Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Making composite or hollow article
Reexamination Certificate
2003-03-19
2004-11-09
Mai, Ngoclan T. (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Making composite or hollow article
C419S010000, C419S018000, C419S023000, C419S037000
Reexamination Certificate
active
06814926
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to the field of solid freeform fabrication (“SFF”) of parts and more specifically to the powder blend for use in the selective laser sintering process utilizing a steel alloy and the method of forming three-dimensional parts employing that powder blend.
2. Description of the Relevant Art
SFF generally refers to the manufacture of articles in a layer-wise or additive fashion directly from computer-aided-design (CAD) databases in an automated fashion, as opposed to conventional machining of prototype articles from engineering drawings in subtractive processes. SFF has, in recent years, made substantial improvements in providing high strength, high density parts for use in the design and pilot production of many useful articles. As a result, the time required to produce prototype parts from engineering designs has reduced from several weeks, using conventional machinery and subtractive processes, to a matter of hours.
One example of an SFF technology is the selective laser sintering process practiced by systems available from 3D Systems, Inc. of Valencia, Calif. According to this technology, articles are produced in layer-wise fashion from a laser-fusible powder that is dispensed one layer at a time. The powder is fused, or sintered, by the application of laser energy that is directed to those portions of the powder corresponding to a cross-section of the article. After the fusing of powder in each layer, an additional layer of powder is then dispensed, and the process repeated, with fused portions of later layers fusing to fused portions of previous layers (as appropriate for the article), until the article is complete. Detailed description of the selective laser sintering technology may be found in U.S. Pat. Nos. 4,863,538 and 5,017,753, both assigned to Board of Regents,
The University of Texas System, and in U.S. Pat. No. 4,247,508, to Housholder. The selective laser sintering technology has enabled the direct manufacture of three-dimensional articles of high resolution and dimensional accuracy from a variety of materials including nylons, polystyrenes, and composite materials such as polymer coated metals and ceramics. Examples of composite powder materials are described in U.S. Pat. Nos. 4,944,817; 5,076,869; and 5,296,062, all assigned to Board of Regents, The University of Texas System, and incorporated herein by reference in pertinent part.
A related SFF technology, referred to as 3-Dimensional (3D) Printing, is described in U.S. Pat. Nos. 5,340,656 and 5,387,380. From a computer (CAD) model of the desired part, a slicing algorithm draws detailed information for every layer. Each layer begins with a thin distribution of powder spread over the surface of a powder bed. Using a technology similar to ink-jet printing, a binder material selectively joins particles where the object is to be formed. A piston that supports the powder bed and the part-in-progress lowers so that the next powder layer can be spread and selectively joined. This layer-by-layer process repeats until the part is completed. Following a heat treatment, unbound powder is removed, leaving the fabricated part.
As SFF technology has evolved, it has increasingly been used not only to make prototype parts but also to make final useful parts as well as tools or molds that can be used to make multiple parts. It is becoming more common to fabricate such parts, tools, or molds with an “indirect” process that uses a powder of metal and/or ceramic particles either coated by or blended with a polymer. The powder is used in the selective laser sintering process to fabricate a “green” article that binds the particles to one another. The green article is then heated to a temperature above the decomposition temperature of the polymer, which both drives off the polymer and also binds the metal and/or ceramic substrate particles to one another to form an intermediate porous article. The porous article can then be infiltrated with another material, such as a lower melting temperature metal to give a fully dense article with desirable properties. The green article can also be fabricated with 3D printing.
Some examples of the use of these approaches for functional applications are described, for example, in U.S. Pat. Nos. 5,433,280: 5,544,550 and 5,839,329 to Smith et al. These describe the use of selective laser sintering a tungsten carbide-polymer composite powder to generate “green” drill bit which is then infiltrated in a furnace cycle with a copper alloy to generate a fully functional drill bit for down hole oil exploration. U.S. Pat. No. 4,554,218 describes the use of a powder mixture having a first metal and a second metal, such as A6 tool steel, and a fugitive binder that is placed in a mold, cured to a green part and then infiltrated with a third metal, preferably a copper or copper-containing alloy, to form an infiltrated, molded metal composite article. Another commercial application of these indirect approaches is a product called ProMetal by ExtrudeHone. Utilizing the 3D Printing technology described above, ProMetal builds metal components by selectively binding metal powder layer by layer. The finished structural skeleton is then sintered and infiltrated with bronze to produce a finished part that is 60% steel and 40% bronze and is used for injection molding tools or final metal parts. Another commercial example is 3D Systems' ST-100 system, which uses selective laser sintering of a steel polymer composite powder to generate a green article. The green article or part is subsequently put through a furnace cycle that removes the polymer binder and infiltrates the metal skeleton with bronze to create a functional fully dense article that can also be used for injection mold tools or final parts.
As is well known in the art, the structural strength of the green article is an important factor in its utility, since weak green articles cannot be safely handled during subsequent operations. Another important factor in the quality of a prototype article is its dimensional accuracy relative to the design dimensions. However, these factors of part strength and dimensional accuracy are generally opposed to one another, since the densification of the powder that occurs in the sintering of the post-process anneal also causes shrinkage of the article. The polymer content of a metal and/or ceramic composite powder described above could be increased in order to provide higher green part strength, but the shrinkage of the part in post-process anneal would increase accordingly. As a result, compromises between article strength and dimensional stability must be made in the design of a composite powder system using a polymeric binder.
Some drawbacks of conventional composite powders incorporating thermoplastic polymer binders have been observed. In the post-process anneal of green articles using such binders, creep deformation has been observed as the article is heated to a temperature above the glass transition temperature of the polymer binder, but below the decomposition temperature at which the binder is released. The viscosity of the polymer decreases to such an extent that the metal or ceramic substrate particles slide past one another under the force of gravity. Not only do the dimensions of the article change as a result of this creep deformation, but also this dimensional change is not uniform. Taller features deform by a larger extent than do shorter features. This non-uniformity in deformation precludes the use of a constant shrinkage correction factor in the selective laser sintering fabrication of the green part, further exacerbating the difficulty of achieving dimensionally accurate articles of high density and strength.
Creep deformation has been observed to deform not only the height but also the shape of vertical features, such as sidewalls. For example, vertical walls of mold cavities formed by selective laser sintering of polymer-coated metal powders, and having a thickness of 0.75 inches and a height of 1.5 inches, have be
Geving Brad
Newell Kenneth J.
Schmidt Kris Alan
3-D Systems, Inc.
D'Alessandro Ralph
Mai Ngoclan T.
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