Patent Grant
12048960
The present invention relates to casting of metals, more specifically, to a novel method of controlled nozzle cooling (CNC) casting using arrays of nozzle embedded in a sand casting mold.
A conventional casting process involves pouring a molten metal in a mold and solidifying the molten metal to produce solid products, i.e., castings. Microstructure and resultant mechanical properties of the casting are controlled by heat removal rates from the molten metal by the mold. Fast heat removal causes fast cooling of the molten metal, resulting in castings of fine microstructure and improved mechanical properties [1-2].
Cooling rates of the molten metal during its solidification process in a mold cavity are affected by thermal diffusivities of the molding materials and an air gap between the mold and the casting [3]. This air gap is formed when the surface of the casting pulls away from the mold surface due to the contraction of the metal on cooling.
Metals or graphite have high thermal diffusivity. These materials are used for making molds for high pressure die casting processes and other permanent mold casting processes, and are excellent in solidifying the molten metal quickly to produce castings of fine microstructure, such as small primary phase grains, dendrite arm spacing (DAS), and fine eutectic phase particles. However, the molten metal has to be forced to flow rapidly to fill the mold cavity before it freezes. Rapid mold filling is always turbulent which causes the formation of defects such as entrapped oxides and gases [4]. Furthermore, metal molds are expensive. Turbulent flow results in severe erosion and soldering damage to metal molds [5].
Sands have a much lower thermal diffusivity than metals and graphite. Mold filling of molten metal in a sand mold cavity can be much smoother than that in a metal mold cavity. However, cooling rates of the molten metal in a sand mold cavity are low. Gap formation further slows down the cooling rate after the fraction solid of the dendrites at the surface of the casting reaches a certain critical value, typically 0.2-0.3 [3]. In cast alloys, most of the eutectic phases and other secondary phase particles are usually formed at a fraction solid much larger than 0.2 [6]. As a result, castings made using sand molds contain coarse eutectic structures, which negatively affect the ductility of the castings [1-2].
U.S. Pat. No. 7,216,691 to Grassi et al. discloses an ablation casting technology which uses a 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, ablating away the molds and cooling the solidifying casting directly using the liquid solvent. Thus, the formation of an air gap at the casting-mold interface is avoided. This innovative technology allows for a smooth molding filling of molten metal in a sand mold cavity but uses a liquid solvent to rapidly cool the casting to achieve cooling rates higher than those in metal molds. Fine solidification microstructure especially that of the eutectic phases and the secondary phases are obtained. Castings made using this technology have much better mechanical properties than those made using the conventional sand casting process [1-2,7-11].
However, there are still a few issues associated with this innovative ablation casting technology. A binder that is rapidly dissolved into a solvent has to be used to hold together the sand particles. As a result, a large variety of sand/binder combinations cannot be used for making sand molds using the ablation casting process. Semi-permanent molds and permanent molds are not suitable for the ablation casting process because these molds cannot be dissolved in a solvent quickly enough. Furthermore, molds contain a soluble binder has to be cured at longer duration than the conventional sand molds using clay as the binder, which extends the mold making cycle. Also, the technology uses flask-less molds to allow for the collapse of the molds because the spray nozzles are located outside the molds and have to travel over the molds. Flask-less molds are usually small but have to be thicker than 70 mm. To ablate away a thick mold from the casting within short time duration, a large amount of liquid solvent is required. This may limit the conditions under which the cooling liquid is allowed to impinge on the surfaces of the solidifying casting. An early impingement of the cooling liquid with its full impacting force of the spraying jet may cause a number of problems including leakage of molten metal from the molds, damaged surfaces of the casting, and distortion of the solidified components. Often, the delivery of the cooling liquid has to be delayed to allow for the formation of a relatively solid skin of the casting before it can withstand the full impact of the spraying jet. Indeed, results shown in the patent to Gassi et al. suggest that DAS of the primary phase in a casting made using the ablation casting process is not changed much compared to that using the conventional sand casting process. This is an indication that water cooling is applied at fairly late stage of the solidification process in the casting, i.e., the casting is cooled in sand molds for its early stage of solidification and then cooled afterwards with the liquid solvent. Fast cooling using the spraying liquid is not fully used during the entire solidification process of the cast metal.
Therefore, there is a need for developing a novel casting process that has the advantage of smooth mold filling of sand molds and rapid solidification of metal molds while also using conventional binders for making the sand molds in flasks. The rapid solidification is achieved by contacting the solidifying metal with a coolant from nozzles embedded in the molds rather than with a solvent sprayed from nozzles traveling over the mold as taught by the U.S. Pat. No. 7,216,691 to Grassi et al.
There is also a need for developing a novel casting process that is suitable for casting processes using semi-permanent molds or metal molds.
There is also a need for developing a process that is capable of making a thin-walled and extremely large casting.
Furthermore, there is a need for developing a process and related apparatus that are retrofittable to existing production lines for making castings.
The invention provides a controlled nozzle cooling casting process using a plurality of nozzles embedded in molds. The process includes the steps of providing at least a mold held at predetermined temperatures, preparing the mold with a plurality of nozzle holes having a thin layer of sand or coating to prevent a direct contact of the nozzle with a molten metal, introducing the molten metal into the mold cavity, embedding nozzles that are connected to a coolant delivery system into the nozzle holes at the pouring station after metal pouring, and delivering a predetermined amount of coolant through each nozzle at predetermined rates, times, and durations to break the layer of sand or coating separating the nozzle to the casting and to cool the external surface of the casting as needed in order to achieve an acceptable level of progressive solidification from the distal end of the casting towards the riser or downsprue until the casting has reached desired temperatures.
In an embodiment of the present invention, a process for reducing the cooling time of a solidifying casting and increasing casting productivity is provided. The process includes the steps of providing at least a mold embedded with a plurality of cooling nozzles, and delivering a predetermined amount of coolant through each nozzle at predetermined rates, times, and durations to eliminate the air gap that usually exists at the interface between the mold and the casting. Eliminating the air gap at the mold-casting interface greatly reduces the cooling time to solidify a casting and increases casting productivity.
In another embodiment of the present invention, a process for reducing the internal defects and increasing the mechanical properties of a casting is provided. The process includes the steps of providing at least a mold embedded with a plurality of cooling nozzles, and delivering a predetermined amount of coolant through each nozzle at predetermined rates, times, and durations to cool the casting as needed just to achieve an acceptable level of progressive solidification from the distal end of the casting towards the riser or downsprue. The cooling of the casting using a coolant produces a fine solidification microstructure and improved mechanical properties.
In yet another embodiment of the present invention, a process for using less or inexpensive molding materials for making a high-quality casting is provided. The process includes the steps of providing a permanent mold lined with a layer of an expendable sand liner, introducing a molten alloy into the mold cavity, embedding a plurality of cooling nozzles in the mold, and delivering a predetermined amount of coolant through each nozzle at predetermined rates, times, and durations to contact the casting to achieve a progressive solidification from the distal end of the casting towards the riser or downsprue until the casting has reached desired temperatures. The use of an expendable sand liner in a permanent mold eliminates the need for using a semi-permanent mold such as a graphite mold for making high quality castings.
In yet another embodiment of the present invention, a process for making a thin-walled large casting is provided. The process includes the steps of providing a permanent mold lined with a layer of an expendable sand liner, heating up the metal mold supported sand liner to high temperatures, supplying a molten alloy into the mold cavity, embedding a plurality of cooling nozzles in the mold, and delivering a predetermined amount of coolant through each nozzle at predetermined rates, times, and durations to contact the casting to achieve a progressive solidification from the distal end of the casting towards the riser or downsprue until the casting has reached desired temperatures. The mold at high temperatures allows for a smooth mold filling of a thin-wall large casting. The controlled nozzle cooling ensures progressive solidification in the casting to achieve the desired performance requirements.
In yet another embodiment of the present invention, a process is provided for making a high-quality casting that can retrofit into existing casting production lines. The process includes the steps of providing molds with a plurality of cavities for cooling nozzles that are molded or machined, introducing a molten metal into the mold cavity, embedding nozzles that are connected to the coolant delivery system in the molds at the pouring station after metal pouring, delivering a predetermined amount of coolant through each nozzle at predetermined rate, time, and duration to contact the casting to achieve a progressive solidification from the distal end of the casting towards the riser or downsprue until the casting has reached desired temperatures, and finally removing the nozzle cooling system out of the mold.
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 invention deals with a controlled cooling sand casting process using an array of nozzles embedded in molds that delivers a desired amount of selected coolant at desired times to contact the surfaces of a casting to ensure progressive solidification from the distal end of a casting to the riser.
The cavity 24 in the mold 20 is used for making a casting. A plurality of nozzle holes 18 are made for hosting the nozzles, 10, 12, 14, and 16 that are to be embedded in the sand mold 20. There is a thin layer 15 of coolant permeable materials such as sand or coating materials that are used for separating the nozzle, 10, 12, 14, or 16, from directly contacting the molten metal during mold filling. Nozzle holes 18 are molded using a pattern or are machined. The nozzle hole 18 can also be a through hole so that the nozzle can be placed in the mold flush with the surface of the cavity 24. Conventional coatings can be applied on the surface of the mold cavity 24, especially if the tip surface of the nozzle 10, 12, 14, or 16 is flush with the surface of the mold cavity 24 if the nozzle hole 18 is a through hole. As a result, there is a thin layer of coolant permeable materials 15 for separating the tip of a nozzle from direct contacting the molten metal during mold filling.
Having made the mold 20, nozzles 10, 12, 14, and 16 mounted on a rigid fixture 48 are placed manually or using a robot in the nozzle holes 18 before or after the molten metal is poured into the mold cavity 24. Each nozzle 10, 12, 14, or 16 can be control individually for translational motions so that it can be placed into or removed out of the nozzle hole at predetermined times. The fixture 48 can also be used to lock the molds in place to prevent the molds from opening and the resultant metal leakage from the molds due to the static pressure that the molten metal in cavity 24 applies on the mold 20. A gap 11 is designed to allow the used coolant and the resultant gases to escape from the tip of the nozzle 10, 12, 14, or 16. A number of venting system 22 (only one is shown in
The sequence of coolant delivery to each array of nozzles is shown in
The delivery of coolant is such that the angle, θ, is greater than the value that is required for an adequate feeding of liquid metal to the solidification shrinkage in the mushy zone at the left side of the freezing front 29. An important feature of coolant delivery is to break the thin layer of coolant permeable materials that separate the molten metal 26 and the tip of the nozzle 10, 12, 14 or 16. As a result, the coolant delivery system is designed to deliver a predetermined amount of a selected coolant including water to the tip of each nozzle, 10, 12, 14 or 16, at predetermined rates, times, and durations to break the thin layer of the coolant permeable materials so that the coolant contacts the external surface of the solidifying casting 26 in order to maintain an acceptable level of progressive solidification from the distal end of the casting to the risers or the downsprue of the casting.
The thickness of the thin layer of coolant permeable materials separating the nozzle, 10, 12, 14 or 16, from the casting 26, has to be thick enough to withhold the static pressure of the molten metal but thin enough so that the nozzle could deliver a coolant to break up this thin layer of coolant permeable materials to directly contact the external surface of the casting 26. It is recommended that the thickness be less than 15 millimeters, preferably less than 3 millimeters.
The spacing or interval between neighboring array of nozzles, for example between nozzle 10 and nozzle 12, is between 4 to 10 times of the local wall-thickness of a steel plate-shaped casting to ensure a progressive solidification from the distal end to the riser or downsprue of the casting. The minimum spacing between the neighboring array of nozzles is dependent on the casting materials and casting methods. Ideally, the spacing should be such that the feeding angle, θ, is greater than the value that is required for an adequate feeding of liquid metal to the solidification shrinkage in the mushy zone of the casting at given rates of coolant delivery from the nozzles.
Venting has to be used in order to release used coolant, mold debris, and resultant moisture from the molds.
One of the nozzles 10 can also be placed at the distal end of the casting as shown in
The sand liner 21 is expendable and used for only once. The metal mold 20 supporting the sand liner 21 can be used for many times. Since a new sand liner 21 needs to be made for each casting 26, the dimensional accuracy of the casting 26 is ensured regardless of deformation/distortion that may occur in the metal molds 20.
The invention further provides examples of the present invention of CNC casting. The examples provided below are meant merely to exemplify several embodiments, and should not be interpreted as limiting the scope of the claims, which are delimited only by the specification.
Mold filling during sand mold casting can be relatively well controlled compared to that during high pressure die casting [12-13]. However, the freezing rate is much lower in sand molds than that in metal molds because sand has a lower thermal diffusivity than metal. As a result, sand castings usually have coarse solidification microstructures and poor mechanical properties. Grassi et al tested making automotive steering knuckles of aluminum A356 alloy using a typical sand casting process and an ablation casting process [1-2]. They found that that the tensile strength, yield strength, and elongation in samples taken from the conventional sand casting were 228 MPa, 179 MPa, and 3.5 respectively. Using water as solvent in the ablation casting process, the tensile strength, yield strength, and elongation in samples taken from the casting were 325 MPa, 261 MPa, and 12.5 respectively, much higher than that in the sand casting. It is expected that castings made using the present invention of CNC casting as shown in
Steel railway wheels were initially made using a sand mold with a metal ring to chill the tread of the wheel to encourage progressive solidification starting from the tread surface to the wheel hub [14]. Later, a graphite mold technology was developed [15]. Steel wheels produced using graphite molds are more consistent in quality than those made using sand molds. U.S. Pat. No. 3,302,919 to Beetle et al. describes a method of using a sand liner in graphite molds to make a cast steel railway wheel. As shown in
In the automotive industry, thin-walled large aluminum castings are usually made using the high pressure die casting (HPDC) process because the sand casting process is not capable of producing such castings. HPDC is also termed as die casting. During die casting, high pressures have to be used to inject molten aluminum at high speeds into the cavity in molds made of steel in order to be able to fill the entire die cavity [13,16]. Still, there is a limit on the size of a casting that the die casting is capable of making. U.S. patent application Ser. No. 15/874,348 by Kallas of Tesla, Inc. discloses a giant die casting machine for the production of the entire body frame of a car in a single press. The body frame part may be the largest thin-walled aluminum casting to be made in the casting industry. The present invention shown in
Because progressive solidification is not achievable in HPDC process, the industry has been using various means to achieve local progressive solidification using cooling lines in the metal mold or cooling pins. Cooling lines are drilled into a block of a metal die so the cooling lines are usually straight. The coolant, usually water or oil, is not in direct contact with the casting. Instead, it is only circulating in the cooling lines to take away heat from the die. To prevent damage to the expensive metal die, the cooling lines are usually drilled at least 10 mm away from the cavity surfaces. Heat extraction of these cooling lines from the solidifying casting is limited by the thermal diffusivity of the at least 10 mm thick steel. It is widely believed that the cooling lines are effective only in maintaining the dies at certain temperatures and are ineffective in reducing local solidification time in the casting. Cooling pins are more effective in achieving local progressive solidification in a casting. The cooling pins are made of metal and have coolant circulating within them as well. Still the chill effect of the cooling pins is limited by the thermal diffusivity of the metal separating the coolant from the casting, although the thickness of this metal layer becomes thinner using 3D printing technologies. By delivering a desired amount of a selected coolant through nozzles to contact the surfaces of the casting, more effective progressive solidification can be achieved at least locally using the present invention shown in
Aluminum automotive wheels are made using permanent mold process. The molds are made of steel. A relative thick coating is applied on the mold surface to protect the mold steel from erosion during mold filling under low pressure or under gravity casting conditions. The use of a thick coating also slows down the flow speed of the metal during mold filling. A photograph of a wheel is shown at top left image in
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.
The present U.S. patent application is a continuous-in-part application of U.S. patent application Ser. No. 16/992,245 filed Jul. 28, 2020. The relevant contents of this prior application are hereby incorporated by reference into the present disclosure.
| Number | Name | Date | Kind |
|---|---|---|---|
| 20090065170 | Vais | Mar 2009 | A1 |
| Number | Date | Country |
|---|---|---|
| 2196281 | Apr 1988 | GB |
| 2003103342 | Apr 2003 | JP |
| WO-2013085401 | Jun 2013 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20240198414 A1 | Jun 2024 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 16992245 | Aug 2020 | US |
| Child | 18401289 | US |
