Abstract:
A camshaft journal and method of producing the same. The method uses dynamic magnetic compaction in conjunction with austenitic manganese steel powder metal precursors. Journals formed along the camshaft are configured to cooperate with complementary bearing surfaces, and can be used in cooperation with one or more sensors such that the journal does not magnetically interfere with signals travelling to such sensors. The journals may also be subjected to machining, sintering or both once the dynamic magnetic compaction has been completed.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to the manufacture of non-magnetic steel automotive components using a powder metallurgy process, and more particularly to the manufacture of austenitic camshaft journals using a dynamic magnetic compaction (DMC) process. 
         [0002]    Automotive engine camshafts are used to open and close valves in synchronization with the movement of pistons, fuel and oxygen in an internal combustion engine. A typical camshaft includes a rotating shaft with a number of lobes arranged in groups (or packs) mounted along the shaft&#39;s length, where each group is configured to cooperate with one or more valves within each of the engine&#39;s cylinders. Upon rotation of the camshaft, the lobes selectively force the spring-loaded valves to open or close, depending on what stage of operation cycle a corresponding piston is in. 
         [0003]    The camshaft is mounted to the camshaft housing, cylinder head or related engine structure through cooperation of numerous journals on the camshaft and a complementary-shaped bearing formed in the housing. The camshaft journals are intermittently spaced along the shaft length such that they segment each of the groups of cams. During operation, the journal and bearing are technically not in contact, as oil or a related lubricant is inserted therebetween to form a thin film in the region between their adjacent surfaces. Throughout the remainder of this disclosure, the placement of the journal and the bearing in contact with one another will be construed to also cover the situation discussed above where the two surfaces are separated only by the thin film of lubricant. 
         [0004]    Even with such lubrication, the environment is harsh, as high temperatures, rotational speeds and attendant radial loads, coupled with the need for long-term care-free operation, dictate that the journal used for a camshaft be made from high-strength materials that can be formed to very tight tolerances. Moreover, large-scale production dictates that the journal be as inexpensive to make as possible. 
         [0005]    Austenitic manganese steels (also known as Hadfield steels or Sheffield steels) exhibit many desirable attributes that, for reasons set forth below, may be useful in camshaft journals. Such attributes include good wear resistance, toughness, ductility and non-magnetic behavior, this last attribute important for allowing the journal to be in the close proximity of one or more magnetic position sensors that can be used to provide information relating to camshaft rotation or related engine operating conditions. Traditionally, manganese austenitic steels have been produced in cast form; however, casting has a tendency to produce numerous brittle carbides at the grain boundaries. Heat treatment (for example, heating to the austenitic region and followed by water quenching) breaks down the carbides to allow for machining and other post-casting operations, but necessitates an additional processing step. Other difficulties also arise in cast austenitic manganese steels. For example, only a minimal amount of grinding is permitted, as such causes the material to go through a significant increase in work hardening and concomitant decrease in machinability. Hot forging of sintered powder compacts at elevated temperatures (for example, up to 1100° C.) may also be used; however, this method is not suitable for large-scale production, and is therefore not commercially viable. To avoid these difficulties in machinability, most components that are cast from austenitic manganese steels forego these extra steps, which unfortunately results in components that cannot take full advantage of the capability of the materials. 
         [0006]    Still other approaches to producing austenitic manganese steels, such as powder metallurgy (PM), have been contemplated. In PM, the processing route typically includes pressing (or compacting) a powder, followed by sintering. Unfortunately, the resulting components tend to have mechanical properties that are inferior to that of the conventional casting discussed above. Specifically, the high oxygen affinity of the alloy&#39;s manganese and chromium results in a material with high porosity and accompanying reduction in mechanical properties. Moreover, some degree of post-sintering machining is required, and as mentioned above, austenitic manganese steels are not amenable to machining. Other processes, such as dynamic hot pressing (DHP), where sintered powder compacts are further processed (such as by forging) at elevated temperatures, may be used. These, too have drawbacks, as problems with production scale-up, dimensional control and uniformity of microstructure may prevent such an approach from gaining acceptance. 
         [0007]    It is therefore desirable to develop a method of producing a manganese austenitic steel that is amenable to large-scale production while being capable of taking full advantage of its structural properties. It is more particularly desirable to produce camshaft journals and other high-volume production components based on manganese austenitic steels to be made using a process that is capable of producing near net shape with minimal or no machining. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    These desires can be met by the present invention, wherein improved engine components and methods of making such components are disclosed. According to a first aspect of the invention, a method of fabricating a non-magnetic camshaft journal using DMC is disclosed. The method includes providing a die or related tool with an interior profile that is substantially similar to the exterior profile of the camshaft journal being formed, where the formulated powder that would ultimately produce an austenitic manganese steel can be used. In the present context, the term “substantially” refers to an arrangement of elements or features that, while in theory would be expected to exhibit exact correspondence or behavior, may, in practice embody something slightly less than exact. As such, the term denotes the degree by which a quantitative value, measurement or other related representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue. 
         [0009]    In one form, the austenitic manganese steel is powdered. In addition, the method may optionally include placing a second material within a part of the die interior profile of the journal. As discussed above, this method is especially relevant to the production of the end journal that either holds (or is close to) the position sensor. By incorporating two different powders (i.e., one that would produce austenitic manganese steel and the other with enhanced machinability), a hybrid approach to creating a journal with tailored properties can be adopted. Such an approach could be used to produce an outer layer that takes advantage of a work hardenable material, while also keeping the non-magnetic properties of the manganese austenitic steel in certain journal locations, such as the aforementioned position sensor. In this way, the austenitic manganese steel can be placed judiciously, thereby allowing a more machinable, less expensive or other second material, which (in addition to possessing different magnetic properties) may possess different wear, friction or related tribological properties from the austenitic manganese steel. Such a second powder can be selected from metal powders, ceramic powders and a combination of both. For example, having a material with better machinability and formability would allow the journal to be assembled on the cam shaft using any conventional assembly methods. 
         [0010]    Other optional steps may be employed. For example, the DMC may be used to compact the austenitic manganese steel into a green (i.e., prior to sintering) precursor, after which such precursor can be consolidated. Such consolidating of the green precursor may include sintering. Furthermore, one or more additional shapes can be formed in the journal prior to the sintering. In one form, machining in the green state may be used to form the lubricant passageways. In configurations where oil passageways may be useful, their formation in a green state may be beneficial. Such machining may take place prior to any heat treating or related consolidation techniques. In one form, the DMC is achieved by placing the austenitic manganese steel in powder form inside an electrically conductive sleeve or related armature, and then passing electric current through a coil that is placed around the powder material and the armature such that the current induces a magnetic field and consequent magnetic pressure pulse that is imparted to the armature and the powder metal contained therein. In another option, machining of the journal after forming can be done prior to sintering in an inert or related protective atmosphere. 
         [0011]    According to another aspect of the invention, a method of fabricating a camshaft journal is disclosed. The method includes providing a die, template or related structure with an interior profile that substantially defines an exterior surface of the camshaft journal, into which a compactable austenitic manganese steel is placed. As with the previous aspect, one significant advantage over the prior art DMC process is that non-axisymmetric and related irregular component shapes can be formed. 
         [0012]    Optionally, the austenitic manganese steel is in powder form prior to placement into the die. In other options, additional steps, such as sintering or related heat treating, machining or a combination of the above can be performed to place the camshaft in a more permanent form. As with the first aspect, this aspect may also include materials with tribologically different properties than the austenitic manganese steel. In this way, metal alloys of specific composition can be strategically placed on portions of the exterior surface of the camshaft journal to tailor the material properties to the camshaft journal&#39;s needs. Alternatively, a more machinable or steel powder of magnetizable composition could be placed in the interior of the journal. 
         [0013]    According to yet another aspect of the invention, a method of making a camshaft journal is disclosed. The method includes providing a die with an interior profile that substantially defines an exterior surface of a journal, placing a compactable austenitic manganese steel within at least a portion of the die interior profile and forming at least the journal using DMC. 
         [0014]    Optionally, the method includes using powdered austenitic manganese steel. In a more particular form, machining, heat treating or both can be performed on the journal after it has been formed by the DMC process. For example, and as discussed above, the passageway can be machined into the journal when the latter is still in the green state. 
         [0015]    According to still another embodiment of the invention, a method of making a journal from multiple precursor composition is described. The method including using steel powders one of which corresponds to a relatively machinable alloy suitable for a portion of the journal, and the other of which corresponds to a substantially non-magnetic alloy suitable for use in another portion of the journal. 
         [0016]    In one optional form, the method includes the connection or related assembly of the journal onto the camshaft. In another option, the relatively machinable composition would be possessive of relatively magnetic properties, and its use would accordingly be limited to an internal portion of the journal. Furthermore, the substantially non-magnetic composition, such as those typical of Hadfield steels, would be used in an exposed, exterior portion of the journal. In another option, the first and second portions of the form are situated along a rotational axis so that when a journal produced by the form is formed, the first and second steel precursors (which correspond to a magnetic and non-magnetic steel, for example) occupy substantially distinct axial portions of the journal. In another form, the magnetic material part may be formed from a separate disk or plate that can be placed at or near one axial end of the journal so that in circumstances where a sensor is used to pick up rotational information about the camshaft, the disk or plate (which can be made to rotate along with the camshaft and journal), it can do so in conjunction with discontinuities, interruptions or related variances formed in the disk or plate periphery. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0017]    The following detailed description of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which: 
           [0018]      FIGS. 1A through 1C  shows a the various steps used in the DMC process of the prior art for making a cylindrical-shaped powder component; 
           [0019]      FIG. 2  shows a top-down view of a cylindrical part made from the DMC process of the prior art; 
           [0020]      FIG. 3  shows a view of a camshaft with journals made by a modified DMC process according to an aspect of the present invention; 
           [0021]      FIG. 4  shows another view of one of the camshaft journals from  FIG. 3 ; 
           [0022]      FIGS. 5A and 5B  show a simplified die and camshaft journal, both prior to ( FIG. 5A ) and after ( FIG. 5B ) the modified DMC process of the present invention; and 
           [0023]      FIG. 6  shows a partial cutaway view of an automotive engine with a camshaft employing one or more journals made by the modified DMC process of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    Referring initially to  FIGS. 1A through 1C  and  2 , a conventional DMC process is shown, where a generally cylindrical-shaped component is produced.  FIG. 1A  shows a powder material  10  placed within an electrically conductive cylindrical armature  20  (also called a sleeve). A coil  30  is connected to a direct current power supply (not shown) such that electric current can be passed through the coil  30 . The powder material  10  substantially fills the electrically conductive armature  20 . Referring with particularity to  FIG. 1B , a large quantity of electrical current  40  is made to flow through the coil  30 ; this current induces a magnetic field  50  in a normal direction that in turn sets up magnetic pressure pulse  60  that is applied to the electrically conductive container  20 . This radially inward pressure acts to compress the container  20 , causing the powder material  10  to become compacted into a fully densified part in a very brief amount of time (for example, less than one second) and at relatively low temperatures. In addition, this operation can (if necessary) be performed in a controlled environment to avoid contaminating the consolidated material. By way of example, the current flow through the coil  30  may be in the order of 100,000 amperes at a voltage of about 4,000 volts, although it will be appreciated that other values of current and voltage may be employed, depending on the characteristics of the container  20  and the powder material  10  inside. Referring with particularity to  FIG. 1C , the armature  20  and powder material  10  are shown compressed (occupying a smaller transverse dimension than previous size of  FIG. 1A ) as a result of the DMC process. 
         [0025]    Referring with particularity to  FIG. 2 , a top-down view of a notional cylindrical DMC containment structure according to the prior art is shown, where the loosely held powder  10  is placed in the electrically conductive round container  20  to await the compacting force that arises from the magnetic field set up by a sudden passage of a large amount of current through the coil  30 . This induced current produces a second magnetic field which, by its magnitude and direction, repels the first magnetic field. This mutual repulsion causes container  20  to be compressed, which in turn applies pressure on the powder  10 , causing its compaction. Coil  30  is placed inside an external containment shell  70  to restrain the coil  30  against radially-outward expansion when repelled by the second magnetic field. 
         [0026]    The chemical composition of austenitic manganese steels is preferably about 1.0 to 1.4 weight percent carbon (C), about 10.0 to 15.0 weight percent manganese (Mn), about 0.3 to 1.5 weight percent silicon (Si), up to about 0.07 weight percent phosphorous (P), and up to about 0.07 weight percent sulfur (S), with a balance of iron (Fe). Use of DMC to compact the specially formulated powders into desirable shape with desirable chemical composition would allow a novel way of processing this hard to process class of materials. As discussed above, the component could be subsequently machined in the green state and later sintered in a protective atmosphere as needed. In addition to avoiding the PM porosity, DMC does not require expensive, time-consuming preheating, making it compatible with green component machining and subsequent heat treating. 
         [0027]    Referring next to  FIG. 3 , a camshaft  100  that defines a generally elongate shaft body with numerous cam groups  110 ,  120 ,  130  and  140  spaced axially along the length of the body is shown. Using the first cam group  110  as an example, numerous cams  110 A,  110 B and  110 C are angularly offset relative to one another to perform the opening and closing of engine valves (not shown) in a manner known to those skilled in the art. As will also be understood by those skilled in the cam art, a lobe extends from each cam  110 A,  110 B and  110 C to define its generally non-axisymmetric axial profile. While the camshaft  100  shown has four separate cam groups  110 ,  120 ,  130  and  140  (which could be used as an intake or exhaust camshaft for a four or eight cylinder engine), it will be appreciated by those skilled in the engine art that different camshaft configurations consistent with the needs of a particular engine are also within the scope of the present invention. 
         [0028]    Journals  150 ,  160 ,  170 ,  180  and  190  are spaced between each of the cam groups  110 ,  120 ,  130  and  140 , and transmit the rotational loads of camshaft  100  to a camshaft housing, cylinder head or related engine structure (none of which are shown) through bearings that define a generally smooth surface formed as part of such structure. Unlike the cams within the various cam groups  110 ,  120 ,  130  and  140 , the journals  150 ,  160 ,  170 ,  180  and  190  define a generally axisymmetric profile to facilitate smooth rotational cooperation with the respective bearings. Cam caps (also not shown) can be used to form the remaining inner race of the bearings as a way to secure the journals  150 ,  160 ,  170 ,  180  and  190  within the bearings. In one form, the journals  150 ,  160 ,  170 ,  180  and  190  can be secured to the camshaft  100  through shrink-fitting or other known methods. Camshaft  100  may include additional components formed on or mounted to the body, such as a gear  200  that can engage a crankshaft gear (not shown), and a gear  210  that can be used to drive a distributor cap or oil pump (neither of which is shown). As discussed above, a premixed powder of the desired composition can be introduced into a die cavity formed in the shape of the various components of camshaft  100 , especially the journals  150 ,  160 ,  170 ,  180  and  190 . 
         [0029]    Referring next to  FIG. 4 , one of the journals  150  is shown coupled to a part of the camshaft  100  where a gear (such as gear  210  shown in  FIG. 3 ) or other component may be placed. A thin disk  300  of conventional steel is placed axially adjacent or in contact with one end of the journal  150 , and includes one or more slots  305  formed at the periphery thereof. In one form, the disk  300  can be rigidly affixed to the shaft by known means so that it rotates at the same rate as the journal  150  and by extension, the camshaft  100 . The disk  300 , which can be made from a conventional powder metal approach or be formed and then have the slots  305  cut into them, can be used in conjunction with magnetic sensor  400  to generate a signal that corresponds to the passage of the slots  305  (and their concomitant magnetic field discontinuity), which in turn provide indicia of the rotational state of camshaft  100 . A wire  405  conveys the sensed signal from sensor  400  to a controller (not shown) or related device to provide timing or other operating information. The manganese steel composition and its attendant non-magnetic property would be beneficial for the journal  150 , as otherwise its presence in a magnetic metal form would interfere with the generation of signals in sensor  400 . In one optional feature, an oil (or lubricant) passageway (not shown) can be formed in journal  150  in order to deliver lubricant to the bearing and journal  150  through an internal fluid coupling formed inside the journal  150 . 
         [0030]    DMC tooling (including the wiring that will allow the passage of electrical current) is placed around the die cavity. Upon the passage of electric current (and the consequent creation of a pair of opposing magnetic fields), the powder in the die cavity compacts into a near-net shape. Likewise, any sintering, if needed, can be achieved in a controlled atmosphere furnace, wherein the amount of oxygen in the furnace is tightly controlled, This step may be followed by controlled cooling that may or may not include water quenching. By performing any of the machining or other post-compaction operations prior to a final sintering step, the present method overcomes the difficulty that fully-processed austenitic manganese steels (with their attendant hardness) of the prior art have experienced. In this way, austenitic manganese steel journals can be made that were hitherto not feasible as a large-scale production material. 
         [0031]    Referring next to  FIGS. 5A and 5B , a setup  500  used to make the camshaft journal using the DMC process according to an aspect of the present invention is shown. An electrically-conducting coil  530  is wound around a sleeve  525  (made from a highly electrical conductive material, such as copper) that is placed circumferentially around a powder mass  540  contained in a die  550 . As shown, a gap (for example, and air gap)  135  is situated between coil  530  and sleeve  525 . As with conventional DMC, the present DMC-based process exploits the electric current flowing through coil  530  in order to impart a magnetically-compressive force onto the sleeve  525 , die  550  and the powder precursor materials  540  within. The inner surface of die  550  is similarly shaped to the desired outer shape of the journal  150  of the camshaft  100  being formed. The die  550  may include numerous reusable segments that can come in various shapes (for example, quartered), not shown). A central bore can be formed in the journal  150  through the inclusion of an appropriately-shaped mandrel or core rod  560  during the forming process. Sleeve  525  is compressed by the magnetic forces generated by coil  530 , as is die  550 ; this in turn causes the powder precursor materials  540  to be deformed by the compressive forces, resulting in formation of a green or un-sintered journal  150  that may subsequently undergo conventional sintering, machining and related finishing steps (none of which are shown). As discussed above, a separate disk  300  (shown in  FIG. 4 ) can be coupled to one or more of the journals  150 ,  160 ,  170 ,  180  and  190  to allow the use of a sensor or related device to derive operational parameters (such as rotational attributes) from the camshaft  100 . 
         [0032]    Referring with particularity to  FIG. 6 , portions of the top of an automotive engine  1000  incorporating a camshaft  100  with one of the cams  110 A of cam group  110  is shown for a notional direct-acting tappet overhead cam design. A cylinder head  1200  includes intake ports  1240  and exhaust ports  1250  with corresponding intake and exhaust valves  1400 ,  1500  to convey the incoming air and spent combustion byproducts, respectively that are produced by a combustion process taking place between the piston  1300  and a spark plug (not shown) in the cylinder. When the lobed portion of the cam  110 A rotates into engagement with top of valve  1500 , it pushes the valve  1500  downwards to overcome the bias formed by spring  1600 , thereby forcing the valve  1500  to open and allowing exhaust gas built up in the cylinder above piston  1500  to escape. As discussed previously, camshaft  100  is driven from an external source, such as a crankshaft (not shown) through the gear  200  depicted in  FIG. 4 . It will be appreciated that similar structure is included for the intake valve  1400 , but is removed from the present drawing for clarity. The hardness of the austenitic manganese steel ensures that the journals  150 ,  160 ,  170 ,  180  and  190  can endure significant loads over prolonged periods of operation, while is nonmagnetic character ensures that it will not interfere with magnetic-based sensors (such as sensor  300  shown in  FIG. 4 ) disposed nearby. It will be appreciated by those skilled in the art that the valve train architecture shown associated with engine  1000 , which includes a direct-acting tappet, is merely representative, and that camshaft  100  and its journals  150 ,  160 ,  170 ,  180  and  190  manufactured using the DMC process as described herein are equally applicable to other valve train architectures (not shown). 
         [0033]    While certain representative embodiments and details have been shown for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes may be made without departing from the scope of the invention, which is defined in the appended claims.