Patent Application
20060070419
The present invention relates to feedstock materials for semi-solid forming, preferably feedstock materials comprising processed, reprocessed or materials to be recycled from semi-solid forming processes.
Metal alloys for semi-solid forming are known in the art. Typically, when magnesium alloys are used for thixomolding, the feedstock is produced by chipping bars comprising magnesium. The chips from these bars are, however, problematic because they are often contaminated with cutting fluid from the chipper and because they often contain metal contaminates from the knives of a chipper. These chips cause additional problems when used as feedstock because the size and shapes of the chips are non-uniform. As a result of the non-uniform size and shape of the chips, (1) voids form in injection molded parts because of non-uniform packing of the chips, (2) bridging may occur at the feed port, and/or (3) non-optimal performance of the injection molding apparatus occurs. Bridging at the feed port occurs because chips are of non-uniform thickness and therefore have non-uniform melting profiles. That is to say, the thinner edges of the chips will melt before the thicker centers. From time to time, plural chips fuse together and become lodged in entry and exit points, thereby blocking the flow of material and interrupting the process. With regard to non-optimal performance of the apparatus, “hiccups” may occur when chips become lodged between an extruder screw and the wall of the extruder. This lodging often creates a whining sound and causes excessive wear to the screw and the barrel wall of the semi-solid forming machine.
Another problem lies with the microstructure of the feedstock materials. Traditional feedstock materials commonly have a dendritic microstructure that is disadvantageous during the thixomolding operation. This problem is common in virgin feedstocks and may be encountered in recycled feedstocks as well.
Another problem is with scraps produced during the thixoforming operation. In a typical operation, scraps, such as sprues and runners are returned to metal foundries, where the scraps are melted to a liquid stage, purified, realloyed, made into billets again (typically with a microstructure containing dendrites not preferred for thixomolding), rechipped and sold back to thixomolder end users. Besides the disadvantages of thixomolding feedstock materials having non-desirable microstructure and the necessity to have to transport scraps back to foundries for reprocessing, the current techniques of recycling provide a disincentive against using proprietary metallurgical compositions.
As a result, there exists a need for a feedstock material that avoids the problems discussed above as well as for an apparatus and process for making such a feedstock.
One object of the present invention is to provide a feedstock that overcomes the problems and disadvantages of present feedstock materials, as well as a process and apparatus for feedstock production. An additional object of the invention is to overcome obstacles and cost associated with the recycling of thixotropic revert due to irregular shape and potential for contamination. Another object of the invention is to produce a feedstock of a uniform shape which will feed well in subsequent processing. Yet another object of the invention is to provide a process and apparatus for removing volatile paints and oils from the recycled material used as raw material to provide the feedstock in a flashing step. Yet another object of the invention is to provide a process and apparatus for further alloying elements into spent alloys in the recycling and/or processing stage. Still another object of the invention is to provide an inexpensive means to reduce raw material, including ingots, to a uniform raw material for processing to feedstock without going through the traditional chipping process and with only minimal contamination from a size reduction process. A further object of the invention is to produce a feedstock having a microstructure that is favorable to thixoforming, which may include a globular structure or a crystalline microstructure.
In order to achieve the foregoing and further objects, there has been provided according to one aspect of the invention a process for producing feedstock materials for semi-solid forming. The process desirably includes providing solid-state raw material, reducing the size of the raw material to produce particles of raw material of a size suitable for further processing, introducing the particles of raw material into an extruder and extruding particles of raw material to form an extrudate feedstock material. It may be desirable to separate the particles of raw material by size, weight, density or other physical characteristics before introducing the particles into the extruder. The extrudate feedstock desirably includes a non-dendritic microstructure such as a globular microstructure.
According to another aspect of the invention, there has been provided an apparatus for producing feedstock materials for semi-solid metal forming. The apparatus generally includes an extruder, a feeder attached to and in communication with the extruder for introducing raw material into the extruder, and means for maintaining the raw material under an inert atmosphere or vacuum in the feeder and as the raw material is fed into the extruder. The apparatus may further include a discharge orifice (e.g., die or nozzle) affixed to the extruder and through which the raw material exits the extruder as extrudate. The apparatus may also include a means for controlling the temperature of the extrudate while in the extruder and while the extrudate is being discharged from the extruder. Preferably, the extrudate reaches a temperature sufficient to transform the raw material to its semi-solid state below its liquefying temperature. The apparatus desirably includes a means for maintaining the extrudate under an inert atmosphere as the raw materials are being extruded into the extrudate, as the extrudate exits the discharge orifice and a pelletizer for dividing extrudate into pellets.
According to yet another aspect of the invention there has been provided a system for producing feedstock materials for semi-solid forming. The system may include raw material, a size reduction station for reducing the raw materials into particles, a magnetic screen for removing particles larger in size than openings in the screen and for removing magnetic contaminants, an extruder comprising a screw and containing semi-solid raw material and a pelletizer.
Further objects, features and advantages of the present invention will become apparent from the detailed description of preferred embodiments that follows when considered together with the accompanying drawings. It is to be understood that both the foregoing summary of the invention and the following description of the drawings and detailed description are of a preferred embodiment, and not restrictive of the invention or other alternate embodiments of the invention.
The invention is explained below with reference to the exemplary embodiments and with reference to the accompanying drawings, in which:
The present invention provides a process to produce feedstock materials for semi-solid forming which have a preferred metallurgy, microstructure, and particle shape. Feedstock materials may comprise magnesium alloys such as AZ91D, AM50, or AM60, and may also include currently non-thixomoldable magnesium alloys as well as currently non-thixomoldable non-magnesium alloys including, without limitation, zinc, aluminum, tin-lead and gallium based alloys. Raw materials for producing the feedstock material commonly are in a solid-state, and may include ingots, scrap from die casters or scrap, including scrap from thixomolders such as sprues or runners. The raw material may be contaminated with organic paints, coatings, oils, and other non-metal contaminates that typically volatilize at temperatures around 500° C. The raw material may comprise metal having a dendritic and/or a non-dendritic structure. In a preferred embodiment, at least some of the raw material may be recycled from previous thixoforming operations.
According to one aspect of the invention, a process for producing a feedstock material from raw materials is provided. A preferred process is shown in
According to the preferred embodiment, raw materials in the form of scrap and/or virgin magnesium may be reduced in size by shredding raw materials in, for example, a shear shredding machine to provide particles of raw material. The particles may be of varying size and are generally no larger than the size of the feedthroat of the extruder, which is commonly no more than about 3″ in size, and more commonly no greater than about 1″ in size, and more desirably no greater than about ⅜″ in size. The particles may also comprise more fine particles of about ¼″ or less in size. Generally, the size of the particles is dependant on the equipment used to further process the raw material to feedstock as would apparent to one skilled in the art. A shearing machine may be provided which is capable of reducing raw materials such as scrap, revert and ingots to a uniform, particle size that may be fed into an extruder. In an embodiment that desires to achieve particle size of about ⅜″ or less, a shear shredding machine may, for example, have intermeshing rotary knives spaced apart by about ⅜″ such that incoming raw material can be reduced to particles of about ⅜″ or less in size. According to another embodiment of the present invention, the raw material may be reduced by the size reduction apparatus to particles of about ¼″ in size or less.
Other embodiments may include a different style of size reduction apparatus. For example, two shearing sets may be arranged in series. In this exemplary embodiment, a first set of blades (e.g., rotary knives) may reduce incoming raw material to particles of a first size (e.g., about ½″ or less in size), and a second set of blades may reduce the particles produced by the first set of blades to particles of a second size (e.g., about ⅜″ or less in size). According to yet another embodiment, raw materials may be reduced by a size reduction apparatus that includes two plates (preferably contoured plates) that can be drawn together to crush the raw materials to a desired size. The crushing apparatus may also include a heating system and an inert or protective atmosphere.
Once incoming raw material has been reduced by the shredder or other size reducing apparatus, the particles may be separated based upon physical characteristics of the particles. Common characteristics include size, weight, shape, density, magnetism, or other physical characteristic. Separation may be performed by one or more steps, as desired. For example, the particles may be reduced and separated by a predetermined size, such as about ⅜″ or less. Separation may be employed with magnetized screens to remove magnetic contaminants, such as ferrous contaminants and prevent passage of particles greater than about ⅜″. The magnetic contaminants may have entered during size reduction or have been a part of the raw materials. If magnetizing screens are used to remove magnetic contaminants, the screens are preferably interchangeable because they can be cleaned frequently and provide for maximum efficiency in removing magnetic contaminants.
In the preferred embodiment as shown in
After separation (e.g., through screening or another separation apparatus), raw material being introduced for processing may consist of highly variable shapes and have a wide variety of surface characteristics. These varied raw materials may include pieces that have open cavities, which may be susceptible to oxidation. The tendency of magnesium or other raw material to oxidize at high temperatures can be minimized by maintaining the raw materials in an inert atmosphere. Non-exhaustive examples of gases that can be used to create the intert atmoshphere include the noble gases such as helium, argon and nitrogen, or other gases including SF6. The inert atmosphere may be provided by introducing a constant flow of inert gas, such as argon, on the raw material as it is introduced to the extruder. To minimize the trapping of air in the cavities, it may be desirable to tumble the raw materials and particles over a distance in an inert atmosphere. The tumbling action may allow the trapped air to be released and provides more comprehensive protection against high temperature oxidation. In one embodiment, a plurality of rotating damper-like obstructions near the feed throat area may be used to provide the tumbling action. As the dampers rotate, the added material is forced to tumble and release any air bubbles contained therein. Another embodiment may require no moving parts, but rather may include a sequence of projections from the feed throat wall that also tumbles the raw material or particles that cause the release of air bubbles from the material.
Microstructural and metallurgical refinement of incoming raw material may be performed, at least partially, in an extruder. Extruders contemplated by the present invention may include, for example, twin-screw extruders, single screw extruders, and stationary screw extruders. Twin-screw extruders may be either co-rotating or counter-rotating extruders with either intermeshing or non-intermeshing screws. In terms of design, twin-screw extruder screws may be either identical or complementary. According to one preferred embodiment, the extruder may be a co-rotating intermeshing twin-screw extruder with identical screws. In the exemplary twin and single screw extruders, a screw may be mounted in a given barrel such that the screw is capable of rotary movement relative to the barrel and is suitable for moving raw material along the barrel and eventually forces the extrudate through discharge orifice. Stationary screw extruders may also be used which includes a screw that does not rotate relative to a barrel. In the stationary screw embodiment, a plunger arranged behind the screw relative to the discharge orifice of the extruder may be employed to force material along the screw and, ultimately through the nozzle of the extruder.
Screws and other components used in suitable extruders may optionally comprise a hardening and/or a non-corrosive coating on their outer surface. Due to the high corrosion potential of processing magnesium alloys in their semi-solid state, it is beneficial if extruder components that contact semi-solid magnesium are resistant to corrosion. According to one preferred embodiment of the invention, extruder components may be manufactured from a cobalt based material such as, for example, STELLITE, a cobalt-chromium alloy, or lined with a corrosion resistant material such as, for example, SIALON, a silicon nitride alloy. Further examples of suitable corrosion resistant materials include INCONEL, a nickel-chromium alloy, and HASTELLOY, a nickel-molybdenum alloy, although other corrosion resistant materials apparent to one skilled in the art may be used.
Extruder screws may be designed in a modular fashion such that several elements line up on a shaft to produce screws of desired length. In a preferred embodiment, such elements may include, for example, high pitch forwarding elements, moderate pitch forwarding elements, low pitch forwarding elements, polygonal elements, distributive combers, kneading blocks, and blister rings. In a preferred embodiment an aspect ratio of a screw may be between about 20:1 and 50:1.
Extruder barrels may also be designed in a modular fashion wherein several segments fit together to make up the full extruder length. Barrel segments are typically manufactured in aspect ratios of 4:1, although other aspect ratios are also suitable depending on the bore diameter. Barrel segments may, optionally, be provided with independent thermal control thereby providing for a plurality of longitudinally spaced heating zones. As used herein, the term “thermal control” includes control of cooling and/or heating capabilities. Cooling capabilities within the barrel may be provided by machined water or oil channels. Heating capabilities may include ceramic band heaters or induction heaters. Ceramic band heaters contemplated by the present invention are typically less expensive and easier to procure than induction heaters. Induction heaters, however, provide the advantage of ensuring that incoming alloy is heated to the core of the extruder. According to one embodiment of the invention, a given barrel may comprise a plurality of heaters. Barrels may be manufactured such that the bulk metallurgy of a given barrel is one that would not react with the raw materials, for example semi-solid magnesium. This may be accomplished by providing a barrel lining. Non-exhaustive examples of barrel linings that may resist corrosion include cobalt based metal or ceramic linings. Should a barrel lining be used, the lining and the barrel desirably are closely fitted to provide more efficient heat transfer to the barrel.
In twin screw extruders, barrel segments are normally bored with two intermeshing channels, resulting in a “figure 8” pattern. Typically, the barrel segments are bored through, with no other ports for materials to exit. In some cases, however, the barrel segment may contain an opening for a vacuum port in order to assist in pulling off volatile components during extrusion. Such an arrangement is typically called a “flash zone”.
Other than flash zone areas which may be under vacuum, all stages of the extruder for the incoming raw materials and additional alloying elements as well as the discharge orifice may, optionally and preferably, be in an inert atmosphere. The inert atmosphere may be created by the addition of an inert gas such as, for example, argon, to prevent oxidation of the semi-solid alloy. In order to avoid oxidation, the alloy preferably is not exposed to ambient atmosphere until after the alloy cools to a temperature substantially below its solidus temperature. Preferably, the apparatus for producing the feedstock may be provided with means for maintaining the raw material under a cover gas in a feeder and as the raw material is fed into the extruder as particles. In addition, an inert atmosphere may be provided with means to maintain the extrudate under an inert gas atmosphere as the extrudate exits the discharge orifice of the extruder. For example, a hood arrangement may be used or, alternatively, a tubular ring comprising holes around its circumference may be used to direct streams of an inert gas onto the extrudate. An inert gas atmosphere may serve to protect the extrudate from oxidation and/or may cool the extrudate. Preferably, the means for maintaining an inert gas atmosphere does not impede the flow of extrudate from the discharge orifice of the extruder.
In a preferred embodiment, raw material may be introduced into the extruder at a rate less than or equal to 100% of the capacity of the extruder. The screw segments may be arranged along a shaft in a manner which promotes rapid acceptance of incoming raw material into the extruder near a feed throat and then compression of the raw material as it begins to heat and become more pliable. The incoming raw material may then be exposed to a series of kneading blocks arranged in a forwarding fashion so that dendrites present in the incoming raw material can be sheared and broken. An example of a dendritic microstructure is shown in
The raw material may then be exposed to polygonal elements or high pitched forwarding elements within the extruder in a vacuum vented barrel segment, e.g. a “flash zone,” where paints, oils, and other organic components become volatile and may be removed by a vacuum. This flash zone may be used alone or in conjunction with other flashing of the raw material, for example, before or after reduction of the raw material. A temperature profile along the length of a given extruder may be controlled such that the temperature increase from a feeder to a flash point, decrease after the flash point and then increases again in the direction of the discharge orifice as more clearly shown in
Additional alloying elements can be added via an additional downstream feed throat. Suitable alloying materials will be apparent to one skilled in the art and may include, without limitation, manganese, zinc, silicon, aluminum, chromium, cobalt, lead, platinum, titanium or a combination thereof. The raw material and any additional alloying elements may be compressed and may pass through a series kneading blocks where the alloying elements are dispersed into the raw material. The resulting, newly formed alloy, then may pass through a series of distributive combers where it is mixed more completely under sufficient shear and temperature to maintain a semi-solid extrudate. After this mixing, the alloy may be forwarded by high pitched forwarding elements, compressed by moderate pitched forwarding elements and fully compacted by low pitched forwarding elements which forward it to the discharge orifice. It is beneficial for the alloy to be compressed immediately before the discharge nozzle so that the extrudate is a continuous strand and/or so that oxygen cannot enter and oxidize the alloy. Preferably, a rate of movement of the material through the extruder is substantially independent of a shearing rate of the material.
During extrusion particles of the raw material may, optionally, be introduced into a given extruder at a rate less than the maximum rate for that extruder. Suitable shear rates include rates of about 5 to 5000 reciprocal seconds, with a rate of about 500 to about 2000 being more preferable, and 1000 to 1500 reciprocal seconds being most preferred. Preferably, a rate of movement of raw material along a barrel of a given extruder is substantially independent of the shearing rate of the raw material. Mean residence times of material in the extruder range from, for example, about 15 to about 200 seconds. In embodiments of the invention comprising two intermeshing screws, an exemplary, non-limiting preferable rate of rotation for the screws is about 25 to 500 rpm, and more preferably about 50 to about 250 rpm, and even more preferably about 125 to about 175 rpm.
Additional alloying elements may be added in order to resolve problems such as, for example, high iron content. In this particular example, manganese may be added as an alloying element because it can isolate iron in the microstructure and renew the corrosion resistance of magnesium if it has been lost due to ferrous contamination. Other alloying elements contemplated by the present invention include manganese, zinc, silicon, aluminum, chromium, cobalt, lead, platinum, and titanium. Desirably, an additional alloying element may be added to the process while the element is at ambient temperature or at a temperature greater than an ambient temperature but less than a solidus temperature of a material in the extruder.
At the discharge orifice the material must be kept sufficiently fluid to flow out of the orifice because there is no longer the capability to shear the alloy. One way to keep the material sufficiently fluid is through temperature control. For example, this may be accomplished by maintaining the temperature of the alloy in the discharge orifice at a temperature within plus or minus 5 degrees Kelvin of the intended discharge orifice temperature. The alloy may then be cooled quickly in the presence of an inert gas such as, for example, argon. Optionally, a gear pump constructed of corrosion resistant components may be fitted to the discharge orifice to assist in pulling the solidifying magnesium from the extruder. A die including as many as ten ports or more through which material can exit may optionally be fitted to the discharge orifice.
In the preferred embodiment, the extrudate exits the discharge orifice and/or die(s) generally in the form of a noodle-like strand or a plurality of noodle-like strands. A noodle-like strand typically comprises a circular cross section, which may have a diameter suitable for final application of the feedstock (e.g., about ¼″ or less for the preferred thixomolding operation). Depending on the final application in which the feedstock material may be employed, other cross sections may also be suitable. An apparatus according to the invention may optionally comprise means for controlling the temperature of the extrudate. The temperature may be controlled by, for example heating bands in the orifice, cooling fluid in the orifice, cooling gas emanating from a ring surrounding the orifice and/or the speed at which extrudate is pulled from or pushed out of the orifice. For example, the extrudate, while in noodle form, may be immediately exposed to cool argon gas to begin cooling and lock a preferred microstructure into place.
Preferred microstructures may comprise, for example, a rosette structure, a globular structure and/or an encapsulated alpha phase which may be preferred in a given processing application because it flows more easily as compared to a dendritic structure if employed in the same processing application. A microstructure other than dendritic microstructure may also be preferred because of its physical properties, such as, for example greater tension strength and/or higher elongation as compared to dendritic materials. When the raw materials are exposed to high shear conditions during extrusion and/or just prior to discharge into extrudate from the nozzle, die, or other discharge orifice, the dendritic structures are generally reduced and/or eliminated. The high shear before discharge can also preferably eliminate the formation of dendrites upon cooling the extrudate. Assuming, for the sake of calculation, that materials primarily exhibit laminar flow characteristics and Newtonian flow behavior, suitable shear rates just prior to entering the discharge orifice include, for example, shear rates of about 500 to about 2000 reciprocal seconds. Finite microstructures can be achieved by controlling the take-off and cooling rate from the extruder. The take off and/or cooling rates may be controlled by the speed of extruder screws or a plunger or by take-off via a gear pump. Take-off of the noodle may occur at speeds equal to or greater than the output of the extruder.
At the take-off point, the extrudate may be divided (most often by cutting) into a desired final size by a pelletizer. Under different embodiments, the extrudate exiting the extruder may be divided shortly after exiting or be transported by a conveyor under an inert gas atmosphere before being divided by the pelletizer. In one preferred embodiment, the pelletizer may be a mechanical cutter where the cutting action may be performed by a knife or blade as shown in
Once the pellets are cooled to solidus state, they can be used as feedstock material for molding or other forming applications. The feedstock material may be made from most any combination of alloys as needed by a desired application. As discussed, the feedstock desirably includes a microstructure other than the dendritic microstructure which can be controlled by the shear and temperature during the semi-solid extrusion process. For example, magnesium alloy feedstock was prepared with the above identified process using a stationary screw extruder having sufficient shear to provide a globular microstructure as shown in
Turning now to the drawings,
The foregoing embodiments have been shown for illustrative purposes only and are not intended to limit the scope of the invention which is defined by the claims.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US02/32721 | 10/15/2002 | WO | 12/13/2004 |
| Number | Date | Country | |
|---|---|---|---|
| 60329338 | Oct 2001 | US |
