Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Metal and nonmetal in final product
Reexamination Certificate
2002-05-29
2004-01-13
Mai, Ngoclan (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Metal and nonmetal in final product
C419S027000, C419S029000, C419S038000, C075S230000, C075S246000
Reexamination Certificate
active
06676894
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to an article formed by compacting and sintering iron powder to form a matrix structure and infiltrating the matrix structure with copper metal. More particularly, this invention relates to such article wherein the iron powder contains nickel to further enhance mechanical properties.
BACKGROUND OF THE INVENTION
U.S. Pat. No. 6,551,373, issued to Alcini et al. in 2003, describes a metal article formed of an iron powder matrix and a copper infiltrant. The matrix is formed by compacting and sintering iron powder containing a phosphorus sintering aid. Molten copper is infiltrated into pores within the matrix. The resulting composite exhibits a superior combination of mechanical properties, including elongation and tensile strength. Nevertheless, it is desired to further improve mechanical properties of the copper-infiltrated iron powder article.
BRIEF SUMMARY OF THE INVENTION
In accordance with this invention, a copper-infiltrated iron powder metal article is ma de by a method that includes compacting and sintering a predominately iron powder that contain significant additions of phosphorus and nickel. Phosphorus is present in an amount effective to produce a concentration in the matrix of between 0.1 and 1.2 weight percent. As used herein, concentrations of constituents of the iron matrix are reported based upon the weight of the matrix without infiltrant, whereas the proportion of copper infiltrant is reported based upon the total weight of iron matrix and infiltrant in the product article. The nickel content is between about 1 and 7 weight percent, and preferably between about 1 and 3 weight percent. The method also comprises infiltrating pores within the iron matrix with a copper metal. Infiltration is preferably carried out during sintering, or may be suitably carried out in a subsequent step. The resulting article thus comprises an iron matrix that contains phosphorus and nickel, and copper metal disposed within pores within the matrix. It is found that the addition of nickel to the matrix, in combination with phosphorus in the matrix and copper metal infiltrant, significantly improves mechanical properties, including tensile strength and elongation.
DETAILED DESCRIPTION OF THE INVENTION
In accordance with the preferred embodiment of this invention, a composite metal article is manufactured by compacting a powder mixture composed of an iron powder, a ferro phosphorus sintering aid and a nickel alloying agent, sintering the compact to form a porous matrix, and infiltrating pores of the matrix with a copper metal.
The powder mixture is preferably a blend composed predominately of iron powder and containing significant amounts of iron phosphorus powder and nickel powder. A preferred iron phosphorus powder contains ferro-phosphorus intermetallic compound. During heating for sintering, the iron phosphorus forms a transient, low melting liquid phase to enhance bonding between iron particles. Upon continued heating, the phosphorus diffuses into the bulk of the iron, causing the liquid phase to solidify and producing iron phosphorus alloy within the matrix. In addition to aiding in sintering of the iron particles, it is believed that the phosphorus aids in wetting of the iron by the molten copper during infiltration, as described in U.S. patent application Ser. No. 09/843,469, incorporated herein by reference. In general, it is desired to add an amount of iron phosphorus powder effective to produce a phosphorus concentration in the matrix between about 0.01 weight percent and 1.2 weight percent, based upon the weight of the iron matrix. A preferred iron phosphorus addition is between 0.4 and 0.9 weight percent phosphorus. While ferro-phosphorus powder is added in the preferred embodiment, phosphorus may be suitably added in other forms, including as a pre-alloy with iron or other metal.
In accordance with the invention, the metal powder also contains nickel, preferably added as nickel powder. In general, a nickel addition of 1 weight percent or more is effective to significantly improve tensile strength while still providing good elongation. Additions greater than about 7 weight percent nickel provide little additional improvement in mechanical properties while adding undesirably to the cost of the product. A preferred range is between about 1.0 and 3.0 weight percent nickel.
In addition to phosphorus and nickel, the iron powder mixture may contain other alloy agents to further enhance desired properties. Such agents may be suitably added as a separate powder or pre-alloyed with the iron, nickel or phosphorus.
By way of an example, in an optional aspect of this invention, molybdenum may be added in the amount up to about 2.0 weight percent, based on the weight of the matrix. When added in combination with nickel, molybdenum is found to further increase tensile strength and to reduce elongation.
It is desired that the mixture contain less than 0.5 weight percent carbon to minimize the substantial effect of carbon on the mechanical properties of iron. Accordingly, in the examples that follow, carbon was limited to less than 0.03 weight percent, based upon the weight of the matrix. The mixture also preferably contains small amounts, typically less than about 2 percent, of fugitive binders and lubricants to facilitate compaction, but which vaporize during sintering and do not affect the product composition or properties.
In preparation for sintering, the powder mixture is filled into a die and compacted to form a green compact. It is an advantage of the invention that the infiltrated product has substantially the size and shape of the iron powder compact, so that the article may be made to near net shape to reduce machining necessary to finish the product. Mechanical properties within preferred ranges may be obtained utilizing a single action press, although the method may be suitably carried out using a double acting press, powder forging or other compaction techniques. The density of the green compact, and thus the volume of pores available for copper infiltration, is determined by the pressure applied to the powder in the die. In general, improved mechanical properties may be obtained for green compacts having a density between about 6.0 and 7.3 grams per cubic centimeter, although densities greater than about 6.8 grams per cubic centimeter are more difficult to compact because of relative hardness of the nickel powder. A preferred compact density is between 6.5 and 6.8 grams per cubic centimeter.
The green compact is heated to sinter the iron powder and form a porous matrix. In general, sintering is carried out in a vacuum at a temperature in the range of about 1100° C. and 1300° C. for a time sufficient to diffusion bond the iron particles into an integral structure. As the temperature is heated above 1,050° C., the iron and phosphorus form a low melting eutectic liquid phase that wets the iron powder surfaces to promote bonding between the particles. Upon continued heating, the phosphorus diffuses into the bulk of the iron, whereupon the liquid phase solidifies. Also during heating, nickel, derived from the nickel powder, diffuses into the iron and thereby forms an alloy that strengthens the matrix.
The sintered product thus comprises an iron matrix that includes pores formed at interstices between particles. In accordance with this invention, a copper metal is infiltrated into the pores of the matrix to produce an infiltrated article having improved mechanical properties. In a preferred embodiment, copper infiltration is carried out concurrent with heating the green iron compact for sintering. For this purpose, metallic copper is arranged in contact with the green compact prior to sintering. During heating above the copper melting temperature of 1083° C., the copper melts and is drawn into the compact by capillary action. While not limited to any particular theory, it is believed that the addition of phosphorus and nickel to the iron powder enhances infiltration by the copper, particularly when infiltration is carried out c
Brinks Hofer Gilson & Lione
Mai Ngoclan
NTN Corporation
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