JP2004536967A - Sintering methods and tools used for metal injection molding of large parts - Google Patents

Abstract

An improved drying, binder evaporation and sintering process that can be used with dedicated sintering tools and can provide shape-stable sintering of large and complex metal injection molded preform parts or flow bodies. Improved processes include a three-stage drying process, a single-stage binder evaporation process, and a two-stage sintering process.

Description

【Technical field】
[0001]
FIELD OF THE INVENTION The present invention relates to techniques for sintering metal injection molded preforms or flow bodies, and more particularly to a two-step sintering method and related methods for controlling the deformation of the flow body that generally occurs during the sintering process. About tools.
[Background Art]
[0002]
Metal injection molding (MIM) is a well-known technique for cost-effectively producing complex multidimensional parts. Generally, such components are relatively small in size, ranging from about 25 to about 250 grams, and are often produced in large quantities. Metal injection molding is most commonly used in the automotive, small arms and medical industries.
[0003]
Generally, in a MIM process, powdered metal, water and a binder are mixed. Generally, the binder is composed of an aqueous organic gel. The mixed powdered metal and binder composition produces a generally flowable mixture at relatively low temperatures and pressures. Generally, the ratio of the binder to the powder metal is about 40 to 60% by volume of the binder. The purpose is to produce a viscous, flowable mixture that fills all the crevasses and small size features of the mold. Generally, the flowable mixture is transferred to a mold via an injection molding machine.
[0004]
Injection molding machines are known in the art and can generally apply hundreds of tons of pressure to a mold. Generally, the mold is configured to include internal cooling passages for solidifying the flowable material prior to removal. Generally, the mold cavity is larger than the desired finished part cavity to compensate for the shrinkage that occurs after binder removal. The mold structure can be formed from a rigid or flexible material, such as metal, plastic or rubber. The mold preferably includes vents or vent lines to allow air to escape from the mold during the molding process. Alternatively, the mold may include a porous metal or ceramic insert that allows air to escape from the mold. After filling the mold with the flowable mixture, pressure is applied to the mold / mixture to form a molded part. This is also known as preform. Typical injection molding pressures for the preform range from about 68,950 to 82,740 kPa (about 10 to 12 ksi). This shaped preform is called a "green" part. The green preform can be dried in an oven that heats to a temperature sufficient to evaporate most of the residual moisture. The preform is then placed in a furnace to evaporate the binder. The preform is then sintered in order to achieve a part having a high density and thus sufficient operating strength.
[0005]
Sintering is a high temperature process for agglomerating a powdered metal preform into a substantially solid form having the same or nearly the same mechanical properties as the cast or refined material. In general, sintering refers to raising the temperature of a powdered metal preform to a temperature close to, but not exceeding, the melting point of the material and holding the temperature there for a defined period of time. Under such circumstances, interparticle melting occurs and the material becomes dense and solid.
[0006]
Generally, complete solidification does not occur, but the sintering density can approach 99% for some materials. As the densification process occurs, the size of interstitial voids in the preform shrinks and their number decreases. As a result, the bulk volume of the sintered preform is significantly smaller than the bulk volume of the preform before sintering. As the preform shrinks, geometric deformation of the preform may occur. This deformation is relatively small for small parts and can easily be improved by secondary machining operations. However, for large parts with a net weight greater than 250 grams, this undesirable deformation is more problematic.
[0007]
In general, during densification, while the preform is exposed to elevated temperatures, some shapes of the preform, such as tubulars and other shapes, are less resistant to the effects of deformation and have a final conforming to the preform. Realizing the shape has been recognized as a challenge in sintering such metal parts. See, for example, U.S. Patent No. 5,710,969. This problem is particularly evident when sintering preforms with large cylindrical cross sections and irregular high mass protrusions. For example, the cross-section of a large cylindrical preform is deformed under the influence of gravity into an oval-shaped, densified cross-section. For this reason, the use of MIM and sintering techniques has not been extended to the production of relatively large parts weighing more than about 250 grams, or parts having a cylindrical cross section greater than about 3.8 cm in diameter.
DISCLOSURE OF THE INVENTION
[Problems to be solved by the invention]
[0008]
An object of the present invention is to provide a sintering method and a tool that can sinter a relatively large part while maintaining the shape stability of the part.
[Means for Solving the Problems]
[0009]
The present invention provides methods and / or tools that can be used to make dimensionally accurate MIM parts of a size and / or complexity not heretofore feasible, which can be used with dedicated sintering tools. Includes improved drying, binder removal, and sintering processes that provide shape stable sintering of large and complex MIM parts.
[0010]
By way of example only, the improved process includes a four-stage drying process, a single-stage binder removal process, and a two-stage sintering process. Drying a wet green preform is particularly important, as cracking often occurs during the drying process, resulting in a large number of scrap parts. This problem is particularly common with large MIM components.
[0011]
The new two-stage sintering process includes a first consolidation stage in which the MIM molded preform is densified at a first sintering temperature to about 60% to 80% of its maximum density. And then allowed to cool. In general, the sintering temperature used in the first sintering stage is much lower than the melting point of the powdered metal material used in the molding process, so that the preform is subject to gravity acting on any large unsupported surface From being improperly distorted. It can be shown that it is desirable to keep the first sintering temperature below the solidus temperature of the alloy (ie, the temperature at which the alloy begins to melt). This first stage serves to fix the overall shape of the preform.
[0012]
In a second stage, the preform is heated to a second sintering temperature near the melting point of the powdered metal material, at which temperature a denser component density is produced.
Generally, in a preform part having both large and small cylindrical features, it is possible to use a refractory sintering tool, such as an insert of a given size, in both the first and second sintering stages. it can. Heat resistant materials such as aluminum oxide ceramic can be used for the insert. In the first sintering stage, inserts are used to support the preform and control the diameter of any small cylindrical features. In a second sintering step, a larger set of cylindrical features can be fitted with a second set of inserts, thereby causing gravity to cause the features to become ovoid or otherwise undesired during sintering. Prevent exaggerated deformation of these features.
[0013]
These and other features of the present invention will become more apparent from the following detailed description of the invention when read in conjunction with the accompanying exemplary drawings.
BEST MODE FOR CARRYING OUT THE INVENTION
[0014]
As used herein, the term "preform" is intended to include conventional powdered metal preforms in which powdered metal is compression molded without the use of a binder. The term "preform" also includes MIM flow bodies, where the flow body is made from a mixture of powdered metal, water and a binder. The flow body is a structure or a part in which a flow path is formed inside, such as a portion of a valve assembly in which a fluid flow path is formed.
[0015]
Throughout the specification, the methods and tools of the present invention have a nominal chemical composition of 52.5Ni-18.5Fe-18.5Cr-5.1Nb-3Mo-0.9Ti-0 mixed with a binder comprising an aqueous agar solution. A specific flow body made from a powdered metal mixture of 0.55 Al-0.4C (wt%) commercial Inconel 718 is described.
[0016]
In general, various temperatures and heating times can be applied to any Inconel alloy composition. Those skilled in the art will appreciate that the sintering method of the present invention can be applied to virtually any metal alloy, including, but not limited to, iron, nickel and titanium based alloys. The sintering temperature and frequency of alloys other than Inconel 718 are of course different from those described. Further, the method of the present invention can be used with virtually any preform or MIM flow body configuration, and the tool of the present invention can be used with any preform or flow body having large and small cylindrical features. Can also be used.
[0017]
Referring to FIG. 1, there is shown an exemplary flow body 26 ready for first stage sintering. The flow body is a butterfly valve housing having a large cylindrical hole 30 having an internal diameter of about 8.8 cm and a pair of smaller cylindrical holes 28 having an internal diameter of about 3.0 cm. A typical flow body feature has a wall thickness of about 3 mm. The weight of the flow body weighs about 1000 grams, which is far in excess of the weight of parts typically manufactured in the MIM process. The flow body includes a diaphragm 20 that helps support the roundness of the flow body formed during the molding process. However, the diaphragm is not required in all applications and is removed before or after sintering, as desired.
[0018]
This flow body is manufactured using the method and tool of the present invention and is dimensionally and geometrically representative of a large flow body of the type that can be successfully manufactured using the method of the present invention. Other large and complex MIM parts can also be manufactured using this method and tool. The method of the present invention is believed to be suitable for sintering flow bodies with cylindrical features having a weight of at least up to 1500 grams and a diameter of more than 8 cm.
[0019]
As shown in FIG. 1, a dedicated sintering tool supports the flow body. Specifically, a ceramic insert, such as a cylinder 34, is placed inside each small hole (see also FIG. 3). Each cylinder retains the shape of the respective hole in which it is located during the first stage sintering and serves to support the flow body via ceramic rods 32. Each of the cylinders may include a through hole 35 (FIG. 3) that slidably receives the ceramic rod 32. The ceramic rod, which may be solid or tubular, is placed in a ceramic support structure 40, such as a refractory brick support structure. The support structure may include a base 42 for receiving a ceramic rod and a pair of V-notch blocks 41 (FIG. 3). The configuration of the first stage sintering tools 32, 34, 41 and 42 is shown more specifically in FIG. Flow body 26 is supported by ceramic rod 32 via cylinder 34 so that it is spaced from base 42.
[0020]
It will be appreciated that for smaller parts having smaller holes, the cylinder 34 can be removed and the part supported by the ceramic rod 32 alone. In this case, a ceramic rod may or may not be used to ensure roundness of the hole. For example, a rod may be used to support the part, but the rod need not retain the relatively small roundness of the hole. Further, the orientation of the flow body with respect to the support structure can be varied as desired. For example, FIG. 1 shows a large cylindrical hole 30 having a horizontal axis. However, it is also possible to rotate the part on the ceramic rod such that the hole 30 has a vertical axis.
[0021]
Referring now to FIG. 2, there is shown a flow body 26 ready for second stage sintering. Two large diameter ceramic inserts, for example a cylinder 38 (see also FIG. 4), are located inside the large bore 30. Like the smaller ceramic cylinders used in the first stage sintering, these cylinders serve to hold the shape of the holes and support the flow body during sintering via ceramic rods 36. The ceramic rod may be the same rod used in the first stage sintering. Referring now to FIG. 4, the second stage sintering tool is shown in detail. Each of the cylinders 38 has a through hole 37 that slidably receives the rod 36. The rod 36 supports the cylinder and therefore supports the flow body in the firebrick support structure 40. The same support structure can be used in the first and second sintering stages. Also in the second sintering stage, the flow body is supported by a ceramic rod via a cylinder so that it is spaced from the base.
[0022]
Preferably, the sintering tool is manufactured from a commercially available aluminum oxide ceramic. Aluminum oxide is a durable material that does not deform during sintering and does not stick to the Inconel 718 metal flow body. The sintered tool can be manufactured by machining a bar of aluminum oxide or by an injection molding process known in the art. Preferably, the outer diameters of cylinders 34 and 38 are machined to match the desired inner diameter of the final dimension of the hole in which they are located. Thus, during sintering, the desired final dimensions of the cylindrical features of the flow body can be more easily controlled as the flow body shrinks around the cylinder. Often, the diameter of cylinders 34 and 38 is machined to a diameter smaller than the final inside diameter of the cylindrical features of the flow body, leaving a small amount of extra material for secondary machining operations. It would be desirable. Otherwise, it should be understood that the insert can take any shape required to form the holes required by the shape of the desired end piece during the sintering process.
[0023]
Next, the sintering process of the present invention will be described in detail with reference to FIG. Steps 12-18 include the drying process of the wet green MIM part. Prior art drying processes require MIM parts that dry rapidly at high temperatures. This procedure is effective for small parts. However, large MIM parts with relatively large cylindrical features can crack during the rapid drying process, resulting in an unacceptably large number of scrap parts. This is due to the rapid evaporation of water from the flow body binder between the thick and thin flow body sections, and between the dry outer (outer) part and the wetter inner part. It is believed to cause a difference in contraction between the two. Therefore, an important step in the successful manufacture of large MIM parts is to remove water from the parts without cracking.
[0024]
At step 12, one or more newly formed green flow bodies are sealed in a container or bag of plastic or any other suitable material. The sealed container is stored at room temperature and atmospheric pressure for a period of 2-3 days. During this time, water vapor evaporates from each flow body and condenses on the walls of the container or bag. At step 14, the sealed container or bag is vented to atmosphere to begin slow rate drying. Next, the flow body is stored in this state for a period of 3 to 5 days. During this period, water evaporates from the previously sealed container or bag and water vapor continues to evaporate from the flow body.
[0025]
At step 16, each flow body is removed from the vented container and dried on a shelf or other support for an additional 2-3 days. In general, tests have shown that it is important to dry the green flow body slowly to prevent cracking. However, the duration of drying the flow bodies in sealed containers, vented containers and on shelves can vary widely depending on factors such as the particular flow body size and wall thickness. Thus, the aforementioned drying times are only examples.
[0026]
Using the times described above, a crack-free flow body of the type shown in FIG. 1 was produced. In step 18, the flow body is fired in an oven at atmospheric pressure at 60 ° ± 5 ° C. for about 24 hours. When firing at a low temperature in an oven, the water remaining in the flow body evaporates. When the drying process is completed, the dried green flow body typically has lost about 7% of its "as molded" weight. In step 20, the flow body is heated in a furnace to about 275 ° C. ± 5 ° C. for about 2 hours. In this step, the non-aqueous portion of the binder evaporates from the flow body. At this point, the dried, unsintered flow body is ready for sintering.
[0027]
Further testing has shown that adding one or more additives to the binder can speed up the drying process, eliminating the need for green flow bodies in containers or bags, and applications. In some cases, it has been suggested that simply drying the unsintered flow body at room temperature for a few days or less will result in a product ready for sintering. However, this faster drying method appears to have a detrimental effect on the surface finish, for example causing pitting. The tests have not been completed and it is not yet concluded whether adding additives to the binder to reduce the drying time is preferred for any particular application. While the drying method shown in FIG. 5 is considered to be an acceptable method, other drying methods are also contemplated and the described sintering method applies to any properly dried green MIM parts. It should be understood that can also be used.
[0028]
In the first stage sintering, the flow body is positioned by the ceramic tools 32, 34, 41 and 42 as described above. In step 22, the flow body is placed in a high vacuum furnace and heated for about 30 minutes, preferably to about 1235 ° C. The purpose of the first stage sintering is to substantially fix the overall shape of the part. Therefore, at 1235 ° C. for 30 minutes, some interparticle melting occurs in the flow body. Generally, this melting occurs on the outer surface of the flow body. Typical densities of the flow body of Inconel 718 after first stage sintering are about 60% to 80% of the maximum achieved density. During the first stage sintering, the flow body is not heated close enough to the melting point of the metal alloy to be sufficiently plastic, so that the gravitational force acting on the flow body can cause significant deformation of the flow body. obtain.
[0029]
Temperature control during the sintering process is important, but some temperature fluctuations are allowed. For example, for the first sintering stage, 1100 ° C. to 1240 ° C. is an acceptable operating range for the flow body. Temperature ranges from 1230 ° C to 1240 ° C can also be used. The duration of heating the flow body also depends on the shape of the flow body. Flow bodies with thin walls will require shorter sintering times, and flow bodies with correspondingly thicker walled cross sections will require longer sintering times.
[0030]
Generally, after the first stage sintering, the flow body is removed from the high vacuum furnace and allowed to cool for several hours between the first and second stage sintering. This cooling period is not critical to the process, but mainly allows the first stage sintering tool to be removed from the flow body and the second stage sintering tool to be mounted in the flow body. Using the methods and tools described herein, one or more flow bodies can be machined simultaneously.
[0031]
At step 24, the second stage sintering tools 36, 38, 41 and 42 are mounted in a flow body also placed in a high vacuum furnace. Next, the flow body is heated to a temperature of about 1280 ° C. ± 5 ° C. for about 30 minutes. Temperatures of about 1270 ° C. or higher may be used. The purpose of the second stage sintering is to achieve an increase in the density of the flow body and even its maximum densification. Temperature control is more important in the second stage sintering because the flow body is heated to a temperature near the melting point of the alloy composition. In this regard, the sintering temperature should not exceed the melting point of the alloy. Test results show that using a second stage sintering at 1280 ° C. ± 5 ° C., densification approaches 99% of the alloy density of the forging part. It is quite possible to carry out the second stage sintering at a temperature lower than 1275 ° C. At a lower second stage sintering temperature, there is less densification of the flow body within a given time, and correspondingly, the finished part has a higher porosity and somewhat reduced use strength . This can be tolerated without problems for parts where maximum strength is not required. After the second stage sintering, the flow body can be machined and / or heat treated as desired. For example, if the flow body is subjected to a solution heat treatment and further a precipitation hardening treatment, desired mechanical properties are achieved. This procedure is known in the art.
[0032]
Cast flow bodies and MIM flow bodies generally have different surface properties. The cast flow body has a surface roughness of about 250 μinch (6.35 μm), while the flow body of the MIM has a surface roughness of less than about 30 μinch (0.762 μm). In the MIM process, less material is wasted and less machining is required than in casting, and therefore, it is less expensive to make parts in the MIM process.
[0033]
It will be appreciated that a new multi-stage MIM part drying and sintering process has been disclosed. These new processes allow relatively large MIM parts to be sintered while retaining good dimensional control of the part shape. In addition, specialized aluminum oxide ceramic sintering tools have been disclosed that assist in maintaining the correct dimensions of large cylindrical features. Although only the presently preferred embodiments have been described in detail, it will be apparent to those skilled in the art that variations and modifications can be made to the systems and methods disclosed herein without departing from the scope of the invention. There will be. Accordingly, the invention is not limited except as by the appended claims.
[0034]
1 is a perspective view showing a valve flow body ready for first stage sintering with a sintering tool according to the present invention.
FIG. 2 is another perspective view showing the flow body of FIG. 1 with an additional sintering tool ready for second stage sintering, in accordance with the present invention.
FIG. 3 is a perspective view showing the sintering tool of FIG. 1 in more detail;
FIG. 4 is a perspective view showing the sintering tool of FIG. 2 in more detail;
FIG. 5 is a flowchart illustrating the steps of the drying, binder evaporation, and sintering processes of the present invention.

Claims (31)

A method of sintering a powder metal preform (26), comprising:
Sintering the preform for a predetermined time within a first sintering temperature range, wherein the preform shrinks;
Sintering the preform for a predetermined time within a second sintering temperature range, wherein a maximum desired density of the preform is obtained in the second sintering temperature range. Including methods. The method of claim 1, wherein the first sintering temperature range is from about 1100C to about 1240C. The method of claim 2, wherein said second sintering temperature range is about 1280C +/- 5C. The method of claim 1, wherein sintering within the first sintering temperature range is continued until inter-particle melting of the preform occurs, thereby fixing the shape of the preform. The method of claim 1 wherein the preform reaches about 60% to about 80% of its maximum density in the first sintering temperature range. A sintering tool assembly for supporting a metal injection molded preform having a surface defining an internal passage, the sintering tool assembly comprising:
A support structure (40);
A rod structure (32) having a first end and a second end, both ends mounted on the support structure, wherein the rod structure is disposed within the passage and supports the preform. Tool assembly. A sintering tool assembly for supporting a metal injection molded preform having a surface defining an internal passage,
An insert (34) disposed in the passage and comprising at least one surface of a predetermined shape;
A support structure (40);
A rod structure (32) mounted on the support structure, the rod structure (32) supporting the insert (34). A sintered tool assembly wherein at least a portion of the surface of the predetermined shape of the insert contacts at least a portion of the surface defining the internal passage of the preform to support the preform. The sintered tool assembly according to claim 7, wherein the rod structure and the insert comprise an aluminum oxide ceramic. The sintering tool according to claim 7, wherein the support structure includes a pair of blocks (41), and the insert is supported between the pair of blocks. 10. The block of claim 9, wherein each of the pair of blocks has a V-shaped notch, and both ends of the rod structure are mounted within the V-shaped notch on respective blocks of the pair of blocks. Sintering tools. The sintered tool according to claim 9, wherein each of the pair of blocks is made of a refractory brick. A method of sintering a powder metal flow body (26), comprising:
Providing a flow body (26) having one or more small cylindrical features (28) and one or more large cylindrical features (30);
Sintering the flow body for a predetermined time within a first sintering temperature range, wherein the flow body contracts;
Sintering the flow body for a predetermined time within a second sintering temperature range, wherein a maximum desired density of the flow body is obtained in the second sintering temperature range. Including methods. Providing a first set of generally cylindrical sintering tools (32, 34) for retaining the shape of the small cylindrical features of the flow body during sintering at the first sintering temperature. When,
Providing a second set of substantially cylindrical sintering tools to retain the shape of the large cylindrical features of the flow body during sintering at the second sintering temperature. The method according to claim 12. The first set of sintering tools includes a rod (32) and one or more cylinders (34), the cylinders having through holes (35) for slidably receiving the rods. The method described in. 15. The sintering tool of claim 14, wherein the second set of sintering tools includes a rod (36) and one or more cylinders (38), the cylinders including through holes (37) for slidably receiving the rods. Method. A method of drying a metal preform in preparation for sintering, wherein said preform (26) comprises a composition of powdered metal and a binder;
Drying the preform in a sealed container;
Drying the preform in a vented container;
Evaporating water from the preform by heating the preform at a predetermined temperature for a predetermined time;
Evaporating the binder from the preform by heating the preform at a predetermined temperature for a predetermined time. 17. The method of claim 16, wherein the preform is dried for about two or three days in a sealed container. 17. The method of claim 16, wherein the preform is dried for about 2 or 3 days in a vented container. 17. The method according to claim 16, wherein water is evaporated from the preform by heating the preform at a temperature of 60 ° C ± 5 ° C for 24 hours. 17. The method of claim 16, wherein the binder is evaporated from the preform by heating the preform at a temperature of 275 ° C ± 5 ° C for 2 hours. Sintering the preform at a first predetermined temperature for a predetermined time;
Sintering said preform at a second predetermined temperature for a predetermined time. A sintered flow body (26) having at least one cylindrical hole (28, 30) and having a weight greater than about 250 grams. 23. The flow body according to claim 22, comprising Inconel 718. 23. The flow body of claim 22, having a weight greater than about 300 grams. 23. The flow body of claim 22, having a weight greater than about 1000 grams. 23. The flow body of claim 22, wherein said holes have a diameter greater than about 8 cm. 23. The flow body of claim 22, wherein said holes have a diameter greater than about 5 cm. 23. The flow body of claim 22, wherein said holes have a diameter greater than about 3.8 cm. 23. The flow body according to claim 22, having at least two cylindrical holes (28, 30). 30. The flow body of claim 29, wherein one of the at least two cylindrical holes (30) is larger than the other of the at least two cylindrical holes (28). 23. The flow body of claim 22, having a surface roughness less than about 30 microns.

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