Systems and Methods for Reducing Metal Fatigue in Forged Parts

Abstract

Systems and methods for flash forging a part, where a first portion of the forging seam is substantially free from grain flow pattern exit points and a second portion includes grain flow pattern exit points. In one embodiment, blocker steps are performed with pressure being applied in a first direction. A finisher step is then performed with pressure being applied in a second direction. In the blocker steps, a first portion of the part is formed without flash by extruding metal into one of the dies. The extruded shape is near the final desired shape of the first portion. In the finisher step, the first portion is pressed between two dies to achieve its final shape without producing a substantial amount of flash. Because there is no substantial flash, grain flow pattern exit points will not be produced when flash is trimmed from the part.

Description

BACKGROUND

1. Field of the Invention

The invention relates generally to forging processes, and more particularly to methods for forging parts which results in improved metallurgical grain structure which causes the parts to the stronger, particularly in areas which are subject to repetitive stress and resultant fatigue cracking.

2. Related Art

Conventional forging techniques are very useful, and can be employed to manufacture a wide variety of metal parts. One of the most useful forging techniques is press forging. In press forging, a piece of metal is placed between two dies, and the dies or pressed together (typically with tens of thousands of pounds of force) to cause the metal to be molded into the shape of the dies.

There are two types of press forging that are commonly used—flash forging and flashless forging. In a flash forging process, some of the excess metal is allowed to squeeze out through a space between the dies when the dies are pressed together. The metal that is squeezed out from between the dies is referred to as “flash.” In a flashless forging process, the two dies fit tightly together so that there is no space between the dies through which excess metal can escape.

Each of these two types of press forging has its own advantages and disadvantages. Because flashless forging does not allow metal to escape from between the dies during the forging process, it requires very high precision in regard to the amount of metal that is used to start the process. If too little is used, there will be avoids in the part. If too much is used, much more pressure is required to force the dies together. Consequently, the dies may not come together completely, and the dimensions of the part may not be within the desired tolerances. Because the high degree of precision that is required, flashless forging is typically used much less often than flash forging.

While flash forging requires less precision and is more generally easier to implement than flashless forging, it also has its own drawbacks. For instance, it is necessary to have some excess metal in the part to provide back pressure which ensures that the metal is completely pressed into the faces of the dies. Because additional material is necessary, the material cost for the part is higher than in the case of flashless forging.

Another problem with flash forging relates to the grain of the metal. When metal is forged, the grain structure of the metal follows the flow of the metal during the forging process. When a flash forging process is used, the grain within the metal part extends into the flash. Then, when the flash is removed from the part during the finishing process, the grain pattern runs to the surface of the part rather than being parallel to the surface, as in portions of the metal which are formed against the faces of the dies. The points at which the grain pattern ends at the surface of the part are prone to fatigue when the metal is repeatedly stressed.

It would therefore be desirable to provide methods for forging metal parts which require less precision than flashless forging processes, but which result in improved metallurgical grain patterns in the finished parts, thereby making the parts stronger and less subject to fatigue cracking.

SUMMARY OF THE INVENTION

This disclosure is directed to systems and methods for flash forging that solve one or more of the problems discussed above. In one particular embodiment, a method is provided for flash forging a part. In this method, one or more blocker steps are performed to form a first portion of the part without flash. A finisher step is then performed to form the first portion of the part into its final shape. In each of the blocker steps, the piece of metal being forged is placed between a pair of dies and pressure is applied to the dies in a first direction. In the finisher step, the pressure is applied to the dies in a second direction which is different from the first direction.

In one embodiment, the part being forged is a steering knuckle. The portion of the steering knuckle that is formed without flash is the spindle of the steering knuckle. The spindle is extruded into one of the dies in a blocker step. The attachment ears are also extruded into one of the dies in a blocker step. In these blocker steps, pressure is applied to the dies along a direction parallel to the axis of the spindle. The steering knuckle is then re-oriented, and in the finisher step, the pressure is applied to the dies in a direction which is perpendicular to the axis of the spindle. Depending upon how closely the spindle produced by the blocker steps approximates the final shape of the spindle, the finisher step may produce very little flash, or no flash at all.

Another embodiment comprises a forged part having a first portion and a second portion, where a forging seam is substantially free from grain flow pattern exit points on the first portion and includes grain flow pattern exit points on the second portion. The forged part may, for example, be a steering knuckle having a flange, a spindle and attachment ears. The spindle extends outward from the flange in a first direction and the attachment ears extend outward from the flange in a second direction which is opposite the first direction. In this embodiment, the spindle has substantially no grain flow pattern exit points, while the attachment ears may have such exit points.

Numerous other embodiments are also possible.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention may become apparent upon reading the following detailed description and upon reference to the accompanying drawings.

FIGS. 1A-1E illustrate a series of shapes corresponding to a conventional flash forging process.

FIGS. 2A-2C illustrate the consequences of flash forging a steering knuckle in a conventional manner.

FIGS. 3A-3D are a series of figures illustrating the forging of a steering knuckle in accordance with one embodiment of the invention.

FIG. 4 is a cutaway view of a spindle formed with no flash, in accordance with one embodiment of the invention.

While the invention is subject to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and the accompanying detailed description. It should be understood, however, that the drawings and detailed description are not intended to limit the invention to the particular embodiment which is described. This disclosure is instead intended to cover all modifications, equivalents and alternatives falling within the scope of the present invention as defined by the appended claims.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

One or more embodiments of the invention are described below. It should be noted that these and any other embodiments described below are exemplary and are intended to be illustrative of the invention rather than limiting.

As described herein, various embodiments of the invention comprise systems and methods for forging metal parts, where a first portion of each part is formed with a grain flow pattern that has a substantially reduced number of exit points at the seam of the part. In one embodiment, a piece of metal is forged in a series of flash forging steps. In the preliminary steps, the piece of metal is formed into a desired shape by placing it between two dies and applying pressure to the dies in a first direction. This is repeated one or more times to bring the shape of the piece of metal closer to the desired shape of the final part. In each of these steps, the first portion of the part is formed against the face of a single one of the dies so that there is no seam, and no flash, on the first portion of the part. In a final forging step, the part is re-oriented so that the pressure is applied to the dies (and applied by the dies to the part) in a direction which is different from (e.g., perpendicular to) the first direction. In this step, the first portion of the part is formed by both dies, so that there is a seam on the first portion corresponding to the seam between the dies. Because the first portion is already in a shape which is very near its final shape, little or no flash is formed on the first portion of the part during the final forging step. As a result, the grain flow pattern within the first portion of the part does not extend to any substantial degree into the flash, and is less prone to fatigue and failure.

In one particular embodiment, a steering knuckle for a heavy truck is manufactured using a forging process. The steering knuckle has a relatively flat, circular flange, with a spindle extending outward from the flange in one direction, and a pair of attachment ears extending outward from the flange in the opposite direction. Rather than using conventional flash forging techniques which would produce flash along a seam which extends all the way around the part, new techniques are used in order to reduce or eliminate the flash at the seam along the spindle, while allowing flash to form at the seam of the remainder of the part.

In this embodiment, the initial shape of the piece of metal to be forged is cylindrical. In a first blocker step, the metal is extruded into a first die to form a portion that will become the spindle of the steering knuckle. Because the spindle portion is extruded into the die, there is no seam and no flash on the spindle portion. In a second blocker step, the metal is extruded away from the spindle portion to form a portion that will become the attachment ears of the steering knuckle. As in the first blocker step, the spindle portion is pressed against a single die so that there is still no seam and no flash on the spindle portion. In both the first and second blocker steps, the dies are pressed against the part along a direction which is parallel to the axis of the spindle portion.

In a final forging step, the dies are applied to the part along a direction which is perpendicular to the axis of the spindle portion. In this step, a first die is applied to one side of the spindle portion while a second die is applied to the opposite side of the spindle portion. There is therefore a seam along the spindle portion. Because the spindle portion has already been formed into a shape which is very close to the final shape, however, little or no flash is formed at the seam when the dies are pressed together to form the part into its final shape. Because very little of the metal in the spindle portion flows out of the seam as flash, the grain flow pattern of the metal does not extend to any substantial degree into the flash. Consequently, when the small amount of flash is trimmed from the spindle of the finished part, the grain flow pattern has far fewer termination points at the surface of the spindle than conventionally forged steering knuckles. This results in a substantially stronger steering knuckle which is much less likely to fatigue and fail when subjected to the cyclical stress caused by normally use of the part.

It should be noted that, while the following description focuses on the manufacture of a steering knuckle, the techniques disclosed herein can be applied to the manufacture of many other, different parts. The example of the steering knuckle should therefore be construed as illustrative, rather than limiting of the scope of the invention. It is contemplated that these techniques will be most useful in the manufacture of parts that are subjected to cyclical stresses that cause fatigue and subsequent failure of the parts.

Before describing the various exemplary embodiments of the invention, it may be helpful to first describe conventional flash forging. Referring to FIGS. 1A-1D, a series of shapes corresponding to a conventional flash forging process are shown. In each step of the forging process, the metal to be forged is heated, and is then pressed between two dies. The faces of the dies are the inverse of the desired shape of the part. Because the shape of the finished part may be very complex, it may be necessary to forge the part in a series of steps, each of which forms the metal into a shape which is closer to the desired final shape.

FIG. 1A shows the shape of the piece of metal to be forged, prior to any of the forging steps. FIG. 1B shows the shape of the forged part after a first blocker step, FIG. 1C shows the shape of the forged part after a second blocker step, and FIG. 1D shows the shape of the forged part after a final forging step. It can be seen in these figures, and particularly in FIG. 1D, that as the part is pressed between the dies, the metal takes the shape of the dies, and any excess metal is squeezed out at the seam between the dies, forming flash. FIG. 1E shows the finished part after the flash has been trimmed from the part.

It should be noted that, in each step of the forging process of FIGS. 1A-1D, the dies are pressed against the part in the same direction. In this instance, the pressure is applied in each step in a direction which is perpendicular to the spindle axis. The spindle axis is indicated in the figures by the dashed line, while the direction in which the pressure is applied is indicated by the arrows. The seam of the forging and the corresponding flash produced in the forging process normally extends, uninterrupted, all the way around the part.

As the part is forged, the pressure applied to the part breaks down the polycrystalline structure of the metal, forming “grains” of the metal. As the grains of metal flow from one position to another, a pattern (a grain flow pattern) is formed. As the metal flows in the forging, the grains become aligned in the direction of the flow. This alignment of the grains can actually increase the strength of the metal. If the grain flow pattern is interrupted, however, stresses on the metal can be effectively concentrated at the point of interruption, which may result in fatigue cracking or other failures.

Referring to FIGS. 2A-2C, the consequences of flash forging a steering knuckle in a conventional manner are shown. FIG. 2A shows a cutaway view of the spindle of the steering knuckle with the flash formed in the forging process. FIG. 2B shows a cutaway view of the steering knuckle spindle after the flash has been trimmed from the spindle. FIG. 2C shows the steering knuckle with an indication of where the flash formed during the forging process has been trimmed.

Referring to FIG. 2A, the cutaway end 210 of spindle 200 is depicted with a set of lines that are similar to the rings in a tree trunk. These lines illustrate the grain flow pattern within the metal of the spindle. As noted above, the grain flow pattern is formed by the alignment of the crystalline grains of the metal in the direction in which the metal flowed during forging. It can be seen that the grain flow pattern in the spindle roughly follows the contours of the spindle, including the flash (220, 221) that is formed on the sides of the spindle.

FIG. 2A shows the spindle following the last forging step, but before being trimmed. The part is finished by trimming the flash, which is simply is excess material. Referring to FIG. 2B, the cutaway spindle 200 is shown after flash 220, 221 has been removed. It can be seen that the grain flow pattern, which previously followed the contours of the forged part into the flash, now has exit points 224, 225 at the surface of the spindle. “Exit points” is used here to refer to places at which a layer of the grain flow pattern terminates at the surface of the part. The presence of exit points 224 and 225 at the surface of a part can make the part more susceptible to fatigue and corrosion. In case of the steering knuckle spindle, which is subject to cyclical stress from the rotation in a wheel on the spindle, the exit points of the grain flow pattern, particularly where spindle 200 joins 230 flange (see FIG. 2C,) often lead to fatigue cracking and failure of the part. Even if the steering knuckle does not fail in use, the increased likelihood of failure resulting from the exit points of the grain flow pattern shorten the useful lifetime of the part.

Referring to FIGS. 3A-3D, a set of figures illustrating the forging of a steering knuckle in accordance with one embodiment of the invention is shown. FIG. 3A shows the shape of the piece of metal to be forged. FIG. 3B shows the shape of the forged part after the first blocker step. FIG. 3C shows the shape of the forged part after a second blocker step. FIG. 3D shows the shape of the part after a final forging step.

As shown in FIG. 3A, the initial shape of the metal to be forged is a cylinder. This shape was selected because the initial blocker step forges the metal into an axially symmetric shape. Referring to FIG. 3B, the metal is extruded (downward) in the first blocker step to form the portion 310 of the part that will become the spindle. Spindle portion 310 is extruded by pressing the metal into a lower die. Pressure is applied to the dies along the direction indicated by the arrows (which is parallel to the spindle axis, as indicated by the dashed line. The seam between the dies in this step is located at the periphery of the flange portion 320, so there are no seams, and no flash, on spindle portion 310.

A second blocker step then forges the metal into the shape shown in FIG. 3C. In this step, the lower die is essentially the same shape as the lower die used in the first blocker step. The upper die is shaped to form the portion of the part that will become the attachment ears (340, 341.) In this second blocker step, pressure is applied to the dies along the same direction as in the first blocker step (i.e., parallel to the spindle axis.) Again, there are no seams and no flash on the spindle portion of the part.

A final forging step is then performed to form the part into the shape shown in FIG. 3D. It should be noted that the part formed by the second blocker step is re-oriented before the final forging step so that the pressure in the final step is applied to the dies in a direction which is perpendicular to the spindle axis, rather than parallel to the axis. Conventionally, pressure is applied in the same direction (with respect to a given axis of the part) in all of the forging steps. Because the spindle portion of the part is formed into a near-final shape without any flash in the two blocker steps, the final forging step may displace little or no metal from this portion of the part. The final step therefore produces a spindle that is substantially flashless. That is, little or no flash is formed along the seam of the spindle. Most, if not all, of the flash is instead formed at the seam around the attachment ears and the side of the flange facing away from the spindle.

The amount of flash formed on the spindle is controlled, at least in part, by how closely the last blocker step conforms the spindle to its desired final shape. If the last blocker step leaves the spindle in almost its final shape, there will be almost no flash. If the last blocker step is not almost the same as the final shape, some metal may have to be displaced during the finisher step, thereby forming some flash. In the case that no flash is formed on the spindle, there is no flash to be trimmed from the spindle, so the grain flow pattern does not have exit points at the surface of the spindle. Referring to FIG. 4, a cutaway view of a spindle formed with no flash is shown. Because the grain flow pattern does not have exit points at the surface of the spindle, the spindle is less susceptible to the fatigue and corrosion than in steering knuckles which are manufactured using conventional forging techniques. In the event that the final forging step does cause some flash to be formed on the spindle, it should be noted that the amount of flash will be substantially less than is formed when conventional forging techniques are used. Because there is less flash, the grain flow pattern does not extend as far into the flash and, when the flash is trimmed, there are substantially fewer exit points at the surface of the spindle than in the case of conventionally manufactured steering knuckles. Again, this results in a steering knuckle which is much less susceptible to fatigue and corrosion than its conventionally manufactured counterparts.

As noted above, the foregoing description of the steering knuckle and corresponding methods of manufacture are provided as examples, and should be construed as illustrative, rather than limiting, of the invention. The techniques disclosed herein can be applied to the manufacture of many other, different parts.

The benefits and advantages which may be provided by the present invention have been described above with regard to specific embodiments. These benefits and advantages, and any elements or limitations that may cause them to occur or to become more pronounced are not to be construed as critical, required, or essential features of any or all of the claims. As used herein, the terms “comprises,” “comprising,” or any other variations thereof, are intended to be interpreted as non-exclusively including the elements or limitations which follow those terms. Accordingly, a system, method, or other embodiment that comprises a set of elements is not limited to only those elements, and may include other elements not expressly listed or inherent to the claimed embodiment.

While the present invention has been described with reference to particular embodiments, it should be understood that the embodiments are illustrative and that the scope of the invention is not limited to these embodiments. Many variations, modifications, additions and improvements to the embodiments described above are possible. It is contemplated that these variations, modifications, additions and improvements fall within the scope of the invention as detailed within the following claims.

Claims

1. A method for flash forging a part comprising:

performing one or more blocker steps to form a first portion of the part without flash, wherein in each of the blocker steps, a piece of metal is placed between a pair of preliminary dies and pressure is applied to the dies in a first direction; and

performing a finisher step to form the first portion of the part into a final shape, wherein in the finisher step, the piece of metal is placed between a pair of final dies and pressure is applied to the dies in a second direction which is different from the first direction.

2. The method of claim 1, wherein the finisher step produces no flash on the first portion of the part.

3. The method of claim 1, wherein the second direction is substantially perpendicular to the first direction.

4. The method of claim 1, wherein:

the part comprises a steering knuckle;

the first portion of the part comprises a spindle of the steering knuckle;

the first direction is parallel to an axis of the spindle; and

the second direction is perpendicular to the axis of the spindle.

5. The method of claim 4, wherein in a first one of the blocker steps, the spindle is extruded into one of the preliminary dies.

6. The method of claim 5, wherein in a subsequent one of the blocker steps, a pair of attachment ears are extruded into one of the preliminary dies.

7. A steering knuckle formed by the process of claim 6.

8. A forged part formed by the process of claim 1.

9. A forged part comprising: a first portion and a second portion, wherein a forging seam of the first portion is substantially free from grain flow pattern exit points and a forging seam of the second portion includes grain flow pattern exit points.

10. The forged part of claim 9,

wherein the forged part comprises a steering knuckle having a flange, a spindle and attachment ears,

wherein the spindle extends outward from the flange in a first direction and the attachment ears extend outward from the flange in a second direction which is opposite the first direction, and

wherein the first portion comprises the spindle and the second portion comprises the attachment ears.

11. The forged part claim 9, wherein the forging seam of the first portion and the forging seam of the second portion form a continuous seam around the periphery of the forged part.