has been widely used for making large thin-walled shape castings [2]. High pressure is used for driving a molten metal into the thin-walled cavity in strong metal molds. The internal quality of the thin-walled castings made by the HPDC process is usually poor due to the entrapped gases and oxides during the turbulent mold filling process associated with the HPDC process [3-4]. Furthermore, solidification shrinkage in thin-walled shape casting is difficult to feed [5]. As a result, porosity is a common defect in products made using the HPDC process, including semisolid recasting, indirect squeeze casting, or even direct squeeze casting, which is difficult to utilize in making complexly shaped thin-walled parts of high internal integrity. In addition, the size of a casting that can be produced by the HPDC process is limited by the size and properties of the metal mold and the fluidity of the alloy [2, 6-7].
Gravity casting processes using metal molds have issues with mold filling for producing large thin-walled shape castings [6] and shrinkage feeding problems [8]. The minimal wall thickness that can be made using these processes is much greater, and the maximum size of a casting is much smaller than those made using HPDC. These processes are difficult for manufacturing large thin-walled castings of high internal integrity.
Sand casting is probably the only cost-effective casting process that is capable of producing porosity-free shape castings of a large size and a complex geometry. However, the minimal wall thickness of a casting produced by the sand casting process is much greater than that by HPDC due to fluidity issues.
Porosity in a sand casting consists of gas porosity and shrinkage porosity [5]. Gas porosity can be removed by careful degassing. To minimize shrinkage porosity, risers must be used. The feeding distance of a riser is about 2 times the wall thickness for a steel casting and about up to 10 times the wall thickness for an aluminum casting [9]. As a result, a large number of risers must be used to make a large thin-walled casting free from shrinkage porosity, leading to extremely low metal yield per mold. These risers must be machined out, resulting in extra labor and costs.
Forced directional solidification from the distal end of the casting to its riser/feeder could be useful in extending the feeding length of the riser. For example, the use of metal chills extends the feeding distance of a riser by two times the wall thickness of a casting [9]. Still, such an increase in feeding distance by using chills is very limited.
U.S. Pat. No. 7,216,691 to Grassi et al. discloses an ablation casting technology which uses a water-soluble binder for making sand molds and nozzles outside of the molds for spraying a liquid solvent over the molds to dissolve the soluble binder, to ablate away the molds and to cool the solidifying casting progressively from the distal end of the casting to the feeder. Such a technology is capable of extending the feeding distance of the feeder, but a unique soluble binder must be used for this technology.
So far, research on ablation casting technology has been focused on shapes of castings that have no issues associated with fluidity and shrinkage porosity [10-21]. The castings tested are either of a relatively thick wall or with risers. Little work on thin-walled casting is available in literature. Porosity was found in A356 alloy castings with an early application of ablation cooling, but a significant amount of porosity was formed in both the sand casting and the castings ablation cooled at later stages of solidification [16]. Porosity was also found in aluminum matrix composite castings solidified under ablation cooling conditions [13].
Therefore, there is a need to develop a novel casting process that is capable of producing thin-walled shape castings free from shrinkage porosity. Such castings would have mechanical properties approaching those of forgings made from the same alloy.
There is also a need to develop a novel casting process that is capable of forming an entire thin-walled casting of high internal quality using a single downsprue, feeder, or riser wherein the downsprue serves as the feeder or the riser.
There is also a need to develop a process that is capable of producing large sand castings with comparable wall thicknesses and greater sizes than those of HPDC castings.
Furthermore, there is a need to develop a process that is capable of manufacturing large thin-walled castings with controlled internal porosity distribution for weight reduction.
Furthermore, there is also a need to develop a process that is capable of using ablation cooling with water spray but does not require the use of a water-soluble binder to make sand molds.
SUMMARY OF THE INVENTION
The invention provides a hot mold progressive cooling (HMPC) sand casting process for the fabrication of thin-walled shape castings of high internal integrity. The process includes the steps of providing at least one sand mold held at elevated temperatures, introducing a molten alloy into the mold cavity, maintaining the mold or molds above predetermined temperatures while the molten alloy undergoes solidification within the mold cavity, and progressively cooling the solidifying alloy using a coolant from the distal end of the casting towards the riser or feeder until the casting has reached desired temperatures.
In an embodiment of the present invention, a process for reducing the use of risers or feeders for the fabrication of thin-walled shape castings of high internal integrity is provided. The process includes the steps of providing at least one mold held at elevated temperatures, introducing a molten alloy into the mold cavity through a downsprue, maintaining the mold or molds above predetermined temperatures while the molten alloy undergoes solidification within the mold cavity, and progressively cooling the solidifying alloy using a coolant from the distal end of the casting towards the downsprue until the entire casting is completely solidified.
In another embodiment of the present invention, a process for the fabrication of extremely large thin-walled shape castings of high internal integrity is provided. The process includes the steps of providing at least one mold held at elevated temperatures, introducing a molten alloy into the mold cavity, maintaining the mold or molds above predetermined temperatures to ensure the fluidity of the alloy to fill the mold cavity, and progressively cooling the solidifying alloy using a coolant from the distal end of the casting towards the feeder/downsprue until the entire casting is totally solidified.
In yet another embodiment of the present invention, a process for the fabrication of thin-walled shape castings with controlled porosity distribution is provided. The process includes the steps of providing at least one mold held at various locally elevated temperatures, introducing a molten alloy into the mold cavity, maintaining the mold or molds above predetermined temperatures while the molten alloy undergoes solidification within the mold cavity, and progressively cooling the solidifying alloy using a coolant with varying speeds from the distal end of the casting towards the downsprue until the entire casting is totally solidified.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of a layout of one embodiment of the present invention.
FIG. 2 is a schematic view of a layout of one embodiment of the present invention.
FIG. 3 is a schematic view of a casting, the molds and the locations where tensile specimens are taken from the plate-shaped casting.
FIG. 4 depicts the relationship between the solid fraction and the temperature of the A356.2 alloy.
FIGS. 5A, 5B, and 5C show the cooling curves, the evolution of fraction solid in a sand casting on cooling, and an SEM image of the fractured surface of a tensile specimen, respectively.
FIGS. 6A, 6B, and 6C show the cooling curves of a sand casting and a HMPC casting, the evolution of fraction solid in the HMPC casting on cooling, and an SEM image of the fractured surface of a tensile specimen taken from the HMPC casting, respectively.
FIGS. 7A, 7B, and 7C show the cooling curves of a sand casting and a HMPC casting, the evolution of fraction solid in the HMPC casting on cooling, and an SEM image of the fractured surface of a tensile specimen taken from the HMPC casting, respectively.
FIGS. 8A, 8B, and 8C show the cooling curves of a sand casting and a HMPC casting, the evolution of fraction solid in the HMPC casting on cooling, and an SEM image of the fractured surface of a tensile specimen taken from the HMPC casting, respectively.
FIG. 9 is a photograph showing the HMPC process in operation.
FIG. 10 is a photograph showing two castings made by the HMPC process.
FIG. 11 shows the tensile strength of selected castings and forgings.
FIG. 12 shows the elongation of selected castings and forgings.
DETAILED DESCRIPTION OF THE INVENTION
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety.
The present invention is also explained in detail in the article recently published in an international journal [22].
In a preferred embodiment, the present invention relates to a method of manufacturing large thin-walled sand castings with high internal integrity using hot mold with progressive cooling (HMPC) process. The term “large thin-wall” or “thin-walled” defines the shape of a casting when its length or width, whichever is the greatest, is 4 times to orders of magnitude greater than its average wall thickness. Such a casting is unable to be fed using a single riser by prior art in the metal casting industry. The conventional wisdom in the art of metal casting is that the feeding distance of a riser is 2.5 times the thickness of steel castings and is less than 10 times the thickness of aluminum castings [8]. The term “high internal integrity” refers to the internal quality of a casting that is free from shrinkage porosity. By such definitions, large thin-walled castings of high internal integrity are unable to be manufactured using a single riser by prior art because the solidification shrinkage of the casting cannot be fed, resulting in the formation of shrinkage porosity in the casting. Hot molds in the present invention include those that are heated up to elevated temperatures using known conventional technologies in the metal casting industry. Hot molds also include molds that contain exothermic materials or insulating materials at their localities. For example, molds can be heated up using furnaces, ovens, or infrared lamps. Exothermic materials can be placed in certain locations in a mold to heat up the mold locally. Insulation materials can be placed in certain locations to maintain the local temperatures in the molds. The term “progressive cooling” refers to cooling methods that maintain the freezing front movement with selected speeds, which are under control, from the distal end of the casting to the riser, feeder, or downsprue. The uniqueness of this present invention is in the use of combined technologies in mold heating and casting cooling to ensure the ease at which molten metal can fill a large thin-walled mold cavity and feed the solidification shrinkage during the solidification of the molten metal using a single downsprue that serves as a feeder or a riser.
Cavities for a large thin-walled casting are usually difficult to fill by molten metal because the molten metal tends to solidify when it flows into the cold cavity. The length of a thin-walled cavity that a molten metal can flow before being frozen depends on the size of the flow channel, the temperature of the molten metal and the molds, the pressure driving the flow, and other factors. Molds with higher temperatures allow a molten metal to flow a greater length than molds with lower temperatures. Given sufficient pressure, a molten metal can flow to fill a cavity of any length provided that the mold temperatures are higher than the freezing temperature of the molten metal and the channel thickness is not extremely thin. Still, the metal casting industry prefers not using molds of high temperatures partly because the hot molds slow down the solidification rates in the casting, promote one-dimensional heat transfer from surfaces to the centerline of the thin-wall casting, and lead to increased formation of porosity in the final product. Molds of high temperatures are usually used for making large thin-walled castings that have no strict requirements on their internal quality.
The new idea of this present invention is to use the hot molds to facilitate the mold filling of a molten metal and to maintain the solidifying metal at temperatures where the colder freezing front can be fed by hotter metal ahead of the front by either a liquid feeding or mass feeding mechanism. To keep the freezing front sufficiently fed, the freezing front has to travel from the distal end of the casting to the feeder/downsprue while the solidifying metal ahead of the front is still at adequate temperatures. Such a condition has to be satisfied by forcing the freezing front to travel through the casting within the time frame where shrinkage feeding can still be maintained. The present invention deals with the utilization of technologies for mold heating and progressive cooling to produce large thin-walled shape castings of high internal integrity. The use of hot molds is 1) to ensure that extremely large thin-walled cavities can be filled by a molten metal or material, 2) to ensure that the solidifying metal in predetermined regions of the mold can be maintained at predetermined temperatures before the coolant is applied locally, and 3) to allow sand mold with certain conventional binders that are not water-soluble after being heated up to high temperatures to be used for controlled penetration of the coolant to cool the solidifying casting. The use of hot mold technology combined with the progressive cooling technology is to ensure that shrinkage porosity can be totally avoided in the solidifying casting using a single downspruce. Furthermore, a controlled mold temperature distribution combined with a controlled variable progressive cooling can be used for controlling porosity distribution in a casting so that critical areas in the casting are porosity free but non-critical areas contain a controlled amount of porosity, leading to a mass reduction of the resultant casting.
Binders that are water-soluble at room temperatures may not be water-soluble after the binder is hardened by chemical reactions or by heating. Sodium silicate is such an example. When the sodium silicate is hardened chemically using CO2, chemical reactions in the form of Na2SiO3+2CO2+2H2O=2NaHCO3+H2SiO3 occur, producing H2SiO3 that is not water-soluble. One of the purposes of using a hot mold is to break the bond between the binding materials and the aggregates using the quenching effect of the coolant so that the hot mold collapse immediately when it is contacted by the coolant, allowing the coolant to direct cool the solidifying casting. This is also an important feature of this disclosure. A direct contact of the coolant to the surfaces of a solidifying casting ensures that the temperature gradient in the casting is high so that the freezing front in the casting can be controlled by the speeds at which the coolant travels over the casting, from its distal end to the feeder/downsprue.
FIG. 1 illustrates a method of one embodiment of the present invention on the production of large thin-walled sand castings with high internal integrity using HMPC technology. Sand molds 12 and 16 are made with or without flasks. The mold, 12 or 16, is composed of an aggregate and a binder that are conventionally used for making green sand molds (most of them are not water-soluble), and can be made using conventional mold making methods including, but not limited to, mold making machine, sand blower, 3D printing, etc. Molds thus made are heated to predesigned elevated temperatures and become hot molds. Cavity 10 in the molds 12 and 16 is used to make a casting. Cavity 14 in the mold 12 is used as a feeder or a downsprue where a molten metal is introduced to form the casting. A coolant is provided through coolant delivery devices 18 and 20 and applied on the solidifying casting. The coolant may first be applied at the distal end of the casting through the coolant delivery device 18 which is fixed between mold 12 and mold 16. The coolant delivered from device 18 to the casting promotes the formation of a freezing front near the distal end of the casting. As soon as the freezing front is formed at the distal end of the casting, the coolant can then be applied through device 18 which travels from the distal end of the casting towards the feeder in cavity 14 to completely solidify the metal. In some cases, device 20 is not needed. Coolant can be directly applied through device 18 as it progressively travels from the distal end of the mold 12 to the feeder of the casting in cavity 14. The speed of the device 18 is defined as a relative translational speed between the device 18 and the molds 12 and 16. Such a relative motion can be caused either by moving the device 18 while the molds are stationary, by moving the molds 12 and 16 while the device 18 is stationary, or by moving both device 18 and molds towards each other. The speed of the relative motion can be either constant or time dependent for either removing porosity completely or for controlling porosity distribution in the solidified casting. The speed is usually between a range of 0 mm/s to 100 mm/s, preferably in a range between 2 to 40 mm/s. The temperature distribution in molds 12 and 16 can also be controlled for controlling porosity distribution in the final solidified casting.
FIG. 2 depicts the operation of producing a large thin-walled sand casting with high internal integrity using the present invention of HMPC technology. When coolant is applied through the hot molds 12 and 16 on the solidifying casting 24 as described in the paragraph [0038], freezing fronts are formed and move towards the feeder in the casting 24. Two important freezing fronts are illustrated in FIG. 2. The solidus front 20 is closer to the coolant delivery device 18 and the rigid dendrite front 22 is further away from the coolant deliver device 18. A dendrite network is considered as rigid when the fraction solid is about 0.25, which is also the dendrite coherency point. Therefore, the rigid dendrite front is also termed as the front of the dendrite coherency point.
There are two features shown in FIG. 2 that are closely related to porosity formation. One is the angle between the normal direction of the front to the central line 26 along the casting wall thickness. The other is the distance between these two fronts, 20 and 22.
For each front, there is an average angle of the front to the central line 26 along the casting thickness. The temperatures of the molds 12 and 16 and the coolant conditions including the coolant amount and the speed at which the coolant delivery device 18 travels should be controlled such that the average angle of the front 20 has to be greater than a critical value. When the average angle is greater than the critical value, solidification shrinkage of the front 20 can be fed by the liquid from the feeder so that shrinkage porosity can be avoided if the distance between these two fronts is small. For a controlled distribution of porosity in the casting, the mold temperatures and the cooling conditions have to be controlled such that the average angles in the regions where shrinkage porosity has to be maintained small or the distance between two fronts has to be large.
The distance between the fronts shown in FIG. 2 affects the ease of liquid feeding to the solidification shrinkage at front 20. Dendrites existing between these two fronts tend to restrict the liquid ahead of front 22 to flow towards front 20. The use of hot molds in this present invention is to reduce the distance between these two fronts so that the liquid ahead of front 22 has less difficulties in feeding the solidification shrinkage at front 20.
FIG. 3 illustrates a plate-shaped casting, the mold assembly, and the locations and orientation of the tensile samples taken from the plate-shaped casting for mechanical property measurement. Only one feeder that served as a downsprue was located near the left end of the casting. The thickness, width, and the length of the casting were 13 mm, 70 mm, and 400 mm, respectively. The length to thickness ratio of the plate-shaped casting was about 31, much greater than 4-10, indicating that shrinkage porosity should form in the casting under conventional sand casting conditions using only one feeder or one riser. A356.2 aluminum-silicon hypoeutectic alloy was used to make the plate-shaped casting to demonstrate the benefits of the present invention of HMPC process over sand casting and ablation casting processes.
FIG. 4 shows the relationship between the fraction solid and temperature for A356.2 alloy. The primary dendrite phase, which is a fcc phase, forms at about 615° C. where the fraction solid is zero. The eutectic silicon phase forms at the eutectic temperature, TEU, about 572° C. where the fraction solid is about 0.5. Below the eutectic temperature, a number of intermetallic phases form. The solidus temperature of the alloy is about 550 to 560° C. where the fraction of solid is 1.0 [23].
A validated model [15, 22] was used to calculate the cooling curves of the plate-shaped casting under conventional sand casting conditions and HMPC conditions by applied ablation cooling at a constant translational velocity of 10 mm/s from the distal end of the casting to the gate/feeder.
FIG. 5A illustrates the cooling curves of a sand casting. The molten alloy at 680° C. is poured into the mold cavity. After mold filling as shown in FIG. 5A, the local temperature at the distal end of the casting drops to the liquidus temperature, TL, in about 18 s, to TEU in about 80 s, then to the solidus temperature, Ts, in about 160 s as local solidification completes. The temperature at the gate/feeder drops slower than that at the distal end of the casting. There is a temperature plateau at TEU on both cooling curves. The evolution of fraction solid in the sand casting during 55 to 80 s after mold filling is shown in FIG. 5B. There are a number of fronts illustrated in FIG. 5B. The solidus front is shown as the gray front where the fraction of solid is 1. The eutectic front is outlined by the boundary between yellow and orange where the fraction solid is 0.5 and the corresponding temperature is TEU. The dendrite coherency point is outlined by the boundary between red and brown where the fraction solid is 0.25 according to Chai et al. [24]. For this alloy casting, the eutectic front almost overlaps with the solidus front. One can use either the solidus front or the eutectic front to represent front 20 and the front of dendrite coherency point to represent front 22 shown in FIG. 2. It can be clearly seen that when the solidus front just reaches the distal end of the plate-shaped casting, the front of dendrite coherency point has already entered the feeder/downsprue of the casting. The distance between these two fronts is the entire length of the casting, and the fraction solid in the entire casting is greater than 0.25. It is extremely difficult for the liquid in the feeder to flow through dendrites over such a long distance to feed the solidification shrinkage of the solidus front. As a result, shrinkage porosity occurs in the casting. FIG. 5C is a scanning electron microscopy (SEM) image of the fracture surface of a tensile specimen taken from the center section of the casting as shown in FIG. 3. The SEM image reveals a few large shrinkage pores on the fracture surface. The fracture surface is widely accepted as an ideal surface to show larger pores that are more detrimental to the mechanical properties, especially ductility, than smaller pores in a casting [25-26].
FIG. 6A illustrates the cooling curves of a sand casting and a HMPC casting when the mold temperature is held at 100° C. The molten alloy at 680° C. is poured into the mold cavity. Ablation cooling traveling at 10 mm/s is then applied from the distal end of the casting starting at about 80 s after the mold cavity is totally filled. Such conditions are similar to ablation casting [10-11] where the mold temperatures are at room temperatures, slightly lower than 100° C. The cooling curves of the sand casting are represented by the black or red dashed lines. The dashed cooling curves are similar to that shown in FIG. 5A except that the solidification times, the times from the moment of pouring to the moments when the cooling curves reach the solidus temperature, are longer because the mold temperatures are slightly higher than those shown in FIG. 5A. The application of a progressive ablation cooling reduces the solidification times significantly as shown in the solid curves in FIG. 6A. The evolution of fraction solid in the HMPC casting during 55 to 80 s after mold filling is shown in FIG. 6B. The solidus front appears at the distal end of the casting at 55 s while the front of dendrite coherency point has already reached the middle of the casting. The distance between these two fronts is increased at 60 s and then is decreased afterwards. It appears that the feeder/downsprue is capable of feeding the solidification of the solidus front when the front moves close to the feeder towards the end of the casting solidification. However, the majority portion of the casting, especially the middle portion of the casting, cannot be sufficiently fed. Shrinkage porosity does exist in the tensile specimens as shown in FIG. 6C.
FIG. 7A illustrates the cooling curves of a sand casting and a HMPC casting when the mold temperature is held at 200° C. The molten alloy at 680° C. is poured into the mold cavity. Ablation cooling traveling at 10 mm/s is then applied from the distal end of the casting starting at about 80 s after the mold cavity is totally filled. The cooling curves of the sand casting, represented by the black or red dashed lines, are similar to those shown in FIGS. 5A and 6A except that the solidification times are longer because the mold temperatures are higher. The application of a progressive ablation cooling reduces the solidification times significantly as shown in the solid curves in FIG. 7A, which is very similar to the curves associated with ablation cooling in FIG. 6A. The evolution of fraction solid in the HMPC casting during 55 to 80 s after mold filling is shown in FIG. 7B. The solidus front reaches the distal end of the casting at 55 s while the front of dendrite coherency point is within 1-2 thickness distance away from the solidus front, indicating that the liquid ahead of the front of dendrite coherency point is capable of feeding the solidification shrinkage of the solidus front. The distance between these two fronts is increased to more than 4 times the wall thickness of the casting when the cooling time is about 70 s, but the distance then is reduced afterwards. Feeding problems could occur for steel casting when the distance between these two fronts is greater than 4 times, but this is not a major concern for aluminum casting. The SEM image, FIG. 7C, of the fracture surface shows only two small pores, indicating the internal quality of the aluminum HMPC casting is reasonably good.
FIG. 8A illustrates the cooling curves of a sand casting and a HMPC casting when the mold temperature is held at 350° C. The molten alloy at 680° C. is poured into the mold cavity. Ablation cooling traveling at 10 mm/s is then applied from the distal end of the casting starting at about 80 s after the mold cavity is totally filled. The cooling curves of the sand casting indicate that the solidification times in the sand casting in molds of higher temperatures are longer than those in molds with lower temperatures. However, there is no significant difference in solidification times of the HMPC castings solidified in molds held at different temperatures. The evolution of fraction solid in the HMPC casting during 55 to 80 s after mold filling is shown in FIG. 8B. The solidus front reaches the distal end of the casting at 55 s while the front of dendrite coherency point is within 1-2 thickness distance away from the eutectic front. In fact, the distance between these two fronts is within 1-2 times the thickness of the wall throughout the entire solidification process of the plate-shaped casting, indicating that the liquid ahead of the front of dendrite coherency point is capable of feeding the solidification shrinkage of the solidus front. Indeed, the SEM image shown in FIG. 8C depicts no shrinkage porosity on the tensile sample taken from the center portion of the casting, indicating that the internal quality of the HMPC casting is excellent, similar to that of forgings.
Results shown in FIGS. 5-8 suggest that the present invention of HMPC technology is capable of producing thin-walled castings free from shrinkage porosity if the mold temperature is sufficiently high. The invention can also be used for controlling porosity distribution in a casting by varying the local temperatures since pores form in regions where the mold temperatures are lower and are eliminated in regions where the mold temperatures are higher. Porosity distribution in a casting can also be controlled by fixing the mold temperature and varying the speeds of progressive cooling. Pores should form when the speed is too low or too high.
The key idea of this present invention of HMPC technology is to control the mold temperature and the delivery of a coolant to the solidifying casting in such a manner that the distance between important solidification fronts, for example the distance between the dendritic front and the solidus front, is within a certain limit to eliminate porosity formation and outside the limit for allowing pores to form in the casting. This limit seems to be in the range of 4 to 10 times the wall-thickness of the thin-walled casting.
The invention further provides examples of the present invention of HMPC technology. The examples provided below are merely meant to exemplify several embodiments and should not be interpreted as limiting the scope of the claims, which are delimited only by the specification.
EXAMPLE
FIG. 3 illustrates a plate-shaped casting, the mold assembly, and the locations and orientation of the tensile samples taken from the plate-shaped casting. The thickness, width, and the length of the casting were 13 mm, 70 mm, and 400 mm, respectively.
2.5 kg of A356.2 alloy was melted in a graphite crucible using electric resistance heating, heated to 720° C. in 20 min, modified with 0.05 wt. % Sr, fully degassed while the melt cooled down from 720° C. to 680° C. before poured into the cavity in steel metal molds for making permanent mold castings, sand molds with sodium silicate as the binder for making sand castings, or preheated sand molds at various temperatures (100, 200, or 350° C.) for making HMPC castings using the HMPC technology described in one embodiment of this present invention. Silica sand with conventional sodium silicate binder was mixed in a sand mixer for making sand molds. The molds were hardened using CO2.
For comparison, forgings of the same dimensions of the plate-shaped casting were obtained. These forgings were plastically deformed, at high forging temperatures, by 70% along its wall thickness to close out any cavities that might exist and to breakup silicon particles into small fragments.
Molds for the HMPC process were preheated in a muffle furnace to desired temperatures. The preheated molds were then filled with the A356.2 alloy and transferred to an ablation cooling setup shown in FIG. 9 for making plate-shaped castings. The ablation cooling setup consisted of an array of water nozzles and a translation conveyer traveling at rate of 10 mm/s over the filled molds, ablating away the sand molds to cool the solidifying casting. Castings made are shown in FIG. 10. Ablation cooling technology [10-11] failed to ablate the sand molds held at room temperatures under controlled manner, probably because the binder was not water-soluble after being hardened using CO2. As a result, no castings with satisfactory surface quality were successfully made by ablation cooling when the molds containing silica sand with a conventional binder were at room temperatures.
FIG. 11 shows the tensile strength of the forgings and castings made using various processes where TM is the mold temperature. Castings with satisfactory surface quality were made when TM was 100° C. or higher. Generally, the tensile strength obtained on this alloy was lower than those reported in the literature partly because the mass of metal melted was only 2.5 kg in this example. It is very difficult to refine molten aluminum of small quantity.
FIG. 12 illustrates the ductility of the forgings and castings made using various processes where TM is the mold temperature. Castings with satisfactory surface quality were made when TM was 100° C. or higher. The elongation of the sand casting and metal mold casting were around 3% or less. The HMPC process with mold temperatures higher than 200° C. is capable of increasing the elongation of the thin-walled plate-shaped casting to 16-19%. Such an elongation level is very close to that of the forgings and is 8 times greater than that of the sand casting.
While the invention has been described in connection with specific embodiments thereof, it will be understood that the inventive methodology is capable of further modifications. This patent application is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to the essential features herein before set forth and as follows in scope of the appended claims.
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Patent Application
20240278315
