Powder Metallurgy Superalloys

History

Nickel-base powder metallurgy (PM) superalloys have been in the hot sections of jet engine aircraft for over twenty five years. They provide improved strength, creep resistance, creep fatigue, and better low cycle fatigue properties at higher temperatures than conventional superalloys. Fracture mechanics, with its emphasis on defect size and crack growth rate, is a major design criterion for their application, and the concern about ways to both define and limit the defects that are inherent in PM products continues. Tight process control limits during processing after atomisation deal very successfully with non-metallic defects. Contamination has been minimised or eliminated through handling in clean room environments, often using vacuum or inert gas screening and loading. To quote one user of PM, `Powder is not the problem. Process control is the key.' Nearly all engine manufacturers agree that PM superalloys will give improved high temperature performance.

Reports have recognised that fatigue life of superalloys was limited by defects, but improved alloys were likely to come from PM. In a 1995 report, the US National Materials Advisory Board stated ‘turbine engine applications account for approximately 90 percent of the market for superalloys’, and it is probable that 100% of applications are in gas turbine engines.

Processing

PM superalloys were initially used as near net shapes in the as-HIP and heat treated condition using coarse powder as the starting material. Although there has been alloy development over the past twenty years, two process changes represent more significant advances in the PM superalloy technology; a shift initiated by Pratt & Whitney to ‘gatorising’ or isothermal forging, and the use of finer powder. All PM superalloys today use powder with a maximum particle size no greater than 106 microns, and in many cases with a maximum size as small as 44 microns. These two changes were performance driven - start with smaller non-metallic defects (finer powder) whose size distribution is tight, or make those defects that are present less vulnerable to fracture initiation and growth (isothermal forging). Atomisation improvements have enabled the powder manufacturer to improve the yield of fine powder with little or no sacrifice in cost, and the low cycle fatigue properties of isothermally forged products are superior to as-HIP product. The fine-grained structure in extruded billets for isothermal forging enables them to be inspected using high sensitivity ultrasonic testing to detect what small defects may be present. Although the vast majority of PM parts in today's engines are isothermally forged, as-HIP parts are still being used especially in those applications where creep strength is the sole design criterion. A third change that appears more frequently in current specifications is a switch to supersolvus heat treatment to increased grain size and damage tolerance. The PM superalloys now in service are all nickel base and gamma prime hardened, table 1. The volume percent of gamma prime varies up to 64% in MERL 76, but lower percentages in the two most recent alloys, N-18 and Rene 88DT suggesting that the optimum may have been exceeded.

Table 1. Compositions of some PM superalloys

Alloy


Element

Rene
95

Rene
88 DT

Astroloy

MERL
76

IN-100

N-18

C

Cr

Co

Mo

W

Ti

Al

B

Zr

Other

0.06

13

7

3.5

3.5

2.5

3.5

0.007

0.005

Nb 3.5

0.03

16

13

4.0

4.0

3.7

2.0

0.015

0.03

Nb 0.7

0.02

14.5

16.5

5.0

-

3.5

4.0

0.03

0.06

-

0.02

12.4

18.5

3.2

-

5.0

4.3

0.02

0.05

Nb 1.65

0.07

12.5

18.5

3.2

-

4.4

5.0

0.02

0.06

V 0.8

0.02

11.5

15.7

6.5

-

4.35

4.35

0.02

0.03

Hf 0.5

The engine manufacturers using components derived from PM technology include Pratt and Whitney, General Electric, MTU, SNECMA, International Aerospace, CFM International, IHI and Allied Signal. PM parts range from a low of about 1.4kg (3.1bs) for cooling plates on the T 700 to a high of 640kg (14001bs) for the compressor disk on the GE 90.

Most of the PM superalloy parts in engines were chosen because of the performance that each one could deliver. While this used to be the sole criterion for alloy selection, the cost factor is now a major factor with many manufacturers.

Alloy development

Alloy development also continues, especially in conjunction with NASA's program on the Mach 2.4 High Speed Civil Transport, and the IHPTET Program where higher temperature capabilities as well as longer times at T3max, are sought. Because two of the immediate goals of IHPTET are an 80% increase in power to weight ratio and a 30% reduction in specific fuel consumption, the use of titanium aluminides is virtually mandated. Interest in alpha-2 alloys has waned because of their temperature limitations, but work continues on monolithic gamma as well as gamma metal matrix composites.

A Ti-48Al-2Nb-2Cr centrifugal compressor diffuser has already been made as a replacement for IN718 with a 45% weight saving. This success has led to incorporating the gamma TiAl diffuser into a future Joint Technology Advanced Gas Generator (JTAGG). Orthorhombic aluminide may prove to be the biggest player in this field.

The specific strength advantages of these materials may push some superalloys out of compressor components, and their high temperature properties may have similar results in other engine parts. Aluminide production applications, which will cut superalloy usage, are still a short way off and changes involving cost reduction and alloy development may affect the scenario for PM superalloy usage quite quickly.

The reasons for alloy development are not new. The push to increase the operating speed and temperature of turbine disks at both the rim and the bore continues. There is work going on independently at Pratt & Whitney, General Electric, Rolls Royce, and SNECMA to develop a new disk alloy or an improved process that will allow rim temperatures to be between 700 and 750°C.

Criteria for development at these temperatures continue to be crack growth tolerance, low cycle fatigue, creep resistance, creep fatigue, and a resistance to environmental attack. Bore temperatures of the turbine disk will also increase, and the major criterion there will be elevated temperature strength. Three of the companies involved explicitly state that any new alloy will be powder. New alloys are also likely to be revisions of existing alloys for these incremental improvements. GE holds patents which modify compositions of existing alloys using tantalum as a major alloying element, and in some cases substitute it completely for tungsten.

Processing

Process changes to improve performance of existing or modified alloys continue. Using finer powder satisfies the demand for improved reliability in civil aircraft. Supersolvus heat treatment to increase grain size has proved a reliable means to improve fatigue life and in the case of N-18 has resulted in only a 5% decrease in tensile strength.

With a single alloy for the disk, selective heat treatment is a process that must be considered. The result is a part with varying grain size from the rim to the bore and a gradation of properties within the same material. The use by Pratt & Whitney of IN-100 `integrally bladed rotors' in the 6th, 7th, 8th, and 9th compressor stages of the F-119 engine is a very specific example where properties are controlled by the process. The first flight of this engine which will power the F-22 advanced fighter was scheduled for early 1996. Compressor blades do not see the high temperatures of those in the turbine section, so this process may be limited to the compressor section. Use of integrally bladed rotors means a thinner, lower weight rim, but this process will probably be limited to military engines since commercial operators would probably prefer to replace a single blade than a whole disk.

Another processing route for higher operating temperatures is the dual alloy approach in which the blade and ring assembly is bonded to the hub of a disk. This enables the use of the cast blade alloy in the hottest section of the disk where creep could cause failure. Depending on temperature requirements, the hub could be a PM alloy. There may also be work being done on the concept of a triple alloy, comprising one for the blades, a second for the rim, and the third for the hub.

Requirements for the NASA High Speed Civil Transport originally focused on the combustor and the exhaust nozzle, but now include air foils which will be cast, the fan container disk and the turbine disk. It is the latter that poses the biggest challenge. Although the rim temperature will be approximately 700°C, it will stay hot for 2 to 3 hours to present engines maximum temperature for only a few minutes during takeoff.

It has also been reported that there will be no temperature gradient between the rim and hub, so it is likely that process innovations and new alloys will be the solution. Because the maximum temperature is only 700°C, this disk will stay nickel based and will be a PM superalloy. NASA's objective is to have all the technology available by 2003 so that a commercial decision can be made to have a plane built by 2005.

Cost reduction

There are three cost improvement processes which may have an early effect on PM superalloy usage. Two of these are very similar. These are atomisation deposition (Spraycast X), and electroslag remelting as a source for spray forming. The third is the replacement of VIM-VAR superalloys with PM as-HIP superalloys.

Spraycast X, developed by Howmet for Pratt & Whitney, is a lower cost method to producing superalloy rings. It provides a direct one step conversion of vacuum melted superalloy to semifinished rings. The production of rings from billet material is compared with that of material from Spraycast. Economically, it appears advantageous, but grain size control presents challenges. At present, P&W is not considering this process as a replacement for IN-100. Its purpose is to replace cast and wrought alloys as a cost reduction.

General Electric envisions its patented process, using the ESR furnace in conjunction with the cold induction guiding nozzle, as a means to manufacture preforms which will be isothermally forged to replace some of its PM superalloy hardware. Whether it can produce the quality necessary to replace parts currently made from powder remains to be seen. Because the product that solidifies on the preform is approximately half liquid and half solid, the necessary grain size control may prove to be a very formidable problem in the development work and could eventually limit the use of this process.

A third cost reduction process is one that Allied Signal used when they first used PM Astroloy in the disks on their APUs. The cost analysis, in which the final component cost was the raw material cost plus fabrication, showed that the uniform fine grained PM product machined much better than wrought Astroloy, and lowered the total cost. The company's experience has been very good over a ten year time period. In response to the question `Why use as-HIP?', the reply was `It is better to know your worst defect and be prepared to deal with it. Know the size of the ceramic defect and deal with this from a design standpoint using fracture mechanics to get defect tolerance. There is then no need to worry about unforeseen forging defects.'

Another very large user of as-HIP PM superalloys is the Russian aerospace industry, all of whose PM superalloys are as-HIP They have fifteen years experience with this product and are happy with as-HIP hardware. It is possible that these experiences may re-open the evaluation of as-HIP hardware as a cost reduction.

Another cost reduction potential, the use of nitrogen instead of argon as the inert gas during atomisation, will probably not succeed. There is a long history of success with argon, and perhaps the only way that nitrogen will be used is with spray deposition to ensure that there is no porosity in the product.

PM Growth

PM superalloys, which were termed `a troubled adolescent' have now fully matured. Growth in the near future will depend on the success of the Boeing 777, since there are significant amounts of powder in both the P&W and GE engines for this aircraft. The GE 90 alone has six PM parts with a total weight of 2100kg (47001bs).

Two other areas for growth are the market for lower thrust engines, where demand for increased efficiencies may bring PM superalloys into play, and land based turbines for power generation. This industry may borrow the technology from the aerospace field.

Key Properties

Compared to conventionally produced superalloys, PM superalloys provide:

        Improved strength

        Improved creep resistance

        Improved creep fatigue

        Better low cycle fatigue properties

Primary author: Dr. William Eisen

Source: Materials World, vol. 4, pp. 22-24, 1996

For more information on Materials World please visit The Institute of Materials

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