Patent Application
20240207930
The present invention relates to methods for making a sintered body, and in particular to a technology for improving the mechanical and physical properties of a powder metallurgy sintered body through mixed powders of different ranges of particle sizes.
Powder metal process is known to the persons skilled to this art and comprises the formation of metal powders, compacting such powders into compacts, and followed by sintering the green compacts at elevated temperatures so as to produce sintered workpiece with good mechanical and physical properties.
The metal powders used in powder metal technology could be elemental powders, such as pure iron, pure copper, and pure nickel powders, or pre-alloyed powders, or a combination of the elemental and pre-alloyed powders.
Yet in another method, the alloying elements, such as nickel 71 and copper, can be coated onto the iron powder 70 by plating. The plated powder as shown in
The alloying elemental powder can also be bonded to the iron powder by mixing the iron powder 60 with fine alloying powders, such as nickel 61 and molybdenum 62, and then heating the mixture at around 800° C. during which interparticle diffusion occurs between the alloying elements and iron. The fine powders are diffusion-bonded firmly to the iron powder and alleviate the segregation problem during handling. However, the cost is high due to the extra mid-temperature diffusion treatment. The thus-made powder is often called diffusion-bonded powder as shown in
Another way to bond the fine alloying powders, such as nickel 52 and copper 53, is to apply a thin polymeric sticky binder coating 51 onto iron powder 50 surfaces, as shown in
Moreover, the alloying elements, such as silicon and manganese, that were alloyed with iron in a ferro master alloy powder can be used as additives. For example,
In addition to the methods described above, U.S. Pat. No. 5,834,640 also teaches a process of mixing elemental coarse iron powders with fine ferro alloys (either singularly or a combination of two or more ferro alloys) having a mean particle size of about 8 to 12 microns. Furthermore, the amounts of alloying elements in ferro alloys are 78 wt. % for Fe—Mn, 65 wt. % for Fe—Cr, 71 wt. % for Fe—Mo, 75 wt. % for Fe—V, and 75 wt. % for Fe—Si. For light alloying element of boron, the amount is 17.5 wt. %. The amounts of these elements are high, similar to using master alloys, which makes it difficult to reduce the surface oxides on these ferro alloy powders and to homogenize the alloying elements compared to those with a smaller amount. Furthermore, the fine ferro alloy additive powders decrease the flowability of the mixed powder and make the die filling difficult during compaction.
Yet another prior art is to combine two or three or more of the above mixed powders. An example is given by MPIF FLN4C-4005 hybrid low-alloy steel, in which pre-alloyed Fe-0.5Mo-0.2Mn low-alloyed powder is mixed with fine nickel powder, fine copper powder and graphite powder. This hybrid powder may also contain up to 5 wt. % of unalloyed iron powder so as to improve the compressibility of the mixed powder.
The powders as described in the prior art still present a room for improvement on cost, flowability, apparent density, compressibility, and sintered properties. It is the objective of this invention to provide a mixed powder with improved powder characteristics and the thus made sintered workpiece.
According to an aspect of some embodiments of the present invention there is provided a method for making a sintered body, comprising the following steps:
According to an aspect of some embodiments of the present invention there is provided a method for making a sintered body, comprising the following steps:
It is to be understood that the terminology used in the description of the various embodiments and examples herein is for the purpose of describing particular examples only and is not intended to be limiting.
As used herein, the singular forms “a”, “an”, and “the” include the plural forms as well, unless the context clearly indicates otherwise, which do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item(s). In addition, the indefinite and definite articles shall include the plural and the singular unless the contrary is clear from the context.
As used herein, the terms “include” and “comprise” indicate the presence of recited features, ingredients, elements and/or compositions, but do not exclude the presence or addition of one or more other features, ingredients, elements, compositions and/or groups thereof.
As used herein, the term “substantially spherical” refers to powders or particles with rounded shapes that are preferably non-faceted or substantially free of sharp corners.
As used herein, the terms “powder” and “powders” are defined as a substance formulated as a large number of finely divided solid particles having a selected median particle size. Although the particulate substance is typically manufactured in a dry state, the particulate substance may, after manufacture, be placed in a wet environment, such as in a slurry.
As used herein, the terms “coarse” and “fine” used herein are used to indicate the particle size of a certain powder relative to the particle size of another powder, and are not used to define the specific range of particle size of the powder. That is, “a coarse powder” as used in the present disclosure means the coarse powder has a median particle size larger than another powder or particle described in the context, and “a fine powder” as used in the present disclosure means the fine powder has a median particle size smaller than another powder or particle described in the context. There is no intent to limit the particle size or median particle size of the powder unless indicated herein or clearly contradicted by context.
As used herein, the term “powder mixture” and “mixed powder” means more than one powder blended or mixed together prior to a sintering process. In an example, the powder mixture and the mixed powder refer to a combination of more than one powder with different compositions, different median particle sizes and/or different powder structures.
As used herein, the term “Fe-based” means a composition comprising Fe as a main component. Herein, the phrase “comprising Fe as a main component” may mean that the content of Fe in the composition is the highest compared to other elements. In one example, Fe-based composition means the content of Fe in the composition is 50 wt. % or more.
The present invention, in some embodiments thereof, relates to the field of powder mixtures for sintering in powder metallurgy.
An example is shown in
In one or more embodiments, the coarse matrix powder 10 may be an elemental powder, admixed powder, a diffusion-bonded powder, a pre-alloyed powder, a binder-bonded powder, a composite powder, as shown in
In one or more embodiments, the fine primary powder of the coarse additive powder 11 is an elemental powder, a pre-alloyed powder, an intermetallic compound powder, a carbides powder, a composite powder, a master alloy powder or a combination thereof, and the median particle size of the fine primary powder is less than 15 μm. In one embodiment, the coarse additive powder 11 is a granulated powder.
In one embodiment, the coarse matrix powder 10 is Fe-based and contains at least one alloying element. The weight of total alloying elements accounts for less than 10% of the coarse matrix powder 10. When the amount of the total alloying elements in the coarse matrix powder 10 exceeds 10%, the compressibility of the powder becomes poor. The alloying elements may include at least one of following: less than 8% nickel by weight, less than 2% molybdenum by weight, less than 1.5% manganese by weight, less than 4% chromium by weight, less than 3% copper by weight, less than 1.5% cobalt by weight, less than 1.5% tungsten by weight, less than 1.5% silicon by weight, less than 1% niobium by weight and less than 1% vanadium by weight. In addition, the coarse matrix powder 10 may further contain unavoidable impurities.
In an example, the coarse matrix powder 10 may contain 0.5% to 2.5% nickel by weight, 0.2% to 1.5% molybdenum by weight, and 0.01% to 1.0% manganese by weight, unavoidable impurities and balance of Fe.
In an example, the coarse matrix powder 10 may contain 0.5% to 4.5% nickel by weight, 0.2% to 1.5% molybdenum by weight, 0.5% to 2% copper by weight, 0.01% to 1.0% manganese by weight, unavoidable impurities and balance of Fe.
In one embodiment, the coarse additive powder 11 is Fe-based and contains at least one of following: 8% to 20% chromium by weight, 5% to 20% nickel by weight, less than 5% molybdenum by weight, less than 2% manganese by weight, less than 5% copper by weight, less than 1.5% silicon by weight and less than 2% niobium by weight. In addition, the coarse additive powder 11 may further contain unavoidable impurities.
In one or more embodiments, the mixture of the coarse matrix powder 10 and the coarse additive powder 11 may further be added with less than 1.5% graphite powder by weight and less than 1.5% lubricant powder by weight. The graphite powder may serve as a carbon source, supplying the necessary carbon content in the sintered workpiece to further strengthening and hardening the sintered compact, in addition to the alloying elements, without sacrificing the compressibility of the powder mixture; while the lubricant powder, such as zinc stearate and ethylene bis-stearamide, is to decrease the friction and wear between the compact and the tools during compaction.
The powder mixture is used for making a sintered body. The powder mixture may be placed into a mold and applied with a forming pressure to obtain a green body. Then the green body is heated and sintered at a temperature higher than 1,100° C. to form a high-density sintered body.
Hereinafter, embodiments will be described in more detail with reference to the experimental examples. However, the scope of the present disclosure is not limited by the experimental examples. Table 1 below shows the composition of the coarse matrix powder 10 of some experimental examples. The compositions of the coarse matrix powder 10 are represented in weight percentage. For example, Fe-1.8Ni-0.5Mo-0.2Mn-0.5C means the coarse matrix powder has the following composition: 1.8 wt. % Ni, 0.5 wt. % Mo, 0.2 wt. % Mn, 0.5 wt. % C and balance of iron.
The addition of chromium, molybdenum, nickel, copper, manganese and other alloying elements may increase the hardening and solid solution effect of sintered products, resulting in improvement of the hardness and strength. The additions of the alloying elements may be achieved by using elemental powder, diffusion bonding, electroplating or any other powder mixtures shown in
The mechanical properties of the matrix powder can be improved by adding a fine pre-alloyed powder or a fine elemental powder. However, addition of the fine powder results in poor flowability as shown in
In order to solve the flowability problem, it was discovered by the inventors that when the coarse and substantially spherical powder (such as a granulated spherical alloy powder) formed by agglomeration of fine powder is added, the problems of low flowability and low apparent density can be solved. As shown in
In one example of the spray drying methods, the fine primary powder is first mixed with water and binders (such as polyvinyl alcohol and polyethylene glycol) to form a slurry. The slurry is sent to the spray drying chamber in which a high-speed rotating disc spins off the slurry and forms droplets. The water in the droplet is dried out by the surrounding hot air and a dry granulated powder then falls to the bottom of the spray drying chamber for collection.
To prepare spherical powders, the fine primary powder may be a pure elemental powder, a pre-alloyed powder, a master alloy powder, an electroplated composite powder, an intermetallic compound powder, a carbide powder or a combination thereof.
In order to further shorten the homogenization time of the alloying elements, it is preferred to use a spherical granulated powder with a smaller particle size as the coarse additive powder 11. For example, the particle size of the coarse additive powder 11 is equal to or less than 45 μm. Since the coarse additive powder 11 exhibits high sphericity, it still has good flowability. In the case of the same amount added, compared with using the powders with larger particle size, the powders with smaller particle size have more particles (that is, under the same weight, the additive powder 11 with smaller particle size will have more particle numbers) dispersed in the matrix of iron powder. Therefore, the inter-diffusion distance is reduced and homogenization of alloying can be further improved.
Table 2 below shows experimental examples of several powder mixtures prepared using the above methods and compositions. No. 1, No. 2 and No. 5 are comparative examples and prepared according to conventional techniques. In examples No. 1 and No. 5, no alloying powder is added, while a fine pre-alloyed powder with a median particle size of 9 μm is mixed with the matrix powder in example No. 2. To demonstrate the advantages of this invention, the fine pre-alloyed powder of example No. 2 is deliberately selected to be finer 5 than the additive granulated powder 11. The experimental examples No. 3, No. 4 and No. 6 are prepared in accordance with the embodiments of this invention, that is, the coarse spray dried powder is mixed with the matrix powder.
No. 1 selects Fe-1.8Ni-0.5Mo-0.2Mn pre-alloyed powder with median particle size of 90 μm as the matrix powder. No alloying powder was added and thus served as a comparative example. No. 2 was prepared following conventional techniques of adding 15% by weight of the fine pre-alloyed powder, which has a median particle size of 9 μm, to the matrix powder that was also used in No. 1. No. 3 was prepared in accordance with the embodiment of this invention. The matrix powder was the same as that used in No. 1. The matrix powder was mixed with 15% by weight of the spray dried pre-alloyed powder (i.e., the coarse additive powder) with a median particle size of 75 μm. Moreover, the fine primary powder in the granulated powder is the same as that used in No. 2. No. 4 selects the spray dried pre-alloyed powder with a median particle size of 45 μm (i.e., the coarse additive powder), which can further improve the distribution of the pre-alloyed powder in the matrix powder and the homogenization of the alloying elements in the sintered workpiece. The difference between No. 2 and No 0.4 is that the mixed powder of No. 4 retains good flowability because the fine spray dried powder is spherical and flowable. In examples No. 2 to No. 4, the final composition of the mixed powder includes 2.5 wt. % Cr, 3.3 wt. % Ni, 0.8 wt. % Mo, 0.17 wt. % Mn, 0.5 wt. % C and balance of iron.
No. 5 selects MPIF FD-0405 (Fe-3.9Ni-0.5Mo-1.5Cu-0.2Mn-0.5C) as the matrix powder and no alloying powder was added. For the purpose of comparison, No. 5 was added with a spray dried pre-alloyed powder with a median particle size of 78 μm to form example No. 6 with a final composition including 2.5 wt. % Cr, 5.1 wt. % Ni, 0.8 wt. % Mo, 1.3 wt. % Cu, 0.17 wt. % Mn, 0.5 wt. % C and balance of iron.
Each example described above was added with 0.6 wt. % lubricant and 0.6 wt. % graphite powder or carbon black powder. The lubricant is added to facilitate press forming, and the graphite powder or carbon black powder is used to provide 0.5 wt. % carbon contained in the sintered product.
The mixed powders of the examples were measured for their flowability based on ASTM B213. To compare their compressibility and sintered properties, the mixed powders were pressed at a pressure of 600 MPa into a disc of 12 mm in diameter and 5 mm in height. The green discs were debound at 500° C. for 30 minutes and then sintered at 1350° C. for 2 hours under vacuum.
When the mixed powders of examples No. 3, No. 4, and No. 6 are pressed into discs, the granulated pre-alloyed powder will break, since the bonding between the fine primary powders in the granulated pre-alloyed powder is weak. The fine primary powders are thus uniformly distributed between the coarse matrix powders as shown in
Table 3 below shows the flowability of the powder, the density and the hardness of the sintered workpiece of examples No. 1 to No. 6. Comparative Example No. 1 has a good flowability of 26.7 s/50 g. However, comparative example No. 2 has no flowability due to the addition of 15 wt. % 9 μm fine pre-alloyed powder (which is not a granulated powder) into example No. 1 and thus cannot be used for production. In comparison to comparative example No. 1, the flowability of example No. 3 of this invention only slightly deteriorates from 26.7 s/50 g to 28.3 s/50 g, which is still suitable for production. The green density of example No. 3 is the same as that of example No. 1, indicating that the compressibility remains good. However, the hardness of example No. 3 increases significantly to 46.5 HRC. These results demonstrate that using the granulated powder for alloying is an effective way of improving sintered properties of the sintered workpiece without sacrificing the powder flowability. When the particle size of the granulated powder decreases, as shown by example No. 4 of this invention, the flowability deteriorates to 34.3 s/50 g. Nevertheless, the mixed powder remains flowable and can be used for production. The compressibility of example No. 4 also remains good and the green density is 6.86 g/cm3. Since the finer spherical spray dried powder is used, the distribution of the spherical powder is more uniform for example No. 4, and therefore the distribution of the primary powders is also better, and the sintered density and hardness are higher than those using the coarse spherical powder (example No. 3 of this invention).
Comparison of comparative examples No. 5 and No. 6 of this invention further shows that adding spherical granulated powder as a source of alloying elements has little negative impact on the flowability and compressibility of the mixed powder. The mixed powder of example No. 6 is suitable for production with an increased sintered hardness from 35 HRC to 43.8 HRC and similar density compared to those of example No. 5.
The above results show that using the mixed powder in accordance with the present invention can effectively improve the hardness of the sintered workpiece without the problems of flowability, apparent density and compressibility that existed in the prior art.
The fine primary powder of the coarse additive powder 11 in examples No. 3, No. 4, and No. 6 is pre-alloyed powder, but it may also be a pure elemental powder, an electroplated composite powder, an intermetallic compound powder, a carbide powder, a master alloy powder, or a combination thereof.
Another aspect of the present invention is to mix a first metal powder with a second metal powder to produce a mixed powder, and then granulate the mixed powder to form a coarse spherical granulated powder. The coarse spherical granulated powder contains both the first metal powder and the second metal powder. The coarse spherical granulated powder can be produced by spray drying, kneading, mixing and other granulating methods known in the art. In the present invention, spray drying is preferably used.
The second metal powder has a finer particle size than the first metal powder. The first metal powder has a median particle size between 50 μm and 110 μm, and the second metal powder has a median particle size less than 15 μm. The second metal powder in the granulated powder is between 5% between 50% by weight. The first metal powder may be an elemental powder, an admixed powder, a diffusion-bonded powder, a pre-alloyed powder, a binder-bonded powder, a composite powder or a combination thereof. The second metal powder may be an elemental powder, a pre-alloyed powder, an intermetallic compound powder, a carbide powder, a composite powder, a master alloy powder or a combination thereof.
In one embodiment, the first metal powder is Fe-based and contains at least one alloying element. The weight of total alloying elements accounts for less than 10% of the first metal powder. The alloying elements may include at least one of following: less than 8% nickel by weight, less than 2% molybdenum by weight, less than 1.5% manganese by weight, less than 4% chromium by weight, less than 3% copper by weight, less than 1.5% cobalt by weight, less than 1.5% tungsten by weight, less than 1.5% silicon by weight, less than 1% niobium by weight and less than 1% vanadium by weight. In addition, the first metal powder may further contain unavoidable impurities.
In an example, the first metal powder is an Fe-based powder and may contain 0.5% to 2.5% nickel by weight, 0.2% to 1.5% molybdenum by weight, and 0.01% to 1.0% manganese by weight, unavoidable impurities and balance of Fe.
In an example, the first metal powder is an Fe-based powder and may contain 0.5% to 4.5% nickel by weight, 0.2% to 1.5% molybdenum by weight, 0.5% to 2% copper by weight, 0.01% to 1.0% manganese by weight, unavoidable impurities and balance of Fe.
In one embodiment, the second metal powder is an Fe-based powder and may contain at least one of following: 8% to 20% chromium by weight, 5% to 20% nickel by weight, less than 5% molybdenum by weight, less than 2% manganese by weight, less than 5% copper by weight, less than 1.5% silicon by weight and less than 2% niobium by weight. In addition, the second metal powder may further contain unavoidable impurities.
The granulated powder is used for making a sintered body. The granulated powder may be placed into a mold and applied with a forming pressure to obtain a green body. Then the green body is heated and sintered at a temperature higher than 1,100° C. to form a high-density sintered body.
The following example No. 7 is provided to illustrate the present invention, but is not meant to be limiting. In example No. 7, the first metal powder is the same matrix powder used in examples No. 2, No. 3, and No. 4 in Table 2, and the fine primary powder is the same alloying powder used in examples No. 2, No. 3, and No. 4 in Table 2. The composition of the mixed powder in example No. 7 is also the same as that of examples No. 2, No. 3, and No. 4. For the granulated powder obtained by spray drying from the coarse matrix powder and the fine primary powder, the flowability is 27.6 s/50 g, which is even better than that of examples No. 3 and No. 4 in Table 2 (the matrix powder and the alloying powder are same for examples No. 3, No. 4 and No. 7). The forming and sintering conditions in example No. 7 are same as those of examples No. 3, No. 4. In example No. 7, the green density is 6.81 g/cm3, the sintered density is 6.97 g/cm3, and the hardness is 49.1 HRC. These results are similar to examples No. 3 and No. 4, which means that granulating the coarse matrix powder (the first metal powder) and fine primary powder (the second metal powder) together as described herein can also solve the problem of poor flowability and still maintain excellent sintered properties.
| Number | Date | Country | Kind |
|---|---|---|---|
| 111149554 | Dec 2022 | TW | national |
