Abstract:
An extrusion die set includes a charging die and forming die that permit the extrusion of seamless hollow products from a solid billet of aluminum or aluminum alloy on a single cylinder horizontal extrusion press. The charging die imparts a rotational component to the portions of a divided extrusion charge. The portions are combined in a zone of high pressure and further blended in a mixing space having a continuously variable configuration in the extrusion direction. The portions of the extrusion charge are so thoroughly blended that the hollow products have an uninterrupted molecular and crystalline configuration.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a hot extrusion process for aluminum alloy, and more particularly to a method and mold group for producing seamless pipe sections of aluminum alloy using a horizontal single-cylinder extrusion press. 
     2. Description of the Related Art 
     Hot extrusion involves pushing a heated billet of metal through a die. The extruded product may be hollow or solid, and the cross section may vary from a simple round to a complicated shape. Direct extrusion, wherein the metal is forced under pressure through a die opening of the desired cross-sectional area and shape is the most common form of extrusion. The die is typically located in the end of a cylinder opposite a hydraulic ram. 
     The extrusion of products having a hollow cross section is complicated by the necessity to form the internal and external configuration of the part simultaneously. The portion of an extrusion die used to define the interior of a hollow extrusion is typically called a mandrel. The mandrel must be rigidly supported within the die to accurately define the interior configuration of the hollow extrusion. 
     The most common and least expensive type of extrusion equipment is the single-cylinder horizontal extrusion press. In such a press, a heated billet of metal is deposited in a cylinder where a ram forces the hot metal through an extrusion die at one end of the cylinder. Hollow products can be extruded on such a press by using a die in which the mandrel is supported within the die by a rigid connection or connections to an outer portion of the die. Such bridge, porthole and spider-type dies are widely used in the extrusion of hollow shapes from aluminum and aluminum alloy. In these types of dies (all modifications of the same basic construction), the metal billet is divided by the mandrel supports during extrusion and is forced back together as the metal “charge” passes through the remainder of the die and the die extrusion orifice. The metal welds under heat and pressure to form an apparently unitary hollow extruded product. 
     The two components of such a die are illustrated in FIG. 3 (mandrel supports not shown). The resulting extruded product, while appearing seamless in fact includes a weld seam or interruption of the molecular/crystalline structure of the metal wherever the material was forced to divide at a mandrel support. The presence of these imperfections at the molecular level can be confirmed by cutting across a pipe produced by such a method, polishing the cross-sectional surface to a high mirror finish, corroding the pipe in an alkali chemical agent heated to a temperature of 70-80° C. for 10-15 minutes and then cleaning the pipe with water. The weld lines can then be clearly identified and appear in a configuration illustrated in FIG.  1 . 
     A hollow extruded product having such structural imperfections does not have the structural integrity of a true seamless product. Typically, the seamed hollow product will fail along the weld lines or crystal interruptions when exposed to high internal pressure or large compressive loads. Therefore, such products are not widely used in industry in spite of the fact that there is great demand for hollow aluminum products in the manufacturing field. Seamless hollow tubular products, because of their strength and integrity are used to form tubular structural assemblies such as bicycle frames and are also able to withstand internal pressure and have improved corrosion resistance and are therefore in high demand for the aeronautics and food service industries. 
     Several methods are known in the art for producing seamless pipe sections of aluminum and aluminum alloy. FIG. 4 illustrates an apparatus for the centrifugal casting of seamless aluminum pipe sections. In this method, molten aluminum is poured into a mold that is rotated at high speed to form cast-aluminum seamless pipe sections of high integrity. 
     FIG. 5 is a functional block diagram of the equipment necessary for casting round aluminum rods. After the aluminum is melted in an oven  5 - 1 , the melted aluminum passes into an insulated purifying oven  5 - 3  through a channel  5 - 2  where the aluminum is purified in an inert atmosphere. The purified molten aluminum then passes through an outlet  5 - 4  into a facility  5 - 5 ,  5 - 6  for producing solid aluminum rod by a water-cooled cooled continuous casting method. To form a seamless hollow tube, the solid rod must now be machined as is illustrated in FIG.  6 . 
     FIG. 7 illustrates a more advanced method involving the continuous horizontal casting of pipe sections. In this process, after being melted in an oven  7 - 1 , the molten aluminum flows into an insulated purifying oven  7 - 3  and then through a high-pressure water-cooled type graphite mold  7 - 4 . The emerging tubular product is gripped by a tension-slip mechanism  7 - 5  that pulls the emerging product away from the graphite mold. While the tubular product is being pulled away from the mold, it is carefully cooled using high-pressure water. The process takes place from right to left in FIG. 7 with the molten aluminum emerging from the purifying oven  7 - 3  by force of gravity to flow through the graphite mold. 
     In a sophisticated extrusion process, illustrated in FIG. 8, two rams are used. After a first ram (sometimes called a floating mandrel) pierces the billet a second ram pressurizes the now hollow billet and forces the aluminum out of a die where the first ram/mandrel forms the internal configuration of the extruded product. 
     There are several disadvantages to the methods for producing seamless pipe sections of aluminum or aluminum alloy illustrated in FIGS. 4-8. The method of FIG. 4 requires the use of expensive centrifugal casting equipment and is limited to the production of pipe sections having a circular cross section. The method illustrated in FIG.  5  and FIG. 6 involves the expensive step of machining illustrated in FIG.  6  and results in excessive waste, i.e., material removed to form the bore of the tubing. The method of FIG.  5  and FIG. 6 is also limited to producing circular tubing. The method illustrated in FIG. 7 requires the use of expensive and not readily available continuous casting equipment. 
     Additionally, the methods illustrated in FIGS. 4-7 result in a cast-aluminum product in which the molecular structure, surface finish and dimensional tolerances require correction prior to use. Cast pipe sections are typically subjected to cold rolling as illustrated in FIGS. 9 and 10. These processes result in an increased molecular density on the surface of the tubing. It may also be necessary to use mechanical processes to reduce the wall thickness of the cast tubing. 
     Such cold working results in internal stresses in the crystal structure of the cast pipe sections. Therefore, the pipe sections must be heat treated to relieve these internal stresses as illustrated in FIG.  11  and avoid the hairline fractures of the tubing caused by such stresses. The heat treating process, while relieving the internal crystal stresses, typically results in pipe sections which are no longer straight or true. To produce straight pipe sections, each section must now be drawn through a stretching mill as illustrated in FIG.  12 . The pipes are drawn through a tungsten carbide alloy mold with a straight stretching machine from which the seamless pipe sections emerge having a precision configuration which meets manufacturing tolerances. Following stretching, another annealing or heat-treating process must be done. The stretching process may be repeated with new tungsten carbide molds to further alter the configuration of the pipe sections. 
     Each step required to produce acceptable seamless pipe sections of aluminum or aluminum alloy adds to the complexity, expense and production losses of the process. While the method illustrated in FIG. 8 is relatively more advanced, the equipment is expensive, the process consumes a high amount of power and the technical requirements for achieving a tubular product having acceptable concentricity are high. 
     As a result, the prior art methods for producing seamless pipe sections of aluminum or aluminum alloy illustrated in FIGS. 4-12 result in an overall final product rate of not more than 70% (i.e. 30% waste). These factors result in very high production costs for acceptable seamless pipe sections of aluminum or aluminum alloy. 
     SUMMARY OF THE INVENTION 
     An extrusion die set in accordance with the present invention includes a charging die and a forming die configured to mate together and define an axially continuous, substantially tubular charge flow passage. 
     A preferred embodiment of the charging die includes a plurality of substantially identical charge distribution passages symmetrically arranged around an axis. Each charge distribution passage extends from an inlet to an angularly and radially offset outlet. Each charge distribution passage has a continuously variable internal configuration where the cross sectional area of the outlet is larger than the cross sectional area of the inlet. The charging die rigidly supports a mandrel at the mandrel end of an axially extending mixing extension having an opposed charging end. The mixing extension is supported at the charging end by webs of die material defined between said charge distribution passages. The mixing extension tapers from a maximum diameter adjacent the charge distribution passage outlets to a minimum diameter adjacent the integrally supported mandrel. 
     The forming die defines an axial opening configured to receive the mixing extension and mandrel. The internal configuration of the forming die cooperates with the external configuration of the mixing extension and mandrel to define a charge flow passage having several distinct portions. A mixing chamber in communication with the charge distribution outlets is defined by an outside surface and pressure surface within the forming die. An annular exit to the mixing chamber is defined between a pressure shoulder on the pressure surface and the outside or mixing surface of the mixing extension. A mixing space is defined beyond the annular exit in the extrusion direction by the inside surface of a tapering mixing cone inside the forming die and the mixing surface of the axially extending mixing extension. This mixing space has a continuously variable configuration in the extrusion direction due to the differing angular orientations of the inside surface and the mixing surface. The mixing space terminates in an extrusion orifice defined between the mandrel and a forming ring in the forming die. 
     A billet is forced through the die set from the charge distribution passage inlets and eventually through the extrusion orifice in an extrusion direction. A rotational component is imparted to the charge portions as they pass through the arcuate charge distribution passages into the mixing chamber. In the mixing chamber the charge portions are combined by being forced, under high pressure, through the restrictive annular exit. The now substantially combined extrusion charge is further blended in the continuously variable mixing space and finally extruded as a seamless hollow product. 
     An object of the invention is to provide a new and improved extrusion die set and method that permit the extrusion of seamless pipe sections of aluminum and aluminum alloy on a conventional horizontal single-cylinder extrusion press. 
     Another object of the invention is to provide an extrusion die set and method which permit the production of seamless pipe sections of aluminum and aluminum alloy not limited to round pipe cross sections. 
     A further object of the invention is to provide an extrusion die set for use in conjunction with a horizontal single-cylinder extrusion press that blends a divided billet so thoroughly during extrusion that the resulting extruded hollow product can be considered seamless. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a cross sectional view of a tubular product formed by a prior art extrusion process; 
     FIG. 2 is a cross sectional view of a seamless tubular product; 
     FIG. 3 is a sectional view of a prior art extrusion die set; 
     FIG. 4 is a schematic diagram illustrating a prior art centrifugal casting apparatus; 
     FIG. 5 is a schematic diagram illustrating the several steps of a prior art casting process; 
     FIG. 6 is a diagram illustrating the production of a hollow product from a casting produced in FIG. 5; 
     FIG. 7 is a schematic diagram illustrating the several steps of a prior art continuous casting process; 
     FIG. 8 is a schematic diagram, partially cut away, illustrating a prior art extrusion apparatus; 
     FIG. 9 is a partial illustration of a prior art cold rolling apparatus; 
     FIG. 10 is a partial illustration of a prior art cold rolling apparatus; 
     FIG. 11 is a cut away illustration of a prior art annealing furnace; 
     FIG. 12 is a schematic diagram, partially cut away, of a prior art tube stretching apparatus; 
     FIG. 13 is a sectional view through the center of an extrusion die set in accordance with the present invention; 
     FIG. 14 is an end view, partially in phantom, of the extrusion die set of FIG. 13 turned 90° to the left; 
     FIG. 15 is a sectional view through the center of a charging die in accordance with the present invention; 
     FIG. 16 is an end view, partially in phantom, of the charging die of FIG. 15 turned 90° to the left; 
     FIG. 17 is a sectional view through the center of a forming die in accordance with the present invention; 
     FIG. 18 is an end view of the forming die of FIG. 17 turned 90° to the left; 
     FIG. 19 is a sectional view through the center of an alternative extrusion die set in accordance with the present invention; 
     FIG. 20 is an end view, partially in phantom, of the extrusion die set of FIG. 19 turned 90° to the left; and 
     FIG. 21 is an end view, partially in phantom, of the extrusion die set of FIG. 19 turned 90° to the right. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIGS. 13-18 wherein like numerals represent like parts throughout the Figures, an extrusion die set in accordance with the present invention is designated by the numeral  10 . A preferred embodiment of an extrusion die set in accordance with the present invention includes a charging die  1  and a forming die  2  mated together along a peripheral shoulder  6 ,  8 . Axial arrows in FIGS. 13,  15  and  17  indicate a process direction from right to left. The mated die set  10  defines a continuous charge flow passage  7  in the process direction beginning at a plurality of charge distribution passage inlets  4  and extending through an extrusion orifice  40 . 
     As is best seen in FIGS. 14 and 16, the charge flow passage  7  begins at inlet  4  of each charge distribution passage  5 . In the illustrated preferred embodiment, three charge distribution passage inlets  4  are symmetrically arranged around a central axis  9 . Each of the charge distribution passages  5  has the same configuration. Each charge distribution passage  5  extends from an inlet  4  to an angularly offset outlet  4 ′. The internal configuration of each charge distribution passage  5  changes from the symmetrical arcuate oval of the inlet  4  to the asymmetrical pointed ovoid of the outlet  4 ′. The configuration of the outlet  4 ′ defines a larger surface area and, in the case of the preferred embodiment, extends the pointed end of the ovoid angularly in a counterclockwise direction. The resulting charge flow passage  5  has a continuously variable internal configuration and defines an arcuate path projecting counterclockwise around the central axis  9  in the process direction. 
     The charging die  1  also includes a centrally supported mixing extension  22  projecting in the process direction. Webs  26  of die material located between the charge distribution passages  5  support the mixing extension  22 . The charge distribution passages  5  illustrated in the preferred embodiment not only change their shape and angular orientation from inlet  4  to outlet  4 ′, but also project radially outwardly from inlet  4  to outlet  4 ′. Thus, the outside surface of the charging end  23  of the mixing extension  22  projects away from the central axis  9  at angle α. The projecting portion  25  of the mixing extension  22  tapers at a constant rate from its widest point  21  to its narrowest point  26 . The widest point  21  of the mixing extension  22  is located immediately adjacent the outlets  4 ′ of the charge distribution passages  5 . The narrowest point  26  of the mixing extension  22  is located at the junction of the mixing extension and the integrally extending mandrel  24 . The mixing extension  22  tapers at an angle β with respect to the central axis  9 . 
     The charging die  1  is configured to mate with a forming die  2  illustrated in FIGS. 17 and 18. The forming die  2  defines an axial through space having several distinct portions. In the process direction, or from right to left of FIG. 17, a peripheral shoulder  8  is configured to mate with and surround the peripheral shoulder  6  located on the charging die  1 . An outside wall  31  substantially parallel to the central axis  9  and a compression surface  32  partially define a mixing chamber  30 . 
     With reference to FIG. 13, the mixing chamber  30  is further defined by the mixing extension  25  as an annular space in communication with the charge distribution passages outlets  4 ′. The diameter of the mixing chamber  30  is slightly larger than the diameter defined by the outlets  4 ′ of the charge distribution passages  5 . The end of the mixing chamber  30  opposite the charge distribution passage outlets  4 ′ is defined by the compression surface  32  as is best seen in FIGS. 17 and 18. The compression surface  32  extends from the outside surface  31  to a compression shoulder  33 . The compression surface is disposed at angle Φ to a plane perpendicular to the central axis  9 . The annular compression shoulder  33  and the adjacent surface of the mixing extension  25  define an annular exit  37  from the mixing chamber  30  in the process direction. The annular exit  37  has a cross sectional area (measured perpendicular to the axis  9 ) approximately 25% of the cross sectional area of the mixing chamber  30 . 
     The forming die  2  defines a mixing cone  34  extending from the compression shoulder  33  to a forming ring  36 . The inside diameter of the mixing cone  34  tapers at a constant rate from the compression shoulder  33  in the process direction to a forming ring  36 . As a result of this constant taper, the inside surface  38  of the mixing cone  34  is disposed at an angle γ with respect to the central axis  9 . The forming ring  36  defines the external configuration of the extruded product (not illustrated). In the illustrated embodiment, the forming ring  36  is a circular cylindrical surface substantially parallel to the central axis  9 . 
     As is best seen in FIG. 13, angle γ is less than angle β, resulting in a mixing space  39  having constantly variable dimensions in the process direction. The mixing space  39  terminates in an extrusion orifice  40  defined between the mandrel  24  and the forming ring  36 . 
     The configuration of the extrusion die set  10  will now be described with respect to a billet of metal (not illustrated) being forced through the die set in the process direction. In the process direction, the billet will first encounter the three charging die inlets  4 . Pressure provided by the ram of the extrusion press (not illustrated) will force the billet to divide in three parts, each part passing through one of the charge distribution passages  5 . 
     The angularly offset and continuously variable configuration of the charge distribution passages  5  induce a counterclockwise rotational motion in each of the three charge portions. The three charge portions enter the mixing chamber  30  where the extra space provided by the mixing chamber&#39;s enlarged outer diameter permit the three portions of the charge to continue their counterclockwise motion until the mixing chamber is filled. The three charge portions then encounter the angled pressure surface  32  and pressure shoulder  33  where they are forced under high pressure to blend together before passing through the restricted annular exit  37 . The angled orientation of the pressure surface  32  guides the charge portions toward the annular exit  37 . Upon passing through the annular exit  37 , the three distinct charge portions have now been at least partially combined to form a single extrusion charge. 
     The now at least partially blended extrusion charge is forced under pressure along the constantly variable internal configuration of the mixing space  39 . The counterclockwise motion induced by the charge distribution passages  5  acts in concert with the constantly varying internal configuration of the mixing space  39  to continue the blending process. At the end of the mixing space  39  the now thoroughly blended extrusion charge encounters the extrusion orifice  40 , another zone of high pressure. At the extrusion orifice  40  the extrusion charge is forced between the mandrel  24  and the forming ring  36  to form a seamless tubular extrusion in a shape defined by the space between the mandrel  24  and the forming ring  36 . 
     The inventive configuration of the charge flow passage  7  through the die set  10  imparts rotational and angular flow forces to the extrusion charge which combine the once separate charge portions so thoroughly that the extruded product can be considered seamless. Furthermore, since the tubular product is extruded and not cast, all of the crystalline and dimensional defects inherent in cast tubular products are not found in the seamless extruded product. 
     The illustrated die set has a substantially cylindrical outer profile and is internally configured to produce a hollow extruded product having a circular cross section. FIGS. 19-21 illustrate an alternative die set  10 ′ configured to produce a square extruded product. The charging die  1 ′ and forming die  2 ′ are substantially identical to those of die set  10 . The internal configurations and principles of operation are also substantially identical except for the shape of the forming ring  36 ′ and the mandrel  24 ′. These have a square shape and together define a square extrusion orifice  40 ′. 
     While preferred embodiments of the foregoing invention have been set forth for purposes of illustration, the foregoing description should not be deemed a limitation of the invention herein. Accordingly, various modification, adaptations and alternatives may occur to one skilled in the art without departing from the spirit and the scope of the present invention.