Abstract:
A method of making a valve for use in a combustion engine includes forming a valve core of a titanium material, the valve core including a head and an elongated stem extending away from the head with a curved region connecting the head and the elongated stem. The elongated stem includes a central guide region that is substantially uniform in outer dimension and free of any undercut. Surface portions of the valve core are finished to a suitable roughness average, the finishing occurring at least in the central guide region of the elongated stem. A coating process is used to apply and bond a multi-layer coating to finished surface portions of the valve core including the central guide region of the elongated stem.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/633,819, filed Dec. 7, 2004, the entirety of which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD  
       [0002]     The present application relates to valves and more particularly to a coated valve for use in an automobile engine.  
       BACKGROUND  
       [0003]     Intake and exhaust valves are used to control flow of fluid into and out of a combustion chamber of an engine utilizing a machined seating surface that can mate with a corresponding valve seat. Typically, the valves include an enlarged head including the seating surface and a stem that can be mechanically linked to a control system for controlling movement of the valves. During use, the valves reciprocate within a guide sleeve, opening and closing respective passageways in communication with the combustion chamber.  
         [0004]     To reduce wear on the valve due to sliding contact with the guide sleeve, valves have been proposed that include a stem having an undercut region that is filled with stainless steel and molybdenum alloy by plasma spraying. This stainless steel and molybdenum alloy spray filling reduces the coefficient of friction between the guide sleeve and the valve.  
       SUMMARY  
       [0005]     In an aspect, a method of making a valve for use in a combustion engine is provided. The method includes forming a valve core of a titanium material, the valve core including a head and an elongated stem extending away from the head with a curved region connecting the head and the elongated stem. The elongated stem includes a central guide region that is substantially uniform in outer dimension and free of any undercut. Surface portions of the valve core are finished to a roughness average of at most about three micro inches, the finishing occurring at least in the central guide region of the elongated stem and in the curved region connecting the head and the elongated stem. A physical vapor deposition process is used to apply and bond a multi-layer coating to finished surface portions of the valve core including the central guide region of the elongated stem and the curved region connecting the head and elongated stem. The multi-layer coating includes a material with a hardness greater than that of the titanium material.  
         [0006]     In another aspect, a valve for use in an engine includes a valve body including a stem portion and an enlarged head portion. The valve body includes a valve core including a titanium material and having a surface polished to a roughness average of at most three micro inches. The valve body further includes a physical vapor deposition coating that has one or more layers of metallic coating material bonded to the polished surface of the valve core.  
         [0007]     The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a side view of an embodiment of a valve;  
         [0009]      FIG. 1A  is a section view of the valve of  FIG. 1  along line A-A;  
         [0010]      FIG. 1B  is a detail view at area B of  FIG. 1A ;  
         [0011]      FIG. 2  is a diagram of an embodiment of a process of making the valve of  FIG. 1 ; and  
         [0012]      FIG. 3  is a section view of the valve of  FIG. 1  located in a guide sleeve within a combustion engine. 
     
    
     DETAILED DESCRIPTION  
       [0013]     Referring to  FIG. 1 , a valve  10  (e.g., suitable for use as an intake or exhaust valve of a combustion engine) has a valve body  12  including an enlarged head  14  connected to a stem  16  that extends to a free distal end  18  opposite the head. The head  14  includes an annular seating surface  26  formed along its periphery that can be used to seal an intake or exhaust passageway of a combustion chamber. Referring also to  FIGS. 1A and 1B , the valve  10  includes a multilayer physical vapor deposition coating  20  deposited on a micro polished surface  50  of a metal core  22 . The multilayer physical vapor deposition coating  20  can improve certain physical characteristics of the metal core, such as lubricity and corrosion resistance, while the micro polished surface can remove surface abnormalities, sometimes referred to as “stress risers” that may lead to valve failure during operation.  
         [0014]     The head  14  includes a face  42  and an opposite side  44  from which the stem  16  extends. A contoured surface  45  extends from the outer periphery of the head  14  to the stem  16  providing a radiused transition therebetween. The face  42 , as shown in  FIG. 1A , has a relatively planar annular portion  46  and a central recessed portion  48  bounded by the planar portion. Recessed portion  48  can be provided to reduce the weight of the valve  10 . Alternatively, the face  42  may be flat, with no recessed portion. The seating surface  26  is designed to mate with a corresponding valve seat (not shown). As shown, the seating surface  26  has contoured regions  26   a  and  26   b  having linear profiles (see  FIG. 1A ) that extend at an angle to each other. The seating surface can be of any suitable design for mating with a valve seat.  
         [0015]     Extending inwardly from an outer surface  30  at the distal end  18  of the stem  16  is an annular groove  28  ( FIG. 1 ). The annular groove  28  can be used to connect the valve  10  to a valve control system of an automobile engine, for example. To provide reinforcement for the stem  16 , for example, when the valve  10  is attached to a rocker arm, an insert  32  (e.g., formed of hardened steel) is disposed within a recess  34  at the distal end  18  of the metal core  22 . In other embodiments, the valve  10  does not include the insert  32  or recess  34 . In cases where no insert  32  is provided, a lash cap (not shown) may be used to reinforce the stem  16 .  
         [0016]     Referring still to  FIG. 1A , metal core  22  provides the structure to which the multilayer physical vapor deposition coating  20  is bonded. The shape of the metal core is similar to that of the finished valve body  12  in that it includes an enlarged head portion  52  and a stem portion  54 . An interface  56  connects the head and stem portions  52  and  54 . In some embodiments, the head and stem portions  52  and  54  are formed separately, at least to some degree, (e.g., by forging) and then connected together (e.g., by welding). While the interface  56  is shown as being substantially perpendicular to the elongated axis of the stem portion  54 , it can be any suitable configuration, such as angled to provide a relatively smooth transition from the head portion  52  to the stem portion  54 .  
         [0017]     The surface  50  of the metal core  22  is micro polished, which can remove stress risers that can lead to valve failure during operation. In some embodiments, surface  50  is micro polished to a roughness average (Ra) of no more than about three micro inches. Providing a suitably microfinished, superfinished or polished surface prior to deposition of the hard coating aids in reducing the wear that the coating will cause on the guide ring or guide sleeve. Roughness average (Ra) variations may be possible depending on the hardness of the guide sleeve that will be used. In some embodiments, only a portion (e.g., only the stem portion) of the surface  50  of the metal core  22  is micro polished. Suitable materials for forming the head and stem portions  52 ,  54  of the metal core  22  may include titanium or titanium alloys, such as Ti-64, Ti-62S, Ti-6246 or Ti-6242-Si.  
         [0018]     Referring to  FIG. 1B , multilayer physical vapor deposition coating  20  is a composite formed of numerous individual layers  60  of material. As shown, the layers  60   a - 60   n  are stacked, one over the other, with each layer bonded to at least one adjacent layer of coating material. In some embodiments, multilayer physical vapor deposition coating  20  includes at least about 1000 layers, such as about 2700 layers. However, the number of layers can be increased or decreased as desired.  
         [0019]     The arrangement of the layers  60   a - 60   n  can also be selected as desired. The arrangement may depend on, for example, the materials used to form the individual layers. For example, in one embodiment, the multilayer coating is a composite formed of alternating layers of titanium, titanium nitride and tungsten carbide, respectively. In other embodiments, multiple adjacent layers may be formed of the same material. One example of a suitable, commercially available composite coating material is Balnit® Futura Nano, available from Balzers, Inc. of Brunswick, Ohio.  
         [0020]     As shown, layers  60   a - 60   n  have substantially the same thickness, however, the thicknesses of the layers can vary. The thickness of the individual layers  60   a - 60   n  may also depend on, for example, the materials used to form the individual layers or the process used to form the individual layers. By way of example, the total thickness of the multilayer physical vapor deposition coating  20  including layers  60   a - 60   n  can be in the range of between about 2×10 −4  and about 16×10 −4  inch (e.g. in the range of between about 5×10 −4  and about 12×10 −4  inch). As one example, the total thickness of the multilayer physical vapor deposition coating is about 8×10 −4  inch. Coating thicknesses of less than and greater than 8×10 −4  inch are also contemplated. A suitable physical vapor deposition coating process is commercially available from Balzers, Inc.  
         [0021]     As noted above, the multilayer physical vapor deposition coating  20  improves physical properties of the metal core  22 . As shown, the multilayer physical vapor deposition coating  20  covers the entire metal core  22 , except at face  42  where the valve body  12  is free of the multilayer physical vapor deposition coating. However, the multilayer physical vapor deposition coating  20  can be bonded to the metal core  22  where desired. In some embodiments, the multilayer physical vapor deposition coating  20  provides a coefficient of friction against steel (dry) of about 0.35, a micro hardness (HV 0.05) of about 3300 and an oxidation temperature of about 1650 degrees Fahrenheit.  
         [0022]     Referring to  FIG. 2 , a flow diagram  70  of a suitable process for forming the valve  10  is shown. In a forging operation  72 , a core head preform and a core stem preform are formed to respective preliminary shapes. The head preform is qualified to a desired shape by a machining operation  74 , for example, using a CNC lathe. The head and stem preforms are then connected together by any suitable connecting method  76  such as by friction welding to form a core preform, and the core preform is heat treated at a desired temperature during a heat treating operation  78 . After heat treating, the core preform is machined during a machining operation  80  (e.g., using a CNC lathe) to preliminary dimensions and the insert  32  is placed in recess  34  at step  82 . Additional material is removed from the stem portion of the core preform by a centerless grinding operation  84  (e.g., to reduce the roughness average (Ra) of the surface to between about 15 and four micro inches, for example, by removing about 4×10 −4  inch of material) and then the core preform is machined at step  86  to final preform dimensions. The stem portion and the curved head portion up to the seating surface  16  are then micro polished during a polishing operation  88 , for example, to a roughness average of no more than three micro inches, which can remove surface stress risers caused by surface abnormalities forming the metal core  22 . Other portions of the core preform may also be micro polished, such as the face  52 . In some embodiments, during micro polishing about 4×10 −4  inch of material is removed. The multilayer coating  20  may then applied to the entire outer surface of the core  22 , except on the face  52  of the head in a coating operation  90  (e.g., using a vapor deposition process) and a final polishing operation  92  is performed in which a small amount (e.g., one micro inch or less) of coating material is removed forming the valve  10 .  
         [0023]     Referring now to  FIG. 3 , during use, the stem  16  of the coated valve  10  may reciprocate within a guide sleeve  100 . The guide sleeve  100  aids in aligning the valve  10  so that the seating surface  26  seats against a valve seat, e.g., to seal a combustion chamber (not shown). The relatively smooth multilayer coating  20 , by providing a relatively low coefficient of friction between the valve and the guide sleeve, reduces the wear on the valve due to sliding contact between a central guide region of the stem and the guide sleeve, which can increase the useful life of the valve. The coated, micro polished surface  50  reduces stress risers that can also lead to valve failure. Additionally, the multilayer coating  20  is applied to the micro polished surface  50  of the valve without any need for undercutting the stem portion  54  in the central region of the stem between the annular groove  28  and the head  14 , which can improve the strength and durability of the valve  10 .  
         [0024]     It is to be clearly understood that the above description is intended by way of illustration and example only and is not intended to be taken by way of limitation, and that changes and modifications are possible. For example, in some embodiments, face  52  may be coated with the multilayer physical vapor deposition coating  20 . In some embodiments, the metal core may be formed as a single piece, e.g., by a forging operation. Although, coatings by physical vapor deposition have been primarily described, other coating processes can be used. Accordingly, other embodiments are contemplated.