The following article is a component of the April 1999 (vol. 51, no. 4) JOM and is presented as JOM-e. Such articles appear exclusively on the web and do not have print equivalents.

The Production-Scale Spray Forming of Superalloys for Aerospace Applications

CONTENTS
INTRODUCTION OSPREY PROCESS THE SPRAYCAST-X PROCESS PROCESS ADVANTAGES Improved Workability Machinability Rapid Prototyping

INTRODUCTION

Globally, superalloy structural rings and cases for aircraft gas turbines represent a $200 million annual market. The Spraycast-X process, a modification of the Osprey Metals Ltd. spray-forming process (described in the sidebar), is licensed by Sprayform Technologies International, a joint venture company developed by Howmet and Pratt & Whitney on April 2, 1997, to develop the Spraycast-X technology to serve this market. The construction of a production facility adjacent to Howmet's hot isostatic pressing (HIPing) capability has been completed. Currently, developmental and initial production ring and case application articles are underway for Pratt & Whitney, Rolls Royce, Rolls Royce-Allison, Solar, and AlliedSignal.

Sprayform Technologies International L.L.C. has two sprayform units, one used for development applications and the other for production. The development unit has a melt capacity of 454 kg and a maximum size capacity of 76 cm diameter. The production unit, which is expected to go on-line in mid-1999, has a melt capacity of 2.7 tonnes and a maximum size capacity of 152 cm diameter (Figure 1). Local sources for HIPing, heat treatment, testing, and machining requirements have been established. Business plans anticipate spraying initial production quantities of 226 tonnes in 1999 and ramping up to 1,814 tonnes annually within the next three years.

OSPREY PROCESS

The Osprey process is a technique for the one-step conversion of molten alloy into a dense, fine-grain, homogeneous product with 0.2-2.0% porosity. The alloy is melted in an induction furnace under argon and atomized with high-velocity nitrogen gas. While this is similar to the argon gas atomization technique for making superalloy powder, the molten droplets are not allowed to solidify in the Osprey process; they are collected on a substrate while still partially molten. The advantage of the Osprey process over conventional powder-metallurgy (P/M) processes is that many of the steps between atomization and consolidation are eliminated, thus reducing not only cost, but also the opportunity for inadvertently contaminating the powder. The shaped products are suitable for direct HIPing densification or subsequent thermomechanical processing. This offers an alternative to materials that cannot be forged as conventionally cast ingot.

THE SPRAYCAST-X PROCESS

The as-sprayed material is extremely dense and homogeneous and exhibits a uniform grain size predominantly dictated by the alloy composition. Although the as-sprayed density of the deposited material is approximately 98%, all remaining porosity must be closed. Consequently, HIPing is an integral step in the process.

To begin, the alloy is vacuum-induction melted (VIM) in a ceramic crucible, delivered to a tundish, and metered from the tundish through a controlled orifice or nozzle (Figure 2). This metered alloy stream is then passed through the two-stage atomizer array (from Osprey Metals Ltd.), within which the stream is broken up into very fine droplets by high-purity argon gas impingement. The resulting spray is then deposited onto a preheated carbon steel mandrel, which rotates under the spray. The shape and thickness of the deposit are controlled by the withdrawal of this mandrel from under the spray plume. The completed superalloy part may then be HIPed only, HIPed and ring rolled, or HIPed and forged.

PROCESS ADVANTAGES

Improved Workability

The spray-forming technology provides improved hot workability because of improved homogenization of alloy chemistry and a reduction of the grain size inherent in gas-atomization processing. The improvement is especially significant when compared to products of VIM-vacuum arc remelting or VIM-electroslag remelting methods.

Figure 1. Details of Sprayform Technologies International LLC's production unit, which will be operational in mid-1999.	Figure 2. The Spraycast-X process. The alloy is first vacuum-melted, then atomized by a stream of high-purity argon gas. In the next step, the molten droplets are sprayed onto a mandrel, converting the superalloy into a semifinished ring shape.

The tensile strength of Spraycast-X + HIPed, Spraycast-X + ring-rolled, and Spraycast-X + HIPing + ring-rolled products made of IN718 and Waspaloy is comparable to conventional wrought material at room temperature and 650°C. Tests show that the ductility of these Waspaloy products is equal to or greater than the wrought counterpart; the ductility of the Spraycast-X IN718 is lower than the wrought counterpart at both temperatures.

Spraycast-X processed material allows some flexibility in determining the post-spray and HIP processing approach. For static components, the spray + HIP preform allows direct component manufacture. For rotating rings, which require hot working, the spray + HIP product may be directly ring rolled or forged as needed.

Qualitative and quantitative information from ring-roll sources indicates that the Spraycast-X spray + HIP preforms are easier to ring roll than cast and wrought counterparts of the same alloy. This workability is evident not only for IN718 alloy, but also for the less hot-workable alloy systems, including Waspaloy and U720.

The increased hot workability derives in part from improved flow stress, which is a result of the higher homogeneity and uniformity of microstructures. These attributes also promote the machinability of typical aerospace alloys, without the need for any chemistry or heat-treatment modifications.

Machinability

An examination of the cast and wrought vs. Spraycast-X U720 microstructures shows that large primary carbides remain in the cast and wrought material, but are not evident in the Spraycast-X form.

Rapid Prototyping

The carbon steel mandrel, which is the typical tooling for making components by the Spraycast-X process, is either spin formed (for production runs) or hand-fabricated and welded (for first article or development parts). In either case, this mandrel is inexpensive compared to the cost of the overall component.

Turnaround time of the Spraycast-X process is often only a few days. For example, two exhaust cases were completed in six days.

ABOUT THE AUTHOR

Gregory A. Butzer is general manager of Sprayform Technologies International.