RACE AND METHOD OF MAKING SAME

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. provisional patent application serial number 61/286,642 titled "Composite Powder Metal Constant Velocity Joint Inner Race and Method" and filed on December 15, 2009. The full contents of that application is incorporated by reference as if set forth in its entirety herein.

STATEMENT OF FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not applicable.

TECHNICAL FIELD

[0003] The invention relates to a forged powder metal part. In particular, this invention relates to a composite powder metal constant velocity joint inner race and a method of manufacture thereof.

BACKGROUND OF THE INVENTION

[0004] A constant velocity joint (CVJ) is a shaft coupling that is widely used in drive systems or automobiles to transmit power through a variable angle from a first rotating shaft to a second rotating shaft. One type of CVJ includes (1) an inner race having an inner spline and an outer surface with ball tracks, (2) an outer joint part also having ball tracks, and (3) bearing balls located between the ball tracks of the inner race and outer joint part.

[0005] With reference to FIG. 1, a conventional CVJ inner race 10 is made by cold forming, forging, and then case carburizing a metal 12 to achieve a nearly uniform effective case depth 14. The case depth 14 for each of the ball tracks 16 and the splines 18 are indicated by the dashed lines in the partial cross-sectional view of FIG. 1. The processing parameters to achieve a nearly uniform carburization of a fully dense part of a specific hardness, case depth and carbon gradient are generally known. Secondary operations, such as machining and grading, may be performed to ensure dimensional accuracy of the various features of the inner race 10.

[0006] Unfortunately, a nearly uniform case depth does not necessarily achieve the desired mechanical properties for a CVJ inner race. Although a relatively deep hardened area in the ball tracks is beneficial to provide a robust load bearing zone and prevent wear, spalling, or brinnelling during use, it is less desirable to have a hardened spline area. If the area around the splines is hardened, then it will be difficult to machine the splines in a cost- effective way to obtain the required dimensional tolerances. [0007] Therefore, there is a need for an improved CVJ inner race. In particular, there is a need for a CVJ inner race exhibiting improved load bearing upon the ball tracks and having improved spline performance.

SUMMARY OF THE INVENTION

[0008] A method of making an inner race for a constant velocity joint from at least two different powder metal materials is disclosed. The method includes forming a composite preform compact from at least two different powder metal materials. A first portion and a second portion of a compaction form are filled with a first powder metal material and a second powder metal material, respectively. The first powder metal material and the second powder metal material are compacted in the compaction form to form the composite preform compact. In the composite preform compact, the first powder metal material at least partially forms a plurality of pre-forged ball tracks in a higher concentration than the second powder metal material. The second powder metal material at least partially forms a pre-forged inner race core in a higher concentration than the first powder metal material. The composite preform compact is sintered and forged to form the inner race. The inner race has an outer section comprising the first powder metal material in a higher concentration than the second powder metal material in which the outer section has a plurality of the ball tracks with land surfaces there between. The inner race also includes an inner section comprising the second powder metal material in a higher concentration than the first powder metal material.

[0009] The inner section and the outer section of the inner race may have different hardness properties. In one form, the first powder metal material may have a greater hardenability than the second powder metal material. If this is the case, the method may further include the step of quenching the inner race after forging to achieve the different hardness properties between the outer section and the inner section of the inner race. In another form, the first powder metal material may have a greater carbon content than the second powder metal material. When the inner section and the outer section have different hardness properties, this may readily accommodate the method further including the steps of machining the ball tracks and cold sizing a splined area of the inner section.

[0010] The method may also include a step of carburizing before forging. At least a portion of the pre-forged inner race core may be masked during the step of carburizing such that a case depth in the pre-forged ball tracks exceeds a case depth in the pre-forged inner race core.

[0011] The step of compacting may form a composite preform compact having an axially-extending opening. If an axially-extending opening is formed in the composite preform compact, then the step of forging the composite preform compact to form the inner race may include forming a splined surface in the axially-extending opening.

[0012] The step of forging may create a variable boundary profile between the inner section and the outer section in which a radial distance from the axis of rotation of the inner race to the variable boundary profile varies over an axial length of the inner race.

[0013] A forged composite inner race for a constant velocity joint is also disclosed. The inner race includes an outer section and an inner section. The outer section has a plurality of ball tracks formed thereon with corresponding lands between adjacent ball tracks. The outer section comprises a first powder metal material in a higher concentration than a second powder metal material. The inner section has an axially-extending splined opening formed therein. The inner section comprises the second powder metal material in a higher concentration than the first powder metal material. The first powder metal material of the outer section has greater hardness than the second powder metal material of the inner section.

[0014] In one form of the inner race, the first powder metal material may have a greater hardenability than the second powder metal material. In another form of the inner race, the first powder metal material may have a higher carbon content than the second powder metal material.

[0015] In still another form of the inner race, the inner race may be carburized to provide an essentially constant case depth in an outer surface of a composite preform compact. When the composite preform compact is forged, the essentially constant case depth may flow to provide a case depth that is greater in the ball tracks than the lands in the inner race.

[0016] The inner race may have a variable boundary profile formed between the outer section and the inner section. In this variable boundary profile, a distance from the axis of rotation of the inner race to the variable boundary profile varies over an axial length of the inner race.

[0017] According to another aspect, a forged inner race for a constant velocity joint is disclosed comprising an outer section integrally formed around an inner section. The outer section comprises a first powder metal material in a concentration higher than a second powder metal material and the inner section comprises the second powder metal material in a concentration higher than the first powder metal material so as to provide a boundary profile between the inner and outer sections. The forged inner race has an axis for rotation wherein, along a given plane that intersects the axis, a radial distance from the axis to the boundary profile varies along the length of the axis and a radial distance from the boundary profile to the outer surface of the outer section varies along the length of the axis. [0018] Again, in some forms, the first powder metal material may be harder than the second powder material and the boundary profile may be formed during a forging operation.

[0019] The outer surface may have a plurality of ball tracks formed there in and lands formed there between. The radial distance from the boundary profile to the outer surface of the outer section may be greater at the ball tracks than at the lands.

[0020] Thus, the present invention provides many advantages over conventional CVJ inner races and processes for making CVJ inner races. As the inner race comprises more than one powder metal material, each of the features (i.e., the ball tracks and splined area) can be formed from a powder metal having material properties that are best suited for the particular demands of the feature. The ball tracks can be made of a hard material to receive the bearing balls while the splined area can be made of a soft material that is better for formation of the splines. Unlike inner races made from a single material, there is no conflict or tradeoff between the material properties of these features. Further, the present invention presents a process that requires significantly fewer steps to create an inner race than conventional processes for forming CVJ inner races.

[0021] These and still other advantages of the invention will be apparent from the detailed description and drawings. The following is a description of some preferred embodiments of the present invention, which are not intended to be the only embodiments within the scope of the claims that define the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 shows a partial cross-sectional view of a case carburized CVJ inner race according to the prior art;

[0023] FIG. 2 shows an isometric view of a composite preform made after compacting and sintering;

[0024] FIG. 3 shows a partial longitudinal cross-sectional view of the composite preform of FIG. 2 taken along line 3-3 of FIG. 2;

[0025] FIG. 4 shows a top front view of an CVJ inner race made from the composite preform of FIGS. 2 and 3;

[0026] FIG. 5 shows a partial longitudinal cross-sectional view of the inner race shown in FIG. 4 taken along line 5-5;

[0027] FIG. 6 shows a partial axial cross-sectional view of the CVJ inner race of FIG.

4; and

[0028] FIG. 7 shows an exemplary process for making the composite CVJ inner race. DETAILED DESCRIPTION

[0029] Referring first to FIGS. 2 and 3, a powder metal composite preform 20 is shown after compacting and sintering, but before forging. This composite preform 20 is a precursor to a final forged inner race for a constant velocity joint. As will be described in more detail below, the composite preform 20 is formed by sintering a composite preform compact. Therefore, it will be readily understood that the composite preform compact will have a shape and sections generally corresponding to those described for the composite preform 20, although for the composite preform compact the sections will be in the form of a compacted powder metal prior to sintering as opposed to a sintered powder metal as illustrated by the crosshatching in FIG. 3. Because compacted powder metal shrinks during sintering, the dimensions of the composite preform compact will be slightly greater than the dimensions of the composite preform 20 and, further, some small amount of dimensional distortion may occur during sintering.

[0030] The composite preform 20 is generally cylindrical in shape and extends along an axis A-A. An opening 22 defined by a radially inward facing inner surface 23 axially extends from a top face 24 to a bottom face 26 though the composite preform 20. On the outer surface of the composite preform 20, a plurality of pre-forged ball tracks 28 are formed. These pre-forged ball tracks 28 are concave and also extend from the top face 24 to the bottom face 26 of the composite preform 20. A plurality of pre-forged lands 30 separate the pre-forged ball tracks 28.

[0031] This composite preform 20 includes two portions made of different powder metal materials. The first portion 32 comprises a first powder metal material and includes the pre-forged ball tracks 28 and the pre-forged lands 30. The second portion 34 comprises a second powder metal material and includes a pre-forged inner race core around the axially- extending opening 22.

[0032] Between the first portion 32 and the second portion 34 is a boundary 36 demarcated by a change in the composition of the powder metal material. Outside of the boundary 36 in the first portion 32, the first powder metal material is in higher concentration than the second powder metal material. Inside of the boundary 36 in the second portion 34, the second powder metal material is in higher concentration than the first powder metal material. In one preferred form, there may be no second material outside of the boundary and no first material inside of the boundary. Although the boundary 36 is shown as a clear line in FIG. 3, it should be appreciated that there may be some mixing of the powders at the boundary 36 during the filling and compaction process. In the composite preform 20 shown, the boundary 36 has a shape approximating the curved face of a cylinder. [0033] Referring now to FIGS. 4 and 5, a forged inner race 120 made by forging the composite preform 20 is shown. During the forging process, the composite preform 20 is subjected to axial forces that reduce the axial height of the inner race 120 relative to the initial axial height of the composite preform 20 and induce the lateral flow of the sintered powder metal material in the forging die. As a result, the forged inner race 120 is shorter and the outer surface 121 is rounder than the composite preform 20. Moreover, forging will typically increase the density of the as-sintered materials because some amount of porosity usually exists in the as -sintered materials.

[0034] The forged inner race 120 has new or modified features in comparison to the composite preform 20. During the forging process or during subsequent cold sizing, a splined area 123 is formed in what was the inner surface 23 of the axially extending opening 22 or pre-forged inner race core of the composite preform 20. This splined area 123 may be shaped for connection to, for example, a shaft. Additionally, during the forging process, the pre-forged ball tracks 28 and the pre-forged lands 30 are shaped into ball tracks 128 and lands 130. As shown, the forged ball tracks 128 and lands 130 generally extend along an axial direction, are curved, and bow outward from the axis A-A near the mid-section of the forged inner race 120.

[0035] These ball tracks 128 can have a number of different configurations. For example, the ball tracks 128 of the forged inner race 120 may be configured for plunging or fixed styles of joint types. When viewed in longitudinal section as in FIG. 5, the ball tracks 128 can be curved in various configurations including widening toward one axial end of the forged inner race 120. If the ball tracks 128 are widened on the axial ends, then it is contemplated that the axial end (e.g., top end or bottom end) on which the ball tracks 128 are widened may alternate from one track to the next as one angularly traverses around the outer surface 121 of the forged inner race 120. The ball tracks 128 can also be angled with respect to the longitudinal axis of the part. Such tracks are common in cross-groove plunging joints, for example. However, for forged inner races that are to be used in constant velocity joints, the ball tracks 128 should be configured to provide a constant velocity plane for articulation.

[0036] Notably, the forged inner race 120 has an outer section 132 and an inner section 134 made from different powder metal materials which generally correspond to the materials used to make the pre-forged sections of the composite preform 20. The outer section 132 of the forged inner race 120, which includes the forged ball tracks 128 and lands 130, corresponds to the first portion 32 of the composite preform 20 and comprises the first powder metal material in higher concentration than the inner section 134. The inner section 134 of the forged inner race 120, which includes the splined area 123, corresponds to the second portion 34 of the composite preform 20 and comprises the second powder metal material in higher concentration than the outer section 132. The outer section 132 is integrally formed around the inner section 134 such that the forged inner race 120 is a unitary component formed from two separate powder metal materials, each having different characteristics as will be described in more detail below.

[0037] A variable boundary profile 136 extends between the outer section 132 and the inner section 134 of the forged inner race 120. The shape of the variable boundary profile 136 is a function of the initial shape of the boundary 36 in the composite preform 20 and the amount of lateral flow that occurs during the forging process. In general, the greater the localized amount of lateral flow during the forging process, the further the variable boundary profile 136 will be from the axis A-A in the forged inner race 120. For example, the tip or outermost circumferential portion 138 of the variable boundary profile 136 will be in an area of the forged inner race 120 that was subjected to the greatest amount of lateral flow during the forging process, as the boundary 36 in the composite preform 20 was radially equidistant from the axis A-A over the height of the composite preform 20.

[0038] The first powder metal material and the second powder metal material may be selected to have advantageous properties for the features of the forged inner race 120 formed of the respective powder metal materials. For example, as the outer section 132 has ball tracks 128 formed thereon that may be prone to wear, spalling, or brinnelling, it may be preferable that the first powder metal material be selected to be a material that is hard or that has high hardenability. As the inner section 134 has a splined area 123 formed therein, the second material may be preferably selected to be a material that is soft to accommodate the forging of fine features or be a material that is easily machined so that fine machining, such as grading, can be preformed on the splined area 123. Preferably, the second powder metal material also provides impact resistance and shear resistance suitable for the application. In one form of the invention, the first powder metal material has a surface hardness of at least 58 HRC and the second powder metal material has a hardness of at most 43 HRC. In another form of the invention, the first powder metal material may be a material having a high hardenability such as a ferrous material having a carbon content of more than 0.5 wt %, while the second powder metal material may be a non-hardening material such as a ferrous material having a carbon content less than 0.3 wt %.

[0039] Moreover, by controlling the shape of the variable boundary profile 136 during forging, it is possible to control the depth of the outer section 132 at various locations in the forged inner race 120. For example, with additional reference to FIG. 6 which shows a cross-sectional view taken perpendicular to the axis of rotation of the forged inner race 120, it is possible to achieve a deeper layer of the harder first powder metal material in the ball tracks 128 than in the lands 130 by variable forging. As it is typically desirable for the ball tracks 128 to have a hard surface that runs deep into the inner race 120 for wear resistance, this form of the variable boundary profile 136 places the hard material where it is most desired, while minimizing the amount of the harder material in the splined area 123 and lands 130, where this harder material may present problems during secondary operations such as machining.

[0040] Given a desired variable boundary profile 136 and forged inner race 120 shape, it is possible within limits to work backwards to determine an initial composite preform shape and forging die shape to achieve the desired variable boundary profile 136.

[0041] Also, additional or multiple material sections (i.e., two or more material sections) may be utilized in a preform in order to obtain multiple composite variable boundary profiles on select portions of a final forged part, thereby obtaining select performance features. For instance, a first material could be used in the region of the ball tracks to provide a hard surface, a second material could be used as the bulk of the of the part, and a third material could be used in the region of the splines to provide desired strength or machining properties.

[0042] While the process is described with respect to a forged inner race 120 for a constant velocity joint, it is contemplated that a variable boundary profile 136 may be achieved on other parts such as an outer joint part for a constant velocity joint or other powder metal components.

[0043] Referring now to FIG. 7, one embodiment of a process or method is shown to make a forged inner race 120 having a variable boundary profile 136. For brevity, and because some of these process steps are known to those in the art of forging powder metals, only certain aspects of the inventive process are discussed below. In this regard, material selection, temperature processing, and compaction pressures are discussed only briefly.

[0044] Prior to compaction, the powder metal materials are separately prepared and then placed into a compaction form such as a tool and die set. The mixing steps 210 and 211 ready each powder metal material separately, including any needed binders or lubricants, by mixing the material until a nearly uniform mixture is achieved. Once mixed, each of the mixed powder metal materials are filled into the compaction form during the filling steps 212 and 213. These filling steps 212 and 213 may be simultaneous or sequential. An optional step of separating 214 may be included during the filling steps 212 and 213, thereby facilitating the desired powder metal material placement into the compaction form. The step of separating 214 may include using a separator or the like to define the portions of the compaction form filled during the filling steps 212 and 213. Once the powders are filled into the compaction form, this separator may be removed.

[0045] While two mixing steps 210 and 21 1 and two filling steps 212 and 213 are shown, it is recognized that additional mixing or filling may be performed for each additional material included.

[0046] After the filling of the die cavity or compaction form is complete, the metal powders are compacted according to a compacting step 216 within the die cavity or compaction form to create a composite preform compact (i.e., an un-sintered powder metal component). The composite preform compact may have a shape and boundary similar to the composite preform 20 shown in FIGS. 2 and 3, albeit slightly larger as the compact will shrink during sintering. However, other shapes and boundaries may be formed during the compaction process by modifying the filling process and die cavity shape. Although not shown in the process flow, after the composite preform compact is compacted, the composite preform compact is ejected from the compaction form.

[0047] After the composite preform compact is formed, a sintering step 218 may be performed to form a composite preform from the composite preform compact. Typically, this sintering step 218 involves heating the composite preform compact to a temperature near, but below, the melting temperature of the powder metal component. As the composite preform compact comprises multiple different powder metal materials, the sintering temperature should generally be selected to be the lower of the sintering temperatures for these powder metals so as not to raise the other powder metal material(s) above its or their melting temperature(s). It is also possible that for certain components and/or compositions, liquid phase sintering may be employed and so solid state diffusion may not be the only mechanism by which sintering occurs.

[0048] Optionally, if one of the powder metal materials is conducive to carburization or "sint-carb" processing, as is know in the art, then the material may include a carburization step to achieve further beneficial results prior to the forging step. Notably, if sint-carb processing occurs prior to the forging step 220, then it is contemplated that the case depth of the carburized sections may be stretched during the lateral flow of forging, resulting in variable case depth thickness in the carburized surfaces.

[0049] Although not required, in some forms of the invention, the sintered composite preform may be separately carburized prior to forging by, for example, gas carburization. If carburized, it may be preferable to mask certain portions of the sintered composite preform to avoid their carburization (e.g., the splined area). Alternatively, if carburization of some features is preferred, while avoiding carburization of other features is also preferred, then the features to be carburized may be formed of a material that is easily carburized while the features that are desirably not carburized may be formed of a material that is less receptive to carburization.

[0050] A variable forging or forging step 220 comprises forging the composite preform at a forging temperature and a forging pressure to obtain an essentially dense, net shape, part. The variable boundary profile 136 is achieved by utilizing a die set of the forge with a form corresponding to the form of the inner race 120 to variably enhance the critical flow of the different powder metal portions of the composite preform 20 during the forging process. For example, for the composite preform 20 described above, the boundary 36 of the composite preform 20 is strategically compressed into the die sections, in which some portions of the composite preform 20 are stretched and thinned during the forging operation, while other portions of the composite preform 20 are thickened and deepened achieving the different powder metal zones from the composite preform 20. Prior to the forging process, a preheating step may be included wherein the composite preform 20 is heated to a pre-forge temperature to enhance the desired metal flow during the forging process.

[0051] It is contemplated that the step of forging may include oscillation of one or more of the tooling components used in the forging process. Oscillation of the components may help to induce lateral flow of the material and deform the material more readily than if only static forces are applied.

[0052] A cooling step 222 may be used to selectively harden certain features of the forged inner race 120 or obtain a particular metallurgy in one of the sections. For example, if a hardenable material is used to form the outer section, while a non-hardenable material is used to form the inner section, quenching the forged part immediately after the forging step 220 can result in the hardening of the outer section relative to the inner section. Cooling of the forged part may be by quenching in oil, water, air or by other methods suitable to the powder metal forging process.

[0053] In one aspect of the inventive process, the composite preform is directly quenched after forging to harden at least one of the powder metal materials. This quench can eliminate the need for a later carburizing step to provide the targeted hardness in the ball tracks 128, reducing cycle time for the part and costs associated with the longer cycle times and post-forging heating processes.

[0054] Prior to cooling, the forged part may be allowed to dwell for a period allowing for enhanced properties by allowing the material temperature to stabilize in the part. The appropriate length of dwell time will depend on a time-temperature-transformation chart for the material to be quenched, as different materials have different responses to different thermal treatments.

[0055] Further post forging operations may include, turning, facing, surface grinding, splining, and broaching of the product depending upon final specification requirements, thereby being ready for washing, packing, or shipping. Because of the multiple materials forming the inner race, the finish class of these steps can be improved. For example, a higher spline class is achievable for the spline because it can comprise the non-hardened second material. This can improve the fabrication process by reducing tool wear and reducing the failure of the component during secondary operations.

[0056] As compared to traditional methods of forging, hardening and machining inner joint parts, the disclosed process eliminates the steps of hard machining the ball tracks and outer land surfaces, and part washing. The resulting inner joint part has a high hardness and full material density in the regions of the ball tracks, and provides a deeper load bearing zone than in carburized steel. The part also exhibits a robust inner spline having a net shape spline and datum geometry. It can be cold sized after quenching, thereby eliminating any post- carburizing distortion and need for spline grading.

[0057] With proper combinations of powder metal, compaction forms, processing times, processing temperatures, processing pressures, forging dies, and cooling methods a near-net shape, fully dense components may be obtained having a variable boundary profile, thereby requiring minimal if any machining operations facilitating cost savings and performance improvements.

[0058] While various process steps have been presented, they are intended only to be limited in scope or order as indicated in the claims of this invention. Further, while the invention has been described in connection with several embodiments, it should be understood that the invention is not limited to those embodiments. Thus, the invention covers all alternatives, modifications, and equivalents as may be included in the spirit and scope of the appended claims.