HOT ISOSTATIC PRESSING PROCESS FOR SUPERALLOY POWDER

Abstract

Hot isostatic press (HIP) process for superalloy powder, to form a superalloy member. A first step HIP temperature is higher than an initial melting temperature of low-melting-point alloy powder and more than 15° C. lower than a solidus of completely homogenized alloy. Pressure is ≧90 MPa, and time is 20 minutes≦t≧1 hour. Heating is stopped after the first step to cool material until temperature is below initial melting temperature of low-melting-point phase. There is temperature keeping for ≧2 hours, to ensure low-melting-point phase, formed during cooling after first step, is completely dissolved. Alloy is cooled after second step to room temperature as furnace pressure keeping continues. Formation of an original particle boundary is prevented or there is significantly reduced the number of precipitated phases on the original particle boundary in HIP procedure, to obtain compact alloy with microscopic structures as equiaxed crystals.

Description

TECHNICAL FIELD

The present invention belongs to the field of powder metallurgy superalloy, and in particular to a hot isostatic pressing process for superalloy powder, which is applicable to the manufacturing of powder metallurgy superalloy components solely by using hot isostatic pressing.

BACKGROUND

Superalloy is a kind of material that is most widely used in aero engines. The mechanical properties and temperature capability of superalloy greatly depend on the amount of strengthening elements added to the alloy. But addition of too many strengthening elements will increase the alloy's macroscopic and microscopic segregation, deteriorate the microstructure homogeneity and hot workability and even lead to the loss of the ability to be processed by hot working. Preparation of superalloy powder particles using rapid solidification can effectively inhibit elemental segregation formed during solidification, thereby allowing more strengthening elements to be added to superalloy without reducing the alloy's microstructure homogeneity. The superalloy prepared by powder compaction using the rapidly solidified powder has homogeneous microstructure and excellent mechanical properties, and is then widely used in the hot section components of aero engines, such as the turbine disks. However, the superalloy prepared by powder metallurgy process still has its shortcomings. Carbide and other precipitates will precipitate on the surface of the powder particle during hot isostatic pressing compaction, which is detrimental because they will influence the ductility of the alloy and the particle boundaries with preferential precipitated precipitates are also potential crack initiation sites, and in turn affect the reliability of powder superalloy prepared by hot isostatic pressing.

To improve the precipitation of the precipitates along the prior particle boundaries in order to increase reliability of the powder metallurgy superalloys, researchers have developed a series of methods that include:

1. Introducing heavy deformation to the hot isostatic pressed billet by using extrusion, cogging, isothermal forging, etc. to change the morphology of the powder prior particle boundary so as to change the distribution of the precipitates thereon.

2. Annealing the hot isostatic pressed superalloy powder billet at high temperature for long time to partially dissolve the precipitates.

3. To improve the phase precipitation along the prior particle boundary by adding other elements, such as Hf.

Undoubtedly, these methods increase the manufacturing cost of the powder metallurgy superalloy.

SUMMARY

The present invention aims to provide a hot isostatic pressing process for superalloy powder, capable of obtaining powder superalloy billets with excellent microstructure and mechanical property directly by hot isostatic pressing.

The present invention has the following technical solution:

A hot isostatic pressing process for superalloy powder, the specific steps of which are as follows:

(1) preparing superalloy powder by gas atomization or other methods; sieving the powder to obtain powder particles with size less than or equal to 155 μm; loading the sieved powder particles into carbon steel or stainless steel capsule; hot degassing and then sealing the capsule;

(2) putting the powder capsule prepared in step (1) into a hot isostatic press apparatus; starting hot isostatic press compaction when the temperature and pressure in the furnace reach the presupposed conditions, the presupposed condition can be obtained either by pressuring and heating simultaneously or by heating to the presupposed temperature and then pressuring to presupposed pressure;

wherein the parameters for the hot isostatic press are: the hot isostatic pressing temperature is within the range between the incipient melting temperature of the low-melting-point phase in the powder particles and the solidus of completely homogenized alloy plus 15° C.; the pressure should be higher than or equal to 90 MPa; the holding time in this stage should be longer than or equal to 20 minutes and shorter than or equal to 1 hour when the temperature in the furnace reaches the presupposed temperature;

(3) stop heating when the holding process of step (2) is finished, cool the powder capsule in the furnace to temperature that is below the incipient temperature of the low-melting-point phase in the powder particles and then hold at that temperature (this temperature is usually 10° C. to 20° C. below the incipient melting temperature of the low-melting-phase), the holding time for this stage should be 2 hours or longer to ensure that the low-melting-point phase formed during cooling can be completely dissolved during the holding process and the pressure in the temperature keeping process should be higher than or equal to 90 MPa; stop heating and cool the capsule in the furnace to room temperature when this holding process is finished.

This hot isostatic pressing process for superalloy powder is applicable to hot isostatic pressing consolidation of nickel-iron based superalloy powder or nickel-based superalloy powder.

In this hot isostatic pressing process for superalloy powder, in step (1), the size of the sieved powder particles is preferably less than or equal to 105 μm; and the preferred size of the sieved powder particles is less than or equal to 55 μm.

In this hot isostatic pressing process for superalloy powder, for GH4169 (Which has similar chemistry to alloy Inconel 718) and its derived alloys powder thereof, the incipient melting temperature of the low-melting-point phase is the Laves-phase melting temperature of GH4169 and its derived alloys thereof; for other γ′ phase strengthened nickel-base superalloy powder, the incipient melting temperature of the low-melting-point phase is γ/γ′ eutectic temperature.

In this hot isostatic pressing process for superalloy powder, in step (2), the preferred range of hot isostatic pressing pressure is 120 MPa to 150 MPa.

In this hot isostatic pressing process for superalloy powder, in step (3), the preferred range of pressure of the holding process is 120 MPa to 150 MPa.

In this hot isostatic pressing process for superalloy powder, in step (2), when the hot isostatic pressing temperature is higher than the incipient melting temperature of the low-melting-point phase in alloy powder and lower than the solidus of the completely homogenized alloy, the amount of precipitates along the prior particle boundaries can be significantly reduced; and when the hot isostatic pressing temperature is higher than the solidus of the completely homogenized alloy and less than solidus of completely homogenized alloy plus 15° C., the formation of the prior particle boundary can be avoided.

The present invention has the following advantages and beneficial effects:

1. The process of the present invention comprises two steps. The temperature range of the first step should be higher than the incipient melting temperature of the low-melting-point phase in alloy powder and lower than the solidus of completely homogenized alloy plus 15° C., the gas pressure should be higher than or equal to 90 MPa, and holding time should be longer than or equal to 20 minutes and shorter than or equal to 1 hour. When the first step is completed, stop heating and cool the capsule within the furnace until the temperature in the furnace is below the incipient melting temperature of the low-melting-point phase, and then hold at that temperature,this is the second step. The holding time of the second step should be 2 hours or longer, so as to ensure that the low-melting-point phase formed during cooling after the first step can be completely dissolved; and when the second step is completed, the capsule is cooled to room temperature, the pressure in the furnace should be maintained as that of the holding stage. The present invention is used for hot isostatic pressing consolidation for rapidly solidified superalloy powder; in combination with a near-net-shape forming technology, this process can also be used for the manufacturing of superally components with complex geometry, so as to increase the utilization rate of material.

2. The present invention can be realized on traditional hot isostatic pressing machine. The process is applicable to hot isostatic pressing consolidation of nickel-iron-base superalloy powder and nickel-base superalloy powder.

3. The present invention is simple and practical and can shorten the manufacturing procedure of powder superalloy components, thereby reducing the manufacturing cost thereof.

DESCRIPTION OF THE DRAWINGS

FIG. 1(a)-FIG. 1(b) are microstructures (optical microscopy images) of powder metallurgy GH4169G alloy prepared by process A of the present invention, wherein FIG. 1(a) has a magnification of ×100, and FIG. 1(b) has a magnification of ×200.

FIG. 2(a)-FIG. 2(b) are tensile fractures (scanning electron microscopy images) of the heat treated powder metallurgy GH4169G prepared by process A of the present invention tested at room temperature and 650° C., wherein FIG. 2(a) is at room temperature and FIG. 2(b) is at

FIG. 3(a)-FIG. 3(b) are microstructures (optical microscopy images) of powder metallurgy GH4169G alloy prepared by process B of the present invention, wherein FIG. 3(a) has a magnification of ×100, and FIG. 3(b) has a magnification of ×200.

FIG. 4(a)-FIG. 4(b) are tensile fractures (scanning electron microscopy images) of the heat treated powder metallurgy GH4169G prepared by process B of the present invention tested at room temperature and 650° C., wherein FIG. 4(a) is at room temperature and FIG. 4(b) is at 650° C.

FIG. 5(a)-FIG. 5(b) are microstructures (optical microscopy images) of ME3 alloy (A nickel-base superalloy) prepared by process C of the present invention, wherein FIG. 5(a) has a magnification of ×100, and FIG. 5(b) has a magnification of ×200.

FIG. 6(a)-FIG. 6(b) are microstructures (scanning electron microscopy images) of ME3 alloy prepared by process D of the present invention, wherein FIG. 6(a) has a magnification of ×100, and FIG. 6(b) has a magnification of ×200.

DETAILED DESCRIPTION

The present invention relates to a hot isostatic pressing process which can prevent the formation of prior particle boundary or significantly reduce the amount of precipitates precipitated along the prior particle boundaries, comprising the specific steps as follows:

1. Preparing superalloy powder by gas atomization or other methods; sieving the powder to obtain powder of with size less than or equal to 155 μm (the preferred size is less than or equal to 105 μm and the best size is less than or equal to 55 μm); filling the powder into a low carbon steel or stainless steel capsule; high-temperature degassing and sealing the capsule. The use of fine powder is aiming at reducing the number of ceramic inclusions and hollow powder in the powder batch. The carbon steel or stainless steel capsule is used because they are solid and have enough strength and will not react with the powder particles in the temperature range used in the present invention. High-temperature degassing is used to minimize the gas absorbed on the surface of the powder particles so as to reduce the tendency of the alloy to form thermal-induced pore during subsequent heat treatment. The degassing temperature is between 180° C. and 500° C.

2. Putting the powder capsule prepared in the first step into a hot isostatic pressing apparatus; heating and pressuring the furnace simultaneously and starting hot isostatic press consolidation when the pressure and temperature reach the presupposed condition. The process parameters of hot isostatic pressing in the first stage are: the temperature should be higher than the incipient melting temperature of the low-melting-point phase of the alloy powder (e.g.: Laves-phase melting temperature for alloy Inconel 718 and its derevatives (i.e., Chinese GH4169 alloy series), and γ/γ′ eutectic temperature of other y′-phase strengthened nickel-base superalloy) and lower than the solidus of completely homogenized alloy plus 15° C.; the pressure should be higher than or equal to 90 MPa; and the holding time should be longer than or equal to 20 minutes and shorter than or equal to 1 hour when the temperature in the furnace reaches the presupposed temperature. There are three reasons for the selection of temperature for the first stage within a temperature range at which a certain amount of liquid phases is formed for three reasons. The first reason is that the solubility of elements carbon, boron, etc. in the alloy matrix at high temperature increases, so precipitates such as carbide, boride, etc. are hard to precipitate on the particle surface. The second reason is that the partial melting of the partial powder surface leads to the decrease of the amount of attachable position for nucleation of phases such as carbide. The third reason is that when the liquid phase forms, the powder partially melts, then the spherical shape of the powder disappears. The holding time for the first stage is selected at longer than or equal to 20 minutes and shorter than or equal to 1 hour because for two reasons: firstly, at least 20 minutes are needed for the completely compaction of the powder compact within the temperature range of the first step selected in the present invention; and secondly, overlong holding time will lead to overlarge grain size of the powder compact, thereby affecting mechanical properties.

3. When the first step is completed, stop heating and cool the powder capsule within the furnace until the temperature of the furnace is below the incipient melting temperature of the low-melting-point phase of the powder particle, and then maintain the temperature for longer time. This is the second step. The holding time in the second step should be 2 hours or longer, so as to ensure that the low-melting-point substance formed during cooling after the first step can be completely dissolved; there can be pressure or no pressure in this stage, but with pressure is preferable. The pressure should be higher than or equal to 90 MPa; when the second step is finished, stop heating and cool the capsule inside the furnace to room temperature. The second step is necessary for the following reasons: in the first step of hot isostatic pressing, certain amount of liquid phase will form in the capsule. The liquid will form Laves phase (Inconel 718 and derived alloys thereof, i.e., GH4169 and derived alloy thereof) and γ/γ′ eutectic (γ′ strengthened nickel-base superalloy) during cooling after the first step. The Laves phase and the γ/γ′ eutectic are brittle and are potential crack sources in service, so they must be eliminated. The method for eliminating the Laves phase and the γ/γ′ eutectic is to hold the alloy at temperature just below the incipient melting temperature of Laves phase or the γ/γ′ eutectic for long time. Pressure is preferable for step two because the formation of thermal-induced pores in hot isostatic pressed billets can be avoided with an external pressure.

The present invention will be further described in detail below in combination with the drawings and the exploit examples

EXAMPLE 1

See Table 1 for chemistry of the alloy:

TABLE 1
Chemistry of alloy GH4169G (wt. %)
Cr	Mo	Al	Ti	Nb	C	B	P	Ni	Fe
19.3	2.98	0.5	1.04	4.94	0.031	0.008	0.023	53.5	Bal-
									ance

In this example, argon atomization was used for powder preparation; the powder with size less than 105 micrometers was filled into a stainless steel capsule; the powder capsule was hot degassed and then hot isostatic pressed. The following process (A) is selected for this alloy:

In the first stage: the capsule was hot isostatic pressed at 1245° C./150 MPa for 0.5 hour, the furnace was heated and pressurized simultaneously to reach these parameters. The capsule was cooled in the furnace to the next stage when this stage is finished.

In the second stage: the capsule was kept at 1110° C./150 MPa for 4 hours, and was cooled to room temperature when this stage is finished.

For this process, the hot isostatic pressing temperature in the first stage is higher than the Laves-phase melting temperature (1160° C.), but lower than the solidus temperature of the alloy (1260° C.).

Microstructures of the alloy prepared by the process are shown in FIG. 1(a) and FIG. 1(b).

It can be seen that alloy prepared using the process has uniform and fine grain structure, and there are few precipitates along the prior particle boundaries.

The alloy prepared by this process was direct aged and its tensile properties at room temperature and 650° C. and 650° C./760 MPa stress rupture property were then tested, and the results are listed in Table 2. It can be seen from the table that the tensile properties at both room temperature and 650 have met the specification of alloy GH4169 (Inconel 718) and it is much higher than those of alloy K4169 (cast Inconel 718) . The stress rupture property of the alloy is excellent. Especially, the stress rupture life of 650° C./690 MPa exceeds 700 hours, which is comparable with wrought GH4169G (a derivative of alloy Inconel 718).

The tensile fracture surfaces of the heat treated sample tested at room temperature and 650° C. are shown in FIG. 2(a) and FIG. 2(b). It can be seen that the fracture mode is dimple ductile dominated fracture, which indicates that the powder particles were well combined during the hot isostatic pressing.

EXAMPLE 2

The alloy used in this example is the same as that in example 1. The difference between this example and example 1 is that the temperature in the first stage of this example is above the solidus of the alloy and there will be more liquid phase in the capsule during hot isostatic pressing. Thus, the temperature in the second stage is increased so as to ensure that the Laves phase formed during cooling after the first stage can be eliminated adequately.

In this example, argon atomization was used for powder preparation; the powder with size less than 105 micrometers was filled into a stainless steel capsule; the powder capsule was hot degassed and then hot isostatic pressed. The following process (B) is selected:

In the first stage: the capsule was hot isostatic pressed at 1265° C./150 MPa for 0.5 hour, and the furnace was heated and pressurized simultaneously to reach these parameters. The capsule was cooled in the furnace to the next stage when this stage is finished;

In the second stage: the capsule was kept at 1140° C./150 MPa for 4 hours, and was cooled to room temperature when this stage is finished.

The hot isostatic pressing temperature in the first stage of this process is higher than the Laves-phase melting temperature (1160° C.) and the solidus temperature of the alloy (1260° C.).

Microstructures of the alloy prepared by this process are shown in FIG. 3(a) and FIG. 3(b). It can be seen that the formation of prior particle boundary was completely avoided using this hot isostatic pressing process, and this alloy has equiaxed grain structure.

The alloy prepared by this process was direct aged and its tensile properties at room temperature and 650° C. and 650° C./760 MPa stress rupture properties were then tested, and the results are listed in Table 2. It can be seen from the table that the tensile properties at both room temperature and 650° C. have met the specification of alloy GH4169 (Inconel 718) and it is much higher than those of alloy K4169 (cast Inconel 718), the stress rupture properties are also very good. However, due to the coarse grain size compared with that of alloy prepared by process A, the strength of alloy prepared using process B is lower than that of process A.

The tensile fracture surfaces of the heat treated sample tested at room temperature and 650° C. are shown in FIG. 4(a) and FIG. 4(b). It can be seen that the tensile fracture mode of samples tested at room temperature and 650° C. are fully dimple ductile fracture, which indicates that the powder particles are well combined.

TABLE 2
Mechanical properties of powder metallurgy GH4169G alloy
prepared by the present invention (after heat treatment)
				Reduction
Tensile Properties	σ0.2, MPa	σb, MPa	Elongation	in Area
25□	Prepared by	1189	1358	20.5%	33%
	process A
	Prepared by	1143	1300	21%	35%
	process B
	GH4169	1035-1167	1275-1400	12%-21%	15%
	specification
	K4169	935	1100	16%	26%
650	Prepared by	947	1088	25%	29%
	process A
	Prepared by	905	1032	21.6%	34%
	process B
	GH4169	860-1000	1000-1200	12%-19%	15%
	specification
	K4169	795	895	10%	28%
	(600° C.)
	Temperature	Load	Process	Life	Elongation
Stress	650° C.	690 MPa	Process A	700.1 hours	8.96%
rupture			Process B	457 hours	6.8%

EXAMPLE 3

Table 3 lists the chemistry of the alloy:

TABLE 3
Chemistry of nickel-cobalt-base alloy (wt. %)
Co	Cr	Mo	W	Al	Ti	Nb	Ta	C	B	Zr	Ni
20.6	12.9	3.96	2	3.6	3.8	1.1	2.4	0.04	0.03	53.5	Bal-
											ance

In this example, argon atomization was used for powder preparation; the powder with size less than 155 micrometers was filled into a stainless steel capsule; the powder capsule was hot degassed and then hot isostatic pressed. The following process (C) is selected according to the characteristic of the alloy:

In the first stage: the capsule was hot isostatic pressed at 1245° C./150 MPa for 1 hour, the furnace was heated and pressurized simultaneously to reach these parameters. The capsule was cooled in the furnace to the next stage when this stage is finished.

In the second stage: the capsule was kept at 1210° C./150 MPa for 4 hours, and was cooled to room temperature inside the furnace when this stage is finished.

The hot isostatic pressing temperature in the first stage of the process is higher than the melting temperature (1220° C.) of the γ/γ′ eutectic, but lower than the solidus of the alloy (1260° C. to 1265° C.).

Microstructures of the alloy prepared by the process are shown in FIG. 5(a) and FIG. 5(b). It can be seen that the alloy prepared by the process has an average grain size of 44 μm. The prior particle boundary can be observed from the alloy, but fewer precipitated phase particles are precipitated thereon.

The alloy prepared by the process is subjected to 1170° C./1 h/air cooling+845° C./4 h/air cooling +760° C./8 h/air cooling heat treatment, and then the tensile properties at room temperature and 760° C. and the stress rupture property at 760° C./690 MPa were tested, and the results are listed in Table 4. It can be seen from the table that the alloy prepared by the process has excellent mechanical properties.

TABLE 4
Tensile properties and stress rupture property of a nickel-
cobalt-base Superalloy Prepared by the Present Invention
(Process C)
	σ0.2,	σb,		Reduction in
Tensile Properties	MPa	MPa	Elongation	Area
25° C.	1263	1546	11.6%	11.8%
760° C.	1134	1265	6.8%	6.5%
	Temperature	Load	Rupture life	Elongation
Stress ruputure	760° C.	690 MPa	81 h	4.3%

EXAMPLE 4

The alloy, particle size and degassing procedure used in this example are the same as that of example 3. The difference between this example and example 3 is that the first step of this process is conducted near the solidus of the alloy. The specific process (D) is:

In the first stage: the capsule was hot isostatic pressed at 1265° C./150 MPa for 1 hour, and the furnace was heated and pressurized simultaneously to reach these parameters. The capsule was cooled in the furnace to the next stage when this stage is finished.

In the second stage: the capsule was kept at 1210° C./150 MPa for 4 hours, and was cooled to room temperature inside the furnace when this stage is finished.

The hot isostatic pressing temperature in the first stage of this process is slightly higher than the solidus temperature of the alloy (1260° C. to 1265° C.).

Microstructures of the alloy prepared by this process are shown in FIG. 6(a) and FIG. 6(b). It can be seen that the process can completely avoids the formation of the prior particle boundaries during hot isostatic pressing, thereby obtaining equiaxed microstructure.

The results of the examples show that the process of the present invention can avoid the prior particle boundry formed during the hot isostatic pressing or significantly reduce the amount of the precipitates along the prior particle boundary, thereby obtaining compact alloy with equiaxed microstructures and with excellent mechanical properties. The process can shorten the manufacturing procedure of powder metallurgy superalloy billets or components. In turn, it can reduce the manufacturing cost. The process is suitable for hot isostatic pressing of superalloy powder of all systems.

Claims

1. A hot isostatic pressing method for superalloy powder, which is characterized in that:

the process comprises the following steps:

(1) preparing superalloy powder by gas atomization or other methods; sieving the powder to obtain the powder with size less than or equal to 155 μm; loading the sieved powder into a carbon steel or stainless steel capsule; hot degassing and then sealing the capsule;

(2) putting the powder capsule prepared in step (1) into a hot isostatic pressing apparatus; starting hot isostatic press compaction after reaching the presupposed conditions in a manner of heating and pressuring simultaneously or heating firstly and then pressuring;

wherein the parameters for the hot isostatic press are: the hot isostatic pressing temperature is within the range between the incipient melting temperature of the low-melting-point phase in the powder particles and the solidus of completely homogenized alloy plus 15° C.; the pressure is higher than or equal to 90 MPa; the holding time in this stage is longer than or equal to 20 minutes and shorter than or equal to 1 hour when the temperature in the furnace reaching the presupposed temperature;

(3) stop heating when the holding process of step (2) is finished, cool the capsule in the furnace to temperature that is below the incipient temperature of the low-melting-point phase in the powder particles and then hold at that temperature, the holding time for this stage is 2 hours or longer to ensure that the low-melting-point phase formed during cooling can be completely dissolved and the pressure in the temperature keeping process is higher than or equal to 90 MPa; stop heating and cool the capsule in the furnace to room temperature when this holding process is finished.

2. The hot isostatic pressing method for superalloy powder according to claim 1, which is characterized in that: the method is applicable to hot isostatic pressing consolidation of nickel-iron-based superalloy powder or nickel-based superalloy powder.

3. The hot isostatic pressing method for superalloy powder according to claim 1, which is characterized in that: in step (1), the size of the sieved powder particles is less than or equal to 105 μm.

4. The hot isostatic pressing method for superalloy powder according to claim 1, which is characterized in that: in step (1), the size of the sieved powder particles is less than or equal to 55 μm.

5. The hot isostatic pressing method for superalloy powder according to claim 1, which is characterized in that: for GH4169 and derived alloys powder thereof, the incipient melting temperature of the low-melting-point phase is the Laves-phase incipient melting temperature of GH4169 and derived alloys thereof; and for other γ′-phase strengthened nickel-base superalloy powder, the incipient melting temperature of the low-melting-point phase is γ/γ′ eutectic temperature.

6. The hot isostatic pressing method for superalloy powder according to claim 1, which is characterized in that: in step (2), the range of hot isostatic pressing pressure is 120 MPa to 150 MPa.

7. The hot isostatic pressing method for superalloy powder according to claim 1, which is characterized in that: in step (3), the range of pressure in the temperature keeping process is 120 MPa to 150 MPa.

8. The hot isostatic pressing method for superalloy powder according to claim 1, which is characterized in that: in step (2), when the hot isostatic pressing temperature is higher than the incipient melting temperature of the low-melting-point phase of alloy powder and lower than the solidus of completely homogenized alloy, this method can reduce the amount of precipitates along the prior particle boundary.

9. The hot isostatic pressing method for superalloy powder according to claim 1, which is characterized in that: in step (2), when the hot isostatic pressing temperature is higher than the solidus of completely homogenized alloy and lower than solidus of completely homogenized alloy plus 15° C., this method can eliminate or avoid the formation of prior particle boundary.