Abstract:
A tubular part having a tubular main body and an integral tubular protrusion projecting laterally from the side of the main body and in fluid-tight communication therewith is superplastically formed by inserting the tube in a cavity of a die base and heating the die to a temperature at which the material of which the tube is made exhibits superplastic properties. The distal end of a pull-rod is extended through an opening in the die base and through a hole in the side wall aligned with the opening. A pull-die is selected having a cross section larger than the hole and about equal to the desired internal cross section of the tubular protrusion. The pull die is attached to the distal end of the rod and (before or after attachment) is heated to about the superplastic temperature of the tubing material. Linear actuators are operated to pull the rod and attached pull die through the hole at a predetermined rate which produces about an optimal superplastic strain rate for the material, thereby superplastically stretching marginal portions of the tubular body around the hole and forming the tubing material in marginal regions around the hole against surfaces defining the opening in the die base into the tubular protrusion integrally joined to the tube around an integral junction region.

Description:
This application claims the benefit of provisional application No. 60/057,153 filed Aug. 28, 1997 
    
    
     FIELD OF THE INVENTION 
     This invention relates to pull-outs in tubing and duct systems for conveying gaseous and liquid fluids, and more particularly, to tubing and duct parts made of materials exhibiting superplastic properties and having integral protrusion formations, formed by superplastic forming, by which other matching parts can be attached to produce a fluid-tight system. 
     BACKGROUND OF THE INVENTION 
     Tubing and duct systems for conveying gaseous and liquid fluids are in widespread use in many industries. In the aerospace industry, welded ducts are used in the environmental control system and in the wing de-icing system for conveying heated air from the engine to the leading edges and nacelle inlet nose to prevent ice from forming on those critical surfaces in icing conditions in flight. These and other duct systems have elbows, “T” ducts, flanges and other components used to assemble the complete system. A “T-duct” is a short length of tubing having an integral tubular protrusion from the duct side wall by which a side duct can be attached, as by welding or coupling hardware, into a duct line. This protrusion is commonly known as a “pull-out”. 
     Two methods for making a tubular part, such as a “T” duct, with an integral pull-out are taught in U.S. Pat. No. 5,649,439 issued on Jul. 22, 1997 to David W. Schulz and entitled “Tool for Sealing Superplastic Tube”. Both methods use gas pressure to superplastically form a portion of a side wall of an end-sealed tube, heated to superplastic temperature in a die, into a side pocket of the die to form the pull-out. The formed tube is cooled and removed from the die, and the end of the pull-out is trimmed off to remove the cap and to give the pull-out a planar lip. 
     These methods reliably and repeatably produce parts as designed, but have one shortcoming that, in aerospace applications in particular, has significant economic consequences. Since the end cap of the pull-out bulge must remain intact to contain the pressurized forming gas, the material in the cap is not available for use in the pull-out side wall. Accordingly, to prevent excessive thinning of the pull-out, a thicker tube than is required by the engineering specifications for that duct system must be used. That thicker tube, carried just to avoid the excessive thinout of the pull-out lip, can add several pounds to an airplane de-icing duct system, for example. In the aerospace industry, in particular, wherein weight is an important factor in the design of any system, even a few pounds of weight in excess of that required by the engineering specifications is looked upon with disfavor. 
     Another problem with excessive thinning of the pull-out on a tubular part occurs when the mating duct is welded to the pull-out. Welding of thin-wall ducts and tubing requires careful control of the welding power and speed to obtain a weld bead with the desired penetration and mass, and to avoid burn-through or other over heating problems. Welding a pull-out joint that has been thinned, to a fresh section of straight tubing with a thicker wall, presents a difficult challenge that requires the skills of a master welder. Oftentimes even the best welders are unable to manage keeping an even weld bead or avoid blow-through holes because of the difference in the amount of parent material being melted around the pull-out. Many parts are scrapped because of non-conforming weld bead width, insufficient weld penetration, blow holes, weld-line porosity, inclusions and other defects that can be attributed to the variation of thickness surrounding the pull-out. 
     The radius area where the pull-out joins the tube is always a high stress area on an airplane de-icing duct system due to bending stresses caused by movement of the wings in flight, thermal stresses and sonic fatigue. All of these factors generate stresses that are transmitted along the spurs of the duct to the joint at the formed pull-out radius where the pull-out meets the mainline section of the straight tube. For this reason, there is a structural benefit in locating the weld bead of the tube welded to the pull-out as far as possible from the pull-out radius, so the stresses that are concentrated at the pull-out radius are not concentrated at the weld bead, since the welding process introduces defects such as porosity, etc. in the weld and decreases the structural load capacity of the duct around the weld. 
     Another existing tube pull-out production technique is a ball pulling process that is used to produce the same type of aerospace ducting tee&#39;s and joints. A round hole is cut in the sidewall of a tube in a position where the pull-out is to be formed. A ball that is slightly larger in diameter than the hole is pulled through the hole to form a pull-out with the same inside diameter as the outside diameter of the ball. The process is designed in such a way that the ram of a hydraulic actuator can be run up inside the tube through the hole, a ball screwed onto the threaded end of the ram, and the ball pulled through the hole using the hydraulic action of the actuator. The pull-out shape is controlled by a die which has a machine cut draw radius around which the pull-out forms as the ball stretches the material outward. 
     An enhanced pull-out method has been used wherein the ball is first heated to a temperature of about 1000° F. When the pulling process commences, heat from the hot ball is conducted to the tubing material in the region that will be stretched into the pull-out, heating it to an elevated temperature, near the temperature of the ball. A slight increase in ductility is realized by heating the ducting material. For example, the possible elongation of commercially pure titanium made in accordance with Mil Standard Mil-T-9046J, CP-1 at room temperature is about 25%; at 1000° F. its possible elongation is about 28%. 
     The problem with the conventional heated ball pull-out process is cracking and excessive thinout around the lip of the pull-out. The forming stresses and elongations that result during forming are very high and often surpass the formability limits of the material. The strain needed to form the pull-out causes a high scrap rate due to cracking. Aerospace ducting systems are usually designed to approach the minimum thickness to save weight, hence thinout at the lip of the pull-out can reduce the lip thickness below the acceptable minimum. Many parts are scrapped because the pull-out lip is thinner than this engineering designed minimum thickness. 
     The conventional pull-out forming process has many variables that contribute to the high scrap rate problem. The ductility of alloys used in ducting systems can vary from lot to lot. Elongation differences of only 1 or 2% in the raw material properties can have a significant impact on cracking and thinout. 
     In addition to variations in the material, it is difficult to precisely locate the hole cut in the tube relative to the position and linear path that the ball travels when the pull-out is made. A misalignment of even 0.005″ can have a significant effect on the elongation of the pull-out sidewalls. Many process failures occur in which the pull-out depth is slightly short on one side and is longer and cracked on the opposite side, resulting from slight misalignment of the hole with the ball travel path. 
     Because the conventional pull-out forming process causes thinout in the same location that is the most highly stressed, welded duct systems in airplanes have always been designed with thicker tube walls than would otherwise be necessary, thereby increasing the weight of the airplane duct system. The weight is especially undesirable in wing de-icing systems because there is a multiplier effect of weight in the wings. 
     Thus, there has long been an unsatisfied need in the industry for a process for making pull-outs that does not suffer from excessive thinning of the rim of the pull-out and which avoids cracking or bursting in the highly strained regions around the rim on the pull-out. The benefits of producing a flange, pull-out, or T-duct with reduced thickness variation would extend to both aerospace manufacturing and design capabilities, and also to commercial and industrial applications. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of this invention to provide an improved method of making a tubular part having a tubular body and a superplastically formed tubular protrusion extending at an obtuse angle from the tubular body and in fluid tight communication therewith. Another object of this invention is to provide an improved reliable method with a low scrap rate of making a tubular pull-out on a duct or other tubular body of superplastic material by which the duct can be connected to adjacent ducts or other tubular members in a fluid conduction system. A further object of this invention is to provide an improved tubular part having an integral pull-out formed by superplastic forming and having an acceptable degree of thin-out at the rim of the pull-out to facilitate connection of ducts or other tubular members to the tubular in an assembly. A still further object of this invention is to provide an improved apparatus for superplastic forming of tubular pull-outs on a tubular part. 
     These and other objects of the invention are attained in a method of making a superplastically formed integral tubular protrusion in a side wall of tubes for making parts such as tubular elbows and tees, including the steps of inserting the tube in a cavity of a die base and heating the die to a temperature at which the material of which the tube is made exhibits superplastic properties. A distal end of a rod is extended through an opening in the die base and through a hole in the side wall of the tube aligned with the opening in the die. A pull die, having a cross section larger than the hole and about equal to the desired internal cross section of the tubular protrusion, is attached to the distal end of the rod, the pull die is heated to about the superplastic temperature and is pulled through the hole, superplastically forming the tubing material in marginal regions around the hole against surfaces defining the opening in the die base into the tubular protrusion integrally joined to the tube with an integral junction region. Optimal elongations are achieved using optimal strain rates that minimize grain growth and achieve economical production rates. Material thinout around the rim of the pull-out is significantly reduced, and the process enables the use of more extreme pull-out designs. Variations of the process include formed pull-outs on flat or contoured flanges for joining ducting components that are non-circular in cross-section. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The invention and its many attendant objects and advantages will become better understood on reading the following description of the preferred embodiments in conjunction with the following drawings, wherein: 
     FIG. 1 is a perspective schematic view of a system, including associated controls and actuators, for supporting a tube while heating it to superplastic temperature, and for pulling a pull die through a hole in the tube to form a pull-out in accordance with this invention; 
     FIG. 2 is a partial sectional elevation of the enclosure shown in FIG. 1, shown with the die in place and holding a tube from which an integral pull-out has been superplastically formed; 
     FIG. 3 is a perspective view, from below, of a die set used in the apparatus of FIGS. 1 and 2 to perform the process of this invention; 
     FIG. 4 is a perspective view of a tube as it lies in the die set shown in FIG. 3 prior to forming the pull-out, but with the die deleted for clarity; 
     FIG. 5 is a perspective view of the tube shown in FIG. 4 after forming the pull-out; 
     FIG. 6 is a sectional perspective view of the lower die half shown in FIGS. 2 and 3; 
     FIGS. 7-9 are perspective views of three tubes, only half of each shown for clarity, showing three different shapes of openings through which the pull-die can be pulled to form the pull-out of this invention; 
     FIG. 10 is a perspective view of a retaining tube used to support the tube during formation of the pullouts in the process of this invention; 
     FIGS. 11 and 12 are perspective views of the retaining tube shown in FIG. 10 in the tube in the apparatus shown in FIG. 2 in the pre-formed and post-formed conditions, respectively; 
     FIG. 13 is a graph showing a representative forming schedule to form the part shown in FIG. 14; 
     FIG. 14 is a perspective view of a part formed in accordance with this invention; 
     FIG. 15 is a perspective view of a part having a pull-out on an oblique angle formed in accordance with this invention; 
     FIG. 16 is a sectional elevation along lines  16 — 16  in FIG. 15; 
     FIG. 17 is a perspective view of an elbow formed in accordance with this invention; 
     FIG. 18 is a perspective view of a tee formed in accordance with this invention; 
     FIG. 19 is a perspective view of a domed-end preform used to make the part shown in FIG. 17; 
     FIG. 20 is a perspective view of a preform used to make the part shown in FIG. 18; 
     FIG. 21 is a perspective view of a round planform flange formed in accordance with this invention; 
     FIG. 22 is a perspective view of a sheet from which the flange shown in FIG. 21 is cut; 
     FIG. 23 is a perspective view of a tooling set in which the sheet shown in FIG. 22 is formed; 
     FIG. 24 is an exploded perspective view of the tooling set shown in FIG. 23; 
     FIG. 25 is a sectional elevation of the draw ring along lines  25 — 25  in FIG. 24; 
     FIG. 26 is a perspective view of a rectangular planform flange formed in accordance with this invention; 
     FIG. 27 is a perspective view of a sheet from which the flange shown in FIG. 26 is cut; 
     FIG. 28 is a perspective view of an apparatus for forming the sheet shown in FIG. 26; 
     FIG. 29 is an exploded perspective view of the apparatus shown in FIG. 28; 
     FIG. 30 is a perspective view of a contoured base flange formed in accordance with this invention; 
     FIG. 31 is a perspective view of an apparatus for forming the part shown in FIG. 30 in accordance with this invention; 
     FIG. 32 is an exploded perspective view of the apparatus shown in FIG. 31; 
     FIG. 33 is a perspective view of a reducing flange formed in accordance with this invention; 
     FIG. 34 is a sectional perspective view of a die used to make the part shown in FIG. 33; 
     FIG. 35 is a superplastically formed, diffusion bonded part formed in accordance with this invention; 
     FIG. 36 is a sectional elevation of the superplastically formed, diffusion bonded part along lines  36 - 36  in FIG. 35; 
     FIGS. 37-39 are sectional elevations of the apparatus and component parts for making the part shown in FIG. 35; and 
     FIGS. 40-43 are sectional elevations of a tube in a die showing several stages of a prethinning process for forming a pull-out in accordance with this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to the drawings, wherein like reference numerals designate identical or corresponding parts, and more particularly to FIGS. 1 and 2 thereof, an automated apparatus for forming a tubular part  25  having a tubular pull-out  27  on a tube  29  in accordance with this invention is shown having an insulated enclosure  30  enclosing an open interior space  32  containing top and bottom heated platens  33  and  35  supported at the top and bottom of the enclosure  30  on insulated ceramic refractory slabs  37  and  39 . The enclosure  30  is similar to a conventional superplastic forming press, but it does not need or have the powerful hydraulic ram and supporting structures necessary to react the gas pressure forces exerted within the die in the course of conventional superplastic forming operations, so the enclosure is far less costly to build and maintain. Instead, a simple frame (not shown) of conventional design supports the upper components of the apparatus on the base. The platens  33  and  35  are heated by electrical rod heaters with electrical power controlled by a Proportioning, Integrating and Derivative (“PID”) three mode controllers  40  in an electrical control cabinet  42 . The PID controllers make possible a rapid heating rate on a controlled heating curve that ensures that the designated temperature will be reached quickly without overshooting. An insulated side wall  44  of the enclosure  30  surrounds the enclosed space  32  on three sides, and an insulated front door (not shown) movable between open and closed positions provides access to the enclosed space  32  for inserting and removing top and bottom halves  46  and  48  of a die  50 , shown in FIG. 2, in which the tubular part  25  is formed. A die lifter  51  of conventional design is provided for lifting the upper half  46  of the die  50  during insertion of the tube  29  and removal of the formed part after forming. 
     A vertically oriented pull-rod  52  extends through aligned holes in the base  54  of the enclosure, the bottom insulated slab  39 , the lower platen  35 , and the die bottom  48 . The pull-rod  52  has a proximal end attached to an activation unit  55  powered by a motor  58 . In FIG. 1, the proximal end is the bottom end, but the rod could alternatively be arranged to enter the enclosure from the top or sides. The activation unit  55  could be a hydraulic or screw drive, or a servomotor with a gear reduction unit, providing precisely controlled vertical displacement of the rod  52  under the power of the motor  58 , controlled by a programmable controller  60 , which also controls the operation of the PID controllers. The position and line of action of the activation unit  55  can be moved to provide off-center and non-vertical lines of action for the pull-rod  52 , for applications described later below. 
     A pull die, represented in FIGS. 1 and 3 as a ball  65 , is removably attached to a distal end of the rod  52 , shown in FIG. 1 as the top end. The ball  65  is the forming die by which the material of the side wall of the tube  29  is formed into the tubular pull-out  27 , as explained in detail below. 
     The die  50  is split along a horizontal center plane  67  through the axis of a cylindrical cavity  70  sized to receive the tube  29  with a snug fit. As shown in FIG. 6, the die lower half  48  has an opening  72  with a flaring lead-in portion  74  providing a draw radius, tapering to a cylindrical bore  75  on a vertical axis  77  intersecting the horizontal axis of the cylindrical cavity  70  and having an internal diameter equal to the desired outside diameter of the pull-out  27 . The vertical axis  77  of the bore  75  coincides with the axis and line of action of the rod  52  when pulled by the activation unit  55 . 
     Referring again to FIG. 1, the opening  79  in the lower platen  35  and the insulated slab  39  is of sufficient diameter to receive the rod  52  and also the pull die  65  when it is retracted by the activation unit  55 . The activation unit may be provided with sufficient range to pull the pull die  65  all the way out of the enclosure  30  so that it may be conveniently disconnected from the rod  52  directly without the use of remote manipulators, as described below. 
     In operation, the upper and lower die halves  46  and  48  are preheated to superplastic forming temperature by contact with the platens  33  and  35  heated with the rod heaters under control of the heater controllers  40 . The upper die half is lifted by the die lifter  51  and a tube  29 , having a pre-cut hole  80  through the side wall, is inserted into the lower die half  48 , with the center of the hole  80  aligned with the vertical bore  75  in the lower die half  48 , which in turn is aligned with the opening  79  in the lower platen  35  and insulated slab  39 . The die  50  is closed by lowering the upper die half  46  onto the lower die half  48 . In some applications, the upper die half  46  may be omitted. 
     The tube  29  is made from a metal such as titanium 6-4 alloy, which has superplastic properties. Superplastic properties include the capability of the metal to develop unusually high tensile elongations and plastic deformation at elevated temperatures, with a reduced tendency toward necking or thinning. The characteristics of superplastic forming and diffusion bonding are now reasonably well understood, and are discussed in detail in U.S. Pat. No. 3,927,817 to Hamilton, U.S. Pat. No. 4,361,262 to Israeli, and U.S. Pat. No. 5,214,948 to Sanders. The diffusion bonding properties are important only in connection with the embodiment illustrated in FIGS. 35-39 and discussed in detail below. 
     The rod  52  is extended upward, with its axis coincident with the aligned axes  70  of the opening  79  in the lower platen  35 , the vertical bore  75  in the lower die half  48  and the hole  80  in the tube  29 . A pull die  65 , preheated by induction heating or the like to superplastic forming temperature, is inserted from the side into the center of the tube  29  and positioned in alignment with the axis of the rod  52  using a manipulator arm (not shown) of conventional design. The rod  52  is advanced and rotated about its axis to engage the threads on the distal end of the rod  52  with corresponding threads in an internally threaded hole in the pull die  65 . The tube  29  is heated in the die  50  to the desired superplastic forming temperature, and the pull die  65  may also be heated by electrical resistance heaters energized by electrical conductors  84  in the rod  52  if it was not heated before attachment to the rod  52 . 
     When the tube  29  and the pull die  65  are at superplastic forming temperature, about 1650° F. for 6-4 titanium alloy, the motor  58  of the activation unit  55  is energized to pull the pull die  65  through the hole  80  in the tube  29  at a controlled rate. The speed of the activation unit  55  is precisely controlled to pull the pull die  65  at a rate that strains the tubing material at a predetermined rate. Hence, it is advisable to quantify the flow of material around the forming radius at the junction of the tube and the pull-out using engineering analysis, such as finite element analysis, to determine the speed at which the pull die  65  is pulled through the hole. The rate that the activation unit  55  pulls the die  65  through the hole is measured by a linear encoder and the motion is precisely controlled during the forming cycle to account for changes in the geometry of the tube in the area adjacent to and within the pull-out  27 . The activation unit  55  has a programmable logic controller, either in the activation unit itself or in the control console  60 , which provides feedback and control to the motor  58  in the activation unit by which the pull die rod  52  is pulled at a precisely controlled rate. The engineering analysis, such as finite element analysis, by which the flow of material around the forming radius is quantified, provides an idealized linear speed schedule to program the linear actuator to match the optimal superplastic strain rate of the tube material. 
     As shown in FIGS. 7-9, the hole  80  can be made in various shapes, depending on the conditions. The best shape for a right angle pull-out having an internal diameter equal to the internal diameter of the tube  29  (shown in FIGS. 7-9 as half tubes for clarity of presentation) is an oval hole as shown in FIG.  7 . The narrow elongated hole  81  shown in FIG. 8 is best for elbows, pull-outs, and material having exceptional super-elastic elongation capabilities. The round hole  82  shown in FIG. 9 is appropriate for material having poor elongation capabilities and for the diffusion bonding embodiment discussed below. 
     The tensile stresses developed in the tube  29  as the pull die  65  is pulled through the hole  80  can be great enough in some materials to pucker the tube material circumferentially adjacent to the pull-out  27 . To support the tube sidewall against such puckering, a retaining sleeve  85 , shown in FIGS. 10-12, is inserted into the tube  29  to hold the tube material against the sides of the die cavity  70 . A hole  88  in the retaining sleeve  85  is large enough to pass the pull die  65 , and the tube material around the hole  88  is sufficient to form the pull-out  27 . The retaining sleeve  88  may be a partial cylinder as shown in FIGS. 10-12, or it may be a complete cylinder. Preferably, the retaining sleeve  85  is high temperature, corrosion resistant, tool steel with a suitable release coating to prevent adhesion to the tube  29 . The steel material of the retaining sleeve  85  has a higher coefficient of thermal expansion than the titanium material of the tube  29 , so the steel retaining sleeve expands to hold the tube firmly between it and the die cavity surface  70 . When the formed part  25  is removed from the die cavity  70  and cools, the steel retaining sleeve  85  contracts more than the tube  29 , and the retaining sleeve  85  can be removed easily from the formed part  25 . 
     EXAMPLE 
     A tube of 6-4 titanium alloy (6 aluminum, 4 vanadium, balance titanium, Mil-T-9046J, type AB-1) having an internal diameter of 10 inches and a wall thickness of 0.041 inches is selected. An oval hole  80  having a major axis 7 inches long and a minor axis 3 inches long is cut in the sidewall of the tube, with the major axis extending parallel to the longitudinal axis of the tube. The tube  29  is inserted in the lower half  48  of a die made of a suitable die material such as cast ceramic as disclosed in U.S. Pat. No. 5,467,626, or corrosion resistant tool steel such as ESCO 49-C or Hayne&#39;s Alloy HN. The die half  48  has a pull-out opening  72 , shown in FIG. 6, having a curved draw radius  74  that tapers in a smooth curve to a cylindrical bore  75 . The tube  29  is positioned with the center of the oval hole  80  aligned with the axis of the bore  75  in the die half  48 . Alignment is by alignment pins or the like in the die cavity engaging holes and/or slots in trim portions of the tube  29 . 
     The pull die  65  is pulled through the hole  80  on a pull schedule graphed in FIG.  13 . The pull rate is initially about 0.5 inches/minute, but slows gradually to about 0.2 inches/minute in the intermediate portions of the cycle. The pull rate is then increased to nearly the same as the initial pull rate. This pull rate schedule produces an optimal strain rate of about 2×10 −4  sec −1  for the material in the marginal regions around the hole  80 . The resulting part  25 , shown in FIG. 14, has a thickness at the trim line  90  that is about 0.030″ which is more than 70% of the original thickness of the tube  29 . 
     Other types of parts may be made using this same process or slight modifications thereof. For example, angled pull-outs of the type shown in FIGS. 15 and 16 are made using a die set having an angled opening. The activation unit  55  is moved laterally and the line of action of the pull-rod  52  is aligned with the axis  95  of the angled opening in the die. Preferably, the pull-rod  52  is guided to ensure that it moves straight into the opening along the axis  95  thereof. Elbows  100  and tees  105  may be made as shown in FIGS. 17 and 18 using a heated pull die pulled through one or two oval holes or slots  107  in a closed end portion  110  of closed-end tubes  115  or  120 , shown in FIGS. 19 and 20, made of superplastic material as described above. 
     Formed flanges of any desired planform and base curvature, from flat to compound curvature, can be made using tooling described below. The formed flanges are generally for the purpose of attaching a tubular part such as a duct to a structure that receives or delivers a liquid supply through the duct. A flange  125  is shown in FIG. 21, having a flat base  127  and an upstanding pull-out  130  with a round planform  130  for attachment to a duct or other tubular structure. A plurality of holes  132  is drilled in the base  127  for attachment to the structure to which the flange is to be connected. 
     The flange  125  is cut out of a sheet  135  shown in FIG. 22 in which the pullout  130  is formed by an apparatus  140 , partially shown in FIG.  23  and shown in exploded form in FIG.  24 . The apparatus  140  includes a die base  142  and a matching draw ring  145  which between them hold the sheet  135  in which the pull-out  130  is to be formed. The die base  142  has a central clearance hole  147  sized to receive and pass a punch  150  on the end of a press ram rod  152 . The draw ring  145  has a rounded tapering opening  155 , shown in FIG. 25, around which the marginal regions  157  of the sheet around a central hole  160  are formed into the pull-out  130  when the punch  150  is pressed into and through the hole  160 . The die base  145  and draw ring  145  are supported in an apparatus similar to the apparatus  30  shown in FIG. 1, but including a central opening in the upper platen  33  and the insulating slab  37  to provide -clearance for the punch  150  when it emerges from the formed hole in the sheet  135 . A manipulator (not shown) of known construction is mounted above the enclosure for griping and removing the punch  150  from the ram rod  152  after the forming operation. 
     The process of forming the flange  125  of FIG. 21 starts with cutting the central hole  160  in the sheet  135 . In this example, the sheet is 6-4 titanium alloy 0.060 inches thick and the hole  160  is circular and one inch in diameter. The die set comprising the die base  142  and the draw ring  145  is installed in the enclosure apparatus and is heated to superplastic forming temperature for the sheet material, or about 1750° F. for the 6-4 titanium material. The die set is opened and the sheet is inserted onto the die base  142  with the central hole  160  of the sheet  135  aligned coaxially with the clearance hole  147  in the die base  142  and the rounded tapering opening  155  in the draw ring  145 . Suitable stops or alignment pins may be attached to or machined in the die base  142  to facilitate such alignment. A mushroom-shaped punch  150  shown in FIGS. 23 and 24 is attached to the ram rod  152  and the punch may be preheated in an induction heater or with internal electrical resistance heaters to shorten the cycle time. When the sheet and the punch are at the desired superplastic forming temperature, the punch  150  is moved with an activation unit (not shown) corresponding to the activation unit  55  shown in FIG. 1 along a line of action coincident with the aligned axes of the openings in the die base  142  and draw ring  145  and the hole  160  in the sheet  135 . The punch  150  is moved on a schedule that produces the optimal superplastic strain rate for the material of the sheet  135 . Alternatively, the punch can be shaped so that the material of the sheet  135  is strained at the optimal superplastic strain rate when the punch  150  is moved at a constant speed. A punch shape intended for this purpose is indicated in FIGS. 23 and 24 wherein the leading and trailing surfaces of the punch are angled from the axis of the ram rod more steeply than the middle portions of the punch  150 . 
     After the pull-out  160  is formed in the sheet  135 , the punch is detached from the ram rod  152  by the manipulator, and the ram rod is retracted back through the die set and the formed part. The draw ring  145  is lifted off the die base  142 , taking the formed part with it. The part can easily be separated from the draw ring  145  and removed for cleaning and final trimming and drilling of holes  132  to complete the manufacturing steps for the flange  125 . 
     The same process used to make the flange  125  shown in FIG. 21 can be used to make a flat, rectangular planform flange  165  shown in FIG. 26 cut from a formed sheet  167  shown in FIG.  27 . The apparatus shown in FIGS. 28 and 29 used to form the pull-out  169  on the sheet  167  is the same as the apparatus shown in FIGS. 23 and 24 except for the shape of the punch and the openings in the die and draw ring, which have a shape corresponding to the rectangular opening of the flange  165  in FIG.  26 . The opening  170  in the sheet  167  (before forming the pull-out  169 ) is shown as oval in shape, but the shape will vary with the shape of the punch, and each pull-out shape requires its own analysis to determine he optimal shape so that that enough material is available to form the pull-out  169  of the desired size and type of material and that the material is not stretched beyond its superplastic forming limits, and further that the thinout around the lip  172  of the pull-out  169  is not excessive. It is noteworthy that the opening  170  is stretched to be much larger during the forming process due to the material being drawn around the punch. This phenomenon reduces the amount of thinning in the pull-out  169 . 
     A contoured, rectangular flange  200 , shown in FIG. 30, has a base  205  having a simple contour, but could be made with a compound contour instead. The apparatus  210  for forming the flange  200  includes a die base  212  and a draw ring  214  similar to the apparatus shown in FIGS. 28 and 29, except that the mating surfaces  215  and  217  of the die base  212  and the draw ring  214 , shown in the exploded view of the apparatus  210  in FIGS. 31 and 32, are shaped with the desired curvature of the flange base  205 . The forming process for making the contoured flange  200  is identical to the process used to make the flange  165  shown in FIG.  26 . 
     The flange forming process and apparatus can be modified to produce a reducing flange  230  shown in FIG.  33 . The reducing flange  230  has a base  232  like the base of the flange  165  shown in FIG. 26, and an upstanding pull-out  234  like the pull-out  169  of the part shown in FIG.  26 . An integral brim  237  projects partially across the top of the pull-out  234 , surrounding a central opening  240 . A series of holes  242  is drilled in the base  232  and another series of holes  244  is drilled in the brim  237  for attachment to mating structures. 
     The apparatus for forming the reducing flange  230  is the same as the apparatus shown in FIGS. 28 and 29, or in FIGS. 31 and 32, depending on whether the reducing flange is to have a flat or contoured base. The punch design is different, however. The punch  250 , shown in FIG. 34, has a lead-in central projection  252  and a flat shoulder section  254  extending around the projection out to the sides of the punch  250 . The flat shoulder section  254  can be shaped to produce any desired contour, parallel or non-parallel to the base  232 . 
     The process for forming the reducing flange  230  is similar to the process used to form the flange  165  shown in FIG. 26, except that the punch  250  is not pushed all the way through the sheet. Instead, the punch is stopped short of full penetration through the sheet, leaving the brim  237  projecting inward. After forming, the part  230  is cooled with a stream of air which causes it to contract around the punch  250 . As the part thermally contracts, it is restrained by the punch  250  which causes the part to stretch or plastically deform to slightly larger dimensions relative to the dimensions it would have if it were removed hot from the punch. The stretched part is now reheated by allowing it to sit on the hot punch until it thermally expands enough to allow the punch to move freely out of the pull-out  234 . 
     Referring now to FIGS. 35 and 36, another embodiment of the invention is shown wherein a part  274  is made having a partial pull-out  275  which is superplastically formed on a tube  29  and is diffusion bonded to a stub tube  278  to form a high strength pull-out of any desired lip thickness and with extra wall thickness in the junction radius  280  where stresses tend to be concentrated. This embodiment removes the weld junction  282  from the vicinity of the junction radius  280  and makes quality welds easier to achieve since the lip  287  of the pull-out can be made any desired thickness. 
     Diffusion bonding refers to metallurgical joining of two pieces of metal by molecular or atomic co-mingling at the faying surface of the two pieces when they are heated and pressed into intimate contact for a sufficient time. It is a solid state process resulting in the formation of a single piece of metal from two or more separate pieces without a discernible junction line between them, and is characterized by the absence of any significant change of metallurgical properties of the metal, such as occurs with other types of joining such as brazing or welding. 
     The superplastically formed and diffusion bonded part  274 , shown in FIGS. 35 and 36, is made in an apparatus shown in FIGS. 37-39. The part  274  has a short integral pull-out  275  formed on a tube  29  with a pull-die  285 . The term “integral” as used herein means that the tube  29  and the pull-out  275  are of a single piece of metal, not separate pieces attached, connected or joined to make the part. An extension or stub tube  278  is diffusion bonded to the end of the pull-out  275  in an overlapping relationship as shown in FIGS. 36 and 39. The thickness of the overlapping region can be made quite thick, as illustrated, without making the other regions of the part unnecessarily thick, so the part is thick where the greatest stresses are encountered and thin elsewhere. The stub tube  278  has a distal end lip  287  that is thick and plane for easy welding into a duct system. The weld region is well removed from the pull-out  275  so there is no problem with weakness in the high stress region caused by weld porosity or other weld defects. 
     The apparatus shown in FIGS. 37-39 for superplastically forming and diffusion bonding tubing pull-outs of the type shown in FIGS. 35 and 36 includes a die set  50  like the die set used in the embodiment shown in FIGS. 1-6, the lower die half  48  of which is shown in FIGS. 37-39. The pull die  285  of modified form as shown in FIGS. 37-39 is designed to form the pull-out  275  and also provide radial pressure to press the pull-out  275  against the upper portion of the stub tube  278  and the wall of the opening  72  in the lower die half  48  to achieve a diffusion bond. 
     In preparation for forming and diffusion bonding, the tube  29  and the stub tube  278  are chemically cleaned by immersion, first in an alkaline bath to remove grease and other such contaminants, and then in an acid bath, such as 42% nitric acid and 2.4% hydrofluoric acid to remove metal oxides from the titanium alloy tube  29 . The cleaned tubes are rinsed in clean water to remove residues of the acid cleaner, but residues from the rinsing solution may remain on the tube after removal from the rinsing bath. These residues are removed from the tube in the region of the diffusion bonding by wiping with a fabric wad, such as gauze cloth, wetted with a reagent grade solvent such as punctilious ethyl alcohol. The tube is wiped until the gauze comes away clean after wiping. The alcohol evaporates leaving no residue and leaving the tube free of contaminants that would interfere with a complete and rapid diffusion bond when the conditions for such a bond are established. 
     Titanium and titanium alloys that are to be diffusion bonded must be protected from exposure to oxidizing materials, such as oxygen in the atmosphere, at all times in the process at which the part is heated to a temperature above 700° F., because titanium oxidizes readily above that temperature. For best results, an inert gas, such as welding quality argon, is used as a cover gas to protect the titanium from oxidation attack when the part is hot. The apparatus shown in FIGS. 1,  2 , and  37 - 39  is closed after the pull-die  285  is positioned and attached to the pull-rod  52 . The tube  29  and the die set  50  are purged of air and contaminants using dry argon flooding or other known oxygen purging techniques in the diffusion bonding art. 
     The tube  29  and the stub tube  278  are heated by conductive and radiant heating from the die set  50  and the pull-die  285  is heated by internal electrical heaters, by absorbing radiant heat from the tube, or is preheated before insertion into the tube  29  and attachment to the pull-rod  52 , or by some combination thereof. When the tube  29  has reached superplastic forming temperature, the pull-die  285  is pulled down with the pull-rod  52 , using an activation unit  55  like the one shown in FIG. 1, and superplastically forms the margin regions  290  around the hole  80  down and outward against the top portion of the stub tube  278 , as shown in FIGS. 38 and 39. The pull die  285  is sized to provide radial pressure against the pull-out  275  and the overlapping portions of the stub tube  278  to provide sufficient pressure to form a good diffusion bond. If additional pressure is needed, an electrical resistance heater in the pull die  285  can be energized to raise the temperature of the pull-die  285  an additional 10-50° F. to increase its diameter by thermal expansion and increase the interference pressure between the pull-out  275  and the stub tube  278 . After diffusion bonding is complete, the electrical power to the pull-die  285  is shut off and the die is allowed to cool, or is actively cooled by gas or liquid cooling passages in the pull-die  285  fed from the pull-rod  52 . The cooled pull-die  285  contracts away from the diffusion bonded pull-out/stub tube and is lifted by the pull-rod  52  and is gripped by the manipulator arm while the pull-rod  52  is rotated and detached from the pull-die  285 . 
     After cooling below superplastic temperature, the part is removed from the die cavity  70  and is recleaned to remove any alpha case that may have formed on the part from high temperature contact with residual air that may not have been purged from the die cavity  70 . After cleaning, the part is finished and ready for welding into a duct system without further trimming or other processing. 
     A prethinning scheme, illustrated in FIGS. 40-42, prethins the tube  29  in the intermediate regions  295  between the restraining sleeve  85  and a lip portion  300  in the region immediately surrounding the hole  80  in the tube  29 . By prethinning the intermediate regions  295 , the portions of the tube  29  that will be superplastically formed into the pull-out  27  are preferentially prestretched so that the lip portions  300 , which ordinarily are stretched the most during a forming operation of the type illustrated in FIGS. 4 and 5, are protected against excessive stretching by focusing the initial stretching initially in the intermediate portions  295 . In the later phases of the cycle following the phases illustrated in FIGS. 42 and 43, the lip portion is released to stretch freely, but at that point is thicker than the intermediate portions  295 , so the stretching in the later phases of the operation continue to be distributed evenly between the intermediate portions  295  and the lip portions  300  even though the lip portions have a smaller radius. 
     As shown in FIG. 40, an apparatus for performing a prethining operation in accordance with this invention includes a pull die  305  having a forming surface  307  by which the tube  29  is formed against the surfaces  74  and  75  of the die half  48 . The pull-die  305  is shaped like the die  285  shown in FIGS. 37-39, but could be shaped like the pull-die  65  in FIG. 1 if it will not be used for diffusion bonding. A clamping tube  310  slides telescopically on the pull-rod  52  under control of the activation unit  55  to releasably clamp the lip portion  300  of the tube  29  around the hole  80  between a disc  315  and a shoulder  320  on the die  305 . 
     In operation, a tube  29  is selected and the restraining sleeve  85  is inserted in the tube  29  with the axes of the holes  88  and  80  of the restraining sleeve  85  and the tube  29  aligned. The tube  29  and its restraining sleeve  85  are inserted into the die cavity  70  of a preheated lower die half  48  with the axis of the opening  80  aligned with the axis  77  of the bore  75  . The die  305  is preheated and inserted through an open end of the tube  29  with a manipulator arm, as described previously, and the pull-rod  52  is extended and rotated to engage the threads on the distal end of the pull-rod  52  with the threaded hole in the bottom of the die  305 . The pull-rod  52  is retracted slightly to engage the shoulder  320  of the pull-die  305  with the hole  80  in the tube  29  and the clamping tube  310  is slid up the pull-rod to clamp the lip portion of the tube  29  around the hole  80  between the die shoulder  320  and the disc  315 . 
     When the temperature of the tube  29  and the die  305  are at the desired superplastic forming temperature, the pull-rod  52  and clamping sleeve  310  are extended upward as shown in FIG. 41, superplastically stretching the intermediate marginal portions  295  around the hole  80  while preventing thinning of the lip portions  300  by virtue of its clamped position. The stretching rate is based on an optimal strain rate for the material of which the tube  29  is made. When the intermediate marginal portions  295  have been stretched to the desired extent, the pull-rod  52  and the clamping tube  310  are retracted downward past the initial position it had in FIG.  40 . As illustrated in FIG. 42, the intermediate marginal portions  295  are now pre-stretched and can be laid over the tapering surfaces  74  of the die half  48  without stretching the lip portion  300  around the hole  80  in the tube  29 , as shown in FIG.  43 . After the position illustrated in FIG. 43 is reached, the lip portion  300  is released by withdrawing the clamping tube  310  and continuing the downward motion of the pull-die  305  to finish stretching the lip portion  300  against the sides of the opening  72  in the lower die half  48 . The die  305  is now pushed back up away from the formed pull-out and is detached from the pull-rod  52  by gripping the pull-die with the manipulator and rotation the pull-rod  52  to unscrew it from the pull-die  305 . The die is opened and the formed part is removed as described earlier. 
     Obviously, numerous modifications and variations of the preferred embodiment described above will occur to those skilled in the art in light of this disclosure. Accordingly, it is my intention that these modifications and variations, and the equivalents thereof, are to be considered to be within the spirit and scope of my invention, wherein: