Patent Publication Number: US-2015075242-A1

Title: Friction-stir extruders and friction-stir extrusion processes

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 61/879,397, filed on Sep. 18, 2013, the disclosure of which is incorporated in its entirety by reference herein. 
    
    
     BACKGROUND 
     Metal extruded products such as tubes are widely used for various applications in both structural and pressure flow applications. Aluminum tubes produced by conventional extrusion processes are a popular material for scaffolding, medical devices, structural framing, bicycle frames, and heat exchangers. Drawn aluminum tubes are widely used for various applications in both structural and pressure flow applications. Similarly, seamless extruded tubes are also widely used for various applications in both structural and pressure flow applications. 
     The use of aluminum tubes in heat exchangers is typically limited to low temperature and cryogenic applications, such as processing liquid natural gas (LNG). However, aluminum tubes have been used in seawater service applications such as desalination with moderate to good success. In addition, aluminum tubes have been tested for decades as a candidate material for ocean thermal energy conversion (OTEC) heat exchangers. OTEC is a method for generating electricity based on the temperature difference that exists between deep water and shallow water of a large body of water, such as an ocean, sea, gulf, or large deep lake. An OTEC system utilizes a heat engine, i.e., a thermodynamic device or system that generates electricity based on a temperature differential, which is thermally coupled between relatively warmer shallow water and relatively colder deep water. 
     Even though aluminum is a good selection from a cost perspective, the poor resistance to corrosive seawater can result in a lower service life than titanium or stainless steel alternatives. However, aluminum tubes produced with conventional extrusion processes have only found limited usages in heat exchanger applications with seawater service. Corrosion testing reveals that conventionally extruded aluminum alloys can exhibit severe pitting corrosion after two to three years of exposure to seawater. The aluminum samples in the surface seawater corrosion tests exhibited much less pitting occurrences with substantially less maximum depth of pits, relative to the aluminum samples in deep seawater. Deep seawater may be pulled from a depth of approximately 1,000 meters and can cause accelerated pitting corrosion in aluminum tubes because the deep seawater has less dissolved oxygen (DO) and a lower pH than surface seawater. The lower values of DO and pH tend to prevent the natural aluminum oxide layer from reforming to stop growth of initiated pits, as well as prevent new pits from forming. Since deep seawater is generally used in the OTEC thermodynamic cycle, this corrosion phenomenon can affect conventionally extruded tubes. 
     SUMMARY 
     Aspects of the disclosure can include a friction-stir mandrel having a textured end portion integral with a body portion. The textured end portion is configured to friction-stir process a starting material that is forced across the textured end portion and through a die in a plasticized state to form a pipe. 
     Embodiments include a method of forming a pipe, having the steps of forcing a starting material across a textured end of a mandrel and through a die in a plasticized state, so that the textured end of the mandrel breaks up existing grains of the starting material. The method also includes the step of forming the pipe from material forced through the die. The formed pipe has smaller resultant grains on an interior surface than the existing grains of the starting material. 
     Embodiments include a porthole die friction-stir extrusion method, having the steps of loading a feedstock billet into a container, and abutting one end of the feedstock billet with a ram and abutting another end of the feedstock billet against a die mandrel. The method also includes rotating the feedstock billet and the container against a die cap while pressure is applied by the ram. The method also includes extruding plasticized feedstock through passages of the die mandrel. Grains of the plasticized feedstock are broken up by a textured mandrel tip of the die mandrel. The method also includes forming a hollow tube from the extruded plasticized feedstock. 
     Embodiments include a seamless tube friction-stir extrusion method, having the steps of loading a feedstock billet into a container, and abutting one end of the feedstock billet with a ram and a concentrically-located mandrel, and abutting another end of the feedstock billet against a die. The method may also include piercing through the feedstock billet with the concentrically-located mandrel up to the die, and applying pressure to the feedstock billet by the ram. The method also includes extruding plasticized feedstock through the die and over a textured portion of the concentrically-located mandrel. Grains of the plasticized feedstock are broken up by the textured portion of the concentrically-located mandrel. The method also includes forming a seamless tube from the extruded plasticized feedstock. 
     Embodiments include a tube friction-stir drawing method, having the steps of loading a first end of a tube work piece into a die tool and tool carrier of a container, and inserting a mandrel tool at a second end of the tube work piece. The method also includes engaging a gripper at the first end of the tube work piece, and drawing a textured portion of the mandrel tool inside the die tool by the gripper while continuously drawing the tube work piece over the textured portion. Grains of the drawn tube work piece are broken up by the textured portion of the mandrel tool. The method also includes forming a drawn tube of smaller diameter and thinner wall thickness. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various exemplary embodiments will be described in detail with reference to the following figures, wherein: 
         FIG. 1  is a schematic diagram of an OTEC power generation system according to one embodiment; 
         FIG. 2  is a cross-sectional view of a heat exchanger according to one embodiment; 
         FIGS. 3A-3B  are illustrations of porthole die extrusion systems according to one embodiment; 
         FIGS. 4A-4D  are illustrations of a porthole die extrusion process according to one embodiment; 
         FIGS. 5A-5C  are illustrations of a porthole friction-extrusion system according to one embodiment; 
         FIG. 6  is an illustration of an extrusion mandrel and die according to one embodiment; 
         FIGS. 7A-7C  are illustrations of an indirect friction-extrusion system according to one embodiment; 
         FIGS. 8A-8D  are illustrations of an integral hollow punch and die cap, and a decoupled die mandrel according to one embodiment; 
         FIG. 9  illustrates a seamless tube extruder and extrusion process according to one embodiment; 
         FIGS. 10A-10D  illustrate a seamless tube extrusion process in detail according to one embodiment; 
         FIGS. 11A-11F  illustrate a friction-extruded seamless tube process according to one embodiment; 
         FIG. 12  illustrates an active end of a mandrel tool according to one embodiment; 
         FIG. 13  illustrates a produced tubing according to one embodiment; 
         FIG. 14  illustrates a rotating mandrel tool and a non-rotating bearing according to one embodiment; 
         FIG. 15  illustrates a mandrel with a threaded extension and cylindrical screw cap according to one embodiment; 
         FIGS. 16A-16C  illustrate a two-piece mandrel according to one embodiment; 
         FIGS. 17A-17C  illustrate an indirect friction-extrusion method for seamless tubes according to one embodiment; 
         FIG. 18  illustrates a tube-drawing extrusion process according to one embodiment; 
         FIGS. 19A-19D  illustrate a tube-drawing extrusion process in detail according to one embodiment; 
         FIGS. 20A-20C  illustrate a friction-extrusion tube drawing process according to one embodiment; 
         FIGS. 21A-21C  illustrate a rotating mandrel and a non-rotational container according to one embodiment; 
         FIG. 22  illustrates a rotating container and a non-rotational mandrel according to one embodiment; 
         FIG. 23  is a flowchart showing an exemplary porthole die extrusion method according to one embodiment; 
         FIG. 24  is a flowchart showing an exemplary seamless tube extrusion method according to one embodiment; 
         FIG. 25  is a flowchart showing an exemplary tube drawing method according to one embodiment; and 
         FIG. 26  is a flowchart showing an exemplary pipe forming method according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Aluminum tubes can be used in heat exchangers, such as those used in an ocean thermal energy conversion (OTEC) operation.  FIG. 1  is a schematic diagram of an exemplary OTEC power generation system according to one embodiment. However, other OTEC power generation systems can be used with embodiments described herein. As shown, the OTEC system  100  can include an offshore platform  102 , a turbo-generator  104 , a closed-loop conduit  106 , an evaporator  110 - 1 , a condenser  110 - 2 , a hull  112 , multiple pumps  114 ,  116 , and  124 , and multiple conduits  120 ,  122 ,  128 , and  130 . 
     Offshore platform  102  is a tension leg offshore platform, which has buoyant hull  112 , and also includes a deck, caissons, and pontoons. The hull  112  is supported above seabed  136  by rigid tension legs that are anchored to the seabed  136  at deployment location  134 . For clarity, the deck, caisson, pontoons, and tension legs are not illustrated in  FIG. 1 . 
     In some embodiments, offshore platform  102  is deployed at a deployment location in a body of water other than an ocean (e.g., a lake, sea, etc.). In some embodiments, offshore platform  102  is an offshore platform other than a tension leg offshore platform, such as a semi-submersible, spar, drill ship, jack-up offshore platform, grazing plant, or the like. Other offshore platform types are contemplated by embodiments described herein. 
     Turbo-generator  104  is a turbine-driven generator mounted on hull  112 . Turbo-generator  104  generates electrical energy in response to a flow of fluid and provides the generated electrical energy on output cable  138 . Closed-loop conduit  106  is a conduit for conveying working fluid  108  through evaporator  110 - 1 , condenser  110 - 2 , and turbo-generator  104 . 
     Evaporator  110 - 1  is a shell-and-tube heat exchanger that is configured to transfer heat from warm seawater in surface region  118  and working fluid  108 , thereby inducing the working fluid  108  to vaporize. Condenser  110 - 2  is a shell-and-tube heat exchanger that is configured to transfer heat from vaporized working fluid  108  to cold seawater from deep-water region  126 , thereby inducing condensation of vaporized working fluid  108  back into liquid form. Evaporator  110 - 1  and condenser  110 - 2  are mechanically and fluidically coupled with offshore platform  102 . 
     Turbo-generator  104 , closed-loop conduit  106 , evaporator  110 - 1 , and condenser  110 - 2  collectively form a Rankine-cycle engine that generates electrical energy based on the difference in the temperature of water in surface region  118  and the temperature of water in deep-water region  126 . In operation, pump  114  pumps working fluid  108  in liquid form through closed-loop conduit  106  to evaporator  110 - 1 . Ammonia is an example of a working fluid  108  that can be used in OTEC systems. However, other fluids that evaporate at the temperature of the water in surface region  118  and condense at the temperature of the water in deep-water region  126  can be used as working fluid  108 , and are contemplated by embodiments described herein. 
     Pump  116  draws warm seawater from surface region  118  into evaporator  110 - 1  via conduit  120 . In some OTEC deployments, the water in surface region  118  is at a substantially constant temperature of approximately 25 degrees centigrade (subject to weather and sunlight conditions). At evaporator  110 - 1 , heat from the warm water is absorbed by working fluid  108 , which induces the working fluid  108  to vaporize. After passing through evaporator  110 - 1 , the now slightly cooler water is ejected back into ocean  140  via conduit  122 . The output of conduit  122  is usually located deeper in ocean  140  than surface region  118  to avoid decreasing the average water temperature in the surface region  118 . 
     The expanded working fluid  108  vapor is forced through turbo-generator  104 , thereby driving the turbo-generator  104  to generate electrical energy. The generated electrical energy is provided on output cable  138 . After passing through turbo-generator  104 , the vaporized working fluid  108  enters condenser  110 - 2 . 
     Pump  124  draws cold seawater from deep-water region  126  into condenser  110 - 2  via conduit  128 . Deep-water region  126  can be approximately 1000 meters below the surface of the body of water, at which depth water is at a substantially constant temperature of a few degrees centigrade. The cold water travels through condenser  110 - 2 , where it absorbs heat from the vaporized working fluid  108 . As a result, working fluid  108  condenses back into liquid form. After passing through condenser  110 - 2 , the now slightly warmer water is ejected into ocean  140  via conduit  130 . The output of conduit  130  is usually located at a shallower depth in ocean  140  than that of deep-water region  126  to avoid increasing the average water temperature in the deep-water region  126 . Pump  114  pumps the condensed working fluid  108  back into evaporator  110 - 1  where it is vaporized again, thereby continuing the Rankine cycle that drives turbo-generator  104 . 
       FIG. 2  is a cross-sectional view of a shell-and-tube heat exchanger according to an embodiment described herein. An exemplary heat exchanger includes a shell  202 , a primary fluid inlet  204 , an input manifold  206 , an output manifold  208 , a primary fluid outlet  210 , a secondary fluid inlet  212 , a secondary fluid outlet  214 , and multiple tubes  216 , tube plates  220 , and baffles  224 . Heat exchanger  110  enables efficient heat transfer between a primary fluid that flows through tubes  216  and a secondary fluid that flows through shell  202 , such that the secondary fluid flows across the outer surface of each of the tubes  216 . With reference to  FIG. 1 , the primary fluid is working fluid  108  and the secondary fluid is seawater. Shell  202  is a housing that includes a material suitable for long-term exposure to seawater. Shell  202  and tube plates  220  collectively define a flow vessel for conveying seawater from secondary fluid inlet  212  to secondary fluid outlet  214 . 
     Working fluid  108  is conveyed to each of tubes  216  by primary fluid inlet  204  and input manifold  206 . In similar fashion, working fluid  108  is collected from each of tubes  216  at output manifold  208  and provided to primary fluid outlet  210 . Primary fluid inlet  204  and primary fluid outlet  210  are fluidically coupled with closed-circuit conduit  106 , such that heat exchanger  110  forms part of the closed-circuit conduit. 
     Seawater is provided to shell  202  at secondary fluid inlet  212 . In evaporator  110 - 1 , secondary fluid inlet  212  is fluidically coupled with conduit  120 . In condenser  110 - 2 , secondary fluid inlet  212  is fluidically coupled with conduit  128 . Seawater exits shell  202  through secondary fluid outlet  214 . In evaporator  110 - 1 , secondary fluid outlet  214  is fluidically coupled with conduit  122 . In condenser  110 - 2 , secondary fluid inlet  214  is fluidically coupled with conduit  130 .  FIGS. 1 and 2  depict secondary fluid inlet  212  and secondary fluid outlet  214  on the same side of the heat exchanger. However, secondary fluid inlet  212  and secondary fluid outlet  214  can be located on opposite sides of the heat exchanger to facilitate efficient heat transfer between the primary and secondary fluids. 
     In one embodiment, each of the tubes  216  is a conduit of aluminum alloy having length, inner diameter, and tube wall thickness that are selected for efficient thermal coupling between seawater and working fluid  108 . A shell-and-tube heat exchanger suitable for a modern OTEC system can include five to six thousand tubes having a length of up to thirty feet. Each of the tube plates  220  is a mechanically rigid circular plate of aluminum alloy having a plurality of holes  218 . Each end of the tubes  216  is joined to a different one of the tube plates  220  at holes  218  to collectively define a tubesheet  222 . 
     Baffles  224  can be transverse baffles that induce a transverse component to the flow of seawater through the heat exchanger. In some embodiments, baffles  224  also provide support for the tubes  216  in the region between the tube plates  220 . Baffles  224  include a plurality of through-holes for the tubes  216 . The number and placement of baffles  224  is a matter of heat exchanger design, and one skilled in the art would recognize that any practical number of baffles  224  can be included in the heat exchanger. Tube plates  220  and baffles  224  hold the tubes in an arrangement that facilitates heat transfer between seawater flowing along the outer surfaces of the tubes  216  and working fluid  108  that flows through the tubes  216 . 
     One method of forming metal tubes, including aluminum tubes is a porthole die extrusion process.  FIG. 3A  is an illustration of a direct porthole die extrusion system in which a die mandrel  310  is coupled with a die cap  320 , and feedstock material  330  such as aluminum material is forced through the die mandrel  310  and through the die cap  320  to form a tubular finished product  340 . The top drawing of  FIG. 3B  illustrates a direct extrusion process in which a ram  350  pushes against a billet  360  to force the billet material through a stationary die  370  to form an extruded hollow tube  380 . The bottom drawing of  FIG. 3B  illustrates an indirect extrusion process in which a hollow punch  390  with an integral die  395  presses against the billet inside a container. The extruded billet material  380  is forced through the orifice within the hollow punch  390 . 
       FIGS. 4A-4D  illustrate a porthole die extrusion process in more detail. A container  410  provides structural support for the dynamic process. A die mandrel  420  and a coupled die cap  430  are inserted into the container  410 . A die carrier  440  and an aluminum billet  450  are inserted into the die carrier  440  in  FIG. 413 . A press disc  460  is butted against the back side of the aluminum billet  450 . In  FIG. 4C , a ram  470  is butted against the press disc  460 .  FIG. 4D  illustrates the tube extrusion  480  as the ram  470  forces the aluminum billet material  450  through the die cap  430  and die mandrel  420  to form a hollow tube. During the extrusion process, the die mandrel  420  and die cap  430  remain stationary while the feedstock material is forced through the die mandrel bridge and separated into four distinct material flow paths. The plasticized material re-welds in the die cap  430  and forms a tubular shape over the die mandrel  420 . 
     One short-coming of the porthole die extrusion process described with reference to  FIGS. 3A-4D  is the finished product has poor resistance to pitting corrosion in a saltwater environment, such as that of an OTEC system. A major cause for the poor corrosion resistance is the presence of a large grain surface. In addition, the products of this extrusion process have low mechanical properties, such as bending, fatigue, and fracture toughness. 
     Incorporating friction extrusion tools and processes break down the original grains of feedstock metal into fine grains. Most or all of the precipitates are dissolved back into the base metal, resulting in extruded products having very fine equiaxed grains and much cleaner grain boundaries with fewer and smaller precipitates on the tube inside surface of the extrusions. The grains are also equiaxed in the direction of extrusion, whereby any cross-section of a friction-extruded tube will show a homogenous grain size. Friction-extruded tubes can still be heat treated after extruding, such as aging to improve mechanical properties like tensile strength, as well as improve corrosion resistance. Friction-extruded products also exhibit better mechanical properties, and therefore have a much longer service life, as compared to conventionally extruded products. 
       FIG. 5A  illustrates an exterior view of a direct friction-extrusion system  500 . A container  510  holds the die mandrel and die cap in place during the extrusion. A motor  520  and a pulley and drive belt system  530  power the extrusion process, which exerts a force against the feedstock metal via a ram  540 . A metal tube will be extruded to the right side of the figure. 
       FIG. 5B  is a cross-sectional view of the friction-extrusion system  500  during the extrusion process. In embodiments described herein, the die mandrel  550  is decoupled from the die cap  560 . The die mandrel  550  and cylinder  570  containing the feedstock  580  (illustrated by the right-pointed arrow) spin together at a high rotational speed, while the die cap  560  remains non-rotational. Frictional heating and considerable force from the ram  540  results in severe plastic deformation of the feedstock  580 . 
     As the plasticized feedstock  580  enters the stationary die cap  560 , the material flowing through the die mandrel  550  is frictionally-processed in the weld chamber  590  when the material comes in contact with the features of the stationary die cap  560 , as illustrated in  FIG. 5C . As a result, the frictionally processed material flows over the male piece of the die mandrel  550  to form an extruded tube  595  with very fine grains.  FIG. 5C  illustrates the extruded tube  595 , produced by a rotating container  570  and feedstock  580 , as well as a rotating die mandrel  550 . However, the die cap  560  is fixed, i.e., not rotating. The decoupled die mandrel  550  and die cap  560  provide a fine-grained surface on the inner surface of the extruded tube  595 . 
     In some embodiments illustrated in  FIG. 6 , the die cap has a conical feature  610  to allow for less pressure and enhanced material flow characteristics in the weld zone. In other embodiments, the die mandrel tip has a textured surface  620  to assist with shearing through material grains and to refine the microstructure for round shape extrusions. The textured surface  620  includes, but is not limited to surfaces containing features, such as threads, ridges, studs, and protrusions. The tip of the mandrel can have a plain cylindrical feature for forming a smooth diameter tube. In still other embodiments, a mandrel bearing  630  can promote a smooth interior finish on the resultant extruded tube. The bearing is held in place with a bearing nut  640  that is attached to the die mandrel, and does not affect the forming of the tube. During operation, the textured die mandrel  620  and feedstock  650  rotate, but the mandrel bearing  630  does not rotate because it has a rotational degree of freedom from the die mandrel and is held stationary by the extrusion forces applied to the bearing exterior. 
       FIGS. 7A-7C  illustrate an indirect friction-extrusion system, according to embodiments described herein.  FIG. 7A  is a cross-sectional view of a setup assembly of an indirect friction-extrusion system in which a container  710  and billet  720  rotate, while die mandrel  730  and die cap  740  remain non-rotational.  FIG. 7B  is a close-up view showing material flowing through the die mandrel  730  to form a tube  750 . The hollow punch  740  is integral with the die cap and is coupled with the die mandrel  730  using threaded fasteners  760 .  FIG. 7C  is an isometric view showing a container  710  moving towards a stationary hollow punch  740  to produce a length of tubing  750 . 
     Two different embodiments are described for an indirect friction-extrusion system. In the first embodiment, the die cap and integral hollow punch rotate, while the container remains non-rotational. The hollow punch pushes the die mandrel and die cap into the billet towards the stationary container. In the second embodiment, the rotating container and billet push the billet against a stationary die mandrel, die cap, and hollow punch. As the container is pushed with ram force, the plasticized aluminum billet is forced through the die mandrel and out the die cap, through the hollow punch aperture as a finished tube. The second embodiment is illustrated in  FIGS. 7A-7C .  FIGS. 7A-7C  also illustrate a hollow punch that is integral with the die cap and is coupled with the die mandrel as a single integral assembly. 
     Another embodiment includes a hollow punch that is integral with the die cap, but is decoupled with the die mandrel and is separated by a thrust bearing, with reference to  FIGS. 8A-8D .  FIG. 8A  illustrates a container  810  with a hex boss  820 , which is configured to receive a hex-shaped billet  830 . Any other polygonal-shaped boss and billet combination is contemplated by embodiments described herein.  FIG. 8B  illustrates that the container  810  and enclosed billet  830 , as well as the die mandrel  840  are rotating, while the die cap and integral hollow punch  850  are stationary. The integrated hollow punch and die cap  850  are coupled with the die mandrel  840  using threaded fasteners, wherein the thrust bearing  860  rotates with the container  810  and billet  830 .  FIG. 8C  illustrates the plasticized billet material is forced through openings in the die cap  850 . The roughened surface of the rotating die mandrel  840  breaks up large grains of billet material  830 . As a result, the extruded tube  870  contains fine-grain material on the interior surface of the tube.  FIG. 8D  illustrates the length of tubing  870  produced as the container and billet material move towards the stationary hollow punch. 
     Some embodiments include removing the male mandrel portion and using a die cap designed with a non-circular geometric shape, such as a square, hexagon, or other polygonal shape. Some embodiments include non-circular geometric shapes that also have a non-circular hollow. The initial circular hollow can be formed into a non-circular shape, such as a square, hexagon, or other polygonal shape through the use of a secondary die. 
       FIGS. 3A-8D  illustrate embodiments for porthole die extruders and porthole die extrusion processes. Embodiments for seamless extruders and seamless extrusion processes are described herein under. 
       FIG. 9  illustrates a seamless tube extruder  900  and extrusion process. A billet  910  is pierced with a mandrel  920 , while pressure is applied to the billet material. The extruded material forms a hollow seamless tube  930  over the mandrel  920 . All of the components illustrated in  FIG. 9  are stationary or only allowed to move in one axis, i.e. towards the die and away from the die. Seamless extruded tubes can be produced in this manner by either a direct process (a moving ram forced against the billet material) or an indirect process (a die is forced against a stationary billet). 
       FIGS. 10A-10D  illustrate the seamless tube extrusion process in detail. A container  1010  receives a die tool  1020  and tool holder  1030  in  FIG. 10A . A press disc  1040  is pressed against a back end of a billet  1050 , and a ram  1060  is positioned against the press disc  1040 . The press disc  1040 , billet  1050 , and ram  1060  are positioned inside the carrier  1010 , as illustrated in  FIG. 10B . A mandrel  1070  pierces the billet  1050 , as illustrated in  FIG. 10C . The ram  1060  continues to press against the billet  1050  to force billet material through the die  1020  and over the surface of the mandrel  1070 , as illustrated in  FIG. 10D . The ram  1060  continues to press against the billet  1050  until the billet material  1050  has been extruded, to form a resultant hollow tube  1080 . The press disc  1040 , scrap billet, and tube  1080  are removed from the container  1010  at the conclusion of the processing. 
     One short-coming of the seamless tube extrusion process described with reference to  FIGS. 10A-10D  is the resultant tubes, particularly aluminum tubes have poor resistance to pitting corrosion, especially in a saltwater environment such as OTEC heat exchangers. Incorporating frictional heating and extensive plastic deformation into a seamless tube extrusion process improves the strength and corrosion resistance of seamless tubing. During friction extrusion, the original grains of the feedstock metal are broken down into fine grains. In addition, most or all of the precipitates are dissolved back into the base metal. The resultant extruded products have very fine grains and much cleaner grain boundaries, as well as fewer and smaller precipitates inside of the tubing. This resultant microstructure exhibits better mechanical properties and much better resistance to corrosive environments, such as seawater. As a result, the service life of the tubing is greatly extended. 
       FIGS. 11A-11F  illustrate how friction processing is incorporated into the seamless extrusion process described above.  FIG. 11A  illustrates that a rotating mandrel  1110  is used against a non-rotating ram  1120 , press disc  1130 , container  1140 , die  1150 , and die carrier  1160 . A lateral force presses the ram  1120  against the billet  1170  as the mandrel  1110  rotates into the billet  1170 , as illustrated in  FIG. 11B . The rotation of the mandrel  1110  stirs the billet  1170  near the die  1150  opening, as illustrated in  FIG. 11C . This refines the grains of the feedstock material before it is formed into a tube. The mandrel  1110  spins at a high rotational speed while all other components remain in a non-rotational state. Frictional heating, as well as a high force from the ram results in severe plastic deformation of the feedstock. As the plasticized feedstock is pressed against the back face of the die  1150 , the material grain structure is broken up as a result of shearing forces from the mandrel  1110 . The ram  1120  continues to press against the press disc  1130  and billet  1170  to extrude a resultant seamless tube  1180 , as illustrated in  FIG. 11D . 
     The frictionally processed material flows through a mandrel tip  1190  to form an extruded tube with a smooth interior finish and very fine grains, as illustrated in  FIG. 11E . In addition, the mandrel can have textured features  1195 , including but not limited to threads, ridges, studs, and protrusions that assist with breaking up grains and causing the material to flow towards the die opening, as illustrated in  FIG. 11F . 
       FIG. 12  illustrates an exemplary active end of a mandrel  1200 . A textured or featured surface  1210  assists with breaking up the large grains in the billet material. Any textured or featured surface that breaks down the grains of the material can be used, including but not limited to a threaded surface, a ridged surface, a studded surface, or other protrusions on an end portion of the mandrel  1200 . The smooth tip  1220  provides a smooth finish on the interior surface of the tubing. The smooth tip also minimizes the amount of excess billet material extruded at end of the resultant tubing. This prevents the excess extruded billet material from forming along the interior walls of the resultant tubing. In addition, the smooth tip  1220  can prevent or reduce a shearing zone on the interior surface near the end of the resultant tubing, caused by the rotating threaded region  1210  of the mandrel  1200 . The mandrel  1200  described herein and illustrated in  FIG. 12  produces continuously extruded tubing, such as twenty to fifty foot length tubing with a fine grain structure. The threaded features  1210  of the mandrel  1200  effectively break up the grain structure in the tube wall and reconsolidate the material to produce a refined grain structure. The seamless friction extrusion process continuously pushes the billet out of the die, as illustrated in  FIG. 13  to produce a fine grained interior surface  1310  along the entire length of the tubing  1320 . 
     In some embodiments as illustrated in  FIG. 14 , the rotating mandrel  1400  is designed with a bearing  1410  on the tip, such that the bearing remains non-rotating as the feedstock flows over the bearing. The inside diameter profile of the tubing is last formed by the mandrel tip  1420 . 
     There may be instances in which an extrusion force is very large, which will lower or completely prevent the bearing from rotating. This can cause an overly preferential interior finish. In order to account for this or counter this effect, the mandrel  1510  has a textured extension  1520  to allow a cylindrical screw cap  1530  to tighten against the bearing  1540  to keep it in place, as illustrated in  FIG. 15 . The bearing  1540  and cylindrical screw cap  1530  are designed with a high strength material to allow piercing of the billet. 
     In some embodiments, the mandrel is a two-piece assembly with one piece rotating and the other piece non-rotating, as illustrated in  FIGS. 16A-16C . The two-piece mandrel  1600  works similar to a retractable pin tool. A pin portion  1610  of the mandrel forms the inside diameter of a tube, while a pin tool shoulder portion  1620  of the mandrel stirs the billet  1630 , as illustrated in  FIG. 16A . The mandrel shoulder portion  1620  can be coupled with a gear or pulley to rotate independent of the mandrel pin portion  1610 . The mandrel pin portion  1610  is kept stationary so that it does not rotate with the mandrel shoulder portion  1620 , as illustrated in  FIG. 16B . This allows the extruded material to form over a stationary mandrel to produce a better surface finish.  FIG. 16C  illustrates the billet extruding over the mandrel pin portion  1610  to form an extruded tube  1640 . 
     An indirect extrusion method can also be used to produce seamless friction extruded tubes with reference to  FIGS. 17A-17C .  FIG. 17A  is a cross-sectional view illustrating a container  1710  holding a billet  1720 . A press disc  1730  and a ram  1740  with a mandrel are butted against one end of the billet  1720 , and a die  1750  and hollow punch  1760  are coupled with the billet  1720  at the other end.  FIG. 17B  illustrates a mandrel  1770  piercing the billet  1720 . In  FIG. 17C , the hollow punch  1760  presses against the billet  1720  to form a continuous extruded tube  1780 . As illustrated in  FIGS. 17A-17C , the hollow punch  1760  pushes against the die  1750  and billet  1720  while the ram  1740  remains stationary in an indirect extrusion method. In addition, the mandrel  1770  retracts as the container punch presses forward to maintain the positioning near the die  1750  in an indirect extrusion process. As the hollow punch  1760  applies force against the die  1750  and billet  1720 , the tube  1780  is extruded over the mandrel  1770  and out through the hollow punch  1760 . 
     In some embodiments of the indirect extrusion method, the die and feedstock billet are heated before the extrusion process begins. In other embodiments, the die and feedstock billet require minimal heating or no heating prior to the extrusion process because frictional heat is generated in the weld chamber. Temperatures of approximately 700-800 degrees F. are needed for aluminum or an aluminum alloy metal to reach a moldable viscosity. 
     Both the direct and the indirect seamless extrusion processes can be implemented with the mandrel tool described above to produce seamless friction-extruded tubing. For example, the mandrel illustrated in  FIGS. 17B-17C  can be rotating while the hollow punch presses against the billet, which would break up the large grains normally present in a seamless extruded tube. In addition, the tip of the mandrel can be threaded to further break up the large grains and produce a fine grained interior finish on the tubing. 
     In some embodiments, the feedstock material can be a billet containing recycled metal, such as machining chips, powder, or scrap. The feedstock is capped with a solid metal cylinder with a hole through the center, which matches the outer diameter of the mandrel tool. Since the ram action pushes the semi-loose metal chips, scrap, and/or powder through the die mandrel without sufficient heating, the metal washers are set on the top and bottom of the feedstock billet to allow sufficient heating of the feedstock before the plasticized material is allowed to enter the weld chamber. The washer on the top of the billet presses against the ram and prevents metal from extruding past the ram in the opposite direction of the die. 
     Another process related to extrusion of tubing is tube drawing. A tube drawing process is usually performed as a secondary operation after a tube has been seamless extruded, porthole die extruded, or electric resistance welded (ERW). The starting work piece can be oversized and drawn down to a smaller diameter and a smaller wall thickness, as illustrated generally in  FIG. 18 . 
     The tube-drawing process is illustrated in detail in  FIGS. 19A-19D . A work piece  1910  is installed against a die  1920  and tool carrier  1930 , as illustrated in  FIG. 19A . A mandrel  1940  is inserted into the back end of the work piece  1910 , as illustrated in  FIG. 19B . The mandrel  1940  is pushed forward until it is located concentrically within the opening of the die  1920 . A gripper  1950  is inserted into the tube from the front end until it is positioned behind the die  1920  and tool carrier  1930 , also illustrated in  FIG. 19B . The gripper  1950  has an expanding mandrel on the end of its rod that will tighten onto the inside of the tube wall, illustrated in  FIG. 19C . The gripper  1950  grips with enough force to pull the tube through the die  1920  and out of the tool carrier  1930 . The gripper  1950  continues to pull the work piece  1910  through the die  1920  and over the mandrel  1940  to reform the tube diameter and wall thickness to its final dimensions, illustrated in  FIG. 19D . All of the components are either stationary or are allowed to move in just one axis, i.e. towards or away from the die  1920 . A tube drawing process can use either a direct process or an indirect process. 
     A modification of the above-described drawing process incorporates friction extrusion into the tube work piece during the drawing process to produce a fine grain interior surface of the drawn tubes.  FIG. 20A  illustrates an exterior view of the container  2010  with a mandrel  2020  inserted into the left-side view of the work piece  2030  and a gripper  2040  inserted into the right-side of the work piece  2030 .  FIG. 20A  illustrates the mandrel  2020  is rotating, while the container  2010  remains stationary. 
       FIG. 20B  is an interior view of the mandrel  2020  end section in near vicinity to the gripper  2040  end section within the work piece tubing. The mandrel  2020  end has a tapered cap design, with a threaded configuration adjacent to the tapered cap. As the mandrel  2020  is rotated, the threads break up the large grains of the interior surface of the work piece. As a result, small fine grains are formed on the interior surface of the reworked work piece. 
       FIG. 20C  illustrates the gripper  2040  pulling on the work piece  2030 . The mandrel  2020  is lodged between the upper and lower sections of the die  2050 , which forces the work piece  2030  to be thinned at the exit point of the tool carrier. As a result, the tube is extended in length and the tube thickness is reduced. In an example, a ten-foot length original work piece can be used to form a thirty-foot length finished tube. The tapered cap of the mandrel tool produces a smooth interior finish. 
     In other embodiments, a textured mandrel  2110  is rotated while pulled from one end of a tube work piece located inside of a stationary container  2120  completely out through the opposite end of the tube work piece, as illustrated in  FIG. 21A . The mandrel  2110  is rotated while it is pulled through the tube work piece.  FIG. 21B  illustrates a textured region  2115  of the mandrel  2110 , which breaks up large grains of the original tube work piece as it is pulled from one end of the work piece to the other end. Fine grains result on the interior surface of the drawn tubing. The mandrel  2110  has a smaller textured portion on one end that is used to rotate the mandrel  2110  and pull the mandrel  2110  through the tube work piece. The non-textured portion of the mandrel  2110  will be the same or almost the same dimension as the inside diameter of the tube. The textured portion of the mandrel  2110  has features, such as threads, ridges, studs, or protrusions that are slightly larger in diameter than the non-textured portion, such that the threads, ridges, studs, or protrusions engage and stir the tube wall without penetrating through the tube wall into the container, as illustrated in  FIG. 21C . 
     The container can be split into two halves and bolted or clamped together, such that the resultant drawn friction-extruded tube can be easily removed. In addition, the smaller diameter sections of the mandrel shaft can be supported with bearings and/or linear bearings that stabilize the mandrel along the length that extends beyond the container. The bearings help control run-out of the mandrel at significant distances away from the rotary motion source, such as a motor or spindle. 
       FIG. 22  illustrates an alternative embodiment in which the container  2210  rotates, while the mandrel  2220  remains non-rotational. The container can have integral features, such as pulley drives that allow a belt drive to rotate the container. The drive mechanism includes, but is not limited to a geared motor or a hydraulic motor. Since the tube needs to rotate with the container, the tube ends are expanded into each end of the container using a mechanical or hydraulic expander tool. Alternatively, the tube work piece can be held in place by a gripper mechanism on each end of the tube, or the tube work piece can be gripped or secured in such a way that the container is no longer required. The tube can be easily removed from the container after it has been expanded and friction-drawn within the container, since the two halves are bolted together. 
     For a substantially long mandrel tool, the shaft can have a hex feature or other torque-driving feature that allows the use of shaft guides along the tube length to assist with transmitting torque, which is applied to the mandrel tool from the spinning container and the tube. The shaft guides can be fixed to a grounded structure and have a matched hex or other torque-driving feature that allows the shaft to move in only one linear direction. 
     The end of the mandrel tool has a textured end and a smooth end cap, as previously described. Therefore, the textured surface of the mandrel tool breaks up the large grains on the interior surface of the original tube work piece. The textured surface of the mandrel includes, but is not limited to features, such as a threaded surface or a surface containing ridges, studs, or other protrusions. The interior of the resultant drawn tube has small grains and a smooth surface. 
     Conventionally-drawn tubing has nominal grain sizes similar to rolled plates and frequently has very large grains on the interior surface. As a result, the tubing has a low resistance to corrosive environments, especially on a large grain surface. The tubing also has low mechanical properties, pertaining to bendability, fatigue, and fracture toughness. 
     By implementing friction extruding and stir welding processes described herein, the inside surface of the tube is treated to produce a fine grain microstructure, which has significant corrosion advantages over conventionally-drawn tubing. It has a high resistance to corrosive environments on the ends and the interior surface. Mechanical properties, such as bendability, fatigue resistance, and fracture toughness are increased when embodiments described herein are practiced. 
     Feedstock material includes, but is not limited to aluminum and aluminum alloys, titanium and titanium alloys, steels and steel alloys including stainless steels, copper and copper alloys, and super alloys containing nickel, molybdenum, chromium, and cobalt. Some embodiments include heating the dies and feedstock billet before the extrusion process begins. However, other embodiments require minimal or no heating prior to the extrusion process because adequate frictional heat is generated within the weld chamber. Still other embodiments include using a billet of recycled metal scrap, machining chips, or powder. 
     One embodiment includes using titanium feedstock chips or powder to form tubing according to embodiments previously described herein. Conventional titanium processing and stainless steel processing are quite costly. However, titanium and stainless steels formed from a billet of scrap metal or powder metal according to the porthole die friction-extruded tube and the seamless friction-extruded tube methods described herein can provide a much more economical mode of tube manufacturing for titanium and stainless steel tubing. 
     Some embodiments include incorporating metal matrix composite particles, such as aluminum oxides, silicon carbides, and boron carbides, as well as carbon nano-particles into a composite billet in conjunction with embodiments described herein for porthole die friction-extruded tubes, seamless friction-extruded tubes, and drawn friction-extruded tubes. The carbon nano-particles can be mixed with a metal feedstock, such as aluminum to form a matrix nano-composite billet. The friction-extrusion mandrels and processes described herein provide smaller finer grains on the interior surface of the tubing. The nano-particles improve the mechanical and metallurgical properties of the tubing for a higher strength-to-weight ratio and high temperature resistance to allow for higher operating temperatures. As a result, the carbon nano-particle matrix friction-extrusion tubing can be extended to conditions comparable to titanium tubing, but at a cost of that for aluminum tubing. In addition, friction extruding enables mass production of the nanocomposite tubing. 
     Embodiments described herein provide corrosion-resistant tubing that can be used in a saltwater environment, such as in OTEC heat exchangers. Another embodiment includes a thermal desalination system and method in which seawater is flash evaporated off the exterior of the heat exchanger tubes. Fresh water is condensed on the inside of the tubes. 
     Embodiments described herein for porthole die friction-stir extruded tubes, seamless friction-stir extruded tubes, and friction-stir drawn tubes provide advantages of a very fine grain size on the interior surfaces of the tubes, high resistance to corrosive environments, both on the surface and the interior of the tubes, and high mechanical properties such as bending, fatigue, and fracture toughness. These advantages are realized by a friction-stir mandrel tool, which includes a textured end portion that is integral with a body portion. The textured end portion is configured to friction-stir process a starting material forced across the textured end portion and through a die in a plasticized state to form a pipe. The textured end portion includes, but is not limited to features, such as threads, ridges, studs, or protrusions. The starting material can include a metal, such as aluminum or an aluminum alloy. 
     The friction-stir mandrel tool can be configured to rotate while the starting material remains rotationally stationary. Likewise, the mandrel tool can be configured to remain rotationally stationary while the starting material rotates. The mandrel tool can also have a smooth cap formed over an end of the textured end portion, wherein the smooth cap is configured to provide a final smooth interior surface on the formed pipe. A diameter of the textured end portion is slightly larger than an inside diameter of the formed pipe, and smaller than an outside diameter of the formed pipe. In some embodiments, the mandrel tool is integral with the die. In other embodiments, the mandrel tool is configured to pierce through the starting material. In a tube-drawn process, the mandrel tool is configured to be drawn into the die in conjunction with drawing the starting material over the mandrel tool. 
       FIG. 23  is a flowchart showing an exemplary porthole die friction-stir extrusion method  2300 . A feedstock billet is loaded into a container in step S 2310 . One end of the feedstock billet is abutted with a ram, and another end of the feedstock billet is abutted against a die mandrel in step S 2320 . The feedstock billet and the container are rotated against a die cap while pressure is applied by the ram in step S 2330 . Plasticized feedstock is extruded through passages of the die mandrel in step S 2340 . Grains of the plasticized feedstock are broken up by a textured mandrel tip of the die mandrel. A hollow tube is formed from the extruded plasticized feedstock in step S 2350 . In some embodiments, the die mandrel rotates while the feedstock billet and the container rotate. In other embodiments, an interior surface of the extruded hollow tube is smoothed by a mandrel bearing attached to an end of the textured mandrel tip. In still other embodiments, the plasticized feedstock is extruded through a hollow punch aperture integrally formed with the die mandrel. The plasticized feedstock can be extruded through a rotating hollow punch aperture. 
       FIG. 24  is a flowchart showing an exemplary seamless tube friction-stir extrusion method  2400 . A feedstock billet is loaded into a container in step S 2410 . One end of the feedstock billet is abutted with a ram and a concentrically-located mandrel. Another end of the feedstock billet is abutted against a die in step S 2420 . The feedstock billet is pierced with the concentrically-located mandrel up to the die in step S 2430 . Pressure is applied to the feedstock billet by the ram in step  2440 . Plasticized feedstock is extruded through the die and over a textured portion of the concentrically-located mandrel in step S 2450 . Grains of the plasticized feedstock are broken up by the textured portion of the concentrically-located mandrel. A seamless tube is formed from the extruded plasticized feedstock in step S 2460 . In some embodiments, the concentrically-located mandrel is rotated during the extruding. In other embodiments, a recrystallized microstructure is formed in an interior wall of the seamless tube. 
       FIG. 25  is a flowchart showing an exemplary tube friction-stir drawing method  2500 . A first end of a tube work piece is loaded into a die tool and tool carrier of a container in step S 2510 . A mandrel tool is inserted at a second end of the tube work piece in step S 2520 . The first end of the tube work piece is engaged in step S 2530 . A textured portion of the mandrel tool is drawn inside the die tool while the tube work piece is continuously drawn over the textured portion in step S 2550 . Grains of the drawn tube work piece are broken up by the textured portion of the mandrel tool. A drawn tube of smaller diameter and thinner wall thickness is formed in step S 2560 . In some embodiments, the mandrel tool is rotated during the drawing. In other embodiments, the container and the tube work piece are rotated during the drawing. 
       FIG. 26  is a flowchart showing an exemplary pipe forming method  2600 . A starting material is forced across a textured end of a mandrel and through a die in a plasticized state in step S 2610 . The textured end of the mandrel breaks up existing grains of the starting material. The pipe is formed from material forced through the die in step S 2620 . The formed pipe has smaller resultant grains on an interior surface than the existing grains of the starting material. The textured end includes, but is not limited to features, such as threads, ridges, studs, or protrusions. 
     In addition to a saltwater environment, embodiments described herein can be implemented in several other corrosion-inducing environments, including but not limited to aircraft hydraulic tubing, liquid natural gas cryogenic heat exchangers, and heat exchangers used in an acidic environment. The pharmaceutical and food processing industries require a high degree of cleanliness. Some instances of pharmaceutical and food processing use marine-grade aluminum, such as 50/52 or 58, titanium, or a high nickel-content stainless steel because of the extremely corrosive environment. Embodiments described herein provide an efficient and economical alternative for these environments. 
     While the invention has been described in conjunction with the specific exemplary embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, exemplary embodiments as set forth herein are intended to be illustrative, not limiting. There are changes that can be made without departing from the spirit and scope of the invention.