METHOD OF FORMING POWDER METAL COMPONENTS HAVING SURFACE DENSIFICATION

Abstract

A method for producing a powder metal article having a three dimensional shape and having at least one densified surface comprising: a) providing a blend of powdered metals; b) compacting said blend to form a pre-form having a general shape of said article; c) sintering said pre-form; d) densifying at least one cylindrical surface region of said pre-form; and, e) forming said pre-form to a final density and into the three dimensional shape of said article.

Description

FIELD OF THE INVENTION

The present invention relates to methods of manufacturing powder metal components. More specifically, the invention provides a method of densifying the core and surface of a powder metal component to achieve a product having an evenly densified surface.

BACKGROUND OF THE INVENTION

Powder metal (PM) technology is a lower cost alternative for producing components that could be made from wrought metal. The use of PM components is precluded in many applications because of inferior mechanical strength caused by residual porosity. Therefore, in the manufacture of PM articles, the achievement of high density, close to that of wrought steel (generally assumed to be approximately 7.86 g/cc), is of significant importance since the strength and durability of a PM article is directly related to its density. Typically, the basic steps involved in the manufacture of a powder metal component are: a) blending the desired metal powders; b) compacting the powder to the desired shape; c) sintering the compact; and d) forming the sintered compact to the desired final shape. The final step is used to impart the required dimensional features of the article. Following the forming step, it is also common to perform a heat treatment on the finished article to impart, where desired, certain mechanical properties as known in the art.

The final density of a PM article is dependent on the characteristics of the powders in the blend, sintering conditions, and the compressive forces applied to the article primarily during the compaction and forming steps. It is common in known methods to compact a powder blend to a moderate initial density, approximately 7.0 g/cc, and further densifying the compact during subsequent forming steps. Various compositions of metal powder blends are known in the art as are methods of compaction and sintering. Examples of known blends and methods are taught in U.S. Pat. No. 5,476,632 (incorporated herein by reference).

For manufacturing articles having a bearing surface and the like, it is known to increase the surface density at the desired locations to provide a densified region that is capable of withstanding the bearing forces. The prior art provides various methods for densifying surface portions of PM articles. For example, U.S. Pat. No. 5,540,883 (incorporated herein by reference), teaches a method of densifying a selected surface portion of a sintered article by applying rolling cylinders or the like to create a bearing surface on an article. The bearing surface, by virtue of having an increased density is better suited to withstand the physical stresses (i.e. rolling or sliding stresses) applied on that portion of the article. In the process taught in the aforementioned reference, a specific section of the article can be provided with a density that approximates the theoretical maximum value while the rest of the article has a density of approximately 90% to 98% of the theoretical maximum value. The reference is directed to providing bearing surfaces or bushings, which are inherently cylindrical and do not have a complex shape. Moreover, the reference does not teach any alteration of the core density during the cylindrical surface densification step.

Various other surface densification methods are known in the art such as, for example, in U.S. Pat. Nos. 6,168,754; 6,013,225; 5,884,527; and, 6,110,419 (all of which are incorporated herein by reference).

For example, U.S. Pat. No. 5,884,527 teaches a method of roll forming a sintered gear comprising meshing a sintered pre-form in interference with a rotating roll forming gear die. This method is mainly suited for surface densifying pre-forms of specific geometries, such as gear teeth, that permit the design of a rolling die to impart the necessary combination of line contact and relative motion between the die and pre-form.

Furthermore, U.S. Pat. No. 6,168,754 teaches a method of densifying the contoured surface of a sintered pre-form comprising forcing said article at ambient temperatures through a series of dies having successively more interference contact with the surfaces to be densified. A disadvantage of this method is the need for multiple dies and the inherent complexity and cost.

Moreover, U.S. Pat. No. 6,013,225 teaches a method of surface densifying a pre-form utilizing a method of selectively heating the surface of said article and forcing it through a die. Inherent disadvantages of this method are the requirement of a separate surface heating step, decreased tool life due to elevated die temperatures and the corresponding detrimental effect on dimensional accuracy.

In the known methods of surface densification, the densification step is typically conducted after the forming step, when the formed article has its final shape and is close to final core density. Moreover, the known methods are generally incapable of surface densifying contoured surfaces, require unduly complex dies, and/or involve high process costs. For example, in published U.S. application Ser. No. 10/767,014 (published under number 2004/0177719), there is taught a method for surface densification. This reference teaches a process wherein a powder metal is compacted to the final form, sintered and then surface densified prior to sizing or forging. This reference stipulates that for any final surface having a complex or irregular shape, special densification processes are required (such as peening). Thus, the less expensive rolling process cannot be used for irregularly shaped articles.

The present invention seeks to mitigate at least some of the deficiencies in the prior art powder metal manufacturing methods.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a method of producing powder metal components with high core and surface densities and with high precision on contoured forms.

In another aspect, the invention provides a method of densifying a sintered powder metal article by first surface densifying a cylindrical surface on a sintered perform and subsequently forming the article to final core density and final shape in a closed die cavity, wherein the formed article has a compressed length of 5 to 30% less than the original sintered length.

In another aspect, the invention provides a method of making a sintered metal article comprising: blending one or more lubricants, carbon, alloys, and iron; pressing the blend to form a compact; sintering the compact to produce a sintered powder metal article; densifying the surface of the article at ambient temperature by relative motion between the article and a densification tool; and, forming the article in a closed die cavity having a clearance for movement of said article so as to allow the article to assume a final shape and final density.

In another aspect, the invention provides a method of densifying a sintered metal article comprising: blending one or more lubricants, graphite, iron and one or more of ferromanganese, ferromolybdenum and ferrochromium; pressing the blend to form a compact; sintering the compact at a temperature of at least 1250° C.; surface densifying at least one cylindrical surface of the sintered article by roller burnishing; forming said article at between 600 and 1300 MPa in a closed cavity so as to produce a final part with core a density of 90 to 98% of the theoretical density, a compressed length of 5 to 30% less than the sintered length and contoured densified surfaces.

In another aspect, the invention provides a method of making a sintered metal article by blending one or more lubricants, carbon, and iron powder pre-alloyed with Mn, Mo, Cr, Ni, etc; pressing the mixture, or blend, to produce a compact; sintering the compact at a temperature of at least 1100° C.; surface densifying at least one cylindrical surface of the sintered article by roller burnishing; forming the article to a final core density and shape in a closed die cavity; wherein the formed article has a compressed length of 5 to 30% less than the original sintered length.

In another aspect, the invention provides a method of making a sintered metal article by: blending one or more lubricants, carbon, and elemental or substantially pure iron and one or more of Mn, Mo, Ni, Cu, etc in elemental form; pressing the mixture, or blend, to produce a compact; sintering the compact at a temperature of at least 1100° C.; surface densifying at least one cylindrical surface of the sintered article by roller burnishing; and subsequently forming the article to a final core density and shape in a closed die cavity, wherein the formed article has a compressed length of 5 to 30% less than the original sintered length.

In a further aspect, the invention provides a method of producing overrunning clutches or the like with high core density and densified contact surfaces.

Thus, in one aspect, the present invention provides a method for producing a powder metal article having a three dimensional shape and having at least one densified surface region, the method comprising:

a) providing a blend of powdered metals;

b) compacting the blend to form a pre-form having a general shape of the article, the preform being generally cylindrically shaped at a region corresponding to the at least one densified surface region;

c) sintering the pre-form;

d) densifying the at least one surface region of the pre-form; and,

e) forming the pre-form to a desired final density and into a desired three dimensional shape of the article.

In another aspect, the present invention provides a method for producing a powder metal article having a three dimensional shape and having at least one densified surface region, the method comprising:

a) providing a blend of powdered metals;

b) compacting the blend to form a pre-form having a general shape of the article, the pre-form having a density of between 70% to 90% of the theoretical maximum density and being generally cylindrically shaped at a region corresponding to the at least one densified surface region;

c) sintering the pre-form;

d) densifying the at least one surface region of the pre-form to at least 90% of the theoretical maximum density; and,

e) forming the pre-form to a desired final density and into a desired three dimensional shape of the article.

In another aspect, the invention provides a powder metal pre-form having a general shape of a desired article, the pre-form having a density of between 70% to 90% of the theoretical maximum density and being generally cylindrically shaped at least one surface region.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the preferred embodiments of the invention will become more apparent in the following detailed description in which reference is made to the appended drawings wherein:

FIG. 1 is a top view of a die, in an open position, with the top punch removed, loaded with a pre-form.

FIG. 2 is a cross sectional view of the die of FIG. 1.

FIG. 3 is a top view of a die, in a closed position, with the top punch removed, loaded with a pre-form.

FIG. 4 is a cross sectional view of the die of FIG. 3.

FIG. 5 is a graph comparing the sub-surface density gradients of a surface densified pre-form and the final C—Mn-Mo one-way clutch outer race formed at 985 MPa.

FIG. 6 is a graph of the formed core density of a C—Mn-Mo one-way clutch outer race.

FIG. 7 is a graph of the formed closure of a C—Mn-Mo one-way clutch outer race.

FIG. 8 is a graph of the formed radial movement of a C—Mn-Mo one-way clutch outer race.

FIG. 9 is a graph comparing the sub-surface density gradients of a surface densified pre-form and the final formed shape for a C—Mo one-way clutch inner race formed at 925 MPa.

FIG. 10 is a graph of the formed core density of a C—Mo one-way clutch inner race.

FIG. 11 is a graph of the formed closure of a C—Mo one-way clutch inner race.

FIG. 12 is a graph of the formed radial movement of a C—Mo one-way clutch inner race.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, the terms indicated below will be understood to have the associated meanings:

“Metal powder”—a metal that is in a fine powder form. The metal may be pure (i.e. iron), pre-alloyed iron (i.e. iron alloyed with other metals such as, but not limited to, one or more of molybdenum, chromium or nickel), or an alloy of one or more metals (i.e. ferro manganese, ferro molydenum or ferro chromium).

“Powder metal article”—an article formed from a metal powder. The metal powder is compressed under high pressures in a die or mould having a desired shape. The compressed article may be subjected to other processes such as sintering etc. to achieve desired physical properties.

“Blend” or “Powder metal blend”—a blend of one or more metal powders and other additives such as lubricants, carbon (e.g. graphite) etc.

“Compacting”—the step of pressing a powder metal blend in a rigid die until the blend assumes a desired shape and density. The compacted shape may be the same or similar to that of the final article.

“Compact” or “pre-form”—the article resulting from compacting a powder metal blend.

“Sintering”—a process wherein a compact is subjected to high temperatures and selected atmospheres (e.g. a reducing atmosphere) to cause the compact to become a coherent mass by heating without melting. Sintering is normally performed on a powder metal compact to impart desired physical properties.

“Surface densification” or “Selective densification”—the step of densifying a select portion of a sintered compact, usually the outer surface or a portion thereof. The densification is preferably conducted by means of rollers and the like as known in the art. However, various apparatus for densifying surfaces will be apparent to persons skilled in the art after reviewing the following description. The densification step can be conducted on the entire surface of the compact or on one or more cylindrical portions thereof. Thus, as used herein, the term “region” or “densified surface region” will be understood to mean the entire surface or a portion thereof. The densified surface region will typically be densified to a density of between 80% and 100%, and preferably at least 98%, of the theoretical maximum density. Further, the depth of the densified region would be at least 0.025 mm (or 0.001 inches) from the surface. In one embodiment, the densified region would extend to a thickness of about 1 mm (or 0.04 inches) from the surface.

“Theoretical maximum density”—refers to the density of the powder metal compact when processed until no pores exist. In the general case, the theoretical maximum density would be the density of wrought steel, i.e. 7.86 g/cc.

“Forming”—a process of providing a sintered compact with its final shape. This step is normally performed in a closed die or mould, wherein the sintered compact is subjected to pressure to result in the final dimensions and density of the final article. The forming step is known by various terms including: sizing, coining, repressing, re-striking and powder forging. In some cases, the sintered compact may also be heated prior to the forming step in order to improve the malleability of the material.

“Annealing”—a process of treating a surface densified or formed sintered article wherein the article is subjected to high temperatures in a select atmosphere (e.g. protective atmosphere, vacuum etc.) to anneal the article to obtain an advantageous microstructure.

“Heat treatment”—a process of treating a formed sintered article wherein the article is subjected to high temperatures, select atmospheres (e.g. protective atmosphere, vacuum, carburizing, etc.), and rapid cooling to obtain desired mechanical properties. Heat treatment methods include, but are not limited to, through-hardening, carburizing and induction hardening which are typically followed with a tempering treatment for optimum properties.

In one embodiment, the present invention provides a method of surface densifying the contoured surfaces of a sintered powder metal article, such as, for example, the cam forms of a one-way clutch, by first surface densifying a cylindrical surface of a lower density pre-form and forming the article to final desired shape and density in a closed die cavity. Thus, according to the invention, the surface densification step is performed prior to the forming step. Therefore, in summary, the steps of the invention are as follows: a) mixing one or more powder metals to form the desired blend; b) compacting the powder to create a pre-form; c) sintering the pre-form; d) performing a surface densification step on the pre-form; and e) forming the article to the desired shape and core density. After the forming step, the formed article may be further shaped and heat treated.

Formation of Powder Metal Blend

It will be appreciated by persons skilled in the art that a wide range of powder blends may be used in the method of the present invention. In one embodiment, the present invention utilizes low alloy steel compositions, where the carbon content is less than 0.7% and preferably below 0.3% by weight of the final sintered article.

In one embodiment of the invention, the powder compositions may comprise low cost iron powders, which are blended with calculated amounts of ferro alloys, graphite and lubricant such that the final desired composition is achieved following sintering and the powder blend is suited to compaction in rigid compaction dies. Examples of these powder blends are provided in U.S. Pat. No. 5,476,632 (incorporated herein by reference). The use of substantially pure iron powder admixed with ferro alloys may be preferred as such powders are relatively highly compressible and are relatively inexpensive as compared to pre-alloyed powders. The powder blend of the invention may comprise elemental or substantially pure iron powder blends, fully pre-alloyed powder blends and partially pre-alloyed powder blends. It will be appreciated that any composition of powder metal blend may be used in the present invention. In one embodiment of the invention, alloys of iron, such as ferro manganese, ferro molydenum and ferro chromium may be used individually, or in combination, as required to achieve desired performance requirements of the final article. For example, one, two, or three ferro alloys may be blended with the base iron powder. A wide range of alloy elements can be used in the process described herein, depending on the final product performance requirements, including: carbon, chromium, copper, manganese, molybdenum, nickel, niobium and vanadium. Alloy elements may be present either singly or in combination.

The base iron powder will generally have a particle size distribution in the range of 10 to 350 μm. This range includes the base iron powder particle sizes of 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, 250, 300, or 350 μm and any size there-between. The alloying additions typically will have a particle size distribution in the range of 2 to 20 μm. This range includes the particle size of alloying additions of 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20 μm and any size there-between. Commercially available lubricant powder is added to the blend to facilitate compaction. Typical lubricants include zinc stearate, stearic acid or ethylene bistearamide. Various particle sizes, lubricants and other additives will be apparent to persons skilled in the art.

As a further alternative, the present invention may be used with pre-alloyed powder metals, some examples being molybdenum or chromium-molybdenum pre-alloys having between 0 to 1.5% of each alloy with the remainder being unavoidable impurities. With such powders, sintering can be conducted at temperatures of 1100° C. to 1150° C., or alternatively at higher temperatures greater than 1250° C. Typical commercial examples of prealloyed molybdenum powders are Quebec Metal Powder sold under the trademarks QMP Atomet™ 4401, and Hoeganaes Ancorsteel™ 85HP, both of which have approximately 0.85% by weight molybdenum. The particle size of the pre-alloyed molybdenum powder metal is typically within the range of 45 to 250 μm. Between 0 to 0.7% carbon by weight may be added. Compacting is facilitated by the addition of the lubricants discussed previously.

Compaction of Powder Metal Blend

The compaction step is performed in the known manner using powder formulated as discussed above, whereby the blended powder is pressed in a rigid die at approximately 400 MPa to between 70 and 90% of the theoretical maximum density. Various other pressures and end product densities will be apparent to persons skilled in the art. After compacting, the shape and dimensions of the resulting compact, or pre-form, are substantially similar to the final article with the exception of allowances for size change due subsequent operations. However, in accordance with the present invention, one or more surface regions to be densified are maintained in a cylindrical form. These areas may, in the final article, be later formed into complex, non-cylindrical shapes, as will be discussed further below. As will be made clear in the following description, by maintaining the surface regions to be densified in a cylindrical form, the surface densification of the compact is facilitated.

Sintering of Compacted Pre-Form

Next, the compacted article, or pre-form, is then sintered using methods commonly known in the art. For example, the sintering process may be conducted in a reducing atmosphere or vacuum at a temperature in excess of 1250° C. such that oxides from both the iron and alloy additions contained in the compact are reduced and metallurgical bonds are formed between contacting particles to impart strength and ductility to the sintered article. The chemical reduction process also allows for uniform diffusion of alloying elements throughout the iron particles resulting in a homogeneous microstructure. Particularly for higher carbon content materials, an isothermal hold or slow cooling treatment may also be utilised to maximize the ferrite content of said article as described, for example, in U.S. Pat. No. 5,997,805 (which is incorporated herein by reference). As will be understood, such isothermal treatment serves to improve the malleability of the sintered article. Further, the isothermal treatment step can be included within the cooling phase of the sintering step (i.e. it can form a part of the sintering step) or can be included as a separate step following sintering. In the case of elemental powder blends and partially or fully pre-alloyed powder metal, sintering may take place at conventional sintering temperatures of 1100° to 1150° C. or at a higher temperature up to 1350° C. As known in the art, no significant densification occurs during the sintering process. As such, the density of the sintered compact will remain substantially the same as that of the compacted pre-form.

Surface Densification of Sintered Pre-Form

Densification of the surface, according to one aspect of the present invention, is generally performed using a plurality of small diameter rollers in a roller burnishing tool to cold roll the one or more cylindrical surfaces of the sintered compact. Examples of such tools and processes are provided in U.S. Pat. No. 5,540,883; however, various other tools and methods known in the art may equally be used. As is known to persons skilled in the art, the application of a roller burnishing tool, for example, to a cylindrical surface, compresses the surface, collapsing the pores contained therein so that the surface of the article has a density approaching the theoretical maximum density.

As mentioned above, the surface densification step of the present invention is conducted on one or more generally cylindrical surfaces (or surface regions) of the pre-form. Thus, as will be apparent to persons skilled in the art, the invention allows the use of less expensive (and easier to use) roller apparatus to achieve the desired densification. Moreover, by not initially forming (to the desired shapes) the surfaces to be densified, the invention makes it possible to achieve a uniform densification over the entire area being densified. Furthermore, the use of a roller densification apparatus, in accordance with a preferred embodiment of the invention, results in an optimum surface finish and dimensional control, which is not possible with known shot peening methods. This, therefore, offers an important advantage over other processes known in the art.

In the surface densification step, the surface regions being densified are provided with densities of at least 80% and up to 100% of the theoretical maximum density. In a preferred embodiment, the densified surface region has an approximate thickness of between 0.025 mm to 1 mm (i.e. 0.001 to 0.04 inches) below the surface. The core density of the pre-form is not significantly altered during the surface densification step and, therefore, the core of the pre-form remains the same as that resulting from the compaction step.

The article may subsequently be annealed, at temperatures between 800° and 1100° C. in a protective atmosphere or vacuum, for the purpose of developing proper metallurgical bonding, re-crystallizing the densified surface material and obtaining an advantageous microstructure for forming or contact fatigue durability.

The surface region being densified by this step may comprise either or both of the inner and outer regions of the pre-formed article. This aspect is described, for example, in U.S. Pat. Nos. 5,540,883 and 5,972,132 (the entire contents of which are incorporated herein by reference)

Forming of Article

The selectively densified article is then subjected to a forming operation to achieve the desired final density, shape and dimensional requirements. The forming step is preferably carried out in a closed die and at ambient temperatures, although, if required, elevated temperatures may also be used. The final density is obtained and closely controlled by the movement of the sintered material during forming and the dimensions are controlled by the rigid die set. Such dies are commonly known in the art. Where the final dimensions are not critical to component functionality, complete filling of the die cavity may not be required. The forming operation is alternatively referred to in the art as, inter alia, sizing, coining, repressing, forging or re-striking. These processes will be known to persons skilled in the art. All of the above mentioned processes involve the application of pressure to a sintered compact enclosed within a rigid die cavity. Conventional rigid dies as used in regular sizing/coining/repressing/restriking presses may be used in the present invention to achieve the final surface configuration and higher density of the final article with precise control. Forming is accomplished by the selection of the composition of the sintered article, by the selection of appropriate sintering temperature and furnace profile, by the selection of pressure used in the forming operation, and the selection of the forming tool to provide the necessary clearance between the tools and the sintered article for movement of the sintered compact to the final shape. The required choice of these parameters will be known to persons skilled in the art. After forming, the article will have a final core density of between 90% and 98% of the theoretical maximum, and the densified surfaces will have assumed the final configuration with overall radial dimensions of the contoured form, differing by 0.1 to 10% as compared to the diameter of the surface densified region of the pre-form. Further, the final article will normally have a length dimension that is approximately between 5 to 30% less than the same dimension measured on the sintered and surface densified pre-form.

Generally, as mentioned above, the article resulting from the surface densification step will have the approximate but not final shape of the desired article. As such, the surface densified article will typically not occupy the entire volume of the forming die. FIGS. 1 to 4 illustrate a die having a punch or ram with walls 12 and 14 and an outer die wall 16. Typically, the outer wall 16 is stationary while the punch walls 12 and 14 are designed to move towards and away from each other. It will be understood that, in some systems, one of punch walls 12 or 14 may also remain stationary. The combination of these elements (12, 14, and 16) form a die cavity 20 into which a sintered pre-form 22 is inserted. According to the present invention, the surface of the pre-form will have a densified layer 23, having a generally cylindrical geometry are described above. As also described above, the die cavity is of the shape of the desired final product. As shown in FIGS. 1 and 2, the pre-form is dimensioned to be smaller than the die cavity, thereby leaving a clearance 24 between the pre-form 22 and the outer walls 16 and 18. As the punch walls 12 and 14 are moved towards each other, the pre-form is compressed and radially expanded until is fills, and assumes the shape of, the die cavity 20. The final punch position is illustrated in FIGS. 3 and 4, which also show the final formed article 26 with the densified surface 28 after having assumed the shape of the die walls 16 and 18.

FIGS. 1 to 4 only illustrate a die for the forming operation. It will be understood by persons skilled in the art that the actual shape and configuration of the die will depend upon the specific article being formed. For example, the die can include core rods, moveable outer walls or other configurations necessary to achieve the final article shape. It will be noted that FIGS. 1 to 4 serve to illustrate a forming operation conducted on the outer surface of a sintered pre-form. However, as will be understood by persons skilled in the art, the forming die can be used to provide the article with a desired outer and/or inner shape also as known in the art. Similarly, the forming operation can be used to form multilevel parts, such as an over-running clutch or other such articles as known in the art.

Heat Treatment of Formed Article

Subsequent to forming, the article may optionally be annealed, at temperatures between 800 and 1300° C. in a protective atmosphere or vacuum and with suitable cooling in order to obtain proper metallurgical bonding and to fully develop the desired mechanical properties.

The final article is usually required to have high wear and fatigue resistance. For this reason, heat treatment such as carburizing, quenching and tempering, etc., may be applied to an article made from a blend with 0.4% or less carbon, while through hardening or induction hardening and tempering, etc., can be performed on a part containing greater than 0.4% carbon. A prior through hardening treatment, either applied as a forced cooling, or quenching following annealing or as a separate heat treatment operation may be applied to increase the core yield strength. Both methods produce an article with a hardened surface case and a hard core that is resistant to wear and exhibits superior fatigue performance. Various other heat treatment methods will be known to persons skilled in the art.

The invention described herein relates to the surface densification of a PM article while it still has a simple cylindrical geometry (i.e. prior to final forming of the article) and utilizes the ductility of the pre-formed material to impart the final contoured shape to the densified surface. As will be appreciated by persons skilled in the art, the invention provides an improvement over previously known methods, which require surface densification of the final formed article. As will be understood by persons skilled in the art, one of the key advantages of the present invention lies in its ability to provide an improved, efficient process for producing a PM article having a complex shape and with specific surface densification. As indicated above, it is often very difficult or impossible to selectively densify complex surfaces since the densification apparatus known in the art can only accommodate simple (i.e. cylindrical bearings) or specific (i.e. gear teeth) shapes, or require multiple passes through different dies. As also described above, the prior art methods require complex densification methods and apparatus to achieve surface densification of irregularly shaped objects. Articles made according to the present invention may include any powder metal article such as gears, bearings, cams etc. as will be apparent to persons skilled in the art.

The invention will now be described with reference to certain specific examples. It will be understood that the following examples are meant only to illustrate the invention and are not intended to limit the scope of the invention in any way.

EXAMPLE 1

Carbon Manganese Molybdenum Outer Race

Iron powder, lubricant, graphite, ferromanganese and ferromolybdenum were blended to achieve a sintered composition of approximately 0.2% carbon, 0.9% manganese and 0.5% molybdenum. The powder was formed into rings, which were compacted to a density of 6.5 g/cc (approximately 83% of the theoretical maximum) with a pressure of about 350 MPa. The compacted rings were sintered at 1280° C. for 20 minutes. A nitrogen/hydrogen atmosphere was maintained throughout the cycle.

The bore of each sintered ring was surface densified by the method described in U.S. Pat. No. 6,110,419 (incorporated herein by reference) thereby achieving a local surface density in excess of 99% of the theoretical maximum density, while the core density remained at 6.5 g/cc. This density profile is illustrated in FIG. 5. The bore surface densified rings were formed in a closed die with a core rod having the geometry of the final cam form. The material exhibited remarkable ductility and densification. At forming pressures of 700 to 1050 MPa, core densities of 7.30 To 7.55 g/cc were obtained as illustrated in FIG. 6. Axial closures over the same pressure range were 15 to 18% of the sintered length (as illustrated in FIG. 7), and radial movement of up to 4% of the sintered outer radius was achieved (as illustrated in FIG. 8). Following the forming step, the densified bore layer was intact (FIG. 5), the core density was increased as indicated above, the surface had substantially assumed the final cam shape and the active cam form exhibited both excellent surface finish and dimensional stability.

EXAMPLE 2

Carbon Molybdenum Inner Race

A blend with a sintered composition of 0.6% carbon and 0.9% molybdenum was prepared by combining iron powder, ferromolybdenum, graphite, and lubricant. Rings of the powder blend were compacted to 85% of theoretical maximum density with pressure of approximately 520 MPa. The rings were sintered in a nitrogen/hydrogen atmosphere for 20 minutes at 1280° C. followed by an isothermal hold (as described in U.S. Pat. No. 5,997,805, incorporated herein by reference) resulting in a malleable sintered article. The cylindrical outer surface of the sintered rings was selectively densified using the roller burnishing method (as described in U.S. Pat. No. 5,540,883, incorporated herein by reference) to achieve a surface density of greater than 99% theoretical maximum as illustrated in FIG. 9. As in Example 1, the material exhibited remarkable ductility and through-densification. Core densities of 7.20 to 7.45 g/cc were achieved at forming pressures of 750 to 1050 MPa (as illustrated in FIG. 10) resulting in axial closures of 12 to 16% of the sintered length (as illustrated in FIG. 11). Radial movement of up to 4% of the sintered inner radius was obtained as shown in FIG. 12. As in Example 1, after the forming step, the densified bore layer was intact (FIG. 9), the core density was increased as said, the surface had substantially assumed the final cam shape and the active cam form exhibited both excellent surface finish and dimensional stability.

Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the scope of the invention as outlined in the claims appended hereto. The disclosures of all references recited above are incorporated herein in their entirety.

Claims

1. A method for producing a powder metal article having a three dimensional shape and having at least one densified surface region, said method comprising:

a) providing a blend of powdered metals;

b) compacting the blend to form a pre-form having a general shape of the article, said preform being generally cylindrically shaped at a region corresponding to said at least one densified surface region;

c) sintering said pre-form;

d) densifying said at least one surface region of said pre-form; and,

e) forming said pre-form to a desired final density and into a desired three dimensional shape of said article.

2. The method of claim 1 wherein said step of densifying comprises cold rolling said at least one surface.

3. The method of claim 1 further comprising:

f) subjecting said article to annealing.

4. The method of claim 1 further comprising:

f) subjecting said article to heat treatment.

5. The method of claim 1 wherein said powder is compacted in step (b) to a density of between 70% and 90% of the theoretical maximum density.

6. The method of claim 1 wherein step (d) comprises densifying said at least one surface region to a density of at least 80% of the theoretical maximum density.

7. The method of claim 1 wherein step (d) comprises densifying said at least one surface region to a density of between 95% and 100% of the theoretical maximum density.

8. The method of claim 1 wherein said forming step includes compressing said pre-form to a core density of at least 90% of the theoretical maximum density.

9. The method of claim 1 wherein said forming step includes compressing said pre-form to a core density of between 90% and 98% of the theoretical maximum density.

10. The method of claim 1 wherein said at least one surface region has a thickness of between 0.001 and 0.04 inches after densification.

11. The method of claim 1 wherein said powder metal blend comprises compressible iron powder, at least one ferro alloy, a lubricant, and carbon in the form of graphite.

12. The method claim 11 wherein said ferro alloy comprises an alloy of iron with a metal chosen from the group consisting of: chromium, copper, manganese, molybdenum, nickel, niobium, vanadium, and combinations thereof.

13. The method of claim 12 wherein said ferro alloy is chosen from the group consisting of: ferro manganese, ferro molybdenum, and ferro chromium.

14. The method of claim 1 wherein said powder blend comprises: elemental or substantially pure powder blends; fully pre-alloyed powder blends; partially pre-alloyed powder blends; or, powder blends containing ferro alloys.

15. The method of claim 1 further including an isothermal treatment step following said sintering step (c).

16. The method of claim 1 further including an isothermal treatment step, wherein said isothermal treatment is included in a cooling phase of said sintering step (c).

17. A method for producing a powder metal article having a three dimensional shape and having at least one densified surface region, said method comprising:

a) providing a blend of powdered metals;

b) compacting the blend to form a pre-form having a general shape of the article, said pre-form having a density of between 70% to 90% of the theoretical maximum density and being generally cylindrically shaped at a region corresponding to said at least one densified surface region;

c) sintering said pre-form;

d) densifying said at least one surface region of said pre-form to at least 80% of the theoretical maximum density; and,

e) forming said pre-form to a desired final density and into a desired three dimensional shape of said article.

18. The method of claim 17 further including an isothermal treatment step following said sintering step (c).

19. The method of claim 17 further including an isothermal treatment step, wherein said isothermal treatment is included in a cooling phase of said sintering step (c).

20. A powder metal pre-form having a general shape of a desired article, said pre-form having a density of between 70% to 90% of the theoretical maximum density and being generally cylindrically shaped at least one surface region.

21. A powder metal article formed from the pre-form of claim 20.