NON-MAGNETIC CAMSHAFT JOURNAL AND METHOD OF MAKING SAME

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

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.

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's length, where each group is configured to cooperate with one or more valves within each of the engine'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.

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.

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.

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.

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'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.

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

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.

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.

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.

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.

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's needs. Alternatively, a more machinable or steel powder of magnetizable composition could be placed in the interior of the journal.

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.

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.

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.

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

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:

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;

FIG. 2 shows a top-down view of a cylindrical part made from the DMC process of the prior art;

FIG. 3 shows a view of a camshaft with journals made by a modified DMC process according to an aspect of the present invention;

FIG. 4 shows another view of one of the camshaft journals from FIG. 3;

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

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

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.

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.

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.

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 110A, 110B and 110C 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 110A, 110B and 110C 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.

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.

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.

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.

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.

Referring with particularity to FIG. 6, portions of the top of an automotive engine 1000 incorporating a camshaft 100 with one of the cams 110A 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 110A 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).

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.

Claims

1. A method of fabricating a camshaft journal using dynamic magnetic compaction, said method comprising:

providing a camshaft die with an interior profile that substantially defines an exterior profile of said journal;

placing, within at least a part of said interior profile a powdered austenitic manganese steel such that upon formation of said journal with said camshaft die, at least the portion of said exterior profile that corresponds to said journal is made from said austenitic manganese steel; and

subjecting said powdered austenitic manganese steel in said camshaft die to said dynamic magnetic compaction.

2. The method of claim 1, wherein said dynamic magnetic compaction comprises compacting said powdered austenitic manganese steel into a green precursor.

3. The method of claim 2, further comprising consolidating said green precursor.

4. The method of claim 3, wherein said consolidating said green precursor comprises sintering.

5. The method of claim 3, wherein said consolidating said green precursor comprises machining.

6. The method of claim 3, further comprising forming an additional shape in said journal prior to said sintering.

7. The method of claim 1, further comprising arranging a position sensor in signal cooperation with said journal such that said journal's substantially lack of magnetic properties prevent it from substantially causing any degradation to a signal extending between a rotated camshaft to which said journal is coupled and said sensor.

8. A camshaft journal made by the method of claim 1.

9. A method of fabricating a camshaft journal, said method comprising:

providing a die with an interior profile that substantially defines an exterior surface of said journal;

placing a compactable austenitic manganese steel within at least a portion of said interior profile of said die; and

forming said journal using dynamic magnetic compaction.

10. The method of claim 9, wherein said austenitic manganese steel is in powder form prior to placement into said die.

11. The method of claim 9, further comprising heat treating said journal after said forming.

12. The method of claim 11, wherein said heat treating comprises sintering.

13. The method of claim 9, further comprising machining said journal after said forming.

14. The method of claim 13, further comprising sintering said journal after said machining and forming.

15. The method of claim 13, wherein said machining said journal after said forming takes place prior to sintering in a protective atmosphere.

16. The method of claim 9, further comprising machining and heat treating said journal after said forming.

17. A method of making a camshaft journal, said method comprising:

providing a die with an interior profile that substantially defines an exterior surface of said journal;

placing a compactable austenitic manganese steel within at least a portion of said interior profile of said die; and

forming at least said journal using dynamic magnetic compaction.

18. The method of claim 17, wherein said compactable austenitic manganese steel is in powdered form prior to said forming.

19. The method of claim 18, further comprising performing at least one of machining and heat treating said journal after said forming.

20. A method of making a camshaft journal from multiple precursor compositions, said method comprising:

defining a form that substantially corresponds to a shape of said journal;

arranging a first steel precursor in a first portion of said form, said first steel precursor configured such that said portion of said journal that corresponds thereto possesses relatively machinable properties;

arranging a second steel precursor in a second portion of said form, said second steel precursor configured such that said portion of said journal that corresponds thereto possesses substantially non-magnetic properties; and

applying dynamic magnetic compaction to said first and second steel precursors in said form.

21. The method of claim 20, further comprising connecting said journal onto a camshaft.

22. The method of claim 20, wherein said first portion of said form corresponds to at least a portion of the outer surface of said journal and said second portion of said form corresponds to at least a portion of the inner surface of said journal.

23. The method of claim 20, wherein said first and second portions of said form are situated along a rotational axis of said journal such that said first and second steel precursors occupy substantially distinct axial portions of said journal.

24. The method of claim 23, further comprising forming at least one signal-generating interruption in a portion of said journal that corresponds to said first portion of said form.

25. The method of claim 24, wherein said at least one signal-generating interruption corresponds to a cutout in a peripheral portion of said journal.