Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Reexamination Certificate
2002-07-12
2004-06-08
Mai, Ngoclan T. (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
Reexamination Certificate
active
06746506
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of powder metallurgy. More particularly, the present invention relates to methods which involve blending a relatively fine metal powder with a relatively coarse prealloyed metal powder to produce a mixture that has a widened sintering temperature window compared to that of the relatively coarse prealloyed metal powder. The present invention also relates to articles made from such powder mixtures.
2. Description of the Related Art
The science and industry of making and using powder metals is referred to as powder metallurgy. Powder metal compositions include elemental metals as well as metal alloys and compounds. A wide variety of processes are used to make powder metals, for example, chemical or electrolytic precipitation, partial vaporization of metal containing compounds, and the solidification of liquid metal droplets atomized from molten metal streams. The shapes of metal powder particles are influenced by the powder making method and range from spherical to irregular shapes. Powder metals particles range in size from submicron to hundreds of microns. Particle size is measured as a diameter for spherical powders or as an effective diameter for non-spherical powders.
Various techniques are employed to consolidate powder metals particles to form useful metal articles through the use of applied pressure and/or elevated temperatures. The powder metal to be consolidated is typically formed into a shape at room temperature and held in place through the use at least one restraining mechanism such as container walls, a fugitive binder material, or mechanical interlocking caused by pressing the powder metal particles together with high pressure in a die press. Examples of specific forming processes include powder containerization, solid free-forming layer-wise buildup techniques (for example, three-dimensional printing (3DP) and selective laser sintering (SLS)), metal injection molding (MIM), and metal powder die pressing. The term “green article” is used herein to refer to the shaped powder metal article produced by this stage of the consolidation process. The green article is then heated to one or more elevated temperatures at which atomic diffusion and surface tension mechanisms become active to consolidate the powder metal by sintering. The term “sintered article” is used herein to refer to the consolidated powder metal article produced by this stage of the consolidation process. Although sintering may occur to some extent over a range of temperatures as the green article is being heated, the peak temperature to which the green article is heated is what is usually referred to as the “sintering temperature.” Generally, the green article is held for a period of time ranging from a few minutes to a few hours at the sintering temperature, the length of time depending upon a variety of process and metallurgical system-related factors.
The heating of the green article is done in a controlled atmosphere or vacuum so as to protect the powder metal from undesired reactions with atmospheric constituents. The heating is also controlled so as to eliminate any fugitive binders from the green article. The consolidation of the green article into a sintered article is typically done at about atmospheric pressure or under vacuum. Some specialized techniques, however, such as hot isostatic pressing, hot uniaxial pressing, and hot extrusion, apply a pressure to the green article while it is hot to aid in the consolidation. In some processes, for example, in some embodiments of 3DP and SLS, consolidation is achieved through an infiltration process by wicking a liquid metal into the pores of the green article from a source external to the green article.
As the consolidation of the powder metal proceeds from green article to sintered article, the density of the article increases as some or all of its porosity is eliminated. Density, in this application, may be defined as “absolute density,” which is the article's mass per unit volume. Absolute density is expressed in terms such as grams per cubic centimeter. Density is also defined as “relative density,” which is the ratio of the absolute density of a powder metal article to the density which that article would have if it contained no porosity. Relative density is expressed in terms of a percentage, with a highly porous article having a low relative density and an article having no porosity having a relative density of 100%. The relative density of a green article depends on many factors and is sensitive to the method by which the green article was formed. Green article densities are generally in the range of about 50-90%. The relative densities of sintered articles also depend on a variety of factors, including parameters of the sintering process. The sintered article relative densities typically are in the range of 75-95%. For applications in which the mechanical strength of a sintered article is of importance, high relative densities are generally desired. The increase in the relative density from the green article stage to the sintered article stage is referred to herein as “densification.”
Densification may proceed by “solid state sintering,” which is a term that describes the phenomena by which solid particles become joined together at contact points through the diffusion of atoms between the contacting particles. The number of point contacts for a given volume of powder and the ratio of surface area to particle volume increase as metal powder particle size decreases, which results in finer powder metals solid state sintering more readily and at lower temperatures than do larger powder metal particles.
Densification of a green article during sintering can be enhanced by the presence of a liquid phase within the green article. The enhancement occurs because of the relatively high atomic diffusion rates through a liquid as compared to a solid and because of the effect that the surface tension of the liquid has in drawing the solid particles together. The sintering that results from the presence of the liquid phase is identified as “liquid phase sintering.” In some powder metallurgical systems, the powder metal in the green article comprises a minor volume fraction of a relatively low temperature melting powder metal and a high volume fraction of a second type of powder metal which remains solid at the sintering temperature. For example, a tungsten carbide-cobalt green article may contain a low volume fraction cobalt powder, which is liquid at the sintering temperature, and a high volume fraction tungsten carbide powder, which remains solid at the sintering temperature.
An important variant of liquid phase sintering is supersolidus liquid phase sintering. Supersolidus liquid phase sintering is possible if a prealloyed powder metal passes into a solid-plus-liquid phase state upon heating. Referring to
FIG. 1
, which depicts a portion of an idealized temperature-composition equilibrium phase diagram
1
for an alloy system consisting of metal Y and metal Z, the horizontal axis
2
relates to composition with the left hand end
4
of horizontal axis
2
representing pure metal Y. The weight percentage of metal Z in the alloy composition increases linearly to the right along the horizontal axis
2
. The vertical axis
6
relates to temperature, which increases in the upward direction. The phase diagram
1
contains two phase boundary lines, liquidus line
8
and solidus line
10
, which divide the illustrated portion of phase diagram
1
into three phase regions: a liquid phase region
12
above liquidus line
8
; a solid-plus-liquid phase region
14
between liquidus line
8
and solidus line
10
; and a solid phase region
16
below solidus line
10
. A pure metal, such as metal Y, upon heating from a temperature at which it is a solid, remains a solid until it reaches its melting point temperature, T
m
18
, at which it melts, and is a liquid at temperatures above T
m
18
. In contrast, a Y-Z alloy of composition X
20
, upon heating from the so
Liu Jianxin
Ryneson Michael L.
Extrude Hone Corporation
Lizzi Thomas
Mai Ngoclan T.
LandOfFree
Blended powder solid-supersolidus liquid phase sintering does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Blended powder solid-supersolidus liquid phase sintering, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Blended powder solid-supersolidus liquid phase sintering will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3358537