Process And Apparatus For Densifying Powder Metal Compact To Form A Gear Havng A Hub Portion, And Preferred Powder Metal Compact Shape For Use Therewith

Abstract

A process and apparatus are described for densifying a hot powder metal compact in incremental stages with a single stroke of a forming or forging press. Flow of metal during densification and deformation of the hot compact are controlled to prevent undesired flow of material between a hub portion and a main body portion of the final product to be produced. A preferred shape of powder metal compact is described for use with the process and apparatus disclosed herein.

Description

BACKGROUND AND BRIEF DESCRIPTION OF INVENTION

This invention relates to improvements in the final forming of metal products having (a) a main body portion, (b) a hub portion formed integrally with the main body portion, and (c) a bore extending through the main body and hub portions. More specifically, the invention is concerned with a process and apparatus for producing high strength gear products from hot powder metal compacts, for use as side gears in present day automobile differentials.

It is known in the powder metal art to form ferrous metal products from a powder metal compact which has been heat treated and forged with known forging equipment. Typically, a cold metal powder is compacted into a preferred shape and coherent form which can be more easily handled during subsequent heating and forming operations. The powder metal can be compacted with a known mechanical or isostatic pressing means which imparts the preferred shape to the compact and which increases the density of the material making up the compact to approximately 75 to 90% of its theoretical value. Then the compact is sintered in a furnace to produce a metallurgically clean compact having improved characteristics for placement in a die cavity of a forge or other forming means. Final forming of the compact imparts a final shape to and increases the density of the final product to about 100% of its theoretical value.

In the production of high strength ferrous metal parts, it is preferred that certain compacting and heat treating processes be applied to the ferrous powder prior to forging, but these preliminary processes do not form a separate part of the present invention.

Certain shapes and forms of gear pieces present greater problems than others for final forming of a high strength, high density product. The present invention is concerned with a type of gear piece which includes, for example, a hub portion formed integrally with a main body portion and which further includes a splined bore extending through the hub and main body portions. The main body portion is shaped to include bevel tooth profiles on one face thereof. A typical application for a gear piece of this type is in an automobile drive train differential. This shape of product is difficult to form because known forming processes produce an undesired degree of material flow between the hub and main body portions of the final product. For example, it would be possible to produce such a part from a relatively long cylindrical compact which is grossly deformed to radially expand one end thereof to form an enlarged main body portion at the end of a cylindrical hub. However, this degree of radial expansion is far in excess of strains which can be tolerated by certain ferrous materials, and fracturing of the final product is likely. Another method of forming a hub on a main body portion of a gear piece would require back-extrusion of the hub from a disc-shaped compact. This would require relatively complex equipment and extreme movements of material to produce the final shape, resulting in a low quality product and excessive wear of a die in which the product is shaped.

In contrast to the processes discussed above, the present invention involves a process which starts with a compact shape having partially formed hub and main body portions with a bore extending therethrough. Final forming or forging of the compact is accomplished through steps of incremental forming which sequentially densify and shape and hub portion and the main body portion of the compact, and the sequential densification and shaping are used to control material flow between the hub and main body portions so that gross displacements of material will not occur.

The apparatus of the invention provides for rapid densification and deforming of hot powder metal compacts into final, high strength products with a single stroke of a forming press. During the single stroke of the forming press, a compound forming tool, comprising a punch having first and second elements for contacting different parts of the hot compact, is brought into sequential engagement with the hub and main body portions of the compact. A first punch element makes contact with the partially formed hub portion of the hot compact so as to press the hot compact onto a core rod support in a closed-end die cavity. The core rod support is provided with a splined configuration on its surface so that splines are formed radially into the bore surface of the hot compact as it is advanced into the die cavity. The first punch element functions to densify and shape the hub portion of the compact without significant movement of material between the hub portion and the main body portion of the compact. The second punch element functions to densify and deform the main body portion of the hot compact after the hub portion has been densified by the first punch element. Sequencing means are provided for delaying the action of the second punch element until the hot compact makes contact with the closed end portion of the die and full densification of the hub portion is substantially completed.

A preferred embodiment of the invention provides for a concentric mounting of the first and second punch elements, and each punch element is positioned and mounted to be reciprocated back and forth in the direction of the central longitudinal axis of a compact being formed. Both punch elements are mounted in a common ram or other driving means and are simultaneously driven, in a forming stroke, toward the closed end of a die so as to engage and move a hot compact into the die cavity. The second punch element is resiliently mounted, in the axial direction, relative to the first punch element, and this provides for a sequential timing of the two punch elements in their respective contacts with the hub and main body portions of the hot compact. In this way, each punch element applies an axial force to a separate portion of the hot compact, and the direction of movement of the axial forces corresponds to the direction of movement of the ram or driving means during a forming stroke of the apparatus. Thus, each forming stroke of the ram imparts incremental steps of forming to a hot compact contained within a fixed-position die cavity.

A preferred shape for a powder metal compact considers the shape of the closed end portion of the die cavity into which the compact is introduced and formed. In the case of a side gear pinion, of the type to be described in this specification, the main body portion of the compact should be provided with a generally curved face for contacting the angle defined by projecting tooth shapes formed in the closed end of the die cavity. This relationship avoids unwanted movements or stresses of the main body portion of the compact during densification of the compact.

These and other features and advantages of the present invention will become apparent in the more detailed discussion which follows. In that discussion reference will be made to the accompanying drawings as briefly described below.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an elevational view in section of a preferred shape of a powder metal compact for use with the process and apparatus of the present invention;

FIG. 2 illustrates an initial step of incremental forming of the compact of FIG. 1;

FIG. 3 illustrates a further step of incremental forming of the compact of FIG. 2;

FIG. 4 is an elevational view, in section, of a relatively simple form of apparatus for carrying out the process of the invention, showing the apparatus in a condition for applying a forming force to a hub portion of a compact contained within a die; and

FIG. 5 illustrates the apparatus of FIG. 4 in a condition for applying a forming force to a main body portion of a compact contained within a die.

DETAILED DESCRIPTION OF INVENTION

FIGS. 1-3 depict various stages of the process of the present invention wherein a hot powder metal compact of the type shown in FIG. 1 is densified and formed to produce the final gear product illustrated in FIG. 3. In each of the views the compact and final gear product are shown as including a main body portion 10, a hub portion 12 formed integrally with the main body portion, and a bore 14 extending through the main body and hub portion. The bore 14 has a smooth surface in the initial compact form of FIG. 1 and is shown as including a splined surface in the views of FIGS. 2 and 3 after subsequent forming steps of the process have been applied thereto. Although this invention will be described in terms of producing the specific gear form illustrated in FIG. 3, it can be appreciated that the principles of the invention generally apply to the manufacture of products having a main body portion 10, a hub portion 12, and bore 14 extending through the main body and hub portions.

In order to produce the specific gear form illustrated in FIG. 3, it is preferred that the initial shape of a hot powder metal compact for use in the process of this invention be of the general form illustrated in FIG. 1. This compact form can be considered as a "stepped" shape or one in which there is a partially formed hub portion 12 and a partially formed main body portion 10, both of which are to be further densified and shaped to establish the final product of FIG. 3. The weight distribution of powder between the hub and main body portions is substantially the same as in the final part to be formed. The preferred compact shape includes a front face portion 18 for being received into a die cavity of a forming apparatus. The front face portion is of a generally hemi-spherical or curved shape for making progressive contact shape with angular projections of tooth profiles in the die cavity. The end portion 16 of the compact is intended to be received against a flat closed-end portion of a die cavity, and an optional counterbore 19 may be included in the end portion 16 to receive an ejector member. The relationship between the shape of the compact and the die cavity into which it is placed will be discussed in greater detail later with reference to FIGS. 4 and 5.

The basic concept of the process of this invention is one of applying incremental axial forces to the compact of FIG. 1 to sequentially densify and shape the hub portion 12 and the main body portion 10 of the compact. The forces which are applied to the separate parts of the compact move in sequence in a common direction in such a way that there is no substantial movement of material of the compact between the hub and main body portions. This prevents the establishment of unwanted stresses between the hub and main body portions of the final product, and the entire process is carried out without gross movements or reverse movements of material relative to the direction of travel of tooling which imparts the axial forces to the hot compact.

FIG. 1 illustrates an initial step in the process wherein an axial force is applied only to a terminal end face 20 of the hub portion 12 of a hot compact which has been placed in a die cavity for a forging or forming operation. The arrows shown in FIG. 1 depict the direction of the axial force which is applied to the hub portion of the compact. This axial force serves to initially densify the hub portion of the compact prior to any significant densification of the main body portion of the compact. During the initial step of densifying the hub portion 12 of the compact, a back face 22 of the main body portion of the compact is constrained from moving toward or away from the hub portion 12. This results in a substantial densification of the material contained in the partially formed hub portion 12 of the compact illustrated in FIG. 1 without any significant movement of that material into the main body portion of the compact.

After the hub portion has been substantially formed, as illustrated in FIG. 2, continued application of the initial axial force to the end face 20, accompanied by an application of an axial force to the back face 22, results in a flow of material from the main body portion of the compact toward the radially outward areas of the die cavity. This flow is illustrated by arrows in FIG. 2. This radial flow of material in the main body of the compact is initiated by a separate step of the process which applies a separate axial force to the back face 22 of the compact. The separate axial force moves in the same direction as did the first axial force which was applied to the hub portion of the compact, and this provides for full densification and shaping of the main body portion of the compact. Since the hub portion 10 is essentially fully densified and completed before forming of the main body portion is initiated, there is little tendency for the material of the main body portion to reverse its flow in the direction of the hub portion. Thus, the hub and main body portions are sequentially densified and formed with complete control of material movement during the incremental steps of forming each portion.

The splined configuration which is imparted to the bore 14 of the hot compact is imparted thereto simultaneously with the initiation of forming of the hub portion of the compact. In fact, the initial step of applying an axial force to the end face 20 of the compact illustrated in FIG. 1 can function to press the hot compact into supported engagement with a core rod means having a splined surface so as to radially densify and form a splined surface in the bore 14 of the hot compact. It is especially advantageous to produce the splined configuration on the inner bore 14 during initial insertion of the hot compact into a die cavity because a more complete and precise configuration can be imparted to the bore while the compact is at its highest temperature in its delivery to the forming apparatus. Relatively high pressure is required to radially densify and shape the bore of a compact, and prior art methods which have attempted this forming step at a later stage in the development of a gear shape have resulted in incomplete or inaccurate spline formations.

A simplified form of apparatus is depicted in the illustrations of FIGS. 4 and 5 to illustrate a basic apparatus for carrying out the process of the present invention. Of course, it is to be understood that apparatus designed for a high speed production system would be relatively more complex and would include structures for automatically handling a hot compact and the finished part in its movement into and out of a die cavity.

As shown in FIGS. 4 and 5 the apparatus is of a type which includes a die 30 having a cavity defined therein for imparting a final shape to the compact being densified. Further, the apparatus includes a core rod means 32 which is supported and contained within the die 30 for being received in the bore 14 of a compact which is inserted into the die cavity. The core rod means 32 is provided with a splined configuration for a major length of its outside surface, and a reduced diameter end portion 33 may be provided for initially positioning a compact on the core rod means prior to forging. The apparatus may also include a known ejector mechanism 34 which comprises a tubular member splined to the outside surface of the core rod means 32 for closing off the end of the die cavity and for ejecting a final-formed product from the cavity by a movement upwardly along the length of the core rod means 32. In the type of apparatus illustrated, the position of the die 30 is fixed, and a separate punch means 36 is moved relative to the die means to close the open end of the die means and to apply an impact to a compact contained therein. Apparatus of the type just described is generally known in the art. The improvement of the present invention is concerned with the punch means 36 and the manner in which it applies an impact force to a hot compact contained within the die cavity of the apparatus. In this regard, it is known to utilize a compound pressing tool, made up of separate punch elements, for initially compacting a powder to produce a stepped compact shape of the type shown in FIG. 1. However, the improved punch means of this invention comprises a compound tool having certain structures and functions which are different from those used in the art of pressing powder into compact forms.

The improved punch is illustrated in FIGS. 4 and 5 as including a first punch element 38 and a second punch element 40. The first punch element functions to apply an axial force to a partially formed hub portion of a hot compact after the hot compact is received on a core rod means 32, as shown in FIG. 4. The second punch element functions to apply a separate axial force to a partially formed main body portion of a hot compact after full densification of the hub portion is substantially completed so as to prevent significant material flow between the hub portion and the main body portion. The functional application of the second punch element is illustrated in FIG. 5. The two punch elements 38 and 40 comprise tubular elements which are concentrically mounted relative to each other so as to define, in combination, a closed end for the die cavity when the punch is inserted into the die. Both punch elements are mounted in a common ram or other driving means and are simultaneously driven back and forth in the direction of the central longitudinal axis of the compact being formed. During a forming stroke, both punch elements are moved in sequence towards the closed end (the bottom end in the orientation of FIGS. 4 and 5) of the die cavity, and incremental forming of the hot compact contained within the die cavity is achieved with each full forming stroke. Thus, it is not necessary to subject the compact to separate forming strokes in order to achieve the type of incremental forming which is preferred by the present invention.

As shown in FIG. 4, the first punch element 38 includes an end face 44 which substantially matches the terminal end face 20 (see FIG. 1) of the powder metal compact being formed. In addition, the inner surface of a bore portion 46 of the first punch member 38 includes a splined configuration which mates with the splined configuration of the core rod means 32 to prevent extrusion of powder metal material between the first punch element and the core rod means.

The second punch element 40 is resiliently mounted, in the axial direction, relative to the first punch element. In the relatively simple apparatus depicted in FIGS. 4 and 5, spring means 48 are compressively loaded between the second punch element 40 and a portion of the ram 42 which fixes the position of the first punch element 38 to thereby allow a limited distance of movement between the two punch elements. The distance of movement is set to correspond to the length of travel desired for the first punch element 38 from the time it makes initial contact with the hot compact to the time it is required that the second punch element 40 begin its densification of the main body portion of the hot compact.

Operation of the apparatus illustrated in FIGS. 4 and 5 involves a placement of a hot compact on the free end of the core rod means 32. Then, the forming stroke of the apparatus is initiated by movement of the ram 42 downwardly so as to insert the free ends of the first and second punch elements into the open end of the die cavity. The free ends of the first and second punch elements are axially offset from one another so that both free ends make simultaneous contact with respective portions of the hot compact. During the insertion movement of the punch means 36 into the die cavity, the hot compact is pressed into supported engagement with the core rod means and fully inserted into the die cavity until contact is made with closed end structures of the die cavity. During this insertion, spline configurations are formed in the inner bore of the hot compact. Continued movement of the punch means 36 results in an application of an axial force to the hub portion of the compact as a result of a continued advancement of the first punch element 38 towards the closed end of the die cavity. At the same time, there is no axial movement of the second punch element 40, and the spring elements 48 are further compressed until a back end 50 of the second punch element 40 engages a surface 52 of the ram 42. During initial densification of the hub portion of the hot compact, the second punch element 40 functions to confine the material of the hot compact to prevent unwanted movement from the hub portion to the main body portion thereof and to prevent radial outward movement of the hub portion. After the hub portion of the compact has been substantially fully formed, the second punch element 40 initiates its axial movement against the back surface 22 (see FIG. 1) of the compact. This applies an axial force in the main body portion of the compact, and moves material within the main body portion to a conforming contact with all exposed portions of the die. Completion of the forming stroke is depicted in FIG. 5 wherein the final product of FIG. 3 has been fully formed. It can be seen that an annular end face 54 of the second punch element 40 defines the back face of the completed gear piece. Upon completion of the forming stroke, the ram 42 is reciprocated in the opposite direction from the forming stroke, and all parts of the punch means 36 are withdrawn from the die cavity. After this, the ejector means 34 can be actuated to lift the completed gear piece out of engagement with the die.

Referring back to the relationships shown in FIG. 4, it can be seen that the curved face portion 18 of the compact, as discussed above with reference to FIG. 1, is shaped to progressively engage the angle defined by a plurality of tooth-shaped projections 56 which extend into the die cavity from the closed end thereof to define a series of tooth sidewalls and bottomlands for a gear. This relationship between the initial shape of a forward end of the compact and the portion of a die which it contacts is very important because a substantial mismatch between the shape of the compact and the shape of the die will produce a stress or bending moment in the compact as it is pressed rapidly into full engagement with the die. This is especially true where the initial diameter of the hot compact is substantially less than the full diameter of the die, thereby leaving considerable space for unwanted movements of the compact before controlled densification and forming have taken place. In addition, it has been found that a tangential engagement of the tooth-shaped projections 56 with a curved surface of a compact is preferred to a "matching" engagement between such projections and a compact having a tapered face without a curved shape. This has the effect of reducing tensile circumferential strains in the tips of the teeth being formed while increasing local compressive strains (axially) in each tooth. Also, it has been found that an increase in diameter of the main body portion of the compact has a similar desirable effect to increasing the compressive strain along the length of the teeth (which in turn increases the tolerable tensile strain transverse to the tooth before cracking).

Having described a specific application of the present invention to a type of gear piece and present day usage, it can be appreciated that the principles of the invention can be applied to other product configurations in which it is desired to control stresses and material density between body portions having substantially different sizes and shapes. Also, it can be appreciated that other designs of apparatus may be used for practising the process of this invention. For example, separate punch elements may be separately controlled with separate mechanical drive elements, if desired. Additional applications of the principles of this invention to usages which would be contemplated by those skilled in this art, in view of the teachings herein, are intended to be included within the scope of the claims which follow.

Claims

What is claimed is:

1. A process for densifying a powder metal compact so as to form a product having (a) a main body portion, (b) a hub portion formed integrally with the main body portion, and (c) a bore extending through the main body and hub portions, said process comprising the steps of:

placing the compact in a die cavity so as to be supported by a core rod means carried in said die cavity,

applying an axial force to a partially formed hub portion of said compact so as to press the compact against a closed end portion of the die cavity and to fully densify the hub portion of the compact prior to full densification of the main body portion of the compact, and

applying a separate axial force to a partially formed main body portion of said compact after full densification of said hub portion is substantially completed so as to prevent significant material flow between the hub portion and the main body portion.

2. The process of claim 1 wherein said axial forces are sequentially applied to said compact in a single stroke of a forming punch.

3. The process of claim 1 wherein said step of placing includes a step of pressing said compact into supported engagement with said core rod means so as to radially densify an inner surface defining the bore of the compact.

4. The process of claim 3 wherein said step of pressing of the compact into engagement with the core rod means forms a spline in the bore surface of the compact.

5. Apparatus for densifying a powder metal compact having (a) a partially formed hub portion, (b) a partially formed main body portion, and (c) a bore extending through the main body and hub portions, said apparatus being of a type which includes a die having a cavity defined therein for imparting a final shape to the compact being densified, a core rod means contained within said die for being received in the bore of the compact and for supporting the compact, and a punch means for applying axial forces to the compact contained within said die, the improvement in said punch means comprising

a first punch element for applying a first axial force to the partially formed hub portion of a compact after the compact is received on said core rod means, said first punch element being positioned to reciprocate in the direction of the central longitudinal axis of the compact so as to apply said first axial force in a direction which presses the compact against a closed end portion of said die cavity to thereby fully densify the hub portion of the compact prior to full densification of the main body portion of the compact,

a second punch element for applying a second axial force to the partially formed main body portion of the compact after full densification of said hub portion is substantially completed so as to prevent significant material flow between the hub portion and the main body portion, said second punch element being positioned to reciprocate in the same direction as said first punch element so as to apply said second axial force in a direction which presses the compact into conforming contact with all parts of said die,

driving means for advancing said first and second punch elements toward said die to apply said axial forces to a compact contained therein, and

sequencing means for delaying the application of said second axial force by said second punch element until full densification of said hub portion is substantially completed.

6. The apparatus of claim 5 wherein said first punch element includes a tubular member having an end face which substantially matches a terminal end face of the hub portion of said powder metal compact.

7. The apparatus of claim 6 wherein said second punch element comprises a tubular element positioned concentrically around said first punch element and wherein said second punch element has an end face which defines a back face of the main body portion of the final product to be produced.

8. The apparatus of claim 5 wherein said core rod means is provided with a spline configuration on its outer surface so as to form a spline configuration along the bore of said compact as the compact is pressed into position of said core rod means.

9. The apparatus of claim 8 wherein said first punch element includes a spline configuration on a surface which mates with the outer surface of said core rod means.

10. The apparatus of claim 7 wherein said first and second punch elements are mounted in a common ram structure driven by said driving means so that both of said punch elements are moved together toward said die with each forming stroke of the ram structure, and wherein said second punch element is resiliently mounted relative to said first punch element so that said second punch element is delayed in its forward movement, upon engaging a portion of said compact, until said first punch element has advanced for a sufficient distance to substantially completely densify the hub portion of the compact.

11. The apparatus of claim 5 wherein said die cavity has a configuration which establishes gear tooth profiles on a compact densified therein.

12. The apparatus of claim 5 wherein said powder metal compact has a shape which allows full insertion of the compact into said die cavity by the action of said first punch element and without a bending moment being applied to the main portion of the compact relative to its hub portion.