Continuous production of large diameter bars for semi-solid forming

FIELD OF THE INVENTION

[0001] The present invention relates to the field of semi-solid forming and, more particularly, to the process and equipment for the production and use of precursor material in the continuous production of large diameter bars for semi-solid forming.

BACKGROUND OF THE INVENTION

[0002] ‘Light’ metals are usually classified as belonging to the families of the following metals: aluminum, magnesium, and titanium (and to a smaller extent, beryllium). Table 1 provides a list of important properties of these three light metals.

1TABLE 1Properties of light metalsPropertyAluminumMagnesiumTitaniumRelative atomic mass (C = 12)272448Melting point (Celsius)6606501675Relative density2.71.74.5Elastic modulus (GPa)7045120Specific modulus262626Abundance on earth's crust (wt %)82.80.9(See Polmear, I. J., Light Alloys, Metallurgy of Light Metals, John Wiley & Sons, 1996, ISBN 0-470-23565-9).

[0003] Light metals have been traditionally used to reduce the weight of structural components in transportation (automotive and aerospace) and architectural industries. It should be noted that over 70% of the magnesium metal production of the world is used to alloy aluminum, steel, and iron.

[0004] Among the above listed three light metals, aluminum is by far the most important in terms of its prevalence, applications, and properties and plays a particularly important role in the transportation industry as shown in Table 2.

2TABLE 2Typical automotive parts made by aluminum (kg finished weight)PartWeight per item (kg)Weight per typical car (kg)Cylinder block16.716.7Cylinder head11.111.1Intake manifold5.65.6Transmission case8.18.1Clutch housing1.81.8Car wheel8.341.5Piston0.83.2Other3.03.0(See Aluminum to 2015, The looming shortage, King James, The Economic Intelligence Unit Research Report, EIU, 15 Regent Street, London SW1Y 4LR, United Kingdom, ISBN 0-85058-990-8, 1997).

[0005] Aluminum alloys can be divided broadly into cast alloys and wrought alloys. It should be noted that the present invention is applicable to both cast and wrought alloys. About 85% of aluminum is used in a wrought form, in products that include plate, sheet, foil (referred to as ‘flats’), extrusions, tube, rod, bar, and wire. The starting material for these wrought forms is cast ingot. The various forming techniques to convert the cast ingot into the wrought form greatly alter the structure of the material. The thermal nature and deformation in the working operation have a profound impact on the structure of the material and its resulting mechanical properties.

[0006] The most important distinction between casting alloys and wrought alloys is the high fluidity of casting alloys which permit the formation of thin walls and complex geometries. It should be noted that high-purity aluminum has low fluidity and inferior mechanical properties. Because of this, various alloying elements are added to pure aluminum, the solubility of which is usually lower in the solid state than in the liquid phase. Addition of alloying elements also normally lowers the melting point of the mixture. The above phenomena cause what is commonly referred to as micro- and macro- ‘segregation’ of the alloying elements in the cast structure.

[0007] Aluminum is high in reactivity among metals. Molten aluminum reacts with oxygen and moisture readily, forming a passive oxide layer which prevents further oxidation. If this adherent oxide layer is disturbed by mixing, pouring, vibration, shock, etc., a fresh oxide layer will immediately form on the newly exposed metal surface. However, this same oxide layer prevents two convergent molten streams to weld or join well. If the alloying elements segregate to the molten metal surface, the composition and properties of the oxide layer will be affected. (See Characterization of metals and alloys, 1993, Paul H. Holloway and P. N. Vaidyanathan, Butterworth-Heinemann, ISBN 0-7506-9246-4).

[0008] Molten aluminum has a high affinity for hydrogen. (See Nonferrous Wire Handbook, vol.2—Bare wire processing, ed in chief Tassi, Otto J., The Wire Association International Inc., Guilford, Conn., 1981, p. 55). Even in liquid aluminum, hydrogen solubility is a strong function of temperature; however, at the melting point, the solubility is 20 to 25 times more in liquid than in solid. The concentration of hydrogen in liquid aluminum is almost a linear relationship, with a slope of 0.006 ml H2 per 0.1 kg aluminum per 1 degree Celsius. The above underscores the need for effective degassing using either inert gases, or a combination of inter and active gases (like chlorine).

[0009] A process known as semi-solid forming is particularly useful when dealing with light metals, and particularly aluminum. The process of semi-solid forming shaped articles utilizing continuous extrusion is described in U.S. Pat. No. 6,120,625 entitled “Processes for Producing Fine Grained Metal Compositions Using Continuous Extrusion For Semi-Solid Forming of Shaped Articles”, the teachings of which are incorporated by reference herein. Briefly, semi-solid metal forming involves forming a metallic alloy at a temperature between its equilibrium liquidus and equilibrium solidus temperatures. It is a hybrid metalworking process combining the elements of both casting and forging/extrusion. One of the key elements for the successful operation of a semi-solid forming process is the microstructure of the metallic alloy being thus formed. By heating the composition above the solidus and below the liquidus, its grain structure transforms to a partially solid, partially liquid mixture comprising of uniform discrete spheroidal particles contained in a lower melting liquid matrix. The heated alloy is then formed and solidified while in a partially solid, partially liquid condition, the solidified article having a uniform, fine grained microstructure.

[0010] The present invention discloses improvements to the process of semi-solid forming.

SUMMARY OF THE INVENTION

[0011] The present invention relates to the process and equipment for the production and use of precursor material for semi-solid formation of shaped articles. More specifically, this invention relates to the production and use of precursor material for the semi-solid formation of light non-ferrous metals. This invention also relates to the production and use of precursor material for the semi-solid formation of non-ferrous copper-based alloys and metal matrix composites (MMC) based on the above metals. The inventive process and equipment allows extrusion of material bars with a negative extrusion ratio for use in semi-solid forming.

[0012] In one aspect, the invention provides for a process for preparing precursor material for semi-solid forming, comprising: adapting a precursor material for manufacture into a shaped article by semi-solid forming; casting a rod of said precursor material; purifying the precursor material rod to remove impurities; and hot rolling the rod into a substantially circular cross section. The step of adapting could comprise alloying a precursor material to a composition suitable for semi-solid forming. The precursor material rod could be wound into coils or reels. The precursor material could be a light metal such as aluminum, magnesium, titanium or an alloy thereof, a non-ferrous copper-based alloy, or a metal matrix composite.

[0013] In another aspect, the invention provides for a process for using precursor material in semi-solid forming, comprising: preparing a precursor material rod; frictionally extruding said precursor material rod to form a precursor material bar; heating said precursor material bar at a selected heating rate to a temperature between the solidus and liquidus temperatures of the material and maintaining the temperature for a specific time wherein the material acquires a microstructure which consists of discrete spheroidal particles suspended in a lower melting liquid matrix.

[0014] In yet another aspect, the invention provides for a continuous extrusion apparatus employed in the integral process for semi-solid forming, comprising: a frictional extrusion source for frictionally extruding a precursor material rod; a transfer chamber in communication with the frictional extrusion source for collecting the frictionally extruded precursor metal from the frictional extrusion source; an accumulation chamber, that is connected to the transfer chamber and which accumulates specific amounts of the frictionally extruded precursor material; and an extrusion die held by a die and chamber holding set and connected with the accumulation chamber, wherein a precursor material bar is extruded from the extrusion die.

[0015] In yet another aspect, the invention provides for an frictionally extruded material bar produced by a process comprising the steps of: preparing a precursor material rod; and frictionally extruding a precursor material rod with a negative extrusion ratio.

BRIEF DESCRIPTION OF THE DRAWING

[0016] The invention is described with reference to the several figures of the drawing, in which:

[0017]
FIG. 1 illustrates one embodiment for the process of precursor material rod production by the continuous casting and hot rolling method;

[0018]
FIG. 2 illustrates another embodiment of the process of precursor material rod production by the continuous casting and hot rolling route emphasizing various purifying and cleaning techniques;

[0019]
FIG. 3 illustrates one embodiment for a cleaning preparation of the precursor rod;

[0020]
FIG. 4 illustrates the process of rod preparation involving nitrogen shrouding, induction heating before rotary extrusion in a reducing or inert atmosphere;

[0021]
FIG. 5 is a schematic illustration of the process of rotary extrusion utilizing an accumulation chamber technique;

[0022]
FIG. 6 shows a plot of mass of a billet versus diameter for billets of increasing length (L);

[0023]
FIGS. 7, 8 and 9 illustrate multiple embodiments of exemplary accumulation chamber designs;

[0024]
FIG. 10 is a schematic cross-sectional side view of a frictional extrusion apparatus according to one embodiment of the invention;

[0025]
FIG. 11 is an enlarged view of the frictional extrusion apparatus showing an accumulation chamber design;

[0026]
FIG. 12 illustrates the results of a tensile test applied to a sample taken from a wheel;

[0027]
FIG. 13 shows the bulk structure of 2″ diameter A356 SANGS™ bar, reheated at 595° C. for 4 minutes and quenched at 100×magnification;

[0028]
FIG. 14 (A, B, and C) shows microstructure evolution of an A356 alloy sample;

[0029]
FIGS. 15 and 16 show heating curves for AA6082 bar;

[0030]
FIG. 17 shows sample No. 63830 after 30 minutes of heating at 638° C.;

[0031]
FIG. 18 shows sample No. 63830 after cleaning and quenching;

[0032]
FIG. 19 shows sample No. 64330 after being heated for 30 minutes at 643° C. and quenched;

[0033]
FIG. 20 shows sample No.530120 after being heated for 2 hours at 530° C. and after quenching;

[0034]
FIG. 21 shows sample No. 530120w, w wrapped in aluminum foil, heated for 2 hours at 530° C. and then quenched.

DETAILED DESCRIPTION OF THE INVENTION

[0035] Definitions

[0036] ‘Casting’ is referred to in this disclosure as the practice of pouring metal into a mold of a desired shape and letting it solidify.

[0037] ‘Drawing stock’ (redraw rod, continuous-cast, or rolled) is referred to in this disclosure as a product which is a solid round product that is long in relation to cross section, produced by continuous casting followed by size-rolling, or by rolling from D.C. cast ingot, suitable for drawing into wire.

[0038] ‘Dross and skimmings’ are referred to in this disclosure as the mixture of oxides and other impurities which float to the surface of molten aluminum and is skimmed off.

[0039] ‘Extruded pipe and tube’ is referred to in this disclosure as a hollow product, formed by extruding, that is long in relation to its cross section, which is round, square, rectangular, hexagonal, octagonal, or elliptical in shape.

[0040] ‘Extruded rod and bar’ is referred to in this disclosure as a solid product, produced by extruding, sometimes brought to final dimensions by drawing, that is long in relation to its cross section, which is round, square, rectangular, hexagonal, or octagonal in shape and having at least one perpendicular distance between parallel faces.

[0041] ‘Extruded shapes’ are referred to in this disclosure as products produced by extruding, that are long in relation to its cross sectional dimensions and has a cross section other than that of rod and bar and pipe and tube.

[0042] ‘Extrusion ingot or billet’ is referred to in this disclosure as a solid or hollow cast form, usually cylindrical, suitable for extruding.

[0043] ‘Forgings’ are referred to in this disclosure as products worked to a predetermined shape by one or more processes such as hammering, upsetting, pressing, etc.

[0044] ‘Ingots’ are referred to in this disclosure as large solid bars of aluminum cast from molten aluminum, its shape being determined by its intended remelting or fabricating use: rolling, forging, extrusion or casting.

[0045] ‘Rod and bar’ is referred to as in this disclosure as a solid, round, square, rectangular, hexagonal, or octagonal in shape produced by continuous casting or rolling, that is long in relation to its cross section, and having at least one perpendicular distance between parallel faces.

[0046] Semi-Solid Precursor Forming

[0047] Semi-solid precursor (S SP) forming refers to any forming technology that heats specially fabricated ‘precursor’ slugs to temperatures below normal casting temperatures for the slug material, and then deforms the said slug to form a net or near-net shape. The slug can be either a light non-ferrous metal such as aluminum, titanium, magnesium or alloys thereof, a copper non-ferrous alloy, or a metal matrix composite (MMC) made of these metals. Usually, the slug is heated to between 40% to 70% volume fraction solid.

[0048] Semi-solid alloys are characterized by unique flow properties due to the fact that their viscosity (in their semi-solid or semi-liquid state) is pseudoplastic (shear rate dependent) and thixotropic (time rate dependent).

[0049] When compared with traditional casting techniques, SSP forming has significant advantages.

[0050] Precursor material for SSP forming can be precisely cut into appropriate lengths, and hence the amount can be very accurately controlled. This precursor material can also be more accurately positioned within the forming machine, and its flow can thus be more carefully regulated and controlled.

[0051] Unlike molten metal front that fills a die in a forming process, which said front is usually turbulent thus entrapping air and other inclusions, the flow front in SSP forming is more uniform and laminar. This results in better quality of the final shape formed by SSP forming techniques.

[0052] Solidification shrinkage in the part being formed is reduced, and it is possible to cast wrought alloys like AA6061 and AA2024. Gas entrapment in the formed part is minimized. Parts can be manufactured in T5 or T6 heat treated condition, unlike parts produced by high-pressure liquid die casting, where heat treatment of the part results in surface blisters and pin holes.

[0053] Because of the very definition of the process of SSP forming, there is reduction of solidification related defects, like hot tearing and shrinkage void formation, in the final formed shape.

[0054] Continuous Extrusion

[0055] The continuous extrusion process, since its invention in 1971, has established a niche in the metals processing industry, especially for copper, lead, and aluminum profiles. This friction actuated process was discovered by Derek Green at the Springfields Nuclear Power development Laboratories of the United Kingdom Atomic Energy Authority. (See U.S. Pat. No. 3,765,216 to Green).

[0056] The basic machinery has a rotating wheel and a stationery shoe overlapping a part of the shoe circumference. The shoe can be held in a tight relationship to the wheel. The wheel can be either one piece or multiple pieces (both two-piece and three-piece shoes are used in construction).

[0057] This extrusion machine takes material feedstock rod at atmospheric pressure and room temperature (though one embodiment of this invention teaches the use of in-line preheated feedstock), passes it continuously through a highly pressurized environment and then out through the extrusion die. Effective use is made of the friction naturally existing between the billet and the ‘container’ in conventional extrusion. In fact, this friction is used as (1) the driving force to feed the stock into the extrusion machine, and (2) as the pressure generator for enabling extrusion.

[0058] One of the challenges is to obtain a ‘continuous’ process, as opposed to conventional extrusion processes. This is possible only if both the feedstock and the ‘container’ are continuous. In practice, the continuous container is accomplished by the use of a rotating endless grooves wheel, with a stationary shoe overlapping a part of the wheel circumference. The groove can be on the face or on the periphery of the wheel, and can be of any shape. In practice, either straight square or radiused grooves are normally used.

[0059] The extrusion container thus formed with the groove walls and the tooling is closed at the die zone end by an ‘abutment’, which is usually mounted on the shoe assembly. The abutment completely fills the cross section of the groove, save a small clearance which causes extrusion of a ‘flash’ through this clearance.

[0060] It should be noted that because of tooling restrictions, the rotary extrusion process is particularly useful for non-ferrous metals, powders, and granules.

[0061] The walls of the groove, together with the tooling supported in the shoe and projecting into the wheel groove, forms an extrusion container. This is closed at the die zone by an abutment, which is usually mounted on the shoe assembly.

[0062] When the extrusion wheel is rotated, the feedstock is pulled by frictional forces between the feedstock and the walls of the groove. If sufficient frictional force is generated, the material of the feedstock yields, and is then plastically deformed or ‘upset’ to fill the groove. This upset usually happens over a short distance in front of the abutment called the grip length. The compressive stress build up finally causes the feedstock material to ‘flow’, causing extrusion to occur through the die orifice.

[0063] Thus the rotary extrusion process offers a versatile technique to obtain large amounts of deformation, considerable changes in shape, and rapid heating in one step. This implies the use of high pressures, and means of controlling the generation and containment of this pressure. Pressures of the order of 1000 N/mm2 and temperatures as high as 600° C. are generated in this ‘pressure vessel’. This also means that a large amount of energy dissipation has to occur over a short distance. The heat (deformation) energy thus generated is imparted to the wheel, tooling (shoe and die assembly). Effective cooling is therefore needed to prevent overheating of parts of the machine and the product.

[0064] Illustrations showing the maximum stress, temperature and strain points in a continuous extrusion process of AA606 1 bar can be found in the following reference: Lu, J., Saluja, N., Riviere A. L., Zhou, Y., Computer modeling of the continuous forming extrusion process of AA606 1 alloy, Journal of Materials Processing Technology, vol. 79, 1998, pp 200-212. From these illustrations, it can be seen that stress, temperature and strain rise rapidly in the grip length zone. The temperature and strain maxima closely match in location which conforms to expected results.

[0065] The continuous nature of the process allows the elimination of the end discards in conventional extrusion, resulting in a product with excellent surface finish and dimensional tolerances. Change of product shape and size entails simply a change of die, making the process flexible, versatile, and adaptive.

[0066] Product cooling thus becomes very important due to the following: (1) high temperatures may cause hot shortness, leading to very high flow stresses and hence process disruption (2) it has a direct bearing on machine throughput, (3) high temperatures may result in excessive grain growth, oxidation, and other contamination, (4) they result in shorter tooling life.

[0067] Parts of the tooling that are overheated may have to be fabricated with high temperature alloys like Inconel and other P/M refractory based tools (for example tungsten carbide-cobalt inserts). This is necessary in order to ensure high-temperature strength.

[0068] Referring now to the figures of the drawing, the figures constitute a part of this specification and illustrate exemplary embodiments to the invention. It is to be understood that in some instances various aspects of the invention may be shown exaggerated or enlarged to facilitate an understanding of the invention.

[0069]
FIG. 1 illustrates one embodiment for the process of preparing precursor material rod by continuous casting and hot rolling. Liquid metal of high purity (for example, at least 99.7% pure) is introduced into an alloying furnace in which the liquid metal is alloyed 6 to a composition suitable for semi-solid forming. Alternatively material in the form of ingots or sows having low Fe and low Cr may be melted 4 and then introduced into the alloying furnace and alloyed 6. The material is then subjected to the flowing steps: introduction into a holding furnace 8 to remove dross and inclusions; degassing 12 to remove hydrogen, other gases, light inclusions and dross; grain modification/refining/fluxing 20; filtering 22; casting 24; scalping 26; hot rolling 30; in-line thermal treatment and quenching 32; and coiling 34. This process prepares a drawing stock of precursor material rod suitable for manufacture into a shaped article by extrusion and the processes associated with semi-solid forming.

[0070]
FIG. 2 illustrates another embodiment of the process of precursor material rod production by continuous casting and hot rolling emphasizing various purifying techniques. In this embodiment, there is a laundering step 10 after the holding furnace step to further remove truss, two degassing steps 12 and 14, a chlorine treatment step 16, another laundering step 18, and a second in-line heat treatment and quenching step 28 between the scalping 26 and hot rolling step 30.

[0071] In multiple embodiments, the precursor material rod can have dimensions ranging anywhere from 7 mm in diameter to 40 mm in diameter, and preferably 15 to 32 mm.

[0072] Of particular importance in the bar extrusion process is the purity and cleanliness of the precursor rod material. The precursor rod should be substantially free of internal impurities such as inclusions or contaminants, and should be substantially free of external impurities such as oil emulsions on the surface that are introduced rod making or storage, transportation and handling. Additionally, after manufacture of the precursor rod, it should be substantially free from cracks which may can contain contaminants even after a surface cleaning process. FIG. 3 illustrates one embodiment for a cleaning preparation of the precursor rod. The rod may optionally be inserted through a shaving die 36 in which a scalping process may be performed on the rod to remove any segregated phases, oxides and accumulated debris from the rod top and sides. The rod could then undergo a hot water rinse and air wipe 38. Various subsequent cleaning techniques are then possible including: chemical cleaning with NaOH/HNO3 40; dry machining 41; shot blasting 42; electrochemical cleaning 43; ultrasonic cleaning 44; and electromagnetic induced abrasive cleaning 45. The rod may further undergo a final rinse and wipe 46 and a vacuum bagging 48 in case it needs to be stored or transported.

[0073]
FIG. 4 illustrates the process of rod preparation involving nitrogen shrouding 50, and induction heating 52 before rotary extrusion in a reducing or inert atmosphere 54.

[0074] The process of rotary extrusion has been adapted for production of extrudates of larger cross sectional dimension than the feedstock (producing a negative extrusion ratio) with the use of an accumulation chamber technique. This is schematically depicted in FIG. 5. After rotary extrusion 54, the extruded rod may then undergo expansion 56 (and potentially contraction and landing (flat) stages) in an accumulation chamber to produce an extruded material bar, mixing 58 with the use of mixing plates or “mixers”, in-site quenching 60, outside quenching, 62, sawing 64 and stacking 66. The extruded bar is then subject to heating, delivering and semi-solid forming processes 68 to produce a shaped article.

[0075]
FIG. 6 shows a plot of mass of a billet versus diameter for billets of increasing length (L). As billet diameter increases, the bar translation rate slows considerably and it becomes incrementally more difficult to make a bar of larger diameter.

[0076] The shape of the accumulation chamber is of particular importance in the design of a continuous extrusion apparatus. The following summarizes what factors are considered when designing the accumulation chamber (in no particular order).

[0077] One consideration is to minimize dead zones. These are zones where the metal has very little motion and is not being frequently replenished. The lack of motion and replenishment translates to a longer residence time, which in turn increase the chance of oxidation of the metal (due to the high temperatures).

[0078] Another consideration is chamber length. This has a dual effect. First, a gradual rate of change of cross section of the metal (expansion), would mean use of a accumulation chamber of large enough length to accommodate that expansion. Second, the chamber needs to be sufficiently long to build up sufficient back pressure in order to fill all the interstices, voids, and gaps to form a uniformly filled cross section.

[0079] A third consideration is the shape of the chamber. This has to allow for gradual change, and non-sharp corners which can act as both stress concentrators and temperature concentrators.

[0080] A fourth consideration is material flow. The best way for material to flow is with a laminar and plane flow front, with minimum of intermixing, and eddies. The shape should promote such a flow. Also, the flow should be as axisymmetric as possible. (It has been noted that when the material enters the accumulation chamber, it tends to go upward. This can be countered by placing the chamber slightly off-centered.)

[0081] A fifth consideration is to minimize pick up of gases and inclusions. The flow of the metal and the shape of the die should not allow layers of metal to flow into swirls and entrap gases and impurities during the transit.

[0082] A sixth consideration is modular design. Rather than have to change the whole accumulation chamber, it should be constructed in modules, so that some parameters or some parts can be changed quickly and inexpensively, for example just the shape of the cone can be changed, or just the landing distance can be changed. Also, this allows for quick and inexpensive change of the diameter of bars being produced.

[0083] A seventh consideration is the prevention of ‘layering’. It should avoid sudden changes in velocity of the metal in various layers, which can cause a ‘layering’ phenomenon to develop in the cross section of bar.

[0084] The accumulation chamber should allow good compaction of metal, especially in the center of the bar. This can be achieved by (a) utilizing the right length of the chamber, (b) carefully controlling the temperature of the metal (controlled cooling, quenching, and the like), and (c) by introducing ‘contraction’ steps in between two expansion steps or one expansion and one landing. By landing, we mean a flat section of the chamber (neither expansion nor contraction).

[0085]
FIGS. 7, 8 and 9 illustrate multiple embodiments of exemplary accumulation chamber designs. When the material is forced out of the first orifice 78, it enters a transfer chamber 80 that abuts the first orifice 78. After the transfer chamber 80, the material enters the accumulation chamber 81, where the material is continuously fed from a rotary extrusion machine, and undergoes gradual modification to achieve the final dimensions. From the accumulation chamber 81 the metal passes through an extrusion die orifice 83 to form a finished product. It should be noted that one could have multiple die orifices both entering and the transfer chamber 80 and exiting the accumulation chamber 81. The accumulation chamber 81 could constitute an array of expansion, contraction, or landing (flat) chambers depending upon the particular application. In one embodiment, the resulting size of the bar as controlled by the accumulation chamber 81 and extrusion die orifice 83 is 1 inch to 6 inches in diameter.

[0086]
FIG. 10 is a schematic cross-sectional side view of a frictional extrusion apparatus according to one embodiment of the invention. FIG. 11 is an enlarged view of the frictional extrusion apparatus showing an accumulation chamber design. The frictional extrusion apparatus 70 includes a frictional extrusion source 71, such as a continuous rotary extrusion device, which extrudes precursor material rod. The frictional extrusion source 71 is in communication with a transfer chamber 80 which collects a frictionally extruded material 79 through the first orifice 78. An exit extrusion die is held by a die and chamber holding set 82 and has a orifice 83 in communication with an accumulation chamber 81, such that the material 79 is pushed by an extrusion pressure generated from the frictional extrusion source 71 through the transfer chamber 80 and the accumulation chamber 81 and exiting through the orifice 83 of the exit extrusion die. The accumulation chamber 81 is connected to the exit extrusion die orifice 83 and transfer chamber 80 and helps controls the dimension and structure of the extruded precursor material bar 85. In one embodiment the accumulation chamber 81 is adapted to frictional extrude the metal with a negative extrusion ratio. The accumulation chamber 81 may comprise an expansion section wherein the frictionally extruded precursor metal is extruded to a dimension which is expanded from that of the precursor material rod.

[0087] Alternatively, the accumulation chamber 81 may comprise a contraction section wherein the frictionally extruded precursor material is extruded to a reduced dimension. The accumulation chamber 81 may also comprise a landing or flat section wherein the extrusion dimension remains unchanged. In other embodiments, the accumulation chamber 81 may comprise multiple chambers of expansion, contraction, landing or any combination thereof.

[0088] The frictional extrusion source 71 suitable for use in the present invention has a rotatable wheel 72 having a circumferential endless groove 73 therein. The groove 73 is engaged with a shoe member 74 having an abutment 76 which intrudes into the groove 73, thereby blocking the whole of passageway 77 which is defined by the groove 73 and shoe member 74. A first orifice 78 is positioned near the abutment 76 for release of the frictionally extruded material 79. The first orifice 78 can be situated in the shoe so that the frictionally extruded metal 79 is extruded either radially or tangentially from the wheel 72.

[0089] In operation according to the present invention, the wheel 72 is rotated in the direction indicated by arrow 84. A precursor material rod 75 moves forward into passageway 77 where it meets abutment 76. The frictional drag on the precursor material rod 75 creates sufficient frictional pressure to extrude the feed material through the first orifice 78. Usually the deformation resulting from friction extrusion contains a large fraction of shear strain. The extrusion apparatus 70 may have one or more passageways 77. The feedstock of the continuous extrusion process can be a solid, powder or granular, or molten metal. In the case of a solid metal feedstock, the continuously fed precursor material rod 75 is first subjected to a small amount of cold working when being dragged in the passageway 77 by the friction. The metal then usually is subjected to a large amount of warm and/or hot extrusion deformation when it is extruded by the frictional pressure through the first orifice 78. By the frictional pressure, the frictionally extruded metal is then continuously pushed through the transfer chamber 80, expanded or contracted in the accumulation chamber 81, and exits out of the extrusion die orifice 83 to obtain a precursor material bar 85 of deformed fine-grain solid metal composition suitable for semi-solid forming.

[0090] In other embodiments (see FIG. 5), the invention also provides means for on-line sawing or shearing the extruded precursor material bar to slugs of required length; means for heating the slugs at a selected heating rate to a temperature between the solidus and liquidus temperatures of the material; means for delivering the extruded and heated slugs; and means for semi-solid forming the extruded and heated slugs to shaped articles, e.g. a high pressure die casting or forging apparatus, which processes are described in U.S. Pat. No. 6,120,625.

[0091] When heating the extruded precursor material bar at a selected heating rate to a temperature between the solidus and liquidus temperatures of the material, the temperature is maintained for a specific time to achieve a deformed fine grain microstructure. In one embodiment, the amount of deformation can result in a deformed fine grain structure having a grain size less than 30 μm and a subgrain size less than 2 μm in the frictionally extruded precursor material. A deformed fine grain microstructure is desired for the deformed material to be used in semi-solid forming, since the structure can be transformed into a microstructure which comprises spheroidal particles uniformly distributed in the lower melting liquid when the material is reheated to a temperature between the solidus and liquidus temperatures of the material. This is because there is distortion energy stored in the deformed fine grain microstructure. Deformation energy stored in the deformed material promotes the microstructure transformation by increasing the diffusion rate of the low melting element to grain and subgrain boundaries, resulting in quick melting near grain or subgrain boundaries when the deformed material is quickly heated to a semi-solid temperature. The stored distortion energy also induces recrystallization when reheating the deformed and quenched material to above the recrystallization temperature of the material. The greater the stored distortion energy, the more recrystallized nuclei can be obtained, leading to finer spheroidal particles when the deformed material is quickly heated to a semi-solid temperature.

[0092] Material that is extruded through the clearance between the abutment and the groove is called ‘flash’. Excessive flash results in wastage of feedstock, whereas too little flash implies that the clearance may be dangerously low (potentially resulting in abutment damage), or that a large amount of surface impurities may be getting into the product. Usual flash is in ribbon or flake form. Flash compactors may be used to compress the flash into manageable bales. Flash measurement involves careful weighing of the flash, and comparing it to the feedstock and product throughput rates.

EXAMPLE 1

[0093] Table 3 shows the chemical compositions for aluminum alloys 356, 357 and 6061 processed by the invention.

3TABLE 3Chemical Compositions for aluminum alloys 356, 357 and 6061OTHERElements [%]ELEMENTSAlloySiFeCuMnMgCrNiZnSnTiEachTotalAlA356.26.5-0.120.10.050.3-—0.05—0.20.050.15rem7.50.45A357.26.5-0.120.10.050.45-—0.05—0.04-0.03(p)0.1rem7.50.70.260610.4-0.70.15-0.150.8-0.04-—0.25—0.150.050.15rem0.80.41.20.35(p)0.04-0.07% Be.

[0094] Table 4 shows the processing conditions for the semi-solid forming of a wheel using an alloy of A356.2.

4TABLE 4Processing conditions for semi-solid forming of wheelPlunger speed˜0.5 m/sMetal front speed1 m/sHeating time of slugs (total, from room temperature)20 minutesNo. of slugs heated4Heating time to go from solidus to casting7-8 minutestemperature (˜585 C.)No. of robots in use2Plunger pressure1500 barRating on each heating station80 kWPower rating on heater1470 kVAEnergy needed to heat 1 kg of aluminum alloy to the˜0.7 kWhrcasting temperatureCycle time to make wheel˜5 minutesAlloy usedA 356.2Slug sizeD: 4″L: 400 mm

[0095] It should be noted that the choice of alloys is based on factors other than fatigue life. The fatigue strength of a material varies with the alloy, but the concentration of stresses on the joints generally varies inconsequentially with different alloys of aluminum. (See Sharpe, Maurice L., Nordmark, Glenn E., and Menzemer, Craig C., Fatigue design of aluminum components and structures, Mac Graw Hill, p. 16, ISBN 0-07-056970-3, 1996). The more important aspect is making the sections that carry the maximum load and stress concentration thick enough. It should also be noted that the fatigue strengths of forgings, plate, sheet, and extrusions of any specific alloy are generally about the same, but usually larger than a casting of the same alloy because of porosities in the latter.

[0096]
FIG. 12 illustrates the results of a tensile test applied to a sample taken from a central portion of the wheel. The UTS was found to be 310 MPa, Yield (0.2% offset) was 29 MPa, and the Elongation (A5 type) was 9.2%.

[0097]
FIG. 13 is a micrograph at 100× magnification of the bulk microstructure of 2″ diameter A356 SANGS™ bar, reheated at 595° C. for 4 minutes and quenched.

[0098]
FIG. 14 (A, B, and C) shows microstructure evolution of an A356 alloy samples. FIG. 14A is the microstructure of a conform-extruded 2″ billet with severely deformed dendrites. FIG. 14B is the microstructure of a rod reheated in a resistance furnace for 140 minutes at 590° C. FIG. 14C is the microstructure of a semi-solid structure of 5″ billet after reheating 5 minutes at 595° C. showing a fine grain microstructure with average grain size of approximately 50 to 100 μm comprising discrete spheroidal grains uniformly distributed.

EXAMPLE 2

[0099] Continuous production of AA6082 bar of diameter 33.8 mm was performed in July 2001 with samples of dimension Ø 33.8×398 mm undergoing rotary frictional extrusion.

[0100] The following tests were performed on the bars:

[0101] (1) Cut 9 samples of 033.8×34 mm for heating. Cut some samples “F” (as fabricated) status, for microscopy. (2) Hardness testing on two samples as above in “F” status.

[0102] The heating patterns for the 9 samples are shown in Table 5:

5TABLE 5Heating patterns for samplesHOLDINGSECONDARYTEMPERATURETIMELABEL[° C.][MIN.]OBSERVATION638056385638156381563830638306430564356431564315643306433064330w64330wrapped in Al foil530120530120530120w530120wrapped in Al foil

[0103] All 9 samples were quenched in water within 15 seconds of taking them out from the furnace. No aging applied after the quenching. Hardness testing performed right after quenching.

[0104] Test Results

[0105] Nine samples were cut as explained above. Hardness tests were performed on two samples in “as fabricated” (“F”) status, on their cross section; results are in Table 6:

6TABLE 6Hardness test resultsSECONDARYPLACE OFVALUECONVERSIONLABELTESTHRETO HB*63830center5473638304 mm from edge48.568.363815center52.5; 51.5; 52.171; 71.4; 71.7638154 mm from edge47.6; 52; 47.5;67.6; 71.3; 67.5;51; 46.8; 47.270.5; 66.9; 67.2*Based on a conversion chart for light alloys, non-mathematical.

[0106] These results indicate a lower hardness on the side and higher hardness toward the center.

[0107] Applying Heating Patterns

[0108] The furnace used is a high precision lab furnace, ATS type, model 3210, three heating zones controlled by three separated PID controllers. One sample per heating session has been used. Only exception 538120 with 538120w together. Each sample held two thermocouples installed in the same hole Ø 3×16 mm drilled in the center of the cross section. One thermocouple was linked to a National Instruments data acquisition system, while the other thermocouple was linked to a temperature an Omega calibrator, model CL23A for detecting the error for each heating. All thermocouples have been calibrated before at 575° C. and each of them had their known error. We assumed that this error didn't change for temperatures like 530, 638 and 643° C.

[0109] In these conditions the temperature reading error has been limited down to about ±1° C. Heating curves are presented as FIGS. 15 and 16 for heating at 638° C. and holding times of 5 and 15 minutes respectively.

[0110] 64330 and 530120 samples were completely wrapped in Al foil to see if there is any important diffusion/contamination during the heating process.

CONCLUSIONS

[0111] (1) Samples are clear of defects after heating and quenching.

[0112] (2) At 638° C., with a holding of 30 min., a collapsed region has been observed on the topside of the sample. Same features are observed for all heating patterns at 643° C. Collapse process observed in the center, when the bar starts to ‘liquefy’.

[0113]
FIG. 17 shows sample No. 63830 after 30 minutes of heating at 638° C. The lower level in the center is showing that the material started collapsing 100. The figure illustrates the position of thermocouples 110 in all samples.

[0114]
FIG. 18 shows sample No. 63830 after cleaning and quenching.

[0115]
FIG. 19 shows sample No. 64330 after being heated for 30 minutes at 643° C. and quenched. The top center of the bar is seen to start to “liquefy”.

[0116]
FIG. 20 shows sample No. 530120 after being heated for 2 hours at 530° C. and after quenching. Zero surface defects are seen.

[0117]
FIG. 21 shows sample No. 530120w. The sample was wrapped in aluminum foil, heated for 2 hours at 530° C. and then quenched. No defects are visible on its surface.

[0118] In other embodiments, this invention may be applied not only to extruded rod and bar but also extruded pipe and tube as well as other extruded shapes.

[0119] 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. It is intended that the specification and examples be considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims.

Continuous production of large diameter bars for semi-solid forming

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