EP1981668B1

EP1981668B1 - Method to prepare metal structure suitable for semi-solid metal processing

Info

Publication number: EP1981668B1
Application number: EP07763446A
Authority: EP
Inventors: Jessada c/o Prince of Songkla Univ. WANNASIN; Raul A. c/o Selmet Inc. MARTINEZ; Merton C. c/o Massachusetts Institute Of Technology FLEMMINGS
Original assignee: National Science and Technology Development Agency
Current assignee: National Science and Technology Development Agency
Priority date: 2006-02-02
Filing date: 2007-01-31
Publication date: 2013-01-16
Anticipated expiration: 2027-01-31
Also published as: JP5242416B2; JP2009525192A; ES2403369T3; EP1981668A2; EP1981668A4; WO2007092203A2; WO2007092203B1; DK1981668T3; WO2007092203A3

Description

This invention relates to a method to prepare non-dendritic, semi-solid metal slurries for semi-solid casting and forming. In particular, the invention flows gas bubbles through at least one solid medium inserted into or accommodates therein the liquid metal or alloy thereby cooling the liquid metal or alloy to a temperature below the melting temperature thereof while agitating the liquid metal or alloy with the gas bobbles and forming solid fractions therein.
The metal structure suitable for semi-solid metal (SSM) processing was first discovered in the early 1970's in a Ph. D. thesis of D.B. Spencer entitled "Rheology of Liquid-Solid Mixtures of Lead Tin," advisor M.C. Flemings, Massachusetts Institute of Technology (June 1971). They mechanically agitated a solidifying alloy in the solid-liquid temperature range and found that the solid phase would not be in the form of dendrites, but instead spheroidal particles. The non-dendritic nature of the solid phase gave these metal "slurries" unique flow properties. A metal slurry containing up to 50% of solid phase flows homogeneously with an "effective viscosity" orders of magnitude greater than liquid alloy. If the metal slurries are formed into parts, the higher viscosity will lead to less turbulent mold filling, thereby producing high quality parts by minimizing the entrapment of air and inclusions.
Since the invention of the SSM processes, industry has grown increasingly aware of their potential. Today SSM research is conducted in academia and industry worldwide. The proliferation of SSM applications in industry is, however, still in its early stages. Aluminum alloy automotive applications for have been the major focus of industrial interest in SSM. Concern about fuel efficiency and the environment has resulted in a drive toward lighter vehicles. This has led to a continual increase in aluminum casting usage in vehicles and an increasing need for processes like SSM which can yield high quality aluminum parts. Some of the automotive parts being considered for aluminum SSM are suspension components, air conditioning compressors, and master brake cylinders as described by S.P. Midson and K. Brissing in an article entitled "Semi-Solid Casting of Aluminum Alloys: A Status Report" (Modem Casting, February, 1997, pgs. 41-43). A commercial car manufacturer used a rheocasting process to manufacture cylinder blocks for the next line of turbo diesel engines. See M. Yamazaki et al. Development of a High-Strength Aluminum Cylinder Block for Diesel Engine Employing a New Production Process, SAE International, Publication 2004-01-1447.
Two semi-solid metal processing routes are industrially feasible: "thixocasting" and "rheocasting." Thixocasting is a process in which a non-dendritic structure is obtained by reheating a fully solidified billet back into the solid-liquid temperature range then forming it into a part. Rheocasting is a process in which a slurry with non-dendritic structure is created from liquid alloy, and then formed into a part.
Over the last 30 years, the SSM processing route used by industry has been thixocasting as presented by M.C. Flemings and W.L. Johnson in an article entitled "High Viscosity Liquid and Semi-Solid Metal Casting: Processes and Products" at World Foundry Conference held in KynogJu, Korea, Oct. 20-24, 2002. Electromagnetically stirred billet is produced by continuous casters as described by C. Vives in an article entitled "Elaboration of Semisolid Alloys by Means of New Electromagnetic Rheocasting"(Processes, Metallurgical Transactions B, (23b), April, 1992, pgs 189-206). The thixocaster would buy these billets, reheat them into the solid-liquid temperature range, and form them into parts. Even though high quality aluminum parts are obtained, issues such as operating cost and process control have prevented wide-spread adoption of thixocasting. In 2000, thixocasting process was estimated to represent only about 1% of the 2.5 million tons of aluminum castings in North America, Europe, and Japan (or about 25,000 tons) in an article by P. Kapranos et al. entitled "Near net shaping by semi-solid metal processing" (Materials and Design, (21), 2000, pgs 387-394).
Therefore, the recent trend in semi-solid metal processing is to advance the rheocasting route. Rheocasting has immediate cost advantages over thixocasting since liquid alloy can be formed into a non-dendritic metal slurry at the production site and scrap metals can be recycled in-house. Today several processes for creating non-dendritic structures from liquid alloy are available. The first approach used was mechanical stirring of metal in the solid-liquid temperature range. Several patents, including US Patent Nos. 5,555,926 , 5,887,640 , and 5,983,978 , describe equipment designed to create metal slurry by mechanical stirring. Also See S. Ji, Z. Fan, M.J. Bevis. Semi-solid processing of engineering alloys by a twin-screw rheomoulding process, Materials Science and Engineering A, (299A), 2001, pgs 210-217. However, the lack of robust stirring materials able to withstand exposure to molten aluminum for long periods of time has limited the use of the mechanical stirring methods in industry.
Up to this point, the challenge for rheocasting has been the limited knowledge about how to efficiently process liquid alloy to create non-dendritic metal slurries. It is now known that by controlling the conditions present during the initial stages of solidification (the formation of only the first few percent of solid phase) non-dendritic structures can easily be created. By combining localized rapid cooling and vigorous agitation in a melt so that the temperature drops from just above to just below the liquidus, a non-dendritic structure can be achieved in a matter of seconds, as described by R.A. Martinez in his MS Thesis entitled "A New Technique for the Formation of Semisolid Structures" (June 2001) and in his Ph.D. Thesis entitled "Formation and Processing of Rheocast Microstructures" (June 2004), Professor M.C. Flemings advisor, Massachusetts Institute of Technology.
US 6,645,322 refers to a method which efficiently rheocasts alloy with spheroidal particles by immersing a cool rotating rod into a melt held above the liquidus temperature. The immersion of the rotating rod simultaneously creates a region of high local cooling, provides vigorous convection, and drops the bulk melt temperature below the liquidus. The process can create slurry from liquid metal with large variability in superheat, making it a robust and efficient method to produce aluminum alloy slurries. However, since the method requires a moving solid medium, some issues may be anticipated. First of all, it may not be simple to apply a water cooling system to the rod continuously while the rod is rotating. Sensors such as temperature sensors may not be simple to be attached inside the rod to measure temperatures. In addition, while the rod is rotating, there is a possibility that a vortex is formed. Formation of a vortex may result in increased metal oxidation.
According to US 2001/020759 A1 , a molten metal vessel includes a first chamber for containing molten metal such as molten aluminum therein, a filtration chamber in which impurities contained in the molten metal are removed, and a second chamber for reserving clean molten metal to be supplied to a casting die. The molten metal flows from the first chamber to the second chamber through the filtration chamber. A pair of filters, preferably a pair of cylindrical filters, are disposed in the filtration chamber, so that the impurities contained in the molten metal can be filtered twice. The filters are attached to the bottom of the filtration chamber where molten metal oxides hardly develop, so that the filters are easily replaced with new ones for the maintenance purpose. A rod-shaped heater may be disposed in the cylindrical filters to keep the molten metal temperature in a strictly controlled range.; Thus, the impurities are effectively removed from the molten metal through the filters which are easily replaceable.
In US 6,521,015 B1 , a method is disclosed for filtering molten aluminum containing suspended particles using an improved filtration media, the method comprising the steps of providing a source of molten aluminum and providing media having a coating thereon, the coating having a softening point at molten aluminum temperatures to provide adhesive properties and bonding of suspended particles in the molten aluminum thereto. The filtration media is contacted with molten aluminum and suspended particles are adhesively bonded thereto to provide molten aluminum having suspended particles removed therefrom.
US 5,846,481 A refers to an improved molten aluminum refining apparatus in which the refining gas passageway is defined by a helical groove in the rotor shaft and sleeve for heating gas by contact with the sleeve as it flows toward the rotor to generally the temperature of the melt; which comprises a stator surrounding the sleeve and occupying a portion of the surface of the molten aluminum, and wherein the vessel comprises side walls that diverge upwardly at an angle of from about 5 degrees to about 16 degrees for permitting the gas bubbles to expand without substantial coalescence as the bubbles move upwardly in the melt reducing the metallostatic pressure on the respective bubbles.
The disclosure of US 4,154,689 A teaches an improvement in the filtration of molten metal especially aluminum using a removable vertically disposed filter plate and at least one fluxing gas inlet positioned in such a manner so as to introduce fluxing gas into the filter plate. The filter plate is provided with a bevelled peripheral surface mating with a like bevelled surface in a filter chamber so that the filter plate can be conveniently inserted in the chamber and removed therefrom. Fluxing gas is provided to the melt through the inlet and flows through the filter plate so as to assure extensive contact with the melt. Dissolved gases and non-metallic inclusions are thereby abstracted and removed from the melt.
According to EP 1004682 A1 , a monolithic, fired ceramic gas diffuser for injecting gas into a molten metal bath, including a first portion, a second portion integrated with the first portion, and a bore passing through the first portion and communicating with the second portion for supplying gas to the second portion, wherein at least the second portion has a network of interconnected pores that provides preferential gas flow from the bore through the second portion to inject gas into the molten metal bath.
EP 1561529 A1 provides a rheoforming apparatus that ensures the manufacture of products with fine, uniform, spherical particles, with improvements in energy efficiency and mechanical properties of the products, cost reduction, convenience of forming, and shorter process duration. The apparatus includes a first sleeve, an end of which is formed with a slurry outlet port for releasing a slurry, a second sleeve for retaining a molten metal, an end of which communicates with the first sleeve, a sealing member for opening or closing the end of the second sleeve, a stirring unit for applying an electromagnetic field to the second sleeve, and a plunger, which is slidably inserted into the other end of the second sleeve to press the slurry manufactured in the second sleeve.
In US 4,108,643 A , a metal composition is characterized by greater than about 65 weight percent degenerate denritic or nodular primary discrete solid particles suspended in a secondary phase having a lower melting point than the primary particles and which secondary phase can be solid or liquid. The method involves raising the temperature of a metal alloy to a value at which the alloy is largely or completely in the molten state. The melt is then subjected to vigorous agitation in an apparatus having an inner surface contacting the liquid-primary solid composition that is not wet by the composition and the heat is extracted to increase the portion of the mixture is in solid degenerate dendrite or nodular form greater than about sixty-five percent while continuing the agitation
In view of the above, it would be desirable to have a method that does not require a solid medium to rotate to avoid the drawbacks mentioned above.
This is achieved by the method of claim 1.
Preferred embodiments of the invention are characterized in the sub-claims.
This invention utilizes the principle presented by Martinez and Flemings that if a combination of localized chill with vigorous convection is applied to a melt held just above its liquidus temperature, a non-dendritic structure can form in a matter of seconds after solidification begins. In this invention, it has been found that by flowing gas bubbles through a solid object into a molten metal alloy held at a temperature above the liquidus temperature, non-dendritic, semi-solid metal slurry is obtained. In this invention, gas bubbles provide vigorous convection while also provide some localized chill. Localized chill is also achieved through the use of a solid object. In contrast to prior inventions, this invention uses gas bubbles as the medium to provide agitation, not solid objects such as impellers or cylindrical rods as in the prior art.
In one aspect, this invention describes a method to prepare non-dendritic, semi-solid metal slurries by introducing gas bubbles through a solid object into a molten metal alloy held at a temperature above the liquidus temperature. In this invention, since the solid object is not rotating, several advantages can be anticipated. With no rotating parts, a cooling system and sensory systems can be applied with simple designs. Since a vortex is not formed in the molten metal alloy, increased metal oxidation due to the vortex is avoided. If a porous solid object is used to provide gas bubbles, wetting and reaction between the molten metal alloy and the immersed medium are avoided since the flow of gas bubbles out of the pores on the solid medium acts as a protective layer between the molten metal alloy and the medium surfaces. In addition, the flow of gas bubbles inside the molten metal alloy help remove slag, dissolved gases, and any impurities from the molten metal alloy, these widely used processes are known as degassing or de-slagging processes.
The foregoing and additional features and characteristics of the present invention will become more apparent from the following detailed description considered with reference to the accompanying drawings in which like reference numerals designate like elements and wherein:
FIG. 1 shows the first embodiment of an apparatus for preparing non-dendritic, semi-solid metal slurries according to the invention.
FIG. 2 shows the second embodiment of an apparatus for preparing non-dendritic, semi-solid metal slurries according to the invention.
FIG. 3 shows the third embodiment of an apparatus for preparing non-dendritic, semi-solid metal slurries according to the invention.
FIG. 4 shows the fourth embodiment of an apparatus for preparing non-dendritic, semi-solid metal slurries according to the invention.
FIG. 5 shows a representative micrograph of a dendritic microstructure provided without applying the invention.
FIG. 6 shows a representative micrograph of the non-dendritic, semi-solid structure provided according to the invention.
FIG. 7 shows a representative micrograph of another microstructure provided according to the invention.
Shown in FIG. 1 is an apparatus for preparing non-dendritic, semi-solid metal slurries in accordance with an embodiment of this invention. The apparatus includes a holding vessel for receiving and hold a molten metal alloy, a lance (a hollow cylindrical tube) for providing inert gas bubbles, and a solid object to provide more localized chill. In one process, the lance is immersed in the molten metal alloy which is held at a temperature above the liquidus temperature. Then, inert gas is flowed through the lance creating gas bubbles while a solid object is lowered into the molten metal alloy. The gas is selected of the group consisting of nitrogen, argon, carbon dioxide, and a mixture of these gases. The metal alloy is selected from the group consisting of aluminum alloys, magnesium alloys, copper alloys, ferrous alloys, zinc alloys, nickel alloys, and titanium alloys.
As the temperature of the metal drops below the liquidus temperature, non-dendritic, semi-solid metal slurry is being formed. When the metal slurry has a solid content of about 1% to about 50% by weight, the solid object is removed and the gas flow is stopped.
Other variations of the invention are possible, such as a pipe, an impeller, a rod, or a container. Some of them are given in FIG. 2 - FIG. 4. FIG. 2 shows another embodiment. In this process, inert gas is flowed through a nozzle of a solid object. In another embodiment, FIG. 3, inert gas is flowed through a porous solid object. In this case, fine and uniform bubbles are obtained. FIG. 4 shows another embodiment. In one process, inert gas is flowed through a porous solid located at the wall. Fine gas bubbles can be introduced on all the wall surfaces and the bottom surfaces or only parts of the surfaces. The solid object may be made of graphite, ceramics, metals, or composites of these materials.
The molten metal or alloy is cooled by the flow of gas bubbles thereinto and by the contact with the solid object. Since more than one discussed-above solid object can be deployed simultaneously, the melted metal or alloy can be cooled by the contact with multiple solid objects. In addition, the solid object is cooled by flowing air, water, or any cooling fluids through itself.
Beside the cooling and agitating functions, the gas bubbles also protect the solid object from reacting with the metal or alloy, and remove slag, dissolved gases, or impurities from the metal or alloy.
Other embodiments of the invention will be apparent to those skilled in the art from a consideration of the specification or practice of the invention disclosed herein.

Example 1

A357 Alloy Processed using a Porous Graphite Diffuser

The following is a detailed description of a method for preparing non-dendritic, semi-solid slurries of A357 alloy, with reference to FIG. 3.
About 520 g of aluminum alloy A357 (Al-7.0 wt% Si-0.5 wt% Mg) was melted in a stainless steel crucible coated with boron nitride in an electric furnace. A porous graphite rod (2.54 cm OD, 1.5 cm OD, 15.24 cm long, 2.6 g/cm³) was machined to form a diffuser. The wall of the graphite diffuser was roughly 10% open porosity. The graphite diffuser was then connected to an argon gas cylinder equipped with a gas flow meter to provide fine gas bubbles into the melt. The alloy was melted and heated to 630 °C. The melt was slowly cooled down to 625 °C, with the cooling rate of about 1 °C/minute, and the diffuser was quickly immersed introducing fine argon gas bubbles with the volumetric flow rate of about 2 liter/min. The bubbling process was carried out until solid phase of about 10% in the melt was achieved before the diffuser was quickly removed and the melt allowed to cool slowly. When the metal temperature reached 580 °C (about 45% solid fraction), a slice of the metal in the crucible was removed and quenched in water.
The sample was then polished and examined under an optical microscope. For comparison, FIG. 5 is given to show a representative micrograph of the un-processed dendritic microstructure. The micrograph shows coarse grain structure with more than 400 µm in size. In contrast, FIG. 6 shows a representative micrograph of the non-dendritic, semi-solid structure processed by this method. In this method, the grain structure is significantly finer with less than 200 µm in size.

Example 2

A357 Alloy Processed using a Lance

The following is a detailed description of a method for preparing non-dendritic, semi-solid slurries of A357 alloy, with reference to FIG. 1.
About 520 g of aluminum alloy A357 (Al-7.0 wt% Si-0.5 wt% Mg) was melted in a stainless steel crucible coated with boron nitride in an electric furnace. A stainless steel tube (0.4 cm ID, 0.6 cm OD) was machined to form a lance as shown in FIG. 1. The end of the tube was mechanically closed and a small nozzle was machined. The lance was coated with boron nitride and then connected to an argon gas cylinder equipped with a gas flow meter to provide fine gas bubbles into the melt. The lance was immersed in the melt while the alloy was being heated to 630 °C. The melt was slowly cooled down to 625 °C, with the cooling rate of about 1 °C/minute, and the solid copper chill coated with graphite was quickly immersed with fine argon gas bubbles being introduced through the lance at the same time, see FIG. 1. The volumetric flow rate was about 1.5 liter/min. The bubbling process was carried out until solid phase of about 5% in the melt was achieved before the solid copper chill was quickly removed and the gas flow was stopped. The melt was then allowed to cool slowly. When the metal temperature reached 580 °C (about 45% solid fraction), a slice of the metal in the crucible was removed and quenched in water. The samples were then polished and examined under an optical microscope. FIG. 7 shows a representative micrograph of the microstructure.

Claims

A method for forming a semi-solid metal or alloy having non-dendritic grain structures using gas bubbles, comprising:
a first step of heating metal or alloy above a melting temperature thereof to provide liquid metal or alloy;

a second step of flowing gas bubbles through at least one solid medium inserted into or accommodates therein said liquid metal or alloy thereby cooling said liquid metal or alloy to a temperature below said melting temperature thereof while agitating said liquid metal or alloy with the gas bubbles and forming solid fractions therein; and

a third step of stopping flowing said gas bubbles therethrough when the solid fraction of said liquid metal or alloy reaches a range of 0.01-0.5 by weight, preferably a range of 0.01-0.2 by weight, thereby providing the semisolid metal or alloy having non-dendritic grain structures.
The method according to claim 1, wherein the cooling is further achieved by contacting said liquid metal or alloy with said solid medium.
The method according to claim 1, wherein said step of flowing gas bubbles provides a cooling rate of at least 1 degree Celsius per minute.
The method according to claim 3, wherein the cooling is further achieved by contacting said liquid metal or alloy with at least one other solid medium which is different form said solid medium.
The method according to claim 3, wherein said solid medium is cooled by means of flowing air or a cooling fluid therethrough.
The method according to claim 1, wherein the solid medium is prevented from stirring or rotating.