Patent Publication Number: US-2006005792-A1

Title: Lightweight valve

Description:
The present invention is directed to a lightweight valve as recited in the definition of the species in claim  1 , as known for example from DE 198 04 053 A1. In addition, the present invention relates to a method for producing such a lightweight valve as recited in the definition of the species in claim  5 .  
      The use of weight-optimized valves in combustion engines renders it possible to significantly reduce the power losses to friction in valve operation. This is especially significant for combustion engines with high rotational speeds, but also plays an important role for alternative valve operating systems that are not based on a conventional camshaft control. Besides the use of light materials (such as silicon nitride ceramic, titanium or aluminum alloys or titanium aluminides), the valve weight may be reduced in particular by incorporating cavities into the valve stem and/or the valve cone.  
      Known from the species-forming German Patent Document DE 198 04 053 A1 is a hollow lightweight valve having a stem, a valve cone, and a valve disk, the valve cone and valve disk together forming a cavity. The valve cone and valve disk are thin-walled individual parts which are joined to each other and to the valve stem by soldering or welding. To achieve high strength and rigidity of the valve—in particular in the area of the valve cone—despite the minimal wall thickness, the cavity of the valve is provided with a support structure which braces the valve disk cover against the stem. This support structure is intended to minimize the deformation of the valve head under load and to inhibit the formation of cracks in the area of the valve head.  
      In the embodiments of the lightweight valve shown in DE 198 04 053 A1, the valve cone and valve disk are joined via fillet welds; possible welding methods for this are in particular the common fusion welding methods such as TIG, laser or electron beam welding. However, these welding methods are only conditionally usable due to the comparatively high heat development when used in the case of thin-walled valve geometries, in particular when the cavity of the valve is filled with a metal cooling medium: Because of the spatially tightly proximate arrangement of cooling metal and welding surface, there is a danger in these cases that the cooling metal will hot-melt and reach the weld area, which may result in a significant reduction in the strength and tightness of the weld. In order to avoid this problem, the cooling medium may be added later—i.e., after the welding is finished; however, this is accompanied by an additional procedural step of closing the cavity, and is therefore very expensive. A further disadvantage of the forenamed welding methods is that the process times (allowing for positioning and aligning the valve disk with respect to the valve cone, possibly subsequently adding the cooling metal and then closing the cavity) are uneconomically long. For some material combinations of the valve disk and valve cone, for example for parts made of titanium-based alloys, a protective gas atmosphere or vacuum is also necessary when using this welding method, which further increases the production expense and hence the costs of such a lightweight valve.  
      The present invention is therefore based on the object of providing a lightweight valve which has high stability in the face of the thermal and mechanical operating loads and is also simple and economical to produce. In addition, the present invention is based on the object of making a production method for such a lightweight valve available which is suitable for large series production.  
      The object is achieved according to the present invention by the features of claims  1  and  5 .  
      Accordingly, the lightweight valve is made up of a valve body—which for its part includes the valve stem and the hollow valve cone—and a valve disk cover, which is joined to the valve cone via a compression connection welding method. Formed between valve cone and valve disk cover when the valve is in the assembled position is a weight-reducing cavity which is provided with a strength-increasing support structure.  
      The compression connection welding methods have the advantage—in contrast to the common fusion welding methods—that these methods involve locally closely limited warming of the welding zone. Therefore, when these methods are used negligible warping of the workpieces occurs. Furthermore, process-secure welding of the thin-walled valve components is therefore possible without the danger of weakening the welding zone (for example through contamination of the welding surface by molten cooling metal). Moreover, with the use of these methods, it is possible to join a broad spectrum of different material combinations without protective gas. Furthermore, with the compression connection welding methods—in contrast to fusion welding—there is no “welding path” in the actual meaning of the word, along which a welding head would have to be guided; therefore when using a material-displacing welding method it is possible to utilize the rotational symmetry of the valve being produced to achieve a very simple—and hence economical—highly precise relative positioning of the individual parts in the employed welding device.  
      Possible welding methods for joining the valve body with the valve cover are in particular friction welding or a resistance compression welding method.  
      In friction welding, the heat needed to weld the valve disk cover to the valve body is produced by relative movement of the individual components, which are pressed against each other (see claim  6 ). To this end, the valve body for example is set in rotation, while the valve disk cover is firmly clamped into an axially movable device and pressed against the rotating valve body. When the temperature and plasticity needed for welding are reached, the rotating valve body is braked and at the same time the contact pressure is increased, so compressing the valve body against the valve disk cover produces a welding of the two parts in a ring-shaped contact zone. The welding parameters (rotational speed, friction force, moment of braking, and compression, etc.) depend on the combination of materials and the geometry of the joining parts in the welding zone.  
      In resistance compression welding (for example projection welding or capacitor discharge welding), the workpieces to be welded—valve body and valve disk cover—are clamped into the welding device in such a way that the two workpieces are touching each other along a ring-shaped contact zone. The valve body and valve disk are welded to each other in this contact zone by the high current flow (caused for example by the discharge of a capacitor), so that a continuous ring-shaped connection is formed between the two workpieces (see claim  7 ). Since the welding pulse is very short (about 10 to 15 milliseconds in capacitor discharge welding), and since the currents are introduced into a locally closely limited area, only slight warping of the workpiece occurs.  
      In both projection welding and capacitor discharge welding, the quality of the resulting weld depends substantially on a continuous ring-shaped contact area being formed between the valve body and valve disk cover, along which the local material heating and welding occur. In an especially easily producible embodiment, the valve disk cover blank has a circumferential edge in the edge area on the side facing the valve body, which meets the valve body on a conically shaped area of the valve body in the assembled position (see claim  8 ). In another easily produced embodiment of the present invention, the valve disk cover blank is provided with an edge area, part of which is the shape of a truncated cone, while the valve body blank has an edge between a hollow cylindrical section and a planar section in the area of contact with the valve disk cover blank (see claim  9 ). In both cases, a ring-shaped edge meets a conically shaped opposite area, resulting in a high-strength ring-shaped weld.  
      Advantageously, a one-piece valve body blank (having a valve stem and a valve cone) is used to make the lightweight valve (see claim  2 ). This has the advantage that the production of the valve body blank does not require an additional procedural step to join the valve stem to the valve cone; furthermore, the risk of a loss of strength due to faulty joining of the individual parts is eliminated in the case of one-piece valve bodies.  
      In an advantageous embodiment of the present invention, the cavity between the valve cone and valve disk cover is filled with a cooling medium, which improves the dissipation of heat from the thermally heavily loaded areas of the valve disk cover and the adjoining zones of the valve cone (see claim  3 ). Sodium is used in particular as the cooling medium. The good heat conductivity of sodium is utilized here, but in particular the transfer of the heat by the vibrating motion of the valve in operation, whereby hot sodium is transported to cooler areas, gives off heat there, and when cooled is available again to absorb heat in the hotter disk area. Instead of sodium, other metals with a low melting point such as potassium or potassium-sodium alloys may also be used.  
      It is particularly advantageous to extend the cavity in the interior of the valve into the valve stem (see claim  4 ). This offers great advantages, in particular when the cavity is filled with a cooling medium, since in this case the cooling medium may be transported by the vibrating motion of the valve from the hot area of the valve disk into the cooler interior of the stem, where it undergoes particularly effective cooling due to the greater temperature differential. 
    
    
      The present invention will be explained in greater detail below on the basis of a plurality of exemplary embodiments depicted in the drawing.  
       FIG. 1   a  shows a lightweight valve according to the present invention;  
       FIG. 1   b  shows an alternative design of the lightweight valve according to the present invention;  
       FIG. 2  shows a schematic representation of the procedural steps in making the lightweight valve of  FIG. 1   a : valve body blank and valve disk cover blank . . .  
       FIG. 2   a  . . . before welding;  
       FIG. 2   b  . . . during welding  
       FIG. 2   c  . . . in the finished welded state.  
       FIG. 3  shows an alternative design of the blanks to be welded. 
    
    
       FIG. 1   a  shows a schematic representation of a lightweight valve  1  according to the present invention including a valve body  2  and a valve disk cover  3 , which are welded together by a compression connection welding method. Valve body  2  is made up of a valve shaft  4  and a hollow valve cone  5 , and has a one-piece design in the present exemplary embodiment. Valve cone  5  and valve disk cover  3  together form valve head  6 . A weight-optimizing cavity  7  is formed between valve cone  5  and valve disk cover  3 . A support structure  8  positioned in cavity  7  supports valve disk cover  3  vis-a-vis stem  4 ; in the present case support structure  8  is formed by a pin  9  centered in cavity  7 . Instead of one-piece valve body  2  shown in  FIG. 1 , a valve body assembled of a plurality of individual parts (for example using different materials for the stem and the valve cone) may be utilized.  
      Valve disk cover  3  may be welded to valve cone  5  according to the present invention, using capacitor discharge welding. The associated procedural steps are represented schematically in  FIGS. 2   a  through  2   c . A valve body blank  10  is assumed that is provided with an internal cavity  11  in the area of valve cone  5 —as depicted in  FIG. 2   a . In interior space  11  of valve cone  5  is a support structure  8 , which projects a predefined depth into interior space  11 . A conically shaped joining zone  13  is provided on wall  12  of interior cavity  11 . Valve body blank  10  may be produced by shaping (forging, extrusion, etc.) and/or by machining. The other part in the joint is a valve disk cover blank  14 , which has the form of a cylindrical disk  15  in the present example; joining zone  16  provided on valve disk cover blank  14  thus has the form of a ring-shaped edge  17  with a right-angled contour.  
      For welding the two joining parts  10 ,  14 , valve disk cover blank  14  is inserted into cavity  11  of valve body blank  10 ; circumferential edge  17  of valve disk cover blank  14  then makes linear contact with conical joining zone  13  in interior cavity  11  of valve cone  5 . Then, using a compressor discharge welding device  18  (indicated schematically by broken lines in  FIG. 2   b ), valve disk cover blank  14  is pressed into interior space  11  of valve body blank  10  (arrow  19  in  FIG. 2   b ), and at the same time the capacitor integrated into the power circuit of welding device  18  is discharged; because of the high currents flowing through joining parts  10 ,  14 , edge  17  is welded to joining area  13  opposite it on valve body blank  10 , so that a continuous ring-shaped connection  20  is formed between valve disk cover blank  14  and valve body blank  10 , and cavity  7  formed between the two joining parts  10 ,  14  is closed off tightly from the outside world. Since the welding pulse in capacitor discharge welding is very short, 10-15 milliseconds, only slight warping of joining parts  10 ,  14  occurs. The flat underside of valve disk cover blank  14  ensures a large contact area  21  with welding die  22  of capacitor discharge welding device  18 . This contact area  21  is parallel to ring-shaped edge  17 , which permits precisely directed and uniform pressing of entire edge  17  against opposing joining surface  13  of valve body blank  10 . The fact that contact area  21  is significantly larger than the (approximately linear) area of contact of edge  17  on joining area  13  of valve body blank  10  ensures that the material heating and plasticizing during welding takes place reliably at edge  17 .  
      Conical angle  23  of conical joining area  13  is preferably between 10° and 80°. The diameter of disk  15  is matched to the diameter and conical angle  23  of joining area  13  and the welding parameters (current strength, contact pressure, etc.) are chosen so that valve disk cover blank  14  penetrates so deeply into interior cavity  11  during welding that it rests against support structure  8 ; this ensures that valve disk cover  3  is braced against valve stem  4  by support structure  8  during later operation.  
      The use of capacitor discharge welding allows a broad spectrum of different materials to be welded, so that the materials of valve body  10 ,  2  and valve disk cover  14 ,  3  may be selected according to the other (e.g., functional) requirements. In particular, all known valve materials, as well as for example titanium aluminides, iron aluminides, metal matrix composite materials, titanium and aluminum alloys etc., may be utilized and combined with each other. The method is thus also usable in particular for applications for which other welding methods are unusable, or usable only with difficulty.  
      An alternative design of joining areas  13 ′,  16 ′ on valve body blank  10 ′ and valve disk cover blank  14 ′ is shown in  FIG. 3 : In this case, joining area  16 ′ on valve disk cover blank  14 ′ has the form of a truncated cone, while a circumferential edge  13 ′ is provided on valve body blank  10 ′. Analogous to the example of  FIGS. 2   a  and  2   b , the contact area between the two blanks  10 ′,  14 ′ is provided here too by a ring-shaped circumferential linear contour. In addition to the exemplary embodiments of joining parts  10 ,  14  shown in  FIGS. 2   a  and  3 , any other desired geometries are possible; it is essential that two joining parts touch each other in a ring-shaped circumferential linear contact zone when in position for assembly.  
      As an alternative to capacitor discharge welding, the two parts to be joined may also be joined by projection welding, circumferential edge  17 ,  13 ′ acting on valve body blank  10 ,  10 ′ or valve disk cover blank  14 ,  14 ′ as an ignition-initiating projection.  
      In addition, the two parts to be joined may be joined to each other by friction welding. In this case, valve disk cover blank  14  for example is held in the friction welding machine so that it is not able to rotate but may move axially, while valve body blank  10  is mounted and driven so that it rotates in a fixed position. First, valve disk cover blank  14  is pressed against conical joining area  13  of valve cone  5 , initially still with moderate axial force, the material of both parts located near the contact zone heating up and becoming soft as a result of the friction. If a temperature appropriate for welding is reached and a doughy condition is reached in the contact zone of the parts, rotating valve body blank  10  is very quickly stopped and at the same time the axial force of valve disk cover blank  14  is increased and the latter is pressed into internal cavity  11  of valve cone  5 , by a certain axial stroke. As that happens, parts  10 ,  14  become deeply welded to each other at the contact zone. In contrast to projection and capacitor discharge welding, friction welding does not require the contact area between the two joining parts  10 ,  14  to be linear; instead it may be useful—depending on the wall thickness and the geometry of joining parts  10 ,  14  in the joining area—to provide a two-dimensional contact zone.  
      After joining parts  10 ,  14  have been welded, valve  1  is machined; an area of valve disk cover blank  14  that projects from valve cone  5  is machined down to the desired dimension (broken line  24  in  FIG. 2   c ) and any remaining welding burr etc. is removed.  
      An alternative embodiment of a lightweight valve  1 ′ according to the present invention is depicted in  FIG. 1   b . In this case, cavity  7 ′ extends into valve stem  4 ′. In this exemplary embodiment, support structure  8  is formed by a plurality of pins  9 ′, which are positioned equidistant on a circular arc. Cavity  7 ′ is filled with a cooling medium  25  (for example sodium), which is present in liquid aggregate state at the normal operating temperatures of valve  1 ′. During operation of valve  1 ′, cooling medium  25  therefore flows through cavity  7 ′ and thus supports the dissipation of heat from the hot area of valve head  6 ′ into cooler stem area  4 ′. To produce valve  1 ′ of  FIG. 1   b , interior space  11  of valve body blank  10 ,  10 ′ is first filled with cooling medium  25 , and then—using one of the methods described above—is welded to valve disk cover blank  14 ,  14 ′. For the welding, the cooling medium is either pressed in its solid aggregate state into interior space  11  of valve body  10 ,  10 ′ and held in place there by support structure  8 , and/or valve body blank  10 ,  10 ′, containing (liquid or solid) cooling medium  25  in its interior space  11 , is oriented vertically during welding in such a way that cooling medium  25  cannot escape.  
      Alternatively to or in addition to pins  9 ′ shown in  FIGS. 1   a  and  1   b , inner support structure  8  may also have ring-shaped circumferential support walls and/or laterally projecting support ribs. The wall thickness of valve cone  5  or of valve disk cover  3  may be purposefully optimized—taking into account the design of support structure  8 —in order to further reduce the weight of valve  1 ,  1 ′.