Abstract:
A billet of an aluminum alloy or magnesium alloy is formed by a combination of forming operations into a desired article of complex, but open shape. In a first step a billet is heated and extruded to form an extruded workpiece profile having at least first and second sections of different thicknesses. The extruded workpiece may be shaped so that the respective sections are at an angle to each other. The extruded workpiece is then further formed against a forming surface so that the shape of least one of the sections is further formed toward the shape of the article. The methods are suitable for efficient manufacture of many like complex shapes such as brackets and reinforcement members, and even container pans for computers and other electronic devices.

Description:
TECHNICAL FIELD 
     This invention pertains to the forming of articles of complex shapes (including sections with varying thicknesses) in light metal alloy starting materials. More specifically, this invention pertains to the use of a combination of extrusion forming and elevated temperature forming against a die surface to make useful structural parts that are difficult to form (or cannot be made) by a single forming operation. 
     BACKGROUND OF THE INVENTION 
     There is a desire to reduce weight in many manufactured articles. The need is acute in the manufacture of automotive vehicles but is not limited to such applications. The ability to integrate multiple thicknesses into a single part is useful for minimizing overall part mass without sacrificing strength or stiffness in selected regions. Also thin gage parts, such as sheet metal, frequently afford limited thread engagement, and thus limited holding power, for threaded fasteners. Thus the ability to selectively introduce thicker regions into a part enables the use of threaded fasteners without compromising retention, thereby facilitating part removal for replacement or service. 
     Aluminum alloys and magnesium alloys are available for automotive body applications and the like but these light weight materials are not usually as formable as many ferrous alloys. Metal sheets of suitable alloys of aluminum and magnesium have been formed into body panels and the like by stamping, warm stamping, fluid forming and hot blow forming. But the starting materials for such forming processes are usually sheets of uniform thickness which limit the shapes of articles that can be formed. Often it is desired to make other more complicated shapes of the same alloys such as reinforcement parts, attachment parts, stiffening parts and the like that have sections of varying thickness. 
     There remains a need for forming practices that can complement stamping, warm stamping or hot blow forming or the like and be applied to a primary shape light metal alloy workpiece like, for example, a billet, bar, strip, or sheet, and form the workpiece into an article having sections of varying thicknesses and, often, at varying and sharp angles. 
     SUMMARY OF THE INVENTION 
     This invention is devised for the forming of suitable light metal alloys, particularly aluminum and magnesium alloys, into unitary formed articles having sections of different thicknesses and sections that need not be co-planar. 
     Embodiments of this invention include one or more extrusion steps by which a starting billet is formed into a precursor workpiece shape with connected sections of desired thicknesses, which may be of different dimensions. As stated, the connected sections may be formed at predetermined angles with respect to each other. The extruded precursor piece is formed as an open shape with at least two edges that are spaced apart from the extrusion axis. In other words, the extruded body is not in the shape of a tube. 
     The extrusion step or steps may be performed at a suitable elevated temperature as required by the respective aluminum or magnesium alloy composition. Typical billet temperatures for extrusion of aluminum alloys range from about 425 to 500° C.; for extrusion of magnesium alloys billet temperatures of between 300 and 450° C. are preferred. The extruded shape may then be heat treated if necessary so that it may be further shaped by hot stamping, hot blow forming, or the like. 
     The temperature of the extrudate will be greater than the temperature of billet from which it formed due to the deformation work required in extrusion and will typically be comparable to that employed in hot forming. Thus, appropriate coupling and sequencing of the extrusion and hot forming operations will at least minimize and may eliminate the need for a separate heating step prior to hot forming. 
     In many embodiments of the invention a long extruded precursor shape may be sheared or cut at desired distances along the axis of extrusion to form blanks suitable for subsequent forming. 
     The extruded precursor shape is then heated to a suitable elevated temperature, if necessary, and further formed by urging it against a heated forming tool or heated die surface. A surface of the extruded article is forced into suitable contact with the heated tool so that the engaging surface of the article acquires the shape of the tool. For example, the extruded shape may be pressed by a complementary tool or die into sliding, metal forming engagement against the forming surface as in a hot stamping practice. Aluminum and magnesium alloy workpieces can be subjected to hot stamping in a temperature range of about 200° C. to about 350° C. In another embodiment, the extruded workpiece is heated to a highly formable state and fluid pressure is applied to one side of the workpiece to force the other side into conforming shape with a heated die or tool. Hot blow forming of aluminum alloy and magnesium alloy workpieces is often performed at workpiece and tool temperatures in the range of about 400° C. to about 500° C. 
     It is contemplated that much of the shaping of the article will be accomplished by a combination of the extrusion steps and hot forming steps or, where applicable, conventional forming steps. However, it is recognized that a suitable starting shape for the extrusion step is required. And it is further recognized that some finishing steps, such as trimming, hole forming, and the like, may be required on the hot formed shape. 
     Other objects and advantages of the invention will be apparent from the following detailed descriptions of illustrative embodiments of practices of the invention. Reference will be made to drawing figures that are described in the following section of this specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-D  show a sequence of operations suitable for forming a fender reinforcement by the practice of the invention.  FIG. 1A  shows a suitable extruded form;  FIG. 1B  shows the form of  FIG. 1A  located in a suitable forming apparatus shown in partial cutaway;  FIG. 1C  shows the formed part; and  FIG. 1D  shows the finished part. 
         FIGS. 2A-D  show a sequence of operations suitable for forming a bracket by the practice of this invention.  FIG. 2A  shows an extruded form;  FIG. 2B  shows the extruded form of  FIG. 2A  after further shaping;  FIG. 2C  shows the formed part; and  FIG. 2D  shows the finished part. 
         FIGS. 3A-D  show a sequence of operation suitable for forming a case for an electronic device by the practice of this invention.  FIG. 3A  shows an extruded form;  FIG. 3B  shows the extruded form of  FIG. 3A  after further shaping;  FIG. 3C  shows the extruded form of  FIG. 3B  and associated die portion after further shaping;  FIG. 3D  shows the finished part. 
         FIG. 3E  shows the finished part of  FIG. 3D  with components assembled in a configuration suitable for use. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Magnesium and aluminum alloys may be readily extruded. The extrusion process, which involves forcing materials through a shaping die, is a well-developed, low-cost process which is inherently capable of producing forms of varying thickness. Further, unlike the flat sheet of uniform thickness which is the starting material for most sheet forming processes, extrusion is capable of forming much more complex geometries along the length of the extrudate. 
     By way of example only, the form shown in  FIG. 1A  which combines segments of unequal thickness inclined at about right angles to one another is entirely suitable for fabrication by extrusion. Further the junction between the inclined sections may be made with only a small radius conferring a ‘crisp’ appearance not always achievable in formed or bent sheet metal components. 
     These characteristics: the suitability of the light metal alloys of aluminum and magnesium for extrusion; the ability of the extrusion process to produce shapes of varying thickness; and the ability of the extrusion process to produce other than planar geometric forms are used in this invention to beneficially complement current sheet forming processes for these light metals. 
     The invention applies sheet forming processes to forms of complex starting geometries developed by extrusion to enable the manufacture of complex structural forms and attachments. These structural forms and attachments may thus be made of alloys identical to or compatible with the alloys employed in stampings formed from current sheet compositions. 
     It is well known that the formability of magnesium and aluminum may be enhanced by conducting forming at elevated temperature and thereby enabling the formation of more complex shapes. In practice of this invention similar elevated temperature forming practices may be followed, but are not required if forming at lower temperatures such as room temperature, generally about 25° C., is adequate for achieving the desired form. 
     The invention may best be understood by consideration of the following examples. It will be appreciated that the output of extrusion processes is a part which is of limited extent in the two dimensions defining the cross-section of the extrusion but is extensive in the third, length dimension. Further it is known that asymmetrical extrusions or of non-uniform cross-section frequently exhibit twist or bend along their length. It is however common practice to straighten or untwist the extruded part by controlled application of plastic deformation in a post-extrusion stretching or twisting process. It is also well known to cut the extrudate into a number of smaller, commonly-dimensioned parts so that a single extruded length, when sectioned, will yield multiple parts. In the practice of the following examples these preliminary operations will not be further discussed. The focus will be on the subsequent processing of these commonly-dimensioned parts which, by analogy with conventional sheet forming terminology will be described as extruded blanks in future descriptions. 
     EXAMPLE 1 
     A Fender Reinforcement 
     In this example an aluminum or magnesium alloy bracket or reinforcement member is made for use, for example, in attaching a polymeric fender to a space frame body structure. Four or more such parts might be used in the making of a vehicle body. 
       FIG. 1A  shows a section  10  of an extruded form comprising two thicknesses of extrudate and generally resembling the letter “Z”. The extrudate comprises a first segment  11 , a second segment  12  and a connecting segment  35  joining segments  11  and  12 . The material thickness in segment  12  is greater than the material thickness in segment  11 . Connecting segment  35  has a portion comprised of material of thickness corresponding to segment  11  and a portion comprised of material of thickness corresponding to segment  12  with a progressive transition from one thickness to the other in region  13 . Segment  11  adjoins segment  35  along line  14  corresponding to a generally 90° change in inclination while segment  12  adjoins transition segment  35  along line  15  again corresponding to a generally 90° change in inclination. If these changes in inclination between the part segments at lines  14  and  15  were formed by bending they would be considered small radius bends, that is bends in which the bend radius is comparable to the metal thickness. The formation of small radius bends is challenging, particularly in materials of limited ductility, and the ability to impart this shape by extrusion appreciably simplifies forming complex geometric forms in thin components. 
       FIG. 1B  shows the extruded form  10  of  FIG. 1A  positioned in a hot forming die  20  comprising an upper section  21  and a lower section  22  with these sections positioned in a press capable of applying mechanical force in a direction indicated by arrow  100 . Forming is accomplished by first heating the die and sheet to a temperature in the range of 400 to 500C and applying gas pressure (up to 500 psi) to one side of the sheet thereby inducing the sheet to bulge and deform into contact with a shape imparting die. In  FIG. 1B , the die section is the lower section  22  and its interior geometry (shown partially in ghost) generally corresponds to the resulting part geometry shown in  FIG. 1C . Upper section  20  is primarily a pressurization chamber, comprising a sealing shell  80  and interior cavity  90 . 
     To assure good gas sealing, the periphery of upper and lower dies  21  and  22  substantially reproduce the upper and lower surfaces  50 ,  60  of extruded blank  10 , see  FIG. 1A . Sealing is further assured by the introduction of a seal  16 , shown in  FIG. 1C , resulting from the cooperative interaction of features on upper die  21  and complementary features on lower die  22  which result in a local offset of the surfaces  50 ,  60  of blank  10  and thereby define a substantially leak-free pressurizable cavity  90  within upper die  21 . 
     Upon exposure to appropriate gas pressure the extruded blank  10  will adopt, with fidelity dependent on the magnitude of the applied pressure, the general form of lower die  22  and the formed part  10 ′, shown in  FIG. 1C , may be removed. Comparison of  FIGS. 1A and 1C  reveals that the general “Z” form of blank  10  has been retained but that the forming step has enabled the formation of ‘pocket’-like features  54 ,  56  comprising substantially vertical surfaces  19 ,  28  and  38  and substantially horizontal features  17  and  18  located on surfaces  11  and  35  of blank  10 . 
     Further processing, trimming of metal excess to the part and fabrication of holes  42 ,  44  and  46  for example, by punching or machining results in the finished part, a bracket  10 ″, shown in  FIG. 1D . Hole  42  is illustrated as a slotted hole which may be desirable for adjustment, particularly for a bracket like that shown. It will be appreciated that the shape of holes  42 ,  44  and  46  is not restricted to the geometries shown but may instead generally adopt any configuration which meets the desired engineering function. 
     It will be appreciated that the selected die geometry was complementary to the desired part geometry and supported thicker section  12  and enabling development of additional shaped features in thinner section  11 . However, by appropriate choice of thicknesses, the strength mismatch between the thick and thin sections may be made sufficient as to render the thick section substantially un-deformable under conditions leading to the deformation of the thinner section. In this situation die support for the thicker section will be unnecessary and simpler dies may be employed. This may be particularly desirable for closed section, or tubular, forming where thickness variations in tube wall thickness could be introduced to suitably channel the deformation when the tubes are subject to internal pressurization at elevated temperature. 
     The process has been depicted as it would be practiced in forming only a single part in each forming operation. However, particularly for small parts, it may be more efficient to employ a longer extruded blank and place it in a die with multiple cavities so that multiple parts may be formed in a single forming operation. After forming they would be separated to create the desired part  10 ″. 
     The forming process for this part has been described as fabrication of an extruded blank and warm or blow forming. However those skilled in the art will appreciate that if the pocket-like features are relatively shallow and easily formed it may be possible to form them using press forming, conventional forming technology employing matched die sets in a mechanical or hydraulic press. Press forming may be carried out at elevated temperature or at room temperature, generally about 25° C. In this case the need to have the direction of the metal flow aligned with the action of the press will necessitate positioning the extruded blank in a different orientation in the press than is shown in  FIG. 1B . However, the importance of appropriate ‘die tip’ in achieving optimal forming performance is well known to those skilled in the sheet forming art. 
     EXAMPLE 2 
     Bracket 
     In this example another aluminum or magnesium alloy bracket or reinforcement member is made for use, for example, in making a vehicle body structure. Several such parts might be used in the making of a vehicle body. 
     In Example 1, the extruded blank was used directly and the required trimming was performed subsequent to the forming operation. The extruded blank however may require additional trimming prior to forming to render its shape suitable for the specific forming operation contemplated. In this example, shown in  FIG. 2A , the extruded blank  110  has a form generally resembling the letter “L” with a first segment or leg  111  of the “L” being thinner than the second segment or the second leg of the “L”  112 . In this example the transition from thick section to thin section occurs generally at small radius outer bend  113 , the specific location being dictated by the need to achieve a smooth transition from the thick to thin sections and therefore dependent on the specific radius selected for inner bend radius  114 . 
     In  FIG. 2B , the extruded blank has been further processed and trimmed, removing a portion of section  112  and thereby creating surfaces  116  and  117  as well as generating radiussed, in plan view, corners  115 ,  124 ,  125  and  126 , here shown as commonly dimensioned for convenience only. This geometry may be developed using a cutting or shearing operation using matched cutting edges urged together by, for example, a press, or alternatively by a machining operation such as sawing or milling. 
       FIG. 2C  illustrates the form of blank  110  after two forming operations in a press: a bending operation to create a bend along bend axis  118  and thereby form a section of horizontal segment  111  into a vertical orientation shown as  111 ′; and an operation to form dart  119  along the bend line  118  and thereby stiffen the bend to render it more resistant to unbending in service. These features may be introduced in a single press operation or in two separate press operations and may be performed with the die and workpiece at room temperature, that is at around 25° C., or at elevated temperature depending on the formability and ductility of the workpiece. 
     In  FIG. 2D , the bracket-like component has been subjected to additional shaping to introduce holes  121 ,  122  and  123 . As was the case for Example 1 the geometry of the holes may be circular (as depicted), slotted, oval or any other configuration which satisfies the engineering requirements and the holes may be fabricated by press operations like punching or by machining, for example by drilling or milling. 
     In this case however, the hole making operation may precede the forming operation and may even be conducted on the original extruded blank shown in  FIG. 2A . Generally introducing holes prior to forming is discouraged due to the difficulty of precisely locating the holes in the finished part, but if the hole positions are loosely toleranced this may be an acceptable procedure. Thus the sequence of operations shown in  FIGS. 2  A-D may be modified without necessarily affecting the utility of the formed part. 
     EXAMPLE 3 
     Case for an Electronic Device 
     In this example a pan structure of light weight aluminum or magnesium alloy is made with integral reinforcing or spacing ribs being formed in the base of the pan. The pan may be used as an enclosure for a computer or other electronic device. 
     In this example, the extruded blank  210  shown in  FIG. 3A  comprises a generally flat horizontal base  211  of generally uniform thickness with a plurality of integral, parallel, stiffening features  212  and  213  that extend upwardly from a surface of base  211 . Stiffening features  212  and  213  may, as shown, in  FIG. 3A  and subsequent, be of differing height and will extend along the length of the blank. 
     In the configuration shown in  FIG. 3A , the blank  210  will be more resistant to deformation in a direction substantially orthogonal to the orientation of ribs  212 ,  213  and less resistant to deformation in a direction generally aligned with the orientation of ribs  212 ,  213 . To enable more homogenous deformation, at least on the periphery of blank  210 , the ribs  212  and  213  may be machined off along lines  214  and  216  to create regions  215  of thickness substantially equal to that of the planar region  211  of extruded blank  210  leaving residual rib sections  212 ′ and  213 ′. Thus, as shown in  FIG. 3B  a perimeter region of substantially equal thickness and hence substantially equal resistance to deformation has been created around the perimeter of extruded blank  210 , thereby creating extruded blank  210 ′. 
     A similar result may be achieved by welding, for example using a laser, additional sections to extend the length of the blank  210  shown in  FIG. 3A  and create an extruded tailor welded blank. Further the process has been described as cutting an extrudate to form an extruded blank followed by machining to create the form of  FIG. 3B . However it will be appreciated that the extrudate may be machined first and then cut to create blank  210 ′. For example: 
     a) face mill to create in the extrudate a uniformly spaced array of stiffening rib-free regions of width (2× “Y”) and of thickness generally equal to that of region  211  a distance “X” apart; and 
     b) repeatedly shear the milled extrudate in the middle of the milled section. A cutting process, for example sawing or milling rather than a shearing process may be employed provided allowance is made for kerf losses 
     Extruded blank  210 ′ exhibits representatively-positioned locator features  218 , here shown as parallel-sided slots terminated in a semi-circular arc, intended to engage with mating locators in the die to enable accurate placement of the blank in the die. Alternate locator geometries are however well known. For example tapered slots terminated in a semi-circular arc are frequently employed. This configuration provides some self-guiding characteristics to the locators if the locating pins in the die, rather than being fixed, shuttle between a blank load/unload position and a stamping position. 
       FIG. 3C  shows a cutaway view of the extruded blank  210 ′ located in the lower, die section  220  of a die-set intended for hot or warm forming. Analogously to Example 1, the die-set comprises a lower, die, section  220  and an upper section (not shown) capable of imparting gas pressure to one side of blank  210 ′ to urge it against forming surface  224 . In common with Example 1, the periphery of the upper and lower dies substantially reproduces the upper and lower surfaces of extruded blank  210 ′. Sealing is further assured by the introduction of a seal bead  219  around the periphery of blank  210 ′. Seal bead  219  is formed by the action of a continuous protruding feature on the upper die intended to at least partially penetrate the upper surface of blank  210 ′ and thereby define a substantially leak-free pressurizable cavity within the upper die. 
     Upon exposure to appropriate gas pressure, typically up to 500 psi at a temperature of between 400 and 500° C. the extruded blank  210 ′ will adopt the general form of lower die forming surface  224  to create the formed part. For the example shown in  FIG. 3C  the originally flat blank has been shaped into the form of a rectangular pan. The central portion where stiffening ribs  212 ′ and  213 ′ remain undergoes only limited deformation, with most of the shape change occurring in the uniform-thickness periphery of the blank. A portion of the periphery  222  supports the sealing bead  219  while the remainder forms wall sections  223  and  215 ′, thereby forming a flat-bottomed rectangular pan with stiffened bottom. 
     While the example shown undergoes only limited deformation in its interior portion, alternate deformation patterns, and in consequence alternate final part geometries, may readily be promoted by adjustment of die surface  224  and by selective removal of ribs  212 ′ and  213 ′ or portions thereof. 
     Trimming off peripheral flanges  222  and  215  results in finished part  210 ″, shown in  FIG. 3D . This part may readily be adapted as a portion of a container for electronic devices as shown in  FIG. 3E , wherein components  32  and  33  are shown attached to stiffening ribs  212 ′ and  213 ′ and hinges  231  are shown attached to stiffening rib  212 ′. Thus, stiffening ribs  212 ′ and  213 ′ convey dual benefit in stiffening the flat bottom of the pan-shaped part and providing sufficient thickness to enable good retention and tensioning of removable threaded or mechanical fasteners  241 . 
     It will be appreciated that insertion of mechanical fasteners will require at least fabrication of holes in the stiffening ribs. Such holes, typically blind holes if appearance is an issue, may be made by drilling. 
     Although the present invention has been described with reference to preferred embodiments and examples, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.