Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Sintering which includes a chemical reaction
Reexamination Certificate
2000-07-07
2003-01-21
Jenkins, Daniel J. (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Sintering which includes a chemical reaction
C419S026000, C419S036000, C419S037000
Reexamination Certificate
active
06508980
ABSTRACT:
BACKGROUND
This invention relates to metal and ceramic containing parts that are formed from powder, and more particularly to a method of making and control of the dimensions of such parts.
Standard practice in powder metallurgy falls into two basic categories. In powder pressing, powder is placed between two hardened steel dies and compacted, typically to densities of 80% or more. This compaction involves the deforming the powder particles so that they mechanically interlock, thus creating a porous skeleton. Often this skeleton is subsequently infiltrated with a lower melting point material in order to form a fully dense part. For example, skeletons of steel powder are often infiltrated with copper alloys.
Alternatively, the techniques of metal injection molding are used, where a powder is mixed with a binder material and is injected into a die. After the binder has solidified, the powder component is removed. This green part is typically approximately 60% dense. The binder is then burned off or removed chemically and the skeleton is then sintered. Generally speaking, the skeleton is sintered to near full density.
In the fabrication of metal components by powder metallurgy, the control of the dimensions of the final component is often an important issue. Such dimensional control becomes an especially important issue when the components being made are to be used as tools and dies for the fabrication of other components by forming processes.
In a known tooling process, described in general in U.S. Pat. No. 4,554,218 entitled INFILTRATED POWDERED METAL COMPOSITE ARTICLE, issued on Nov. 19, 1985, in the name of Gardner, et al., a skeleton is formed by packing powder around a form and holding it together with a polymeric binder. After removal from the form, the polymeric binder is burned off and the skeleton is lightly sintered. A subsequent infiltration with a low melting point alloy provides a fully dense part.
The two most common approaches for densifying a powder skeleton are either to sinter it to full density or to fill the voids in the skeleton with a second material. These voids may be filled by infiltration of a lower melting point metal, or by infiltration of a polymeric material such as an epoxy.
The skeleton that is formed in the first step of the process may range in density from 55-85%. If sintering to full density is chosen, a significant amount of additional shrinkage must be incurred. The shrinkage arises because material migrates from within the bodies of particles to form larger necks between particles. For example, if a skeleton of 60% density is sintered to full density, the shrinkage must be approximately 18% linear. This large amount of shrinkage can cause significant problems if the goal is to maintain good dimensional accuracy. For example, if a 1% variation in shrinkage is encountered then a dimension which requires a 15% shrink will have an uncertainty of 0.15% of original. Thus, a 10 cm dimension will be uncertain by 0.15 mm, a very significant error when precision components are considered. For this reason, the method of creating a skeleton and then sintering the skeleton to full density in a secondary operation is not attractive when precision parts are concerned.
A processing technique that uses powders has become known as “three-dimensional printing” (“3D Printing”) and is described in general in numerous patents, including: U.S. Pat. No. 5,204,055, entitled THREE-DIMENSIONAL PRINTING TECHNIQUES, by Sachs, Haggerty, Cima, and Williams; U.S. Pat. No. 5,340,656, entitled THREE-DIMENSIONAL PRINTING TECHNIQUES, by Sachs, Haggerty, Cima, and Williams; U.S. Pat. No. 5,387,380, entitled THREE-DIMENSIONAL PRINTING TECHNIQUES, by Cima, Sachs, Fan, Bredt, Michaels, Khanuja, Lauder, Lee, Brancazio, Curodeau, and Tuerck; U.S. Pat. No. 5,490,882, entitled PROCESS FOR REMOVING LOOSE POWDER PARTICLES FROM INTERIOR PASSAGES OF A BODY, by Sachs, Cima, Bredt, and Khanuja; and U.S. Pat. No. 5,660,621, entitled BINDER COMPOSITION FOR USE IN THREE-DIMENSIONAL PRINTING, by James Bredt; U.S. Pat. No. 5,771,402, issued Jul. 7, 1998, entitled ENHANCEMENT OF THERMAL PROPERTIES OF TOOLING MADE BY SOLID FREE FORM FABRICATION TECHNIQUES, by Allen, Michaels, and Sachs; and U.S. Pat. No. 5,807,437, issued on Sep. 15, 1998, entitled HIGH SPEED, HIGH QUALITY THREE DIMENSIONAL PRINTING, by Sachs, Curodeau, Fan, Bredt, Cima, and Brancazio. All of the foregoing 3D Printing patents are incorporated herein fully by reference.
3D Printing is also disclosed and discussed in co-pending, co-assigned applications, including: U.S. Ser. No. 08/600,215, filed Feb. 12, 1996, entitled CERAMIC MOLD FINISHING TECHNIQUES FOR REMOVING POWDER, by Sachs, Cima, Bredt, Khanuja, and Yu; U.S. Ser. No. 08/856,515, filed May 15, 1997, entitled CONTINUOUS INK-JET DROPLET GENERATOR, by Sachs and Serdy; U.S. Ser. No. 08/831,636, filed Apr. 9, 1997, entitled THREE DIMENSIONAL PRODUCT MANUFACTURE USING MASKS, by Sachs and Cima; U.S. Ser. No. 60/060,090, filed Sep. 26, 1997, entitled REACTIVE BINDERS FOR METAL PARTS PRODUCED BY THREE DIMENSIONAL PRINTING, by Sachs, Yoo, Allen, and Cima (provisional application); U.S. Ser. No. 60/094,288, filed Jul. 27, 1998, entitled METHOD OF MAKING INJECTION MOLDS HAVING COOLING CHANNELS THAT ARE CONFORMAL TO THE BODY CAVITY, by Xu and Sachs (provisional application); and PCT application PCT/US98/12280, filed Jun. 12, 1998, which designates the U.S., entitled JETTING LAYERS OF POWDER AND THE FORMATION OF FINE POWDER BEDS THEREBY, by Sachs, Caradonna, Serdy, Grau, Cima, and Saxton. All of the foregoing 3D Printing patent applications (and provisional application) are incorporated herein fully by reference.
The flexibility of the 3D Printing process makes it possible to construct a part out of any material available in powdered form. Such a part can possess almost any geometry, including overhangs, undercuts, and internal volumes. The 3D Printing process was initially developed for the production of ceramic shells and is also useful in making metal parts. One key use for such a system is the production of injection molding tooling for plastic parts. Injection molds are used to make a vast array of items, ranging from toys to floppy disks. The lead times for the production of such tools generally range from a few weeks to several months. The rapid production available via the 3D Printing process can greatly reduce this lead time, thereby alleviating a bottleneck and reducing the duration of product development.
Designers of plastic parts often call for fairly tight part tolerances. Therefore, the tolerances of the injection molds are also critical. In a known 3D Printing process, metal parts are produced by printing a polymer binder into stainless steel powder. The bound parts are subsequently furnace-treated to lightly sinter and debind. This debinding step requires burning out the polymer binder, which is typically a messy process that raises maintenance challenges. Once lightly sintered and debound, the parts are then infiltrated with a molten metal alloy. Sometimes, in order to prevent gravitational slumping and other forms of part distortion, the green part is loosely repacked in refractory material to support unsupported sections. This repacking is called “settering”. In some cases, the infiltration step is accompanied by a second, more severe sintering step.
Such a more severe sintering step is provided to achieve certain mechanical properties, such as higher impact toughness, yield and tensile strengths. It is believed that these properties improve due to increased necking between particles. However, it is just this necking during sintering that causes shrinkage.
These post-printing processes cause a total linear dimensional change of approximately −1.5%±0.2%. In other words, the average shrinkage is 1.5%. However, there is an uncertainty in the value of shrinkage of ±0.2%. For large parts, these uncertainties become extremely significant (in the absolute). This uncertainty results in the loss of some dimensional control of the parts. Furthermore, growt
Allen Samuel
Hadjiloucas Constantinos
Sachs Emanuel M.
Yoo Helen J.
Jenkins Daniel J.
Massachusetts Institute of Technology
Weissburg Steven J.
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