Patent Application
20150251246
This disclosure relates to cooling systems for use in tools that shape materials and methods of making and using such cooling systems and tools.
Heat related defects are an ongoing problem in the shaping of heated materials and in casting processes in particular. Increased complexity of cast parts, demands for higher productivity, and demands for lower scrap rates all tend to place higher thermal loads on die cooling systems. Typical cooling systems usually consist of a number of channels drilled or cut into the tool through which a liquid coolant is intended to flow. This is a single-phase system, where heat is transferred from the die surface (which is in contact with the material being shaped), through the material of the die, and to the surface of the cooling channels, where the liquid cooling fluid absorbs heat and carries it away from the die.
Heat is removed from the cooling channel walls by the cooling fluid by forced convection in an attempt to maintain an isothermal condition at the shaping surface. However, due to the high heat flux at the interface between the heated material and the die, high heat diffusion resistance of the die material (e.g., tool steel), and the convective resistance of the cooling fluid, more heat is sometimes transferred into the die material than is removed by the cooling fluid during shaping and cooling portions of the process. This leads to an increase in the overall temperature of the die, which can cause of a number of heat related problems, such as die soldering, shrink porosity, and/or surface cracks in the shaped part. With conventional cooling systems, excess heat is sometimes removed using die spray after the solidification portion of the casting process. This can lengthen cycle times, increase lubricant use, increase liquid reclaim cost, and reduce die tool life via failure of the tool steel.
In accordance with one embodiment, a tool for shaping a heated material includes a tool portion and a cooling passage formed in the tool portion. The tool portion includes a shaping surface that has a three-dimensional contour and contacts the heated material during shaping. The cooling passage is spaced from the shaping surface and includes a two-phase portion that is shaped to follow the three-dimensional contour of the shaping surface. The two-phase portion of the cooling passage is configured such that a cooling fluid flowing along the cooling passage undergoes two-phase fluid flow when extracting heat from the heated material.
In accordance with another embodiment, a tool for shaping a heated material includes a tool portion and a cooling passage formed in the tool portion. The tool portion includes a plurality of separately formed pieces with passage surfaces and a shaping surface that contacts the heated material during shaping. The cooling passage is spaced from the shaping surface, and opposing walls of the cooling passage are provided by the passage surfaces of the separately formed pieces. The opposing walls are spaced apart such that a cooling fluid flowing along a two-phase portion of the cooling passage undergoes two-phase fluid flow when extracting heat from the heated material.
In accordance with another embodiment, a method of shaping a heated material into a shaped article comprises the steps of: (a) introducing the heated material into a shaping tool; (b) contacting the heated material with a shaping surface of the shaping tool such that the heated material conforms to the shaping surface; and (c) extracting heat from the heated material while the shaping tool is in a closed condition via latent heat cooling. The extracted heat transforms liquid cooling fluid flowing along a cooling passage of the shaping tool to the vapor phase.
Preferred exemplary embodiments will hereinafter be described in conjunction with the appended drawings, wherein like designations denote like elements, and wherein:
Described below is a tool that shapes and cools heated material to form a manufactured article. The tool includes a cooling system with one or more cooling passages formed in the tool. A cooling fluid flows along the cooling passages while the heated material is in contact with a shaping surface of the tool and extracts heat from the heated material to help transform the material into the desired shape and/or state. The cooling passages can be configured as described below to greatly enhance the rate of heat extraction from the heated material by bringing the cooling fluid to its boiling point and employing the latent heat of evaporation of the cooling fluid to cool the heated material.
Referring now to
The illustrated tool 10 includes a plurality of tool portions 14-20 and is shown in a closed condition in
The cooling system 12 includes one or more cooling passages 26 and may include or employ one or more of the following features in various combinations: microchannels, two-phase fluid flow, conformal or three-dimensional cooling channels, bidirectional fluid flow, latent heat cooling, cooling channels with non-uniform cross-sectional areas along their length, a flow regime with non-negligible surface tension effects, opposing walls provided by separately formed pieces of the tool, or spacing between opposing passage walls that is at least an order of magnitude less than the length and width of the passage. This list of features is non-exhaustive and some of these features are described in further detail below.
In the example of
Two-phase fluid flow is a characteristic of latent heat cooling. As used herein, latent heat cooling is a type of cooling process that uses the latent heat of evaporation of the cooling fluid to extract heat from the heated material to solidify or otherwise change the phase or state of the heated material while the material is in contact with the shaping surface 24 of the tool 10. The latent heat of evaporation is the heat energy per unit mass of cooling fluid required to change the cooling fluid from liquid to vapor phase. Latent heat cooling thus extracts heat from the heated material via phase change of the cooling fluid at a constant temperature—i.e., at the boiling point of the fluid—and does not merely rely on thermal energy transfer to a liquid phase cooling fluid that only raises the temperature of the cooling fluid in accordance with the specific heat of the cooling fluid. Under certain conditions, such as in sufficiently configured cooling channel portions, bidirectional fluid flow may occur, where cooling fluid in the liquid phase is flowing in one direction and at least some cooling fluid in the vapor phase is flowing in the opposite direction.
The cooling passage 26 is a hollow internal portion of the insert 32 and is spaced from the shaping surface 24. In the illustrated embodiment, the shaping surface 24 has a three-dimensional contour, and the two-phase portion 28 of the cooling passage 26 is shaped to follow this three-dimensional contour. The two-phase portion 28 of the illustrated passage 26 is defined between opposing walls or passage surfaces 42, 44. In this example, each of the passage surfaces 42, 44 is a three-dimensional surface that is shaped to follow the three-dimensional contour of the corresponding shaping surface 24 in the two-phase portion 28 of the passage 26.
Cooling passages that generally follow the contour of the shaping surface(s) of the tool may be referred to as conformal cooling passages. Conformal cooling passages may be configured to maintain a relatively constant thickness of tool material (e.g., tool steel) between the cooling passage and the shaping surface, changing direction along their length within the tool to follow the contour of the shaping surface.
As shown in
The passage walls 42, 44 oppose each other across the cooling passage 26 and are spaced from each other by a distance D, which may vary along the flow direction of the passage. The two-phase portion 28 of the cooling passage has a length L defined between the first and second openings 38, 40 and measured along the contour of the passage as shown. A width of the passage is measured in a direction transverse or perpendicular to both the lengthwise direction and the direction in which the distance D is measured. In the particularly illustrated embodiment, where the core insert 32 is round and symmetric about a central axis A, the passage width is measured in the circumferential direction. This is evident in
The cross-section of
As is apparent from
In various embodiments, the distance D between the opposing walls of the passage is at least one order of magnitude less than both the length L and the width W of the passage in the two-phase portion. In some embodiments, the distance D is at least two orders of magnitude less than both the length L and the width W. For instance, the distance D may be in a range from about 0.5 mm to about 4 mm, while the length L and the width W may both be in a range from 5 mm to 40 mm or from 50 mm to 400 mm. These ranges are non-limiting and are provided to highlight certain structural distinctions over traditional cooling channels.
As shown in the figures, the two-phase portion 28 may have smaller dimensions than other portions of the cooling passage 26 and, in some cases, smaller dimensions that traditional drilled cooling channels. In particular, the distance D between the opposing walls of the passage may be relatively small in the two-phase portion 28. These portions of the cooling passage may be referred to as microchannels, which is a non-limiting term with regard to passage size. Microchannels may be generally smaller in cross-section than traditional cooling channels provided in prior art tools for a given cooling fluid. For instance, traditional casting die cooling channels may be 9 mm to 10 mm in diameter with water-based cooling fluids, while comparable microchannels may range in size from about 0.5 mm to about 4 mm for a similar cooling fluid.
The two-phase portion 28 of the cooling passage may also be characterized by the fluid flow therein. The two-phase portion 28 can be sized or otherwise configured such that surface tension effects associated with vapor bubble formation in the cooling fluid play a relatively significant role in the fluid flow along the passage. In contrast to larger cooling channels, in which vapor bubbles are formed at and released from cooling channel walls, the two-phase portion of the cooling passage may be made sufficiently small that vapor bubbles quickly reach a size that spans the distance D between opposing passage walls, making surface tension effects appreciable as cooling fluid in the liquid phase makes its way through the bubble-filled passage. Other parameters useful for determining the characteristics a two-phase portion of the cooling passage are given below. Passage dimensions are also dependent on properties of the cooling fluid, and may thus be very different for water-based cooling fluids and oil-based cooling fluids, for example.
In some cases, the two-phase portion 28 of the cooling passage has a distance D between opposing wall that is sufficiently small that the pressure drop along the length of the passage must be accounted for in order to maintain cooling fluid flow in the desired flow regime. Thus, as noted above, the two-phase portion 28 may have a non-uniform cross-sectional area, or distance D between opposing walls, along its length to maintain the desired fluid velocity and flow rate along the passage.
Certain dimensionless quantities associated with fluid flow and surface tension effects can be combined and used to help achieve the desired two-phase fluid flow regime in the cooling passage. The Bond number (Bo) and the Reynolds number (Re) are two of these quantities. In some embodiments, the two-phase portion of the cooling passage is configured such that the following relationship is satisfied:
Bo
0.5
Re≦160.
The Bond number is defined as:
where g is gravitational acceleration, ρf is fluid density, ρg is gas density, σ is surface tension, and D is the depth of the passage (i.e., distance between opposing passage walls.
The Reynolds number is defined as:
where m′ is the mass flow rate, and μf is dynamic fluid viscosity. Further explanation of the significance of the interaction between the Bond number and the Reynolds number is described below.
The tool of
In the illustrated example, the lower tool portion 116 includes a base 130 and an insert 132. The insert 132 includes a cooling passage 126 with a portion 128 configured for enhanced cooling. The portion 128 may be a two-phase portion of the cooling passage.
As with the example of
As applied to a hot stamping process, the enhanced cooling system may be useful to quench the heated sheet metal and/or transform or control the microstructure of the metal. The process may include heating the material above a transition temperature, forming the material while heated, and quenching the material to achieve a particular microstructure. The rate of cooling in the quenching step can be critical to achieve the desired microstructure. Certain boron-based steel, for example, may be heated to a temperature near, at, or above an austenitic transition temperature for forming, then quenched at a rate sufficient to form a martensite microstructure. If the cooling rate during quenching is too slow, the desired mechanical properties may not be achieved in the finished article.
In one particular example, the heated material is sheet steel heated to a temperature in a range from 650° C. to 850° C., or to some other suitable temperature at which the sheet material has enhanced formability thereby allowing complex shapes to be formed in a single stroke. The heated material is pressed between the tool portions, where it is cooled under pressure for a specific amount of time with the help of the above-described cooling system 112. The time required for cooling may vary based on the sheet thickness after full draw depth is reached. During the cooling period, the formed part is quenched in the closed tool at a cooling rate of at least 50° C./s or at a cooling rate in a range from about 50° C./s to 100° C./s, resulting in at least some martensitic transformation in the material. The total cycle time for transferring, stamping, and cooling may range from 15 to 25 seconds. The formed article exits the hot stamping tool at approximately 150° C. The above-described enhanced cooling system, with two-phase cooling passages and water as the cooling fluid, has been determined to increase the heat transfer coefficient from the heated material and through the shaping surface to the cooling passage to 1,000,000 W/m2K, compared with approximately 90,000 W/m2K with traditional cooling lines.
The remaining figures and description summarize some of the principles of enhanced cooling system design, along with a description of the design and successful implementation of an illustrative enhanced cooling system in a steel casting die used in an aluminum die casting process. Skilled artisans will recognize specific examples of additional features of the cooling system and methods of using the cooling system in this non-limiting example. It is intended that such features may also be combined in various ways with the above-described features to realize the benefits of these teachings.
Successful prevention of high temperature concentration during a metal casting process has been achieved by design and implementation of the above-described enhanced (i.e., high-performance) cooling system. The cooling system employed two-phase fluid flow along a portion of a cooling passage of a die casting tool and achieved higher heat dissipation rates that simple liquid phase cooling. The two-phase fluid flow takes advantage of the latent heat of vaporization of the cooling fluid to provide relatively high heat transfer rates. As described below, the high-performance cooling system has been modeled with commercial software (e.g., FLOW3D®). A working example of the cooling system was implemented in a production casting die and the results correlated with numerical analysis.
The efficiency of a cooling system can typically be improved by: decreasing the thermal resistance of the die by moving the cooling channels or lines closer to the casting surface or by using a casting die material with a higher heat transfer coefficient; increasing the convective heat transfer coefficient between the cooling channel walls and the cooling fluid; and/or decreasing the temperature of the cooling fluid.
In some heated material shaping processes, such as high pressure aluminum casting, high pressure and high molten metal velocity compound the problem of excess heat during the casting process. Mechanical and thermal stresses are factors that can limit the minimum distance required between cooling channel and the casting surface of the die, for example.
Heat flux in a single-phase system (where liquid coolant is kept below its boiling point in the liquid phase) can be expressed as:
q
s-p
={dot over (m)}C
P(Tf−TW) (1)
where, {dot over (m)}, CP, Tf, and TW are mass flow rate, specific heat, temperature of the cooling fluid, and temperature of the cooling passage wall, respectively. Changing the temperature of the cooling fluid has a limited effect on the efficiency of the cooling system.
Heat flux in a two-phase system, where the cooling fluid is kept at or above its boiling point as a mixture of liquid and vapor phases, can be expressed as:
q
m-p
={dot over (m)}xh
lg (2)
where, x and klg are quality and latent heat of vaporization, respectively.
Boiling is a phase change process that occurs when a fluid reaches its saturation temperature and is a relatively effective mode of heat transfer. Various stages of boiling are depicted in
As shown in
When the surface of the wall becomes isolated, heat cannot be transferred efficiently into the liquid cooling fluid. This causes the reduction of the heat flux as shown in Zone IV of
An advantage of the two-phase cooling system is the ability to maintain a quasi-stable channel wall temperature in the direction of fluid flow in combination with a high heat transfer coefficient. Two-phase flow can occur in the cooling passages of the die during the cooling or solidification portion of the shaping process any time the cooling fluid reaches its boiling point. However, there are certain differences between the controlled two-phase flow in specially configured coolant passage portions and boiling cooling fluid in conventional die cooling channels. In the enhanced cooling system described herein, a portion of the cooling passage may be configured such that surface tension forces have a more pronounced influence on the fluid flow characteristics. Bubbly flow can be rarely observed in a microchannel because bubbles grow and quickly reach size of the passage. In a relatively small sized passage, Taylor instability is not typically present.
Typical stages of the boiling process in microchannels are shown in
Cooling passages of a casting die or other shaping tool play an important role in extracting the heat transferred into the bulk tool material from the heated material (e.g., molten metal) during the cooling stage of the process. Increased capacity of the cooling passages can help reduce or prevent a number of the casting problems listed above. The ability to dissipate a larger heat flux can also reduce the cooling time of the heated material. Attempts have been made to optimize die cooling channels by providing specific channel shapes or sizes based on calculated thermal load—i.e., predicted hotter regions of the tool surface had an associated higher coolant flow rate, for example. However, in these efforts, cooling channel position and spacing were kept in line with conventional drilled cooling channel design.
The high-efficiency cooling system described herein, which may employ three-dimensional cooling passages and/or microchannels, has been modeled with commercial CFD software (e.g., FLOW3D®) in proof-of-concept experiments. A comparison of the results between experimentally derived (published) and numerically approximated relations between dimensionless Nu and Re numbers for water flowing in D=0.2 mm passage is illustrated in
When employing a relatively small distance D between opposing passage walls, the pressure drop along the length L of the passage should be considered when designing the cooling system. Pressure variation in a cooling passage is plotted in
In some cases, unconfined bubble flow is the preferred flow regime in order to maximize the heat transfer coefficient in the cooling passage. The transition between confined and unconfined bubble flow in a microchannel is expressed as follows:
Bo
0.5
Re=160. (3)
where, Bo is the Bond number and Re is the Reynolds number. The transition between Zones IV and V of
A metal die casting tool with a core portion in front of the gate was chosen to verify the effectiveness of the enhanced cooling system. The embodiments depicted in
The cooling passage 26 depicted in
After verifying by simulation that the stresses would not exceed the fatigue stress of the tool steel, the insert 32 was fabricated and implemented in the core portion 14 of the casting die 10 of
It is to be understood that the foregoing description is not a definition of the invention, but is a description of one or more preferred exemplary embodiments of the invention. The invention is not limited to the particular embodiment(s) disclosed herein, but rather is defined solely by the claims below. Furthermore, the statements contained in the foregoing description relate to particular embodiments and are not to be construed as limitations on the scope of the invention or on the definition of terms used in the claims, except where a term or phrase is expressly defined above. Various other embodiments and various changes and modifications to the disclosed embodiment(s) will become apparent to those skilled in the art. All such other embodiments, changes, and modifications are intended to come within the scope of the appended claims.
As used in this specification and claims, the terms “for example,” “e.g.,” “for instance,” “such as,” and “like,” and the verbs “comprising,” “having,” “including,” and their other verb forms, when used in conjunction with a listing of one or more components or other items, are each to be construed as open-ended, meaning that that the listing is not to be considered as excluding other, additional components or items. Other terms are to be construed using their broadest reasonable meaning unless they are used in a context that requires a different interpretation.
This application claims the benefit of U.S. Provisional Ser. No. 61/949,480 filed on Mar. 7, 2014, the entire contents of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61949480 | Mar 2014 | US |
