TWI904397B - A mixed powder, sintered body and its preparation method for powder metallurgy - Google Patents

Abstract

A mixed powder for powder metallurgy includes a first coarse metal powder and a second coarse metal powder. The first coarse metal powder is a matrix powder, and the second coarse metal powder is composed of aggregates of multiple fine primary powders. The weight percentage of the second coarse metal powder in the mixed powder is between 5% and 50%, and the second coarse metal powder has a near-spherical powder morphology and a median particle size of less than 100 μm.

Description

A mixed powder, sintered body and its preparation method for powder metallurgy

This invention relates to a mixed powder, a sintered body, and a method for preparing the same for powder metallurgy, and particularly to a technique for improving the mechanical and physical properties of a powder metallurgy sintered body by using mixed powders of different particle sizes.

Powder metallurgy is a technology well-known to technicians in this field. Generally speaking, the powder metallurgy process includes the preparation of metal powder, the pressing of powder into green compacts, and then the sintering of the green compacts at high temperatures to produce sintered workpieces with good mechanical and physical properties.

The metal powders used in powder metallurgy can be elemental powders, such as pure iron, pure copper, or pure nickel powder, or pre-alloyed powders, or combinations of elemental powders and pre-alloyed powders. Figures 1A to 1F are schematic diagrams of powder mixtures used in the prior art. Figure 1A shows a mixture of coarse iron elemental powder 90 and fine alloy powder additives such as nickel elemental powder 91 and copper elemental powder 92. The reason for adding fine powder is to ensure that these alloying elemental powders are more uniformly distributed before sintering, thus improving the homogenization of the alloying elements compared to adding coarse powder. These powders are soft and can be pressed into high-density green blanks. Because of the large particle size of iron powder (median number of about 60μm-110μm), the alloy additives need a long time to fully diffuse into the core of the iron powder during sintering. Therefore, the microstructure of the sintered workpiece is not uniform.

However, another drawback is that fine alloy powder additives easily pass through the pore channels between large particles and settle to the bottom during processing and transportation, causing segregation problems, which in turn leads to inhomogeneity in microstructure and mechanical properties. Furthermore, the presence of fine powder results in poor flowability of the mixed powder. This is because, at the same addition amount, the number of fine powder particles is greater than that of coarse powder, increasing the number of contact points between powder particles and thus increasing interparticle friction. This increased internal friction also leads to a decrease in apparent density and an increase in forming pressure. In other words, compared to metal mixed powders without added fine powder, the addition of fine powder impairs compressibility.

Here are some examples of the mixed powders shown in Figure 1A. FC-0205, a standard from the Metal Powder Industries Federation (MPIF), is an example of a mixed powder like the one in Figure 1A. The raw material for FC-0205 is a mixture of crude iron powder, 2 wt.% fine copper powder, and some graphite powder. The amount of graphite powder is such that after green body sintering, the green body contains approximately 0.5 wt.% carbon. Another example is FN-0408, whose raw material contains crude iron powder, 4 wt.% fine nickel powder, and approximately 0.8 wt.% graphite powder.

Figure 1B shows a pre-alloyed powder 80, in which the alloying elements are dissolved in the iron during the melting and atomization powdering process. An example is FL-4205 in the MPIF standard. FL-4205's raw material powder is a pre-alloyed low-alloy steel powder containing 0.35 wt.%-0.55 wt.% nickel, 0.50 wt.%-0.85 wt.% molybdenum, 0.2 wt.%-0.4 wt.% manganese, and approximately 0.5 wt.% graphite powder. Due to the solid solution effect of the alloying elements, the pre-alloyed powder is relatively hard. Therefore, this powder is difficult to compress into high-density green bodies. Although the compressibility of this powder is poor, its advantage is that the microstructure in the sintered workpiece is uniform, resulting in uniform mechanical properties.

In another method, alloying elements (such as nickel and copper) can be electroplated onto iron powder to form a coated powder, as shown in Figure 1C. The nickel and copper coating 71 is formed on the iron powder 70, and this is commonly referred to as a composite powder. Compared to the aforementioned mixed powders, the alloying element distribution in the coated composite powder is more uniform. However, electroplating is very expensive due to environmental concerns.

Alternatively, alloying element powders can be produced by mixing iron powder with fine alloy powders (such as nickel and molybdenum), and then heating the mixture to approximately 800°C. At this temperature, cross-diffusion occurs between the alloying elements and iron, thereby binding the alloying element powder to the iron powder. The fine powder is firmly fixed to the iron powder, alleviating the segregation problem. However, this requires additional mid-temperature diffusion treatment, leading to increased manufacturing costs. Furthermore, the compressibility of the mixed powder is still slightly reduced due to mid-temperature diffusion. The aforementioned powders are commonly referred to as diffusion-bonded powders. As shown in Figure 1D, nickel powder 62 and molybdenum powder 61 are respectively bonded to iron powder 60. One example is FD-0405 in the MPIF standard, which contains 3.6 wt.%-4.4 wt.% nickel, 0.4 wt.%-0.6 wt.% molybdenum, 1.3 wt.%-1.7 wt.% copper, 0.05 wt.%-0.30 wt.% manganese and about 0.5 wt.% graphite powder.

Another method involves coating the iron powder surface with a thin polymer adhesive coating, known as binder-treated powder. This coating is viscous enough to bind fine additive powders without being too sticky and significantly reducing the flowability and apparent density of the mixed powder. Binder-treated powder also mitigates the segregation problem of fine alloy powders. As shown in Figure 1E, nickel powder 52 and copper powder 53 are bonded to iron powder 50 respectively via a polymer adhesive coating 51. However, although the fine powders are well bonded to the iron powder, it still affects particle flow and thus reduces flowability. Furthermore, when using elemental powders as alloying elements, the homogenization time for these alloying elements is quite long.

Furthermore, silicon and manganese, as alloying elements, can first be combined with iron to form a master alloy before being used as additives. For example, Figure 1F shows a mixture of iron powder 40 and master alloy powder 41, which comprises 40 wt.% manganese, 15 wt.% silicon, and 1 wt.% carbon, with the remainder being iron. When using high alloy content, especially highly reactive elements such as manganese and silicon, the oxides on the surface of the master alloy powder are very stable and difficult to reduce during sintering, resulting in low sintering density and poor mechanical properties. In addition to adding master alloys, intermetallic compounds or carbides can also be used; however, these additives are expensive.

In addition to the methods described above, US Patent No. 5,834,640 teaches a method for mixing coarse elemental iron powder with fine iron alloy powder (single or a combination of two or more iron alloys) with a median particle size of approximately 8 μm-12 μm. Furthermore, in the taught iron alloys, the contents of the alloying elements are as follows: manganese content of 78 wt.% for Fe-Mn alloys, chromium content of 65 wt.% for Fe-Cr alloys, molybdenum content of 71 wt.% for Fe-Mo alloys, vanadium content of 75 wt.% for Fe-V alloys, and silicon content of 75 wt.% for Fe-Si alloys. The content of the light element boron is 17.5 wt.%. These alloys have a high content of alloying elements, similar to using a master alloy. This makes it difficult to reduce the surface oxides on these ferroalloy powders, thus making it difficult to homogenize the alloying elements. In addition, the added fine powder reduces the flowability of the mixed powder and makes it more difficult to fill the mold cavity during the compaction process.

Another existing technique involves mixing two, three, or more of the aforementioned powders. For example, FLN4C-4005 hybrid low-alloy steel in the MPIF standard contains 3.6 wt.%-4.4 wt.% nickel, 0.4 wt.%-0.6 wt.% molybdenum, 1.3 wt.%-1.7 wt.% copper, 0.05 wt.%-0.30 wt.% manganese, and approximately 0.5 wt.% graphite powder. It is prepared by mixing a pre-alloyed Fe-xNi-yCu-0.5Mo-0.2Mn powder with fine nickel powder, fine copper powder, and graphite powder, wherein the total proportion of fine nickel powder and fine copper powder is approximately 4 wt.%. This mixed powder may also contain up to 5 wt.% elemental iron powder to improve its compressibility. However, the powder still contains fine nickel and copper powder, resulting in poor flowability.

In summary, existing powders still have room for improvement in terms of cost, flowability, apparent density, environmental friendliness, compressibility, and sintering performance. Therefore, the object of this invention is to provide a mixed powder with improved powder properties and a sintered workpiece made therefrom.

According to one aspect of the present invention, a method for preparing a powder metallurgy sintered body is provided, comprising the following steps: Step 1-1: providing a mixed powder for pressing and sintering, the mixed powder comprising a first coarse metal powder and a second coarse metal powder, wherein the first coarse metal powder is a matrix powder, the median particle size of the first coarse metal powder is between 50 μm and 110 μm, and the second coarse metal powder is a spray-granulated spherical powder formed by the aggregation of multiple fine primary powders, the spray-granulated spherical powder being prepared by spray granulation of the fine primary powders with water and a binder, the median particle size of the fine primary powders being less than 15 μm. μm, the second coarse metal powder accounts for between 5% and 50% of the weight of the mixed powder, and the second coarse metal powder has a nearly spherical powder morphology and a median particle size of less than 100 μm; wherein, the first coarse metal powder is mainly composed of iron and contains at least one alloying element, the total weight of the alloying element accounts for less than 10% of the weight of the mixed powder, and the second coarse metal powder is mainly composed of iron; Step 1-2: fill the mixed powder into a mold and apply a forming pressure to the mixed powder to form a green body; and Step 1-3: heat the green body to a temperature higher than 1,100°C to sinter it into a sintered body with high density.

According to one aspect of the present invention, a mixed powder for pressing and sintering is provided, comprising a first coarse metal powder and a second coarse metal powder, wherein the first coarse metal powder is a matrix powder with a median particle size between 50 μm and 110 μm, and the second coarse metal powder is a spray-granulated spherical powder composed of multiple fine primary powders aggregated together, the spray-granulated spherical powder being prepared by spray granulation of the fine primary powders with water and a binder, the median particle size of the fine primary powders being less than 15 μm, the second coarse metal powder accounting for a weight percentage of the mixed powder between 5% and 50%, and the second coarse metal powder having a near-spherical powder morphology and a median particle size less than 100 μm. μm, the first coarse metal powder is mainly composed of iron and contains at least one alloying element, the total weight of which accounts for less than 10% of the weight of the mixed powder, the second coarse metal powder is mainly composed of iron, wherein the mixed powder is used for sintering at temperatures above 1,100°C.

According to one aspect of the present invention, a method for preparing a powder metallurgy sintered body is provided, comprising the following steps: Step 1-1: providing an aggregated powder with a near-spherical powder morphology for pressing and sintering, the aggregated powder being formed by mixing and aggregating multiple first metal powders and multiple second metal powders with a particle size finer than the first metal powders, the aggregated powder being a spray-granulated spherical powder, the spray-granulated spherical powder being prepared by spray granulation of the first metal powder, the second metal powder, water, and a binder, wherein the median particle size of the first metal powder is between 50 μm and 110 μm, and the median particle size of the second metal powder is less than 15 μm. μm, and the second metal powder accounts for between 5% and 50% of the weight percentage of the aggregated powder; wherein, the first metal powder is mainly composed of iron and contains at least one alloying element, the total weight of which accounts for less than 10% of the weight percentage of the mixed powder, and the second metal powder is mainly composed of iron; Step 1-2: Fill the aggregated powder into a mold and apply a forming pressure to the aggregated powder to form a green body; and Step 1-3: Heat the green body to a temperature higher than 1,100°C to sinter it into a sintered body with high density.

According to one aspect of the invention, a powder for pressing and high-temperature sintering is provided, which is an aggregated powder with a near-spherical powder morphology. The aggregated powder is formed by mixing and agglomerating multiple first metal powders and multiple second metal powders with particle sizes finer than the first metal powders. The aggregated powder is a spray-granulated spherical powder, which is prepared by spray granulation of the first metal powder, the second metal powder, water, and a binder. The median particle size of the first metal powder is between 50 μm and 110 μm, and the median particle size of the second metal powder is less than 15 μm. The first metal powder is mainly composed of iron and contains at least one alloying element. The total weight of the alloying element accounts for less than 10% of the weight of the aggregated powder. The second metal powder is mainly composed of iron. The aggregated powder is used for sintering at temperatures above 1,100°C.

It should be understood that the terminology used in the description of the various embodiments herein is for the purpose of describing particular examples only and is not intended to be limiting. Unless the context clearly indicates otherwise, or intentionally limits the number of elements, the singular forms "a" and "the" used herein also include the plural forms. It will be further understood that the terms "comprising" and/or "including," as used herein, indicate the presence of the described features, ingredients, elements, and/or composition, but do not preclude the addition or presence of one or more other features, ingredients, elements, compositions, and/or groups thereof. Indefinite and definite articles should include both plural and singular forms unless the opposite is clearly apparent from the context. Furthermore, the terms "coarse" and "fine" used herein are used to indicate the particle size of one powder relative to the particle size of another powder, and are not intended to absolutely limit the particle size of the powder.

This article discloses a mixed powder for powder metallurgy, referring to Figure 2, comprising a base powder 10 (i.e., a first coarse metal powder) and a spherical coarse additive powder 11 (i.e., a second coarse metal powder). The median particle size of the base powder 10 is between 50 μm and 110 μm. The base powder 10 can be elemental iron powder or ferroalloy powder with good compressibility. In one example, the base powder 10 can be elemental powder, mixed powder, diffusion bonding powder, pre-alloyed powder, binder powder, composite powder, or a combination thereof. In this invention, the coarse additive powder 11 is formed by the aggregation of multiple fine primary powders and has a nearly spherical powder morphology. In contrast, the coarse matrix powder 10 is not formed by the aggregation of multiple fine primary powders, but is a powder obtained by, for example, atomization, mechanical method, or reduction method. In terms of powder morphology, the coarse matrix powder 10 has a less regular powder morphology than the coarse additive powder 11. On the other hand, the weight percentage of the coarse additive powder 11 in the mixed powder is between 5% and 50%, and the median particle size of the coarse additive powder 11 is less than 100 μm. The fine primary powder of the coarse additive powder 11 is an elemental powder, a pre-alloyed powder, a compound powder, a carbide powder, a composite powder, a master alloy powder, or a mixture 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 (i.e., the first coarse metal powder) is mainly composed of iron and contains at least one alloying element. The total weight of the alloying element accounts for less than 10% of the weight of the mixed powder. The alloying element includes less than 8% nickel, less than 2% molybdenum, less than 1.5% manganese, less than 4% chromium, less than 3% copper, less than 1.5% cobalt, less than 1.5% tungsten, less than 1.5% silicon, less than 1% niobium, less than 1% vanadium, and unavoidable impurities.

Furthermore, the first coarse metal powder also contains nickel at a weight percentage between 0.5% and 2.5%, molybdenum at a weight percentage between 0.2% and 1.0%, manganese at a weight percentage between 0.01% and 1.0%, and unavoidable impurities.

Furthermore, the first coarse metal powder contains nickel at a weight percentage between 0.5% and 4.5%, molybdenum at a weight percentage between 0.2% and 1.0%, copper at a weight percentage between 0.5% and 2%, manganese at a weight percentage between 0.01% and 1.0%, and unavoidable impurities.

In one embodiment, the crude additive powder 11 (i.e., the second crude metal powder) is mainly composed of iron and contains chromium at a weight percentage between 8% and 20%, nickel at a weight percentage between 5% and 20%, molybdenum at a weight percentage less than 5%, manganese at a weight percentage less than 2%, copper at a weight percentage less than 5%, silicon at a weight percentage less than 1.5%, niobium at a weight percentage less than 2%, and unavoidable impurities.

Table 1 below shows the composition of the crude matrix powder 10 in certain experimental examples according to the present invention:

Table 1 Substitute Content of alloying elements other than iron Fe <2 wt.% FL-4605 (Fe-1.8Ni-0.5Mo-0.2Mn-0.5C) 3 wt.% FL-5608 (Fe-1.5Cr-0.25Mo-0.2Mn-0.7C) 2.65 wt.% FD-0405 (Fe-4Ni-0.5Mo-1.5Cu-0.2Mn-0.5C) 6.7 wt.% FLN6-4405 (Fe-6Ni-0.8Mo-0.2Mn-0.5C) 7.5 wt.%

The addition of alloying elements such as chromium, molybdenum, nickel, copper, and manganese is to increase the hardening and solid solution effects of the material, thereby improving the hardness and strength of the sintered workpiece. These alloying elements can be added in the form of elemental powder, diffusion bonding, electroplating, or any other form as shown in Figures 1A to 1F. In this invention, the pre-alloyed powder shown in Figure 1B is preferably used as the matrix powder. However, the solid solution of these alloying elements can lead to powder hardening, thus reducing the compressibility of the powder. Therefore, the total content of alloying elements in the matrix powder of this invention is designed to be less than 10 wt.%. For example, pre-alloyed MPIFs FL-4605 (Fe-1.8Ni-0.5Mo-0.2Mn-0.5C) and FL5608 (Fe-1.5Cr-0.25Mo-0.2Mn-0.7C) can be used; another example is the diffusion-bonded MPIF FD-0405 (Fe-4Ni-0.5Mo-1.5Cu-0.2Mn-0.5C). Furthermore, the matrix powder can also be a blended low-alloy steel, such as FLN6-4405 (Fe-6Ni-0.8Mo-0.2Mn-0.5C), which involves adding nickel and iron powder to the pre-alloyed matrix powder to improve compressibility. However, other powders with good flowability and compressibility used in the powder metallurgy industry can also be used as matrix powders.

To improve the mechanical properties of the matrix powder, fine pre-alloyed powder or elemental powder can be added, and the final properties are indeed improved. However, as shown in Figure 1A, the fine powder results in poor flowability, making it difficult for the mixed powder to be uniformly filled into the mold cavity, resulting in inconsistent weights of the blanks during mass production.

To address the flowability issue, it was discovered incidentally that adding coarse spherical powder (e.g., granulated spherical alloy powder) composed of aggregates of multiple fine raw powders could resolve the problems of low flowability and low apparent density. As shown in Figure 3, the granulated spherical powder, being spherical and coarse, maintains good flowability when filling mold cavities, unlike the low flowability of mixed powders containing fine powders. This powder can be produced by combining large quantities of fine raw powders into coarse spherical powders through spray drying, kneading, mixing, and other methods known in the art. In this invention, spray drying is preferred because the spherical powder produced by spray drying exhibits minimal variation in particle size and sphericity between different production batches. When pressure molding is applied, the spherical coarse additive powder 11 will separate into a large number of fine raw powders due to pressure or extrusion, and these fine raw powders will fill the gaps between the coarse matrix powder 10.

In the spray drying process, the fine raw powder is first mixed with water and a binder (such as polyvinyl alcohol and polyethylene glycol) to form a slurry. The resulting slurry is then fed into the spray drying chamber. After passing through a high-speed rotating disc, the slurry is spun out and forms droplets. The moisture in the droplets is dried by the surrounding hot air, thus producing dried spherical powder that falls to the bottom of the chamber and is collected.

To formulate spherical powders, the fine raw powder can be pure elemental powder, pre-alloyed powder, diffusion-bonded powder, electroplated composite powder, dielectric compound powder, carbide powder, or a combination of these powders. Figure 4A shows an example of Fe-5Ni mixed powder formed by adding 5 wt.% pure nickel spherical aggregate powder 13 to pure iron powder (matrix powder) 12; in contrast, Figure 4B shows a mixed powder formed by replacing pure nickel spherical aggregate powder 13 with spherical aggregate powder 15 made from Fe-10Ni pre-alloyed powder, and mixing it with an equal amount of pure iron powder (matrix powder) 14, while still maintaining the Fe-5Ni composition. As can be seen from the structure of the mixed powder in Figure 4B, the alloying or homogenization time of nickel is much shorter than in Figure 4A. In the example shown in Figure 4B, because the nickel-containing spherical powder is pre-distributed evenly in the iron powder, the nickel diffuses into the iron matrix over a shorter distance, making it a more cost-effective and efficient alloying method. Other forms of powder may also be used in this invention, such as combinations of elemental powders, pre-alloyed powders, master alloy powders, carbide powders, composite powders, or intermetallic compound powders.

To further shorten the homogenization time of alloying elements, spherical powders with smaller particle sizes can be used as coarse additive powder 11, such as 45 μm or smaller. Due to their spherical shape, they still maintain good flowability. With the same addition amount, compared to using coarser coarse additive powder 11, finer coarse additive powder 11 will have more particles (i.e., more particles of finer coarse additive powder 11 per unit weight) dispersed in the iron powder matrix. Therefore, the mutual diffusion distance is reduced, and alloy homogenization can be further improved.

Table 2 below provides experimental examples of several powder mixtures prepared using the above methods and compositions. Alloys 1, 2, and 5 were prepared using existing techniques. Alloys 1 and 5 used only coarse matrix powder without adding alloy powder, while Alloy 2 added fine pre-alloyed powder (not the coarse aggregated alloy powder of the present invention) in addition to the coarse matrix powder. Alloys 3, 4, and 6 were prepared using the method of the present invention, i.e., by adding coarse spray-granulated alloy powder.

Table 2 Number coarse matrix powder Added alloy powder The final components of the mixed powder 1 Fe-1.8Ni-0.5Mo-0.2Mn pre-alloyed powder without Fe-1.8Ni-0.5Mo-0.2Mn-0.5C 2 Fe-1.8Ni-0.5Mo-0.2Mn pre-alloyed powder 9 μm pre-alloyed powder Fe-2.5Cr-3.3Ni-0.8Mo-0.17Mn-0.5C 3 Fe-1.8Ni-0.5Mo-0.2Mn pre-alloyed powder 75 μm spray-granulated pre-alloyed powder Fe-2.5Cr-3.3Ni-0.8Mo-0.17Mn-0.5C 4 Fe-1.8Ni-0.5Mo-0.2Mn pre-alloyed powder <45 μm spray granulation pre-alloyed powder Fe-2.5Cr-3.3Ni-0.8Mo-0.17Mn-0.5C 5 Dispersion bonding powder of Fe-3.9Ni-0.5Mo-1.5Cu-0.2Mn without Fe-3.9Ni-0.5Mo-1.5Cu-0.2Mn-0.5C 6 Dispersion bonding powder of Fe-3.9Ni-0.5Mo-1.5Cu-0.2Mn 78 μm spray-granulated pre-alloyed powder Fe-2.5Cr-5.1Ni-0.8Mo-1.3Cu-0.17Mn-0.5C

Alloy 1 uses Fe-1.8Ni-0.5Mo-0.2Mn pre-alloyed powder with a median particle size of 90 μm as the matrix powder, without adding any other alloy powders, and is therefore used as a comparative example. Alloy 2 is prepared by adding approximately 15 wt.% of pre-alloyed fine powder with a median particle size of 9 μm to the matrix powder used in Alloy 1, according to existing technology. Alloy 3 is prepared using the method of this invention, using the same matrix powder as Alloy 1. This matrix powder is mixed with approximately 15 wt.% of spray-granulated pre-alloyed powder (i.e., coarse added alloy powder) with a median particle size of 75 μm. Furthermore, the original powder in this spherical powder is the same as the added alloy powder used in Alloy 2. Alloy 4 uses spray-granulated pre-alloyed powder (i.e., coarse-added alloy powder) with a median particle size of 45 μm, which further improves the distribution of the pre-alloyed powder in the matrix powder and the homogenization of alloying elements in the sintered workpiece. Unlike Alloy 2, the mixed powder of Alloy 4 retains fluidity because the finely spray-granulated powder is spherical. In Alloys 2 to 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, and 0.5 wt.% C, with the remainder being iron.

Alloy 5 uses MPIF FD-0405 (Fe-3.9Ni-0.5Mo-1.5Cu-0.2Mn-0.5C) as the matrix powder, without adding any alloy powder, with a median particle size of 85 μm. For comparison, Alloy 6 is Alloy 5 with the addition of spray-granulated pre-alloy powder (i.e., coarse added alloy powder) with a median particle size of 78 μm. The final composition of the mixed powder includes 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 the remainder is iron.

Alloys 1 through 6 were supplemented with 0.6 wt.% lubricant to facilitate pressing, and approximately 0.6 wt.% graphite powder or carbon black powder to provide the required carbon content of approximately 0.5 wt.% in the sintered blank. To test the flowability of these sintered parts, the flowability of the mixed powder of each alloy was measured using the method of ASTM B213. Furthermore, to compare the compressibility and sintering performance of the mixed powder of each alloy, the mixed powder was pressed into discs with a diameter of 12 mm and a thickness of 5 mm under a pressure of 600 MPa. The discs were then degreased at 500°C for 30 minutes and sintered under vacuum at 1350°C for 2 hours.

When the mixed powders of Alloys 3, 4, and 6 are pressed into ingots, the spherical pre-alloyed granules break due to the low bonding force between the original powders. This allows the finer original powders to be uniformly distributed among the coarser matrix powders, as shown in Figure 5. The photograph in Figure 5 shows that when using spherical aggregated powders, the finer original powders can be uniformly dispersed among the coarser matrix powders, and the mixed powders do not exhibit segregation or flowability issues. Furthermore, by adjusting the type and content of the binder used in the spherical aggregated powders, the yield strength and hardness of the spherical aggregated powders can be reduced. Through this adjustment, the spherical aggregated powders become more ductile, allowing the finer original powders to be extruded into the pores between particles under pressure, thereby further improving their distribution.

Table 3 below shows the powder flowability, sintering density, and hardness of the sintered ingots. Alloy 1 exhibits good flowability of 26.7 s/50 g. However, when 15 wt.% of ungranulated fine pre-alloyed powder (Alloy 2) is added, the mixed powder becomes non-flowable and therefore unsuitable for production. When Alloy 1 is mixed with spray-granulated pre-alloyed powder to form Alloy 3, the flowability deteriorates only slightly from 26.7 s/50 g to 28.3 s/50 g, but remains perfectly usable for production. Alloy 3 has the same green density as Alloy 1, indicating good compressibility, but the hardness after sintering increases significantly from 3.9 HRC to 46.5 HRC. These results demonstrate that alloying with granulated spherical powders is an effective way to improve the sintering performance of workpieces without sacrificing powder flowability. When the particle size of the spherical powder decreases, as shown in Alloy 4, the flowability deteriorates to 34.3 s/50g. Nevertheless, the mixed powder of Alloy 4 remains flowable and therefore suitable for routine production; it also maintains good compressibility, with a green density of 6.86 g/cm³. Due to the use of finer spherical powders, the distribution of the spherical powders is more uniform, resulting in better original powder distribution and higher sintering density and hardness compared to using coarser spherical powders (Alloy 3).

Table 3 Number Flowability (s/50g) Embryo density (g/ ^cm3 ) Sintering density (g/ ^cm3 ) Hardness (HRC) 1 26.7 6.87 7.12 3.9 2 Not flowing 6.94 7.16 51.2 3 28.3 6.87 7.11 46.5 4 34.3 6.86 7.16 50.7 5 23.6 7.15 7.28 35.0 6 25.2 7.13 7.25 43.8

A comparison of the data for Alloy 5 and Alloy 6 further shows that adding spherical powder as a source of alloying elements has little negative impact on the flowability and compressibility of the resulting mixed powder, and the mixed powder of Alloy 6 remains very suitable for routine production. However, the sintering hardness can be increased from 35 HRC to 43.8 HRC.

The above results show that the method of this invention can effectively improve the hardness of sintered workpieces without the problems of fluidity, apparent density, and compressibility encountered in the prior art. The original powder of the coarse additive powder 11 in Examples 3, 4, and 6 above is a pre-alloyed powder, but it can also be a pure element powder, a diffusion bonding powder, an electroplating composite powder, a dielectric metal compound powder, or a carbide powder, or a combination of these powders.

Another method of this invention involves mixing the aforementioned coarse matrix powder with fine primary powder for agglomerating into coarse spherical powder, and then granulating the mixture together to form coarse, nearly spherical powder. The resulting coarse, nearly spherical powder contains both coarse matrix powder and fine primary powder. This powder can be produced by spray drying, kneading, mixing, and other methods known in the art. In this invention, spray drying is preferably used.

In Alloy 7 of the following experimental examples, the coarse matrix powder is the matrix powder of Alloy 2 in Table 2, and the fine raw powder is the additive alloy powder of Alloy 2 in Table 2, in the same proportion. After mixing these two powders, the flowability of the granulated powder obtained by spray drying is 27.6 s/50g, which is better than the data of Alloys 3 and 4 in Table 2 (which use the same matrix powder and additive alloy powder as Alloy 7, but adopt the first method of this invention). In addition, the green density under the same forming conditions is 6.81 g/ ^cm3 , the sintering density under the same sintering conditions is 6.97 g/ ^cm3 , and the hardness is 49.1 HRC. These results are similar to the data for alloys 3 and 4 in Table 2, which use the same powder. This indicates that the second method of the present invention can also solve the problem of poor flowability caused by adding fine alloy powder, while still maintaining excellent sintering properties.

10: Coarse matrix powder 11: Coarse additive powder 12: Pure iron powder (matrix powder) 13: Pure nickel spherical aggregate powder 14: Pure iron powder (matrix powder) 15: Spherical aggregate powder 40: Iron powder 41: Master alloy powder 50: Iron powder 51: Polymer adhesive coating 52: Nickel element powder 53: Copper element powder 60: Iron powder 61: Molybdenum element powder 62: Nickel element powder 70: Iron powder 71: Plating layer 80: Pre-alloy powder 90: Iron element powder 91: Nickel element powder 92: Copper element powder

Figures 1A to 1F are schematic diagrams of different mixed powders in the prior art. Figure 2 is a schematic diagram of a mixed powder according to an embodiment of the present invention. Figure 3 is a scanning electron microscope image of spherical powder according to an embodiment of the present invention. Figure 4A is a schematic diagram of a mixed powder of pure nickel powder and pure iron powder according to an embodiment of the present invention. Figure 4B is a schematic diagram of a mixed powder of pure iron powder and Fe-10Ni pre-alloyed powder according to an embodiment of the present invention. Figure 5 is a scanning electron microscope image of the fracture surface of a pressed billet according to an embodiment of the present invention.

10: Coarse matrix powder

11: Coarse Added Powder

Claims (17)

A method for preparing a powder metallurgy sintered body includes the following steps: Step 1-1: Providing a mixed powder for pressing and sintering, the mixed powder comprising a first coarse metal powder and a second coarse metal powder, wherein the first coarse metal powder is a matrix powder, the median particle size of the first coarse metal powder is between 50 μm and 110 μm, and the second coarse metal powder is a spray-granulated spherical powder composed of multiple fine original powders aggregated together, the spray-granulated spherical powder being prepared by spray granulation of the fine original powders with water and a binder, the median particle size of the fine original powders being less than 15 μm. The second coarse metal powder accounts for between 5% and 50% of the weight of the mixed powder, and the second coarse metal powder has a nearly spherical powder morphology and a median particle size of less than 100 μm; wherein, the first coarse metal powder is mainly composed of iron and contains at least one alloying element, the total weight of which accounts for less than 10% of the weight of the mixed powder, and the second coarse metal powder is mainly composed of iron; Step 1-2: Fill the mixed powder into a mold and apply a forming pressure to the mixed powder to form a green body; and Step 1-3: Heat the green body to a temperature higher than 1,100°C to sinter it into a sintered body with high density. The method of claim 1, wherein the fine original powder in the second coarse metal powder is an elemental powder, a pre-alloyed powder, a compound powder, a carbide powder, a composite powder, a master alloy powder, or a mixture thereof. The method of claim 1, wherein the median particle size of the second coarse metal powder is less than 45 μm. The method of claim 1, wherein the first coarse metal powder may be an elemental powder, a mixed powder, a diffusion bonding powder, a pre-alloyed powder, a binder powder, a composite powder, or a combination thereof. The method of claim 1, wherein the alloying elements include less than 8% nickel, less than 2% molybdenum, less than 1.5% manganese, less than 4% chromium, less than 3% copper, less than 1.5% cobalt, less than 1.5% tungsten, less than 1.5% silicon, less than 1% niobium, less than 1% vanadium, and unavoidable impurities. The method of claim 5, wherein the first coarse metal powder further comprises nickel in a weight percentage between 0.5% and 2.5%, molybdenum in a weight percentage between 0.2% and 1.0%, manganese in a weight percentage between 0.01% and 1.0%, and unavoidable impurities. The method of claim 5, wherein the first coarse metal powder comprises nickel in a weight percentage between 0.5% and 4.5%, molybdenum in a weight percentage between 0.2% and 1.0%, copper in a weight percentage between 0.5% and 2%, manganese in a weight percentage between 0.01% and 1.0%, and unavoidable impurities. The method of claim 1, wherein the second coarse metal powder further comprises chromium at a weight percentage between 8% and 20%, nickel at a weight percentage between 5% and 20%, molybdenum at a weight percentage less than 5%, manganese at a weight percentage less than 2%, copper at a weight percentage less than 5%, silicon at a weight percentage less than 1.5%, niobium at a weight percentage less than 2%, and unavoidable impurities. A mixed powder for pressing and sintering includes a first coarse metal powder and a second coarse metal powder. The first coarse metal powder is a matrix powder with a median particle size between 50 μm and 110 μm. The second coarse metal powder is a spray-granulated spherical powder composed of multiple fine primary powders aggregated together. The spray-granulated spherical powder is obtained by spray granulation of the fine primary powders with water and a binder. The median particle size of the fine primary powders is less than 15 μm. The second coarse metal powder accounts for between 5% and 50% of the weight of the mixed powder, and has a near-spherical powder morphology with a median particle size less than 100 μm. μm, the first coarse metal powder is mainly composed of iron and contains at least one alloying element, the total weight of which accounts for less than 10% of the weight of the mixed powder, the second coarse metal powder is mainly composed of iron, wherein the mixed powder is used for sintering at temperatures above 1,100°C. A method for preparing a powder metallurgy sintered body includes the following steps: Step 1-1: Providing an aggregated powder with a near-spherical powder morphology for pressing and sintering, the aggregated powder being formed by mixing and aggregating multiple first metal powders and multiple second metal powders with a particle size finer than the first metal powders, the aggregated powder being a spray-granulated spherical powder, the spray-granulated spherical powder being prepared by spray granulation of the first metal powder, the second metal powder, water and a binder, wherein the median particle size of the first metal powder is between 50 μm and 110 μm, the median particle size of the second metal powder is less than 15 μm, and the weight percentage of the second metal powder in the aggregated powder is between 5% and 50%; The first metal powder is mainly composed of iron and contains at least one alloying element, the total weight of which accounts for less than 10% of the weight of the mixed powder; the second metal powder is mainly composed of iron; Step 1-2: The aggregated powder is filled into a mold and a forming pressure is applied to the aggregated powder to form a green body; and Step 1-3: The green body is heated to a temperature above 1,100°C and sintered into a sintered body with high density. The method of claim 10, wherein the first metal powder may be an elemental powder, a mixed powder, a diffusion bonding powder, a pre-alloyed powder, a binder powder, a composite powder, or a combination thereof. The method of claim 10, wherein the second metal powder is an elemental powder, a pre-alloyed powder, a compound powder, a carbide powder, a composite powder, a master alloy powder, or a mixture thereof. The method of claim 10, wherein the alloying elements include less than 8% nickel, less than 2% molybdenum, less than 1.5% manganese, less than 4% chromium, less than 3% copper, less than 1.5% cobalt, less than 1.5% tungsten, less than 1.5% silicon, less than 1% niobium, less than 1% vanadium, and unavoidable impurities. The method of claim 13, wherein the first metal powder further comprises nickel in a weight percentage between 0.5% and 2.5%, molybdenum in a weight percentage between 0.2% and 1.0%, manganese in a weight percentage between 0.01% and 1.0%, and unavoidable impurities. The method of claim 13, wherein the first metal powder comprises nickel in a weight percentage between 0.5% and 4.5%, molybdenum in a weight percentage between 0.2% and 1.0%, copper in a weight percentage between 0.5% and 2%, manganese in a weight percentage between 0.01% and 1.0%, and unavoidable impurities. The method of claim 10, wherein the second metal powder further comprises chromium at a weight percentage between 8% and 20%, nickel at a weight percentage between 5% and 20%, molybdenum at a weight percentage less than 5%, manganese at a weight percentage less than 2%, copper at a weight percentage less than 5%, silicon at a weight percentage less than 1.5%, niobium at a weight percentage less than 2%, and unavoidable impurities. A powder for pressing and high-temperature sintering is an aggregated powder with a near-spherical morphology. This aggregated powder is composed of multiple first metal powders and multiple second metal powders with a particle size finer than the first metal powders. The aggregated powder is a spray-granulated spherical powder, prepared by spray granulation of the first metal powder, the second metal powder, water, and a binder. The median particle size of the first metal powder is between 50 μm and 110 μm, and the median particle size of the second metal powder is less than 15 μm. The first metal powder is mainly composed of iron and contains at least one alloying element. The total weight of the alloying element accounts for less than 10% of the weight of the aggregated powder. The second metal powder is mainly composed of iron. The aggregated powder is used for sintering at temperatures above 1,100°C.