Patent Application
20210316357
This invention relates to the direct chill (DC) casting of metals and metal alloys, particularly aluminum and aluminum alloys wherein a homogeneous product suitable to form metal products such as sheet and plate articles is directly obtained.
Metals and metal alloys, particularly aluminum and aluminum alloys, are cast from a molten phase to produce ingots or billets that are subsequently subjected to further processing such as rolling or hot working to produce sheet or plate articles which may be converted to final products. Throughout the following description the term billet will be used to describe the product of the DC casting process. A billet represents an elongated metal casting product, usually cylindrical in shape, and having a small diameter in comparison to its length. However, the principles and operations applied herein may also be applicable to the production of ingots. DC casting to produce billets or ingots is conventionally carried out in a shallow, open-ended, axially vertical mold which is initially closed at its lower end by a downwardly movable platform (often referred to as a bottom block). The mold is surrounded by a cooling jacket through which a cooling fluid such as water is continuously circulated to provide external chilling of the mold wall. The molten aluminum (or other metal) is introduced into the upper end of the chilled mold and, as the molten metal solidifies in a region adjacent to the inner periphery of the mold, the platform is moved downwardly. With an effectively continuous movement of the platform and correspondingly continuous supply of molten aluminum to the mold, a billet of desired length may be produced.
The billet emerging from the lower (output) end of the mold in vertical DC casting is externally solid but is still molten in its central core. In other words, the pool of molten metal within the mold extends downwardly into the central portion of the downwardly-moving ingot for some distance below the mold as a sump of molten metal. This sump has a progressively decreasing cross-section in the downward direction as the ingot solidifies inwardly from the outer surface until its core portion becomes completely solid.
Direct chill cast billets produced in this way are generally subjected to hot and cold rolling steps, or other hot-working procedures, in order to produce articles of desired form. However, a homogenization treatment is conventionally necessary in order to convert the metal to a more usable form. During the solidification of DC cast alloys multiple events are taking place within the microstructure. Firstly, the metal phase is nucleating in grains which may be cellular, dendritic or a combination thereof and conventionally chemical grain refining chemicals are added to assist this process. Such chemicals add cost and create problems in operation and even may adversely affect the properties of the final product. Further, where non-equilibrium solidification conditions exist alloy components may be rejected from the forming grains and are concentrated in pockets in the microstructure, thus also adversely affecting the performance properties of the product. The result of these events is compositional variances across not only the grain but also in the regions adjacent to the intermetallic phases where relatively soft and hard regions co-exist in the structure and, if not modified or transformed, will create property variances unacceptable to the final product.
Homogenization conventionally involves heat treatment to correct the microscopic deficiencies described above in the cast microstructure. Homogenization involves heating the cast billet to an elevated temperature (generally a temperature above a transition temperature, e.g. a temperature close to the liquidus temperature of the aluminum or aluminum alloy for from a few hours to as many 24 hours or even longer. As a result of the homogenation treatment grain distribution becomes more uniform. Further, low melting point constituent particles that may have formed during casting are dissolved back into the grains. Additionally, any large intermetallic particles that formed during casting may be fractured. Finally, precipitates of chemical additives for the purpose of strengthening the material which may have formed are dissolved and then evenly redistributed as the billet cools. The homogenation operation is a high energy consumption operation and has a direct cost effect on the operation in consideration of the present high cost of energy.
It is an object of the present invention to provide a DC casting method and apparatus which directly provides a cast metal billet having a homogenous microstructure without need for a homogenation heat treatment or requiring only minimal homogenation treatment.
It is a further object of the present invention to provide a DC casting method and apparatus which directly provides a cast metal billet having a homogenous microstructure without need for inclusion of a grain refining chemical, or only minimal grain refining chemical.
These and other objects are provided by the present invention, the first embodiment of which provides a method for direct chill casting of a metal or metal alloy, comprising:
supplying a fluid melt comprising a molten metal or molten metal alloy to a direct chill (DC) mold having an inlet and an outlet;
cooling the fluid melt in the mold to obtain a billet having a molten core forming an inwardly tapering sump and a solid outer shell that thickens at increasing distance from the mold outlet;
applying vibrational energy to the fluid melt in the molten core sump of a billet exiting the mold with a device positioned within the mold;
injecting a flow of a purge gas into the fluid melt in the molten core sump of the billet;
applying vibrational energy to the solid outer shell of the billet beyond the outlet of the mold in a region of the tapering sump;
removing the billet from the mold outlet; and
further cooling the billet beyond the mold outlet to obtain a solid billet.
In one aspect of the first embodiment, applying vibrational energy to the solid outer shell of the billet in the region of the tapering sump includes applying the vibrational energy from a plurality of vibrational energy sources located in a plurality of positions around the circumference of the billet.
In another aspect of the first embodiment, applying ultrasound vibrational energy to the solid outer shell of the billet in the region of the tapering sump includes applying the vibrational energy through a layer of coolant sprayed on the outer surface of the billet beyond the outlet of the mold.
In another aspect of the first embodiment, the direct chill mold is a vertical DC mold.
In another aspect of the first embodiment, the direct chill mold is a horizontal DC mold.
In a second embodiment, the present invention provides a direct chill (DC) casting mold, comprising:
a vertically oriented open-ended mold having an upper positioned inlet and lower positioned outlet;
a feed trough for supply of a fluid melt to the upper inlet of the mold;
a liquid cooling system providing a fluid cooling jacket at the outlet of the mold;
a vibrational energy source positioned vertically above the mold inlet and extending into the mold;
a purge gas feed unit positioned vertically above the mold inlet and extending into the mold; and
a plurality of vibrational energy sources circumferentially arranged beneath the outlet of the mold;
wherein the vertical position of the circumferentially arranged plurality of vibrational energy sources is located in close proximity of the mold outlet such that the vibrational energy is applied to a billet exiting the mold in a region of an inwardly tapering melt sump within the billet.
In an aspect of the second embodiment, the vertically positioned vibrational energy source comprises at least one ultrasonic transducer, at least one mechanically-driven vibrator, or a combination thereof.
In another aspect of the second embodiment, the vertically positioned vibrational energy source and the purge gas feed unit are combined as an ultrasonic degasser unit wherein the ultrasonic degasser comprises: an elongated probe comprising a first end and a second end, the first end attached to an ultrasonic transducer and the second end comprising a tip located at the outlet of the mold, and a purging gas delivery comprising a purging gas inlet and a purging gas outlet, the purging gas outlet disposed at the tip of the elongated probe for introducing a purging gas into the region at the outlet of the mold.
In another aspect of the second embodiment each of the plurality of vibrational energy sources circumferentially arranged beneath the outlet of the mold comprise at least one ultrasonic transducer, at least one mechanically-driven vibrator, or a combination thereof.
In another aspect of the second embodiment, each of the plurality of vibrational energy sources circumferentially arranged beneath the outlet of the mold are positioned to directly contact a solid surface of a billet exiting the mold.
In another aspect of the second embodiment, each of the plurality of vibrational energy sources circumferentially arranged beneath the outlet of the mold are positioned to contact a cooling fluid jacket on a solid surface of a billet exiting the mold.
In a third embodiment, the present invention provides a direct chill (DC) casting mold, comprising:
a horizontally oriented open-ended mold having an inlet and outlet;
a feed trough for supply of a fluid melt to the inlet of the mold;
a liquid cooling system providing a fluid cooling jacket at the outlet of the mold;
a vibrational energy source positioned at the mold inlet and extending into the mold;
a purge gas feed unit positioned at the mold inlet and extending into the mold; and
a plurality of vibrational energy sources circumferentially arranged beyond the outlet of the mold;
wherein the position of the circumferentially arranged plurality of vibrational energy sources is located in close proximity of the mold outlet such that the vibrational energy is applied to a billet exiting the mold in a region of an inwardly tapering melt sump within the billet.
In an aspect of the third embodiment, the vibrational energy source positioned at the mold inlet comprises at least one ultrasonic transducer, at least one mechanically-driven vibrator, or a combination thereof.
In another aspect of the third embodiment, the vibrational energy source positioned at the mold inlet and the purge gas feed unit are combined as an ultrasonic degasser unit wherein the ultrasonic degasser comprises: an elongated probe comprising a first end and a second end, the first end attached to an ultrasonic transducer and the second end comprising a tip located at the outlet of the mold, and a purging gas delivery comprising a purging gas inlet and a purging gas outlet, the purging gas outlet disposed at the tip of the elongated probe for introducing a purging gas into the region at the outlet of the mold.
In another aspect of the third embodiment each of the plurality of vibrational energy sources circumferentially arranged betond the outlet of the mold comprise at least one ultrasonic transducer, at least one mechanically-driven vibrator, or a combination thereof.
In another aspect of the second embodiment, each of the plurality of vibrational energy sources circumferentially arranged beyond the outlet of the mold are positioned to directly contact a solid surface of a billet exiting the mold.
In another aspect of the third embodiment, each of the plurality of vibrational energy sources circumferentially arranged beyond the outlet of the mold are positioned to contact a cooling fluid jacket on a solid surface of a billet exiting the mold.
In a fourth embodiment, the present invention provides a metal or metal alloy billet obtained by the method of the first embodiment wherein the billet does not comprise a grain refining chemical and the billet has not been subjected to a thermal homogenation treatment.
In a special aspect of the fourth embodiment, the billet is an aluminum or aluminum alloy billet.
The foregoing paragraphs have been provided by way of general introduction, and are not intended to limit the scope of the following claims. The described embodiments, together with further advantages, will be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings.
In the following description the words “a” and “an” and the like carry the meaning of “one or more.” The phrases “selected from the group consisting of,” “chosen from,” and the like include mixtures of the specified materials. Terms such as “contain(s)” and the like are open terms meaning ‘including at least’ unless otherwise specifically noted. All references, patents, applications, tests, standards, documents, publications, brochures, texts, articles, etc. mentioned herein are incorporated herein by reference. Where a numerical limit or range is stated, the endpoints are included. Also, all values and subranges within a numerical limit or range are specifically included as if explicitly written out.
Throughout the remainder of this description aluminum alloy will be discussed. However, it is to be understood that the gist of the embodiments described may not be limited to aluminum alloy and may be equally applicable to any metal or metal alloy cast in a DC casting operation. Further, although billets are described in the embodiments, the method may also be considered applicable to the casting of ingots. Thus, according to the present method embodiments application of vibrational or ultrasound energy in a two tier system to a DC casting process is provided. In the first tier, a combination of ultrasonic energy and/or purge gas is directly inserted into the melt sump of a billet formed in an open mold of a DC casting system at a point where the billet is outside the outlet of the mold. This combination of vibrational energy and purge gas serves to evenly distribute the alloying elements in the melt sump region and to remove entrapped gases in the melt. Additionally, it is believed that grain refining also results from this direct application of vibrational energy into the melt sump region. Because this melt sump area is adjacent to the solidification boundary of the cooling billet, the even distribution of the alloying elements may be retained in the solidified billet. Further in a second tier, by application of the vibratonal energy, especially ultrasonic energy, across the billet wall in the region of the melt sump, crystals solidifying at the solidification front are broken away from the front in smaller crystal units and become more evenly distributed in the solidified alloy.
Thus as a result of the two tiers of treatment a billet having a homogeneous distribution of alloying elements and fine grains is obtained. This is exactly the result targeted in the thermal homogenation process as previously described and thus the energy and operational costs of a homogenation operation may be avoided. Additionally, as fine grains are generated by the application of the ultrasonic energy at the solidification front as described above a fine grain structure is obtained without the need for inclusion of grain refining chemicals such as titanium boride (TIBOR) or the titanium carbon mixture (TiCar).
A high quality DC aluminum alloy cast billet may thus be obtained without the use of refining chemicals and at significant reduction of production time and energy cost. Such improvement in quality and cost in the DC casting of billets was highly unexpected and offers significant advantage over conventional DC casting system now in operation.
Thus in the first embodiment, the present invention provides a method for direct chill (DC) casting of a metal or metal alloy, comprising:
supplying a fluid melt comprising a molten metal or molten metal alloy to a direct chill mold having an inlet and an outlet;
cooling the fluid melt in the mold to obtain a billet having a molten core forming an inwardly tapering sump and a solid outer shell that thickens at increasing distance from the mold outlet;
applying vibrational energy to the fluid melt in the molten core sump of a billet exiting the mold with a device positioned within the mold;
optionally, injecting a flow of a purge gas into the fluid melt in the molten core sump of the billet;
applying vibrational energy to the solid outer shell of the billet beyond the outlet of the mold in a region of the tapering sump; and
removing the billet from the mold outlet;
further cooling the billet beyond the mold outlet to obtain a solid billet.
The DC casting mold may be vertically or horizontally oriented.
The preparation and supply of the fluid melt of the molten metal or metal alloy is conventionally known and any of the known systems may be employed with the present invention. Further, handling of the solidified billet is also conventionally known and any such systems may be suitably combined with the present invention.
The application of the ultrasound vibrational energy to the solid outer shell of the billet in the region of the tapering sump includes applying the vibrational energy from a plurality of vibrational energy sources located in a plurality of positions around the circumference of the billet. In theory, the greater the number of vibrational energy inputs applied the more effective the generation of fine grains from the solidification front. However, in practice the maximum number may be limited by the spacial configuration of the DC molding unit. Generally, at least two vibrational energy sources may be employed, preferably 2 to 8 vibrational energy sources, more preferably 3 to 6 and most preferably 4 vibrational energy devices may be employed.
The purge gas may be any gas suitable for use with molten metal or molten metal alloy. Generally, an inert gas such as nitrogen or argon is preferred. However, in specific applications other gases may be employed as the purge gas, including combinations of gases.
In an aspect of the invention where a purge gas is employed, the vibrational energy source positioned in the mold and the purge gas feed unit are combined as an ultrasonic degasser unit wherein the ultrasonic degasser comprises: an elongated probe comprising a first end and a second end, the first end attached to a ultrasonic transducer and the second end comprising a tip located at the outlet of the mold, and a purging gas delivery comprising a purging gas inlet and a purging gas outlet, the purging gas outlet disposed at the tip of the elongated probe for introducing a purging gas into the region at the outlet of the mold.
As indicated previously, the billet below or beyond the mold outlet is coated with a coolant jacket, preferably a jacket of water. Thus there are two configurations for application of vibrational energy to the billet outer shell. In one configuration, the vibrational energy source may be inserted through the coolant jacket and directly contact the billet surface. In a second configuration the vibrational energy source contacts the water jacket and the ultrasound energy is conveyed by the coolant to the billet surface.
In either configuration, in consideration that the vibrational energy is dampened by the structure of the solid billet, the position of the vibrational energy device relative to the tapering sump may be located close to the mold outlet where the thickness of the solid wall is minimal. In certain arrangements the positioning of the plurality of the vibrational energy devices may be arranged at differing locations of the sump such that the ultrasound energy is applied across a maximum area of the solidification front.
The vibrational energy device may be any such device suitable for utilization in the DC casting mold as described. A broad range of powers and ultrasonic frequencies can be used with the DC casting method as described herein and may be adjusted for optimal performance depending upon the particular alloy being cast and the depth, shape and dimensions of the mold. In one aspect the source of ultrasonic vibrations may provide a power of 1.5 kW at an acoustic frequency of 20 kHz.
In general, the power of the probe (vibrational energy device) may range between 50 and 5000 W depending on the dimensions of the probe. These powers are typically applied to the probe to ensure that the power density at the end of the probe is higher than 100 W/cm2, which may be considered the threshold for grain cleavage at the solidification front. The powers at this area can range from 50 to 5000 W, 100 to 3000 W, 500 to 2000 W, 1000 to 1500 W or any intermediate or overlapping range. Higher powers for larger probes and lower powers for smaller probes are possible. In various embodiments of the invention, the applied vibrational energy power density can range from 10 W/cm2 to 500 W/cm2, or 20 W/cm2 to 400 W/cm2, or 30 W/cm2 to 300 W/cm2, or 50 W/cm2 to 200 W/cm2, or 70 W/cm2 to 150 W/cm2, or any intermediate or overlapping ranges thereof.
In general, a frequency of from 5 to 400 kHz (or any intermediate range) may be used. Alternatively, 10 and 30 kHz (or any intermediate range) may be used. Alternatively, 15 and 25 kHz (or any intermediate range) may be used. The frequency applied can range from 5 to 400 KHz, 10 to 30 kHz, 15 to 25 kHz, 10 to 200 KHz, or 50 to 100 kHz or any intermediate or overlapping ranges thereof.
The vibrational energy device may be any of such devices known in the art and may be an ultrasonic wave probe (or sonotrode), a piezoelectric transducer, an ultrasonic radiator, or a magnetostrictive element. In a case where the vibrational energy is conveyed through the cooling medium, an ultrasonic transducer may be preferred. In one embodiment of the invention, ultrasonic energy is supplied from a transducer that is capable of converting electrical currents to mechanical energy thus creating vibrational frequencies above 20 kHz (e.g., up to 400 kHz), with the ultrasonic energy being supplied from either or both piezoelectric elements or magnetostrictive elements.
In an embodiment where an ultrasonic wave probe contacts the liquid cooling medium a separation distance from a tip of the ultrasonic wave probe to the solid billet wall may be variable. The separation distance may be for example less than 1 mm, less than 2 mm, less than 5 mm, less than 1 cm, less than 2 cm, less than 5 cm, less than 10 cm, less than 20, or less than 50 cm.
In one aspect of the invention, the vibrational energy device may be a piezoelectric transducer formed of a ceramic material that is sandwiched between electrodes which provide attachment points for electrical contact. Once a voltage is applied to the ceramic through the electrodes, the ceramic expands and contracts at ultrasonic frequencies.
As known in the art, an ultrasonic booster may be used to amplify or intensify the vibrational energy created by a piezoelectric transducer. The booster does not increase or decrease the frequency of the vibrations; it increases the amplitude of the vibration. In one embodiment of the invention, a booster connects between the piezoelectric transducer and the probe.
Magnetostrictive transducers are typically composed of a large number of material plates that will expand and contract once an electromagnetic field is applied. More specifically, magnetostrictive transducers suitable for the present invention can include in one embodiment a large number of nickel (or other magnetostrictive material) plates or laminations arranged in parallel with one edge of each laminate attached to the bottom of a process container or other surface to be vibrated. A coil of wire is placed around the magnetostrictive material to provide the magnetic field. For example, when a flow of electrical current is supplied through the coil of wire, a magnetic field is created. This magnetic field causes the magnetostrictive material to contract or elongate, thereby introducing a sound wave into a fluid in contact with the expanding and contracting magnetostrictive material. Typical ultrasonic frequencies from magnetostrictive transducers suitable for the invention range from 20 to 200 kHz. Higher or lower frequencies can be used depending on the natural frequency of the magnetostrictive element.
For magnetostrictive transducers, nickel is one of the most commonly used materials. When a voltage is applied to the transducer, the nickel material expands and contracts at ultrasonic frequencies. In one embodiment of the invention, the nickel plates are directly silver brazed to a stainless steel plate. The stainless steel plate of the magnetostrictive transducer is the surface that is vibrating at ultrasonic frequencies and is the surface (or probe) coupled directly to the cooling medium. The cavitations that are produced in the cooling medium via the plate vibrating at ultrasonic frequencies, then impact the solid surface of the billet.
Mechanical vibrators useful for the invention can operate from 8,000 to 15,000 vibrations per minute, although higher and lower frequencies can be used. In one embodiment of the invention, the vibrational mechanism is configured to vibrate between 565 and 5,000 vibrations per second. Accordingly, ranges suitable for the mechanical vibrations that may be used in the invention include: 0 to 10 KHz, 10 Hz to 4000 Hz, 20 Hz to 2000 Hz, 40 Hz to 1000 Hz, 100 Hz to 500 Hz, and intermediate and combined ranges thereof, including a preferred range of 565 to 5,000 Hz.
While described above with respect to ultrasonic and mechanically driven embodiments, the invention is not so limited to one or the other of these ranges, but can be used for a broad spectrum of vibrational energy up to 400 KHz including single frequency and multiple frequency sources. Additionally, a combination of sources (ultrasonic and mechanically driven sources, or different ultrasonic sources, or different mechanically driven sources or acoustic energy sources to be described below) may be used.
In a second embodiment, the present invention provides a direct chill (DC) casting mold, comprising:
a vertically oriented open-ended mold having an upper positioned inlet and lower positioned outlet;
a feed trough for supply of a fluid melt to the upper inlet of the mold;
a liquid cooling system providing a fluid cooling jacket at the outlet of the mold;
a vibrational energy source positioned vertically above the mold inlet and extending into the mold;
optionally, a purge gas feed unit positioned vertically above the mold inlet and extending into the mold; and
a plurality of vibrational energy sources circumferentially arranged beneath the outlet of the mold;
wherein the vertical position of the circumferentially arranged plurality of vibrational energy sources is located in close proximity of the mold outlet such that the vibrational energy is applied to a billet exiting the mold in a region of an inwardly tapering melt sump within the billet.
The mold may be constructed of any material compatible with the molten metal composition to be cast. Generally, the mold may be constructed of copper or graphite.
In one aspect, the vertically positioned vibrational energy source comprises at least one ultrasonic transducer, at least one mechanically-driven vibrator, or a combination thereof.
In a further aspect the vertically positioned vibrational energy source and the purge gas feed unit are combined as an ultrasonic degasser unit wherein the ultrasonic degasser comprises: an elongated probe comprising a first end and a second end, the first end attached to an ultrasonic transducer and the second end comprising a tip located at the outlet of the mold, and a purging gas delivery comprising a purging gas inlet and a purging gas outlet, the purging gas outlet disposed at the tip of the elongated probe for introducing a purging gas into the region at the outlet of the mold.
A schematic visual concept of the DC casting mold system is shown in
As previously described each of the plurality of vibrational energy sources circumferentially arranged beneath the outlet of the mold comprise at least one ultrasonic transducer, at least one mechanically-driven vibrator, or a combination thereof. Further, each of the plurality of vibrational energy sources circumferentially arranged beneath the outlet of the mold may be positioned to directly contact a solid surface of a billet exiting the mold. In another aspect as shown in
In a third embodiment, the present invention provides a direct chill (DC) casting mold, comprising:
a horizontally oriented open-ended mold having an inlet and an outlet;
a feed trough for supply of a fluid melt to the inlet of the mold;
a liquid cooling system providing a fluid cooling jacket at the outlet of the mold;
a vibrational energy source positioned at the mold inlet and extending into the mold;
optionally, a purge gas feed unit positioned at the mold inlet and extending into the mold; and
a plurality of vibrational energy sources circumferentially arranged beyond the outlet of the mold;
wherein the position of the circumferentially arranged plurality of vibrational energy sources is located in close proximity of the mold outlet such that the vibrational energy is applied to a billet exiting the mold in a region of an inwardly tapering melt sump within the billet.
The mold may be constructed of any material compatible with the molten metal composition to be cast. Generally, the mold may be constructed of copper or graphite.
In one aspect, the vibrational energy source positioned within the mold comprises at least one ultrasonic transducer, at least one mechanically-driven vibrator, or a combination thereof.
In a further aspect when purge gas is employed, the vibrational energy source positioned within the mold and the purge gas feed unit are combined as an ultrasonic degasser unit wherein the ultrasonic degasser comprises: an elongated probe comprising a first end and a second end, the first end attached to an ultrasonic transducer and the second end comprising a tip located at the outlet of the mold, and a purging gas delivery comprising a purging gas inlet and a purging gas outlet, the purging gas outlet disposed at the tip of the elongated probe for introducing a purging gas into the region at the outlet of the mold.
In a fourth embodiment the present invention is drawn to a cast alloy billet obtained by the method of the present invention. The billet does not comprise a grain refining chemical, or a significantly reduced quantity of grain refining chemical, and the billet has not been subjected to a thermal homogenization treatment. In a preferred aspect the billet is an aluminum or aluminum alloy billet.
The above description is presented to enable a person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the preferred embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Thus, this invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. In this regard, certain embodiments within the invention may not show every benefit of the invention, considered broadly.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2019/043445 | 7/25/2019 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62703035 | Jul 2018 | US |
