GB2536483B

GB2536483B - A method of Forming a Metal Component

Info

Publication number: GB2536483B
Application number: GB1504682.4A
Authority: GB
Inventors: Bai Qian; Jiang Jiaying; Tian Gaofeng; Wang Shuyun; Zou Jinwen; Lu Feng; Lin Jianguo; Dean Trevor
Original assignee: Avic Beijing Institute Of Aeronautical Mat Avic Biam
Current assignee: Avic Beijing Institute Of Aeronautical Mat Avic Biam
Priority date: 2015-03-19
Filing date: 2015-03-19
Publication date: 2019-10-09
Anticipated expiration: 2035-03-19
Also published as: GB2536483A; GB201504682D0

Description

A Method of Forming a Metal Component

The present invention relates to a method of forming a metal component.

Powder metallurgy is the process of blending fine powdered metal, pressing it into a desired shape or form (compacting), and then heating the compressed metal in a controlled atmosphere to bond the material (sintering). The range of products that can be manufactured by powder metallurgy includes alloys, created from high melting point metals, metal/non-metal composite materials, composites of metals that do not dissolve into each other and porous materials.

Powder metallurgy allows products to be manufactured while avoiding segregation associated with melting and reducing manufacturing cost by limiting the amount of machining and welding. In addition, alloys with significantly improved mechanical properties can also be produced as the desired microstructure can be achieved by adjusting the composition of metal powders. There are several processes typically used in conventional powder metallurgy, including: powder compaction and sintering, hot isostatic pressing, powder forging, powder rolling, powder extrusion, powder injection, spray forming, etc., and a wide range of powders are available.

Nickel-based superalloys are commonly used in gas turbine engines which are subject to high temperatures and require high strength, excellent creep resistance, as well as corrosion and oxidation resistance, due to the presence of y’ (Ni3Al) precipitates which are coherent and remain stable to relatively high temperatures. In the mid-1960s alloy designers were focused on increasing the y’ volume fraction for increased high-temperature strength, y’ volume fractions of 60% are now common. Unfortunately increasing y’ volume fractions generally brought with them the difficulty of increased macrosegregation in large ingot castings. Powder metallurgy offers a method for overcoming the severe macrosegregation, since the material is divided into small droplets while it is a homogeneous liquid, and thus the maximum segregation distance is restricted by the size of the solidified droplets.

Hot isostatic pressing is often used for the manufacture of components using nickel-based superalloys. However, final components manufactured using hot isostatic pressing typically require a substantial amount of time to produce, expending high levels of energy in the process to keep the powders at high temperatures for hours.

Exposing the powder components to high temperatures for sustained periods of time in this way also causes prior particle boundary (PPB) to form within the final components which can negatively impact the mechanical properties of the material. Further, low compacting pressure can cause insufficient deformation of the metal powders, which may lead to a weakening of hardness and other properties of the final component.

Figure l shows a conventional process for producing powder metallurgy superalloys using hot isostatic pressing.

At step Si, powdered metal is packed into a purpose built stainless steel cylindrical container which is then evacuated of air and sealed (step S2). Then, at step S3, the cylindrical container with the powder is hot isostatically pressed. In the hot isostatically pressing process the powder is heated and simultaneously subjected to a gas pressure (normally argon) in a pressure vessel for several hours depending on the size of the components.

It will be appreciated that the hot isostatic pressing process leads to both relatively high cost and relatively low product efficiency. Furthermore, an intrinsic problem with such processes is that prior particle boundary (PPB) precipitate networks can emerge in the hot isostatic pressing process due to extended periods of exposure to elevated temperature and the small amount of plastic deformation experienced by the powder particles. PPB precipitation continues to pose a major problem that is preventing the application of net-shaped hot isostatic pressing of nickel-based superalloys. It is generally considered that PPBs are caused by particle contamination and atomic segregation during the long heating time. Both reactions result in precipitation at PPBs, either as carbides, oxides, or oxy-carbides. Since PPB precipitates are brittle and thus provide an easy fracture path, the undesirable PPB networks can compromise the mechanical properties of the hot isostatic pressed product. In order to break up the PPB networks, the hot isostatic pressed product is subsequently hot extruded and hot forged.

In a typical process for powder metallurgy superalloys, the stainless steel container is removed after hot isostatic pressing (step S4) to leave cylindrical metal bar. This cylindrical metal bar is then heated (step S5). At step S6, the cylindrical metal bar is hot extruded, after which the extruded cylindrical metal bar is cut into billets (step S7) based on the volume of the finished component. Each billet (step S8) is heated and preforged (step S9). A finish-forging process (step S10) allows further deformation and the final shape of component can be obtained.

The present invention sets out to provide an improved method of forming a metal component compared to conventional methods.

According to a first aspect of the invention there is provided a method of forming a metal component as set out in claim 1. Optional method features are set out in claims 2- 12.

Embodiments of the invention will now be described, by way of example and with reference to the accompanying drawings in which:-

Figure 1 shows a flow chart of a conventional method for forming a metal component; Figure 2 shows a flow chart of a method for forming a metal component according to a first embodiment;

Figure 3 shows metal powder in a preform container according to the method of the first embodiment;

Figure 4 shows metal powder in a preform container with a die set according to the method of the first embodiment; and

Figure 5 shows a formed metal component.

Embodiments of the invention are suitable for forming metal components from metal powder.

Figure 2 shows a flow chart of a method for forming a metal component according to a first embodiment. Figure 2 will be discussed along with Figures 3, 4 and 5.

At step Sil, a preform container 1 is filled with metal powder 2. In this embodiment, the preform container 1 is made out of stainless steel with a predetermined shape and thickness. In some embodiments, the shape and thickness of the stainless steel preform container 1 is optimised using finite element methods, however other suitable methods could be used in other embodiments.

In this embodiment, the metal component comprises a nickel based superalloy. As a result, the metal powder is the FGH96 nickel-based superalloy, which has the compositions in Table 1 (wt%)

Table 1

In this embodiment, the preform container 1 is formed from sheet metal using stamping. However, in other embodiments, the preform container 1 maybe formed using other methods, using formed from sheet metal using processes such as: forming, drawing, machining and welding.

At step S12, the preform container 1 containing the metal powder 2 is evacuated and sealed. In this embodiment, the preform container 1 is evacuated of air and sealed by welding the end of the connection to the vacuum pump, but other embodiments could use alternative methods.

In some embodiments, glass lubricant may be applied to the outer surface of the preform container 1 to reduce the friction and act as thermal insulation during the transferring from the furnace to the die-set.

Also glass lubricant can act as a barrier between the stainless steel preform and the tools (i.e. the die-set) in order to avoid the diffusion bonding between them at elevated temperature and high forging load. However, embodiments are not limited thereto. For example, lubricants for conventional hot forging process can be used.

At step S13, the preform container 1 containing the metal powder 2 is heated. This step bonds the metal powder 2 to form a metal object within the preform container 1 haring a shape defined by the shape of the insider of the preform container 1. This can be considered to be a first shape. After this heating step, the combination of the preform container 1 surrounding the metal object haring a first shape can be considered to be a work-piece 4 (see Figure 4).

In some embodiments of the invention, the heating time can be considerably shorter than for conventional hot isostatic pressing processes. For example, in this embodiment, the heating time comprises one minute heating for one millimetre depth of material (‘material’ referring to both the perform container 1 and the metal powder 2). In other embodiments, the heating time maybe a few seconds by using resistance heating.

As discussed in more detail below, the first shape of the metal object (see step S13) is a near net shape.

In this embodiment, the preform container 1 containing the metal powder 2 is heated to 1100°C or thereabouts in a furnace (not shown). The work-piece is then soaked at this temperature to ensure a thermal equilibrium state. At this temperature, which is slightly lower than the gamma prime solution temperature, the metal material is soft and easy to form, while the un-dissolved gamma prime hinders abnormal grain growth and ensures fine grain structure. In other embodiments, for similar alloys, a temperature of range of temperature of from 1000 °C to 1150 °C may be used.

In other embodiments, depending on the material, the heating temperature maybe that used for conventional hot forging of that material.

In this embodiment, the furnace (not shown) acts as a heating mechanism arranged to heat the preform container 1 with the metal powder 2 so as to bond the metal powder to form a metal object within the preform container l having the first shape. However, in other embodiments, other suitable ways of heating the preform container 1 could be used. For example, induction heating or electrical resistance heating could be used. Then, at step S14, the work-piece 4 (i.e. the preform container 1 including the metal object having a first shape) is forged to form a formed work-piece. In this embodiment, this is done using a die set that comprises a punch 3, a constraining member 5 and a bottom die 6. The formed work-piece (not shown) includes the forged shape of the preform container and the metal object. Prior to forging the metal object within the preform container 1 had the first shape. After forging the metal object will have a second shape within the forged preform container.

In this embodiment, the forging comprises placing a first end (e.g. the bottom) of the work-piece 4 adjacent the bottom die 6 and placing the constraining member 5 around the work-piece 4. A second end of the work-piece 4 that is opposite the first end (e.g. the top) is then pressed by the punch 3. The constraining member 5 limits deformation of work-piece 4 during punching in a direction normal to a direction connecting the first and second ends of the preform container including the metal object (e.g. a lateral direction).

In this embodiment, a punch velocity of 100 mm/s is used. A relatively high forming speed is desirable in order to reduce the heat loss during hot forging. Other embodiments could use other suitable ranges, for example from 50 mm/s to 200 mm/s. This range has the advantage of minimising heat loss during powder forging since the temperature of the dies are lower.

In this embodiment, prior to forging the die set is heated to 250 °C. Heating to a temperature higher than room temperature can minimise heat loss of the powder, also a temperature lower than the powder temperature can extend the die life and reduce the die material cost since cheaper die material can be used. In other embodiments, other heating temperatures may be used, for example the heating may be done in a range 200-300°C. This temperature range can be changed according to different materials.

In this embodiment, the heating of the die set prior to forging is done using heating bands (not shown) around the periphery of punch 3, constraining ring 5 and bottom die 6. Hence, the heating bands (not shown) act as a heating mechanism for the die set. In other embodiments, other suitable heating methods maybe used. For example, induction heating or electrical resistance heating could be used.

In this embodiment, graphite lubricant is applied to the surface of dies contacting to the work-piece to reduce the friction. However, embodiments are not limited thereto. Other embodiments could use lubricants for conventional hot forging processes.

It will be appreciated that the forging in step S14 of the work-piece 4 involves both the deformation of the preform container 1 and the metal object having the first shape within the preform container 1. In other words, the preform container 1 is forged along with the metal object having the first shape to form a metal object having the second shape within the forged preform container (not shown).

The formed work-piece can then transferred from the die-set and cooled to room temperature.

At step S15, the forged preform container is removed from the metal object having the second shape. This produces the metal component 7 as shown in Figure 5. In this embodiment, the forged preform container is removed from the metal object having the second shape by machining. However, other embodiments could use other methods such as chemical etching. Other material removal methods could be used, such as grinding and electrolytic etching.

It will be appreciated that the metal component 7 may be a final component. Alternatively, further machining or other process steps may be applied to the metal component 7. In other words, machining or other process steps maybe applied to component 7 as required.

In this embodiment, the die set acts as forging mechanism arranged to forge the preform container including the metal object to form a formed work-piece.

As discussed, this embodiment uses a die set comprising a bottom die 6, a constraining member 5 and a punch 3. However, it will be appreciated that other embodiments could use dies having different forms.

As discussed, embodiments of the invention can provide a method of forming a metal component comprising filling a preform container with a metal powder and heating the preform container with the metal powder so as to bond the metal powder to form a metal object within the preform container having a first shape. The preform container including the metal object is then forged to form a formed work-piece. This formed work-piece includes the forged preform container and the metal object now having a second shape within the forged preform container. Then the forged preform container is removed from the second metal object (e.g. by machining or etching) so as to produce the metal component. The first shape may be a near net shape of the metal component.

The concept of bonding metal powder in a preform container that is then directly forged (later removing the forged preform container) can be termed direct powder forging. Direct powder forging is associated with numerous advantages compared to conventional methods.

For example, embodiments of the invention can be associated with low energy consumption, high productivity and high efficiency compared to the conventional powder metallurgy using hot isostatic pressing. This advantage stems from the greatly reduced number of processing steps in such embodiments of the invention when compared to conventional processes.

In addition, embodiments of the invention can use relatively simple hot forging presses can be used for direct powder forging. Embodiments of the invention do not require expensive facilities for hot isostatic pressing. Furthermore, the need for hot extrusion is eliminated, further simplifying the process.

In addition, a near net-shaped component can be made by the direct powder forging methods of the present invention. In other words, the first shape of the metal object (see step S13) can be a near net shape. Embodiments of the invention can achieve this near net shape prior to the forging step (step S14) by optimising the geometry and the thickness of the preformed container 1. Hence, material waste can be reduced.

In some embodiments, the evacuated and sealed preform container 1 can protect the metal powder from oxidation, which can reduce the consequential formation of PPB networks, and thus improve mechanical properties of final components.

In this embodiment, the heating time comprises one minute heating for one millimetre depth of material. As a result of the shorter exposure time to high temperature and larger amount of deformation, the effect of PPB networks in the component is weakened significantly when compared to hot isostatically pressing.

Since the formation of PPB networks is a time-dependent diffusion process, far fewer PPBs are formed in direct powder forging due to a much shorter exposure time to high temperature compared with that in hot isostatic pressing processes. The existing inevitable PPB networks are broken up by the large plastic deformation in the direct powder forging process.

Due to the severe plastic deformation and thus recrystallization, the microstructure can be easily controlled at different locations in the finished component compared with hot isostatic pressing, and thus the mechanical properties are improved. In hot isostatically pressing processes, plastic deformation is less since a much lower pressure is used, the dominated powder bonding mechanism is material diffusion, and recrystallization is insufficient. However in the powder forging process using embodiments of the present invention, it is possible to design the deformation extent and thus the recrystallization of the material.

Many further variations and modifications will suggest themselves to those versed in the art upon making reference to the foregoing illustrative embodiments, which are given by way of example only, and which are not intended to limit the scope of the invention, that being determined by the appended claims

Claims

1. A method of forming a metal component (7) comprising: filling a preform container (1) with a metal powder (2); heating the preform container with the metal powder so as to bond the metal powder to form a metal object within the preform container having a first shape; forging the preform container including the metal object to form a formed work-piece (4), the formed work-piece including the forged preform container and the metal object having a second shape within the forged preform container; and removing the forged preform container from the second metal object so as to produce the metal component, wherein the first shape is a near net shape of the metal component.

2. A method according to claim 1, wherein once the preform container is filled with the metal powder, air is evacuated from the preform container and the preform container is sealed.

3. A method according to claim 1 or 2, wherein the heating the preform container with the metal powder further comprises heating the preform container with the metal powder so as to achieve thermal equilibrium.

4. A method according to any one of claims 1 to 3, wherein the metal component is formed of a nickel based superalloy, and the heating the preform container with the metal powder comprises heating at a temperature lower than a gamma prime solution temperature, wherein the temperature is 1000 °C to 1150 °C, optionally wherein the temperature is 1100 °C.

5. A method according to any one of claims 1 to 4, wherein the forging the preform container including the metal object to form the formed work-piece comprises: placing the preform container including the metal object adjacent a die; using a punch (3) to press the preform container including the metal object against the die, the punch and the die comprising a die set.

6. A method according to claim 5, wherein the die set is heated before the forging the preform container including the metal object to form the formed work-piece, optionally wherein the die set is heated to a temperature of from 200 to 3OO°C.

7- A method according to claim 5 or 6, wherein the die comprises a bottom die (6) and a constraining member (5), wherein a first end of the preform container including the metal object is placed adjacent the bottom die and a second end of the preform container including the metal object that is opposite to the first end is pressed by the punch, wherein the constraining member is placed around the preform container including the metal object so as to limit deformation during the forging of the preform container including the metal object in a direction normal to a direction connecting the first and second ends of the preform container including the metal object.

8. A method according to any one of claims 5 to 7, wherein the using a punch to press the preform container including the metal object against the die comprises using a punch velocity of from 50 mm/s to 200 mm/s.

9. A method according to any one of claims 5 to 8, wherein lubricant is applied to outer surfaces of the die set prior to the forging, optionally wherein the lubricant is graphite lubricant.

10. A method according to any one of claims 1 to 9, wherein lubricant is applied to an outer surface of the preform container including the metal object prior to the forging, optionally wherein the lubricant is glass lubricant.

11. A method according to any one of claims 1 to 10, wherein the removing the forged preform container from the second metal object comprises machining or chemical etching.

12. A method according to any one of claims 1 to 11, wherein the heating the preform container with the metal powder so as to bond the metal powder to form the metal object within the preform container having the first shape comprises using a heating time of one minute heating for one millimetre depth of material.