US20200360986A1

US20200360986A1 - Casting metals

Info

Publication number: US20200360986A1
Application number: US16/412,204
Authority: US
Inventors: Franz-Josef Wöstmann; Christoph Pille; Matthias Busse
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Priority date: 2019-05-14
Filing date: 2019-05-14
Publication date: 2020-11-19
Also published as: EP3969205A1; US20220266331A1; WO2020229634A1; CN113811408A

Abstract

The casting of metals or salt and products made by casting metals or salts are described. For example, a method for casting metals or salts includes evacuating a mold that is pressurized with an inert gas and coupled to a pressurized source of molten metal or salt, wherein the pressurization of the source is sufficiently high to drive the molten metal or salt into the mold in response to the evacuation.

Description

BACKGROUND

This invention relates to processes for casting metals and the resulting products.
In metalworking, casting is process in which molten metal is added to a mold (also known as a tool or a die) that defines a hollow cavity. The molten metal is then allowed to cool and solidify. The solid casting is then removed from the mold. If the casting is to have a relatively complex geometry (e.g., with holes, undercuts, and/or integrated channels) cores and/or multi-slide techniques can be used. Castings can be used to create a variety of different products.

SUMMARY

The casting of metals and products made by casting metals are described. For example, a method for casting metals or salts includes evacuating a mold that is pressurized with an inert gas and coupled to a pressurized source of molten metal or salt, wherein the pressurization of the source is sufficiently high to drive the molten metal or salt into the mold in response to the evacuation.
This and other methods can include one or more of the following features. Evacuating the mold can include coupling the mold to a vacuum chamber, e.g., wherein the vacuum chamber has a volume that is 30 or 50 times a volume of the mold. The source of molten metal or salt can be pressurized to a pressure of less than 0.2 MPa gage or less, e.g., to about 0.1 MPa gage. The source of molten metal or salt can be pressurized with the inert gas. The molten metal or salt is one of copper, steel, aluminum, alloys, salt or salt mixtures. The mold can be configured to cast a coil, for example, a coil as described in this application. The mold can be evacuated, e.g., in 0.1 seconds or less. In some implementations, prior to the evacuation, the pressurization of the mold and the pressurization of the source are comparable and molten metal or salt is exposed to the inert gas pressurizing the mold. The mold can include a core, multiple slides, or both a core and multiple slides. The mold can include a gas-permeable core. Evacuating the mold can include evacuating at least part of the mold by drawing gas through the gas-permeable core. The pressurized source can include a crucible that includes resistive or inductive heating elements integrated therein or disposed along a surface thereof.
As another example, a coil includes a cast unitary metal strip covered by an insulator and layered to form a plurality of turns about an axis, the metal strip having a width generally perpendicular to the axis and a height generally parallel to the axis, wherein the width is larger than the height.
This and other coils can include one or more of the following features. The cast unitary metal strip can have a generally rounded rectangular cross-section parallel to the axis. The insulation of adjacent turns of the metal strip contact each other over substantially the entire width of the metal strip. The coil can display physical characteristics that are characteristic being cast in a relatively low pressure difference casting process, for example, wherein the physical characteristics is dimensions of microbubbles in the coil. The coil can display physical characteristics that are characteristic being cast directly in a shape that is nearly suitable for use. For example, a the metal strip can be free from stress-induced microdefects resulting from an application of a force perpendicular to the axis to shape the metal strip. The metal strip can include aluminum and the insulator can include aluminum oxide. The metal strip can include copper. The cast unitary metal strip coil can have a generally trapezoidal cross section parallel to the axis, wherein the cross section is oriented to intersect comparable cross sections of other coils of a motor or generator arranged generally concentrically about a center. The generally trapezoidal cross section can be formed by a single winding of the cast unitary metal strip. In at least one cross section parallel to the axis, regions of the cast unitary metal strip can have non-uniform widths. For example, the regions progressively increase or decrease in width along the axis. In the at least one cross section parallel to the axis, regions of the cast unitary metal strip can have non-uniform heights, for example, wherein the regions can progressively increase or decrease in height along the axis. The coil can be cast in an extended shape that forms a plurality of turns about an axis with a space between adjacent turns. For example, the space between adjacent turns can be 5 to 10 times wider than the thickness of the metal strip along the axis. In some cases, the space between adjacent turns can be, e.g., 1 to 20 mm.
The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic representation of a casting system.

FIG. 2 is a schematic representation of the casting system in the casting process.

FIG. 3 is a schematic representation of a conductive coil that can be cast using, e.g., the casting system of FIGS. 1 and 2.

FIG. 4 is a schematic representation of one possible cross-section of the conductive coil of FIG. 3.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

FIG. 1 is a schematic representation of a casting system 100. Casting system 100 includes a crucible 105, a crucible pressure container 110, a casting mold 115, a feed pipe 120, a pressurized inert gas source 125, and a vacuum chamber 130. Feed pipe 120 extends into crucible 105 and forms a fluid flow path suited for pressure-driven flow of molten metal(s) 135 from crucible 105 into the interior 117 of mold 115. Molten metal(s) 135 can be, e.g., aluminum, magnesium, zinc, copper, iron, or their alloys, including steel. Mold 115 can be selected based on the metal(s) 135 to be cast and can be, e.g., a steel or sand die. In some implementations, ceramic molds can be used. In some implementations, cores and sliders can be used. For example, the cores can be permanent or non-permanent. Casting system 100 can be operated to drive metal(s) 135 from crucible 105 through feed pipe 120 into the interior volume 117 of mold 115 using a relatively low pressure differential.
Gas source 125 can contain a relatively high pressure inert gas such as nitrogen or a noble gas. In some implementations, the inert gas can be dried. Gas source 125 can be, e.g., a pressurized gas canister or line. Gas source 125 is coupled to one or both of the interior volume 117 of casting mold 115 and the interior volume 112 of crucible pressure container 110 via one or more lines 140, 145. Lines 140, 145 can be opened and closed using one or more valves 150, 151. As discussed further below, gas source 125, lines 140, 145, and valves 150, 151 are configured to pressurize the interior volume 117 of casting mold 115 and the interior volume 112 of crucible pressure container 110 at approximately the same rate.
As discussed further below, line 145 is dimensioned and gas source 125 is able to provide a sufficient volume flow of inert gas to rapidly fill the interior volume 112 of crucible pressure container 110. For example, in some implementations, line 145 and gas source 125 are able to fill the interior volume 112 of crucible pressure container 110 in less than 1 sec. In general, interior volume 112 can be filled without associated turbulence in the metal bath. To assist in rapid filling, valve 151 is generally positioned immediately adjacent the interior volume 112 of crucible pressure container 110. With such a positioning, the volume of line 145 between valve 151 and interior volume 112 is relatively small and the rate of pressure change in interior volume 112 is increased.
In some implementations, the interior of crucible pressure container 110 can be tailored to conform to the exterior of crucible 105. For example, in some instances, the exterior of crucible 105 can be generally cylindrical or conical in shape, and the interior of crucible pressure container 110 can define a generally cylindrical or conical volume that conforms to the exterior of crucible 105 with a small tolerance. Such a tailoring can reduce the size of interior volume 112 and speed changes in pressure.
In some implementations, resistive or inductive heating elements can be integrated into crucible 105, integrated into crucible pressure container 110, and/or disposed along a surface of either. Such heating elements can facilitate reductions in the size of interior volume 112.
Vacuum chamber 130 is coupled to the interior 117 of casting mold 115 via a line 155. Line 155 be opened and closed using one or more valves 160. Vacuum chamber 130 has a volume that is significantly larger than the volume of the interior 117 of casting mold 115. For example, the volume of vacuum chamber 130 can be 50 or more times the volume of the interior 117 of casting mold 115. Valve 160 can be a quick acting valve, e.g., a valve that can complete the transition from a closed state to an open state in less than 100 milliseconds.
As discussed further below, line 155 is dimensioned and chamber 130 is able to withdraw a sufficient volume flow of inert gas to rapidly empty the interior 117 of casting mold 115. For example, in some implementations, line 155 and chamber 130 are able to empty the interior 117 of casting mold 115 in under 0.1 seconds. To assist in emptying, valve 150 is generally positioned immediately adjacent the interior 117 of casting mold 115. With such a positioning, the dead volume of line 140 between valve 150 and interior 117 is relatively small and little or no additional air must be withdrawn through line 155. Also, valve 160 is generally positioned immediately adjacent the interior 117 of casting mold 115.
In implementations where casting mold 115 includes a sand core or other gas-permeable core, line 155 can be coupled to withdraw gas through a solid (but gas-permeable) portion of the core. This would allow gas to be withdrawn from the entire casting volume of mold 115, as well as avoid gas emissions from the core.
In some implementations, the interior 117 of casting mold 115 is coupled, e.g., to atmosphere via one or more lines 165. In the illustrated implementation, line 165 includes a check valve 170 that only allows unidirectional flow out of the interior of casting mold 115. In other implementations, one or more lines 165 can be unobstructed and un-valved passages that allow relatively small flow rates compared to the flow rates through lines 140, 155 when one or more of valves 150, 160 is open.
In preparation for casting, casting mold 115 and crucible pressure container 110 are filled with inert gas from gas source 125, with the pressures in casting mold 115 and crucible pressure container 110 ultimately rising above atmospheric pressure. For example, casting mold 115 and crucible pressure container 110 can be pressurized to 1 MPa gage, although generally casting mold 115 and crucible pressure container 110 2will be pressurized to 0.2 MPa gage or less. For example, casting mold 115 and crucible pressure container 110 can ultimately be pressurized to 0.02-0.08 MPa gage. The pressurization process can insure that reactive gases (e.g., atmospheric oxygen) do not remain in casting mold 115 and crucible pressure container 110 during casting. For example, relatively pure inert gas can flow from gas source 125 into casting mold 115 and crucible pressure container 110 at the same time that a mixture of inert gas and reactive gases escapes from casting mold 115 and crucible pressure container 110 through one or more lines 165. Depending upon the respective flow rates and volumes, the reactive gases in casting mold 115 and crucible pressure container 110 will eventually be depleted and the gas in casting mold 115 and crucible pressure container 110 will have a composition that resembles the composition of the gas from gas source 125.
Before, during, and/or after casting mold 115 and crucible pressure container 110 are filled with inert gas, crucible 105 can be heated to melt metal(s) 135. Feed pipe 120 extends into the molten metal(s) 135 in crucible 105. As or after metal(s) 135 melts, the pressures of the interior 117 of casting mold 115 and the interior 112 of crucible pressure container 110 can be regulated to form a head 175 of molten metal(s) 135 that fills the gating system of casting mold 115. In the illustrated implementation, molten metal(s) 135 rises to a level 180 that is slightly below portion of casting mold 115 where the final product is to be cast. In some implementations, feed pipe 120 include a heating system to control the temperature of molten metal(s) 135 in feed pipe 120 (not shown).
The particular approach for regulating the pressures in casting mold 115 and crucible pressure container 110 can depend upon the particular structure of casting system 100. For example, in some implementations, the pressures in casting mold 115 and crucible pressure container 110 may inherently result from the respective flow rates through lines 140, 145, and 165. For example, the sizes of lines 140, 145, and 165 can be selected so that the escape of gas from casting mold 115 through line 165 results in the pressure of casting mold 115 being lower than the pressure in crucible pressure container 110 by a desired amount. Other implementations—including implementations that include pressure sensors and active feedback control—are possible.
In general, casting mold 115 will be heated to a temperature suitable for good form filling behavior and a minimum fatigue fretting of the die. For example, when copper is being cast, permanent casting mold 115 will be heated to temperatures between 100-350° C. In contrast, when aluminum is being cast, permanent casting mold 115 will be heated to temperatures between 250-350° C.
FIG. 2 is a schematic representation of casting system 100 in the casting process. With casting mold 115 and crucible pressure container 110 both pressurized, valve 150 is closed and valve 160 opened. Since vacuum chamber 130 has a volume that is significantly larger than the volume of the interior 117 of casting mold 115, pressure in the interior 117 of casting mold 115 falls nearly to zero very rapidly. For example, the pressure in the interior 117 of casting mold 115 can fall nearly to zero in under 0.1 seconds. In contrast, crucible pressure container 110 remains pressurized, and the head 175 of molten metal(s) 135 rises to a level 185, filling casting mold 115.
As discussed above, the pressure in casting mold 115 and crucible pressure container 110 can, in some implementations, be 0.1 MPa or so above atmospheric pressure. In the context of casting metals, this is a relatively low pressure difference, i.e., around two atmospheres. By way of contrast, high pressure die casting can be performed at pressures up to, e.g., 120 MPa or so, i.e., around 120 atmospheres pressure difference.
At such high pressures, the velocity of molten metals in the casting mold reaches a high velocity, e.g., 200 m/s.
By operating at such a relatively low pressure, the present casting process is both safer and quicker. Indeed, a relatively low pressure casting process may not be subject to stringent regulatory requirements. Also, the melt need never be exposed to an oxidative atmosphere. Rather, the melt is exposed only to an inert atmosphere and (essentially) vacuum.
In some instances, products cast using such a relatively low pressure difference may display physical characteristics that are characteristic of the casting process. For example, in some instances, products may be less likely to include microbubbles than products that are cast using high pressure casting. In high pressure casting, any gas trapped in the molten metal will expand significantly once the high casting pressure is released. In contrast, with a relatively low pressure casting, the volume expansion of gas in the molten metal is significantly smaller.
As another example, in some instances, products cast using such a relatively low pressure difference may display surface structuring characteristic of the casting process.
In some implementations, during or after the evacuation of the interior 117 of casting mold 115, the pressure applied to molten metal 135 can be increased. For example, in some implementations, an additional source of pressurized inert gas can be coupled to the interior 112 of crucible pressure container 110 by a valve/line system (not shown). Such a valve can be opened in response to the opening of valve 160 to increase the pressurization in interior 112 of the interior volume 112 of crucible pressure container 110. As another example, in some implementation, interior volume 112 of crucible pressure container 110 can be pressurized through valve 151. In either case, pressurization of interior volume 112 combined with evacuation of the interior 117 of casting mold 115 can increasing filling speed and the mass inertia, allowing smaller wall thicknesses in the casting.
The casting process illustrated schematically in FIGS. 1 and 2 can be used to cast a variety of different products—including products that are difficult to cast using other casting techniques.
FIG. 3 is a schematic representation of a conductive coil 300 that can be cast using, e.g., casting system 100. Conductive coil 300 is a unitary cast piece that can be used, e.g., in a motor or an electric generator. The surface of metal strip 310 is covered with a relatively thin insulator so that metal strip 310 forms a coil-shaped conductive path.
Conductive coil 300 defines a gap 305 around an axis 307 about which a metal strip 310 is “wound” in a series of layers 312 that each form a turn of coil 300. Metal strip 310 has a height 315 that is less than a width 320, where width 320 is oriented generally perpendicular to axis 307 and height 315 is oriented generally parallel to axis 307. As discussed in further detail below, the dimensions of metal strip 310 are generally not be uniform—either within a single layer 312 or from one layer 312 to another layer 312. In the illustrated implementation, metal strip 310 is generally rectangular in cross-section with generally flat insulated surfaces of adjacent layers 312 in contact with one another. This is not necessarily the case. For example, in some implementations, the surface of metal strip 310 may be intentionally structured, e.g., so that adjacent layers 312 engage with one another to increase the area of contact.
Although layers 312 in conductive coil 300 are shown in the schematic representation as having generally sharp edges 325 and gap 305 is shown as having a cuboid shape, in general, both edges 325 and the edges of gap 305 will be at least somewhat rounded. Further, although the cross-sectional area of metal strip 310 is illustrated as generally rectangular with sharp edges, this is generally not the case.
Conductive coil 300 can be cast from one or more metals, including aluminum, copper, manganese, or steel. Conductive coils cast from aluminum are lighter that comparably dimensioned conductive coils cast or otherwise formed from copper. The insulator can be formed from one or more layers of different materials and can include, e.g., metal oxides such as aluminum oxide.
In some implementations, conductive coil 300 can be cast directly in a shape that is nearly suitable for use, e.g., in a motor or generator. In other implementations, conductive coil 300 can be cast in an extended shape that forms a plurality of turns about axis 307 but must be compressed along the direction of axis 307 prior to use. For example, the space between adjacent turns can be 5 to 10 times wider than the thickness of the metal strip along the axis. In some cases, the space between adjacent turns can be, e.g., 1 to 20 mm. Regardless of whether coil 300 is cast in an extended shape or in a shape that is nearly suitable for use, it is not necessary to bend or otherwise shape metal strip 310 by applying a force perpendicular to axis 307, e.g., to wind metal strip 310 about a bobbin. The physical structure of metal strip 310 may be characteristic of such a casting. For example, when conductive coil 300 is cast directly in a shape that is nearly suitable for use, metal strip 310 may maintain this nearly suitable shape without the application of external forces. As another example, the surface of metal strip 310 may be free from stress-induced microdefects associated with such a shaping by application of forces perpendicular to axis 307.
FIG. 4 is a schematic representation of one possible cross-section of conductive coil 300. The cross-section—which is generally parallel to axis 307 and arranged to cut through a concentric arrangement of coils—cuts across metal strip 310 of coil 300 at different regions 405, 410, 415, 420, 425, 430, 435, 440.
In the illustrated implementation, coil 300 is dimensioned to fill a large percentage of the design space of an electric motor or generator within the illustrated cross-section with a single metal strip. In more detail, a motor or generator can include several coils arranged concentrically, for example, mounted on a stator disposed around a rotor. In the perspective of FIG. 4, the coils would be arranged concentrically about a center that is positioned below coil 300. Additional coils would thus be positioned, e.g., below and to the left and right of coil 300, oriented so that their respective axes are directed to the center of the concentric arrangement. With such a disposition, less space is available for each coil toward the center of the concentric arrangement than is available to toward the outside of the concentric arrangement.
Motor and generator coils generally have a trapezoidal cross section that is narrower on the side which is to be disposed toward the center of the concentric arrangement and wider on the side which is to be disposed toward the outside of the concentric arrangement. In coils formed by winding a wire around a bobbin or other member, such a trapezoidal cross section is generally achieved by winding the wire more times toward the outside of the concentric arrangement than toward the center of the concentric arrangement.
In contrast, coil 300 can be dimensioned to have a trapezoidal cross section with a single metal strip wound about axis 307. This dimensioning is reflected in the non-uniform sizing of regions 405, 410, 415, 420, 425, 430, 435, 440. For example, region 420 has a width 445 that is smaller than a width 450 of region 405. Regions 410, 415 have widths that are intermediate to widths 445, 450. Similarly, region 440 has a width that is smaller than the width of region 425, with regions 430, 435 having intermediate widths.
In some implementations, the height of regions 405, 410, 415, 420, 425, 430, 435, 440 can also be non-uniform. For example, a height 455 of region 425 can be less than a height 470 of region 440. Regions 430, 435 can have heights 460, 465 that are intermediate to heights 455, 470. Similarly, region 405 can have a height that is smaller than the height of region 420, with regions 410, 415 having intermediate heights.
The respective heights and widths of different regions 405, 410, 415, 420, 425, 430, 435, 440 fill a larger percentage of the design space of an electric motor or generator than can be filled using wires that have either a round or other cross section. A cast conductive coil may thus be advantageous in a number of ways. For example, casting can provide relatively precise control over the dimensions and positioning of metal strip 310. This allows a relatively large portion of the cross-sectional area (such as shown in FIG. 4) to be filled with conductive metal. By way of comparison, coils formed from a wire that has a circular cross-section necessarily includes larger gaps where current is not conducted. As a result, the maximum current density per cross-sectional area is reduced, as is, e.g., the strength of the magnetic field that can be generated and the power density of a motor.
Also, since metal strip 310 contacts itself in adjacent layers 312 over a relatively large area, heat can be transferred relatively efficiently across coil 300. This is beneficial, e.g., in applications where current density is relatively high and the heat from resistive heating is to be conducted away from coil 300.
A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made. For example, other products, including, e.g., vehicle suspension systems, tire rims, an structural parts can be cast. As another example, rather than casting molten metal 135, salts or salt mixtures can be cast. Cast salts can be used, e.g., as cores in high pressure die casting.
Accordingly, other embodiments are within the scope of the following claims.

Claims

What is claimed is:

1. A method for casting metals or salts, the method comprising:

evacuating a mold that is pressurized with an inert gas and coupled to a pressurized source of molten metal or salt, wherein the pressurization of the source is sufficiently high to drive the molten metal or salt into the mold in response to the evacuation.

2. The method of claim 1, wherein evacuating the mold comprises coupling the mold to a vacuum chamber, wherein the vacuum chamber has a volume that is 10 to 100 time a volume of the mold.

3. The method of claim 1, wherein the source of molten metal or salt is pressurized to a pressure of less than 0.2 MPa gage or less.

4. The method of claim 1, wherein the source of molten metal or salt is pressurized with the inert gas.

5. The method of claim 1, wherein the mold is configured to cast a coil.

6. The method of claim 1, wherein the mold is evacuated in 0.1 seconds or less.

7. The method of claim 1, wherein:

prior to the evacuation, the pressurization of the interior of the mold and the pressurization of the source are comparable and molten metal or salt is exposed to the inert gas pressurizing the mold.

8. The method of claim 1, further comprising:

during the evacuation, after the evacuation, or both during and after the evacuation, increasing pressurization of the source of molten metal or salt.

9. The method of claim 1, wherein the mold comprises a core, multiple slides, or both a core and multiple slides.

10. The method of claim 9, wherein the mold comprises a gas-permeable core and wherein evacuating the mold comprises evacuating at least part of the mold by drawing gas through the gas-permeable core.

11. The method of claim 1, wherein the pressurized source comprises a crucible that includes resistive or inductive heating elements integrated therein or disposed along a surface thereof.

12. A coil comprising:

a cast unitary metal strip covered by an insulator and layered to form a plurality of turns about an axis, the metal strip having a width generally perpendicular to the axis and a height generally parallel to the axis, wherein the width is larger than the height.

13. The coil of claim 12, wherein the cast unitary metal strip has a generally rounded rectangular cross-section parallel to the axis.

14. The coil of claim 12, wherein the insulation of adjacent turns of the metal strip contact each other over substantially the entire width of the metal strip.

15. The coil of claim 12, wherein the coil displays physical characteristics that are characteristic being cast in a relatively low pressure difference casting process.

16. The coil of claim 15, wherein the physical characteristics include dimensions of microbubbles in the coil.

17. The coil of claim 12, wherein the coil displays physical characteristics that are characteristic being cast directly in a shape that is nearly suitable for use, for example, wherein the metal strip is free from stress-induced microdefects resulting from an application of a force perpendicular to the axis to shape the metal strip.

18. The coil of claim 12, wherein the metal strip comprises aluminum and the insulator comprises aluminum oxide.

19. The coil of claim 12, wherein the cast unitary metal strip coil has a generally trapezoidal cross section parallel to the axis, wherein the cross section is oriented to intersect comparable cross sections of other coils of a motor or generator arranged generally concentrically about a center.

20. The coil of claim 20, wherein the generally trapezoidal cross section is formed by a single winding of the cast unitary metal strip.

21. The coil of claim 12, wherein, in at least one cross section parallel to the axis, regions of the cast unitary metal strip have non-uniform widths.

22. The coil of claim 21, wherein the regions progressively increase or decrease in width along the axis.

23. The coil of claim 21, wherein, in the at least one cross section parallel to the axis, regions of the cast unitary metal strip have non-uniform heights, for example, wherein the regions progressively increase or decrease in height along the axis.

24. The coil of claim 12, wherein the coil is cast in an extended shape that forms a plurality of turns about an axis with a space between adjacent turns, for example, wherein the space between adjacent turns is 5 to 10 times wider than the thickness of the metal strip along the axis.